Target poisoning during reactive magnetron sputtering: Part I: the influence of ion implantation

Abstract

Gettering plays a minor role during reactive sputtering of silicon in a nitrogen/argon mixture. However, an abrupt increase of the target voltage as a function of the nitrogen mole fraction is noticed which is not expected from the classical models explaining reactive magnetron sputtering. To explain the target voltage behaviour during DC magnetron sputtering of silicon in an argon/nitrogen mixture, a model is proposed which is based on the reactive ion implantation into the subsurface region of the silicon target. The model calculates the concentration of the nitrogen ions implanted into the target and assumes three possible pathways for these implanted ions. A first pathway is the chemical reaction between the implanted nitrogen ions and the target material to form silicon nitride. The implanted nitrogen can also remain in the target as non-reacted nitrogen atoms. Or, the nitrogen atoms can recombine in the target and diffuse from the target. The compound formation results in a decrease of the target surface recession speed or target erosion rate. As the surface concentration of the implanted ions is inversely proportional to the surface recession speed, an avalanche situation becomes possible. This abrupt transition in recession speed is accompanied with a sudden increase of the concentration of non-reacted nitrogen atoms in the target. In this way, the abrupt target voltage change, noticed at a given mole fraction of nitrogen in the plasma, can be understood.