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Research Paper

Applied Science and Convergence Technology 2021; 30(4): 102-106

Published online July 30, 2021

https://doi.org/10.5757/ASCT.2021.30.4.102

Copyright © The Korean Vacuum Society.

Characteristics of SiO2 Etching by Capacitively Coupled Plasma with Different Fluorocarbon Liquids (C7F14, C7F8) and Fluorocarbon Gas (C4F8)

Seung-Wan Yooa, Chulhee Chob,d, Kyungtae Kimb, Hyochang Leec, and Shinjae Youb,d,*

aMemory Division, SAMSUNG Electronics, Hwasung 18448, Republic of Korea
bDepartment of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
cAdvanced Instrumentation, Korea Research Institute of Standards and Science, Daejeon 34113, Republic of Korea
dInstitute of Quantum System (IQS), Chungnam National University, Daejeon 34134, Republic of Korea

Correspondence to:E-mail: sjyou@cnu.ac.kr

Received: June 25, 2021; Revised: July 27, 2021; Accepted: July 29, 2021

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 non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Fluorocarbon (C7F14, C7F8) plasmas are investigated to verify their etching characteristics as an alternative etchant of SiO2 etch process because C7F8 and C7F14 precursors are expected to have low Global warming potentials. Comparing the etch results of C4F8, C7F14, and C7F8 plasmas, C7F8 provides the highest selectivity for etching SiO2 at a moderate etching rate of the three fluorocarbons. C4F8 and C7F14 plasmas show similar magnitudes of selectivity at the same O2 injection. O2 addition is used to control densities of carbon species and optimize etching conditions. From comparison of the species existing in the C4F8, C7F14, and C7F8 plasmas by the electron-emitting source, CF radicals and carbon atoms are important in determining the remarkable selectivity of C7F8 plasma. This understanding is verified using X-ray photoelectron spectroscopy analysis.

Keywords: Silicon oxide etching, Liquid fluorocarbon precursor, Global warming potential, Etch selectivity, Capacitively coupled plasma

In the ultralarge-scaled integrated devices fabrication, as SiO2 etching process for manufacturing 3D semiconductor devices demands high aspect ratio and small feature size, etching characteristics such as high SiO2 etch rate and highly selective etch of SiO2 over Si3N4 or Poly-Si are getting more important. On the other hand, regarding the global warming problems, perfluorinated compounds (PFCs) like C2F6, C4F8, which are used as precursors for SiO2 etching process, generally have high Global Warming Potential (GWP) and been considered to raise greenhouse effect [1]. Therefore, it is important to develop an alternative etching process using low GWP precursor and showing competent etching properties compared with conventional SiO2 etch process.

In an effort to develop an alternative etchant with low GWP, C5F8 had been investigated to evaluate its etching performances as an alternative precursor for the SiO2 etch process [2, 3]. Because C5F8 has a double bond in its a cyclic-structure so that C5F8 shows shorter atmospheric lifetime and low GWP. Therefore, if C5F8 is effused from reactor without being dissociated in plasma, the C5F8 effused will not worsen global warming.

Considering the issues, C7F8 (octafluorotoluene) and C7F14 (perfluoro-methylcyclohexane) are investigated in this study as an alternative fluorocarbon precursor for C4F8 which is used conventionally in the industry. Although the values of GWPs of C7F8 and C7F14 are not calculated and are unknown, the C7F8 and C7F14 liquid precursors are considered to have same potential compared with C5F8 as an alternative fluorocarbon precursor regardless of GWP. As for the reason, from the fact that a portion of parent PFC is known to be effused out of chamber without being dissociated in the plasma, the liquid PFC effused can be condensed and retrieved easily [4, 5].

Using the C7F14 and C7F8 as liquid fluorocarbon precursors, the etch rate and the etch selectivity of SiO2 over resist are compared with C4F8 in the plasmas. Dissociative characterizations of the precursors were investigated using the quadrupole mass spectrometer (QMS) both in plasma process and plasma-off condition. This analysis method is enable us to interpret the etching characters of the liquid precursor.

For verifying etch characteristics of the precursors, a radio frequency (RF, 13.56 MHz) capacitively-coupled plasma (CCP) was used to etch SiO2 layer. The CCP chamber has a diameter of 34 cm and a height of 14.5 cm and the plasma was maintained between two parallel plate electrodes separated by 5 cm. During the etch processes, the RF power was pulsed at 500 Hz with a 50 % duty cycle at 300 W. The process pressure and substrate temperature were maintained at 20 mTorr and room temperature, respectively.

Mixtures of PFC/Ar/O2 were injected to the chamber through the nozzle installed at the top electrode of the chamber. Flow rates of the PFCs were equally 6 sccm during their respective process and flow rate of Ar was kept 6 sccm. It is widely known that oxygen can be used to control CxFy polymer formation and The CxFy polymer at surface play an important role in the SiO2 selective etching [6, 7]. From this understanding, it is considered that controlling amount of oxygen addition is the primary method to find optimal conditions of each PFCs. In order to optimize the process condition of the precursors, the flow rates of O2 were changed in range of 0–4 sccm for the experiments of C4F8 and C7F14. C7F8 precursors deposit more polymer than other precursors, so SiO2 was not etched in same condition of other precursors. Therefore, the flow rates of O2 were changed in range of 6–12 sccm for the experiment of C7F8. For the etch process, substrate was composed of 1 µm thick SiO2 layer masked with a 200 nm thick poly-Si, which layers were deposited on silicon wafer.

To measure the radical species in the plasmas, the ordinary quadrupole mass spectrometer system (SRS-200) was installed at the chamber. The radical species were extracted from the chamber through metallic tube (3.5 cm in diameter, 2 cm in length) and introduced to the sampling orifice of the QMS, which orifices was 0.1 µm in diameter. Operating pressure of the QMS was maintained bellow 1×10−6 Torr for being free from the collision of gases. Meanwhile, gases decomposed from the precursors using electron emitting source were measured by the another QMS (PSM, Hiden Analytical, England) which is capable of appearance potential mass spectroscopy (APMS) analysis.

In order to examine the etch rate and the etch selectivity of SiO2 over resist, profiles of SiO2 layer etched in the plasma were examined by field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan). To measure the plasma density, a cut-off probe was used [8, 9], which was placed at center of the chamber. And using an oscilloscope installed at bottom electrode, self-bias voltage was measured.

Figure 1(a) shows the etch rates of SiO2 and Fig. 1(b) shows etch selectivities of SiO2 over poly-Si as a function of O2 flow rate, which are processed in the C4F8, C7F14, and C7F8 plasmas. It is known that carbon and fluorine arrived at etching surface form fluorocarbon film and etch SiO2 film by reacting with Si and oxygen of the surface, but, if the carbon and fluorine arrive to the surface with so large flux and form a thick film, etch rate will be decreased and etch reaction can be stopped [7]. Therefore, controlling the thickness of fluorocarbon on surface is a main method to optimize etch processes [10]. On this understanding, the etch rate and the etch selectivity were investigated at various oxygen flow rates for comparison between the optimal conditions of the C4F8, C7F14, and C7F8 plasmas.

Figure 1. (a) Etch rates of SiO2 and (b) etch selectivities of SiO2 over poly-Si as a function of O2 flow rates obtained in the C4F8, C7F14, and C7F8 plasmas.

As the oxygen flow rate is increased from 0 to 6 sccm for the experiments of C4F8 and C7F14 and increased from 6 to 12 sccm for the experiment of C7F8, all the etch rates of the precursors are increased. In the range of oxygen flow rate from 0 to 6 sccm, C4F8 and C7F14 show similar values of selectivities, but the etching rates of the C4F8 plasmas were higher than that of the C7F14 results except for the oxygen flow rate of 2 sccm. At the oxygen flow rate of 2 sccm, selectivity of the C7F14 results show slightly larger value than that of the C4F8 results.

In the C7F8 plasma process, oxygen was injected to the chamber with the flow rates of 6–12 sccm. Because, at the oxygen flow rate under 6 sccm, deposition of CxFy polymer on the wafer was so large that etching of SiO2 layer was not progressed. Etch rates of C7F8 for oxygen flow rate of 6–12 sccm were lower than those of other precursor for oxygen flow rates of 0–6 sccm, in contrast, etch selectivies of C7F8 plasma are larger than those of C4F8 and C7F14 results. Especially, at the oxygen flow rates of 8 and 10 sccm, etch selectivities were significantly larger than those of C4F8 and C7F14 plasma.

In order to explain the differences between the etch properties of C4F8, C7F14, and C7F8 shown in Fig. 1, it can be analyzed in two aspects. First, the electrical properties of plasma enable to affect etch characteristics can be examined. Flux and density of ions arriving the surface of substrate are related with the electrical property of the plasma such as self-bias voltage of plasma and plasma density. In this regard, electron density and self-bias voltage were analyzed.

In Fig. 2, the plasma density and the self-bias voltage were measured at the same condition of Fig. 1. In Fig. 2(a), plasma density was also not different between the precursors. Therefore, the plasma density is not related with the difference of the etch results in Fig. 1. From the Fig. 2(b), the self-bias voltage is slightly changed with varying precursors, but this variation is not enough to change the etch characteristics for each precursor. Thus, it is considered that self-bias voltage is not related with the difference of the etch results in Fig. 1(a). Therefore, it can be deduced that the differences of etch results between the precursors are not attributed to the change of electrical properties of the plasmas. Meanwhile, despite the oxygen addition, the plasma density and the self-bias voltage were steady. Therefore, it can be also deduced that the electrical properties caused by oxygen addition does not affect etch results of Fig. 1(a).

Figure 2. (a) Plasma density and (b) self-bias measured as a function of O2 flow rates in the C4F8, C7F14, and C7F8 plasmas.

The differences among the etch properties of the C4F8, C7F14, and C7F8 plasmas shown in Fig. 1 can be analyzed in another aspect. Second, plasma species were analyzed by the quadrupole mass spectrometer (QMS). Figure 3 shows (a) C, (b) CF, (c) CF2, (d) CF3, and (e) F radical densities measured by QMS under the same conditions used to obtain the data in Fig. 1.

Figure 3. (a) C, (b) CF, (c) CF2, (d) CF3, and (e) F radical densities measured by a QMS(SRS-200) in the C4F8, C7F14, and C7F8 plasmas.

In the C4F8 plasma process, both CF and CF2 radicals were most frequently observed than the other precursor plasma. The C7F14 plasma has a lower CF2 radical density than the C4F8 plasmas, but larger CF2 radical density than that of the C7F8 plasmas. Regarding CF radical, the C4F8 and C7F14 plasmas show the similar magnitude of densities each other. The C7F8 plasmas show smaller CF and CF2 radical densities than those of the other precursor plasmas. In addition, C7F8 results show slightly larger fluorine radical density than C7F14 results. From these facts, it is expected that etch results of C7F8 plasmas would show smaller etch selectivity than the C4F8 and C7F14 plasma processes. However, in comparison with the C4F8 and C7F14 etch results, the selectivities of the C7F8 etch results are larger and the etch rate of C7F8 results are smaller. Meanwhile, it needs to be mentioned that the carbon densities of the C7F8 plasmas were somewhat larger than that of the C4F8 and C7F14 plasmas.

The results, like above mentioned, in which tendencies of etch properties were not consistent with changes of CxFy density measured by QMS are also found in the other report [11]. This is because radical density measured by QMS during plasma process does not represent the information about the species arriving the surface, but represent the species existing in the plasma. Plasma state is normally determined by reactions between species like electron, ion and neutral and by the interaction between the species and solid surface after plasma is initiated by electron impact ionization. Therefore, so as to elucidate plasma state and the etch result accurately, it will be necessary to distinguish the species arriving the surface from the species existing in the plasma.

In order to obtain the information about the species which affects the etching surface, the fluorocarbon films deposited in the C4F8, C7F14, and C7F8 plasmas are analyzed by XPS. Figures 4(a)–(c) are the XPS spectra of the fluorocarbon films deposited in the C4F8, C7F14 and C7F8 plasmas, respectively. The photoelectron C 1s peaks observed in Fig. 4 are C-C (283.8 eV), C-CF (284.7 eV), CF (286.5 eV), CF2 (289.2 eV), and CF3 (291.5 eV). The pick positions can be shifted due to the charging effect of polymer film [12, 13]. In Fig. 4, as the oxygen flow rate increased, decreasing of C 1s peaks were observed for all three precursors. C 1s peaks of C7F8 results were mostly larger than those of the C4F8 and C7F14 results. And C 1s peaks of C7F14 are larger than those of C4F8 at oxygen flow rate of 2–6 sccm. These results are consistent with the selectivities shown in Fig. 1(b).

Figure 4. XPS spectra of photoelectron C 1s peaks of the fluorocarbon film de-posited in the (a) C4F8, (b) C7F14, and (c) C7F8 plasmas.

For better understanding of the species which affect the etching surface, after C4F8, C7F14, and C7F8 are dissociated using electron emitting source, the dissociated species were measured using QMS so that the density of the species decomposed from the precursors (C4F8, C7F14, C7F8), which are shown in Fig. 5, would be compared with the species existing in the plasmas as shown in Fig. 3.

Figure 5. (a) C, (b) CF, (c) CF2, (d) CF3, and (e) F radical densities measured as a function of electron impact energy using a QMS (PSM) after C4F8, C7F14, and C7F8 are decomposed by the electron-emitting source.

Figure 5 shows the (a) C, (b) CF, (c) CF2, (d) CF3, and (e) F radical densities measured at various electron impact energies using a QMS after C4F8, C7F14, and C7F8 are dissociated using an electron-emitting source [14, 15]. In this study, because the filament is used only for gas dissociation, the angle of the electron beam is not con-sidered and the gas is decomposed by the electron-emitting source of the ionizer in the QMS. For resolution and accuracy of the electron energy, the PSM apparatus was used, allowing APMS analysis.

In Figs. 5(b) and (c), the gases decomposed from the precursors show larger densities of CF than of CF2, while Figs. 3(b) and (c) show no significant differences between CF and CF2. According to the results, CF radicals are considered to be involved in the surface reactions and to have more influence on the etch process than CF2 radicals.

In Figs. 5(b) and (c), the gases decomposed from C4F8 show the highest densities of both CF and CF2, in agreement with Fig. 3. The C7F14 plasma shows lower CF2 radical density than the C4F8 plasma, but higher CF2 radical density than the C7F8 plasmas, which also agrees with Fig. 3(c). However, C7F8 plasma shows greater CF radical density than C7F14 plasma, which does not match the results of Fig. 3(b). This is considered to be related with the larger C-CF and CF peaks from the C7F8 results compared to those of the other precursor. It can be deduced that the CF radical is more important in determining the etching characteristics of the C7F8 plasma process than it is with C7F14.

On the other hand, in Fig. 5(a), the carbon atom density measured from dissociated C7F8 gas is one order of magnitude higher than that from the other precursors. This result indicates that the C7F8 plasma produces more carbon as carbon atoms, instead of fluorocarbons. It can be also inferred that the carbon atoms and molecules generated by the C7F8 plasma enhance the magnitude of selectivity, accompanying the noticeable formation of C-CFx bonds shown in Fig. 4(c).

Considering the compatibility of C7F14 and C7F8 as alternatives for C4F8, C7F14 plasma in the SiO2 etch process is expected to show etching characteristics similar to those of C4F8 plasma. The C7F8 plasma process shows a lower etch rate than C4F8 and C7F14. However, C7F8 plasma achieves remarkable selectivity with a moderate etch rate by adjusting the amount of carbon produced through applying the appropriate O2 flow rate. C7F8 plasma processing is therefore appropriate for high-aspect-ratio etching processes.

This research was supported by a National Research Council of Science & Technology (NST) grant from the Korean government (MSIP) (No. CAP-17-02-NFRI), by the Industrial Strategic Technology Devel-opment Program-Next Generation Semiconductor R&D (20010412, Development of Metal Oxide Carbon Layer Strip Process and Com-mercial Equipment for EUV Mask) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea), by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the MOTIE of the Republic of Korea (No. 20172010105910), by the MOTIE (20009818, 20010420) and KSRC (Korea Semiconductor Research Consortium) support program for the development of future semiconductor de-vices, by a Korea Institute for Advancement of Technology (KIAT) grant funded by the Korean government (MOTIE) (P0008458, The Competency Development Program for Industry Specialist), and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2020R1A6A1A03047771).

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