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.\ud \ud 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.\ud \ud 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 Thornton's structure zone model

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

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.\ud \ud These films are applied as piezoelectric transducers in micromechanical sensors and actuators. Their piezoelectric behaviour strongly depends on the electric properties.\ud \ud 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

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

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