Simple optical characterisation for biomimetic micromachined silicon strain-sensing structure

This paper presents an on-going work to develop micromachined silicon-based strain sensor inspired from the campaniform sensillum of insects. We present simple optical setup for the characterisation of a membrane-in-recess structure as an early stage in mimicking the natural sensor. The microstructure is a 500 nm-thick SiO2/SiN circular membrane, burried 13 μm from the surface of a 3x3 mm, 525 μm thick Si-chip. The chip was attached to a 45x10x0.525 mm Si beam. The simple optical characterisation setup is based on imaging the reflected laser beam from the biomimetic structure. Since an optical cavity between the membrane and the Si beams beneath was formed, ideal flat-parallel FabryPerot interferometer equation was applied to interpret the results semi-quantitatively. We obtained 2-D interference fringe pattern having 3 orders of maxima from the middle to the edge of the circular apperture, as a result of an initial internal membrane stress. The pattern changed non-linearly as we applied flexural strain from behind the beam up to 50 μm, most probably caused by nonlinear deflection of the membrane (i.e. the membrane did not deflect similarly as the beam beneath it). This phenomena might explain one of the strain-amplifying properties of this biomimetic strain sensing microstructure.