Understanding the Nonlinear Behavior of a new z-Axis MEMS Accelerometer with In-Plane Readout

This work presents a comprehensive study of the source of nonlinearities in a novel z-axis MicroElectroMechanical Systems (MEMS) accelerometer fabricated using a two silicon layers fabrication process. The device features a unique mechanical architecture that converts the out-of-plane motion of the proof mass into linear in-plane displacement of the sensing frames, enabling efficient capacitive readout. Initial experimental characterization revealed an unexpected non-linearity, exceeding predictions of the ideal mechanical model. To investigate the origin of this behavior, a detailed 3D finite element method (FEM) analysis was performed, incorporating fabrication-induced effects such as substrate deformation and residual stresses. Simulations demonstrated that substrate deformation has negligible impact within the operational range, while residual pre-stresses on the structural silicon layer strongly influences the device response, producing non-linearity levels consistent with experimental measurements. The close agreement between FEM predictions and experimental data validates the model and identifies residual pre-stresses on the structural silicon layer as the dominant factor affecting the device linearity. These insights provide a clear pathway for future design optimization, suggesting that careful control of residual stress and potential structural modifications can significantly improve the performance, linearity, and reliability of subsequent generations of z-axis MEMS accelerometers.