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Thermal decomposition and structural variation by heating on hydrogenated amorphous carbon films
Diamond & Related Materials 101 (2020) 107609
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Thermal decomposition and structural variation by heating on hydrogenated amorphous carbon films
T
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Hiroki Akasakaa, , Sarayut Tunmeeb, Ukit Rittihongb, Masashi Tomidokoroa, Chanan Euaruksakulb, Souta Norizukia, Ratchadaporn Supruangnetb, Hideki Nakajimab, Yuki Hirataa, Naoto Ohtakea a b
Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Nakhon Ratchasima 30000, Thailand
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogenated amorphous carbon film Thermal decomposition Near edge X-ray absorption fine structure X-ray photoemission electron microscopy Uniformity of carbon bonding
Hydrogenated amorphous carbon (a-C:H) films are only empirically known to be decomposed by heating, and this heat resistance is expected to be dependent on the structure of the film. To understand the decomposition processes and their uniformity, the thermal decomposition characteristics of two types of a-C:H film were investigated after the structural determination of the sp2/(sp2 + sp3) ratios and hydrogen contents. The mass of the gas released by thermal decomposition of the a-C:H films was detected by evaluation of the thermal desorption spectroscopy to determine the starting temperature the film with high hydrogen content started to desorb hydrogens at 400 °C and hydrocarbons at 750 °C. The film with low hydrogens started to desorb decomposition gases at 750 °C. Moreover, uniform decomposition of the films was observed using synchrotron-based X-ray photoemission electron microscopy (X-PEEM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The thermal decomposition of a-C:H films with uniform film structure proceeds relatively uniformly, and the mechanism of thermal decomposition of the films depends on the source materials of the films. The thermal decomposition proceeds uniformly at the micron scale when the initial structure is uniform.
1. Introduction A hydrogenated amorphous carbon film (a-C:H) is composed of sp2 and sp3 bonding carbon and hydrogen, and the characteristics of these films are dominated by factors such as the sp2/(sp2 + sp3) ratio and the hydrogen content. Thus, the structure is often defined by a ternary diagram proposed by Robertson [1,2]. Hydrogenated amorphous carbon films have been mainly applied to mechanical parts due to their excellent mechanical properties such as their high hardness and low friction coefficients [1,3]. In the automotive field, in particular, diamond-like carbon (DLC) film which is a hard a-C:H film, has been applied to sliding parts to reduce the fuel consumption by frictional loss [3]. Because of its hardness and low adhesion to metals, a-C:H also has been applied to the cutting-edges of cutting tools for aluminum [4]. In these applications, a-C:H is heated by friction and exposed to high temperatures in several cases. These a-C:H films are empirically known to be decomposed by heat, and this heat resistance is expected to be dependent on the structure of the film. Some reports have shown that the film undergoes heat
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decomposition at 400–800 °C [5,6]. Bourgoin et al. reported that the heat resistance of films was more than 400 °C [5]. Although these films are known to decompose and undergo thermal damage at 300–900 °C, the relationship between the starting temperature of decomposition and the a-C:H film structure is not clear. In addition, Akasaka et al. previously reported that the temperature at which decomposition starts differs only by the method of film formation [6]. Regarding the thermal stability of a-C:H films, Tallant et al. indicated that thermal desorption analysis detected the onset of hydrogen evolution from an a-C:H film deposited from methane carbon in a vacuum at 260 °C. Moreover, they reported that the conversion to nano-graphite occurred at 450–600 °C under air with atmospheric pressure [7]. These reports did not present the structure of the film, such as the sp2/(sp2 + sp3) bonding carbon ratio or the hydrogen content. Therefore, it is unknown which structure on Robertson's ternary diagram corresponds to these films. Few studies have investigated the relationship between the film structure and the decomposition temperature. In addition, these decomposition temperatures are known to be related to the frictional heating damage on the sliding part, which is the largest application area of the a-C:H film,
Corresponding author. E-mail address: [email protected] (H. Akasaka).
https://doi.org/10.1016/j.diamond.2019.107609 Received 30 July 2019; Received in revised form 24 October 2019; Accepted 2 November 2019 Available online 06 November 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
Diamond & Related Materials 101 (2020) 107609
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Although both the starting temperature of decomposition and the degree of uniformity of the decomposition process are significant in these applications, it is not clear whether heat decomposition of the aC:H film proceeds uniformly or locally. Several deposition methods have shown that structures are often not completely uniform during the deposition of a-C:H films. Thus, it is necessary to investigate the structural change due to heating for both uniform and non-uniform film structures. To understand the decomposition processes and their degree uniformity, we investigate the thermal decomposition of a-C:H films after structural determination of the sp2/(sp2 + sp3) carbon bonding ratio and the hydrogen contents. Moreover, the masses of gas released by thermal decomposition of the a-C:H films were detected to determine the starting temperature and the decomposition uniformity of a-C:H films. The local change in the distribution of the sp2/(sp2 + sp3) ratio in the film was observed using synchrotron-based spectromicroscopy technique, which is the combination of X-ray photoemission electron microscopy (X-PEEM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy.
Fig. 1. C K-edge NEXAFS spectra of the a-C:H films before heating.
and are very important. In addition, the a-C:H film is also used as a protective hard coating for adhesion resistance of cutting tools [8,9]. The heat resistance temperature of the film is an essential factor in this application of a-C:H films, and it is important to indicate which structure is suitable for such applications in industry.
2. Experimental Hydrogenated amorphous carbon films were prepared from methane (CH4) and acetylene (C2H2) via pulsed plasma chemical vapor deposition (CVD) [12]. A substrate of silicon (100) was placed on the
Fig. 2. Thermal desorption spectra of mass number, m/z = 1, 2, 12, 17, 18, and 26, 28, 32, 44 for the a-C:H film was deposited from hydrocarbons gases: (a) C2H2 gas and (b) CH4 gas. 2
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Fig. 3. X-PEEM images were captured at the photon energies of 284.6, 287.2, and 289.8 eV for a-C:H films as-deposition and after heating of 500, 600, 700, and 800 °C.
used to observe the X-PEEM images of the C K-edge of 275–330 eV at a resolution of the sub-micrometer resolution. Thus, the NEXAFS and XPEEM were used to detect the change in the distribution of the sp2/ (sp2 + sp3) carbon bonding ratio due to heating. Samples were heated to 800 °C by a tungsten heater located inside the X-PEEM system, and XPEEM images were observed at 100 °C increments. Heating significantly shortens the mean free path of the low energy photo-electrons generated by X-ray irradiation due to the desorption of the decomposition gas. Thus, it is hard to obtain X-PEEM images with in-situ observation at high temperature. The samples were heated to the target temperature, cooled to less than 200 °C, and then an X-PEEM image was obtained. The elemental contrast in the X-PEEM image was accomplished by tuning the photon energy through the absorption edges of the elements [17,18]. Because the photoelectron emission intensity is enhanced at the absorption edges, local areas on the surface with the matching element emit more photoelectrons and present bright in the X-PEEM image [19].
negative electrode in a vacuum chamber. Before the deposition of DLC, the natural oxidation layer on the silicon surface was removed by argon (Ar) plasma irradiation for 30 min. Ar gas was introduced at 20 cm3/ min, and the pressure was maintained at 3 Pa. The applied voltage was −2.4 kV at a frequency of 14.4 kHz. Then, hydrocarbon gas as the source material was introduced into the vacuum chamber. Hydrocarbon (CH4 or C2H2) gas was introduced at 15 cm3/min, and the pressure was maintained at 3 Pa. The applied voltage was maintained at −4.0 kV and at a frequency of 14.4 kHz. The deposition time of a-C:H films on Si substrate was set at 60 min. The thickness was approximately 1 μm. The film structure was determined using glow discharge optical emission spectrometry (GD-OES) and NEXAFS techniques. In GD-OES measurement, a calibration curve was created from the relationship between the hydrogen content and the optical emission intensity using the three types of a-C:H films whose hydrogen contents were previously known. These contents were measured by Rutherford back scattering (RBS) with elastic recoil detection analysis (ERDA). The hydrogen content of each sample was determined using this calibration curve. A beamline 3.2Ua & b (BL 3.2Ua & b) of Synchrotron Light Research Institute (SLRI) in Thailand was used for X-ray generation by synchrotron radiation for the NEXAFS measurements. Carbon K-edge NEXAFS spectra were obtained in the energy range of 275–330 eV at a resolution of 0.5 eV (FWHM) in the total electron yield mode [13,14]. The relative sp2 content was determined by comparison with the highly oriented pyrolytic (HOPG) spectrum [15,16]. The thermal desorption spectroscopy system composed of an infrared lamp, an ultrahigh vacuum chamber, and a quadrupole mass spectrometer (QMS: Qulee BGM-202, Ulvac Ltd.), was used to estimate the mass and desorption temperature of the decomposing gas by thermal decomposition of the a-C:H films. The samples with the size of 15 mm2 were kept at 100 °C for 12 h in a vacuum chamber at 10−5 Pa. Then, the temperature was increased at 15 °C/min until a temperature of 800 °C was reached. The mass of the desorbed molecules at each temperature increment was detected by QMS. To understand the initial condition of the a-C:H film, the sp2/ (sp2 + sp3) carbon bonding ratio of each deposited film was determined before heating by NEXAFS spectroscopy. Also, the BL3.2Ua & b was
3. Results and discussion Fig. 1 shows the C K-edge NEXAFS spectra of the a-C:H films. Fig. 1 was used as a reference of the formation of the bonding configuration and the sp2/(sp2 + sp3) ratio of the a-C:H film. The sp2/(sp2 + sp3) ratio are 0.46 and 0.47 for the films deposited from C2H2 and CH4, respectively. The hydrogen contents analyzed by GD-OES are 9.2 and 23.3 at. % for C2H2 and CH4, respectively. The hydrogen contents of the film depend on the concentration of hydrogen atoms in the source materials in the reaction atmosphere, and the sp2/(sp2 + sp3) bonding ratio of carbon in the film also changed accordingly. Amorphous C:H films with two different structures were prepared from these source materials. These structural values provide information about the structure of the films before heating. How these structures affect the structural change by heating was investigated. Fig. 2 shows the thermal desorption spectra (TDS) of the a-C:H films for mass numbers, m/z = 1, 2, 12, 17, 18, 26, 28, 32, and 44. In the spectrum of the a-C:H film deposited by CVD from C2H2, desorption peaks of m/z = 1, 2, 12, 18, 28, and 44 were observed at 750 °C, which 3
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m/z = 1, 12, 18, and 28 started at 750 °C. The thermal decomposition of this film shows that hydrogens in the film were first desorbed at 450 °C, and then the hydrocarbon components were desorbed at 750 °C. These spectra indicated that hydrogen atoms are desorbed from the aC:H film as hydrogen gas around 450 °C when the hydrogen content is large, whereas when the amount of hydrogen is small, hydrogen atoms are not desorbed from the a-C:H films at temperature up to 750 °C, because CeH bonding is maintained. To observe the uniformity of the structural change by heating, the observations of X-PEEM with NEXAFS techniques on a-C:H film deposited from CH4 were conducted after heating at each temperature. Fig. 3 shows the X-PEEM images for the a-C:H films deposited from CH4before and after heating of 500–800 °C. The images were obtained at photon energies of 284.6, 287.2 and 289.8 eV, representing the classification of π* (C]C), σ*(CeH), and σ*(CeC) states, respectively. X-PEEM images at 284.6 eV represent the transition of 1 s → π* which corresponds to the absorption of the sp2 bonding carbon atoms, and the X-PEEM images at 287.2 eV represent the transition of 1 s → σ* which is attributed to CeH bonding. Images at 289.8 eV represent the transition of 1 s → σ*, which corresponds to the absorption of the sp3 bonding carbon atoms. The bright and dark areas in the X-PEEM images are correlated to the contrast mechanisms, such as elemental, chemical, work function, and topographic contrasts as reported previous works [17,18]. In these images, the elemental and chemical contrast mechanisms were considered to describe the behavior of the uniformity of the structural variation by heating, and to explain the species of the local defects in the a-C:H films. Although this image should indicate an islands structure when there is a sub-micron cluster, micrometer-sized clusters were not observed in all X-PEEM images. This result indicates the uniformity of the a-C:H structure, which displays bright images for all samples. Therefore, the evidence suggests that the growth of local sp2 and sp3 structures by heating TDS does not occur in the X-PEEM image if the distribution of sp2 and sp3 structures in the films is uniform before heating. Since the emitted electrons from the film surface are deflected inside the X-PEEM observation system and collide with the fluorescent plate and because we observe this fluorescence contrast as an X-PEEM image, the relationship between the fluorescence intensity in the X-PEEM images and the incident photon energy cannot be used as the usual NEXAFS spectrum. On the other hand, the relative change in the fluorescence intensity within the X-PEEM images can be used to evaluate the change in the structure of the film. Hence, the influence of heating is a key factor for determining the atomic bonding and distribution of sp2/(sp2 + sp3) ratios in the a-C:H films after this determination the NEXAFS technique can be used for characterization. Fig. 4 shows the C K-edge NEXAFS spectra before and after heating at each temperatures. These spectra were normalized using the clean Si (100) and were calibrated to their peak position by HOPG. The presence of pre-edge resonance at a photon energy of 284.6 eV is assigned to the transition from the 1 s orbital to the unoccupied π* orbitals that principally originate from the sp2 site (C]C), and also include the contribution of the sp sites (C^C) if present [19]. The broad band of energy-edge from 288.0 to 330.0 eV is related to 1 s orbital to the unoccupied σ* transition at the sp, sp2 and sp3 sites in the films, promoting the broad structure of a-C:H film due to different superposition of the sp2 and sp3 arrangements and their atomic intermixing [20,21]. The other peak positions at 286.4, 287.2, 288.6, 289.8, 291.8, and 303.8 eV are attributed to the π*(C-OH), σ*(CeH), π*(C]O), σ* (CeC), σ* (C]C), and σ* (C^C) states, respectively. In addition, the peak positions at 288.0 eV indicates the σ* (CeH) state due to the bonding between the carbon and hydrogen atoms in the films, which confirmed the type of a-C:H film [19]. The prominent feature of the σ*(CeH) states illustrates the C-sp3 coordination bonded by hydrogen atoms in the local structure of the a-C:H films. Note that the intensity of such σ*(CeH) states is closely related to the hydrogen content evaluated from the GEOES technique. To clarify all features of
Fig. 4. C K-edge NEXAFS spectra of Si (100), HOPG, and as-deposited a-C:H film, and after the heating of a-C:H film of 500, 600, 700, and 800 °C. The black solid lines, red solid line, and gray solid lines are experimental data, the fitting curve, and deconvolution of each peak. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Comparison of the areal intensities of the π* (C]C), σ* (CeH), σ*(CeC) and sp2/(sp2 + sp3) ratio were evaluated from C K-edge NEXAFS spectra of a-C:H films at different temperatures. Samples
a
As-deposition 500 °C 600 °C 700 °C 800 °C a
sp2/(sp2 + sp3) (relative value)
Areal intensities π* (C]C)
π* (C]C)
π* (C]C)
0.82 0.95 1.02 1.12 1.17
0.23 0.38 0.44 0.53 0.66
0.10 0.20 0.28 0.41 0.40
0.65 0.69 0.76 0.83 0.86
Room temperature (RT).
may be assigned to H, H2, C, O, C2H4, and C3H8, respectively. Although it is hard to separate from m/z = 44, m/z = 44 was also assigned to CO2 when a surface oxidized area or an adsorbed oxygen and related molecules was present due to the reaction with carbon. This result indicates that film decomposition started at 750 °C for all structures. The spectrum for the film deposited from CH4 indicates that desorption of molecules with m/z = 2 started at 450 °C, and that of molecules with 4
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Fig. 5. X-PEEM images of as-deposited a-C:H film and after heating of 400 °C, and 500°C, which were captured with photon energies of 284.6, 287.2, and 289.8 eV at the areas of local defects (bright areas) in the a-C:H film. The red arrows represent the local defects in the film and the red circles present the selected areas for NEXAFS measurement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
deposition, after heating of 400, and 500 °C at photon energies of 284.6287.2 and 289.8 eV, relating three local bonding structures π* (C]C), σ* (CeH), and σ*(CeC) states, respectively. The signal of brightness in the surface-region indicates the relationship between the elemental or chemical contrasts and the photon energy change. The uniformity in area A appears as a weak bright area, while the local areas of B and C show a strong brightness at 284.6 eV. This strong brightness indicates that sp2 is locally rich in these areas. The X-PEEM images after annealing for the 400 and 500 °C samples do not show strong brightness. On the X-PEEM images at 287.2 eV, bright contrast that can be assigned to CeH bond was clearly observed before heating, but the bright contrast after heating almost disappeared. In addition, the area C in the image at 287.2 eV was still observed as a weak contrast in the image after heating of 400 °C, and became unobservable after heating at 500 °C. In X-PEEM images at the 289.8 eV, the contrast can be assigned to the CeC bond; however, all contrasts disappeared by heating at 400 °C at the same point as that in the image at 287.2 eV before heating. To clarify the local bonding structures and the sp2/(sp2 + sp3) ratios in the a-C:H films, the uniformity (area A) and local defects (areas B and C) are characterized. The C K-edge NEXAFS spectra for as-deposition and after heating of 400, and 500 °C were calculated for areas A, B, and C from the relationship between the fluorescence intensity in the XPEEM images and the incident photon energy, as shown in Fig. 6. The local bonding structures in Fig. 6(a)–(c) can be assigned to similar structures as shown in Fig. 4. The π* (C]C), σ* (CeH) and σ*(CeC) peaks and the sp2/(sp2 + sp3) ratios are presented in Table 2. For A–C areas, the areal intensities and the sp2/(sp2 + sp3) ratios of the as-deposition sample are smaller than those for heat-treated samples to 400 and 500 °C. In addition, the sp2 content in area A in the a-C:H film has a lower value than that in areas B and C. These results indicate that heating causes the structural changes supporting the X-PEEM results.
the C K-edge NEXAFS spectra, these peaks in spectra were assigned to each structure as shown in Fig. 3 [19,22,23]. The peaks of π* (C]C), π*(C-OH), σ*(CeH), π*(C]O), σ* (CeC), σ* (C]C), and σ* (C^C) are observed. Although not quantitative, to present the structural change due to heat, their relationship to the absolute sp2 contents in the a-C:H film were calculated as the relative sp2 contents by fitting the area under the curve of the pre-edge resonance π* (C]C) peak. However, the HOPG spectrum as of a sample has not been used to determine the sp2 contents with the X-PEEM system because the surface had a very sharp tip due to the graphene edge and bring arc discharge. The relative sp2 content was calculated by the empirical formulas in the Reference using a HOPG spectrum which was obtained by NEXAFS measurement at BL.3.1 [24]. Therefore, the values obtained by XPEEM were shown to be relative value. The areal intensities of the π* (C]C), σ* (CeH), σ*(CeC) and the sp2/(sp2 + sp3) ratios for a-C:H films are presented in Table 1. The sp2/(sp2 + sp3) relative ratios during deposition and after heating at 500, 600, 700, and 800 °C are 0.65, 0.69, 0.76, 0.83, and 0.86, respectively. The area intensities of each peak and the sp2/ (sp2 + sp3) ratios increased due to heating. The bright and dark contrasts of the images in Fig. 3 show that the transition of 1 s → π* with a photon energy as low at 288.0 eV exhibit a bright appearance, while the transition of 1 s → σ* with a photon energy over 288.8 eV presents a near black appearance. These findings provide evidence for the structural changes that occur in the a-C:H films as the temperature increases. Next, we looked for areas with defects, and local defects were found on the surface in several areas of the a-C:H film deposited from CH4. How the structure in the sp2/(sp2 + sp3) ratio distribution changed by heating was evaluated from observation of this area when the initial carbon bonding distribution was not uniform. Thus, areas with local defects or different structures were observed using the X-PEEM technique in this study. Fig. 5 shows the stack of X-PEEM images from as5
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The X-PEEM image eventually changed to a uniform image, and even if the sp2/(sp2 + sp3) ratio was locally distributed after the deposition, it eventually changed to a uniform structure of sp2/(sp2 + sp3). These results indicate that the heat resistance depends on the ratio of the sp2/(sp2 + sp3) carbon bond ratio, and suggests that it changes to the sp2/(sp2 + sp3) ratio which can withstand heating when the temperature exceeds the heat resistance temperature for the mixed structure of the sp2 and sp3 carbon ratios. The TDS results for the smaller hydrogen content a-C:H film with depositions from C2H2 up to 700 °C do not indicate the desorption peaks of m/z = 1 or 2 which are related to hydrogen. However, the TDS results for the higher hydrogen content a-C:H film deposited from CH4 show the desorbed peaks for these masses at approximately 400 °C. Therefore, in these films, the sp2/ (sp2 + sp3) distribution images in Fig. 3 observed by X-PEEM occur after the start of the hydrogen desorption. This indicates that these desorption reactions proceeded uniformly for the sp2 and sp3carbon bonding distribution. This result is similar to that due to the elimination of hydrogen from hydrocarbon gases, such as methane which can be uniformly distributed in space. For example, at the methane equilibrium, 93.1% hydrogen atoms are eliminated at 600 °C, and more than 99% of hydrogens are converted to hydrogen and carbon at 800 °C, theoretically [25]. The a-C:H film contains many hydrocarbon functional groups such as methyl and ethyl groups, and as the hydrogen content increases, the number of functional groups with large numbers of hydrogen atoms increases. Hence, hydrogen atoms contained in each functional group are desorbed by the application of thermal energy higher than their binding energy, as is the case for the aforementioned hydrocarbon gas. The desorption peaks in the TDS spectra related to carbon atoms were observed only at temperature of 750 °C or higher in all samples. Since linear hydrocarbon solids such as olefin polymers usually decompose at lower temperatures in a vacuum, for example poly-vinyl chloride decompose at 200 °C [26], the component of the aC:H film is likely not a paraffin or an olefin structure, but a higherdimensional structure. The thermal decomposition of these higher dimensional structures may also change uniformly since no local contrast was observed in the X-PEEM images. Hence, the reason for the low heat resistance of the a-C:H film with high hydrogen content is the elimination of hydrogen at low temperature, and it is thought that the elimination of carbon atoms is initiated when the temperature exceeds 750 °C. Therefore, the results indicate that the heat resistance of the film was greatly affected by the structure. 4. Conclusions In conclusion, the thermal decomposition of a-C:H films with a uniform film structure proceeds relatively uniformity, and the thermal decomposition mechanism of the films depends on the source material of the films. When the amount of hydrogen in the source material in the CVD method is large, the decomposition temperature of the film decreased. Moreover, the thermal decomposition proceeds uniformly in the order of “μm” if the initial structure is uniform. If initial structure is not uniform, the film was decomposed when the decomposition temperature of that structure was reached, and the carbon bonds will change to sp2 bonds, and eventually the structure will change to a sp2 rich uniform structure. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 6. C K-edge NEXAFS spectra of (a) area A, (b) area B, and (c) area C at different temperatures. The NEXAFS spectra taken from the areas into X-PEEM images in Fig. 5.
Acknowledgments This work was supported by Japan Society for the Promotion of Science KAKENHI Grant Number 17H03142 (Grant-in-Aid for Scientific 6
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Table 2 Comparison of the areal intensities of the π* (C]C), σ* (CeH), σ*(CeC) and sp2/(sp2 + sp3) ratio for the areas of A, B, and C at different temperatures, which was evaluated from C K-edge NEXAFS spectra in Fig. 6. Area
Samples
A
a
B
C
a
As-deposition 400 °C 500 °C a As-deposition 400 °C 500 °C a As-deposition 400 °C 500 °C
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sp2/(sp2 + sp3) (relative value)
Areal intensities π* (C]C)
σ* (CeH)
σ*(CeC)
0.83 1.05 1.10 0.83 1.16 1.18 0.86 1.20 1.23
0.21 0.35 0.26 0.20 0.42 0.35 0.24 0.45 0.38
0.12 0.24 0.17 0.08 0.37 0.27 0.12 0.38 0.31
0.64 0.81 0.85 0.64 0.86 0.88 0.65 0.88 0.90
Room temperature (RT).
Research (B)) and 17KK0111 (Fund for the Promotion of Joint International Research). And this collaborative research between Tokyo Inst. of Tech. and SLRI was supported by FY2019 Research Abroad and Invitational Program for International Collaboration and Competitive research funds international collaborative research at the School of Engineering, Tokyo Inst. of Tech. References [1] J. Robertson, Electronic processes in hydrogenated amorphous carbon, J. NonCrystal. Solids 198–200 (1996) 615–618. [2] J. Robertson, Property of diamond like carbon, Surf. Coat. Technol. 50 (1992) 185–203. [3] M. Kano, DLC coating technology applied to sliding parts of automotive engine, New Diam. Front. Carbon Technol. 16 (2006) 201–210. [4] H. Fukui, J. Okida, N. Omori, H. Moriguchi, K. Tsuda, Cutting performance of DLC coated tools in dry machining aluminum alloys, Surf. Coat. Technol. 187 (2004) 70–76. [5] D. Bourgoin, S. Turgeon, G.G. Ross, Thin Solid Films 357 (1999) 246–253. [6] H. Akasaka, S. Kawaguchi, S. Ohshio, H. Saitoh, Thermal decomposition on amorphous carbon films, Trans. Mater. Res. Soc. Japan 36 (2011) 505–508. [7] D.R. Tallant, J.E. Parmeter, M.P. Siegal, R.L. Simpson, The thermal stability of
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Thermal decomposition and structural variation by heating on hydrogenated amorphous carbon films
T
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Hiroki Akasakaa, , Sarayut Tunmeeb, Ukit Rittihongb, Masashi Tomidokoroa, Chanan Euaruksakulb, Souta Norizukia, Ratchadaporn Supruangnetb, Hideki Nakajimab, Yuki Hirataa, Naoto Ohtakea a b
Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Nakhon Ratchasima 30000, Thailand
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogenated amorphous carbon film Thermal decomposition Near edge X-ray absorption fine structure X-ray photoemission electron microscopy Uniformity of carbon bonding
Hydrogenated amorphous carbon (a-C:H) films are only empirically known to be decomposed by heating, and this heat resistance is expected to be dependent on the structure of the film. To understand the decomposition processes and their uniformity, the thermal decomposition characteristics of two types of a-C:H film were investigated after the structural determination of the sp2/(sp2 + sp3) ratios and hydrogen contents. The mass of the gas released by thermal decomposition of the a-C:H films was detected by evaluation of the thermal desorption spectroscopy to determine the starting temperature the film with high hydrogen content started to desorb hydrogens at 400 °C and hydrocarbons at 750 °C. The film with low hydrogens started to desorb decomposition gases at 750 °C. Moreover, uniform decomposition of the films was observed using synchrotron-based X-ray photoemission electron microscopy (X-PEEM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The thermal decomposition of a-C:H films with uniform film structure proceeds relatively uniformly, and the mechanism of thermal decomposition of the films depends on the source materials of the films. The thermal decomposition proceeds uniformly at the micron scale when the initial structure is uniform.
1. Introduction A hydrogenated amorphous carbon film (a-C:H) is composed of sp2 and sp3 bonding carbon and hydrogen, and the characteristics of these films are dominated by factors such as the sp2/(sp2 + sp3) ratio and the hydrogen content. Thus, the structure is often defined by a ternary diagram proposed by Robertson [1,2]. Hydrogenated amorphous carbon films have been mainly applied to mechanical parts due to their excellent mechanical properties such as their high hardness and low friction coefficients [1,3]. In the automotive field, in particular, diamond-like carbon (DLC) film which is a hard a-C:H film, has been applied to sliding parts to reduce the fuel consumption by frictional loss [3]. Because of its hardness and low adhesion to metals, a-C:H also has been applied to the cutting-edges of cutting tools for aluminum [4]. In these applications, a-C:H is heated by friction and exposed to high temperatures in several cases. These a-C:H films are empirically known to be decomposed by heat, and this heat resistance is expected to be dependent on the structure of the film. Some reports have shown that the film undergoes heat
⁎
decomposition at 400–800 °C [5,6]. Bourgoin et al. reported that the heat resistance of films was more than 400 °C [5]. Although these films are known to decompose and undergo thermal damage at 300–900 °C, the relationship between the starting temperature of decomposition and the a-C:H film structure is not clear. In addition, Akasaka et al. previously reported that the temperature at which decomposition starts differs only by the method of film formation [6]. Regarding the thermal stability of a-C:H films, Tallant et al. indicated that thermal desorption analysis detected the onset of hydrogen evolution from an a-C:H film deposited from methane carbon in a vacuum at 260 °C. Moreover, they reported that the conversion to nano-graphite occurred at 450–600 °C under air with atmospheric pressure [7]. These reports did not present the structure of the film, such as the sp2/(sp2 + sp3) bonding carbon ratio or the hydrogen content. Therefore, it is unknown which structure on Robertson's ternary diagram corresponds to these films. Few studies have investigated the relationship between the film structure and the decomposition temperature. In addition, these decomposition temperatures are known to be related to the frictional heating damage on the sliding part, which is the largest application area of the a-C:H film,
Corresponding author. E-mail address: [email protected] (H. Akasaka).
https://doi.org/10.1016/j.diamond.2019.107609 Received 30 July 2019; Received in revised form 24 October 2019; Accepted 2 November 2019 Available online 06 November 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.
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Although both the starting temperature of decomposition and the degree of uniformity of the decomposition process are significant in these applications, it is not clear whether heat decomposition of the aC:H film proceeds uniformly or locally. Several deposition methods have shown that structures are often not completely uniform during the deposition of a-C:H films. Thus, it is necessary to investigate the structural change due to heating for both uniform and non-uniform film structures. To understand the decomposition processes and their degree uniformity, we investigate the thermal decomposition of a-C:H films after structural determination of the sp2/(sp2 + sp3) carbon bonding ratio and the hydrogen contents. Moreover, the masses of gas released by thermal decomposition of the a-C:H films were detected to determine the starting temperature and the decomposition uniformity of a-C:H films. The local change in the distribution of the sp2/(sp2 + sp3) ratio in the film was observed using synchrotron-based spectromicroscopy technique, which is the combination of X-ray photoemission electron microscopy (X-PEEM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy.
Fig. 1. C K-edge NEXAFS spectra of the a-C:H films before heating.
and are very important. In addition, the a-C:H film is also used as a protective hard coating for adhesion resistance of cutting tools [8,9]. The heat resistance temperature of the film is an essential factor in this application of a-C:H films, and it is important to indicate which structure is suitable for such applications in industry.
2. Experimental Hydrogenated amorphous carbon films were prepared from methane (CH4) and acetylene (C2H2) via pulsed plasma chemical vapor deposition (CVD) [12]. A substrate of silicon (100) was placed on the
Fig. 2. Thermal desorption spectra of mass number, m/z = 1, 2, 12, 17, 18, and 26, 28, 32, 44 for the a-C:H film was deposited from hydrocarbons gases: (a) C2H2 gas and (b) CH4 gas. 2
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Fig. 3. X-PEEM images were captured at the photon energies of 284.6, 287.2, and 289.8 eV for a-C:H films as-deposition and after heating of 500, 600, 700, and 800 °C.
used to observe the X-PEEM images of the C K-edge of 275–330 eV at a resolution of the sub-micrometer resolution. Thus, the NEXAFS and XPEEM were used to detect the change in the distribution of the sp2/ (sp2 + sp3) carbon bonding ratio due to heating. Samples were heated to 800 °C by a tungsten heater located inside the X-PEEM system, and XPEEM images were observed at 100 °C increments. Heating significantly shortens the mean free path of the low energy photo-electrons generated by X-ray irradiation due to the desorption of the decomposition gas. Thus, it is hard to obtain X-PEEM images with in-situ observation at high temperature. The samples were heated to the target temperature, cooled to less than 200 °C, and then an X-PEEM image was obtained. The elemental contrast in the X-PEEM image was accomplished by tuning the photon energy through the absorption edges of the elements [17,18]. Because the photoelectron emission intensity is enhanced at the absorption edges, local areas on the surface with the matching element emit more photoelectrons and present bright in the X-PEEM image [19].
negative electrode in a vacuum chamber. Before the deposition of DLC, the natural oxidation layer on the silicon surface was removed by argon (Ar) plasma irradiation for 30 min. Ar gas was introduced at 20 cm3/ min, and the pressure was maintained at 3 Pa. The applied voltage was −2.4 kV at a frequency of 14.4 kHz. Then, hydrocarbon gas as the source material was introduced into the vacuum chamber. Hydrocarbon (CH4 or C2H2) gas was introduced at 15 cm3/min, and the pressure was maintained at 3 Pa. The applied voltage was maintained at −4.0 kV and at a frequency of 14.4 kHz. The deposition time of a-C:H films on Si substrate was set at 60 min. The thickness was approximately 1 μm. The film structure was determined using glow discharge optical emission spectrometry (GD-OES) and NEXAFS techniques. In GD-OES measurement, a calibration curve was created from the relationship between the hydrogen content and the optical emission intensity using the three types of a-C:H films whose hydrogen contents were previously known. These contents were measured by Rutherford back scattering (RBS) with elastic recoil detection analysis (ERDA). The hydrogen content of each sample was determined using this calibration curve. A beamline 3.2Ua & b (BL 3.2Ua & b) of Synchrotron Light Research Institute (SLRI) in Thailand was used for X-ray generation by synchrotron radiation for the NEXAFS measurements. Carbon K-edge NEXAFS spectra were obtained in the energy range of 275–330 eV at a resolution of 0.5 eV (FWHM) in the total electron yield mode [13,14]. The relative sp2 content was determined by comparison with the highly oriented pyrolytic (HOPG) spectrum [15,16]. The thermal desorption spectroscopy system composed of an infrared lamp, an ultrahigh vacuum chamber, and a quadrupole mass spectrometer (QMS: Qulee BGM-202, Ulvac Ltd.), was used to estimate the mass and desorption temperature of the decomposing gas by thermal decomposition of the a-C:H films. The samples with the size of 15 mm2 were kept at 100 °C for 12 h in a vacuum chamber at 10−5 Pa. Then, the temperature was increased at 15 °C/min until a temperature of 800 °C was reached. The mass of the desorbed molecules at each temperature increment was detected by QMS. To understand the initial condition of the a-C:H film, the sp2/ (sp2 + sp3) carbon bonding ratio of each deposited film was determined before heating by NEXAFS spectroscopy. Also, the BL3.2Ua & b was
3. Results and discussion Fig. 1 shows the C K-edge NEXAFS spectra of the a-C:H films. Fig. 1 was used as a reference of the formation of the bonding configuration and the sp2/(sp2 + sp3) ratio of the a-C:H film. The sp2/(sp2 + sp3) ratio are 0.46 and 0.47 for the films deposited from C2H2 and CH4, respectively. The hydrogen contents analyzed by GD-OES are 9.2 and 23.3 at. % for C2H2 and CH4, respectively. The hydrogen contents of the film depend on the concentration of hydrogen atoms in the source materials in the reaction atmosphere, and the sp2/(sp2 + sp3) bonding ratio of carbon in the film also changed accordingly. Amorphous C:H films with two different structures were prepared from these source materials. These structural values provide information about the structure of the films before heating. How these structures affect the structural change by heating was investigated. Fig. 2 shows the thermal desorption spectra (TDS) of the a-C:H films for mass numbers, m/z = 1, 2, 12, 17, 18, 26, 28, 32, and 44. In the spectrum of the a-C:H film deposited by CVD from C2H2, desorption peaks of m/z = 1, 2, 12, 18, 28, and 44 were observed at 750 °C, which 3
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m/z = 1, 12, 18, and 28 started at 750 °C. The thermal decomposition of this film shows that hydrogens in the film were first desorbed at 450 °C, and then the hydrocarbon components were desorbed at 750 °C. These spectra indicated that hydrogen atoms are desorbed from the aC:H film as hydrogen gas around 450 °C when the hydrogen content is large, whereas when the amount of hydrogen is small, hydrogen atoms are not desorbed from the a-C:H films at temperature up to 750 °C, because CeH bonding is maintained. To observe the uniformity of the structural change by heating, the observations of X-PEEM with NEXAFS techniques on a-C:H film deposited from CH4 were conducted after heating at each temperature. Fig. 3 shows the X-PEEM images for the a-C:H films deposited from CH4before and after heating of 500–800 °C. The images were obtained at photon energies of 284.6, 287.2 and 289.8 eV, representing the classification of π* (C]C), σ*(CeH), and σ*(CeC) states, respectively. X-PEEM images at 284.6 eV represent the transition of 1 s → π* which corresponds to the absorption of the sp2 bonding carbon atoms, and the X-PEEM images at 287.2 eV represent the transition of 1 s → σ* which is attributed to CeH bonding. Images at 289.8 eV represent the transition of 1 s → σ*, which corresponds to the absorption of the sp3 bonding carbon atoms. The bright and dark areas in the X-PEEM images are correlated to the contrast mechanisms, such as elemental, chemical, work function, and topographic contrasts as reported previous works [17,18]. In these images, the elemental and chemical contrast mechanisms were considered to describe the behavior of the uniformity of the structural variation by heating, and to explain the species of the local defects in the a-C:H films. Although this image should indicate an islands structure when there is a sub-micron cluster, micrometer-sized clusters were not observed in all X-PEEM images. This result indicates the uniformity of the a-C:H structure, which displays bright images for all samples. Therefore, the evidence suggests that the growth of local sp2 and sp3 structures by heating TDS does not occur in the X-PEEM image if the distribution of sp2 and sp3 structures in the films is uniform before heating. Since the emitted electrons from the film surface are deflected inside the X-PEEM observation system and collide with the fluorescent plate and because we observe this fluorescence contrast as an X-PEEM image, the relationship between the fluorescence intensity in the X-PEEM images and the incident photon energy cannot be used as the usual NEXAFS spectrum. On the other hand, the relative change in the fluorescence intensity within the X-PEEM images can be used to evaluate the change in the structure of the film. Hence, the influence of heating is a key factor for determining the atomic bonding and distribution of sp2/(sp2 + sp3) ratios in the a-C:H films after this determination the NEXAFS technique can be used for characterization. Fig. 4 shows the C K-edge NEXAFS spectra before and after heating at each temperatures. These spectra were normalized using the clean Si (100) and were calibrated to their peak position by HOPG. The presence of pre-edge resonance at a photon energy of 284.6 eV is assigned to the transition from the 1 s orbital to the unoccupied π* orbitals that principally originate from the sp2 site (C]C), and also include the contribution of the sp sites (C^C) if present [19]. The broad band of energy-edge from 288.0 to 330.0 eV is related to 1 s orbital to the unoccupied σ* transition at the sp, sp2 and sp3 sites in the films, promoting the broad structure of a-C:H film due to different superposition of the sp2 and sp3 arrangements and their atomic intermixing [20,21]. The other peak positions at 286.4, 287.2, 288.6, 289.8, 291.8, and 303.8 eV are attributed to the π*(C-OH), σ*(CeH), π*(C]O), σ* (CeC), σ* (C]C), and σ* (C^C) states, respectively. In addition, the peak positions at 288.0 eV indicates the σ* (CeH) state due to the bonding between the carbon and hydrogen atoms in the films, which confirmed the type of a-C:H film [19]. The prominent feature of the σ*(CeH) states illustrates the C-sp3 coordination bonded by hydrogen atoms in the local structure of the a-C:H films. Note that the intensity of such σ*(CeH) states is closely related to the hydrogen content evaluated from the GEOES technique. To clarify all features of
Fig. 4. C K-edge NEXAFS spectra of Si (100), HOPG, and as-deposited a-C:H film, and after the heating of a-C:H film of 500, 600, 700, and 800 °C. The black solid lines, red solid line, and gray solid lines are experimental data, the fitting curve, and deconvolution of each peak. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Comparison of the areal intensities of the π* (C]C), σ* (CeH), σ*(CeC) and sp2/(sp2 + sp3) ratio were evaluated from C K-edge NEXAFS spectra of a-C:H films at different temperatures. Samples
a
As-deposition 500 °C 600 °C 700 °C 800 °C a
sp2/(sp2 + sp3) (relative value)
Areal intensities π* (C]C)
π* (C]C)
π* (C]C)
0.82 0.95 1.02 1.12 1.17
0.23 0.38 0.44 0.53 0.66
0.10 0.20 0.28 0.41 0.40
0.65 0.69 0.76 0.83 0.86
Room temperature (RT).
may be assigned to H, H2, C, O, C2H4, and C3H8, respectively. Although it is hard to separate from m/z = 44, m/z = 44 was also assigned to CO2 when a surface oxidized area or an adsorbed oxygen and related molecules was present due to the reaction with carbon. This result indicates that film decomposition started at 750 °C for all structures. The spectrum for the film deposited from CH4 indicates that desorption of molecules with m/z = 2 started at 450 °C, and that of molecules with 4
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Fig. 5. X-PEEM images of as-deposited a-C:H film and after heating of 400 °C, and 500°C, which were captured with photon energies of 284.6, 287.2, and 289.8 eV at the areas of local defects (bright areas) in the a-C:H film. The red arrows represent the local defects in the film and the red circles present the selected areas for NEXAFS measurement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
deposition, after heating of 400, and 500 °C at photon energies of 284.6287.2 and 289.8 eV, relating three local bonding structures π* (C]C), σ* (CeH), and σ*(CeC) states, respectively. The signal of brightness in the surface-region indicates the relationship between the elemental or chemical contrasts and the photon energy change. The uniformity in area A appears as a weak bright area, while the local areas of B and C show a strong brightness at 284.6 eV. This strong brightness indicates that sp2 is locally rich in these areas. The X-PEEM images after annealing for the 400 and 500 °C samples do not show strong brightness. On the X-PEEM images at 287.2 eV, bright contrast that can be assigned to CeH bond was clearly observed before heating, but the bright contrast after heating almost disappeared. In addition, the area C in the image at 287.2 eV was still observed as a weak contrast in the image after heating of 400 °C, and became unobservable after heating at 500 °C. In X-PEEM images at the 289.8 eV, the contrast can be assigned to the CeC bond; however, all contrasts disappeared by heating at 400 °C at the same point as that in the image at 287.2 eV before heating. To clarify the local bonding structures and the sp2/(sp2 + sp3) ratios in the a-C:H films, the uniformity (area A) and local defects (areas B and C) are characterized. The C K-edge NEXAFS spectra for as-deposition and after heating of 400, and 500 °C were calculated for areas A, B, and C from the relationship between the fluorescence intensity in the XPEEM images and the incident photon energy, as shown in Fig. 6. The local bonding structures in Fig. 6(a)–(c) can be assigned to similar structures as shown in Fig. 4. The π* (C]C), σ* (CeH) and σ*(CeC) peaks and the sp2/(sp2 + sp3) ratios are presented in Table 2. For A–C areas, the areal intensities and the sp2/(sp2 + sp3) ratios of the as-deposition sample are smaller than those for heat-treated samples to 400 and 500 °C. In addition, the sp2 content in area A in the a-C:H film has a lower value than that in areas B and C. These results indicate that heating causes the structural changes supporting the X-PEEM results.
the C K-edge NEXAFS spectra, these peaks in spectra were assigned to each structure as shown in Fig. 3 [19,22,23]. The peaks of π* (C]C), π*(C-OH), σ*(CeH), π*(C]O), σ* (CeC), σ* (C]C), and σ* (C^C) are observed. Although not quantitative, to present the structural change due to heat, their relationship to the absolute sp2 contents in the a-C:H film were calculated as the relative sp2 contents by fitting the area under the curve of the pre-edge resonance π* (C]C) peak. However, the HOPG spectrum as of a sample has not been used to determine the sp2 contents with the X-PEEM system because the surface had a very sharp tip due to the graphene edge and bring arc discharge. The relative sp2 content was calculated by the empirical formulas in the Reference using a HOPG spectrum which was obtained by NEXAFS measurement at BL.3.1 [24]. Therefore, the values obtained by XPEEM were shown to be relative value. The areal intensities of the π* (C]C), σ* (CeH), σ*(CeC) and the sp2/(sp2 + sp3) ratios for a-C:H films are presented in Table 1. The sp2/(sp2 + sp3) relative ratios during deposition and after heating at 500, 600, 700, and 800 °C are 0.65, 0.69, 0.76, 0.83, and 0.86, respectively. The area intensities of each peak and the sp2/ (sp2 + sp3) ratios increased due to heating. The bright and dark contrasts of the images in Fig. 3 show that the transition of 1 s → π* with a photon energy as low at 288.0 eV exhibit a bright appearance, while the transition of 1 s → σ* with a photon energy over 288.8 eV presents a near black appearance. These findings provide evidence for the structural changes that occur in the a-C:H films as the temperature increases. Next, we looked for areas with defects, and local defects were found on the surface in several areas of the a-C:H film deposited from CH4. How the structure in the sp2/(sp2 + sp3) ratio distribution changed by heating was evaluated from observation of this area when the initial carbon bonding distribution was not uniform. Thus, areas with local defects or different structures were observed using the X-PEEM technique in this study. Fig. 5 shows the stack of X-PEEM images from as5
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The X-PEEM image eventually changed to a uniform image, and even if the sp2/(sp2 + sp3) ratio was locally distributed after the deposition, it eventually changed to a uniform structure of sp2/(sp2 + sp3). These results indicate that the heat resistance depends on the ratio of the sp2/(sp2 + sp3) carbon bond ratio, and suggests that it changes to the sp2/(sp2 + sp3) ratio which can withstand heating when the temperature exceeds the heat resistance temperature for the mixed structure of the sp2 and sp3 carbon ratios. The TDS results for the smaller hydrogen content a-C:H film with depositions from C2H2 up to 700 °C do not indicate the desorption peaks of m/z = 1 or 2 which are related to hydrogen. However, the TDS results for the higher hydrogen content a-C:H film deposited from CH4 show the desorbed peaks for these masses at approximately 400 °C. Therefore, in these films, the sp2/ (sp2 + sp3) distribution images in Fig. 3 observed by X-PEEM occur after the start of the hydrogen desorption. This indicates that these desorption reactions proceeded uniformly for the sp2 and sp3carbon bonding distribution. This result is similar to that due to the elimination of hydrogen from hydrocarbon gases, such as methane which can be uniformly distributed in space. For example, at the methane equilibrium, 93.1% hydrogen atoms are eliminated at 600 °C, and more than 99% of hydrogens are converted to hydrogen and carbon at 800 °C, theoretically [25]. The a-C:H film contains many hydrocarbon functional groups such as methyl and ethyl groups, and as the hydrogen content increases, the number of functional groups with large numbers of hydrogen atoms increases. Hence, hydrogen atoms contained in each functional group are desorbed by the application of thermal energy higher than their binding energy, as is the case for the aforementioned hydrocarbon gas. The desorption peaks in the TDS spectra related to carbon atoms were observed only at temperature of 750 °C or higher in all samples. Since linear hydrocarbon solids such as olefin polymers usually decompose at lower temperatures in a vacuum, for example poly-vinyl chloride decompose at 200 °C [26], the component of the aC:H film is likely not a paraffin or an olefin structure, but a higherdimensional structure. The thermal decomposition of these higher dimensional structures may also change uniformly since no local contrast was observed in the X-PEEM images. Hence, the reason for the low heat resistance of the a-C:H film with high hydrogen content is the elimination of hydrogen at low temperature, and it is thought that the elimination of carbon atoms is initiated when the temperature exceeds 750 °C. Therefore, the results indicate that the heat resistance of the film was greatly affected by the structure. 4. Conclusions In conclusion, the thermal decomposition of a-C:H films with a uniform film structure proceeds relatively uniformity, and the thermal decomposition mechanism of the films depends on the source material of the films. When the amount of hydrogen in the source material in the CVD method is large, the decomposition temperature of the film decreased. Moreover, the thermal decomposition proceeds uniformly in the order of “μm” if the initial structure is uniform. If initial structure is not uniform, the film was decomposed when the decomposition temperature of that structure was reached, and the carbon bonds will change to sp2 bonds, and eventually the structure will change to a sp2 rich uniform structure. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 6. C K-edge NEXAFS spectra of (a) area A, (b) area B, and (c) area C at different temperatures. The NEXAFS spectra taken from the areas into X-PEEM images in Fig. 5.
Acknowledgments This work was supported by Japan Society for the Promotion of Science KAKENHI Grant Number 17H03142 (Grant-in-Aid for Scientific 6
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Table 2 Comparison of the areal intensities of the π* (C]C), σ* (CeH), σ*(CeC) and sp2/(sp2 + sp3) ratio for the areas of A, B, and C at different temperatures, which was evaluated from C K-edge NEXAFS spectra in Fig. 6. Area
Samples
A
a
B
C
a
As-deposition 400 °C 500 °C a As-deposition 400 °C 500 °C a As-deposition 400 °C 500 °C
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sp2/(sp2 + sp3) (relative value)
Areal intensities π* (C]C)
σ* (CeH)
σ*(CeC)
0.83 1.05 1.10 0.83 1.16 1.18 0.86 1.20 1.23
0.21 0.35 0.26 0.20 0.42 0.35 0.24 0.45 0.38
0.12 0.24 0.17 0.08 0.37 0.27 0.12 0.38 0.31
0.64 0.81 0.85 0.64 0.86 0.88 0.65 0.88 0.90
Room temperature (RT).
Research (B)) and 17KK0111 (Fund for the Promotion of Joint International Research). And this collaborative research between Tokyo Inst. of Tech. and SLRI was supported by FY2019 Research Abroad and Invitational Program for International Collaboration and Competitive research funds international collaborative research at the School of Engineering, Tokyo Inst. of Tech. References [1] J. Robertson, Electronic processes in hydrogenated amorphous carbon, J. NonCrystal. Solids 198–200 (1996) 615–618. [2] J. Robertson, Property of diamond like carbon, Surf. Coat. Technol. 50 (1992) 185–203. [3] M. Kano, DLC coating technology applied to sliding parts of automotive engine, New Diam. Front. Carbon Technol. 16 (2006) 201–210. [4] H. Fukui, J. Okida, N. Omori, H. Moriguchi, K. Tsuda, Cutting performance of DLC coated tools in dry machining aluminum alloys, Surf. Coat. Technol. 187 (2004) 70–76. [5] D. Bourgoin, S. Turgeon, G.G. Ross, Thin Solid Films 357 (1999) 246–253. [6] H. Akasaka, S. Kawaguchi, S. Ohshio, H. Saitoh, Thermal decomposition on amorphous carbon films, Trans. Mater. Res. Soc. Japan 36 (2011) 505–508. [7] D.R. Tallant, J.E. Parmeter, M.P. Siegal, R.L. Simpson, The thermal stability of
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