WO2002031600A1

WO2002031600A1 - Deep grayscale etching of silicon

Info

Publication number: WO2002031600A1
Application number: PCT/US2001/042629
Authority: WO
Inventors: Michael Ray Whitley; Rodney L. Clark; Russell Jay Shaw; David Renick Brown; Peter Scott Erbach; Gregg T. Borek
Original assignee: Mems Optical, Inc.
Priority date: 2000-10-10
Filing date: 2001-10-10
Publication date: 2002-04-18
Also published as: AU2002211892A1

Abstract

Curved structures are etched into silicon suitable for utilization in meso-machines. The curved structures are obtained by patterning photoresist using gray scale technology, followed by an ICP-machining process of alternating sequential etching and polymerization steps. The curved structures can also be obtain using a two step etching process where the gray scale patterned photoresist is first etched using reactive ion etching (RIE), followed by ICP machining by alternating sequential etching and polymerization steps.

Description

DEEP GRAYSCALE ETCHING OF SILICON

FIELD OF THE INVENTION

[0001] Grayscale photolithography and ICP-machining are used to deeply etch curved structures and can be utilized to construct a variety of meso-scale machines.

BACKGROUND OF THE INVENTION

[0002] There is a need in a variety of MEMS (micro- electro-mechanical systems) for small structures that are curved or non-linear in a direction orthogonal to the direction of the substrate. Up to now, there has been no known way to etch such small curved structures deeply into silicon. These small curved structures are less than 100 μ in size, and curved structures of this magnitude have heretofore not been amenable to traditional etching process such as RIE (Reactive Ion Etch) and ICP-machining (Inductively Coupled Plasma-Machining) . These curved structures would be desirably used to produce components such as turbine rotors and micro-lenses necessary to form micro-scale or meso-scale machines in silicon or other desirable materials.

[0003] ICP-machining has been used to perform highly anisotropic etch of large structures on a scale larger than 100 μm in size, and usually on a scale of several hundreds of μm in size. However, ICP-machining has only been used to make binary, i.e., anisotropic, structures because of the pulsed mode of operation of this technique. ICP-machining works in alternating pulses of high rate etch in a plasma, followed by passivation in the presence of a polymer former. Multiple cycles of etch and passivation are repeated until the desired etch depth is attained. These repeated cycles result in. artifacts characterized by the formation of steps or ridges in ^"the finished workpiece. As a result, ICP machining has not been utilized in the formation of small curved structures orthogonal to the direction of the substrate.

[0004] . The highly anisotropic ICP-machining method of etching silicon is typified by U.S. Patent 5,501,893 to Laermer et al. U.S. Patent 5,501,893 anisotropically etches silicon to provide a laterally defined recess structure through an etching mask employing a plasma. The method of U.S. Patent 5,501,893 includes an etching step on a surface of the silicon by contacting a reactive etching gas to remove material from the surface of the silicon and provide exposed surfaces. Etching with an etchant gas is usually performed in a timed pulse. Following the etching step is a passivation step using a polymer former. A passivation time of one minute is required to form an approximately 50 nm thick polymer layer on the substrate, as is discussed at column 5, lines 3-7 of U.S. Patent 5,501,893. Polymerization of at least one polymer former contained in the plasma applied onto the surface of the silicon covers the etched surface of the silicon with a polymer layer to thereby form a temporary etching stop. Alternately repeating the etching step- and the masking step provides selectivity combined with anisotropy of the individual etching steps, as is discussed in column 4, lines 49-52 of U.S. Patent 5,501,893. However, the anisotropy is accompanied by the formation of steps or ridges, which is described as a partially stripped polymer at column 4, lines 38-44 of U.S. Patent 5,501,893.

[0005] ICP machining has not heretofore been considered suitable for etching curved features into the substrate. It has heretofore been believed that ICP machining is limited to the etching of digital features. The step-forming phenomenon has lead the inventors to believe that ICP machining is inappropriate for utilization in forming deep etching curved structures orthogonal to the surface of the substrate.

[0006] RIE (reactive ion etch) uses a radio frequency signal to generate a plasma of chemically reactive gas to etch the substrate. Because the substrate is placed directly onto the cathode, momentum transfer plays a significant role in the etching process. Thus by varying the process parameters, etching may be performed either isotropically or anisotropically. RIE has been used in analog photoresist applications, but RIE etch rates are very low. As a result, RIE is* not a practical method to etch curved structures having much depth.

[0007] An additional disadvantage of RIE is a low selectivity to silicon. RIE will typically have a selectivity of silicon to photoresist to 4:1 to 10:1. On the other hand, ICP-machining has a selectivity of 70:1 to 100:1. Since a plasma etch will typically etch at a rate of 2.5 μm per minute, a high selectivity such as is shown by ICP machining will result in a reduced thickness of photoresist being required.

[0008] A typical application using RIE is discussed in U.S Patent 5,628,917 to MacDonald et al. MacDonald discusses a masking process for fabricating ultra-high aspect ratio, wafer-free MOEM structures. A single mask process allows the formation of releasable three dimensional frame-like objects which can be up to about half the wafer thickness in depth, and which can be lifted off the substrate. However, this is a trench RIE process which is not suitable to the formation of small curved structures. [0009] The micro-machining of deeply curved small objects such as gigahertz lenses or turbine rotors requires the utilization of an analog patterning method such as grayscale. However, etching methods such as RIE or ICP- machining have characteristic anisotropies that were believed to only be amenable to non-analog binary applications. Further, ICP-machining produces artifacts such as ridges and step formation that are inimical to the formation of smooth, deeply curved structures. As a result, RIE and ICP-machining have been considered totally unsuitable to create the smooth, deeply etched curved surfaces required in many micro-machine applications . [0010] Micro-machining is based upon MEMS (Micro-

Electro-Mechanical Systems) or MOEMS (Micro-Opto-Electro- Mechanical Systems) , though both refer to essentially the same technology. The basis for these technologies is micromachining, which involves the manufacture of mechanical structures in the micron to millimeter range. Precision machining is typically work done above the mm range and molecular machining is work done in the nanometer or molecular regime. When one combines micro-machined devices with micro-electronics and micro-optics, MEMS/MOEMS technology is the result. However, these technologies lack the finesse to produce the small, deeply curved structures necessary for micro-optics less than 100 μm in size. [0011] As discussed above, an additional phenomenon associated with ICP-machining arises from the alternating pulses of plasma etch and passivation in the presence of a polymer former. These alternating pulses result in the formation of anisotropic "steps" or ridge structures in the surface of the silicon substrate. The formation of these steps prevent the formation of the smooth curved surfaces necessary for applications such as micro-lenses and turbine rotors.

[0012] The anisotropy and step formation associated with traditional etch process such as RIE and ICP-machining has heretofore prevented the utilization of these etch processes in the manufacture of smooth curved artifacts such as micro-lenses. RIE and ICP-machining have heretofore been used in macro-type processes where the feature size is in excess of 100 μm and is frequently at the scale of 100' s of μm. As a result, the application of RIE and ICP-machining to micro-scale processing of smooth curved surfaces less than 100 μm has heretofore been considered counterintuitive by the inventors .

[0013] The utilization of gray scale optics in the formation of products such as microlenses is typified in U.S. Patent 5,310,623 to Gal. Gal pertains to a microlens replica formed in the photoresist material with a gray scale mask of a selected wavelength (usually in the uv) , and the material replica is subsequently used to reproduce the replica directly in the substrate by differential ion milling. Differential ion milling entails shooting a stream of argon atoms at the substrate, and it can be considered to be analogous to sandblasting. As a result, differential ion milling is amenable to the formation of curved micro- structures. However, differential ion milling has the disadvantage of not being able to readily perform the deep etching necessary to form the structures necessary for micro-lenses or turbine rotors.

[0014] Standard gray scale techniques such as that of Gal allow the etching of shallow (< 100 μm) structures, but have, the disadvantage of needing thick layers of photoresist, where an approximately 20 μm layer of photoresist is required to produce a 100 μm structure. [0015] As has been shown, there has heretofore been no process that can readily form smooth curved micro- structures necessary for miniaturized electro-optics and turbine rotors. Traditional etching method such as ICP- machining and RIE are highly anisotropic and leave artifacts which infer that these methods are not suitable for the formation of smooth curved surfaces less than 100 μm. [0016] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

SUMMARY OF THE INVENTION

[0017] The inventors have discovered that the combination of plasma etching with gray scale technology unexpectedly allows the formation of uniform curved structures necessary for the manufacture of advanced micro machines and meso machines.

[0018] The invention, in part, pertains to a method to etch a smooth curved structure which includes coating a substrate with photoresist, patterning the photoresist using at least one gray scale mask to produce a photomask having the smooth curved structure orthogonal to a surface of the substrate, developing the photoresist, and etching the substrate using ICP-machining.

[0019] The invention, in part, pertains to a method to etch a smooth curved structure that includes coating a substrate with photoresist, patterning the photoresist using at least one gray scale mask to produce a photomask having the smooth curved structure orthogonal to a surface of the substrate, developing the photoresist, etching the substrate using reactive ion etching, and etching the substrate using ICP-machining.

[0020] The invention, in part, pertains to the ICP- machining being a plasma etching process performed by contacting - the substrate with a reactive etchant gas to remove material from the surface of the substrate and provide exposed surfaces, and polymerizing using at least one polymer former contained in the plasma.

[0021] The invention, in part, pertains to a micro- machined device which has a smooth curved structure orthogonal to a surface of a substrate, and the smooth curved structure is formed by coating the substrate with photoresist, patterning the photoresist using at least one gray scale mask to produce a photomask having the smooth curved structure orthogonal to the surface of the substrate, developing the photoresist, and etching the substrate using ICP-machining.

[0022] The invention, in part, pertains to a micro- machined device which has a smooth curved structure orthogonal to a surface of a substrate, and the smooth curved structure is formed by coating the substrate with photoresist, patterning the photoresist using at least one gray scale mask to produce a photomask having the smooth curved structure orthogonal to a surface of the substrate, developing the photoresist, etching the substrate using reactive ion etching, and etching the substrate using ICP- machining.

[0023] The invention, in part, allows the etching of curved, non-binary, structures deeply into silicon. The invention combines gray scale technology with ICP-machining to result, in smooth curved structures suitable for use as micro-lenses br turbine rotors. Alternatively, the invention, in part, combines gray scale photolithography with a two step etch of standard RIE followed by ICP- machining.

[0024] The inventors have discovered that despite the drawbacks of anisotropy and ridge formation associated with ICP-machining, the combination of ICP-machining with gray scale etch can successfully form smoothly curved features of a size less than 100 μm or even 50 μm. A short pulse cycle time of, e.g., six seconds, of the invention minimizes step formation so as to very closely approximate the topography of a smooth curve. As a result, micro- turbine blades and micro-machines incorporating sophisticated electro-optic components are now feasible. As been demonstrated by the inventors, ICP-machining can be used to etch isotropic micro-scale features in addition to the traditional etching of anisotropic macro-scale features.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are included to provide a further understanding of the invention. The drawings illustrate embodiments of the invention and together with the description serve to explain the principles of the embodiments of the invention.

[0026] Figure 1 is a diagram showing a curved structure orthogonal to the surface of the substrate.

[0027] Figure 2 is a photomicrograph showing 164 μm deep structures etched into silicon according to an embodiment of the present invention. [0028] Figure 3 is a photomicrograph showing a side view of 164 μm deep structures etched into silicon according to an embodiment of the present invention.

[0029] Figure 4 shows a two step etch process according to an embodiment of the present invention. [0030] Figure 5 is a photomicrograph showing 150 μm deep structures with photoresist plateaus etched into silicon according to the two step etch process of the present invention.

[0031] Figure 6 is a photomicrographic image of an impeller implant showing the test quadrant for etch development .

[0032] Figure 7 is an SEM image showing a side view of the impeller rib quadrant.

[0033] Figure 8 shows the metrology, i.e., measurement, locations on the impeller rib.

[0034] Figure 9 shows a slice profile across section

7ΛA of the impeller rib quadrant bisecting the impeller rib.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0035] Advantages of the present invention will become more apparent from the detailed description given herein. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modification within the spirit and scope of the invention will become apparent to those skilled in the art form this detailed description.

[0036] The invention incorporates gray scale photolithography with ICP-machining techniques to allow the formation of curved, non-binary structures orthogonal to the substrate. surface and are deeply etched into the substrate. Alternately, the invention uses gray scale photolithography followed by standard RIE (Reactive Ion Etching) to form shallow curved, non-binary structures, and a subsequent ICP- machining etch deeply etches the structures into the substrate.

[0037] Regarding orthogonality, Fig. 1 shows a curved structure 1, orthogonal to the surface 2 of a substrate 3. The orthogonality is shown where the arc 4 of the curved structure 1 lies in a plane perpendicular to the surface 2. In contrast, a non-orthogonal curve (not shown) amenable to known anisotropic etch processes would have an arc in the plane of the surface of the substrate. [0038] Applications of the invention include utilization of the technology to fabricate turbine rotors and micro-lenses The lenses can include GHz lenses and high frequency lenses .

[0039] Standard gray scale techniques allow the etching of shallow curved structures of a magnitude of less than 100 μm using an etch method such as ion milling with argon. Standard gray scale techniques require thick layers of photoresist which are usually approximately 20 μm deep. In contrast, the invention utilizes ICP-machining or RIE/ICP-machining which can etch deeply using a photoresist layer between 1 μm and 40 μm in depth. The ability of ICP- machining to use a thinner photoresist layer arises from the high selectivity of ICP-machining to silicon, which can be from about 70:1 to 100:1. In contrast, RIE has a selectivity to silicon ranging from about 4:1 to 10:1. [0040] The differential between the selectivities of ICP-machining and RIE to silicon can be advantageously utilized in the invention. RIE can be used to etch a shallow curved structure by utilizing RIE's comparatively more linear feature selectivity. Following the RIE etch, ICP machining can be used to perform a' deep anisotropic etch to form a deeply curved structure. The etch sequence can also be reversed so that an initial deep structure can be etched using ICP-machining, and then a slightly curved surface to the structure can be added using RIE. Additionally, photoresist can be stripped and/or added between the two etch steps. As a result, the invention can be used to manufacture highly asymmetric structural features.

[0041] Gray scale photolithography of the invention uses an exposure mask, which can be constructed with a plurality of precisely located and sized light transmitting openings. This technology is typified by using a chrome mask having small openings. The openings are formed with sufficiently small specific opening sizes and are located at a sufficiently large number of specific locations which correlate to related locations on the desired object, to allow an image of the designed object to be produced in a photoresist material.

[0042] Gray scale photolithography typified by U.S.

Patent 5,482,800 and 5,310,623 to Gal uses a single pixel exposure mask subdivided into subpixels. Each subpixel is in turn subdivided into gray scale resolution elements. A typical pixel can be 80 μm on each side, each subpixel can be 2 μm on each side, and each gray scale resolution element can be 0.2 μm on each side. The exposing light is uv light of 0.3 μm wavelength. The resolution elements can be arranged in groups so as to enable a full wavelength of the uv light to pass through an opening formed by the aligned resolution elements. Infrared light exposure is used with a 128 shade, gray scale. However, uv light is preferably used with a 9000 shade gray scale.

[0043] Patterning the photoresist to form a photomask layer can be performed using a single gray scale mask. Alternatively, patterning the photoresist to produce a variable thickness photomask layer can be accomplished by exposing with 2 or more gray scale masks.

[0044] By using an appropriate pattern, an exposure in a photoresist material can be created which will cause the height of the processed photoresist material to replicate the height of the desired workpiece. The exposed photoresist can then be processed by developing using known methods to produce an impression of the desired pattern in the developed photoresist.

[0045] The image is produced by exposing the photoresist material to light of a selected wavelength through the gray scale mask, transmitted through openings in the exposure mask for a selected time period. The light is usually ultraviolet light. The exposed photoresist material is subsequently processed to procure the desired object on a substrate material using an etching method such as ICP- machining or RIE/ICP-machining.

[0046] In an embodiment of the invention, a thin film of photoresist is applied to the substrate. One method of applying photoresist entails placing a drop of liquid photoresist onto a silicon wafer, then rapidly spinning the wafer to achieve a uniform thin coating of photoresist on the surface of the substrate. The photoresist is then exposed to ultraviolet light through a gray scale mask. The photoresist is then developed using known methods to produce an impression of the desired workpiece in the photoresist layer. The developed photoresist is then treated with ICP- machining or, alternately, by RIE followed by ICP-machining. [0047] The substrate material is preferably silicon. However, the substrate may be selected from any number of materials, which can be silicon, GaAs, plastic, glass, quartz or metals such Cu, Al and Ge.

[0048] The photoresist can be a positive or negative photoresist. The positive photoresist material can be a novalak or phenyl-formaldehyde resin. The negative photoresist materiel can be a polyimide. Epoxy based negative resists have been used in MEMS processing. The preferred photoresist is a positive novalak pho6toresist . The specific type of photoresist is selected for, among other characteristics, the desired depth of the photoresist layer. The photoresist layer can be from 1 μm to 40 μm in thickness.

[0049] RIE (Reactive Ion Etch) uses a radio frequency signal to generate a plasma of chemically reactive gas used to etch various materials. Because the surface to be etched is placed directly onto the cathode, momentum transfer plays a significant role in the etching process. Thus by varying the process parameters, etching may be done either isotropically or anisotropically.

[0050] RIE uses mass flow controllers (MFCs) to assure precise control of the process gases. An exhaust valve controller controls chamber pressure. A power supply provides power to generate plasma. The plasma generating power can typically can be up to 300 watts at 30KHz. [0051] For standard RIE etching of grayscale photoresist structures, the choice of process gases is dependent on the material being etched. For silicon, SF₆ and 0₂ are typically used, although other gases can and are used depending, of the application. Other process gases used include, but are not limited to CF₄ and CHF₃, which can also be mixed with 0₂. The variation of process parameters allows a desired selectivity for the etch. Selectivity is defined as the etch rate of the photoresist versus the etch rate of the substrate.

[0052] The ICP-machining method permits an etching rate of between 2 and 20 μm/min and a polymer layer that can be an approximately 50 nm thick TEFLON-like polymer (polytetrafluoroethylene-like polymer) layer. The polymerization step can be performed using a mixture of Ar and CHF₃. The etching step can be performed for a sufficient duration to attain an etching depth of approximately 2-3 μm. The ICP-machining method uses a plasma which can be generated using microwave energy at outputs between 300 and 1200W (2.45 GHz). During the etching steps, an ion bombardment with energies between 5 and 30 eV can be used so as to have the structure base completely free form deposits from the plasma.

[0053] Using a standard RIE, selectivities of from

4:1 to 10:1 can be achieved for silicon (different materials have different selectivities) . This means that at a selectivity of 5:1, for every 5 microns of silicon etched, 1 micron of photoresist will be etched. This limits the depth of structures that can be etched, because of the limits in patterning photoresist to greater and greater thicknesses. With the use ICP-machining, selectivities of 70:1 up to over 100:1 can be achieved. As a result, ICP machining requires a shallower depth of photoresist for a given substrate etch depth.

[0054] ICP-machining is a method of anisotropic plasma etching to provide laterally defined recess structures through an etching mask employing a plasma. ICP- machining includes plasma etching by contact with a reactive etchant gas to remove material from the surface of the substrate and provide exposed surfaces. ^'Passivating in a polymerizing step, using at least one polymer former (CHF₃) contained in the plasma, covers the exposed substrate with a polymer to from a temporary etching stop. The etching step and the polymerizing step can be repeated to provide a high mask selectivity combined with a very high anisotropy of the etched structures.

[0055] The ICP-machining process is performed separately in separate, alternating sequential etching and polymerization steps. During the etching step, chemically active species and electrically charged particles (ions) are generated in the reactor with the aid of an electrical discharge in a mixture of sulfur hexafluoride (SFe) and Ar. Subsequently a polymerization step is performed with a mix of, for example, trifluoromethane (CHF₃) and Ar. The etching and polymerization steps are repeated until the desired structure and etch depth is obtained. In the invention, the best results were observed for polymerization and step times of about 6 seconds.

[0056] During the ICP-machining etch step, a mixture of SF_δ and Ar can be used at a gas flow between 0 and 100 seem and a processing pressure between 10 and 100 μbar. Other common etchant gases that can be used include NF₃, CF₄, or other materials that can release fluorine. Plasma generation can take place with microwave irradiation at outputs between 300 and 1200W at 2.45 GHz. At the same time, a substrate prestress for ion acceleration is applied to the substrate electrode. The substrate prestress is preferably between 5 and 30V and can be achieved with a high-frequency supply (13.56 MHz) at outputs between 2 and 10 W.

[0057] The passivation or polymerization step is performed with a mixture of CHF₃ and Ar at a gas flow between 0 and 100 seem and a processing pressure between 10 and 100 μbar. At an output preferably between 300 and 1200W, microwave radiation and thus a plasma are generated by means of a resonator. During the polymerization step, an approximately 50 nm thick, TEFLON-like (polytetrafluoroethylene-like) polymer layer is precipitated on the side walls or on the etching base. A time period of up to 1 minute is required for the polymer formation. Preferably, a polymerization step time of 6 seconds is used. [0058] During the polymerization step of the ICP- machining process, it is advantageous to perform an ionic effect on the silicon substrate. For this, the substrate electrode is acted upon by a high frequency output of, for example 3 to 5W, which results in a substrate prestress of approximately 5V. Because the polymer layers that precipitated during the polymerization step without the ionic effect are only very slowly etched (only a few nm per minute) during the etching step, the simultaneous ion effect during the etching step offers the advantage that the polymer etching rate can be dramatically increased to over 100 nm/ in. This is even achieved when the silicon substrate is bombarded with a low ionic energy, e.g., 5 eV. During the etching steps, an ion bombardment with energies between 5 and 30 eV is recommended in order to leave the structure base completely free from deposits form the plasma, so that no roughness of the etching based is established. [0059] . The anisotropic etching process of ICP- machining has^' a very high selectivity of silicon to photoresist, generally 70:1 to 100:1. This high selectivity allows the etching of very deep structures with minimal photoresist thickness. The anisotropic etching process can be used to form vertical sidewalls or lightly off angle sidewalls. However, the inventors have discovered that the combination of gray scale photolithography with the anisotropic etching process can yield curved, deeply etched structures .

[0060] Although the mechanism of achieving a uniform curved structure is not definitely known, it is believed that the short cycle times during ICP-machining results in minimized step formation so as to approximate a smooth curve. For example, a 6 second pulse combined with a 2.5 μm per minute etch rate results in an etch depth of approximately 0.4 μm per cycle. Any step built up during the following passivation step must be less than the 0.4 μm etch. As a result, the imperfections accompanying ICP- machining will be minuscule and the resulting structure will approximate a smooth curve. The invention is preferably practiced using 6 second pulses of plasma etch and passivation.

[0061] In one embodiment of the invention, photoresist is patterned using the gray scale techniques discussed above. Following patterning, the structure is etched using the ICP-machining process of alternating sequential etching and polymerization steps. Figs. 2 and 3 are photomicrographs showing views of 164 μm deep structures etched using this method.

[0062] Fig. 4 shows another embodiment of the invention. In this embodiment, the photoresist is patterned using the gray scale techniques discussed above. The structure is etched using a two step process: RIE followed by ICP-machining. The RIE is a standard RIE etch used to etch the curved surfaces into the substrate, leaving plateau areas of photoresist. Following the isotropic etch, an ICP- machining anisotropic etch of alternating sequential etching and polymerization steps is performed.

[0063] As shown in Fig. 4, the RIE is used to perform a shallow etch. Following RIE, ICP-machining performs the deep etching to create the deeply etched finished structure. In the two-step embodiment of the present invention, the anisotropic ICP-machining etch does not etch the curved surfaces into the silicon, but merely makes them deeper. The photoresist plateau allows the formation of deep structures with straight sidewalls and curved bottoms. Fig. 5 is a photomicrograph showing the photoresist plateaus formed using the two-step RIE/ICP- machining etch process.

[0064] An alternative embodiment of the invention can be used to create a deeply etched structure having a slightly curved surface. The deeply etched structure can first be made using ICP machining. After the deep structure is formed, RIE can be used to add a slightly curved surface to the structure. In this embodiment, it may be necessary to strip and/or add a layer of photoresist between the ICP- machining and the RIE step.

[0065] Experimental results illustrate the effectiveness of the technology of the invention. Figure 6 is a photomicrographic image of an impeller implant showing the test quadrant for etch development. Figure 7 is an SEM image showing a side view of the impeller rib quadrant. The metrology, i.e., measurement, locations on the impeller rib quadrant are shown in Figure 8. The metrology locations include inner blade rib 5, outer blade rib 6 and flat area 7. The relationship between the inner blade rib 5 and the outer blade rib 7 is shown in Figure 9, which is a slice profile across section AA of the impeller rib quadrant bisecting the impeller rib. Here, it is seen that the inner blade rib has a higher elevation from the flat area than does the outer blade rib.

[0066] A gray scale photoresist sacrificial etch mask was used to form the impeller on a single crystal silicon wafer. The ICP machining of the invention was used for an etch time of 50 minutes at an etch temperature of 50 °C. The approximate etch depth was 300 μm. A Zygo New View surface mapper was used to characterize the surface of the impeller. Measurements were performed on four different wafers: Tl which was coated with photoresist, T2 which was coated with a smoothed photoresist, 13 an impeller etched in accordance with the invention, and T4 a second impeller etched according to the invention. The results are in Table 1.

Table 1 .

[0067] The roughness results in Table 1 are given as

Ra, the average of perturbations of the mean surface depth, and RMS is the lσ standard deviation. Examples 1 and 2 show the dramatic difference between smoothed and unsmoothed photoresist. However, example 3 shows that this smoothing of the photoresist has no equally dramatic effect on the surface roughness of the impeller rib. Example 4 demonstrates the smoothness of the flat area of the impeller.

[0068] Examples 5 and 6 were on a rib made in accordance with the invention using a 300 x 200 μm viewing area. In, contrast, Examples 7 and 8 were on a similar wafer made according to the invention, but the viewing area was carefully selected so as not to incorporate one of the residual step or ridge areas of the impeller rib structure. As a result, the contribution to the roughness by the steps can be observed. Consequently, the results indicate the improvement in surface roughness that can be obtained by the minimizing step formation.

[0069] It is to be understood that the foregoing descriptions and specific embodiments shown herein are merely illustrative of the best mode of the invention and the principles thereof, and that modifications and additions may be easily made by those skilled in the art without departing for the spirit and scope of the invention, which is therefore understood to be limited only by the scope of the appended claims.

Claims

We claim:

1. A method to etch a smooth curved structure which comprises : providing a substrate; coating the substrate with photoresist; patterning the photoresist using at least one gray scale mask to produce a photomask having the smooth curved structure orthogonal to a surface of the substrate; developing the photoresist; and etching the substrate using ICP-machining.

2. A method to etch a smooth curved structure which comprises : providing a substrate; coating the substrate with photoresist; patterning the photoresist using at least one gray scale mask to produce a photomask having the smooth curved structure orthogonal to a surface of the substrate; developing the photoresist; etching the substrate using reactive ion etching; and etching the substrate using ICP-machining.

3. The method according to claim 1 or 2, wherein the substrate is silicon.

4. The method according to claim 1 or 2, wherein the photoresist is a positive novalak photoresist.

5. The method according to claim 1 or 2, wherein the photoresist is less than 20 μm thick.

6. The method according to claim 1 or 2, wherein the mask is constructed with a plurality of precisely located and sized light transmitting openings.

7. The method according to claim 1 or 2, wherein the patterning is performed using ultraviolet light.

8. The method according to claim 1 or 2, wherein the ICP-machining is a plasma etching process performed using the following steps: contacting the substrate with a reactive etchant gas to remove material from the surface of the substrate and provide exposed surfaces; and polymerizing using at least one polymer former contained in the plasma.

9. The method according to claim 8, wherein the plasma is generated using microwave radiation at outputs between 300 and 1200W at 2.45 GHz.

10. The method according to claim 8, wherein the reactive etchant gas comprises SF₆ and Ar.

11. The method according to claim 8, wherein the polymer former comprises CF₃ or CHF₃.

12. The method according to claim 8, wherein the plasma etching process is accompanied by ion bombardment with energies between 5 and 30 eV.

13. The method according to claim 8, wherein the contacting the substrate step lasts about 6 seconds and the polymerization step lasts about 6 seconds.

14. The method according to claim 8, wherein the polymerization is performed for a sufficient time to form an approximately 50 nm thick polymer layer.

15. The method according to claim 14, wherein the polymer layer comprises polytetrafluoroethylene.

16. The method according to claim 8, wherein the contacting step and polymerizing step are repeated until a desired etch depth is obtained.

17. The method according to claim 2, wherein deep reactive ion etching the substrate deepens the curved surfaces .

18. The method according to claim 2, wherein the reactive ion etching is performed at a power of up to 300 watts at 30KHZ.

19. The method according to claim 2, wherein the reactive ion etching is performed using a gas mixtures selected from CF₄ and 0₂, CHF₄ and 0₂, or SF₆ and 0₂.

20. The method according to claim 1 or 2, wherein the method forms turbine rotors or high frequency lenses.

21. The method according to claim 20, wherein the turbine rotors^", GHz lenses or high frequency lenses are smaller than about 50 μm.

22. The method according to claim 1 or 2, wherein the ICP-machining has a selectivity to silicon of about 70:1 to 100:1.

23. The method according to claim 1 or 2, wherein the patterning the photoresist is performed using 1 gray scale mask.

24. The method according to claim 1 or 2, wherein the patterning the photoresist is performed using 2 or more gray scale masks.

25. A micro-machined device which comprises: a smooth curved structure orthogonal to a surface of a substrate, the smooth curved structure being formed by coating the substrate with photoresist, patterning the photoresist using at least one gray scale mask to produce a photomask having the smooth curved structure orthogonal to the surface of the substrate, developing the photoresist, and etching the substrate using ICP-machining.

26. A micro-machined device which comprises: a smooth curved structure orthogonal to a surface of a substrate, the smooth curved structure being formed by coating the substrate with photoresist, patterning the photoresist using at least one gray scale mask to produce a photomask having the smooth curved structure orthogonal to a surface of the substrate, developing the photoresist, etching the substrate using reactive ion etching, and etching the substrate using ICP-machining.