Because the structure of semiconductor devices has changed to miniaturized three-dimensional (3D) structures, etching technologies that enable 3D atomic-scale control have received increasing interest. Therefore, atomic level/layer etching (ALEt), which is a counterpart of atomic layer deposition (ALD), has attracted much attention due to its potential in atomic-scale 3D nanofabrication. ALEt typically involves a cyclic repetition of self-limiting formation and desorption of surface modified layers. Thus, geometric loading effects are mitigated when using ALEt because of the self-limiting behavior.

The authors have developed a novel isotropic ALEt process for SiN using cyclic repetitions of hydrofluorocarbon-based plasma exposure and thermal annealing [1]. The first step of the etching cycle for SiN is exposing the sample surface at room temperature to hydrofluorocarbon-based radicals that form ammonium hexafluorosilicate ((NH₄)₂SiF₆) on SiN. The second step is thermal annealing above 100°C to sublimate the ammonium hexafluorosilicate. SiN film with an atomically thin layer can be etched away by repeating the cycle of forming and by removing the ammonium hexafluorosilicate. This cyclic etching shows self-limiting behavior because the formation of the ammonium hexafluorosilicate saturates as the plasma exposure time increases. This technology is applicable to other nitride films besides SiN. The authors have demonstrated selective isotropic ALEt of TiN [2], which is widely used as a gate electrode and barrier metal in semiconductor devices.

In this paper, we present our results with the isotropic ALEt process for SiN and TiN. This method features a variety of promising characteristics, such as high selectivity with respect to SiO₂and poly Si, atomic-level control of the etching amount, and isotropic etched profiles. The authors developed a novel etching apparatus as an ALEt tool for 300-mm wafers. This apparatus utilizes an infrared (IR) lamp for thermal annealing to achieve rapid temperature cycles. Particular attention is devoted to the surface reaction mechanism in the novel isotropic ALEt process for SiN. The results of in situ X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS) are described; they reveal the formation and decomposition of ammonium hexafluorosilicate on SiN during the cycle of plasma exposure and annealing. The potential industrial applications of these two processes are also described.

[1] K. Shinoda et al., Appl. Phys. Express 9, 106201 (2016).

[2] K. Shinoda et al., AVS Symposium and Exhibition, PS+TF-WeM10 (2016).