MEMS—suspended gate field effect transistor (SGFET) accelerometer process integration with parylene flexure and plasma doping

This manuscript presents the design, process integration, fabrication, and characterization of an out-of-plane suspended gate field-effect transistor (SGFET)-based parylene-C MEMS accelerometer. A comprehensive analysis and simulation were conducted in semiconductor, mechanical, electromechanical, and circuit domains. The multidisciplinary analysis was aimed to optimize key specifications of the SGFET and MEMS structures, including substrate doping, pull-in voltage and range, supply voltage, parylene-C polymer thickness, load resistance, and other critical parameters. The parylene-C polymer is used as the MEMS structural material, providing low Young’s modulus, better thermal stability, and chemical inertness. Surface micromachining process is developed to realize parylene MEMS suspended gate of SGFET. Plasma doping process was adopted to realize shallow source and drain junction at low doping energy. The threshold voltage (V_TH) of SGFET with 1 µm air-gap (t_air) was 1.7 V and the effective charge density (Q_eff) was 1.26 × 10⁻⁸ C cm⁻². The laser Doppler vibrometer and nanoindenter based experiments were carried out on the fabricated accelerometer shows a resonant frequency of 7.2 kHz and a stiffness of 6 N m⁻¹, respectively. A common source amplifier with SGFET realized on a printed circuit board followed by mounting the complete sensor on a custom fixture for a tilting experiment to observe the response of the accelerometer in the dynamic range of ±1 g. The sensitivity and the non-linearity at the output were 4.67 mV g⁻¹ and ∼5%, respectively.