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Study of Surface Reaction and Gas Phase Chemistries in High Density C4F8/O2/Ar and C4F8/O2/Ar/CH2F2 Plasma for Contact Hole Etching
Study of Surface Reaction and Gas Phase Chemistries in High Density C4F8/O2/Ar and C4F8/O2/Ar/CH2F2 Plasma for Contact Hole Etching
Transactions on Electrical and Electronic Materials. 2015. Apr, 16(2): 90-94
Copyright © 2015, The Korean Institute of Electrical and Electronic Material Engineers
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : March 03, 2015
  • Accepted : April 04, 2015
  • Published : April 25, 2015
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About the Authors
Gwan-Ha Kim
gwanha@ddc.ac.kr
Abstract
In this study, the characterizations of oxide contact hole etching are investigated with C 4 F 8 /O 2 /Ar and CH 2 F 2 /C 4 F 8 /O 2 / Ar plasma. As the percent composition of C 4 F 8 in a C 4 F 8 /O 2 /Ar mixture increases, the amount of polymer deposited on the etched surface also increases because the C x F y polymer layer retards the reaction of oxygen atoms with PR. Adding CH 2 F 2 into the C 4 F 8 /O 2 /Ar plasma increases the etch rate of the oxide and the selectivity of oxide to PR. The profile of contact holes was close to 90°, and no visible residue was seen in the SEM image at a C 4 F 8 /(C 4 F 8 +O 2 ) ratio of 58%. The changes of chemical composition in the chamber were analyzed using optical emission spectroscopy, and the chemical reaction on the etched surface was investigated using X-ray photoelectron spectroscopy.
Keywords
1. INTRODUCTION
SiO 2 (oxide) etch processes having high selectivity over photoresist (PR), silicon, and Si 3 N 4 (nitride) are required to manufacture ultra-large-scale integrated-circuit devices. Generally, oxide etching is performed by using fluorocarbon gases to deposit a fluoro-polymer on the underlying silicon or mask materials. This deposit enhances the etching selectivity of oxide over silicon or nitride. However, in the patterning of high-aspect-ratio contact holes or self-aligned contacts, these plasmas have serious problems related to the low selectivity of the underlying or mask material, the micro-loading effect, and the etch stop interrupting the etching [1] .
Previous studies focused on either the anisotropic etching or selective etching of oxide over silicon, nitride, or PR using conventional low-density plasma sources. Recently, high-density-plasma (HDP) systems have attracted considerable attention, primarily because HDP sources operate at low pressure and allow independent control of ion flux and ion energy [2 - 5] . Consequently, the plasma-surface interactions become significant, and surface conditions such as the temperature and cleanliness of the reactor wall play an important role in determining both the gas-phase chemistries and the surface-reaction deposition and etching on the wafer surface.
We studied the fluorocarbon layer formation that appears to dominate the surface reaction in order to understand the etch mechanism for highly selective etching using C 4 F 8 and CH 2 F 2 additive chemistry in contact hole etching. The relationship between gas chemistry and the fluorocarbon film was systematically investigated. Optical emission spectroscopy (OES) and X-ray photoelectron spectroscopy (XPS) were used to quantitatively analyze the gas chemistry and etched surfaces, respectively.
2. EXPERIMENTAL DETAILS
The HDP etching system is an APTC SELEX200, as shown in Fig. 1 . Multi-spiral coils and a bushing, connected to the 12.56-MHz RF power generator, are located above a thick horizontal ceramic window. This plasma source shows characteristics of both traditional sources of inductively coupled plasma (ICP) and capacitively coupled plasma (CCP), as well as its own characteristics. Some of the basic properties and process performances were previously reported [6] . A Semisysco smart-EPD TM system of optical emission spectroscopy was used to observe specific plasma, chemical, and wafer behaviors in the process chamber. Through the extraction of the characteristic behaviors from the optical spectrum, a representative value of a certain process can be obtained.
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Schematic diagram of etching system.
On a 200-mm wafer, 600-nm-thick TEOS (oxide) was deposited using a low-pressure chemical vapor deposition (LPCVD) and PR films were prepared using a spin-coating system. The etch rates of non-pattern oxide and PR were measured as a function of the C 4 F 8 /O 2 gas mixing ratio and CH 2 F 2 addition into the C 4 F 8 /O 2 /Ar plasma, with the process pressure of 16 Pa, RF power of 100 W, bias power of 1,800 W, and Ar flow rate of 300 sccm. The total flow rate is 331 sccm. The pattern samples were prepared to have contact hole structures by using the KrF photoresist that was used to check the etch profile. The PR thickness for the contact-hole etching mask was 200 nm, and a 58-nm-thick organic bottom anti-reflected coating (OBARC) was used for the photo-mask process.
To analyze the effect of the C 4 F 8 /O 2 gas mixing ratio and addition of CH 2 F 2 gas into the C 4 F 8 /O 2 /Ar plasma, we investigated the chemical species in the gas phase with an optical emission spectroscopy. The surface analysis of polymer on oxide films was etched in various C 4 F 8 /O 2 gas mixing ratios and was performed using X-ray photoemission spectroscopy (XPS, VG Scientific ESCALAB 200R). Etched contact hole patterns were characterized using field emission scanning electron microscopy (FE-SEM, FEI Sirion 400) for the etch profile.
3. RESULTS AND DISCUCSSION
Figure 2 shows the etch rates for both oxide and PR as functions of the C 4 F 8 /O 2 mixing ratio in the C 4 F 8 /O 2 /Ar plasma. Also, the etch selectivity of oxide over PR is shown. As the C 4 F 8 fractional flow rate increases, the oxide etch rate also increases, while the etch rate of PR decreases. Accordingly, higher etch selectivity is obtained for higher C 4 F 8 concentration in the C 4 F 8 /O 2 /Ar plasma. The behaviors of etch rates can be simply associated with the behaviors of the corresponding active species that include fluorine atoms and CF x radicals for the oxide and oxygen atoms for the PR. However, it is reported that the selective etching of oxide over PR using various fluorocarbon plasmas is influenced not only by the fluxes of active species, but also by the thickness of the fluorocarbon C x F y polymer layer formed over the etched structure. Evidently, an increase in the C 4 F 8 /O 2 mixing ratio increases the thickness of the fluorocarbon C x F y polymer layer due to both the increasing deposition rate (through the increasing flux of fluorocarbon radicals) and decreasing destruction rate that is supported by the chemical reaction with oxygen atoms. At the same time, the C x F y polymer layer deposited on the oxide can be effectively destroyed by the oxygen released from the oxide. As such, the polymer layer on the PR is expected to be thicker than that for oxide and to play the role of inhibitor for the reaction of oxygen atoms with PR. Together with the decreasing flux of oxygen atoms, the effect of C x F y polymer layer also decreases the etch rate of PR and increases the etch selectivity of oxide over PR with increasing C 4 F 8 /O 2 mixing ratio.
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Etch rates of oxide and PR, and selectivity of oxide to PR as a function of C4F8/O2 gas mixing ratio.
We used OES analysis to investigate the behaviors of the plasma active species. The emission intensities of F (703.7 nm), CF (247.5 nm), CF 2 (251.9 nm), O (844.6 nm), and CO (483.1 nm) radicals were measured as functions of the C 4 F 8 /O 2 gas mixing ratio [7] .
Figure 3 shows that as the C 4 F 8 /O 2 mixing ratio increases, the emission intensities for both the CF and CF 2 radicals increase, but the intensities of the atomic lines corresponding to the O and F atoms decrease. Assuming that the emission intensities are proportional to the volume densities of the corresponding species in a gas phase, the observed changes in plasma composition can result from a combination of several factors. Particularly, increases in both CF and CF 2 densities are potentially caused by two effects: (i) the increasing density of C 4 F 8 molecules that are the only source of CF x radicals in our system and (ii) the decrease in oxygen partial pressure, which lowers the destruction rate for CF x radicals (CF x + O → COF x−1 + F) both in bulk plasma and on the etched surface [8] . This also explains the decreasing tendency mentioned for the volume density of F atoms. Finally, comparing the data in Figs. 2 and 3 , one can assume that the etching of oxide is controlled more by CF x radicals than by F atoms.
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Optical emission intensities of F, CF, CF2, O, and CO radicals as a function of C4F8/O2 gas mixing ratio.
We used XPS measurements to analyze the chemical composition of the etched surfaces.
Figure 4 shows C 1 s and Si 2 p XPS narrow-scan spectra for surfaces etched with various gas mixing ratios. A wide C 1s peak can be deconvoluted into five peaks according to well-known data on binding energies for carbon-containing compounds. These peaks are 284.3 eV (C-C), 286.6 eV (C-CF x ), 288.8 eV (C-F), 290.1 eV (C-F 2 ), and 293.2 eV (C-F 3 ) [9 , 10] . Each spectrum shown in Fig. 4(a) shows a similar qualitative composition, except for peak intensities. As the C 4 F 8 /O 2 ratio increases, the CF 2 and CF 3 bonding increases significantly and their intensities reach the same value as the C-C and C-F peaks. This indicates a growth of the CF x polymer layer, which has a fluorine-rich and low-carbon composition at high C 4 F 8 contents. Figure 4(b) shows that the Si 2 p narrow-scan spectra cannot be resolved in the SiO (102.7 eV), SiO 2 (103.6 eV), and SiC (100.7 eV) compounds due to their adjacent binding energies. However, as the C 4 F 8 /O 2 gas mixing ratio increases, the intensity of Si 2 p narrow-scan spectra decreases because the increased CF x radical obstructs the reaction between the O species and silicon. This phenomenon was related to the change of the etch property.
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XPS spectra of (a) C 1 s and (b) Si 2 p after etching as a function of C4F8/O2 gas mixing ratio.
Figure 5 shows the cross-sectional SEM image of the etched contact holes pattern as a function of the C 4 F 8 /O 2 mixing ratio in the C 4 F 8 /O 2 /Ar plasma. The profile of holes is close to 90° in Fig. 5(a) , and the etch residue is not found at the sidewall. As the C 4 F 8 fractional pressure increases, the oxide etch rate and selectivity of oxide to PR increases, but it has a disadvantage at the etch profile.
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Cross-sectional images of SEM in the contact hole etching. (a) C4F8/O2 = 18/13 sccm (58%), (b) C4F8/O2 = 22/9 sccm (71%), and (c) C4F8/O2 = 26/5 sccm (84%).
Although an adequate etch profile was obtained, a higher etch selectivity is required to make self-aligned contacts and to provide deeper etching. To find the process condition that provides better etch selectivity, we investigated the effect of CH 2 F 2 addition in C 4 F 8 /O 2 /Ar (C 4 F 8 /O 2 /Ar=18/13/300) plasma.
Figure 6 shows that the addition of CH 2 F 2 results in increasing the etch rate of SiO 2 , but in decreasing the etch rate of PR, so that the etch selectivity of oxide over PR increases. These effects may be associated with the role of hydrogen atoms formed in plasma due to the dissociation of CH 2 F 2 , and at least two mechanisms can be proposed. First, H atoms can attach to atomic fluorine to form HF molecules. This results in decreasing the volume density of F atoms, decreasing the recombination rate of F with CF x , and thus increasing the density of CF x , which is the main chemically active species for oxide. This mechanism is in good agreement with the OES data shown in Fig. 7 .
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Etch rates of oxide and PR and selectivity of oxide to PR as a function of CH2F2 additive flow ratio into the 58% C4F8/O2 plasma.
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Optical emission intensities of F, CF, CF2, O, Ar, C2, CO, and H radicals as a function of CH2F2 additive flow ratio into the 58% C4F8/ O2 plasma.
From Fig. 7 , it can be seen that the addition of CH 2 F 2 is accompanied by an increase in the emission intensity of the CF 2 band that can be associated with increasing volume density for these species. Second, hydrogen can assist the chemical etching of oxide by extracting oxygen atoms to form OH radicals.
Figure 8 shows the C 1s and Si 2 p XPS narrow-scan spectra of the etched oxide surface with and without an admixture of CH 2 F 2 in the C 4 F 8 /O 2 /Ar plasma. The addition of CH 2 F 2 increases the density of the CF x bonds on the surface, which can be associated with the increasing flux of these species coming from bulk plasma. The intensity of the Si 2 p XPS narrow-scan spectra is shown in Fig. 8(b) . This peak can comprise SiC, SiO, and SiO 2 . As mentioned above, in Fig. 4 , the intensity change of this peak affects the etch property. Therefore, Fig. 8 demonstrates the increasing volume density of CF x radicals (as previously mentioned with the OES measurements) and confirms the mechanism proposed to explain the acceleration of the oxide etch rate in the presence of CH 2 F 2 molecules.
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XPS spectra of (a) C 1 s and (b) Si 2 p after etching. (a) 0 sccm CH2F2 addition and (b) 6 sccm CH2F2 addition.
Figure 9 shows the cross-sectional SEM image of the etched contact holes pattern as a function of CH 2 F 2 addition in the C 4 F 8 /O 2 /Ar plasma. When the CH 2 F 2 gas was added, a slight bowing profile was shown, but higher selectivity of oxide to PR could be obtained. Moreover, the bowing profile can be removed by changing the gas mixture. However, the excessive CH 2 F 2 gas addition induces sidewall roughness of the contact hole, as confirmed in Fig. 9(b) .
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Cross-sectional images of SEM in the contact hole etching. (a) 6 sccm CH2F2 addtion and (b) 12 sccm CH2F2 addition.
4. CONCLUSIONS
In this work, we investigated the etching of contact holes through a PR mask using C 4 F 8 /O 2 /Ar and CH 2 F 2 /C 4 F 8 /O 2 /Ar gas mixtures. As the mixing ratio of C 4 F 8 on the C 4 F 8 /O 2 increases, the amount of polymer deposited on the etched surface also increases. This leads to an increase in the etch selectivity of oxide over PR because the C x F y polymer layer retards the reaction of oxygen atoms with PR. The profile of the contact holes was found to be close to 90°, and the etch residue is not found at the sidewall at a C 4 F 8 /(C 4 F 8 +O 2 ) ratio of 58%. We also found that the etch selectivity of oxide over PR can be improved by the addition of CH 2 F 2 in the C 4 F 8 /O 2 /Ar plasma because the CF x radical volume density is increased.
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