Optimization of submicron deep trench profiles with the STiGer cryoetching process: reduction of defects

Abstract

The STiGer process is a time-multiplexed cryogenic etching method designed to achieve high aspect ratio structures on silicon. SF6 or SF6/O2 plasmas are used as etch cycles and SiF4/O2 plasmas are used as passivation cycles. Trenches with a critical dimension of 0.8 µm have been etched to a depth of 38 µm with an average etch rate of 1.8 µm min−1. These features exhibit both undercut and a defect which is called extended scalloping. We describe this defect specific to the STiGer process and we discuss its origin: the extended scalloping is composed of anisotropic cavities developed on the sidewalls of the feature top (typically in the first 2–3 µm below the mask). It originates from ions scattered at the feature entrance that hit the top profile and remove the passivation layer where it is weakest. Then, we propose two methods to reduce this extended scalloping. The first consists in adding a low oxygen flow in the etching cycle. It favors a low additional passivation which reduces scalloping. The second technique consists in gradually increasing the SF6 flow from a low value to the nominal value. Consequently, the process starts with a low etch rate and an efficient passivation.