Abstract:
Accelerometers and methods of forming accelerometers are described. The accelerometers are provided with electrically conductive structure configured for connection with external circuitry. The electrically conductive structure has a folded-back architecture which reduces temperature-induced anomalies which can adversely impact acceleration-sensing function of the accelerometer.

Description:
TECHNICAL FIELD 
     The present invention relates to accelerometers and methods of forming the same. In particular, the invention relates to vibrating beam accelerometers and methods of forming the same. 
     BACKGROUND OF THE INVENTION 
     Vibrating beam accelerometers are generally known in the art. Exemplary accelerometers and/or component parts such as force sensors for use with accelerometers are disclosed in U.S. Pat. Nos. 5,339,698, 5,501,103, 5,456,110, 5,450,762, 5,331,242, 5,367,217, and 5,456,111, the disclosures of which are expressly incorporated herein by reference. 
     A typical vibrating beam accelerometer can be etched from a silicon wafer using micromachining techniques which are generally known. The vibrating beam or beams of such accelerometers are used to form one or more resonators which control the frequency of one or more oscillator circuits. The vibrating beam or beams are generally connected between a frame and one or more proof masses and are configured so that an acceleration results in a tension or compression force along the beam or beams. Accordingly, changes in the resonant frequency of the beams occur and exemplary signals from the oscillators are then frequency modulated which indicates acceleration. 
     For additional background material on accelerometers, and in particular vibrating beam accelerometers, the reader is referred to a text by Anthony Lawrence entitled, Modern Inertial Technology-Navigation, Guidance, and Control, the disclosure of which is expressly incorporated herein by reference. 
     Some accelerometers can be formed in crystalline quartz. Such quartz possesses piezoelectric properties which can be utilized in connection with one or more vibrating beams to measure acceleration. Unlike crystalline quartz however, silicon is not piezoelectric. Accordingly, piezoelectric drive cannot be used or incorporated into a silicon-based system for measuring acceleration. One practical drive method suitable for use with silicon-based systems is electromagnetic drive. For electromagnetic drive, the vibrating beam or beams are placed in a magnetic field. Electrical current passed over or through the beam or beams exerts a force on the beams while the motion of the beam or beams in the magnetic field generates an electrical voltage. The resistivity of silicon, however, makes it impractical to use the conductivity of silicon to conduct the appropriate electrical current. One past solution has been to form or provide a layer of conductive material having a sufficiently low resistance over the beam or beams. An exemplary material is gold which can be readily patterned to have separate conductive layers on the different beams of the accelerometer. U.S. Pat. No. 5,501,103 incorporated by reference above describes such solutions. One particular drive circuit configuration requires electrical leads, in addition to those on the force sensing beams, between the proof mass and the frame. Separate silicon beams or struts having a metal disposed over an oxide have been used for this purpose. 
     One problem associated with the use of a metal layer over the vibrating beams or struts is that the metal material undergoes irreversible changes with temperature variations. As a result, changes in frequency which are not a true indication of the acceleration can be experienced by the proof mass. The exemplary gold material mentioned above exhibits this problem. While other metals and combinations of metals have been tried, none have resulted in sufficiently stable frequency over operating temperature ranges. 
     FIG. 1 shows a silicon micromachined vibrating beam accelerometer generally at 20. The accelerometer comprises a frame 22 and a first proof mass 24. Proof mass 24 includes a mounted end 26 and a distal end 28 away from or opposite mounted end 26. A flexure 30 is provided and extends between mounted end 26 and frame 22. As used in the context of this document, &#34;flexure&#34; will be understood to mean one or more flexure portions which are joined with a proof mass. Flexure 30 defines a hinge axis HA 1  about which proof mass 24 can be moved in relation to an acceleration experienced by accelerometer 20 along an input or sensitive axis which is generally into the plane of the page upon which FIG. 1 appears. A vibrating beam assembly 32 is connected between frame 22 and proof mass 24. Assembly 32 includes a pair of vibratable beams 34, 36. A strut assembly 38 is provided and is connected between frame 22 and proof mass 24. Strut assembly 38 includes individual struts 40, 42. 
     In the illustrated example, a second proof mass 44 is provided and includes a mounted end 46 and a distal end 48 away from or opposite mounted end 46. A flexure 50 is provided and is connected between mounted end 46 and frame 22. Flexure 50 defines a hinge axis HA 2  about which second proof mass 44 can be moved in relation to an experienced acceleration. A vibrating beam assembly 52 is provided and is connected between frame 22 and proof mass 44. Vibrating beam assembly 52 includes individual vibratable beams 54, 56. A strut assembly 58 is provided and connected between frame 22 and proof mass 44. Strut assembly 58 includes individual struts 60, 62. 
     Accelerometer 20 is etched from a wafer of silicon crystal with surfaces disposed in the 1,0,0 crystal planes. The accelerometer in practice is mounted directly or indirectly to a vehicle the acceleration of which is to be measured. Frame 22 and proof masses 24, 44 typically have thicknesses (into the plane of the page upon which FIG. 1 appears) which are generally comparable to the thickness of a silicon wafer, i.e., typically around 400 to 525 microns. Flexures 30, 50 have respective transition areas 29, 31, and 49, 51 which extend toward a central portion of each flexure which has a thickness of around 20 microns. In the illustrated example, vibrating beam assemblies 32, 52 comprise double ended tuning forks with respective end parts 35, 55 providing for good mechanical coupling of the vibrating beams. 
     FIG. 2 shows electrically conductive structure disposed over vibrating beam assemblies 32, 52, proof masses 24, 44, and strut assemblies 38, 58. The conductive material defines first and second conductive paths 64, 66 which extend between respective pairs of bond pads 68, 70 and 72, 74. Third and fourth conductive paths 76, 78 are provided and extend over vibrating beam assembly 52, proof mass 44, and strut assembly 58 as shown. Conductive paths 76, 78 extend between respective pairs of bond pads 80, 82, and 84, 86. An exemplary conductive material comprising the conductive structure defining paths 64, 66, 76, and 78 is gold which can be provided to a thickness of around 0.5 microns and which can be separated from the underlying silicon by a layer of silicon oxide which is typically 0.5 microns thick. 
     Vibrating beam assemblies 32, 52 are arranged so that an acceleration causes a tension force on one of the assemblies and a compression force on the other of the assemblies. A difference in frequencies between the vibrating beam assemblies provides an indication of acceleration. The electrically conductive structure defining the conductive paths, and in particular bond pads 68, 70, 72, 74, 80, 82, 84, and 86 are used to couple the vibrating beam assemblies with which each is associated to an external oscillator circuit. In the illustrated example in FIG. 2, first conductive path 64 is provided over one vibrating beam and one strut, and second conductive path 66 is provided over the other vibrating beam and the other strut. Similarly, third conductive path 76 is provided over one vibrating beam and one strut while fourth conductive path 78 is provided over the other vibrating beam and other strut. One of the vibrating beams for each proof mass is driven by a current, while motion of the other beam produces a voltage. Mechanical coupling between the vibrating beams of each beam assembly makes it possible to drive one beam and sense the motion of the other. In the illustrated example, hinge axes HA 1  and HA 2  are disposed on a common side of frame 22. 
     FIG. 3 shows an alternate accelerometer design. Like numerals from the above-described embodiment have been utilized with the suffix &#34;a&#34;. In this example, proof mass 44a is rotated 180° from that shown in FIGS. 1 and 2. Accordingly, the respective hinge axes of the proof masses are now disposed on different or opposite sides of frame 22a. This configuration has been found to have advantages which relate to near perpendicular alignment of the combined sensitive axes of the proof masses with the front and back surfaces of the accelerometer. 
     FIG. 4 shows an alternate accelerometer design. Like numerals from the above-described embodiment have been utilized with the suffix &#34;b&#34;. In this example, there is no strut assembly connected to proof masses 24b, 44b and frame 22b. The conductive structure which forms conductive paths 64b, 76b over the vibrating beam assemblies of each proof mass are connected together at each respective proof mass end. The equivalent circuit of this configuration with the vibrating beams immersed in a magnetic field is a resistor, an inductor, and a capacitor connected in parallel. 
     FIGS. 5 and 6 show an accelerometer 20c which utilizes only one proof mass 24c and includes a pair of vibrating beam assemblies and strut assemblies. 
     FIG. 7 shows an embodiment which utilizes only one proof mass 24d and no strut assembly. Additionally, the conductive structure is connected together at the proof mass end as in FIG. 4. 
     The implementations described just above are susceptible to temperature-induced effects which can cause inaccuracies in the sensed acceleration of each proof mass. Specifically, temperature changes can cause the conductive structure defining each of the conductive paths over the accelerometer to expand and contract differently than silicon. This causes a change in length of the conductive structure which does not match the dimensional change in the remaining silicon structure. Accordingly, a force is generated on the vibrating beam assemblies, the strut assemblies (where incorporated into a particular design), and the associated proof mass or masses. Over extended temperature ranges, for example from -40° C. to 100° C., the metal or conductive structure undergoes irreversible changes so that even if the accelerometer is calibrated over several temperatures and corrections are made for temperature effects, the irreversible changes still cause errors. Similarity of the errors in the vibrating beam assemblies can result in less error in the frequency difference, but the error is still too great for some applications. 
     This invention arose out of concerns associated with providing accelerometers and methods of forming the accelerometers which are directed to solving problems associated with temperature changes and the effects such changes have on the corresponding structure of accelerometers. 
     SUMMARY OF THE INVENTION 
     Accelerometers and methods of forming accelerometers are described. In one implementation, an accelerometer includes a frame, a proof mass, a flexure connected between the frame and the proof mass and defining a hinge axis about which the proof mass can be moved. A vibrating beam assembly is provided and connected between the frame and the proof mass. In one aspect, a strut assembly is provided and connected between the frame and the proof mass. Electrically conductive structure is supported over the vibrating beam assembly and the strut assembly and configured for connection to an external electrical circuit. The electrically conductive structure defines a first conductive path having two pairs of first conductive path portions. One of the pair of first conductive path portions is supported over the vibrating beam assembly and extends along individual lines which define a first set of lines which are generally parallel with one another. The other of the pair of first conductive path portions is supported over the strut assembly and extends along individual lines which define a second set of lines which are generally parallel with one another. The electrically conductive structure further defines a second conductive path having two pairs of second conductive path portions. One of the pair of second conductive path portions is supported over the vibrating beam assembly and extends along individual lines which define a third set of lines which are generally parallel with one another. The other of the pair of second conductive path portions is supported over the strut assembly and extends along individual lines which define a fourth set of lines which are generally parallel with one another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a known accelerometer. 
     FIG. 2 is a top plan view of the FIG. 1 accelerometer with electrically conductive structure disposed thereon. 
     FIG. 3 is a top plan view of another known accelerometer. 
     FIG. 4 is a top plan view of another known accelerometer. 
     FIG. 5 is a top plan view of another known accelerometer. 
     FIG. 6 is a top plan view of the FIG. 5 accelerometer with electrically conductive structure disposed thereon. 
     FIG. 7 is a top plan view of another known accelerometer. 
     FIG. 8 is a top plan view of an accelerometer in accordance with one implementation of the present invention. 
     FIG. 9 is a top plan view of the FIG. 8 accelerometer with electrically conductive structure disposed thereon. 
     FIG. 10 is a top plan view of an accelerometer in accordance with another implementation of the present invention. 
     FIG. 11 is a top plan view of an accelerometer in accordance with another implementation of the present invention. 
     FIG. 12 is a top plan view of an accelerometer in accordance with another implementation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 8 and 9 show an accelerometer in accordance with a first implementation of the invention generally at 20e. Like numerals from the above-described accelerometers have been utilized with the suffix &#34;e&#34;. Differences are indicated with different numerals or letters. Accordingly, accelerometer 20e includes vibrating beam assemblies 32e, 52e. Accelerometer 20e also includes strut assemblies 38e, 58e. The beam assemblies and strut assemblies are connected between frame 22e and proof mass 24e. Vibrating beam assembly 32e comprises a pair of vibratable beams 88, 90, and a pair of generally non-vibratable beams 92, 94. Non-vibratable beams 92, 94 are tied together with a pair of cross pieces 96, 98. The beams comprising vibrating beam assembly 32e are generally elongate and are formed along individual longitudinal axes defining lines l 1 , l 2 , l 3 , and l 4 . Specifically, beam 94 is formed along line l 1 , beam 92 is formed along line l 2 , beam 88 is formed along line l 3 , and beam 90 is formed along line l 4 . In the illustrated example, the lines are generally parallel with one another. 
     Strut assembly 38e includes first and second struts 40e and 42e. Strut 40e is generally elongate and formed along a longitudinal axis which defines a pair of lines l 5 , l 6  which are generally parallel with one another. Strut 42e is generally elongate and formed along a longitudinal axis which defines a pair of lines l 7 , l 8  which are generally parallel with one another. Each individual strut of the pair of struts include strut portions which extend from the proof mass along the associated longitudinal axis and to the frame in directions which are generally opposite one another. For example, first strut 40e includes a pair first strut portions which correspond to those portions of the strut which are formed along lines l 5 , l 6  respectively. As one proceeds along strut 40e from proof mass 24e to frame 22e, the strut is seen to extend in a first direction (to the right as viewed in FIG. 8 and generally parallel with the hinge axis) along line l 6 . The strut then bends and extends in a second direction away from line l 6  and toward line l 5 . The strut then bends again to extend in a third direction (to the left as viewed in FIG. 8) along line l 5  and finally connects with the frame. The first and third directions are generally opposite one another. The path just described can be considered as a folded path in which the strut is folded back on itself to define the opposing directions. Similarly, second strut 42e includes third and fourth strut portions which extend in generally opposite directions and which correspond with lines l 7 , l 8 . 
     Vibrating beam assembly 52e includes vibratable beams 100, 102, and generally non-vibratable beams 104, 106. The beams of vibrating beam assembly 52e are generally elongate and formed along individual longitudinal axes which define lines l 9 , l 10 , l 1 , and l 12 . Specifically, beam 106 is formed along line l 9 , beam 104 is formed along line l 10 , beam 100 is formed along line l 11 , and beam 102 is formed along line l 12 . Non-vibratable beams 104, 106 are tied together with a pair of cross pieces 108, 110. 
     Strut assembly 58e includes a pair of generally elongate first and second struts 60e, 62e. Strut 60e is formed along a longitudinal axis which defines lines l 13 , l 14  which are generally parallel with one another. Strut 62e is formed along a longitudinal axis which defines lines l 15 , l 16  which are generally parallel with one another. The individual struts of the pair of struts include respective strut portions which extend from the proof mass along the associated longitudinal axis and to the frame in directions which are generally opposite one another as described above in connection with strut assembly 38e. 
     FIG. 9 shows electrically conductive structure which has been formed over vibrating beam assemblies 32e, 52e, and strut assemblies 38e, 58e. A first conductive path 112 is defined by the electrically conductive structure and extends over vibrating beam assembly 32e and strut assembly 38e between a pair of spaced-apart first bond pads 68e, 70e. First conductive path 112 includes two pairs of first conductive path portions, with one of the pair being supported over vibrating beam assembly 32e, and the other of the pair being supported over strut assembly 38e. Specifically, first conductive path portions 114, 116 are disposed over and supported by beams 94, 90 (FIG. 8) of vibrating beam assembly 32e and extend along lines l 1 , l 4  respectively. Accordingly, lines l 1 , l 4  define a first set of lines which are generally parallel with one another. A pair of first conductive path portions, 118, 120 are supported over common strut 42e (FIG. 8) of strut assembly 38e and constitute the other of the pair of conductive path portions. Path portions 118, 120 extend along lines l 7 , l 8  with such lines defining a second set of lines which are generally parallel with one another. 
     The electrically conductive structure further defines a second conductive path 122 having two pairs of second conductive path portions, one of which being disposed over and supported by vibrating beam assembly 32e, the other of which being disposed over and supported by strut assembly 38e. The second conductive path extends between a pair of spaced-apart second bonds pads 72e, 74e. Specifically, second conductive path portions 124, 126 are supported over beams 92, 88 (FIG. 8) respectively. Accordingly, such conductive path portions are formed along individual lines l 2 , l 3  which define a third set of lines which are generally parallel with one another. A pair of second path portions 128, 130 are provided over common strut 40e (FIG. 8) and are formed along lines l 5 , l 6 . Path portions 128, 130 constitute the other of the pair of second conductive path portions which are supported over strut assembly 38e. Lines l 5 , l 6  constitute a fourth set of lines which are generally parallel with one another. 
     The electrically conductive structure further defines a third conductive path 132 having two pairs of third conductive path portions, one of the pair being supported over vibrating beam assembly 52e, the other of the pair being supported over strut assembly 58e. The third conductive path extends between a pair of spaced-apart third bond pads 80e, 82e. Specifically, third conductive path portions 134, 136 are supported over beams 102, 106 (FIG. 8) respectively. Accordingly, path portions 134, 136 extend along lines l 12 , l 9  respectively. Lines l 12 , l 9  comprise a fifth set of lines which are generally parallel with one another. A pair of third conductive path portions 138, 140 are supported over strut 62e (FIG. 8). Path portions 138, 140 constitute the other of the pair of third conductive path portions and are formed along lines l 16 , l 15  respectively. Accordingly, lines l 16 , l 15  constitute a sixth set of lines which are generally parallel with one another. 
     The electrically conductive structure further defines a fourth conductive path 142 having two pairs of fourth conductive path portions, one pair of which being supported over vibrating beam assembly 52e, the other pair of which being supported over strut assembly 58e. The fourth conductive path extends between a pair of spaced-apart fourth bond pads 84e, 86e. Specifically, a pair of fourth conductive path portions 144, 146 are disposed over beams 104, 100 (FIG. 8) respectively, and accordingly along lines l 10 , l 11 . Lines 1 10 , l 11  constitute a seventh set of lines which are generally parallel with one another. Fourth path portions 148, 150 are supported over common strut 60e of strut assembly 52e and constitute the other of the pair of fourth conductive path portions. Such path portions are formed along lines l 14 , l 13 , with such lines constituting an eighth set of lines which are generally parallel with one another. 
     In the illustrated example, both strut assemblies 38e, 58e are disposed intermediate distal end 28e of the proof mass and frame 22e. Additionally, first set of lines l 1 , l 4  are generally parallel with third set of lines l 2 , l 3 . Further, second set of lines l 7 , l 8  are generally parallel with fourth set of lines l 5 , l 6 . Also, the first set of lines l 1 , l 4  are generally perpendicular to second set of lines l 7 , l 8  ; and third set of lines l 2 , l 3  are generally perpendicular to the fourth set of lines l 5 , l 6 . Other positional relationships between the struts and the beams are evident in this embodiment. For example, the first, third, fifth, and seventh sets of lines mentioned above are generally parallel with one another. Additionally, the second, fourth, sixth, and eighth sets of lines mentioned above are generally parallel with one another. 
     The above-described construction has advantages over the previously-described embodiments in that any temperature-induced bending or deformation exerts an order of magnitude less force between the frame and the proof mass, and an order of magnitude less force on the vibrating beam assemblies and strut assemblies. In addition, this embodiment constitutes one in which. the electrically conductive structure which is formed over the beams and struts is folded back on itself over both the beams and the struts. It is possible, however, for the beams and/or struts to be configured so that the electrically conductive structure folds back over only either the beams or the struts, but not both. Moreover, it is possible for the struts to be eliminated all together from the construction such that only the beam assemblies provide the desired folded-back architecture. In this illustrated example, the conductive bond pads with which each conductive path is connected are supported over the frame on one side of hinge axis HA 1e . 
     FIG. 10 shows an alternate embodiment of an accelerometer generally at 20f. Like numbers from the above-described embodiment have been utilized with the suffix &#34;f&#34;. Strut assemblies 38f, 58f are disposed intermediate mounted end 26f of proof mass 24f and frame 22f. Accordingly, the strut assemblies are disposed proximate flexure 30f. By locating the strut assemblies near the effective center of rotation of proof mass 24f, stiffening of the proof mass by the struts is diminished by a factor of 2 or more. Additionally, any force exerted by the struts on the proof mass as a result of temperature changes are diminished. In this example, the respective bond pads with which each conductive path is connected are disposed on different or opposite sides of hinge axis HA 1f . 
     FIG. 11 shows an embodiment in which the strut assemblies are disposed intermediate mounted end 26g of proof mass 24g and frame 22g. Like numerals have been used with the suffix &#34;g&#34;. The strut assemblies do not, in this embodiment, have the folded-back architecture of the above strut assembly. Accordingly, the folded-back architecture is provided by vibrating beam assemblies 32g, 52g. 
     FIG. 12 shows an implementation in which accelerometer 20h includes two proof masses 24h, 44h. Like numerals from the above-described embodiments have been utilized with the suffix &#34;h&#34;. In the illustrated example, hinge axes HA 1h  and HA 2h  are disposed on a common side of frame 22h. It will be appreciated, however, that the proof masses could be formed with their associated hinge axis-defining flexures disposed oppositely one another as in FIG. 3. In that case, the corresponding strut assemblies would be disposed oppositely one another as well. In this example, the folded-back architecture is provided by both the vibrating beam assemblies and the strut assemblies. It is possible, however, for only one of the beam assemblies or the strut assemblies to provide the folded-back architecture. Moreover, it is possible for the struts to be eliminated from the two-proof mass implementations so that only the vibrating beam assemblies would provide the desired architecture. Further, the folded back architecture of the present invention could be incorporated into each of the embodiments described and illustrated in FIGS. 1-7. 
     The accelerometers described above can be fabricated through various silicon processing techniques. For example, the sloping surfaces or transition areas of the accelerometer can be formed through anisotropic etching of the silicon with, for example, potassium hydroxide. The flexures and vibrating beam assemblies can be formed from epitaxially grown silicon which is oppositely doped relative to the substrate. The epitaxial layers can be protected during etching through provision of a suitable voltage or back bias using known techniques. In another method, there is no epitaxial layer on the backside and the flexure is formed by etching from both sides of the wafer. This places the flexure closer to the center plane of the wafer so that the sensitive axis of the accelerometer is more nearly aligned generally normal to the front and back surfaces of the silicon. In yet another method, there are no epitaxial layers and the vibrating beam assemblies are etched from a silicon layer which is separated from the substrate by a thin, e.g. about 2 microns, layer of silicon oxide. For a detailed discussion of silicon processing techniques, reference is made to the following publications, which are incorporated herein by reference: VLSI Fabrication Principles by Sorab K. Ghandhi, and Silicon Processing for the VLSI Era., Vols. 1-3, by S. Wolf &amp; R. J. Tauber 
     The invention has been described in compliance with the applicable statutes. Variations and modifications will be readily apparent to those of skill in the art. It is therefore to be understood that the invention is not limited to the specific features shown and described, since the disclosure comprises preferred forms of putting the invention into effect. The invention is, therefore, to be interpreted in light of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.