WO2002019509A2

WO2002019509A2 - Comb drive assembly having comb drive fingers with varying cross-sections and methods of making same

Info

Publication number: WO2002019509A2
Application number: PCT/US2001/026775
Authority: WO
Inventors: Kenneth M. Hays
Original assignee: Honeywell International Inc.
Priority date: 2000-08-28
Filing date: 2001-08-28
Publication date: 2002-03-07
Also published as: WO2002019509A3

Abstract

A MEMS comb drive assembly and associated methods of fabrication are provide that are capable of gap variance between adjacent interdigitated comb drive fingers during comb drive operation. Gap variance allows for an increase in motive force by increasing the average capacitance variation of the comb structure as the distance between the comb fingers varies. The comb drive assembly includes first and second comb drive members that each comprise a series of comb drive fingers. The comb drive fingers will characteristically have cross-sections that vary in area along any lengthwise position of the comb drive finger. This variance in cross-sectional area provides for a gap differential between adjacent interdigitated comb drive fingers when the comb drive assembly is operational. As the gap varies during comb drive operation, the average variance in capacitance between opposing comb drive fingers increase, thereby increasing the motive force capabilities of the comb drive assembly. In one embodiment of the invention, the comb drive fingers are configured as wedge shaped structures in a plan view; moreover the comb drive fingers are frusto wedge shaped structures in a plan view. The wedge shaped configuration allows for the comb drive fingers of opposing comb drive members to be interdigitated such that the gap between adjacent comb fingers varies as the fingers are placed into motion. The frusto wedge shaped embodiment allows for the end of the body of the comb finger furthest from the base of the comb drive member to be truncated, so that ends of the comb drive fingers do not repeatedly contact the base of the opposing comb drive member during normal operation. In a further embodiment of the invention the comb drive assembly is a rotary comb drive assembly capable of imparting rotary motion to a torsional resonance plate. The invention further provides for methods of fabricating comb drive fingers.

Description

COMB DRIVE ASSEMBLY HAVING COMB DRIVE FINGERS WITH VARYING CROSS-SECTIONS AND METHOD OF MAKING SAME

FIELD OF THE INVENTION The present invention relates generally to comb drive assemblies for providing motion in microelectromechanical devices. More particularly, the present invention provides a comb drive assembly having comb drive fingers with non-uniform cross- sections which can thereby increase the force generated by the comb drive fingers with less input voltage.

BACKGROUND OF THE INVENTION . An important development in the area of electronics has been the application of traditional silicon integrated circuit fabrication techniques to fabricate microelectromechanical system (MEMS) devices. Specifically, an important design goal for many electronic applications has been the miniaturization of the components of the devices and circuit used in the electronic application. While the advent of silicon integrated circuit fabrication techniques for fabricating integrated circuits has helped to decrease the size of electronic circuits, until recently, electronic applications that also included machines or movable parts were limited in their miniaturization by manufacturing constraints related to manufacturing these parts from traditional mechanical materials. For this reason, traditional silicon integrated circuit fabrication techniques for fabricating integrated circuits are now also used to fabricate MEMS devices from silicon wafers. Compared to more traditional micro-machines manufactured from metallic materials, MEMS devices are typically several times smaller, more cost effective and provide higher reliability.

An important aspect of many MEMS devices is the drive assembly used to create movement or motion in MEMS devices. One drive assembly that is used for a wide variety of applications is a comb drive assembly. Comb drive assemblies are described in detail in United States Patent No. 5,025,346, entitled "Laterally Driven Resonant Microstructures", issued on June 18, 1991, in the name of inventors Tang et al. The contents of this patent are hereby incorporated by reference as if setforth fully herein. With reference to the plan view perspective of FIG. 1 a typical, prior art, comb drive assembly 10 includes first and second comb drive members, 12 and 14, respectively, that are positioned such that they face each other. Importantly, the first and second comb drive members of the comb drive assembly include a series of comb drive fingers 16 that are interdigitated with the comb drive fingers of the other drive member. Further, the first and second comb drive members also include electrical contacts 18 for making an electrical connection between a voltage source, not shown, and the comb drive fingers which serve as electrodes. hi operation, to induce motion in the MEMS device, an electrical signal is applied across the comb drive fingers. This electrical signal energizes a capacitance in the gap between the comb drive fingers. This energized capacitance provides a motive force to drive the comb drive fingers of the first and second drive assemblies together. Either one or both of the comb drive members are connected to a moveable mass 20. In the comb drive assembly shown in FIG.l, the energized capacitance in the gap 30 between the comb drive fingers provides the motive force necessary to set the moveable mass into motion. The movable mass is in mechanical communication with the second comb drive member 14 and is anchored to the underlying substrate 22 by anchors 24. Suspension springs 26, or another type of bias force, connect the moveable mass to the anchors and are used to allow for the linear motion of the moveable mass and to drive the comb drive assemblies away from each other as the voltage of the electrical signal provided to the comb drive decreases. Additionally, as depicted, the first comb drive member 12 is a stationary structure that is anchored to the underlying substrate by anchor 28. The linear motion provided by comb drive assemblies of this nature can be used for various purposes including switches, relays, resonators and other similar devices.

Although comb drive assemblies, similar to those described above, are advantageous for providing motion in MEMS devices, there are some drawbacks and limitations associated with current comb drive assemblies. Specifically, as discussed above, the motive force generated by the drive comb fingers is due to the energized capacitance in the gap between the individual comb drive fingers. The force generated by the comb drive fingers is inversely proportional to the width of the gap that exists between the interdigitated teeth of the respective comb drive members. Since the gap between the comb teeth is constant and does not vary as voltage is applied to the comb drive members (i.e. as the comb drive teeth move in response to the voltage supplied), so too is the amount of force generated by the overall comb drive assembly. Therefore, it is necessary to increase the amount of voltage supplied to the comb drive assembly or provide for a larger comb drive assembly in order to increase the motive force that is generated by the comb drive assembly.

However, providing for larger comb drive assemblies and/or increasing the amount of voltage supplied to the comb drive assembly does not aid in producing an efficient MEMS device. Larger comb drive assemblies dictate more space on the substrate and, thus, lead to larger MEMS chip sizes. As MEMS device chip size increases, so too does the attendant per-chip cost. Additionally, while higher voltages can be applied to the comb devices as a means of providing for greater motive force, higher voltages will typically mean more expensive electronics that characteristically will occupy more space on the chip or require off-chip electronics at additional expense. Previous attempts at higher efficiency comb drive designs have resulted in marginal success. Most of these alternative designs have focused on the overall geometry of the comb drive assembly, the height-to-width ratio of the comb drive teeth, or the composition of the comb drive electrodes. See for example, United States Patent No. 6,000,280, entitled "Drive Electrodes for Microfabricated Torsional Cantilevers", issued on December 14, 1999, in the name of inventors* Miller et al. or United States Patent No. 5,969,848, entitled "Micomachined Electrostatic Vertical Actuator", issued on October 19, 1999, in the name of inventor Lee et al. These patents disclose various alternative comb drive designs that attempt to provide for higher efficiency comb drive assemblies. However, these designs provide for more intricate comb drive assemblies or comb drive teeth. As such, they have shown to be difficult to fabricate due to more stringent fabrication tolerances (i.e. higher aspect ratios) and the involvement of more precise masking and etching techniques.

A need exists to develop a comb drive assembly that increases the motive force per applied voltage ratio. This would allow for a reduction in the complexity and expense of drive electronics and/or an increase in the number of MEMS devices per wafer, thus increasing yield and decreasing fabrication costs. Additionally, it would be highly advantageous to increase the motive force provided by the comb drive assembly without increasing the complexity of the fabrication process. A comb drive that exhibits increased motive force without the need to impart greater voltage or complex fabrication would have widespread use with any MEMS fabrication suitable for comb drive implementation. Comb drive assemblies that feature such attributes are highly desirable in high volume commercial applications, such as sensors for the automotive industry.

SUMMARY OF THE INVENTION A MEMS comb drive assembly and associated methods of fabrication are therefore provided that are capable of providing for gap variance between adjacent interdigitated comb drive fingers during comb drive operation. Gap variance allows for an increase in motive force by more rapidly increasing the average capacitance across the comb structure as the distance between the comb fingers decreases.

The comb drive assembly includes first and second comb drive members that each comprise a series of comb drive fingers. The comb drive fingers will characteristically have cross-sections that vary in area along any lengthwise position of the comb drive finger. This variance in cross-sectional area provides for a gap differential between adjacent interdigitated comb drive fingers when the comb drive assembly is operational. As the gap varies during comb drive operation, the average variance in capacitance between opposing comb drive fingers increases nonlinearly, thereby increasing the motive force capabilities of the comb drive assembly.

In one embodiment of the invention, the comb drive fingers are configured as wedge shaped structures in a plan view, moreover the comb drive fingers are frusto wedge shaped structures in a plan view. The wedge shaped configuration allows for the comb drive fingers of opposing comb drive members to be interdigitated such that the gap between adjacent comb fingers varies as the fingers are placed into motion. The frusto wedge shaped embodiment allows for the end of the body of the comb finger furthest from the base of the comb drive member to be truncated, so that ends of the comb drive fingers avoid repeatedly contacting the base of the opposing comb drive member during typical operation, thereby avoiding mechanical damage. In a further embodiment of the invention the comb drive assembly is a rotary comb drive assembly capable of imparting rotary motion to a torsional resonance plate. The comb drive fingers of this embodiment extend in an arc such that the lengthwise extension of the body of the comb drive finger is arcuate. Additionally, the comb drive fingers of this embodiment may be wedge-shaped structures and, moreover, frusto wedge-shaped structures.

The invention is additionally embodied in a method for fabricating comb drive fingers for a comb drive assembly. The method comprises providing for a suitable workpiece, applying a layer of photoresist to a surface of the workpiece, patterning the photoresist to, define at least one comb drive finger having a varying cross-section in the lengthwise direction and etching away portions of the workpiece defined by the patterned photoresist to thereby form comb drive fingers. Additionally, the method may comprise attaching a substrate to the workpiece, etching through the workpiece into the substrate when etching the pattern defined by the patterned photoresist to thereby form comb drive fingers and dissolving the unetched portions of the substrate to remove the substrate from the comb drive fingers.

The comb drive assemblies of the present invention provide for a significant increase in the motive force per applied volt. This increase in force is accomplished by achieving an increase in capacitance variation between comb drive members. The increased capacitance variance results from the gap variance between adjacent fingers during comb drive operation. As a result of this increase in motive force it is possible to reduce the complexity of drive electronics, thereby minimizing area consumption on the wafer and minimizing material costs. By changing the geometric configuration of the comb drive fingers to allow for gap variance, these benefits can be realized without increasing the complexity of the comb drive fabrication. MEMS devices employing the use of the comb drive assemblies of the present invention have wide spread potential for use in many applications that require increased motive force without adding additional complexity or cost to the overall MEMS device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.l is a plan view illustration of a MEMS comb drive assembly used to provide for linear motion, in accordance with the prior art.

FIG. 2 is a plan view illustration of a MEMS comb drive assembly used to provide for linear motion, in accordance with an embodiment of the present invention. FIG. 3 is an enlarged plan view diagram of opposing MEMS comb drive members having comb drive fingers that vary in cross-section along the lengthwise direction of the finger, in accordance with an embodiment of the present invention. FIG. 4 is an enlarged plan view diagram of opposing MEMS comb drive members having comb drive fingers that have constant cross-sections along the lengthwise direction of the fmger, in accordance with the prior art.

FIG. 5 is a plan view illustration of half portions of two adjacent MEMS comb drive fingers and corresponding dimensional indicators, in accordance with an embodiment of the present invention.

FIG. 6 is a plan view illustration of a MEMS rotary comb drive assembly having comb drive fingers that vary in cross-section along the lengthwise direction of the finger, in accordance with an embodiment of the present invention. FIG. 7 is an enlarged plan view illustration of opposing MEMS rotary comb_' drive members having comb drive fingers ,that vary in cross-section along the i ^■ > v ^• . > lengthwise direction of the fmger, in accordance with an embodiment of the present invention. .:

FIGS. 8 A - 8D are isometric diagrams of various stages in the fabrication - process of MEMS comb drive assemblies having comb drive fingers that vary in cross- section along the lengthwise direction of the fmger, in accordance with an embodiment of the present invention.

FIG. 9 is a plan view illustration of alternate shapes of comb drive fingers in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

FIG. 2 is. a plan view of a comb drive assembly 40 having comb drive fingers 42 that have a varying cross-section in the lengthwise direction, in accordance with an embodiment of the present invention. The comb drive assembly, which is fabricated on a substrate 44, includes first and second comb drive members, 46 and 48, respectively, that are positioned such that they face each other. The first and second comb drive members of the comb drive assembly include, at least one and, typically, a series of comb drive fingers 42 that are interdigitated with the comb drive fingers of the other drive member. The comb drive fingers of the present invention are shaped so as to have a varying cross-section in the lengthwise direction. In one embodiment of the invention the comb drive fingers are characteristically wedge shaped and, moreover, frusto wedge-shaped (i.e. the wedge shaped finger is truncated or truncated at the end farthest from the base of the comb drive).

The comb drive assembly may further comprise electrical contacts 50 formed on the substrate 44 for making an electrical connection between a voltage source (not shown in FIG 2) and the comb drive members 46 and 48. As electrical voltage is supplied to the comb drive members, the comb drive fingers act as electrodes and the capacitance is energized across the gap 52 between the comb drive fingers. Either one or both of the comb drive members are connected to a moveable mass. In the comb drive assembly shown in FIG. 2, the capacitance across the gap between the comb drive fingers provides the motive force necessary to set the moveable mass 54 into motion. The movable mass is in mechanical communication with the second comb drive member 48 and is anchored to the underlying substrate 44 by anchors 56. Suspension springs 58 or another type of bias force, connect the moveable mass to the anchors and are used to allow for the linear motion of the moveable mass. Additionally, as depicted, the first comb drive member 46 is a stationary structure that is anchored to the underlying substrate by anchor 60.

In accordance with the present invention, the comb drive fingers 42 have varying cross-sections in the lengthwise direction, as shown in the enlarged plan view perspective of FIG. 3. As depicted the comb drive fingers are frusto wedge-shaped having a truncated end 62 furthest from the base of the comb drive member. The truncated end of the fingers do not repeatedly contact the base of the opposing comb drive member during normal operation. In comparison, prior art, comb drive fingers 16 having a constant cross-section in the lengthwise direction are shown in the plan view perspective of FIG. 4. In operation, to induce motion in the MEMS device, an electrical signal is applied across the interdigitated comb drive fingers by applying a corresponding electrical signal across the electrical contacts 50. This electrical signal energizes a capacitance across the gap between the comb drive fingers. This capacitance provides a motive force to drive the interdigitated comb drive fingers of the first and second drive assemblies together. In the present invention, as the comb drive members move relative to each other the capacitance between the interdigitated comb drive fingers changes nonlinearly per unit of displacement the comb drive members. In this fashion, the magnitude of the electrostatic force supplied by the comb drive assembly is inversely proportional to the width of the gap between the interdigitated comb drive fingers.

As shown in FIG. 3, in the present invention the gap 52 between the comb drive fingers 42 decreases as the comb drive members 46 and 48 are drawn closer to one another and the gap becomes larger as the comb drive members are moved apart. As shown in FIG. 4, in the prior art comb drive assemblies the gap 30 between the comb drive fingers 16 remains constant as the comb drive members 12 and 14 are drawn closer to one another or moved apart, hi the present invention the varying gap between the comb drive fingers increases the motive force that can be applied by the comb drive assembly as the interdigitated fingers move closer together (i.e. become more intermeshed). This increase in motive force is due to the increase in the average capacitance across the gap as the gap distance between the fingers decreases. As previously stated, in principle the motive force exerted by the comb drive assembly is inversely proportional to the gap between the comb drive fingers. As such, as the gap decreases, the motive force increases. This increase in motive force due to comb drive finger geometry is accomplished without a need to increase the voltage that is applied to the comb drive members. As a result, comb drive assemblies can be fabricated that result in heightened motive forces, thus, allowing for a reduction in drive electronics complexity and/or an increase in the number of MEMS devices per semiconductor wafer.

Geometric Design Criteria

FIG. 5 is a plan view of half-portions of adjacent comb drive fingers having varying cross sections in the lengthwise direction, in accordance with an embodiment of the present invention. FIG. 5 is provided to assist in understanding the dimensional relationship that exists between the fingers in a properly designed comb drive assembly having frusto wedge shaped comb drive fingers. Proper attention to the design dimensions of the comb drive fingers is necessary to insure that the gap (g) between adjacent comb drive fingers can be minimized during operation to the furthest degree possible without having opposing comb drive members become non-operational (i.e. jam) during maximum amplitude operation. As shown, p is the distance between the centerlines of adjacent comb drive fingers, t is the width at the truncated end of the comb drive finger, w is the width at the base of the comb drive finger and / is the length of the comb drive fmger from the base to the truncated end. Additionally, g defines the gap between adjacent comb drive fingers, x is the peφendicular length from the base of a finger of the first comb drive member to the base of a finger of a second comb drive member, s is the sloped distance by which opposing fingers overlap and θ is the angle of slope for the comb drive fingers. The angle of slope of the comb drive finger is defined in relationship to width at the base of the comb drive finger, the width at the truncated end of the comb drive fmger and the length of the comb drive finger, as:

w-t tan # =

21

Therefore, the geometric equivalent relationships for cosine and sine are defined as:

Having defined the angle of slope of the comb drive finger it is then possible to derive an equation to define the gap between adjacent comb drive fingers as a relationship of the distance between centerlines of adjacent fingers, the width at the truncated end of the comb drive, the length of the comb drive finger from the base to the truncated end, and the varying length from the base of the finger of the first comb drive member to the base of the finger of the second comb drive member, as:

In theoretical operation when the comb drive fingers reach a position of movement such that the tips of the comb drive fingers on one side contact the anchor to which the drive fingers on the opposite side are attached, then the gap between the fingers would be minimum (g=g_m) and the length from the base of the finger of the first comb drive member to the base of the fmger of the second comb drive member would equal the length of a comb drive finger (x=l). (In practice, to prevent damage to the comb fingers, the drive amplitude should not be high enough to repeatedly cause such contact.) In such an instance, the minimum gap g_m between the fingers can be defined as:

W+ t^λ

&- m = cos θ In the non-operational state, prior to applying an electrical excitation across the capacitance between the comb drive members the gap between adjacent comb drive fingers would rest at its equilibrium value (g=gi) the length from the base of the fmger of the first comb drive member to the base of the fmger of the second comb drive member would rest at its equilibrium value (x=xi). In such an instance, the equilibrium value g_t of the gap between the fingers can be defined as:

The two above equations that define the minimum and equilibrium values (g_m andgv, respectively) of the gap between adjacent comb drive fingers can be combined with the cosine and sine relationships and solved for the width of a comb drive finger and at its base w as follows:

where the subscript i refers to the equilibrium (undisplaced) value of the associated parameter and where g_m is the value of g when the truncated ends of the comb teeth on one side contact the plate to which the opposing teeth are joined (i.e. x = I).

Correspondingly, the width t of the comb drive finger at the truncated end is defined by:

Electrostatic Analysis

The significant increase in the motive force generated by comb drive assemblies utilizing comb drive fingers having varying cross-sections in the lengthwise direction can be analytically compared to prior art comb drive filters. For a comb drive finger having a thickness, λ, (i.e. the height of the finger, not shown in the plan view) the capacitance (C(x)) can de defined in relation to the thickness (λ), the sloped distance by which opposing fingers overlap (s), the gap between adjacent comb drive fingers (g), and the electrical permitivity of space (ε₀), as follows:

C(x) = ε₀ g

The varying sloped distance by which opposing fingers overlap (s) can be derived from the length of the comb drive finger from the base to the truncated end (I), the angle of slope for the comb drive fingers (θ) and the length from the base of a finger of the first comb drive member to the base of a finger of a second comb drive member (x), as follows:

21 — x s =

COS 0 The initial (undisplaced) sloped distance by which opposing comb drive fingers overlap (si) is defined as follows:

As such, the varying sloped distance by which opposing fingers overlap (s) can be defined in terms of the initial (undisplaced) sloped distance (si), as follows:

X — X; s - s_; — cos#

The varying gap between adjacent comb drive fingers (g) can be derived from the initial gap between adjacent comb drive fingers (gi), the initial and varying length from the base of a finger of the first comb drive member to the base of a finger of a second comb drive member (x,* and x) and the angle of slope for the comb drive fingers (θ), as follows:

Therefore, the capacitance that exists across the gap between adjacent comb drive fingers can be defined as:

21 — x

C(^χ) = ^£°^λ

COS 0 g_i + (x-x_i) sinθ

Thus, the net mechanical energy output (AE) due to electrostatic forces drawing the opposing sets of comb teeth together as the base-to-base separation goes from x=x_t+ Ax to x=Xj-Δx and applied voltage (V) remains constant can be derived from the variation in the capacitance (C(x)) foraied by the gap (g). This relationship is defined as follows (Note: This equation neglects fringing of the electric field):

(To avoid damage caused by comb-to-comb contact, D Δx D <x,-l.) Combining the above capacitance (C(x)) equation and net change in energy equation (AE), the net mechanical output energy can be defined as:

For prior art comb drive assemblies having comb drive fingers of constant cross- section in the lengthwise direction, the angle of slope for the comb drive fingers equals zero (θ= 0). This reduces the net mechanical output energy equation to the following:

Comparing the change in energy exhibited by a comb drive assembly having fingers with varying cross-sections in the lengthwise direction versus the change in energy exhibited by a comb drive assembly having fingers with constant cross-section in the lengthwise direction the resulting comparison will, typically, show a significant increase in the average force (i.e. mechanical energy output over a given comb finger displacement) that can be applied by the comb drive assembly of the present invention. The quantitative value of the increase will depend on the geometrical parameters used in a particular comb drive application.

In application, both analytically and experimentally, a greater than two-fold increase in the force per applied volt can be achieved using a truncated comb drive finger, in accordance with the present invention. For example, analysis indicates that 3.16 times as much mechanical energy would be output by a comb drive with frusto- wedge shaped fingers as described herein with/.=8 (microns or arbitrary units), /=35, x_;*=50, gr=2.5, g_m=0.375, closing from x=60 to x=40 (Ax=10) than by a similar comb drive with similar design parameters but following the teaching of the prior art with a constant gap (θ=0).

While the depicted comb drive finger has a single straight-line wedge shape in the lengthwise direction it is also possible and within the inventive concepts herein disclosed to form the comb drive fingers having non-straight-line wedge shapes or shapes other than wedges. For example, the comb drive finger may a wedge shaped body that extends arcuately from the base of the comb drive member or the finger may be fabricated so that wedge has predetermined steps along the lengthwise direction of the body. Some examples of comb teeth with shapes other than simple wedges appear in plan view in FIG. 9. Shapes other than simple straight-line wedges allowing variation of the inter-finger gap during comb operation would be useful in tailoring the comb drive's force-versus-displacement characteristics to suit applications where mechanisms of varying mechanical advantage (e.g., a linear-to-rotary reciprocating mechanism) must be driven, in addition to the overall advantage of reduced operating voltage. Generally, the comb drive finger will define a body that extends lengthwise from the base of the comb drive member to the opposing truncated end furthest from the base of the comb drive member. A cross-section of the body taken in a plane perpendicular to the lengthwise extension of the body differs from a cross section of the body in a plane perpendicular to the lengthwise extension of the body at a different position along the lengthwise extension of the comb drive finger. Typically, the cross- section of the finger taken at the base of the comb drive member will have the greatest cross-sectional area and will gradually decrease until the minimal cross-sectional area is attained at the end of the fmger furthest from the base.

FIG. 6 is a plan view illustration of a comb drive assembly that provides rotary motion, in accordance with an embodiment of the present invention. The rotaiy comb drive assembly can be utilized in various applications including low frequency resonators, rate gyros, and rotary-to-linear transducers. The comb drive assembly 100 includes four sets of comb drive members 102 and 104 com ected as part of a torsional resonant plate 106 that is suspended above an underlying substrate 108. The number of comb drive member sets is shown by way of example only, other rotary configurations of comb drive member sets, typically a concentric configuration, are also possible.

Each set of comb drive members has a series of interdigitated comb drive fingers 110. A suspension spring mechanism 112, or other type of bias force, is in mechanical communication with hub 114 of the rotary comb drive assembly and allows for the comb drive members to provide for torsional resonance. The hub typically provides the only means of support between the rotary comb drive assembly and the underlying substrate. Electrical contacts 116, typically disposed on the substrate, are in electrical communication with the hub and the comb drive members. As electrostatic voltage is applied to the rotary comb drive assembly, resulting energized capacitance across the gap between the interdigitated comb drive fingers 110 provides the impetus for motion of the exterior ring 118 in either a clockwise or counterclockwise direction.

The interdigitated comb drive fingers of the rotary motion embodiment are shown in more detail in FIG. 7. In accordance with the present invention, the comb drive fingers have a varying cross-section in the lengthwise direction even though the comb drive fingers are arcuate. As shown, the comb drive fingers are characteristically wedge shaped and, more over, frusto wedge-shaped (i.e. the wedge shaped fmger is truncated or truncated at the end farthest from the base of the comb drive). The truncated structure of the comb drive fingers provides for a varying gap between adjacent comb drive fingers. This variance in the gap increases the force that can be applied by the rotary comb drive assembly by increasing the average change in capacitance of the comb drive member sets as the distance between the fingers decreases. In the rotary embodiment the comb drive fingers extend from the base 120 of the comb drive member in arc-like fashion. The tapering in the fingers allows for the interdigitated fingers to move in an arcuate path while varying the gap 122 between adjacent comb drive fingers.

The balanced concentric placement of comb drive member sets in the rotary embodiment provide for a high degree of flexibility for differential drive and sense. Since both the drive and sense ports are differentially balanced, excitation of undesired oscillation modes is avoided and signal corruption by feedthrough is minimized.

FIGS. 8 A - 8D are isometric diagrams of various stages of the fabrication of the comb drive fingers of the comb drive assembly in accordance with an embodiment of the present invention. While the embodiment shown may be one preferred method for fabricating the comb drive assembly, it also possible and within the inventive concepts herein disclosed to fabricate the comb drive assembly using any known MEMS fabrication process that will provide for comb drive fingers having generally large aspect ratios.

FIGS. 8A-8C show a pair of substrates, each of which may be a portion of a larger substrate wafer from which multiple micromechanical devices might be created. Substrate regions corresponding to a single, simplified comb drive assembly device are shown only for clarity. A single device can occupy from a small fraction of a square millimeter to many square millimeters, depending on overall device design. Finger-to- fmger pitches for comb drives can range from a few microns to several tens of microns, although values outside this range are also possible.

FIG. 8A shows a pair of substrate sections that will be patterned and joined to form a device. The upper substrate 200 is typically made of silicon, and is typically prepared (e.g., using thermal diffusion or epitaxial deposition) with a highly doped surface layer 210 containing a high concentration of a known dopant or dopants (e.g. typically boron) to make it electrically conductive and resistant to selective etch chemicals. Comb drive fingers and other structures will be formed in this highly doped layer. The lower substrate 220 is typically made of glass (e.g., Pyrex®). Its surface is typically of low electrical conductivity. Photoresist patterns 230 created on the lower substrate's surface (e.g., typically using standard photolithography) define the locations of mesas to which some structures formed in the upper substrate will ultimately be joined.

FIG. 8B shows the same pair of substrate sections 200 and 220. The upper substrate's surface has been patterned with photoresist structures 240 (e.g., typically using standard photolithography) that will define eventual structures such as comb fingers 250 and spring flexures 260. The bond mesas 270 formed in the lower substrate (e.g., typically using reactive ion etching) follow the photoresist patterns 230 shown earlier in FIG. 8 A. The photoresist on the lower substrate has also been removed (e.g, using an oxygen plasma) from the now-formed bond mesas.

FIG. 8C again shows both substrate sections 200 and 220. The upper substrate section is shown following sidewall 280 formation (e.g., typically using inductively- coupled reactive ion etching) and subsequent photoresist removal (e.g., using an oxygen plasma). The sidewalls are etched deep enough to penetrate the highly doped layer 210 on the surface of the upper substrate. The lower substrate section is shown following the deposition and patterning of metallic conductive paths 290 and electrodes 300 (e.g., typically using standard metallic sputtering, photolithography and etching techniques). These conductive paths will allow the comb drive to be energized, and will also allow for other features of device operation that may be desired. FIG. 8D shows a finished comb drive device chip 400. The chip is formed by joining the upper substrate and lower substrates face-to-face at the bond mesas 270 (e.g., typically by anodic bonding) and then removing the region of the upper substrate that is not highly doped (e.g., typically using a doping-selective chemical such as pyrocatechol ethylenediamine). The comb drive assemblies of the present invention provide for a significant increase in the motive force per applied volt. This increase in force is accomplished by achieving an increase in capacitance variation between comb drive members as a result of the decrease in the gap between adjacent fingers during comb drive operation. As a result of this increase in motive force it possible to reduce the complexity of drive electronics, thereby minimizing area consumption on the wafer and minimizing material - costs. By changing the geometric configuration of the comb drive fingers to allow for gap variance, these benefits can be realized without increasing the complexity of the comb drive fabrication. MEMS devices employing the use of the comb drive assemblies of the present invention have wide spread potential for use in many applications that require increased motive force without adding additional complexity or cost to the overall MEMS device. For example, some advantageous applications include rate gyros, resonators, and switches. Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

WHAT IS CLAIMED IS :

1. A comb drive assembly for use in providing movement in microelectromechanical devices, said comb drive assembly comprising: first and second comb drive members each having at least one comb drive finger, wherein at least one of said comb drive fingers of at least one of said comb drive members has a varying cross-section for different positions along a length of said comb drive finger.

2. The comb drive assembly according to Claim 1, wherein at least one of said comb drive fingers of at least one of said comb drive members has opposed ends and a body extending lengthwise between said opposed ends, wherein a cross-section of said body of said comb drive fmger in a plane perpendicular to the lengthwise extension of said body is different from a cross-section of said body in a plane peφendicular to the lengthwise extension of said body at a different position along the lengthwise extension of said body.

3. The comb drive assembly according to Claim 2, wherein said at least one comb drive finger has a base end connected to said first comb drive member and an opposed distal end extending outwardly lengthwise toward said second comb drive member, wherein the cross-section of said base end in a plane peφendicular to the lengthwise extension of said comb drive finger is greater in area than the cross-section of said distal end in a plane peφendicular to the lengthwise extension of said comb drive finger.

4. A comb drive assembly according to Claim 2, wherein said body has a pair of opposed sides extending lengthwise between the opposed ends, wherein said at least one comb drive fmger is wedge-shaped such that a pair of opposed sides of said body of said comb drive fmger converge from one end to an opposed end of said body.

5. A comb drive assembly according to Claim 2, wherein said body has a pair of opposed sides extending lengthwise between the opposed ends, wherein said at least one comb drive fmger is frusto wedge-shaped such that a pair of opposed sides of said body of said comb drive finger converge from one end to an opposed end of said body, and wherein said body of said comb drive finger is truncated at the opposed end.

6. A comb drive assembly according to Claim 2, wherein said at least one comb drive finger extends in an arc such that the lengthwise extension of said body of the said at least one comb drive finger is also arcuate.

7. A comb drive assembly according to Claim 2, wherein said first and second comb drive members are positioned relative to each other such that at least one comb drive finger of said first comb drive member is interdigitated with at least one comb drive finger of said second comb drive assembly, wherein at least one of said comb drive fingers of at least one of said first and second comb drive members has a differing cross-section for different positions along a length of said comb drive finger, such that as said first and second comb drive members move relative to each other a gap between said interdigitated drive comb fingers of said first and second comb drive members varies.

8. A comb drive assembly accordmg to Claim 7, wherein said comb drive fingers of said first and second comb drive members have a base end connected to said respective comb drive members and an opposed truncated end extending outwardly lengthwise toward the other comb drive member, and wherein said base end of at least one of said drive comb fingers has a width defined by the equation:

where: w = width of the base end of said comb drive fmger; p = distance between an adjacent pair of lengthwise centerlines of said comb drive fingers; and

I = length of said body of said comb drive finger; and

Xi = the initial length between the base ends of said comb drive fingers; and g ■= varying position gap between said comb drive fingers where: gi = position gap between said comb drive fingers at an initial position of said comb drive members; and g_m = position gap between said comb drive fingers when the length between the base ends of said comb drive fingers equals the length of said body of said comb drive fmger.

9. A comb drive assembly according to Claim 8, wherein said truncated end of at least one of said comb drive fingers has a width defined by the equation:

where: t = width of the truncated end of said comb drive finger; p = distance between an adjacent pair of lengthwise centerlines of said comb drive fingers; and

1 = length of said body of said comb drive fmger; and

Xi = the initial length between the base ends of said comb drive fingers; and g ^■= varying position gap between said comb drive fingers where: gi = position gap between said comb drive fingers at an initial position of said comb drive members; and g_m = position gap between said comb drive fingers where the length between the base ends of said comb drive fingers equals the length of said body of said comb drive finger.

10. A comb drive assembly for use in providing movement in microelectromechanical devices, said comb drive assembly comprising:

first and second comb drive members each having at least one comb drive fmger that is interdigitated with a comb drive finger from the other comb drive member, wherein as said first and second comb drive members move relative to each other a gap between said interdigitated comb drive fingers varies.

11. A comb drive assembly according to Claim 10, wherein as said first and second comb drive members move closer to each other the gap between said interdigitated comb drive fingers decreases.

12. A comb drive assembly according to Claim 10, wherein an electrical signal is applied to said interdigitated comb drive fingers, and wherein as said first and second comb drive members move relative to each other a capacitance between said interdigitated comb drive fingers changes nonlinearly with respect to displacement of said first and second drive comb members.

13. A comb drive assembly according to Claim 10, wherein when an electrical signal is applied to said interdigitated comb drive fingers, said first and second comb drive members are driven apart by electrostatic force, and wherein the magnitude of the electrostatic force is inversely proportional to the size of the gap between said interdigitated comb drive fingers.

14. A comb drive assembly according to Claim 12, wherein at least one of said comb drive fingers of at least one of said comb drive members has opposed ends and a body extending lengthwise between said opposed ends, wherein said body tapers from one opposed end to the other opposed end by a predetermined angle defined as θ.

15. A comb drive assembly according to Claim 14, wherein a net mechanical output energy due to electrostatic force across the gap between said interdigitated comb drive fingers as said first and second comb drive members are moved relative to each other is defined by the equation:

where:

Ε = energy stored in capacitance across the gap between said interdigitated drive comb fingers; V = voltage applied to said interdigitated drive comb fingers;

C(x) = capacitance across the gap between said interdigitated drive comb fingers as a function of x; εo = electrical permitivity of space in the gap; λ = thickness of said interdigitated comb drive fingers;

1 = length of said body of said comb drive finger; and gi = position gap between said comb drive fingers at an initial position of said comb drive members; and x = varying length between the base ends of said interdigitated comb drive fingers; and

Xi = length between the base ends of said comb drive fingers at an initial position of said interdigitated comb drive members.

16. A comb drive assembly according to Claim 10, wherein at least one of said comb drive fingers of at least one of said comb drive members has a differing cross-section for different positions along a length of said comb drive finger.

17. A comb drive assembly according to Claim 16, wherein at least one of said comb drive fingers of at least one of said comb drive members has opposed ends and a body extending lengthwise between said opposed ends, wherein a cross-section of said body of said comb drive finger in a plane peφendicular to the lengthwise extension of said body is different from a cross-section of said body in a plane peφendicular to the lengthwise extension of said body at a different position along the lengthwise extension of said body.

18. A comb drive assembly according to Claim 17, wherein said body has a pair of opposed sides extending lengthwise between the opposed ends, wherein said at least one comb drive finger is frusto wedge-shaped such that a pair of opposed sides of said body of said comb drive finger converge from one end to an opposed end of said body, and wherein said body of said comb drive finger is truncated at the opposed end.

19. A method for forming comb drive fingers for a comb drive assembly, wherein said method comprises: providing a workpiece suitable for forming the comb drive fingers; applying a layer of photoresist to a surface of the workpiece; patterning the photoresist to define the comb drive fingers of the comb drive assembly such that at least one of the comb drive fingers of the comb drive assembly has a different cross-section for different positions along a length of the comb drive fmger; and etching portions of the workpiece defined by said patterned photoresist to thereby form the comb drive fingers.

20. A method according to Claim 19, wherein said patterning comprises patterning at least one of the comb drive fingers such that the comb drive finger has opposed ends and a body extending lengthwise between the opposed ends, wherein a cross-section of the body of the comb drive finger in a plane peφendicular to the lengthwise extension of the body is different from a cross-section of the body in a plane peφendicular to the lengthwise extension of the body at a different position along the lengthwise extension of the body.

21. A method according to 20, wherein said patterning comprises patterning at least one comb drive finger to have a pair of opposed sides extending lengthwise between the opposed ends, and wherein said patterning comprises patterning the at least one comb drive finger in a wedge-shaped pattern such that the pair of opposed sides of the body of the comb drive finger converge from one end to an opposed end of the body.

22. A method accordmg to 20, wherein said patterning comprises patterning at least one comb drive fmger such that the comb drive finger extends in an arc and such that the lengthwise extension of the body of the comb drive fmger is also arcuate.

23. A method according to Claim 19, wherein said patterning comprises patterning first and second comb drive members each having at least one comb drive finger interdigitated with one comb drive finger of the other comb drive assembly, and wherein said patterning step patterns at least one of the comb drive fingers of at least one of the first and second comb drive members such that the comb drive finger has a differing cross-section for different positions along a length of the comb drive finger such that during operation of the comb drive assembly as the first and second comb drive members move relative to each other, a gap between the drive comb fingers of the first and second comb drive members varies.

24. A method according to Claim 19, wherein said providing comprises providing a workpiece having opposed surfaces, wherein a substrate is attached to one of the surfaces, wherein said etching step comprises etching through portions of the workpiece and into the substrate to thereby form the comb drive fingers, and wherein said method further comprises dissolving the unetched portions of the substrate to remove the substrate from the comb drive fingers defined by the workpiece.