At sub-10 nm technology nodes, silicon etching is critical to FEOL processes such as FinFET thinning, gap filler removal, pre-III-V deposition etch, and nanowire release etching. The key to developing solutions for these applications is being able to control the anisotropy of the Si etch, or the etch selectivity between Si (111), Si (100), and Si (110). For this study, we focused on the impact of modifications to tetramethylammonium hydroxide (TMAH) on Si etch selectivity, with the ultimate goal of developing an isotropic Si etchant based on TMAH.   

Initial studies focused on etch selectivity testing of TMAH based chemistries on Si (SOI) wafers of varying orientations. Etch rates were based on measuring thickness changes by cross-sectional SEM. Differences in surface roughness were also characterized by atomic force microscopy. Finally, solutions were evaluated for anisotropy by performing an etch of patterned pSi wafers.

As expected, aqueous TMAH solutions showed a high level of anisotropy (111/100/110 = 1:5:5) and significant level of surface roughening. In contrast, some of the modified TMAH solutions tested had decreased surface roughening and an decreased level of anisotropy (111/100/110 = 1:2:2). Pattern etching on implanted pSi wafers is used to highlight the differences in anisotropy between TMAH and the different modifications.