P. Hudek

MEMS pressure sensor fabricated by advanced bulk micromachining techniques

We present the design and implementation of a MEMS pressure sensor with an operation potential under harsh conditions at high temperatures (T = 300 – 800°C). The sensor consists of a circular HEMT (C-HEMT) integrated on a circular AlGaN/GaN membrane. In order to realize MEMS for extreme conditions using AlGaN/GaN material system, two key issues should be solved: (a) realization of MEMS structures by etching of the substrate material and (b) formation of metallic contacts (both ohmic and Schottky) to be able to withstand high thermal loads. In this design concept the piezoresistive and piezoelectric effect of AlGaN/GaN heterostructure is used to sense the pressure under static and/or dynamic conditions. The backside bulk micromachining of our SiC wafer in the first experiment started with FS-laser ablation down to ~200 -270μm deep holes of 500μm in diameter. Because no additional intermediate layer can stop the ablation process, the number of laser pulses has to be optimized in order to reach the required ablation depth. 2D structural-mechanical and piezoelectric analyses were performed to verify the mechanical and piezoelectric response of the circular membrane pressure sensor to static pressure load (in the range between 20 and 100kPa). We suggested that suppressing the residual stress in the membrane can improve the sensor response. The parameters of the same devices previously fabricated on bulk substrates and/or membranes were compared. The maxima of drain currents of our C-HEMT devices on SiC exhibit more than four times higher values compared to those measured on silicon substrates.

Laser ablation: A supporting technique to micromachining of SiC

We present an effective fabrication method of AlGaN/GaN membrane on SiC for MEMS sensors applications. It employs laser ablation as a supporting technique to the plasma enhanced etching methods. Circular patterns transferred deeply into bulk SiC substrates fabricated by ablation using (1) excimer laser and (2) femtosecond (fs) laser tools were compared. We found that the fs laser tool is more suitable for bulk micromachining of SiC because of the clearness of the process. The additional higher thermal load can be also suppressed. A simple laser cleaning procedure was found allowing us to fabricate deep structures without the ablation process retardation by debris formation.