Applied Science and Convergence Technology 2024; 33(1): 27-31
Published online January 30, 2024
Copyright © The Korean Vacuum Society.
aDepartment of Control and Instrumentation Engineering, Korea University, Sejong 30019, Republic of Korea
bDepartment of Computer and Information Science, Korea University, Sejong 30019, 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.
In this study, we evaluate the etch damage of silicon oxycarbide (SiOC) films in an inductively coupled plasma using a 2 MHz bias power frequency and compare it to the damage in a 13.56 MHz bias power system. For this study, CF4/He/Ar or CF4/C4F8/Ar mixed gas plasmas were used. To evaluate the plasma-exposed damage, ellipsometric spectroscopy (ellipsometry) and Fourier-transform infrared (FT-IR) spectroscopy were performed. The dielectric constant and Si-O/C-O area % ratios were extracted from ellipsometry and FT-IR results, respectively. We confirmed that ions among the plasma parameters, such as ions, ultraviolet, and radicals, have a significant impact on thin-film properties. Although the etching rate of the oxide film at 2 MHz was higher than that at a 13.56 MHz bias frequency, it was confirmed that the damage to the SiOC thin film at 2 MHz was lower than that at 13.56 MHz. In addition, FT-IR analysis proved to be a useful tool for evaluating the plasma damage in SiOC thin films. The polymer thickness was calculated through X-ray photoelectron spectroscopy. Based on these results, the effect of the polymer on the change in the Si-O/C-O ratio is discussed.
Keywords: Silicon oxycarbide, Dielectric constant, Fourier-transform infrared plasma etching damage, Surface characteristics
To reduce the resistance-capacitance delay due to the reduction of the feature size and wiring spacing of the integrated circuit, the SiO2 thin film was replaced with a low dielectric constant (low-
The high aspect ratio oxide etching process has been entirely performed by the capacitively coupled plasma reactive ion etching (RIE) process so far. Recently, we suggested that the etching process can be successfully performed by the inductively coupled plasma (ICP) RIE process using a low frequency (2 MHz) bias power system . However, the etching damage to SiOC thin films has not yet been examined. Accordingly, in this study, the etching damage caused by the 13.56 MHz bias power was also investigated, and these results were compared with the etching damage caused by a 2 MHz bias power.
Plasma etching process was performed using an ICP etcher. The reactor consists of a cylindrical chamber made of anodized aluminum and a five-turn copper coil placed on a horizontal quartz window. The coil was connected to a 13.56 MHz power supply. A 13.56 or 2 MHz RF generator was applied to the bottom electrode to control the negative DC bias [14,15].
To minimize changes in the radicals formed in the plasma, CF4 was fixed at 50 sccm, and the ratios of Ar and He were adjusted. To maintain a constant ion density, the Ar flow rate was fixed at 50 sccm and the ratios of CF4 and C4F8 were adjusted.
As described previously , two types of masks were used: MgF2 and Si. The support for the optical mask was printed with a 3D printer, and the length of the support was 1.7 mm. As the sidewalls of the mask support were open, some radicals could freely diffuse into the samples. Previous studies  also confirmed that the degree of influence of radicals and photons is proportional to the distance between the sample and optical mask.
The SiOC thin films were deposited using a magnetron sputtering system. A 900 × 129 mm2 SiC target was used. A 400 nm thick SiOC film was deposited on a Si wafer by applying a power of 3 kW under 300 sccm Ar and 6 sccm O2.
The dielectric constant was calculated as the square of the refractive index, and the refractive index of the SiOC thin film was measured using spectroscopic ellipsometry (M-2000V, J. A Woollam). The chemical bond changes on the surfaces of the thin films were evaluated using FT-IR spectroscopy (IFS66v/S and Hyperion 3000, Bruker).
To confirm the plasma properties, plasma diagnosis was performed using a dual Langmuir probe (DLP 2000; Plasmart Inc., Korea). Optical emission spectroscopy (OES) (Avaspec-3648, JinYoung Tech, Korea) and a UV detector (YK-35UV) were used to measure the light intensity generated from the plasma.
The flow of the CF4 gas was kept constant, and the experiment was conducted by changing the flow rate ratio of Ar to He. Ar has lower ionization energy than He . This indicates that when He is replaced with Ar, the density of ions in the plasma increases.
Figure 1 shows the SiOC etch rates and VDC(self-bias voltage) at bias power supply frequency of 13.56 or 2 MHz. In the case of a 2 MHz bias power, the process was performed under process conditions in which the source power, bias power, and pressure were 150 W, 600 W, and 6 mTorr, respectively. Meanwhile, at 13.56 MHz, the source power, bias power, and pressure were 500 W, 100 W, and 10 mTorr, respectively. Figure 1(a) shows that in the 2 MHz system, much higher etch rates were observed compared to the 13.56 MHz system. As shown in Fig. 1(b), the difference in etching rate is largely due to the difference in
Figure 2 shows the positive ion density at bias power frequency of 13.56 and 2 MHz. In the 13.56 MHz system, much higher ion densities were observed compared to the 2 MHz system. This phenomenon seems to be due to the higher source power in the 13.56 MHz system compared with that in the 2 MHz system. However, as the Ar fraction increased, the lower ionization energy of Ar (1520.571 kJ/mol) compared with He (2372.322 kJ/mol) increased the density of ions formed in the plasma .
Figure 3 shows the change in the dielectric constant ϵ with the Ar fraction. As expected, no change in the dielectric constant was observed for the Si mask sample. When the Ar ratio changed from 0 to 100 % at 2 MHz bias power, the ion density changed from 3.109 to 3.59 × 109 cm−3 by approximately 10.6 %. This is due to the low ionization energy of the Ar gas. In other words, the ion density increased as the Ar fraction increased. In this study, the intensity of UV also increased from 2.05 to 2.25 mW/cm2 by approximately 9.7 %, showing that by changing the Ar fraction, not only the density of ions but also the UV intensity changed. Without a mask, that is, when samples were exposed to ions, UV irradiation, and radicals, the change in the dielectric constant increased from 2.937 to 2.943 as Ar fraction increased from 0 to 100 % at a bias power of 2 MHz. When using the MgF2 mask, that is, for samples exposed to UV irradiation and radicals, the dielectric constant increased from 2.915 to 2.917. For the Si mask, as the ratio of the Ar gas varied, there was small change in the dielectric constant. At 13.56 MHz, the dielectric constant of the no mask samples changed by approximately 0.23 %, whereas it changed by approximately 0.2 % at 2 MHz. A slight change was identified; however, the difference was so small that it was difficult to clearly identify the change. However, at 13.56 MHz, the dielectric constant was larger. This can be interpreted as a result of the higher ion density in the case of 13.56 MHz, as shown in Fig. 2. This implied that the change in the dielectric constant value was less when using 2 MHz compared with that at 13.56 MHz. Although the etching rate of the oxide film at 2 MHz was higher than that at a 13.56 MHz bias frequency, the damage to the SiOC thin film at 2 MHz was lower than that at 13.56 MHz.
The change in the dielectric constant due to plasma exposure was very small. To further clarify the changes in SiOC damage owing to plasma exposure, FT-IR analysis was performed. To quantify the degree of etching damage, FT-IR spectra were extracted in the range of 800–1,250 cm−1. Using the C–O (1,030 cm−1) and Si–O bonds (1123 cm−1) functional groups [4,14,17,18] from the FT-IR spectra, the area % of the Si-O and C-O bonds were calculated. Figure 4 shows the deconvolution of the FT-IR spectra into Si-O and C-O bonds.
Figure 5(a) shows the ratio of the Si-O and C-O area % with the increasing Ar fraction in the plasma. When no mask was used, that is, the samples were exposed to ions, radicals, and UV irradiation at 2 MHz bias power, the change in ratio of the Si-O and C-O area % increased from 0.821 to 0.881 by approximately 7.3 %. In the case of the MgF2 mask, that is, when the samples were exposed to radicals and UV irradiation, the change in ratio of Si-O and C-O area % increased from 0.603 to 0.626 by approximately 3.8 %. As shown in Fig. 3, the dielectric constant of the no-mask sample changed by approximately 0.2 %, whereas the FT-IR results changed by approximately 7.3 %. In the MgF2-masked sample, the dielectric constant changed by less than 1 %, but the Si-O and C-O area % changed by approximately 3.8 %. These results indicate that FT-IR analysis is more effective than dielectric constant measurements for detecting damage to SiOC thin films caused by plasma exposure. Meanwhile, at 13.56 MHz, the Si-O / C-O ratio of the no-mask and MgF2-masked samples changed by 8.6 and 5.0 %, respectively. The SiOC thin film was less damaged by plasma exposure at 2 MHz than that at 13.56 MHz, even though the etch rate of SiOC films at 2 MHz was much higher than that at 13.56 MHz. For the samples with no mask, the change in the Si-O/C-O ratio was greater than that of the MgF2-masked sample because UV irradiation, radicals, and ions affected the change in the Si-O/C-O ratio. To separate the effects of UV irradiation and ion exposure on the dielectric constant changes, the ion and UV effects were recalculated. The UV effect was calculated using the difference between the Si-O/C-O ratios of the Si-masked and MgF2-masked samples, and the ion effect was determined using the difference between the Si-O/C-O ratios of the MgF2-masked and unmasked samples . For instance, the impact of ion bombardment in Ar plasma can be evaluated using
Next, we investigated the effect of the polymer on the Si-O/C-O ratio. CF4 and C4F8 gases, which are fluorocarbons, were used to confirm the effect of the polymer. While the total flow rates of CF4 and C4F8 gases were fixed at 10 sccm, the amount of polymer was varied by changing the flow rate ratio of the CF4 and C4F8 gases. The flow rate of Ar gas was fixed at 50 sccm.
When the CF4 gas ratio changed from 0 to 100 % at 2 MHz bias power, the ion density increased from 5.61 × 109 to 5.68 × 109 cm−3 by approximately 1.2 % and UV intensity increased from 9.63 to 9.72 mW/cm2 by approximately 0.93 %. This indicates that when the Ar flow rate was fixed, even when the CF4/C4F8 ratio increased, the ion density and UV intensity in the plasma did not increase significantly. In contrast, a change in the CF4/C4F8 ratio influenced the radical species formed in the plasma . OES was performed to confirm the changes in these radicals. Figure 6 shows the extracted
As reported in a previous study, the energy applied to the sample surface is reduced by the polymer formed . To confirm this effect, the thickness of the polymer formed on the sample surface was calculated through XPS analysis. Calculations were performed using the following equation :
The thickness of the fluorocarbon layer (
Figure 8(a) shows the change in the Si–O/C–O ratio with the change in the CF4/C4F8 ratio. When 2 MHz was used, the change in Si-O/CO ratio with changes in etching conditions was smaller compared to that of 13.56 MHz. Overall, the Si-O/C-O ratio of the samples exposed to 2 MHz plasma was lower than that of the samples exposed to 13.56 MHz plasma. Although a vertical etched profile could be obtained when the process was performed at 2 MHz, the damage to the SiOC thin film formed was not larger compared to that at 13.56 MHz, as mentioned above. To isolate the effect of the plasma parameters on etch damage, the method shown in Fig. 5 was applied, as shown in Fig. 8(b). At 2 MHz system, UV effect increased the Si-O/C-O ratio from 0.21 to 0.22 by approximately 5 % and ion effect increased it from 0.23 to 0.25 by approximately 9 %. Moreover, as the mixing ratio of CF4 gas changed from 0 to 100 %, the UV intensity and ion density varied by almost 1 %, that is, almost no changes were observed. Based on this result, it could be confirmed that the increase in the Si-O/C-O ratio was relatively large compared to that when the ratio of Ar and He was changed. Although the change in the ion density and UV intensity decreased by approximately 1/10 in the CF4 ratio change experiments compared to that in the Ar ratio; the change in the Si-O/C-O ratio was almost similar. These results cannot be explained by the changes in ion density and UV intensity. This indicates that the thickness of the polymer layer formed on the sample surface influences the Si-O/C-O ratio. As the ratio of CF4 gas increased, the ion density and UV intensity slightly increased, and the thickness of the polymer formed on the sample surface decreased. Consequently, the energy of the ions and UV absorbed by the polymer decreased, and the Si-O/C-O ratio formed on the sample surface was relatively large. Furthermore, the Si-O/C-O ratio change in the ion effect at 2 MHz was greater than that at 13.56 MHz. This seems to be due to the thinner polymer formed at 2 MHz compared with that at 13.56 MHz. The thinner polymer formation appears to be related to the higher
The properties of SiOC thin films damaged by plasma exposure are investigated using a CF4/C4F8/Ar/He mixed gas plasma in an ICP system with a bias power of 2 MHz. To evaluate the etching damage, FT-IR spectra are measured, and the ratio of the Si-O/C-O area % is calculated.
The change in the Si-O/C-O ratio extracted from the FT-IR analysis was more sensitive than the dielectric constant measurement. Although a vertical etching cross section could be obtained when the process was performed at 2 MHz, the damage to the formed SiOC thin film was not larger than that at 13.56 MHz. However, as the ratio of CF4 to C4F8 gases changed, the density of ions and UV intensity changed slightly, whereas the CFx polymer thickness changed considerably. As the proportion of CF4 gas increased, polymer formation on the sample significantly influenced the Si-O/C-O ratio. This phenomenon occurred as the thick polymer absorbed the energy caused by the UV and ions.
This work was partly supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant RS-2020-KA157018) and ‘Regional Innovation Strategy (RIS)’ through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE)(2021RIS-004).
The authors declare no conflicts of interest.