TRIDYN simulation of target poisoning in reactive sputtering

During reactive sputter deposition, target "poisoning", i.e. the formation of a compound layer at the target surface, may reduce the sputter erosion rate substantially and thereby represent a major limitation to achieve high deposition rates. In order to investigate the formation of the poisoned layer, the TRIDYN program has been employed to simulate the processes that take place at the target surface during sputtering at ion energies which are typical for a magnetron discharge, in a typical gas mixture of Ar with a small (<10%) addition of a reactive gas (e.g. oxygen). The bulk of the sputtering results from Ar ion bombardment, while the reactive gas ions contribute to compound formation due to implantation into the subsurface layer. In addition, reactive gas molecules are adsorbed on the surface and react with target metal atoms to add to the formation of the compound layer. Thus, both chemisorption and ion implantation of energetic reactive ions are the two main mechanism! s for the formation of the poisoned layer.TRIDYN simulations have been performed at varying reactive ion to total ion flux ratio, and at varying ion to reactive neutral flux ratio, for fluences which are sufficiently large to achieve a stationary deposition/erosion balance. The results illustrate that the two mechanisms will generate almost identical shapes of the poisoned layer. They also demonstrate the significance of recoil implantation from the chemisorbed layer for the formation of the compound layer. In agreement with experimental findings, the calculated sputter erosion rate of the target is predicted to decrease monotonically as the partial pressure of the reactive gas increases. The shape of the sputter erosion curve hardly changes between conditions dominated by ion implantation or chemisorption. We therefore conclude that ion implantation basically acts as an additional source of reactive atoms to the target surface.