Mechanical characterisation of a polysilicon accelerometer

This paper describes a measurement system which has been developed to determine, experimentally, the dynamic characteristics of microstructures. These characteristics include dynamic mode shape of free vibration, natural frequencies and modal damping factors. The system is demonstrated by determining the dynamic characteristics of a polysilicon accelerometer designed and produced at DRA Malvern. The accelerometer takes the form of a 2 μm thick polysilicon plate supported at as corners by beam like ligaments. Acceleration causes bending defections of the plate against the elasticity of the ligaments and this is taken as a measure of acceleration. The mechanical design of the sensor was carried using finite element analysis (ANSYS). In the experimental evaluation of the dynamics of the accelerometer the sensor was mounted within a general purpose piezoelectric staging and excited into vibration by applying a swept sine excitation to the piezoelectric. Using a laser vibrometer the notion of an array of selected test points on the accelerometer was measured and point frequency response functions calculated over a frequency range of 0 to 100 kHz. Using this data the dynamic mode shapes, natural frequencies and modal damping factors were calculated and the experimental behaviour of the sensor observed by animating a wire framed drawing of the structure. The mechanical damping of the accelerometer is determined by the behaviour of the squeeze film between the plate and the supporting substrate