Performance characteristics of second generation polysilicon resonating beam force transducers

Abstract

Polysilicon resonating beam force transducers and their performance characteristics are studied. Doubly clamped beams in vacuum display a shift in resonant frequency with applied axial load. Functionality, miniaturization, and batch fabrication were accomplished using surface micromachining techniques with tensile, fine-grained polysilicon as the construction material. The device could be a very accurate, cost-effective alternative to presently available force transducers if it displays sharply defined and stable resonant frequencies. The devices are driven into resonance by means of electrothermomechanical or capacitive forces. Analytical expressions are derived to illustrate the detrimental effect the drive mechanisms have on the resonant frequency. Material and process sensitivities are calculated from a closed-form expression for the fundamental resonant frequency. Experimental beams have typical dimensions of 200- mu m length, 45- mu m width, and 2- mu m thickness. The fundamental unloaded resonant frequency is near 650 kHz and can be adjusted by processing. It will shift to nearly 900 kHz with an applied strain level of +0.1%. A temperature coefficient of the frequency of -75 p.p.m./ degrees C for the finished batch-fabricated device was demonstrated. Theory and experiment are in agreement, showing that the resonant frequency stability and not the ability to measure frequency limits the force resolution.