Abstract:
A disc resonator gyroscope (DRG) and method of manufacture. The DRG has a surrounding pattern of bond metal having a symmetry related to the symmetry of a resonator device wafer that enables more even dissipation of heat from a resonator device wafer of the DRG during an etching operation. The metal bond frame eliminates or substantially reduces the thermal asymmetry that the resonator device wafer normally experiences when a conventional, square bond frame is used, which in turn can cause geometric asymmetry in the widths of the beams that are etched into the resonator device wafer of the DRG.

Description:
FIELD 
     The present disclosure relates to disc resonator gyroscopes and their manufacture, and more particularly for a disc resonator gyroscope and method of making same that reduces or eliminates the frequency offset between the radial vibrational modes of the gyroscope, and thus improves frequency coincidence of the two vibrational axes of the gyroscope used for driving and sensing the device, respectively. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     MEMS (micro-electromechanical system) gyroscopes, such as the silicon disc resonator gyroscope (DRG) are typically operated with a single mechanical resonant frequency on both the drive and sense axes of the DRG. Due to manufacturing variations, there is typically a frequency offset between the two vibrational modes of the DRG used for driving and sensing the motion, respectively, that must be corrected by a means such as electrostatic tuning or mass trimming to bring the two frequencies into coincidence. The inventors have discovered that during etching of the resonator pattern on the resonator wafer of the DRG, asymmetric conduction of heat during the etching process can cause subtle dimensional variations in the arcuate slots created during the etching process. More specifically, the inventors have discovered that during the etching process, hotter regions of the resonator wafer tend to etch faster and have more mask undercut than cooler areas. This difference in temperature can result in etching that produces arcuate slots in the resonator wafer that are either slightly larger or smaller in cross sectional dimension, than a desired design dimension. Put differently, the difference in various regions of the resonator wafer during the etching process can cause geometric asymmetry of the resulting slots, and thus the beams that are formed in the resonator wafer. These differences (i.e., variations) in cross sectional dimension for the slots result in beams being created that can be either slightly thicker than desired, or slightly thinner than desired, which introduces a variation in the mechanical compliance of the beams, and hence vibrational frequency, for different radial orientations of the gyroscope. This variation in sensitivity is manifested in the above-mentioned frequency offset that is present between the two vibrational modes of the DRG used for drive and sense, respectively, that must be corrected. Slight differences in beam thickness, on the order of tens of nanometers, can give rise to frequency offsets of order 20-40 Hz for DRG structures designed for a resonant frequency of order 14 kHz. This asymmetry can arise even for a fully symmetric resonator pattern. 
     Correction of the frequency offset introduces additional cost and complexity into the DRG, or into its manufacture. Correction typically has been accomplished by subsequent electronic tuning of the DRG or physically trimming portions of the resonator wafer to bring the two vibrational modes into coincidence. Electronic tuning adds to complexity in the electronic circuitry, and tuning voltage instability can degrade apparent device performance. Physical trimming of the resonator requires additional processing, such as laser trimming, that is both expensive and time consuming. 
     SUMMARY 
     The present disclosure is directed to a disc resonator gyroscope and a method for manufacturing same that provides for reduced geometric asymmetry, and thus improved performance. 
     In one implementation, a method of manufacturing a disc resonator gyroscope (DRG) that produces improved geometric symmetry of the gyroscope is disclosed. The method includes the operations of forming a substrate base wafer having a patterned metal bonding layer on one surface of the substrate base wafer, with the patterned metal bond layer having a first symmetry. A resonator device wafer is bonded to the patterned metal bonding layer. Etching of the resonator device wafer is then performed in accordance with a pattern having a second symmetry to remove regions of material from the resonator device wafer to define the vibratory structure for the resonator device wafer. The second symmetry has a relationship to the first symmetry that significantly enhances thermal symmetry in the resonator device wafer during the etching process. In addition to providing mechanical attachment, the metal bonding layer operates to provide a heat conduction path from the resonator device wafer to the substrate base wafer. This metal layer can thus impact the thermal symmetry created in the resonator device wafer during the etching operation. Hence, the patterned metal bonding layer is formed to reduce the geometric asymmetry created during the etching operation. 
     In various implementations the patterned metal bonding layer may be formed by gold or a combination of gold with appropriate adhesion layers (such as chromium or titanium) and diffusion barrier layers (such as titanium tungsten or tungsten nitride). The bonding of the substrate base wafer to the resonator device wafer may be accomplished by thermocompression bonding, fusion bonding, transient liquid phase bonding and eutectic bonding, or by any other suitable bonding means. 
     In one specific implementation the patterned metal bonding layer comprises a plurality of arcuately shaped metal bonding pads disposed uniformly about a peripheral area of the substrate base wafer. In another implementation a plurality of circular metal bonding pads are arranged circumferentially about the resonator device wafer. In still another implementation the patterned metal bonding layer comprises an overall square shape but with the corners thereof filled in with metal to produce a generally circular opening within the metal bonding layer. In each of the foregoing arrangements, the metal bonding pads help to reduce thermal-asymmetry in the resonator device wafer during etching of the resonator device wafer, which in turn helps to reduce geometric asymmetry of the etched slots in the resonator device wafer. 
     In one specific embodiment a disc resonator gyroscope is formed that has a substrate base wafer having a patterned metal bonding layer about a peripheral area of the substrate base wafer, on one surface of the substrate base wafer. The patterned metal bonding layer has a first symmetry. A resonator device wafer is bonded to the patterned metal bonding layer, with the resonator device wafer subsequently being etched to remove material in select areas. The etched areas have a second symmetry. The first and second symmetries have a relation to enable generally uniform thermal conductivity in the resonator device wafer during etching of the patterned areas of the resonator device wafer. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a perspective view of a disc resonator generator (DRG) in accordance with one embodiment of the present disclosure; 
         FIGS. 2A-2G  are a series of simplified side views that illustrate a series of manufacturing operations in forming one embodiment of a substrate base wafer for a disc rate gyroscope of the present disclosure; 
         FIGS. 3A and 3B  illustrate exemplary operations for forming a resonator device wafer of the present gyroscope; 
         FIGS. 4A-4C  illustrate exemplary operations for thermocompression bonding the substrate base wafer to the resonator device wafer; 
         FIG. 5  is an illustration showing, in a plan view, a layout of a prior art resonator bond frame and the areas where the resonator bond frame acts to significantly dissipate heat from the resonator device wafer during etching of the slots in the resonator device wafer; 
         FIG. 6  is a plan view of one exemplary layout for the metal bonding layer used with the disc resonator gyroscope of the present disclosure; 
         FIG. 6A  is a highly enlarged, cross-sectional view of one metal bond frame pad formed on the substrate base wafer, in accordance with section line  6 A- 6 A in  FIG. 1 , and also showing the alignment and attachment of its associated mating bond pad on the inner surface of the resonator device wafer; 
         FIG. 7  is a plan view of an inside surface of the resonator device wafer prior to securing to the substrate base wafer; 
         FIG. 8  is a plan view of another layout for the metal bonding layer in which the corner portions of the generally square shaped pattern are filled in with metal; 
         FIG. 9  is a plan view of another layout for the metal bonding layer in which a plurality of generally circular metal bond pads are arranged in a circumferential pattern; and 
         FIG. 10  is a graph illustrating the improvement in frequency co-incidence between the orthogonal vibratory modes of a prior art DRG and a DRG of the present disclosure having the metal bond frame described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     Referring to  FIG. 1 , there is shown a silicon disc resonator gyroscope (hereinafter “DRG”)  10  in accordance with one embodiment of the present disclosure. The DRG  10  includes a substrate base wafer  12 , a resonator device wafer  14  (shown only in phantom lines) and a cap  16 . As will be explained more fully in the following paragraphs, the substrate base wafer  12  and the resonator device wafer  14  may be bonded together, for example by thermocompression bonding, fusion bonding, transient liquid phase bonding, eutectic bonding, or by any other suitable bonding means. Similarly, the cap  16  may be secured to the resonator device wafer  14  by any of the above-mentioned methods. 
     Referring to  FIGS. 2A-2G , a brief overview of one exemplary method for forming the substrate base wafer  12  will be described. It will be appreciated that the following description describes a known process for forming the substrate base wafer  12 , and is being provided simply to aid the reader with a basic understanding of the major operations in forming the various layers of the substrate  12 . The substrate base wafer  12  begins with a substrate wafer  18 , which may be silicon, quartz or any other suitable material, but simply for the purpose of discussion will be a silicon substrate wafer. A pair of alignment features  20  are formed on or in a first surface  22  of the substrate wafer  18 . In  FIG. 2B  a second pair of alignment features  24  are formed on or in an opposing second surface  26  of the substrate wafer  18  and are aligned to the first pair of alignment features  20 . In  FIG. 2C , recesses  28  having a depth of typically about 1.0 um-5.0 um are formed in the first surface  22 . In  FIG. 2D , an insulator layer  30  is formed on to the first surface  22 , preferably by thermal oxidation of the silicon (Si) substrate to an oxide thickness of approximately 1.0 um. In  FIG. 2E , a first layer of metal  32  (typically gold) on the order of 0.7 um thick is deposited over the insulator layer  30  at selected locations. In  FIG. 2G , a 1.0 um insulator layer  34  is formed over portions of the first metal layer  32 . In  FIG. 2G , a second (or “top”) metal layer  36  is formed over portions of the insulator layer  34 . The result is the substrate base wafer  12  that forms a portion of the DRG  10 . 
       FIGS. 3A and 3B  illustrate operations in forming the resonator device wafer  14 . This begins by forming front to back alignment features  42 , preferably by etching, on or in upper and lower surfaces  40   a  and  40   b , respectively, of a resonator wafer  40 . In  FIG. 3B , metal  44 , typically gold or gold/chromium may be deposited onto one surface of the wafer  40 . The metal  44  will be used in subsequent operations to bond the two wafers  40  and  18  together. 
     Referring now to  FIGS. 4A-4C , exemplary operations for bonding the substrate base wafer  12  and the resonator device wafer  14  together will be described. In  FIG. 4A , the wafers  12  and  14  are aligned using the alignment features  20 ,  24  and  42  and secured together, for example by thermocompression bonding, fusion bonding, transient liquid phase bonding, eutectic bonding, or by any other suitable securing operation. During this operation the metal layers of the resonator device wafer  14  are physically bonded to the top metal layer  36  of the substrate base wafer  12 . In  FIG. 4B  metal cap bond material  43  is formed on an upper surface  43   a  of the resonator wafer  40 . In  FIG. 4C , optical lithography methods may be used to create a photoresist pattern on surface  40   b . This pattern may be used as an etch mask to selectively remove portions of the resonator wafer  40  in a micromachining operation. The micromachining operation creates, by etching, a precision pattern of slots  46  in the resonator wafer  40  that enables the wafer  14  to operate as the resonator device wafer  14 , and to thus form the movable portion of the DRG  10  and interleaved static electrodes. The slots  46  in this example are about 25 um in width and etched to a depth of about 270 um. During the etching process the substrate base wafer  12  is typically positioned on a cold plate  48  to help cool the resonator device wafer  14 . Although not illustrated, the last operation is the etching and bonding, using bonding techniques such as described above, of a cap (such as cap  16  in  FIG. 1 ) to physically cover selected portions of the resonator device wafer  14 . The formation and attachment of the cap over the resonator device wafer  14  prevents dirt and contaminants from entering into the slots  46  and may alternately be used to create a hermetic vacuum environment around the device. Additional details on the basic construction process described above are available from U.S. patent application Ser. No. 10/639,135 to Shcheglov et al., entitled “Integral Resonator Gyroscope”, filed Aug. 12, 2003, the disclosure of which is hereby incorporated by reference in the present disclosure. The substrate base wafer  12  may optionally also include layers to improve adhesion and layers to prevent interdiffusion. Such adhesion layers may be formed using materials such as chromium or titanium and the diffusion barrier layers may be selected from among Titanium Tungsten (TiW), Titanium Nitride (TiN), Tungsten Nitride (WN), Platinum (Pt), Molybdenum (Mo), and Molybdenum Nitride (MoN). It will be appreciated that the diffusion barrier layers help to prevent the diffusion of the material (e.g., gold) into the substrate material (often silicon) of the substrate base wafer  12  and the resonator device wafer  14  during the bonding of the layers  12  and  14 . 
     With brief reference to  FIG. 5 , a plan view of a typical prior art metal-resonator bonding layer pattern showing in particular the region of the bond frame  50 . The region of the bond frame  50  circumscribes the patterned (i.e., etched) area of a prior art substrate base wafer. The bonding layer pattern defines the locations where the resonator device wafer  14  is mechanically bonded to the substrate base wafer  12 . The resonator bond frame  50  essentially forms a peripheral metal ring around the patterned portion (i.e., rings and stationary electrodes) of the resonator device wafer and is formed or deposited on the substrate base wafer prior to the securing of the substrate base wafer and the resonator wafer together. The resonator bond frame  50  effectively forms a peripheral wall for the DRG. 
     During the etching of the resonator device wafer (not shown in  FIG. 5 ), heat generated in the resonator wafer by etching is dissipated through the metal-to-metal bonded areas between the base substrate and resonator wafers, through the base wafer, and ultimately to the cold plate  48  onto which the wafers are supported during etching. It has also been found by the inventors that a tangible amount of heat is also dissipated through the resonator bond frame  50 , particularly for those rings that are in close proximity to the areas  52 ,  54 ,  56  and  58  in  FIG. 5 ; in other words, for those rings that are closely adjacent the resonator bond frame  50 . This variation in proximity of the heat conduction path to the etched rings can cause thermal asymmetry in the resonator device wafer during the etching operation explained in connection with  FIG. 4C . The thermal asymmetry can in turn result in geometric asymmetry of the slots that are etched in to the resonator device wafer  14 , and thus variations in the widths of the beams that are formed between the slots. The asymmetry in beam width can cause a corresponding asymmetry in mechanical compliance, and can cause the beams to vibrate differently along different axial vibrational axes. The geometric asymmetry necessitates frequency tuning, which may require a relatively high tuning bias voltage to be applied to a DRG to account for the geometric asymmetry of the mechanical compliance. This increases the cost and complexity of the tuning circuit for the DRG and can degrade device performance by virtue of fluctuations of the tuning voltage. 
     Referring to  FIG. 6 , one embodiment of a bond frame  60  that is used in the DRG  10  is shown. Again, the illustration is a plan view of the substrate base wafer  12  with the bond frame  60  thereon.  FIG. 7  shows the actual resonator device wafer  14 , which has a circular shape, and has a diameter large enough to overlay the bond frame  60 . 
     Referring further to  FIGS. 6 and 6A , the bond frame  60  in this example is comprised of a plurality of arcuate shaped metal bond pads or layers  60   a  ( FIG. 6 ) that are formed on the upper surface  22  of the substrate base wafer  12 , and in a circle around the pattern  62   a  of grooves, and mating arcuate shaped metal bond pads  60   b  ( FIG. 6A ) that are formed on the lower surface  40   b  of the resonator device wafer  14  around the pattern  62   b  of slots  46 . The arcuate shaped metal bond pads  60   a  and  60   b  are aligned and secured together during the bonding of the two wafers  12  and  14  ( FIG. 6A ) to form the bond frame  60 . Thus, when the wafers  12  and  14  are bonded together, the metal bond pads  60   a  and  60   b  cooperatively form the metal bond frame  60 , which represents a series of arcuate metal pads having a desired height (typically on the order of 0.5 um to 2.0 um), and which are separated from the pattern  62   b  of slots  46  by a uniform distance at all points around the circumference of the pattern  62   b  of slots  46 . This uniform spacing of the metal bond pads  60   a , 60   b  from the periphery of the pattern  62   b  of slots  46  ensures that the metal bond frame  60  acts uniformly in dissipating heat from the outermost slots  46  of the pattern  62   b  during the etching process. The metal bond frame  60  enables excellent geometric symmetry to be achieved in the etched pattern  62   b  of slots  46 . The metal bond frame  60  may be made from gold, a combination of gold and chromium, or any other suitable material. 
     With brief reference to  FIG. 10 , the significant improvement in frequency co-incidence between the two radial vibratory modes corresponding to the drive and sense orientations is illustrated. The dashed line  100  in  FIG. 10  illustrates the frequency co-incidence of a silicon DRG formed with the rotationally symmetric metal bond frame  60  matched to the symmetry of the resonator pattern. The frequency mismatch between the two radial vibrational modes is reduced to only about 0.025% for devices from a wafer incorporating this feature. The frequency co-incidence of devices from a similar wafer fabricated without the symmetric metal bond frame  60 ,  70  or  80  of the present application is substantially higher and is illustrated by the bar graphs. 
       FIG. 8  shows a different embodiment of the bond frame  70 . In this embodiment the bond frame has a generally square outer periphery, but inside corners  72  are filled with metal material to keep the spacing between the inner periphery  74  of the bond frame  70  the outer periphery of the pattern  62   a  of slots uniform around the entire pattern  62   a . The bond frame  70  is preferably formed from gold, a combination of gold and chromium, or any other suitable metal. In this embodiment it will be appreciated that the resonator device wafer  14  will have a metal bond frame layout symmetrical to that of bond frame  70 , which is bonded to the bond frame  70  during bonding of the two wafers  12  and  14 . 
       FIG. 9  shows a bond frame  80  in accordance with another embodiment. The bond frame  80  is formed by a plurality of generally circular metal pads  80   a  that form an overall circular ring around the outer periphery of the pattern  62  of slots. The circular metal pads  80   a  serve to maintain thermal symmetry of the resonator device wafer  14  during the etching process. The bond frame  80  may be formed from gold, a mixture of gold and chromium, or any other suitable metal or metallic compound. In this embodiment it will be appreciated that the resonator device wafer  14  will include a similar plurality of circular metal pads that bond to the metal pads  80   a  during the bonding of the wafers  12  and  14 . 
     From the foregoing discussion, it will be apparent that the various metal bond frames discussed herein contribute to a construction for the DRG  10  that enables the DRG to be made with significantly enhanced thermal symmetry during the etching of the resonator device wafer  14 . It will also be appreciated that while the discussion above has been limited to the pattern of the bond metal used to adhere the resonator device wafer  14  to the substrate base wafer  14 , other patterns and photomask designs (such as for the resonator etching, cap bond metal and cap etch) will likely also need to be modified accordingly for self-consistent construction of the DRG  10 . For example, the pillar etch (i.e., the pattern of recesses in the substrate base wafer  12 ), the resonator device wafer bond metal  44  ( FIG. 3B ), the top base substrate metal  36  ( FIG. 2G ), the resonator cap metal  43  ( FIG. 3B ), the etch for the resonator device wafer  14 , and the etch for the cap device (not shown) may all need to be modified. 
     While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.