(12) United States Patent
`Christenson et al.
`
`(54) MEMS SENSOR STRUCTURE AND
`MICROFABRICATION PROCESS
`THEREFOR
`
`(*) Notice:
`
`(75) Inventors: John Carl Christenson, Kokomo,
`Steven Edward Staller, Russiaville;
`John Emmett Freeman, Kempton;
`Troy Allan Chase, Kokomo; Robert
`Lawrence Healton, Kokomo; David
`Boyd Rich, Kokomo, all of IN (US)
`(73) Assignee: Delphi Technologies, Inc., Troy, MI
`(US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`(21) Appl. No.: 09/410,713
`(22) Filed:
`Oct. 1, 1999
`(51) Int. Cl. ................................................ H01L 21/00
`(52) U.S. Cl. ............................... 216/2: 216/33; 216/41;
`216/59; 216/67; 216/79; 438/719, 438/735
`(58) Field of Search ................................ 216/2, 33, 41,
`216/59, 67, 79; 438/14, 719, 723, 735,
`743; 73/517 R, 517 A
`References Cited
`U.S. PATENT DOCUMENTS
`
`(56)
`
`5,251,484 A 10/1993 Mastache .................. 73/517 A
`5,447,601. A
`9/1995 Norris ........................... 216/2
`5,840,199 A 11/1998 Warren .......................... 216/2
`6,174.820 B1 * 1/2001 Habermehl et al. ......... 216/2 X
`OTHER PUBLICATIONS
`“Rejecting Rotational Disturbances on Small Disk Drives
`Using Rotational Accelerometers' Daniel Y. Abramovitch,
`1996 IFAC World Congress in San Francisco, CA 1996, pp.
`1-6.
`
`
`
`USOO642871 3B1
`(10) Patent No.:
`US 6,428,713 B1
`(45) Date of Patent:
`Aug. 6, 2002
`
`“Increased Disturbance Rejection in Magnetic Disk Drives
`by Acceleration Feed forward Control and Parameter Adap
`tion” M.T. White and M. Tomizuka, vol. 5, No. 6, 1997, pp.
`741-751.
`"Embedded Interconnect and Electrical Isolation for
`High-Aspect-Ratio, SOI Inertial Instruments' T. J. Brosni
`han, J.F. Bustillo, A.P. Pisano and R.T. Howe, 1996 Inter
`national Conference on Solid-State Sensors and Actuators,
`Chicago, Jun. 16-19, 1997, pp. 637-640.
`* cited by examiner
`Primary Examiner William A. Powell
`(74) Attorney, Agent, or Firm Jimmy L. Funke
`(57)
`ABSTRACT
`A micro-electro-mechanical Structure including a Semicon
`ductor layer mounted to an annular Support Structure via an
`isolation layer wherein the Semiconductor layer is micro
`machined to form a Suspended body having a plurality of
`Suspension projections extending from the body to the rim
`and groups of integral projections extending toward but
`Spaced from the rim between Said Suspension projections.
`Each projection in Said groupS has a base attached to the
`body and a tip proximate the rim. The Structure includes a
`plurality of inward projections extending from and Sup
`ported on the rim and toward the body. Each Such projection
`has a base attached to the rim and a tip proximate the body;
`wherein the grouped projections and the inward projections
`are arranged in an interdigitated fashion to define a plurality
`of proximate projection pairs independent of the Suspension
`elements Such that a primary capacitive gap is defined
`between the projections of each projection pair. Also, a
`process is disclosed for fabricating the micro-electro
`mechanical Structure including the Steps of removing a
`highly doped etch termination layer and thereafter etching
`through a lightly doped epitaxial layer to thereby define and
`release the Structure.
`
`10 Claims, 4 Drawing Sheets
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`Abbott
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`Page 001
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`Aug. 6, 2002
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`Aug. 6, 2002
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`US 6,428,713 B1
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`1
`MEMS SENSOR STRUCTURE AND
`MICROFABRICATION PROCESS
`THEREFOR
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`This application discloses Subject matter which is dis
`closed and claimed in co-pending U.S. application Ser. No.
`09/411,339, filed Oct. 1, 1999, in the names of John Carl
`Christenson et al., and entitled “Method and Apparatus for
`Electrically Testing and Characterizing Formation of Micro
`electronic Features, the entire contents of which are incor
`porated herein by reference. This application is also related
`to co-pending application Ser. No. 09/410,712, entitled
`“Angular Accelerometer,” filed Oct. 1, 1999, in the name of
`David Boyd Rich.
`TECHNICAL FIELD
`The present invention relates to micro-electro-mechanical
`systems (MEMS) and in particular to an accelerometer and
`related microfabrication processes for the high-volume
`manufacture of Such a device.
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`cally integrated with the Structure to provide electrical
`communication between the Structure and other microelec
`tronic circuits. See FIG. 1 of the Mastache patent identified
`above. Furthermore, Such a device is typically encapsulated
`and hermetically sealed within a microShell (i.e., a cap). The
`microShell Serves many purposes, Some of which include,
`for example, Shielding the micro-mechanical parts of the
`MEMS device from particle (such as dust) contamination,
`Shielding the micro-mechanical parts from corrosive
`environments, shielding the MEMS device from humidity
`(Stiction) and H2O (in either the liquid or vapor phase),
`shielding the MEMS structure from mechanical damage
`(Such as abrasion), and accommodating the need for the
`MEMS device to operate in a vacuum, at a particular
`pressure, or in a particular liquid or gas (Such as, for
`example, dry nitrogen) environment.
`A typical MEMS device has a size on the order of less
`than 10 meter, and may have feature sizes of 10 to 10
`meter. This poses a challenge to the Structural design and
`microfabrication processes associated with these Small
`Scale, intricate and precise devices in View of the desire to
`have fabrication repeatability, fast throughput times, and
`high product yields from high-volume manufacturing.
`However, the achievement of these goals often primarily
`depends upon the ability to Successfully execute the critical
`etching process Step in accordance with a desired predeter
`mined shape of the body mass and the micro-mechanical
`parts of a proposed MEMS device.
`MEMS devices such as rotary accelerometers having
`opposing projections (fingers) which are interdigitated can
`present a challenge in the microfabrication processes par
`ticularly where dimensionally different but equally critical
`gap spacings must be etched at the same time. This is a result
`of the fact that wider gaps typically etch faster than narrower
`gapS.
`There is a need in the art for an improved Structural design
`for a MEMS device having interdigitated elements such as
`projections which will reduce or eliminate the adverse
`effects associated with the etch process. There is also a need
`in the art for an improved implementation of the etch process
`which can be utilized to specifically fabricate the above
`mentioned improved structural design for a MEMS device
`having opposing, interposed and interSpaced projections
`which will circumvent and thereby negate the adverse
`effects associated with the etch process.
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`BACKGROUND OF THE INVENTION
`Presently, micro-structure devices called MEMS (micro
`electro-mechanical Systems) are gaining popularity in the
`microelectronics industry. Such MEMS devices include, for
`example, micro-mechanical filters, pressure micro-Sensors,
`micro-gyroscopes, micro-resonators, actuators, rate Sensors,
`and acceleration sensors. These MEMS devices are created
`by microfabrication processes and techniques Sometimes
`referred to as micromachining. These processes involve the
`formation of discrete Shapes in a layer of Semiconductor
`material by trenching into the layer with an etch medium.
`Because MEMS typically require movement of one or more
`of the formed shapes relative to others, the trenching is done
`in part over a cavity and in part over a Substrate or bonding
`layer.
`MEMS technology can be used to form rotary acceler
`ometers. The main structure of a typical MEMS rotary
`accelerometer comprises a proof mass Supported by a flex
`ure Suspension that is compliant for rotation but Stiff for
`translation. In a known device, the Suspension comprises
`fingers extending radially from the body straddled by
`inwardly projecting capacitor plates mechanically grounded
`to Surrounding annular Substrate area; see U.S. Pat. No.
`5,251,484, “ROTATIONAL ACCELEROMETER issued
`Oct. 12, 1993 to M. D. Mastache and assigned to Hewlett
`Packard Co. of Palo Alto.
`Forming the body mass and micro-mechanical parts of the
`MEMS device can generally be accomplished, for example,
`by a process of anisotropically etching through one or more
`upper layers of Semiconductor material(s) which are situated
`above a cavity previously etched into a lower Semiconductor
`Substrate. Such a process for forming the body mass and
`micro-mechanical suspension parts of a MEMS device is
`often referred to as a “bond/etch-back” process. Other
`processes, however, can instead be utilized to form and/or
`release the body mass and micro-mechanical parts of a
`MEMS device. Such other processes can include a through
`the-wafer etch process; a lateral release etch (confined or
`isotropic) process; or a lateral Selective undercut etch of a
`buried layer, a film, or a buried etch-stop layer after a MEMS
`delineation etch has been performed.
`65
`In addition to properly forming the main Structures of the
`MEMS accelerometer, electrically conductive lines are typi
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`SUMMARY OF THE INVENTION
`The present invention provides a micro-electro
`mechanical Sensor Structure with an improved design com
`prising rigid interdigitated projections forming capacitive
`plate elements and, in a preferred embodiment, flexible
`projections forming a rotationally compliant Suspension.
`According to the invention, the micro-electro-mechanical
`Structure basically comprises a Semi-conductor layer which
`is micromachined to define a proof mass Suspended relative
`to a Support Substrate by one or more flexible Suspension
`projections extending from the proof mass to a Substrate
`based Support area. Between these Suspension projections
`and also extending outwardly from the proof mass are Sets
`of additional rigid, Spaced apart projections which move
`with the proof mass according to a compliance mode estab
`lished by the Suspension elements, e.g., at right angles to the
`longitudinal axes of the finger-like projections. Interdigi
`tated with Such projections are complemental projections
`extending from the Support area toward the proof mass and
`defining, in combination with the rigid body projections,
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`US 6,428,713 B1
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`3
`narrow Sensor gaps of uniform width and larger, parasitic
`capacitive gaps. The Sensor gaps are formed to exhibit
`essentially constant gap widths. Such that the etch process is
`easily geared to their formation with no loSS of accuracy due
`to different etch rates in other areas of the film.
`In the illustrative embodiment, the proof mass is generally
`circular and the Suspension elements and interdigitated
`capacitance elements are radially arranged. The compliance
`mode in this embodiment is circular or rotary. However,
`linear devices using the principles hereafter explained are
`readily designed.
`The present invention further provides an improved pro
`ceSS for fabricating the micro-electro-mechanical Structure
`with its improved design for opposing, interdigitated pro
`jections consistent with general bond/etch-back methods of
`fabrication. The proceSS basically includes the Steps of
`providing a first Substrate, etching a cavity within the first
`Substrate, and forming an isolation layer on the first Sub
`Strate. Further Steps include providing a Second Substrate,
`doping the top portion of the Second Substrate to thereby
`form an etch termination layer, forming all doped epitaxial
`layer on the etch termination layer portion of the Second
`Substrate Such that the etch termination layer portion of the
`Second Substrate has a higher doping concentration than the
`epitaxial layer. Then, the Second Substrate is bonded to the
`first Substrate Such that the epitaxial layer covers the cavity
`and is bonded to the isolation layer at the periphery of the
`cavity of the first Substrate. Then, the non-termination layer
`portion of the second substrate is removed from the etch
`termination layer portion of the Second Substrate, and the
`etch termination layer portion of the Second Substrate is
`removed from the epitaxial layer. A photoresist is then
`applied on the epitaxial layer, and the photoresist is pat
`terned according to a predetermined shape of the micro
`electro-mechanical Structure. Thereafter, a step of anisotro
`pically etching through Sections of the epitaxial layer, as
`revealed by the patterned photoresist, is performed to
`thereby define and release the micro-electro-mechanical
`Structure above the cavity,. The remaining patterned photo
`resist is then removed.
`According to a preferred process of the present invention,
`the Step of doping the top portion of the Second Substrate to
`thereby form an etch termination layer preferably includes
`the Step of doping the top portion of the Second Substrate
`with a p-type dopant comprising boron and germanium. In
`addition, the Step of forming a doped: epitaxial layer pref
`erably includes the Step of doping the layer with a p-type
`dopant. Furthermore, the first Substrate and the Second
`Substrate preferably comprise Silicon, and the isolation layer
`preferably comprises Silicon dioxide.
`Also according to the preferred process of the present
`invention, the Step of applying photoresist on the epitaxial
`layer includes the Step of utilizing a positive photoresist. In
`addition, the Step of anisotropically etching through the
`epitaxial layer to, define and release the micro-electro
`mechanical Structure above the cavity preferably includes
`the Step of contacting the epitaxial layer with a plasma
`comprising Sulfur hexafluoride and oxygen, and the Step of
`cooling the epitaxial layer to a cryogenic temperature of leSS
`than about 173 EK.
`Further, according to the preferred process of the present
`invention, the Step of pattering the photoresist according to
`a predetermined shape preferably includes the Steps of
`determining a minimum capacitive gap between the inter
`digitated projections of the micro-electro-mechanical Struc
`ture which are nearest to each other, defining the predeter
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`4
`mined shape Such that each base of each projection is
`proximate to at least one tip of another projection by a
`distance Substantially equal to the minimum capacitive gap,
`and Selectively removing the photoresist to reveal bare
`Sections of the epitaxial layer according to the predeter
`mined shape.
`Other advantages, Structural and process design
`considerations, and applications of the present invention will
`become apparent to those skilled in the art when the detailed
`description of the best mode contemplated for practicing the
`invention, as Set forth hereinbelow, is read in conjunction
`with the accompanying drawings.
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`BRIEF DESCRIPTION OF THE DRAWINGS
`The present invention will now be described, by way of
`example, with reference to the following drawings.
`FIG. 1 is a top view of a Sensing element for a rotational
`accelerometer MEMS device;
`FIGS. 2(A) through 2(D) are cross-sectional views of the
`structure illustrated in FIG. 1 along section lines A-A, B-B',
`C-C and D-D', respectively;
`FIG. 3 is a partial top view of the structure illustrated in
`FIG. 1, particularly highlighting the cantilevers, and
`FIGS. 4(A) through 4(M) illustrate the primary steps and
`stages of the preferred process for fabrication of a MEMS
`Structure having opposing, interposed and interSpaced pro
`jections according to the present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`Referring to FIGS. 1 and 2, a rotary accelerometer sensor
`40 comprises a generally circular, Semiconductor mass 10
`Suspended relative to an annular Support layer 54 by four
`equally spaced radially extending, flexible Suspension pro
`jections 18. The projections extend into the body 10, are of
`relatively thin Section, terminate in large-area tabs 19 and
`provide both rotary compliance and translational Stiffness.
`Projections 18 form the Suspension system for body 10 and
`do not, for all practical purposes, affect the capacitance as
`hereinafter explained.
`Between each suspension projection 18, body 10 is
`formed to define a group of equally Spaced and essentially
`constant width capacitive projections 20 which are integral
`with body 10 but extend radially outwardly therefrom.
`Projections 20 have rounded tips 24 which lengthen the
`Sensor gap as hereinafter explained. The projections 20
`effectively form one of two opposed capacitor plates as
`hereinafter explained. The Suspension projections 18, for all
`practical purposes, do not form capacitor elements.
`The FIG. 1 structure further comprises a four-piece rim
`Structure collectively defining the Second capacitor plate.
`The rim Structure comprises four identical quadrants each
`including a rim element 16 having opposite end areas
`adjacent but Spaced from a tab 19 and tapering, triangular,
`inwardly-projecting capacitive projections 30 having wide
`base areas 32 and rounded tips 34. Each projection 20 has
`one side lying adjacent and in closely and uniformly spaced
`relationship to a complemental side of a projection 30 to
`form a primary capacitive gap. Moreover, the rim Structure
`is etched Such that the capacitive gap continues around the
`tips 24 and 34 to define an S-shape. The other sides of the
`projections 20 and 30 are more widely spaced from each
`other; i.e., two or three times the Spacing of the primary gap,
`to greatly reduce the capacitive coupling therebetween. The
`circular body 10 is widely spaced from rim 16 so as to
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`reduce capacitive coupling in the radial direction except at
`the tips of the projections.
`The result is a thin-film structure in which the four rim
`quadrants with their projections 30 can be electrically con
`nected to form one plate of a capacitor while the body 10
`with its projections 20 form the other plate. Complementary
`external electrical connections and components may be as
`disclosed in the Mastache patent the disclosure of which is
`incorporated herein by reference. When subjected to rota
`tional acceleration about the center axis of the proof mass,
`the Suspension projections or tethers 18 fleX to permit
`angular movement of the proof mass and the outwardly
`extending fingers 20 relative to the rim structure of the
`inwardly extending fingers 30. This produces capacitive
`changes due to Spacing variations in the primary gaps. The
`Suspension elements 18 function essentially Solely in a
`mechanical Support, flexure Suspension role and do not
`materially contribute to output Signal quality.
`The structure of FIGS. 1 and 2 incorporates two structural
`design advantages for a MEMS device having opposing
`interposed and interSpaced projections which will circum
`vent the adverse effects associated with the etch lag phe
`OCO.
`Concerning the first design advantage, the projections 20,
`according to the present invention, are relatively uniform in
`width along their lengths and have a high length to width
`ratio. The projections 30, on the other hand, are pyramidic
`in shape Such that their sides are not parallel. As a result,
`however, one side of the first projection 20 and one side of
`the Second projection 30 of each projection pair is Substan
`tially uniformly spaced apart from each other, along the
`length of the first projection 20, by a distance substantially
`equal to the minimum capacitive gap. Projections 20 and 30
`can, of course, have Various shapes, e.g., Straight, angled and
`curved, So long as the Sensor gaps between them are of
`uniform width. These projections 20 and 30, having such
`desirable dimensions and features, are prepared by the
`method of the invention which-avoids the over-etching
`asSociated with the etch lag phenomenon along wide
`trenches. As a result, the electrical characteristics (such as
`resistance and capacitance levels) inherent in the thicker and
`wider Structure of each projection.(finger) are at desired
`levels and are no longer adversely affected due to exceSS
`thinning of each projection due to the over-etching associ
`ated with the etch lag phenomenon.
`AS a Second design advantage, the tips 24 of the projec
`tions 20, according to the present invention, are proximate to
`the rim 16. In particular, the tips 24 are spaced from the rim
`16 by a distance Substantially equal to the capacitive gaps
`36. The rim 16 is preferably shaped such that the circum
`ference of each tip 24 of each first projection 20 is substan
`tially uniformly spaced from the rim 16 by a distance
`Substantially equal to the primary capacitive gap. Such
`relatively close spacing between the tips 24 and the rim 16
`is made possible by the method of the invention which
`avoids the over-etching associated with the etch lag phe
`nomenon. The method of the invention avoids the tendency
`of prior art methods to excessively etch away tipS 24.
`Advantageously, the tips 34 of the projections 30, accord
`ing to the present invention, are proximate to the body mass
`10 to circumvent the effects associated with etch lag. In
`particular, the tips 34 are spaced from the body mass 10 by
`a distance Substantially equal to the capacitive gaps 36. Such
`a close spacing between the tips.34 and the body mass 10
`ensures that over-etching associated with the etch lag, phe
`nomenon will neither excessively etch away the tips 34 nor
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`excessively etch into the body mass 10. The body mass 10
`is preferably shaped Such that the circumference of each tip
`34 of each second projection 30 is substantially uniformly
`spaced from the body mass 10 by a distance substantially
`equal to the primary capacitive gap.
`Furthermore, concerning the Second design advantage,
`the natural Structural consequence of the closer spacings
`between the tips 24 and the rim 16 is that at least one side
`of each of the bases 32 associated with the projections 30 is
`proximately located near one of the tips 24 by a distance
`Substantially equal to the capacitive gap 36. Such a closer
`spacing ensures that over-etching associated with the etch
`lag phenomenon will neither excessively etch into each of
`the bases 32 nor excessively etch away the tips 24.
`Likewise, the natural Structural consequence of the closer
`spacings between the tips 34 and the body mass 10 is that at
`least one side of each of the bases 22 associated with the
`projections 20 is proximately located near one of the tips 34
`by a distance Substantially equal to the capacitive gap 36.
`Such a closer spacing ensures that over-etching associated
`with the etch lag phenomenon will neither excessively etch
`into each of the bases 22 nor excessively etch away the tips
`34.
`Ultimately, as a result of the preferred structure in FIG. 1,
`the bases 22 of the projections 20 and the bases 32 of the
`projections 30 no longer have the tendency to be extraordi
`narily thin and fragile due to the etch lag phenomenon. Thus,
`the preferred Structure according to the present invention
`helps eliminate the possibility that the projections 20 and the
`projections 30 may break off.
`FIGS. 2(A) through 2(D) are cross-sectional views of the
`structure illustrated in FIG. 1 positioned over a cavity 52 in
`a substrate 50. An isolation layer 54 covers the substrate 50
`as well as the lining of the cavity 52. The semiconductor leer
`14 is mounted on the isolation layer 54 at the periphery of
`the cavity 52 such that the body mass 10 is suspended above
`the cavity 52 via cantilever suspension projections 18.
`Capacitive gaps 36 between the tipS 24 of the first projec
`tions 20 and the rim 16, and capacitive gaps 36 between the
`tips 34 of the second projections and the body 10 are
`particularly highlighted in FIGS. 2(A) and 2(B). The canti
`levers 18 are attached to the body mass 10 at points 76. The
`first projections 20 and the second projections 30 are defined
`in the Semiconductor layer 14.
`FIG. 3 is a partial top view of the structure illustrated in
`FIG. 1, particularly highlighting the cantilever 18. Accord
`ing to the preferred embodiment of the present invention, the
`sense structure 40 has at least one cantilever 18 connected
`between the body 10 and the rim 16. Each cantilever 18
`thereby flexibly mounts the body 10 to the rim 16 Such that
`the body 10 along with the rigid projections 20 are capable
`of rotational movement relative to the fixed Surrounding
`Structure including the projections 30 extending from the
`rim 16. The ideal gap Surrounding the Suspension projec
`tions 18 is greater than the minimum (sensor) gap between
`projections 20, 30 and equal to or Smaller than the parasitic
`gap. Each cantilever 18, the Semiconductor layer 14, the
`body 10, the first projections 20, and the second projections
`30 are comprised of an electrically conductive, doped Semi
`conductor material Such that the differential capacitance
`between the first projections 20 and the Second projections
`30 can be electrically measured whenever the MEMS sense
`Structure 40 experiences rotational acceleration caused by an
`external Stimulus.
`It is to be understood that the particular sense structure 40
`for use in a capacitive rotational accelerometer, as illustrated
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`in the figures, is only one of many different possible MEMS
`Structures that can incorporate and benefit from the teach
`ings of the present invention. In general, the novel aspects of
`the present invention can be utilized and incorporated in
`other MEMS structures having interdigitated projections as
`well.
`The structure of a MEMS device may generally be
`fabricated by a bond/etch back technique. According to a
`past implementation of this technique, a first Semiconductor
`Substrate is formed and a cavity is thereafter etched into this
`first SubStrate. Next, an oxidation Step is carried out to
`thereby form an oxide layer (that is, an, isolation layer) over
`the surface and cavity of the first substrate. In addition to this
`first Substrate, a Second Semiconductor Substrate is formed
`separately from the first Substrate. The top portion of this
`Second Substrate is typically very highly doped (that is, is
`highly concentrated) with p-type impurities, Such as boron
`and/or germanium, to thereby create an etch termination
`layer (also sometimes referred to as an etch stop layer or a
`barrier layer). From a semiconductor fabrication and pro
`cessing standpoint, attempted etching with an ICP DRIE
`(inductively coupled plasma deep reactive ion etch) machine
`through Such a termination layer comprised of highly
`p-doped Silicon, for example, is greatly attenuated. Next, a
`lightly doped epitaxial Semiconductor layer (Sometimes
`referred to as an “epi-layer') is grown on top of the Second
`substrate. This epitaxial layer is to be the layer from which
`the structure of the MEMS device is ultimately defined and
`released.
`Further regarding the past implementation of the bond/
`etch back technique, once the epitaxial layer is properly
`formed on the Second Substrate, the Second Substrate along
`with its epitaxial layer is then inverted and bonded over the
`cavity in the first Substrate Such that the epitaxial layer
`covers the cavity and is bonded to the oxide layer (that is,
`isolation layer) at the periphery of the cavity. In this inverted
`configuration, the epitaxial layer is thus situated directly
`above the cavity, and the highly p-doped portion (that is, the
`etch termination layer portion) of the Second Substrate is on
`top of the epitaxial layer. After bonding and etch back is
`completed, an etch process Step is then typically attempted
`to precisely etch deep trenches through both the highly
`p-doped portion (the etch termination layer portion) of the
`Second Substrate and the epitaxial layer until the cavity
`underneath these layerS is breached. In this way, the remain
`ing portions of the etch termination layer portion of the
`Second Substrate and the remaining portions of the epitaxial
`layer are together released and Suspended above the cavity.
`These remaining unetched portions will then Serve as the
`micro-machined structure of a MEMS device.
`A significant problem with the particular bond/etch back
`technique described above is that attempting to etch through
`both the highly p-doped portion (that is, the etch termination
`layer) of the Second Substrate and the lightly doped epitaxial
`layer Simultaneously, during the same etching proceSS Step,
`often produces very poor and uneven Sidewall profiles in the
`trenches being etched through these two layers. This is
`especially the case for the Sidewalls of the trenches etched
`into the epitaxial layer. In particular, once the highly
`p-doped portion of the Second Substrate is etched through,
`the sidewall profiles of the trenches etched into the epitaxial
`layer are typically not anisotropic in form. That is, the
`Sidewalls of the trenches are not Substantially vertical and
`Smooth, but are instead heavily striated or Somewhat iso
`tropic in form with undesired lateral etching into the. Side
`walls of the trenches. Additionally, the Silicon (for example)
`of the epitaxial layer may be undesirably micro-masked as
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`a result of the etch termination layer being incompletely
`etched, thereby undesirably causing Silicon “spires' or
`“grass” to be formed on the sidewalls and bottoms within the
`epitaxial layer trenches. Such uneven etching through the
`epitaxial layer is most likely attributable to the disparity in
`the etch rates inherent in the highly p-doped portion (that is,
`the etch termination layer portion) of the Second Substrate
`and the lightly doped epitaxial layer. Of most concern,
`however, is that Such lateral etching into the Sidewalls of the
`trenches formed in the epitaxial layer ultimately produces a
`MEMS device structure which is malformed and rendered
`unfit for customer use. For instance, any Silicon “spires' or
`“grass” undesirably formed within the trenches of the epi
`taxial layer often become particulates when the cavity
`underneath the epitaxial layer is breached during etching.
`These particulates can prevent or interfere with rotational
`translation of the Suspended body, thereby directly hindering
`or preventing proper operation of the MEMS structure.
`Furthermore, these particulates can also undesirably physi
`cally bridge the gaps between the “capacitor plates' of the
`first projections and the Second projections, thereby electri
`cally shorting the first projections and the Second projections
`together and rendering the MEMS structure useless. Thus, as
`a result, utilization of the particular technique described
`above can produce a relatively low product yield. The
`method according to the present invention significantly
`improves upon past implementations of the bond/etch back
`technique and produces anisotropic etching (that is, Vertical
`and Smooth trench Sidewalls) through the epitaxial layer
`from which a MEMS structure is to be formed. FIGS. 4(A)
`through 4(M) illustrate the primary steps and stages of the
`preferred method/process for fabrication of the preferred
`MEMS structure according to the present invention.
`As illustrated in FIG. 4(A), a first substrate 50 made from
`Semiconductor material(s) is initially formed and provided.
`According to the preferred embodiment of the present
`invention, the first Substrate 50 is made primarily of silicon.
`However, the first Substrate 50 can instead be comprised
`with other materials as well, Such as, for example, glass,
`ceramic, Sapphire, and Stainless Steel. Furthermore, the first
`substrate 50 can be doped (or not doped at all) with either
`n-type or p-type impurities at any doping concentration
`level. This first substrate can be formed by any acceptable
`method known in the art.
`As illustrated in FIG. 4(B), a cavity 52 is then etched into
`the first substrate 50. The cavity 52 can be formed by any
`known conventional mean