M. Belmahi

Diamond thin film growth by pulsed microwave plasma at high power density in a CH4–H2 gas mixture

Abstract Microwave plasma assisted chemical vapour deposition in hydrogen–methane gas mixture is one of the most widespread methods used for the growth of diamond thin films. It has been proved that at low microwave power (300 W) the modulation of the power can lead to a significant improvement of the growth rate or the quality of diamond films. In this work, we show that this advantage can be validated at an industrial scale in reactors working at higher power (peak power up to 6 kW). The pulsed mode operation shows that whatever the frequency or the duty cycle, the quality of diamond (according to the micro-Raman spectroscopy) can be appreciably improved at the same growth rate. An optimal range of temporal parameters as the pulse and afterglow duration, corresponding to an optimal range of frequency and duty cycle has been determined. The main reactive species observed by optical emission spectroscopy in such plasmas are the CH and C 2 radicals as well as atomic hydrogen. The chemistry of CH 4 /H 2 discharge is very rich in hydrocarbon radicals. Nevertheless, some works have shown that the CH x radicals are the key species for the diamond deposit whereas C 2 H y is rather responsible for the graphite growth and the atomic hydrogen preferentially for the graphite etching. Time resolved optical emission spectroscopy and double pulse technique have been carried out to investigate the evolution of the active species during the discharge on-time as well as during the afterglow. These plasma studies lead to a better understanding of the advantages in modulating the power and allow establishing some correlations between the temporal evolution of the observed species and the characteristics of the deposited films.

Investigations on nitrogen addition in the CH4–H2 gas mixture used for diamond deposition for a better understanding and the optimisation of the synthesis process

Abstract In this study, we report on some investigations on the effects of nitrogen addition in the conventional CH 4 H 2 gas mixture employed for diamond synthesis by microwave plasma assisted chemical vapour deposition process. Diamond films have been achieved for CH 4 concentrations ranging between 0.5 and 3%, and nitrogen amounts introduced in the feed gas between 0 and 3000 ppm, for an averaged power density of 15 W cm −3 . The analysis of the deposited films has been carried out using SEM, micro-Raman spectroscopy, profilometry, XRD and electrical characterisations. For the experimental conditions employed we have pointed out that the main and conjugated effects of nitrogen are to enhance the secondary germination formation and the {100} faces development. Consequently, these effects widely depend on CH 4 and N 2 concentrations and the use of an optimal gas mixture composition leads to a maximal growth rate, chemical purity and surface electrical resistance, with a {100}〈100〉 surface global morphology. Furthermore, the achievement of thicker diamond films using the optimal gas composition has permitted to confirm few mechanisms forecasted for thin films. In particular, these mechanisms are related to the {100}〈100〉 morphology formation and to the influence of nitrogen atom incorporation in the films on the surface electrical properties. Finally, the interest of nitrogen addition has been discussed in terms of process optimisation for the production of good quality diamond films for specific applications.

a-SiC x N y thin films deposited by a microwave plasma assisted C VD process using a CH 4 /N 2 /Ar/HMDSN mixture: methane rat e effect

Amorphous silicon carbonitride thin films were deposited using a microwave plasma assisted chemical vapour deposition process fed with a mixture of methane, nitrogen, argon and hexamethyldisilazane (Si2C6H19N). Effects of the methane rate on thin films composition, n anostructuration and characteristics are investigated by means of various techniques such as X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, Transmission Electron Microscopy and UV-Visible absorption. The raise of the methane rate results in less organic, denser films and in an increase of refractive index.

Growth of piezoelectric aluminium nitride for layered SAW devices

A piezoelectric film combined with a high velocity substrate such as diamond seems very promising for SAW devices operating at high frequencies. In this work, we investigate the optimisation of growth parameters to produce AlN films with required properties for SAW devices: high resistivity, low roughness and good piezoelectricity coupling. AlN films are deposited by reactive DC magnetron sputtering on [100] silicon substrates under various deposition conditions including N/sub 2/ concentration in Ar-N/sub 2/ gas mixture, sputtering pressure (3/spl times/10/sup -3/ to 9/spl times/10/sup -3/ mbar), DC power (100-400 W) and substrate temperature (100 to 600/spl deg/C). The growth duration was modulated to obtain a constant film thickness (2 /spl mu/m) to permit better comparison. X-ray diffraction shows that the AlN films deposited in the range of 60-80% N/sub 2/, 400/spl deg/C, and 6/spl times/10/sup -3/ mbar, exhibit columnar structure textured in [002] orientation corresponding to wurtzite structure with the c-axis oriented perpendicular to the surface. AlN films elaborated in optimum conditions exhibit low surface roughness (<5 nm) and high electrical resistivity (>10/sup 14/ /spl Omega/.cm). The stoichiometric composition determined by energy dispersive X-ray spectroscopy (EDXS) reveals a weak presence of oxygen in the Al/sub 1/N/sub 1/ films. The best compromise is obtained for the sample grown with 75% N/sub 2/. A SAW filter is formed by development of IDT of 32 /spl mu/m wavelength on AlN/Si and AlN/sapphire structures. For AlN film grown with 75% N/sub 2/, frequency responses measured by network analyser show a central frequency of 156.4 MHz and 176 MHz corresponding to phase velocities of 5004.8 and 5632 m/s respectively for silicon and sapphire substrates. Also, we have formed a layered structure AlN/diamond SAW device which does not exhibit an operating frequency due to high surface roughness of the diamond film.