PECVD 방법으로 제작된 a-CN_(χ):H 박막의 구조적 특성 조사
Investigation on structural properties of a-CN_(χ):H thin films deposited by PECVD
PECVD a-CN_(χ):H 박막 a-CN_(χ):H thin films;
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In this work, we manufactured plasma-enhanced chemical vapor deposition(PECVD) system and generated reactive plasma using gaseous mixture of CH_(4)/N_(2)/Ar by PECVD. The PECVD method using low-frequency(60 Hz), which is Korean commercial power frequency, has some advantages compared with that using RF(13.56 MHz); the plasma CVD apparatus is simple to operate due to the simplicity of the power part and the use of a low power density causes less sample damage. Hydrogenated amorphous carbon(a-C:H) and carbon nitride(a-CN_(χ):H) thin films are deposited by LF-PECVD as a function of the deposition time, CH_(4) fraction(CH_(4)/[CH_(4)+Ar]), input power, N_(2) fraction(N_(2)/[N_(2)+Ar]), and deposition temperature. In order to compare their structural properties, we deposited the carbon thin films by RF-PECVD at the optimum conditions of LF-PECVD, and then diagnosed the plasma characteristics of LF-PECVD system using optical emission spectroscopy(OES). We characterized a-C:H and a-CN_(χ):H thin films with respect to their thickness, surface morphology, composition, bonding structure, and electric property by using various methods including scanning electron microscopy(SEM), atomic force microscopy(AFM), UV-VIS-NIR spectroscopy, Fourier transform infrared(FTIR) spectroscopy, visible(514.5 nm) and UV(257 nm) Raman spectroscopy, X-ray photoelectron spectroscopy(XPS), X-ray diffraction(XRD), X-ray reflectivity(XRR), and field emission. In the OES measurement of CH_(4)/Ar plasma, the intensities of all radicals due to the electrically excited species decrease as CH_(4) fraction increases, while they increase as input power increases. This behavior of radical intensity affects to the deposition rate of the thin films. In the OES measurement of CH_(4)/N_(2)/Ar plasma, the most radicals decrease as N_(2) fraction increases, then deposition rate of the thin films decreases. However, the intensities of radicals due to CN(388 nm) and N_(2)^(+)(358.50 nm) increase as N_(2) fraction increases. These trends of radicals contribute to the structural change of the thin films. Also, the N/C ratio of a-CN_(χ):H thin films decrease due to the decrease of CN(388 nm) radical and the deposition rate decreases due to the re-etching of H_(α)(656.26 nm) radical as deposition temperature increases. RMS roughness of a-C:H films increases as Ar fraction increases, which has a minimum value at the input power of 20 W. RMS roughness of a-CN_(χ):H films increases more than that of a-C:H because of the sputtering due to CN and N^(+) radicals, which increase drastically up to the deposition temperature of 400℃. The optical gap of a-C:H has a maximum value at the CH_(4) fraction of 25% and the input power of 20W. These behaviors in the optical gap are associated with the increase of sp^(3)/sp^(2) ratio and hydrogen content in the thin films, which is confirmed by the results of FTIR spectra. The optical gap of a-CN_(χ):H has a maximum value at the N_(2) fraction of 53%, which increase as the deposition temperature increases. From the visible(514.5 nm) Raman, we could find that the I_(D)/I_(G) ratio of a-C:H thin films has a minimum value at the CH_(4) fraction of 25% and the input power of 20W. The position of G-peak also shifts toward lower wavelength. These results indicate that graphitic crystallite size(L_(a)) decreases and the sp^(3) content of the thin films increases, which are also consistent with the increase of I_(T)/I_(G) ratio in UV Raman. The increase of T-peak(980 cm^(-1)) means the increase of sp^(3) structure in the thin films. In the case of a-CN_(χ):H(χ∼0.22) thin films, the nanocrystalline diamond(due to sp^(3) structure) and C≡N bonds peaks in visible Raman present at 1110 and 2220 cm^(-1), respectively. The area of the (1110 cm^(-1)+D) and 2220 cm^(-1) peak normalized to the G-peak has a maximum value when the N_(2) fraction is 53%. These results are the evidence of local crystallization with amorphous phase as the hybrid nitrogen to carbon. The trends of 2220 cm^(-1) peak are also consistent with the results of FTIR spectra. UV Raman of a-CN_(χ):H(χ∼0.13) shows that three characteristics peaks present at approximately 1060∼1070 cm^(-1)(T), 1220 cm^(-1)(P1), and 2240 cm^(-1)(P2). The T- and P1 peak due to β-C_(3)N_(4) phase of C-N bonds increases as deposition temperature increases, while the P2 peak due to sp^(3)-type C≡N bonds decreases. It is thought that the increase of T- and P1 peak is the evidence of the local crystallization with nanosized β-C_(3)N_(4) phase in amorphous matrix. The detailed chemical bonding of the deposited films is found by curve fitting the C 1s and N 1s XPS peaks, and various characteristic peaks, such as the C-C, the C=N, and the C≡N or C-N bonds, are found. Analysis of the deconvoluted C 1s and N 1s peaks reveals that the relative intensity ratio of sp^(3)-type C≡N reaches a maximum value at N_(2) fraction of 53%(N/C=0.22). The C-C bonds decrease and the C-N(or C≡N) bonds increase as deposition temperature increases. It is difficult to distinguish between C-N and C≡N bonding configurations since their binding energies are very close. However, it should be mentioned that the Raman and FTIR spectra identifies the presence of C-N bonds in a-CN_(χ):H(χ∼0.13) thin films. Therefore, the nanosized β-C_(3)N_(4) phase with amorphous phase increases as deposition temperature increases. This evidence is confirmed by the SEM images and XRD pattern. In this work, the comparison of the thin films deposited by RF-PECVD reveals that LF-PECVD exhibits similar material quality. Therefore, LF-PECVD is an useful method for the deposition of the carbon thin films. From the above results, we could find that the changes in the structural properties of a-C:H thin films are influenced by the CH_(4) fraction and input power. Moreover, we could find that the characteristics of field emission in the films are developed with the addition of nitrogen and the deposition temperature. In particular, the nanosized β-C_(3)N_(4) crystalline phase in a-CN_(χ):H thin films deposited by PECVD increases with decreasing amorphous phase due to the increase of the deposition temperature.