In this work, we present a pressure sensor based on diamond coated AlGaN/GaN diaphragm with integrated high electron mobility transistor (HEMT). The influence of the diamond film thickness (in the range of 1 μm to 50 μm) on the properties of the AlGaN/GaN diaphragm is studied by finite element simulation method (FEM). The effect of thermal buckling as well as the induced piezoelectric charge of HEMTs as a function of pressure and temperature is investigated. It was found out that diamond coated sensor better prevents the effect known as thermal buckling of the diaphragm at elevated temperature. Thermal buckling of diaphragms with 1, 5, 10 μm diamond coating occurs at temperature 40, 73 and 142 °C, respectively. Compared with original GaN diaphragm, diamond expanded the operational temperature range of the pressure sensor. Moreover, compared with the operational range of pressure sensor based on pure GaN diaphragm (up to 30 kPa), diamond coated modified MEMS sensors withstand relatively higher pressures (2.2 MPa). The maximum load on the diaphragm increased two times by adding only 1 μm of diamond coating.
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 necessary for further improvement of sensing properties of such MEMS sensors. For example sensitivity of the AlGaN/GaN based MEMS pressure sensor can be modified by membrane thickness. We demonstrated that a 4H-SiC 80μm 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 process by fabrication of micromechanical membrane structures also in bulk 3C-SiC, borosilicate glass, sapphire and Al2O3 ceramic substrates by femtosecond laser (520nm) ablation. On a 350μm thick 4H-SiC substrate we produced an array of 275μm deep and 1000μm to 3000μm of diameter blind holes without damaging the 2μm 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 process 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.
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.
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