Postdeposition Processing

Postdeposition heating of films can be done in a furnace, by flash lamp heating such as is used in RTP techniques, or by laser irradiation. In the extreme, the heating can be used to melt and "reflow" the film to planarize the surface.

Postdeposition heating can create film stresses due to differences in the coefficient of thermal expansion between the film and substrate and between different phases in the film. These stresses can result in plastic deformation of the film or substrate material, create stress-related changes in the film properties such as voids (Ref 54), or create interfacial fractures.

Diffusion. Heating is used to promote mass transport so as to anneal the residual stress and defect structure in deposited films. For example, it has been shown that glass films exhibit strain points far lower than those of the bulk material (Ref 55), that grain growth can take place in sputter-deposited copper films at very low temperatures (Ref 56), and that stress relief in TiB2 films occurs far below the annealing temperature of the bulk material. Postdeposition heating has been shown to modify the structure and electrical properties of deposited SiO2 films. These effects are probably due to the residual film stress and high defect concentrations in the deposited films.

Postdeposition heat treatments can be used to induce grain growth or phase changes, but care must be taken because the changes can result in increased film stress or fracture. The substrate material and structure can influence the kinetics of the phase change by influencing the nucleation of the new phase. Postdeposition heating rarely allows densification of columnar films, because the surfaces of the columnar structure react with the ambient and the surface layers that are formed prevent the diffusion needed for densification. Typically, heating during deposition or in situ heating in the deposition chamber is more effective in densifying deposits than ex situ heating.

Agglomeration. Postdeposition heating can cause the film structure to agglomerate into islands generating porosity and changing the optical and electrical properties of the films (Ref 57). Agglomeration also occurs by grain boundary grooving of the film material (Ref 58).

Heating with Reaction and Diffusion. Postdeposition heat treatments are used to promote reaction between unreacted codeposited materials and to promote reaction of the deposited material with an ambient gas. For instance, it is common practice to heat deposited high-temperature oxide superconductor films in an oxygen atmosphere to improve their performance, and transparent, electrically conductive indium-tin-oxide films are heated in "forming gas" to increase their electrical conductivity. Heating can also cause the formation of internal dispersed phases (e.g., Ni-B, Cu-Al) to give dispersion strengthening.

Heating is used to alloy the deposited material with the substrate surface. Postdeposition diffusion and reaction can form a more extensive interfacial region and induce compound formation in semiconductor metallization. Postdeposition heating and diffusion can be used to completely convert the deposited material to interfacial material. For example, a platinum film on silicon can be heated to form a platinum silicide layer. Postdeposition interdiffusion can result in the failure of metallized semiconductor devices by diffusion and shorting of the junctions.

Alloying and reaction between films and substrates can be limited by:

• Deposition of refractory metal diffusion barriers such as tungsten or W-Ti alloy

• Deposition of electrically conductive diffusion barrier layers of compounds such as carbide (e.g., TiC), nitrides (e.g., TiN), or silicides (Ref 59)

• Formation of compounds that act as diffusion barriers when the film and substrate materials react

• Doping of the film or substrate materials with a material that retards mass-transport (e.g., rare earth metals in aluminum)

• Rapidly heating and cooling the surface region

Melting. The XeCl (308 nm) excimer laser has been used to melt and planarize thin films of gold, copper, and aluminum on silicon devices with submicron features.

Postdeposition ion bombardment using reactive or nonreactive bombarding species can be used to change the composition or properties of the film material or to increase the interfacial adhesion by interfacial mixing or "stitching" (Ref 60).

To "recoil mix" or "stitch" an interface, the films must be rather thin (<100 nm) and the ion energies are selected to give the peak range just beyond the interface. In recoil mixing at an interface, if the materials involved are miscible, the ion mixing results in interfacial reaction and diffusion. If the materials are not miscible, the interfacial region is not mixed but the adhesion is increased. Generally adhesion improvement is dose dependent, with the best result being for doses of 1015 to 1017 ions/cm2, while excessive bombardment induces interfacial voids. Part of the observed increase in adhesion may be due to the elimination of interfacial voids by "forward sputtering."

Deformation. In the case of films of soft materials, the film structure can be densified and porosity closed by postdeposition burnishing (Ref 61) or shot peening of the surface. For example, the MCrAl (where M can be a metal of various types) films deposited on turbine blades are routinely shot peened to increase their corrosion resistance.

Chemical and Electrochemical Treatments. Deposited films may be subjected to various chemical and electrochemical treatments to convert all or part of the film to another material. For example, aluminum films can be anodized (Ref 62) or chemically converted by a chromate conversion process for increased corrosion resistance.

Pore Filling. Porosity of the deposited films is often a limiting factor in their use. Various techniques may be used to fill the pores in the film. For example, electrophoretic deposition of polymer particles has been used to selectively fill the pores in a dielectric film on a conductive substrate (Ref 62), and corrosion of the substrate through the pores has been used to plug the pores with corrosion products.

Topcoats are often applied to deposited films to increase abrasion and corrosion resistance. An example of a fluid topcoat is the dip-coated polysiloxane coating used on aluminum-coated polycarbonate automotive headlight reflectors.

Fluid Topcoats. Often deposited coatings are overcoated with a thin (<1pm) protective film of highly cross-linked polymeric material. Many topcoat materials require heat for curing. The heating is necessary to remove the solvents and cross-link the polymeric materials. Due to environmental concerns this type of topcoat material is being modified to give a higher "solids content" (i.e., less solvent will be released into the environment). Another class of polymeric topcoat materials are the organo-siloxanes, which consist of an organic host polymer containing siloxane coupling agents and colloidal silica. The coatings can be applied by spraying, spinning, or dip coating and can be cross-linked by UV radiation or electron irradiation (e.g., UV-curable acrylics).

Plasma-Deposited Topcoats. Some topcoat material can be deposited by plasma polymerization. For example, plasma polymerization of polysiloxane is used to form organo-silicon coatings, which are sometimes heat treated in oxygen to increase the Si-O content of the films. The plasma-polymerized organo-silicon films have excellent surface coverage ability and are hydrophobic, hard, and relatively pinhole-free. The films are being used as clear protective topcoats on optical reflective films. Plasma-deposited organo-carbon films are used as conductive films.

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