High temperature piezoresistive β-SiC-on-SOI pressure sensor with on chip SiC thermistor
Introduction
Cubic silicon carbide, heteroepitaxially grown on silicon on insulator (SOI), is a promising material for the development of micromechanical high temperature devices such as pressure sensors [1,2, this volume]. Using single crystalline β-SiC piezoresistors, which have a higher gauge factor than polycrystalline SiC piezoresistors [3], [4], results in a high output signal of the pressure sensor. However, tests of the pressure sensor in a combustion engine showed [5], [6] that for the operation of the pressure sensor over a wide temperature range it is necessary to compensate for the temperature dependence of sensitivity and offset voltage. Therefore the measurement of the temperature can be realized using an on chip polycrystalline SiC thermistor.
Section snippets
Silicon carbide on insulator (SiCOI) material system
For the fabrication of the SiCOI material system, a n-type β-SiC film with a thickness of 1,1 μm was heteroepitaxially grown by Daimler-Benz Frankfurt on a 〈100〉-oriented SOI substrate (UNIBOND) from SOITEC, France [7, this volume]. During oxidation in a dry oxygen ambient at 1423 K for 2 h an insulating capping oxide layer was grown on the SiCOI substrate. Afterwards a 2.5 μm thick polycrystalline SiC film was deposited by Daimler Benz Munich onto the capping oxide layer.
Device structure
The fabricated sensor
Material characterization
The general view of the layer structure is shown in the cross-section transmission electron microscope (TEM) micrograph in Fig. 2. The SiC/Si-overlayer is perfect and no cavities were observed in the Si-overlayer and the buried oxide (BOX). Only some strain was produced at the SiC/Si interface (denoted by arrows) as a result of the lattice mismatch of the SiC/Si system. In the planar view TEM micrograph typical defects of the β-SiC layer can be seen (Fig. 3). The density of stacking faults in
Conclusions
A piezoresistive high temperature pressure sensor with β-SiC piezoresistors on a SOI substrate and with an on chip SiC thermistor was developed and fabricated. The results show the capability of the SiCOI pressure sensor to monitoring the cylinder pressure of combustion engines. The temperature dependence of the polycrystalline SiC thermistor allows for the realization of an external compensation of the temperature dependence of sensitivity and offset voltage of the pressure sensor over a wide
Acknowledgements
The authors would like to thank J. Stoemenos, Aristotle University of Thessaloniki, Greece, for the TEM analysis. This work was financially supported by the German Ministry of Education and Research (BMBF) under contract number 414-4013-16 SV 023.
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