Patent Publication Number: US-2005133479-A1

Title: Equipment and process for creating a custom sloped etch in a substrate

Description:
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 (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.  
     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 3 ) 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  
       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; 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.  
    
    
     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 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 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  FIG. 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 .  
       FIG. 1B  is a schematic view showing the formation of a patterned photomask  18  onto the control layer  16  of  FIG. 1A . As shown in  FIG. 1B , 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 Å 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 . In certain embodiments, for example, the second photomask layer  22  can include a relatively thin (e.g. 5 Å 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  FIG. 1B , 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.  
       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. As can be seen in  FIG. 1D , 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.  
      As can be further seen in  FIG. 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 (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 . 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 .  
       FIGS. 3A-3C  are schematic views illustrating the creation of a custom sloped etch in the substrate  12  of  FIG. 1 . At a first time t 1  depicted in  FIG. 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,  FIG. 3A  may depict an initial view of the substrate  12  after being immersed within the etching apparatus  38  of  FIG. 2 . It should be understood, however, that 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 .  
      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  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 . As can be seen in  FIG. 3B , 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 . 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 .  
       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. As shown in  FIG. 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  FIG. 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  FIG. 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 .  
       FIG. 4  is a graph showing an illustrative custom sloped etch  68  formed in accordance with the illustrative process of  FIGS. 3A-3C . As shown in  FIG. 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 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 . In certain embodiments, for example, the first curved region  68  can be formed by adding an initial amount of HNO 3  and HF within the etch bath  40  (at time t=0), and then steadily increasing the amount of HF between times t 1  and t 2  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 2  and t 3 . 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 3  contained within the etch bath to gradually decrease the vertical etch rate within the substrate  12 . As can be seen at time t 2  in  FIG. 4 , for example, an inflection  66  ( FIG. 3C ) is created at time t 2  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.  
       FIG. 5  is a schematic view showing the masked substrate of  FIG. 1  placed within another illustrative etching apparatus  74  containing a single etchant. As shown in  FIG. 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 .  
       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 . 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  FIGS. 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. In 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  FIG. 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 .  FIG. 6B  is a top view of the substrate  94 , showing the shape of the opening  104  in greater detail. As can be seen in  FIG. 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  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 , however, 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 . As shown in a first position in  FIG. 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 .  
       FIG. 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  FIG. 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.  
       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 . 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  FIGS. 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  FIG. 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 .  
       FIGS. 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  FIG. 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  FIG. 7D , 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. 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  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.