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Research Paper

Applied Science and Convergence Technology 2023; 32(5): 114-117

Published online September 30, 2023


Copyright © The Korean Vacuum Society.

Spectroscopic Analysis of Effects of Additive Nitrogen on Atmospheric Pressure Ar/HMDS Plasma

Jonggu Hana , Rodolphe Mauchaufféb , and Se Youn Moona , c , d , *

aDepartment of Applied Plasma and Quantum Beam Engineering, Jeonbuk National University, Jeonju 54896, Republic of Korea
bMaterials Research and Technology Department, Luxembourg Institute of Science and Technology, Esch-sur-Alzette L-4362, Luxembourg
cDepartment of Quantum System Engineering, Jeonbuk National University, Jeonju 54896, Republic of Korea
dHigh-Enthalpy Plasma Research Center, Jeonbuk National University, Wanju 55317, Republic of Korea

Correspondence to:symoon@jbnu.ac.kr

Received: June 12, 2023; Revised: August 16, 2023; Accepted: August 25, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.

Hexamethyldisilazane (HMDS) is a cost-effective and stable compound used to synthesize thin films for applications such as silicon nitride thin-film deposition. Plasma-enhanced chemical vapor deposition is commonly used to deposit thin films from HMDS because plasma offers sufficient energy to effectively decompose HMDS and facilitates the formation of films. Various gases are added to the plasma to modulate its characteristics. This study investigated the effect of N2 addition to atmospheric-pressure Ar/HMDS plasma. Optical emission spectroscopy measurements provided valuable insights into the characteristics of the plasma under different N2 gas flow rates, and the importance of N2 as a deposition parameter was discussed. The introduction of N2 in the Ar/HMDS plasma decreased the excitation, vibrational, and rotational temperatures, significantly changing the composition of reactive species in the plasma.

Keywords: Hexamethyldisilazane, Plasma enhanced chemical vapor deposition, Nitrogen plasma, Synthetic spectra, Optical emission spectroscopy

Hexamethyldisilazane (HMDS) is commonly used for surface modification and manufacturing of low-dielectric-constant (κ) materials in various fields, such as semiconductors [13]. HMDS has nonpolar methyl groups that contribute to the formation of porous thin films with unique properties, such as high hydrophobicity and low permittivity [1,4]. However, synthesizing films from HMDS is challenging because it is a stable compound requiring high activation energy to dissociate and form beneficial radicals such as CHx and Si-CHx that are attributed to thin film deposition [5].

Plasma-enhanced chemical vapor deposition (PECVD) is widely used to decompose stable compounds and generate fluent reactive species. The HMDS in plasma is easily decomposed through various pathways, such as electron impact and penning ionization [6]. Thus, PECVD is an efficient and effective alternative to conventional wet chemical processes for thin-film deposition [2,7]. PECVD offers control over the reactive species in the plasma and, consequently, over the deposited film properties by tuning the electron temperature, i.e., adjusting operational parameters such as the input power and plasma gas composition [1,8,9]. Kim et al. [9] reported that increasing the HMDS weight percentage in Ar/HMDS/O2 plasma increased both the impurities and surface roughness of the deposited films. In a previous study [1], we discovered that the amount of skeleton structure molecules, such as Si- CH3, in thin films could be tuned by controlling the flow rate of N2 gas in atmospheric pressure Ar/HMDS/N2 plasma. However, further investigation of plasma characterization is required to fully understand the influence of additive gas effects on the plasma process.

Owing to the distinct physical properties of N2, such as its high vibrational temperature and many metastable states, it significantly impacts its plasma characteristics and surface reactions [1012]. Therefore, it is widely utilized in various processes to control plasma, leading to beneficial changes in processes such as deposition, etching, and gas conversion [11,13]. This indicates that understanding the influence of N2 on plasma is crucial for understanding plasma characteristics and optimizing plasma processes.

Hence, in this study, we investigated the correlation between the plasma thermal characteristics and reactive species to understand the influence of N2 on atmospheric-pressure Ar/HMDS plasma. Accordingly, reactive species in the plasma were also investigated using optical emission spectroscopy and various temperatures of plasma, i.e., rotational temperature (Trot), vibrational temperature (Tvib), and excitation temperature (Texc), were estimated using the synthetic spectra technique.

The atmospheric-pressure plasma source was composed of a cylindrical electrode (diameter = 12 mm) in a dielectric tube (alumina; thickness = 2 mm) and powered by a 13.56 MHz radio-frequency power generator via a matching network, as illustrated in Fig. 1. A detailed description of the experimental setup is provided [1]. Ar (99.999 %) gas with a flow rate of 15 L⋅min−1 was introduced as the main plasma gas. HMDS (99.9 %, Sigma-Aldrich) vapor was generated and carried to the plasma with a 0.02 L·min−1 Ar carrier gas passing through a bubbler system. The influence of N2 on the Ar/HMDS plasma was studied by varying the N2 gas flow rate from 0 to 0.1 L⋅min−1. The emission spectra obtained from the plasma center were analyzed using a spectrometer (Princeton Instruments, IsoPlane SCT320) equipped with a charge-coupled device (Princeton Instruments, PIXIS400B) and a grating (1,800 grooves/mm, blaze wavelength = 500 nm) in the spectral range of 300–900 nm. The wavelength and sensitivity calibrations of the spectrometer were performed using a standard lamp source, and the full width at half maximum of the instrumental broadening of the spectrometer was estimated to be 0.084 nm.

Figure 1. Scheme of the atmospheric pressure-PECVD setup equipped with a visible spectrometer.

Three temperatures (Trot, Tvib, and Texc) were calculated by fitting numerically simulated spectra to the observed spectra. The theoretical emission intensity of N2 molecules, considering the collisional quenching rate between Ar and N2 [15,16] as follows:

InvJn'v'J'=64π4cN034πRe2qv,vSJ,Jτeffλ04Q×expEekTe expEvkTvibexpErkTrot ,

where c is the speed of light, k is the Boltzmann constant, Q represents the partition functions, N0 is the density of particles, λ0) is the line position in nm unit, Ee, Ev, and Er are the electronic, vibrational, and rotational energies, respectively, R¯e2 is the electronic transition moment, q(v′,v″) is the Franck–Condon factor, S(J′,J″) is the Hönl–London factor, and τeff is the effective lifetime. Single prime (′) and double prime (″) denote the upper and lower energy levels [14]. Based on Eq. (1), theoretical N2 spectra at different temperatures were obtained by considering an instrumental broadening of 0.084 nm, as illustrated in Fig. 2(a). Figure 2(b) depicts an example of the measurement of temperatures (Trot, Tvib, and Texc) by comparing the best-fitted synthetic N2 spectra to the observed N2 spectra of the Ar/HMDS/N2 plasma (for 0.005 L•min-1 N2) at 100 W.

Figure 2. (a) Simulated synthetic N2 spectra for different temperature cases. The instrumental broadening for spectra simulation is 0.084 nm. (b) Comparison of the observed spectra (BLACK) with the synthetic spectra of N2 (RED) for a 100 W Ar/HMDS plasma with the addition of 0.05 L⋅min−1 N2.

Optical emission spectroscopy was employed to monitor the reactive species and determine the characteristics of Ar/HMDS/N2 plasma. Typical optical emission spectra of Ar/HMDS/N2 discharge at 100 W with Ar (15 L⋅min−1) and HMDS (0.1 L⋅min−1) in the ultraviolet to visible (VIS) ranges are illustrated in Fig. 3. For clarity, the spectra are presented in two ranges, 380–900 and 300–550 nm, with different exposure times. Strong Ar atomic lines and N2 first positive system (FPS, B3ΠgA3Σu+) were mainly observed in the range of 550–900 nm, whereas N2 second positive system (SPS, C3ΠuB3Πg), OH 3064 Å system (A2Σ+X2Π), and various carbon-containing diatomic molecular bands such as CH 4300 Å system (A2ΔX2Π), C2 swan system (A3ΠgX'3Πu) and CN violet system (B2ΣX2Σ) were observed in the shorter wavelength region of 380–550 nm. The observed reactive species originated from the dissociation of gases (N2, HMDS, and ambient gas) and the recombination of their fragmented species. For example, CH primarily originated from the dissociation of HMDS in plasma, whereas CN resulted from the recombination of dissociated carbon and nitrogen.

Figure 3. (a) Emission spectrum in the range of 380–900 nm and (b) 300–550 nm of Ar/HMDS/N2 plasma at 100 W with 0.005 L⋅min−1 of N2.

Based on the synthetic spectra technique [1416], the plasma temperatures were obtained by analyzing the observed spectra. Rotational and vibrational motions deliver energy and facilitate chemical reactions in the plasma [17]. In addition, the energy distribution of the excitation states enables the estimation of changes in the energy path channel and/or electron temperature [14,18]. Consequently, the temperature information must be obtained to understand the plasma characteristics. The Trot was determined by analyzing the rotational band systems of the N2 SPS (1,4), while the Tvib was obtained by observing three N2 SPS rovibrational bands (Δv = 3, v′ = 0, 1, and 2). The Texc was evaluated using two electronic band systems: N2 SPS and N2 FPS. As observed in Fig. 4, as the N2 flow rate increased from 0 to 0.1 L⋅min-1, Trot, Tvib, and Texc reduced from 696–538 K, 2372–1978 K, and 5659–5584 K, respectively. The presence of N2 provided additional electron energy loss channels, such as molecular vibrations and rotations, which could have decreased the temperature in the Ar/HMDS plasma. Notably, the addition of trace amounts of N2 (≤0.01 L⋅min-1) substantially influenced the thermal characteristics of the Ar/HMDS plasma, as well as the composition ratio of reactive species in it [Fig. 5(a)]. Conversely, Trot and Texc at an N2 flow rate of over 0.01 L⋅min-1 increased marginally, which could be attributed to the charge exchange between Ar ions and the increased number of N2 molecules [18]. In general, the additional N2 molecules in the plasma can be converted into N2+ via charge transfer with Ar ions (Ar+ + N2 → Ar + N2+ ) owing to the approximately equal ionization potentials of Ar (15.76 eV) and N2 (15.58 eV). Because the produced N2+ ions convert into two nitrogen atoms by exothermic dissociative recombination, the temperature can be increased [18].

Figure 4. Influence of N2 on the rotational temperature (triangles), vibrational temperature (stars) and excitation temperature (circles).

Figure 5. (a) Normalized intensities of molecular and atomic emission spectra and (b) their intensity ratios as a function of N2 flow rate in Ar/HMDS/N2 plasma.

Figure 5(a) illustrates the intensities of the emitted species, such as N2 (C-B), CN (B-X), CH (A-X), C2 (A-X), OH (A-X), and Ar I (2p3→1s5, 706.7 nm), plotted for various N2 flow rates. The N2 emission intensity IN2 increased with increasing N2 flow rate up to 0.05 L⋅min-1 and then declined. Likewise, the CN emission intensity ICN soared at a low N2 flow rate of ≤ 0.005 L⋅min-1 and plummeted at higher rates. The emission intensities of CH, OH, C2, and Ar (denoted by ICH, IOH, IC2, and IAr, respectively) decreased by 63, 70, 89, and 64 %, respectively, as the flow rate increased from 0.0 to 0.1 L·min-1. The overall emission intensities, except for that of the N2 molecule, decreased owing to the weaker ionization (lower electron density) caused by N2, which provided additional electron energy loss channels and reduced the dissociation of HMDS-generating atomic carbons. Because of this reduced dissociation of the molecules, the number of carbon-containing molecules (CN and C2) rapidly decreased, as shown in Fig. 5(a). The OES results closely matched the variations in the chemical compositions of the deposited films [1]. The addition of N2 led to changes in the degree of dissociation of the HMDS precursor forming more Si-CH3 in the deposited films.

To compare the relative concentrations of reactive species in the plasma, the intensity of the Ar atom (706.7 nm) was employed as a base species because the Ar gas flow rates did not change and Ar was not involved in the chemical reactions. The emission intensity ratios of ICN, ICH, IC2, and IOH to IAr as functions of the N2 flow rate are presented in Fig. 5(b). Notably, we observed an inflection point for each emission intensity at 0.01 L⋅min-1, which was the same for the changes in temperature (Fig. 4). Interestingly, we confirmed that N2 flow dependence differed for each species. For example, when the N2 flow rate was less than 0.01 L⋅min-1, the intensity ratio ICN/IAr increased with increasing N2 flow rate, while ICH/IAr and IOH/IAr decreased. Conversely, when the N2 flow rate was above 0.01 L⋅min-1, the intensity ratios ICH/IAr and IOH/IAr exhibited an upward trend and eventually saturated. However, the ratios of carbon-containing species, such as ICN/IAr and IC2/IAr, rapidly decreased as the N2 flow rate increased. This indicates that the CN and C2 species originating from the recombination between the highly dissociated components of HMDS significantly decreased because the electron energy dispersed to various loss channels by N2 introduction was insufficient to form carbon atoms from HMDS, as shown in Fig. 4.

In this study, the influence of N2 as an additive on the temperature and reactive species in Ar/HMDS plasma was studied. Plasma properties such as chemical composition and thermal characteristics were investigated using optical emission spectroscopy. In particular, by simulating the synthetic spectra of N2 molecules, such as N2 FPS and N2 SPS, plasma temperatures such as the excitation (Texc), vibrational (Tvib), and rotational temperature (Trot) were estimated. From the results, we discovered that each energy state showing different temperatures revealed that the plasma was not in thermal equilibrium. In addition, even in small quantities, N2 addition cooled the Ar/HMDS plasma. Interestingly, a drastic change in the composition ratio of the plasma was observed when the additive N2 flow rate was below or above approximately 0.01 L⋅min-1. For example, at lower N2 flow rates (0.005–0.010 L⋅min-1), more CN molecules were observed. Conversely, at higher N2 flow rates (>0.05 L⋅min-1), CH and OH became the majority molecules, while the number of CN and C2 species progressively decreased because of the reduction in the electron temperature caused by N2 addition. The presence of N2, which provides more energy loss channels, reduced the decomposition rate of HMDS, which explains the effects of additive N2 on the properties of thin films, such as changes in morphology and composition ratio [1,9,19]. Furthermore, we determined that N2 could modulate the plasma temperature, playing an important role in the production and loss of species, which can be useful for efficiently controlling the properties of the deposited film.

This work was supported by a Korea Basic Institute (National Research Facilities and Equipment Center) under a grant funded by the Ministry of Education (2021R1A6C101B383), by the R&D program of Plasma Equipment Intelligence Convergence Research Center (No. 1711121944), and by SAMSUNG Electronics (No. IO210129-08345-01).

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