Jan Söderkvist

Gallium arsenide as a mechanical material

The aim of this work is to introduce GaAs as a mechanical material to those working primarily with silicon in micromechanics, and to give an update of the micromechanical properties of GaAs. Mechanical properties, some promising response mechanisms for micromechanical sensors, and recent micromechanical applications are reviewed for GaAs, and its best developed alloying system, the AlxGa1-xAs ternary.

High-sensitivity surface micromachined structures for internal stress and stress gradient evaluation

The internal stress and stress gradient of thick () and thin () polysilicon films were evaluated with surface micromachined test structures. The structure that measured internal stress consisted of actuator beams rotating an indicator through an angle corresponding to the stress. The indicator deflection was measured in an SEM. Finite element analysis (FEA) was used both to optimize the design and to calibrate the structure. A folded beam design was used to minimize the total area the structure occupied so that it could be incorporated in the wafer layout of other surface micromachined details, and used for online process diagnostics. The indicator was provided with a Vernier scale to facilitate quick evaluation in an optimal microscope. The stress gradient was measured from the deflection of long () cantilever beams. The deflection was measured in an optical microscope and the output was calibrated with FEA calculations.

Piezoelectric beams and vibrating angular rate sensors

A tuning fork angular rate sensor made out of a single piece of quartz has been studied. The piezoelectric effect is used both to excite a reference vibration in the plane of the tuning fork and to detect a vibration normal to this plane. The amplitude of the second vibration is directly proportional to the applied angular velocity. The structure is made rigid in order for it to survive in a harsh environment. This implies that the only vibrationally active areas are the tines of the tuning fork. The performance of the sensor is predicted with the help of a phenomenological piezoelectric beam theory. This theory shows that it suffices to study the two-dimensional (2-D) dielectric field in the cross-sections of the beams in order to obtain the values of the piezoelectric equivalent components. Estimates of these values can be obtained without the use of special computer programs. The predictions are shown to be in agreement with measurements.<<ETX>>

The piezoelectric effect of GaAs used for resonators and resonant sensors

The piezoelectric effect in GaAs is investigated for potential use in sensor and resonator applications. Flexural resonant vibrations of GaAs tuning-fork structures have been excited at 33 kHz, with measured performance in excellent agreement with predictions. Longitudinal vibrations can equally well be activated. High Q-values at atmospheric pressure (Q=570000 at 4 MHz) have been measured for thickness shear vibrations of (100) GaAs wafers, using lateral electric field excitation. The temperature and stress dependences of the resonance frequency are important features when designing GaAs sensors. The temperature dependences of the frequency range from -59 ppm degrees C-1 to -48 ppm degrees C-1, and the stress dependences are at least 200 ppm MPa-1 and 39 ppm MPa-1 for tensile stress of flexural and shear vibrations, respectively. The semiconducting nature of GaAs affects the piezoelectric behaviour only at high temperatures. The successful experiments open up many possibilities of new resonator and resonant sensor concepts.

Anisotropic etching of Z-cut quartz

The etch rate in monocrystalline quartz depends on the crystalline orientation. Etch-rate diagrams for micromachining of monocrystalline quartz in, for instance, hydrofluoride-based etchants, are a necessity if one requires the best manufacturing conditions for an etched structure. In this paper we use the development of side-wall profiles in etched grooves, on a Z-cut quartz wafer, to produce two-dimensional etch diagrams. The etch conditions are eight combinations of temperature, from 22 degrees C to 80 degrees C, and etchant mixtures of HF and NH4F diluted in water.

Similarities between piezoelectric, thermal and other internal means of exciting vibrations

There is a striking similarity between piezoelectric, thermal and other internal volume forces. Thus, methods used for understanding and predicting piezoelectric vibrations can be used for, for instance, thermally activated vibrations in silicon structures. The author shows such a method for beam-shaped structures. A comparison between the efficiency of thermal and piezoelectric excitations, as well as with external electrostatic excitation, is made. The complex problem of correct choice of elastic constants is mentioned.

Micromachined sensor structures with linear capacitive response

Abstract Capacitive membrane-type pressure sensors normally have nonlinear pressure-capacitance relationships, implying the need for sophisticated electronics. Various capacitive pressure sensor structures with improved linearity are suggested in this article. These include membranes that touch the bottom of the cavity, corrugated or thinned membrane edges with stiffened membrane centers, as well as a sensor structure with a double-comb design. The design possibilities increase with these methods. Most work was carried out with the help of finite element analysis (FEA). Pressure sensors have been manufactured based on the FEA simulations, and the experiments verify the calculations. The results show very promising linearity properties, with nonlinearities less than 1%, and sensitivities around 10 pF/bar. Weaknesses introduced in the membrane can improve performance if they are located correctly.

Design of a solid-state gyroscopic sensor made of quartz

Abstract An angular rate sensor made out of a single piece of quartz has been studied. The piezoelectric effect in quartz is used both to excite a reference vibration in the plane of a tuning fork and to sense the vibration normal to this plane due to an externally applied rotation. The amplitude of the second vibration is directly proportional to the angular velocity of the applied rotation. In con- trast to ordinary types of tuning-fork gyroscopes, this sensor uses its stem only as a support. It also utilizes a fairly large resonance frequency. This enables the sensor to survive in a vibration-rich enviromnent. The well-established means of manufacturing used by watch crystal manufacturers can be used.

Characterization of an inchworm prototype motor

Abstract A new driving principle for an active joint intended for, e.g., a microrobot, has been evaluated. Piezoelectric bimorphs are used as actuator elements, and quasistatic positioning in combination with an inchworm type of repetition produces the rotation. The performance of a macroscopic prototype has been investigated and compared with an analytical model and with finite-element analysis. The agreement between the model and the prototype is good, and it is believed that the characteristics of a microsized active joint can be estimated from the analytical model. The difference in size between the prototype and the proposed micromotor is a factor of 100. A micromotor is expected to have a very high torque-to-volume ratio (3 kN m −2 ). Its power and speed limits are believed to be sufficient for an operational micromotor. Other characteristics that make it well suited for microrobotics are: no gliding contact causing wear, simple design and locked rotor when the voltage is turned off. A possible fabrication process, microassembly, is demonstrated. A bimorph microelement has been successfully bonded to a support on a substrate and the expected behaviour has been verified.

Electric equivalent circuit for flexural vibrations in piezoelectric materials

General expressions for the values of the components of an electric equivalent circuit are derived for flexural vibrations. These expressions are applied to some interesting electrode configurations. The obtained values are in good agreement with experimental values and values specified by manufacturers of watch crystals. It is noted that solving Laplaces equation for the dielectric field is sufficient in order to obtain the values of the vibration amplitude, the piezoelectric current, and the equivalent components. The piezoelectric part of the electric field need only be considered if very accurate values of the resonance frequencies are desired. It is shown how accurate estimates can be obtained without the need of advanced calculation tools.<<ETX>>