Deposition of Composite Materials

Composite materials are materials that consist of several phases in the form of either layers or phases dispersed in a matrix.

Layered Composites. In many applications, multilayer film structures are used. One of the most common uses is to produce an adhesion layer next to the substrate with the functional layer(s) on top, such as the Ti-Au metallization on oxides. In some cases several layers are used. Examples are Ti-Pd-Au (Ref 47, 48), where the palladium inhibits corrosion, and the Ti-Pd-Cu-Au metallization, where the copper acts as an economical electrical conductor and the gold is present to prevent surface reaction.

Dispersed Phase Composites. Dispersed phase materials can be formed by codepositing insoluble materials. If the temperature is high enough for mass transport, the phases will separate, resulting in a two-phase material. Composite materials can also be formed by codepositing materials where the phase formed by reaction is dispersed in a matrix of the unreacted material (Ref 49). For example, a reactive material such as titanium can be codeposited with a less reactive material such as nickel in a reactive environment of oxygen or carbon to give dispersed phases of oxides (TiO2) or carbides (TiC) in nickel. Composite films can be formed by a minor constituent reacting with the major constituent to form an intermetallic phase that is dispersed in the major phase. For example, in the Al-2Cu metallization, on heating, the Al2Cu phase will precipitate to form a dispersion in the aluminum. This precipitate phase then acts as segregation sites for voids formed due to film stress. In cases where two or more materials are depositing at the same time on nonreactive surfaces, there may be changes in composition in the early stages of nucleation due to differing segregation effects.

The presence of second-phase materials in a film can lead to galvanic corrosion problems when an electrolyte is present. For example, Al-Al2Cu composite films have been found to be more susceptible to intergranular and pitting corrosion than pure aluminum films (Ref 50). The Al2Cu acts as a cathode (-0.73 volts) while the aluminum acts as the anode (-0.85 volts). The corrosion effects become more important with increasing copper concentration, so the copper in the Al-Cu metallization is limited to 2 to 4% when a homogeneous distribution of the Al-Cu particles is desirable.

Composite materials of metal particles in a polymer matrix can be formed by deposition of the metallic phase during plasma polymerization. Such a composite film has been shown to have a better wear durability than the polymer film alone (Ref 51) and to have interesting optical properties.

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