Thin Film Deposition Processes

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t available, molecular dynamic studies of film growth were only in the minds of physicists and materials scientists. Only within this decade are computers with sufficient speed and capacity available, and interestingly they are based in significant part on the development of thin film coatings used in microelectronic chips. The level of sophistication that has been achieved over this time is best seen in the molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE) deposition processes. In both the deposition process concept is simple, but it required developments in vacuum technology, sophisticated electronic controllers, purity of gases, solids and liquids used in the processes, new materials (e.g., high vapor pressure organometallic precursors), etc., before useful thin film materials could be achieved. The degree of crystal perfection and purity in each of these processes is excellent, and because they are layer-by-layer deposition processes, the ability to design new multilayer materials and electronic devices of practical and scientific importance is limited only by our imagination and ingenuity. And yet MBE and OMVPE take radically different approaches to achieving similar results of controlled epitaxial growth. In MBE, temperature is the primary parameter controlling high adatom mobility, and ultraclean and ultrahigh vacuum conditions allow the atoms sufficient time to do their predictable merry dance on the growing film surface, unimpeded by impurity atoms. OMVPE, on the other hand, substitutes chemistry for temperature and pure gases for ultrahigh vacuum. With OMVPE the ability to produce high purity organometallic precursor gases of sufficient purity, low dissociation temperature, and low cost are limiting factors — but once achieved, OMVPE has the advantages of low capital equipment cost and relatively high material throughput.

Not all thin film applications require, or even desire, crystalline perfection. Chemical vapor deposition (CVD) processes carried out under more "normal" conditions of chemical purity are still widely used despite the high temperatures required to dissociate the input gases because CVD has the potential for a high degree of control, conformal coating, and low cost. And thermal evaporation has been historically the most widely used deposition technique, with substrate temperatures varying from very low to high values, and a resulting wide range of crystallite sizes and properties. Deposition processes involving plasmas have become more prominent since the mid-1960s — first with rf sputtering, then plasma-assisted CVD, followed by magnetron sputtering, plasma spraying, ion beam deposition, and several variations and combinations (e.g., activated reactive evaporation, ion plating, ion-assisted deposition). We have seen how bombardment processes can augment thermal and chemical processes in achieving specific goals — such as films with controlled characteristics and resulting properties, usually at low substrate temperatures. Unfortunately, measurement and control of

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