Germanium ion implantation efficiency improvement with use of germanium tetrafluoride

Abstract

Ion implantation of germanium in silicon wafers is often troubled by reduced ion source life due to use of germanium tetrafluoride (GeF4) as a source material. The problem is mainly due to tungsten re-deposition, a result of a fluorine-induced halogen cycle initiated within the ion source. The halogen cycle is particularly pronounced in the case of GeF4 by easy fragmentation of the molecule, as well as low utilization of germanium due to wide isotopic distribution of natural abundance GeF4. Through isotopic enrichment of 72GeF4, benefits such as enhanced ion implantation beam current, lower gas flow, and longer source life can be achieved versus natural GeF4. Additional benefits can be realized when using mixtures of GeF4 with hydrogen (H2). Data are presented that show the effect of H2 content on beam current as well as on tungsten transport. Lastly, thermodynamics and stability results are presented for a single cylinder mixture of GeF4 with H2.