Studies on the influence of argon flow rate on PECVD grown diamond-like nanocomposite film

Optik 124 (2013) 6915–6918

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Studies on the influence of argon flow rate on PECVD grown diamond-like nanocomposite film R. Chakraborty ∗ , R. Mandal 1 , R. Das Department of Applied Optics & Photonics University of Calcutta, 92 A.P.C. Road, Kolkata 700009, India

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Article history: Received 16 January 2013 Accepted 26 May 2013

Keywords: Diamond-like nanocomposite PECVD Argon flow rate Interferometric technique Surface quality

a b s t r a c t Diamond like nanocomposite film is grown using PECVD method for its use as IR window. It is seen that the argon flow rate variation can change the surface quality and the film thickness and these changes are also not linear. So, depending upon application, argon flow rate is to be optimized. Moreover, an interferometric technique to measure the phase variation of the film surface has been proposed. This phase variation is comparable with the surface quality as obtained from AFM images and can thus be considered as a cheap alternative technique to measure surface quality. © 2013 Elsevier GmbH. All rights reserved.

1. Introduction Various devices which move at high speeds, such as aircraft and guided missiles, use infrared (IR) “windows” to receive signals for their control remotely. Thus they are to be protected against abrasion, excessive rise in temperature caused by friction or biological attack. Known materials which are IR transmissive, like zinc sulfide (ZnS), zinc selenide (ZnSe), germanium (Ge), gallium arsenide (GaAs), gallium phosphide (GaP), fused silica (SiO2 ), aluminum oxynitride (AlON), etc., however cannot satisfy all these criterions. Diamond-like Carbon (DLC) films can be an alternative but their processing requires high temperature which can degrade the substrate. This problem can be overcome by using interlayers but that leads to degradation of optical transmissivity [1,2]. Diamond-like nanocomposite (DLN) can be recommended as a novel class of diamond related material whose coating can be used to solve all the above mentioned problems, It comprises of two amorphous interpenetrating network structures: one is “diamondlike” (a-C:H) network and the other is “glass-like” (a-Si:O) network in adjustable proportion [3,4]. These films exhibit excellent physical and chemical properties similar to diamond films but are easier to produce compared to later [5,6]. The presence of the glasslike network distinguish it from the conventional DLC films and is also referred as SiOx containing DLC film [7,8]. Although DLN

∗ Corresponding author. Tel.: +91 33 23522411x482; fax: +91 33 23519755. E-mail addresses: [email protected], [email protected] (R. Chakraborty). 1 Present address: School of Mechanical & Manufacturing Engineering, Loughborough University, UK. 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.05.178

have lower hardness compared to DLC but its lower residual stress, better adherence to any material, high wear resistance and lower frictional coefficient has made it a very important material for tribological applications [9–12]. DLN films are also biocompatiable and have been studied for stent coating and cell and protein responses [13,14]. In this work, the influence of the argon flow rate on the properties of the DLN film grown by standard PECVD method has been studied. Determination of surface phase profile using an interferometric technique along with the concept of digital holography is also proposed. It is seen that this technique can give an idea of surface morphology without the use of sophisticated instruments. In the next section, the experimental work done is described. This involves a description of the deposition process of DLN film using a PECVD system. The set-up of measurement of phase variation is also described there as a separate sub-section. The results of different characterizations/analysis have been discussed in Section 3 which is followed by conclusion.

2. Experimental details 2.1. Deposition of DLN film The PECVD system used in the present study is illustrated schematically in Fig. 1. It is a rugged system where it can separately be used as a PECVD, Sputtering and Thermal Evaporation system. The substrates are cleaned by conventional cleaning method followed by drying in a nitrogen jet. Cleaned substrates are loaded into the PECVD chamber followed by evacuation up to a pressure

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80 Rotating substrate Holder

Water cooled Chamber

Tungsten Filament

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Control Unit

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Resistive heating unit

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Vacuum System

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2.2. Interferometric setup for surface phase profile evaluation A simple Mach-Zehnder interferometer is used to get the interference pattern and is shown in Fig. 2. Light from a He-Ne laser is focused by a microscope objective (20×) on a pinhole (A). The light is then collimated and incident on a cubic beam splitter (B1) which splits the light into two parts propagating in two orthogonal directions. The two parts are then reflected by the plane mirrors M1 and M2 and combined by the second beam splitter B2 to create the interference pattern. The sample under test is placed in any one arm of this Mach-Zehnder interferometer and the interference pattern is captured on a CCD. Since the intensity of the interference pattern is too high, the CCD gets saturated giving a bright uniform white picture. To avoid this, a polarizer and an analyzer pair is placed before beam splitter B1 and optic axis is set such that a good contrast is achievable at the CCD. The sample which is basically the DLN coated glass sample is placed in the path M1B2, such that in

Fig. 2. The Mach-Zehnder interferometer setup used for analysis of surface phase variation.

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80

100

120

Grazing angle

Fig. 1. Schematic diagram of the PECVD system used for DLN film deposition.

of about 10−5 mbar. Then argon gas is introduced into the chamber via a mass flow controller until the chamber pressure reaches approximately 10−4 mbar. The argon flow rate is varied in order to find its effect of the deposited DLN thin film. The deposition time is however kept constant. The samples are further cleaned by in situ argon-plasma cleaning prior to DLN film deposition. The precursor flow is adjusted via a needle valve and by using gravity control. The liquid precursor is vaporized as soon as it enters the vacuum chamber. The RF substrate bias power supply is concurrently switched on. The precursor ions form the stable plasma, which are accelerated toward the substrates due to the negative DC substrate bias induced by the RF power. In the present study, DLN film has been deposited on glass substrates in this specially designed PECVD system, using a liquid precursor (2,3,4-triphenyl nonamethyl pentasiloxane) containing C, H, Si and O as constituents. The substrate temperature during deposition process is maintained at ∼85 ◦ C.

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Fig. 3. Plot of XRD data of DLN film on glass.

CCD we get both the background and the change due to the sample is seen. This well known method in optics along with the basic principles of holography [15,16] is used to evaluate the differences in surface quality between samples. Here the hologram is recorded first, which is basically an interference pattern of the light diffracted from the sample and an off axis reference wave. That hologram is reconstructed numerically later. The main concentration is on the phase part of the reconstructed wave front, which clearly gives indication about the surface of the sample. If there is any surface undulation or roughness present in the sample, the diffracted beam from the surface will change accordingly, due to the different amount of phase introduced by different portions of the sample. For different sample, the phase introduced in the light diffracted from the sample is also changed, thus signifying the difference between different samples. 3. Results and discussion XRD analysis of the DLN coated samples has been done to show the nature of the thin film. Fig. 3 shows the XRD data of the DLN film coating on glass for grazing angle of incidence. As expected no peak is observed for coatings on glass substrate, establishing the amorphous nature of DLN thin film. The surface morphology of the DLN coated samples is observed using AFM. The imaging is done in Tapping Mode using Digital Instruments Nanoscope AFM from Veeco Instruments Inc. As shown in Fig. 4, the surface is relatively smooth. However, the

Fig. 4. AFM image of DLN coated glass showing its surface quality for argon flow rate of (a) 50 ml/min (Sample 4) (b) 75 ml/min (Sample 1) (c) 100 ml/min (Sample 2) and (d) 150 ml/min (Sample 3).

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Fig. 5. Transmission characteristic of DLN films on glass for argon flow rate of (a) 50 ml/min (Sample 4) (b) 75 ml/min (Sample 1) (c) 100 ml/min (Sample 2) and (d) 150 ml/min (Sample 3).

surface quality varies with the percentage of argon introduced. In Fig. 4(a–d), AFM images of DLN coated glass surfaces are shown for flow rate of 50 ml/min (Sample 4), 75 ml/min (Sample 1), 100 ml/min (Sample 2) and 150 ml/min (Sample 3) respectively. The image reveals a distinct difference in surface quality with 75 ml/min argon flow rate giving the best surface quality, i.e. having least roughness. At higher argon flow rate, it appears that DLN film tends to form clusters. Clusters can destroy the amorphous nature of the structure, can serve as active centers of degradation, and in the case of optical components, can act as light scattering centers. The transmission of the DLN film at different wavelength is to be known to find its applicability in the IR region. So the transmittance studies of the coated DLN films are carried out using the Perkin Elmer Spectrophotometer. The transmission characteristics are shown in Fig. 5 and reveal that that the transmission increases with wavelength and at short and mid-IR region the transmission is almost 90%. The transmission after 2000 nm wavelength couldn’t be measured because of the limitation of the instrument used. Nevertheless, the transmission characteristics reveal that a very good transmission property of DLN in the IR wavelength. It is also seen that the argon concentration have almost no effect on the nature of transmission curve for Sample 1 (argon flow rate: 75 ml/min), Sample 3 (argon flow rate: 150 ml/min) and Sample 4 (argon flow rate: 50 ml/min) but the nature is distinctly different for Sample 2 (argon flow rate: 100 ml/min). To find out the reason behind this, the variation of refractive index (r.i.) of the DLN film coated samples in the IR region and hence its thickness are calculated using Swanpoel Method [17,18], taking into account the interference due to multiple refractions at film/substrate and air/film interfaces. The Table 1 Values of refractive index of DLN film for different wavelengths and the film thickness calculated using Swanpoel Method from data obtained from Fig. 5 for different samples. Argon flow rate (Sample #) Wavelength (nm)

Refractive index

Average thickness (nm)

50 ml/min (Sample 4)

1908 1460 1146 1018

1.88 1.835 1.842 1.856

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75 ml/min (Sample 1)

1904 1500 1178 1050

1.795 1.812 1.825 1.890

1781

100 ml/min (Sample 2)

1906 1254 1020

1.812 1.894 2.108

896

150 ml/min (Sample 3)

1904 1440 1112 1020

1.858 1.812 1.852 1.896

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Fig. 6. Phase pattern of DLN films of three different types of surface morphology for argon flow rate of (a) 75 ml/min (Sample 1) (b) 100 ml/min (Sample 2) and (c) 150 ml/min (Sample 3).

substrate (soda-lime glass in this case) r.i. is considered as 1.51. Their calculated values are tabulated in Table 1. As can be seen from the Table, Sample 2 (argon flow rate: 100 ml/min) has the film thickness which is not only the lowest of all the samples but its value is also distinctly less (896 nm). It can be established from the calculated data that the film thickness decreases with argon flow rate and then start increasing. This data can also give some insight of the cluster like morphology of the AFM image of Sample 2 as seen in the Fig. 4(c). The film growth process occurs in a form of cluster which becomes smooth with increased film thickness. As the thickness of Sample 2 is very less, the clusters can be seen in its AFM image. In Fig. 6, the phase patterns of three DLN coated samples are shown. The analysis is done by simple hologram reconstruction technique. If O(x, y) is the object wavefront and R(x, y) is the reference wavefront which is considered as plane wave, then the intensity distribution of the recorded hologram, h(x, y), can be represented by h(x, y) = |R(x, y) + O(x, y)|2 = (R(x, y) + O(x, y))(R(x, y) + O(x, y))∗ = R(x, y)R(x, y)∗ + O(x, y).O(x, y)∗ + R(x, y)O(x, y)∗ + R(x, y)∗ .O(x, y)

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where the first two terms represent the zero-order while the third and fourth represents virtual and real image respectively. Representing zero order term, virtual and real images as a(x, y), i*(x, y) and i(x, y) respectively, the Fourier Transform of the hologram is FT [h(x, y)] = H(u, v) = A(u, v) + I(u, v)ı(u − u0 , v − v0 ) + I ∗ (u, v)ı(u + u0 , v + v0 ) where the Fourier spectrum I*(u, v) contains the desired information of the virtual image. u and v are the spatial frequencies in the u and v directions, uo and vo are the carrier frequencies in the u and v directions. One of the diffraction orders can be easily extracted eliminating the undiffracted component A(u, v) and complementary image spectra. In digital holography the numerical reconstruction method consists of calculating the Fresnel diffraction pattern of the hologram. The hologram used here is a Fresnel type hologram, as the recording plane lies within the region of Fresnel diffraction of the illuminated object. At the time of reconstruction, only the I*(u + uo , v + vo ) or I(u − uo , v − vo ) spectrum is selected by applying a simple filter. The reconstruction process, further analysis and simulation were carried out in MATLAB. Holograms for different samples are recorded, by placing them in the same position, without changing any other parameters of the experimental set up. The image processing toolbox in MATLAB stores images as two dimensional arrays, in which each element of the matrix corresponds to a single pixel in the displayed image. This software implements a two dimensional Fast Fourier Transform (FFT) in its fft2 function. The reconstructed phase is plotted by the surf function in the MATLAB. In the reconstructed phase, it is difficult to differentiate between the sample phase and background phase. So, the background fringe was recorded by CCD and converted into background phase using MATLAB. This background phase is then subtracted from the reconstructed image phase, which gives the information about the phase introduced by the sample. In Fig. 6 the phase patterns are plotted for DLN coated glass substrates obtained by introduction of three different argon concentrations. The phase patterns shown in Fig. 6(a–c) are for the three samples formed by flowing argon rate of 75 ml/min (Sample 1), 100 ml/min (Sample 2) and 150 ml/min (Sample 3) respectively. The unit-step size for the co-ordinates is kept same for all figures. It is seen that the phase patterns are distinctly different with Fig. 6(a) (Sample 1) having the least variation and Fig. 6(c) (Sample 3) having the most variation. If the phase patterns are compared with the AFM images, it is seen that they have a similar nature as the surface quality of the films. Thus a cheap and simple optical technique can be used to test the surface quality of DLN films which can be an alternative to sophisticated imaging methods like AFM to qualitatively study the quality of nano films. 4. Conclusion Diamond-like-nanocomposite has been coated on glass substrates using a PECVD system for use as IR transmissive window. The variation of the film quality due to change in Ar flow rate have been studied. While the AFM results of the surface quality shows that the surface quality improves and then degrades with argon

flow rate increment, the film thickness data obtained from spectrometer readings however shows that thickness decreases and then increases with the increase in flow rate. Thus the argon flow rate is critical for DLN film growth and needs to be optimized for different applications. Moreover, a very simple technique of surface phase variation measurement has been proposed which can give a qualitative idea of DLN film surface morphology. Some parameters like vibration isolation, proper placing of the samples in the same position should be carefully monitored to get better phase variation results. Acknowledgement This work is part of the project funded by Centre for Research in Nanoscience & Nanotechnology, University of Calcutta. References [1] L.M. Goldman, S.K. Jha, N. Gunda, R. Cooke, N. Agarwal, S.A. Sastri, A. Harker, J. Kirsch, Durable Coatings for IR Windows, Proc. SPIE-Window Dome Technol. Mater. III 5786 (2005) 381–392. [2] M.P. Siegal, D.R. Tallant, P.N. Provencia, D.L. Overmyer, R.L. Simpson, L.J. Martinez-Miranda, Ultrahard carbon nanocomposite films, Appl. Phys. Lett. 76 (2000) 3052–3054. [3] V.F. Dorfman, Diamond-like nanocomposites (DLN), Thin Solid Films 212 (1992) 267–273. [4] C. Venkatraman, A. Goel, R. Lei, D. Kester, C. Outten, Electrical properties of diamond like nanocomposite coatings, Thin Solid Films 308–309 (1997) 173–177. [5] V.F. Dorfman, B.N. Pypkin, US Patent, (1994) Patent No: 5352493. [6] D. Neerinck, P. Persoone, M. Sercu, A. Goel, D. Kester, D. Bray, Diamond-like nanocomposite coatings (a-C:H/a-Si:O) for tribological applications, Diamond Relat. Mater. 7 (1998) 468–471. [7] W.J. Yang, T. Sekino, K.B. Shim, K. Niihara, K.H. Auh, Microstructure and tribological properties of SiOx/DLC films grown by PECVD, Surf. Coat. Technol. 194 (2005) 128–135. ˇ Meskinis, A. Tamuleviciene, Structure, properties and applications of dia[8] S. mond like nanocomposite (SiOx containing DLC) films: a review, Mater. Sci. (MEDZˇ IAGOTYRA) 17 (2011) 358–370. [9] A. Pandit, N.P. Padture, Interfacial toughness of diamond-like nanocomposite (DLN) thin films on silicon nitride substrates, J. Mater. Sci. Lett. 22 (2003) 1261–1262. [10] W.J. Yang, Y.-H. Choa, T. Sekino, K.B. Shim, K. Niihara, K.H. Auh, Structural characteristics of diamond-like nanocomposite films grown by PECVD, Mater. Lett. 57 (2003) 3305–3310. [11] T.S. Santra, C.H. Liu, T.K. Bhattacharyya, P. Patel, T.K. Barik, Characterization of diamond-like nanocomposite thin films grown by plasma enhanced chemical vapor deposition, J. Appl. Phys. 107 (2010) 124320. [12] S.V. Prasad, T.W. Scharf, P.G. Kotula, J.R. Michael, T.R. Christenson, Application of diamond-like nanocomposite tribological coatings on LIGA microsystem parts, J. Microelectromech. Syst. 18 (2009) 695–704. [13] I.D. Scheerder, M. Szilard, H. Yanming, X.B. Ping, E. Verbeken, D. Neerinck, E. Demeyere, W. Coppens, F. Van de Warf, Evaluation of the biocompatibility of two new diamond-like stent coatings (Dylyn) in a porcine coronary stent model, J. Invasive Cardiol. 12 (2000) 389–394. [14] T. Das, D. Ghosh, T.K. Bhattacharyya, T.K. Maiti, Biocompatibility of diamondlike nanocomposite thin films, J Mater. Sci. Mater. Med. 18 (2007) 493–500. [15] M. Born, E. Wolf, Principles of Optics, 6th ed., Pergamon Press, New York, 1980. [16] J. Goodman, Introduction to Fourier Optics, 3rd ed., Roberts & Co, Colorado, 2004. [17] J. Sánchez-González, A. Díaz-Parralejo, A.L. Ortiz, F. Guiberteau, Determination of optical properties in nanostructured thin films using the Swanepoel method, Appl. Surf. Sci. 252 (2006) 6013–6017. [18] B. Swatowska, T. Stapinski, S. Zimowski, Properties of a-Si:N:H films beneficial for silicon solar cells applications, Opto-Electron. Rev. 20 (2012) 168–173.