Preferential sputtering effects in thin film processing

Predicting the partial sputtering yield (sputtered atoms of one element/incident energetic ion) for different elements during sputtering from multielement targets is not a straightforward task. It is commonly observed that ion bombardment of composite targets exhibits preferential sputtering. Such processes are normally studied by computer simulations since theory gives qualitative results only. This work presents a systematic computational study of the major ballistically induced preferential sputtering effects taking place during ion bombardment of composite materials. It is shown that in a vast number of cases ballistic preferential sputtering is a dominant process particularly at moderate temperatures where thermodynamic effects are suppressed. In particular, sputtering from composite targets, resputtering during ion assisted deposition of composite films, sputtering of thin films onto heavy substrates, ion assisted deposition onto different substrate materials, sputter depth profiling, etc. are technological processes shown to commonly exhibit preferential sputtering. It is further shown that a number of sputtering situations exhibit along with the preferential sputtering the so called sputter yield amplification effect. The latter consists in that the partial sputtering yield of a given species is larger than the sputtering yield of the pure material. As a consequence, the sputtering yield of certain materials can be enhanced substantially by introducing small concentrations (a few percent) of impurity species in the target. The work provides ample examples of various situations of academic and technological interest where preferential sputtering and sputter yield amplification occur. The major ballistic mechanisms leading to preferential sputtering and sputter yield amplification are also briefly discussed.