WO1995034943A1

WO1995034943A1 - Micromotors with utilitarian features and methods of their fabrication

Info

Publication number: WO1995034943A1
Application number: PCT/US1994/014799
Authority: WO
Inventors: Mehran Mehregany; Francis L. Merat
Original assignee: Case Western Reserve University
Priority date: 1994-06-10
Filing date: 1994-12-22
Publication date: 1995-12-21

Abstract

Micromotors are fabricated with utilitarian features (21) on their rotors (23). In some embodiments, the features are formed by the molded addition of material on top of a rotor surface. In other embodiments, the features are formed by the provision of an additional layer on top of the rotor, and the selective removal of material therefrom. In yet other embodiments, the features are defined by the selective removal of material from the rotor itself. The disclosure is particularly illustrated with reference to the fabrication of polygon (nickel) mirror (21) on a polysilicon, electrostatic micromotor rotor (23) for use in scanning applications. However, the principles of the invention can likewise be applied to fabrication of a variety of other features, such as optical gratings, shutters, mechanical actuators, pump impellers and fins, and to a variety of different micrometer constructions.

Description

MICROMOTORS WITH UTILITARIAN FEATURES AND METHODS OF THEIR FABRICATION

Field of the Invention

The present invention relates to micromotors, and more particularly relates to the fabrication of features on micromotor rotors wherein the rotors serve as prime movers.

Background and Summary of the Invention

Applicants' copending application, cited above and attached hereto as an Appendix, reviews various work in the field of micromotor design and fabrication, and discloses a number of important improvements thereto. The present application is directed to further improvements in micromachine technology, particularly to the design and fabrication of micromotors wherein the rotors are provided with working features (i.e. mirrors, gratings, fins, pump impellers, optical shutters, etc.) that are driven by the micromotor.

The present application is illustrated with reference to a particular application, namely the provision of mirrors on a micromotor rotor so as to enable micromachined optical scanning. While this is one important application of the disclosed technology, it should be recognized that the invention is not so limited. Instead, the invention finds application whenever the rotor of a micromotor is to be used as a prime mover in a system. (By prime mover, applicants mean that the rotor directly moves an element that performs the ultimate work of a given assembly. This is in contrast to use of a rotor simply to actuate a further assembly (as by intermeshed gears) wherein an element in that further assembly serves as the prime mover.)

In accordance with one embodiment of the present invention, a micromotor is fabricated using the same general process detailed in the Appendix. However, before the rotor release step, a thick layer of photoresist is applied and exposed to define areas on the rotor where metal features (e.g. nickel mirrors) are to be attached. The substrate is then developed to remove the photoresist from the defined areas, leaving mold cavities. Electroless-plating is performed, filling the mold cavities with metal. The molding photoresist is thereafter removed, leaving metal plate structures extending from the rotor.

More generally, micromotors according to the present invention are fabricated with utilitarian features on their rotors. In some embodiments, the features are formed by the molded addition of material on top of a rotor surface. In other embodiments, the features are formed by the provision of an additional layer on top of the rotor, and the selective removal of material therefrom. In yet other embodiments, the features are defined by the selective removal of material from the rotor itself.

While illustrated with reference to polysilicon surface micromachined electrostatic motors, the invention finds applicability to all manner of micromachinery, including electromagnetic devices, and devices based on other structural materials and fabrication technologies.

The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. Brief Description of the Drawings Fig. 1 is a schematic drawing of a rotating polygon scanner with optically reflective rotor segments, in accordance with one embodiment of the present invention.

Fig. 2 is an illustration of an optical microscanner fabricated by electroless-plating of nickel reflecting surfaces on the rotor of a salient-pole micromotor, in accordance with one embodiment of the present invention. Fig. 3 is a sectional view (not to scale) of the microscanner of Fig. 2 after release, showing illumination of the nickel surface with a light beam. Fig. 4 is a schematic drawing showing the concept of a micromotor grating scanner in the reflection mode.

Fig. 5 is an illustration of an optical microscanner made by electroless-plating of nickel reflecting surfaces on the rotor of a wobble micromotor, in accordance with one embodiment of the present invention.

Fig. 6 is a sectional view (not to scale) of a micromotor according to one embodiment of the present invention during its fabrication. Figs. 7A-7C are sectional views depicting further fabrication steps of the micromotor shown in Fig. 6.

Fig. 8 is a chart depicting gear ratio versus excitation voltage for a micromotor fabricated according to one embodiment of the present invention, with and without load applied.

Fig. 9 is a chart depicting step transient data for salient-pole micromotors fabricated according to one embodiment of the present invention, with and without load applied.

Fig. 10 depicts instrumentation used for optical measurement on microscanners fabricated according to one embodiment of the present invention.

Figs. 11A and 11B are graphs of detected optical radiation from a rotating polygon scanner for: (A) a

0.5 mm-diameter rotor with constant rotation speed; and (B) a 1.0 mm-diameter rotor with different operating rotor speeds during measurement.

Fig. 12 is an optical radiation pattern resulting from a rotating polygon scanner in accordance with one embodiment of the present invention. Detailed Description The present invention is illustrated with reference to an exemplary application, namely the fabrication of mirrors on a micromotor rotor. However, as detailed below, the invention finds a great variety of diverse applications.

Microscanners The development of polysilicon micromotors for optical scanning applications is a good match between the small size and low cost of batch fabricated micromotors, and the low-load requirements imposed by optical elements such as polygon mirrors. Typical optical scanners (e.g. supermarket scanners) are large, complex systems requiring careful alignment. A micromechanical scanner implementation reduces the weight and size of existing scanners by two orders of magnitude with significant decrease in cost due to batch fabrication.

In the following discussion, we address two types of microscanners based on polysilicon micromotors: a micromotor polygon scanner and a micromotor grating scanner. Fig. 1 is a schematic drawing of a polygon scanner 20 with optically reflective rotor segments. Figs. 2 and 3 show a microscanner 21 fabricated on a rotor 23 of a salient-pole micromotor 22, and a cross- sectional view of same. (The rotor 23 is fabricated in perforated/spoked fashion to facilitate its quick release. The depicted spokes have a width of 10 to 15 microns, as compared with the substantially larger dimensions of the rotor itself. Also shown in Figs. 2 and 3 are the stator poles 24, and the bearing 25.)

The polygon scanner can scan in a straight line in a plane, but requires high-aspect-ratio fabrication of metallic microstructures for the reflective surfaces and careful optical design in order to efficiently transmit optical radiation through small dimensions of the microstructure. A polygon scanner such as described herein can also be used for inexpensive optical switches.

The micromotor shown in Figs. 2 and 3 has a 0.5 millimeter out rotor diameter. The thickness of the nickel is 20 microns and the width of the nickel lines is 10 microns.

A micromotor grating scanner is similar to the foregoing but replaces the plated optical element with a diffractive element. Fig. 4 is a schematic drawing of a micromotor grating scanner 30. The source illumination 32 comes from out of the plane of the substrate 31 and can be perpendicular to the plane of the substrate depending upon the application.

Depending on the optical wavelength and the substrate material, a rotating grating scanner 30 is capable of being operated in either the transmission mode or reflection mode. Because the optical source can be perpendicular to the substrate plane 31 and the grating structure will be of millimeter dimensions, the optical design is not as complex as that for the micromotor polygon scanner. In addition, arrays of gratings rotated by synchronized micromotors are capable of scanning large diameter optical beams (larger than 1 millimeter) . In general, the micromotor polygon scanner scans in a straight line in a plane, and the micromotor grating scanner scans in a curved line out of the plane of the grating. The former requires high-aspect-ratio optical elements to limit diffraction losses, and the source must lie in the same plane. This also poses strict requirements on the optical source in terms of efficiently coupling the optical energy to the reflecting elements of the polygon. The most promising sources are optical fibers or integrated optical waveguides.

The grating scanner requires well-defined grating lines and precise control of the grating depth during fabrication. However, coupling the optical energy to the grating is relatively simple. Processing of a grating scanner for transmission mode requires that the substrate be transparent to the optical radiation. For expository convenience, we focus the following discussion on fabrication of polygon scanners. Artisans will recognize that the same principles can be used to realize grating scanners.

Fabrication of the Microscanners The microscanner structures described herein are based on the development of millimeter-sized polysilicon micromotors and the integration of a high-aspect-ratio electroless nickel plating process with polysilicon surface micromachining to fabricate the optical elements. Millimeter-Sized Polysilicon Micromotor

In order to support optical elements large enough to have acceptable diffraction losses, we developed large diameter (up to one millimeter) micromotors. Fig. 5 shows a microscanner 40 fabricated on the rotor 41 of a 1.0 mm-diameter wobble micromotor 42. In our work, typical motor dimensions are rotor diameters of 0.5 to 1.0 millimeters, rotor/stator gaps of 1.5 to 2.5 microns, and rotor/stator thicknesses of 5 microns. Again, the thickness of the nickel plating 43 defining the mirror is 20 microns and the width of the nickel is 10 microns.

Fig. 6 presents a cross-sectional schematic of a millimeter-sized micromotor 50 before release. Millimeter-sized motors were fabricated using the rapid prototyping process described in the Appendix, resulting in flange bearing wobble and salient-pole motors. The rotor 51, stator 52, and bearing 53 were fabricated from a 5 micron-thick phosphorous-doped polysilicon film. A 2.4 micron-thick low temperature oxidation (LTO) film 54 is used for substrate/stator isolation, as well as the sacrificial layer under the rotor. The bearing clearance 55 is created by a 0.5 micron thermal oxidation layer 56. Fabrication of the Reflector A high-aspect-ratio lithography process is used in conjunction with an electroless nickel plating process to fabricate the polygon reflectors 57 on the rotor of the polysilicon micromotor 50 prior to release. Of key importance is the compatibility of the reflector fabrication process with that of the micromotor process, in particular during the release step. Additionally, a large area smooth reflective surface is desired. Figs. 7A-7C further detail the reflector fabrication process. After the motor is fabricated (Fig. 6) , the oxide layer 56 on top of the rotor 51 and stator 52 is removed in BHF. A thick (up to 23 microns) photoresist layer 58 is then spun cast and photolithographically patterned to define mold voids 59 for plating (Fig. 7A) . Electroless plating of nickel is then performed (Fig. 7B) , and the photoresist mold 58 is removed. A 10 minute hydrofluoric acid (HF) bath performs the final release, freeing the rotor and underetching to form the anchors (Fig. 7C) . The release process does not alter the reflective properties of the nickel structures in our work.

In the photolithography process for the plating mold (Fig. 7A) , large heights, smooth vertical sidewalls and material compatibility with the electroless plating chemicals were required. Our photolithography process used a positive photoresist 58 of high transparency and high viscosity with multiple coats to obtain these photoresist films and planarized surfaces. The softbase, exposure, and development conditions were controlled to obtain near vertical sidewalls. A standard high pressure mercury ultraviolet source is used for exposure. In the illustrated embodiment, the thickness of the photoresist 58 is 23 microns; the mold opening has a width of 7 microns.

Before plating, the polysilicon surface on which the features (mirrors) are to be formed is pretreated. In our work, this pretreatment includes: (a) a 45 second etch in a solution of HN0₃, HG, and H₂0; (b) a nine minute deposition of Pd as a starting catalyst from a solution of PdCl₂, SnCl₂, and HCl; (c) 15 second in HCl to remove the Sn oxide complex; and (d) a two minute rinse in deionized water. In (a) etching of the polysilicon surface takes place, which has been determined to be important for the uniformity of the plating process and for adhesion. The adhesion force between nickel and polysilicon seems to be primarily mechanical (not chemical) in nature. The etch rate and surface morphology vary depending on the polysilicon grain size which is, in turn, affected by the LPCVD deposition conditions, as well as subsequent thermal treatment. For short etch times, plating will not be initiated uniformly. For long times, the polysilicon film thickness is reduced significantly, affecting the mechanical integrity of the rotor.

After the surface treatment step, electroless plating is carried out in a mixture of NiS0₄, NaH₂P0₂ (as reducing agent) , and CH₃C00Na (as a buffer and mild complexing agent for Ni. A fresh plating solution is used for each batch to keep the nickel concentration constant and to avoid contamination from the solution itself, photoresist, and/or substrate. In the illustrated embodiment, electroless plating on polysilicon is conducted at 90°C in an electroless plating solution with a pH of 5. In this embodiment, the thickness of the nickel is 20 microns. The polygon beam width is 7 microns. Polysilicon Microscanners

Preliminary Mechanical Measurements The microscanners fabricated from millimeter- sized motors operate smoothly and reproducibly in room air for extended periods (e.g., several months) after release. Minimum voltages can be as low as 12V, while maximum rotor speeds have been 100 rpm for microscanners fabricated on millimeter-sized wobble motors and 2500 rpm for those fabricated on millimeter-sized salient- pole motors. These large motors have motive torques over an order of magnitude larger than previously reported polysilicon micromotors.

Fig. 8 presents typical experimental gear ratios as a function of excitation voltage for loaded (e.g., nickel plated) and unloaded (e.g. unplated) wobble motors. As expected, the gear ratio increases with increased load since the load increases friction at the flange contact by increasing the normal contact force. The increased flange friction leads to increased rotor slip. The load is estimated near 72 μN using pure nickel density of 8.9 g/cm³. The increase in the gear ratio at the smaller excitation voltages (and hence smaller motive torques) is due to increased rotor slip as well.

Fig. 9 presents typical step transient data for salient-pole motors with load and without load. Stroboscopic dynamometry is used to measure the step response of the motors. The step response is overdamped for these motors due to the increased viscous drag caused by the larger rotor as compared to our previous micromotors. The rise time of the step response is one order of magnitude larger than that for our previous micromotors which are one-tenth the size of these motors. As expected, the step response of the motor with load is slower due to the increased inertia and friction.

Preliminary Optical Measurements We have performed preliminary optical testing of the microscanners using a 633 nm He-Ne laser coupled into a multi-mode optical fiber. The core of the optical fiber is 20 microns in diameter and the cladding thickness is 125 microns. The laser beam is mechanically chopped to facilitate subsequent optical detection. The optical fiber is positioned approximately in the plane of the substrate 1.5 millimeters from the axis of rotation of the motor. The scanned optical radiation is detected using a photo detector (Milles Griot #13 DSI 007) . Absolute power measurements have not yet been performed to determine the optical efficiency of the microscanners. Fig. 10 presents a schematic diagram of the instrumentation for optical measurements on the microscanners.

Figs. 11A and 11B show the detected radiation reflected by microscanners fabricated on wobble micromotors. The diffraction pattern of the scanned radiation is repeatable. The intensity variation in the scanned radiation may be due to wobbling of the micromotor on its rotational axis. The potential of the micromotor scanner is most clearly seen in Fig. 12 which is a digitized photo of a laser beam reflected from the microscanner. The image is captured using a common camcorder with a macro lens approximately in the plane of the scanner and about six inches from the micromotor. The diffraction pattern is observed in Fig. 14.

Discussion Our results clearly demonstrate the ability of the micromotors to rotate optical elements capable of directing laser beams. However, polygon reflectors which are much taller, i.e. on the order of 100-300 microns are more desirable. Such structures can be implemented by the LIGA process. Structures of this size entail additional weight and friction, but these considerations can readily be dealt with by artisans in the field.

The optical scanners described herein have the polygon reflectors located a considerable distance from the outer dimensions of the rotor. Such arrangements are not usually efficient for coupling the source radiation to the reflector for either scanning or switching applications. A better mode is to place the reflecting surfaces near the outer rotor of the micromotor. For optical switching applications, the source and detected radiation can be transmitted through optical waveguides fabricated on the substrate, dra atically improving the optical efficiency of the device.

Additional information on electroless plating of metals in micromachinery can be found in Furukawa et al, "Electroless Plating of Metals for Micromechanical

Structures," Proc. 7th Int. Conf. Solid-state Sensors and Actuators, June, 1993, pp. 66-69.

Alternative Embodiments Having described and illustrated the principles of our invention with reference to illustrative embodiments and methods, and several variations thereof, it should be apparent that the disclosed embodiments and methods can be modified in arrangement and detail without departing from such principles. For example, while the detailed embodiment defined the utilitarian features on the rotor by the addition of molded material thereon, in other embodiments the utilitarian features can be provided by providing a further layer on top of the rotor and selectively removing portions therefrom. For example, a film (which can be an insulator, a metal, polysilicon, or silicon carbide, to name but a few) can be deposited on top of the rotor. The thickness of the film depends on the particular application. Optical gratings may require a thickness of a few thousand Angstroms; fluid pumping elements may require a thickness of tens or hundreds of microns. Photoresist is then applied to the top of the film and exposed through a mask defining a desired pattern. After developing, the exposed photoresist is removed, and the substrate etched to remove the film thereby exposed. After etching, the remaining photoresist is removed, leaving the patterned film on top of the rotor to serve its utilitarian function.

Still further, the utilitarian features can be defined not just by the addition of material on top of the rotor, but alternatively by the removal of part of the top surface of the rotor itself. Grooves, gratings, shutters, blades and fins can thereby be provided, limited only by the thickness of the rotor and electrostatic operational constraints. Indeed, in some applications, the removal of material can extend all the way through the rotor, resulting in shaped utilitarian perforations therethrough.

It is generally preferred to define the utilitarian features before the rotor is released, regardless of which of the foregoing approaches is used. While the detailed embodiment involved the provision of utilitarian features on an inner-rotor micromotor, it will be recognized that the same principles can be applied to other micromotor designs, such as outer-rotor micromotors. Further, while the invention has been illustrated with reference to polysilicon, electrostatic motors, it will be recognized that the same principles are likewise applicable to micromotors fabricated with other surface micromachining techniques (e.g. with nickel as a structural material, to name but one) , and relying on other actuation principles (e.g. electromagnetic) .

Yet further, the illustrated embodiment made use of electroless plating to fill a mold on top of the rotor. In other embodiments, electroplating can be used for this purpose. In such other embodiments, a seed layer is first applied (as by sputtering or evaporation) , and this layer is then used as the cathode in the electroplating process.

While the preferred embodiment made use of a micromotor rotor as a prime mover (i.e., the rotor directly moves an element that performs the ultimate work of a given assembly) , the same principles can be used to achieve other ends. For example, a micromotor rotor can be provided with features such as mechanical actuator elements or a crown of vertically defined gear teeth that simply serve to drive further elements of an assembly (as opposed to performing the assembly's ultimate work) . Finally, while the foregoing description focused on the provision of mirrors on micromotor rotors, it will be recognized that a tremendous number of other features can likewise be fabricated. Optical shutters, gratings, mechanical actuators, and fluidic pumping elements are but a few of a lengthy catalog of such elements.

In view of the wide variety of embodiments to which the principles of our invention can be applied, it should be apparent that the detailed embodiments are illustrative only and should not be taken as limiting the scope of our invention. Rather, we claim as our invention all such modifications as may come within the scope and spirit of the following claims and equivalents thereto.

MICROMOTORS AND METHODS OF FABRICATION

Background and Summary of the Invention Micromotors have been the subject of extensive academic and commercial investigation, both in the United States and abroad, for several years. However, despite this focused effort, a number of problems remain in the basic design of micromotors, and in methods of their fabrication.

Before proceeding further it should be noted that there are two basic types of micromotors: inner rotor and outer rotor. Inner rotor micromotors are more conventional in design and use a rotor centrally positioned within a circular array of stator elements. Outer rotor micromotors, in contrast, position the stator elements in the center, and the rotor takes the form of a ring or like shape mounted for rotation about the central stator. For many applications, outer rotor micromotors offer the most promise.

Illustrative of the former state of the art in outer rotor motors is Furuhata et al, "Outer Rotor

Surface-Micromachined Wobble Micromotor," Proc. IEEE Conf. on Micro Electro Mechanical Systems, 1993, pp. 161-166. However, Furuhata suffers from a number of drawbacks. A major drawback is that the fabrication of the micromotor requires manual placement of the rotor, as by a micro manipulator.

Once installed, the Furuhata rotor lies unsecured on the substrate and can be dislodged if the substrate is inverted.

U.S. Patents 5,093,594 and 5,043,043 disclose inner rotor micromotors which don't require manual rotor placement, and which secure the rotor in-place by a central bearing. However, as noted, inner rotor micromotors are disadvantageous in many applications. Further, the fabrication techniques disclosed in these patents are still quite complex, often requiring many APPENDIX A - Page 1 masking steps (e.g. more than five) .

It is an object of the preferred embodiment of the present invention to overcome the above-noted and other drawbacks of the prior art. In accordance with the preferred embodiment of the present invention, a micromotor is fabricated using just three mask steps. In addition to simplicity, this fabrication process also results in in-place formation of the micromotor rotor. The rotor is secured in-place by a flange bearing which is fabricated as part of the three mask process. In outer rotor embodiments, the flange bearing can provide electrical insulation between the rotor and stator, thereby increasing motive torque. Outer rotor embodiments can also be of the exposed periphery type, allowing the rotor to be toothed to permit driving of other mechanisms fabricated on the same substrate.

The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. Brief Description of the Drawings Fig. 1 is a top plan view of an outer rotor micromotor according to one embodiment of the present invention.

Figs. 2A - 2E are cross-sectional views showing steps in the fabrication of the micromotor of Fig. 1.

Fig. 3 is a cross sectional view showing an inner rotor micromotor according to one embodiment of the present invention.

Detailed Description Referring to Fig. 1, an outer rotor micromotor 10 according to one embodiment of the present invention includes a stator 12, a flange bearing 14, and a rotor 16. The stator 12 includes a plurality of poles, such as 12a-12f, which are driven in sequence to electrostatically attract the rotor 16 and effect APPENDIX A - Page 2 rotation. (For clarity of illustration, the rotor and bearing in Fig. 1 are depicted as centered on the stator — the position in which they are fabricated.)

Fabrication Process Referring to Figs. 2A - 2E, a silicon wafer 18 serves as the substrate for the illustrated micromotor 10. The thickness of the substrate is not particularly critical; since the process of fabrication is surface micromachining, the substrate simply acts as a mechanical support for the motor. However, the use of a semiconductor substrate allows for integration of electronics which allows the integration of micromotors and mechanisms with drive and detection circuitry.

On substrate 18 is deposited a layer of silicon dioxide 20. In the preferred embodiment, this deposition is accomplished by low pressure chemical vapor deposition (LPCVD) using an LTO process with silane and oxygen, and results in a layer 2.4 microns thick. This LTO layer 20 serves as substrate/stator isolation, as well as a sacrificial layer under the rotor.

Next, a 3-5 micron layer of polysilicon 22 is deposited, again via LPCVD. To make this layer more conductive, the polysilicon is heavily doped with phosphorous. In the preferred embodiment, this is done by a thermal diffusion process, although other techniques can be used.

Before patterning the top polysilicon layer 22, a hard mask layer 24, having a thickness of 0.3 to 1 micron, is first formed. This is done by a thermal oxidation process.

After formation of the hard mask layer 24, the wafer is coated, as by spin coating, with a 1 micron layer of photoresist. The wafer is then exposed by a conventional exposure technique through a mask that defines the stator poles 12a - 12f, and the rotor 16.

APPENDIX A - Page 3 (A 5X wafer stepper is used in the preferred process; in other embodiments contact printing or other exposure techniques could of course be used.) The wafer is then developed, dissolving the exposed photoresist and uncovering selected areas of the hard mask layer 24.

The hard mask layer 24 is etched by reactive ion etching (aka plasma etching or dry etching) using the photoresist as a mask. The hard mask layer could be etched by wet chemical means, with a slight loss in definition accuracy.

The wafer is next etched to remove the polysilicon layers 24, 22, thereby defining the rotor 16, stator 12, and rotor/stator gap 26. This etching is done by reactive ion etching, although wet chemical etching could again be used with some compromise of definition.

Next, photoresist is again applied, and exposed through a mask that exposes only the region where the flange bearing 14 is to be formed. After developing, wet chemical etching (e.g. HF) is used to form the undercut 28 under the polysilicon layer 22 (Fig. 2B) .

The resist is then removed and the wafer put in a thermal oxidation furnace to form a layer 30 of silicon dioxide, approximately 0.3 microns thick, over the entire wafer surface. This layer determines the bearing clearance 32 (Fig. 2C) .

Formation of the bearing 14 begins by depositing a 1.5 micron thick silicon-rich silicon nitride layer by LPCVD. (Silicon-rich silicon nitride is used to lessen internal stresses in the bearing material and for its very small etch rate in HF.) Photoresist is again applied, exposed, and developed, and the wafer is then etched to form the bearing 14, as shown in Fig. 2D.

(Alternatively, the bearing 14 can be formed of polyimide. In this case the polyimide is spin cast and permitted to dry, letting the solvent evaporate. The wafer is then soft-baked in air for approximately 30 APPENDIX A - Page 4 inutes at 350°C to cure. A layer of aluminum, 0.1 - 0.2 microns thick, is next evaporated over the polyimide. Photoresist is then applied and the wafer exposed. After developing, the wafer is wet etched in H₃P0₄/HN0₃/acetic acid solution to remove aluminum where polyimide is not desired. Finally, the wafer is treated with an oxygen plasma, turning the unmasked polyimide into C0₂ and N0₂. The polyimide bearing then remains.)

It will be recognized that, since fabrication of the bearing 14 is the last step of the process before release, other bearing materials (discussed below) can be substituted with no change in the earlier fabrication steps.

(The HF etch rates of LPCVD silicon-rich nitride and polyimide are very small, making them compatible with polysilicon surface micromachining. Additionally, both materials can be deposited in processes that fill the flange mold, including the rotor/stator gap.)

Finally, a HF bath is used to etch away the silicon dioxide LTO 20 underneath the rotor, freeing it for rotation (Fig. 2E) . The oxide bearing clearance layer 30 over all the wafer is removed in this same process. The bath is timed so that the rotor is freed, while some of the LTO layer 20 underneath the stator poles persists, serving as anchors 34 to hold the stator in place. (A key to the successful fabrication of micromotors utilizing the present process is selection of the geometry of parts that are to be released (e.g. the rotor) such that they are released long before significant underetching of the remainder of the device (e.g. the stator).)

Outer-Rotor Micromotor Characteristics In a presently preferred embodiment, that stator 12 has a radius of 100 microns. The rotor 16 has an inner radius of 101.5 microns, resulting in a rotor/stator gap of 1.5 microns. The rotor has a maximum outer radius (i.e. to the gear teeth) of 121 APPENDIX A - Page 5 microns. The thickness of both the rotor and the stator is 5 microns. The flange bearing 14 has a mean radius of 100.75 microns and overhangs the rotor and stator approximately 4 to 5 microns. In the central portion of its "I" cross-sectional shape, bearing 14 has a thickness of 0.9 microns. (All dimensions are approximate.)

The above-described micromotor 10 operates smoothly and reproducibly in room air with a minimum operating voltage of approximately 14 volts. Power supply constraints have limited rotor speeds to 35 rpm, although higher speeds should be possible. If the rotor/stator thickness is decreased, the excitation voltage must be increased commensurately. A rotor/stator thickness of 2.5 microns, for example, may require a 100 volt excitation signal.

In the preferred embodiment, the motors operate successfully without grounding the rotor (i.e., the rotor is floating) . In other embodiments, other approaches to ground the rotor are possible, such as a sliding contact to the rotor or by a grounded driven gear which the rotor drives.

The gear ratio (i.e. ratio of rotation of the stator field to the resulting physical rotation of the rotor) of an outer-rotor micromotor is given by dividing the bearing radius (against which the rotor rotates) by the bearing clearance. Since the bearing clearance is nominally 0.3 microns in the above-described process, a gear ratio of 333 would be expected for a micromotor having a bearing radius of 100 microns. In practice, the gear ratio is higher due to rotor slip. (As the excitation voltage increases, slip decreases, and the gear ratio more nearly conforms to its theoretical value.) Thus, to achieve a 35 rpm rotor speed, the stator poles 12a - 12f are excited at a rotational rate of about 12KHz.

APPENDIX A - Page 6 The illustrated micromotor 10 achieves motive torques over 100 times larger than previously reported polysilicon micromotors. This substantial increase is due to the larger gear ratios (about 5 times) , in addition to the larger motor radii (about 2 times) , the thicker rotor/stator polysilicon (about 2 times) , and the dielectric effect in the rotor/stator gap (about 5 times) .

One feature of micromotor 10 is that the rotor/stator gap 26 is partly filled by the bearing material, which is electrically insulating. (In the illustrated embodiment, the gap is mostly filled.) Since the relative dielectric constant of an insulator is larger than 1, the motive torque is increased in comparison with conventional micromotor designs in which the rotor/stator gap is filled with air. The increase in the micromotor motive torque is nearly by as much as the dielectric constant if the bearing clearance is a small fraction of the rotor/stator gap size. Therefore, by using a material with a very high dielectric constant it is possible to increase the motive torque substantially. The silicon nitride used in the illustrative embodiment has a dielectric constant of about 7. However, materials with dielectric constants two to three orders of magnitude higher (e.g. thin-film PZT (lead zironate titanate) , which has a dielectric constant of 800 - 1300, or barium-strontium titanate) can alternatively be utilized. Silicon nitride or polyimide is presently preferred by the inventors due to its ease of micromachining.

Another feature of the above-described fabrication procedure is that the rotor/stator gap is defined in the first photolithography step. Since the lithography is performed on a flat surface, definition of rotor/stator features (e.g. gap) with 1 micron resolution in photoresist can be obtained. (Thermal oxidation for the bearing clearance creation consumes APPENDIX A - Page 7 so e of the rotor/stator sidewalls, leading to increased gap size in the final device. If a minimum gap is desired, the thermal oxidation step can be replaced with LTO deposition, readily eliminating the sidewall consumption effect.)

Inner Rotor Micromotor An inner rotor micromotor 36 (Fig. 3) can be fabricated using the same basic process as was employed to fabricate the outer rotor micromotor 10. In the illustrated inner rotor micromotor 36, a flange bearing 38 couples the rotor 40 to a bearing post 42. Surrounding the rotor are the poles that comprise the stator 44. The difference in fabrication arises in formation of the bearing 38. Instead of using a silicon rich silicon nitride (or polyimide) material for the bearing, the illustrated micromotor 36 uses polysilicon. As in the outer rotor micromotor, a layer of the bearing material is applied over all surfaces of the wafer, and is then photographically patterned and etched to create the flange bearing. In this instance the bearing is conducting, rather than insulating. Again, a HF bath follows to release the rotor while keeping the stator and bearing post 42 anchored.

Conclusion In keeping with the maxim that a patent should not teach, but preferably omit , what is known in the prior art, we have not belabored the steps and elements (e.g. generation of driving signals, connection of driving signals to stator poles, photolithographic processes, etc.) which are taken directly from the prior art. The reader who is not adequately versed in these areas may wish to consult the References Cited to gain additional understanding. Additional information on the preferred embodiments, such as experimental test data, can be found in Deng et al, "A Simple Fabrication Process for Side-Drive Micromotors," Proc. 7th International Conference on Solid-state Sensors and APPENDIX A - Page 8 Actuators, June, 1993, pp. 756-759, and Deng et al, "Outer Rotor Polysilicon Wobble Micromotors," Proc. IEEE Micro Electro Mechanical Systems, January, 1994, pp. 269-272. Having described and illustrated the principles of our invention with reference to illustrative embodiments and methods, and several variations thereof, it should be apparent that the disclosed embodiments and methods can be modified in arrangement and detail without departing from such principles. For example, while the preferred embodiment made use of a silicon wafer as a substrate, a variety of other materials (e.g. quartz, silicon carbide, fused silica, alumina, sapphire, and other silicon on insulator wafers, to name a few) could obviously be substituted therefor.

Likewise with the constitution of the other component layers and bearing.

Similarly, while the illustrative embodiments made use of flange bearings, it will be recognized that other bearing shapes can be utilized in other applications. For example, the lower extension on flange 14 (i.e. next to the substrate) can be omitted, and bushings can be formed on the bottom of the rotor to prevent adhesion of the rotor to the substrate. Still further, it will be appreciated that the fabrication processes and/or structures described above can be used as foundations on which more complex fabrication technologies and/or micromachined assemblies and/or mechanisms can based. Indeed, a plurality of micromotors can be fabricated on a single substrate and operated in conjunction to effect a variety of mechanical systems. Further, electronic circuitry can be integrated with the micromotor(s) on the substrate to effect additional sensing and processing operations. In one such embodiment, a speed sensor is provided on- substrate and senses rotation of the rotor. This sensor

APPENDIX A - Page 9 can be implemented to utilize the stator poles as sensing elements.

While the illustrative embodiment includes 6 stator poles, it will be recognized that a greater or lesser number can be used in alternate embodiments. Similarly, while the illustrative embodiment has been described with reference to one particular set of dimensions, it will be recognized that physically larger or smaller embodiments can readily be realized. Even within the embodiment illustrated, the dimensions can be varied as application and fabrication needs dictate (e.g. in some circumstances it may be desirable to have a rotor/stator gap of up to 3 microns in the illustrated embodiment) . The geometrical arrangements described above are presently preferred, but a variety of other geometrical arrangements can also be utilized. For example, the insulated bearing, outer rotor structure can have its stators folded outside the rotor to result in an inner- rotor micromotor in which the rotor is coupled to the stators by a flange bearing between the outer edges of the rotor and the inner edge of the stators. This embodiment does not require a center bearing post and takes advantage of the dielectric constant of the flange bearing material to increase the motive torque.

APPENDIX A - Page 10

Claims

1. A micromotor fabricated on a substrate and having a rotor and a stator, the stator including a plurality of stator poles, the micromotor characterized by a flange bearing coupling the rotor and stator.

2. An outer rotor micromotor according to claim 1 in which an outer periphery of the rotor is exposed.

3. The micromotor of claim 1 in which the rotor is fabricated in-place.

4. The micromotor of claim 1 in which an electrically insulating material having a dielectric constant greater than two is disposed in a gap defined between the stator and rotor, wherein motive torque is increased by reason of the dielectric constant of the insulating bearing material.

5. The micromotor of claim 4 in which the flange bearing is said insulating bearing material.

6. The motor of claim 1 in which the insulating member is a flange bearing that mechanically couples the rotor to the stator so as to mechanically constrain the rotor to planar rotational motion.

7. The motor of claim l in which an outer periphery of the rotor is non-circular.

8. The motor of claim 7 in which the outer periphery of the rotor defines a plurality of gear teeth.

9. A mechanical system including a plurality of micromotors according to claim 1.

10. In a micromotor having a substrate and first and second elements, one of said elements being movable relative to the other, an improvement comprising a flange bearing coupling the two elements together, wherein said flange bearing is not affixed to either element nor to the substrate.

11. An electrostatic micromotor comprising a rotor and stator formed in-place by a series of deposition and etching steps, the micromotor further APPENDIX A - Page 11 including a material with a dielectric constant of at least two disposed in a gap defined between the rotor and stator.

12. The micromotor of claim 11 in which said material has a dielectric constant of more than 500.

13. In a process of photolithographically fabricating a micromotor having a rotor and stator, an improvement wherein exactly three masks are used in the fabrication process, and the rotor is fabricated in- place.

14. In a process of fabricating a micromotor by a series of photolithographic steps, the micromotor having a rotor and a stator, an improvement wherein a gap between the rotor and stator is defined in a first of said photolithographic steps.

15. The process of claim 14 in which the gap has a width of less than 3.0 microns.

16. In a process of fabricating an outer rotor micromotor having a rotor and a stator on a substrate, an improvement comprising: fabricating the rotor in place; and defining a rotor/stator gap of less than 3.0 microns.

17. The process of claim 16 which further includes integrating electronic circuitry for use with the micromotor on the same substrate as the micromotor.

18. An outer rotor micromotor fabricated according to claim 16.

19. The micromotor of claim 18 which further includes an electrically insulating member having a dielectric constant greater than two disposed in said gap.

20. The micromotor of claim 19 in which the electrically insulating member is a flange bearing that serves to mechanically couple the rotor and stator.

APPENDIX A - Page 12

21. A mechanical system including a plurality of micromotors according to claim 18.

22. An outer rotor micromotor having a plurality of centrally disposed stator elements and an annular rotor positioned therearound, wherein the rotor is polysilicon.

APPENDIX A - Page 13