Extraction of residual stress and dimensions from electrical measurements on surface micromachined test structures

The deposited thin films in surface micromachining have a lot of residual stress, and it is essential to measure this for both process development and monitoring. We estimate residual stress by electrical measurements on a series of fixed-fixed polysilicon beams designed to deflect laterally due to stress. To minimize errors in estimation during parameter extraction, the device dimensions also have to be measured accurately. Surface micromachining of an oxide-anchored polysilicon cantilever beam can result in beam undercut, reduction in beam thickness, and increase in the gap between the beam and the substrate. The undercut in the beam is estimated from the resonance frequency of the cantilever beam and also by using polysilicon resistors. Final device thickness is obtained by measuring the resistance in fixed-fixed beams.