Ion-beam assisted etching of semiconductors

Abstract

Many groups have investigated the interaction of surfaces with ion beams, with and without simultaneous exposure to a reactive gas, as a model simulation system for plasma etching. On the basis of their experimental evidence current understanding of ion beam assisted etching of semiconductors is reviewed. The discussion is restricted to technologically important ions with atomic number Z⪆6 in the energy range 0.25–25 keV. This allows for a meaningful comparison with predictions from linear cascade theory. First, physical sputtering by noble gas ions will be discussed. Next, reactive and molecular ions are treated. It is shown that theur sputtering yield behaviour is not dramatically different from that of inert gas ions and that chemical contributions can reasonably be accounted for. Finally, chemical sputtering of materials by simultaneous exposure to ion bombardment and a reactive gas flow is examined. Observed systematics in absolute etch rates and energy distributions of emitted reaction products point to a number of parallel acting mechanisms. Adsorption and subsequent ion beam mixing of the reactive gas into the surface, ion induced desorption, electronic excitation of the target, surface temperature and the chemical activity of the gas can each dominate the detailed etch behaviour. In spite of the fact that the broad trends in chemical sputtering apparently agree with a collision-cascade-like ejection mechanism, this multitude of interactions presently prohibits development of quantitative models.