Atomic layer etching of 3D structures in silicon: Self-limiting and nonideal reactions

Abstract

Current (and future) microelectronics fabrication requirements place unprecedented demands on the fidelity of plasma etching. As device features shrink to atomic dimensions, the plasma etching processes used to define these devices must resolve these scales. By separating etching processes into cycles of multiple, self-limited steps, different physics processes which are closely coupled in traditional plasma etching can be largely decoupled and separately optimized. This technique, atomic layer etching (ALE), can ideally remove uniform layers of material with consistent thickness in each cycle. ALE holds the promise of improving uniformity, reducing damage, increasing selectivity, and minimizing aspect ratio dependent etching (ARDE) rates. The practical implementation of ALE depends on how close to ideal the system can be operated and the tolerance to nonideal conditions. In this paper, results are discussed from a computational investigation of the consequences of nonidealities in the ALE of silicon using Ar/Cl 2 plasmas for both two dimensional trenches and three dimensional features. The authors found that ideal ALE requires self-limited processes during all steps of the ALE cycle. Steps that include continuous (non-self-limited) etching reactions reduce the ability of ALE to decouple process parameters. In addition to an etch depth that depends on pulse length per cycle, non-self-limited processes can reintroduce ARDE and produce surface roughening. By controlling subcycle pulse times, these deleterious effects can be minimized, and many of the benefits of ALE can be restored. Even nonideal ALE processes, when properly optimized, still provide benefits over continuous etching with similar chemistries and ion energy distributions. Using fluxes generated by a conventional inductively coupled plasma reactor, an example ALE process is able to clear the corners in a three-dimensional fin based field effect transistor case study with significantly less over-etch than the continuous process.

Atomic layer etching of gallium nitride (0001)

“…This recently triggered industrial interest in atomic layer etching (ALE) of traditional semiconductors. [1][2][3] ALE is a cyclic thin film etching process that uses self-limiting reactions, 4,5 in order to obtain a well defined etch per cycle. This can be a single atomic (or molecular, as in this work) layer etched in one etch cycle.…”

State of the Art and Future Perspectives in Advanced CMOS Technology

“…The above-mentioned etching techniques offer good etching selectivity, but the etching accuracy is not high enough to meet the demand of cavity etching in the inner spacer and the precision control of the diameter of the vertical nanowire preparation. Therefore, it is necessary to develop new etching methods e.g., digital atomic layer etching (ALE) or quasi-atomic layer etching (QALE) techniques [ 215 , 216 , 217 ]. Pargon et al proposed digital etching with alternating O 2 /He gas oxidation and NF 3 /NH 3/ O 2 etching to obtain a high SiGe etching selectivity to silicon (60: 1), but the accuracy is not high enough (about 19 nm/cycle) [ 218 ].…”

Transient behavior in quasi-atomic layer etching of silicon dioxide and silicon nitride in fluorocarbon plasmas

“…This effect was previously observed using our model for Cl 2 /Ar plasma ALE of silicon when clearing residual silicon from the tapered sidewalls between fins during gate etching. 62 In fluorocarbon plasma ALE of SiO 2 , sensitivity to the angular dependence of sputtering yield during the ion bombardment phase is exacerbated by polymer buildup on the walls. Ions striking the tapered sidewall at a near-grazing angle must penetrate the polymer layer to interact with the underlying selvedge layer.…”