Inertial sensors may be formed using micromachining processes and may include microelectromechanical systems (MEMS) devices. In MEMS devices, certain micromachined structures are designed to move relative to a substrate and other micromachined structures in response to forces applied. The operation of the MEMS device is dependent on the ability of the device to sense acceleration, motion and tilt. This may be achieved by suspending a proof mass with sense fingers or beams that are free to move between a set of fixed fingers. When the MEMS device undergoes acceleration, the proof mass moves resulting in a change in capacitance between the fixed and moving fingers. This change in capacitance is a measure of the acceleration applied. The sensitivity of the MEMS device is dependent on the capacitance change per applied g-force and the ability of the design to measure this change. Consequently, the sensitivity may be dependent on the area of the beam sidewalls (e.g., thickness, length and number of beams), spacing between the beams and the stiffness of restoring springs. Thus, one way to improve the MEMS device performance is to increase the thickness of the fingers or beams. The thicker films improve sensor signal to noise performance and also provide greater vertical stiction performance.
Standard MEMS device processing may involve thin film deposition, photolithography and etching techniques that are well known to those skilled in the art. During MEMS processing, multiple layers of material may be deposited on a substrate and then selectively etched away. A sacrificial layer is then removed leaving some structures suspended above or formed in the surface of the substrate. In MEMS inertial sensors, the sensor material used to make the sensor element is typically polysilicon and the sacrificial material that is etched away to release the polysilicon beams are typically deposited oxides. As a result of the manufacturing process, internal stresses in the various MEMS device components may cause their relative position to change after the release compared to their position before it. In order to have a properly functioning inertial sensor, the beams should remain substantially flat after the release. For the polysilicon beams to remain flat after release, the film layer should have a residual tensile stress and a low stress gradient through the thickness of the film. For example, a residual tensile stress prevents buckling of bridge structures that are anchored at both ends. Also, a low stress gradient prevents bending of the released beams. However, depositing a thick polysilicon layer for the beams may lead to an undesirable stress or stress gradient within the thick polysilicon layer, and/or may even result in film cracking.