Si3N4 Etching Research Articles

Unveiling polycrystalline silicon channel dissolution mechanism in wet etching process of 3D NAND fabrication

3D NAND is one of the most promising storage devices due to its high storage density and low energy consumption. Selective wet etching of Si3N4 by hot-concentrated phosphoric acid (PA) solution is a key process for 3D NAND fabrication. However, the channel material, polycrystalline silicon (poly-Si), is unexpectedly dissolved due to its inevitable exposure to the PA etching solution, which causes device failure and thus limits the stacking density of 3D NAND. Herein, to avoid the detrimental dissolution of poly-Si, we unveil the mechanism of poly-Si dissolution in PA solution from kinetic and thermodynamic perspectives. We find that the etch process shows a two-stage reaction, in which H2O plays a key role in lowering the activation barrier for poly-Si etching. Further thermodynamic analysis demonstrates that poly-Si prefers to be oxidized by H2O to suboxide rather than to SiO2. Combined with surface chemistry characterization, we propose the dissolution mechanism as a two-stage process involving the dissolution of suboxide interface and bulk poly-Si, where the nucleophilic substitution of Si–H with Si–OH on bulk poly-Si surface determines the overall etch rate. This study advances the 3D NAND development by establishing a theoretical foundation for identifying and implementing efficacious strategies to impede detrimental dissolution.

Preventing oxide precipitation in selective wet etching of Si3N4 for 3D-NAND structure fabrication: The role of bubbles

Selective wet etching of Si3N4 is a critical process in the fabrication of 3D-NAND structures; however, it faces a oxide precipitation problem that significantly deteriorates the remaining structure morphology. A recent study by Kim et al.(Kim et al., 2022 [1]) showed that generating CO2 bubbles during the wet etching process efficiently solves the precipitation problem in the fabrication of a 128 multi-layer 3D-NAND structure. In this study, we numerically investigated the multiscale diffusive transport of oxides in the etching process via three different simplified simulations at different scales to reveal the underlying mechanism. We found that mass transport within the multilayer structures alone cannot contribute to the oxide precipitation behavior. Macroscopic transport from the wafer-etchant interface to the bulk must be considered since it contributes to a high oxide concentration at the wafer-etchant surface, which further increases the concentration within the trenches, leading to the precipitation problem. Through the conjecture oxide transport simulation, we found that the large bubbles generated from the reaction agitate the surrounding liquid and dramatically reduce the oxide concentration at the wafer-etchant surface by one order of magnitude, thereby solving the precipitation problem. Our findings clearly explain the experimental results reported by Kim et al. and will further benefit the development of process-intensification technologies in wet etching.

Behavior of oxide regrowth during the selective Si3N4 etching process on 3D NAND structures using finite element computational simulations

Oxide regrowth should be suppressed during the etching of silicon nitride, which is used to fabricate three-dimensional Not AND (3D NAND) devices. This requires a detailed understanding on the mass-transfer characteristics of Si3N4 etching byproducts that cause oxide regrowth. Finite element method (FEM) simulations are performed to investigate the mass transfer of Si3N4 etching byproducts (EBSi3N4) and oxide regrowth during Si3N4 etching on a 3D Si3N4/SiO2 multistack. In particular, the concentration profiles of the byproducts under various etching conditions, such as different numbers of Si3N4/SiO2 multistacks, diffusion coefficients of the byproducts, etching rates of Si3N4, and initial concentrations of Si-based additives, are monitored. Furthermore, the in-situ generation of oxide regrowth under various etching conditions is evaluated. Consequently, an increase in the diffusion coefficient of the byproducts is proposed as the most promising method for suppressing oxide regrowth during Si3N4 etching on high-numbered 3D NAND stacks. The FEM simulation results are strongly supported by data from Si3N4 etching experiments on Si3N4/SiO2 multistacks.

Selective Si<sub>3</sub>N<sub>4</sub> Etching for 3D NAND Integration by Using Low Concentration of H<sub>3</sub>PO<sub>4</sub>

This study investigated the etching kinetics of Si3N4 in various concentration of H3PO4 solution and the effect of Si3N4 etching enhancers on the etching process, particularly for 3D NAND trench structures. 30 wt% H3PO4 was used to etch Si3N4, which can produce higher Si3N4/SiO2 etching selectivity and similar Si3N4 etching rate compared to a conventional 85 wt% H3PO4. 30 wt% H3PO4 showed significantly improved etching performance for the Si3N4/SiO2 3D NAND structure as compared to 85 wt% H3PO4. In particular, the transportation ability of H3PO4 into 3D NAND trench structures can be improved by reducing viscosity of etchant, which can be obtained by reducing the concentration of H3PO4. In addition, Si3N4 etching enhancers were introduced to accelerate the Si3N4 etching kinetics in 30 wt% H3PO4. Addition of such additives improved the Si3N4 etching rate and Si3N4/SiO2 etching selectivity while suppressing oxide regrowth. The results provide valuable insights for optimizing selective Si3N4 etching process in 3D NAND structures.

Investigation of Oxide Regrowth in the Selective Si<sub>3</sub>N<sub>4</sub> Etching Process for 3D NAND Fabrication by Using Finite Element Modeling Simulation

In the process of manufacturing three-dimensional Not AND (3D NAND) flash memory devices, oxide regrowth occurs during the selective Si3N4 etching process on the Si3N4/SiO2 multi-stack structures. The oxide regrowth issue must be suppressed. Thus, effects of two factors, the mass transfer ability of etching byproduct and the stack number of 3D NAND structure, on oxide regrowth were investigated in this study using finite element method (FEM) simulation. Using FEM simulation, we predicted that increase in the stack number of the 3D NAND structure and decrease in the diffusivity of Si3N4 etching byproducts highly aggravated the oxide regrowth. Therefore, it was suggested that an etchant capable of promoting H2SiO3 diffusion behavior would inhibit the oxide regrowth during the selective Si3N4 etching process of the 3D NAND structure.

Effect of addition of amino acids with hydrophilic side chains to low-concentration H3PO4 for improvement of Si3N4 etching kinetics and oxide regrowth phenomena on multi-layered Si3N4/SiO2 structures

Improvement of Si3N4 etching kinetics and suppression of oxide regrowth phenomena are critical in the selective Si3N4 etching process on multi-layered Si3N4/SiO2 structures for the fabrication of three-dimensional Not AND (3D NAND) devices. To control mass transfer of the solution and the Si3N4 etching by-product, various concentrations of H3PO4 solutions were used to remove Si3N4 layers of a patterned multi-layered Si3N4/SiO2 structure. It was observed that oxide regrowth was suppressed by reducing the concentration of H3PO4, and 30 wt% H3PO4 produced the highest Si3N4/SiO2 etching selectivity. In addition, amino acids with hydrophilic side chains, such as L-histidine, L-glutamic acid, L-proline, and L-lysine, were introduced into 30 wt% H3PO4. The introduction of amino acids increased the Si3N4 etching rate by providing hydrophilic side chains and polarizing silicon atoms on the Si3N4 surface. Furthermore, the addition of amino acids suppressed oxide regrowth by dehydration of the silanol group of the SiO2-like Si3N4 etching by-product. Finally, Si3N4 could be selectively and uniformly etched in 30 wt% H3PO4 containing amino acids without oxide regrowth on multi-layered Si3N4/SiO2 structures.

Highly selective Si3N4 etching on Si using pulsed-microwave CH3F/O2/Ar plasma

Highly selective Si3N4 etching on Si was achieved in a CH3F/O2/Ar plasma using pulsed-microwave plasma and time-modulation bias. The Si3N4/Si selectivity reached infinity at a peak-to-peak voltage (V pp) of 240 V. The effect of pulsed-microwave on CH3F gas dissociation for highly selective Si3N4 etching was investigated by deposited film analysis, optical emission spectroscopy, and ion current flux measurements. As the duty cycle of the pulsed-microwave was decreased, the plasma density during the pulse on period decreased and the CH/H ratio increased. The pulsed-microwave plasma produced low-dissociation radicals by providing a low plasma density. The low-dissociation radicals in the CH3F plasma formed a fluorine (F)-rich hydrofluorocarbon (HFC) layer on the Si3N4 wafer surface. The F-rich HFC layer promotes Si3N4 etching even at low ion energy, where Si etching does not proceed, and enables highly selective Si3N4 etching on Si.

In Situ Monitoring of Etching Characteristic and Surface Reactions in Atomic Layer Etching of SiN Using Cyclic CF4/H2 and H2 Plasmas.

Cyclic atomic layer etching (ALE) of SiN with high selectivity to SiO2, utilizing a hydrofluorocarbon deposition followed by exposure to hydrogen plasma, is presented. The surface reaction mechanism and etching behavior were investigated with in situ attenuated total reflectance Fourier transformation infrared spectroscopy (ATR-FTIR) and spectroscopic ellipsometry. In the deposition step, the hydrofluorocarbon film was deposited on top of the SiN films using the CF4/H2 plasmas with varying H2 contents (33 to 85%). Subsequently, the surface-modified SiN film was exposed to a hydrogen plasma for etching. The self-limiting SiN etching was observed, where the etch depth solely depended on the F concentration of the deposited hydrofluorocarbon layer once its thickness exceeded a critical value. A high selectivity of approximately 8.6 for SiN over SiO2 was achieved. The in situ ATR-FTIR spectra revealed that during the deposition step, besides the formation of the C-H peak associated with hydrofluorocarbon deposition, the appearance of the N-H4 absorbance band indicated the formation of an ammonium fluorosilicate layer on top of SiN. In the subsequent H2 plasma etching step, both the surface modification layer and the pre-deposited hydrofluorocarbon layer were removed. The removal of the surface-modified layer and hydrofluorocarbon layer was associated with the etch rate during H2 plasma exposure. These findings indicate the importance of the formation and removal of the surface modification layer for achieving ALE of SiN. The dissociation of the hydrofluorocarbon layer by the H2 plasma released reactants that interacted with SiN, leading to the formation of a new surface modification layer. The etching process significantly slowed down once the hydrofluorocarbon deposition and surface modification layer were completely removed.

ON THE COMPARISON OF REACTIVE-ION ETCHING MECHANISMS FOR SiO2 AND Si3N4 IN HBr + Ar PLASMA

This work investigated the influence of component ratio in the HBr + Ar gas mixture on electro-physical plasma parameters, steady-state densities of active species and reactive-ion etching (RIE) kinetics for SiO2 and Si3N4 under conditions of inductive RF (13.56 MHz) discharge. The combination of plasma diagnostics by Langmuir probes and plasma modeling indicated that an increase in Ar content at constant gas pressure and input power a) caused an increase in electron temperature and densities of charged species; b) results in increasing ion bombardment intensity; and c) leads to the nearly proportional decrease in Br atoms density and flux. It was found that variations of SiO2 and Si3N4 etching rates vs. mixture composition are qualitatively similar while the maximum difference in corresponding absolute values takes place in pure HBr plasma. The analysis of RIE mechanisms was carried out using model-predicted data on fluxes of ions and bromine atoms. It was found that the dominant SiO2 etching mechanism is the ion-assisted chemical reaction which is characterized by the nearly-constant rate in the range of 0–80% Ar due to an increase in the effective reaction probability. That is why the noticeable intensification of physical sputtering with increasing Ar fraction in a feed gas causes the only weak growth of obtained SiO2 RIE rate. Oppositely, the Si3N4 etching process is mainly contributed by the physical sputtering while the efficiency of ion-stimulated chemical reaction is limited by the low reaction probability. This provides both slower etching process (especially in Ar-poor plasmas) and stronger sensitivity of etching rate to the change in mixture composition. For citation: Efremov A.M., Betelin V.B., Kwon K.-H. On the comparison of reactive-ion etching mechanisms for SiO2 and Si3N4 in HBr + Ar plasma. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2023. V. 66. N 6. P. 37-45. DOI: 10.6060/ivkkt.20236606.6786.

Selective functionalization of silicon nitride with a water-soluble etch-resistant polymer

In semiconductor device manufacturing, it is typically necessary to process devices incorporating both Si3N4 and SiO2 with etching solutions. However, several commercially ubiquitous etchants affect both these dielectrics simultaneously; therefore, high-cost protection solutions or highly selective etchants are required to mitigate this issue. To address this problem, this study examined a simple method of selectively functionalizing Si3N4 with a protective polymer film, thus reducing its etching rate relative to that of SiO2. Several polymers were screened for their selectivity and resistance to buffered hydrofluoric acid (BHF) etching. A water-soluble cellulose-based polymer, DC01, was subsequently found to reduce the etching of Si3N4 selectively, owing to strong interactions between the polymer and underlying bulk material once the native oxide is removed. In addition, X-ray photoelectron spectroscopy results confirmed the retention of DC01 on this substrate after etching, while quartz crystal microbalance measurements indicated the stability of the polymer film after multiple rounds of rinsing with aqueous surfactant solutions. Thus, this polymer has considerable potential as an additive for the simplification of Si3N4/SiO2 composite processing.

Plasma induced damage on AlGaN/GaN heterostructure during gate opening for power devices

During the fabrication of metal oxide semiconductor high electron mobility transistor based on AlGaN/GaN heterostructure, gate patterning is recognized as the most critical step that can lead to electrical degradation of the transistor. In this work, we performed the SiN cap layer plasma etching processes by two fluorine-based plasma processes (SF6/Ar and CHF3/CF4/Ar) with low (≈15 eV) and high (≈260 eV) ion energies. Moreover, we investigate the postetching treatment using a KOH solution in order to restore the quality of the AlGaN barrier surface after etching. The objective of this article is to evaluate the AlGaN barrier surface damage after the listed plasma etching processes and postetching strategies by using quasi-in situ angle-resolved x-ray photoelectron spectroscopy, transmission electron microscopy, and atomic force microscope. Accordingly, it is found that both high ion energy plasma processes lead to a significant stoichiometric change and modification of the AlGaN barrier layer into a 1.5 nm F-rich AlGaNFx subsurface reactive layer. The decrease in ionic energy leads to a decrease in the SiN etch rate and a significant improvement in the SiN/AlGaN etch selectivity (which becomes infinite) for both plasma chemistries. Moreover, the decrease in ion energy decreases the depth of the modification (about 0.5 nm) and reduces the stochiometric change of the AlGaN barrier layer. However, both low and high ion energy SF6/Ar plasma lead to 0.8 eV Fermi level shift toward the valence band. Furthermore, the KOH postetching treatment demonstrates complete and effective removal of the AlGaNFx subsurface reactive layer and restoration of the surface properties of the AlGaN layer. However, this removal leads to AlGaN recesses that are correlated to the thickness of the reactive layer formed during the etching.

Plasma Parameters and Kinetics of Reactive Ion Etching of SiO2 and Si3N4 in an HBr/Cl2/Ar Mixture

Plasma Parameters and Kinetics of Reactive Ion Etching of SiO2 and Si3N4 in an HBr/Cl2/Ar Mixture

Role of SiCl4 addition in CH3F/O2 based chemistry for Si3N4 etching selectively to SiO2, SiCO, and Si

Dry etching of amorphous silicon nitride (Si3N4) selectively toward silicon dioxide (SiO2), silicon oxicarbide (SiCO), and crystalline silicon (c-Si) in an inductive coupled plasma reactor using CHF3/O2/He chemistry with SiCl4 addition is studied. Plasma exposure of c-Si, SiO2, and SiCO leads to an oxifluoride deposition. The deposition rate is the same for all these materials and increases linearly with the amount of SiCl4 added. On the other hand, Si3N4 etching is observed at very small amount of SiCl4 added (2 SCCM), while oxide deposition takes place at higher SiCl4 flow (10 SCCM). Quasi-in situ angle resolved x-ray photoelectron spectroscopy investigation shows oxifluoride SiOxFy deposition on c-Si and SiCO, while a thin F-rich reactive layer is observed on Si3N4. The oxidation of the Si3N4 surface with O2 plasma prior to CHF3/O2/He with small SiCl4 addition plasma treatment showed that the oxidation state plays a significant role in the etching/deposition equilibrium. In addition, it is found that oxifluoride deposition on Si3N4 is driven by ion energy, with deposition observed at 0 V substrate bias voltage, while etching is observed for values higher than 150 V. All these results show that a competition takes place between silicon oxifluoride deposition and etching, depending on the substrate material, surface oxidation, and ion energy. Based on the additional optical emission spectroscopy data, we proposed insights to explain the different etching and deposition behaviors observed. It is focused on the crucial role of ion energy and the nitrogen presence in Si3N4 leading to the formation of CN and HCN, helping to get a thinner reactive layer and to evacuate etch by-products on Si3N4 while an oxifluoride buildup on the other materials takes place.

Cryogenic etching of silicon compounds using a CHF3 based plasma

Cryogenic etching of a-Si, SiO2, and Si3N4 materials by CHF3/Ar inductively coupled plasma is investigated in a range of temperature from −140 to +20 °C. Samples of the three different materials are placed together on the same silicon carrier wafer. Depending on the experimental conditions, etching or deposition regimes were obtained on the samples. The thickness variation was measured by spectroscopic ellipsometry. A process window between −120 and −80 °C was found in which the Si3N4 surface is etched while CFx deposition is obtained on a-Si and SiO2 surfaces, resulting in the infinite etching selectivity of Si3N4 to the other materials. At high enough self-bias (−120 V) and very low temperature (<−130 °C), Si3N4 etch is reduced down to a very low value, while a-Si and SiO2 are still being etched, which inverses the selectivity between Si3N4 and the two other materials. EDX analyses of a Si3N4/a-Si/SiO2 layer stack after the same etching process carried out at 20 and −100 °C confirm the presence of carbon and fluorine on a-Si at low temperature, showing the effect of the low temperature to switch from the etching to deposition regime on this material.

Plasma Parameters and Kinetics of Reactive Ion Etching of SiO2 and Si3N4 in an HBr/Cl2/Ar Mixture

The parameters of the gas phase and the kinetics of reactive ion etching of SiO2 and Si3N4 under conditions of an induction RF (13.56 MHz) discharge with a varying HBr/Cl2 ratio is studied. The study includes plasma diagnostics using Langmuir probes, plasma modeling to find stationary concentrations of active particles, measuring velocities, and analyzing etching mechanisms in the effective interaction prob-ability approximation. It is found that the substitution of HBr by Cl2 at a constant argon content (a) is accompanied by a noticeable change in the electrical parameters of the plasma; (b) leads to a weak increase in the intensity of ion bombardment of the treated surface; and (c) causes a significant increase in the total concentration and flux density of reactive particles. It is shown that the etching rates of SiO2 and Si3N4 increase monotonically as the proportion of Cl2 increases in a mixture, while the main etching mechanism is an ion-stimulated chemical reaction. The model description of the kinetics of such a reaction in the first approximation assumes (a) the additive contribution of bromine and chlorine atoms and (b) the direct pro-portional dependence of their effective interaction probabilities on the intensity of ion bombardment. The existence of an additional channel of heterogeneous interaction with the participation of HCl molecules is proposed.

Improvement of Si3N4/SiO2 etching selectivity through the passivation of SiO2 surface in aromatic carboxylic acid-added H3PO4 solutions for the 3D NAND integration

Selective Si3N4 etching becomes challenging as the number of stacks in three-dimensional Not AND (3D NAND) flash memory devices increases. To suppress changes in the thickness of SiO2 layers in Si3N4/SiO2 multi-stack structures, etching inhibitors should be added to H3PO4 solutions. Notably, carboxylic acids can be adsorbed on OH-terminated substances, such as SiO2, by forming Si-O-C bonds. Therefore, carboxylic acids added to H3PO4 solutions can be adsorbed on SiO2 surfaces, and passivation layers can be formed, suppressing SiO2 etching. When a Si3N4/SiO2 multi-stack structure is dipped in a H3PO4 solution, the Si3N4 layers are removed, and the SiOxNy surfaces on the SiO2 layers are converted into SiOx surfaces with positively charged oxygen vacancies. For aromatic carboxylic acids, C=O bonds in their carboxyl groups can be converted to C-O-, causing electrostatic attraction between the C-O- groups and oxygen vacancies on SiOx surfaces. Consequently, aromatic carboxylic acids can be adsorbed on SiOx surfaces via the electrostatic attraction and Si-O-C covalent bond formation, creating robust passivation layers and suppressing SiO2 etching. However, aliphatic carboxylic acids form only Si-O-C bonds on SiOx surfaces and hence cannot realize the same functions as their aromatic counterparts.

Applications and mechanisms of anisotropic two-step Si3N4 etching with hydrogen plasma conditioning

The ability to precisely form Si3N4 spacers is critical to the success of dynamic random-access memory and NAND (NOT AND) flash memory technology development. In this study, we investigated the mechanisms and process windows of an innovative two-step nitride (Si3N4) etch consisting of H2 plasma processing in an inductively coupled plasma chamber followed by either buffered oxide etch (BOE, a mix of NH4F and HF solution) wet clean or in situ NF3 plasma etch. We obtained layer–by-layer removal with each layer’s removal capable of more than 10 nm. We revealed that H can penetrate more than 20 nm deep into the nitride film to transform pristine Si3N4 into SiON after air exposure, which can be subsequently removed by BOE wet clean. The H2 and BOE steps do not need to run back-to-back; the modified SiON layer is stable enough to sustain elevated temperature processing and can be removed by BOE later down-the-line integration. We also demonstrated that using NF3 plasma can have highly selective etch of nitride over oxide due to the incubation time difference between these two types of films. It takes much longer time to initiate the chemical reaction for oxide compared with nitride. Critically, the role of H2 is not the key for high selectivity; instead, it provides an etch directionality and shortens the incubation time for both nitride and oxide.

Electron beam-induced etching of SiO2, Si3N4, and poly-Si assisted by CF4/O2 remote plasma

Electron-stimulated etching of surfaces functionalized by remote plasma is a flexible and novel approach for material removal. In comparison with plasma dry etching, which uses the ion-neutral synergistic effect to control material etching, electron beam-induced etching (EBIE) uses an electron-neutral synergistic effect. This approach appears promising for the reduction of plasma-induced damage (PID), including atomic displacement and lateral straggling, along with the potential for greater control and lateral resolution. One challenge for EBIE is the limited selection of chemical precursor molecules that can be used to produce functionalized materials suitable for etching under electron beam irradiation. In this work, we studied a new experimental approach that utilizes a remote plasma source to functionalize substrate surfaces in conjunction with electron beam irradiation by an electron flood gun. Etching rates (ERs) of SiO2, Si3N4, and poly-Si are reported in a broad survey of processing conditions. The parametric dependence of the ER of these Si-based materials on the operating parameters of the flood gun and the remote plasma source is evaluated. We also identified the processing parameters that enable the realization of material selective removal, i.e., the etching selectivity of Si3N4 over SiO2 and poly-Si over SiO2. Additionally, surface characterization of etched materials is used to clarify the effects of the co-introduction of particle fluxes from the remote plasma and flood gun sources on surface chemistry.

Redeposition mechanism on silicon oxide layers during selective etching process in 3D NAND manufacture

3D NAND flash memory with vertically stacked cells has been developed to break through the limits of technology nodes. However, during the selective etching process, it is difficult to ensure the byproducts diffuse away from the trenches in the multistacked layers. Once saturated, the byproduct causes abnormal redeposition on the SiO2 layers. This problem has restricted the development of high-density 3D NAND memory. To solve this problem, the composition and formation mechanism of the redeposited layer must be clarified. In this study, a ternary-wafer system comprising a Si3N4/SiO2/Si3N4 stack was fabricated to study the redeposition mechanism, and the morphology, elastic properties, and chemical composition of the redeposited layer were clarified. The redeposited layer consists of spherical particles with elastic surfaces (average Young’s modulus of 24.17 GPa). The particles were confirmed to comprise colloidal silica gel covered by silanols. By considering the chemistry of silica, the redeposition mechanism was proposed as follows: colloidal silica gel is formed by the aggregation of silicic acids from Si3N4 etching, which adsorb onto the SiO2 layer through oxide bridges and hydrogen bonding. Our work will contribute to the development of high-density 3D NAND memory.

Improvement of SiO2 surface morphology during the selective Si3N4 etching in the multi-layered 3D NAND Si3N4/SiO2 stack structures by the generation of CO2 gas through the control of redox reaction

Selective removal of silicon nitride (Si3N4) layers from multilayer Si3N4/silicon dioxide (SiO2) stack structures is essential for fabricating 3-dimensional (3D) NAND flash memory devices. Although phosphoric acid (H3PO4) selectively etches Si3N4 layers to a certain degree, the tip thicknesses of the SiO2 layers increase owing to oxide regrowth, and the SiO2-layered trenches become clogged. In particular, the oxide regrowth phenomenon worsens as it progresses to the bottom of the structure. In this study, carbon dioxide (CO2) gas was generated by adding acidic reducing agents to H3PO4 to suppress the oxide regrowth in the stack structure. CO2 gas creates a turbulent flow in the slits and SiO2/Si3N4/SiO2 trenches, promoting the diffusion of Si3N4 etch byproducts from the cul-de-sac of the SiO2/Si3N4/SiO2 trench structures to the outside of the stack structure. The addition of oxalic acid to H3PO4 generated a high concentration of CO2 gas and selectively etched Si3N4 layers without oxide regrowth in the stack structure. Finally, the selective etching of Si3N4 layers, which is faster and more uniform than that in pure 85 wt.% H3PO4, was performed without oxide regrowth in the entire 128-layer Si3N4/SiO2 multi-stack structure.