Length effect on the plastic deformation of SiO2 microcantilevers

Abstract

SiO2 films are of much interest for the realization of suspended micromechanical structures such as microcantilevers and micro-bridges owing to their superior characteristics when compared to conventional c-Si. However, presence of residual stress induced bending and structural plastic deformation in these devices especially at elevated temperatures hinder their potential for real-time field applications. Moreover, the impact of micro-scale dimensional effects on the magnitude of deformation in these devices is not well understood. In this work, we have investigated the effect of post-release high temperature annealing on the bending and resonance frequency characteristics of SiO2 microcantilevers (MCs) with a specific focus on the length effect. For this purpose, SiO2 MCs of various lengths (Length: 50–190 μm, Width: 37 μm and thickness: 0.98 μm) were fabricated using direct laser writer and wet chemical etching methods and were subjected to high temperature annealing under N2 and O2 environments. We demonstrate through our experiments that the curvature of SiO2 MCs increases with both annealing temperature and length square of MCs. Our experiments revealed that, apart from onset of plastic deformation, stress accumulation at the fixed end of SiO2 MCs is also responsible for the observed curvature increase. From the measured curvature, mean (σ0) and gradient (σ1) residual stress values were estimated and found to increase with annealing temperature. Interestingly, there exists a critical length of MCs (90 μm) below which both σ0 and σ1 were found to increase further with reducing MC length and is shown to be due to the existence of large transverse curvature at these dimensions. High temperature annealing was also found to increase the resonance frequency of MCs and the increase was maximum ∼ 8% for 130 μm long MC. Various possible reasons for the frequency shift is discussed in detail.