Laser fabrication of vibrating microcavities on silicon: Formation dynamics and frequency tunability

Abstract

Laser processing of silicon has long since become a standard method for wafer dicing, surface texturing, re-crystallization after ion implantation, as well as surface amorphization, to name a few examples. Here, we introduce a novel concept, the fabrication of vibrating microcavities in a commercial platform, Si wafers covered by a thick SiO2 layer, acting as a membrane. We demonstrate that by exposing our material system to single ultrashort laser pulses above the evaporation threshold of Si, a controlled delamination of the cover layer without fracture can be achieved, leading to the formation of a microcavity accompanied by a surface bulging up to 1.5 micrometer. The formation dynamics of the microcavities has been investigated by time-resolved optical probe techniques. Ultrafast pump-probe microscopy resolves the initial stages of substrate melting, the onset of layer expansion followed by relaxation. Real-time reflectivity measurements with nanosecond resolution resolve the long lived, highly complex oscillation process of the membrane, extending over tens of microseconds. The formed cavities can be re-excited at low fluence without damage or degradation, featuring vibrations at single frequencies with high Q factors. The frequencies can be tuned continuously from 8 MHz up to 19 MHz by adjusting the fabrication fluence. A simple classical model is found to well describe the behavior of the membranes. Our results pave the way for applications in micro- and nanooptics and acoustics, in great part due to the commercial availability of the Si-SiO2 platform and the simplicity of the fabrication method. An extension to the field of microelectromechanical system (MEMS) of this technology is proposed.