ABSTRACT
Thin film technology is commonly used in many applications including microelectronics, optics, hard, and corrosion resistant coatings or micromechanics due to a wide variety of properties of thin films. Since the macroscopic properties of a material strongly depend on the film structure characterized in terms of grain size and crystallographic orientation, lattice defects, phase composition, and surface morphology, it is necessary to control the film growth at the atomic level. Indeed, progress in thin film technology depends upon the ability to selectively and controllably deposit the films with required physical properties. Very promising way how to engineer the properties of thin films has been found in manipulating the real structures of materials. Hence, understanding of the structure-property relationships is recently among the most important issues in material science. It is even more important for thin film processing since thin films are typically formed far away from equilibrium conditions. This is also a reason why thin films in most cases contain a much higher content of defects than conventionally prepared bulk materials. Due to the highly nonequilibrium state of the deposition process, besides vacancies, dislocations, grain boundaries, or precipitates, thin films are also characteristic for their anisotropic properties. Although it is very difficult to predict the structure and properties of deposited films due to a high complexity of the deposition process, it is simultaneously a reason for a high variety of films with different elemental
and phase compositions and microstructures that can be synthesized simply by varying the deposition conditions. Thin films can be prepared using a large number of deposition methods developed recently to cover all the demands of industry applications. They combine various physical principles for the film deposition so that the whole spectra of microstructures and physical properties of thin films are obtained. Due to specific limitations of every deposition process, there does not exist a universal technique for preparation films for any application. Especially the needs for low processing temperatures require very specific deposition conditions since in that case films are formed far away from thermodynamic equilibrium. Indeed, it is always necessary to optimize deposition conditions in order to enhance surface and bulk diffusion as the determinant atomic processes controlling structure evolution during the film growth. For this purpose, energetic particle bombardment has been shown very useful as an essential tool for enhancing adatom mobility and manipulating nucleation rates. On the other side, a competitive fashion and the kinetic limitations induced by low-temperature growth allows the controlled synthesis of unique metastable structures and structures of a high disorder showing completely new properties. The restricted surface and bulk diffusion results in limited migration of adatoms from the random points of impingement onto energetically favorable sites and thus films are formed in a different manner compared to equilibrium conditions. The film formation is reviewed as a process starting with condensation and nucleation of the adatoms on the surface followed by the nuclei growth, coalescence, interface formation, and subsequent thickness growth by the continued nucleation of depositing atoms on previously deposited material. All the above given stages of film growth can be effectively influenced by deposition parameters. The nucleation and growth kinetics, microstructural evolution, and consequently physical properties of films are determined mainly by the composition and energy distribution of the incident sputtered flux, energy of species bombarding the growing film surface, and the deposition temperature. Also the substrate material and its condition play a very important role. Surface roughness, crystallinity, and orientation significantly influence the growth and the final film structure. In principal, these control variables can be used to tailor the film structure and physical properties of the as-
deposited material. Another approach to vary film properties has been shown by addition of alloying elements into the film that tends to modify the structure evolution already at their extremely low concentrations. Hence, various elements are now generally applied as additives into thin films in order to form high sophisticated structures in polycrystalline thin films with unique physical and chemical properties. It, however, requires extensive studies of the correlation between film structure and deposition parameters. Only after the basic film formation processes are fully understood new materials can be designed for specific technological applications.
