A Technique for Noncontact Measurement of Cavity Lengths and Deflection Shape in Optical MEMS Pressure Transducer

Noncontact measurements are very important for the characterization of MEMS devices. In this paper, a novel technique for noncontact measurement of cavity length (CL) and deflection shape (DS) in an optical MEMS pressure transducer is reported. The technique introduces a multimode optical fiber terminated with an `angle polished connector (APC)' for noncontact and convenient measurement of cavity length in the device. An interferometer is formed between two nearest surfaces of glass wafer and the silicon diaphragm in the device. The diaphragm is already in deflected position under the normal ambient pressure. The APC based `fiber probe' is positioned at some gap above the glass surface and scanned in X-Y plane to measure the cavity length at various positions over the deflected silicon diaphragm. The signals are achieved very easily even with a millimeter gap of the probe above the glass and hence noncontact scanning in X-Y plane becomes possible. The optical detection is based on white light interferometry (WLI) for absolute measurement of cavity length. The deflection shape of square anisotropic silicon diaphragm was derived from measured CL values. A deflection shape function (DSF) was fitted on experimental data. The presented technique facilitates nondestructive evaluation of the device post-fabrication and offers a low cost metrology system.