This work investigates the mechanism of polycrystalline silicon (poly-Si) dissolution in H3PO4. The etching rate of poly-Si was more rapid than that of silicon dioxide (SiO2) in H3PO4 solution, because poly-Si surface possesses unstable Si1+, Si2+, and Si3+ states - rather than Si4+, which is more stable. Additives such as epichlorohydrin, 1,2-epoxy-3-phenoxypropane, vinyl bromide, and phenyl vinyl ether with epoxy or vinyl groups were introduced into the H3PO4 to passivate the poly-Si surface. This significantly increased the presence of the Si4+ state and provided hydrophobic functional groups on the poly-Si surface to reduce the wettability of the poly-Si surface. The introduction of these additives significantly reduced the etching rate of poly-Si. 3-glycidyloxypropyl tri-methoxysilane, 3-glycidyloxypropyl dimethoxymethylsilane, allyltrimethoxysilane, and vinyltrimethoxysilane with Si-O bonds, as well as epoxy or vinyl groups in their molecular structures, were demonstrated to suppress the etching rate of both SiO2 and poly-Si. The introduction of these additives to H(3)PO(4 )promoted the selective removal of silicon nitride (Si3N4) layers from multi-layered Si3N4/SiO2 trench structures without the loss of either poly-Si or SiO2.

Subcritical water was used to selectively remove Si3N4 layers in the 3D NAND fabrication. Various fluorine compounds were added to subcritical water. When sulfonyl fluoride-based additives were added to subcritical water, the dissolution of Si3N4 was accelerated by F-, and the Si3N4-to-SiO2 removal selectivity increased owing to the high [F-]/[HF2-] ratio and low OH- concentration. Furthermore, the addition of H2SiO3 to the subcritical water resulted in selective and uniform Si3N4 removal without the loss of SiO2 layers in the 128-layer Si3N4/SiO2 stack structure. However, the addition of ammonium fluoride compounds to subcritical water could not achieve selective removal of Si3N4. The addition of HCl to the ammonium fluoride-containing subcritical water suppressed the dissolution of Si and SiO2. Further addition of H2SiO3 and optimization of the additive concentrations led to selective and uniform Si3N4 removal in the 128-layer Si3N4/SiO2 stack structure without the loss of the Si substrate and SiO2 layers.

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.

We develop stable and printable inks consisting of mixtures of binary CuBr and CuI salts. The precursor inks show a good jetting ability to form crystalline Cu(Br x I 1– x ) alloys after printing and annealing at a low temperature. The printed films show targeted elemental composition and crystal structures as dictated by the designed ink stoichiometry. Increasing the amount of Br reduces the carrier density of the alloys. The low-temperature printing capability enables us to fabricate a functioning p-type TFT on polyester fabrics .

The etching mechanism of the Si3N4 in H3PO4 solution is proposed. The effect of H3PO4 concentrations on Si3N4 etching rate is investigated. The Si3N4 etching rate is not simply proportional to H3PO4. Both H3PO4 and H2O participate in the Si3N4 etching reaction and play their roles. First, H2PO4 - which is a weak nucleophile generated from H3PO4 replaces -NH2 on the Si3N4 surface with -H2PO4 by an SN1-like reaction. Next, N-Si which is backbone of Si3N4 network is substituted by H2O with Si-OH by an SN2-like reaction. From the Si3N4 etching mechanism and kinetic studies, it is concluded that the Si3N4 etching rate is proportional to product of concentration of H2O and H3PO4. These results suggest that consumption of H3PO4 can be reduced by using a lower concentration of H3PO4 between two H3PO4 concentrations that can produce a similar Si3N4 etching rates.

Silicon nitride (Si3N4) has been widely used as an insulating or sacrificial layer in various electronic devices. A removal of Si3N4 is also required in the semiconductor manufacturing process. In particular, in the 3D NAND flash memory manufacturing processes, selective removal of Si3N4 from the Si3N4/SiO2 stack structure is one of the very critical processes. It is known that Si3N4 is etched in high temperature phosphoric acid (H3PO4). Si3N4 is etched by nucleophilic attack of H2PO4 - and H2O in H3PO4 solution [1]. However, as the number of Si3N4/SiO2 stacked layers increases to improve the memory density of 3D NAND, oxide regrowth on the SiO2 layer and non-uniform top-to-bottom etching performance during Si3N4 etching process are the issues that need to be solved [2]. In this study, a novel Si3N4 etching in superheated water is investigated. As the temperature of H2O increases, the self-ionization of H2O is accelerated, thus the concentrations of H3O+ and OH- ion increase. It was previously reported that Si3N4 is etched by nucleophilic attack of OH- in superheated water [3]. However, in general, superheated water produced a low Si3N4 etching rate with the attack of Si substrate, making it difficult to apply to the Si3N4 etching process. Therefore, in this study, various additives were added to superheated water to selectively etch Si3N4 without material loss of the SiO2 and Si substrate. LPCVD Si3N4 film was prepared on Si wafer. A patterned Si3N4/SiO2 multi-stack structure was prepared the selective etching of Si3N4. Deionized (DI) water with a resistivity of 18.25 M was used to prepare superheated water in the reactor. Acetic acid, butyric acid, citric acid, formic acid, hexanoic acid, lactic acid or tartaric acid was added to DI water before heating. The etching of process was conducted 160 °C for 20 min. The thickness of the Si3N4 film was measured using a spectroscopic ellipsometry and a field-emission scanning electron microscope. To increase Si3N4 etching rate and suppress the material loss of the Si substrate during the Si3N4 etching process, various carboxylic acids, which are acidic and nucleophiles, were added to superheated water. The etching rates of Si3N4 in 1 M carboxylic-acid-containing superheated water at 160 °C were shown in Fig. 1(a). To reduce the effect of OH- and investigate the effect of carboxylic acid on Si3N4 etching, HCl was added to superheated water, adjusting the pH of the solution at 1, 2 and 3. Etching rates of Si3N4 are shown in Fig. 1(a). When either of tartaric acid, citric acid, lactic acid, or acetic acid was added to superheated water, Si3N4 etching rate was increased at a given pH, as compared to that obtained in HCl-added superheated water. However, when butyric acid, hexanoic acid or formic acid was added to superheated water, similar or lower etching rates of Si3N4 were observed as compared to the HCl-added superheated water. The results in Fig. 1(a) suggest that Si3N4 etching kinetics differs depending on the type of carboxylic acid added to superheated water. To investigate the reason for different Si3N4 etching rate with the type of carboxylic acid, the concentrations of carboxylate (RCOO-) ion were calculated from the pK a values of carboxylic acids at 25 °C. As shown in Fig. 1(b), the etching rates of Si3N4 increased with the concentration of carboxylate ions. Based on these results, it is thought that carboxylate ions play an important role in Si3N4 etching in superheated water. To investigate the selective Si3N4 etching ability of carboxylic-acid-containing superheated water, a Si3N4/SiO2 multi pair-layered structure on Si wafer was etched in tartaric-acid-containing superheated water. The cross-sectional FE-SEM images before and after the Si3N4 etching process were shown in Fig. 2. It is clearly shown that Si3N4 was selectively etched without material loss of SiO2 and Si substrate. Finally, eco-friendly and environmentally friendly carboxylic-acid-containing superheated water process can be a strong candidate that can replace the conventional Si3N4 etching process. References [1] T. Park, C. Son, T. Kim, S. Lim, J. Ind. Eng. Chem., 102, 146−154 (2021). [2] T. Kim, C. Son, T. Park, S. Lim, Microelectron. Eng., 221, 111191 (2020). [3] C. Son, S. Lim, ECS J. Solid State Sci. Technol., 8, N85−N91 (2019). Figure 1

Silicon nitride (Si3N4) has been widely used as an insulating or etch stop layer in the semiconductor devices [1]. For example, in a 3D V-NAND flash memory manufacturing process, Si3N4 is alternately stacked with silicon oxide (SiO2) to form a NAND structure. Here, it is necessary to selectively etch Si3N4 avoiding etching of SiO2 in the repeated Si3N4/SiO2 multi-stack layers. In general, phosphoric acid (H3PO4) is used to selectively etch Si3N4 against SiO2 [2]. However, since the concentration of H3PO4 solution changes with the evaporation of H2O, it is not easy to determine the concentration dependence of Si3N4 etching rate in H3PO4 solution. In order to properly control the shape of the 3D V-NAND structure, it is necessary to extensively understand the effect of H3PO4 concentration on the Si3N4 etching. The study of Si3N4 etching has been mainly focused on the effect of additives in H3PO4 on the behavior of Si3N4 etching to solve the issue such as oxide regrowth so far [3, 4]. Fundamental study of the Si3N4 etching reaction mechanism in H3PO4 is lacking. In this study, the Si3N4 etching mechanism in H3PO4 solution is elucidated systematically. For the investigation of Si3N4 etching reaction mechanism, LPCVD Si3N4 blanket wafer and Si3N4/SiO2 multi-stack layered trench wafer were used. Etching experiments of of Si3N4 were conducted in 10-95 wt% H3PO4 solutions at 160 °C. Spectroscopic ellipsometry and field emission scanning electron microscopy were used to measure the etching rate of Si3N4 after etching process. First, the changes in etching rate of Si3N4 in the various concentrations of H3PO4 solution were investigated. As shown in Fig. 1, according to our kinetic calculation and experiments, etching rate of Si3N4 increased as the H3PO4 concentration increased until the concentration of H3PO4 reaches a certain concentration that produced the highest etching rate. However, etching rate of Si3N4 decreased with the concentration of H3PO4 beyond the critical H3PO4 concentration. According to the results, there may be two concentrations of H3PO4 solution which produce an identical etching rate of Si3N4. It is also suggested that not only H3PO4 but also H2O plays an important role in the Si3N4 etching kinetics. To investigate the role of H2O and H3PO4 in the etching reaction of Si3N4, kinetic isotope effect (KIE) of Si3N4 etching reaction in H3PO4 solution was used. KIE provides information about which reactant determines the rate-limiting step. In this study, each reactant of Si3N4 etching reaction, H2O or H3PO4, was substituted with D2O and D3PO4, respectively. Then, etching of Si3N4 was conducted in four different solutions. As shown in Fig. 2, when H3PO4 was replaced by D3PO4, the Si3N4 etching rate was not significantly changed. However, when H2O was replaced by D2O, the Si3N4 etching rate decreased by 30 %. Therefore, it is thought that rate-limiting step of the Si3N4 etching reaction is determined by an elementary reaction related to H2O rather than H3PO4. Based on the above results, Si3N4 etching reaction mechanism was suggested. First, the Si3N4 surface termination (-NH2) is substituted with H2PO4 -, which is a weak nucleophile, by an SN1-like reaction. When H2PO4 - binds to Si, the backbone of Si3N4 is weakened because the electronegativity of O (3.44) is greater than N (3.04). Next, the Si-N of the weakened backbone of Si3N4 is substituted with Si-OH by an SN2-like reaction of H2O. This SN2-like reaction is considered as a rate-limiting step. It is believed that understanding of etching mechanism of Si3N4 in H3PO4 solution will improve selective etching process of Si3N4 for the integration of 3D V-NAND. References [1] A. Grill, P.R. Aron, Thin Solid Films, 96, 25-30 (1982). [2] W.V. Gelder, V.E. Hauser, J. Electrochem. Soc., 114, 869-872 (1967). [3] D. Seo, J. Bae, E. Oh, S. Kim, S. Lim, Microelectron. Eng., 118, 66-71 (2014). [4] T. Kim, C. Son, T. Park, S. Lim, Microelectron. Eng., 221, 111191 (2020). Figure 1

A selective Si3N4 etching is required in the semiconductor manufacturing process. In general, Si3N4 was selectively etched to SiO2 in hot H3PO4. However, since the existing Si3N4 etching process used a high-temperature and high-concentration acid solution, environment, health, and safety issues may occur. In this study, a new Si3N4 etching process based on superheated water was studied. In superheated water, Si3N4 was etched, but also Si and SiO2 were etched, making it difficult to apply to actual Si3N4 etching process. By adding carboxylic acid to superheated water, Si3N4 was selectively etched without material loss of Si and SiO2 in a Si3N4/SiO2 repeated stack structure. The Si3N4 etching process using superheated water with addition of carboxylic acid is an eco-friendly, safe and new etching process that can solve various problems that may occur in the existing H3PO4 process.

We develop stable and printable precursor inks from binary metal halides; the inkjet-printed textile-based CuBrI thin-film transistors at a low temperature of 60 °C demonstrated the potential for printing complementary circuits in wearable electronic textiles.

This work proposes the mechanism of the silicon nitride (Si3N4) dissolution reaction in phosphoric acid (H3PO4) solution and shows how to kinetically control the reaction. The various H3PO4 concentrations and temperature dependencies of Si3N4 dissolution rates and the behavior of Si3N4 dissolution with different acids were investigated. First, the dissolution of Si3N4 in H3PO4 begins with an S(N)1-like process composed of an NH2-leaving step from Si and an H2PO4-coupling step. Next, an S(N)2-like reaction occurs, in which N-Si of N-Si-H2PO4 is broken by H2O and replaced with Si-OH. The latter S(N)2-like reaction is thought to determine the overall dissolution rate of Si3N4. When a weak nucleophile, such as H2PO4-, is coupled with Si, the rate of the SN2-like reaction increases where the Si-N of the backbone of Si3N4 is broken. From the reaction rate based on those reaction mechanisms, the overall dissolution rate of Si3N4 is determined by the product of the H3PO4 and H2O concentrations, and that two different concentrations of H3PO4 produce identical dissolution rates at a given temperature. It is also indicated that the Si3N4 dissolution rate compared to the SiO2 dissolution rate is higher at a lower concentration of the two H3PO4 solutions. (C) 2021 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

The conventional Si3N4 etching process using H3PO4 or HF may cause environment, health, and safety issues. In this study, by adding ionic compounds and carboxylic acids to superheated water, the etching of Si3N4 was demonstrated without the use of H3PO4 and HF. In ionic-compound-containing superheated water, the etching rate of Si3N4 (R-Si3N4) showed a strong dependence on the OH- concentration: R-Si3N4 approximate to 170[OH-] (0.12). However, the material loss of the Si substrate is inevitable because of the high OH- concentration in aqueous solution. The addition of carboxylic acid to superheated water also increased the etching rate of Si3N4. In carboxylic-acid-containing superheated water, the etching rate of Si3N4 was dependent on the concentration of the carboxylate ion, RCOO-, rather than the OH- concentration: R-Si3N4 approximate to 351[RCOO-](0.49) = 7.6 x 10(-4)[OH-](-0.49). It is suggested that RCOO- breaks the Si-N bond of Si3N4 and forms Si-(OOCR) n in an S(N)2-like reaction. Finally, Si3N4 is etched in the form of Si(OH)(4) in the superheated water containing carboxylic acids. In addition, this new aqueous process significantly reduced the material loss of the Si substrate that can occur during the etching of Si3N4. Since the use of H3PO4 or HF is avoided entirely, the ecofriendly Si3N4 etching process described in this study may reduce the potential environmental and safety problems associated with the conventional Si3N4 etching process.

Wet etching of Si3N4 was conducted in superheated water at 160 °C with different additives type and concentration. In general, etching rate of Si3N4 increased with the pH of solution. However, it is difficult to fully explain the Si3N4 etching behavior just with the pH of solution. The OH- concentration (or pH) in superheated water at 160 °C are different from the pH of solution at room temperature. Therefore, the OH- concentrations in superheated water at 160 °C were calculated using van't Hoff equation, equilibrium constant equations, mass and charge balance equations. The calculated OH- concentration at 160 °C showed better correlation with Si3N4 etching rate than that of initial pH of solution.

Indium gallium arsenide (InGaAs) is a promising candidate for high-performance complementary metal-oxidesemiconductor (CMOS) channel materials. In this study, self-assembled monolayer passivation was performed on the surface of p-type Zn-doped InGaAs to improve the semiconductor/dielectric interfacial electrical properties. In particular, the oxidation behavior and surface state change that occurred with hexene passivation were analyzed. A relatively thin oxide was formed on the hexene-passivated InGaAs surface and compared to the unpassivated surface after exposure to air over time. It was observed that oxidation was effectively suppressed for all the In, Ga, and As elements. It is considered from the relationship between the oxidation time and the oxide thickness change that the initial oxidation surface reaction is hindered by hexene passivation. In addition, the depletion depth was reduced from 28.2 to 24.4 nm and the interface trapped charge density of InGaAs MOS capacitor was decreased from 4.29 x 10(13) to 2.24 x 10(13) cm(-2)eV(-1) after hexene passivation. The improvement in the interfacial electrical performance of the MOS capacitor may result from the suppressed oxidation caused by the passivation with hexene on the InGaAs surface.

To fabricate a n-ZnO/p-CuI heterojunction diode with a high rectification ratio at a large junction area, gamma-CuI film was spin-coated on ZnO. Since the n-ZnO/p-CuI diode showed a rectification ratio as small as 2.38 x 10(2), the ZnS interfacial layer was prepared by successive ionic layer adsorption and reaction. By placing a ZnS layer between the n-ZnO/p-CuI, the rectification ratio from the diode increased to 1.71 x 10(7) at a junction area of 1 cm(2). Modification of the valence band offset and the conduction band offset of the n-ZnO/p-CuI junction in the presence of the interfacial ZnS layer may facilitate carrier transport between ZnO and CuI. In addition, it was confirmed that two types of defects (oxygen vacancy and unpaired electron trapped on an oxygen vacancy) were reduced by insertion of an interfacial ZnS layer. A significant improvement in the rectification ratio resulted from the large decrease in the reverse current in the ZnO/CuI diode by a reduced amount of oxygen vacancies. Therefore, the improvement of band alignment with the decrease in the interfacial defects by the introduction of wide bandgap ZnS passivation layer enhanced the electrical characteristics of ZnO/CuI diode.

It is necessary to selectively etch Si3N4 in Si3N4/SiO2 multi-stack structure for the fabrication of 3D NAND [1]. The etching reaction of Si3N4 in phosphoric acid is known as 3Si3N4 + 4H3PO4 + 27H2O → 4(NH4)3PO4 + 9H2SiO3 [2]. However, as the number of Si3N4 etching batch process increases, the Si3N4 etching performance of H3PO4 decreases. Therefore, a fresh H3PO4 solution should be used for each process. As a result, the consumption of H3PO4 and process cost increase. In addition, the etching by-products could be reattached to SiO2 layers, which causes problems in the subsequent processes. In this study, the etching of Si3N4 and SiO2 in superheated water was investigated. In addition, by adding proper additives to superheated water, the Si3N4 etching rate and Si3N4/SiO2 etch selectivity were controlled. Blanket Si3N4 and SiO2 films on Si wafer were used. A patterned Si3N4/SiO2 pair-layered structure was also used. HCl, NH4OH, H2SiO3 and HF were used as additives. The thicknesses of the Si3N4 and SiO2 films were measured using spectroscopic ellipsometry and FE-SEM. The etching rates of Si3N4 and SiO2 were shown in Figure 1. The etch rate of Si3N4 and SiO2 in superheated water (pH 7) was 23 and 1.5 Å/min, respectively. When HCl was added, the etching rate of Si3N4 was decreased. On the other hand, when NH4OH was added, the etching rate of Si3N4 was increased. Therefore, it is believed that OH- generated from the self-ionization of water in the superheater water plays an important role in the etching of Si3N4. In addition, proper additives were added to superheated water to improve the etching performance. The result of patterned Si3N4/SiO2 structure using superheated water with additive was shown in Fig. 2(a). For comparison, the result of the 85 wt% H3PO4 process was shown in Fig. 2(b). In the superheated water with additive and the H3PO4 process, the lateral etched depths of the Si3N4 were similar. However, in the superheated water process, the thickness of the remaining SiO2 layers was maintained without thinning. Therefore, optimized superheated water without H3PO4 show a very promising selective etch performance at the 3D Si3N4/SiO2 pair-layer structure. References [1] S. Aritome, NAND flash memory technologies, p. 273, John Wiley & Sons, Hoboken, NJ (2015). [2] K.B. Sundaram, R.E. Sah, H. Baumann, K. Balachandran, R.M. Todi, Microelectron. Eng., 70, 109 (2003). Figure 1

For the integration of 3D NAND, which has multiple Si 3 N 4 and SiO­­ 2 pair-layer stack structure, highly selective etching of Si 3 N 4 to SiO­­ 2 is required without decrease in the etching rate of Si 3 N 4 . While phosphoric acid is widely used as an etchant of Si 3 N 4 in actual processes [1], chemical additives are added to the phosphoric acid as accelerators or inhibitors to control the kinetics of the reaction. In general, etching of Si 3 N 4 is done at around 160 °C, but in the meantime, the process temperature may increase with the introduction of single wafer tool. In this study, we investigated the effects of various additives in H 3 PO 4 on the Si 3 N 4 etch rate and Si 3 N 4 /SiO 2 etch selectivity at a higher temperature. LPCVD Si 3 N 4 and SiO 2 blanker wafers were etched in H 3 PO 4 with the addition of various etching accelerators and inhibitors such as HF, NH 4 F, Si(OH) 4 , Si(OC 2 H 5 ) 4 , and H 2 SiF 6 at a temperature range from 160 to 200 °C. The thickness of the wafer before and after etching was measured by ellipsometry to measure etching rates of the films. When experiments were carried out with additives containing F, the etching rates of Si 3 N 4 and SiO 2 were increased by more than 30%. On the other hand, the Si 3 N 4 and SiO 2 etching rates decreased with the addition of the Si-based additives, but the decrease was remarkable in SiO 2 . In addition, the Arrhenius plots of Si 3 N 4 and SiO 2 were plotted to obtain activation energies of the reactions. As shown in Fig. 1(a), the F-containing additives act as catalyst in the Si 3 N 4 etching reaction, reducing activation energy than the conventional phosphoric acid process (54 kJ/mol·K). On the other hand, Si-based additive show no significant change in the activation energy for the etching of Si 3 N 4 (Fig. 1(b)). This is because addition of Si-based material to H 3 PO 4 does not change reaction pathway, but suppresses the rates of etching reaction by Le-Chateliers principle [2]. On the other hand, SiO 2 shows slightly different etching tendency. As shown in Fig. 2(a), the activation energy of the SiO 2 etching reaction decreased with the addition of F-containing additives than that of phosphoric acid process (81 kJ/mol·K). However, Si-containing additives increased activation energy as inactivating agents. In addition, fluorosilicic acid (H 2 SiF 6 ) was selected as an additive because it was expected that it reacts with water in aqueous solution to produce hydrates of Si and HF, giving effects of both F-containing and Si-containing additives. It was observed that the etching rate of Si 3 N 4 was increased, but the etching rate of SiO 2 was decreased (Data not shown here). In addition, the activation energy of the etching of Si 3 N 4 was lower than that of phosphoric acid. On the other hand, the activation energy of the SiO 2 etching increased. Therefore, it is suggested that the Si-inactivating effect is greater than the catalytic etching effect of fluorine. Based on the current study, it is concluded that a higher etch selectivity with an increased Si 3 N 4 etch rate is achievable with the addition of proper additives by controlling the activation energy. References [1] S. Aritome, NAND flash memory technologies, p. 273, John Wiley & Sons, Hoboken, NJ (2015). [2] D. Seo, J. S. Bae, E. Oh, S. Kim, S. Lim, Microelectron. Eng., 118, 66 (2014). Figure 1

Selective etching of Si3N4 to SiO2 is essential in the semiconductor fabrication process. In particular, as the number of alternating Si3N4/SiO2 multi-layered stacks increases, selective removal of Si3N4 without loss of SiO2 becomes difficult. In this study, the dissolution of Si3N4 was demonstrated in superheated water without addition of H3PO4, which has been widely used to etch Si3N4. The dissolution rates of Si3N4 and SiO2 in the superheated water depended strongly on the concentration of OH-, and the activation energy obtained for the dissolution of Si3N4 was 72.65 +/- 0.95 kJ/mol. It is believed that the attack of the partially delta(+) charged Si atoms in the Si3N4 by nucleophilic OH- was the key step in the dissolution of Si3N4 in the superheated water. Because a tradeoff between the dissolution rate of Si3N4 and the Si3N4-to-SiO2 etching selectivity was observed, H2SiO3 and HF were added to HCl-based superheated water for optimization. The HCl-based superheated water with the addition of 0.005 vol% HF and 0.01 M H2SiO3 allowed successful fabrication of a horizontal SiO2 trench structure on a patterned Si3N4/SiO2 15 pair-layered stack through selective etching of Si3N4 without thinning of the SiO2 layer. (C) The Author(s) 2019. Published by ECS.

It is essential to selectively etch Si3N4 in the presence of SiO2 during the process of fabricating vertical 3D NAND structures. SiO2 etching inhibitors can be added to H3PO4 to increase Si3N4-to-SiO2 etch selectivity; however, the addition of SiO2 etching inhibitors to H3PO4 generates oxide regrowth issues on the SiO2 etch stop layers of the Si3N4/SiO2 pair-layer stacks. It is also observed that generation rate of Si3N4 etching byproduct as well as addition of SiO2 etching inhibitor strongly affects oxide regrowth behavior on the Si3N4/SiO2 multipair-layered structure. In addition, the limited mass transfer of Si3N4 etching byproduct produced heavier amounts of oxide regrowth on the corners of the SiO2 etch stop layers and at the stack structure bottoms. Modification of surface reactivity and the adhesion properties of silica-like monomers on the SiO2 etch stop layer may be important factors for reducing oxide regrowth during the selective etching of Si3N4.

For the integration of 3D NAND, which has multiple Si3N4 and SiO­­2 pair-layer stack structure, highly selective etching of Si3N4 to SiO­­2 is required without decrease in the etching rate of Si3N4. While phosphoric acid is widely used as an etchant of Si3N4 in actual processes [1], chemical additives are added to the phosphoric acid as accelerators or inhibitors to control the kinetics of the reaction. In general, etching of Si3N4 is done at around 160 °C, but in the meantime, the process temperature may increase with the introduction of single wafer tool. In this study, we investigated the effects of various additives in H3PO4 on the Si3N4 etch rate and Si3N4/SiO2 etch selectivity at a higher temperature. LPCVD Si3N4 and SiO2 blanker wafers were etched in H3PO4 with the addition of various etching accelerators and inhibitors such as HF, NH4F, Si(OH)4, Si(OC2H5)4, and H2SiF6 at a temperature range from 160 to 200 °C. The thickness of the wafer before and after etching was measured by ellipsometry to measure etching rates of the films. When experiments were carried out with additives containing F, the etching rates of Si3N4 and SiO2 were increased by more than 30%. On the other hand, the Si3N4 and SiO2 etching rates decreased with the addition of the Si-based additives, but the decrease was remarkable in SiO2. In addition, the Arrhenius plots of Si3N4 and SiO2 were plotted to obtain activation energies of the reactions. As shown in Fig. 1(a), the F-containing additives act as catalyst in the Si3N4 etching reaction, reducing activation energy than the conventional phosphoric acid process (54 kJ/mol·K). On the other hand, Si-based additive show no significant change in the activation energy for the etching of Si3N4 (Fig. 1(b)). This is because addition of Si-based material to H3PO4 does not change reaction pathway, but suppresses the rates of etching reaction by Le-Chateliers principle [2]. On the other hand, SiO2 shows slightly different etching tendency. As shown in Fig. 2(a), the activation energy of the SiO2 etching reaction decreased with the addition of F-containing additives than that of phosphoric acid process (81 kJ/mol·K). However, Si-containing additives increased activation energy as inactivating agents. In addition, fluorosilicic acid (H2SiF6) was selected as an additive because it was expected that it reacts with water in aqueous solution to produce hydrates of Si and HF, giving effects of both F-containing and Si-containing additives. It was observed that the etching rate of Si3N4 was increased, but the etching rate of SiO2 was decreased (Data not shown here). In addition, the activation energy of the etching of Si3N4 was lower than that of phosphoric acid. On the other hand, the activation energy of the SiO2 etching increased. Therefore, it is suggested that the Si-inactivating effect is greater than the catalytic etching effect of fluorine. Based on the current study, it is concluded that a higher etch selectivity with an increased Si3N4 etch rate is achievable with the addition of proper additives by controlling the activation energy. References [1] S. Aritome, NAND flash memory technologies, p. 273, John Wiley & Sons, Hoboken, NJ (2015). [2] D. Seo, J. S. Bae, E. Oh, S. Kim, S. Lim, Microelectron. Eng., 118, 66 (2014). Figure 1

As the number of stacked layers of the 3D NAND structure increases, the memory density of 3D NAND flash memory device increases [1]. In the fabrication of 3D NAND device, it is essential to selectively etch Si3N4 by an etchant flowing along a narrow slit followed by metal deposition. However, as the number of Si3N4/SiO2 multi-stack increases, selective etching of Si3N4 becomes difficult. In order to increase the Si3N4-to-SiO2 etch selectivity, SiO2 etching inhibitor should be added to H3PO4. Unfortunately, most of SiO2 etching inhibitors added to H3PO4 generate oxide regrowth around SiO2 layer on the Si3N4/SiO2 multi-stack structure. In this study, the mechanism of oxide regrowth occurred on the 3D NAND structure was investigated. Patterned Si3N4/SiO2 multi-stack structures were prepared. Etching experiments were performed using coupon wafers in 85% H3PO4 or etching inhibitor-added H3PO4 at 160 °C. The Si3N4 and SiO2 etching rates and Si3N4-to-SiO2 etch selectivity were measured using spectroscopic ellipsometry and oxide regrowth on the Si3N4/SiO2 multi-stack structure was analyzed using FE-SEM and HR-TEM. When the Si3N4/SiO2 multi-stack structure was etched in the SiO2 etching inhibitor-added H3PO4, oxide regrowth was observed on the SiO2 layered trenches on the Si3N4/SiO2 multi-stack structure, as shown in Fig. 1. The oxide regrowth occurred intensively at the corner of SiO2 layered trench and oxide regrowth occurred heavily at the bottom of Si3N4/SiO2 multi-stack structure as compared to the top. The overall amount of oxide regrowth on the SiO2 layered trench increased as the concentration of SiO2 etching inhibitor added to H3PO4 increased. In addition, it is shown that generation rate of etching product is an important factor to occur oxide regrowth. Based on the results, it is suggested that etching inhibitor addition and high generation rate of etching product and mass transfer limitation are responsible for the oxide regrowth. Through their optimization, selective Si3N4 etching on the Si3N4/SiO2 multi-stack structure without oxide thinning and regrowth could be obtained, as shown in Fig. 2. References [1] S. Aritome, NAND flash memory technologies, p. 273, John Wiley & Sons, Hoboken, NJ (2015). [2] J. Jang, et al, in Symp. VLSI Tech. Dig., 192 (2009). Figure 1