Manufacturing of membranes by laser ablation in SiC, sapphire, glass and ceramic for GaN/ferroelectric thin film MEMS and pressure sensors

Abstract

AlGaN/GaN-based high electron mobility transistors (HEMTs), Schottky diodes and/or resistors have been presented as sensing devices for mechanical or chemical sensors operating in extreme conditions. Creation of appropriate diaphragms and/or cantilevers out of substrate materials is necessary for further improvement of sensing properties of such MEMS sensors. The sensitivity of the AlGaN/GaN based MEMS pressure sensor can be modified by membrane thickness. In case of SiC as substrate material of the epitaxial AlGaN/GaN heterostructure layers, we applied laser ablation technique for micromachining of the membranes. We were able to verify the feasibility of this process by fabrication of micromechanical membrane structures also in bulk 3C-SiC, borosilicate glass, sapphire and Al2O3 ceramic substrates by femtosecond laser (520 nm) ablation. On a 350 µm-thick 4H-SiC substrate we produced an array of 275 µm deep and 1000---3000 µm in diameter blind holes without damaging the 2 µm GaN layer at the back side. Our experiments indicate that pinhole defects in the ablated membranes are affected by ripple structures related to the polarization of the laser. We developed an ablation technique inhibiting the formation of pin holes caused by laser induced periodic surface structures (LIPSS).