Plasma Atomic Layer Etching of SiO2 and Si3N4 with Low Global Warming

Introduction

Atomic layer etching (ALE) is an important process for creating nano scale patterns as ALE can remove layers with atomic-scale precision, excellent uniformity, and atomic level surface roughness. ALE processes are typically operated with self-limiting cyclic processes that consist of surface modification and removal steps in a cycle accomplished via a variety of surface reactions. The conventional Perfluorocarbons used in ALE cycle have a very high global warming potential (GWP). The current study features a low-GWP C₄H₃F₇O isomer plasmas for ALE process on SiO₂ and Si₃N₄. Impedans made Octiv VI probe was used to measure bias potential on the substrate and build up a relationship between the etch rate and bias voltages. 

Experimental setup

The cyclic ALE was studied in an inductively coupled plasma (ICP) reactor with a source power of 13.56 MHz and a bias power of 12.56 MHz, as shown in Figure 1 where the bias voltage was measured using a VI probe (Impedans, Octiv Poly). This study uses three isomers of C₄H₃F₇O which are heptafluoropropyl methyl ether (HFE-347mcc3), heptafluoroisopropyl methyl ether (HFE-347mmy) and Perfluoropropyl carbinol (PPC) and argon plasma for fluorination and etching steps.

 

 

 

Figure 1 (above) Schematic of the ICP reactor and (below) atomic layer etching process

Results

As shown in Figure 2, the etch per cycle (EPC) of SiO₂ was investigated as a function of the bias voltage of the Argon plasma. Depending on the ion energy during the etching step, three distinct regions were identified: the incomplete etch region, the ALE window region, and the physical sputtering region. At a bias voltage of less than 50 V, an incomplete etch region was observed for all of the C₄H₃F₇O isomers. At bias voltages ranging between 50−60 V, the ALE window regions or constant EPC were observed in all C₄H₃F₇O isomers. The physical sputtering region was observed at 60V, which is comparable to the previous SiO₂ etching result using Ar plasma.

Figure 2 Etch per cycle of SiO₂ in the fluorination step using C₄H₃F₇O isomers as a function of bias voltage.

The EPC of Si₃N₄ for C₄H₃F₇O isomer plasmas was also investigated as a function of Ar plasma bias voltage, as shown in Figure 3. The chemical sputtering threshold energy of Si₃N₄ was determined to be 15 V for the PPC plasma and 25V for the HFE-347mcc3 and HFE-347mmy plasmas. The ALE window regions were observed at bias voltages ranging from 50−60 V, as in the case of SiO₂. Because of the hydrogen content, all C₄H₃F₇O isomers have a higher EPC of Si₃N₄ than SiO₂.

Figure 3 Etch per cycle of Si₃N₄ using C₄H₃F₇O isomers in the fluorination step as a function of bias voltage in the etching step using Ar plasma. The etch per cycle of HFE-347mcc3 and HFE-347mmy is shown on the left y-axis, and that of PPC is shown on the right y-axis.

In the ALE process, high selectivity is one of the most critical factors in the etching process, and it is affected by the precursor, fluorocarbon film thickness, ion energy, and etching time. As shown in Figure 4, the etch selectivity of SiO₂/poly-Si and Si₃N₄/poly-Si was investigated depending on the C₄H₃F₇O isomer plasmas used in the fluorination step. The high etch selectivity of SiO₂/poly-Si can be applied to the SiO₂ etching process of fin field-effect transistor fin formation, and the high etch selectivity of Si₃N₄/ poly-Si can be applied to the Si₃N₄ spacer etching process.

Figure 4 Etch selectivity of SiO₂/poly-Si and Si₃N₄/poly-Si depending on the precursor in the fluorination step.

Summary

In this study, an ALE process for SiO₂ and Si₃N₄ was developed and characterized as a cyclic process consisting of surface fluorination and etching steps using low-GWP C₄H₃F₇O isomer plasmas. Owing to the hydrogen contained in the C₄H₃F₇O isomers, the chemical sputtering threshold energy of Si₃N₄ is observed to be 5−10 V lower than that of SiO₂. The ALE window region was identified in all C₄H₃F₇O isomers at bias voltages ranging from 50−60V and the EPC of Si₃N₄ was higher than SiO₂. The C₄H₃F₇O isomers can be used to reduce the global warming effect in the ALE process while also achieving high etch selectivity of SiO₂/poly-Si and Si₃N₄/poly-Si.

Reference

Yongjae Kim, Hojin Kang, Changkoo Kim and Heeyeop Chae, ACS Sustainable Chem. Eng. 2023, 11, 6136−6142  https://doi.org/10.1021/acssuschemeng.2c05186

 

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