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The effect of thickness on the properties of titanium films deposited by dc magnetron sputtering
Materials Science and Engineering A 458 (2007) 361–365
The effect of thickness on the properties of titanium films deposited by dc magnetron sputtering Y.L. Jeyachandran, B. Karunagaran, Sa.K. Narayandass ∗ , D. Mangalaraj Department of Physics, Bharathiar University, Coimbatore 641046, Tamil Nadu, India Received 29 September 2006; received in revised form 13 December 2006; accepted 19 December 2006
Abstract Titanium (Ti) films of thickness in the range of 101–254 nm were prepared onto glass and silicon substrates by direct current magnetron sputtering method. The effect of thickness on the electrical, structural, optical and surface properties of the films was studied. The room temperature sheet resistance decreased from 14.3 to 3.6 / and the temperature coefficient of resistance value increased from 0.14 to 0.20% K−1 with increase in the thickness of the film. The optical and surface composition characteristics of the films were least influenced by the thickness. The films of all thickness exhibited hexagonal closed packed crystalline structure with predominant (0 0 2) crystallite orientation and an additional (1 0 0) orientation particularly in the films of thickness in the range of 153–205 nm. The average particle size in the films varied from 9 to 40 nm with the thickness. © 2007 Elsevier B.V. All rights reserved. Keywords: Titanium films; Thickness; Properties
1. Introduction The present report is a continuation of our earlier work [1]. In our previous report, a systematic investigation on the properties of the titanium (Ti) films with respect to the deposition parameters under more conventional condition has been presented. The Ti films prepared at conventional conditions, that is, at a vacuum of 4 × 10−4 Pa or less, may contain impurities in form of carbide, oxide and nitride that makes them less suitable for high sensitive application such as in microelectronics and sensors. However, this processing condition could be considered for protective and hard coating applications. In protective applications such as corrosion resistance or biomedical coatings, the thickness of the films has a greater effect in terms of substrate bonding, structural and surface properties [2,3]. Hence the present work is aimed at to investigate the effect of thickness on the properties of Ti films. 2. Experimental details Ti films were sputter deposited onto cleaned glass and silicon substrates kept at room temperature from a Ti metal target
∗
Corresponding author. Tel.: +91 422 2425458; fax: +91 422 2422387. E-mail address: [email protected] (Sa.K. Narayandass).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.088
(75 mm diameter × 5 mm thick, 99.995% pure, PI-KEM, England) mechanically clamped to the magnetron cathode of the sputtering system. Commercial argon was used as the sputtering gas. The target-to-substrate separation was 100 mm. Prior to the film deposition vacuum and target conditioning were performed. The deposition chamber was pumped down to the ultimate vacuum and repeatedly charged with argon and pumped down in order to minimize the residual gas components. The Ti target was pre-sputtered at a sputtering pressure of 2 Pa and cathode power of 125 W to sputter out the surface oxide layer. The presputtering was done until the Ti characteristic plasma glow (dark blue colour) appeared. The Ti films of thickness in the range of 101–254 nm were prepared onto glass and silicon substrates under an optimum condition of base vacuum 4 × 10−4 Pa, sputtering pressure 1.1 Pa and cathode power 125 W. The silicon and glass substrates were cleaned according the procedures described elsewhere [1]. The thickness of the films were obtained using multiple beam interferometer and cross-checked by spectroscopy ellipsometer (SE) measurements. All the characterisation experiments were performed ex situ and other than the resistance–temperature (R–T) measurements all the other characterisations were made at room temperature. For SE measurements, the films prepared on silicon substrates were used and for all other characterisations the films prepared on glass substrates were used.
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Fig. 1. Sheet resistance vs. temperature plots of the Ti films of different thicknesses.
Fig. 3. Basic spectroscopy ellipsometry spectra of the Ti films of thickness 101, 153, 205 and 254 nm.
Fig. 2. Temperature coefficient resistance values of the Ti films of different thicknesses.
Fig. 4. Refractive index and extinction coefficient spectra of the Ti films of thickness 101, 153, 205 and 254 nm.
Y.L. Jeyachandran et al. / Materials Science and Engineering A 458 (2007) 361–365
Fig. 5. Light penetration depth vs. wavelength plot measured for the Ti film of thickness 205 nm.
Sheet resistance (R) and R–T measurements for the films were made using a four-point probe system (Scientific Equipments, Roorkee, India). Four contacts in linear geometry were made with the films using adjustable spring loaded pointers. A constant current source (100 A resolution) and a digital microvoltmeter (1 V resolution) were used to apply current and measure the voltage, respectively, and a PID controlled oven (0.1 ◦ C resolution) was used as the temperature source. The chemical nature of the outermost part of the films was obtained by X-ray
363
photoelectron spectroscopy (XPS, Specs Sage 150 spectrometer). The measurements were performed at a base pressure of 10−7 Pa using an Mg K␣ X-ray (λ = 1253.6 eV) source under a resolution of 0.1 eV. The electron analyser pass energy in the XPS high-resolution scans was 35.75 eV. The take-off angle of the photoelectrons was 45◦ . The Unifittu (version 2.1) [4] software was used for peak fitting and quantitative chemical analysis, applying sensitivity factors given by the manufacturer of the instrument. The high-resolution spectra were charge compensated by setting the binding energy (BE) of the carbon (C 1s) contamination peak to 284.4 eV. The optical constants of the films were derived from the SE results. Ellipsometry data of films were obtained using Jobin-Yvon Uvisel spectroscopy ellipsometer in the wavelength range of 300–800 nm. It is a phase-modulation ellipsometer. The obtained data were fitted into a model defined by the Forouhi–Bloomer (F–B) equations derived from the Kramers–Kronig relationship [5]. The structural properties of the films were studied using Xray diffractometer (XRD). The XRD patterns were obtained from Philips PW 3040 XRD using Cu K␣1 (λ = 0.1542 nm) radiation operated at 30 kV and 30 mA. The scan was performed in continuous mode for a 2θ range of 20–60◦ with a step of 0.02◦ . The scan speed was 2◦ min−1 . and the sample spin speed applied was 30 rpm. The morphological properties of the films were studied using the micrographs obtained from a scanning electron microscope (SEM, Siemens, UK). The images were taken at an accelerating voltage of 5 kV.
Fig. 6. X-ray photoelectron spectroscopy survey spectra and high-resolution Ti 2p peaks of the Ti films of thickness 153, 173 and 254 nm.
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Table 1 Refractive index and extinction coefficient values of the Ti films at a typical wavelength of 632.8 nm Thickness (nm)
n
k
101 153 205 254
2.55 2.64 2.57 2.59
3.12 3.18 3.17 3.13
3. Results and discussion The behaviour of R of the Ti films in the temperature range of 303–373 K is shown in Fig. 1. The films showed metallic behaviour, that is, an increase in R with temperature. The temperature coefficient of resistance (TCR) values of the films of different thicknesses as evaluated from the R–T plots are presented in Fig. 2. With the decrease of surface-to-volume ratio, the room temperature R and TCR values followed the general rule (R ∝ 1/thickness and TCR ∝ 1/R (303 K)) whereby R (303 K) decreased from 14.3 to 3.6 / and TCR increased from 0.14 to 0.20% K−1 with increase in thickness of the films. The tan Ψ and cos Δ spectra of the Ti films obtained from SE are shown in Fig. 3. No significant difference in the tan Ψ and cos Δ patterns among the films of different thickness was observed. This characteristic may be due to the existence of nearly similar surface chemical composition and uniform microstructure in the films. The detailed discussion on the SE spectra correlated with the surface composition and microstructure of Ti films may be found in our previous publication [1]. The spectra of the optical constants such as refractive index (n) and extinction coefficient (k) of the films derived from the SE spectra are shown in Fig. 4 and the n and k values of the films at a typical wavelength of 632.8 nm are presented in Table 1. The films exhibited least variations in their n and k spectra and the n and k values were found to be in the metallic Ti range [6]. The observed optical features may be the characteristics of the top 20 nm of the Ti films, as the penetration depth measurements showed that the light of wavelength range of 300–800 nm could penetrate up to a depth of 14–19.2 nm of Ti films. The penetration depth versus wavelength graph for the Ti film of typical thickness 205 nm is shown in Fig. 5. The XPS survey spectra of the surface of the Ti films showed two essential peaks at 530.7 and 458.3 eV corresponding to Ti 2p and O 1s electron energy levels. The typical XPS survey spectra and the peak fitted high resolution scans of the Ti 2p peak of the films of thickness 153, 173 and 254 nm are shown in Fig. 6. The
Fig. 7. X-ray diffraction patterns of the Ti films of different thicknesses.
Ti 2p peaks resolved into two spin orbit coupling components Ti 2p1/2 and Ti 2p3/2 at binding energies 464.5 and 458.8 eV, respectively, mainly due to the presence of surface native oxide [3]. The Ti 2p3/2 peak showed three component resolutions centered at 458.7 (I), 455.6 (II) and 453.9 eV (III). The probable assignment to the origin of the components [7] and the evaluated relative percentage of the components are given in Table 2. The variation in the percentage of the components among the film was found to be least as evidenced in SE studies (tan Ψ spectra). The I (TiO2 ), II (Ti sub-oxide phase, Ti2 O3 ) and III (Ti metallic) components varied in a narrow range between 81.4 and 84.5%, 4.5–5.9% and 10.8–14.1%, respectively, with an evaluation error within 1%. The XRD patterns of the Ti films are shown in Fig. 7. Two diffraction peaks were observed in the patterns. The peak at 2θ ∼ 37.8◦ that corresponds to the (0 0 2) orientation of the hexagonal closed packed (hcp) structure [8] was observed in films of all thicknesses and was the preferred orientation
Table 2 Assignment to the Ti 2p3/2 peak (Fig. 6) components and relative percentage of the components on the surface of the Ti films of thickness 153, 173 and 254 nm Component
I II III a
Binding energy (eV)
458.7 455.6 453.9 The component may probably be assigned to the Ti2 O3 phase.
Assignment
TiO2 Sub-oxidea Ti-metallic
Relative percentage (%) 153 nm
173 nm
254 nm
81.4 4.5 14.1
81.7 5.9 12.4
84.5 4.7 10.8
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Fig. 8. Scanning electron microscopy images of the Ti films of thickness 153 and 254 nm with average particle size of 9 and 35 nm, respectively.
of the Ti crystallites. The hcp structure is the characteristic crystalline phase of Ti at room temperature. The (0 0 2) orientation was reported as the preferred one in Ti films on large mismatch substrates like glass [8]. This is because (0 0 2) orientation has low surface energy and thus will be the preferred orientation in terms of surface free energy [9]. In addition to the preferred (0 0 2) orientation, another peak at 2θ ∼ 34.78◦ that corresponds to the (1 0 0) orientation [10] was observed in the films of thickness 153, 173 and 205 nm. The intensity of the (1 0 0) peak decreased with increase in thickness. The XRD results of the films may be interpreted to some extent on the basis of stress and strain evolution mechanism in the films. In general, the (0 0 2) orientation will be preferred in Ti films on glass substrates. With increase in thickness the development of (1 0 0) orientation in the films may be due to the stress activated mechanisms. It has been reported that at initial stage of the Ti film growth tensile stress develop in the film that changes to compressive mode during growth and again change to tensile mode at the final stage of growth [2]. Therefore, a competition would have existed between the tensile and compressive stress modes in the films with a specific stress mode dominating at a specific or a range of thickness. At lower thickness the (0 0 2) orientation would have been supported by the dominant tensile stress [2,3]. With increase in thickness the compressive stress would have evolved in the films resulting the development of (1 0 0) orientation and that would have relaxed to tensile mode at higher thickness favouring the (0 0 2) preferred orientation. The morphology of the Ti films obtained by SEM was found to be void free and uniform as evidenced from of SE studies (cos Δ spectra). Additionally, the SEM micrographs exhibited the existence of densely pack microstructure in the films with better crystallite resolution. The average particle size in the films as evaluated from the micrographs was found to be in the range of 9–40 nm with increase in thickness. The typical SEM micrographs of the films of thickness 153 and 254 nm are shown in Fig. 8.
4. Conclusions The effect of thickness on the electrical, optical, structural and surface composition properties of the titanium films in the thickness range of 101–254 nm was studied. The variation in chemical composition and optical constants of the films with thickness was least significant. The room temperature sheet resistance and TCR values of the films varied as 14.3 to 3.6 / and 0.14 to 0.20% K−1 , respectively, with thickness. A notable effect of thickness was observed in the structural properties. Although the crystallite orientation (0 0 2) of hcp structure was predominant in the films, an additional (1 0 0) orientation developed in the films specifically of thickness in the range of 153–205 nm. The stress activation or relaxation mechanisms were supposed to be the reason for these structural features. Acknowledgement We sincerely thank Dr. T.E. Jenkins, University of Wales, for his kind help in performing penetration depth measurements and for the valuable suggestions. References [1] Y.L. Jeyachandran, B. Karunagaran, Sa.K. Narayandass, D. Mangalaraj, T.E. Jenkins, P.J. Martin, Mater. Sci. Eng. A 431 (2006) 277. [2] M. Poppeller, R. Abermann, Thin Solid Films 295 (1997) 60. [3] K. Cai, M. Muller, J. Bossert, A. Rechtenbach, K.D. Jandt, Appl. Surf. Sci. 250 (2005) 75. [4] R. Hesse, T. Chasse, P. Streubel, R. Szargan, Surf. Interf. Anal. 36 (2004) 1373. [5] A.R. Forouhi, J. Bloomer, Phys. Rev. B 38 (1989) 1865. [6] H.G. Tompkins, R. Gregory, B. Boeck, Surf. Interf. Anal. 17 (1991) 22. [7] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mulengreg, Handbook of X-ray Photoelectron Spectroscopy, Physical Electrons Division, Perkin-Elmer Corp., Eden Praire, MN, 1979. [8] R. Checcbetto, Thin Solid Films 302 (1997) 77. [9] T. Suzuki, Y. Kamimura, K.O.K. Kirchiner, Phil. Mag. A 79 (1999) 1629. [10] S.R. McCarthy, North Dakota University, US, ICDD Grant-in-Aid, 1993 (JCPDS No. 44-1294).
The effect of thickness on the properties of titanium films deposited by dc magnetron sputtering Y.L. Jeyachandran, B. Karunagaran, Sa.K. Narayandass ∗ , D. Mangalaraj Department of Physics, Bharathiar University, Coimbatore 641046, Tamil Nadu, India Received 29 September 2006; received in revised form 13 December 2006; accepted 19 December 2006
Abstract Titanium (Ti) films of thickness in the range of 101–254 nm were prepared onto glass and silicon substrates by direct current magnetron sputtering method. The effect of thickness on the electrical, structural, optical and surface properties of the films was studied. The room temperature sheet resistance decreased from 14.3 to 3.6 / and the temperature coefficient of resistance value increased from 0.14 to 0.20% K−1 with increase in the thickness of the film. The optical and surface composition characteristics of the films were least influenced by the thickness. The films of all thickness exhibited hexagonal closed packed crystalline structure with predominant (0 0 2) crystallite orientation and an additional (1 0 0) orientation particularly in the films of thickness in the range of 153–205 nm. The average particle size in the films varied from 9 to 40 nm with the thickness. © 2007 Elsevier B.V. All rights reserved. Keywords: Titanium films; Thickness; Properties
1. Introduction The present report is a continuation of our earlier work [1]. In our previous report, a systematic investigation on the properties of the titanium (Ti) films with respect to the deposition parameters under more conventional condition has been presented. The Ti films prepared at conventional conditions, that is, at a vacuum of 4 × 10−4 Pa or less, may contain impurities in form of carbide, oxide and nitride that makes them less suitable for high sensitive application such as in microelectronics and sensors. However, this processing condition could be considered for protective and hard coating applications. In protective applications such as corrosion resistance or biomedical coatings, the thickness of the films has a greater effect in terms of substrate bonding, structural and surface properties [2,3]. Hence the present work is aimed at to investigate the effect of thickness on the properties of Ti films. 2. Experimental details Ti films were sputter deposited onto cleaned glass and silicon substrates kept at room temperature from a Ti metal target
∗
Corresponding author. Tel.: +91 422 2425458; fax: +91 422 2422387. E-mail address: [email protected] (Sa.K. Narayandass).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.088
(75 mm diameter × 5 mm thick, 99.995% pure, PI-KEM, England) mechanically clamped to the magnetron cathode of the sputtering system. Commercial argon was used as the sputtering gas. The target-to-substrate separation was 100 mm. Prior to the film deposition vacuum and target conditioning were performed. The deposition chamber was pumped down to the ultimate vacuum and repeatedly charged with argon and pumped down in order to minimize the residual gas components. The Ti target was pre-sputtered at a sputtering pressure of 2 Pa and cathode power of 125 W to sputter out the surface oxide layer. The presputtering was done until the Ti characteristic plasma glow (dark blue colour) appeared. The Ti films of thickness in the range of 101–254 nm were prepared onto glass and silicon substrates under an optimum condition of base vacuum 4 × 10−4 Pa, sputtering pressure 1.1 Pa and cathode power 125 W. The silicon and glass substrates were cleaned according the procedures described elsewhere [1]. The thickness of the films were obtained using multiple beam interferometer and cross-checked by spectroscopy ellipsometer (SE) measurements. All the characterisation experiments were performed ex situ and other than the resistance–temperature (R–T) measurements all the other characterisations were made at room temperature. For SE measurements, the films prepared on silicon substrates were used and for all other characterisations the films prepared on glass substrates were used.
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Fig. 1. Sheet resistance vs. temperature plots of the Ti films of different thicknesses.
Fig. 3. Basic spectroscopy ellipsometry spectra of the Ti films of thickness 101, 153, 205 and 254 nm.
Fig. 2. Temperature coefficient resistance values of the Ti films of different thicknesses.
Fig. 4. Refractive index and extinction coefficient spectra of the Ti films of thickness 101, 153, 205 and 254 nm.
Y.L. Jeyachandran et al. / Materials Science and Engineering A 458 (2007) 361–365
Fig. 5. Light penetration depth vs. wavelength plot measured for the Ti film of thickness 205 nm.
Sheet resistance (R) and R–T measurements for the films were made using a four-point probe system (Scientific Equipments, Roorkee, India). Four contacts in linear geometry were made with the films using adjustable spring loaded pointers. A constant current source (100 A resolution) and a digital microvoltmeter (1 V resolution) were used to apply current and measure the voltage, respectively, and a PID controlled oven (0.1 ◦ C resolution) was used as the temperature source. The chemical nature of the outermost part of the films was obtained by X-ray
363
photoelectron spectroscopy (XPS, Specs Sage 150 spectrometer). The measurements were performed at a base pressure of 10−7 Pa using an Mg K␣ X-ray (λ = 1253.6 eV) source under a resolution of 0.1 eV. The electron analyser pass energy in the XPS high-resolution scans was 35.75 eV. The take-off angle of the photoelectrons was 45◦ . The Unifittu (version 2.1) [4] software was used for peak fitting and quantitative chemical analysis, applying sensitivity factors given by the manufacturer of the instrument. The high-resolution spectra were charge compensated by setting the binding energy (BE) of the carbon (C 1s) contamination peak to 284.4 eV. The optical constants of the films were derived from the SE results. Ellipsometry data of films were obtained using Jobin-Yvon Uvisel spectroscopy ellipsometer in the wavelength range of 300–800 nm. It is a phase-modulation ellipsometer. The obtained data were fitted into a model defined by the Forouhi–Bloomer (F–B) equations derived from the Kramers–Kronig relationship [5]. The structural properties of the films were studied using Xray diffractometer (XRD). The XRD patterns were obtained from Philips PW 3040 XRD using Cu K␣1 (λ = 0.1542 nm) radiation operated at 30 kV and 30 mA. The scan was performed in continuous mode for a 2θ range of 20–60◦ with a step of 0.02◦ . The scan speed was 2◦ min−1 . and the sample spin speed applied was 30 rpm. The morphological properties of the films were studied using the micrographs obtained from a scanning electron microscope (SEM, Siemens, UK). The images were taken at an accelerating voltage of 5 kV.
Fig. 6. X-ray photoelectron spectroscopy survey spectra and high-resolution Ti 2p peaks of the Ti films of thickness 153, 173 and 254 nm.
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Y.L. Jeyachandran et al. / Materials Science and Engineering A 458 (2007) 361–365
Table 1 Refractive index and extinction coefficient values of the Ti films at a typical wavelength of 632.8 nm Thickness (nm)
n
k
101 153 205 254
2.55 2.64 2.57 2.59
3.12 3.18 3.17 3.13
3. Results and discussion The behaviour of R of the Ti films in the temperature range of 303–373 K is shown in Fig. 1. The films showed metallic behaviour, that is, an increase in R with temperature. The temperature coefficient of resistance (TCR) values of the films of different thicknesses as evaluated from the R–T plots are presented in Fig. 2. With the decrease of surface-to-volume ratio, the room temperature R and TCR values followed the general rule (R ∝ 1/thickness and TCR ∝ 1/R (303 K)) whereby R (303 K) decreased from 14.3 to 3.6 / and TCR increased from 0.14 to 0.20% K−1 with increase in thickness of the films. The tan Ψ and cos Δ spectra of the Ti films obtained from SE are shown in Fig. 3. No significant difference in the tan Ψ and cos Δ patterns among the films of different thickness was observed. This characteristic may be due to the existence of nearly similar surface chemical composition and uniform microstructure in the films. The detailed discussion on the SE spectra correlated with the surface composition and microstructure of Ti films may be found in our previous publication [1]. The spectra of the optical constants such as refractive index (n) and extinction coefficient (k) of the films derived from the SE spectra are shown in Fig. 4 and the n and k values of the films at a typical wavelength of 632.8 nm are presented in Table 1. The films exhibited least variations in their n and k spectra and the n and k values were found to be in the metallic Ti range [6]. The observed optical features may be the characteristics of the top 20 nm of the Ti films, as the penetration depth measurements showed that the light of wavelength range of 300–800 nm could penetrate up to a depth of 14–19.2 nm of Ti films. The penetration depth versus wavelength graph for the Ti film of typical thickness 205 nm is shown in Fig. 5. The XPS survey spectra of the surface of the Ti films showed two essential peaks at 530.7 and 458.3 eV corresponding to Ti 2p and O 1s electron energy levels. The typical XPS survey spectra and the peak fitted high resolution scans of the Ti 2p peak of the films of thickness 153, 173 and 254 nm are shown in Fig. 6. The
Fig. 7. X-ray diffraction patterns of the Ti films of different thicknesses.
Ti 2p peaks resolved into two spin orbit coupling components Ti 2p1/2 and Ti 2p3/2 at binding energies 464.5 and 458.8 eV, respectively, mainly due to the presence of surface native oxide [3]. The Ti 2p3/2 peak showed three component resolutions centered at 458.7 (I), 455.6 (II) and 453.9 eV (III). The probable assignment to the origin of the components [7] and the evaluated relative percentage of the components are given in Table 2. The variation in the percentage of the components among the film was found to be least as evidenced in SE studies (tan Ψ spectra). The I (TiO2 ), II (Ti sub-oxide phase, Ti2 O3 ) and III (Ti metallic) components varied in a narrow range between 81.4 and 84.5%, 4.5–5.9% and 10.8–14.1%, respectively, with an evaluation error within 1%. The XRD patterns of the Ti films are shown in Fig. 7. Two diffraction peaks were observed in the patterns. The peak at 2θ ∼ 37.8◦ that corresponds to the (0 0 2) orientation of the hexagonal closed packed (hcp) structure [8] was observed in films of all thicknesses and was the preferred orientation
Table 2 Assignment to the Ti 2p3/2 peak (Fig. 6) components and relative percentage of the components on the surface of the Ti films of thickness 153, 173 and 254 nm Component
I II III a
Binding energy (eV)
458.7 455.6 453.9 The component may probably be assigned to the Ti2 O3 phase.
Assignment
TiO2 Sub-oxidea Ti-metallic
Relative percentage (%) 153 nm
173 nm
254 nm
81.4 4.5 14.1
81.7 5.9 12.4
84.5 4.7 10.8
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Fig. 8. Scanning electron microscopy images of the Ti films of thickness 153 and 254 nm with average particle size of 9 and 35 nm, respectively.
of the Ti crystallites. The hcp structure is the characteristic crystalline phase of Ti at room temperature. The (0 0 2) orientation was reported as the preferred one in Ti films on large mismatch substrates like glass [8]. This is because (0 0 2) orientation has low surface energy and thus will be the preferred orientation in terms of surface free energy [9]. In addition to the preferred (0 0 2) orientation, another peak at 2θ ∼ 34.78◦ that corresponds to the (1 0 0) orientation [10] was observed in the films of thickness 153, 173 and 205 nm. The intensity of the (1 0 0) peak decreased with increase in thickness. The XRD results of the films may be interpreted to some extent on the basis of stress and strain evolution mechanism in the films. In general, the (0 0 2) orientation will be preferred in Ti films on glass substrates. With increase in thickness the development of (1 0 0) orientation in the films may be due to the stress activated mechanisms. It has been reported that at initial stage of the Ti film growth tensile stress develop in the film that changes to compressive mode during growth and again change to tensile mode at the final stage of growth [2]. Therefore, a competition would have existed between the tensile and compressive stress modes in the films with a specific stress mode dominating at a specific or a range of thickness. At lower thickness the (0 0 2) orientation would have been supported by the dominant tensile stress [2,3]. With increase in thickness the compressive stress would have evolved in the films resulting the development of (1 0 0) orientation and that would have relaxed to tensile mode at higher thickness favouring the (0 0 2) preferred orientation. The morphology of the Ti films obtained by SEM was found to be void free and uniform as evidenced from of SE studies (cos Δ spectra). Additionally, the SEM micrographs exhibited the existence of densely pack microstructure in the films with better crystallite resolution. The average particle size in the films as evaluated from the micrographs was found to be in the range of 9–40 nm with increase in thickness. The typical SEM micrographs of the films of thickness 153 and 254 nm are shown in Fig. 8.
4. Conclusions The effect of thickness on the electrical, optical, structural and surface composition properties of the titanium films in the thickness range of 101–254 nm was studied. The variation in chemical composition and optical constants of the films with thickness was least significant. The room temperature sheet resistance and TCR values of the films varied as 14.3 to 3.6 / and 0.14 to 0.20% K−1 , respectively, with thickness. A notable effect of thickness was observed in the structural properties. Although the crystallite orientation (0 0 2) of hcp structure was predominant in the films, an additional (1 0 0) orientation developed in the films specifically of thickness in the range of 153–205 nm. The stress activation or relaxation mechanisms were supposed to be the reason for these structural features. Acknowledgement We sincerely thank Dr. T.E. Jenkins, University of Wales, for his kind help in performing penetration depth measurements and for the valuable suggestions. References [1] Y.L. Jeyachandran, B. Karunagaran, Sa.K. Narayandass, D. Mangalaraj, T.E. Jenkins, P.J. Martin, Mater. Sci. Eng. A 431 (2006) 277. [2] M. Poppeller, R. Abermann, Thin Solid Films 295 (1997) 60. [3] K. Cai, M. Muller, J. Bossert, A. Rechtenbach, K.D. Jandt, Appl. Surf. Sci. 250 (2005) 75. [4] R. Hesse, T. Chasse, P. Streubel, R. Szargan, Surf. Interf. Anal. 36 (2004) 1373. [5] A.R. Forouhi, J. Bloomer, Phys. Rev. B 38 (1989) 1865. [6] H.G. Tompkins, R. Gregory, B. Boeck, Surf. Interf. Anal. 17 (1991) 22. [7] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mulengreg, Handbook of X-ray Photoelectron Spectroscopy, Physical Electrons Division, Perkin-Elmer Corp., Eden Praire, MN, 1979. [8] R. Checcbetto, Thin Solid Films 302 (1997) 77. [9] T. Suzuki, Y. Kamimura, K.O.K. Kirchiner, Phil. Mag. A 79 (1999) 1629. [10] S.R. McCarthy, North Dakota University, US, ICDD Grant-in-Aid, 1993 (JCPDS No. 44-1294).