5.2 Chemical vapor deposition  (Page 6/6)

Plasma CVD has numerous advantages over thermal CVD. Obviously the reduced deposition temperature is a bonus for the semiconductor industry which must worry about dopant diffusion and metal interconnects melting at the temperatures required for thermal CVD. Also, the low pressures (between 0.1 - 10 Torr) required for sustaining a plasma result in surface kinetics controlling the reaction and therefore greater film uniformity. A disadvantage of plasma CVD is that it is often difficult to control stoichiometry due to variations in bond strengths of various precursors. For example, PECVD films of silicon nitride tend to be silicon rich because of the relative bond strength of N ₂ relative to the Si-H bond. Additionally, some films may be easily damaged by ion bombardment from the plasma.

Photochemical cvd

Photochemical CVD uses the energy of photons to initiate the chemical reactions. Photodissociation of the chemical precursor involves the absorption of one or more photons resulting in the breaking of a chemical bond. The most common precursors for photo-assisted deposition are the hydrides, carbonyls, and the alkyls. The dissociation of dimethylzinc by [link] , a photon creates a zinc radical and a methyl radical ( ^. CH ₃ ) that will react with hydrogen in the reactor to produce methane.

Like several metal-organics, dimethylzinc is dissociated by the absorption of only one UV photon. However, some precursors require absorption of more than one photon to completely dissociate. There are two basic configurations for photochemical CVD. The first method uses a laser primarily as a localized heat source. The second method uses high energy photons to decompose the reactants on or near the growth surface.

In thermal laser CVD, sometimes referred to as laser pyrolysis, the laser is used to heat a substrate that absorbs the laser photons. Laser heating of substrates is a very localized process and deposition occurs selectively on the illuminated portions of the substrates. Except for the method of heating, laser CVD is identical to thermal CVD. The laser CVD method has the potential to be used for direct writing of features with relatively high resolution. The lateral extent of film growth when the substrate is illuminated by a laser is determined not only by the spot size of the laser, but by the thermal conductivity of the substrate. A variation of laser pyrolysis uses a laser to heat the gas molecules such that they are fragmented by thermal processes.

Photochemical effects can be induced by a laser if the precursor molecules absorb at the laser wavelength. UV photons have sufficient energy to break the bonds in the precursor chemicals. Laser-assisted CVD (LACVD) uses a laser, usually an eximer laser, to provide the high energy photons needed to break the bonds in the precursor molecules. [link] shows two geometries for LACVD. For the perpendicular illumination the photochemical effects generally occur in the adsorbed adlayer on the substrate. Perpendicular irradiation is often done using a UV lamp instead of a laser so that unwanted substrate heating is not produced by the light source. The parallel illumination configuration has the benefit that reaction by-products are produced further from the growth surface and have less chance of being incorporated into the growing film. The main benefit of LACVD is that nearly no heat is required for deposition of high quality films.

An application of laser photolysis is photonucleation. Photonucleation is the process by which a chemisorbed adlayer of metal precursors is photolyzed by the laser to create a nucleation site for further growth. Photonucleation is useful in promoting growth on substrates that have small sticking coefficients for gas phase metal atoms. By beginning the nucleation process with photonucleation the natural barrier to surface nucleation on the substrate is overcome.

Bibliography

• J. G. Eden, in Thin Film Processes II , Eds. J. L. Vossen and W. Kern, Academic Press, New York (1991).
• T. T. Kodas and M. J. Hamton-Smith, The Chemistry of Metal CVD , VCH, New York (1994).
• M. Konuma, Film Deposition by Plasma Techniques , Springer-Verlag, New York (1992).
• H. O. Pierson, Handbook of Chemical Vapor Deposition , Noyes Publications, Park Ridge (1992).
• R. Reif and W. Kern, in Thin Film Processes II , Eds. J. L. Vossen and W. Kern, Academic Press, New York (1991).
• G. B. Stringfellow, Organometallic Vapor-Phase Epitaxy: Theory and Practice , Academic Press, New York (1994).