Development of a novel structure zone model relating to the closed-field unbalanced magnetron sputtering system

Abstract

It is well established that the microstructure of a thin film strongly influences its physical and chemical properties. Microstructure, in turn, is determined by a number of deposition and process parameters which control the energy delivered to the growing film. The closed-field unbalanced magnetron sputtering (CFUBMS) process has now been developed to the stage where it can be routinely used to deposit very high quality, well adhered coatings of a wide range of metals and ceramics. A key factor in the success of this process is the ability to transport large ion currents to the substrate. This can enhance the formation of fully dense coating structures at relatively low homologous temperatures, compared to other sputtering systems. The importance of microstructure on the performance of a coating has led to the development of models designed to describe coating structure in terms of specific deposition parameters. Several such structure zone models (SZMs) relating to various physical vapor deposition (PVD) processes have been published. However, because of the advantages of operating in the CFUBMS mode, the structure of coatings deposited in this mode do not conform to those predicted by existing SZMs relating to other PVD processes. Also, in most existing SZMs, the final coating structure is described in terms of the homologous temperature of the coating and one other parameter which attempts to describe the additional influence on the structure of the simultaneous ion bombardment of the growing film. Several parameters have been used to fill this role including coating pressure, substrate bias voltage, and an energy parameter defined as the average energy carried by the arriving ions per condensing atom. However, other studies have shown that ion energy and ion flux are fundamental parameters in ion-assisted PVD processes and their effects must be considered separately when describing coating structures. A detailed investigation has now been carried out into the CFUBMS process. As a result of this, a SZM relating to the CFUBMS system has been developed, in which coating structures are described in terms of homologous temperature, bias voltage and the ion-to-atom ratio incident at the substrate. This is a novel model which allows the influence of ion flux and ion energy to be considered separately. This study has also highlighted a number of other characteristics of the CFUBMS system. For example, both ion current density and deposition rate are directly proportional to the target current, although their coefficients of proportionality differ. Deposition rate decreases more rapidly with increasing substrate-to-target separation than ion current. Consequently the ion-to-atom ratio incident at the substrate increases with separation. Indeed, with magnetrons of fixed magnetic configuration, in order to increase the ion-to-atom ratio for any set of deposition parameters, it is necessary to increase the substrate-to-target separation.