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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 (fO N 6 kHz) is measured with an external load force up to 0.4 N. The absohite 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. electrical excitation and detection of the vibration of the sensor beam using the piezoelectrical effect of a ZnO layer [ 1, 31. The bottom electrode is a highly boron doped (p++-)silicon layer and the top one is an aluminium layer. In order to improve the efficiency of the electromechanical transduction of the ZnO at low frequencies, a p + +-ZnO structure is used. This structure will have a better efficiency than the pi + SiO,-ZnO (MOS) structure presented in ref. 4 because of the complete depletion of the ZnO layer [5] and the absence of a voltage drop across the oxide layer in the MOS structure.

An important design consideration in the development of two-port resonant sensors is the electrical cross-talk between the input port and the output port. The overall transfer function if(jo) of the two-port sensor is equal to the vectorial sum of a transfer function representing the mechanical behavior and a transfer function representing the electrical cross-talk. The resonant silicon beam force sensor with a piezoelectric driver and a piezoelectric detector is analyzed. Two solutions to reduce the level of cross-talk are presented. Proper adjustment of the value of relevant components in the sensor geometry and/or electrical interruption of the bottom electrode result in a cross-talk level close to the noise level for the frequency range of interest.