Structure and composition of zirconium carbide thin

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May 12, 2014 - Figure 2 of the original article PDF file, as supplied to AIP Publishing, contained a PDF- processing error. .... V beam voltage and a gas flow rate of 3 standard cubic centimeters gives .... A small peak at binding energy 343.4eV corresponds to. Zr 3p1/2. ... Dr. V. Ganesan and Mr Mohan Gangrade for AFM.

Structure and composition of zirconium carbide thin-film grown by ion beam sputtering for optical applications Amol Singh, Mohammed H. Modi, Rajnish Dhawan, and G. S. Lodha Citation: AIP Conference Proceedings 1591, 869 (2014); doi: 10.1063/1.4872785 View online: http://dx.doi.org/10.1063/1.4872785 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1591?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Compound-target sputtering for niobium carbide thin-film deposition J. Vac. Sci. Technol. B 22, L24 (2004); 10.1116/1.1800491 Structural and optical properties of thin zirconium oxide films prepared by reactive direct current magnetron sputtering J. Appl. Phys. 92, 3599 (2002); 10.1063/1.1503858 Structural characteristics and hardness of zirconium carbide films prepared by tri-ion beam-assisted deposition J. Vac. Sci. Technol. A 16, 2337 (1998); 10.1116/1.581349 Optical properties of dense thinfilm Si and Ge prepared by ionbeam sputtering J. Appl. Phys. 58, 954 (1985); 10.1063/1.336172 Optical recording applications of reactive ion beam sputter deposited thinfilm composites Appl. Phys. Lett. 44, 1023 (1984); 10.1063/1.94631

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Corrigendum: Structure and Composition of Zirconium Carbide Thin-Film Grown by Ion Beam Sputtering for Optical Applications, Amol Singh, Mohammed H. Modi, Rajnish Dhawan and G. S. Lodha, AIP Conf. Proc. 1591, 869 (2014) Figure 2 of the original article PDF file, as supplied to AIP Publishing, contained a PDFprocessing error. This article was updated on 12 May 2014 to correct that error.

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Structure and Composition of Zirconium Carbide Thin-Film Grown by Ion Beam Sputtering for Optical Applications Amol Singh1, Mohammed.H.Modi1,*, Rajnish Dhawan1, G.S.Lodha1 1

X-ray Optics Section, ISU Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, India * Email Corresponding Author: [email protected]

Abstract. Thin film of compound material ZrC was deposited on Si (100) wafer using ion beam sputtering method. The deposition was carried out at room temperature and at base pressure of 3×10−5 Pa. X-ray photoelectron spectroscopy (XPS) measurements were performed for determining the surface chemical compositions. Grazing incidence x-ray reflectivity (GIXRR) measurements were performed to study the film thickness, roughness and density. From GIXRR curve roughness value of the film was found less than 1 nm indicating smooth surface morphology. Films density was found 6.51 g/cm3, which is close to bulk density. Atomic force microscopy (AFM) measurements were performed to check the surface morphology. AFM investigation showed that the film surface is smooth, which corroborate the GIXRR data. Keywords: Thin films, Ion beam evaporation, X-ray reflectivity, Multilayers, X-ray optics. PACS: 68.60.Bs, 81.15.Jj, 61.05.cm, 78.67.Pt, 41.50.+h

INTRODUCTION

Deposition of high quality ZrC thin films are difficult due to its high melting point, low evaporation and sputtering rate, high reactivity of Zr with oxygen and water vapours. Earlier several techniques are used for the deposition of ZrC thin films such as physical vapor deposition, chemical vapor deposition, dc magnetron sputtering, ebeam evaporation, pulsed laser deposition (PLD) etc. In PLD technique deposition of such material is quite challenging because of very demanding deposition condition such as excess substarte temperature and very high laser fluence. A laser fluence around 10J/cm2 is required which is quite challenging to be implemented. ZrC thin film of thickness 300Ǻ was grown on Si (100) substrate using ion beam sputtering method. Film thickness, roughness and film density was measured using Grazing incidence x-ray reflectivity (GIXRR) technique. X-ray photoelectron spectroscopy (XPS) technique was used for the surface elemental composition; AFM measurements were performed to investigate surface morphology.

Multilayers comprised of low Z/ high Z elements are widely used in variety of applications in soft x-ray region. Such multilayer poses a severe drawback of high reactivity among the constituent elements. Formation of silicide in Si based multilayers (Mo/Si, W/Si, Nb/Si etc) is a common problem. These multilayers have very poor thermal stability because of intermixing of layers in high heat load environment. To overcome the inter-diffusion a barrier layer (of carbon) is deposited in between Mo and Si, which reduces reflectivity in significant amount. Apart from a barrier layer many authors have used compound materials to prevent interfacial reaction. Recently NbC/Si multilayer was proposed as a replacement of Mo/Si multilayer. This new combination shows high thermal stability and equivalent reflectivity performance as one obtained with Mo/Si structure1. The NbC/Si multilayer structure was reported to be stable upto 700°C annealing temperature. A. F. Jankowski et al. have reported W/B4C a better combination over W/C multilayers in terms of thermal stability2. Qi Zhong et al. have reported Zr/Al multilayer a good combination for the use of EUV applications, but Zr/Al has very poor thermal stability after 200°C3. Intermixing and Al-Zr alloy formation are reported after 200°C in Zr/Al multilayers. ZrC can be a good substitute for Zr. In optical applications composition and microstructure of the deposited films are critical and must be investigated to improve the quality of thin films. ZrC have some excellent properties which makes it very attractive in variety of demanding applications, such as very high melting point (Tm=3445°C), very high hardness (30-35 GPa), high thermo chemical stability, high wear resistance, high optical emissivity, etc.4.

EXPERIMENTAL Thin film of ZrC of 300Ǻ was deposited on Si (100) wafer using ion beam sputtering setup. Commercially available sputtering target of ZrC having 99.99% purity was used for the deposition. Prior to deposition the system was evacuated to the base pressure of 3×10−5 Pa and deposition was carried out at constant pressure of 6×10−2 Pa using Ar ions. The process parameters for obtaining smooth and uniform ZrC film were optimized by depositing various thin film samples where it was found that the films deposited at 1000 V beam voltage and a gas flow rate of 3 standard cubic centimeters gives low roughness and uniform thickness film.

Solid State Physics AIP Conf. Proc. 1591, 869-871 (2014); doi: 10.1063/1.4872785 © 2014 AIP Publishing LLC 978-0-7354-1225-5/$30.00

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X-ray photoelectron spectroscopy (XPS) measurements were carried using an Omicron EA-125 photo electron spectrometer working at a base pressure of ~6.66×10-8 Pa. Al Kα radiation was used for the analysis, with the source operated at 10 kV anode voltage and 10 mA emission current. 100

ZrC 300 Angstrom Measured Fitted

-1

10

Reflectivity

This decrement in film density can be explained by smaller crystalline grains compared with epitaxial films and increased grain boundary volume fraction and voids for the films. V. Cracium et al. have deposited ZrC thin films for application in hard coating8. They achieved density of 5.80 gm/cm3 at 30°C substrate temperature, which is very low compared to bulk value. They have achieved highest density of 6.60 gm/cm3 at substrate temperature 500°C. Near bulk desnty in ZrC film was obtained in thick samples of 1650Å by PLD technique9 . In the present case near bulk density was obtained in ZrC film for very low thickness value of 300Ǻ. This shows that the ion beam sputtering technique can be used to achieve near bulk density in the ZrC thin film after systematic process optimization. RMS surface roughness of ZrC film from the fitting of GIXRR data was found less than 1nm.

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Figure 1 Measured and fitted GIXRR data of ZrC thin film. Vertical line shows the critical angle for ZrC. GIXRR measurements in hard x-ray region were performed using a BRUKER D-8 system consisting of θ θ goniometer and x-ray source of Cu target at λ =1.54 Ǻ Cu-Kα. Parratt formalism5 was used to analyse the reflectivity data. Effect of surface roughness was taken in account using Novet-Croce model6. A nonlinear leastsquare refinement routine based on the Genetic algorithm was used to refine fitting parameters7. AFM measurements were performed with a Nanoscope III from Digital Instrument with super sharp silicon nitride cantilever to probe different portion of the film surface. Measurements were carried out over 1 μm, 2 μm and 5 μm scale in close contact mode over 256×256 pixel area. The probe fixed on a small spring is scanned over the surface of the sample. The force between tip and the surface was ~10-8 N. The relative position of tip and the surface was adjusted by a feedback system in such a way that the force remains constant.

800 700 ZrC 3p1/2

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Figure 2 Measured and fitted XPS spectrum for Zr 3p region acquired from as deposited ZrC thin film of thickness 300 Å. 500

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RESULTS AND DISCUSSION Figure 1 shows the measured and simulated GIXRR curves of ZrC thin film. The well resolved Kiessig fringes (shown in Figure 1) suggest that the interfaces in the film have a sharp density gradient and low roughness. GIXRR data were fitted with assuming three layer model in Motofit software. The three layer model comprised of a ~30-40 Ǻ native oxide SiO2 layer on the substarte, a ion beam deposited ZrC layer and a third surface layer formed due to contamination/oxidation in the ambient environment. After rigorous fitting the thickness of the ZrC film (including surface layer) was found to be 343Ǻ The density of the film evaluated from the critical angle position as marked by vertical dashed line in the Figure 1 was found to be ~6.51 gm/cm3 which is slightly lower than the bulk value (6.73 gm/cm3) of epitaxial ZrC films.

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Figure 3 Measured and fitted XPS spectrum for C 1s region acquired from as deposited ZrC thin film of thickness 300 Å. Chemical composition of the film was analysed using XPS measurement. Figure 2 shows the high resolution 870

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XPS spectra corresponding to Zr 3p region acquired from the sample. XPS spectrum is fitted with four components. A small peak at binding energy 343.4eV corresponds to Zr 3p1/2. Peak position is taken from the elemental library supplied with the XPS instrument. The second peak with binding energy 346.96eV corresponds to ZrC 3p1/2. Another small peak with binding energy 329.7eV (from elemental library) corresponds to Zr 3p3/2. Similarly fourth peak with binding energy 333.23eV corresponds to ZrC 3p3/2. No other peaks were observed in the XPS spectrum indicating very less contamination on the surface. Carbon 1s core spectrum of the same sample is shown in Figure 3. The spectrum was fitted with two components. One with binding energy 285.19eV corresponding to C-C sp2 hybridization and other with binding energy 282.10eV which corresponds to carbon bonded to Zr. XPS investigation clearly indicates very less amount of contamination in the films. For analysing surface morphology, AFM measurements were performed and are shown in Figure 4. AFM images were taken at 1μ, 2μ and 5μ scales respectively. AFM data clearly shows smooth surface morphology. RMS roughness value was calculated from all the images using WSXM software10. Roughness value for 1μ×1μ image was 6.5Ǻ, for 2μ×2μ image roughness was 7.1Ǻ and for 5μ×5μ roughness was found 8.1Ǻ. Roughness calculated from GIXRR data and AFM images are in close agreement. AFM data confirms that film surface is smooth which is required for optical elements. Our detailed study of GIXRR, XPS and AFM data clearly indicates that ZrC thin film is grown with smooth suface morphology with density very close to bulk value. Further ZrC thin films of different thickness are grown to study the growth kinetics of ZrC and results will be published elsewhere.

Figure 4 AFM 2D topography (left panel) and corresponding 3D reconstructed (right panel) images of ZrC thin film. Scan is performed in three ranges 1μ, 2μ and 5μ.

REFERENCES 1

Mohammed H. Modi et al. Opt Exp. 20, 15114-15120 (2012). 2 A. F. Jankowski et al. J. Appl. Phys. 68, 5162-5168 (1990). 3 Qi Zhong et al. J. Phys. Conf. Ser. 425, 152010 (1-5) (2013). 4 V.Cracuin et al. J Eur. Cer. Soc. 33, 2223-2226 (2013). 5 L.G. Parratt, Phys. Rev. 95, 359-369 (1954). 6 L.Novet and P.Croce, Rev. Phys. Appl. 15, 761-779 (1980). 7 Nelson A, J. App. Cryst. 39, 273-276 (2006). 8 V. Cracium et al. J. Eur. Cer. Soci. 133, 2223-2226 (2013). 9 D. Cracium et al. Appl. Sur. Sci. 255, 5260-5263 (2009). 10 I. Horcas et al. Rev. Sci. Instr. 78, 013705 (1-8) (2007).

CONCLUSIONS ZrC thin film was deposited on Si substrate in high vacuum conditions by ion beam sputtering technique. GIXRR spectra indicated that the film density was around 6.51 gm/cm3, which is 95-97% of the tabulated bulk value. AFM data suggest that the film has smooth surface morphology. The study suggest that the ion beam sputtering technique can be used to grow compound film of ZrC material having near bulk density and smooth surface morphology.

ACKNOWLEDGEMENTS Authors are thankful to Dr. D. M. Phase Mr. A. D. Wadikar for XPS measurements. Authors are thankful to Dr. V. Ganesan and Mr Mohan Gangrade for AFM measurements.

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