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1. (WO2005061378) MATERIEL ET PROCEDE PERMETTANT DE FORMER UNE GRAVURE INCLINEE PERSONNALISEE DANS UN SUBSTRAT
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EQUIPMENT AND PROCESS FOR CREATING A CUSTOM SLOPED ETCH
IN A SUBSTRATE
Field of the Invention
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.

Background of the Invention
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 (UN) 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.

Summary of the Invention
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 (HNO ) 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.

Brief Description of the Drawings Figures 1A-1D are schematic views illustrating the foπnation of a control layer and a photomask on a substrate;
Figure 2 is a diagrammatic view showing the masked substrate of Figure 1 placed within an etch bath containing multiple etchants;
Figures 3A-3C are schematic views illustrating the creation of a custom sloped etch in the masked substrate of Figure 1 ;
Figure 4 is a graph showing an illustrative custom sloped etch formed in accordance with the process of Figures 3A-3C.
Figure 5 is a schematic view showing the masked substrate of Figure 1 placed within another illustrative etching apparatus containing a single etchant;
Figures 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
Figures 7A-7D are schematic views illustrating the creation of a custom sloped etch using a control layer and a photomask having multiple openings.

Detailed Description of the Invention
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 maybe utilized.
Referring now to Figures 1 A-1D, an illustrative process of forming a control layer and photomask on a substrate will now be described. The process, represented generally by reference number 10, may begin with the step of providing a substrate 12 having a surface 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 Figure 1A, 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. In certain embodiments, for example, 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. In some embodiments, 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. In certain embodiments, for example, the control 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 the surface 14. As is discussed in further steps below, 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.

Figure IB is a schematic view showing the formation of a patterned photomask 18 onto the control layer 16 of Figure 1A. As shown in Figure IB, 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. In certain embodiments, the first photomask layer 20 can include a relatively thin (e.g. 5 A 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. To facilitate adhesion of the SiN film in those embodiments wherein the control layer 16 is formed of nickel, a thin layer of chrome may be used as an intermediate layer to bond the two layers 16,20 together.
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 the surface 14 of the substrate 12 during the etching process. 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. i certain embodiments, for example, the second photomask layer 22 can include a relatively thin (e.g. 5 A thick) layer of polysilicon applied over the first photomask layer 20 at room temperature. To bimorph the photomask 18, 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.
While the application of a second photomask layer 22 is specifically illustrated in Figure IB, it should be understood that other methods may be employed to bimorph the photomask 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 the control layer 16 could be used to bimorph the photomask 18. In use, 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.
Figures IC 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.
Figure ID is a top view of the substrate 12 of Figure IC, showing the shape of the opening 24 in greater detail. As can be seen in Figure ID, 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.
Figure 2 is a diagrammatic view showing the masked substrate 12 of Figure 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.
As can be further seen in Figure 2, 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. In certain embodiments, for example, the first etchant 54 can include a fast-rate etchant solution of nitric acid (HNO3) 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. In contrast to the first etchant 54, 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. In certain embodiments, for example, 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.
Figures 3A-3C are schematic views illustrating the creation of a custom sloped etch in the substrate 12 of Figure 1. At a first time ti depicted in Figure 3A, 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. In certain embodiments, for example, Figure 3 A may depict an initial view of the substrate 12 after being immersed within the etching apparatus 38 of Figure 2. It should be understood, however, that the various illustrative etching stages depicted in Figures 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 Figure 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 the control layer 16 is greater than the etch rate within the substrate 12. In certain embodiments, for example, 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, whereas 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 Figure 3 A, this initial combination of first etchant 54 and second etchant 58 results in the formation of a gap 62.
Figure 3B is a schematic view showing the etching of substrate 12 and control layer 16 at a second time t2. As can be seen in Figure 3B, the relative concentrations of the first and second etchants 54,58 causes the gap 62 to significantly widen between times ti and t2, forming a curved surface 64 within the surface 14 of the substrate 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 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.
Figure 3C is a schematic view showing the substrate 12 at a third time t at or near the conclusion of the etching process. As shown in Figure 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 the gap 62. In certain embodiments, 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. When bimorph properties are imparted to the photomask layers 20,22, for example, 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.
As can be further seen in Figure 3C, 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. In the illustrative slope depicted in Figure 3C, for example, 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. In certain embodiments, for example, a depth D of about 4 to 8 μm 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. Once the desired shape has been formed within the surface 14 of the substrate 12, the photomask 18 and remaining control layer 16 (if any) can then removed, leaving intact the custom sloped etch formed in the substrate 12.
Figure 4 is a graph showing an illustrative custom sloped etch 68 formed in accordance with the illustrative process of Figures 3A-3C. As shown in Figure 4, 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 ti and t2. 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. In certain embodiments, for example, the first curved 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 ti and t2 to gradually increase the vertical etch rate within the substrate 12.
A second curved region 72 can also be formed in the substrate 12 between times t and t3. In contrast to the first curved region 70, 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 contained within the etch bath to gradually decrease the vertical etch rate within the substrate 12. As can be seen at time t2 in Figure 4, for example, an inflection 66 (Figure 3C) is created at time t2 when the flow rates of the first and second etchants 54,58 are adjusted to gradually decrease the vertical etch during this time. By adjusting the relative concentrations of the first and second etchant solutions 54,58 in this manner, 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. 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.
Figure 5 is a schematic view showing the masked substrate of Figure 1 placed within another illustrative etching apparatus 74 containing a single etchant. As shown in Figure 5, 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. In certain embodiments, 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.
To create a custom sloped etch in the substrate 12, 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. In certain embodiments, for example, 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). In general, 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. Thus, by selectively increasing and/or decreasing the temperature within the etch bath 76, a desired sloped etch can be formed in the substrate 12.
Figures 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 Figures 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 Figures lA-lC. In certain embodiments, for example, control layer 98 can include a layer of nickel or other suitable material applied to the surface of a quartz substrate 94.

In the illustrative embodiment of Figures 6A-6D, the photomask 100 includes a single layer 102 of silicon nitride (SiN) film or other suitable mask material. As with other embodiments discussed herein, 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, hi certain embodiments, for example, 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. Alternatively, 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.
In a first step depicted in Figure 6A, an opening 104 can be formed through the single photomask layer 102 to expose at least a part of the underlying control layer 98. Figure 6B is a top view of the substrate 94, showing the shape of the opening 104 in greater detail. As can be seen in Figure 6B, 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 Figure ID, 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, however, can be made greater than the width W of the longitudinal slit 32 to expose more of the underlying control layer 98.
Figures 6C-6D illustrate the steps of creating a custom sloped etch within the surface 96 of the substrate 94. As shown in a first position in Figure 6C, 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.

Figure 6D is a schematic view showing the substrate 94 at a second time at or near the conclusion of the etching process. As can be seen in Figure 6D, 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.
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 the substrate 94 and/or control layer 98. In either case, the photomask layer 120 can be configured to bimorph away from the surface 96 of the substrate 94 during the etching process, if desired.
Figures 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 Figures lA-lC. In certain embodiments, for example, the control layer 124 can include a layer of nickel or other suitable material applied to the surface of a quartz substrate 120.
In the illustrative embodiment of Figures 7A-7D, the photomask 126 includes a single, thin layer 128 of silicon nitride (SiN) film or other suitable mask material. As with other embodiments discussed herein, 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. As can be seen in greater detail in Figure 7B, 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.
Figures 7C-7D illustrate the steps of creating a custom sloped etch within the surface 122 of the substrate 120. In a first position illustrated in Figure 7C, 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. As can be seen at a later time in Figure 7D, the existence of multiple openings 130,132 withm 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. In certain embodiments, for example, the distance D between each of the longitudinal slits 134,136 can be made greater to increase the height of the kink 252. Alternatively, 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 Figures 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 maybe made in details, particularly in matters of shape, size and arrangement of parts without exceeding the scope of the invention.