Formation of titanium nitride layers from a gaseous phase with the participation of a chemical reaction: contribution of the temperature deposition

Abstract

The processes of formation of titanium nitride layers by chemical vapor deposition techniques are, as a rule, conducted in a TiCl4-H2-N2 atmosphere at pressures ranging from 40 to 80 kPa, the treatment temperature being 1123 to 1273 K. The formation of titanium nitride layers on an iron substrate begins with the reduction of titanium chlorides, this being the result of a chemical reaction of these chlorides with iron contained in the substrate. The titanium thus liberated reacts with nitrogen to form titanium nitride. The above-mentioned reactions can take place until a compact TiN layer has been formed. The further growth of the layer proceeds owing to the reduction of titanium chloride, TiCl4, by means of hydrogen down to TiCl3 or TiCl2. As a result of disproportionation of lower chlorides, titanium appears which reacts with nitrogen to form TiN [1, 2]. It is assumed that in the process of formation of titanium nitride from chloride atmospheres, as long as a compact layer has not yet been formed, the most probable reactions are those of reduction of titanium chloride by iron, as a result of which FeCl2 appears [3]. In the literature no unambiguous statement can be found regarding the chemical reactions responsible for the occurrence of titanium on the growth surface at temperatures below 1273 K. The production of TiN layers by the low-pressure chemical vapor deposition (LPCVD) technique is, as a rule, performed in a steel reactor. In such reactors, chemical transport of iron from the reactor walls towards the surface of the layer under formation is likely to occur. Iron, present on the growth surface, may under such conditions continue to act as a reducer of titanium chlorides, this leading to the appearance of titanium on the compact titanium nitride layer. In order to verify the correctness of this assumption, the process of growth of TiN layers in a TiCl4-H2-N2 atmosphere was investigated in the presence of a source of iron, namely a cylindrical screen made of carbon steel (0.10 wt% C), as well as without any source of iron, i.e. by applying an Al2O3 reactor with no screen. The layers were deposited on Armco iron and Al2O3 substrates in the form of plates of dimensions 30 mm× 110 mm× 2 mm. The process of deposition of TiN layers was carried out on an apparatus used for LPCVD treatment [1]. Parameters of the deposition process are given in Table I. The layers produced in temperatures from 1073 K to 1423 K in the presence of a source of iron on an Armco iron as well as an Al2O3 substrate were golden-colored, characteristic of titanium nitride. The investigation of the phase composition, performed by X-ray diffraction analysis (Philips diffractometer, CuKα radiation), has shown that the layers produced on the Al2O3 as well as those on the Armco iron substrate consist of the TiN phase. In the case of Armco iron specimens for temperature up to 1350 K a loss in weight is observed, whereupon its value increases. In contrast to this, the Al2O3 substrate is the only substrate on which an increase in weight is observed (Fig. 1). A loss in weight of the Armco iron specimens may be indicative of the participation of iron, contained in the substrate, in chemical reactions with the components of the gaseous phase. Fig. 2 shows the surface appearance of the layer deposited on the Armco iron substrates at temperatures of 1073 K and 1233 K it confirms that in the deposition process of TiN on Armco iron chemical reactions occur in which iron contained in the substrate participate, and clear etching of the grain boundaries are seen. Surface analysis of the Al2O3 substrate, performed prior to the deposition of the layer, has demonstrated