US20050133479A1 - Equipment and process for creating a custom sloped etch in a substrate - Google Patents
Equipment and process for creating a custom sloped etch in a substrate Download PDFInfo
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- US20050133479A1 US20050133479A1 US10/739,521 US73952103A US2005133479A1 US 20050133479 A1 US20050133479 A1 US 20050133479A1 US 73952103 A US73952103 A US 73952103A US 2005133479 A1 US2005133479 A1 US 2005133479A1
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- substrate
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- control layer
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
- B81C1/00436—Shaping materials, i.e. techniques for structuring the substrate or the layers on the substrate
- B81C1/00523—Etching material
- B81C1/00547—Etching processes not provided for in groups B81C1/00531 - B81C1/00539
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
- B81C1/00388—Etch mask forming
- B81C1/00396—Mask characterised by its composition, e.g. multilayer masks
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81C—PROCESSES OR APPARATUS SPECIALLY ADAPTED FOR THE MANUFACTURE OR TREATMENT OF MICROSTRUCTURAL DEVICES OR SYSTEMS
- B81C1/00—Manufacture or treatment of devices or systems in or on a substrate
- B81C1/00388—Etch mask forming
- B81C1/00412—Mask characterised by its behaviour during the etching process, e.g. soluble masks
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
- H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
- H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
- H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
- H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
- H01L21/30604—Chemical etching
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
- H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
- H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
- H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
- H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
- H01L21/308—Chemical or electrical treatment, e.g. electrolytic etching using masks
- H01L21/3083—Chemical or electrical treatment, e.g. electrolytic etching using masks characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane
- H01L21/3085—Chemical or electrical treatment, e.g. electrolytic etching using masks characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane characterised by their behaviour during the process, e.g. soluble masks, redeposited masks
Definitions
- the present invention relates generally to the field of semiconductor manufacturing and microelectromechanical systems (MEMS). More specifically, the present invention pertains to equipment and processes for creating a custom sloped etch in a substrate.
- MEMS microelectromechanical systems
- a sloped surface may, for example, allow an electrode that is positioned on the sloped surface to be near one or more electrodes on a beam or diaphragm at one location. The electrode on the sloped surface may then slope away from the beam or diaphragm. This may allow the beam or diaphragm to be initially actuated with a relatively small voltage, and then roll down along the sloped surface to provide the desired displacement.
- the absence of such sloped surfaces can increase the voltage necessary to displace actuatable surfaces, and can cause a decrease in actuation speed.
- the shape of the sloped surface can also limit the amount of travel or displacement of the actuatable surface(s), further reducing the effectiveness of the device.
- the creation of a sloped surface in a substrate has many other useful applications including, for example, the formation of optical lens, as well as other such device having a desired contour or shape.
- an optical mask and a photolithography stepper system can be used to locally modulate the frequency of an ultraviolet (UV) light source, forming a graduated pattern of photo-resist in a photomask layer.
- UV ultraviolet
- a dry or wet-etch step containing a single etchant solution capable of selectively etching the substrate material is then used to transfer the graduated pattern of photo-resist to the substrate.
- the resolution of many prior art methods prohibit the creation of certain custom sloped etches.
- the depth at which the slope can be formed within the substrate is often limited to only a few microns, preventing the formation of deep slopes useful in many conventional MEMS devices.
- the ability to vary the steepness of the contoured slope and or shape may be limited by the resolution of the etching method employed, further preventing the formation of certain slopes in the substrate.
- An illustrative process for creating a custom sloped etch may include the steps of providing a substrate having a surface to be etched, providing a control layer on or above the surface of the substrate, providing at least one patterned mask layer onto or above the control layer, and then selectively etching each of the control layer and the substrate surface, at varying and/or controlled rates, to form a sloped etch in the substrate surface.
- the patterned mask layer can include one or more openings exposing the control layer to etchant contained, for example, in an etch bath or other suitable etching apparatus. The geometry and/or shape of the openings can be modified to alter the depth, steepness, shape, and other various characteristics of the slope, as desired.
- the process of selectively etching the control layer to form the sloped etch can be accomplished by immersing the substrate in an etch bath containing one or more etchants adapted to selectively etch each of the substrate and the control layer materials.
- a relatively fast-rate etchant solution of nitric acid (HNO 3 ) can be used to selectively etch the control layer material
- a relatively slow-rate etchant solution of hydrofluoric acid (HF) can be used to selectively etch the substrate material.
- the relative concentrations of the two etchants can be varied throughout the etching process to alter the etch rate of the substrate and/or control layer, allowing the creation of a custom sloped etch having a particular shape or profile.
- the temperature of the etch bath may also be varied and/or controlled throughout the etching process to help alter the etch rate of the substrate and/or control layer.
- a single etchant capable of selectively etching each of the control layer and substrate at different temperatures, and thus at different etch rates can be used to form a custom sloped etch in a substrate.
- the materials forming the substrate and control layer can be selected to exhibit different etch rates at various temperature ranges.
- the temperature of the etchant can be varied in a manner that alters the etch rate in one material (e.g. the substrate material) more or less relative to the other material (e.g. the control layer material). By adjusting the temperature of the etch bath during the etching process, any number of desired shapes can be formed on the substrate.
- FIGS. 1A-1D are schematic views illustrating the formation of a control layer and a photomask on a substrate
- FIG. 2 is a diagrammatic view showing the masked substrate of FIG. 1 placed within an etch bath containing multiple etchants;
- FIGS. 3A-3C are schematic views illustrating the creation of a custom sloped etch in the masked substrate of FIG. 1 ;
- FIG. 4 is a graph showing an illustrative custom sloped etch formed in accordance with the process of FIGS. 3A-3C .
- FIG. 5 is a schematic view showing the masked substrate of FIG. 1 placed within another illustrative etching apparatus containing a single etchant;
- FIGS. 6A-6D are schematic views illustrating the creation of a custom sloped etch using a control layer and a photomask having a rectangular slot.
- FIGS. 7A-7D are schematic views illustrating the creation of a custom sloped etch using a control layer and a photomask having multiple openings.
- Substrate 12 may include, for example, a thin wafer of quartz sometimes used in the construction of a MEMS electrostatic actuator, optical lens, or other such device having a desired contour or shape.
- substrate 12 may be provided as part of the bottom and/or top curved surfaces of an electrostatic actuator, as part of an optical lens, or any other suitable device. While quartz may be used for the substrate material in the illustrative embodiment, it should be understood that other materials such as silicon, gallium, arsenide, germanium, glass, etc. could also be used, if desired.
- a sacrificial control layer 16 can be applied onto the surface 14 of the substrate 12 .
- the control layer 16 can be formed on the substrate 12 using any number of suitable deposition techniques known in the art.
- the control layer 16 can be formed by sputtering metallic (e.g. Nickel) particles onto the surface 14 using a suitable sputtering process such as laser sputtering. Other methods such as vapor deposition or adhesion could also be utilized, if desired.
- the control layer 16 may include more than one layer, with at least some of the layers exhibiting different etch characteristics.
- the control layer 16 should typically include a material different from that used in forming the substrate 12 .
- the control layer 16 can include a layer of nickel having a thickness of approximately 1 to 2 ⁇ m.
- Other materials and/or dimensions are also possible, however, depending on the particular slope characteristic desired in the surface 14 .
- the various properties of the materials used in forming the substrate 12 and control layer 16 can be used to control the etch rate within the surface 14 of the substrate 12 , allowing a custom sloped etch to be formed in the substrate 12 .
- FIG. 1B is a schematic view showing the formation of a patterned photomask 18 onto the control layer 16 of FIG. 1A .
- the photomask 18 can include a first photomask layer 20 disposed over the control layer 16 , and a second (optional) photomask layer 22 disposed over the first photomask layer 20 .
- the first photomask layer 20 can include a relatively thin (e.g. 5 ⁇ thick) layer of silicon nitride (SiN) film or other suitable material that acts as a mask to prevent the flow of etchant into the control layer 16 .
- SiN silicon nitride
- a thin layer of chrome may be used as an intermediate layer to bond the two layers 16 , 20 together.
- the second photomask layer 22 can include a material similar to that of the first photomask layer 20 , or can include a material having different mechanical and/or thermal properties than that of the first photomask layer 20 .
- the second photomask layer 22 can include a relatively thin (e.g. 5 ⁇ thick) layer of polysilicon applied over the first photomask layer 20 at room temperature.
- the first photomask layer 20 can be applied to the control layer under compression whereas the second photomask layer 22 can be applied under tension, imparting a residual stress within the photomask 18 that causes it to curl and/or displace in a particular manner as the control layer 16 is being removed.
- a second photomask layer 22 is specifically illustrated in FIG. 1B , it should be understood that other methods may be employed to bimorph the photomask 18 , if desired.
- a single photomask layer having a coefficient of thermal expansion different than that of the material forming the control layer 16 could be used to bimorph the photomask 18 .
- the difference in thermal coefficients causes the photomask 18 to thermally expand at a greater or lesser rate than the control layer 16 , imparting a bias to the two materials that causes the photomask 18 to curl and/or displace during etching.
- FIG. 1C is a schematic view showing the formation of an opening 24 through the photomask layers 20 , 22 to expose at least a portion of the underlying control layer 16 . Formation of the opening 24 can be accomplished using any suitable technique such as photolithography.
- FIG. 1D is a top view of the substrate 12 of FIG. 1C , showing the shape of the opening 24 in greater detail.
- the opening 24 may define a longitudinal slit 32 having a width W and a length L. In other embodiments, however, the dimensions of the opening 24 can be arranged to form some other desired arrangement.
- FIG. 2 is a diagrammatic view showing the masked substrate 12 of FIG. 1 placed within an etching apparatus 38 containing multiple etchant solutions.
- Etching apparatus 38 includes an etch bath 40 containing one or more heater elements 42 and one or more temperature sensors 44 electrically connected to a controller 46 that can be used to monitor and/or regulate the temperature of fluid within the etch bath 40 .
- An optional overflow tube 48 can also be provided to maintain the fluid level within the etch bath 40 at a particular level, if desired.
- a number of pipes 50 , 52 can be used to deliver a number of etchants into the etch bath 40 .
- a first etchant 54 adapted to selectively etch the control layer 16 can be delivered through pipe 50 and into the etch bath 40 .
- the first etchant 54 can include a fast-rate etchant solution of nitric acid (HNO 3 ) that can be used to etch the nickel forming the control layer 16 in some embodiments.
- the flow of first etchant 54 can be varied using a flow control valve 42 or other suitable flow control means.
- a second etchant 58 adapted to selectively etch the substrate 12 can also be delivered into the etch bath 40 via a second pipe 52 .
- the second etchant 58 may be a relatively slow rate-etchant configured to etch the substrate 12 at a slower rate than the first etchant 54 .
- a diluted solution of hydrofluoric acid (HF) can be utilized to etch the substrate 12 at a rate of approximately 1 to 400 times slower than the etch rate of the first etchant 54 .
- a flow control valve 60 or other suitable flow control means can be used to adjust the flow of second etchant 58 into the etch bath 40 .
- FIGS. 3A-3C are schematic views illustrating the creation of a custom sloped etch in the substrate 12 of FIG. 1 .
- substrate 12 is shown immediately after the initiation of the etching process, wherein the substrate 12 is immersed in an etching apparatus containing one or more etchants configured to selectively etch each of the substrate 12 and the control layer 16 .
- FIG. 3A may depict an initial view of the substrate 12 after being immersed within the etching apparatus 38 of FIG. 2 .
- the various illustrative etching stages depicted in FIGS. 3A-3C can be accomplished using other methods and/or techniques described herein, including the use of a single etchant solution as discussed herein with respect to FIG. 5 .
- the etch rate within the control layer 16 is greater than the etch rate within the substrate 12 .
- the relatively fast-rate first etchant 54 can be configured to etch the control layer 16 at a rate of about 1 to 10 microns/min
- the relatively slow-rate second etchant 58 can be configured to etch the substrate 12 at a rate of about 0.01 to 1.0 microns/min. As shown in FIG. 3A , this initial combination of first etchant 54 and second etchant 58 results in the formation of a gap 62 .
- FIG. 3B is a schematic view showing the etching of substrate 12 and control layer 16 at a second time t 2 .
- the relative concentrations of the first and second etchants 54 , 58 causes the gap 62 to significantly widen between times t 1 and t 2 , forming a curved surface 64 within the surface 14 of the substrate 12 .
- the vertical etch rate will vary based on factors such as the size and geometry of the mask opening 24 , the concentration and temperature of etchant(s) within the etch bath, and the material characteristics of the substrate 12 and control layer 16 .
- FIG. 3C is a schematic view showing the substrate 12 at a third time t 3 at or near the conclusion of the etching process.
- the relative concentrations of the etchant(s) within the etch bath have increased the width and, to a lesser degree, the depth D of the gap 62 .
- the etching process can be continued for a duration sufficient to etch away all or a portion of the control layer 16 . The duration necessary to accomplish this will depend in part on the material of the substrate 12 and control layer 16 , the concentrations of the etchant(s) used, and the dimensions of the substrate 12 .
- the amount of etching occurring within the substrate 12 can also be made to depend on the characteristics of the photomask 18 used.
- the photomask 18 can be configured to curl upwardly away from the surface 14 of the substrate 12 , allowing more etchant to become entrained within the gap 62 .
- the existence of more etchant within the gap 64 tends to accelerate the vertical etch rate of the substrate 12 during the etch, in some cases forming a slope having a greater depth D.
- the slope of the curve 64 can be varied during the etching process to form a contour within the surface 14 of the substrate 12 .
- the relative concentrations of the etchant(s) used during the etching process can be adjusted to create a number of inflection points 66 within the curved surface 64 , forming an S-shaped slope.
- the location of the inflection points 66 and the steepness of the curved surface 64 can be varied to alter the shape of the slope, as desired.
- the depth D of the slope can also be varied, as desired, to produce a particular profile or shape.
- a depth D of about 4 to 8 ⁇ m may be achieved into the surface 14 of the substrate 12 using the methods discussed herein. However, other depths can also be achieved, as desired.
- FIG. 4 is a graph showing an illustrative custom sloped etch 68 formed in accordance with the illustrative process of FIGS. 3A-3C .
- the relative concentration of the first etchant 54 is significant in comparison to the concentration of the second etchant 58 , causing a greater amount of lateral etching than vertical etching.
- a first curved region 70 can be formed in the substrate 12 between times t 1 and t 2
- the first curved region 70 can be formed by varying relative concentrations and/or temperature of first and second etchants 54 , 58 contained within the etch bath 40 .
- a second curved region 72 can also be formed in the substrate 12 between times t 2 and t 3 .
- the second curved region 72 can be formed, for example, by shutting-off the flow of HF into the etch bath 40 and gradually increasing the amount of HNO 3 contained within the etch bath to gradually decrease the vertical etch rate within the substrate 12 .
- an inflection 66 FIG. 3C
- the steepness of the formed slope etch 68 can be made gradual, in some cases on the order of only a few degrees.
- the characteristics of the sloped etch 68 can further be altered by the selection of etchants used.
- an anisotropic etchant exhibiting crystallinity dependence can be utilized to produce other desired profiles in a crystalline substrate such as silicon, if desired.
- Other factors such as the concentration of the etchant can also be exploited to create a desired slope in the substrate.
- FIG. 5 is a schematic view showing the masked substrate of FIG. 1 placed within another illustrative etching apparatus 74 containing a single etchant.
- etching apparatus 74 includes an etch bath 74 having one or more heater elements 78 and one or more temperature sensors 80 electrically connected to a controller 82 that can be used to regulate and/or monitor the temperature at selective times during the etching process.
- a single etchant 84 capable of etching both the substrate 12 and control layer 16 can be delivered through a pipe 86 and into the etch bath 76 .
- a flow control valve 90 can be further provided to control the flow of etchant 84 into the etch bath 76 .
- An optional overflow tube 88 can also be utilized to maintain the fluid level within the etch bath 76 at a particular level, if desired.
- the temperature within the etch bath 76 can be varied at one or more times during the etching process to alter the respective etch rates of the substrate 12 and control layer 16 .
- the steepness of the slope imparted to the substrate 12 will depend on the relative etch rates of the substrate 12 and control layer 16 at various temperatures.
- the etch rate of the control layer 16 can be configured to increase at a greater rate at a particular temperature or temperature range (e.g. at 100° C.).
- the greater the difference in relative etch rates between the two materials the more gradual the slope that can be imparted to the substrate 12 , all other factors being the same.
- a desired sloped etch can be formed in the substrate 12 .
- FIGS. 6A-6D are schematic views illustrating the creation of a custom sloped etch using a control layer and a patterned photomask having a rectangular slot.
- the process represented generally by reference number 92 , is similar to that described above with respect to FIGS. 3A-3C , beginning with the step of providing a substrate 94 having a surface 96 to be etched.
- Substrate 94 may include, for example, a thin wafer of quartz used in the construction of a MEMS electrostatic actuator, optical lens, or other similar device having a desired contour or shape.
- a control layer 98 and photomask 100 can also be applied to the surface 96 of the substrate 94 in a manner similar to that described above in FIGS. 1A-1C .
- control layer 98 can include a layer of nickel or other suitable material applied to the surface of a quartz substrate 94 .
- the photomask 100 includes a single layer 102 of silicon nitride (SiN) film or other suitable mask material.
- the single photomask layer 102 can be configured to bimorph, causing the layer 102 to curl upwardly away from the surface 96 of the substrate 94 during the etching process.
- the photomask layer 102 can be configured to bimorph by applying a stretching (i.e. tensile) force to the photomask layer 102 while it is being applied to the control layer 98 .
- the photomask layer 102 can include a material having a different coefficient of thermal expansion than that of the material forming the control layer 98 , causing the photomask layer 102 to shrink at a greater or lesser rate than the control layer 98 .
- an opening 104 can be formed through the single photomask layer 102 to expose at least a part of the underlying control layer 98 .
- FIG. 6B is a top view of the substrate 94 , showing the shape of the opening 104 in greater detail.
- the opening 104 may define a rectangular slot 107 having a width W and a length L. Similar to the longitudinal slit 32 discussed above with respect to FIG. 1D , the rectangular slot 107 can be configured to form a contoured slope or profile along the length of the substrate 94 .
- the width W of the rectangular slot 106 can be made greater than the width W of the longitudinal slit 32 to expose more of the underlying control layer 98 .
- FIGS. 6C-6D illustrate the steps of creating a custom sloped etch within the surface 96 of the substrate 94 .
- the existence of the rectangular slot 107 forms a channel 112 having a substantially flat region 114 .
- the dimensions of the flat region 114 will typically depend in part on the width W and length L of the rectangular slot 107 .
- FIG. 6D is a schematic view showing the substrate 94 at a second time at or near the conclusion of the etching process.
- one or more curved surfaces 116 can also be formed within the surface 96 of the substrate 94 .
- the curved surfaces 116 can be formed by selectively etching each of the substrate 94 and the control layer 98 using multiple etchants having differing relative etch rates.
- the temperature of the etch bath may also be controlled during the etching process to help increase and/or decrease the etch rate of the substrate 94 and/or control layer 98 .
- the curved surfaces 116 can be formed using single etchant by adjusting the temperature within the etch bath at various times during the etching process to increase and/or decrease the etch rate of the substrate 94 and/or control layer 98 .
- the photomask layer 120 can be configured to bimorph away from the surface 96 of the substrate 94 during the etching process, if desired.
- FIGS. 7A-7D are schematic views illustrating the creation of a custom sloped etch using a control layer and a patterned photomask having multiple openings.
- the process represented generally by reference number 118 , can begin with the step of providing a substrate 120 having a surface 122 to be etched.
- Substrate 120 may include, for example, a thin wafer of quartz or other suitable material.
- a control layer 124 and photomask 126 can also be applied to the substrate 120 in a manner similar to that described above with respect to FIGS. 1A-1C .
- the control layer 124 can include a layer of nickel or other suitable material applied to the surface of a quartz substrate 120 .
- the photomask 126 includes a single, thin layer 128 of silicon nitride (SiN) film or other suitable mask material.
- the single photomask layer 128 can be configured to bimorph during etching, causing the layer 128 to curl upwardly away from the surface 122 of the substrate 120 .
- the photomask 126 may define a plurality of openings 130 , 132 that expose the control layer 124 to etchant contained, for example, in an etch bath.
- the openings 130 , 132 can each define a longitudinal slit 134 , 136 spaced apart from each other a distance D on the photomask layer 128 .
- FIGS. 7C-7D illustrate the steps of creating a custom sloped etch within the surface 122 of the substrate 120 .
- the existence of the openings 130 , 132 through the photomask 126 initially creates a number of gaps 138 , 140 within the control layer 124 and substrate 120 .
- the existence of multiple openings 130 , 132 within the photomask 126 creates a curved surface 142 having a kink 144 .
- the distance D between the longitudinal slits 134 , 136 can be varied to alter the characteristics of the kink 144 formed.
- the distance D between each of the longitudinal slits 134 , 136 can be made greater to increase the height of the kink 252 .
- the distance D between each of the longitudinal slits 132 , 134 can be made smaller to decrease the height of the kink 252 .
- Other factors such as the dimensions of the longitudinal slits 132 , 134 can also be adjusted to produce a desired contour in the substrate 120 . While the use of two openings 130 , 132 is specifically illustrated FIGS. 7A-7D , it should be understood that any number of openings could be employed to alter the shape of the slope, as desired.
Abstract
Description
- The present invention relates generally to the field of semiconductor manufacturing and microelectromechanical systems (MEMS). More specifically, the present invention pertains to equipment and processes for creating a custom sloped etch in a substrate.
- The creation of custom sloped etches is important in the manufacture of microelectromechanical system (MEMS) devices and other small-scale devices. In the construction of MEMS devices, for example, such custom sloped etches can be useful in helping to reduce the voltage necessary to electrostatically actuate small structures such as beams or diaphragms, or to perform some other desired function. A sloped surface may, for example, allow an electrode that is positioned on the sloped surface to be near one or more electrodes on a beam or diaphragm at one location. The electrode on the sloped surface may then slope away from the beam or diaphragm. This may allow the beam or diaphragm to be initially actuated with a relatively small voltage, and then roll down along the sloped surface to provide the desired displacement.
- In certain devices, the absence of such sloped surfaces can increase the voltage necessary to displace actuatable surfaces, and can cause a decrease in actuation speed. In certain cases, the shape of the sloped surface can also limit the amount of travel or displacement of the actuatable surface(s), further reducing the effectiveness of the device. The creation of a sloped surface in a substrate has many other useful applications including, for example, the formation of optical lens, as well as other such device having a desired contour or shape.
- To overcome these shortcomings, several processes have been developed to form slope etches within a substrate that are adapted to contour to the size and shape of the actuatable surfaces. In a gray-scale lithography process, for example, an optical mask and a photolithography stepper system can be used to locally modulate the frequency of an ultraviolet (UV) light source, forming a graduated pattern of photo-resist in a photomask layer. Once formed thereon, a dry or wet-etch step containing a single etchant solution capable of selectively etching the substrate material is then used to transfer the graduated pattern of photo-resist to the substrate.
- The resolution of many prior art methods prohibit the creation of certain custom sloped etches. In a gray-scale lithography process, for example, the depth at which the slope can be formed within the substrate is often limited to only a few microns, preventing the formation of deep slopes useful in many conventional MEMS devices. Moreover, the ability to vary the steepness of the contoured slope and or shape may be limited by the resolution of the etching method employed, further preventing the formation of certain slopes in the substrate. As a result, there is a need in the art for equipment and processes for creating custom sloped etches in a substrate.
- The present invention pertains to equipment and processes for creating a custom sloped etch in a substrate. An illustrative process for creating a custom sloped etch may include the steps of providing a substrate having a surface to be etched, providing a control layer on or above the surface of the substrate, providing at least one patterned mask layer onto or above the control layer, and then selectively etching each of the control layer and the substrate surface, at varying and/or controlled rates, to form a sloped etch in the substrate surface. The patterned mask layer can include one or more openings exposing the control layer to etchant contained, for example, in an etch bath or other suitable etching apparatus. The geometry and/or shape of the openings can be modified to alter the depth, steepness, shape, and other various characteristics of the slope, as desired.
- The process of selectively etching the control layer to form the sloped etch can be accomplished by immersing the substrate in an etch bath containing one or more etchants adapted to selectively etch each of the substrate and the control layer materials. In certain embodiments, for example, a relatively fast-rate etchant solution of nitric acid (HNO3) can be used to selectively etch the control layer material, whereas a relatively slow-rate etchant solution of hydrofluoric acid (HF) can be used to selectively etch the substrate material. The relative concentrations of the two etchants can be varied throughout the etching process to alter the etch rate of the substrate and/or control layer, allowing the creation of a custom sloped etch having a particular shape or profile. In some cases, the temperature of the etch bath may also be varied and/or controlled throughout the etching process to help alter the etch rate of the substrate and/or control layer.
- In another illustrative embodiment of the present invention, a single etchant capable of selectively etching each of the control layer and substrate at different temperatures, and thus at different etch rates, can be used to form a custom sloped etch in a substrate. In certain embodiments, for example, the materials forming the substrate and control layer can be selected to exhibit different etch rates at various temperature ranges. When placed within an etch bath including one or more heaters, for example, the temperature of the etchant can be varied in a manner that alters the etch rate in one material (e.g. the substrate material) more or less relative to the other material (e.g. the control layer material). By adjusting the temperature of the etch bath during the etching process, any number of desired shapes can be formed on the substrate.
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FIGS. 1A-1D are schematic views illustrating the formation of a control layer and a photomask on a substrate; -
FIG. 2 is a diagrammatic view showing the masked substrate ofFIG. 1 placed within an etch bath containing multiple etchants; -
FIGS. 3A-3C are schematic views illustrating the creation of a custom sloped etch in the masked substrate ofFIG. 1 ; -
FIG. 4 is a graph showing an illustrative custom sloped etch formed in accordance with the process ofFIGS. 3A-3C . -
FIG. 5 is a schematic view showing the masked substrate ofFIG. 1 placed within another illustrative etching apparatus containing a single etchant; -
FIGS. 6A-6D are schematic views illustrating the creation of a custom sloped etch using a control layer and a photomask having a rectangular slot; and -
FIGS. 7A-7D are schematic views illustrating the creation of a custom sloped etch using a control layer and a photomask having multiple openings. - The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of construction, dimensions, and materials are illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized.
- Referring now to
FIGS. 1A-1D , an illustrative process of forming a control layer and photomask on a substrate will now be described. The process, represented generally byreference number 10, may begin with the step of providing asubstrate 12 having asurface 14 to be etched in accordance with several steps discussed herein.Substrate 12 may include, for example, a thin wafer of quartz sometimes used in the construction of a MEMS electrostatic actuator, optical lens, or other such device having a desired contour or shape. In certain embodiments, for example,substrate 12 may be provided as part of the bottom and/or top curved surfaces of an electrostatic actuator, as part of an optical lens, or any other suitable device. While quartz may be used for the substrate material in the illustrative embodiment, it should be understood that other materials such as silicon, gallium, arsenide, germanium, glass, etc. could also be used, if desired. - As can be further seen in
FIG. 1A , asacrificial control layer 16 can be applied onto thesurface 14 of thesubstrate 12. Thecontrol layer 16 can be formed on thesubstrate 12 using any number of suitable deposition techniques known in the art. In certain embodiments, for example, thecontrol layer 16 can be formed by sputtering metallic (e.g. Nickel) particles onto thesurface 14 using a suitable sputtering process such as laser sputtering. Other methods such as vapor deposition or adhesion could also be utilized, if desired. In some embodiments, thecontrol layer 16 may include more than one layer, with at least some of the layers exhibiting different etch characteristics. - The
control layer 16 should typically include a material different from that used in forming thesubstrate 12. In certain embodiments, for example, thecontrol layer 16 can include a layer of nickel having a thickness of approximately 1 to 2 μm. Other materials and/or dimensions are also possible, however, depending on the particular slope characteristic desired in thesurface 14. As is discussed in further steps below, the various properties of the materials used in forming thesubstrate 12 andcontrol layer 16 can be used to control the etch rate within thesurface 14 of thesubstrate 12, allowing a custom sloped etch to be formed in thesubstrate 12. -
FIG. 1B is a schematic view showing the formation of a patternedphotomask 18 onto thecontrol layer 16 ofFIG. 1A . As shown inFIG. 1B , thephotomask 18 can include afirst photomask layer 20 disposed over thecontrol layer 16, and a second (optional)photomask layer 22 disposed over thefirst photomask layer 20. In certain embodiments, thefirst photomask layer 20 can include a relatively thin (e.g. 5 Å thick) layer of silicon nitride (SiN) film or other suitable material that acts as a mask to prevent the flow of etchant into thecontrol layer 16. To facilitate adhesion of the SiN film in those embodiments wherein thecontrol layer 16 is formed of nickel, a thin layer of chrome may be used as an intermediate layer to bond the twolayers - In certain embodiments, it may be desirable to bimorph the
photomask 18 to cause it to curl and/or displace in a direction away from thesurface 14 of thesubstrate 12 during the etching process. Thesecond photomask layer 22 can include a material similar to that of thefirst photomask layer 20, or can include a material having different mechanical and/or thermal properties than that of thefirst photomask layer 20. In certain embodiments, for example, thesecond photomask layer 22 can include a relatively thin (e.g. 5 Å thick) layer of polysilicon applied over thefirst photomask layer 20 at room temperature. To bimorph thephotomask 18, thefirst photomask layer 20 can be applied to the control layer under compression whereas thesecond photomask layer 22 can be applied under tension, imparting a residual stress within thephotomask 18 that causes it to curl and/or displace in a particular manner as thecontrol layer 16 is being removed. - While the application of a
second photomask layer 22 is specifically illustrated inFIG. 1B , it should be understood that other methods may be employed to bimorph thephotomask 18, if desired. In one alternative method, for example, a single photomask layer having a coefficient of thermal expansion different than that of the material forming thecontrol layer 16 could be used to bimorph thephotomask 18. In use, the difference in thermal coefficients causes thephotomask 18 to thermally expand at a greater or lesser rate than thecontrol layer 16, imparting a bias to the two materials that causes thephotomask 18 to curl and/or displace during etching. -
FIG. 1C is a schematic view showing the formation of anopening 24 through the photomask layers 20,22 to expose at least a portion of theunderlying control layer 16. Formation of theopening 24 can be accomplished using any suitable technique such as photolithography. -
FIG. 1D is a top view of thesubstrate 12 ofFIG. 1C , showing the shape of theopening 24 in greater detail. As can be seen inFIG. 1D , theopening 24 may define alongitudinal slit 32 having a width W and a length L. In other embodiments, however, the dimensions of theopening 24 can be arranged to form some other desired arrangement. -
FIG. 2 is a diagrammatic view showing themasked substrate 12 ofFIG. 1 placed within anetching apparatus 38 containing multiple etchant solutions.Etching apparatus 38 includes anetch bath 40 containing one ormore heater elements 42 and one ormore temperature sensors 44 electrically connected to acontroller 46 that can be used to monitor and/or regulate the temperature of fluid within theetch bath 40. Anoptional overflow tube 48 can also be provided to maintain the fluid level within theetch bath 40 at a particular level, if desired. - As can be further seen in
FIG. 2 , a number ofpipes etch bath 40. Afirst etchant 54 adapted to selectively etch thecontrol layer 16 can be delivered throughpipe 50 and into theetch bath 40. In certain embodiments, for example, thefirst etchant 54 can include a fast-rate etchant solution of nitric acid (HNO3) that can be used to etch the nickel forming thecontrol layer 16 in some embodiments. The flow offirst etchant 54 can be varied using aflow control valve 42 or other suitable flow control means. - A
second etchant 58 adapted to selectively etch thesubstrate 12 can also be delivered into theetch bath 40 via asecond pipe 52. In contrast to thefirst etchant 54, thesecond etchant 58 may be a relatively slow rate-etchant configured to etch thesubstrate 12 at a slower rate than thefirst etchant 54. In certain embodiments, for example, a diluted solution of hydrofluoric acid (HF) can be utilized to etch thesubstrate 12 at a rate of approximately 1 to 400 times slower than the etch rate of thefirst etchant 54. Aflow control valve 60 or other suitable flow control means can be used to adjust the flow ofsecond etchant 58 into theetch bath 40. -
FIGS. 3A-3C are schematic views illustrating the creation of a custom sloped etch in thesubstrate 12 ofFIG. 1 . At a first time t1 depicted inFIG. 3A ,substrate 12 is shown immediately after the initiation of the etching process, wherein thesubstrate 12 is immersed in an etching apparatus containing one or more etchants configured to selectively etch each of thesubstrate 12 and thecontrol layer 16. In certain embodiments, for example,FIG. 3A may depict an initial view of thesubstrate 12 after being immersed within theetching apparatus 38 ofFIG. 2 . It should be understood, however, that the various illustrative etching stages depicted inFIGS. 3A-3C can be accomplished using other methods and/or techniques described herein, including the use of a single etchant solution as discussed herein with respect toFIG. 5 . - Based on the relatively weak concentration of the second etchant 58 (e.g. hydrofluoric acid (HF)) contained within the
etch bath 40, the etch rate within thecontrol layer 16 is greater than the etch rate within thesubstrate 12. In certain embodiments, for example, the relatively fast-ratefirst etchant 54 can be configured to etch thecontrol layer 16 at a rate of about 1 to 10 microns/min, whereas the relatively slow-ratesecond etchant 58 can be configured to etch thesubstrate 12 at a rate of about 0.01 to 1.0 microns/min. As shown inFIG. 3A , this initial combination offirst etchant 54 andsecond etchant 58 results in the formation of agap 62. -
FIG. 3B is a schematic view showing the etching ofsubstrate 12 andcontrol layer 16 at a second time t2. As can be seen inFIG. 3B , the relative concentrations of the first andsecond etchants gap 62 to significantly widen between times t1 and t2, forming acurved surface 64 within thesurface 14 of thesubstrate 12. In contrast to the lateral etch rate, which remains substantially constant during the etching process, the vertical etch rate will vary based on factors such as the size and geometry of themask opening 24, the concentration and temperature of etchant(s) within the etch bath, and the material characteristics of thesubstrate 12 andcontrol layer 16. -
FIG. 3C is a schematic view showing thesubstrate 12 at a third time t3 at or near the conclusion of the etching process. As shown inFIG. 3C , the relative concentrations of the etchant(s) within the etch bath have increased the width and, to a lesser degree, the depth D of thegap 62. In certain embodiments, the etching process can be continued for a duration sufficient to etch away all or a portion of thecontrol layer 16. The duration necessary to accomplish this will depend in part on the material of thesubstrate 12 andcontrol layer 16, the concentrations of the etchant(s) used, and the dimensions of thesubstrate 12. - The amount of etching occurring within the
substrate 12 can also be made to depend on the characteristics of thephotomask 18 used. When bimorph properties are imparted to the photomask layers 20,22, for example, thephotomask 18 can be configured to curl upwardly away from thesurface 14 of thesubstrate 12, allowing more etchant to become entrained within thegap 62. The existence of more etchant within thegap 64 tends to accelerate the vertical etch rate of thesubstrate 12 during the etch, in some cases forming a slope having a greater depth D. - As can be further seen in
FIG. 3C , the slope of thecurve 64 can be varied during the etching process to form a contour within thesurface 14 of thesubstrate 12. In the illustrative slope depicted inFIG. 3C , for example, the relative concentrations of the etchant(s) used during the etching process can be adjusted to create a number ofinflection points 66 within thecurved surface 64, forming an S-shaped slope. The location of theinflection points 66 and the steepness of thecurved surface 64 can be varied to alter the shape of the slope, as desired. The depth D of the slope can also be varied, as desired, to produce a particular profile or shape. In certain embodiments, for example, a depth D of about 4 to 8 μm may be achieved into thesurface 14 of thesubstrate 12 using the methods discussed herein. However, other depths can also be achieved, as desired. Once the desired shape has been formed within thesurface 14 of thesubstrate 12, thephotomask 18 and remaining control layer 16 (if any) can then removed, leaving intact the custom sloped etch formed in thesubstrate 12. -
FIG. 4 is a graph showing an illustrative custom slopedetch 68 formed in accordance with the illustrative process ofFIGS. 3A-3C . As shown inFIG. 4 , the relative concentration of thefirst etchant 54 is significant in comparison to the concentration of thesecond etchant 58, causing a greater amount of lateral etching than vertical etching. - A first
curved region 70 can be formed in thesubstrate 12 between times t1 and t2 The firstcurved region 70 can be formed by varying relative concentrations and/or temperature of first andsecond etchants etch bath 40. In certain embodiments, for example, the firstcurved region 68 can be formed by adding an initial amount of HNO3 and HF within the etch bath 40 (at time t=0), and then steadily increasing the amount of HF between times t1 and t2 to gradually increase the vertical etch rate within thesubstrate 12. - A second
curved region 72 can also be formed in thesubstrate 12 between times t2 and t3. In contrast to the firstcurved region 70, the secondcurved region 72 can be formed, for example, by shutting-off the flow of HF into theetch bath 40 and gradually increasing the amount of HNO3 contained within the etch bath to gradually decrease the vertical etch rate within thesubstrate 12. As can be seen at time t2 inFIG. 4 , for example, an inflection 66 (FIG. 3C ) is created at time t2 when the flow rates of the first andsecond etchants second etchant solutions slope etch 68 can be made gradual, in some cases on the order of only a few degrees. - The characteristics of the sloped
etch 68 can further be altered by the selection of etchants used. In certain embodiments, for example, an anisotropic etchant exhibiting crystallinity dependence can be utilized to produce other desired profiles in a crystalline substrate such as silicon, if desired. Other factors such as the concentration of the etchant can also be exploited to create a desired slope in the substrate. -
FIG. 5 is a schematic view showing the masked substrate ofFIG. 1 placed within anotherillustrative etching apparatus 74 containing a single etchant. As shown inFIG. 5 ,etching apparatus 74 includes anetch bath 74 having one ormore heater elements 78 and one ormore temperature sensors 80 electrically connected to acontroller 82 that can be used to regulate and/or monitor the temperature at selective times during the etching process. Asingle etchant 84 capable of etching both thesubstrate 12 andcontrol layer 16 can be delivered through apipe 86 and into theetch bath 76. In certain embodiments, aflow control valve 90 can be further provided to control the flow ofetchant 84 into theetch bath 76. Anoptional overflow tube 88 can also be utilized to maintain the fluid level within theetch bath 76 at a particular level, if desired. - To create a custom sloped etch in the
substrate 12, the temperature within theetch bath 76 can be varied at one or more times during the etching process to alter the respective etch rates of thesubstrate 12 andcontrol layer 16. The steepness of the slope imparted to thesubstrate 12 will depend on the relative etch rates of thesubstrate 12 andcontrol layer 16 at various temperatures. In certain embodiments, for example, the etch rate of thecontrol layer 16 can be configured to increase at a greater rate at a particular temperature or temperature range (e.g. at 100° C.). In general, the greater the difference in relative etch rates between the two materials, the more gradual the slope that can be imparted to thesubstrate 12, all other factors being the same. Thus, by selectively increasing and/or decreasing the temperature within theetch bath 76, a desired sloped etch can be formed in thesubstrate 12. -
FIGS. 6A-6D are schematic views illustrating the creation of a custom sloped etch using a control layer and a patterned photomask having a rectangular slot. The process, represented generally byreference number 92, is similar to that described above with respect toFIGS. 3A-3C , beginning with the step of providing asubstrate 94 having asurface 96 to be etched.Substrate 94 may include, for example, a thin wafer of quartz used in the construction of a MEMS electrostatic actuator, optical lens, or other similar device having a desired contour or shape. Acontrol layer 98 andphotomask 100 can also be applied to thesurface 96 of thesubstrate 94 in a manner similar to that described above inFIGS. 1A-1C . In certain embodiments, for example,control layer 98 can include a layer of nickel or other suitable material applied to the surface of aquartz substrate 94. - In the illustrative embodiment of
FIGS. 6A-6D , thephotomask 100 includes asingle layer 102 of silicon nitride (SiN) film or other suitable mask material. As with other embodiments discussed herein, thesingle photomask layer 102 can be configured to bimorph, causing thelayer 102 to curl upwardly away from thesurface 96 of thesubstrate 94 during the etching process. In certain embodiments, for example, thephotomask layer 102 can be configured to bimorph by applying a stretching (i.e. tensile) force to thephotomask layer 102 while it is being applied to thecontrol layer 98. Alternatively, thephotomask layer 102 can include a material having a different coefficient of thermal expansion than that of the material forming thecontrol layer 98, causing thephotomask layer 102 to shrink at a greater or lesser rate than thecontrol layer 98. - In a first step depicted in
FIG. 6A , anopening 104 can be formed through thesingle photomask layer 102 to expose at least a part of theunderlying control layer 98.FIG. 6B is a top view of thesubstrate 94, showing the shape of theopening 104 in greater detail. As can be seen inFIG. 6B , theopening 104 may define arectangular slot 107 having a width W and a length L. Similar to thelongitudinal slit 32 discussed above with respect toFIG. 1D , therectangular slot 107 can be configured to form a contoured slope or profile along the length of thesubstrate 94. The width W of therectangular slot 106, however, can be made greater than the width W of thelongitudinal slit 32 to expose more of theunderlying control layer 98. -
FIGS. 6C-6D illustrate the steps of creating a custom sloped etch within thesurface 96 of thesubstrate 94. As shown in a first position inFIG. 6C , the existence of therectangular slot 107 forms achannel 112 having a substantiallyflat region 114. The dimensions of theflat region 114 will typically depend in part on the width W and length L of therectangular slot 107. -
FIG. 6D is a schematic view showing thesubstrate 94 at a second time at or near the conclusion of the etching process. As can be seen inFIG. 6D , one or morecurved surfaces 116 can also be formed within thesurface 96 of thesubstrate 94. Thecurved surfaces 116 can be formed by selectively etching each of thesubstrate 94 and thecontrol layer 98 using multiple etchants having differing relative etch rates. The temperature of the etch bath may also be controlled during the etching process to help increase and/or decrease the etch rate of thesubstrate 94 and/orcontrol layer 98. - Alternatively, the
curved surfaces 116 can be formed using single etchant by adjusting the temperature within the etch bath at various times during the etching process to increase and/or decrease the etch rate of thesubstrate 94 and/orcontrol layer 98. In either case, thephotomask layer 120 can be configured to bimorph away from thesurface 96 of thesubstrate 94 during the etching process, if desired. -
FIGS. 7A-7D are schematic views illustrating the creation of a custom sloped etch using a control layer and a patterned photomask having multiple openings. The process, represented generally byreference number 118, can begin with the step of providing asubstrate 120 having asurface 122 to be etched.Substrate 120 may include, for example, a thin wafer of quartz or other suitable material. Acontrol layer 124 andphotomask 126 can also be applied to thesubstrate 120 in a manner similar to that described above with respect toFIGS. 1A-1C . In certain embodiments, for example, thecontrol layer 124 can include a layer of nickel or other suitable material applied to the surface of aquartz substrate 120. - In the illustrative embodiment of
FIGS. 7A-7D , thephotomask 126 includes a single,thin layer 128 of silicon nitride (SiN) film or other suitable mask material. As with other embodiments discussed herein, thesingle photomask layer 128 can be configured to bimorph during etching, causing thelayer 128 to curl upwardly away from thesurface 122 of thesubstrate 120. Thephotomask 126 may define a plurality ofopenings control layer 124 to etchant contained, for example, in an etch bath. As can be seen in greater detail inFIG. 7B , theopenings longitudinal slit photomask layer 128. -
FIGS. 7C-7D illustrate the steps of creating a custom sloped etch within thesurface 122 of thesubstrate 120. In a first position illustrated inFIG. 7C , the existence of theopenings photomask 126 initially creates a number ofgaps control layer 124 andsubstrate 120. As can be seen at a later time inFIG. 7D , the existence ofmultiple openings photomask 126 creates acurved surface 142 having a kink 144. The distance D between thelongitudinal slits longitudinal slits longitudinal slits longitudinal slits substrate 120. While the use of twoopenings FIGS. 7A-7D , it should be understood that any number of openings could be employed to alter the shape of the slope, as desired. - Having thus described the several embodiments of the present invention, those of skill in the art will readily appreciate that other embodiments may be made and used which fall within the scope of the claims attached hereto. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size and arrangement of parts without exceeding the scope of the invention.
Claims (45)
Priority Applications (2)
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US10/739,521 US20050133479A1 (en) | 2003-12-19 | 2003-12-19 | Equipment and process for creating a custom sloped etch in a substrate |
PCT/US2004/041864 WO2005061378A2 (en) | 2003-12-18 | 2004-12-14 | Equipment and process for creating a custom sloped etch in a substrate |
Applications Claiming Priority (1)
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US10/739,521 US20050133479A1 (en) | 2003-12-19 | 2003-12-19 | Equipment and process for creating a custom sloped etch in a substrate |
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WO2005061378A3 (en) | 2005-11-10 |
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