Abstract:
A micro electro mechanical system device has a first subassembly having sensor element including a coupler, and a second subassembly including a comb drive. The comb drive having stator plates and rotor plates and the coupler configured to displace the rotor plates relative to the stator plates providing a variable capacitance dependent on the displacement of the rotor plate.

Description:
REFERENCE TO EARLIER FILED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/441,760, filed Feb. 11, 2011, the disclosures of which is incorporated, in its entirety, by this reference. 
     
    
       [0002]    The present application relates to the field of micro electro mechanical system sensors. 
       BACKGROUND 
       [0003]    This disclosure relates to apparatus and methods for improving the precision of capacitive nanoscale measurements of pressure and other physical variables using micro electro mechanical systems, commonly referred to as MEMS. A particular focus is in developing features affording linear performance characteristics of such devices, which may be further varied by application of electrostatic, thermal, or physical displacement biasing. Another focus is in developing novel structural component elements designed to simplify the manufacture of MEMS devices having the desired features. 
         [0004]    The use of MEMS devices for measurement of pressure and other physical variables is known, for example, from U.S. Pat. No. 7,721,587 and the prior art references cited therein. It is well known to researchers in the area of micro and nano-electromechanical systems (M/NEMS) that mechanical performance strongly depends on geometric and material properties. These fabricated properties are difficult to predict and difficult to measure. The problem with prediction is that, given any fabrication recipe, the geometric and material properties of the devices that result from that recipe will vary between fabrication facilities, between fabrication runs, and even across a given wafer itself. A problem with many measurement methods is that they often yield uncertainties that are of the same order as the property being measured. 
         [0005]    Regarding material properties, for a given displacement, Young&#39;s modulus is often used to determine force in MEMS by Hooke&#39;s law. However, the Young&#39;s modulus of fabricated MEMS devices is often unknown. Although many in the field use lookup tables to determine the Young&#39;s modulus, such values are usually averages of measurements that vary by 10 percent or more. Since there is currently no standard for measuring Young&#39;s moduli, the true accuracy of such measurements is unknown. It has been shown that standard overetch errors in fabrication can increase system stiffness as high as 98%. Including the uncertainty in Young&#39;s modulus increases the relative error in stiffness to 188%. Thus, there remains a need for MEMS measurement devices that can be reliably calibrated and operate on a linear slope to simplify the calibration and scaling of the movement of the MEMS device in relation to the variable sought to be measured. 
       SUMMARY 
       [0006]    An embodiment includes a microfabricated variable capacitor comprising a stator and a rotor. The stator includes a plurality of electrically conductive plates each spaced apart from one another and each pair of adjacent plates form a channel therebetween. Each of the plates is in a first common electrical communication. The rotor includes a central hub and first and second arms extending in cantilever manner from opposite sides of the hub. A first plurality of electrically conductive blades is coupled to the first arm and a second plurality of electrically conductive blades is coupled to the second arm. Each of the first plurality and the second plurality of blades are in a second common electrical communication. The hub is suspended from the stator by first and second springs, such that each of the blades is received within a corresponding channel and a portion of each blade coacts with an adjacent said plate to store and electrical charge, and the capacitance between the first electrical communication and the second electrical communication varies as the first and second springs bias the rotor to different positions relative to said stator. 
         [0007]    Another embodiment includes a microfabricated variable capacitor comprising a stator, a rotor, and a suspension system. The stator has a width and includes a plurality of electrically conductive plates each spaced apart from one another with each plate having a top, a bottom, and a midsection therebetween. Adjacent plates form a channel between opposing midsections and each of the plates is in a first common electrical communication. The rotor has a length and includes a plurality of electrically conductive blades with each blade having a top, a bottom, and a midsection therebetween. The rotor is suspended relative to the stator such that each of the blades is received within a corresponding channel and the midsection of each said blade includes an area that overlaps with an area of an adjacent plate. Each of the blades being in a second common electrical communication. The suspension system flexibly couples the rotor relative to said stator. The system flexibly couples to the stator at a location about midway across the width and to the rotor at a location about midway along the length. The capacitance between the first electrical communication and the second electrical communication varies in correspondence to different overlapping areas. 
         [0008]    Other embodiments include a micro electro mechanical system sensor. The micro electro mechanical system sensor comprises a comb drive and a membrane suspended proximate the cone drive. The cone drive has a stator portion with a plurality of stator plates each spaced apart from one another and a rotor portion including a plurality of rotor plates spaced apart from one another. The rotor portion is elastically suspended from the stator portion and the plurality of stator plates and the plurality of rotor plates form a capacitor having a capacitance that varies with the position of the rotor portion relative to the stator portion. The membrane suspended proximate the comb drive has a stub in physical contact with the rotor portion of the comb drive that biases the rotor portion to a first position having a first comb drive capacitance and a movement of the membrane causes the stub to bias the rotor portion to a second position having a second comb capacitance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    To further clarify the above and other advantages and features of the one or more present inventions, reference to specific embodiments thereof are illustrated in the appended drawings. The drawings depict only typical embodiments and are therefore not to be considered limiting. One or more embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0010]      FIG. 1  is a schematic orthogonal view of the upper surfaces of two sub-assemblies designed to form an embodiment of a MEMS device. 
           [0011]      FIG. 2  is a schematic orthogonal view of the lower surfaces of the two sub-assemblies shown in  FIG. 1 . 
           [0012]      FIG. 3   a  is a schematic sectional view of the two sub-assemblies shown in  FIG. 1  spaced apart. 
           [0013]      FIG. 3   b  is a schematic sectional view of the two sub-assemblies shown in  FIG. 1  assembled together causing a physical displacement of one element of the comb drive. 
           [0014]      FIG. 4  is a schematic orthogonal sectional view of the comb drive sub-assembly identifying a region of interest. 
           [0015]      FIG. 5  is a graph of the capacitance of the comb drive in relation to the relative displacement of the elements of the comb drive. 
           [0016]      FIG. 6  is a schematic orthogonal view of a MEMS device including a trench and dimple alignment feature. 
           [0017]      FIG. 7   a  is a schematic sectional view similar to  FIG. 3   a  of the device shown in  FIG. 6 . 
           [0018]      FIG. 7   b  is a schematic sectional view similar to  FIG. 3   b  of the device shown in  FIG. 6 . 
           [0019]      FIG. 8  is schematic orthogonal view similar to  FIG. 1  of the upper surfaces of two sub-assemblies designed to form another embodiment of a MEMS device. 
           [0020]      FIG. 9  is schematic orthogonal view similar to  FIG. 2  of the lower surfaces of two sub-assemblies shown in  FIG. 8 . 
           [0021]      FIG. 10  is a plan view of a circular comb drive included in the second embodiment MEMS device. 
           [0022]      FIG. 11  is a schematic sectional view of a MEMS device included in packaging defining a gas pressure port confronting the MEMS membrane. 
           [0023]      FIG. 12  is an experimental assembly for evaluating the performance of the MEMS devices in relation to gas pressure. 
           [0024]      FIG. 13  is a thermal sensor self-calibrated using Electro Micro-Metrology methods. 
           [0025]      FIG. 14  is another thermal sensor including a Chevron thermal actuator self-calibrated using Electro Micro-Metrology methods. 
       
    
    
       [0026]    The drawings are not necessarily to scale. 
       DETAILED DESCRIPTION 
       [0027]    A MEMS sensor device  10  is shown in  FIG. 1  in an exploded view to include two sub-assemblies  12  and  14 . An upper or membrane sub-assembly  12  includes a SiO 2  membrane  16 , which may have a thickness of about 2 μm. A much thicker upper handle layer  18  surrounds the perimeter of the SiO 2  membrane  16 . The upper handle layer  18  may be formed of single crystal silicon (SCS) and may have a thickness of about 400 μm in a vertical dimension. The lower or comb drive sub-assembly  14  includes a lower handle layer  20 , which may also be formed of SCS and be of similar dimension as the upper handle layer  18 . An outside perimeter of a conductive SCS layer  22  is supported on the lower handle layer  20 . The conductive SCS layer  22  is etched or otherwise fabricated by conventional silicon-on-insulator (SOI) technology to form a comb drive  24 . The comb drive  24  is comprised of a stator having stator plates and a rotor having rotor plates (see  FIG. 4 ). A lower view of the MEMS sensor device  10  is shown in  FIG. 2  to reveal a stub  26  situated at a central location of the membrane  16  and extending toward the comb drive  24 . 
         [0028]      FIG. 3   a  and  FIG. 3   b  are sectional views of the sub-assemblies  12 ,  14  of the embodiment of  FIG. 1 .  FIG. 3   a  shows the two sub-assemblies  12 ,  14  apart, while  FIG. 3   b  shows the two sub-assemblies  12 ,  14  joined. The effect of the joining of the two sub-assemblies  12 ,  14  can be seen by comparing  FIGS. 3   a  and  3   b . In both  FIGS. 3   a  and  3   b , the section is taken through a rotor plate parallel to the stator plates. In  FIG. 3A  a stator portion  28  of the comb drive  24  is visible in the region of the lower handle layer  20  supporting the comb drive  24  while a movable rotor portion  30  of the comb drive  24  is visible above the lower opening provided by the handle layer  20 . As shown in  FIG. 3B , when the two sub-assemblies  12  and  14  are joined, the stub  26  on membrane  16  contacts the movable rotor portion  30  of the comb drive  24  causing a downward displacement of the rotor portion  30  in relation to the stator portion  28 . The two sub-assemblies  12  and  14  are retained together by a SiO 2  junction around the perimeter of the two sub-assemblies. When joined, the resistance to displacement provided by the flexure of supporting portions (not shown) of the rotor portion  30  can at least partially offset the downward force provided by the stub  26  and supporting membrane  16  so that the supporting membrane  16  can become upwardly bowed as shown. 
         [0029]      FIG. 4  is a schematic orthogonal sectional view of the comb drive sub-assembly  14 , the section being taken perpendicularly to the sectional view provided by  FIGS. 3   a  and  3   b  through both the stator portions  28  and rotor portions  30 . Upon assembly of the two sub-assemblies  12  and  14 , the rotor portions  30  are displaced downward relative to the stator portions  28 . Further displacement of the rotor portions  30  relative to the stator portions  28  can occur as a result of a displacement of the membrane  16  due to gas pressure or other forces. The displacement of the rotor portions  30 , which can result from the displacement of the membrane  16 , can cover a range of distances shown in three images on the graph shown in  FIG. 5 . As the rotor portion  30  and stator portion  28  are initially formed, they appear as in image A in  FIG. 5 . When the rotor portion  30  and stator portion  28  are displaced relative to each other they appear, more or less, as shown in image B in  FIG. 5 . As the rotor portion  30  and stator portion  28  become fully displaced relative to each other they may achieve a relative position as shown in image C in  FIG. 5 . The graph in  FIG. 5  shows the capacitance, measured in picofarads, of the parallel plates forming the comb drive  24  in the various positions of relative displacement. The greatest capacitance is, of course, exhibited when the parallel plates of the comb drive  24  have their maximum confronting area to each other as in image A, while the least capacitance is exhibited when the parallel plates of the comb drive  24  have a minimum confronting area to each other as in image C. It is important to note that over a significant range of relative displacement, the change in capacitance is linearly related to the extent of relative displacement. A MEMS sensor device  10  of the present disclosure uses the stub  26  on membrane  16  dimensioned to cause an initial displacement of the rotor portions  30  relative to the stator portions  28 . The dimension is selected such that any further relative displacement of the two portions of the comb drive is in the linear portion of the capacitance/displacement curve. 
         [0030]      FIG. 6  is a schematic orthogonal view of a MEMS device  110  having features similar to the device  10  shown in  FIGS. 1-4 , and including an alignment feature that ensures correct assembly of the device  110 . The MEMS device  110  shown in  FIG. 6  includes membrane sub-assembly  112  having a membrane  116 , with a stub  126  centrally position on the lower surface of the membrane  116 , surrounded by a much thicker upper handle layer  118 . The comb drive sub-assembly  114  includes a lower handle layer  120  in the same general manner as shown in  FIGS. 1-4 . The perimeter portion of the two sub-assemblies  112  and  114  provide a bonding area  21  to physically secure the two sub-assemblies  112  and  114  to each other. An alignment feature comprises at least one trench or ditch  132  provided in the perimeter bonding area of one of the sub-assemblies  112  and  114 . A corresponding dimple or post feature  134  is provided in the perimeter bonding area of the other one of the sub-assemblies  112  and  114 . The trench  132  and dimple  134  can each include a shape characteristic so as to provide a unique alignment relation between the two sub-assemblies  112  and  114 . Additionally, the vertical dimension of the trenches  132  and dimples  134  can be sufficient to provide a tactile sensory input to an assembler assuring correct relative alignment of the two sub-assemblies  112  and  114 .  FIGS. 7   a  and  7   b  are schematic sectional views similar to  FIG. 3   a  and  FIG. 3   b , respectively of the MEMS device  110  shown in  FIG. 6  during assembly. 
         [0031]    An alternative embodiment of a MEMS device  810  shown in  FIGS. 8-10  includes a membrane sub-assembly  812  having a membrane  816 , with a stub  826  centrally positioned on the lower surface of the membrane  816 , surrounded by a much thicker upper handle layer  818 . A comb drive sub-assembly  814  includes a lower handle layer  820 . The perimeter portion of the two sub-assemblies  812  and  814  provide a bonding area to physically secure the sub-assemblies to each other. A comb drive  824  includes a stator portion  828  and a movable rotor portion  830 , both of which are confined within a generally circular perimeter formed by the perimeter portion of the comb drive  824 . Plates forming the two portions of the comb drive  824  are shown in plan view in  FIG. 10 . The plates comprise arcuate elements positioned at spaced distances from a common center  836 , which is also the contact point of the stub  826 . One end of each of the arcuate elements is coupled to a radially extending portion of either the stator portion  828  or the movable rotor portion  830  of the comb drive  824 . The circular comb drive configuration shown in  FIGS. 8-10  is resistant to in-plane translation and insensitive to incidental comb drive rotation during assembly. 
         [0032]      FIG. 11  is a schematic sectional view of a MEMS device  1110 , which can be of any of the previously illustrated embodiments, included in packaging  1138  defining a gas pressure port  1140  opposing a MEMS membrane  1116 . Although shown opposing the MEMS membrane  116 , the port  1140  need not oppose the MEMS membrane  116  in all embodiments. The packaging  1138  preferably defines a fluid impervious environment for the MEMS device  1110 , except for the port  1140 . The material characteristics of the packaging  1138  can be chosen based on the expected environment for the device  1110 . 
         [0033]      FIG. 12  illustrates in block form an experimental assembly  1242  for evaluating the performance of a MEMS device, such as MEMS device  10  of  FIG. 1 , in relation to gas pressure. The experimental assembly  1242  includes a micro probe station  1244  designed to receive the MEMS device in a controlled environmental chamber  1246 . The environmental chamber  1246  can be coupled to a vacuum pump, not shown, for reducing the gas pressure experienced by the MEMS device. A pressure sensor  1248  can be coupled to the environmental chamber  1246  to measure the pressure within the environmental chamber  1246 . An output of the pressure sensor  1248  can be coupled to a pressure controller  1250 , which is in turn coupled to a gas flow/pressure regulator  1252 . The gas flow/pressure regulator  1252  can be coupled to a source of gas, such as a nitrogen container, not shown. The gas flow/pressure regulator  1252  can, in response to signals provided by the pressure controller  1250 , admit a flow of a desired gas to exert pressure on the membrane of the MEMS device being evaluated within the controlled environmental chamber  1246 . The mechanical performance determined from the measured electrical characteristics of the MEMS device  1210  can be tracked by suitable metering equipment  1254 , such as an HP™ model 4284 LCR meter. 
         [0034]    In one example of the Electro Micro-Metrology method, width can be measured in terms of changes in capacitance, w(ΔC); and the uncertainty in width can be measured by multiplying the uncertainty in capacitance by the sensitivity in width to capacitance, ∂C×(∂w/∂ΔC). While the sensitivity is typically large, ˜10 8  m/F, the uncertainty in capacitance is ˜10 −18  F or smaller. Hence, the uncertainty in width is on the order of an angstrom. 
         [0035]    A comb drive microstructure can be fabricated to intentionally include two unequal gap-stops, gap 1  and gap 2 . The two intentionally unequal gaps provide a structure that allows one to eliminate from consideration unknown geometric and material properties. By measuring the change in capacitance required to close the two gaps with an applied voltage, one can obtain the structure&#39;s geometry, electrostatic force, and system stiffness as follows. The measured change in capacitance required to traverse each gap, ΔC 1 , and ΔC 2 , may be respectively expressed as: ΔC 1 =2Nβεh gap 1 /g=2Nβεh (gap 1,layout +Δgap)/g, and ΔC 2 =2Nβεh gap 2 /g=2Nβεh (gap 2,layout +Δgap)/g, where N is the number of comb fingers in the comb drive microstructure, ε is the unknown permittivity of the medium, h is the unknown layer thickness of the microstructure, g is the unknown gap distance between comb fingers, β is the unknown electrostatic fringing field factor, and Δgap is the unknown difference in gap-stop size between the intended design layout and actual fabrication. A layout parameter n is chosen such that gap 1,layout ≠gap 2,layout =n gap 1,layout . Taking the ratio ΔC 1 /ΔC 2  of the above expressions yields Δgap=gap 1,layout (n ΔC 1 /ΔC 2 −1)/(ΔC 1 /ΔC 2 −1). For isotropic fabrication processes within close proximity, Δgap is locally consistent and provides a measure for all planar geometries of the structure. That is, fabricated gaps are gap layout +Δgap, flexure widths are width layout −Δgap, flexure lengths are length+Δgap, etc. 
         [0036]    Another unique attribute of the Electro Micro-Metrology method is the ability to directly quantify the uncertainty of measurement. The uncertainties in the measured capacitance ∂C and voltage ∂V, i.e. order of readout resolution due to an accumulation of noise sources, yield corresponding uncertainties in mechanical properties. That is, by replacing all instances of capacitance and voltage with ΔC±∂C and ∂V±∂V in the above expressions, multivariate Taylor expansions about the electrical uncertainties yield mechanical uncertainties as the first order terms of the form x i  (ΔC)∂C for uncertainty in displacement, F 1 (ΔC,V)∂C±F 2 (ΔC,V)∂V for the uncertainty in force, and K 1 (ΔC,V)∂C±K 2 (ΔC,V)∂V for uncertainty in stiffness. 
         [0037]    Additionally, the Electro Micro-Metrology method can also be used to effectively select the system stiffness for a MEMS device to be a particular amount of N/m. The change in capacitance can be used to measure the fabricated geometry, the comb drive force, mechanical stiffness, and displacement. Specifically, the Electro Micro-Metrology comb drive force is given by F E   =½ΨV   2 , the stiffness is given by KM=½Ψ 2 V 2 /ΔC, and the displacement x=ΔC/Ψ, where Ψ=ΔC gap /gap, which is the comb drive constant. 
         [0038]    The Electro Micro-Metrology method can be used for an autonomous self-calibrated temperature sensor  1300  having a linear response curve. In this application, changes in electrical capacitance are used to sense thermally-induced vibrations or static deformations. A resonator  1302  shown in  FIG. 13  can incorporate a fixed-fixed active or passive resonator  1302  for measuring planar oscillation frequencies. The fixed-fixed oscillator experiences a change in resonance frequency due to thermal expansion. The change in resonance frequency is significant due to the fixed-fixed configuration. After system mass and stiffness are determined by the Electro Micro-Metrology method, measurement of resonant frequency is used to determine temperature by the change in stiffness due to thermal expansion of the fixed-fixed oscillator. This resonator  1302  may be driven actively by applying a suitable oscillating voltage for large displacement amplitudes, or the resonator  1302  may be driven passively due to thermally-induced vibrations at the expense of much smaller amplitudes. 
         [0039]    The resonator  1400  shown in  FIG. 14  incorporates a “Chevron” electro-thermal actuator for measuring planar deflections. Static thermal expansion of the Chevron actuator is used to deflect the differential comb drive. The Chevron actuator consists of one or more angled flexures to create a preferential magnified deflection. More flexures can be used to increase stiffness and to reduce thermal noise. Such an Electro Micro-Metrology based approach allows the performance and design space to be pushed to achieve maximum thermal sensitivity. That is, capacitance is the most precise mode of measurement to date. For example, a change in capacitance on the order of zeptofarads (10 −12  F) correlates to a comb drive displacement on the order of 10 −13  m. It is well known that the relationship between stiffness and temperature is given by: (½)K(x 2 )=(½)k B T, where K is the stiffness, x is the amplitude of vibration, k B  is Boltzmann&#39;s constant, and T is the temperature. However, unlike the previous efforts of others, by using Electro Micro-Metrology methods one is able to determine accurate and precise measurements of stiffness and displacement, which can be used to measure the absolute temperature T. The Electro Micro-Metrology methods render the use of any external reference temperature standard unnecessary.