SiAlON Substrates Research Articles

Structure of thin polycrystalline silicon films on ceramic substrates

The structure of polycrystalline silicon films with thickness of 10 μm, which were deposited by thermal chemical vapor deposition on ceramic substrates, was investigated. In this article we report results of silicon films deposited on SiAlON, mullite, and alumina substrates. The type of substrate strongly influences the stress and the texture of the films. In the silicon films deposited on SiAlON substrate tensile stress is observed, while in the films deposited on mullite and alumina substrates compressive stress is present. In all deposited films the observed stress is higher than the thermal stress, which has been determined from the differences in the thermal expansion coefficients of the film and substrate materials. We observe in the films grown on SiAlON substrate that the total stress decreases with increasing deposition temperature, which implies a reduction of the intrinsic stress in the films. We ascribe the reduction of the intrinsic stress to a lower defect formation at higher deposition temperature. The silicon films exhibit a preferential texture, which becomes more random at higher deposition temperatures. We also observe that at higher deposition temperatures the grain size is larger and the crystal growth is columnar. We propose that the observed structural coherence between the alumina substrate and the silicon film results in an epitaxial growth that is responsible for a good adhesion of the film to the substrate.

Nondestructive evaluation of adhesive strength of CVD polycrystalline diamond coatings deposited on SIALON substrates : Conway, J.C.; Cohen, P.H.; Tittmann, BY.; Drawl, W.R.; Meloncelli, M.; Poeth, D.F.; Kropiewnicki, R. Nondestructive Characterization of Materials IV. Edited by C.O. Ruud, J.F. Bussiere and R.E. Green. pp. 49–57. Plenum Press (1991)

Nondestructive evaluation of adhesive strength of CVD polycrystalline diamond coatings deposited on SIALON substrates : Conway, J.C.; Cohen, P.H.; Tittmann, BY.; Drawl, W.R.; Meloncelli, M.; Poeth, D.F.; Kropiewnicki, R. Nondestructive Characterization of Materials IV. Edited by C.O. Ruud, J.F. Bussiere and R.E. Green. pp. 49–57. Plenum Press (1991)

Comparative measurement of residual stress in diamond coatings by low-incident-beam-angle-diffraction and micro-Raman spectroscopy

Two experimental techniques for the quantitative measurement of residual stress in thin polycrystalline diamond coatings have been developed. The x-ray low-incident-beam-angle-diffraction (LIBAD) allows one to measure the lattice strain with well-defined in-depth information, while micro-Raman spectroscopy permits one to accurately measure the frequencies of the zone-center optical phonons of diamond which are related to the lattice strain. The interpretation of the measured information in terms of residual stress is outlined for both techniques. The residual stress data obtained by either method in thin CVD diamond coatings were found to be in excellent agreement. The sign and magnitude of the balanced biaxial stress in the coating plane depend mainly on the substrate material used for the diamond deposition. Compressive stress was present in diamond coatings deposited on WC-Co substrates, whereas tensile stress was found in those on SiAlON substrates.

Cathodoluminescence (CL) and CL spectra of microwave plasma-enhanced CVD diamond

Recent research work is reported on cold cathode luminescence (CL) microscopy and CL microspectrophotometry of diamond produced by means of plasma-activated mixtures of hydrogen and methane gases. Diamond layers of octahedral, cubooctahedral or cubic habit on SiAlON substrates as well as ballas-type layers were investigated. Diamond was also deposited on sheet substrates of titanium, niobium, tantalum, chromium, molybdenum, tungsten, iron (spheroidal and lamellar cast iron), cobalt and nickel. Studying CL colors together with CL spectra has proved to be a reliable diagnostic method for differentiating between the morphological properties of CVD diamond on SiAlON substrates. It is however not possible with this method to clearly differentiate all diamond layers on any one of the sample substrates investigated. These findings could justify a future application of both CL microscopy and CL microspectrophotometry in production control.

Hot-filament chemical vapour deposition of diamond on SiAlON at pressures up to 500 Torr

Diamond layers were grown by hot-filament-activated chemical vapour deposition at total gas pressures ranging from 5 to 500 Torr, using hydrogen-methane gas mixtures, filament temperatures of 2200–2500°C and SiAlON substrates. In the lower pressure range from 5 to 10 Torr, 0.5 vol.% CH 4 was the optimum concentration for producing well-faceted diamonds, while concentrations above 1.0 vol.% CH 4 led to ballas-type diamonds. Between 20 and 300 Torr, faceted diamond layers grew also at CH 4 concentrations of 1.0 vol.%, whereby the highest growth rate (1.44 μm h −1) was obtained at 20 Torr, with a filament temperature of 2500°C. From 50 to 500 Torr the deposition rate decreased with increasing gas pressure, even falling below 0.05 μm h −1 at the highest pressure. This strong decrease in the growth rate with pressure was linked to a strong decrease in the diamond nucleation rate. Increasing filament temperature led to a higher nucleation density and a higher deposition rate over the whole pressure range investigated. The observed effects of gas pressure on diamond nucleation and growth can be explained with changes in the formation and recombination rates and the resulting concentrations of the atomic hydrogen.

Methodology for adhesion characterization of microwave PECVD diamond films on SiAlON tool inserts

The silicon nitride/SiAlON family of ceramic tool materials provides substrates with low thermal expansion coefficients for the deposition of continuous diamond films for prospective metal-cutting applications. Adhesion of diamond films grown by processes such as microwave plasma-enhanced chemical vapor deposition (MPECVD) to these ceramics can, however, be erratic. The characteristics of the diamond-ceramic interface which evolves during the deposition process are thus significant in the understanding of film adhesion. In this study, Auger electron spectroscopy (AES) and scanning electron microscopy were used to elucidate the nature of this interface, revealed by indentation fractures that produced interfacial cracks between diamond films and SiAlON substrates. The films were prepared with comparable methane-hydrogen feedstocks, but in two different microwave plasma deposition systems. AES showed, in some cases, apparent carburization of the SiAlON surface, rendering it less insulating and thus amenable to characterization by electron spectroscopy and scanning microscopy. Improved adhesion of the diamond films was observed when carbon enrichment of the SiAlON surface was apparently achieved during the growth process.

Analysis of hydrogen in low pressure diamond layers

The hydrogen concentration of diamond layers produced by a low pressure chemical vapour deposition method on SiAlON substrates has been determined by means of the 15N nuclear reaction analysis technique. The results show distinct differences in the hydrogen content which depend on the gas pressure and on the acetone-hydrogen gas mixture used in the deposition. The resulting diamond morphology is found to be related to the hydrogen content, well-faceted layers containing about 0.2–0.9 at % H, and balas-type layers with about 1.4 at.%H. Contrary to expection, no relation between the crystal size in the well-faceted layers and the average hydrogen content was found. Additional Rutherford backscattering measurements revealed a high purity of the films with respect to elements heavier than carbon.

Effects of microwave plasma deposition parameters on diamond coating formation on SiAlON substrates

Diamond films were grown on SiAlON substrates from different CH 4/H 2 gas mixtures by microwave-activated chemical vapor deposition (MW-CVD), and the effects of gas pressure, methane concentration, microwave power and substrate temperature on growth rate and morphology were examined. Scanning electron microscope pictures showed that at low methane concentrations the synthesized diamond crystals were characterized mainly by triangular (111) facets, whereas at higher methane concentrations (100) facets were predominant. Correlations between diamond morphology, diamond growth rates, homogeneity of the deposition and the size of the plasma ball are given. The effects on diamond growth and diamond morphology with respect to deposition area, gas pressure and the variations in plasma intensity will be discussed.

Nucleation of diamond on silicon, SiAlON, and graphite substrates coated with an a-C:H layer

We investigated the influence of a hydrogenated disordered carbon (a-C:H) layer on the nucleation of diamond. Substrates c-Si〈100〉, SiAlON, and highly oriented pyrolytic graphite {0001} were used in this study. The substrate surfaces were characterized with Auger electron spectroscopy (AES) while diamond growth was followed with Raman spectroscopy and scanning electron microscopy (SEM). It was found that on silicon and SiAlON substrates the presence of the a-C:H layer enabled diamond to grow readily without any polishing treatment. Moreover, more continuous diamond films could be grown when the substrate was polished with diamond powder prior to the deposition of the a-C:H layer. This important result suggests that the nucleation of diamond occurs readily on disordered carbon surfaces, and that the formation of this type of layer is indeed one step in the diamond nucleation mechanism. Altogether, the data refute the argument that silicon defects play a direct role in the nucleation process. Auger spectra revealed that for short deposition times and untreated silicon surfaces, the deposited layer corresponds to an amorphous carbon layer. In these cases, the subsequent diamond nucleation was found to be limited. However, when the diamond nucleation density was found to be high; i.e., after lengthy deposits of a-C:H or after diamond polishing, the Auger spectra suggested diamondlike carbon layers.

Aspects of diamond deposition by anomalous pulsed d.c. glow-discharge-activated chemical vapor deposition

Diamond was deposited onto SiAlON substrates by glow-discharge-activated chemical vapor deposition (CVD) using hydrogen-methane gas mixtures. The formation of diamond depends strongly on the plasma parameters (current, voltage, pressure, gas mixture) and the substrates used. Diamond deposition occurred in a d.c. glow discharge at a pressure of 30 mbar, a voltage of 660 V and current density of 0.5 A cm 2, and a series of bright and dark striations in the Faraday dark space was observed. As the presence of illumination in the interelectrode space is apparently important for successful diamond deposition, its dependence on the various luminous effects, and the voltage distribution in the interelectrode space are discussed.