J.H.J. Fluitman

Dependence of the quality factor of micromachined silicon beam resonators on pressure and geometry

An experimental study of damping and frequency of vibrating small cantilever beams in their lowest eigenstate is presented. The cantilever beams are fabricated from monocrystalline silicon by means of micromachining methods. Their size is a few millimeters in length, a few 100 µm in width, and a few 10 µm in thickness. Damping and resonance frequency are studied as a function of the ambient pressure p (1–105 Pa) and the geometry of the beam. The purpose of this research was to obtain design rules for sensors employing vibrating beams. The analysis of the experimental results in terms of a semiqualitative model reveals that one can distinguish three mechanisms for the pressure dependence of the damping: viscous, molecular, and intrinsic. For viscous damping a turbulent boundary layer dominates the damping at high pressures (105 Pa), while at smaller pressure laminar flow dominates. In the latter region, this leads to a plateau for the quality factor Q and in the former to Q p. The pressure pc at which the transition from laminar flow dominated damping to turbulent flow dominated damping occurs depends on the geometry of the beams. pc is independent on the length and decreases with both, the width and the thickness of the beams.

A thermopneumatic micropump based on micro-engineering techniques

The design, working principle and realization of an electro-thermopneumatic liquid pump based on micro-engineering techniques are described. The pump, which is of the reciprocating displacement type, comprises a pump chamber, a thin silicon pump membrane and two silicon check valves to direct the flow. The dynamic pressure of an amount of gas contained in a cavity, controlled by resistive heating, actuates the pump membrane. The cavity, chambers, channels and valves are realized in silicon wafers by wet chemical etching. Experimental results are presented. Maximum yield and built-up pressure equal 34 ?l/min and 0.05 atm, at a supply voltage of 6 V. Results of simulations show good agreement with the actual dynamic behaviour of the pump.

A thermo-pneumatic actuation principle for a microminiature pump and other micromechanical devices

A new type of actuator for a microminiature pump is presented. The pressure of air in a cavity, raised by resistive heating, deflects a thin silicon membrane. The dynamics of membrane deflection is studied experimentally, the results being in excellent agreement with simulation. We conclude that the device is suitable as an actuator in a micro-miniature pump.

Performance of thermally excited resonators

A study of electrothermal excitation of micromachined silicon beams is reported. The temperature distribution is calculated as a function of the position of the transducer, resulting in stress in the structure which reduces the resonance frequency. Test samples are realized and measurements of resonance frequency, vibration shape and vibration amplitude are carried out. There is a satisfactory agreement between theory and experiment at small thermal stresses. Near the buckling load we find distinct deviations from theory which are ascribed to mechanical imperfections of the beams.

High-resolution shadow-mask patterning in deep holes and its application to an electrical wafer feed-through

The paper presents a technique to pattern materials in deep holes and/or on non-planar substrate surfaces. A rather old technique, namely, electron-beam evaporation of metals through a shadow mask, is used. The realization of high-resolution shadow masks using micromachining techniques is described. Further, a low ohmic electrical wafer foed-through with a small parasitic capacitance to the substrate and a high placing density is presented.

Thin-film ZnO as micromechanical actuator at low frequencies

A new model is proposed for the low-frequency piezoelectric activity of ZnO films grown on CVD SiO2. In this MOS structure, with ZnO as the semiconductor, a depletion layer is induced by means of a d.c. bias voltage. Using standard semiconductor theory, an expression is derived relating the electric field in this depletion layer with the driving a.c. and d.c. voltages. Due to the built-in charge at the ZnO-SiO2 interface, a depletion layer exists, even when no d.c. bias is applied. We measured the vibration amplitude at resonance of the tip of a silicon cantilever, upon which the MOS structure was deposited, as function of a.c. and d.c. voltages. The results show good agreement with calculated curves. Therefore, it can be concluded that thin-film ZnO can be used as a piezoelectric actuator for micromechanical devices working at low frequencies.

Selective mode excitation and detection of micromachined resonators

Distributed mechanical systems, such as micromachined resonant strain gages, possess an infinite number of modes of vibration. Generally, one is interested in only one or a few modes. A method is described with which only the desired modes are excited and detected. This is achieved by geometrically shaping the elements used for excitation and detection of the vibration. The method is based on the orthogonality principle, which is valid for a variety of structures and vibrations. The work described is restricted to transversal vibrations of prismatic beams, clamped on both sides (microbridges). The effect of axial stress on the suppression of unwanted modes is discussed and the design rules for obtaining the shapes for most commonly used excitation and detection mechanisms are deduced. Experimental results for some excitation mechanisms are presented.<<ETX>>

Frequency dependence of thermal excitation of micromechanical resonators

theoretical and experimental study is presented on thermal excitation of micromechanical resonators. It is shown that there is a turnover frequency, ωt, which is related to the square of the ratio of the penetration depth of the thermal wave into the structure and its thickness. For constant power dissipation at the surface of the structure, the bending moment is independent of the frequency ω at ω > ωt, it is proportional to ω-1. At high frequency we obtain a phase shift of -π/2. Vibration amplitudes and turnover frequencies measured on clamped beam structures agree well with the theory.

Microsystems technology: objectives

This contribution focuses on the objectives of microsystems technology (MST). The reason for this is two fold. First of all, it should explain what MST actually is. This question is often posed and a simple answer is lacking, as a consequence of the diversity of subjects that are perceived as MST. The second reason is that a map of the somewhat chaotic field of MST is needed to identify sub-territories, for which standardization in terms of system modules an interconnections is feasible. To define the objectives a pragmatic approach has been followed. From the literature a selection of topics has been chosen and collected that are perceived as belonging to the field of MST by a large community of workers in the field (more than 250 references). In this way an overview has been created with `applications? and `generic issues? as the main characteristics.

Sacrificial wafer bonding for planarization after very deep etching

A technique is presented that provides planarization after a very deep etching step in silicon. This offers the possibility for not only resist spinning and layer patterning but also for realization of bridges and cantilevers across deep grooves or holes. The technique contains a standard dry film lamination step to cover a wafer with a 38 mu m thick foil. Next the foil is etched back to the desired thickness of a few micrometres. This thin film facilitates resist spinning and high-resolution patterning. The planarization method is demonstrated by the fabrication of aluminium bridges across a deep groove in silicon.