Role of neutral transport in aspect ratio dependent plasma etching of three-dimensional features

Abstract

Fabrication of semiconductor devices having three-dimensional (3D) structures places unprecedented demands on plasma etching processes. Among these demands is the frequent need to simultaneously etch features with a wide variety of aspect ratios (AR) on the same wafer. Many plasma etching processes exhibit aspect ratio dependent etching (ARDE)—different etch rates for features that have different aspect ratios, usually slower for larger AR. Processes subject to ARDE require over-etch to clear the larger AR features, which increases the need for high selectivity and low damage. Despite these issues, the physical processes which contribute to ARDE are not well understood. In this paper, results are discussed from a computational investigation on the root causes of ARDE during Ar/Cl2 plasma etching of Si, and, in particular, the role which neutral transport plays in this process. Parametric studies were performed varying neutral-to-ion flux ratios, surface recombination rates of atomic Cl, and neutral and ion angular distributions to determine their influence on neutral transport of Cl to the etch front and ARDE. It was found that each parameter has a significant influence on neutral transport to the etch front (with the exception of the ion angular distribution). Methods for increasing neutral flux (for a given set of ion fluxes) to the etch front were found to push the system toward a neutral saturated, ion starved regime which alleviates ARDE for some range of AR. Increased neutral flux is also correlated with more tapered features, which tend to exhibit more significant ARDE. This coupling of neutral transport with feature profiles makes it difficult to alleviate all ARDE in this system. However, it is possible to optimize parameters in such a way to postpone the onset of ARDE to fairly large AR (>8).