M. Eickhoff

Silicon compatible materials for harsh environment sensors

The wide bandgap semiconductors silicon carbide and diamond and the material system Silicon On Insulator (SOI) are compared regarding their suitability as silicon compatible materials to extend the application fields of micromachined sensors to harsh environment conditions especially to high temperatures. The harsh environment conditions are specified by analyzing the demands of automotive and aerospace applications. The physical properties of the material systems are discussed and their technological stage of development is evaluated, especially with respect to the compatibility to standard silicon processes. Furthermore, the commercial availability of the different materials in the form of substrates is considered and a forecast of future developments is attempted. As an example of a harsh environment sensor, a combustion pressure sensor is presented and characterized.

A high temperature pressure sensor prepared by selective deposition of cubic silicon carbide on SOI substrates

Abstract A high temperature pressure sensor with 3C–SiC piezoresistors as sensing elements was prepared. For the first time the sensing elements were structured by selective deposition of 3C–SiC on a patterned Si/SiO 2 surface. To ensure dielectric isolation SOI substrates were used. The effectiveness of the selective deposition process is demonstrated by REM-photographs. Characterisation of the sensing elements shows the good crystal quality of the sensing elements as indicated by the gauge factor of −18 at room temperature which decreases to −10 at 200°C. As a benefit of the deep dry etching process the related sensitivity is 3.5 mV/V bar at room temperature decreasing to 2.1 mV/V bar at 200°C for a 100-μm thick circular center boss diaphragm.

High temperature piezoresistive β-SiC-on-SOI pressure sensor with on chip SiC thermistor

Abstract This paper reports about a piezoresistive β-SiC-on-silicon on insulator (SOI) pressure sensor with an on chip polycrystalline SiC thermistor for high operating temperatures. The β-SiC film was characterized by TEM-analysis, X-ray diffraction and Hall measurements. The investigations show a good single crystal quality of the β-SiC film and a reliable electrical isolation by the buried oxide layer from the substrate at temperatures up to 673 K. The fabricated pressure sensor chip was tested in the temperature range between room temperature and 573 K. The sensitivity at room temperature is S =2.0 mV V −1 bar −1 . The temperature coefficient of the sensitivity (TCS) between room temperature and 573 K is TCS=−0.16 %K −1 . The temperature coefficient of the resistivity (TCR) of the polycrystalline SiC thermistor is TCR=−0.17 %K −1 .

Heteroepitaxial growth of 3C-SiC on SOI for sensor applications

Typical industrial high temperature sensor applications are reviewed and a short overview of the different high temperature sensor technologies is given. The pros and cons are weighted. Silicon carbide on insulator (SiCOIN) technology comes out to be the most attractive, provided the state of development can be brought up to the one of silicon and silicon on insulator (SOI). Due to the lack of commercially available SiC on SOI wafers, a new SiC on SOI technology has been developed. It is based on the precursor gas methylsilane. The low temperature growth process is described and in-situ n-type doping, which is necessary for sensor applications, has been carried out successfully over a wide range of concentrations without loosing the good crystal properties. Actually the full process is being transferred from a test reactor to a 4 inch machine. This should provide 3C-SiC on SOI wafers for commercial sensor applications. A demonstrator of combustion pressure sensor dedicated to pressure-based engine control is shown. Results of the pressure sensor fitted in a motor-test setup are summarized.

Rapid plasma etching of cubic SiC using NF3/O2 gas mixtures

Abstract SiC is known as a chemically inert material. Therefore structuring of SiC by dry chemical processes is difficult and the reported etch rates are usually low. For sensor and micro-machining applications, however, three-dimensional structuring processes of bulk SiC with high etch rates are needed. We made a systematic study of plasma etching processes with NF 3 /O 2 gas mixtures. Silicon substrates with epitaxial β -SiC layers on top and poly-crystalline, but well orientated β -SiC bulk substrates were used for the etching experiments. As mask materials both evaporated and sputtered aluminium layers, partly with chromium adhesive layers, were applied. We investigated the dependence of the etch rate on the O 2 content in the gas mixture, the substrate temperature and the gas pressure. We reached etch rates up to 1 μm min −1 at the best etching conditions. The etching selectivity of β -SiC compared to other semiconductor materials was studied. An etch rate ratio of SiC to Si and SiO 2 of 4–5 was achieved. Furthermore differences in the etch rates of p- and n-doped β -SiC were observed.

Selective growth of high-quality 3C-SiC using a SiO2 sacrificial-layer technique

Abstract The selectivity of a LPCVD growth process for cubic silicon carbide with respect to the underlying substrate was investigated, and a highly selective growth process on structured Si/SiO 2 substrates was found at temperatures above 1150°C. The temperature dependence of this effect was systematically studied. In the temperature range between 1075–1150°C, polycrystalline SiC was grown on SiO 2 surfaces. At temperatures above 1150°C, no growth on SiO 2 occurred, whereas high-quality 3C-SiC layers were deposited onto the opened SiO 2 mask areas. This was shown by REM pictures and by removal of the SiO 2 layers in wet HF-etchant after the deposition. The selectively deposited 3C-SiC-structures were grown at a rate of 4.4 μm/h.

Measurement of the cylinder pressure in combustion engines with a piezoresistive /spl beta/-SiC-on-SOI pressure sensor

For measuring the cylinder pressure in combustion engines a high temperature pressure sensor has been developed. The sensor is made of a piezoresistive /spl beta/-SiC-on-SOI (SiCOI) sensor chip and a specially designed housing. The SiCOI sensor was characterized under static pressure up to 20 MPa. The sensitivity of the sensor at room temperature is approximately 1.9 mV/MPa and decreases to about 1.2 mV/MPa at 300/spl deg/C. For monitoring the dynamic cylinder pressure in the combustion chamber the SiCOI sensor was placed into the cylinder head of a gasoline engine. The measurements were performed at 1500 rpm under different loads, and for comparison a quartz pressure transducer from Kistier was used as a reference. The maximum pressure at partial load operation amounts to about 2.0 MPa. The difference between the calibrated SiCOI sensor and the reference sensor is significantly less than 0.1 MPa during the whole operation.

The evolution of cavities in Si at the 3C-SiC/Si interface during 3C-SiC deposition by LPCVD

The evolution of cavities in the Si-overlayer (SOL) of the silicon on insulator (SOI) wafers, which are formed during the epitaxial growth of 3C-SiC, was studied by combined Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) observations. The effect of the SiC and the SOL thickness, as well as the SiC rate of growth, on the formation of cavities is discussed. The volume of the missing silicon due to the formation of the cavities in the SOL was measured by AFM. It was shown that a small portion of Si ball up at the edges of the cavities inside the buried oxide (BOX) due to instability of the Si/SiO 2 system at high temperatures. Another part is consumed in order to form SiC during the carbonization process and finally a significant portion is out-diffused during the deposition of the first 200 nm of the SiC film.

Influence of the silicon overlayer thickness of SOI unibond substrates on β-SiC heteroepitaxy

Abstract The present paper describes the epitaxial growth of high quality 3C–SiC on the top of silicon on insulator (SOI) UNIBOND substrates, to achieve an electrical insulation. The process was performed at 1200°C using methylsilane as the precursor gas. The crystal quality was proved using X-ray analysis. The FWHM of the [200] rocking curve reflex was determined to 0.31° using a 200 nm thick SOL. A serial resistance of the SiO2 layer of 2.5·1012 Ω mm2 was obtained at RT which proofs the insulation to the substrate. A technique based on sacrificial oxidation was applied to thin the silicon overlayer (SOL). SOLs between 15 and 200 nm could be prepared. The influence on the structural properties of the SiC film was studied using X-ray, AFM and TEM measurements. It was found that structural properties are dependent on the deposition process and on the SOL thickness. High quality SiC can be grown on SOLs thicker than 50 nm. Possibilities for the growth of highest quality SiC even on much thinner SOLs are discussed.

The Application and Evaluation of a Novel Engine Management System Based on Intelligent Control and Diagnostics Algorithms and Utilising Innovative Sensor Technology

Conventional Engine Management Systems (EMS) are primarily parameter based systems in which there is no underlying model to describe the behaviour of the engine. Such systems therefore require many parameters to control the engine under steady-state, transient and varying ambient conditions. The introduction of Model Based Control in order to overcome this problem has been hindered mainly by the lack of inexpensive sensing devices which can closely monitor the engine’s performance. In particular, the measurement of cylinder pressure which would give valuable information on the engine’s performance, has been very expensive due to the harsh environment of the combustion chamber. In recent years, attention has focused upon the development of inexpensive cylinder pressure sensors in order to realise a system more appropriate for the application of Model Based control. This paper describes the application of new robust pressure sensing technology coupled with Model Based control and diagnostics algorithms.