119
`United States Patent
`5,563,343
`Oct. 8, 1996
`(45) Date of Patent:
`Shawetal.
`
`[11] Patent Number:
`
`NA ARY
`
`US005563343A
`
`seseeeseeesees 73/517 R
`.............
`Tsuchitani
`vee 250/306
`Ameyet al.
`...
`teveee 156/643
`Core oe
`
`we THSATR
`Sherman ....
`
`w» 73/514.18
`
`Tang ....
`LANG ooececccesscssetsceeseeereneeteee
`we T3/514.18
`
`711993
`8/1993
`5/1994
`9/1994
`0/1994
`0/1994
`
`1]
`
`5,228,341
`5,235,187
`5,314,572
`5,345,824
`5,353,641
`5,357,803
`
`OTHER PUBLICATIONS
`
`[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. Checen GOLP 15/08
`[51]
`[52] U.S. C1. es eccessssscssceeenreseee 23/514,18; 73/514.24
`[58] Field of Search 00.0.0...ccc 73/517 R, 514,
`73/515, 516 R, 517 B, 514.18, 514.21,
`$14.24, 514.32, 514.35, 514.38, 514.36,
`514.17
`
`(56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`[57]
`
`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 aspectratio
`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 providesstiffness in the
`movable portion for accuracy.
`
`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-
`eter Sees 50G. Electronic Design, Aug. 8, 1991, pp. 45-56.
`Zhanget al., “A RIE Process for Submicron, Silicon Elec-
`tromechanical Stnictures”, IOP Publishing Ltd., 1992, pp.
`31-38.
`Wilsonet al., “Highly Selective, High Rate Tungsten Depo-
`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.
`Ameyet al., “Formation of Submicron Silicon—on—Insulator
`Structures by Lateral Oxidation of Substrate—Silicon
`Islands”, J. Vac. Sci. Techno]. B 6(1), Jan./Feb. 1988, pp.
`3,835,338
`9/1974 Martin 002.ecenesersesenseenee 310/331
`341-345.
`
`4,381,672 5/1983 O’Conmoret al.oweee 73/505
`
`Payne, R. S., et al. “Surface Micromachined Accelerometer:
`eccesceeseeeeseconeceseees 437/55
`4,437,226
`3/1984 Sochof oc.
`
`A Technology Update’. SAE Intemational, pp. 127-135
`. 73/514.33
`4,553,436
`11/1985 Hansson.......
`(Feb. 25—Mar. 1, 1991).
`........
`ws. 156/643
`4,670,092
`6/1987 Motamedi
`
`
`4,685,198 8/1987 Kawakita et al.oo...ee 437/73
`
`
`Primary Examiner—Hezron E. Williams
`11/1987 Murakami............
`wee 437/225
`4,706,374
`ceeeeseees 437/24
`Assistant Examiner—Christine K. Oda
`4,746,621
`5/1988 Thomasetal. ou...
`Attorney, Agent, or Firm—Jones, Tullar & Cooper, P.C.
`4,750,363
`6/1988 Norling .........
`« THS17 R
`
`wee 257/254
`4,772,928
`9/1988 Dietrich etal. ..
`ABSTRACT
`10/1988 Delapierre ........
`we 156/647
`4,776,924
`
`7/1989 Tamaki et al. oo... 437/62
`4,845,048
`
`
`we 156/647
`7/1989 Howeetal......
`4,851,080
`
`wee 156/647
`4,867,842
`9/1989 Bohreret al.
`8/1990 Roszhart 0.0.0... 73/514.29
`4,945,765
`
`ve 156/647
`4,981,552
`141991 Mikkor.....
`
`
` Sickafus....... vee 156/653
`
`5,045,152
`9/1991
`12/1991 MacDonald......
`vee 257/420
`5,072,288
`
`3/1992 Suzuki etal. ........
`wee 73/514.32
`5,095,752
`
`wee T34514.34
`5,121,180
`6/1992 Beringhause etal.
`
`6/1992 Greiff 0.
`wee 25417
`5,126,812
`
`....
`vee 437/192
`5,149,673
`9/1992 MacDonaldet al.
`
`....
`ve 361/313
`5,179,499
`1/1993 MacDonald et al.
`
`
`we 437/203
`5,198,390
`3/1993 MacDonaldet al.
`....
`4/1993 O’Brien et al. oo...eee 73/514.18
`5,205,171
`
`§2 Claims, 5 Drawing Sheets
`
`de
`
`60
`
`Backbone
`Restoring
`
`4 85 og
`
`Align EX1033
`Align v. 3Shape
`IPR2022-00144
`
`Align EX1033
`Align v. 3Shape
`IPR2022-00144
`
`

`

`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 1 of 5
`
`5,563,343
`
`Z+40
`
`
`FIG.9
`
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`VA
`Iss“
`SSS SeneePIOTTOTOLOLZL.
`
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`

`

`U.S. Patent
`
`5,563,343
`
`Oct. 8, 1996
`
`Sheet 2 of 5
`
`Backbone
`Restoring
`
`Springs
`
`

`

`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 3 of 5
`
`5,563,343
`
`282
`
`

`

`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 4 of 5
`
`5,563,343
`
`10
`
`FIG.
`
`FIG. 13
`
`

`

`U.S. Patent
`
`Oct. 8, 1996
`
`Sheet 5 of 5
`
`5,563,343
`
`554
`
`3520a
`™—348
`
`a 350
`
`524
`
`546
`
`FIG. 14
`
`362
`REF.
`336 —
`
`
`=
`366
`
`Current
`Sensor
`
`568
`
`340
`
`328
`
`338 —»
`
`360
`
`Suppl
`
`FIG. 15
`
`FIG.16
`
`

`

`5,563,343
`
`1
`MICROELECTROMECHANICAL LATERAL
`ACCELEROMETER
`
`BACKGROUND OFTHE INVENTION
`
`The present invention relates, in general, to microelectro-
`mechanical accelerometers and more particularly to accel-
`erometers fabricated from single crystal silicon beams hay-
`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-
`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-
`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 relatively large amount of power
`for operation, and this is not suitable for microelectrome-
`chanical devices. For example, accelerometers have been
`developed whichutilize a solid block of material as the proof
`mass, with piezoelectric supports for ihe block connected in
`a bridge circuit. Such devices can require as much as 2
`milliamps of current for operation.
`
`SUMMARYOF THE INVENTION
`
`therefore, an object of the present invention to
`It is,
`provide improved microelectromechanical accelerometer
`structures.
`
`It is a further object of the invention to provide acceler-
`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 powerfor operation, and which provides a high
`degree of sensitivity.
`Briefly, the present invention is directed to an accelerom-
`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 extendsaxially along the accelerometer, andis
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`2
`supported al each endbythe restoring spring structure which
`tends to maintain the support beam in a rest location with
`respect to the fixed sensorstructure and restrains the motion
`of the support beam.
`The laterally extending movable fingers form movable
`capacitor plates which face corresponding opposed sensor
`capacitor plates onthe stationary interleaved fingers. When
`the structure is subject to an acceleration force having a
`component in a direction parallel to the backbone, each
`movable finger tends to move toward or away from the
`adjacentfixed sensor finger under therestraint of the support
`spring structure. This motion results in a variable capaci-
`tance between adjacent opposed plates which is used to
`measure therelative displacementof the backbonestructure.
`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-
`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 andare 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 aspectratio to provide stability in the vertical direction
`while allowing mechanical compliance in the horizontal
`plane, while the double beam structure and the lateral
`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
`which it can respond andto vary its resonant point.
`The accelerometerstructure is fabricated utilizing a modi-
`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,
`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
`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
`predominately vertical side walls and which define the
`desired structure. Next, a conformal coating of PECVD
`oxide is deposited for protecting the side wails of the
`trenches during the following release etch. The trench bot-
`tom oxide is removed within an isotropic RIE, and a second
`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
`
`

`

`5,563,343
`
`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
`capacitor plates for the accelerometer of the present inven-
`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
`and covered by a layer of oxide, which vias can be used in
`connecting the accelerometer components to external cir-
`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
`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 aspectratios,
`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-
`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 ofthe released accelerometer. Although axial motion
`of the movable structure is of primary importance, a modi-
`fication of the process allows the deposition of metal on the
`substrate surface beneath the beamsso as to permit detection
`of, as well as control of, vertical motion of the released
`structure with respect to the fixed fingers.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The foregoing, and additional objects, features and advan-
`tages ofthe present invention will become apparentto those
`of skill
`in the art from a consideration of the following
`detailed description of preferred embodimentsthereof, 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
`embodimentof 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
`Testoring 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
`embodimentof the invention,illustrating a modified control
`structure;
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`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
`fabricated,
`are
`Capacitance-based accelerometers
`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-
`matically illustrated in FIGS. 1-5, begins with a single
`crystal silicon wafer 10 which provides the substrate in
`which the accelerometeris to be constructed. The wafer may
`be a 1 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 10 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 borontrichlo-
`ride, 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 bythis
`step. The ratio of the silicon etched depth to sidewall
`widening is typically 40 to 1 and etch selectivity (i.e., the
`ratio ofthe 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
`as established by the mask 12.
`After formation of the trenches, a conformal coating of
`PECVDoxide 30 is deposited to protect the side walls of the
`trenches during the subsequent release etch. Thereafter, the
`PECVDoxide layer 30 is removed from the trench bottoms
`to expose floors 24 and 26, by means of an isotropic RIE
`using, for example, CF,. Thereafter, a second deep silicon
`trench etch using, for example, C],RIE deepensthe trenches
`14 and 16,asillustrated at 32 and 34,respectively, to expose
`the silicon substrate 10 below the protective oxide layer 30
`on the side walls. This exposessilicon 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.
`Asillustrated in FIG. 4, the next step is to etch away the
`exposed silicon underneath the oxide layer 30 to thereby
`release the mesa 28 to form a beam such as thatillustrated
`at 42 in FIG. 4. This release step is carried out by means of
`an isotropic SF, etch which releases the structure from the
`underlying substrate. The beam 42 maybe 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.
`
`

`

`5,563,343
`
`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-
`sition also provides a coating 48 on the floor of the trenches
`which are beneath, and spaced from,the released beam. The
`undercutting effect of the release etch step, which is illus-
`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
`wails 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-
`ometer of the present invention preferably is incorporated,
`contact pads are provided for use in connecting the accel-
`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-
`necting the accelerometerto 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
`masking step, which utilizes a resist layer and photolithog-
`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
`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
`sensing the motion of the accelerometer.
`A structure fabricated in accordance with the foregoing
`process isillustrated 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
`producesvertical 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-
`rated as a part of the side walls, or which may be cantile-
`vered therefrom to extend inwardly into the cavity 62. Beam
`74 is connectedto 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
`beamsofthe 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 abovethe 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 87are fixed so that
`they are relatively stiff and infiexible, have a high aspect
`ratio, and, as illustrated in FIG. 5, are covered with a coating
`ofelectrically conductive material such as aluminum to form
`vertical capacitive plates. The aluminum coating also pro-
`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 securedto 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 90-97lie in the horizontal 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-
`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-
`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 betweenthe
`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
`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 are parallel to, and
`extend substantially the entire length of, the corresponding
`adjacent inwardly extending fingers, the terminal endsofthe
`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
`may be fabricated in accordance with the process described
`above with respect to FIGS. 1—5, and thusare 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
`movable supports at opposite endsof 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 whichis flexible in the planeof the
`accelerometer structure to permit axial motion of beam 102
`in the direction of arrow 134. The beam 132is sufficiently
`long andits 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
`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
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`7
`movable fingers spaced apart as required. The spring pro-
`vides a predeterminedresistance 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 are
`insulated from the underlying substrate 64.
`In similar manner, the opposite end 140 of beam 102is
`connectedto 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
`conductive coating for electrical connection of the beam to
`suitable external circuitry.
`Asis illustrated in FIG. 7, which is an expanded view of
`a portion of the device of FIG. 6, the movable assembly 100
`of the accelerometer 60 is made up of a double beam
`structure to provide extra stiffness and strength to this part
`of the device. Asillustrated, the axial support beam 102, or
`backbone, consists of two parallel longitudinal beams 160
`and 162 joined by a multiplicity of interconnecting bridges
`164 which serve as cross-braces. Similarly, each of the
`laterally extending fingers consists of a pair of parallel,
`closely-spaced beams such as the beams 166 and 168
`jllusirated for beam 118 and a multiplicity of cross braces,
`or bridges 170. This double-beam construction provides a
`high degree of stiffness for the moving element 100 so that
`the entire element movesunitarily under accelerating forces
`without flexure of the beam 102 or the fingers 110-117 or
`120-127 to provide accurate measurements of the change of
`capacitance between opposed plates on the adjacent fixed
`and movable finger side walls.
`Typically, in the microstructure of the present invention,
`each of the fixed fingers 80-87 and 90-97 is between about
`S5—15 pm in height depending on whetherit is a mesa-type
`structure or is cantilevered, is about 4 um in width, and may
`be 300 or more micrometers in length. If these fingers are
`cantilevered they preferably are spaced about 2 to 10 um
`above the floor of the cavity 62. The spacing between
`adjacent fingers can be unequal, so that between adjacent
`fixed and movable fingersat rest, such as fingers 80 and 110,
`the spacing may be about 2 ym, while the distance between
`fingers 110 and 81 may be 8 um. The high aspect ratio
`individual beams making up the parallel sets of beamsin the
`movable element 100 preferably will be of somewhat thin-
`ner than the fixed beams and maybeless than 1} pm in width,
`with a preffered minimum dimension in the range of 1-3 ym,
`although the total width of the pair of beams for each
`movable finger may be somewhat greater than the width of
`a fixed finger. In one form of the invention, as many as 80
`individual interleaved capacitor fingers may be provided.
`The end return springs 132 and 142 may have the same
`general thickness and height and thus the same cross-section
`and aspect ratio as the released fingers. The two return
`springs are dimensionedto providethe resilience desired for
`holding the movable element 100 in place, while allowing
`quick and sensitive response to accelerating forces. The high
`aspectratio of the springs insures that they will have greater
`flexibility in the axial direction indicated by arrows 134,
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`than in the vertical direction, perpendicular to the common
`plane of the axial beam 102 andthe 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 N/m
`while the out-of-plane stiffness, resulting from the high
`aspect ratio springs, was 128 N/m.
`following the
`If desired, an additional masking step,
`process of FIGS. 1-5, can be used to removethe side wall
`oxide and metal films from the linear springs 132 and 142.
`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
`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
`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
`example, which may be an alternating excitation source.
`Whenthe 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
`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 displacementof the
`mass of the movable element 100.
`
`The massof 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 magnitudestiffer than
`the restoring springs 132 and 142, the change in separation
`between opposed plates on the fixed and movable fingers
`can be approximated with a simple spring-mass model and
`Newton’s second Jaw. From this a deformation of 13 nm for
`an acceleration of 9.8 m/s” can be expected, and since this
`deformation is small compared to the initial
`two-micron
`separation between the capacitor plates, the parallel plate
`approximation for
`the
`capacitance can be accurately
`expressed