Applied Science and Convergence Technology 2024; 33(1): 1-6
Published online January 30, 2024
Copyright © The Korean Vacuum Society.
aSchool of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
bSKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea
cDepartment of Semiconductor Convergence Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
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In this paper, atomic layer etching (ALE) processes for SiO2 are reviewed and categorized into two distinct group of anisotropic and isotropic ALE processes. Anisotropic ALE typically involves the fluorination of the silicon dioxide (SiO2) surface by fluorocarbon deposition during surface modification, followed by the removal of fluorinated layers by relatively low-energy ions. The impacts of the precursor, ion energy, selectivity, and chamber wall conditions on anisotropic ALE processes are reviewed. Isotropic ALE involves the conversion of SiO2 surfaces into a fluorinated layer or ammonium salt. This layer is subsequently eliminated through various chemical reactions, such as sublimation, fluorination, and ligand exchange. The mechanisms of etching in isotropic ALE are reviewed and classified into two subcategories of thermally isotropic and plasma-assisted isotropic ALE.
Keywords: Anisotropic etching, Atomic layer etching, Isotropic etching, Silicon oxide
Silicon dioxide (SiO2) has been applied as an insulator in various nanoscale semiconductor devices, playing a crucial role in the semiconductor industry for the past 50 years [1,2]. SiO2 exhibits exceptional insulating performance, a bulk resistivity of 1015 Ω cm, and a dielectric breakdown strength of 107 V cm−1. Additionally, it is cost effective and easy to manufacture and demonstrates excellent compatibility with silicon bulk .
Plasma etching is a crucial and essential processing technique in semiconductor device fabrication. Its application to next-generation semiconductor devices is becoming increasingly challenging as the critical dimension (CD) of semiconductors decreases to 10 nm level [4–7]. With decreasing CD and the adoption of three-dimensional structures, conventional reactive-ion etching processes are facing limitations in thickness controllability, etch selectivity, and surface roughness at the nanoscale [6–8]. Consequently, atomic layer etching (ALE) processes are under active development, offering atomic-level precision in layer removal, minimized surface roughness, and exceptional uniformity [4–15].
ALE is a cyclic process that facilitates the atomic-level removal of various layers through a modification step involving radicals or molecules, followed by a removal step utilizing ions or chemical reactions as shown in Fig. 1. ALE processes can provide precise thickness control, excellent surface roughness, and high uniformity at both atomic and nanometer scales [16–21]. A typical ALE process comprises four steps. Initially, the precursor chemisorbs onto the substrate surface through a chemical reaction, which may or may not be self-limiting. The second step involves purging to remove any physically adsorbed reactants. In the third step, modified surface layers are removed, forming volatile etch products via energetic ions or chemical reactions. It is classified as anisotropic ALE when the products are removed faster in one direction by directional energetic ions and as isotropic ALE when removal occurs uniformly in all directions. The final step involves purging the chamber with inert gases, similar to the second step. This sequence is then repeated in subsequent cycles.
In this review, recent advancements in the ALE of SiO2 are categorized into anisotropic and isotropic ALE processes [8,22–43]. For anisotropic SiO2 ALE, we summarize the effects of precursors, ion energy, selectivity, and chamber wall conditions. Isotropic SiO2 ALE processes are reviewed in terms of various etching mechanisms such as surface modification and the formation of volatile products, encompassing conversion, fluorination, and ligand exchange.
For anisotropic SiO2 ALE, the SiO2 surface undergoes fluorination through the deposition of a fluorocarbon layer, using a range of fluorocarbon-based precursors, as summarized in Table I. Various precursors, including C4F8, C4F6, CHF3, C3H3F3, CF3I, and C4H3F7O isomers, are employed in SiO2 ALE. The chemical composition of the deposited fluorocarbon film, influenced by the precursor type, significantly impacts the etching characteristics. Studies have compared the ALE processes using C4F8/Ar and CHF3/Ar plasmas, focusing on fluorocarbon and hydrofluorocarbon precursors . These studies analyze the composition of the fluorocarbon film and the etching rate in relation to the C4F8 and CHF3 precursors, establishing a correlation between the F/C ratio in the fluorocarbon film and the SiO2 etching rate with different precursors.
The addition of hydrogen atoms is known to reduce the fluorine radicals in fluorocarbon plasmas, whereas oxygen atoms enhance it [44–46]. SiO2 ALE processes using C4F8, C4F8/H2, and C3H3F3 plasmas were investigated to determine the impact of hydrogen addition . The fluorocarbon films generated by C4F8/H2 and C3H3F3 plasmas exhibited a lower F/C ratio than those produced by C4F8 plasma. Consequently, the SiO2 etching rate was higher in the C4F8 plasma compared to the C4F8/H2 and C3H3F3 plasmas, due to a higher F/C ratio of fluorocarbons on the SiO2 surface. Furthermore, the SiO2 ALE process utilizing C4F8, CHF3, and C3F7OCH3 plasmas has been examined to assess the effects of both hydrogen and oxygen additions . The fluorocarbon film formed by the C3F7OCH3 plasma exhibited the lowest F/C ratio compared to those produced by the C4F8 and CHF3 plasmas.
The SiO2 ALE process utilizing C4H3F7O isomer plasmas was explored to elucidate the impact of chemical structure . The composition of the resulting fluorocarbon film is dictated by the chemical bonding structure of the C4H3F7O isomers, leading to variations in the F/C ratios as shown in Fig. 2. The F/C ratios of fluorocarbons generated by n-C3F7OCH3 and i-C3F7OCH3 plasmas are lower compared to those formed by CF3CF2CF2CH2OH plasma, which can be attributed to the CH3 radicals stemming from the presence of -OCH3 in the molecule. Correspondingly, the etching rate of SiO2 is highest in the CF3CF2CF2CH2OH plasma and lowest in the i-C3F7OCH3 plasma, mirroring the F/C ratio of the fluorocarbon films. It is observed that the addition of hydrogen to the plasma reduces the F/C ratio of fluorocarbon films and subsequently diminishes the etching rate of SiO2.
In the anisotropic SiO2 ALE process, the parameters defining the ALE window include the precursor used in the fluorination step, the type of gas employed in the removal step, and the ion energy, as summarized in Table II. The threshold physical sputtering energies identified are 50 eV for SiO2 , 20 eV for Si3N4 , and 20 eV for Si . Investigations into the physical sputtering of SiO2, based on the Ar bias voltage, revealed sputtering at ion energies above 65 eV . Surface modification allows for etching with lower ion energy and enables self-limiting removal of the modified layer . In the ALE window region, selective removal of the modified layer from the bulk material occurs. Three distinct regions are discerned based on ion energy: the incomplete etching region, the ALE window region, and the physical sputtering region, as shown in Fig. 3 . The removal of the modified layer is not complete at low ion energies, while sputtering of the underlying material occurs at higher ion energies.
The SiO2 ALE window region has been reported with varying parameters. A previous study reported a window region of 50–60 V with 15 W source power of Ar plasma . Another study identified an SiO2 ALE window region with a bias voltage of approximately 15 eV at 1 kW of Ar source plasma power, highlighting a very low ion energy range for the ALE window due to the high source power . These findings imply that interactions between chemical structure, ion energy, and process conditions determine the efficacy and characteristics of SiO2 ALE processes.
The etching selectivity of SiO2/Si and SiO2/Si3N4 can be enhanced through various approaches, including the choice of precursors, control of fluorocarbon film thickness, ion energy management, etch step time adjustment, and selective deposition techniques, as outlined in Table III. The selectivity in anisotropic SiO2 ALE is influenced by the thickness of the fluorocarbon film, which in turn is determined by the choice of precursor . Improvements in SiO2/Si and SiO2/Si3N4 etch selectivity have been achieved using C4F8, C4F8/H2, and C3H3F3 plasmas by studying the impact of hydrogen addition . The etching selectivity of SiO2/Si and SiO2/Si3N4 were notably improved using the C3H3F3 precursor, attributed to the reduction of fluorine concentration in the fluorocarbon film due to hydrogen. Additionally, the enhancement of SiO2/Si etch selectivity has been discussed using C4H3F7O isomer plasma, as shown in Fig. 4 . A higher presence of Si-C bonds was observed in the fluorocarbon films generated by the i-C3F7OCH3 plasma, compared to those from n-C3F7OCH3 and CF3CF2CF2CH2OH plasmas. These Si-C bonds act as inhibitors for Si etching, thereby decreasing the etching rate of Si and enhancing the SiO2/Si etch selectivity. This evidence suggests that carefully selecting precursors can lead to high etch selectivity in anisotropic ALE processes. Moreover, selective functionalization of the SiNx surface with benzaldehyde has been shown to improve the etch selectivity of SiO2/SiNx, as shown in Fig. 5 . Benzaldehyde selectively deposits on SiNx surfaces featuring -NHx (x = 1, 2) groups, but not on SiO2 surfaces with -OH groups. This selective deposition of benzaldehyde on SiNx surfaces fosters the formation of a hydrofluorocarbon film, which serves as a barrier against the etching of SiNx. The implication of these findings is that through strategic precursor selection and surface functionalization, the etching selectivity for different material combinations in anisotropic ALE processes can be effectively manipulated and optimized.
In the anisotropic SiO2 ALE process, the etching rate is influenced by the fluorocarbon film left on chamber walls, as summarized in Table IV. Achieving a consistent etching rate is crucial in ALE and the fluorocarbon film on the chamber walls plays a significant role in the repeatability of the etching rate. Variations in the SiO2 etching rate across multiple ALE cycles have been reported using
In isotropic SiO2 ALE processes, the SiO2 surface undergoes modification through either thermal reactions or plasma-assisted methods. The etching mechanisms for thermal isotropic SiO2 ALE are summarized in Table V. Here, the SiO2 surfaces are modified by conversion to Al2O3 or by formation of ammonium salt, and then the modified layers are removed through various chemical reactions including sublimation, fluorination, and ligand exchange. These etching mechanisms are categorized into two groups: conversion of SiO2 into Al2O3 or ammonium fluorosilicate (AFS) during the modification step. An example of the isotropic SiO2 ALE process using triethylaluminium (TMA) and hydrogen fluoride (HF) precursors was reported . In this process, SiO2 is converted to Al2O3 utilizing the TMA precursor, as indicated in Eq. (1), which represents the reaction for modification step. Subsequently, Al2O3 undergoes fluorination to AlF3 by HF, as shown in Eq. (2). Finally, AlF3 is removed via a ligand exchange reaction forming volatile AlF(CH3)2 with TMA, as shown in Eq. (3), which represents the reaction for removal step. These reactions occur spontaneously at temperatures as high as 300 °C without plasma assistance.
Isotropic SiO2 ALE process using HF and NH3 gas has been also reported . The AFS layer is formed by the reaction of NH3 molecules with the adsorbed HF on SiO2 surface, as shown in Eq. (4). In the removal step, the AFS layer decomposes at temperatures above 140 °C, forming volatile reaction products such as NH3, HF, and SiF4, as shown in Eq. (5).
The etching mechanisms of plasma-assisted isotropic SiO2 ALE are comprehensively summarized in Table VI. A specific isotropic SiO2 ALE process employing CF4/NH3 or NF3/NH3 plasma has been examined. In this process, SiO2 is converted to AFS in CF4/NH3 or NF3/NH3 plasma as shown in Eqs. (6) and (7). Subsequently, the AFS layer decomposes at temperatures above 160 °C, forming volatile substances such as NH3, HF, and SiF4, as shown in Eq. (8). The selflimiting property of AFS formation was confirmed to be dependent on the plasma duration, with the etching rate escalating from 2.7 to 7.0 nm/cycle based on the gas ratio, as shown in Fig. 7. The etching rate observed in the NF3/NH3 plasma was approximately threefold higher than that in the CF4/NH3 plasma. This discrepancy is attributed to the different bonding energies of F (~506 kJ/mol) and N-F (~239 kJ/mol). NF3 is more prone to dissociation than CF4 under similar conditions, leading to the generation of a greater number of fluorine atoms in the NF3/NH3 plasma compared to the CF4/NH3 plasma. This results in more extensive AFS formation and consequently, a higher etching rate. This step is crucial as it ensures the complete removal of the modified layer, enabling the process to proceed to the next cycle effectively. The distinction in etching rates between the two plasma types underscores the importance of gas selection and plasma conditions in optimizing the isotropic ALE process for SiO2.
In this review, we categorized recent research on the SiO2 ALE process into anisotropic and isotropic processes. For anisotropic SiO2 ALE processes, the effects of the precursor, ion energy, selectivity, and chamber wall conditions were examined. The choice of precursor influenced the F/C ratio in the film deposited on the SiO2 surface, which subsequently affected the etching rate and selectivity. In the anisotropic ALE process, changes in ion energy affected the etching rate, and a consistent etching rate was observed within the defined ALE window. For isotropic SiO2 ALE, we elucidated two types of mechanisms are summarized. SiO2 surface into a fluorinated layer or ammonium salt, followed by removal through various chemical reactions, including sublimation, fluorination, and ligand exchange. Given the increasing complexity and three-dimensional nature of semiconductor device integration, the necessity for both anisotropic and isotropic ALE processes is evident. The selection of specific SiO2 ALE mechanisms should be tailored to the device architecture. Future research on the nuances of the SiO2 ALE mechanism is essential to further enhance the precision and efficiency of these processes in semiconductor fabrication.
This work was supported by the Technology Innovation Program (RS-2022-00155706), funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
The authors declare no conflicts of interest.