F.R. Blom

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.

R.F. planar magnetron sputtered ZnO films I: structural properties

The structural properties of r.f. planar magnetron sputtered ZnO films are studied as a function of deposition parameters: substrate type, substrate temperature, sputter gas pressure, growth rate and sputtering power. These films are applied as piezoelectric transducers in micromechanical sensors and actuators. The electric properties, and consequently the piezoelectric behaviour, depend strongly on the structural properties of the layers. All films are polycrystalline. The individual grains are highly oriented with their crystallographic c axis perpendicular to the substrate. Crystalline substrates such as silicon or SiO2 induce a growth of small grains, a few hundredths of a micron wide and long. Amorphous substrates such as metals or amorphous SiO2 induce a growth of broad columnar grains extending through the film thickness and a few tenths of a micron wide. Trends in density and grain size are in agreement with Thorntons structure zone model.

R.F. planar magnetron sputtered ZnO films II: Electrical properties

The electrical properties of r.f. planar magnetron sputtered ZnO films are studied by means of current-voltage, capacitance-voltage and Van der Pauw measurements. These films are applied as piezoelectric transducers in micromechanical sensors and actuators. Their piezoelectric behaviour strongly depends on the electric properties. A conduction model for the polycrystalline ZnO layers is presented. This model gives a good description of the electrical behaviour, and is useful in understanding the piezoelectric properties of the films studied.

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.

Resonating silicon beam force sensor

A resonating silicon-beam force sensor is being deveoped using micro-machining of silicon and IC-compatible processes. Results are reported here of measurements on the force-to-frequency transfer of bare silicon prototypes. The measurements with forces on the sensor beam up to 0.4 N shows a frequency shift of 3.1 to 5.2 times the unloaded resonance frequency f0(f0 congruent with 3 to 5 kHz), depending on the exact dimensions. Considering these figures, we can predict a frequency shift of 18.3 to 27.6 kHz at the maximum load of 1.0 N for the measured samples. Due to the sample lay-out, a force transfer is present from the externally applied force to the actual pulling force on the sensor beam. Using a simple model to calculate this reduction, we obtain good agreement between the measurements and predictions.

Piezoelectrically driven silicon beam force sensor

A resonant silicon beam force sensor with piezoelectric excitation and detection is being developed. The realization is based on IC and thin-film technology with ZnO as the piezoelectrical layer. The theory, realization and measurements of a bent-frame sensors are described. A frequency shift of about 3.3 times the unloaded resonance frequency f0 (f0 congruent 6 kHz) is measured with an external load force up to 0.4 N. The absolute sensitivity of the force sensor is 64 kHz/N and the full-scale sensitivity is 29 kHz/N. Using a simple model for the load-force transduction from external to sensor force, the measurements are in good agreement with the theory.

Excitation and detection of vibrations of micromechanical structures using a dielectric thin film

A new technique is introduced for both the excitation and the detection of vibrations of micromechanical structures. This makes use of a dielectric thin film, sandwiched between lower and upper electrodes, on top of the vibrating structure. The excitation is based on electrostatic forces between the charged electrodes, causing deformation of the dielectric film and bending of the multilayer structure. The detection of the vibration is capacitive, based on the fluctuation of the capacitance due to the deformation of the dielectric film. Experimental results for a stoichiometric silicon nitride dielectric film on top of a silicon cantilever agree well with predicted values. The yield of the electrostatic excitation as well as of the capacitive detection are satisfactory.

Transduction mechanisms and their applications in micromechanical devices

Transduction mechanisms and their applications in micromechanical actuators and resonating sensors are presented. They include piezoelectric, dielectric, electro-thermo-mechanic, opto-thermo-mechanic, and thermo-pneumatic mechanisms. Advantages and disadvantages with respect to technology and performance are discussed.<<ETX>>

Optically activated ZnO/SiO2/Si cantilever beams

The photomechanical effect induced by periodically varying sub-bandgap illumination in thin ZnO films deposited on oxidized Si has been demonstrated for the first time. The efficiency of this effect is at least one order of magnitude higher as compared to the photothermal activation of Si. Thus it can be considered as a powerful optical drive for resonant sensors. A phenomenological model of the mechanisms involved in the process is proposed. The optomechanical effect can also be used as a complementary method in determination of the surface state parameters of ZnO films.