WO2001031757A1 - Line-narrowing module for high power laser - Google Patents

Line-narrowing module for high power laser Download PDF

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Publication number
WO2001031757A1
WO2001031757A1 PCT/IB2000/001722 IB0001722W WO0131757A1 WO 2001031757 A1 WO2001031757 A1 WO 2001031757A1 IB 0001722 W IB0001722 W IB 0001722W WO 0131757 A1 WO0131757 A1 WO 0131757A1
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Prior art keywords
laser
less
line
etalon
narrowing
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PCT/IB2000/001722
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French (fr)
Inventor
Uwe Stamm
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Lambda Physik Ag
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Filing date
Publication date
Priority claimed from US09/599,130 external-priority patent/US6381256B1/en
Priority claimed from US09/657,396 external-priority patent/US6426966B1/en
Application filed by Lambda Physik Ag filed Critical Lambda Physik Ag
Priority to JP2001533604A priority Critical patent/JP2003513437A/en
Publication of WO2001031757A1 publication Critical patent/WO2001031757A1/en

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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/22Gases
    • H01S3/223Gases the active gas being polyatomic, i.e. containing two or more atoms
    • H01S3/225Gases the active gas being polyatomic, i.e. containing two or more atoms comprising an excimer or exciplex
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08Construction or shape of optical resonators or components thereof
    • H01S3/08004Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/02Constructional details
    • H01S3/03Constructional details of gas laser discharge tubes
    • H01S3/0315Waveguide lasers
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08Construction or shape of optical resonators or components thereof
    • H01S3/08004Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection
    • H01S3/08009Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection using a diffraction grating
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08Construction or shape of optical resonators or components thereof
    • H01S3/08018Mode suppression
    • H01S3/08022Longitudinal modes
    • H01S3/08031Single-mode emission
    • H01S3/08036Single-mode emission using intracavity dispersive, polarising or birefringent elements
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/136Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling devices placed within the cavity
    • H01S3/137Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling devices placed within the cavity for stabilising of frequency
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/22Gases
    • H01S3/223Gases the active gas being polyatomic, i.e. containing two or more atoms
    • H01S3/225Gases the active gas being polyatomic, i.e. containing two or more atoms comprising an excimer or exciplex
    • H01S3/2258F2, i.e. molecular fluoride is comprised for lasing around 157 nm

Definitions

  • TITLE Line-Narrowing Module for High Power Laser
  • the invention relates to a narrow band laser, and particularly to a high power excimer or molecular fluorine laser operating at a high repetition rate (e.g., 1 -4 kHz or more).
  • a high repetition rate e.g. 1 -4 kHz or more.
  • VUV deep ultraviolet
  • the laser system exhibit greater absolute wavelength stability. For example, a laser output beam wavelength stability around or below 0.1 pm is desired.
  • a narrow band 248 nm lithography laser system operating at low power, e.g., less than 1 kHz, and having a bandwidth around 1 pm or more, includes within its resonator a line-narrowing module, such as that shown by example at Fig. 1 (see also, U.S. patent no. 5, 1 50,370, which is hereby incorporated by reference).
  • the line-narrowing module shown includes a prism beam expander including three prisms 30, 30 and 36, an etalon 34 and a grating 38.
  • Each of the prisms 30, 32 and 36, and the plates of the etalon 34 of a line-narrowing module of a conventional KrF laser comprises fused silica due to its transparency at 248 nm.
  • the wavelength stability is, however, substantially above 0.1 pm when the laser is operated at 1 kHz or higher power.
  • Line-narrowing modules vary in their response to the exposure to high power or high repetition rate laser beams that cause heating and aging of the optical components.
  • nonuniform heating of the optical elements of the line-narrowing module may substantially degrade their quality by, for example, disrupting the planarity of optical surfaces by localized expansion and causing fluctuations in the thermally dependent refractive index, thereby distorting the wavefront of the retroreflected or transmitted beam and detuning the wavelength.
  • Wavefront distortions lead to changes in the output bandwidth of the laser system which is a parameter that it is desired to keep constant.
  • Wavelength detuning may be compensated by rotating an optical element, typically the grating or HR mirror, as mentioned above.
  • absorption by optical components results in reduced overall efficiency of the laser.
  • Parameters of the line-narrowing module that depend on the "quality" of the optical components such as the magnitude of angular dispersion, reflectivity for specific wavelengths, linearity (i.e., absence of wavefront distortions), scattering of the beam, etc., will thus affect the bandwidth, linewidth and overall performance of the laser.
  • the quality of the optical components is generally tested by measuring the absorption of DUV (or VUV for F 2 lasers) radiation that leads to heating of the optical components and thermally induced distortions and defects.
  • a conventional " low power" laser might operate at a repetition rate around 600 Hz. It is desired to have a laser operating above 1 kHz, and particularly around 2-4 kHz, or more. For 1 93 nm lasers, a repetition rate around 4 kHz will be used. To achieve sufficient throughput, the expectation for the molecular fluorine laser is to have a repetition rate greater than 4 kHz. At these repetition rates, greater laser power is incident on surfaces of optical components within the laser resonator that can serve to enhance the effects of thermally induced distortions or defects on the bandwidth of the laser.
  • Such high power narrowband laser exhibit a wavelength stability of 0.1 pm or less.
  • a laser including a gain medium surrounded by a resonator and including a line-narrowing module including a prism beam expander and one or more etalons and/or a grating or grism within the resonator.
  • the material of transmissive portions of the line- narrowing module including the prisms and the plates of the etalons comprises a material having an absorption coefficient of less than 5 x 1 0 3 /cm at 248 nm incident radiation, less than 1 0 x 1 0 3 /cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm.
  • the material also has a thermal conductivity greater than 2.0 W/m°C.
  • that material is calcium fluoride (CaF 2 ).
  • Fig. 1 schematically illustrates a line-narrowing module for a KrF excimer laser.
  • Figs. 2a-2b illustrate how the gap spacing of an etalon changes as the etalon is heated due to absorption.
  • Figs. 2c-2d illustrate how localized heating can distort the planarity of a prism surface.
  • Figs. 2e-2f illustrate how absorption of laser radiation can distort the surface contour of a grating.
  • Figs. 3a-3e schematically illustrate further line-narrowing module configurations, in addition to that shown at Fig. 1 , according to preferred embodiments.
  • Fig. 4 schematically illustrates a line-narrowing resonator in accord with a further preferred embodiment.
  • Fig. 5 schematically shows an excimer or molecular fluorine laser system according to a preferred embodiment.
  • transmissive line-narrowing components of a conventional line- narrowing module including fused silica plates of conventional etalons, fused silica beam expanding or dispersive prisms and fused silica layers of diffraction gratings will absorb radiation at an undesirably high rate, causing these materials to heat up too much.
  • a line-narrowing module in shown particularly configured for use with a KrF laser.
  • the line narrowing module includes a first beam expanding prism 30 and a second beam expanding prism 32, an etalon 34, and a third beam expanding prism 36 before a grating 38.
  • the grating may be substituted by a highly reflective (HR) mirror. More or fewer than three prisms may be included, and two or more etalons or no etalon may be included.
  • a dispersive prism or a grism may also be included (see U.S. patent applications no. 09/602, 1 84 and 60/1 70,91 9, which are each assigned to the same assignee as the present application and are hereby incorporated by reference).
  • Each of the prisms 30, 32 and 36 serves to expand the beam, such that by the time the beam reaches the etalon 34 (prisms 30 and 32) and grating 38, the beam is expanded to encompass a larger portion of the etalon 34 and grating 38.
  • the prisms 30, 32 and 36 expand the beam owing to their refractive nature.
  • the index of refraction of each prism 30, 32 and 36 is in turn dependent on the temperature of the prism, particularly at the area on the surface of each prism 30, 32 and 36 that is impinged upon by the beam.
  • n gap is the temperature dependent index of refraction in the gap (not to be confused with n e (T) and n p (T) which are the indices of refraction of the material of the etalon plates and the prisms, respectively), and ⁇ is the angle the ray makes with the planes of the etalon plates.
  • fused silica may not be used because the absorption coefficient of fused silica is about 5.3 x 1 0 3 /cm for incident 248 nm radiation, and for 1 93 nm incident radiation, the absorption coefficient of fused silica is between 20- to-30 x
  • the absorption coefficient of the material of the prisms 30, 32 and 36 in accord with the preferred embodiment is less than 5 x 1 0 "3 /cm at 248 nm incident radiation, less than 1 0 x 1 0 "3 /cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm.
  • the preferred material meeting these specifications is calcium fluoride.
  • Alternative materials may include BaF 2 , LiF, SrF 2 , fluorine-doped quartz and MgF 2 , although MgF 2 is birefringent; generally a property of a material that is not preferred.
  • either or both of the etalon 34 and the grating 38 may be rotated to compensate the wavelength changes just described (see U.S. patent application no. 60/1 78,440, which is assigned to the same assignee as the present application and is hereby incorporated by reference).
  • One or more of the prisms may also be rotated (see U.S. patent application no. 09/244,554, which is assigned to the same assignee as the present application and is hereby incorporated by reference).
  • Figs. 2a-2f illustrate further changes that may be induced by the absorption of the higher power beam by the etalon 34, grating 38 and prisms 30, 32 and 36.
  • Fig. 2a shows an etalon 34 such as may be used in the line-narrowing module as shown in Fig. 1 or Figs. 3c-3e below, or in the line-narrowing resonator shown at Fig. 4, below.
  • Fig. 2a shows an etalon before any thermally induced changes are incurred.
  • the etalon 34 comprises two parallel plates 40 separated by a gap d e .
  • the etalon 34 has two spacers 42 typically comprising zerodurTM, quartz, invarTM or Cordierite (see U.S. patent application no. 60/1 58,808, which is assigned to the same assignee as the present application and is hereby incorporated by reference).
  • Fig. 2b illustrates what happens to the etalon 34 when absorption occurs as a high power beam passes through the etalon 34.
  • the distortion is most pronounced where the absorption of the incident beam occurs which is the critical location in the gap determining the selected wavelength as understood from equation ( 1 ).
  • This surface distortion serves to broaden the beam because the plates are less planar than those shown in Fig. 2b and a greater range of wavelengths are selected due to the spatial dependence of the gap spacing d e '.
  • the center wavelength in the interference spectrum as understood from equation ( 1 ) is also changed.
  • the etalon 34 For the etalon 34, the greater expansion at the surface due to surface heating is followed by greater contraction of the plates during pauses between bursts when the laser is operated in burst mode. This phenomenon which may occur when an etalon 34 is in a high power laser resonator may be referred to as "breathing" . Since the output emission wavelength of the narrow bandwidth laser system is determined by the gap spacing d e of the etalon 34, the result is reduced wavelength stability. Therefore, the etalon 34 of the preferred embodiment has plates 40 comprising a material with a reduced absorption coefficient.
  • the thermal conductivity of the material of the plates of the etalon 34 will effect its performance, particularly at high repetition rates. That is, incident laser radiation is absorbed in a localized region of the etalon plates 40 based on the size of the incident beam. It is this localized region that expands as it heats up effecting the etalon gap spacing and the output emission wavelength of the laser system. The absorbed energy will thermally diffuse away as phonons from this localized region at a rate that depends on the thermal conductivity of the material of the etalon plates 40. A material having a higher thermal conductivity would allow the heat to propagate to less critical portions of the etalon 34 away from the surfaces of the plates 40 upon which the high power beam impinges.
  • the etalon plates 40 comprise a material having a greater thermal conductivity, then the wavelength stability can be further improved.
  • the absorption coefficient of fused silica is about 5.3 x 1 0 3 /cm for incident 248 nm radiation. For 1 93 nm incident radiation, the absorption coefficient of fused silica is between 20-to-30 x 1 0 3 /cm. As described above, these absorption coefficients are larger than would meet the objects of the invention as they provide greater distortion than is tolerable when a high power laser beam is generated for microlithographic applications.
  • the absorption coefficient of the material of the plates of the etalon 34 in accord with the preferred embodiment is less than 5 x 1 0 3 /cm at 248 nm incident radiation, less than 1 0 x 1 0 3 /cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm.
  • the preferred material meeting these specifications is calcium fluoride (CaF 2 ).
  • Alternative materials may include BaF 2 , LiF, SrF 2 , fluorine-doped quartz and MgF 2 , although MgF 2 is birefringent; generally a property of a material that is not preferred, as mentioned above.
  • the thermal conductivity of fused silica at 248 nm and 1 93 nm incident radiation is around 1 .38 W/m°C.
  • This thermal conductivity is too low such that energy of the beam absorbed and converted to heat cannot travel quickly enough away to less critical areas of the plates 40 of the etalon 34.
  • the material has a thermal conductivity greater than 2.0 W/m°C, and more preferably greater than 5.0 W/m°C.
  • the etalon plates 40 preferably comprise calcium fluoride CaF 2 to improve the thermal conductivity of the etalon 34 and reduce distortions at the critical location of the etalon plates 40 wherein the beam is incident.
  • Fig. 2c shows a beam expanding prism such as the prisms 30, 32 and 36 of Fig. 1 .
  • the discussion below is also relevant to a line- narrowing module including one or more dispersive prisms (see, e.g., U.S. patent no. 5,761 ,236, which is hereby incorporated by reference).
  • the prism shown at Fig. 2c has an apex angle ⁇ and smooth flat surfaces.
  • Fig. 2d shows how absorption of a high power laser beam causes the planarity of the surface of the prism 30, 32, or 36, or a dispersive prism, to distort.
  • the distortion of the surface is caused by absorption of the energy of an incident high power beam such as an excimer or molecular fluorine laser operation at 1 -4 kHz or more.
  • the distortion causes an undesirable broadening of the bandwidth and geometry of the beam, in addition to the refractive angle changes described above based on the temperature dependent index of refraction n p (T).
  • Directions of normals to the surface of the prism shown at Fig. 2d vary across the surface of the prism, particularly around the critical portion of the surface where the high power beam is incident.
  • angles of rays travelling within the prism thus vary depending on where those rays strike the distorted surface. This causes increased beam divergence, which is a parameter that the beam expanding prisms 30, 32 and 36 are inserted into the line-narrowing module to reduce.
  • beam divergence which is a parameter that the beam expanding prisms 30, 32 and 36 are inserted into the line-narrowing module to reduce.
  • rays of different wavelengths will cross over each other allowing some wavelengths that would ordinarily be outside the acceptance angle of the resonator to re-enter and broaden the overall bandwidth.
  • a material is selected that has a lower coefficient of absorption, such as less than 5 x 1 0 3 /cm at 248 nm incident radiation, less than 1 0 x 1 0 "3 /cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm.
  • the material preferably has a thermal conductivity greater than 2.0 W/m°C, and more preferably greater than 5.0 W/m°C, so that the heat is carried away from the critical region of the surfaces of the prisms more quickly.
  • the preferred material meeting these specifications is calcium fluoride (CaF 2 ).
  • Alternative materials may include BaF 2 , LiF, SrF 2 , fluorine-doped quartz and MgF 2 , although MgF 2 is birefringent; generally a property of a material that is not preferred, as mentioned above.
  • Figs. 2f and 2g illustrate how a thermally induced curvature of the grating may also be effected by an incident high power laser beam.
  • the grating of Fig. 2f is shown prior to being hit by a high power laser beam.
  • the preferred grating has a blazed structure such as for a typical eschelle grating, or more preferably, such as the grating shown and described in either or both of U.S. patent applications no. 60/1 70,342 and 60/1 67,835, each of which is assigned to the same assignee and is hereby incorporated by reference (see also U.S. patent no. 5,999,31 8, which is hereby incorporated by reference).
  • Undesirable effects on the wavelength may be caused if thermally induced distortions of the surface contour of the grating such as shown in Fig. 2f are caused by an incident high power beam.
  • a preferred grating is generally described in the '835 application, wherein the blazed rulings of the preferred grating are etched directly into the substrate of the grating, rather than being formed in a separate layer and then adhered to onto the substrate.
  • Figs. 3a-3e show some alternative configurations of line-narrowing modules that would benefit by incorporating one or more etalons 34, 35 and/or prisms 30, 32 and/or a grating 38 according the preferred embodiments described above.
  • Fig. 3a schematically shows a line- narrowing module for use with a line-narrowed laser system according to a preferred embodiment including a prism beam expander 30, 32 and a diffraction grating 38 arranged in a Littrow configuration.
  • the beam expander 30, 32 may include the two prisms shown, or may includes only one prism or more than two prisms.
  • a line-narrowing module for an ArF laser preferably includes four prisms.
  • the beam expander 30, 32 serves to reduce divergence of the beam incident onto the grating 38, thus improving wavelength resolution of the wavelength selector.
  • the grating 38 is preferably as described above, or is a high blaze angle echelle grating such as is described at the 60/1 70,342 application, incorporated by reference above.
  • An aperture (not shown) may be included preferably before the beam expander prism 30 to reject stray light and reduce broadband background, and can also serve to reduce the linewidth of the beam by lowering the acceptance angle of the resonator. Exemplary apertures are set forth at U.S. patent no. 5, 1 61 ,238, which is assigned to the same assignee and is hereby incorporated by reference (see also the 09/1 30,277 application incorporated by reference above) .
  • the system of Fig. 3b includes the beam expander 30, 32 described above with respect to Fig. 3a, and also includes a diffraction grating 39 and a highly reflective mirror 41 .
  • the grating 39 preferably differs from the grating 38 of Fig. 3a either in its orientation with respect to the beam, or its configuration such as its blaze angle, etc., or both.
  • the laser beam is incident onto the grating 39 at an angle closer to 90° than for the grating 38.
  • the incidence angle is, in fact, preferably very close to 90°. This arrangement is referred to as the Littman configuration.
  • the Littman configuration serves to increase the wavelength dispersion of the grating 39.
  • the positions of the grating 39 and mirror 41 may be reversed, as well, such as is described at U.S. patent no. 5,970,082, which is hereby incorporated by reference.
  • the diffracted beam After passing through (for a transmission grating, not specifically shown) or reflecting from the diffraction grating 39, the diffracted beam is reflected by the highly reflective mirror 41 .
  • the tuning of the wavelength is preferably achieved by tilting the highly reflective mirror 41 . Tuning may be achieved otherwise by rotating another optic or by pressure tuning one or more optics, or otherwise as described below, or as may be understood by one skilled in the art.
  • Fig. 3c schematically shows another embodiment including beam expanding prisms 30, 32 and Littrow diffraction grating 38, preferably as described above.
  • the system of Fig. 3c includes one or more etalons 34, 35, e.g, two etalons are shown, which provide high-resolution line narrowing, while the grating 38 serves to select a single interference order of the etalons 34, 35.
  • the etalon or etalons 34, 35 may be placed in various positions in the resonator, i.e., other than as shown.
  • a prism or prisms of the beam expander 30 may be positioned between an etalon or etalons 34, 35 and the grating 38.
  • An etalon may also be used as an output coupler, as will be described in more detail below with reference to Fig. 4a.
  • the arrangement of Fig. 3c (as well as Figs. 3d and 3e below) including an etalon or etalons 34, 35 may be varied as described at any of U.S. patent applications no. 60/1 62,735, 60/1 78,445, 60/21 2, 1 83, 09/657,396 or 60/1 58,808, each of which is assigned to the same assignee and is hereby incorporated by reference.
  • Fig. 3d shows another embodiment of the laser system having one or more etalons 34, 35, e.g., two etalons are shown.
  • the system of Fig. 3d is the same as that of Fig. 3c except that the grating 38 of Fig. 3c is replaced with a highly reflective mirror 43 in the embodiment of Fig. 3d, and the etalons 34, 35 are differently configured owing to the omission of the grating 38 which is not available, as in the system of Fig. 3c, to select a single interference order of the etalons 34, 35.
  • the free spectral ranges of etalons 34, 35 are instead adjusted in such a way that one of the etalons 34 or 35, preferably the first etalon 34 after the beam expanding prism 32, selects a single order of the other etalon 35, e.g., the second etalon 35.
  • the second etalon 35 of the preferred arrangement is, therefore, allowed to have a smaller free spectral range and higher wavelength resolution.
  • Fig. 3e Fig. 1 1 shows a seventh preferred embodiment of a resonator arrangement for a molecular fluorine (F2) laser including a line- narrowing module 50 having a prism beam expander 46 and an etalon 48 for line selection and/or line narrowing.
  • the resonator arrangement of the seventh embodiment includes a discharge chamber 2 filled with a gas mixture including molecular fluorine and one or more buffer gases such as neon and/or helium.
  • a pair of main discharge electrodes 3a, 3b are in the discharge chamber and connected to a pulser circuit (not shown) for energizing the molecular fluorine.
  • One or more preionization units is also within the discharge chamber (not shown, but see the 60/1 62,845 application, mentioned above). Other aspects of the discharge chamber 2 may be found at the 09/453,670 application and otherwise as set forth above and/or as understood by those skilled in the art.
  • One window 1 40 is a piano-parallel optical window which serves as an outcoupler.
  • a partially reflective mirror may alternatively be used separate from the chamber window 1 40 for outcoupling the beam, and the window 1 40 serving the dual role of sealing the chamber 2 and outcoupling the beam reduces the number of optical surfaces within the resonator and thereby enhancing its efficiency.
  • the window 140 is preferably aligned perpendicular to the optical axis, but may be oriented at an angle such as 5° or at Brewster's angle or another selected angle for achieving desired reflectivity and transmissivity characteristics and/or to prevent parasitic oscillations.
  • Vacuum bellow 56 or similar arrangement provides versatility in degrees of freedom of adjustment and a vacuum-tight seal.
  • the other window 44 is preferably a plano-convex lens serving to compensate the wavefront curvature as described in U.S. patent no. 6,061 ,382 and European Patent Application EP 00955706 A1 , each of which is assigned to the same assignee and is hereby incorporated by reference. Tightness of the seal is ensured by seals 42.
  • the lens 44 may or may not be adjustable.
  • the lens 44 may be tilted slightly off the normal to the beam (e.g., 5° or so) in order to avoid parasitic oscillations.
  • this lens 44 is anti-reflectively coated.
  • the resonator of an F 2 -laser is shown at Fig. 3e including a preferred line-selection and/or line-narrowing unit coupled with a discharge chamber 2 via bellows 56 and seals 1 42.
  • the beam is preferably expanded by one or more beam expansion prisms 30, 32, preferably as described above, in order to reduce its divergency.
  • prisms 30, 32 and/or 36 may be used, e.g., four prisms are preferred for the ArF laser and Fig. 1 sets forth a preferred KrF laser arrangement, and alternatively, a converging diverging lens pair may be used or other beam expansion means known to those skilled in the art.
  • a transmissive etalon 34 is preferably positioned after the beam expander prisms 30, 32 and serves as a wavelength selector. Finally, highly reflecting mirror 43 returns the beam back through the resonator.
  • the line-narrowing unit is within an enclosure 50 that is coupled to the laser chamber 2 using a bellows 56 and seals 1 42 such that the line- narrowing unit of the laser resonator is sealed from the outer atmosphere.
  • An inert purge gas is flowed through the enclosure 50 via inert and outlet ports 52, 54.
  • a vacuum port may be included or a pump can be connected via either port 52 or 54.
  • a method for controlling the atmosphere in the enclosure may be to maintain the enclosure under evacuated conditions using a pump, or to flow the purge gas rapidly through the enclosure 50, or preferably to first evacuate the enclosure and then to flow the purge gas at a low flow rate.
  • An evacuation may be followed by a backfill of the purge gas and the procedure may be repeated a number of times, e.g., 3-1 0 times, before the purge gas is flowed at a low rate.
  • the entire beam path between the chamber 2 and the highly reflective mirror 43 is in fact preferably enclosed within the enclosure 50 which is purged with some inert gas such as preferably nitrogen, or helium, argon, krypton or the like. High purity nitrogen is preferably used for this purpose.
  • the pressure of the nitrogen in the enclosure 50 may be adjustable, and by changing the pressure of nitrogen, one can adjust the transmission spectrum of the etalon 34 (see, e.g., U.S. patent application 09/1 78,445, relating to pressure tuning a line-narrowing unit including a grating and beam expander, which is assigned to the same assignee and is hereby incorporated by reference).
  • the maximum of transmission of the etalon 34 is preferably adjusted to coincide with a maximum in the primary line at ⁇ i around 1 57.62 nm, among multiple lines around 1 57 nm including also a secondary line ⁇ 2 around 1 57.52 nm which is preferably suppressed, of the natural emission spectrum of the free-running F 2 laser.
  • the etalon 48 may be tuned piezo-electrically, rotationally and/or by adjusting the pressure within the gap between its plates.
  • One way of varying the pressure is to control intake and escape rates of the nitrogen purge gas shown at Fig. 3e using needle valves 52 and 54.
  • Closing output valve 54 and/or opening inlet valve 52 e.g., causes the pressure to rise, and visa versa.
  • Variations of the embodiment of Fig. 3e may include having an outcoupler as a separate optical element from the window 140, with the output window 140 of the chamber 2 being a plano-plano window. This provides an opportunity to mount resonator optics onto a mechanically stable structure, decoupled from the chamber 2. However, this increases optical losses and reduces the efficiency of the laser.
  • a second etalon 35, a grating 38 or 39 and/or one or more dispersive prisms may be included with the line-narrowing unit of Fig. 3e.
  • the etalon 34 may be replaced with a reflective etalon to serve a dual role as a line selection element and as a resonator reflector, such as a high finesse etalon.
  • the laser oscillator of Fig. 3e typically produces lower output power, as compared to a free-running F 2 laser resonator. Therefore, an amplifier may be used in this embodiment, as well as with any of the line- narrowing module embodiments described herein particularly for the F 2 laser, such as is described at U.S. patent application no. 09/599, 1 30, which is assigned to the same assignee as the present application and is hereby incorporated by reference.
  • Fig. 4a schematically shows an embodiment that is a variation of any of the arrangements described above with reference to Figs. 1 and 3a-3e, respectively.
  • the line-narrowing modules shown and described in those embodiments also have included for additional line-narrowing and/or line selection in laser resonators which include them in this embodiment, a reflective etalon outcoupler 46 which replaces the partially reflecting outcoupler mirror typically otherwise included in their resonators.
  • the etalon outcoupler 46 may be used in combination with the grating 38 or 39 and beam expander 30, 32 of Figs. 1 and 3a-3e, respectively, wherein the grating 38 or 39 selects a single interference order of the etalon outcoupler 46.
  • one or more dispersive prisms or another etalon may be used in combination with the etalon outcoupler 46 for selecting a single interference order of the etalon 46.
  • the grating 38 or 39 restricts the wavelength range to a single interference order of the outcoupler etalon 46.
  • the material used for the prisms of the beam expanders 30, 32, 36, etalons 34, 35, 46 and also for the laser windows 1 40, 240 is preferably one that is highly transparent at DUV and/or VUV emission wavelengths of the KrF, ArF and/or F 2 laser at 248 nm, 1 93 nm and 1 57 nm, respectively, with which the line-narrowing optics are being used.
  • the materials are also capable of withstanding long-term exposure to ultraviolet light with minimal degradation effects.
  • the material of the transmissive optical components of the line-narrowing module including the prisms and the plates of the etalons advantageously comprises a material having an absorption coefficient of less than 5 x 1 0 "3 /cm at 248 nm incident radiation, less than 1 0 x 1 0 3 /cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm.
  • the material also has a thermal conductivity greater than 2.0 W/m°C.
  • that material is calcium fluoride (CaF 2 ).
  • Figs. 2a-2f Other materials may include BaF 2 , LiF, MgF 2 , fluorine-doped quartz and SrF 2 . Also, in all of the above embodiments of Figs. 2a-2f, many optical surfaces, particularly those of the prisms, preferably have an anti-reflective coating on one or more optical surfaces, in order to minimize reflection losses and prolong their lifetime.
  • a line-narrowing module in accord with the preferred embodiment will have beam expanding prisms and/or one or more etalons as the only transmissive optical components; the grating 38, 39 and HR mirror 43, if included, being reflective components.
  • a transmission grating or a partially or fully transmissive grism may be included in the resonator.
  • Other transmissive optical components within the laser resonator and also subject to heat induced distortions, as described above, include the windows on the laser chamber and may include portions of the front optics module located on the other side of the discharge chamber as the line-narrowing module described above (one such component may include the etalon outcoupler 46 of Fig.
  • each of the transmissive or substantially transmissve components is comprised of a material having an absorption coefficient that is less than 5 x 1 0 3 /cm at 248 nm incident radiation, less than 1 0 x 1 0 "3 /cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm, and preferably also having a thermal conductivity greater than 2.0 W/m°C, and more preferably greater than 5.0 W/m°C.
  • substantially transmissive it is meant that a reflection grating would not be substantially transmissive.
  • a grism having a reflective coating on a surface for high reflection or partial reflection of the beam would be substantially transmissive.
  • the plates of an etalon output coupler 46 and the blocks 1 30, 1 36 described below would also represent substantially transmissive optics.
  • Figs. 4b-4c show resonator arrangements in accord with a first of two additional embodiments, particularly suited for the F 2 laser.
  • the resonator includes the discharge chamber 2 preferably configured as set forth above, e.g., having a pair of elongated main electrodes 3 and a preionization unit (not shown), and being connected to a pulser circuit 4 and gas supply system 6 (see below) .
  • the resonator of Fig. 4b further includes a rear optics module 1 50 including a line-selection package such as has been described above.
  • a line-selection package that does not considerably extend the resonator length, such as by using too many prisms in the beam expander, or simply by using the prism-grating combination, or a combination having more than one dispersive prism, whereby the rear optics module becomes a large extension of the overall resonator package, may be used in the embodiments of Figs. 4b-4c.
  • An alternative to the rear optics module 1 50 is to provide the line-selection component or components in contact with or in a housing connected to the discharge chamber 2 or laser tube atmosphere (see U.S. patent application no. 09/31 7,695, which is assigned to the same assignee and is hereby incorporated by reference).
  • any of these alternative embodiments of the line-selection package at the opposite side of the resonator as the outcoupler may be advantageously combined with the output coupler 1 30 of Fig. 4b (or the output coupler 1 36 of Fig. 4c, below). That is, the line-selection package of the rear optics module 1 50 (or as set forth in the '695 application), improves the contrast ratio to perhaps 20-1 00, which is advantageous over the characteristic contrast ratio of 7. However, the output coupler 1 30 provides a still higher contrast ratio, e.g., above 100 and perhaps as high as 500 or above.
  • Line narrowing optics may be included within the resonator on the outcoupling side of the chamber 2, such as in the front optics module as set forth in U.S. patent applications no. 60/1 66,967, 60/1 66,277 and 60/1 73,993, each of which is assigned to the same assignee, and in U.S. patent no. 5,852,627, each of which is hereby incorporated by reference.
  • careful additional beam steering means are used for aligning the optics with the rest of the resonator. This beam steering implies losses, and so it is preferred to use an amplifier after the outcoupler in this embodiment to increase the power of the beam (see the 60/140,531 application, incorporated by reference above).
  • the outcoupler of the embodiment of Fig. 4b generally has a periodic transmission function having a maximum at or near ⁇ , (see above for the F 2 laser) and a minimum at or near ⁇ 2 .
  • the preferred outcoupler and those shown at Figs. 4b and 4c, i.e., outcouplers 1 30 and 1 36, has a partially reflecting surface 31 which serves as a resonator reflector of the resonator.
  • the partially reflecting resonator reflector surface is formed after the line-selection providing portion such as at the back surface of the outcoupler 1 30 or 1 36, or as provided by an etalon outcoupler 46 (see Fig.
  • the outcoupler 1 30 is preferably made of a block of VUV transparent material 1 32 such as CaF 2 or one of those other preferred materials mentioned above with advantageous absorption and thermal conductivity characteristics.
  • the material block 1 32 has a special coating 1 34 formed on it.
  • the coating 1 34 is preferably highly compounded with the block 1 32.
  • a preferred thickness of the coating 1 32 between 30 to 50 microns may be, in this way, more easily manufacturable than, e.g., the etalon outcoupler 46, described above.
  • An etalon-like interference effect is produced by the outcoupler 1 30 having the coating 1 32 to perform efficient wavelength selection. That is, the block 1 32 with the coating 1 34 provides the advantageous transmission function described above.
  • the coating is made of MgF 2 .
  • Fig. 4c shows a second embodiment including the discharge chamber 2 and line-selection elements 1 50 preferably as set forth above, and an outcoupler 1 36.
  • the outcoupler 1 36 has a partially reflective surface 1 31 forming the resonator reflector of the resonator, as in the embodiment of Fig. 4b.
  • the outcoupler 1 36 is advantageously formed of MgF 2 which provides the periodic transmission function for suppressing the secondary line ⁇ 2 and improving the contrast ratio.
  • the outcoupler 1 36 forms a highly stable, highly transparent and compact outcoupler. That is, the contrast ratio is advantageously increased for the two characteristic emission lines of the F 2 laser, particularly due to the suppression of the radiation generated in the chamber 2 and being directly outcoupled without traversing the rear module 1 50.
  • a gas discharge laser system preferably a DUV or VUV laser system, such as an excimer, e.g., ArF or KrF, or molecular fluorine (F 2 ) laser system for deep ultraviolet (DUV) or vacuum ultraviolet (VUV) lithography, is schematically shown.
  • an excimer e.g., ArF or KrF
  • the system shown in Fig. 5 generally includes a laser chamber 202 having a pair of main discharge electrodes 3a, 3bconnected with a solid- state pulser module 204, and a gas handling module 206.
  • the solid-state pulser module 204 is powered by a high voltage power supply 208.
  • the laser chamber 202 is surrounded by optics module 21 0 and optics module 21 2, forming a resonator.
  • the optics modules 21 0 and 21 2 are controlled by an optics control module 21 4, or may be alternatively directly controlled by a computer 21 6.
  • the computer 21 6 for laser control receives various inputs and controls various operating parameters of the system.
  • a diagnostic module 21 8 receives and measures one or more parameters of a split off portion of the main beam 220 via optics for deflecting a small portion of the beam toward the module 21 8, such as preferably a beam splitter module 222, as shown.
  • the beam 220 is preferably the laser output to an imaging system (not shown) and ultimately to a workpiece (also not shown), and may be output directly to an application process.
  • the laser control computer 21 6 communicates through an interface 224 with a stepper/scanner computer 226 and other control units 228.
  • the laser chamber 202 contains a laser gas mixture and includes one or more preionization electrodes (not shown) in addition to the pair of main discharge electrodes 3a, 3b.
  • Preferred main electrodes 3a, 3b are described at U.S. patent applications no. 09/453,670, 60/1 84,705 and 60/1 28,227, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference.
  • Other electrode configurations are set forth at U.S. patents no. 5,729,565 and 4,860,300, each of which is assigned to the same assignee, and alternative embodiments are set forth at U.S. patents no.
  • the solid-state pulser module 204 and high voltage power supply 208 supply electrical energy in compressed electrical pulses to the preionization and main electrodes 3a, 3b within the laser chamber 202 to energize the gas mixture.
  • Components of the preferred pulser module and high voltage power supply may be described at U.S. patent applications 60/1 49,392, 60/1 98,058, 60/204,095, 09/432,348 and 09/390, 146, and 60/204,095, and U.S. patents no. 6,005,880 and 6,020,723, each of which is assigned to the same assignee as the present application and which is hereby incorporated by reference into the present application.
  • Other alternative pulser modules are described at U.S. patents no.
  • a conventional pulser module may generate electrical pulses in excess of 1 Joule of electrical power (see the '988 patent, mentioned above).
  • the laser resonator which surrounds the laser chamber 202 containing the laser gas mixture includes optics module 21 0 including line-narrowing optics for a line narrowed excimer or molecular fluorine laser, which may be replaced by a high reflectivity mirror or the like in a laser system wherein either line-narrowing is not desired, or if line narrowing is performed at the front optics module 21 2, or a spectral filter external to the resonator is used, or if the line-narrowing optics are disposed in front of the HR mirror, for narrowing the linewidth of the output beam.
  • optics module 21 0 including line-narrowing optics for a line narrowed excimer or molecular fluorine laser, which may be replaced by a high reflectivity mirror or the like in a laser system wherein either line-narrowing is not desired, or if line narrowing is performed at the front optics module 21 2, or a spectral filter external to the resonator is used, or if the
  • the laser chamber 202 is sealed by windows transparent to the wavelengths of the emitted laser radiation 220.
  • the windows may be Brewster windows or may be aligned at another angle, e.g., 5 °, to the optical path of the resonating beam.
  • One of the windows may also serve to output couple the beam.
  • beam splitter module 222 which includes optics for deflecting a portion of the beam to the diagnostic module 21 8, or otherwise allowing a small portion of the outcoupled beam to reach the diagnostic module 21 8, while a main beam portion 220 is allowed to continue as the output beam 220 of the laser system.
  • Preferred optics include a beamsplitter or otherwise partially reflecting surface optic.
  • the optics may also include a mirror or beam splitter as a second reflecting optic. More than one beam splitter and/or HR mirror(s), and/or dichroic mirror(s) may be used to direct portions of the beam to components of the diagnostic module 21 8.
  • a holographic beam sampler, transmission grating, partially transmissive reflection diffraction grating, grism, prism or other refractive, dispersive and/or transmissive optic or optics may also be used to separate a small beam portion from the main beam 220 for detection at the diagnostic module 21 8, while allowing most of the main beam 220 to reach an application process directly or via an imaging system or otherwise.
  • the output beam 220 may be transmitted at the beam splitter module 222 while a reflected beam portion is directed at the diagnostic module 21 8, or the main beam 220 may be reflected, while a small portion is transmitted to the diagnostic module 21 8.
  • the portion of the outcoupled beam which continues past the beam splitter module 222 is the output beam 220 of the laser, which propagates toward an industrial or experimental application such as an imaging system and workpiece for photolithographic applications.
  • Variations of beam splitter modules 222 particularly for a molecular fluorine laser system are set forth at U.S. patent applications no. 09/598,552 and 60/1 40,530, which are each assigned to the same assignee as the present application and are hereby incorporated by reference.
  • an enclosure (not shown, but see Fig. 3e, e.g.) may seal the beam path of the beam 220 such as to keep the beam path free of photoabsorbing species. Smaller enclosures may seal the beam path between the chamber 202 and the optics modules 210 and 21 2 and between the beam splitter module 222, which itself may be within the same or a separate enclosure, and the diagnostic module 21 8.
  • the preferred enclosure is described in detail in U.S. patent applications no. 09/343,333, 09/598,552, 09/594,892, 09/1 31 ,580 and 60/1 40,530, each of which is assigned to the same assignee and is hereby incorporated by reference, and U.S. patents no. 5,559,584, 5,221 ,823, 5,763,855, 5,81 1 ,753 and 4,61 6,908, all of which are hereby incorporated by reference, and as set forth above with respect to Fig. 3e.
  • the diagnostic module 21 8 preferably includes at least one energy detector. This detector measures the total energy of the beam portion that corresponds directly to the energy of the output beam 220 (see U.S. patent no. 4,61 1 ,270 and U.S. patent application no. 09/379,034, each of which is assigned to the same assignee and is hereby incorporated by reference.
  • An optical configuration such as an optical attenuator, e.g., a plate or a coating, or other optics may be formed on or near the detector or beam splitter module 222 to control the intensity, spectral distribution and/or other parameters of the radiation impinging upon the detector (see U.S. patent applications no. 09/1 72,805, 60/1 72,749, 60/1 66,952 and 60/1 78,620, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference).
  • One other component of the diagnostic module 21 8 is preferably a wavelength and/or bandwidth detection component such as a monitor etalon or grating spectrometer (see U.S. patent applications no. 09/41 6,344, 60/1 86,003, 60/1 58,808, 60/1 86,096, 60/1 86,096 and 60/1 86,096 and 60/202,564, each of which is assigned to the same assignee as the present application, and U.S. patents no. 4,905,243, 5,978,391 , 5,450,207, 4,926,428, 5,748,346, 5,025,445, and 5,978,394, all of the above wavelength and/or bandwidth detection and monitoring components being hereby incorporated by reference.
  • the spectrometer may be within a temperature and pressure controlled housing such as is described in the 60/1 58,808 application.
  • a pulse shape detector or ASE detector such as are described at U.S. patent applications no. 09/484,81 8 and 09/41 8,052, respectively, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference, such as for gas control and/or output beam energy stabilization, or to monitor the amount of amplified spontaneous emission (ASE) within the beam to ensure that the ASE remains below a predetermined level, as set forth in more detail below.
  • ASE amplified spontaneous emission
  • the processor or control computer 21 6 receives and processes values of some of the pulse shape, energy, ASE, energy stability, energy overshoot for burst mode operation, wavelength, spectral purity and/or bandwidth, among other input or output parameters of the laser system and output beam.
  • the processor 21 6 also controls the line narrowing module to tune the wavelength and/or bandwidth or spectral purity, and controls the power supply and pulser module 204 and 208 to control preferably the moving average pulse power or energy, such that the energy dose at points on the workpiece is stabilized around a desired value.
  • the computer 21 6 controls the gas handling module 206 which includes gas supply valves connected to various gas sources.
  • control processor 21 6 for performing burst overshoot control and controlling the gas supply unit by monitoring total input energy to the discharge, among other paramters, for determining the timing and amounts of gas replenishment actions, are described at U.S. patent application no. 60/1 59,525, and U.S. patent application of Rebhan and Nowinski, serial number not yet assigned, entitled Energy Control for an Excimer or Molecular Fluorine Laser, filed October 1 6, 2000, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference.
  • the laser gas mixture is initially filled into the laser chamber 202 during new fills.
  • the gas composition for a very stable excimer or molecular fluorine laser in accord with the preferred embodiment uses helium or neon or a mixture of helium and neon as buffer gas(es), depending on the particular laser being used.
  • Preferred gas compositions are described at U.S. patents no. 4,393,405 and 4,977,573 and U.S. patent applications no. 09/31 7,526, 09/51 3,025, 60/1 24,785, 09/41 8,052, 60/1 59,525 and 60/1 60, 1 26, each of which is assigned to the same assignee and is hereby incorporated by reference into the present application.
  • the concentration of the fluorine in the gas mixture may range from 0.003% to 1 .00%, and is preferably around 0.1 %.
  • An additional gas additive such as a rare gas, such as xenon, may be added for increased energy stability and/or as an attenuator as described in the 09/51 3,025 application incorporated by reference above.
  • an addition of xenon and/or argon may be used for the F2-laser.
  • the concentration of xenon or argon in the mixture may range from 0.0001 % to 0.1 %.
  • an ArF-laser an addition of xenon or krypton may be used also having a concentration between 0.0001 % to 0.1 %.
  • an addition of xenon or argon may be used also having a concentration between 0.0001 % to 0.1 %.
  • Halogen and rare gas injections, total pressure adjustments and gas replacement procedures are performed using the gas handling module 206 preferably including a vacuum pump, a valve network and one or more gas compartments.
  • the gas handling module 206 receives gas via gas lines connected to gas containers, tanks, canisters and/or bottles.
  • Some prefered and alternative gas handling and/or replenishment procedures are described at U.S. patents no. 4,977,573 and 5,396,514 and U.S. patent applications no. 60/1 24,785, 09/41 8,052, 09/379,034, 60/1 59,525, 60/1 71 ,71 7, and 60/1 59,525, and U.S.
  • Exemplary line-narrowing optics contained in the optics module 21 0 include a beam expander, an optional etalon and a diffraction grating, which produces a relatively high degree of dispersion, for a narrow band laser such as is used with a refractive or catadioptric optical lithography imaging system.
  • the front optics module 21 2 may include line-narrowing optics (e.g., etalon, birefringent plate, grating, grism) as well (see the 60/1 66,277, 60/1 73,993 and 60/1 66,967 applications, each being assigned to the same assignee and hereby incorporated by reference).
  • line-narrowing optics e.g., etalon, birefringent plate, grating, grism
  • the grating may be replaced with a highly reflective mirror, and a lower degree of dispersion may be produced by a dispersive prism.
  • a semi-narrow band laser would typically have an output beam linewidth in excess of 1 pm and may be as high as 100 pm in some laser systems, depending on the characteristic broadband bandwidth of the laser.
  • the beam expander of the above exemplary line-narrowing optics of the optics module 21 0 preferably includes one or more prisms.
  • the beam expander may include other beam expanding optics such as a lens assembly or a converging/diverging lens pair.
  • the grating or a highly reflective mirror is preferably rotatable and in Littrow configuration so that the wavelengths reflected into the acceptance angle of the resonator can be selected or tuned.
  • the grating, or other optic or optics, or the entire line-narrowing module may be pressure tuned, such as is set forth in the 60/1 78,445 and 09/31 7,527 applications, each of which is assigned to the same assignee and is hereby incorporated by reference.
  • the grating may be used both for dispersing the beam for achieving narrow bandwidths and also preferably for retroreflecting the beam back toward the laser tube.
  • a highly reflective mirror may be positioned before or after the grating which receives a reflection from the grating and reflects the beam back toward the grating, such as in a Littman configuration, or the grating may be a transmission grating.
  • One or more dispersive prisms may also be used, and more than one etalon may be used.
  • Optics module 21 2 preferably includes means for outcoupling the beam 220, such as a partially reflective resonator reflector.
  • the beam 220 may be otherwise outcoupled such as by an intra-resonator beam splitter or partially reflecting surface of another optical element, and the optics module 21 2 would in this case include a highly reflective mirror.
  • the optics control module 214 preferably controls the optics modules 210 and 21 2 such as by receiving and interpreting signals from the processor 21 6, and initiating realignment or reconfiguration procedures (see the '241 , '695, '277, '554, and '527 applications mentioned above).
  • any localized heating is further reduced in the preferred embodiment over fused silica etalons and prisms because the energy absorbed is dissipated more quickly away from the localized area of photon incidence because of the increased thermal conductivity of the etalon plate and prism bulk materials.
  • operation at higher laser repetition rates such as 1 -4 kHz or more is facilitated, wherein the wavelength stability is maintained at 0.1 pm or less.
  • a laser system may utilize an optical component other than an etalon, or an etalon exhibiting reduced finesse may be used, or greater beam expansion may be used to reduce the laser power density on the etalon plates.

Abstract

An excimer or molecular fluorine laser includes a gain medium surrounded by a resonator and including a line-narrowing module preferably including a prism beam expander and one or more etalons and/or a grating or grism within the resonator. The material of transmissive portions of the line-narrowing module including the prims and the plates and the etalons comprises a material having an absorption coefficient of less than 5 x 10-3/cm at 248 nm incident radiation, less than 10 x 10-3/cm at 193 nm incident radiation, and less than 0.1/cm at 157 nm. Preferably the material also has a thermal conductivity greater than 2.0 W/m°C.

Description

TITLE: Line-Narrowing Module for High Power Laser
Priority
This application claims the benefit of priority to U.S. provisional patent applications no. 60/1 62,735, filed October 29, 1 999, and 60/1 78,440, filed January 27, 2000, and is a Continuation-in-Part of U.S. patent application no. 09/599, 1 30, filed June 22, 2000, which claims the benefit of U.S. provisional patent application no. 60/140,531 , filed June 23, 1 999, and is a Continuation -in-Part of U.S. patent application no. 09/657,396, filed September 8, 2000, each application of which is hereby incorporated by reference.
Background of the Invention
1 . Field of the Invention
The invention relates to a narrow band laser, and particularly to a high power excimer or molecular fluorine laser operating at a high repetition rate (e.g., 1 -4 kHz or more).
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems which emit around 248 nm and which will be followed by the next generation of ArF- excimer laser systems operating around 1 93 nm. Vacuum UV (VUV) lithography uses the molecular fluorine (F2) laser operating around 1 57 nm.
Fabrication of integrated circuit devices of a quarter-micron or less, requires very fine photolithographic techniques. Bandwidths around 1 .0 pm have been deemed sufficient for 248 nm lithography in the past. Today, it is desired to have an excimer or molecular fluorine laser with an output emission bandwidth of less than 0.6 pm and preferably less than 0.4 pm. It is specifically desired to have such a laser today emitting a laser beam with a bandwidth of less than 0.4 pm. Today's knowledge of derived bandwidth depends on wavelength, and the above is thus less than 0.4 pm for the 248 nm KrF laser, less than 0.3 pm for the 1 93 nm ArF laser, and less than 0.1 pm for the 1 57 nm F2 laser.
To produce smaller features on silicon chips using the exposing radiation of a laser operating at the above-mentioned narrower bandwidth, e.g., less than around 0.6 pm, it is desired that the laser system exhibit greater absolute wavelength stability. For example, a laser output beam wavelength stability around or below 0.1 pm is desired.
A narrow band 248 nm lithography laser system operating at low power, e.g., less than 1 kHz, and having a bandwidth around 1 pm or more, includes within its resonator a line-narrowing module, such as that shown by example at Fig. 1 (see also, U.S. patent no. 5, 1 50,370, which is hereby incorporated by reference). The line-narrowing module shown includes a prism beam expander including three prisms 30, 30 and 36, an etalon 34 and a grating 38. Each of the prisms 30, 32 and 36, and the plates of the etalon 34 of a line-narrowing module of a conventional KrF laser comprises fused silica due to its transparency at 248 nm. The wavelength stability is, however, substantially above 0.1 pm when the laser is operated at 1 kHz or higher power.
Line-narrowing modules vary in their response to the exposure to high power or high repetition rate laser beams that cause heating and aging of the optical components. For example, nonuniform heating of the optical elements of the line-narrowing module may substantially degrade their quality by, for example, disrupting the planarity of optical surfaces by localized expansion and causing fluctuations in the thermally dependent refractive index, thereby distorting the wavefront of the retroreflected or transmitted beam and detuning the wavelength. Wavefront distortions lead to changes in the output bandwidth of the laser system which is a parameter that it is desired to keep constant. Wavelength detuning may be compensated by rotating an optical element, typically the grating or HR mirror, as mentioned above. In addition to wavefront distortions and detuning, absorption by optical components results in reduced overall efficiency of the laser.
Parameters of the line-narrowing module that depend on the "quality" of the optical components such as the magnitude of angular dispersion, reflectivity for specific wavelengths, linearity (i.e., absence of wavefront distortions), scattering of the beam, etc., will thus affect the bandwidth, linewidth and overall performance of the laser. The quality of the optical components is generally tested by measuring the absorption of DUV (or VUV for F2 lasers) radiation that leads to heating of the optical components and thermally induced distortions and defects.
It is desired to operate the laser at a higher repetition rate than is typical. For example, a conventional " low power" laser might operate at a repetition rate around 600 Hz. It is desired to have a laser operating above 1 kHz, and particularly around 2-4 kHz, or more. For 1 93 nm lasers, a repetition rate around 4 kHz will be used. To achieve sufficient throughput, the expectation for the molecular fluorine laser is to have a repetition rate greater than 4 kHz. At these repetition rates, greater laser power is incident on surfaces of optical components within the laser resonator that can serve to enhance the effects of thermally induced distortions or defects on the bandwidth of the laser. It is desired to have a narrow band laser operating at high power and having a wavelength stability at or less than substantially 0.1 pm. In addition to KrF (248 nm) lasers, it is desired to have an efficient line- narrowing module for use with ArF ( 1 93 nm) and F2 ( 1 57 nm) lasers.
Summary of the Invention
It is therefore an object of the invention to provide a high power laser, i.e., 1 -2 kHz or more, exhibiting a narrow bandwidth, such as is provided when a line-narrowing module including a prism beam expander and one or more etalons and/or a grating is included within the laser resonator, preferably less than 0.6 pm.
It is a further object of the invention that such high power narrowband laser exhibit a wavelength stability of 0.1 pm or less.
The above objects are met by a laser including a gain medium surrounded by a resonator and including a line-narrowing module including a prism beam expander and one or more etalons and/or a grating or grism within the resonator. The material of transmissive portions of the line- narrowing module including the prisms and the plates of the etalons comprises a material having an absorption coefficient of less than 5 x 1 0 3/cm at 248 nm incident radiation, less than 1 0 x 1 0 3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm. Preferably the material also has a thermal conductivity greater than 2.0 W/m°C. Preferably, that material is calcium fluoride (CaF2).
Brief Description of the Drawings
Fig. 1 schematically illustrates a line-narrowing module for a KrF excimer laser.
Figs. 2a-2b illustrate how the gap spacing of an etalon changes as the etalon is heated due to absorption.
Figs. 2c-2d illustrate how localized heating can distort the planarity of a prism surface. Figs. 2e-2f illustrate how absorption of laser radiation can distort the surface contour of a grating.
Figs. 3a-3e schematically illustrate further line-narrowing module configurations, in addition to that shown at Fig. 1 , according to preferred embodiments.
Fig. 4 schematically illustrates a line-narrowing resonator in accord with a further preferred embodiment.
Fig. 5 schematically shows an excimer or molecular fluorine laser system according to a preferred embodiment.
Incorporation by Reference
What follows is a cite list of references each of which is, in addition to those references cited above and below, including that which is described as background, and in the priority section, and in the above invention summary, are hereby incorporated by reference into the detailed description of the preferred embodiment below, as disclosing alternative embodiments of elements or features of the preferred embodiments not otherwise set forth in detail below. A single one or a combination of two or more of these references may be consulted to obtain a variation of the preferred embodiments described in the detailed description below. Further patent, patent application and non-patent references are cited in the written description and are also incorporated by reference into the detailed description of the preferred embodiment with the same effect as just described with respect to the following references:
U.S. patent applications no. 60/1 78,440, 60/1 67,835, 60/1 78,445, 09/31 7,695, 09/244,554, 09/31 7,527, 60/21 2, 1 83, 09/594,892, 09/602, 1 84, 09/599, 1 30, 09/598,552, 09/629,256, 60/1 62,735, 60/1 66,277, 09/454,803, 60/1 70,91 9, 60/200, 1 63, 60/21 2,257, 60/21 5,933 and 60/1 70,342, each of which is assigned to the same assignee as the present application; U.S. patents no. 5,999,31 8, 5,970,082, 5,095,492, and 6,094,448;
European patent application no. EP 778,642 A2;
Japanese patent application no. 1 0-209533; and
E.G. Loewen, E. Popov, Diffraction Gratings and Applications 1 997 (Marcel Dekker) .
Detailed Description of the Preferred Embodiments
It is recognized in the present invention that when inserted into a resonator of a narrow band laser operating at high power, e.g., 1 -4 kHz or more, transmissive line-narrowing components of a conventional line- narrowing module including fused silica plates of conventional etalons, fused silica beam expanding or dispersive prisms and fused silica layers of diffraction gratings will absorb radiation at an undesirably high rate, causing these materials to heat up too much. The result of this increased absorption, which is greatly enhanced over that incurred components in low power systems, is surface distortion of surfaces of the prisms and grating, and greater fluctuation of the size of the etalon gap due to greater thermal expansion of surfaces of the etalon plates. This greater thermal expansion occurs when a same percentage (based on the absorption coefficient of the material of the surfaces such as those of the etalon plates) of the total number of incident photons is absorbed, but the total absorption is increased due to the increased number of incident photons per time interval from the high power, or higher repetition rate, laser, whether operating in burst or cw mode, over that of the conventional low power laser.
Referring again to Fig. 1 , a line-narrowing module in shown particularly configured for use with a KrF laser. The line narrowing module includes a first beam expanding prism 30 and a second beam expanding prism 32, an etalon 34, and a third beam expanding prism 36 before a grating 38. The grating may be substituted by a highly reflective (HR) mirror. More or fewer than three prisms may be included, and two or more etalons or no etalon may be included. A dispersive prism or a grism may also be included (see U.S. patent applications no. 09/602, 1 84 and 60/1 70,91 9, which are each assigned to the same assignee as the present application and are hereby incorporated by reference).
Each of the prisms 30, 32 and 36 serves to expand the beam, such that by the time the beam reaches the etalon 34 (prisms 30 and 32) and grating 38, the beam is expanded to encompass a larger portion of the etalon 34 and grating 38. The prisms 30, 32 and 36 expand the beam owing to their refractive nature. The index of refraction of each prism 30, 32 and 36 is in turn dependent on the temperature of the prism, particularly at the area on the surface of each prism 30, 32 and 36 that is impinged upon by the beam.
At those areas of the surfaces of the prism, as well as the volumes, of the prisms 30, 32 and 36, absorption occurs. The energy of the absorbed portion of the beam is converted to heat, and the index of refraction changes as the prism temperatures rise. The refractive index changes result in changes in the refraction angles of the beam at those surfaces. The solid and dashed lines shown in Fig. 1 illustrate how a ray of a certain wavelength travels along two different paths for two different temperature states of the prisms 30, 32 and 36. Other effects such as bandwidth broadening due to distortions of the planarity of the surfaces of the prisms 30, 32 and 36 are also induced by the temperature change.
At least two significant effects on the wavelength that will be reflected back out of the line-narrowing module occur. First, the angle at which rays traverse the gap spacing of the etalon 34 is changed. This changes the spectral positions of interference maxima according to:
2ngapdecosθ mλ ( 1 ); where de is the etalon gap spacing, ngap is the temperature dependent index of refraction in the gap (not to be confused with ne(T) and np(T) which are the indices of refraction of the material of the etalon plates and the prisms, respectively), and θ is the angle the ray makes with the planes of the etalon plates.
Second, the angle at which rays strike the grating 38 is changed. This changes the central wavelength reflected from the grating 38 back into the acceptance angle of the resonator according to:
2dgsinφ = pλ (2);
where dg is the groove spacing of the grating, φ is the blaze angle for a blazed echelle grating, and λ is the wavelength reflected back at angle φ. It is therefore in accord with the objects of the invention above to provide prisms 30, 32 and 36 with greater thermal stability, such that the index of refraction np(T) exhibits reduced variance caused by absorption of an incident high power laser beam. To achieve this object, fused silica may not be used because the absorption coefficient of fused silica is about 5.3 x 1 0 3/cm for incident 248 nm radiation, and for 1 93 nm incident radiation, the absorption coefficient of fused silica is between 20- to-30 x
1 0 3/cm, which is too high permitting large temperature fluctuations and greater changes in np(T) than would meet the objects of the invention. Instead, and advantageously, the absorption coefficient of the material of the prisms 30, 32 and 36 in accord with the preferred embodiment is less than 5 x 1 0"3/cm at 248 nm incident radiation, less than 1 0 x 1 0"3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm. The preferred material meeting these specifications is calcium fluoride. Alternative materials may include BaF2, LiF, SrF2, fluorine-doped quartz and MgF2, although MgF2 is birefringent; generally a property of a material that is not preferred.
As shown in Fig. 1 by the arrows near the etalon 34 and grating 38, either or both of the etalon 34 and the grating 38 may be rotated to compensate the wavelength changes just described (see U.S. patent application no. 60/1 78,440, which is assigned to the same assignee as the present application and is hereby incorporated by reference). One or more of the prisms may also be rotated (see U.S. patent application no. 09/244,554, which is assigned to the same assignee as the present application and is hereby incorporated by reference).
Figs. 2a-2f illustrate further changes that may be induced by the absorption of the higher power beam by the etalon 34, grating 38 and prisms 30, 32 and 36. Fig. 2a shows an etalon 34 such as may be used in the line-narrowing module as shown in Fig. 1 or Figs. 3c-3e below, or in the line-narrowing resonator shown at Fig. 4, below. Fig. 2a shows an etalon before any thermally induced changes are incurred. The etalon 34 comprises two parallel plates 40 separated by a gap de. The etalon 34 has two spacers 42 typically comprising zerodur™, quartz, invar™ or Cordierite (see U.S. patent application no. 60/1 58,808, which is assigned to the same assignee as the present application and is hereby incorporated by reference).
Fig. 2b illustrates what happens to the etalon 34 when absorption occurs as a high power beam passes through the etalon 34. The surfaces of the plates that define the gap distort due to thermal expansion caused by absorption and changing the gap spacing to de'. The distortion is most pronounced where the absorption of the incident beam occurs which is the critical location in the gap determining the selected wavelength as understood from equation ( 1 ). This surface distortion serves to broaden the beam because the plates are less planar than those shown in Fig. 2b and a greater range of wavelengths are selected due to the spatial dependence of the gap spacing de'. In addition, the center wavelength in the interference spectrum as understood from equation ( 1 ) is also changed.
For the etalon 34, the greater expansion at the surface due to surface heating is followed by greater contraction of the plates during pauses between bursts when the laser is operated in burst mode. This phenomenon which may occur when an etalon 34 is in a high power laser resonator may be referred to as "breathing" . Since the output emission wavelength of the narrow bandwidth laser system is determined by the gap spacing de of the etalon 34, the result is reduced wavelength stability. Therefore, the etalon 34 of the preferred embodiment has plates 40 comprising a material with a reduced absorption coefficient.
It is also recognized that in addition to the absorption coefficient, the thermal conductivity of the material of the plates of the etalon 34 will effect its performance, particularly at high repetition rates. That is, incident laser radiation is absorbed in a localized region of the etalon plates 40 based on the size of the incident beam. It is this localized region that expands as it heats up effecting the etalon gap spacing and the output emission wavelength of the laser system. The absorbed energy will thermally diffuse away as phonons from this localized region at a rate that depends on the thermal conductivity of the material of the etalon plates 40. A material having a higher thermal conductivity would allow the heat to propagate to less critical portions of the etalon 34 away from the surfaces of the plates 40 upon which the high power beam impinges.
Thus, if the etalon plates 40 comprise a material having a greater thermal conductivity, then the wavelength stability can be further improved.
The absorption coefficient of fused silica is about 5.3 x 1 0 3/cm for incident 248 nm radiation. For 1 93 nm incident radiation, the absorption coefficient of fused silica is between 20-to-30 x 1 0 3/cm. As described above, these absorption coefficients are larger than would meet the objects of the invention as they provide greater distortion than is tolerable when a high power laser beam is generated for microlithographic applications. Advantageously, the absorption coefficient of the material of the plates of the etalon 34 in accord with the preferred embodiment is less than 5 x 1 0 3/cm at 248 nm incident radiation, less than 1 0 x 1 0 3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm. The preferred material meeting these specifications is calcium fluoride (CaF2). Alternative materials may include BaF2, LiF, SrF2, fluorine-doped quartz and MgF2, although MgF2 is birefringent; generally a property of a material that is not preferred, as mentioned above. The thermal conductivity of fused silica at 248 nm and 1 93 nm incident radiation is around 1 .38 W/m°C. This thermal conductivity is too low such that energy of the beam absorbed and converted to heat cannot travel quickly enough away to less critical areas of the plates 40 of the etalon 34. Preferably, the material has a thermal conductivity greater than 2.0 W/m°C, and more preferably greater than 5.0 W/m°C. According also to this specification, the etalon plates 40 preferably comprise calcium fluoride CaF2 to improve the thermal conductivity of the etalon 34 and reduce distortions at the critical location of the etalon plates 40 wherein the beam is incident.
Fig. 2c shows a beam expanding prism such as the prisms 30, 32 and 36 of Fig. 1 . The discussion below is also relevant to a line- narrowing module including one or more dispersive prisms (see, e.g., U.S. patent no. 5,761 ,236, which is hereby incorporated by reference). The prism shown at Fig. 2c has an apex angle γ and smooth flat surfaces.
Fig. 2d shows how absorption of a high power laser beam causes the planarity of the surface of the prism 30, 32, or 36, or a dispersive prism, to distort. The distortion of the surface is caused by absorption of the energy of an incident high power beam such as an excimer or molecular fluorine laser operation at 1 -4 kHz or more. The distortion causes an undesirable broadening of the bandwidth and geometry of the beam, in addition to the refractive angle changes described above based on the temperature dependent index of refraction np(T). Directions of normals to the surface of the prism shown at Fig. 2d vary across the surface of the prism, particularly around the critical portion of the surface where the high power beam is incident. The angles of rays travelling within the prism thus vary depending on where those rays strike the distorted surface. This causes increased beam divergence, which is a parameter that the beam expanding prisms 30, 32 and 36 are inserted into the line-narrowing module to reduce. In addition, as the beam exits the prisms 30, 32 and 36 after impinging upon the grating 38 (see Fig. 1 ), rays of different wavelengths will cross over each other allowing some wavelengths that would ordinarily be outside the acceptance angle of the resonator to re-enter and broaden the overall bandwidth.
In order to reduce the distortion of the surfaces of the prisms 30, 32 and 36, a material is selected that has a lower coefficient of absorption, such as less than 5 x 1 0 3/cm at 248 nm incident radiation, less than 1 0 x 1 0"3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm. In addition, the material preferably has a thermal conductivity greater than 2.0 W/m°C, and more preferably greater than 5.0 W/m°C, so that the heat is carried away from the critical region of the surfaces of the prisms more quickly. The preferred material meeting these specifications is calcium fluoride (CaF2). Alternative materials may include BaF2, LiF, SrF2, fluorine-doped quartz and MgF2, although MgF2 is birefringent; generally a property of a material that is not preferred, as mentioned above.
Figs. 2f and 2g illustrate how a thermally induced curvature of the grating may also be effected by an incident high power laser beam. The grating of Fig. 2f is shown prior to being hit by a high power laser beam. The preferred grating has a blazed structure such as for a typical eschelle grating, or more preferably, such as the grating shown and described in either or both of U.S. patent applications no. 60/1 70,342 and 60/1 67,835, each of which is assigned to the same assignee and is hereby incorporated by reference (see also U.S. patent no. 5,999,31 8, which is hereby incorporated by reference). Undesirable effects on the wavelength may be caused if thermally induced distortions of the surface contour of the grating such as shown in Fig. 2f are caused by an incident high power beam. A preferred grating is generally described in the '835 application, wherein the blazed rulings of the preferred grating are etched directly into the substrate of the grating, rather than being formed in a separate layer and then adhered to onto the substrate.
Figs. 3a-3e show some alternative configurations of line-narrowing modules that would benefit by incorporating one or more etalons 34, 35 and/or prisms 30, 32 and/or a grating 38 according the preferred embodiments described above. Fig. 3a schematically shows a line- narrowing module for use with a line-narrowed laser system according to a preferred embodiment including a prism beam expander 30, 32 and a diffraction grating 38 arranged in a Littrow configuration. The beam expander 30, 32 may include the two prisms shown, or may includes only one prism or more than two prisms. For example, a line-narrowing module for an ArF laser preferably includes four prisms. The beam expander 30, 32 serves to reduce divergence of the beam incident onto the grating 38, thus improving wavelength resolution of the wavelength selector.
The grating 38 is preferably as described above, or is a high blaze angle echelle grating such as is described at the 60/1 70,342 application, incorporated by reference above. An aperture (not shown) may be included preferably before the beam expander prism 30 to reject stray light and reduce broadband background, and can also serve to reduce the linewidth of the beam by lowering the acceptance angle of the resonator. Exemplary apertures are set forth at U.S. patent no. 5, 1 61 ,238, which is assigned to the same assignee and is hereby incorporated by reference (see also the 09/1 30,277 application incorporated by reference above) .
The system of Fig. 3b includes the beam expander 30, 32 described above with respect to Fig. 3a, and also includes a diffraction grating 39 and a highly reflective mirror 41 . The grating 39 preferably differs from the grating 38 of Fig. 3a either in its orientation with respect to the beam, or its configuration such as its blaze angle, etc., or both. The laser beam is incident onto the grating 39 at an angle closer to 90° than for the grating 38. The incidence angle is, in fact, preferably very close to 90°. This arrangement is referred to as the Littman configuration. The Littman configuration serves to increase the wavelength dispersion of the grating 39. The positions of the grating 39 and mirror 41 may be reversed, as well, such as is described at U.S. patent no. 5,970,082, which is hereby incorporated by reference. After passing through (for a transmission grating, not specifically shown) or reflecting from the diffraction grating 39, the diffracted beam is reflected by the highly reflective mirror 41 . The tuning of the wavelength is preferably achieved by tilting the highly reflective mirror 41 . Tuning may be achieved otherwise by rotating another optic or by pressure tuning one or more optics, or otherwise as described below, or as may be understood by one skilled in the art.
Fig. 3c schematically shows another embodiment including beam expanding prisms 30, 32 and Littrow diffraction grating 38, preferably as described above. In addition, the system of Fig. 3c includes one or more etalons 34, 35, e.g, two etalons are shown, which provide high-resolution line narrowing, while the grating 38 serves to select a single interference order of the etalons 34, 35. The etalon or etalons 34, 35 may be placed in various positions in the resonator, i.e., other than as shown. For example, a prism or prisms of the beam expander 30 may be positioned between an etalon or etalons 34, 35 and the grating 38. An etalon may also be used as an output coupler, as will be described in more detail below with reference to Fig. 4a. The arrangement of Fig. 3c (as well as Figs. 3d and 3e below) including an etalon or etalons 34, 35 may be varied as described at any of U.S. patent applications no. 60/1 62,735, 60/1 78,445, 60/21 2, 1 83, 09/657,396 or 60/1 58,808, each of which is assigned to the same assignee and is hereby incorporated by reference.
Fig. 3d shows another embodiment of the laser system having one or more etalons 34, 35, e.g., two etalons are shown. The system of Fig. 3d is the same as that of Fig. 3c except that the grating 38 of Fig. 3c is replaced with a highly reflective mirror 43 in the embodiment of Fig. 3d, and the etalons 34, 35 are differently configured owing to the omission of the grating 38 which is not available, as in the system of Fig. 3c, to select a single interference order of the etalons 34, 35. The free spectral ranges of etalons 34, 35 are instead adjusted in such a way that one of the etalons 34 or 35, preferably the first etalon 34 after the beam expanding prism 32, selects a single order of the other etalon 35, e.g., the second etalon 35. The second etalon 35 of the preferred arrangement is, therefore, allowed to have a smaller free spectral range and higher wavelength resolution. Some further alternative variations of the etalons 34, 35 of the system of Fig. 3d may be used as set forth in U.S. patent no. 4,856,01 8, which is hereby incorporated by reference.
Fig. 3e Fig. 1 1 shows a seventh preferred embodiment of a resonator arrangement for a molecular fluorine (F2) laser including a line- narrowing module 50 having a prism beam expander 46 and an etalon 48 for line selection and/or line narrowing. The resonator arrangement of the seventh embodiment includes a discharge chamber 2 filled with a gas mixture including molecular fluorine and one or more buffer gases such as neon and/or helium. A pair of main discharge electrodes 3a, 3b are in the discharge chamber and connected to a pulser circuit (not shown) for energizing the molecular fluorine. One or more preionization units is also within the discharge chamber (not shown, but see the 60/1 62,845 application, mentioned above). Other aspects of the discharge chamber 2 may be found at the 09/453,670 application and otherwise as set forth above and/or as understood by those skilled in the art.
One window 1 40 is a piano-parallel optical window which serves as an outcoupler. A partially reflective mirror may alternatively be used separate from the chamber window 1 40 for outcoupling the beam, and the window 1 40 serving the dual role of sealing the chamber 2 and outcoupling the beam reduces the number of optical surfaces within the resonator and thereby enhancing its efficiency. The window 140 is preferably aligned perpendicular to the optical axis, but may be oriented at an angle such as 5° or at Brewster's angle or another selected angle for achieving desired reflectivity and transmissivity characteristics and/or to prevent parasitic oscillations.
Vacuum bellow 56 or similar arrangement provides versatility in degrees of freedom of adjustment and a vacuum-tight seal. The other window 44 is preferably a plano-convex lens serving to compensate the wavefront curvature as described in U.S. patent no. 6,061 ,382 and European Patent Application EP 00955706 A1 , each of which is assigned to the same assignee and is hereby incorporated by reference. Tightness of the seal is ensured by seals 42. The lens 44 may or may not be adjustable. The lens 44 may be tilted slightly off the normal to the beam (e.g., 5° or so) in order to avoid parasitic oscillations. Preferably, this lens 44 is anti-reflectively coated.
The resonator of an F2-laser according to a preferred embodiment is shown at Fig. 3e including a preferred line-selection and/or line-narrowing unit coupled with a discharge chamber 2 via bellows 56 and seals 1 42. The beam is preferably expanded by one or more beam expansion prisms 30, 32, preferably as described above, in order to reduce its divergency. Several prisms 30, 32 and/or 36 (see Fig. 1 ) may be used, e.g., four prisms are preferred for the ArF laser and Fig. 1 sets forth a preferred KrF laser arrangement, and alternatively, a converging diverging lens pair may be used or other beam expansion means known to those skilled in the art.
A transmissive etalon 34 is preferably positioned after the beam expander prisms 30, 32 and serves as a wavelength selector. Finally, highly reflecting mirror 43 returns the beam back through the resonator. The line-narrowing unit is within an enclosure 50 that is coupled to the laser chamber 2 using a bellows 56 and seals 1 42 such that the line- narrowing unit of the laser resonator is sealed from the outer atmosphere. An inert purge gas is flowed through the enclosure 50 via inert and outlet ports 52, 54. A vacuum port may be included or a pump can be connected via either port 52 or 54.
A method for controlling the atmosphere in the enclosure may be to maintain the enclosure under evacuated conditions using a pump, or to flow the purge gas rapidly through the enclosure 50, or preferably to first evacuate the enclosure and then to flow the purge gas at a low flow rate. An evacuation may be followed by a backfill of the purge gas and the procedure may be repeated a number of times, e.g., 3-1 0 times, before the purge gas is flowed at a low rate. For more details and alternative procedures for preparing and maintaining the atmosphere of the enclosure, see U.S. patent applications no. 09/343,333, 09/594,892 and 09/598,522, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference (relating to an enclosure for a beam delivery path and diagnostic beam splitter module for an F2-laser) .
The entire beam path between the chamber 2 and the highly reflective mirror 43 is in fact preferably enclosed within the enclosure 50 which is purged with some inert gas such as preferably nitrogen, or helium, argon, krypton or the like. High purity nitrogen is preferably used for this purpose.
The pressure of the nitrogen in the enclosure 50 may be adjustable, and by changing the pressure of nitrogen, one can adjust the transmission spectrum of the etalon 34 (see, e.g., U.S. patent application 09/1 78,445, relating to pressure tuning a line-narrowing unit including a grating and beam expander, which is assigned to the same assignee and is hereby incorporated by reference). The maximum of transmission of the etalon 34 is preferably adjusted to coincide with a maximum in the primary line at λi around 1 57.62 nm, among multiple lines around 1 57 nm including also a secondary line λ2 around 1 57.52 nm which is preferably suppressed, of the natural emission spectrum of the free-running F2 laser.
The etalon 48 may be tuned piezo-electrically, rotationally and/or by adjusting the pressure within the gap between its plates. One way of varying the pressure is to control intake and escape rates of the nitrogen purge gas shown at Fig. 3e using needle valves 52 and 54. Closing output valve 54 and/or opening inlet valve 52, e.g., causes the pressure to rise, and visa versa. Variations of the embodiment of Fig. 3e may include having an outcoupler as a separate optical element from the window 140, with the output window 140 of the chamber 2 being a plano-plano window. This provides an opportunity to mount resonator optics onto a mechanically stable structure, decoupled from the chamber 2. However, this increases optical losses and reduces the efficiency of the laser. A second etalon 35, a grating 38 or 39 and/or one or more dispersive prisms may be included with the line-narrowing unit of Fig. 3e. The etalon 34 may be replaced with a reflective etalon to serve a dual role as a line selection element and as a resonator reflector, such as a high finesse etalon.
The laser oscillator of Fig. 3e typically produces lower output power, as compared to a free-running F2 laser resonator. Therefore, an amplifier may be used in this embodiment, as well as with any of the line- narrowing module embodiments described herein particularly for the F2 laser, such as is described at U.S. patent application no. 09/599, 1 30, which is assigned to the same assignee as the present application and is hereby incorporated by reference.
Fig. 4a schematically shows an embodiment that is a variation of any of the arrangements described above with reference to Figs. 1 and 3a-3e, respectively. The line-narrowing modules shown and described in those embodiments also have included for additional line-narrowing and/or line selection in laser resonators which include them in this embodiment, a reflective etalon outcoupler 46 which replaces the partially reflecting outcoupler mirror typically otherwise included in their resonators. The etalon outcoupler 46 may be used in combination with the grating 38 or 39 and beam expander 30, 32 of Figs. 1 and 3a-3e, respectively, wherein the grating 38 or 39 selects a single interference order of the etalon outcoupler 46. Alternatively, one or more dispersive prisms or another etalon may be used in combination with the etalon outcoupler 46 for selecting a single interference order of the etalon 46. The grating 38 or 39 restricts the wavelength range to a single interference order of the outcoupler etalon 46.
Variations of the systems of Figs. 1 and 3a-3e that may be used in combination with the systems set forth at Fig. 4a are set forth at the U.S. patent applications no. 09/31 7,527 and 60/1 66,277, each of which is assigned to the same assignee, and U.S. patents no. 6,028,879, 3,609,586, 3,471 ,800, 3,546,622, 5,901 , 1 63, 5,856,991 , 5,440,574, and 5,479,431 , and H. Lengfellner, Generation of tunable pulsed microwave radiation by nonlinear interaction of Nd:YAG laser radiation in GaP crystals, Optics Letters, Vol. 1 2, No. 3 (March 1 987), S. Marcus, Cavity dumping and coupling modulation of an etalon-coupled CO2 laser, J. Appl. Phys., Vol. 53, No. 9 (September 1 982), and The physics and technology of laser resonators, eds. D.R. Hall and P.E. Jackson, at p. 244, and Japanese patent publications no. 3-821 83, 1 -1 79477, 44- 81 47, 62-1 60783, 2-1 52,288, 8-274,399, and 63- 45875, each of which is hereby incorporated by reference.
In all of the above embodiments shown and described with reference to Figs. 1 , 3a-3e and 4a, the material used for the prisms of the beam expanders 30, 32, 36, etalons 34, 35, 46 and also for the laser windows 1 40, 240 is preferably one that is highly transparent at DUV and/or VUV emission wavelengths of the KrF, ArF and/or F2 laser at 248 nm, 1 93 nm and 1 57 nm, respectively, with which the line-narrowing optics are being used. The materials are also capable of withstanding long-term exposure to ultraviolet light with minimal degradation effects.
For the reasons set forth in detail above with particular reference to Figs. 2a-2d, and in accord with the objects of the invention, the material of the transmissive optical components of the line-narrowing module including the prisms and the plates of the etalons advantageously comprises a material having an absorption coefficient of less than 5 x 1 0"3/cm at 248 nm incident radiation, less than 1 0 x 1 0 3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm. Preferably the material also has a thermal conductivity greater than 2.0 W/m°C. Preferably, that material is calcium fluoride (CaF2). Other materials may include BaF2, LiF, MgF2, fluorine-doped quartz and SrF2. Also, in all of the above embodiments of Figs. 2a-2f, many optical surfaces, particularly those of the prisms, preferably have an anti-reflective coating on one or more optical surfaces, in order to minimize reflection losses and prolong their lifetime.
In general, a line-narrowing module in accord with the preferred embodiment will have beam expanding prisms and/or one or more etalons as the only transmissive optical components; the grating 38, 39 and HR mirror 43, if included, being reflective components. In alternative embodiments, a transmission grating or a partially or fully transmissive grism may be included in the resonator. Other transmissive optical components within the laser resonator and also subject to heat induced distortions, as described above, include the windows on the laser chamber and may include portions of the front optics module located on the other side of the discharge chamber as the line-narrowing module described above (one such component may include the etalon outcoupler 46 of Fig. 4a or the birefringent or partially birefringent outcoupler described with respect to Figs. 4b-4c, below. These other components can undergo thermally induced distortions, as well as the prisms and etalon(s), although they do not produce effects on the beam bandwidth and wavelength stability as drastic as those described above.
Thermally induced distortions may for each of these optical components, and particularly the etalon(s) and prism(s), are advantageously reduced in accord with the preferred embodiments, where in the line-narrowing module, and preferably the entire excimer or molecular fluorine laser resonator, each of the transmissive or substantially transmissve components is comprised of a material having an absorption coefficient that is less than 5 x 1 0 3/cm at 248 nm incident radiation, less than 1 0 x 1 0"3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm, and preferably also having a thermal conductivity greater than 2.0 W/m°C, and more preferably greater than 5.0 W/m°C. The preferred material meeting these specifications is, in general and subject to the discussion below with respect to Figs. 4b-4c, calcium fluoride (CaF2), and alternative materials may include BaF2, LiF, SrF2, fluorine-doped quartz and MgF2. By substantially transmissive, it is meant that a reflection grating would not be substantially transmissive. A grism having a reflective coating on a surface for high reflection or partial reflection of the beam would be substantially transmissive. The plates of an etalon output coupler 46 and the blocks 1 30, 1 36 described below would also represent substantially transmissive optics.
Figs. 4b-4c show resonator arrangements in accord with a first of two additional embodiments, particularly suited for the F2 laser. In Fig. 4b, the resonator includes the discharge chamber 2 preferably configured as set forth above, e.g., having a pair of elongated main electrodes 3 and a preionization unit (not shown), and being connected to a pulser circuit 4 and gas supply system 6 (see below) . The resonator of Fig. 4b further includes a rear optics module 1 50 including a line-selection package such as has been described above.
Although many line-selection packages are possible, a line-selection package that does not considerably extend the resonator length, such as by using too many prisms in the beam expander, or simply by using the prism-grating combination, or a combination having more than one dispersive prism, whereby the rear optics module becomes a large extension of the overall resonator package, may be used in the embodiments of Figs. 4b-4c. An alternative to the rear optics module 1 50 is to provide the line-selection component or components in contact with or in a housing connected to the discharge chamber 2 or laser tube atmosphere (see U.S. patent application no. 09/31 7,695, which is assigned to the same assignee and is hereby incorporated by reference).
Any of these alternative embodiments of the line-selection package at the opposite side of the resonator as the outcoupler may be advantageously combined with the output coupler 1 30 of Fig. 4b (or the output coupler 1 36 of Fig. 4c, below). That is, the line-selection package of the rear optics module 1 50 (or as set forth in the '695 application), improves the contrast ratio to perhaps 20-1 00, which is advantageous over the characteristic contrast ratio of 7. However, the output coupler 1 30 provides a still higher contrast ratio, e.g., above 100 and perhaps as high as 500 or above. Line narrowing optics (not shown) may be included within the resonator on the outcoupling side of the chamber 2, such as in the front optics module as set forth in U.S. patent applications no. 60/1 66,967, 60/1 66,277 and 60/1 73,993, each of which is assigned to the same assignee, and in U.S. patent no. 5,852,627, each of which is hereby incorporated by reference. When these line-narrowing optics, or variations thereof as understood by one skilled in the art, are used, careful additional beam steering means are used for aligning the optics with the rest of the resonator. This beam steering implies losses, and so it is preferred to use an amplifier after the outcoupler in this embodiment to increase the power of the beam (see the 60/140,531 application, incorporated by reference above).
The outcoupler of the embodiment of Fig. 4b generally has a periodic transmission function having a maximum at or near λ, (see above for the F2 laser) and a minimum at or near λ2. As such, the preferred outcoupler, and those shown at Figs. 4b and 4c, i.e., outcouplers 1 30 and 1 36, has a partially reflecting surface 31 which serves as a resonator reflector of the resonator. An alternative would be that the partially reflecting resonator reflector surface is formed after the line-selection providing portion such as at the back surface of the outcoupler 1 30 or 1 36, or as provided by an etalon outcoupler 46 (see Fig. 4a), and the outcoupler has a reflectivity maximum at or near λ., and a minimum at or near λ2. The transmission function embodiment is, however, preferred since it is the radiation that outcouples directly from the chamber 2, without traversing the rear optics line-selection package 1 50, in which it is preferred to suppress λ2. The outcoupler 1 30 is preferably made of a block of VUV transparent material 1 32 such as CaF2 or one of those other preferred materials mentioned above with advantageous absorption and thermal conductivity characteristics. The material block 1 32 has a special coating 1 34 formed on it. The coating 1 34 is preferably highly compounded with the block 1 32. Advantageously, a preferred thickness of the coating 1 32 between 30 to 50 microns may be, in this way, more easily manufacturable than, e.g., the etalon outcoupler 46, described above. An etalon-like interference effect is produced by the outcoupler 1 30 having the coating 1 32 to perform efficient wavelength selection. That is, the block 1 32 with the coating 1 34 provides the advantageous transmission function described above. Preferably the coating is made of MgF2.
Fig. 4c shows a second embodiment including the discharge chamber 2 and line-selection elements 1 50 preferably as set forth above, and an outcoupler 1 36. The outcoupler 1 36 has a partially reflective surface 1 31 forming the resonator reflector of the resonator, as in the embodiment of Fig. 4b. The outcoupler 1 36 is advantageously formed of MgF2 which provides the periodic transmission function for suppressing the secondary line λ2 and improving the contrast ratio. The small refractive index difference between the ordinary and extraordinary polarizations n0 and ne, respectively, of the MgF2, owing to its birefringent nature, is small enough (i.e., around 0.01 4) that the material thickness of the outcoupler 1 36 is of the order of several millimeters. Preferably, the thickness is around 8.4 mm for producing the desired line-selective transmission.
As set forth above with reference to Fig. 4b, the outcoupler 1 36 forms a highly stable, highly transparent and compact outcoupler. That is, the contrast ratio is advantageously increased for the two characteristic emission lines of the F2 laser, particularly due to the suppression of the radiation generated in the chamber 2 and being directly outcoupled without traversing the rear module 1 50.
Excimer or Molecular Fluorine Laser System The details of the laser resonator, and particularly relating to the line-narrowing and/or selection optics, described above, of the preferred embodiments, may be advantageously used to achieve an excimer or molecular fluorine laser for such industrial applications as microlithography, TFT annealing and micromachining. Referring to Fig. 5, a gas discharge laser system, preferably a DUV or VUV laser system, such as an excimer, e.g., ArF or KrF, or molecular fluorine (F2) laser system for deep ultraviolet (DUV) or vacuum ultraviolet (VUV) lithography, is schematically shown. Alternative configurations for laser systems for use in such other industrial applications as TFT annealing and/or micromachining, e.g., are understood by one skilled in the art as being similar to and/or modified from the system shown in Fig. 5 to meet the requirements of that application. For this purpose, alternative DUV or VUV laser system and component configurations are described at U.S. patent applications no. 09/31 7,695, 09/31 7,526, 09/1 30,277, 09/244,554, 09/452,353, 09/31 7,527, 09/343,333, 60/1 22, 1 45, 60/1 40,531 , 60/1 62,735, 60/1 66,952, 60/1 71 , 1 72, 60/1 41 ,678, 60/1 73,993, 60/1 66,967, 60/147,21 9, 60/1 70,342, 60/1 62,735, 60/1 78,445, 60/1 66,277, 60/1 67,835, 60/1 71 ,91 9, 60/202,564, 60/204,095, 60/1 72,674, 09/574,921 and 60/1 81 , 1 56, and U.S. patents no. 6,005,880, 6,061 ,382, 6,020,723, 5,946,337, 6,014,206, 5,559,81 6, 4,61 1 ,270, 5,761 ,236, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference.
The system shown in Fig. 5 generally includes a laser chamber 202 having a pair of main discharge electrodes 3a, 3bconnected with a solid- state pulser module 204, and a gas handling module 206. The solid-state pulser module 204 is powered by a high voltage power supply 208. The laser chamber 202 is surrounded by optics module 21 0 and optics module 21 2, forming a resonator. The optics modules 21 0 and 21 2 are controlled by an optics control module 21 4, or may be alternatively directly controlled by a computer 21 6.
The computer 21 6 for laser control receives various inputs and controls various operating parameters of the system. A diagnostic module 21 8 receives and measures one or more parameters of a split off portion of the main beam 220 via optics for deflecting a small portion of the beam toward the module 21 8, such as preferably a beam splitter module 222, as shown. The beam 220 is preferably the laser output to an imaging system (not shown) and ultimately to a workpiece (also not shown), and may be output directly to an application process. The laser control computer 21 6 communicates through an interface 224 with a stepper/scanner computer 226 and other control units 228.
Laser Chamber
The laser chamber 202 contains a laser gas mixture and includes one or more preionization electrodes (not shown) in addition to the pair of main discharge electrodes 3a, 3b. Preferred main electrodes 3a, 3b are described at U.S. patent applications no. 09/453,670, 60/1 84,705 and 60/1 28,227, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference. Other electrode configurations are set forth at U.S. patents no. 5,729,565 and 4,860,300, each of which is assigned to the same assignee, and alternative embodiments are set forth at U.S. patents no. 4,691 ,322, 5,535,233 and 5,557,629, all of which are hereby incorporated by reference. Preferred preionization units are described at U.S. patent applications no. 60/1 62,845, 60/1 60, 1 82, 60/1 27,237, 09/535,276 and 09/247,887, each of which is assigned to the same assignee as the present application, and alternative embodiments are set forth at U.S. patents no. 5,337,330, 5,81 8,865 and 5,991 ,324, all of the above patents and patent applications being hereby incorporated by reference. Power Supply Circuit and Pulser Module
The solid-state pulser module 204 and high voltage power supply 208 supply electrical energy in compressed electrical pulses to the preionization and main electrodes 3a, 3b within the laser chamber 202 to energize the gas mixture. Components of the preferred pulser module and high voltage power supply may be described at U.S. patent applications 60/1 49,392, 60/1 98,058, 60/204,095, 09/432,348 and 09/390, 146, and 60/204,095, and U.S. patents no. 6,005,880 and 6,020,723, each of which is assigned to the same assignee as the present application and which is hereby incorporated by reference into the present application. Other alternative pulser modules are described at U.S. patents no. 5,982,800, 5,982,795, 5,940,421 , 5,91 4,974, 5,949,806, 5,936,988, 6,028,872 and 5,729,562, each of which is hereby incorporated by reference. A conventional pulser module may generate electrical pulses in excess of 1 Joule of electrical power (see the '988 patent, mentioned above).
Laser Resonator, General
Several laser resonator configurations may be arranged in accord with the preferred embodiments set forth above. In general, the laser resonator which surrounds the laser chamber 202 containing the laser gas mixture includes optics module 21 0 including line-narrowing optics for a line narrowed excimer or molecular fluorine laser, which may be replaced by a high reflectivity mirror or the like in a laser system wherein either line-narrowing is not desired, or if line narrowing is performed at the front optics module 21 2, or a spectral filter external to the resonator is used, or if the line-narrowing optics are disposed in front of the HR mirror, for narrowing the linewidth of the output beam. The laser chamber 202 is sealed by windows transparent to the wavelengths of the emitted laser radiation 220. The windows may be Brewster windows or may be aligned at another angle, e.g., 5 °, to the optical path of the resonating beam. One of the windows may also serve to output couple the beam.
Extra-Resonator Features
After a portion of the output beam 220 passes the outcoupler of the optics module 21 2, that output portion impinges upon beam splitter module 222 which includes optics for deflecting a portion of the beam to the diagnostic module 21 8, or otherwise allowing a small portion of the outcoupled beam to reach the diagnostic module 21 8, while a main beam portion 220 is allowed to continue as the output beam 220 of the laser system. Preferred optics include a beamsplitter or otherwise partially reflecting surface optic. The optics may also include a mirror or beam splitter as a second reflecting optic. More than one beam splitter and/or HR mirror(s), and/or dichroic mirror(s) may be used to direct portions of the beam to components of the diagnostic module 21 8. A holographic beam sampler, transmission grating, partially transmissive reflection diffraction grating, grism, prism or other refractive, dispersive and/or transmissive optic or optics may also be used to separate a small beam portion from the main beam 220 for detection at the diagnostic module 21 8, while allowing most of the main beam 220 to reach an application process directly or via an imaging system or otherwise.
The output beam 220 may be transmitted at the beam splitter module 222 while a reflected beam portion is directed at the diagnostic module 21 8, or the main beam 220 may be reflected, while a small portion is transmitted to the diagnostic module 21 8. The portion of the outcoupled beam which continues past the beam splitter module 222 is the output beam 220 of the laser, which propagates toward an industrial or experimental application such as an imaging system and workpiece for photolithographic applications. Variations of beam splitter modules 222 particularly for a molecular fluorine laser system are set forth at U.S. patent applications no. 09/598,552 and 60/1 40,530, which are each assigned to the same assignee as the present application and are hereby incorporated by reference.
Beam Path Enclosure
Also particularly for the molecular fluorine laser system, and for the ArF laser system, an enclosure (not shown, but see Fig. 3e, e.g.) may seal the beam path of the beam 220 such as to keep the beam path free of photoabsorbing species. Smaller enclosures may seal the beam path between the chamber 202 and the optics modules 210 and 21 2 and between the beam splitter module 222, which itself may be within the same or a separate enclosure, and the diagnostic module 21 8. The preferred enclosure is described in detail in U.S. patent applications no. 09/343,333, 09/598,552, 09/594,892, 09/1 31 ,580 and 60/1 40,530, each of which is assigned to the same assignee and is hereby incorporated by reference, and U.S. patents no. 5,559,584, 5,221 ,823, 5,763,855, 5,81 1 ,753 and 4,61 6,908, all of which are hereby incorporated by reference, and as set forth above with respect to Fig. 3e.
Diagnostic Module
The diagnostic module 21 8 preferably includes at least one energy detector. This detector measures the total energy of the beam portion that corresponds directly to the energy of the output beam 220 (see U.S. patent no. 4,61 1 ,270 and U.S. patent application no. 09/379,034, each of which is assigned to the same assignee and is hereby incorporated by reference. An optical configuration such as an optical attenuator, e.g., a plate or a coating, or other optics may be formed on or near the detector or beam splitter module 222 to control the intensity, spectral distribution and/or other parameters of the radiation impinging upon the detector (see U.S. patent applications no. 09/1 72,805, 60/1 72,749, 60/1 66,952 and 60/1 78,620, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference).
One other component of the diagnostic module 21 8 is preferably a wavelength and/or bandwidth detection component such as a monitor etalon or grating spectrometer (see U.S. patent applications no. 09/41 6,344, 60/1 86,003, 60/1 58,808, 60/1 86,096, 60/1 86,096 and 60/1 86,096 and 60/202,564, each of which is assigned to the same assignee as the present application, and U.S. patents no. 4,905,243, 5,978,391 , 5,450,207, 4,926,428, 5,748,346, 5,025,445, and 5,978,394, all of the above wavelength and/or bandwidth detection and monitoring components being hereby incorporated by reference. The spectrometer may be within a temperature and pressure controlled housing such as is described in the 60/1 58,808 application.
Other components of the diagnostic module may include a pulse shape detector or ASE detector, such as are described at U.S. patent applications no. 09/484,81 8 and 09/41 8,052, respectively, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference, such as for gas control and/or output beam energy stabilization, or to monitor the amount of amplified spontaneous emission (ASE) within the beam to ensure that the ASE remains below a predetermined level, as set forth in more detail below. There may be a beam alignment monitor, e.g., such as is described at U.S. patent no. 6,01 4,206 which is assigned to the same assignee and is hereby incorporated by reference.
Control Processor The processor or control computer 21 6 receives and processes values of some of the pulse shape, energy, ASE, energy stability, energy overshoot for burst mode operation, wavelength, spectral purity and/or bandwidth, among other input or output parameters of the laser system and output beam. The processor 21 6 also controls the line narrowing module to tune the wavelength and/or bandwidth or spectral purity, and controls the power supply and pulser module 204 and 208 to control preferably the moving average pulse power or energy, such that the energy dose at points on the workpiece is stabilized around a desired value. In addition, the computer 21 6 controls the gas handling module 206 which includes gas supply valves connected to various gas sources.
Further details of the control processor 21 6 such as for performing burst overshoot control and controlling the gas supply unit by monitoring total input energy to the discharge, among other paramters, for determining the timing and amounts of gas replenishment actions, are described at U.S. patent application no. 60/1 59,525, and U.S. patent application of Rebhan and Nowinski, serial number not yet assigned, entitled Energy Control for an Excimer or Molecular Fluorine Laser, filed October 1 6, 2000, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference.
Gas Mixture
The laser gas mixture is initially filled into the laser chamber 202 during new fills. The gas composition for a very stable excimer or molecular fluorine laser in accord with the preferred embodiment uses helium or neon or a mixture of helium and neon as buffer gas(es), depending on the particular laser being used. Preferred gas compositions are described at U.S. patents no. 4,393,405 and 4,977,573 and U.S. patent applications no. 09/31 7,526, 09/51 3,025, 60/1 24,785, 09/41 8,052, 60/1 59,525 and 60/1 60, 1 26, each of which is assigned to the same assignee and is hereby incorporated by reference into the present application. The concentration of the fluorine in the gas mixture may range from 0.003% to 1 .00%, and is preferably around 0.1 %. An additional gas additive, such as a rare gas, such as xenon, may be added for increased energy stability and/or as an attenuator as described in the 09/51 3,025 application incorporated by reference above. Specifically, for the F2-laser, an addition of xenon and/or argon may be used. The concentration of xenon or argon in the mixture may range from 0.0001 % to 0.1 %. For an ArF-laser, an addition of xenon or krypton may be used also having a concentration between 0.0001 % to 0.1 %. For the KrF laser, an addition of xenon or argon may be used also having a concentration between 0.0001 % to 0.1 %.
Gas Replenishment, General
Halogen and rare gas injections, total pressure adjustments and gas replacement procedures are performed using the gas handling module 206 preferably including a vacuum pump, a valve network and one or more gas compartments. The gas handling module 206 receives gas via gas lines connected to gas containers, tanks, canisters and/or bottles. Some prefered and alternative gas handling and/or replenishment procedures are described at U.S. patents no. 4,977,573 and 5,396,514 and U.S. patent applications no. 60/1 24,785, 09/41 8,052, 09/379,034, 60/1 59,525, 60/1 71 ,71 7, and 60/1 59,525, and U.S. patent application of Rebhan and Nowinski, serial number not yet assigned, entitled Energy Control for an Excimer or Molecular Fluorine Laser, filed October 1 6, 2000, each of which is assigned to the same assignee as the present application, and U.S. patents no. 5,978,406, 6,01 4,398 and 6,028,880, all of which are hereby incorporated by reference. A xenon gas supply may be included either internal or external to the laser system according to the '025 application, mentioned above. Line-Narro wing
A general description of the line-narrowing features of the preferred embodiment is provided here, followed by a listing of patent and patent applications being incorporated by reference as describing variations and features that may used with the preferred embodiments described above for providing an output beam with a high spectral purity or bandwidth (e.g., below 0.6 pm) . Exemplary line-narrowing optics contained in the optics module 21 0 include a beam expander, an optional etalon and a diffraction grating, which produces a relatively high degree of dispersion, for a narrow band laser such as is used with a refractive or catadioptric optical lithography imaging system. As referred to above, the front optics module 21 2 may include line-narrowing optics (e.g., etalon, birefringent plate, grating, grism) as well (see the 60/1 66,277, 60/1 73,993 and 60/1 66,967 applications, each being assigned to the same assignee and hereby incorporated by reference).
For a semi-narrow band laser such as is used with an all-reflective imaging system, the grating may be replaced with a highly reflective mirror, and a lower degree of dispersion may be produced by a dispersive prism. A semi-narrow band laser would typically have an output beam linewidth in excess of 1 pm and may be as high as 100 pm in some laser systems, depending on the characteristic broadband bandwidth of the laser.
The beam expander of the above exemplary line-narrowing optics of the optics module 21 0 preferably includes one or more prisms. The beam expander may include other beam expanding optics such as a lens assembly or a converging/diverging lens pair. The grating or a highly reflective mirror is preferably rotatable and in Littrow configuration so that the wavelengths reflected into the acceptance angle of the resonator can be selected or tuned. Alternatively, the grating, or other optic or optics, or the entire line-narrowing module may be pressure tuned, such as is set forth in the 60/1 78,445 and 09/31 7,527 applications, each of which is assigned to the same assignee and is hereby incorporated by reference. The grating may be used both for dispersing the beam for achieving narrow bandwidths and also preferably for retroreflecting the beam back toward the laser tube. Alternatively, a highly reflective mirror may be positioned before or after the grating which receives a reflection from the grating and reflects the beam back toward the grating, such as in a Littman configuration, or the grating may be a transmission grating. One or more dispersive prisms may also be used, and more than one etalon may be used.
Depending on the type and extent of line-narrowing and/or selection and tuning that is desired, and the particular laser that the line- narrowing optics are to be installed into, there are many alternative optical configurations that may be used. For this purpose, those shown in U.S. patents no. 4,399,540, 4,905,243, 5,226,050, 5,559,81 6, 5,659,41 9, 5,663,973, 5,761 ,236, and 5,946,337, and U.S. patent applications no. 09/31 7,695, 09/1 30,277, 09/244,554, 09/31 7,527, 09/073,070, 60/1 24,241 , 60/1 40,532, 60/1 47,21 9 and 60/1 40,531 , 60/1 47,21 9, 60/1 70,342, 60/1 72,749, 60/1 78,620, 60/1 73,993, 60/1 66,277, 60/1 66,967, 60/1 67,835, 60/1 70,91 9, 60/1 86,096, each of which is assigned to the same assignee as the present application, and U.S. patents no. 5,095,492, 5,684,822, 5,835,520, 5,852,627, 5,856,991 , 5,898,725, 5,901 , 1 63, 5,91 7,849, 5,970,082, 5,404,366, 4,975,91 9, 5, 142,543, 5,596,596, 5,802,094, 4,856,01 8, 5,970,082, 5,978,409, 5,999,31 8, 5, 1 50,370 and 4,829,536, and German patent DE 298 22 090.3, are each hereby incorporated by reference into the present application.
Optics module 21 2 preferably includes means for outcoupling the beam 220, such as a partially reflective resonator reflector. The beam 220 may be otherwise outcoupled such as by an intra-resonator beam splitter or partially reflecting surface of another optical element, and the optics module 21 2 would in this case include a highly reflective mirror. The optics control module 214 preferably controls the optics modules 210 and 21 2 such as by receiving and interpreting signals from the processor 21 6, and initiating realignment or reconfiguration procedures (see the '241 , '695, '277, '554, and '527 applications mentioned above).
Advantageously, the localized heating of transmissive optics of the laser resonator, and particularly the plates of etalons and prisms used in the system of the preferred embodiments, the etalon plates and prisms preferably comprising a material having an absorption coefficient of less than 5 x 1 0 3/cm at 248 nm incident radiation, less than 1 0 x 1 0"3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm, and preferably also having a thermal conductivity greater than 2.0 W/m°C, is reduced over that of transmissive components such as etalons and prisms conventionally having fused silica plates for the following reasons. First, a reduced percentage of the incident photon energy is absorbed by the plates and prisms because of the reduced absorption coefficient of the material of the etalon plates and bulk prism materials. In addition, any localized heating is further reduced in the preferred embodiment over fused silica etalons and prisms because the energy absorbed is dissipated more quickly away from the localized area of photon incidence because of the increased thermal conductivity of the etalon plate and prism bulk materials. Thus, operation at higher laser repetition rates such as 1 -4 kHz or more is facilitated, wherein the wavelength stability is maintained at 0.1 pm or less.
Those skilled in the art will appreciate that the just-disclosed preferred embodiments are subject to numerous adaptations and modifications without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope and spirit of the invention, the invention may be practiced other than as specifically described above. For example, the present invention may be incorporated into laser arrangements disclosed in U.S. patents no. 5,856,991 , 5,901 , 1 63 or 5,898,725, each of which is hereby incorporated by reference into the present application. In addition, an etalon similar to that described above may be used for wavelength, bandwidth or energy detection such as are described in U.S. patent no. 4,905,243, which assigned to the same assignee as the present invention, and which is hereby incorporated by reference into the present application. Also, to meet the above objects of the invention, a laser system may utilize an optical component other than an etalon, or an etalon exhibiting reduced finesse may be used, or greater beam expansion may be used to reduce the laser power density on the etalon plates.

Claims

I claim:
1 . An excimer or molecular fluorine laser, comprising: a discharge chamber containing a laser gas mixture; a plurality of electrodes within the discharge chamber and coupled to a pulse power circuit for energizing the gas mixture at a repetition rate of at least 1 kHz; and a resonator for generating a laser beam, wherein the resonator comprises line-narrowing and/or selection optics for reducing the output emission bandwidth of the laser beam including at least one etalon having a pair of plates and at least one beam expanding prism including a bulk refractive material, and wherein said plates and said bulk refractive material are formed from a material having an absorption coefficient of less than 5 x 10 3/cm at 248 nm incident radiation, less than 1 0 x 1 0"3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm incident radiation, such that said bandwidth is reduced to less than substantially 0.6 pm and a wavelength of the line- narrowed output beam exhibits a wavelength stability of less than substantially 0.1 pm.
2. The laser of Claim 1 , wherein said material of said plates of said at least one etalon and bulk material of said one or more prisms has a thermal conductivity greater than 2.0 W/m°C.
3. The laser of Claim 1 , wherein said material of said plates of said at least one etalon and bulk material of said one or more prisms has a thermal conductivity greater than 5.0 W/m°C.
4. The laser of Claim 1 , wherein said at least one etalon includes a transmission etalon, and at least one of said one or more prisms of said beam expander is disposed between said discharge chamber and said transmission etalon.
5. The laser of Claim 1 , wherein said at least one etalon includes an etalon output coupler.
6. An excimer or molecular fluorine laser, comprising: a discharge chamber containing a laser gas mixture; a plurality of electrodes within the discharge chamber and coupled to a pulse power circuit for energizing the gas mixture at a repetition rate of at least 1 kHz; and a resonator for generating a laser beam, wherein the resonator comprises line-narrowing and/or selection optics for reducing the output emission bandwidth of the laser beam, and wherein each at least substantially transmissive optical component of the line-narrowing and/or selection optics is fabricated of a material having an absorption coefficient of less than 5 x 1 0"3/cm at 248 nm incident radiation, less than 1 0 x 1 0"3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm incident radiation, such that said bandwidth is reduced to less than substantially 0.6 pm and a wavelength of the line- narrowed output beam exhibits a wavelength stability of less than substantially 0.1 pm.
7. The laser of Claim 6, wherein said material of said each at least substantially transmissive optical component has a thermal conductivity greater than 2.0 W/m°C.
8. The laser of Claim 6, wherein said material of said each at least substantially transmissive optical component has a thermal conductivity greater than 5.0 W/m°C.
9. The laser of Claim 6, wherein said line-narrowing and/or selection optics include a transmission etalon and one or more beam expanding prisms disposed between said discharge chamber and said transmission etalon.
1 0. The laser of Claim 6, wherein said line-narrowing and/or selection optics include an etalon output coupler.
1 1 . The laser of Claim 6, wherein said laser is a molecular fluorine laser, and an output coupler of said laser has a periodic transmission function for selecting one of two closely-spaced lines around 1 57 nm, and said output coupler is fabricated of a material having an absorption coefficient of less than 5 x 1 0"3/cm at 248 nm incident radiation, less than 10 x 1 0" 3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm incident radiation.
1 2. The laser of Claim 1 1 , wherein said output coupler is fabricated of MgF2.
1 3. The laser of Claim 1 1 , wherein said output coupler is fabricated of CaF2 and MgF2.
14. An excimer or molecular fluorine laser, comprising: a discharge chamber containing a laser gas mixture; a plurality of electrodes within the discharge chamber and coupled to a pulse power circuit for energizing the gas mixture at a repetition rate of at least 1 kHz; and a resonator for generating a laser beam, wherein the resonator comprises line-narrowing and/or selection optics for reducing the output emission bandwidth of the laser beam, and wherein each at least substantially transmissive optical component of the resonator is fabricated of a material having an absorption coefficient of less than 5 x 1 0 3/cm at 248 nm incident radiation, less than 1 0 x 1 0 3/cm at 1 93 nm incident radiation, and less than 0.1 /cm at 1 57 nm incident radiation, such that said bandwidth is reduced to less than substantially 0.6 pm and a wavelength of the line-narrowed output beam exhibits a wavelength stability of less than substantially 0.1 pm.
1 5. The laser of Claim 1 4, wherein said material of said each at least substantially transmissive optical component has a thermal conductivity greater than 2.0 W/m°C.
1 6. The laser of Claim 14, wherein said material of said each at least one substantially transmissive optical component has a thermal conductivity greater than 5.0 W/m°C.
1 7. A KrF excimer laser, comprising: a discharge chamber containing a laser gas mixture including krypton, fluorine and a buffer gas of neon; a plurality of electrodes within the discharge chamber and coupled to a pulse power circuit for energizing the gas mixture at a repetition rate of at least 1 kHz; and a resonator for generating a laser beam, wherein the resonator comprises line-narrowing and/or selection optics for reducing the output emission bandwidth of the laser beam, and wherein each at least substantially transmissive optical component of the line-narrowing and/or selection optics is fabricated of a material having an absorption coefficient of less than 5 x 1 0"3/cm at 248 nm incident radiation of said KrF laser, such that said bandwidth is reduced to less than substantially 0.6 pm and a wavelength of the line-narrowed output beam exhibits a wavelength stability of less than substantially 0.1 pm.
1 8. The KrF laser of Claim 1 7, wherein said line-narrowing and/or selection optics include a transmission etalon and one or more beam expanding prisms disposed between said discharge chamber and said transmission etalon.
1 9. The laser of Claim 1 7, wherein said line-narrowing and/or selection optics include an etalon output coupler.
20. An ArF excimer laser, comprising: a discharge chamber containing a laser gas mixture including argon, fluorine and a buffer gas; a plurality of electrodes within the discharge chamber and coupled to a pulse power circuit for energizing the gas mixture at a repetition rate of at least 1 kHz; and a resonator for generating a laser beam, wherein the resonator comprises line-narrowing and/or selection optics for reducing the output emission bandwidth of the laser beam, and wherein each at least substantially transmissive optical component of the line-narrowing and/or selection optics is fabricated of a material having an absorption coefficient of less than 1 0 x 1 0"3/cm at 1 93 nm incident radiation of said ArF laser, such that said bandwidth is reduced to less than substantially 0.6 pm and a wavelength of the line-narrowed output beam exhibits a wavelength stability of less than substantially 0.1 pm.
21 . A molecular fluorine laser, comprising: a discharge chamber containing a laser gas mixture including fluorine and a buffer gas; a plurality of electrodes within the discharge chamber and coupled to a pulse power circuit for energizing the gas mixture at a repetition rate of at least 1 kHz; and a resonator for generating a laser beam, wherein the resonator comprises line-narrowing and/or selection optics for reducing the output emission bandwidth of the laser beam, and wherein each at least substantially transmissive optical component of the line-narrowing and/or selection optics is fabricated of a material having an absorption coefficient of less than 0.1 /cm at 1 57 nm incident radiation of said molecular fluorine laser, such that said bandwidth is reduced to less than substantially 0.6 pm and a wavelength of the line-narrowed output beam exhibits a wavelength stability of less than substantially 0.1 pm.
22. The molecular fluorine laser of Claim 21 , wherein said line-narrowing and/or selection optics include a transmission etalon and one or more beam expanding prisms disposed between said discharge chamber and said transmission etalon.
23. The molecular fluorine laser of Claim 21 , wherein said line-narrowing and/or selection optics include an etalon output coupler.
24. The laser of any of Claims 1 7-23, wherein said material of said each at least substantially transmissive optical component has a thermal conductivity greater than 2.0 W/m°C.
25. The laser of Claim 24, wherein said material is selected from the group of materials consisting of CaF2, BaF2, LiF, SrF2, MgF2 and fluorine- doped quartz.
26. The laser of Claim 24, wherein said material is CaF2.
27. The laser of any of Claims 1 7-23, wherein said material of said each at least one substantially transmissive optical component has a thermal conductivity greater than 5.0 W/m°C.
28. The laser of any of Claims 1 -1 0 or 14-23, wherein said material is selected from the group of materials consisting of CaF2, BaF2, LiF, SrF2, MgF2 and fluorine-doped quartz.
29. The laser of any of Claims 1 -1 0 or 1 4-23, wherein said material is CaF2.
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US09/599,130 2000-06-22
US09/599,130 US6381256B1 (en) 1999-02-10 2000-06-22 Molecular fluorine laser with spectral linewidth of less than 1 pm
US09/657,396 2000-09-08
US09/657,396 US6426966B1 (en) 1999-02-10 2000-09-08 Molecular fluorine (F2) laser with narrow spectral linewidth
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