Applied Science and Convergence Technology 2023; 32(5): 114-117
Published online September 30, 2023
Copyright © The Korean Vacuum Society.
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
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 [1–3]. 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 CH
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 . 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
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 [10–12]. 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 (
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 . 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.
Three temperatures (
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,
Based on the synthetic spectra technique [14–16], the plasma temperatures were obtained by analyzing the observed spectra. Rotational and vibrational motions deliver energy and facilitate chemical reactions in the plasma . 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
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 (2
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 (
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).
The authors declare no conflicts of interest.