Quasi-atomic 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 a previous 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 of the pressure during the SF6 step from 500 mT to 20 mT, to allow compatibility with modern ICP-RIE 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 side wall homogeneity, and a decrease of 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.