`Shaw et al.
`
`[54) MICROELECTROMECHANICAL LATERAL
`ACCELEROMETER
`
`[75]
`
`Inventors: Kevin A. Shaw; Scott G. Adams; Noel
`C. MacDonald, all of Ithaca, N.Y.
`
`[73] Assignee: Cornell Research Foundation, Inc.,
`Ithaca, N.Y.
`
`[21] Appl. No.: 67,264
`
`[22] Filed:
`
`May 26, 1993
`
`Int. Cl.6
`...................................................... G0lP 15/08
`[51]
`...................................... 73/514.18; 73/514.24
`[52] U.S. Cl .
`[58] Field of Search .................................. 73/517 R, 514,
`73/515, 516 R, 517 B, 514.18, 514.21,
`514.24, 514.32, 514.35, 514.38, 514.36,
`514.17
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,835,338
`4,381,672
`4,437,226
`4,553,436
`4,670,092
`4,685,198
`4,706,374
`4,746,621
`4,750,363
`4,772,928
`4,776,924
`4,845,048
`4,851,080
`4,867,842
`4,945,765
`4,981,552
`5,045,152
`5,072,288
`5,095,752
`5,121,180
`5,126,812
`5,149,673
`5,179,499
`5,198,390
`5,205,171
`
`9/1974 Martin ..................................... 310/331
`5/1983 O'Connor et al ......................... 73/505
`3/1984 Soclof ....................................... 437/55
`11/1985 Hansson ............................... 73/514.33
`6/1987 Motamedi ............................... 156/643
`8/1987 Kawakita et al ......................... 437n3
`11/1987 Murakami ............................... 437/225
`5/1988 Thomas et al. ........................... 437/24
`6/1988 Norling ................................. 73/517 R
`9/1988 Dietrich et al .......................... 257/254
`10/1988 Delapierre ............................... 156/647
`7/1989 Tamaki et al ............................. 437/62
`7/1989 Howe et al ............................. 156/647
`9/1989 Bohrer et al. ........................... 156/647
`8/1990 Roszhart .............................. 73/514.29
`1/1991 Mikkor .................................... 156/647
`9/1991 Sickafus ...... .... ........................ 156/653
`12/1991 MacDonald ............................. 257/420
`3/1992 Suzuki et al ......................... 73/514.32
`6/1992 Beringhause et al. ............... 73/514.34
`6/1992 Greiff ...................................... 257/417
`9/1992 MacDonald et al .................... 437/192
`1/1993 MacDonald et al. ................... 361/313
`3/1993 MacDonald et al .................... 437/203
`4/1993 O'Brien et al ....................... 73/514.18
`
`I 1111111111111111 11111 111111111111111 IIIII IIIII IIIII IIIII 111111111111111111
`5,563,343
`Oct. 8, 1996
`
`US005563343A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,228,341
`5,235,187
`5,314,572
`5,345,824
`5,353,641
`5,357,803
`
`7/1993 Tsuchitani ............................. 73/517 R
`8/1993 Arney el al. ............................ 250/306
`5/1994 Core ........................................ 156/643
`9/1994 Sherman ............................... 73/517 R
`10/1994 Tang ..................................... 73/514.18
`10/1994 Lane ..................................... 73/514.18
`
`OTHER PUBLICATIONS
`
`Accelerometer's Micromachined Mass "Moves" In Plane of
`IC; On-Chip Circuit Controls It And Senses G With Force- -
`Balance Techniques. Airbags Boom When IC Accelerom(cid:173)
`eter Sees 500. Electronic Design, Aug. 8, 1991, pp. 45-56.
`Zhang et al., "A RIE Process for Submicron, Silicon Elec(cid:173)
`tromechanical Structures", IOP Publishing Ltd., 1992, pp.
`31-38.
`Wilson et al., "Highly Selective, High Rate Tungsten Depo(cid:173)
`sition", Materials Research Society, 1985, pp. 35-43.
`Zhang et al. "An RIE Process for Submicron, Silicon
`Electromechanical Structures", IEEE, May 24, 1991, pp.
`520-523.
`Amey et al., "Formation of Submicron Silicon-on-Insulator
`Structures by Lateral Oxidation of Substrate-Silicon
`Islands", J. Vac. Sci. Technol. B 6(1), Jan./Feb. 1988, pp.
`341-345.
`Payne, R. S., et al. "Surface Micromachined Accelerometer:
`A Technology Update". SAE International, pp. 127-135
`(Feb. 25-Mar. 1, 1991).
`
`Primary Examiner-Hezron E. Williams
`Assistant Examiner-Christine K. Oda
`Attome), Agent, or Finn-Jones, Tullar & Cooper, P.C.
`
`[57]
`
`ABSTRACT
`
`A microelectromechanical accelerometer having submicron
`features is fabricated from a single crystal silicon substrate.
`The accelerometer includes a movable portion incorporating
`an axial beam carrying laterally-extending high aspect ratio
`released fingers cantilevered above the floor of a cavity
`formed in the substrate during the fabrication process. The
`movable portion is supported by restoring springs having
`controllable flexibility to vary the resonant frequency of the
`structure. A multiple-beam structure provides stiffness in the
`movable portion for accuracy.
`
`52 Claims, 5 Drawing Sheets
`
`88
`
`0001
`
`Exhibit 1026 page 1 of 16
`DENTAL IMAGING
`
`
`
`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 1 of 5
`
`5,563,343
`
`WL27~
`FIG. I
`
`12
`
`10
`
`12
`
`16
`
`FIG.2
`
`16
`
`14
`
`~=;::
`
`~~=:.....~-
`
`12
`
`FIG. 3
`
`44
`
`FIG. 4
`
`I
`
`42
`
`4
`
`FIG.5
`
`FIG.9
`
`0002
`
`Exhibit 1026 page 2 of 16
`DENTAL IMAGING
`
`
`
`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 2 of S
`
`5,563,343
`
`FIG. 6
`
`88
`
`dll 11 '
`
`60
`
`Restoring
`Springs
`70
`
`111
`
`11111 111:,38
`
`91
`
`98
`
`FIG.7
`
`100
`
`I
`16
`166--~~:~
`118
`85
`116
`84
`114
`
`83 82
`II
`
`11
`
`64
`
`111I
`
`62 i111il
`146
`142
`66
`
`100
`
`125
`126
`
`125 94
`
`95
`
`123
`93 92
`
`0
`
`64
`
`2
`
`0003
`
`Exhibit 1026 page 3 of 16
`DENTAL IMAGING
`
`
`
`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 3 of 5
`
`5,563,343
`
`FIG. 8
`
`182
`I I
`
`""
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`266
`
`270
`278
`280
`
`276
`274
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`258
`
`260
`
`250
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`/
`
`268
`
`264 ~n 272
`~ 282
`
`' 258
`
`252
`
`Fl G. II
`
`0004
`
`Exhibit 1026 page 4 of 16
`DENTAL IMAGING
`
`
`
`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 4 of 5
`
`5,563,343
`
`FI G. 10
`
`22
`
`240
`
`222
`
`236
`
`210
`
`242
`
`~~~--~~~c:::-:-=---=--_ """"=--~ .... '=="~~ V 2
`
`224
`
`230
`
`270
`
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`
`272
`
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`
`FIG. I 2
`
`FIG. 13
`
`0005
`
`Exhibit 1026 page 5 of 16
`DENTAL IMAGING
`
`
`
`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 5 of 5
`
`5,563,343
`
`326
`
`322
`
`)
`
`324
`
`346
`
`FIG. 14
`
`362
`
`REF.
`
`328
`
`364
`
`Current
`Sensor
`
`Variable
`Supply
`
`360
`
`366
`
`368
`
`FIG. 15
`
`FIG.16
`
`0006
`
`Exhibit 1026 page 6 of 16
`DENTAL IMAGING
`
`
`
`1
`MICROELECTROMECHANICAL LATERAL
`ACCELEROMETER
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates, in general, to microelectro(cid:173)
`mechanical accelerometers and more particularly to accel(cid:173)
`erometers fabricated from single crystal silicon beams hav(cid:173)
`ing high aspect ratios for high sensitivity, and to a method
`for fabricating such devices.
`Various techniques and processes have been devised for
`fabricating micromachined structures such as accelerom(cid:173)
`eters, and these prior techniques have been discussed in the
`literature. However, most such processes require multiple
`masking steps, wafer-to-wafer bonding, or the use of wet
`chemistry. It has been found, however, that the use of such
`multiple masks and bonding techniques can introduce align(cid:173)
`ment errors which reduce yield and increase. device cost,
`making such processes unsuitable for sub-micron structures.
`In addition, prior accelerometers have presented problems
`in that they have required a rel a ti vel y large amount of power
`for operation, and this is not suitable for microelectrome(cid:173)
`chanical devices. For example, accelerometers have been
`developed which utilize a solid block of material as the proof
`mass, with piezoelectric supports for the block connected in
`a bridge circuit. Such devices can require as much as 2
`milliamps of current for operation.
`
`SUMMARY OF THE INVENTION
`
`It is, therefore, an object of the present invention to
`provide improved microelectromechanical accelerometer
`structures.
`It is a further object of the invention to provide acceler(cid:173)
`ometer structures fabricated by a process which provides
`submicron feature sizes and high aspect ratios to permit a
`close spacing of relatively movable parts and which results
`in high sensitivity.
`It is another object of the present invention to provide
`microelectromechanical accelerometer structures having
`relatively movable component parts having submicron
`dimensions and having control mechanisms for regulating
`the motion therebetween to reduce mechanical resonance
`and to thereby provide improved accuracy.
`A still further object of the invention is to provide a
`microelectromechanical accelerometer which requires low
`current and power for operation, and which provides a high
`degree of sensitivity.
`Briefly, the present invention is directed to an accelerom(cid:173)
`eter which includes three major components: a central, or
`proof, mass, an adjacent capacitive sensor, and a restoring
`spring. The central mass, which is relatively movable with
`respect to the sensor, includes a longitudinally extending
`beam which forms a primary support, or backbone, for a
`plurality of parallel, laterally extending fingers lying in a
`common plane. This central mass will be referred to herein
`as the movable portion of the accelerometer. The fingers
`extending from the longitudinal beam are interleaved with
`corresponding parallel fingers forming an adjacent sensor,
`these sensor fingers being connected to a support such as a
`substrate adjacent the location of the longitudinal support
`beam structure and lying in the plane of the movable fingers.
`The sensor fingers and support will be referred to herein as
`the fixed portion of the accelerometer. The longitudinal
`support beam extends axially along the accelerometer, and is
`
`20
`
`25
`
`30
`
`55
`
`5,563,343
`
`5
`
`15
`
`2
`supported at each end by the restoring spring structure which
`tends to maintain the support beam in a rest location with
`respect to the fixed sensor structure and restrains the motion
`of the support beam.
`The laterally extending movable fingers form movable
`capacitor plates which face corresponding opposed sensor
`capacitor plates on the stationary interleaved fingers. When
`the structure is subject to an acceleration force having a
`component in a direction parallel to the backbone, each
`10 movable finger tends to move toward or away from the
`adjacent fixed sensor finger under the restraint of the support
`spring structure. This motion results in a variable capaci(cid:173)
`tance between adjacent opposed plates which is used to
`measure the relative displacement of the backbone structure.
`In a preferred form of the invention, the support beam and
`its laterally extending fingers are double beam structures
`which are two orders of magnitude stiffer than the restoring
`springs which support the structure. Furthermore, the sup(cid:173)
`port beam and the movable and the stationary fingers have
`a high aspect ratio so that they are relatively stiff in the
`vertical direction. This high aspect ratio also provides large
`capacitor surfaces for the fingers, while the double beam
`structure provides the mass required for efficient operation
`as an accelerometer. If desired, additional mass can be added
`to the movable portion of the structure.
`The restoring springs which support the movable structure
`of the accelerometer preferably each have a single beam
`structure. The springs extend laterally with respect to the
`backbone and are relatively flexible so as to permit longi-
`tudinal motion of the backbone through bending of the
`springs in the horizontal plane. The support springs have a
`high aspect ratio to provide stability in the vertical direction
`while allowing mechanical compliance in the horizontal
`plane, while the double beam structure and the lateral
`35 extension of the spring support beams provide stiffness in
`the lateral direction. The spring constants of the support
`springs can be adjusted electromechanically or electrically
`so as to adjust the resilience of the accelerometer moving
`structure to thereby adjust the bandwidth of frequencies to
`40 which it ~an respond and to vary its resonant point.
`The accelerometer structure is fabricated utilizing a modi(cid:173)
`fied version of the Single Crystal Reactive Etching And
`Metalization (SCREAM-I) process which is described in
`U.S. application Ser. No. 08/013,319, filed Feb. 5, 1993,
`45 now abandoned, and its continuation U.S. application Ser.
`No. 08/312,797 filed Sept. 27, 1994, which application is
`hereby incorporated herein by reference. As stated in that
`application, the SCREAM-I process is a single mask, single
`wafer, dry etch process which uses optical lithography for
`50 fabricating submicron microelectromechanical devices. In
`accordance with the teachings of that application, a silicon
`dioxide layer is deposited on a single crystal silicon wafer,
`this oxide layer serving as the single etch mask throughout
`the process. Photolithography is used to pattern a resist, and
`then dry etching, such as magnetron ion etching, is used to
`transfer the pattern of the accelerometer structure into the
`oxide. Once the resist material is removed, the patterned
`oxide masks the silicon substrate to allow a deep vertical
`silicon RIE on exposed surfaces to produce trenches having
`60 predominately vertical side walls and which define the
`desired structure. Next, a conformal coating of PECVD
`oxide is deposited for protecting the side walls of the
`trenches during the following release etch. The trench bot(cid:173)
`tom oxide is removed within an isotropic RIE, and a second
`65 deep silicon trench etch deepens the trenches to expose the
`side wall silicon underneath the deposited side wall oxide.
`The exposed silicon underneath the defined structure is
`
`0007
`
`Exhibit 1026 page 7 of 16
`DENTAL IMAGING
`
`
`
`3
`etched away, using an isotropic dry etch such as an SF6 etch
`to release the structure and leave cantilevered beams and
`fingers over the remaining substrate. In the SCREAM-I
`process, aluminum is deposited by sputtering to coat the side
`walls of the released beams and fingers to thereby form the 5
`capacitor plates for the accelerometer of the present inven(cid:173)
`tion.
`In the modified process of the invention, prior to the
`sputtering of aluminum, an etching step is used to open vias
`leading to contact pads formed in the surrounding substrate 10
`and covered by a layer of oxide, which vias can be used in
`connecting the accelerometer components to external cir(cid:173)
`cuitry. Thereafter, the sputtering step covers the entire top
`surface with aluminum, which also contacts the exposed
`pads. Finally, the aluminum is masked, patterned and etched l5
`back so that the aluminum provides connections between the
`pads and corresponding circuit elements.
`Devices constructed in accordance with the foregoing
`process have extremely small dimensions, high aspect ratios,
`and can provide released, movable metal-coated structures
`closely adjacent to corresponding metal-coated stationary
`structures, whereby relative movement can be detected by
`the capacitance between opposed surfaces. The close spac(cid:173)
`ing of the opposed fingers provides high sensitivity, while
`the modified SCREAM process permits fabrication of such
`devices on existing integrated circuits for interconnection
`with existing circuitry. The photolithographic step allows
`fabrication of a variety of accelerometer shapes as well as
`the formation of control structures for use in regulating the
`motion of the released accelerometer. Although axial motion 30
`of the movable structure is of primary importance, a modi(cid:173)
`fication of the process allows the deposition of metal on the
`substrate surface beneath the beams so as to permit detection
`of, as well as control of, vertical motion of the released
`structure with respect to the fixed fingers.
`
`35
`
`25
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The foregoing, and additional objects, features and advan(cid:173)
`tages of the present invention will become apparent to those
`of skill in the art from a consideration of the following
`detailed description of preferred embodiments thereof, taken
`in conjunction with the accompanying drawings, in which:
`FIGS. 1-5 are diagrammatic illustrations of a fabrication
`process in accordance with the present invention;
`FIG. 6 is a diagrammatic top plan view of a preferred
`embodiment of the accelerometer structure of the present
`invention;
`FIG. 7 is an enlarged perspective view of a portion of the
`structure of FIG. 6;
`FIG. 8 is a diagrammatic top plan view of a second
`embodiment of the invention;
`FIG. 9 is a diagrammatic cross-sectional view of a portion
`of the device of FIG. 8;
`FIG. 10 is a diagrammatic top plan view of a third
`embodiment of the invention;
`FIG. 11 is a diagrammatic top plan view of a fourth
`embodiment of the invention, illustrating a control for a
`restoring spring structure;
`FIG. 12 is a partial top plan view of the embodiment of
`FIG. 11, illustrating the effect of an accelerating force on the
`control of FIG. 11;
`FIG. 13 is a diagrammatic top plan view of a fifth 65
`embodiment of the invention, illustrating a modified control
`structure;
`
`50
`
`5,563,343
`
`4
`FIG. 14 is a diagrammatic top plan view of a sixth
`embodiment of the invention, illustrating another modified
`control structure;
`FIG. 15 is a diagrammatic partial top plan view of the
`embodiment of FIG. 14, with the control activated; and
`FIG. 16 is a diagrammatic illustration of a feedback
`control system for the present invention.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`in
`Capacitance-based accelerometers are fabricated,
`accordance with the present invention, utilizing a modified
`version of the process known as Single Crystal Reactive
`Etching And Metalization (SCREAM-I), which is a low
`temperature, single-mask bulk micromachining process
`which permits construction of high aspect ratio single crystal
`silicon beams. The SCREAM-I process, which is diagram(cid:173)
`matically illustrated in FIGS. 1-5, begins with a single
`crystal silicon wafer 10 which provides the substrate in
`20 which the accelerometer is to be constructed. The wafer may
`be a I mohm cm arsenic silicon, for example, on which is
`deposited a PECVD silicon dioxide layer 12. This oxide
`layer serves as the etch mask throughout the fabrication
`process. Photolithography is used to pattern a resist, and
`then magnetron ion etching is used to transfer the pattern
`into the oxide, FIG. 1 illustrating a simple patterned oxide
`layer.
`After the foregoing pattern transfer, the resist is removed
`and the patterned oxide 12 serves as a mask for a deep
`vertical silicon trench etch. This step, which may be an RIE
`process which uses a mixture of chlorine and boron trichlo-
`ridc, etches the substrate 10 in the exposed regions around
`the mask 12 to produce corresponding trenches such as those
`illustrated at 14 and 16 in FIG. 2. The trenches are defined
`by substantially vertical side walls such as the side walls 18
`and 20 which define trench 16 and the side wall 22 which
`defines one side of trench 14. Generally horizontal floor
`surfaces 24 and 26 are formed in trenches 14 and 16 by this
`40 step. The ratio of the silicon etched depth to sidewall
`widening is typically 40 to 1 and etch selectivity (i.e., the
`ratio of the silicon etch rate to the oxide etch rate) is typically
`20 to 1. This allows formation of deep trenches defining
`mesas, such as the mesa 28 in FIG. 2, in any desired pattern
`45 as established by the mask 12.
`After formation of the trenches, a conformal coating of
`PECVD oxide 30 is deposited to protect the side walls of the
`trenches during the subsequent release etch. Thereafter, the
`PECVD oxide layer 30 is removed from the trench bottoms
`to expose floors 24 and 26, by means of an isotropic RIE
`using, for example, CF4 • Thereafter, a second deep silicon
`trench etch using, for example, Cl2RIE deepens the trenches
`14 and 16, as illustrated at 32 and 34, respectively, to expose
`the silicon substrate 10 below the protective oxide layer 30
`55 on the side walls. This exposes silicon sidewall portions 36
`and 38 on opposite sides of the mesa 28 and side wall 40 on
`the substrate portion surrounding the trenches and mesas
`formed by the pattern 12.
`As illustrated in FIG. 4, the next step is to etch away the
`60 exposed silicon underneath the oxide layer 30 to thereby
`release the mesa 28 to form a beam such as that illustrated
`at 42 in FIG. 4. This release step is carried out by means of
`an isotropic SF6 etch which releases the structure from the
`underlying substrate. The beam 42 may be cantilevered from
`the side wall, as illustrated in FIG. 4, or may be supported
`by suitable posts extending from the floor 44 of the trenches
`upwardly to the beam.
`
`0008
`
`Exhibit 1026 page 8 of 16
`DENTAL IMAGING
`
`
`
`5,563,343
`
`10
`
`15
`
`5
`Following the release step, a layer of aluminum 46 (FIG.
`5) is applied, as by sputter deposition, to coat the side walls
`of the released beam and the adjacent substrate. This depo(cid:173)
`sition also provides a coating 48 on the floor of the trenches
`which are beneath, and spaced from, the released beam. The 5
`undercutting effect of the release etch step, which is illus(cid:173)
`trated at 50 in FIG. 5, electrically isolates the layer 48 on the
`floor of the trenches from the side wall coating 46. The
`aluminum coating can be selectively etched to provide
`suitable electrical isolation so that the coating on the side
`walls of the beam and on the substrate can act as capacitor
`plates in the accelerometer. The silicon in the beam 42 serves
`as the mechanical support for the corresponding capacitor
`plate.
`In typical integrated circuit wafers, in which the acceler(cid:173)
`ometer of the present invention preferably is incorporated,
`contact pads are provided for use in connecting the accel(cid:173)
`erometer to surrounding circuitry. The contact pads are
`metal, typically aluminum, formed on an oxide insulating
`layer on the substrate top surface and covered by a protective
`oxide layer. The modified fabrication process of the present
`invention takes advantage of such contact pads for intercon(cid:173)
`necting the accelerometer to the circuitry on the substrate by
`interposing a second masking and etching step prior to the
`sputter deposition step illustrated in FIG. 5. This second 25
`masking step, which utilizes a resist layer and photolithog(cid:173)
`raphy, locates the contact pads so that a following etch step
`through the oxide protective layer opens vias to them.
`After the vias are opened, the sputter deposition step
`applies an aluminum layer which contacts the pads through 30
`the vias. Thereafter, a third masking step is used to etch
`away undesired aluminum to thereby provide conductive
`paths from the sidewall capacitive plates to the contact pads
`on the surrounding substrate, and from there to the circuitry
`on the substrate for providing control potentials and for 35
`sensing the motion of the accelerometer.
`A structure fabricated in accordance with the foregoing
`process is illustrated in simplified form in FIGS. 1-5, and
`includes only a single beam adjacent a nearby substrate.
`However, more complex structures can easily be fabricated
`using the process described above. Such a structure is
`illustrated in top plan view in FIG. 6, to which reference is
`now made. The first lithography step described above
`defines the complex structural features of the accelerometer
`of the present invention, generally indicated at 60, and these
`features are produced within a cavity 62 formed in substrate
`64 during the trench etch steps described above. The process
`produces vertical side walls 66, 68, 70, and 72 for the cavity
`62, with
`the surrounding substrate 64 providing the
`mechanical support for the fixed sensor portion of the
`accelerometer, as well as desired circuitry (not shown) for
`controlling and/or sensing the motion of the movable portion
`of the accelerometer. The sensor portion is connected to the
`side walls 68 and 72 of the cavity 62 and includes a pair of
`support beams 74 and 76, respectively, which are incorpo(cid:173)
`rated as a part of the side walls, or which may be cantile(cid:173)
`vered therefrom to extend inwardly into the cavity 62. Beam
`74 is connected to a plurality of stationary fingers 80 through
`87, which may be solid mesa structures extending upwardly
`from the floor of cavity 62, or may be cantilevered, released
`beams of the type illustrated in FIG. 5. Beams 80-87 extend
`perpendicularly to support beam 74, are parallel to each
`other, and their top surfaces lie in a common horizontal
`plane above the floor of cavity 62. These fingers include a
`layer of oxide so that the capacitive plates which they carry
`are electrically isolated from the substrate floor, and the
`fingers extend toward and terminate near an axial center line
`
`6
`of the cavity 62. The beams 80 through 87 are fixed so that
`they are relatively stiff and inflexible, have a high aspect
`ratio, and, as illustrated in FIG. 5, are covered with a coating
`of electrically conductive material such as aluminum to form
`vertical capacitive plates. The aluminum coating also pro(cid:173)
`vides electrical connection between the side wall capacitive
`plates on each of the beams and an outlet connector pad 88,
`preferably located on the top surface of substrate 64 and
`insulated therefrom, as described above.
`In similar manner, beam 76 is secured to wall 68 of cavity
`62, preferably at a location diametrically opposite to the
`location of beam 74, with a plurality of laterally extending
`spaced beams 90---97 extending inwardly from beam 76
`toward the center axis of the cavity 62. As with beams
`80-87, the beams 90-97 are parallel to each other, have high
`aspect ratios, and may be mesa structures extending
`upwardly from the cavity floor, or may be released from the
`floor so as to be cantilevered to the beam 76. The top
`surfaces of beams 90-97 lie in the horirontal plane of beams
`80--87. Each of the beams 90--97 is covered with a suitable
`insulating coating and a conductive coating such as alumi(cid:173)
`num, whereby the side walls of the beams form vertical
`capacitive plates. Again, the conductive coating on each of
`the beams 90-97 is connected electrically to an output
`connector pad 98 on the top surface of substrate 64 by which
`an electrical connection can be made from the fixed capaci(cid:173)
`tive plates to suitable external electrical circuitry which may
`be incorporated in the substrate wafer 64 or which may be
`external thereto.
`The accelerometer 60 also includes a movable central
`mass portion generally indicated at 100 which is fabricated
`at the same time the inwardly-extending stationary fingers
`80--87 and 90--97 are fabricated. The movable portion 100
`includes an axially-extending support beam 102 between the
`terminal ends of the fingers 80--87 and 90--97. Beam 102
`serves as a backbone supporting a plurality of laterally
`outwardly extending fingers 110-117 which are interleaved
`with the inwardly extending fingers 80--87, and further
`includes outwardly extending fingers 120--127 interleaved
`40 with inwardly extending fixed fingers 90--97. Preferably,
`fingers 110--117 are laterally opposed to fingers 120-127 and
`lie in the horizontal plane of fingers 80-87 and 90-97, but
`it will be apparent that they can be relatively offset, if
`desired. The outwardly extending fingers arc parallel to, and
`45 extend substantially the entire length of, the corresponding
`adjacent inwardly extending fingers, the terminal ends of the
`outwardly extending fingers terminating near, but just short
`of, the support beams 74 and 76.
`Axial beam 102 and lateral fingers 110--117 and 120--127
`50 may be fabricated in accordance with the process described
`above with respect to FIGS. 1-5, and thus are released from
`the underlying substrate 64 for free movement with respect
`thereto. The movable structure 100, consisting of the axial
`beam and lateral fingers, is suspended within cavity 62 by
`55 movable supports at opposite ends of the structure. Thus, the
`left-hand end 130, as viewed in FIG. 6, is secured to, or
`preferably is fabricated integrally with, a laterally extending
`spring support beam 132 which is flexible in the plane of the
`accelerometer structure to permit axial motion of beam 102
`in the direction of arrow 134. The beam 132 is sufficiently
`long and its cross-section is selected to provide the degree of
`flexibility required to enable the accelerometer to respond to
`selected forces, and is secured at its outermost ends to the
`substrate adjacent connector pads 136 and 138. The beam
`65 132 serves as a restoring spring which tends to hold the
`beam 102, and thus the accelerometer moving structure 100,
`in a predetermined rest position with the opposed fixed and
`
`20
`
`60
`
`0009
`
`Exhibit 1026 page 9 of 16
`DENTAL IMAGING
`
`
`
`5,563,343
`
`5
`
`8
`than in the vertical direction, perpendicular to the common
`plane of the axial beam 102 and the lateral fingers 110-117
`and 120-127. In one embodiment of the invention, the
`stiffness in the axial direction of beam 102 was 0.97 Nim
`while the out-of-plane stiffness, resulting from the high
`aspect ratio springs, was 128 N/m.
`If desired, an additional masking step, following the
`process of FIGS. 1-5, can be used to remove the side wall
`oxide and metal films from the linear springs 132 and 142.
`10 Because the lateral spring constant (in the direction of
`arrows 134 and 144) varies with the cube of the beam width,
`the removal of the side wall films can reduce the lateral
`spring constant by up to 2 orders of magnitude. A further
`control over the spring constant is obtained by varying the
`15 width of the spring beams by regulating the width of the
`silicon.
`The capacitor plates on the side walls of the fixed fingers
`are connected to each other and to connectors 88 and 98, as
`noted above, while the capacitors formed on the side walls
`20 of the movable fingers are connected to each other and
`through the metal coating on the axial beam 102 and the
`metallized top surface of spring beam 132 to connector pad
`136. The connectors 88,98 and 136 are connected to suitable
`circuitry, which may include a source of potential 172, for
`25 example, which may be an alternating excitation source.
`When the structure is subjected to an accelerating force, the
`movable element 100 shifts with respect to the fixed fingers
`80-87and 90-97, the movable fingers each moving toward
`or away from the corresponding adjacent fixed fingers. This
`30 motion results in a variable capacitance which can be
`determined by measuring changes in the output voltage at
`connectors 88 and 98 to determine the displacement of the
`mass of the movable element 100.
`The mass of the movable element 100 preferably is on the
`order of 1.0 nkg. If it is assumed that the frequency of the
`accelerating force is significantly lower than the natural
`frequency of the device, and that the lateral movable fingers
`and the axial beam 102 are orders of magnitude stiffer than
`the restoring springs 132 and 142, the change in separation
`40 between opposed plates on the fixed and movable fingers
`can be approximated with a simple spring-mass model and
`Newton's second law. From this a deformation of 13 nm for
`an acceleration of 9.8 m/s2 can be expected, and since this
`deformation is small compared to the initial two-micron
`separation between the capacitor plates, the parallel plate
`the capacitance can be accurately
`approximation for
`expressed using the following equation:
`
`7
`movable fingers spaced apart as required. The spring pro(cid:173)
`vides a predetermined resistance to motion, depending upon
`the dimensions and thus the flexibility of beam 132.
`Beam 132 is fabricated in the manner described above
`with respect to FIGS. 1-5, and is released from the under-
`lying substrate material in the cavity 62 for free motion
`above the floor of the cavity. The spring support beam is
`coated with an insulating layer and a conductive material
`such as aluminum so that it can be connected to external
`circuitry by way of connector pads 136 and 138, which arc
`insulated from the underlying substrate 64.
`In similar manner, the opposite end 140 of beam 102 is
`connected to a laterally extending spring support beam 142
`which serves as a second spring for suspending the movable
`element 100 in its rest position and for allowing axial motion
`in the direction of arrows 134. The outermost ends of spring
`142 are connected to the substrate 64 adjacent corresponding
`connector pads 146 and 148 which are insulated from the
`substrate and are provided with an aluminum or other
`cond