WO2006114764A2

WO2006114764A2 - Position measuring system

Info

Publication number: WO2006114764A2
Application number: PCT/IB2006/051283
Authority: WO
Inventors: Cristian Presura
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-04-27
Filing date: 2006-04-25
Publication date: 2006-11-02
Also published as: WO2006114764A3; TW200643369A

Abstract

Known position measuring systems must be well aligned to provide the necessary resolution for applications such as lithography. The proposed position measuring system (1) comprises a radiation emitting element (2) emitting a radiation beam (3). The radiation beam (3) is diffracted by a diffraction grating (7). First and second diffracted beams (10, 11) having symmetric diffraction orders are reflected back to the radiation-emitting element (2). Meanwhile, the actual frequency of the radiation beam (3) emitting from the radiation-emitting element (2) has been changed so that a beat pattern is produced in the radiation-emitting element (2). By means of a measuring beam (23) this beat pattern is detected in a detector (25).

Description

Position measuring system

The present invention relates to a position measuring system of a diffraction element mounted on a lithographic apparatus. More particularly, the present invention relates to an alignment sensor for a lithographic projection apparatus.

State of the art document US 2003/0090814 Al describes a grating scale position measuring system for measuring a movement of an object by observing the light diffracted from a grating attached to the object. In this known system aspheric lenses provide a subsystem with magnification of 1, and a magnifying system including negative lenses increases the magnification of an intensity pattern to enable accurate measurement of the phase of the intensity pattern. Thereby, a laser diode transmits a laser beam and the reflected light corresponding to the first order maxima of the grating passes through the magnifying system and is detected by a detector.

The grating scale position measuring system known from US 2003/0090814 Al has the disadvantage that the alignment of its components must be very accurate. This is because the components must be arranged so that their reflected light beams corresponding to the first order maxima of the grating impinge on the same point on the surface of the detector. Further, the known system is very susceptible to shocks and such.

It is an object of the invention to provide a position measuring system with a rugged construction and a reduced construction size.

This object is solved by a position measuring system as defined in claim 1. Advantageous developments of the invention are mentioned in the dependent claims. An advantage of the present invention is its brought field of application.

Possible uses of the position measuring system of the invention concern to a lithographic apparatus, but also to alignment sensors for rough environmental conditions, where the measuring system is exposed to shocks, vibrations and such. Also, readjusting the system is rarely necessary and the reliability is high. It is to be noted that symmetric diffraction orders are orders of diffraction that are symmetric with respect to the order of the radiation beam incident on the diffraction element. For example, the +1 and -1 diffraction orders are of symmetric diffraction order, when the radiation beam is incident on the diffraction element in zeroth order. Further, it is to be noted that the diffraction element is not necessarily a part of the position measuring system as claimed. The position measuring system can be sold without a diffraction element. The diffraction element is then mounted on a lithographic apparatus or some other device. Also, the diffraction element can be a part of or mounted on each of a plurality of substrates, respectively. The measure as defined in claim 2 has the advantage that the interference is also periodically repeated so that the detection of the interference in the radiation element is simplified and can be provided with increased accuracy.

According to the measure as defined in claim 3 the frequency of the radiation beam is controlled by an electric current. For example, the radiation element can be build up or comprise a semiconductor laser with temperature-dependent frequency in free running mode. Then, the control unit controls the frequency of the radiation beam by the electric current flowing throw the semiconductor laser. Thereby it is advantageous that the current is controlled on the basis of a triangular waveform.

The measure as defined in claim 7 has the advantage, that the accuracy is enhanced. For example, depending on the application, diffracted beams of one or more higher orders can be selected, for instance the diffracted beams of the tenth order. The resolution is then enhanced.

It is further advantageous that the reflecting element is adapted according to the measure as defined in claim 8. With this measure, the amplitude of the beat signal can be increased to improve detection.

The measures as defined in claims 9 and 10 have the advantage of and simple construction, for example, the detector can be build up or comprise a semiconductor photodiode.

The measure as defined in claim 11 has the advantage that an analysis chosen from a great variety of analyzing methods can be selected to enhance the accuracy of the position measuring system. Therefore, digital sampling of the electric signal can be provided.

The measure as defined in claim 12 has the advantage, that the analysis unit can focus on the beat signal, whereby interferences are suppressed. According to the measure as defined in claim 13 a high reliability of the system is achieved.

According to the measure as defined in claim 14 the modulation frequency is selected on the basis of the length of the light path between the radiation element end the diffraction element. Thereby, the modulation frequency is increased, when the length of the light path is decreased, that is in generally, that the distance between the radiation element and the diffraction element is decreased.

The measures as defined in claims 15 and 16 have the advantage that the resolution for detecting a movement of the diffraction element is optimized. The device manufacturing method according to claim 19 can comprise the further steps of: providing a substrate that is at least partially covered by a layer of radiation- sensitive material; providing a projection beam of radiation using a radiation system; - using patterning means to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation, using at least one optical member, onto a target portion of the layer of radiation-sensitive material; and measuring the position of the optical member with a position measuring system of the invention.

In such a method, the position measuring system of the invention can also be used to adjust a substrate to a mask.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereinafter.

The present invention will become readily understood from the following description of a preferred embodiment thereof made with reference to the accompanying drawings, in which like parts are designated by like reference signs and in which: Fig. 1 shows a position measuring system according to a preferred embodiment of the present invention; and

Fig. 2 shows graphs illustrating the operation of the position measuring system of the preferred embodiment of the invention. Fig. 1 shows a position measuring system 1 according to a preferred embodiment of the invention. The position measuring system 1 can be used as an alignment sensor for a lithographic apparatus. But, the position measuring system 1 of the invention is not limited to such an alignment sensor and can also be used in other applications.

The position measuring system 1, as shown in Fig. 1, comprises a radiation- emitting element 2. The radiation-emitting element 2 can be build up by or comprise a semiconductor laser 2. The radiation-emitting element 2 is emitting a radiation beam 3 that passes through a collimating lens 4 and a collimating lens 5, and is then incidented on a diffraction element 6. The diffraction element 6 comprises a diffraction grating 7 and is mounted on an object (not shown) of a lithographic apparatus.

A blocking element 8 is arranged between the collimating lens 4 and the collimating lens 5, and the radiation beam 3 passes also through an opening 9 of the blocking element 8. The radiation beam 3 incidenting on the diffraction element 6 is diffracted by the diffraction grating 7 adapted as a reflection grating 7 so that a diffracted beam 10 of +1 order, a diffracted beam 11 of-1 order, a diffracted beam 12 of +2 order and a diffracted beam of -2 order diffraction are reflected back towards the radiation emitting element 2.

It is to be noted, that the diffraction grating 7 of the diffraction element 6 can also be adapted as a Bragg grating so that the radiation beam 3 is diffracted while passing through the diffraction element 6.

Further it is to be noted, that the radiation beam 3 is also reflected in zeroth order from the diffraction element 6. In the embodiment of the position measuring system 1 the zeroth order reflection is reflected back to the radiation-emitting element 2. But, in an other arrangement it is also possible to block or suppress the zeroth order reflection. Further order diffractions can also occur, but to simplify the illustration only diffractions up to the second order are shown in Fig. 1.

The diffracted beams 12 and 13 of second order diffraction are passing through the collimating lens 5, and are then blocked by the blocking element 8. The opening 9 of the blocking element 8 is adapted so that only the diffracted beams 10, 11 of first order diffraction are passing through the collimating lens 5 and the opening 9 of the blocking element 8. Hence, the diffracted beams 10, 11 are selected, and they pass further trough the collimating lens 4 to be focused on a front surface 14 of the radiation emitting element 2. Therefore, the diffracted beams 10, 11 are reflected back to the radiation-emitting element 2. At the front surface 14, the diffracted beams 10, 11 are coupled in the radiation-emitting element 2. In the radiation-emitting element 2 the diffracted beams 10, 11 interfere with each other. Depending on the specific position of the refection element 6, which is moveable in a direction 15, the phase difference between the diffracted beams 10, 11 varies so that a constructive, partly constructive, a destructive or a partly destructive interference is caused between the diffracted beams 10, 11 in the radiation emitting element 2. Hence, starting with a position of the diffraction element 6, where a constructive interference between the diffracted beams 10, 11 is caused, the amplitude of the interference between the diffracted beams 10, 11 declines, until it vanishes, and is than rising up to the maximum amplitude again. The process is repeated with the further movement of the diffraction element 6 in the direction 15. Thereby, one period is correlated to a movement of the diffraction element 6 by a pitch distance 16 of the diffraction grating 7.

The radiation-emitting element 2 is connected with a control unit 20 via lines 21, 22. The control unit 20 applies a varying current to the radiation-emitting element 2 to modulate the frequency of the radiation beam 3 emitted from the radiation-emitting element 2. When the diffracted beams 10, 11 are reflected back into the radiation-emitting element 2, the frequency of the radiation beam 3 has changed. In the radiation-emitting element 2, then a superposition of the diffracted beams 10, 11 interfering with each other, and the actual mode of the radiation-emitting element 2 occurs. This superposition produces a beat pattern, wherein the amplitude of the beat pattern depends also on the amplitude of the interference between the diffracted beams 10, 11. Hence, when the diffracted beams 10, 11 are destructively interfering, only a noisy interference signal is produced by the zeroth order diffraction of the radiation beam 3. Otherwise, with constructive interference of the diffracted beams 10, 11, the amplitude of the beat pattern rises to its maximum. The interference of the actual mode in the radiation emitting element 2 and the diffracted beams 10, 11 produces a modulated radiation beam 23 that is decoupled at a rear surface 24 of the radiation emitting element 2 as a measuring beam 23. The measuring beam 23 is detected by a detector 25 that can be built up by a semiconductor photodiode. The detector 25 measures the power of the measuring beam 23. This power is modulated with the beat pattern described above. Further, a higher power of the measuring beam 23 is detected, when the frequency of the actual mode of the radiation emitting element 2 is higher due to a higher current applied by the control unit 20 and vice versa.

The detector 25 outputs an electric signal on the basis of the power measured towards an analysis unit 26. Thereby, the electric signal first passes through a high-pass filter 27 to band pass the low frequency part of the electric signal based on the current modulation from the control unit 20. Therefor, the high-pass filter 27 is connected on one side with the detector 25 over a line 28, and is connected on the other side with the analysis unit 26 over the line 29. Fig. 2 shows graphs illustrating the operation of the position measuring system

1 of the invention. Thereby, in each of the graphs, a time coordinate is displayed on a horizontal axis 30, 31, 32. And, each shown vertical axis 33, 34, 35 shows the amplitude of the respective signal in suitable units.

The control unit 20 applies a current to the radiation-emitting element 2 having a waveform 36 of a triangular shape. Thereby, the current varies between a minimum value 37 and a maximum value 38. Thereby, the range from the minimum value 37 to the maximum value 38 of the current lies within the range of a current for a normal operation of the radiation-emitting element 2. The frequency of the radiation beam 3, of the radiation emitting element 2 follows the waveform 36. Hence, the radiation beam 3 has a maximum frequency at time instant tl and a minimum frequency at time instant t2.

A waveform 39 shows the electric signal 39 output from the detector 25 over line 28. The waveform 39 comprises a low frequency component in coincidence with the waveform 36 superimposed by a beat signal from the beat pattern described above. The low frequency component is band filtered by the high-pass filter 27 so that only the beat signal passes to the analysis unit 26. As shown in Fig. 2, the maxima and minima of the electric signal, as shown by the waveform 39, do not necessarily correspond to the maxima and minima of the modulated current, as shown by the waveform 36. In Fig. 2, a minimum of the beat pattern occurs at time instant tl, so that a local minimum for the power of the measuring beam 23 is detected by the detector 25. For the same reason, the electric signal has a noticeable minimum at time instant t2.

A waveform 40 of the resulting signal, as received by the analysis unit 26 over line 29, shows a regular pattern similar to a sinus waveform. The electric signal shown by the waveform 40 has an amplitude 41. The analysis unit 26 analyzes this electric signal and measures, for instance, the amplitude 41. When the diffraction element 6 is moved in the direction 15, the amplitude 41 changes. In case of a continued movement, as shown by the three dots 42, the beat pattern will at least nearly vanish, as shown by the waveform 43. The analysis unit 26 takes the waveform 43 according to the at least nearly vanishing beat signal and the waveform 40 of a maximum amplitude 41 as reference points for the analysis. Therewith, the analysis unit 26 can easily detected a one pitch 16 movement of the diffraction element 6. Further, the analysis unit interpolates between the reference waveforms 40, 43 to provide a high resolution.

Therefore, the position measuring system 1 can operate without a reference mark. Although an exemplary embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. Such modifications to the inventive concept are intended to be covered by the appended claims in which the reference signs shall not be construed as limiting the scope of the invention. Further, in the description and the appended claims the meaning of "comprising" is not to be understood as excluding other elements or steps. Further, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfill the functions of several means recited in the claims. Also, the wavelength of the radiation beams is not limited to the visible spectrum.

Claims

CLAIMS:

1. Position measuring system (1) comprising: a radiation emitting element (2) for emitting a radiation beam (3) to a diffraction element (6) for diffracting the radiation beam (3) in at least a first diffracted beam (10) and a second diffracted beam (11), the first diffracted beam (10) and the second diffracted beam (11) having symmetric diffraction orders, the radiation emitting element (2) being arranged to receive the first diffracted beam (10) and the second diffracted beam (11) such that the first diffracted beam (10) and the second diffracted beam (11) interfere in said radiation emitting element (2), a control unit (20) for modulating a frequency of the radiation beam (3), and - a detector (25) for detecting the interference of the first diffracted beam (10) and the second diffracted beam (11) in the radiation emitting element (2) in dependence of the modulation of the frequency of the radiation beam.

2. Position measuring system according to claim 1, characterized in that the control unit (20) is adapted to achieve a periodic variation of said frequency of said radiation beam (3).

3. Position measuring system according to claim 1 or 2, characterized in that said radiation emitting element (2) is adapted to emit said radiation beam (3) with a frequency that depends on an actual value of a electric current flowing through said radiation emitting element, and that said control unit (20) is adapted to control said electric current flowing through said radiation emitting element.

4. Position measuring system according to claim 3, characterized in that said control unit (20) is adapted to control said current flow on the basis of a triangular waveform.

5. Position measuring system according to claim 1, characterized in that said radiation emitting element (2) comprises a semiconductor laser.

6. Position measuring system according to claim 1 or 5, characterized in that said diffraction element (6) comprises a diffraction grating (7).

7. Position measuring system according to claim 1, characterized by a blocking element (8) for selecting beams (10, 11) diffracted by said diffraction element (6), wherein only beams (10, 11) selected by said blocking element are reflected back to said radiation emitting element (2), and said blocking element selects at least said first diffracted beam (10) and said second diffracted beam (11).

8. Position measuring system according to claim 7, characterized in that said diffraction element (6) is adapted so that an intensity of at least said first and second beams (10, 11) selected by said blocking element (8) is relatively high.

9. Position measuring system according to claim 1, characterized in that said detector (25) is arranged to detect a measuring beam (23) decoupled from said radiation emitting element (2).

10. Position measuring system according to claim 9, characterized in that said detector (25) measures an intensity of said measuring beam (23).

11. Position measuring system according to claim 10, characterized in that said detector (25) outputs an electric signal on the basis of said intensity measured, that said electric signal is output to an analysis unit (26), and that said analysis unit analyzes said electric signal to determine a relative position of said diffraction element (6).

12. Position measuring system according to claim 11, characterized by a high-pass filter (27) arranged between said detector (25) and said analysis unit (26) for filtering said electric signal, wherein said control unit (20) modulates said frequency of said radiation beam with a modulation frequency, and wherein said high-pass filter is adapted to band pass a beat signal of said electric signal having a beat frequency that is higher than said modulation frequency.

13. Position measuring system according to claim 12, characterized in that said analysis unit analyzes an amplitude of said beat signal.

14. Position measuring system according to claim 12 or 13, characterized in that said control unit (20) is adapted to provide a modulation frequency that is great enough to provide a substantial frequence difference between the actual frequency of said radiation beam (3) of said radiation emitting element (2) and a frequency of said first and second diffracted beams (10, 11), when they return to the radiation emitting element.

15. Position measuring system according to claim 1, characterized in that said diffraction element (6) is arranged so that an angle of incidence of said radiation beam (3) is at least nearly 90 degree.

16. Position measuring system according to claim 1 or 15, characterized in that said diffraction element (6) is moveable in a direction (15) that is at least nearly perpendicular to a direction of incidence of said radiation beam (3) on said diffraction element (6).

17. Position measuring system according to claim 1 for measuring a displacement of a diffraction element in a direction perpendicular to said radiation beam.

18. Lithographic projection apparatus comprising a position measuring system according to one of claims 1 to 17.

19. Device manufacturing method for manufacturing an electronic device, in which a position of a substrate is adjusted by use of a position measuring system according to one of claims 1 to 17.