Characterization of membrane curvature in micromachined silicon accelerometers and gyroscopes using optical interferometry

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Abstract
Micromachined silicon sensors often exhibit curvature of released membrane structures due to internal stresses, doping gradients, and crystalline defects. This curvature can be a significant source of error in inertial sensors such as accelerometers and gyroscopes. Development of process conditions that reduce curl requires a rapid, accurate method for obtaining high-resolution flatness data over a complex two-dimensional surface. This work reports on the use of a commercially-available, nondestructive optical characterization tool that provides high-resolution profiles of micromachined structures. This interferometry technique is shown to be a significant extension of traditional process development tools, such as scanning electron microscopy (SEM) and test structures. Statistical information on flatness of test structures, accelerometers, and gyroscopes is reported as a function of processing conditions. Unexpected and previously undetected phenomena are revealed by the interferometry measurement. Optimization of the diffusion-annealing cycle provides structures that are flat to within 0.1 micrometers. The flatter parts now being produced have contributed to recent advances in the performance of Draper Laboratory's inertial sensors.

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