Characterization of crystallized thick, highly doped PECVD a-Si:H layers for fabrication of surface micromachined MEMS inertial sensors

This study evaluates the feasibility of thick crystallized amorphous silicon (14–21 µm) layers deposited by a plasma-enhanced chemical vapor deposition process (PECVD) as a structural material for the realization of MEMS inertial sensors. The aim of this study was to evaluate the thickness uniformity, sheet resistance uniformity, and mechanical stress characteristics of crystallized amorphous silicon. The fabrication process involves the deposition of amorphous silicon layers using PECVD processes with PH<inf>3</inf>:H<inf>2</inf> flow rates of 200 sccm and 500 sccm, followed by controlled crystallization through annealing at temperatures ranging from 750 °C to 850 °C. Both sacrificial silicon oxide and structural amorphous silicon layers are processed on a single PECVD equipment platform. Characterization techniques include wafer-level thickness measurements, sheet resistance mapping, and stress-deflection analysis to identify relations between the process and the material properties of the crystallized a-Si:H films. The results show that 200 sccm PH<inf>3</inf>:H<inf>2</inf> flow rate delivers significantly superior thickness and sheet resistance uniformity compared to 500 sccm conditions. Crystallization temperatures above 800 °C substantially improve the uniformities on the wafer and enable a precise stress and stress-deflection tuning. The crystallized material exhibits nearly isotropic characteristics with a controllable transition from tensile to compressive stress by adjusting the crystallization temperature. Near-zero stress conditions are achieved for the samples deposited with 200 sccm PH<inf>3</inf>:H<inf>2</inf> addition at temperatures between 750 °C and 800°C. Additionally, chemical mechanical planarization and deep reactive ion etching processing times are notably reduced while maintaining excellent compatibility with established micro surface micromachining processes. In conclusion, the new crystallized PECVD amorphous silicon process offers a viable and advantageous alternative to epitaxial polysilicon, providing superior uniformity and valuable engineering flexibility for the design of stress-compensated devices for high-volume manufacturing of consumer inertial sensors.