<i>In\nSitu</i> Monitoring Surface Reactions\nin Cryogenic Atomic Layer Etching of Silicon Nitride by Alternating\nSurface Modification with Hydrogen Fluoride Dose and Ar Plasmas

Cryogenic\natomic layer etching (ALE) represents a promising\ntechnique\nfor achieving subnanoscale material removal in semiconductor processes,\nowing to its unique self-limiting surface-adsorbing reactions. This\npaper presents a cryogenic ALE method for SiN, utilizing surface modification\nwith a hydrogen fluoride (HF) dose and an Ar etch step for removing\nthe modification layer. The surface reactions and etching mechanism\nwere examined using <i>in situ</i> monitoring techniques,\nincluding spectroscopic ellipsometry and attenuated total reflectance\nFourier transform infrared spectroscopy (ATR-FTIR). Our observations\nreveal a self-limiting etching behavior for SiN and a reduction in\nthe etch amount per cycle (EPC) with a decreasing substrate temperature.\nDuring the HF dose step, in situ ATR-FTIR spectra indicate the formation\nof a surface-adsorbed layer containing ammonium fluorosilicate (AFS)\non the SiN surface. Subsequently, during the Ar plasma etching step,\nboth the AFS layer and the surface-adsorbed species were removed.\nAt lower substrate temperatures, the stability of the AFS layer and\nsurface-absorbed species increased, resulting in a reduction in EPC.\nThrough the control of Ar ion energy and substrate temperature, the\nmanipulation of EPC ranging from several nanometers to a few angstroms\nin atomic layer etching is achieved, offering potential utility in\nnanoscale device applications utilizing silicon nitride.