The deposition of thin metallic films on substrates by the decomposition of metallo-organic solutions is a technology which has been known for many years. Although the technique has been known for a long time, its primary application has been in the decorative trade, with the main emphasis on producing films of precious metals on ceramic and glass articles. The earliest known application was bright gold for decorating porcelain and this process had been used since 1830.
Almost all of the work prior to 1980 used metallo-organic compounds derived from resins or other natural products and were commonly called metal resinates. These resinates were and still remain suitable for most applications in the decorative trade, but the variability of chemistry, which is an inescapable result of the preparation from natural products, was a major impediment to the extensive development of technical applications of the metallo-organic decomposition (MOD) process. It has only been in recent years that MOD films have been produced from pure, well characterized compounds.
Recent applications of MOD processing are in processing technical ceramics where most of the emphasis is on fine particle processing. Sol-gel methods use metal alkoxides as the starting materials and have general formula M(OR).sub.n where M and R are metals and alkyl groups, respectively. Metal alkoxides are hydrolyzed easily and yield oxides, hydroxides, and hydrated oxides in crystalline and amorphorous forms. The fine powders derived from the metal alkoxides have high surface reactivity, making it possible to use low sintering temperatures and obtain uniform grain size bodies with the desired electrical properties. Others have prepared films from the metal alkoxide solutions by dipping substrates into the solution. The barium titanate films prepared by this method have had very good electrical properties. The metal alkoxide method, however, is a sol-gel method and involves several lengthy steps in processing.
The metallo-organic decomposition technology investigated by R. W. Vest, G. M. Vest and others produce the metal oxide films or powders directly and more quickly than those produced form the sol-gel methods. For example, silver films were fabricated by thermally decomposing the MOD silver inks on silicon for use as collector grids for photovoltaic cells. These collector grids required good adhesion, low contact resistance, low sheet resistance, and long term stability. Also, their deposition methods should not degrade the n-p junction. The fired films were 100% solder leach resistant, had good line definition and excellent long term adhesion. The resistivity was very close to that of bulk silver, indicating the films were very dense. This method of metallizing solar cells was also very inexpensive as compared to sputtering or vapor depositing on Si.
There are many advantages of MOD processing compared to alternate techniques for producing metal and ceramic films. The MOD process yields the equilibrium phases of the desired systems at relatively low temperatures, which circumvents the problem of selective volatility of different species. In general, the low temperature processing yields extremely find grain size polycrystalline films; in many cases the initial inorganic films are amorphous to X-rays. This allows for precise control of grain size by annealing after preparation of the films. The low temperature processing and the achievement of equilibrium phases is primarily due to the fact that the formulation that is deposited on the substrate is a true solution, and hence, the mixing of the various ingredients is on the molecular level. This ultimate mixing and high reactivity also can be used to an advantage in preparing very dense films. In most cases, the films with near theoretical density can be achieved. Starting from solution also leads to films with extremely uniform composition over large areas, and allows for uniform doping in the ppm range. High purity can be maintained during MOD processing by appropriate care in the various processing steps.
There are some intrinsic limitations to MOD processing, however. The volume change in going from the deposited wet film to the fired inorganic films is always large. In addition to requiring care during thermal processing, this large volume change means that the fired films will always be thin. This limitation of film thickness can be overcome by the multilayer approach. A second intrinsic limitation is one of the advantages cited in the previous paragraph, namely, that thermodynamic equilibrium is achieved very rapidly because of the extremely high reactivity upon thermal decomposition. Many of the electronic films in use today have their desirable properties because of the non-equilibrium microstructure. These non-equilibrium microstructures cannot be duplicated by MOD technology but alternate approaches to achieve equivalent electrical properties with an equilibrium microstructure can often be utilized. Another limitation due to the thermodynamic equilibrium achieved is that only oxides or only metals of certain elements can be produced.