Chemical deposits from the gas phase

Among the coatings capable of imparting new properties to the surfaces they cover, those obtained by the Chemical Vapor Deposition (CVD) process are undoubtedly enjoying considerable growth.

This process consists in bringing a volatile compound of the material to be deposited into contact either with another gas in the vicinity of the surface to be coated, or with the surface in question, so as to provoke a chemical reaction resulting in at least one solid product.

Initially, the substrate temperature provides the activation energy required to trigger the chemical reaction and, when sufficiently high (800 to 1,000°C), promotes diffusion of the atoms brought to the surface into the substrate. This solid-state diffusion modifies the reaction products and generally ensures good adhesion to the coating. This process can be used to produce coatings of varying thicknesses of virtually any metal, alloy or metal compound, on conductive or insulating materials. In addition to coatings, it can also be used to produce fine powders or solid samples.

Since this original process, many variants have been developed, aiming either to reduce processing temperatures by using non-thermal energy sources such as plasmas or lasers, or labile precursors such as organometallics, or to increase deposition yields by using spray techniques, for example, or to work in a "localized" manner, i.e. on surfaces small enough to consider the deposits as points that can serve as basic units for the construction of two- or three-dimensional micro-objects.

The equipment used to obtain these deposits varies considerably depending on the type of deposit required (isothermal, non-isothermal, static, dynamic, low-pressure processes, etc.).

Coating properties (adhesion, porosity, stoichiometry, purity, etc.) are generally satisfactory enough to open the door to industrial applications.

The main areas of application for the CVD process can be divided into two categories, depending on whether the product formed is a coating (or layer) on a substrate, or a solid product independent of a substrate.

Obtaining a coating enables protection against mechanical wear (TiC, TiN, Al ₂ O ₃ , etc.), protection against high-temperature corrosion and oxidation (Cr, Al, Si, etc.), the production of components for microelectronics (GaAs, Si, AlN, etc.).

The formation of solid products leads to high-purity powder products and single crystals, and enables porous or sintered materials to be consolidated.

CVD techniques are an ideal complement to other deposition methods...