Comparisons of atomic layer etching of silicon in Cl2 and HBr-containing plasmas

This paper reports an experimental investigation of Cl2 versus HBr for plasma atomic layer etching of silicon. An inductively coupled plasma (ICP) source with a constant flow of Ar carrier gases and HBr or Cl2 as a dosing gas was used for etching Si (100). Two modes of dosing were investigated: plasma gas dosing, in which pulsed flows of Cl2 or HBr are partially dissociated with the ICP with no substrate bias, and gas dosing, where the ICP is off during the dosing step. Following either dosing mode, a purge step of up to 5 s is followed by a 1 s period of ICP and substrate bias power, leading to etching of the halogenated surface layer. Optical emission spectroscopy was used to follow relative yields of SiCl, SiCl2, and SiBr, and scanning electron microscopy and profilometry were used to measure etching rates. Plasma gas dosing resulted in etching rates three to four times higher than gas dosing. Small differences were found between the two etchant feed gases, with Cl2 exhibiting about 3%–15% higher etching rate. Etched profiles for HBr plasma gas dosing produced little or no microtrench adjacent to the SiO2-masked line, while HBr gas dosing or Cl2 with either mode of dosing produced microtrenches at the bottom of the Si sidewall.