Large-Area and Low-Temperature Nanodiamond

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We have developed a low-temperature and large-area nanodiamond coating method by microwave-plasma-assisted chemical vapor deposition (MWPCVD) ...
New Diamond and Frontier Carbon Technology Vol. 16, No. 6 2006 MYU Tokyo NDFCT 527

Large-Area and Low-Temperature Nanodiamond Coating by Microwave Plasma Chemical Vapor Deposition K. Tsugawa*, M. Ishihara, J. Kim, M. Hasegawa and Y. Koga Research Center for Advanced Carbon Materials, National Institute of Advanced Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan (Received 13 March 2007; accepted 19 March 2007) Key words:

diamond, nanocrystal, glass substrate, MWPCVD, surface wave

We have developed a low-temperature and large-area nanodiamond coating method by microwave-plasma-assisted chemical vapor deposition (MWPCVD) sustained using surface waves. A highly transparent and smooth nanodiamond film was successfully grown on borosilicate glass, soda-lime glass and quartz substrates by this method. The uniformity of the plasma and the low temperature of the substrates were attained using the microwave plasma, which was sustained using surface waves radiated from two sets of eight parallel coaxial antennas. A relatively low gas pressure of approximately 100 Pa was used to avoid the heating of the substrate by the plasma. Under these conditions, a homogeneous plasma over a 30×30 cm2 area and a substrate temperature of less than 100°C were successfully obtained. We applied a hydrogen and methane plasma to form the nanodiamond coating, which is a conventional gas mixture for diamond CVD growth. CO2 was also added to the gas mixture to improve the film properties, such as transparency. The deposited diamond films are transparent, smooth and reasonably uniform over an area of 30×30 cm2 on the glass substrate. UV-excited Raman spectroscopy shows the formation of diamond film by the sharp peak at 1333 cm–1. X-ray diffraction analysis and transmission electron microscope measurements indicate that the film consists of nanocrystalline diamond grains of sizes ranging from 5 to 20 nm. Optical applications are expected to be developed from the film properties of transparency to visible light, high refractive index and small degree of double refraction.

Corresponding author: e-mail: [email protected]

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Introduction

Diamond exhibits excellent properties for a wide-range optical window such as high transparency to visible light as well as mechanical properties such as its extremely high resistance to wear and chemicals and its thermal conductivity. Diamond deposition over a large area is important for industrial applications including large-area protective coatings, electrodes for electrochemical processes or waste-water treatment in industrial use, and the mass production of coatings for cutting tools. Several types of commercial chemical vapor deposition (CVD) reactor for large-area diamond coating are available at present, applying the DC arc, microwave plasma and hot filament methods. A microcrystalline diamond film has a rough surface due to its highly faceted columnar grains. Thus, it requires a polishing process to obtain a smooth surface and to improve its friction, wear and optical characteristics. However, the polishing of a large area is timeconsuming and expensive. In contrast, a nanodiamond film possesses a flat and smooth surface without polishing, since it consists of nanosized diamond grains.(1,2) This leads a great advantage for applications to low-cost tribological or optical materials due to its low friction, and excellent wear and scattering properties. Diamond exhibits wide optical transmission with a high resistance to mechanically and chemically harsh environments. This means that diamond is an attractive material for a protective coating for optical materials such as glass. However, the adhesion of a deposited diamond film to a glass substrate is poor in most cases, because of the difference between their thermal expansion coefficients and heat-induced strain in the glass.(3) Thus, the temperature has to be maintained lower than that of the strain point of the glass substrate during the diamond deposition process. During the plasma CVD process, the substrate is heated by the plasma. To realize a low substrate temperature, the plasma heating of the substrate must be suppressed. If the gas pressure is low, the plasma heating of the substrate is also suppressed. Also, the low energy (low temperature) of the plasma is effective for suppressing the heating of the substrate. In this study, nanodiamond films were grown on commercially available borosilicate and soda-lime glass substrates, as well as on a quartz substrate with an area of over 30×30 cm2 by microwave plasma chemical vapor deposition (MWPCVD) sustained using surface waves. The properties of the films coated on the glass substrates were investigated.

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Experimental Procedure

An MWPCVD reactor,(2) which is illustrated in Fig. 1, was used for the nanodiamond deposition on glass substrates. This machine was equipped with two microwave power supplies of 2.45 GHz. A microwave power of 20 kW at a maximum was applied to two sets of eight coaxial linear antennas and emitted through quartz tubes that cover the antennas into the reactor chamber to excite the plasma. The antennas were air-cooled in the quartz tubes. For each antenna in the quartz tube, a taper-shaped outer shield, which worked as the reflector of the microwave, was installed. Using this tapered outer shield, a uniform surfacewave plasma was generated under the antenna along the quartz tube. This equipment can

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Fig. 1. Schematic drawing of the microwave MWPCVD reactor sustained using surface waves.

be easily scaled up by increasing the antenna length and increasing the number of antennas. The source gas of the CVD process was a mixture of H2 and CH4, which is common for conventional diamond synthesis by CVD. CO2 was also added to the source gas to enhance the etching of the nondiamond content, particularly at lower temperatures. The concentration of CH4 and CO2 was approximately 1%. The gas pressure during the CVD process was about 100 Pa. The chamber was evacuated from the bottom using a rotary pump. The substrate was placed on a sample stage under the antennas. Using a novel waterbuffer substrate cooling system, the substrate was maintained at temperatures from 100 to 500°C. The temperature of the substrates was measured using an infrared pyrometer. The sample stage underwent linear motion during the deposition to enhance the homogeneity of the deposited films. The plasma that was generated along the quartz tube reached the substrate by diffusion. We confirmed the formation of the surface-wave plasma by measuring the plasma parameter using a Langmuir probe. When a surface-wave plasma is generated, the plasma density exceeds the critical density of a plasma excited using a 2.45GHz microwave, which is 7.4×1010 /cm3. The plasma density at a typical position on the substrate was more than 1011 /cm3, which exceeded the critical density of a 2.45 GHz excitation. In this case, a low electron temperature of about 1.5 eV was also confirmed. Two types of commercially available glass were employed as substrates in the present study: Pyrex glass and soda-lime glass. Pyrex, for which the strain point is above 500°C, is a borosilicate glass in widespread use. Soda-lime glass, for which the strain point is 473°C, is a silica-based glass commonly used as window glass. Before the diamond deposition, the substrates were sonicated for 15 min in an ultrasonic bath in a mixture of ethanol and a 5nm graphite-cluster diamond film synthesized by shock compression, followed by a 3-min ultrasonic ethanol wash. After this pretreatment process, the nucleation density and uniformity of the deposited diamond film were much improved compared with the case without pretreatment. After the deposition, the deposited films were analyzed by scanning electron microscopy (SEM), Raman scattering spectroscopy, X-ray diffraction (XRD) measurements, atomicforce microscopy (AFM) and transmission electron microscopy (TEM). The optical properties of the film were also measured.

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Results and Discussion

Figure 2 shows a diamond film on a Pyrex glass substrate with a 30×30 cm2 area in front of the reaction chamber of the MWPCVD system after a 10-h deposition. The deposited film is smooth, transparent and uniform from the center to the edges of the glass substrate. The typical deposition rate was about 50 nm/h. The adhesion of the nanodiamond films on the Pyrex substrate was examined by scrubbing with SiC sandpaper of #400. The film exhibited good adhesion on the substrate and high scratch resistance. In this test, almost no peeling of the film off the substrate was observed. The obtained diamond films were investigated by ultraviolet (UV)-excited Raman spectroscopy using a 244 nm wavelength. The UV Raman spectrum indicates the formation of a diamond film by the sharp peak at 1333 cm–1 as shown in Fig. 3. The broader band with lower intensity at approximately 1590 cm–1 represents sp2-bonded carbon, which is often observed together with nanodiamond films. On the nanocrystalline scale, the sp2-bonded carbon coexists in the diamond film at the boundaries between the diamond grains.(4) For this film, the relative intensity of the peak at 1333 cm–1 was much higher than that at 1590 cm–1, and the film was highly transparent because of a lower sp2 component. Figure 4 shows a typical XRD pattern of the deposited film. The relative heights of the diamond (111) and (220) peaks in the XRD pattern of the film indicate a random crystallite orientation. The full width at half maximum (FWHM) of the diamond peaks indicates that the crystallites range in size from 5 to 20 nm according to Scherrer’s equation. Figure 5 shows the details of the (111) reflection peak. The peak consists of two

Fig. 2. Diamond film on glass over a 30×30 cm2 area after a 10-h deposition.

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Fig. 3. Typical Raman spectrum of the nanodiamond film (excitation wavelength: 244 nm).

Fig. 4. Typical X-ray (Cu K_1) diffraction pattern of the nanodiamond film on glass.

component peaks. One is a symmetrical peak, which is exactly located at the angle of the diamond (111) reflection. Also, an additional broad peak was seen at approximately 41.7°, a lower reflection angle than that of the (111) reflection, which is characteristic of our nanodiamond films grown at low temperatures. For the peak fitting of this additional reflection peak, an asymmetrical fitting function fitted much more closely to the data than a symmetrical one. This additional peak has been attributed to 6H stacking faults in the direction in the grains.(5) In the case of the nanodiamond films in this study, the formation of this structural defect is attributed to the low deposition temperature. The concentration of this stacking fault in the film has not been clarified from the XRD peak

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Fig. 5. Details of the X-ray (Cu K_1) diffraction pattern of the nanodiamond film on glass around the (111) reflection peak. The peak fitting curves using symmetrical (A) and asymmetrical (B) fitting functions, as well as a baseline, are also shown in the figure.

intensity, however, it could be significant. Thus, the XRD signal attributed to this type of stacking fault was intense, as shown in Fig. 5. For further investigation of the morphology of the nanodiamond film, cross-sectional TEM observations were carried out. Figure 6 shows a typical high-resolution crosssectional TEM image of the nanodiamond film on glass. The TEM observation reveals that the film consists of continuous nanocrystalline diamond grains with sizes in the range from 5 to 20 nm. This result is in agreement with the crystallite size estimated from the XRD pattern. The electron diffraction patterns of the diamond (111), (220) and (311) reflections are also observed. Their ring pattern also indicates the random orientation of the nanocrystalline diamond grains. Figure 7 shows a typical SEM image of the surface of the obtained diamond film. The surface of the film is flat and smooth so that no grains can be distinguished at this magnification. Figure 8 shows an AFM image of the surface of the nanodiamond film, which shows the typical surface structure of the films. The substrate was quartz, which was polished to a roughness (Ra) of less than 1 nm. The thickness of the nanodiamond film was 1.6 +m. Although the thickness of the film is relatively large, a small surface roughness of 3.1 nm was obtained. This can be attributed to the small grain size and its uniformity throughout the whole film. Basic optical properties, such as transmittance, refractive index and double refraction, were examined for the nanodiamond films on Pyrex glass substrates. The optical transmittance of a nanodiamond film with a thickness of 500 nm is shown in Fig. 9. The average transmittance of the visible light (wavelengths 400 to 800 nm) is 90%. This high transmittance in the visible-light region makes it possible to coat glass windows with a

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Fig. 6. High-resolution cross-sectional TEM image of the nanocystalline diamond film on a glass substrate.

Fig. 7. Typical SEM image of the surface of the nanodiamond film on a glass substrate.

nanodiamond film for use in abrasive or other extreme environments, possibly replacing the more expensive materials currently in use. The refractive index and extinction coefficient of the nanodiamond film are shown in Fig. 10. At the standard wavelength of 589 nm (Na D-line) the refractive index is 2.11. Although this value was slightly smaller than that of 2.42 for a diamond film, applications based on high refractive-index nanodiamond coatings on a glass substrate, for which the

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Fig. 8. AFM image of the surface of the nanodiamond film on a quartz substrate. The surface of the quartz substrate was polished to a Ra of 0.87 nm. The thickness of the nanodiamond film was 1.6 +m.

Fig. 9. Wavelength dependence of the transmittance of a nanodiamond film of 500 nm thickness deposited on a Pyrex glass substrate.

refractive index is about 1.5, are expected to be developed. Figure 11 shows the wavelength dependence of the double refraction of the nanodiamond film of 200 nm thickness on the Pyrex glass substrate. The contribution from the substrate

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Fig. 10. Wavelength dependences of the refractive index and extinction coefficient of nanodiamond film of 500 nm thickness on Pyrex glass substrate.

Fig. 11. Wavelength dependence of the double-refraction retardation, 6n, multiplied by the film thickness, d = 200 nm, of a nanodiamond film deposited on Pyrex glass substrate. The contribution from the Pyrex substrate to the double refraction was subtracted in the figure.

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to the measured double refraction was subtracted in the figure. In the case of our nanodiamond film coating, the double-refraction retardation, 6n, was on the order of 10–4. The degree of double refraction of the nanodiamond films on the glass substrate is relatively small, and such a film shows potential for optical applications.

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Conclusions

We have developed a low-temperature and large-area nanodiamond coating method by microwave-plasma-assisted chemical vapor deposition sustained by surface waves using coaxial parallel linear antennas. A surface-wave plasma with a low electron temperature was successfully excited. The effective coating area for the equipment used was 30×30 cm2, however, because of the scalability of the equipment, nanodiamond coatings are possible over much larger areas. A highly transparent and smooth nanodiamond film was successfully grown on glass substrates at substrate temperatures from 100 to 500°C.

Acknowledgements The authors would like to thank M. Liehr and H. Kawarada for the development of the MWPCVD equipment and M. Shelby for help with the experiments. This work was partially supported by the Frontier Carbon Technology Project (NEDO), by the Research and Development of Nanodevices for Practical Utilization of Nanotechnology (NEDO), and by the Task Force Program of Innovation Center for Start-ups (AIST).

References 1) D. M. Gruen, X. Z. Pan, A. R. Krauss, S. Z. Liu, J. S. Luo and C. M. Foster: J. Vac. Sci. Technol., A 12 (1994) 1491. 2) K. Tsugawa, M. Hasegawa and Y. Koga: New Diamond 20 (2004) 16. 3) M. J. Ulczynski, B. Wright and D. K. Reinhard: Diamond Relat. Mater. 7 (1998) 1639. 4) L. C. Nistor, J. Van Landuyt, V. G. Ralchenko, E. D. Obraztsova and A. A. Smolin: Diamond Relat. Mater. 6 (1997) 159. 5) H. Tateyama, H. Noma, Y. Adachi and M. Komatsu: Chem. Mater. 9 (1997) 766.