Tuning residual stresses in ultrathin membranes for resonant frequency control

This master thesis proposes a design and fabrication process for ultrathin CMOS-compatible membranes with resonant frequency tuning enabled by residual stress control in thin film stack. In recent years, interest towards non-invasive monitoring of industrial high-voltage power electronics equipment has strongly increased, especially in the field of partial discharge detection. Partial discharges are incomplete dielectric breakdown in electric insulation material causing long term deterioration and eventually destruction of the affected piece of equipment. Such events can be detected using membrane based ultrasound microphones. This works explores the possibility to increase the sensitivity and the embeddability of membranes. A simple analytical design process based on Matlab modeling is proposed and confirmed by Comsol simulations of multilayer membranes. Eight designs are proposed, four square and four circular designed for resonant frequencies ranging from 70kHz to 300kHz. The design parameters are the thickness of the membrane, its width and the equivalent residual stress across the full thin film stack and are respectively fixed at 1µm, 400 to 1600µm and 50MPa. Simulations and models show close alignement within less than 5% for first mode resonant frequency. Fabricated membranes have measured thickness around 975nm, width within 40µm margin above the design and measured equivalent residual stress in stack around 70MPa. They are characterized using laser vibrometry. Their measured first mode resonant frequency lies within a 5% error margin around the expected value from models. Comparison between detected frequency for higher mode and Comsol simulation results show maximum 20% of error.