Abstract:
A micoroelectromechanical system (MEMS) includes a housing defining an enclosed cavity, stator tines extending from the housing into the cavity, a MEMS device located within the cavity, the MEMS device including a proof mass and rotor tines extending from the proof mass, each rotor tine being positioned at a capacitive distance from a corresponding stator tine. The rotor tines include a first section extending a first distance from an insulating layer of the rotor tines and a second section extending a second distance from the insulating layer in an opposite direction from the first section. The stator tines include a first section extending a first distance from an insulating layer of the stator tines and a second section extending a second distance from the insulating layer in an opposite direction from the first section, the stator tine first distance being greater than the rotor tine first distance.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    High performance Micro-Electro-Mechanical Systems (MEMS) inertial instruments (accelerometers and gyros) require closed-loop operation. Good performance under vibration requires that the magnitude of the electrostatic feedback force be highly insensitive to the position of the instrument&#39;s inertial mass relative to it&#39;s null position. Because of their high force dependency on position, the typical electrostatic vertical comb drive as employed in MEMS actuators cannot be applied to precision MEMS inertial instruments. There have been attempts to provide an electrostatic MEMS drive with very low position sensitivity, but they have been costly due to the numerous manufacturing steps required. 
         [0002]    Therefore, there exists a need for an electrostatic MEMS drive, which has very low position sensitivity. 
       SUMMARY OF THE INVENTION 
       [0003]    An example embodiment of the present invention includes a micoroelectromechanical system (MEMS) that includes a housing formed from a double layer silicon on insulator (SOI) material having an insulating layer separating a first section from a second section, the housing defining an enclosed cavity such that the insulating layer from the housing SOI material runs along a length of each stator tine and is disposed between a first section of each stator tine extending a first distance from the insulating layer and a second section of each stator tine extending a second distance from the insulating layer in an opposite direction from the first section, the first section of each stator tine being electrically connected to the first section of the housing and the second section of each stator tine being electrically connected to the second section of the housing; one or more stator tines extending from the housing into the cavity; a MEMS device located within the cavity, the MEMS device including a proof mass formed from a double layer SOI material having an insulating layer separating a first section from a second section, the proof mass coupled to the housing; and one or more rotor tines extending from the proof mass such that the insulating layer from the proof mass SOI material runs along a length of each rotor tine and is disposed between a first section of each rotor tine extending a first distance from the insulating layer and a second section of each rotor tine extending a second distance from the insulating layer in an opposite direction from the first section, each rotor tine being positioned at a capacitive distance from a corresponding stator tine, the first section of each rotor tine being electrically connected to the first section of the proof mass and the second section of each rotor tine being electrically connected to the second section of the proof mass. In an example embodiment, the proof mass is configured to deflect in a direction approximately orthogonal to a vector normal between the corresponding tines and the stator tine first distance is greater than the rotor tine first distance. 
         [0004]    In accordance with further aspects of the invention, the stator tine second distance is approximately equal to the rotor tine second distance. 
         [0005]    In accordance with other aspects of the invention, the system includes a contact structure that electrically connects the first section of the rotor tines to the second section of the rotor tines through at least one of the insulating layer of the proof mass or the insulating layer of the rotor tines. 
         [0006]    In accordance with still further aspects of the invention, the insulating layer of the rotor tines and the insulating layer of the stator tines are oxide layers. 
         [0007]    In accordance with yet other aspects of the invention, the system includes a contact structure that electrically connects a first portion of the first section of the housing to a first portion of the second section of the housing through a region of the insulating layer of the housing. The example system also includes an isolation trench that electrically isolates the first portion of the first section of the housing from a second portion of the first section of the housing, the first section of the stator tines remaining electrically connected to the second portion of the first section of the housing and the second section of the stator tines remaining electrically connected to the second portion of the second section of the housing while the second portion of the first section of the housing remains electrically isolated from the second portion of the second section of the housing. 
         [0008]    In accordance with still further aspects of the invention, the MEMS device includes a sensor such as an accelerometer or a gyro. 
         [0009]    As will be readily appreciated from the foregoing summary, the invention provides a microelectromechanical system including a MEMS device using position-independent drive electrodes in a multi-layer substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0011]      FIG. 1  illustrates a block diagram of a sensor system formed in accordance with an embodiment of the invention; 
           [0012]      FIG. 2  illustrates a cross-sectional view of components used in an embodiment of the invention; 
           [0013]      FIGS. 3-14  illustrate cross-sectional views of steps in a trench filling process in accordance with an embodiment of the invention; 
           [0014]      FIGS. 15-18  illustrate cross-sectional views of steps in a device mask patterning process in accordance with an embodiment of the invention; 
           [0015]      FIGS. 19-24  illustrate cross-sectional views of steps in a Silicon-On-Insulator (SOI) etching process in accordance with an embodiment of the invention; and 
           [0016]      FIGS. 25-28  illustrate cross sectional views of steps performed after SOI etching in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]      FIG. 1  illustrates a block diagram of a sensor system  38  formed in accordance with an embodiment of the invention. The system  38  includes a housing  40  defining an enclosed cavity  42 . A plurality of stator tines  46  extend from the housing  40  into the enclosed cavity  42 . A proof mass  48  is suspended within the enclosed cavity  42  and includes a plurality of rotor tines  50  that are spaced apart at a capacitive distance from the stator tines  46 . In an example embodiment, the housing  40 , the stator tines  46 , the proof mass  48 , and the rotor tines  50  are formed of a double layer silicon on insulator (SOI) material having an insulating layer separating a first section from a second section. In an example embodiment, the sensor system  38  includes a microelectromechanical system (MEMS) accelerometer and/or gyro. 
         [0018]    The range of the total thickness is 24 to 100 microns. The stator is the full height dimension (24 to 100 microns), and the rotor is roughly ¾ of that height. The stator and rotor tines would be approximately ¾ of the total thickness, with a ¼ offset. For Example, if the total thickness is 24 microns, then the thickness of the rotor and stator combs would be 18 microns, with a 6 micron offset. 
         [0019]    A sensor/controller device  51  is in signal communication with the sensor system  38 . In an example embodiment, the device  51  senses vertical relative motion between the proof mass  48  and the housing  40 . Typically, this sense, or pick-off, would operate capacitively. In closed-loop operation, once the device  51  senses a proof mass movement, the device  51  sends a signal to the stator tines  46  in order to force the rotor tines  50  back to a null position. The signal (e.g., voltage value) that is sent to the stator tines  46  for forcing the rotor tines  50  back to null position is used to calculate the amount of acceleration that the proof mass  48  is experiencing. 
         [0020]    The housing  40  is shown with a filled isolation trench  52  enclosing a first volume  53  of the first section of the SOI material of the housing  40  that is separated from the remainder of the first section of the SOI material of the housing  40 . A conducting structure  54  is located within the first volume  53  and extends into the housing  40 , connecting both sections (layers) of the SOI material of which the housing  40  is formed through the insulating layer of the housing  40 . The conducting structure  54  is connected to ground in an example embodiment, but is connected to a voltage source in other embodiments. A contact pad  56  is attached to an area of the first section of the SOI material of the housing  40  that is electrically isolated from the first volume  53  by the isolation trench  52 . In an example embodiment, the contact pad  56  does not extend through both layers of SOI material of which the housing  40  is formed. The contact pad  56  is connected to a voltage VB in an example embodiment. This allows the voltage VB to be applied to one layer of the SOI material, while ground or the other applied voltage source is applied to the other layer of the SOI material, resulting in differing voltages on each section (layer) of the stator tines  46 . Although not shown, additional structures similar to the isolation trench  52 , the conducting structure  54 , and the contact pad  56  are also present in an embodiment. 
         [0021]    A conducting structure  58  is located on part of the proof mass  48  in an example embodiment. The conducting structure  58  extends into the proof mass  48 , connecting both layers of the SOI material of which the proof mass  48  and rotor tines  50  are formed. The conducting structure  58  is connected to a voltage V A  in an example embodiment. This allows the voltage V A  to be applied to both layers of the SOI material, so that both layers of the SOI material that form the rotor tines  50  receive the voltage V A . Although the conducting structure  58  is shown located on the proof mass  48  in this example, the conducting structure  58  could be located on other structures made of the same SOI material as the rotor tines  50 . 
         [0022]      FIG. 2  illustrates a cross-sectional view of components and structures used in an embodiment of the invention. An example embodiment is formed in a fabrication process from a starting material  98 . The starting material  98  includes a silicon handle wafer  100 , a first silicon on insulator (SOI) layer  102 , and a second SOI layer  104 . The first SOI layer  102  is separated from the handle wafer  100  by a first buried silicon oxide (BOX) layer  106  and from the second SOI layer  104  by a second BOX layer  108 . In an example embodiment, the SOI layers  102 ,  104  are silicon on oxide layers. 
         [0023]    The fabrication process produces various structures in the two SOI layers  102 ,  104  including a Vertical Comb actuator (VCA)  110  with offset stationary and movable comb fingers. The process also produces open trenches and/or structures  112  through both SOI layers  102 ,  104  and the second BOX layer  108 ; open structures with the second SOI layer  104  being partially or fully removed; trenches and/or structures in the second SOI layer  104  which are filled with an electrically isolating material  114 ; and structures connecting the first SOI layer  102  and the second SOI layer  104  through the second BOX layer  108  by being filled with an electrically conducting material  116 . 
         [0024]    In an example embodiment, the forming of structures in the SOI layers  102 ,  104  of the silicon wafer can be divided into fabrication steps that include trench filling, device mask patterning, and SOI etching, which are described in additional detail below. 
         [0025]      FIGS. 3-14  illustrate cross-sectional views of steps in a trench filling process in accordance with an embodiment of the invention. A double SOI wafer  130  is used as starting material. In similar fashion to that described for  FIG. 2 , the wafer  130  includes a silicon handle wafer  132 , a first SOI layer  134 , and a second SOI layer  136 . The first SOI layer  134  is separated from the handle wafer  132  by a first BOX layer  138  and from the second SOI layer  136  by a second BOX layer  140 . 
         [0026]      FIG. 3  illustrates a first masking layer  142  being deposited on the wafer  130  surface and patterned with an isolation trench layout  144 , such as by being photo-resist patterned using photo-lithography, for example. The patterned first masking layer  142  is then used as a mask for etching the second SOI layer  136  in the exposed areas to form a trench  146 , as shown in  FIG. 4 . In an example embodiment, the etching is anisotropic (into the wafer only), which is commonly accomplished by Deep Reactive Ion etching (DRIE) and stops on the second BOX layer  140 . After silicon etching, the remaining masking layer  142  is stripped as shown in  FIG. 5 , such as by plasma ashing or in a solvent solution, for example. 
         [0027]      FIGS. 6-8  illustrate that the trench  146  is then filled with an electrically isolating and mechanically connecting material. The material can be a single material or multiple materials, such as silicon dioxide, silicon nitride, poly-silicon, or materials with similar properties. If poly-silicon is used in an example embodiment, sidewalls of the trench  146  are lined with oxide or nitride before the trench center is filled with poly-silicon. 
         [0028]      FIG. 6  illustrates one variation, where the trench  146  sidewalls are first lined with an electrically isolating material, shown as a first iso-layer  148  that is formed of thermally grown silicon dioxide in an example embodiment. Then, as shown in  FIG. 7 , the center of the trench  146  is filled with another electrically isolating material, shown as a second iso-layer  150  such as Low Pressure Chemical Vapor Deposition (LPCVD) silicon nitride. Next, as illustrated in  FIG. 8 , the second iso-layer  150  material is removed from the wafer  130  surface, stopping on the first iso-layer  148 . 
         [0029]      FIGS. 9-14  illustrate additional steps that electrically connect the first and second SOI layers  134 ,  136  such as may be used in a portion of a moveable comb electrode.  FIG. 9  shows that a resist mask  152  is patterned on the wafer surface with a contact trench/hole layout  154 . In an example embodiment, this is formed using photolithography with the pattern being transferred into the first iso-layer  148  below, such as by reactive ion etching (RIE), for example. Next, as shown in  FIG. 10 , a silicon etching step follows where the second SOI layer  136  is etched in an exposed area  156  to the second BOX layer  140 . In an example embodiment, etching is anisotropic (into the wafer only), such as by DRIE. Next, as shown in  FIG. 11 , the second BOX layer  140  is removed in an unmasked area to form an open contact trench  158  such as by RIE or in a diluted hydrogen fluoride (HF) solution. Then, as shown in  FIG. 12 , the remaining resist mask  152  on the wafer surface is stripped such as by plasma ashing or in a solvent solution, for example. As shown in  FIG. 13 , the open contact trench  158  is filled with an electrically conductive material  160 , such as insitu doped poly-silicon deposited by LPCVD, electrically connecting the second SOI layer  136  and the first SOI layer  134 . Next, as shown in  FIG. 14 , the conductive layer and the first isolayer  148  are removed from the wafer surface. In some embodiments, this is done by silicon RIE and oxide RIE, or by wafer surface grinding and polishing. The resulting wafer is a double SOI material with an embedded isolation trench  162  and a contact trench  164 . The wafer may include various numbers and structures of isolation and contact trenches  162 ,  164 . 
         [0030]      FIGS. 15-18  illustrate cross-sectional views of steps in a device mask patterning process in accordance with an embodiment of the invention. The starting material shown in  FIG. 15  is a double SOI wafer  170 . In similar fashion to that described for  FIG. 3 , the wafer  170  includes a silicon handle wafer  172 , a first SOI layer  174 , and a second SOI layer  176 . The first SOI layer  174  is separated from the handle wafer  172  by a first BOX layer  178  and from the second SOI layer  176  by a second BOX layer  180 . 
         [0031]    A first mask  182 , such as an oxide mask is deposited and patterned with a plurality of stationary elements  184 , such as stator components of a VCA, and with patterns defining the mechanical structure of the sensor (not shown). A second mask  186 , such as a resist mask, is deposited and patterned with stationary  188  and movable elements  190  of the VCA as well as with sensor patterns requiring etching through both SOI layers  174 ,  176  and the second BOX layer  180 , such as a proof mass, beams, flexures, proof mass anchors, and proof mass perforations (all not shown). The stationary comb elements  188  defined in the second mask  186  are smaller than the stationary comb elements  184  defined in the first mask  182 . 
         [0032]    After patterning of the second mask  186 , the pattern of the second mask  186  is transferred into the underlying first mask  182  to form a modified first mask  192  as shown in  FIG. 18 , such as by RIE. The resulting multi-layer mask includes the second mask material (resist) for the movable elements of the VCA, and the modified first mask  192  and second mask  186  material for the stationary elements of the VCA and for the other sensor structure patterns (not shown). The masking method can be repeated to deposit and pattern additional masks, defining multiple etch depths into the second SOI layer  176  or to completely remove the second layer  176  in open areas. 
         [0033]    The definition of the stator and rotor elements by the same mask (second mask  186 ) self-aligns the stator to rotor pattern by centering a rotor element  194  between two stator elements  196 . 
         [0034]      FIGS. 19-24  illustrate cross-sectional views of steps in a Silicon-On-Insulator (SOI) etching process in accordance with an embodiment of the invention. During a first silicon etching step, the second SOI layer  176  is etched to the second BOX layer  180  using the multi-layer mask including masks  186 ,  192  as an etch mask to form the structure shown in  FIG. 19 . Etching is anisotropic, into the wafer only, which is commonly accomplished by DRIE using sulfur hexafluoride (SF 6 ) and octafluorocyclobutane (C 4 F 8 ) as precursors. Next, the second BOX layer  180  is removed in exposed areas  198 , such as by RIE or in a diluted HF solution to form the structure shown in  FIG. 20 . 
         [0035]    During a second silicon etching step, the first SOI layer  174  is etched to the first BOX layer  178  in exposed areas  200  to form the structure shown in  FIG. 21 . Etching is anisotropic as during the first silicon etching step. Next, the remaining second mask  186  (resist) of the double-layer surface mask is selectively removed, such as by plasma ashing using an oxygen precursor or in a solvent solution to form the structure shown in  FIG. 22 . This exposes the movable comb elements  194  of the VCA as well as other device structures requiring a thinned second SOI layer  176 , while the stationary comb elements  196  of the VCA are still masked by the modified first mask layer  192  (oxide). During a third silicon etching step, the second SOI layer  176  is etched in the exposed areas of the movable comb elements  194  of the VCA to form the structure shown in  FIG. 23 . The etching direction is into the wafer again, as during the previous silicon etching steps, preserving the previously etched structures. The etching is timed such that the movable comb elements  194  as well as other device structures (not shown) requiring a thinned second SOI layer  176  are shortened to a pre-defined height. 
         [0036]    If additional masking layers (not shown) were deposited and patterned, the method of etching the second SOI layer  176  would be modified such that the deepest structure is opened first (by removing its mask) and then etched to a depth of its final etch depth minus the etch depths of the following second SOI layer  176  etches. The process is repeated for other structures with different etch depths in the second SOI layer  176 , with the most shallow etch being executed last. 
         [0037]    Then, the remaining modified first mask layer  192  (oxide) is removed from the surface of the processed wafer  170  to form the structure shown in  FIG. 24 , such as by RIE or etching in a diluted HF solution. During this mask-stripping step, care must be taken to not completely remove the first BOX layer  178  which would release the SOI structures prematurely. Therefore, anisotropic reactive ion etching is the preferred method, taking advantage of its higher surface etch rate compared to the first BOX layer  178  etch rate, and a nearly zero etch rate of the second BOX layer  180 . 
         [0038]      FIGS. 25-28  illustrate cross sectional views of steps performed after SOI etching in accordance with an embodiment of the invention. After mechanical structures are fabricated in the SOI layers  174 ,  176 , the processed SOI wafer  170  is bonded to a first cover  210  to form the structure shown in  FIG. 25 . The cover  210  is formed of a glass substrate, such as pyrex, in which various structures (not shown) have been separately fabricated in an example embodiment of the invention. As examples, anodic bonding or bonding using an inter-layer adhesive such as glass-frit, polymer, or metal can be used for attaching the cover  210  to the processed SOI wafer  170 . Sets of electrodes of the parallel plate type are formed between the processed SOI wafer  170  and the cover  210  by using silicon structures (not shown) as a movable electrode and metal pads (not shown) on the cover  210  as a stationary electrode. The electrodes are separated by a gap which is formed by etching a recess into the cover  210 . In an example embodiment, the cover  210  also provides metallized contact pads (not shown) connected to the second SOI layer  176 , metallized contact pads (not shown) in recessed areas for wire-bonding to a sensor package, and metal lines (not shown) between the contact pads. 
         [0039]    After the processed SOI wafer  170  with the SOI layers  174 ,  176  has been bonded to the first cover  210 , the silicon handle wafer  172  is removed, stopping on the first BOX layer  178  to form the structure shown in  FIG. 26 . As examples, this can be done by a combination of processes or by a single process that include grinding, polishing, and etching processes selective to oxide, including wet chemical etching in alkaline solutions such as potassium hydroxide (KOH), ethylene diamine-pyrocatechol (EDP), and tetramethyl ammonium hydroxide (TMAH) as well as plasma etching using fluorine radicals. 
         [0040]    Next, the exposed oxide layer of the first BOX layer  178 , is removed stopping on the first SOI layer  174  to form the structure shown in  FIG. 27 . As examples, this can be done by an etching process selective to silicon, such as by plasma etching using carbon tetrafluoride (CF 4 )/carbon hydro-trifluoride (CHF 3 ) precursors or by wet chemical etching in a diluted hydrogen fluoride (HF) solution. This process releases movable silicon structures previously formed in the SOI layers. 
         [0041]    In some embodiments, a second cover  220  is bonded to the first SOI layer  174  to form the structure shown in  FIG. 28 . The second cover  220  is formed of a glass substrate, such as pyrex, in an example embodiment of the invention. Anodic bonding, or bonding using an interlayer adhesive such as glass frit, polymer, or metal can be used for attaching the second cover  220  to the first SOI layer  174 . In some embodiments, the second cover  220  includes structures (not shown) previously formed in it, including recesses, metal electrode pads, metal pads used to connect to the first SOI layer  174 , metal lines connecting the pads, and/or open access areas for wire bonding to the second cover  220 . 
         [0042]    Devices are then singulated from the wafer assembly such as by dicing, for example. This may require protecting the devices from process residues during singulation and/or cleaning of the devices. 
         [0043]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.