Atomic Layer Etching of Silicon in HBr-Containing Plasmas

Atomic layer etching (ALE) of Si(100) was investigated in an inductively-coupled plasma (ICP), with a constant flow of He/Ar/O₂ and periodic 2s injections of HBr. The ICP and bias powers were on for 4s and 3s, respectively. Optical emissions above the radio frequency bias substrate surface were detected and monitored for Si, SiBr, OH, HBr⁺, Br, O and Ar. Etching rates were measured on SiO₂-masked substrates by laser interferometry. Optical emission spectra and etching rates were obtained over the 7s ALE cycle as a function of the time between HBr exposure and energetic ion bombardment, O₂ addition between 0 and 1%, addition of He, and DC self-bias voltage between -30 and -175 V_DC. At any bias power, below a threshold level of O₂ addition, steady etching of Si was accompanied by a periodic instantaneous rise and then decay in emissions from Si, SiBr and HBr⁺ during each of the energetic ion bombardment steps, with little OH emission. The presence of HBr⁺ emission after the flow of HBr had left the chamber suggests that HBr desorbed during the energetic ion bombardment step. As O₂ flow was increased at a constant bias power, etching stopped and emission from OH became the dominant feature in the UV. The amount of O₂ added before etching stopped was an increasing function of increasing bias power. For conditions near the transition between etching and no etching, the etching per ALE cycle decreased to immeasurably slow over multiple cycles as the surface becomes oxidized. The etching rate under conditions with minimal oxidation was 3 nm/cycle, even with minimal overlap between HBr injection and ICP power. ALE with HBr will also be compared with Cl₂ as the reactive gas.