Patent Publication Number: US-2019185315-A1

Title: Comb MEMS Device and Method of Making a Comb MEMS Device

Description:
This application is a divisional of U.S. patent application Ser. No. 15/345,179, filed Nov. 7, 2016, which application is a divisional of U.S. application Ser. No. 13/743,306, entitled “Comb MEMS Device and Method of Making a Comb MEMS Device” which was filed on Jan. 16, 2013, now U.S. Pat. No. 9,487,386, which applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a comb MEMS device and a method for making a comb MEMS device. In particular embodiments, the invention relates to a silicon comb microphone. 
     BACKGROUND 
     Over the past years a desire for smaller electronic form factors, smaller power consumption and increased performance has driven an integration of MEMS devices. In particular, MEMS microphones may become smaller and smaller because electronic devices such as, e.g., cell phones, laptops, and tablets become smaller and smaller. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the present invention, a method for making a MEMS device comprises forming trenches in a first main surface of a substrate, forming conductive fingers by forming a conductive material in the trenches and forming an opening from a second main surface of the substrate thereby exposing the conductive fingers, the second main surface being opposite the first main surface. 
     In accordance with an embodiment of the present invention, a MEMS device comprises a stator comprising a first set of fingers, a movable element comprising a second set of fingers, wherein the first set of fingers and the second set of fingers are interdigitated and an anti-sticking mechanism between the first set of fingers and the second set of fingers. 
     In accordance with an embodiment of the present invention, a method of forming a MEMS device comprises forming trenches in a first main surface of a substrate, the trenches being spaced apart by rims and forming an insulating layer on bottom surfaces and sidewalls of the trenches. The method further comprises forming first conductive fingers in the trenches, replacing the rims with second conductive fingers and forming an opening from a second main surface of the substrate thereby exposing the first conductive fingers and the second conductive fingers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1 a    shows an embodiment of a MEMS device; 
         FIG. 1 b    shows a top view of interlocked or interdigitated conductive fingers; 
         FIG. 2  shows another embodiment of a MEMS device; 
         FIG. 3  shows yet another embodiment of a MEMS device; 
         FIG. 4  shows a further embodiment of a MEMS device; 
         FIGS. 5 a -5 j    show an embodiment of a method to manufacture a MEMS device; and 
         FIGS. 6 a -6 m    show another embodiment of a method to manufacture a MEMS device. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to preferred embodiments in a specific context, namely a silicon comb microphone. The invention may also be applied, however, to other microphones and MEMS devices. 
     Standard condenser microphones comprise a parallel plate capacitance. A change in distance (gap) between a membrane (the first plate) and a backplate (second plate) measures an incoming sound signal. Based on this construction air moves through the perforations in the backplate an unavoidable generates noise and, therefore, limits signal to noise ratio improvement. 
     Embodiments of the invention provide comb MEMS devices. Further, embodiments of the invention provide comb MEMS devices having an anti-sticking mechanism such as an anti-sticking layer or an anti-sticking structure on the stator and/or movable element fingers. Further embodiments provide methods to manufacture a comb MEMS device with an anti-sticking mechanism. 
       FIG. 1 a    shows a perspective view of an embodiment of a MEMS device  100 . The MEMS device  100  comprises a stator  110 , a movable electrode such as a membrane (or diaphragm)  120 , an opening or cavity  130  beneath the membrane  120  and a support  140 . In some embodiments the movable electrode  120  comprises a cantilever. The membrane  120  comprises membrane fingers  125  and the stator  110  comprises stator fingers  115 . The membrane fingers  125  and the stator fingers  115  are interlocked or interdigitated. The membrane fingers  125  and the stator fingers  115  are arranged such that two membrane fingers  125  are the outermost fingers or two stator fingers are the outermost fingers. 
     The membrane fingers  125  and the stator fingers  115  are configured to move vertical against each other thereby generating an electrical signal which can be measured. Under normal operation, the membrane fingers  125  move relative to the stator fingers  115  and don&#39;t touch each other. 
     In various embodiments, the membrane fingers  125  and/or the stator fingers  115  may comprise an anti-sticking coating so that the membrane fingers  125  and the stator fingers  115  do not stick to each other even when the fingers  115 ,  125  touch each other due to collapse, humidity or dirt. 
     In other embodiments the membrane fingers  125  and/or the stator fingers  115  comprise anti-sticking structures  116  as shown in  FIG. 1 b   . In various embodiments, the anti-sticking structures  116  are arranged at the stator fingers  115 . In alternative embodiments the anti-sticking structures  116  are arranged at the membrane fingers  125 . In yet other embodiments the anti-sticking strictures  116  are disposed on the stator fingers  115  and the membrane fingers  125 . The anti-sticking structures  116  may be anti-sticking bumps protruding out from a finger  115 ,  125  towards a neighboring finger  115 ,  125  or each neighboring finger  115 ,  125 . The anti-sticking structures are configured to prevent stiction of the fingers  115 ,  125 . 
     The MEMS devices  100  may comprise a sound transducer such as a microphone. Alternatively, the MEMS device  100  may comprise other transducers. For example, another transducer may be a sensor such as a pressure sensor, an accelerometer or a RF MEMS. The MEMS device  100  may be a stand-alone device or alternatively may comprise additional circuit elements such as a pre-amplifier and input/output terminals. The MEMS device  100  may comprise an integrated circuit comprising for example an A/D converter and/or a plurality of transistors. 
       FIG. 2  shows a top view of an embodiment of a MEMS device  200 . The MEMS device  200  comprises a stator  210 , a membrane  220 , an opening or cavity  230  beneath the membrane  220  and a support  240 . The membrane  220  is movably connected to the support  240 . The membrane  220  comprises membrane fingers  225  and the stator  210  comprises stator fingers  215 . The membrane fingers  225  and/or the stator fingers  215  may comprise an anti-sticking coating and/or anti-sticking structure so that the membrane fingers  225  and the stator fingers  215  do not stick to each other. 
     The membrane fingers  225  and/or the stator fingers  215  may be conductive. In an embodiment, the membrane fingers  225  and/or the stator fingers  215  may comprise a conductive material. In an embodiment, the conductive material may comprise a metallic material. For example, the metallic material may comprise a pure metal, an alloy and/or a compound. It is understood that any pure metal may include some amount of trace impurities. In an embodiment, the conductive material may comprise a non-metallic material. In an embodiment, the conductive material may comprise a conductive polymer. In an embodiment, the conductive material may comprise a semiconductor material. For example, the semiconductor material may be a doped semiconductor material such as doped silicon. The doped semiconductor material may, for example, be in situ doped. The doped silicon may, for example, comprise a doped monocrystalline silicon and/or a doped polycrystalline silicon. Hence, in an embodiment, the conductive material may comprise a polycrystalline silicon such as a doped polycrystalline silicon. 
     In an embodiment, the membrane fingers  225  and/or the stator fingers  215  may comprise a semiconductor material. The semiconductor material may be a doped semiconductor material or an undoped semiconductor material. The semiconductor material may comprise silicon. The silicon may comprise monocrystalline silicon and/or polycrystalline silicon. In an embodiment the stator fingers  215  and the membrane fingers  225  comprise the same material. In an embodiment, the stator fingers  215  and the membrane fingers  225  may comprise a different material. 
     In an embodiment, the membrane fingers  225  and/or the stator fingers  215  may further comprise dielectric layers. For example, the membrane fingers  225  and/or the stator fingers  215  may comprise a conductive layer sandwiched between two dielectric layers or a dielectric layer sandwiched between two conductive layers. Hence, in an embodiment, the membrane fingers and/or the stator fingers may comprise an alternating arrangement of conductive layers and dielectric layers. The conductive layers may comprise any conductive material such as the conductive materials described above. The membrane  220  may further comprise corrugation lines  221 ,  222 . The corrugation lines  221 ,  222  may be configured to stiffen the inner region of the membrane  220 . The membrane  220  may comprise only corrugation lines  221  along an x-direction or only corrugation lines  222  along a y-direction. Alternatively, the membrane  220  comprises corrugation lines  221 ,  222  in x- and in y-direction (e.g., forming a criss-cross pattern). In one or more embodiments, the corrugation lines  221 ,  222  may be replaced with other forms of elongated protrusions such as ridges, e.g., areas where one surface is flat and the opposite surface is elevated. The MEMS device  200  (e.g., the membrane  220 ) may further comprise corrugation lines  227 ,  228 . The corrugation lines  227 ,  228  may be disposed in a peripheral region of the membrane  220 . The corrugation lines  227 ,  228  may make the membrane  220  more flexible and easier to move. 
       FIG. 3  shows a top view of yet another embodiment of a MEMS device  300 . The MEMS device  300  comprises two stators  310 A,  310 B, a membrane  320 , an opening or cavity  330  beneath the membrane  320  and a support  340 . The membrane  320  comprises a first set of fingers  325 A on a first side and a second set of fingers  325 B on a second side. Similarly, the first stator  310 A comprises a first set of fingers  315 A and the second stator  310 B comprises a second set of fingers  315 B. The first and second set of membrane fingers  325 A,  325 B, and/or the first and second set of stator fingers  315 A,  315 B may comprise an anti-sticking coating and/or anti-sticking structures so that the membrane fingers  325 A,  325 B and the stator fingers  315 A,  315 B do not stick together. The membrane  320  may be anchored  355  at two side surfaces to the support  340 . The anchors  355  may comprise a spring support. The anchor  355  comprising a spring support may be configured to operate in a torsional mode. The spring support may comprise polysilicon. In one embodiment the membrane  320  may comprise corrugation lines or ridges in x-direction and/or y-direction. In a further embodiment the membrane  320  may comprise one or more corrugation lines in a peripheral region of the membrane, the one or more corrugation lines configured make the membrane  320  more flexible. 
     An advantage of this embodiment may be that a phase difference proportional the direction of the sound signal can be detected. For example, if a sound signal hits the top surface of the membrane  320  from a direction which has an angle or is titled relative to a normal of that top surface the membrane ends  320 A,  320 B deflect differently and the MEMS device  300  can detect the phase difference. 
       FIG. 4  shows a top view of another embodiment of a MEMS device  400 . The MEMS device  400  comprises four stators  410 A,  410 B,  410 C,  410 D, a membrane  420 , a cavity or opening  430  beneath the membrane  420 . The membrane  420  comprises a first set of fingers  425 A on a first side, a second set of fingers  425 B on a second side, a third set of fingers  425 C on a third side, and a fourth set of fingers  425 D on a fourth side. Similarly, the first stator  410 A comprises a first set of fingers  415 A, the second stator  410 B comprises a second set of fingers  415 B, the third stator  410 C comprises a third set of fingers  415 C and the fourth stator  410 D comprises a second set of fingers  415 D. The sides of the membrane  420  may comprise the same length or a different length. The number of fingers on the sides may be the same or different between two sides. 
     The membrane fingers  425 A,  425 B,  425 C,  425 D and/or the stator fingers  415 A,  415 B,  415 C,  415 D comprise an anti-sticking coating and/or anti-sticking structures so that the membrane fingers  425 A,  425 B,  425 C,  425 D and the stator fingers  415 A,  415 B,  415 C,  415 D do not stick to each other. The membrane  420  may be anchored  455  at the four corners of the membrane  420  to the support  440 . The anchors  455  may comprise a spring support. The spring support may be a polysilicon. 
     The membrane  420  may comprise corrugation lines or ridges in x-direction and/or y-direction. In a further embodiment the membrane  420  may comprise one or more corrugation lines in a peripheral region of the membrane, the one or more corrugation lines configured make the membrane  420  more flexible. The membrane  420  may comprise a square, a rectangle, a circle or an oval. Alternatively, the membrane  420  comprises any other suitable geometrical form. 
     An advantage of this embodiment may be that the MEMS device  400  yields an increased sensitivity because the sound signal is sensed along the whole perimeter of the membrane  420 . For example, the MEMS device  400  may be advantageous for high mechanical sensitivity applications. 
       FIGS. 5 a -5 j    show an embodiment of a method of forming a MEMS device. In a first step, as shown in  FIGS. 5 a  and 5 b   , trenches  530  are formed in a first main surface of the substrate  510 . The substrate  510  may comprise a semiconductive material such as silicon or germanium, or a compound semiconductor such as SiGe, GaAs, InP, GaN or SiC. Alternatively, the substrate may comprise organic materials such as glass or ceramic. The substrate  500  may be a wafer. 
     The trenches  530  may be etched in a first main surface  515  of the substrate  510 . The trenches  530  may be etched applying a wet etch chemistry or a dry etch chemistry. For example, the trenches  530  may be etched applying RIE. The trenches  530  may be staggered with respect to line  535 . The line  535  separates the stator from the membrane. The trenches  530  are separated by rims or fins  520 . 
     Referring now to  FIG. 5 c   , in various embodiments, the trenches  530  may comprise sidewall structures  532  such as extensions, cut outs or notches. The sidewall structures  532  may eventually form anti-sticking structures such as anti-sticking bumps on the fingers. The sidewall structures  532  may comprise a square shape, oval shape, rectangular shape or triangle shape. Alternatively, the sidewall structures  532  may comprise any geometrical structure which is configured to reduce a contact area when neighboring fingers touch each other. The sidewall structures  532  may be placed on one sidewall of the trench. Alternatively, the sidewall structures  532  may be placed on two or three sidewalls of the trench. In some embodiments, two or more sidewall structures  532  are placed on a sidewall instead of just one. 
     The sidewall structures  532  may be placed on some of the trenches but not on others. For example, the sidewall structures  532  may be placed only in the trenches which eventually develop into stator fingers or may be placed in the trenches which eventually develop into membrane fingers. Alternatively, the sidewall structures  532  are placed in all trenches. The sidewall structures  532  may be formed over the entire height of the trench. Alternatively, the sidewall structures  532  may be formed over a portion of the height of the trenches. The sidewall structures  532  may be a design choice and structured in the photoresist layer before forming the trenches.  FIG. 5 c    shows a top view of an embodiment of these sidewall structures  532 . 
     In the next step, shown in  FIG. 5 d   , the bottom surface and the sidewalls of the trenches  530  and the top surface of the substrate  510  is covered with an insulating layer  540 . The insulating layer  540  may comprise an oxide layer, a nitride layer and/or an oxynitride layer. For example, the insulating layer  540  may be a silicon oxide or TEOS. Alternatively, the insulating layer  540  may be a silicon nitride layer. The insulating layer  540  may be deposited or grown as a conformal layer. The insulating layer  540  may be deposited such that the insulating layer  540  covers only the bottom surface and the sidewalls of the trenches  530  but not a central portion of the trenches  530 . In one embodiment the trenches  530  are partially filled with the insulating layer  540 . In one embodiment the insulating material of the insulating layer  540  may be deposited with applying a CVD process, a PVD process, an ALD process or a wet oxidation of the substrate  510 . 
     In the next step, shown in  FIG. 5 e   , a finger material  550  may be formed in the trenches  530 . The finger material  550  may be a conductive material  550 . In an embodiment, the conductive material  550  may fill (e.g. completely fill) the trenches. The conductive material may be a metallic material. The metallic material may comprise a pure metal, an alloy and/or a compound. The metallic material may, for example, comprise one or more of the elements chosen from the group consisting of Al, Cu, Ni and Si. Examples, include pure aluminum, aluminum alloy, aluminum compound, pure copper, copper alloy, copper compound, pure nickel, nickel alloy and nickel compound. Examples include AlSiCu. The conductive material  550  may comprise a conductive polymer. The conductive material  550  may comprise a doped semiconductor such as doped silicon. The doped silicon may comprise doped polysilicon and/or doped monocrystalline silicon. The doped silicon may be in situ doped. The conductive material  550  may be deposited in different ways such as sputtering, PVD, CVD or ALD. The conductive material may be deposited as a single step (for example, the trenches may be filled (e.g. completely filled) or in two or more steps. When the conductive material  550  comprises a metallic material, it is possible that the conductive material  550  is deposited by a galvanic deposition. The conductive material  550  may be directly deposited onto the insulating layer  540 . 
       FIG. 5 f    shows an embodiment having specific dimensions. The finger material may be removed from over the top surface of the substrate of the insulating layer  540 . The finger material  550  (e.g. the conductive material  550 ) forms fingers for the stator and the membrane. For example, the fingers may be stator fingers  555  and membrane fingers  557 . Two stator fingers  555  may have a membrane finger  557  in between them and two membrane fingers  557  may have a stator finger  555  in between them. Two stator fingers  555  are spaced apart by a pitch A and the two membrane fingers  557  are spaced apart by the pitch A. Two neighboring fingers  555 , 557  may be spaced apart by a spacing B and each finger may comprise thickness C. In one embodiment half a pitch A is calculated as the spacing B+thickness C. 
     In one embodiment the fingers  555 ,  557  may comprise a height H of about 8 μm to about 12 μm, e.g., about 10 μm. The pitch A may be about 4 μm to about 6 μm, e.g., about 5 μm. The spacing B may be about 1 μm and about 2 μm, e.g., about 1.5 μm, and the thickness C may be about 0.5 μm to about 2 μm, e.g., about 1 μm. The thickness D of the trench  530  may be about 1 μm and about 2 μm, e.g., about 1.5 μm. And the thickness E of the insulating layer  540  may be about 0.1 μm to about 0.5 μm, e.g., about 0.25 μm. 
     Then the conductive material  550  may be removed and pads  552  may be formed as shown in  FIG. 5 g   . In one embodiment a photoresist is disposed over the conductive material  550  and then structured. The exposed portions of the conductive material  550  are then removed. The conductive material  550  may be etched down to the insulating layer  540 . The conductive material  550  disposed in the trenches  530  may not be removed. The conductive material  550  in the trenches may form fingers  555 ,  557 . The conductive material  550  may be removed applying a wet etch or a dry etch chemistry. For example, when the conductive material  550  comprises a semiconductor (e.g. a doped semiconductor such as doped silicon), the conductive material  550  may be etched with KOH or acid solutions of HNO 3  plus HF. In another embodiment a plasma process with chlorine or fluorine delivered by SF 6  or Cl 2  may be used. 
     The etch process may be stopped when the top surface of the insulating layer  540  is reached. The etch process is stopped either by end point detection or by timing (the layer thickness of the insulating layer  540  is much lower than the depth of the fingers). 
     Then pads  552  may be formed. The pads  552  may be formed in or on the substrate  520 . The pads  552  can be disposed according to design specifications (e.g., practically anywhere on the top surface of the substrate  520 ). The pads  552  may comprise the conductive material  550 . Alternatively, the pads  552  may be silicided at the pad  552  location. The silicided pads may formed by forming a metallic material on the conductive material ( 550 ). The metallic material may include one or more of the elements from the group consisting of Ni, Co, and Ti. The conductive material  550  and the metallic material may be annealed to form the silicide. In some embodiments the pads  552  are passivated. 
     As shown in  FIG. 5 h   , the substrate  510  is then etched from the second main surface. The substrate  510  is etched with a directional etch. For example, the substrate  510  is etched with a Bosch® etch. The backside etch is applied such that the substrate  510  is removed under the membrane  516  and that the substrate remains under the stator and the support  514 . The backside etch is stopped by the insulating layer  540 . The fingers  555 ,  557  encoded with the insulating layer  540  remain standing and are not etched. 
     Alternatively, the substrate backside is etched with a wet etch comprising, for example, KOH. In another embodiment the substrate backside is etched with a combination of dry etch to the level of the trenches and subsequent wet etching with a higher selectivity of e.g. substrate (e.g., silicon) etching against the insulating layer (e.g., stopping etch layer of oxide) etching. 
     In the next step shown in  FIG. 5 i    the insulating layer  540  is removed. The insulating layer  540  is removed with a wet etch or a dry etch. For example, the insulating layer  540  is etched applying using a HF based solution or vapor. Then the fingers  555 ,  557  are coated with an anti-sticking coating  560 . For example, the anti-sticking coating  560  is deposited by vapor phase or gas phase deposition. Alternatively, the anti-sticking coating  560  is deposited applying a wet deposition such as a spin on coating. The anti-sticking coating  560  may comprise hydrophobic layer. The anti-sticking coating  560  may be a monolayer. For example, the anti-sticking coating  560  may comprise alkylsilane or a perhaloalkylsilane. Alternatively, the anti-sticking coating  560  may comprise HDMS (Hexamethyldisilazane) or SAM (self-assembling monolayers) based on octadecyltrichlorosilance (OTS) or perflourodecyltrichlorosilance (FDTS). 
     In various embodiments, the anti-sticking coating is a single layer or a plurality of layers. Alternatively, the anti-sticking coating comprises anti-sticking bumps. The anti-sticking bumps may be protruding from the fingers  555 ,  557  surface having a tip in the middle or to the side of each bump. In one embodiment the anti-sticking coating may comprise a combination of an anti-sticking layer and anti-sticking bumps. For example, the combination may be disposed on all fingers. Alternatively, the anti-sticking layer(s) may be disposed on the stator fingers  555  (or membrane fingers  557 ) and the anti-sticking bumps are disposed on the membrane fingers  557  (stator fingers  555 ). The stator fingers  555  and the membrane fingers  557  may be coated with a different type of anti-sticking coating  560  materials or with the same type of anti-sticking coating  560  materials. 
     In various embodiments, the stator fingers  555  and/or the membrane fingers  557  comprise the anti-sticking coating and no anti-sticking structures (step in  FIG. 5 c    is omitted). In alternative embodiments the stator fingers  555  and/or the membrane fingers  557  comprise the anti-sticking structures and no anti-sticking coating (the coating step in  FIG. 5 i    is omitted). In further embodiments the stator fingers  555  and/or the membrane fingers  557  comprise the anti-sticking coating and the anti-sticking structures. In yet other embodiments, some fingers comprise the anti-sticking coating and the other fingers comprise the anti-sticking structure. Any combination of anti-sticking coating and anti-sticking structure is possible. 
       FIG. 5 j    shows an embodiment of a MEMS device according to the manufacturing process described with regard to  FIGS. 5 a -5 i   . The stator fingers  555  disposed on the stator  514  may be interlocked or interdigitated with the membrane fingers  557  in the membrane  570 . A cavity  516  is located underneath the membrane  570  so that the membrane  570  can move up and down relative to the stator  514 . 
       FIGS. 6 a -6 m    show another embodiment of a method of making a MEMS device. In a first step, as shown in  FIG. 6 a   , trenches  630  are formed in a first main surface of the substrate  610 . The substrate  610  may comprise a semiconductive material such as silicon or germanium, or a compound semiconductor such as SiGe, GaAs, InP, GaN or SiC. Alternatively, the substrate  610  may comprise organic materials such as glass or ceramic. The substrate  610  may be a wafer. 
     The trenches  630  may be etched into substrate  610 . The trenches  610  may be etched applying a wet etch chemistry or a dry etch chemistry. For example, the trenches  630  may be etched applying RIE. The trenches  630  are separated by rims or fins  620 . In one embodiment the trenches are wide and the rims or fins  620  are narrow. 
     In the next step, shown in  FIG. 6 b   , the bottom surface and the sidewalls of the trench  630  and the top surface of the substrate  610  are covered with an insulating layer  640 . The insulating layer  640  may be an oxide layer or a nitride layer. For example, the insulating layer  640  may be a silicon oxide or TEOS. The insulating layer  640  may be a conformal layer. The insulating layer  640  may be deposited such that the insulating layer  640  covers only the bottom surface and the sidewalls of the trench  630  but not a central portion of the trench  630 . In one embodiment the trenches  630  are partially filled with the insulating layer  640 . In one embodiment, the insulating layer  640  may be deposited with a CVD process, a PVD process or an ALD process. 
     As shown in  FIG. 6 c   , the trenches  640  are filled with a semiconductive material  650 . For example, the trenches  630  are filled with polysilicon or in situ doped polysilicon. Alternatively, the trenches  630  are filled with a metal or otherwise conductive material. The trenches  630  are completely filled in a single step or in a plurality of steps. The conductive material  650  may completely cover the insulating layer  640 . The conductive material  650  may be directly disposed on the insulating material  640 . The conductive material  650  may be removed from over the top surface of the insulating layer  640  so that individual conductive fingers  655  are formed in the trenches  630 . The fingers  655  may be stator fingers or membrane fingers. 
     The trenches  630  are completely filled in a single step or in a plurality of steps by conformal coating such as CVD or ALD. Alternatively, the trenches  630  are filled with a metal  650  such as Al, AlSiCu, or Ni via sputtering or galvanic deposition or an otherwise conductive material. The conductive material  650  may be directly disposed on the insulating layer  540  and may completely cover the insulating layer  640 . 
     In the next step, shown in  FIG. 6 d   , a resist  660  is formed over the substrate  610 . The resist  660  is structured such that the resist  660  is opened over the rims or fins  620 . The resist  660  remains over the trenches  630 . The insulating layer  640  is removed over the top surface of the rims or fins  620  but not over or in the trenches  630 . The insulating layer  640  is removed by an anisotropic etch. For example, the insulating layer  640  is removed by a plasma etch such as CF 4 /CHF 3 . Next, the rims and fins  620  are removed forming trenches  670 . The rims and fins  620  may be removed down to the bottom surface of the trench  630 . The rims and fins  620  may be etched with an anisotropic or isotropic etch. The etch process may be a dry etch process or a wet etch process. For example, the etch process comprises KOH or acid solutions of HNO 3  plus HF, a plasma process with chlorine or fluorine delivered by SF 6  or Cl 2 . This is shown in  FIG. 6   e.    
     Then, as shown in  FIGS. 6 f -6 h   , the resist  660  is removed and the trenches  670  are filled with a semiconductive material  680 . For example, the trenches  670  are filled with polysilicon or in situ doped polysilicon. Alternatively, the trenches  670  are filled with a metal or otherwise conductive material. The trenches  670  are completely filled in a single step or in a plurality of steps. The conductive material  680  may completely cover the insulating layer  640  and the conductive material  655 . In a subsequent step the conductive material  680  may be removed from over the top surface of the insulating layer  640  so that individual conductive fingers  685  are formed in the trenches  670 . The conductive fingers  655  may comprise the same conductive material or a different conductive material as the conductive fingers  685 . 
       FIG. 6 i    shows an embodiment having specific dimensions. Two stator fingers have a membrane finger in between and two membrane fingers have a stator finger in between. Two stator fingers are spaced apart by a pitch A and the two membrane fingers are spaced apart by the pitch A. Two neighboring fingers may be spaced apart by a spacing B and each finger may comprise thickness C. In one embodiment half a pitch A is calculated as the spacing B+thickness C. 
     In one embodiment the fingers comprise a height H of about 8 μm to about 12 μm, e.g., about 10 μm. The pitch A may be about 3 μm to about 5 μm, e.g., about 4 μm. The spacing B may be about 0.5 μm and about 2 μm, e.g., about 1 μm, and the thickness C may be about 0.5 μm to about 2 μm, e.g., about 1 μm. And the thickness E of the insulating layer may be about 0.1 μm to about 0.5 μm, e.g., about 0.25 μm. 
     Then pads may be formed. The pads may be formed as described with respect to  FIG. 5   g.    
     The substrate  610  is then etched from the second main surface. The substrate  610  is etched with a directional etch. For example, the substrate  610  is etched with a Bosch® etch. The backside etch is applied such that the substrate  610  is removed along a length of the membrane  614  and that the substrate  610  remains standing at the stator and support  616 . The backside crates a cavity  614  beneath the membrane. The etch process is stopped by the insulating layer  640  and the fingers  655 ,  685 . This is shown in  FIG. 6   j.    
     As shown in  FIG. 6 k   , a protection layer or a material layer  690  is disposed on the first main surface of the substrate  610 . The protection layer  690  may be a negative photoresist or a positive photoresist. The protection layer  690  is configured to protect the front side or main surface. In the next step, as shown in  FIG. 6 l   , the insulating layer  640  is removed such that the fingers  655 ,  685  are free standing. The insulating layer  640  is removed with a wet etch or a dry etch. For example, the insulating layer  640  is etched applying a buffered HF, HF vapor or gas phase. 
     Then the fingers  655 ,  685  are coated with an anti-sticking coating  695 . For example, the anti-sticking coating  695  is deposited by vapor phase or gas phase deposition. Alternatively, the anti-sticking coating  695  is deposited applying a wet deposition such as a spin on coating. The anti-sticking coating  695  may comprise hydrophobic layer. The anti-sticking coating  695  may be a monolayer. For example, the anti-sticking coating  695  may comprise alkylsilane or a perhaloalkylsilane. Alternatively, the anti-sticking coating  695  may comprise HDMS (Hexamethyldisilazane) or SAM (self-assembling monolayers) based on octadecyltrichlorosilance (OTS) or perflourodecyltrichlorosilance (FDTS). 
     In one embodiment the anti-sticking coating is a single layer or a plurality of layers. Alternatively, the anti-sticking coating comprises anti-sticking bumps. The anti-sticking bumps may be protruding from the conductive fingers  655 ,  685  surface having a tip in the middle or to the side of each bump. In one embodiment the anti-sticking coating  695  may comprise a combination of an anti-sticking layer and anti-sticking bumps. For example, the combination may be disposed on all fingers  655 ,  685 . Alternatively, the anti-sticking layer(s) may be disposed on the stator fingers (or membrane fingers) and the anti-sticking bumps are disposed on the membrane fingers (stator fingers). The stator fingers and the membrane fingers may be coated with a different type of anti-sticking coating materials or with the same type of anti-sticking coating material. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.