Overview of Chemical Vapour Deposition

ANTHONY C. JONES AND MICHAEL L. HITCHMAN

1.1 Basic Definitions

In the broadest sense chemical vapour deposition (CVD) involves the formation of a thin solid film on a substrate material by a chemical reaction of vapour-phase precursors. It can thus be distinguished from physical vapour deposition (PVD) processes, such as evaporation and reactive sputtering, which involve the adsorption of atomic or molecular species on the substrate. The chemical reactions of precursor species occur both in the gas phase and on the substrate. Reactions can be promoted or initiated by heat (thermal CVD), higher frequency radiation such as UV (photo-assisted CVD) or a plasma (plasma-enhanced CVD). There is a sometimes bewildering array of acronyms covered by the overall cachet of CVD and the interested reader is referred to several reviews. Some of the more commonly used acronyms are defined below.

Metal-organic chemical vapour deposition (MOCVD) is a specific type of CVD that utilizes metal-organic precursors. In the strictest sense a metal-organic (or organometallic) compound contains a direct metal-carbon bond (σ or π) (e.g. metal alkyls, metal carbonyls). However, the definition of MOCVD has broadened to include precursors containing metal-oxygen bonds (e.g. metal-alkoxides, metal-β-diketonates) or metal-nitrogen bonds (e.g. metal alkylamides), and even metal hydrides (e.g. trimethylamine alane).

Metal-organic vapour phase epitaxy (MOVPE) or organometallic vapour phase epitaxy (OMVPE) is an MOCVD process that produces single crystal (i.e. epitaxial) films on single crystal substrates from metal-organic precursors. In MOCVD and MOVPE gas-phase reactions can sometimes play a significant role in the deposition chemistry.

Plasma-assisted or plasma-enhanced CVD (PECVD) is a technique in which electrical energy rather than thermal energy is used to initiate homogeneous reactions for the production of chemically active ions and radicals that can participate in heterogeneous reactions, which, in turn, lead to layer formation on the substrate. A major advantage of PECVD over thermal CVD processes is that deposition can occur at very low temperatures, even close to ambient, which allows temperature-sensitive substrates to be used.

Atomic layer deposition (ALD), sometimes called atomic layer epitaxy (ALE), alternatively-pulsed CVD, or atomic layer chemical vapour deposition (ALCVD), is a modification of the CVD process in which gaseous precursors are introduced sequentially to the substrate surface and the reactor is purged with an inert gas, or evacuated, between the precursor pulses. The chemical reactions leading to film deposition in ALD occur exclusively on the substrate at temperatures below the thermal decomposition temperature of the metal-containing precursor and gas-phase reactions are unimportant.

Chemical beam epitaxy (CBE) is high vacuum CVD technique that uses volatile metal-organic precursors and gaseous co-precursors. The closely related technique of metal-organic molecular beam epitaxy (MOMBE) uses volatile metal-organic precursors and co-precursor vapour derived from the solid element. In CBE and MOMBE the chemical reactions occur only on the substrate, leading to single crystal films and so gas-phase reactions play no significant role in film growth. Section 1.3 gives a more detailed description of these processes.

1.2 Historical Perspective

In common with many technologies, developments in CVD have largely arisen out of the requirements of society. These developments have been most rapid when other thin film deposition technologies have proved problematic or inadequate, for instance in the production of multiple thin films, as in modern semiconductor devices, or when the coating of large surface areas is required, as in large-scale functional coatings on glass. Several excellent reviews describe the historical development of CVD processes, and the published literature from the earliest days to the mid-1960s is covered by a comprehensive review by Powell et al. Therefore, this section gives only a brief description, highlighting some key advances.

Probably the earliest patent describing a CVD process was taken out by a certain John Howarth, for the production of "carbon black" for use as a pigment. Unfortunately, due to rather lax health and safety standards, the process only succeeded in burning down the wooden plant. The early electric lamp industry provided another early impetus for CVD, and a patent issued in 1880 to Sawyer and Mann describes a process for the improvement of carbon fibre filaments. However, these proved too fragile and later patents describe CVD processes for the deposition of various metals to produce more robust lamp filaments.

One of the earliest examples of the CVD of metals is the deposition of tungsten, reported as early as 1855. Wohler used WCl6 with hydrogen carrier gas to deposit tungsten metal. Later in the century (1890), the famous Mond Process was developed. This describes the deposition of pure nickel from nickel tetracarbonyl, Ni(CO)4, and was used for the refinement of nickel ore.

The first reports of the deposition of silicon by CVD by the hydrogen reduction of SiCl4 appear as early as 1909 and 1927, and the widespread use of thin silicon films in the electronics industry is anticipated by the CVD of Si-based photo cells and rectifiers just after World War II.

During the late 1950s, triisobutylaluminium, [But3Al] began to be used extensively to catalyze the polymerization of olefins by the Ziegler–Natta process. At around the same time, it was found that the pyrolysis of [But3Al] gave high purity Al metal (>99 at.%). This led to its use in the early 1980s as a CVD precursor to Al metal for very large scale integration (VLSI) applications. In patent literature of the late 1960s, aluminium trihydride (AlH3, alane) was found to be...