G. Krötz

Structural and electronic characterization of β-SiC films on Si grown from mono-methylsilane precursors

Abstract This paper presents the results of an in-depth investigation into the chemical vapour deposition (CVD) growth of β-SiC on Si from H 3 SiCH 3 precursors. In agreement with previous work, we find an onset of CVD growth at substrate temperatures in the order of 750–800 °C. Higher temperatures lead to exponentially increasing growth rates until diffusion limitations set in at about 1000 °C. The highest quality films, with structural characteristics typical of single-crystal material, were deposited at about 1050°C. Substrate pretreatments, except for a pre-deposition HF dip, had surprisingly little influence on the crystal quality. Films deposited at substrate temperatures lower than 1000°C exhibited substantially broader IR absorption peaks and a higher degree of misorientation than those deposited at high temperature.

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.

Formation of epitaxial diamond-silicon carbide heterojunctions

Abstract Heteroepitaxial β-SiC thin films have been deposited on to silicon substrates using methyl silane and 1,2-bis (bromosilyl) ethane Si-C bonded precursors at temperatures in the range 650–1000 °C. The silicon carbide layers were polycrystalline and epitaxially oriented with respect to the silicon substrate. These films have been used as substrates for the deposition of oriented boron doped diamond films using a bias enhanced microwave assisted CVD process. The formation of heterojunctions consisting of p-type diamond and intrinsic or n-type silicon carbide is considered and the electrical properties of the junctions are discussed.

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.

Stabilization of the 3C-SiC/SOI system by an intermediate silicon nitride layer

Abstract The material system 3C–SiC on SOI (silicon on insulator) offers outstanding possibilities for high temperature applications. In order to fully benefit from these possibilities, the quality of especially the SOI substrate must be maintained throughout all process steps of device fabrication. This work demonstrates the stabilizing effect of a 6 nm thin silicon nitride layer at the interface silicon overlayer/buried oxide layer (SOL/BOX) of a UNIBOND–SOI substrate. This layer was formed by implantation of nitrogen into a SOI substrate and subsequent annealing for 2 h at 1100°C. The improved stability of the SOI system during SiC deposition is explained by the following fact: the wetting angle of silicon at the melting point is decreased from 87° (Si on SiO2) to 25° (Si on Si3N4). The improved wetting behavior decreases the tendency of silicon to redistribute and to form cavities. A 2.6 μm thick layer of cubic SiC was deposited at 1200°C on a UNIBOND SOI substrate. This sample was only partly implanted with nitrogen. At the non-implanted part, a significant redistribution of the SOL occurred and cavities were extended from the SOL deep into the BOX. Whereas at the N-implanted part, the BOX was completely protected by the silicon nitride layer and only small cavities were extended into the SOL (thicknesses of SOL and BOX were about 150 and 400 nm, respectively). The crystal quality of the SiC layer was almost the same in both cases. Furthermore, the results indicate that an optimization of the whole process will even lead to better results.

Reversible and irreversible structural changes in amorphous silicon

Abstract The paper attempts to provide an overall view of the hierarchy of structural and configurational equilibria that can be supported by hydrogenated random Si networks. In order to identify intrinsic and H-related structural degrees of freedom, experiments on chemically pure amorphous Si (a-Si) and on hydrogenated amorphous Si (a-Si: H) are discussed in parallel. The comparison shows that both kinds of amorphous material are able to support irreversible and relatively longrange relaxation processes in which the bond-angle disorder is fixed and in which the density of stable danglingbond-defects is established. Because of the rigidity of the random Si networks the corresponding equilibria are frozen in at effective temperatures T∗ > Td (Td is the deposition temperature of the a-Si: H films). Hydrogenated random Si networks, in addition, are able to support reversible valence alternation reactions in which the local coordination of the dopant and defect sites is changed and in which their charge state...

Dopant‐defect interactions in hydrogen‐free amorphous silicon

Abstract Substitutional (B, P, As and Ga) and interstitial (K) dopants have been incorporated into H‐free amorphous Si (a‐Si) films produced by ion beam amorphization of crystalline silicon material. X‐ray absorption fine‐structure, photothermal deflection spectroscopy and electronic transport measurements have been performed on these films to monitor the annealing‐induced ordering phenomena around the implanted dopant impurity sites. We find that, in thermally relaxed a‐Si, substitutional dopant impurities have a strong tendency to enter the Si random network in the form of threefold‐coordinated, electrically inactive, alloying sites. It is shown that the bonding constraints associated with these sites retard the structural relaxation process of the a‐Si films and the crystallization of the a‐Si network in the immediate neighbourhood of these sites. In agreement with previous work, we find that high‐defect‐density a‐Si films can be electrically doped with interstitial K impurities. In such interstitially...

Preparation of crystalline SiC thin films by plasma-enhanced chemical vapour deposition and by ion beam modification of silicon

Abstract Thin crystalline films of SiC have been produced by ion beam modification of heteroepitaxial silicon thin films on sapphire and by plasma-enhanced chemical vapour deposition on crystalline silicon and sapphire substrates. Both methods yield highly transparent crystalline material. The preparation procedures and the optical properties of these materials are described and applications to optical lithography are pointed out.