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A study on the reaction between chlorine trifluoride gas and glass-like carbon
Applied Surface Science 240 (2005) 381–387 www.elsevier.com/locate/apsusc
A study on the reaction between chlorine trifluoride gas and glass-like carbon Yoji Saitoa,*, Takashi Nishizawab, Maki Hamaguchib a
Department of Electrical Engineering and Electronics, Seikei University, 3-3-1 Kichijoji-Kitamachi, Musashino-shi, Tokyo 180-8633, Japan b Materials Research Laboratory, Kobe Steel Ltd., 1-5-5 Takatsukadai, Nishi-ku, Kobe, Hyogo 651-2271, Japan Received in revised form 7 June 2004; accepted 8 July 2004 Available online 14 August 2004
Abstract The reaction between glass-like carbon (GC) and chlorine trifluoride (ClF3) gas was investigated with weight measurements, surface analysis, and gas desorption measurements, where the ClF3 gas is used for the in situ cleaning of tubes in silicon-related fabrication equipment. From Auger electron spectroscopy and X-ray photoelectron spectroscopy measurements, a carbon monofluoride, –(CF)n–, film near the surface of GC is considered to be grown onto the GC surface above 400 8C by the chemical reaction with ClF3, and this thickness of the fluoride film depends on the temperature. The grown fluoride film desorbs by annealing in a vacuum up to 600 8C. Although GC is apparently etched by ClF3 over 600 8C, the etch rate of GC is much lower than that of SiC and quartz. # 2004 Elsevier B.V. All rights reserved. PACS: 81.65.Cf; 81.65.Kn; 82.80.Pv Keywords: Glass-like carbon; Chlorine trifluoride; Photoelectron spectroscopy; Temperature-programmed desorption
1. Introduction Quartz and silicon carbide are widely used in semiconductor equipments, but their corrosion resistance is insufficient especially at high temperatures [1]. Glass-like carbon (GC), which is obtained by * Corresponding author. Tel.: +81 422 37 3725; fax: +81 422 37 3871. E-mail address: [email protected] (Y. Saito).
carbonizing thermosetting resin at high temperatures, is generally an excellent corrosion-resistant and heatresistant bulk material. GC is as hard as quartz and is a good thermal and electrical conductor [2]. The microscopic structure of GC is considered to be polycrystalline rather than amorphous [3]. Although GC is extremely light in weight with a specific gravity of 1.5–1.8 g/cm3, it has a dense structure and is practically impermeable to gas and liquid. GC can, therefore, be an alternative to quartz and silicon carbide for
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.07.006
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semiconductor industries. However, the chemical properties of GC are not well known especially at high temperatures under a corrosive atmosphere. It is known that graphite can be fluorinated by fluorine or fluorine-contained compounds and graphite intercalation compounds are often formed [4]. A chemical reaction between GC and fluoride can also occur but will be different from that for graphite, because the macroscopic crystal structure in GC is much different from those in graphite. GC includes layered structures microscopically, but the layers are greatly folded [5,6] and the arrangements are disordered. This is the first report for the chemical properties of GC against the ClF3. Chlorine trifluoride (ClF3) gas is a strong fluorinating agent and has been used for the in situ cleaning of quartz tubes without plasma in silicon-related chemical vapor deposition (CVD) equipments over the past ten years. Silicon, silicon nitride, tungsten are spontaneously etched by ClF3 near room temperature [7,8]. The ClF3 gas has a suitable vapor pressure (1.5 atm at room temperature) in comparison with very low vapor pressure of the other highly-reactive halogen compounds, for example XeF2 [9] and BrF3 [10], which are sometimes used for micromachining of silicon. The flow rate of ClF3 can be controlled by a conventional mass flow controller. The furnace tubes are usually cleaned by ClF3 at temperatures between 300 and 600 8C. GC should be, therefore, resistant against the ClF3 gas at high temperatures above 400 8C for practical use. In this study, the chemical reaction between GC and ClF3 gas was mainly investigated with weight measurements, surface analysis, and gas-desorption measurements.
2. Experimental We used GC substrates with a thickness of 1.0 mm, which are manufactured by Kobe Steel Ltd. and Unitika Corp. The GC substrates were made by carbonizing phenol resin plates in inert atmosphere at 1600 8C, and purified by treatment in chlorine at 2100 8C. The GC substrates were then mirror-polished by fine alumina powder. We also used amorphous SiC substrates with a thickness of 0.6 mm, which were deposited by the CVD technique and were purchased
Fig. 1. Schematic diagram of a reaction chamber and the related apparatus.
from Hitachi Zosen Ltd. All substrates were cut into 10 10 mm2 pieces before the following treatments. The apparatus for the ClF3 treatments is schematically shown in Fig. 1. The substrates were mounted onto an electrically resistive heater plate, which was also made of GC. The base pressure of the reaction chamber was about 10 3 Torr. ClF3 gas with a purity of 99.9% and Ar gas were introduced into the reaction chamber at flow rates of 20 standard cubic cm per min (sccm) and 10 sccm, respectively. The substrate temperature was controlled by the heater current between room temperature and 650 8C, where the substrate temperature was measured with a pyrometer. The total pressure was maintained at 1.5 Torr by evacuating the chamber with a multi-stage dry pump. The partial pressure of ClF3 was 1.0 Torr. After the ClF3 treatment, the weight of the GC and the SiC substrates was measured and compared to that before the treatment. The surface reaction products were investigated with X ray photoelectron spectroscopy (XPS) measurements. We used a PHI ESCA spectrometer with a double-pass cylindrical mirror analyzer (CMA), Model 15-255GAR, and a Mg anode (1253.6 eV) as a X-ray source. The depth profiles of fluorine were investigated by Auger electron spectroscopy (AES), using a PHI Model 10–155 CMA module. Tempera-
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ture-programmed desorption (TPD) measurements were also carried out for the ClF3-treated GC substrates The TPD system was assembled by ourselves using a mass spectrometer, ULVAC Model QMS-400. The surface roughness of the GC substrates was investigated by the atomic force microscopy (AFM) observation, using a Model Nanoscope E by Digital Instrument Co.
3. Results and Discussion The weight measurements of GC and SiC substrates were carried out before and after the ClF3 treatments. The dependence of the weight change of GC on the temperature is shown as circle plots in Fig. 2, where the ClF3-treatment time was 30 min. The weight change (%) is defined as (the weight after the ClF3-treatment the initial weight)/(the initial weight). The weight of GC is slightly increased by the ClF3 treatment between 400 and 600 8C and substantially decreased above 650 8C. The error bars for the plots above 600 8C mean non-uniform temperature distribution in the sample induced by damage to the GC heater in the ClF3 atmosphere. The increase in weight implies the possibility of the adsorption and the diffusion of fluorine near the surface of GC. The decrease of the weight means the apparent etching phenomenon. GC is estimated to be etched above 550 8C, considering the sign of the slope of the tangent in the temperature dependence of the weight change as shown in Fig. 2. The measurement error of the weight
Fig. 2. Weight change of GC and SiC (square plots) as a function of process temperature.
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change is within 0.05%. On the other hand, the weight of SiC is decreased above 400 8C. SiC is apparently etched above 400 8C from this figure. SiO2 (quartz) is slowly etched by ClF3 above room temperature [1]. The etch rate of thermally grown oxide is 10 nm/min at 400 8C in ClF3 with a partial pressure of 1 Torr. The activation energy of the SiO2 etch rate is about 0.12 eV below 400 8C [1]. GC has, therefore, much better corrosion resistance than SiC and SiO2 at high temperatures. The time dependence of the GC weight change induced by the treatment at 500 8C was investigated. The weight increase almost saturates within 10 min. This implies that the fluorinated layers would grow rapidly in the initial stage of the treatment, and the grown films would prevent the subsequent reaction. Surface analysis was performed to clarify the cause of the weight increase of GC induced by the ClF3 treatment around 500 8C, as indicated in Fig. 2. We found only fluorine and carbon elements with AES measurements on the surface of the ClF3 treated GC. In the analogy of silicon etching by ClF3 [8], the active species will be atomic fluorine from ClF3 molecules. The depth profiles of the incorporated fluorine were investigated by AES measurements using an argon ion sputtering technique for each GC substrate treated with ClF3 for 10 min. The etch rate by the sputtering was roughly estimated to be 0.1 nm/s for the bulk. Fig. 3 shows the ratio of the AES signal intensity
Fig. 3. Depth profiles of the ratio of fluorine with respect to carbon near the surfaces of the GC, which are treated in ClF3 for 10 min at various temperatures, obtained from the Auger electron spectroscopy measurements with Ar ion sputtering technique.
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derived from fluorine with respect to that from carbon, F/C, as a function of sputtering time. It is found that the fluorine is confined to within 5 nm from the surface of the GC treated below 300 8C. At 400 8C, the thickness of the fluorinated layer and the density of the incorporated fluorine increase. The thickness of the fluorinated layer remarkably increases at 500 8C. We consider that the incorporated fluorine would cause the weight increase of GC around 500 8C as indicated in Fig. 2. However, if the sputter rate in the AES analysis is as large as the bulk and the density ratio of the fluorinated layer with respect to the bulk is supposed to be about 2, the estimated weight change will be only 0.01% for one surface at 500 8C. The surfaces of the side walls of the GC substrate are very rough and can form the fluorinated layer with a large area. Unfortunately, it is difficult to explain the cause of weight increase quantitatively at this time. XPS measurements were performed to clarify the chemical structure in the fluorinated region. We found the XPS signals derived from C and F atoms in the survey spectra. No signal derived from chlorine was found in the XPS spectra for all samples, consistent with the AES data. Fig. 4 shows the photoelectron spectra of a C1s core level from the surfaces of the GC,
Fig. 4. X-ray photoelectron spectra of a C1s core level from the surfaces of the GC substrates, which are treated in ClF3 for 10 min at various temperatures and as-received. Note that the binding energy of the spectrum from GC treated at 500 8C is not corrected (see the text).
which were treated in ClF3 at various temperatures. The spectra, which are obtained from the surface of the as-received GC, and the GC treated below 300 8C, consist of mainly the peak derived from C–C bonds. A peak ascribed to C–F bonds appeared at the binding energy of 289 eV in the spectrum after the ClF3 treatments at 400 8C. This indicates that a carbon mono-fluoride layer is formed onto the GC surfaces by the treatment above 400 8C. The GC surface may be slightly fluorinated even at 300 8C. The thickness of the fluorinated region, which contains covalent C–F bonds, formed at 400 8C, is estimated to be a few atomic layers because the C–C bulk is observed in the spectrum. This interpretation is consistent with the depth profiles of fluorine as shown in Fig. 3. At 500 8C, only a peak at the binding energy around 292 eV is found in Fig. 4. The peak derived from C–C bonds is not found in the spectrum at 500 8C, because the photoelectrons cannot escape from the bulk through the fluoride film with a thickness of 50 nm as estimated from Fig. 3. The origin of this peak will be discussed later, because the spectra can be shifted by the surface charge. Nakajima et al. reported the XPS data, which were obtained from the fluorine-intercalated graphite [11]. The reported spectra, however, are much different from ours because of the different crystal structure and the temperature from ours. Fluorine intercalation into graphite can be carried out below room temperature and occurs throughout the bulk. On the other hand, fluorine atoms hardly diffuse into GC below 300 8C, and gradually diffuse and react with GC at high temperatures above 400 8C. The fluorinated region in GC is formed only near surfaces. Fig. 5 indicates the photoelectron spectra of an F1s core level obtained from the surfaces of the GC, treated in ClF3 at various temperatures. Although the fluorine-related components are not found in the C1s spectra below 300 8C in Fig. 4, the fluorine peaks are found in all spectra above 100 8C in Fig. 5. In the spectrum obtained from the GC, treated at 100 8C, the main component is located at a binding energy of 687 eV, which is derived from the adsorbed hydrogen fluoride molecules. [12] The hydrogen fluoride is considered to be generated by the chemical reaction between the adsorbed fluorine and the water vapor, which exists in air, when the substrate was taken out from the reaction chamber. The component at a bind-
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Fig. 5. X-ray photoelectron spectra of a F1s core level from the surfaces of the GC substrates, which are treated in ClF3 for 10 min at various temperatures. Note that the binding energy of the spectrum from GC treated at 500 8C is not corrected.
ing energy of 688 eV was found in the spectra obtained from the GC, treated at between 200 and 400 8C as shown in Fig. 5. The previous report [11] supposes that the fluorine, semi-covalently bound to the carbon atoms, would cause a binding energy of 688 eV. Nevertheless, the peak at 688 eV, which is found from the GC treated at 400 8C, should include the large component derived from covalent F–C bonds [13] also, considering the C–F bonds appeared at 400 8C in Fig. 4. On the other hand, the predominant peak at a binding energy of 691 eV was found in the spectra obtained from the GC, treated at 500 8C as shown in Fig. 5. This peak is also estimated to be derived from covalent F–C bonds. It is found that the peak shift at 500 8C with respect to the peak at 400 8C is 3 eV as shown in Fig. 5. Incidentally, the peak shift in the C1s spectrum between the fluoride at 500 8C and that at 400 8C is also 3 eV as shown in Fig. 4, and is as large as that of the F1s spectra. The spectra of C1s and F1s at 500 8C are likely shifted by the surface charge with respect to those between 100 and 400 8C. The surface charge may be induced during the XPS measurements because of the insulating fluoride film. We carried out, moreover, another XPS measurement on the GC with a thin fluoride film to avoid the surface charge up. Such an additional peak shift with respect to that of C–F
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bonds is hardly observed in the C1s and F1s spectra from the GC treated at 500 8C for 3 min (not shown). Thus, the peaks, which appeared in the C1s and F1s spectra at 500 8C in Figs. 4 and 5, are estimated to be ascribed to the carbon mono-fluoride. The TPD analysis was performed for the fluorinated GC samples, where the temperature-rise rate was about 3 8C/s. The ion currents corresponding to CF+, CF2+, and CF3+ ions were mainly detected, where the ionization energy was 50 eV. The fragment patterns obtained from the TPD measurements around 500 8C were different from those from the pure CF4 gas, in which the signal ratio of CF2+ ions with respect to CF+ ions is as large as about 3 in our system. The ion current corresponding to the CF+ ions was comparable to that of CF2+ ions during the TPD measurements. Most of the desorbed molecules, therefore, are considered to be CF2 and CF3 molecules. Sato et al. considered that the main desorbed molecules would be CF2 molecules from the fluorine intercalated compounds near 500 8C [14]. The etching phenomena of GC by the ClF3 above 600 8C as shown in Fig. 2 is probably induced by mainly the desorption of CF2 and CF3 molecules. Further experiments are needed to determine the main desorbed products. Fig. 6 shows the TPD spectra from the GC samples treated at 400 8C (a), and 500 8C (b), for 10 min. A large and relatively-sharp peak around 500 8C is found in the spectra of the CF+, CF2+, and CF3+ ions for the GC treated at 500 8C as shown in Fig. 6 (b). These peaks are due to the decomposed products from the fluorinated carbon on the GC substrates, taking the XPS data as shown in Fig. 4 into consideration. The decomposition of the fluorides are almost terminated around 550 8C in high-vacuum. After annealing at 600 8C in either high vacuum or Ar ambient for 10 min, the residual fluorine is hardly detected on the surface of the GC with the XPS measurements. Relatively small and broad peaks are found in the spectrum from the GC fluorinated at 400 8C in Fig. 6 (a). This result is consistent with the corresponding AES data as shown in Fig. 3. It should be noted that the peak area in the XPS spectra does not reflect the total amount of the incorporated fluorine when the escape depth of the detected photoelectrons, only a few nm from the surfaces, is less than the thickness of the fluorinated region.
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Fig. 7. Root mean square values of the roughness of the GC surfaces after the ClF3 treatment at 400 8C for 10 min and the subsequent annealing in Ar at 600 8C for 20 min. Process step: (1) as received GC; (2) after the first ClF3 treatment; (3) after the first subsequent annealing in Ar; (4) after the second ClF3 treatment; (5) after the second annealing in Ar, after the third ClF3 treatment. Fig. 6. TPD spectra of CF+ (m/e = 31), CF2+ (m/e = 50), and CF3+ (m/e = 69) ions from GC treated in ClF3: (a) at 400 8C and (b) at 500 8C for 10 min.
We feel that the broad peak in Fig. 6 (a) would include three components, the center of which locates at about 330, 450, and 600 8C. The main component of the peak, located at around 450–500 8C, may be due to the decomposition of the fluorides. The component at low temperature may reflect the other adsorption states, but has not well been clarified at this time. The component around 600 8C is considered to be derived from the fluorinated carbon which is formed during the TPD measurement. The extra fluorine, which is not covalently bound to carbon and has been confined near the surface of GC during the ClF3 treatment at 400 8C and is assumed in Fig. 5, can react with GC at the high temperatures. This component is located at the temperature higher than that in Fig. 6 (b), probably because the process time is required to form the additional fluorinated carbon by the extra fluorine. Finally, the surface morphology of the GC substrates was investigated by the AFM observation. Only random roughness is observed in all AFM images. Fig. 7 shows the variation of the root mean square values of the roughness induced by the ClF3 treatment and the subsequent anneal in Ar. The ClF3
treatment was carried out at 400 8C for 10 min and the annealing was carried out at 600 8C for 20 min in Ar with the partial pressure of 0.5 Torr. The roughness greatly increases after the first ClF3 treatment, probably because of the grown fluorides onto the surface of GC. The apparent interference colors are found in macroscopic observation after the ClF3 treatment above 500 8C. By the subsequent annealing at 600 8C, the roughness decreases and the interference colors diminishes. During the annealing, fluorine desorbs from the GC surface as mentioned above. The removal of the fluorinated carbon, therefore, causes the decrease of the surface roughness. The similar behavior of the roughness is observed also in the following ClF3 treatment and annealing process as shown in Fig. 7. The increase of the surface roughness is suppressed by this recovery phenomenon.
4. Conclusions The chemical reaction between GC and ClF3 was investigated with weight measurements, AES measurements, XPS measurements, TPD measurements, and AFM measurements. Fluorine is adsorbed near the GC surface above 100 8C. Below 400 8C, molecular fluorine is considered to be adsorbed near the GC surfaces. A mono-fluorinated carbon, –(CF)n–, film is
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estimated to be grown onto the GC surfaces treated above 400 8C and its thickness strongly depends on the temperature. The grown carbon fluoride desorbs as mainly CF2 and CF3 molecules by the subsequent annealing up to 600 8C in a vacuum. GC is apparently etched by ClF3 over 600 8C and may be etched slowly near 550 8C. GC has much better corrosion resistance than SiC and quartz at high temperatures. The etched depth and the increase of the surface roughness can be very small for the practical condition. GC will be a peculiar and useful material, which has inactive chemical properties, in the application to corrosive ambient at high temperatures.
Acknowledgements The authors would like to thank T. Momma, H. Takaoka, J. Omokawa, and K. Tana of Seikei University for their technical assistance. The authors are grateful to Central Glass Co. Ltd. for providing ClF3 gas.
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References [1] Y. Saito, Sens. Mater. 14 (2002) 231. [2] M. Kuwabara, M. Sato, M.R. Visokay, H. Hayashi, H. Inoue, K. Muramatsu, J. Appl. Phys. 73 (1993) 6686. [3] M. Shiraishi, Tanso Zairyo Nyumon, Kagaku-Gijutsu-sha, 1984, p. 33 (in Japanese). [4] W. Rudorff, G. Rudorff, Chem. Ber. 80 (1947) 417. [5] F. Rousseaux, D. Tchoubar, Carbon 15 (1977) 63. [6] P. Lamidieu, M. Monthioux, A. Oberlin, Carbon 29 (1991) 239. [7] D.E. Ibottoson, J.A. Mucha, Flamm, J.M. Cook, J. Appl. Phys. 56 (1984) 2939. [8] Y. Saito, O. Yamaoka, A. Yoshida, J. Vac. Sci. Tech. B9 (1991) 2503. [9] J.J. Choi, K. Minami, M. Esashi, Trans. IEE Japan 118 (1998) 437, (in Japanese). [10] X.Q. Wang, X. Yang, K. Walsh, Y.C. Tai, in: The Eight International Conference on Solid-State Sensors and Actuators, Chicago, USA, 1997, p. 1505. [11] T. Nakajima, Y. Matsuo, B. Zemva, A. Jesih, Carbon 34 (1996) 1595. [12] Y. Saito, Surf. Sci. Spectra. 6 (1999) 228. [13] A. Tressaud, M. Chanbon, V. Gupta, S. Flandrois, O.P. Bahl, Carbon 35 (1995) 1339. [14] Y. Sato, R. Hagiwara, Y. Ito, J. Fluorine Chem. 110 (2001) 31.
A study on the reaction between chlorine trifluoride gas and glass-like carbon Yoji Saitoa,*, Takashi Nishizawab, Maki Hamaguchib a
Department of Electrical Engineering and Electronics, Seikei University, 3-3-1 Kichijoji-Kitamachi, Musashino-shi, Tokyo 180-8633, Japan b Materials Research Laboratory, Kobe Steel Ltd., 1-5-5 Takatsukadai, Nishi-ku, Kobe, Hyogo 651-2271, Japan Received in revised form 7 June 2004; accepted 8 July 2004 Available online 14 August 2004
Abstract The reaction between glass-like carbon (GC) and chlorine trifluoride (ClF3) gas was investigated with weight measurements, surface analysis, and gas desorption measurements, where the ClF3 gas is used for the in situ cleaning of tubes in silicon-related fabrication equipment. From Auger electron spectroscopy and X-ray photoelectron spectroscopy measurements, a carbon monofluoride, –(CF)n–, film near the surface of GC is considered to be grown onto the GC surface above 400 8C by the chemical reaction with ClF3, and this thickness of the fluoride film depends on the temperature. The grown fluoride film desorbs by annealing in a vacuum up to 600 8C. Although GC is apparently etched by ClF3 over 600 8C, the etch rate of GC is much lower than that of SiC and quartz. # 2004 Elsevier B.V. All rights reserved. PACS: 81.65.Cf; 81.65.Kn; 82.80.Pv Keywords: Glass-like carbon; Chlorine trifluoride; Photoelectron spectroscopy; Temperature-programmed desorption
1. Introduction Quartz and silicon carbide are widely used in semiconductor equipments, but their corrosion resistance is insufficient especially at high temperatures [1]. Glass-like carbon (GC), which is obtained by * Corresponding author. Tel.: +81 422 37 3725; fax: +81 422 37 3871. E-mail address: [email protected] (Y. Saito).
carbonizing thermosetting resin at high temperatures, is generally an excellent corrosion-resistant and heatresistant bulk material. GC is as hard as quartz and is a good thermal and electrical conductor [2]. The microscopic structure of GC is considered to be polycrystalline rather than amorphous [3]. Although GC is extremely light in weight with a specific gravity of 1.5–1.8 g/cm3, it has a dense structure and is practically impermeable to gas and liquid. GC can, therefore, be an alternative to quartz and silicon carbide for
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.07.006
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semiconductor industries. However, the chemical properties of GC are not well known especially at high temperatures under a corrosive atmosphere. It is known that graphite can be fluorinated by fluorine or fluorine-contained compounds and graphite intercalation compounds are often formed [4]. A chemical reaction between GC and fluoride can also occur but will be different from that for graphite, because the macroscopic crystal structure in GC is much different from those in graphite. GC includes layered structures microscopically, but the layers are greatly folded [5,6] and the arrangements are disordered. This is the first report for the chemical properties of GC against the ClF3. Chlorine trifluoride (ClF3) gas is a strong fluorinating agent and has been used for the in situ cleaning of quartz tubes without plasma in silicon-related chemical vapor deposition (CVD) equipments over the past ten years. Silicon, silicon nitride, tungsten are spontaneously etched by ClF3 near room temperature [7,8]. The ClF3 gas has a suitable vapor pressure (1.5 atm at room temperature) in comparison with very low vapor pressure of the other highly-reactive halogen compounds, for example XeF2 [9] and BrF3 [10], which are sometimes used for micromachining of silicon. The flow rate of ClF3 can be controlled by a conventional mass flow controller. The furnace tubes are usually cleaned by ClF3 at temperatures between 300 and 600 8C. GC should be, therefore, resistant against the ClF3 gas at high temperatures above 400 8C for practical use. In this study, the chemical reaction between GC and ClF3 gas was mainly investigated with weight measurements, surface analysis, and gas-desorption measurements.
2. Experimental We used GC substrates with a thickness of 1.0 mm, which are manufactured by Kobe Steel Ltd. and Unitika Corp. The GC substrates were made by carbonizing phenol resin plates in inert atmosphere at 1600 8C, and purified by treatment in chlorine at 2100 8C. The GC substrates were then mirror-polished by fine alumina powder. We also used amorphous SiC substrates with a thickness of 0.6 mm, which were deposited by the CVD technique and were purchased
Fig. 1. Schematic diagram of a reaction chamber and the related apparatus.
from Hitachi Zosen Ltd. All substrates were cut into 10 10 mm2 pieces before the following treatments. The apparatus for the ClF3 treatments is schematically shown in Fig. 1. The substrates were mounted onto an electrically resistive heater plate, which was also made of GC. The base pressure of the reaction chamber was about 10 3 Torr. ClF3 gas with a purity of 99.9% and Ar gas were introduced into the reaction chamber at flow rates of 20 standard cubic cm per min (sccm) and 10 sccm, respectively. The substrate temperature was controlled by the heater current between room temperature and 650 8C, where the substrate temperature was measured with a pyrometer. The total pressure was maintained at 1.5 Torr by evacuating the chamber with a multi-stage dry pump. The partial pressure of ClF3 was 1.0 Torr. After the ClF3 treatment, the weight of the GC and the SiC substrates was measured and compared to that before the treatment. The surface reaction products were investigated with X ray photoelectron spectroscopy (XPS) measurements. We used a PHI ESCA spectrometer with a double-pass cylindrical mirror analyzer (CMA), Model 15-255GAR, and a Mg anode (1253.6 eV) as a X-ray source. The depth profiles of fluorine were investigated by Auger electron spectroscopy (AES), using a PHI Model 10–155 CMA module. Tempera-
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ture-programmed desorption (TPD) measurements were also carried out for the ClF3-treated GC substrates The TPD system was assembled by ourselves using a mass spectrometer, ULVAC Model QMS-400. The surface roughness of the GC substrates was investigated by the atomic force microscopy (AFM) observation, using a Model Nanoscope E by Digital Instrument Co.
3. Results and Discussion The weight measurements of GC and SiC substrates were carried out before and after the ClF3 treatments. The dependence of the weight change of GC on the temperature is shown as circle plots in Fig. 2, where the ClF3-treatment time was 30 min. The weight change (%) is defined as (the weight after the ClF3-treatment the initial weight)/(the initial weight). The weight of GC is slightly increased by the ClF3 treatment between 400 and 600 8C and substantially decreased above 650 8C. The error bars for the plots above 600 8C mean non-uniform temperature distribution in the sample induced by damage to the GC heater in the ClF3 atmosphere. The increase in weight implies the possibility of the adsorption and the diffusion of fluorine near the surface of GC. The decrease of the weight means the apparent etching phenomenon. GC is estimated to be etched above 550 8C, considering the sign of the slope of the tangent in the temperature dependence of the weight change as shown in Fig. 2. The measurement error of the weight
Fig. 2. Weight change of GC and SiC (square plots) as a function of process temperature.
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change is within 0.05%. On the other hand, the weight of SiC is decreased above 400 8C. SiC is apparently etched above 400 8C from this figure. SiO2 (quartz) is slowly etched by ClF3 above room temperature [1]. The etch rate of thermally grown oxide is 10 nm/min at 400 8C in ClF3 with a partial pressure of 1 Torr. The activation energy of the SiO2 etch rate is about 0.12 eV below 400 8C [1]. GC has, therefore, much better corrosion resistance than SiC and SiO2 at high temperatures. The time dependence of the GC weight change induced by the treatment at 500 8C was investigated. The weight increase almost saturates within 10 min. This implies that the fluorinated layers would grow rapidly in the initial stage of the treatment, and the grown films would prevent the subsequent reaction. Surface analysis was performed to clarify the cause of the weight increase of GC induced by the ClF3 treatment around 500 8C, as indicated in Fig. 2. We found only fluorine and carbon elements with AES measurements on the surface of the ClF3 treated GC. In the analogy of silicon etching by ClF3 [8], the active species will be atomic fluorine from ClF3 molecules. The depth profiles of the incorporated fluorine were investigated by AES measurements using an argon ion sputtering technique for each GC substrate treated with ClF3 for 10 min. The etch rate by the sputtering was roughly estimated to be 0.1 nm/s for the bulk. Fig. 3 shows the ratio of the AES signal intensity
Fig. 3. Depth profiles of the ratio of fluorine with respect to carbon near the surfaces of the GC, which are treated in ClF3 for 10 min at various temperatures, obtained from the Auger electron spectroscopy measurements with Ar ion sputtering technique.
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derived from fluorine with respect to that from carbon, F/C, as a function of sputtering time. It is found that the fluorine is confined to within 5 nm from the surface of the GC treated below 300 8C. At 400 8C, the thickness of the fluorinated layer and the density of the incorporated fluorine increase. The thickness of the fluorinated layer remarkably increases at 500 8C. We consider that the incorporated fluorine would cause the weight increase of GC around 500 8C as indicated in Fig. 2. However, if the sputter rate in the AES analysis is as large as the bulk and the density ratio of the fluorinated layer with respect to the bulk is supposed to be about 2, the estimated weight change will be only 0.01% for one surface at 500 8C. The surfaces of the side walls of the GC substrate are very rough and can form the fluorinated layer with a large area. Unfortunately, it is difficult to explain the cause of weight increase quantitatively at this time. XPS measurements were performed to clarify the chemical structure in the fluorinated region. We found the XPS signals derived from C and F atoms in the survey spectra. No signal derived from chlorine was found in the XPS spectra for all samples, consistent with the AES data. Fig. 4 shows the photoelectron spectra of a C1s core level from the surfaces of the GC,
Fig. 4. X-ray photoelectron spectra of a C1s core level from the surfaces of the GC substrates, which are treated in ClF3 for 10 min at various temperatures and as-received. Note that the binding energy of the spectrum from GC treated at 500 8C is not corrected (see the text).
which were treated in ClF3 at various temperatures. The spectra, which are obtained from the surface of the as-received GC, and the GC treated below 300 8C, consist of mainly the peak derived from C–C bonds. A peak ascribed to C–F bonds appeared at the binding energy of 289 eV in the spectrum after the ClF3 treatments at 400 8C. This indicates that a carbon mono-fluoride layer is formed onto the GC surfaces by the treatment above 400 8C. The GC surface may be slightly fluorinated even at 300 8C. The thickness of the fluorinated region, which contains covalent C–F bonds, formed at 400 8C, is estimated to be a few atomic layers because the C–C bulk is observed in the spectrum. This interpretation is consistent with the depth profiles of fluorine as shown in Fig. 3. At 500 8C, only a peak at the binding energy around 292 eV is found in Fig. 4. The peak derived from C–C bonds is not found in the spectrum at 500 8C, because the photoelectrons cannot escape from the bulk through the fluoride film with a thickness of 50 nm as estimated from Fig. 3. The origin of this peak will be discussed later, because the spectra can be shifted by the surface charge. Nakajima et al. reported the XPS data, which were obtained from the fluorine-intercalated graphite [11]. The reported spectra, however, are much different from ours because of the different crystal structure and the temperature from ours. Fluorine intercalation into graphite can be carried out below room temperature and occurs throughout the bulk. On the other hand, fluorine atoms hardly diffuse into GC below 300 8C, and gradually diffuse and react with GC at high temperatures above 400 8C. The fluorinated region in GC is formed only near surfaces. Fig. 5 indicates the photoelectron spectra of an F1s core level obtained from the surfaces of the GC, treated in ClF3 at various temperatures. Although the fluorine-related components are not found in the C1s spectra below 300 8C in Fig. 4, the fluorine peaks are found in all spectra above 100 8C in Fig. 5. In the spectrum obtained from the GC, treated at 100 8C, the main component is located at a binding energy of 687 eV, which is derived from the adsorbed hydrogen fluoride molecules. [12] The hydrogen fluoride is considered to be generated by the chemical reaction between the adsorbed fluorine and the water vapor, which exists in air, when the substrate was taken out from the reaction chamber. The component at a bind-
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Fig. 5. X-ray photoelectron spectra of a F1s core level from the surfaces of the GC substrates, which are treated in ClF3 for 10 min at various temperatures. Note that the binding energy of the spectrum from GC treated at 500 8C is not corrected.
ing energy of 688 eV was found in the spectra obtained from the GC, treated at between 200 and 400 8C as shown in Fig. 5. The previous report [11] supposes that the fluorine, semi-covalently bound to the carbon atoms, would cause a binding energy of 688 eV. Nevertheless, the peak at 688 eV, which is found from the GC treated at 400 8C, should include the large component derived from covalent F–C bonds [13] also, considering the C–F bonds appeared at 400 8C in Fig. 4. On the other hand, the predominant peak at a binding energy of 691 eV was found in the spectra obtained from the GC, treated at 500 8C as shown in Fig. 5. This peak is also estimated to be derived from covalent F–C bonds. It is found that the peak shift at 500 8C with respect to the peak at 400 8C is 3 eV as shown in Fig. 5. Incidentally, the peak shift in the C1s spectrum between the fluoride at 500 8C and that at 400 8C is also 3 eV as shown in Fig. 4, and is as large as that of the F1s spectra. The spectra of C1s and F1s at 500 8C are likely shifted by the surface charge with respect to those between 100 and 400 8C. The surface charge may be induced during the XPS measurements because of the insulating fluoride film. We carried out, moreover, another XPS measurement on the GC with a thin fluoride film to avoid the surface charge up. Such an additional peak shift with respect to that of C–F
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bonds is hardly observed in the C1s and F1s spectra from the GC treated at 500 8C for 3 min (not shown). Thus, the peaks, which appeared in the C1s and F1s spectra at 500 8C in Figs. 4 and 5, are estimated to be ascribed to the carbon mono-fluoride. The TPD analysis was performed for the fluorinated GC samples, where the temperature-rise rate was about 3 8C/s. The ion currents corresponding to CF+, CF2+, and CF3+ ions were mainly detected, where the ionization energy was 50 eV. The fragment patterns obtained from the TPD measurements around 500 8C were different from those from the pure CF4 gas, in which the signal ratio of CF2+ ions with respect to CF+ ions is as large as about 3 in our system. The ion current corresponding to the CF+ ions was comparable to that of CF2+ ions during the TPD measurements. Most of the desorbed molecules, therefore, are considered to be CF2 and CF3 molecules. Sato et al. considered that the main desorbed molecules would be CF2 molecules from the fluorine intercalated compounds near 500 8C [14]. The etching phenomena of GC by the ClF3 above 600 8C as shown in Fig. 2 is probably induced by mainly the desorption of CF2 and CF3 molecules. Further experiments are needed to determine the main desorbed products. Fig. 6 shows the TPD spectra from the GC samples treated at 400 8C (a), and 500 8C (b), for 10 min. A large and relatively-sharp peak around 500 8C is found in the spectra of the CF+, CF2+, and CF3+ ions for the GC treated at 500 8C as shown in Fig. 6 (b). These peaks are due to the decomposed products from the fluorinated carbon on the GC substrates, taking the XPS data as shown in Fig. 4 into consideration. The decomposition of the fluorides are almost terminated around 550 8C in high-vacuum. After annealing at 600 8C in either high vacuum or Ar ambient for 10 min, the residual fluorine is hardly detected on the surface of the GC with the XPS measurements. Relatively small and broad peaks are found in the spectrum from the GC fluorinated at 400 8C in Fig. 6 (a). This result is consistent with the corresponding AES data as shown in Fig. 3. It should be noted that the peak area in the XPS spectra does not reflect the total amount of the incorporated fluorine when the escape depth of the detected photoelectrons, only a few nm from the surfaces, is less than the thickness of the fluorinated region.
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Fig. 7. Root mean square values of the roughness of the GC surfaces after the ClF3 treatment at 400 8C for 10 min and the subsequent annealing in Ar at 600 8C for 20 min. Process step: (1) as received GC; (2) after the first ClF3 treatment; (3) after the first subsequent annealing in Ar; (4) after the second ClF3 treatment; (5) after the second annealing in Ar, after the third ClF3 treatment. Fig. 6. TPD spectra of CF+ (m/e = 31), CF2+ (m/e = 50), and CF3+ (m/e = 69) ions from GC treated in ClF3: (a) at 400 8C and (b) at 500 8C for 10 min.
We feel that the broad peak in Fig. 6 (a) would include three components, the center of which locates at about 330, 450, and 600 8C. The main component of the peak, located at around 450–500 8C, may be due to the decomposition of the fluorides. The component at low temperature may reflect the other adsorption states, but has not well been clarified at this time. The component around 600 8C is considered to be derived from the fluorinated carbon which is formed during the TPD measurement. The extra fluorine, which is not covalently bound to carbon and has been confined near the surface of GC during the ClF3 treatment at 400 8C and is assumed in Fig. 5, can react with GC at the high temperatures. This component is located at the temperature higher than that in Fig. 6 (b), probably because the process time is required to form the additional fluorinated carbon by the extra fluorine. Finally, the surface morphology of the GC substrates was investigated by the AFM observation. Only random roughness is observed in all AFM images. Fig. 7 shows the variation of the root mean square values of the roughness induced by the ClF3 treatment and the subsequent anneal in Ar. The ClF3
treatment was carried out at 400 8C for 10 min and the annealing was carried out at 600 8C for 20 min in Ar with the partial pressure of 0.5 Torr. The roughness greatly increases after the first ClF3 treatment, probably because of the grown fluorides onto the surface of GC. The apparent interference colors are found in macroscopic observation after the ClF3 treatment above 500 8C. By the subsequent annealing at 600 8C, the roughness decreases and the interference colors diminishes. During the annealing, fluorine desorbs from the GC surface as mentioned above. The removal of the fluorinated carbon, therefore, causes the decrease of the surface roughness. The similar behavior of the roughness is observed also in the following ClF3 treatment and annealing process as shown in Fig. 7. The increase of the surface roughness is suppressed by this recovery phenomenon.
4. Conclusions The chemical reaction between GC and ClF3 was investigated with weight measurements, AES measurements, XPS measurements, TPD measurements, and AFM measurements. Fluorine is adsorbed near the GC surface above 100 8C. Below 400 8C, molecular fluorine is considered to be adsorbed near the GC surfaces. A mono-fluorinated carbon, –(CF)n–, film is
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estimated to be grown onto the GC surfaces treated above 400 8C and its thickness strongly depends on the temperature. The grown carbon fluoride desorbs as mainly CF2 and CF3 molecules by the subsequent annealing up to 600 8C in a vacuum. GC is apparently etched by ClF3 over 600 8C and may be etched slowly near 550 8C. GC has much better corrosion resistance than SiC and quartz at high temperatures. The etched depth and the increase of the surface roughness can be very small for the practical condition. GC will be a peculiar and useful material, which has inactive chemical properties, in the application to corrosive ambient at high temperatures.
Acknowledgements The authors would like to thank T. Momma, H. Takaoka, J. Omokawa, and K. Tana of Seikei University for their technical assistance. The authors are grateful to Central Glass Co. Ltd. for providing ClF3 gas.
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