Plasma diagnostics of r.f. PACVD of boron nitride using a BCl 3−N 2−H 2−Ar gas mixture

Abstract

Boron nitride coatings have been synthesised by plasma-assisted chemical vapour deposition (PACVD) from a BCl 3/N 2/H 2/Ar gas mixture in a hot wall capacitively coupled r.f. (13.56 MHz) system. The influence of the gas composition has been investigated in terms of the nature of active species in the plasma phase using optical emission spectroscopy and mass spectrometry. These characterisations have been correlated with microstructure, and the type of bonding in the deposited films was determined by scanning electron microscopy and Fourier transformed infrared spectroscopy. The study is focused on the major role of hydrogen on the possible mechanisms leading to BN deposition from this complex mixture. The characterisation was performed by steps. First, an Ar/H 2 plasma was studied in order to understand the influence of molecular hydrogen content. To this discharge were then added separately the two precursors N 2 and BCl 3. Finally the deposition plasma was investigated. These characterisations have shown that: (a) a maximum of H atom density is obtained for a 15–30% H 2 in the Ar/H 2 plasma mixture; (b) the introduction of hydrogen in Ar/N 2 controls the nature of the NH x (from N to NH 3) species in the gas phase. These results are correlated with the relative amount of NH bonding in the films; (c) by a modification of the excitation state of the plasma ( ▪) the introduction of H 2 can increase the dissociation rate of the boron precursor BCl 3, and, reacting with chlorine, leads to the formation of HCl. This corresponds to an increase of the growth rate of the coatings. A high content of H 2 in the discharge (H 2>30%) is, however, detrimental to their quality; (d) we were also able to propose possible mechanisms leading to BN formation.