Laser ablation for membrane processing of AlGaN/GaN- and micro structured ferroelectric thin film MEMS and SiC pressure sensors for extreme conditions

Abstract

? COPYRIGHT 2015 SPIE.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. In addition we investigate ferroelectric thin films for integration into micro-electro-mechanical-systems (MEMS). Creation of appropriate diaphragms and/or cantilevers out of SiC is necebary for further improvement of sensing properties of such MEMS sensors. For example sensitivity of the AlGaN/GaN based MEMS prebure sensor can be modified by membrane thickneb. We demonstrated that a 4H-SiC 80?m thick diaphragms can be fabricated much faster with laser ablation than by electrochemical, photochemical or reactive ion etching (RIE). We were able to verify the feasibility of this proceb by fabrication of micromechanical membrane structures also in bulk 3C-SiC, borosilicate glab, sapphire and Al2O3 ceramic substrates by femtosecond laser (520nm) ablation. On a 350?m thick 4H-SiC substrate we produced an array of 275?m deep and 1000?m to 3000?m of diameter blind holes without damaging the 2?m AlN layer at the back side. In addition we investigated ferroelectric thin films as they can be deposited and micro-patterned by a direct UV-lithography method after the ablation proceb for a specific membrane design. The risk to harm or damage the function of thin films was eliminated by that means. Some defects in the ablated membranes are also affected by the polarisation of the laser light. Ripple structures oriented perpendicular to the laser polarisation promote creation of pin holes which would perforate a thin membrane. We developed an ablation technique strongly inhibiting formation of ripples and pin poles.