Quasiatomic layer etching of silicon nitride enhanced by low temperature

Plasma atomic layer etching is a dry etching process using a dose step to modify a material’s surface chemistry and an etch step to remove the modified surface layer. This method of etching has certain advantages over reactive ion etch due to its self-limiting etch process for highly controllable etch depth and reduced surface roughness. In this paper, we expand upon an anisotropic, plasma atomic layer etch recipe used to etch thin films of silicon nitride, which uses an H₂ plasma to modify the surface layer of the material and an SF₆ etch step to remove the modified surface. Several modifications are made to the recipe, including a reduction in the pressure during the SF₆ step from 500 to 20 mT, to allow compatibility with modern inductively coupled plasma-reactive ion etch systems. We then explore this recipe at low wafer temperature and find a reduction of spontaneous isotropic SF₆ etching. This results in an enhancement in the self-limiting aspect of the etch process, an improvement of the etched sidewall homogeneity, and a decrease in the etched surface roughness, which has the potential to be useful for reducing optical loss in silicon nitride waveguides and other nanoscale devices made in silicon nitride.