Richard S. Muller

Etch rates for micromachining processing

The etch rates for 317 combinations of 16 materials (single-crystal silicon, doped, and undoped polysilicon, several types of silicon dioxide, stoichiometric and silicon-rich silicon nitride, aluminum, tungsten, titanium, Ti/W alloy, and two brands of positive photoresist) used in the fabrication of microelectromechanical systems and integrated circuits in 28 wet, plasma, and plasmaless-gas-phase etches (several HF solutions, H/sub 3/PO/sub 4/, HNO/sub 3/+H/sub 2/O+NH/sub 4/F, KOH, Type A aluminum etchant, H/sub 2/O+H/sub 2/O/sub 2/+HF, H/sub 2/O/sub 2/, piranha, acetone, HF vapor, XeF/sub 2/, and various combinations of SF/sub 6/, CF/sub 4/, CHF/sub 3/, Cl/sub 2/, O/sub 2/, N/sub 2/, and He in plasmas) were measured and are tabulated. Etch preparation, use, and chemical reactions (from the technical literature) are given. Sample preparation and MEMS applications are described for the materials.

IC-processed electrostatic micromotors

Abstract We describe the design, fabrication and operation of several micromotors that have been produced using integrated-circuit processing. Both rotors and stators for these motors, which are driven by electrostatic forces, are formed from polycrystalline silicon 1.0–1.5 μm thick. The diameters of the rotors in the motors tested are between 60 and 120 μm. Motors with several friction-reducing designs have been fabricated using phosphosilicate glass (PSG) as a sacrificial material and either one or three polysilicon depositions.

Integrated movable micromechanical structures for sensors and actuators

Movable pin-joints, gears, springs, cranks, and slider structures with dimensions measured in micrometers have been fabricated using silicon microfabrication technology. These micromechanical structures, which have important transducer applications, are batch-fabricated with an IC-compatible process. The movable mechanical elements are built on layers that are later removed so that they are freed for translation and rotation. An undercut-and-refill technique, which makes use of the high surface mobility of silicon atoms undergoing chemical vapor deposition, is used to refill undercut regions in order to form restraining flanges. Typical element sizes and masses are measured in micrometers and nanograms. The process provides the tiny structures in an assembled form avoiding the nearly impossible challenge of handling such small elements individually. >

Magnetically actuated, addressable microstructures

Surface-micromachined, batch-fabricated structures that combine plated-nickel films with polysilicon mechanical flexures to produce individually addressable, magnetically activated devices have been fabricated and tested. Individual microactuator control has been achieved in two ways: (1) by actuating devices using the magnetic field generated by coils integrated around each device and (2) by using electrostatic forces to clamp selected devices to an insulated ground plane while unclamped devices are freely moved through large out-of-plane excursions by an off-chip magnetic field. The present application for these structures is as micromirrors for microphotonic systems where they can be used either for selection from an array of mirrors or else individually for switching among fiber paths.

Resonant-microbridge vapor sensor

A novel integrated vapor sensor is described that incorporates a polycrystalline silicon microbridge coated with a thin polymer film. The microbridge is resonated electrostatically and its vibration is detected capacitively using an integrated NMOS circuit. Vapor uptake by the polymer increases the mass-loading on the microbridge, thereby perturbing the first resonant frequency of the microbridge. In the prototype device, a 150-nm-thick layer of negative photoresist coats a 153-µm-long 1.35-µm-thick polycrystalline silicon microbridge. The phase between the excitation and output voltages at resonance is monitored as the sensor output signal. Exposure to saturated xylene vapor produces a phase shift of -8° with a response time of less than 7 min.

Silicon-processed overhanging microgripper

A silicon-processed microgripper, suitable for mounting on a micropositioner, has been designed and fabricated by combining surface and bulk micromachining. The microgripper consists of a silicon die (7 mm*5 mm), a 1.5 mm long support cantilever, made from boron-doped silicon substrate material (protruding from the die), and a 400 mu m long polysilicon overhanging gripper extending from the end of the support cantilever. The microgripper is electrostatically driven by flexible, interdigitated comb pairs and has significantly smaller feature sizes than have been reported previously for overhanging microstructures. Problems addressed successfully in the microgripper fabrication include the protection of surface-micromachined fine structures during bulk-silicon etching and rinsing. The microgripper has successfully seized several microscopic objects in laboratory experiments. >

IC-processed electrostatic micro-motors

The authors describe the design, fabrication, and operation of several micromotors that have been produced using integrated-circuit processing. Both rotors and stators for these motors, which are driven by electrostatic forces, are formed from 1.0-1.5- mu m-thick polycrystalline silicon. The diameters of the rotors in the motors tested are between 60 and 120 mu m. Motors with several friction-reducing designs have been fabricated using phosphosilicate glass (PSG) as a sacrificial material and either one or three polysilicon depositions. Examples of stepping and three-phase synchronous drive micromotors are described. Typical drive voltages for present designs exceed 100 V. Manually switched motors have tested at speeds up to 12 r.p.m. Synchronous motors have been driven at speeds to 500 r.p.m.<<ETX>>

Silicon micromechanics: sensors and actuators on a chip

The techniques used to fabricate micromechanical structures are described. Bulk micromachining is routinely used to fabricate microstructures with critical dimensions that are precisely determined by the crystal structure of the silicon wafer, by etch-stop layer thicknesses, or by the lithographic masking pattern. Silicon fusion bonding has been used to fabricate micro silicon pressure sensor chips. Surface micromachining, based on depositing and etching structural and sacrificial films, allows the designer to exploit the uniformity with which chemical vapor deposition (CVD) films coat irregular surfaces as well as the patterning fidelity of modern plasma etching processes. Silicon accelerometers, resonant microsensors, motors, and pumps made by these techniques are discussed. Measuring the mechanical properties of silicon, which are important to these applications, is examined.<<ETX>>

IC-processed electrostatic synchronous micromotors

Abstract Micromotors having rotors with a diameter of 120 μm have been fabricated and driven electrostatically to continuous rotation. These motors are built using processes derived from IC micro-circuit fabrication techniques. Initial tests on the motors show that friction plays a dominant role in their dynamic behavior. Observed rotational speeds have thus far been limited to several hundred rpm, which is a small fraction of what should be achievable if only natural frequency were to limit the response. Experimental starting voltages (60 V at minimum and above 100 V for some structures) are at least an order of magnitude larger than had been expected. Continuous motor motion has been observed for as long as one minute under three-phase bias at 200 V. Observations of reverse as well as forward rotor rotation with respect to the driving fields can be explained in terms of the torque/rotor-angle characteristics and friction for the motors.

Magnetic microactuation of polysilicon flexure structures

A microactuator technology that combines magnetic thin films with polysilicon flexural structures is described. Devices are constructed in a batch-fabrication process that combines electroplating with conventional lithography, materials, and equipment. A microactuator consisting of a 400/spl times/(47-40)/spl times/7 /spl mu/m/sup 3/ rectangular plate of NiFe attached to a 400/spl times/(0.9-1.4)/spl times/2.25 /spl mu/m/sup 3/ polysilicon cantilever beam has been displaced over 1.2 mm, rotated over 180/spl deg/, and actuated with over 0.185 nNm of torque. The microactuator is capable of motion both in and out of the wafer plane and has been operated in a conductive fluid environment. Theoretical expressions for the displacement and torque are developed and compared to experimental results.