Deformation Reduction of a MEMS Sensor by Stress Balancing of Multilayer

Stress balancing of a multilayer is presented to reduce out-of-plane deformation of a MEMS structure for infrared (IR) sensor application. The MEMS structure is designed to have a suspended membrane with two supporting legs, and fabricated by surface micromachining using a sacrificial polyimide. The uncontrolled stress of Si₃N₄/a-Si:H/Si₃N₄ multilayer resulted in a large deformation of the membrane, likely causing problems in IR absorption and thermal isolation of the sensor. Flat membrane was successfully achieved by minimizing the residual stress of individual films and adjusting their relative thickness and distribution. Based on simple stress analyses, the deformation of sensor structure is suggested to be dominated by gradient stress rather than mean stress of the multilayer, likewise that of a monitoring cantilever.