Remote Plasma Atomic Layer Deposition of SiN x Using Cyclosilazane and H 2 / N 2 Plasma

: Silicon nitride (SiN x ) thin ﬁlms using 1,3-di-isopropylamino-2,4-dimethylcyclosilazane (CSN-2) and N 2 plasma were investigated. The growth rate of SiN x thin ﬁlms was saturated in the range of 200–500 ◦ C, yielding approximately 0.38 Å / cycle, and featuring a wide process window. The physical and chemical properties of the SiN x ﬁlms were investigated as a function of deposition temperature. As temperature was increased, transmission electron microscopy (TEM) analysis conﬁrmed that a conformal thin ﬁlm was obtained. Also, we developed a three-step process in which the H 2 plasma step was introduced before the N 2 plasma step. In order to investigate the e ﬀ ect of H 2 plasma, we evaluated the growth rate, step coverage, and wet etch rate according to H 2 plasma exposure time (10–30 s). As a result, the side step coverage increased from 82% to 105% and the bottom step coverages increased from 90% to 110% in the narrow pattern. By increasing the H 2 plasma to 30 s, the wet etch rate was 32 Å / min, which is much lower than the case of only N 2 plasma (43 Å / min).


Introduction
Dielectric films, such as silicon nitride (SiN x ), are extensively studied as etch stop layers, gate dielectrics, stress liners, charge trap layers, and as spacer applications in front-end-of-line (FEOL) semiconductor wafer processing. The main applications utilize SiN x films as gate spacers in dynamic random access memory, logic devices, and the charge trap layer of vertical NAND flash devices [1,2]. These spacer films act as an oxygen or dopant out-diffusion barrier and control the source/drain doping profiles. They also act as a film to prevent etching damage during later processing. The requirements of a suitable SiN x spacer film include resistance to etching and high conformality [3][4][5][6]. However, it has been a challenge to develop new methods to ensure these requirements. Recently, gate spacer research has considered methods to control the dielectric constant by doping carbon into SiN x thin films to reduce the resistive-capacitive (RC) delay [7].
A variety of methods exist for depositing SiN x thin films, including low-pressure chemical vapor deposition (LPCVD), plasma-enhanced CVD (PECVD), and atomic layer deposition (ALD). Commonly used LPCVD is capable of producing highly conformal films at high temperature (approximately at 700 • C), with excellent etching properties and low hydrogen content [1,8,9]. However, the high deposition temperature exceeds the thermal budget of the gate spacer fabrication process. SiN x films using PECVD can be grown at low temperatures (≤400 • C); however, these films show low quality and poor step coverage [8][9][10].

Materials and Methods
The Si wafer was a two-inch P-type Si (100) substrate for the deposition of SiN x , cleaned with diluted hydrofluoric acid for 2 min to remove native oxides. After cleaning, the substrate was loaded in the RPALD chamber. At first, SiN x films were deposited by RPALD with CSN-2 as the precursor and N 2 plasma as the reactant (two-step). Precursors were heated at 60 • C using a heating jacket to obtain sufficient vapor pressure and Ar (50 sccm) was used as the carrier gas in the line during the precursor dosing and purging step. The delivery lines were heated to 70 • C to prevent precursor condensation. N 2 plasma was generated by inductively coupled plasma (ICP) generated at a radio frequency of 13.56 MHz (Figure 1a). Deposition temperature was set in a wide range from 100 • C to 600 • C and the plasma power was fixed at 100 W. The recipe was selected as a standard which is composed of five stages (Figure 1b): CSN-2 dose time (3 s), precursor purge time (30 s), N 2 reactant gas injection time (4 s), N 2 plasma exposure time (20 s), and purge time (30 s). As shown in Figure 1c, we have developed the three-step process mentioned above, which adds H 2 gas injection (4 s) and an H 2 plasma exposure step (10, 20, and 30 s time variables) between the precursor purge and N 2 gas injection step. In the previous ALD cycle, the precursor delivery lines were purged with 700 sccm of Ar to remove byproducts and residual gas. A base pressure in the reactor chamber of~10 -6 torr was obtained using a turbo-molecular pump.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 10 and N2 plasma as the reactant (two-step). Precursors were heated at 60 °C using a heating jacket to obtain sufficient vapor pressure and Ar (50 sccm) was used as the carrier gas in the line during the precursor dosing and purging step. The delivery lines were heated to 70 °C to prevent precursor condensation. N2 plasma was generated by inductively coupled plasma (ICP) generated at a radio frequency of 13.56 MHz (Figure 1a). Deposition temperature was set in a wide range from 100 °C to 600 °C and the plasma power was fixed at 100 W. The recipe was selected as a standard which is composed of five stages ( Figure 1b): CSN-2 dose time (3 s), precursor purge time (30 s), N2 reactant gas injection time (4 s), N2 plasma exposure time (20 s), and purge time (30 s). As shown in Figure 1c, we have developed the three-step process mentioned above, which adds H2 gas injection (4 s) and an H2 plasma exposure step (10, 20, and 30 s time variables) between the precursor purge and N2 gas injection step. In the previous ALD cycle, the precursor delivery lines were purged with 700 sccm of Ar to remove byproducts and residual gas. A base pressure in the reactor chamber of ~10 -6 torr was obtained using a turbo-molecular pump. The thickness and refractive index of the deposited SiNx films were measured by spectroscopic ellipsometry (SE) using a Nano-View SE MG-1000 operated at an incident angle of 70° (1.5-5.0 eV). To investigate the surface morphology of SiNx thin films, atomic force microscope (AFM; Park Systems, XE-7) was used. The chemical compositions of the SiNx films were investigated with auger electron spectroscopy (AES). The chemical bonding state was determined by X-ray photoelectron spectroscopy (XPS) using a PHI 700Xi with Mg Kα X-ray source (E = 1.254 keV). Film wet etch rates were evaluated in a diluted HF solution (H2O / HF = 100 : 1). Film thickness and step coverage were examined by transmission electron microscopy (TEM).

Process Window
The deposition rate of SiNx thin films was investigated as a function of precursor dosing time and plasma exposure time at 400 °C. The growth rate per cycle (GPC) increased and saturated at 0.38 Å/cycle, as CSN-2 dosing time increased to 5 s. An apparent saturation for GPC can also be investigated when the plasma exposure time was 20 s or longer. This GPC saturation indicates that The thickness and refractive index of the deposited SiN x films were measured by spectroscopic ellipsometry (SE) using a Nano-View SE MG-1000 operated at an incident angle of 70 • (1.5-5.0 eV). To investigate the surface morphology of SiN x thin films, atomic force microscope (AFM; Park Systems, XE-7) was used. The chemical compositions of the SiN x films were investigated with auger electron spectroscopy (AES). The chemical bonding state was determined by X-ray photoelectron spectroscopy (XPS) using a PHI 700Xi with Mg Kα X-ray source (E = 1.254 keV). Film wet etch rates were evaluated in a diluted HF solution (H 2 O / HF = 100:1). Film thickness and step coverage were examined by transmission electron microscopy (TEM).

Process Window
The deposition rate of SiN x thin films was investigated as a function of precursor dosing time and plasma exposure time at 400 • C. The growth rate per cycle (GPC) increased and saturated at 0.38 Å/cycle, as CSN-2 dosing time increased to 5 s. An apparent saturation for GPC can also be investigated when the plasma exposure time was 20 s or longer. This GPC saturation indicates that the SiN x thin film RPALD process using CSN-2 and N 2 plasma is a self-limited reaction with no thermal decomposition of the precursor.
We investigated the growth rate of SiN x thin films at various temperatures (100-600 • C) as shown in Figure 2. The growth rate of the SiN x thin film was nearly constant at 0.38 Å/cycle in the deposition temperature range of 200-500 • C, known as the ALD process window. In the temperature regions below 200 • C and above 500 • C, the self-limited reaction is disturbed depending on process temperature. The former is due to precursor condensation on the substrate and the latter is due to thermal decomposition of the precursor; a "CVD-like" process [25][26][27]. As the deposition temperature increased, the refractive index of SiN x increased to 1.98 which is similar to 2.01 of the stoichiometric SiN x thin film, indicating that the quality of the film was improved. It can be also assumed that the film density is improved by increasing the refractive index [28].
Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 10 We investigated the growth rate of SiNx thin films at various temperatures (100-600 °C) as shown in Figure 2. The growth rate of the SiNx thin film was nearly constant at 0.38 Å/cycle in the deposition temperature range of 200-500 °C, known as the ALD process window. In the temperature regions below 200 °C and above 500 °C, the self-limited reaction is disturbed depending on process temperature. The former is due to precursor condensation on the substrate and the latter is due to thermal decomposition of the precursor; a "CVD-like" process [25][26][27]. As the deposition temperature increased, the refractive index of SiNx increased to 1.98 which is similar to 2.01 of the stoichiometric SiNx thin film, indicating that the quality of the film was improved. It can be also assumed that the film density is improved by increasing the refractive index [28].

Characteristics of SiNx Thin Films
AFM was used to analyze the roughness of the SiNx thin film. Figure 3 shows AFM images of SiNx thin film deposited at 250 °C, 350 °C, and 500 °C, respectively. Figure 3a has an RMS (route mean square) value of 0.077 nm, if SiNx thin film was deposited at 500 °C, the value of the SiNx film is 0.058 nm. Compared with the SiNx thin film deposited at low temperature, the roughness was slightly decreased as the process temperature increased. However, the width of the increase is so small that all thin films seem to have high surface quality. Even though the thin film is processed by various temperature conditions, good uniformity in the film can be observed.

Characteristics of SiN x Thin Films
AFM was used to analyze the roughness of the SiN x thin film. Figure 3 shows AFM images of SiN x thin film deposited at 250 • C, 350 • C, and 500 • C, respectively. Figure 3a has an RMS (route mean square) value of 0.077 nm, if SiN x thin film was deposited at 500 • C, the value of the SiN x film is 0.058 nm. Compared with the SiN x thin film deposited at low temperature, the roughness was slightly decreased as the process temperature increased. However, the width of the increase is so small that all thin films seem to have high surface quality. Even though the thin film is processed by various temperature conditions, good uniformity in the film can be observed.
To investigate the chemical compositions of the SiN x thin films, depth profiles were measured by AES, as shown in Figure 4. According to the AES analysis, there was an absolute decrease in carbon content with deposition temperature, which means that sufficient thermal energy was contributed to the reaction. Also, the films deposited at low temperature were vulnerable to oxidation due to low film density. SiN x thin films deposited below 250 • C contain a considerable amount of oxygen, which was reduced with increasing deposition temperature to 500 • C and measured to be less than 2%. The constant N and Si atomic contents indicate a uniform film stoichiometry throughout the entire film thickness. The ratio of N to Si was 1.34, which is nearly a stoichiometric Si 3 N 4 film.
AFM was used to analyze the roughness of the SiNx thin film. Figure 3 shows AFM images of SiNx thin film deposited at 250 °C, 350 °C, and 500 °C, respectively. Figure 3a has an RMS (route mean square) value of 0.077 nm, if SiNx thin film was deposited at 500 °C, the value of the SiNx film is 0.058 nm. Compared with the SiNx thin film deposited at low temperature, the roughness was slightly decreased as the process temperature increased. However, the width of the increase is so small that all thin films seem to have high surface quality. Even though the thin film is processed by various temperature conditions, good uniformity in the film can be observed. To investigate the chemical compositions of the SiNx thin films, depth profiles were measured by AES, as shown in Figure 4. According to the AES analysis, there was an absolute decrease in carbon content with deposition temperature, which means that sufficient thermal energy was contributed to the reaction. Also, the films deposited at low temperature were vulnerable to oxidation due to low film density. SiNx thin films deposited below 250 °C contain a considerable amount of oxygen, which was reduced with increasing deposition temperature to 500 °C and measured to be less than 2%. The constant N and Si atomic contents indicate a uniform film stoichiometry throughout the entire film thickness. The ratio of N to Si was 1.34, which is nearly a stoichiometric Si3N4 film. The chemical binding states in each film were investigated using XPS analysis. With the XPS scans of Si 2p and N 1s peaks, we obtained information regarding the bonding behavior of SiNx, as shown in Figure 5 (N 1s peak not shown). The XPS spectrum corresponding to the Si 2p with peak at 101.82 eV is the signature of the Si-N bond. The spectra corresponding to the N 1s content with peaks centered at 397.72 eV are the signature of the N-Si bond. The Si 2p spectra showed that all the samples had peaks at 101.82 eV. In the N 1s spectra, all the samples exhibited a peak at 397.72 eV [16]. In other words, the deposited samples were nearly stoichiometric Si3N4 and no difference in binding energy state was shown when the deposition temperature increased from 250 to 500 °C. As shown in Figure  5, the Si 2p peak deconvolution was performed with the Si-N binding energy corresponding to 101.82 eV and the Si-O binding energy corresponding to 103.27 eV. References to XPS and AES data confirmed that the oxygen content decreased sharply as the deposition temperature increased [16]. The step coverage of SiNx thin film was investigated using trench-patterned wafers, as shown in Figure 6. The aspect ratio of the trench patterned wafer is 2.7 with a top trench width of 31.2 nm in a narrow pattern. SiNx thin film thickness was measured at the top, side, and bottom of the trench with film conformality. The step coverage of the films improved with increasing deposition temperature. The chemical binding states in each film were investigated using XPS analysis. With the XPS scans of Si 2p and N 1s peaks, we obtained information regarding the bonding behavior of SiN x , as shown in Figure 5 (N 1s peak not shown). The XPS spectrum corresponding to the Si 2p with peak at 101.82 eV is the signature of the Si-N bond. The spectra corresponding to the N 1s content with peaks centered at 397.72 eV are the signature of the N-Si bond. The Si 2p spectra showed that all the samples had peaks at 101.82 eV. In the N 1s spectra, all the samples exhibited a peak at 397.72 eV [16]. In other words, the deposited samples were nearly stoichiometric Si 3 N 4 and no difference in binding energy state was shown when the deposition temperature increased from 250 to 500 • C. As shown in Figure 5, the Si 2p peak deconvolution was performed with the Si-N binding energy corresponding to 101.82 eV and the Si-O binding energy corresponding to 103.27 eV. References to XPS and AES data confirmed that the oxygen content decreased sharply as the deposition temperature increased [16].
Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 10 the reaction. Also, the films deposited at low temperature were vulnerable to oxidation due to low film density. SiNx thin films deposited below 250 °C contain a considerable amount of oxygen, which was reduced with increasing deposition temperature to 500 °C and measured to be less than 2%. The constant N and Si atomic contents indicate a uniform film stoichiometry throughout the entire film thickness. The ratio of N to Si was 1.34, which is nearly a stoichiometric Si3N4 film. The chemical binding states in each film were investigated using XPS analysis. With the XPS scans of Si 2p and N 1s peaks, we obtained information regarding the bonding behavior of SiNx, as shown in Figure 5 (N 1s peak not shown). The XPS spectrum corresponding to the Si 2p with peak at 101.82 eV is the signature of the Si-N bond. The spectra corresponding to the N 1s content with peaks centered at 397.72 eV are the signature of the N-Si bond. The Si 2p spectra showed that all the samples had peaks at 101.82 eV. In the N 1s spectra, all the samples exhibited a peak at 397.72 eV [16]. In other words, the deposited samples were nearly stoichiometric Si3N4 and no difference in binding energy state was shown when the deposition temperature increased from 250 to 500 °C. As shown in Figure  5, the Si 2p peak deconvolution was performed with the Si-N binding energy corresponding to 101.82 eV and the Si-O binding energy corresponding to 103.27 eV. References to XPS and AES data confirmed that the oxygen content decreased sharply as the deposition temperature increased [16]. The step coverage of SiNx thin film was investigated using trench-patterned wafers, as shown in Figure 6. The aspect ratio of the trench patterned wafer is 2.7 with a top trench width of 31.2 nm in a narrow pattern. SiNx thin film thickness was measured at the top, side, and bottom of the trench with film conformality. The step coverage of the films improved with increasing deposition temperature. The step coverage of SiN x thin film was investigated using trench-patterned wafers, as shown in Figure 6. The aspect ratio of the trench patterned wafer is 2.7 with a top trench width of 31.2 nm in a narrow pattern. SiN x thin film thickness was measured at the top, side, and bottom of the trench with film conformality. The step coverage of the films improved with increasing deposition temperature. In the narrow pattern, the temperature condition of 250 • C yielded side and bottom coverages of 73% and 80%. As the deposition temperature increased to 500 • C, improved film conformality was obtained with side and bottom coverages of 81% and 87%.
increasing deposition temperature, considering the fact that the film density is inversely proportional to WER [17]. The physical properties of the SiNx film based on the CSN-2 precursor have been studied. Thin films with good step coverage and excellent etching properties were obtained at 500 °C. In order to improve the progress made through the previously described two-step process (CSN-2/purge/N2 plasma/purge), an H2 plasma step was added between the CSN-2 and N2 plasma step to improve the characteristics of the thin film used as a gate spacer. In logic devices, the actual gate spacer process proceeds at temperatures below 400 °C to prevent implant out-diffusion and unwanted metal oxidation in the high-k metal gate (HKMG). Thus, we performed a comparative study between the two-step and three-step processes at 350 °C. N2 plasma not only removes the ligand of the precursor, but also facilitates the chemisorption of the silicon precursor. However, the N radicals have an exceptionally short life time compared to H radicals or O radicals. Due to this short lifetime, recombination loss occurs and the ligand is not completely removed from the precursor, such that the step coverage and quality of the thin film deteriorates. By introducing the H2 plasma before the N2 plasma, efficient ligand removal from the surface can be achieved by H radicals with a long lifetime. The SiNx films with an excellent step coverage and wet etch rate were obtained [21][22][23][24]. The wet etch rate (WER) test was performed to investigate SiN x etching properties. A diluted HF solution (1:100) was used. The LPCVD of Si 3 N 4 film at 730 • C was referenced for the WER test. As shown in Table 1, the wet etch rate decreased with increasing deposition temperature. In the above-mentioned Figure 2, as the deposition temperature increases, the film density also improved due to the increase of the refractive index [28,29]. It is understandable that the wet etch rate decreases with increasing deposition temperature, considering the fact that the film density is inversely proportional to WER [17]. The physical properties of the SiN x film based on the CSN-2 precursor have been studied. Thin films with good step coverage and excellent etching properties were obtained at 500 • C. In order to improve the progress made through the previously described two-step process (CSN-2/purge/N 2 plasma/purge), an H 2 plasma step was added between the CSN-2 and N 2 plasma step to improve the characteristics of the thin film used as a gate spacer. In logic devices, the actual gate spacer process proceeds at temperatures below 400 • C to prevent implant out-diffusion and unwanted metal oxidation in the high-k metal gate (HKMG). Thus, we performed a comparative study between the two-step and three-step processes at 350 • C. N 2 plasma not only removes the ligand of the precursor, but also facilitates the chemisorption of the silicon precursor. However, the N radicals have an exceptionally short life time compared to H radicals or O radicals. Due to this short lifetime, recombination loss occurs and the ligand is not completely removed from the precursor, such that the step coverage and quality of the thin film deteriorates. By introducing the H 2 plasma before the N 2 plasma, efficient ligand removal from the surface can be achieved by H radicals with a long lifetime. The SiN x films with an excellent step coverage and wet etch rate were obtained [21][22][23][24].
As shown in Figure 7a, as the H 2 plasma exposure time increases in the three-step process, the deposition rate decreases because the influence of the H 2 plasma forms an NH 2 or SiH group, which make reactions with the precursor difficult. In contrast, the deposition rate improved by fixing the H 2 plasma and increasing the N 2 plasma time as seen in Figure 7b. This is because the N 2 plasma regenerates the reaction sites on the surface where the -H or -NH 2 group is terminal and forms an undercoordinated surface. Therefore, the precursor easily reacts with the surface [16].
Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 10 As shown in Figure 7a, as the H2 plasma exposure time increases in the three-step process, the deposition rate decreases because the influence of the H2 plasma forms an NH2 or SiH group, which make reactions with the precursor difficult. In contrast, the deposition rate improved by fixing the H2 plasma and increasing the N2 plasma time as seen in Figure 7b. This is because the N2 plasma regenerates the reaction sites on the surface where the -H or -NH2 group is terminal and forms an undercoordinated surface. Therefore, the precursor easily reacts with the surface [16]. As shown in Figure 8, TEM images show step coverage with various H2 plasma exposure times. Compared with the case of As-dep films, the step coverage improves with increasing H2 plasma exposure time. In general, we can observe that the step coverage is greatly improved at the bottom/top compared to the side/top. In the narrow pattern, the side step coverage increased from 82% to 105% and the bottom step coverage increased from 90% to 110%. The narrow pattern is shown in Figure 8d-f. When the H2 plasma exposure time is 20 s, conformality was as good as the thin film proceeded at 500 °C. In Table 2, the WER was investigated at the same condition. As observed, the longer the H2 plasma exposure time, the lower the etch rate, which decreased from 43 to 32 Å/min. As shown in Figure 8, TEM images show step coverage with various H 2 plasma exposure times. Compared with the case of As-dep films, the step coverage improves with increasing H 2 plasma exposure time. In general, we can observe that the step coverage is greatly improved at the bottom/top compared to the side/top. In the narrow pattern, the side step coverage increased from 82% to 105% and the bottom step coverage increased from 90% to 110%. The narrow pattern is shown in Figure 8d-f. When the H 2 plasma exposure time is 20 s, conformality was as good as the thin film proceeded at 500 • C. In Table 2, the WER was investigated at the same condition. As observed, the longer the H 2 plasma exposure time, the lower the etch rate, which decreased from 43 to 32 Å/min. It was confirmed that the step coverage and wet etching characteristics were improved with H 2 plasma time by applying the low temperature process, which is necessary for the deposition of the gate spacer. Through process improvement, we were able to concentrate on the physical properties of the SiN x thin film. It was found that a high quality thin film could be obtained for the gate spacer depending on how the unique characteristics of the plasma reactant are utilized in the ALD process. Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 10  It was confirmed that the step coverage and wet etching characteristics were improved with H2 plasma time by applying the low temperature process, which is necessary for the deposition of the gate spacer. Through process improvement, we were able to concentrate on the physical properties of the SiNx thin film. It was found that a high quality thin film could be obtained for the gate spacer depending on how the unique characteristics of the plasma reactant are utilized in the ALD process.

Conclusions
In this work, physical and chemical properties of SiNx as a gate spacer were evaluated. The process environment was implemented in a remote plasma ALD system designed to overcome the limitations of the high thermal budget of thermal ALD and to minimize the thin film damage that can occur in direct plasma ALD [19]. The atomic concentration and chemical binding state were confirmed by deposition of SiNx with CSN-2 and N2 plasma. The fundamental properties of step coverage and wet etch rate were analyzed. The H2 plasma step was introduced before the N2 plasma step, which is a three-step process resulting in a reduction of growth rate. Many studies using NH3 plasma as a reactant have been performed. In our research, the H2 plasma and N2 plasma were used as reactants, respectively, instead of NH3 plasma. We compared the effect of the two-step process (only N2 plasma) with the three-step process at low temperature (≤400 °C) [22]. According to the results of the study for the narrow pattern, the longer H2 plasma exposure time increased the side step coverage from 81% to 105%, and bottom step coverage increased from 90% to 110%. In addition, the wet etch rate was reduced from 43.2 to 32.4 Å/min. Based on the results of this study, it was confirmed that the step coverage and the wet etching characteristics of the gate spacer thin film can be appropriately controlled by the optimization process.

Conclusions
In this work, physical and chemical properties of SiN x as a gate spacer were evaluated. The process environment was implemented in a remote plasma ALD system designed to overcome the limitations of the high thermal budget of thermal ALD and to minimize the thin film damage that can occur in direct plasma ALD [19]. The atomic concentration and chemical binding state were confirmed by deposition of SiN x with CSN-2 and N 2 plasma. The fundamental properties of step coverage and wet etch rate were analyzed. The H 2 plasma step was introduced before the N 2 plasma step, which is a three-step process resulting in a reduction of growth rate. Many studies using NH 3 plasma as a reactant have been performed. In our research, the H 2 plasma and N 2 plasma were used as reactants, respectively, instead of NH 3 plasma. We compared the effect of the two-step process (only N 2 plasma) with the three-step process at low temperature (≤400 • C) [22]. According to the results of the study for the narrow pattern, the longer H 2 plasma exposure time increased the side step coverage from 81% to 105%, and bottom step coverage increased from 90% to 110%. In addition, the wet etch rate was reduced from 43.2 to 32.4 Å/min. Based on the results of this study, it was confirmed that the step coverage and the wet etching characteristics of the gate spacer thin film can be appropriately controlled by the optimization process.