Table of contents

Conjugated polymers have a number of interesting properties that can be exploited in microfabricated sensors and actuators. For example, polypyrrole is a conjugated polymer that can change volume to deliver significant stresses and strains. These materials can be patterned using conventional microfabrication techniques. The procedures for doing this are described in this paper, focusing on the microfabrication of polypyrrole microactuators. In addition, other methods for the deposition and patterning of conjugated polymers are reviewed. A special technique for releasing actuators, differential adhesion, is also detailed.

A model of thermomechanically driven plates as used, e.g., for acoustic microresonators is reported. The model describes the quasibuckling, excitation, fundamental frequency and vibration mode of multilayered plates under plane strain. The plates consist of stacked thin films with individual elastic modulus, Poisson's ratio, prestress, heat capacity and coefficient of thermal expansion. Membrane edges are elastically clamped to lateral supports. In thin plates two driving mechanisms are identified: the first couples the thermomechanical bending moment to the curvature of the vibration mode; the second couples the thermomechanical line stress to the periodic length change of the vibrating quasibuckled structure. Results of the model include vibration amplitude, sound field pressure, emitted power and quality factor as functions of the membrane properties. Maximum output is obtained close to the quasibuckling transition of the structures, where their resonance frequency is minimum. Largest sound pressures are generated by structures with rigid supports or with vanishing initial bending moment. The model shows excellent agreement with experimental data from micromachined resonant silicon membranes.

A novel design of micromachined capacitive microphone with inherently considerably higher sensitivity than traditional designs is presented in this paper. A process sequence is suggested and means to protect the structure against contamination with dust and humidity are considered. The structure is modelled numerically to calculate the optimum design and to compare with traditional microphone designs.

This paper presents a lumped-mass model especially developed for valveless diffuser pumps. It is implemented using MATLAB. The model is tested for different previously reported valveless diffuser pumps and shows good agreement with the experimental results. The model predicts the flow-pressure characteristics for different excitation levels. The model makes it possible to study flows and pressures inside the pump. The simulations show that the maximum excitation level for the valveless diffuser pump is probably limited by low chamber pressure. Modified designs are tested and it is shown that a pump with two serially connected pump chambers working in anti-phase is advantageous compared with a single chamber pump for both the maximum volume flow and maximum pump pressure. The simulations also indicate that scaling down the diffuser elements from an throat cross-sectional area to a throat cross-sectional area probably increases the attainable pressure head.

An adjustable inductor which is digitally controlled by microrelays has been made using combined surface and bulk micromaching technology. The microrelays were fabricated using a TaSi₂/SiO₂ bimorph cantilever beam, a gold-to-gold electrical contact, aluminum as sacrificial layer and a combined thermal and electrostatic actuation mechanism. The silicon substrate under the inductor region was etched out to reduce the parasitic oxide capacitors and the eddy current power loss in the substrate semiconductor bulk. Sixteen different inductance values ranging from 2.5 nH to 324.8 nH were obtained using a planar rectangular spiral coil and four microrelays. The minimum self-resonant frequency is 1.9 GHz. The lowest measured combined thermal power and electrostatic voltage for the actuation of microrelays are 8.0 mW and 20 V, respectively. The highest operation frequency of microrelays is 10 kHz limited by the mechanical self-resonance. The measured contact resistance typically ranges from 0.6 ohms to 0.8 ohms. The dimensions of the chip measure 3150×930 µm².

The anisotropic etching behavior of (110) silicon wafers in KOH and TMAH was studied with emphasis on ultra-deep microchannels. Effects which degrade the etching behavior when etching to a depth of the order of a millimeter were encountered and investigated. In particular, oxygen precipitates and their growth during high temperature processing apparently strongly affect the etching of the (111) and (110) planes and reduce the achievable anisotropy ratio. A new 1300 °C high temperature step significantly reduces these negative effects by dissolving oxygen precipitates back into the crystal. Additionally, the influence of pattern alignment, solution concentration and the solution dissolved silicon content as well as the influence of varying masking layers on the achievable anisotropy ratio were investigated and optimized.

The ultra-deep (UD) LIGA strategy for die fabrication that has features up to two millimeters in thickness and 100 microns in linewidth has been developed at SRRC, Taiwan. Here, a successive exposure strategy has been demonstrated to overcome the shortage of hard x-rays generated by a medium energy light source such as the 1.5 GeV Taiwan Light Source. Furthermore, this successive exposure process accumulates much more irradiation dosage in the photoresist that leads to a reduction of the developing time and thus produces a UD microstructure with very high aspect ratio. The present process makes use of a conformal mask technology that permits the sidewalls of the microstructure to be perfectly aligned after multiple exposures and developments. Since the total reflection of x-rays inhibits further dosage deposition on the sidewall after the first exposure, the successive exposure process improves the precision of the microstructure. The main concerns of fabricating a UD microstructure include the low diffusion speed involved in developing a high-aspect-ratio microstructure and the high residual stress between photoresist and substrate. A deeper microstructure can be achieved by choosing proper photoresist material, increasing the diffusion speed of development and designing a suitable structure to balance residual stress. A low temperature process is essential to keep the thermal stress from destroying UD microstructures.

An analytical model that can accurately predict the performance of a polysilicon thermal flexure actuator has been developed. This model is based on an electrothermal analysis of the actuator, incorporating conduction heat transfer. Heat radiation from the hot arm of the actuator to the cold arm is also estimated. Results indicate that heat radiation becomes significant only at high input power, and conduction heat losses to both the substrate and the anchor are mainly responsible for the operating temperature of the actuator under routine operations. Actuator deflection is computed based on elastic analysis of structures. To verify the validity of the model, polysilicon thermal flexure actuators have been fabricated and tested. Experimental results are in good agreement with theoretical predications except at high input power. An actuator with a 240 µm long, 2 µm thick, 3 µm wide hot arm and a 180 µm long, 12 µm wide cold arm deflected up to 12 µm for the actuator tip at an input voltage of 5 V while it could be expected to deflect up to 22 µm when a 210 µm long cold arm is used.

We have developed an integrated optical tracking and focusing sensor, which is a chip 1 mm by 0.5 mm. It can travel along guide patterns keeping a certain gap between itself and the guide pattern. It consists of a laser diode, monolithically fabricated photodiodes and microlenses having a graded refractive index distribution in the thickness direction. Experimental results showed that this integrated sensor can be used as a focusing and tracking sensor with sub-micron accuracy.

A design is proposed for a high-Q rf band-pass filter whose active element is a micromachined mechanical resonator. The device is located in a large magnetic field created by a miniature permanent magnet and can be configured to operate at frequencies extending into the low GHz regime.

A concept for the fabrication of low cost resonant scanning mirrors using standard microstructure technology is proposed. To demonstrate the process a mirror device for a head mounted display system was designed and fabricated. The mirror was successfully actuated using a post-mounted piezoelectric device. The monolithic design provides potential for extremely low cost replicated devices. We argue that microstructure technology can also benefit the production of relatively large scale devices.

A qualitative analysis is first carried out to determine the slip coefficient as a function of two dimensionless parameters. An intensive computation using the direct simulation Monte Carlo method is then carried out to simulate the Couette flows between two walls for different gas, number density, wall (plate) velocity (U_w), wall temperature (T_w) and distance between the two walls. Numerical results show that the slip coefficient is proportional to the mean free path _gw of molecules colliding with the wall, which is affected by the number density, the wall temperature and gas mass. The slip coefficient is finally found to be 1.125 _gw. This slip coefficient is verified for five gases, and validated under the conditions of U_w300 m s^-1 and T_w500 K. This slip coefficient is very useful for slip flow analysis in micro-electro-mechanical systems using N-S equations.

This paper describes the development of a fabrication and etching process for the in-situ formation of sacrificial layers in electrodeposited NiFe magnetic alloys. Sacrificial layers consist of iron-rich material electrodeposited in a nickel-rich matrix. The iron-rich layers are formed using a pulsed electrolyte agitation scheme. The removal of sacrificial layers is investigated using a concentrated acid etching procedure as well as a potential- enhanced etching technique. The formation of sacrificial layers in electrodeposited microgears and planar films is demonstrated. We find that glacial acetic acid preferentially removes the sacrificial layers at a rate of 0.5 µm hr^-1 while leaving the remaining nickel-rich structure unaffected. An applied potential is used to accelerate the etch rate of sacrificial material in dilute acetic acid as well as in a chloride-based etching solution. Under potential control, sacrificial layers are etched at rates approaching 80 µm hr^-1. The remaining nickel-rich matrix is not significantly affected during etching and retains its structural fidelity. NiFe sacrificial layers of varying compositions are shown to etch at rates that depend on the iron content. The implications for using these techniques in conventional through-mold plating applications are discussed.