DEVELOPMENT OF A NOVEL STRUCTURE ZONE MODEL RELATING TO THE CLOSED-FIELD UNBALANCED MAGNETRON SPUTTERING SYSTEM

It is well established that the microstructure of a thin film strongly influences its physical and chemical properties. Microstructure, in t urn, is determined by a number of deposition and process parameters wh ich control the energy delivered to the growing film. The closed-field unbalanced magnetron sputtering (CFUBMS) process has now been develop ed to the stage where it can be routinely used to deposit very high qu ality, 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 o f fully dense coating structures at relatively low homologous temperat ures, compared to other sputtering systems. The importance of microstr ucture on the performance of a coating has led to the development of m odels designed to describe coating structure in terms of specific depo sition parameters. Several such structure zone models (SZMs) relating to various physical vapor deposition (PVD) processes have been publish ed. However, because of the advantages of operating in the CFUBMS mode , the structure of coatings deposited in this mode do not conform to t hose predicted by existing SZMs relating to other PVD processes. Also, in most existing SZMs, the final coating structure is described in te rms of the homologous temperature of the coating and one other paramet er which attempts to describe the additional influence on the structur e of the simultaneous ion bombardment of the growing film. Several par ameters have been used to fill this role including coating pressure, s ubstrate bias voltage, and an energy parameter defined as the average energy carried by the arriving ions per condensing atom. However, othe r studies have shown that ion energy and ion flux are fundamental para meters in ion-assisted PVD processes and their effects must be conside red separately when describing coating structures. A detailed investig ation 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 whic h coating structures are described in terms of homologous temperature, bias voltage and the ion-to-atom ratio incident at the substrate. Thi s is a novel model which allows the influence of ion Aux and ion energ y To be considered separately. This study has also highlighted a numbe r 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-t arget separation than ion current. Consequently the ion-to-atom ratio incident at the substrate increases with separation. Indeed, with magn etrons of fixed magnetic configuration, in order to increase the ion-t o-atom ratio for any set of deposition parameters, it is necessary to increase the substrate-to-target separation. (C) 1998 American Vacuum Society. [S0734-2101(98)01805-3].