Fabrication and characterization of SiO2 microcantilevers by direct laser writing and wet chemical etching methods for relative humidity sensing

Abstract

In the present work, SiO2 microcantilevers (MCs) of various lengths (50–330 μm, width – 37 μm, thickness – 0.98 μm) were fabricated using Direct Laser Writing (DLW) and wet chemical etching methods. An effective method to avoid “stiction” while releasing these MCs by wet chemical etching of underlying Si is presented. It is shown that introducing a sharp convex tip at the free end of the MC along with post-etch rinsing in boiling DI water, substantially reduces the stiction. Successfully released MCs were characterized using a 3D optical microscope (3D OM) and Nano Vibration Analyzer (NVA), for initial bending and resonance frequency measurements, respectively. 3D OM measurements revealed that released MCs are bending upwards (i.e. away from the wafer) and the peak deflection (zmax) value increases with increasing MC length (L). This is attributed to the presence of gradient residual stress (σmax) in SiO2 film and from zmax vs. L2 plot σmax was estimated to be 15.14 (± 0.32) MPa. The resonance frequency of the MCs was measured using NVA and found to be lower than the analytically estimated values, especially at smaller lengths. It is explained on the basis of the increase in the effective length of MCs due to undercutting. Finally, relative humidity (RH) sensitivity of the released MCs is demonstrated in dynamic mode, without any functionalization.