Etching Mechanism Based on Hydrogen Fluoride Interactions with Hydrogenated SiN Films Using HF/H₂ and CF₄/H₂ Plasmas

Hsiao S., Sekine M., Hori M.

Hsiao S., Britun N., Nguyen T., Tsutsumi T., Ishikawa K., Sekine M., Hori M.

The effects of substrate temperature (Ts) on the etch rate (ER) of the PECVD-prepared SiN, SiO2 and amorphous carbon (a-C) films, and their selectivity were investigated with a CF4/H2 plasma. The ERs for the SiN at all Ts were higher than that for the SiO2 films. As Ts was decreased from 50 to −20 °C, the ER for the SiN decreased. Contrarily, the ER of the SiO2 films increased. The etching selectivity of SiN over SiO2 reached to near unity when the Ts was −20 °C. At the same time, the ER of for the a-C films was found to be around 0.1 nm/s and irrespective of Ts. The fluorocarbon (FC) thickness was greater for the SiO2 films than that of the SiN. The lower ER for the SiO2 was therefore attributed to the thicker FC layer and resultant etching mechanism. As the Ts was decreased, the FC thickness on the SiO2 films decreased, which led to the ER decrease. The decrease of ER for the SiN etching at the low temperature was likely due to the higher stability of the surface N–H modification layer, compared with that processed at 20 °C, which was confirmed by the in situ FTIR.

Boulard F., Bacquié V., Tavernier A., Possémé N.

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.

Dussart R., Ettouri R., Nos J., Antoun G., Tillocher T., Lefaucheux P.

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.

Hattori T., Kobayashi H., Ohtake H., Akinaga K., Kurosaki Y., Takei A., Sekiguchi A., Maeda K., Takubo C., Yamada M.

Abstract The gas-phase etching of SiO2 was examined using HF and methanol vapor at temperatures below 0 °C and at low pressure. The etching rate of SiO2 increased with decreasing temperature and showed a maximum around –30 °C. The obtained etching rate was a maximum of 40 nm min−1 at plasma-enhanced chemical vapor deposition SiO2. The etching rate of SiN examined for comparison was more than ten times smaller than that of SiO2 under the same condition. As a result, the etching selectivity of SiO2 to SiN was found to be over 20 at –40 °C. When utilizing a low temperature of less than –30 °C, gas-phase etching of SiO2 showing a high etching rate and selectivity was achieved.

Hidayat R., Kim H., Khumaini K., Chowdhury T., Mayangsari T.R., Cho B., Park S., Lee W.

Selective etching of silicon oxide (SiO2) against silicon (Si) using anhydrous hydrogen fluoride (HF) vapor has been used for semiconductor device fabrication. We studied the underlying mechanism of the selective...

Hori M.

In plasmas, a variety of radicals which are defined as electrically neutral radicals in this article are efficiently produced by collisions between electrons and gas molecules. These radicals can subsequently undergo gas phase reactions with solids, liquids and living organisms that result in non-equilibrium surface/interface physicochemical processes. The specific phenomena produced by these reactions remain largely unknown, even though these plasma-based processes could lead to disruptive technological innovations. As an example, in the case of semiconductor microfabrication processes, the density, energy and lifetime of individual radicals, as well as the reaction time constants of these species with various materials should be ascertained. This would allow the identification and control of the effective radical species during processes, such as the high-precision etching and deposition of functional thin films. In addition, the type of reactions occurring between radicals generated in plasmas with liquids or living organisms is still an unexplored area. Establishing a theoretical system for these radical reactions and controlling the associated mechanisms could lead to innovations in the fields of functional devices and materials as well as in the areas of environmental protection, medicine and agriculture/fisheries. Focusing on the non-equilibrium surface/interface physicochemical reactions between radicals and solids occurring in semiconductor plasma processing, this paper describes the formation of nanostructured thin films by top-down mechanisms based on controlled radical production and bottom-up processes involving radical-induced self-organization. As well, this review examines next-generation medical and agricultural applications, such as the selective killing of cancer cells and plant growth promotion and functionalization. These systems result from the interactions of radicals generated in atmospheric-pressure, low-temperature plasmas with liquids, or the interactions of gas or liquid phase radicals with biological species. Finally, the importance of academic research into radical-controlled plasma processes and potential future technologies based on this interdisciplinary field are examined.

Tak H.W., Lee H.J., Wen L., Kang B.J., Sung D., Bae J.W., Kim D.W., Lee W., Lee S.B., Kim K., Cho B.O., Kim Y.L., Song H.D., Yeom G.Y.

• Different dissociated species and ion species in the plasma was shown by using isomers while it has same chemical compounds of C 3 H 2 F 6 . • More CF 2 and H, and less F related to the formation of a fluorocarbon polymer layer on the surface lead lower etch rate HFC-236fa, HFC-236ea, and HFC-236ca consequently. • Etch selectivity and etch profile was also affected as well as aspect ratio dependent etching (ARDE). • It eventually shows that the chemical branch structure of the compound affected the plasma properties and surface polymer formation affected high aspect ratio contact (HARC) etch characteristics significantly depending on the chemical branches in the compounds of C 3 H 2 F 6 . In this study, using three isomers (1,1,1,3,3,3-hexafluoropropane (HFC-236fa), 1,1,1,2,3,3-hexafluoropropane (HFC-236ea), 1,1,2,2,3,3-hexafluoropropane (HFC-236ca)) having the same chemical composition of C 3 H 2 F 6 , effects of chemical branch structure of three C 3 H 2 F 6 isomers on the plasma characteristics and etch characteristics of high aspect ratio ACL patterned SiO 2 were investigated. During the etching of SiO 2 and amorphous carbon layer (ACL) using the three isomers mixed with oxygen, different etch characteristics and plasma characteristics were observed. In the plasmas, more CF 2 and H but, less F were related to the formation of a fluorocarbon polymer layer on the surface, while lower high mass ion species such as C 3 HF 4 + and, C 3 H 2 F 5 + were related to the ion bombardment in the order of HFC-236fa, HFC-236ea, and HFC-236ca consequently leading to a lower SiO 2 etch rate. Therefore, when C 3 H 2 F 6 was used, even with the same chemical composition, the chemical branch structure of the compound affected the plasma properties and etch characteristics significantly depending on chemical branches in the compound. We believe that, for other hydrofluorocarbon compounds mixed with a critical oxygen flow rate, plasma properties and SiO 2 etch characteristics can be estimated through properties of chemical branches attached in these compounds.

Kim Y., Kim S., Kang H., You S., Kim C., Chae H.

Miyoshi N., Kobayashi H., Shinoda K., Kurihara M., Kawamura K., Kouzuma Y., Izawa M.

Thermal atomic layer etching (ALE) for SiO2 films with self-limiting behavior on the surface modification step was developed using sequential exposure to HF and NH3 gases followed by infrared (IR) annealing. X-ray photoelectron spectroscopy analysis showed that an (NH4)2SiF6-based surface-modified layer was formed on the SiO2 surface after gas exposures and that this layer was removed using IR annealing. The etch per cycle (EPC) of the ALE process saturated at 0.9 nm/cycle as the gas exposure times increased. With this self-limiting behavior, SiO2 was etched with high selectivity to poly-Si and Si3N4. The dependence of the EPC on the partial pressures of HF and NH3 was found to be in good agreement with the Langmuir adsorption model. This indicated that the HF and NH­3 molecules were in equilibrium between adsorption and desorption during the exposure, which resulted in the self-limiting formation of the modified layer. In addition to the process with an HF gas flow, it was demonstrated that an H2/SF6 plasma can replace the HF gas exposure step to supply the SiO2 surfaces with HF molecules. The EPC saturated at 2.7 nm/cycle, while no measurable thickness change was observed for poly-Si and Si3N4 films.

Hsiao S., Nguyen T., Tsutsumi T., Ishikawa K., Sekine M., Hori M.

With the increasing interest in dry etching of silicon nitride, utilization of hydrogen-contained fluorocarbon plasma has become one of the most important processes in manufacturing advanced semiconductor devices. The correlation between hydrogen-contained molecules from the plasmas and hydrogen atoms inside the SiN plays a crucial role in etching behavior. In this work, the influences of plasmas (CF4/D2 and CF4/H2) and substrate temperature (Ts, from −20 to 50 °C) on etch rates (ERs) of the PECVD SiN films were investigated. The etch rate performed by CF4/D2 plasma was higher than one obtained by CF4/H2 plasma at substrate temperature of 20 °C and higher. The optical emission spectra showed that the intensities of the fluorocarbon (FC), F, and Balmer emissions were stronger in the CF4/D2 plasma in comparison with CF4/H2. From X-ray photoelectron spectra, a thinner FC layer with a lower F/C ratio was found in the surface of the sample etched by the CF4/H2 plasma. The plasma density, gas phase concentration and FC thickness were not responsible for the higher etch rate in the CF4/D2 plasma. The abstraction of H inside the SiN films by deuterium and, in turn, hydrogen dissociation from Si or N molecules, supported by the results of in situ monitoring of surface structure using attenuated total reflectance-Fourier transform infrared spectroscopy, resulted in the enhanced ER in the CF4/D2 plasma case. The findings imply that the hydrogen dissociation plays an important role in the etching of PECVD-prepared SiN films when the hydrogen concentration of SiN is higher. For the films etched with the CF4/H2 at −20 °C, the increase in ER was attributed to a thinner FC layer and surface reactions. On the contrary, in the CF4/D2 case the dependence of ER on substrate temperature was the consequence of the factors which include the FC layer thickness (diffusion length) and the atomic mobility of the etchants (thermal activation reaction).

Hsiao S., Britun N., Nguyen T., Tsutsumi T., Ishikawa K., Sekine M., Hori M.

Hsiao S., Nakane K., Tsutsumi T., Ishikawa K., Sekine M., Hori M.

The dependence of substrate temperatures (50 to −20 °C) on etch rate in two kinds of PECVD SiN films were investigated by a CF4/H2 mixture plasma. The XRR and XPS results indicate that the chemical composition and film density were almost identical for the films. The FTIR shows that the ratio of N H and Si H groups were found to be significantly different in the SiN films. The N H rich films exhibited a lower etch rate at −20 °C than that observed at room temperature or higher, whereas the Si H rich films showed a higher etch rate at −20 °C. We found that the fluorocarbon thickness was thicker in the Si H rich samples than N H rich samples. The fact suggests that hydrogen originated from the broken Si H bonds enhanced the polymerization, which causes the decrease of etch rate. A thinner fluorocarbon thickness was found in the Si H rich samples at low temperature, which results in the higher etch rate. Angular-resolved XPS indicates that N H bonding formed easier on film surface at −20 °C. These results indicate that the bonding structure and substrate temperature affected the fluorocarbon thickness, fluorine reaction probability and hydrogen dissociation during the SiN etching.

Hsiao S., Ishikawa K., Hayashi T., Ni J., Tsutsumi T., Sekine M., Hori M.

Gas chemistry has a significant impact on etch selectivity in semiconductor device fabrication, which is important for realization of atomic-scale removal and formation of high-aspect ratio features. To widen the controllable changes in the etchant composition in etching processes, our previous calculation showed the possibility of the controllable generation of CH2F and CHF2 related ions and radicals from a 1,1,2-trifluoroethane (CH2FCHF2) parent gas. The etch selectivity among silicon nitride (SiN), silicon dioxide (SiO2) and poly-Si films using CH2FCHF2 plasma mixed with O2 and Ar was investigated. The effects of the CH2FCHF2 and O2 partial pressures on the composition of CHF2+ and CH2F+ ions, which were measured with a quadrupole mass spectrometer, and on the possible reactions with respect to the CH2FCHF2 and O2 mixed gas phase were investigated using quantum chemical calculations. The etch selectivity was investigated through surface etching reactions for SiN, SiO2, and poly-Si films.

Kim Y., Lee S., Cho Y., Kim S., Chae H.

In this work, atomic layer etching (ALE) with heptafluoropropyl methyl ether (C3F7OCH3) plasma was developed for SiO2 and Si3N4 and compared with the results of C4F8 or CHF3 plasmas. C3F7OCH3 has a shorter life time and lower global warming potential (GWP) than CHF3 and C4F8. SiO2 and Si3N4 surfaces were fluorinated with fluorocarbons generated from C4F8 or CHF3 or C3F7OCH3 plasmas, and the fluorinated surface was then removed by ions or radicals generated from Ar or O2 plasma in the following step. Atomic scale etch rates were achieved with cyclic etch rates of 5.8 Å/cycle for C4F8/Ar, 4.1 Å/cycle for CHF3/Ar, and 2.1 Å/cycle for C3F7OCH3/Ar. In case of etching with oxygen, atomic scale etch rates were achieved with cyclic etch rates of 2.9 Å/cycle for C4F8/O2, 1.7 Å/cycle for CHF3/O2, and 1.1 Å/cycle for C3F7OCH3/O2. The etch rate was correlated with the F1s/C1s ratio of the fluorocarbon layers; C3F7OCH3 plasmas generated fluorocarbon layers having the lowest F1s/C1s ratio, and C4F8 plasmas produced the highest F1s/C1s ratio. Constant etch rates were observed in the bias voltage range of 55–60 V, which is identified as the ALE window. In the etching step, Ar and O2 plasmas were applied to remove the fluorocarbon layers. A saturated etch rate with etching time, i.e., a self-limited etching rate, was obtained for all the fluorocarbon gases with both Ar and O2 plasmas. The high etch selectivity of 17.5 was achieved for SiO2/Si and 26.6 for Si3N4/Si with C3F7OCH3/Ar. These high selectivities are attributed to Si–C bonds that act as inhibitors during Si etching.

Shim H., Park Y., Hong S.

Miakonkikh A., Kuzmenko V., Efremov A., Rudenko K.

Trung Nguyen T., Hayashi T., Iwayama H., Sekine M., Hori M., Ishikawa K.

Tak H.W., Choi C.H., Kim S.B., Park M.H., Lee J.S., Sato A., Kim B.S., Jang J.K., Kim E.K., Kim D.W., Yeom G.Y.

Kim D.W., Gil H.S., Park W.C., Lee J.Y., Kim D.S., Jang Y.J., Kim K.C., Pyun D.S., Kim J.Y., Kim Y., Yeom G.Y.

Lill T., Wang M., Wu D., Oh Y., Kim T.W., Wilcoxson M., Singh H., Ghodsi V., George S.M., Barsukov Y., Kaganovich I.

Etching of high aspect ratio features into alternating SiO2 and SiN layers is an enabling technology for the manufacturing of 3D NAND flash memories. In this paper, we study a low-temperature or cryo plasma etch process, which utilizes HF gas together with other gas additives. Compared with a low-temperature process that uses separate fluorine and hydrogen gases, the etching rate of the SiO2/SiN stack doubles. Both materials etch faster with this so-called second generation cryo etch process. Pure HF plasma enhances the SiN etching rate, while SiO2 requires an additional fluorine source such as PF3 to etch meaningfully. The insertion of H2O plasma steps into the second generation cryo etch process boosts the SiN etching rate by a factor of 2.4, while SiO2 etches only 1.3 times faster. We observe a rate enhancing effect of H2O coadsorption in thermal etching experiments of SiN with HF. Ammonium fluorosilicate (AFS) plays a salient role in etching of SiN with HF with and without plasma. AFS appears weakened in the presence of H2O. Density functional theory calculations confirm the reduction of the bonding energy when NH4F in AFS is replaced by H2O.

Hsiao S., Sekine M., Iijima Y., Hori M.

Britun N., Mo M.K., Hsiao S., Arellano F.J., Sekine M., Hori M.

Number density of plasma-generated atoms or molecules is an important parameter for both fundamental research and applications. It can be measured in a straightforward manner, using vacuum-ultraviolet absorption spectroscopy, which is mainly possible in laboratory conditions as it may require bulky equipment, such as lasers. By contrast, optical actinometry is an alternative approach that only uses spontaneous emission from the plasma. This technique relies on the so-called corona excitation and uses emission line ratios between the gases with unknown and known concentrations (called actinometer in the last case). As a result of using line ratios, the additional density calibration is not required if the excitation cross sections are known. This study discusses Ar-based actinometry in low-pressure (roughly &lt;1 kPa) plasma discharges with an emphasis on multiple line ratios. The work is particularly focused on the method’s applicability, the choice of Ar cross sections, and potential error sources. The influence of the additional excitation mechanisms is analyzed based on both experiments and modeling. The optical transitions for F, O, H, N, and P atoms along with expressions for their number density are presented, not requiring high optical resolution for measurements. For the sake of method validation, it is shown that in low-pressure radiofrequency discharges, a nearly excellent agreement between the actinometry data and the calibrated measurements can be achieved by careful selection of optical transitions.

TAKAGI S., Hsiao S., Chih-Yu M., SEKINE M., Matsunaga F.

Abstract For the 3D NAND memory hole with a high aspect ratio above 100, the etching process with hydrogen-fluoride (HF) contained plasmas has been proposed. We have developed a simulation model for gas-phase reactions that reproduces the HF plasma in experiments. The HF plasma was generated using a power supply of 100 MHz frequency, and electron and F densities were measured. The simulation model was constructed on the basis of the collision cross sections and reaction constants reported in the previous papers, and the F density in the simulation was calibrated by comparing it with that in the experiments. As a result of the plasma simulation, the densities of F and the electrons were determined to be 7.52 × 1016 m–3 and 8.50 × 1016 m–3, respectively. Taking into consideration the errors in the experiment, we considered that the simulation model is able to reproduce the experimental HF plasma well.

Ning J., Tang Z., Chen L., Li B., Wu Q., Sun Y., Zhou D.

SiNx:H film deposition via plasma-enhanced chemical vapor deposition has been widely used in semiconductor devices. However, the relationship between the chemical bonds and the physical and chemical properties has rarely been studied for films deposited using tools in terms of the actual volume production. In this study, we investigated the effects of the deposition conditions on the H-related chemical bonding, physical and chemical properties, yield, and quality of SiNx:H films used as passivation layers at the 28 nm technology node. The radiofrequency (RF) power, electrode plate spacing, temperature, chamber pressure, and SiH4:NH3 gas flow ratio were selected as the deposition parameters. The results show a clear relationship between the H-related chemical bonds and the examined film properties. The difference in the refractive index (RI) and breakdown field (EB) of the SiNx:H films is mainly attributed to the change in the Si–H:N–H ratio. As the Si–H:N–H ratio increased, the RI and EB showed linear growth and exponential downward trends, respectively. In addition, compared with the Si–H:N–H ratio, the total Si–H and N–H contents had a greater impact on the wet etching rates of the SiNx:H films, but the stress was not entirely dependent on the total Si–H and N–H contents. Notably, excessive electrode plate spacing can lead to a significant undesired increase in the non-uniformity and surface roughness of SiNx:H films. This study provides industry-level processing guidance for the development of advanced silicon nitride film deposition technology.

Hattori T., KOBAYASHI H., Ohtake H., Akinaga K., Kurosaki Y., Takei A., Sekiguchi A., Maeda K., Takubo C., Yamada M.

Abstract The isotropic gas-phase etching of SiO2 was examined using HF and methanol vapor while changing the pressure from 300 Pa to 900 Pa. The temperature dependence of the etching rate of SiO2 showed a broad maximum around –30 °C, and the rate increased with increasing the pressure. The etching rate of plasma-enhanced chemical vapor deposition (PE-CVD) SiO2 became more than 60 nm/min at 900 Pa at –30 °C. When the pressure was increased from 300 Pa to 900 Pa, the temperature range that indicates the SiO2 etching was shifted to a higher temperature. The etching of SiO2, which did not proceed at 300 Pa, was found to proceed even at 0 °C at 900 Pa. The etching rate of PE-CVD SiN was also found to slightly increase with the pressure. At the higher pressure of 900 Pa, the formation of ammonium hexafluorosilicate, which is byproduct of SiN, was found to increase. As a result, the high selectivity of more than twenty was obtained at the lower pressure of less than 600 Pa and the lower temperature of less than –40 °C.

Hsiao S., Sekine M., Britun N., Mo M.K., Imai Y., Tsutsumi T., Ishikawa K., Iijima Y., Suda R., Yokoi M., Kihara Y., Hori M.

AbstractManufacturing semiconductor devices requires advanced patterning technologies, including reactive ion etching (RIE) based on the synergistic interactions between ions and etch gas. However, these interactions weaken as devices continuously scale down to sub‐nanoscale, primarily attributed to the diminished transport of radicals and ions into the small features. This leads to a significant decrease in etch rate (ER). Here, a novel synergistic interaction involving ions, surface‐adsorbed chemistries, and materials at cryogenic temperatures is found to exhibit a significant increase in the ER of SiO2 using CF4/H2 plasmas. The ER increases twofold when plasma with H2/(CF4 + H2) = 33% is used and the substrate temperature is lowered from 20 to −60 °C. The adsorption of HF and H2O on the SiO2 surface at cryogenic temperatures is confirmed using in situ Fourier transform infrared spectroscopy. The synergistic interactions of the surface‐adsorbed HF/H2O as etching catalysts and plasma species result in the ER enhancement. Therefore, a mechanism called “pseudo‐wet plasma etching” is proposed to explain the cryogenic etching process. This synergy demonstrates that the enhanced etch process is determined by the surface interactions between ions, surface‐adsorbed chemistry, and the material being etched, rather than interactions between ion and gas phase, as observed in the conventional RIE.

Khumaini K., Kim Y., Hidayat R., Chowdhury T., Kim H., Cho B., Park S., Lee W.

We report the etching mechanism of amorphous hydrogenated silicon nitride by hydrogen fluoride (HF) gas using density functional theory (DFT) calculations. Since silicon nitride films are deposited as amorphous with a significant amount of hydrogen, we constructed an amorphous substrate model with a hydrogen concentration of 25 at.% using molecular dynamics simulation and DFT calculations. We then created slab models with different degrees of fluorination and simulated all possible fluorination pathways. The pathways involving cleavage of Si–N or Si–Si bonds showed low activation energies of 0.90 eV or lower, while the pathways involving cleavage of a Si–H bond showed high activation energies of 1.54 eV or higher·NH3, SiF4, SiH2F2, and SiHF3 were released with low activation energies, indicating that etching would be favorable. Next, we modeled the formation and desorption of the (NH4)2SiF6 salt on the fluorinated surface. The salt formation was exothermic with low activation energies, consistent with self-limited etching at low temperatures. At temperatures of 152 °C or higher, (NH4)2SiF6 would desorb, leaving no solid residue, consistent with the high etch rate at elevated temperatures. Our DFT calculations using the amorphous hydrogenated slab model successfully explained the silicon nitride etching process, which could not be explained by a crystalline Si3N4 slab model.