Parametric study of STiGer etching process in order to reduce extended formation of scalloping defects on the sidewalls of silicon submicron trenches

Abstract

A first study was carried out to define the appropriate parameters to create a passivation layer by SiF4/O2 plasma that resists lateral chemical etching by SF6 plasma via the STiGer process at a substrate temperature of −85 °C. The most efficient passivation layer was obtained for a SiF4/O2 gas flow ratio of 0.65, a pressure close to 1 Pa, and high RF source power.Submicron trenches with a critical aperture of about 0.8 μm were etched by the STiGer cryoetching process, which consists of alternating etch (SF6 or SF6/O2 chemistry) and passivation (SiF4/O2 chemistry) steps. The obtained trenches, observed by SEM, were vertical with a high aspect ratio equal to 46 with an average etches rate of about 1.8 μm/min. These features exhibit both an undercut and special defect, which called “extended scalloping”. This defect is composed of anisotropic cavities developed on the feature sidewalls, just below the mask and originates from scattered ions located at the feature entrance, which hit the top profile and locally remove the passivation layer. The formation of these defects, as well as trench profiles, strongly depends on the STiGer process parameters, especially chamber pressure and cycle times (passivation/etch steps), and with the optimization of these parameters, defect-free sidewall trenches are obtained with a high aspect ratio.