Real Time Ellipsometric Study of Boron Nitride Thin Film Growth
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INTRODUCTION Boron nitride (BN) is a suitable material for many optical, electrical and mechanical applications because of its singular properties, such as hardness, chemical inertness, optical transparency, high electrical resistivity, low dielectric losses and high thermal conductivity [1]. In the last years an important research activity has been carried out to prepare BN thin films at reduced pressures by several PVD and CVD methods [2,3]. The structure of BN films can be polycrystalline (cubic and/or hexagonal), turbostratic (a disordered modification of hexagonal structure) or amorphous, depending on deposition conditions. In particular, cubic BN (c-BN) is extremely hard and more chemically stable than diamond at high temperatures. However, to date, the films that have a high proportion of the cubic phase show high compressive stress, which limits the film thickness and frequently leads to poor adherence and even delamination of the film. Furthermore, the growth mechanism of cubic phase in the deposited BN material is not yet well understood. The use of in situ time resolved characterization techniques would be very valuable to understand the BN growth process associated with different deposition techniques. Among the non-intrusive optical characterizations, ellipsometry has been described as a powerful technique for the real time monitoring of the growth of thin film materials because of its sensitivity to very thin layers and density changes [4,5]. In this paper we present a real-time ellipsometric study of the growth of BN films prepared by rf plasma enhanced chemical vapour deposition (PECVD). EXPERIMENTAL Near-stoichiometric BN films were prepared in an RF-PECVD reactor [6] by glow discharge decomposition of a gas mixture of NH3 (pure) and B2H 6 (1% in H2). The films were deposited on Ni-Cr coated polished c-Si substrates, which were placed vertically either on the 307 Mat. Res. Soc. Symp. Proc. Vol. 410 0 1996 Materials Research Society
plasma powered electrode (cathodic samples) or on the grounded electrode (anodic samples). Fixed deposition conditions were: 300'C of substrate temperature, 30 Pa of total pressure and 5 sccm of NH 3 gas flow rate. Cathodic BN films were obtained using 50 sccm gas flow rate of B2H 6-H 2 and applying an electrical rf power of 30 W. For the preparation of anodic films, we increased both the gas flow rate of B2H6 -H2 (100 sccm) and the rf power supplied to discharge (150 W) in order to enhance the film deposition rate. In some discharges, an Ar gas flow rate of 30 sccm was added to gas mixture to activate the gas ionization. The reactor chamber was provided with a fast phase-modulated ellipsometer (PME) operating in spectroscopic mode (1.5-5.4 eV) or in real time mode at a fixed wavelength 171. In this study, the growth of BN films was monitored by real time ellipsometry at a fixed wavelength of 350 nm. From the experimental data, measured at 50 kHz of phase modulation frequency with an integration time of 1.8 s, the ellipsometric (TI, A) trajectories corresponding to the
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