- Email: [email protected]
Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses
Available online at www.sciencedirect.com
Applied Surface Science 254 (2008) 4013–4017 www.elsevier.com/locate/apsusc
Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses S. Petrovic´ a,*, B. Gakovic´ a, D. Perusˇko a, M. Trtica a, B. Radak a, P. Panjan b, Sˇ. Miljanic´ c ˇ A, P.O. Box 522, 11001 Belgrade, Serbia Institute of Nuclear Sciences, VINC b ‘‘Jozˇef Stefan’’ Institute, Jamova 39, 1000 Ljubljana, Slovenia c Faculty of Physical Chemistry, Studenski trg 14-16, 11000 Belgrade, Serbia
a
Received 29 November 2007; received in revised form 18 December 2007; accepted 18 December 2007 Available online 4 January 2008
Abstract Interaction of a nanosecond transversely excited atmospheric (TEA) CO2 laser, operating at 10.6 mm, with tungsten–titanium thin film (190 nm) deposited on silicon of n-type (1 0 0) orientation, was studied. Multi-pulse irradiation was performed in air atmosphere with laser energy densities in the range 24–49 J/cm2. The energy absorbed from the laser beam was mainly converted to thermal energy, which generated a series of effects. The following morphological changes were observed: (i) partial ablation/exfoliation of the WTi thin film, (ii) partial modification of the silicon substrate with formation of polygonal grains, (iii) appearance of hydrodynamic features including nano-globules. Torch-like plumes started appearing in front of the target after several laser pulses. # 2007 Elsevier B.V. All rights reserved. PACS : 68.55.Jk; 52.38.Mf; 79.20.Ds Keywords: WTi thin films; Laser ablation; Nanosecond laser pulse
1. Introduction The research into laser-induced solid surface modifications is significant in many aspects. Understanding of physical and chemical mechanisms of interaction can be important both at the fundamental and technological levels. Some important aspects of the use of lasers in material processing, particularly those that involve material removal and heat driven processes, are: melting, vaporization, condensation of vapor in the gas phase, diffusion, segregation, resolidification, laser–plume interaction, etc. All of these factors are responsible for changes of the structure, composition and chemical state of the irradiated surface. Laser irradiation may modify surface composition and structure of metals and alloys, increasing their resistance to wear and corrosion [1,2]. With laser irradiation it is possible to process and modify localized surface areas [3]. Laser ablation has attracted much attention as a relatively new technique for material processing in forming
* Corresponding author. Tel.: +381 11 8066425; fax: +381 11 8066425. E-mail address: [email protected] (S. Petrovic´). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.12.041
stable nano-sized metal particles [4]. Nanoparticles have shown very promising applications in lubrication and magnetic recording media production [5]. Electrical contact and diffusion barrier of tungsten (W) and tungsten–titanium (WTi) thin films have been studied for microelectronic applications [6,7], development of gas sensors [8,9], functional coatings such as smart windows [10] and protective anticorrosive and oxidation resistant coatings [11]. Nanoparticles made of tungsten and titanium can enhance mechanical properties of metallic alloys, such as steel, by dispersion strengthening of the metallic matrix [5]. Earlier results with a WTi coating on a steel substrate [12] showed noticeable effects of the substrate on the morphology of the damage induced, even though the thickness of the coating was larger than in the present case. This indicates the importance of the substrate material in such systems. Laser-induced surface modifications of a WTi thin film with thicknesses in the nano-domain have not been sufficiently reported in literature so far. The main objective of the present work was to study the surface modification induced by a TEA CO2 laser (at 10 mm wavelength) of a WTi thin film deposited on a silicon substrate. Morphological changes at laser energy
4014
S. Petrovic´ et al. / Applied Surface Science 254 (2008) 4013–4017
densities (fluences) ranging from 24 J/cm2 to 49 J/cm2 were investigated as a function of the number of accumulated laser pulses. 2. Experimental The tungsten–titanium thin films were deposited by dc sputtering of the 90%W–10%Ti wt. target by Ar+ ions. The substrate used in the experiment was silicon of n-type (1 0 0) orientation. Silicon wafers (0.5 mm thick) were polished and cleaned before deposition. Additional cleaning was also performed in the vacuum chamber by electron heating up to a temperature of 130 8C. The deposition was carried out by a Balzers Sputtron II vacuum system. The conditions during the deposition process were: acceleration voltage and current 1.5 kV and 0.7 A, respectively, partial pressure of argon 1.33 10 1 Pa. Under these experimental conditions the constant deposition rate was 0.14 nm/s, which produced a WTi thin film of 190 nm thickness, as measured by a Talystep I profilometer. The laser used in the experiment was an ultraviolet preionized TEA CO2 system [12]. It operated with a nontypical CO2/X, X = H2, H2/N2 gas mixture, which increased the efficiency of the laser. The laser pulse shape can generally be controlled by adjusting the gas mixture content [13,14]. Characteristics of the
laser pulses were the following: working gas mixture CO2/N2/ H2 = 1/2.1/1.3 (1000 mbar total), output pulse energy up to 170 mJ, initial spike 120 ns at FWHM (with about 2 ms tail), transversally multi-mode (approx. 1 cm square array), wavelengths 10.5709 mm and 10.5909 mm, repetition rate 2 Hz. Irradiation of the WTi thin film/Si system was carried out in ambient air atmosphere at a pressure of 1013 mbar. The laser beam was focused through a KBr lens of 6 cm focal length in a direction perpendicular to the sample surface. Various analytical techniques, before and after laser irradiation, were used for target characterization. Optical microscopy (OM) was used for initial general analysis of the modifications obtained. Surface morphology was further observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Topographic changes were characterized by a profilometer and AFM. Reflectivity of the target was observed in the wavelength region from 6.5 mm to 14 mm, by a Specord 75 IR spectrophotometer. 3. Results and discussion An X-ray analysis of the initial non-irradiated WTi thin film showed its polycrystalline structure, composed of a bcc tungsten (W) phase with a presence of a hcp titanium (Ti)
Fig. 1. Optical microscopy analyses of the WTi thin film/Si system after irradiation with TEA CO2 laser. The laser operated in multi-mode regime, with fluences ranging from 24 J/cm2 to 49 J/cm2 in individual peaks: (a) WTi thin film on Si prior laser action; WTi/Si system after (b) 1, (c) 20, (d) 50, (e) 100, (f) 200 laser pulses and (g) schematic of the laser cross-section intensity.
S. Petrovic´ et al. / Applied Surface Science 254 (2008) 4013–4017
phase [15]. The presence of titanium in the WTi thin film resulted in an expanded crystal lattice of W, in which the lattice ˚ . The main grain size of the WTi thin parameter was 3.2237 A film was estimated at about 14 nm, according to the tungsten peak width [16]. The reflectivity measurements in the spectral region around 10.6 mm showed that the thin film of WTi had an initial reflectivity of 87%. It is well known that the reflectivity of a target surface depends on the state of its surface, number of previously accumulated laser pulses, radiation wavelength, etc. [17]. Initially absorbed laser pulses change surface morphology and can cause physico-chemical transformations. These changes additionally increase target absorptivity. Consequently, the level of the target damage after hundreds of accumulated laser pulses can be significant. In the present experiments, the laser beam had a whole range of fluences at each pulse, due to its multi-mode
4015
cross-section. It is observable in Fig. 1 that fluences of the central peaks (estimated at 49 J/cm2) induced damage even at the first pulse, but the surrounding peaks (estimated at fluences of 24 J/ cm2) induce noticeable damage only after about 10 pulses. Generally, laser radiation is converted into thermal energy at the target, which generates series of effects. In case of complex targets with different thermophysical properties of the materials, as in the present experiment, exfoliation is possible. Difference in the thermal expansion coefficients between the WTi thin film and silicon substrate generates additional stress at the interface. A direct consequence of this is the appearance of microcracks and exfoliation of the thin film. Appearance of torch-like plume in front of the target was observed, but only after four successive pulses. The surface morphological changes can be summarized as the following.
Fig. 2. SEM analyses of the WTi/Si system induced by the TEA CO2 laser. (a–c) The WTi/Si system after irradiation with 10, 50, and 100 laser pulses, respectively; (a1, b1, c1) partially ablated regions; (a2, b2, c2) central parts of ablations; (a3, b3, c3) details of the same (central) areas with a different magnification; (d) WTi thin film surface before irradiation. Experimental conditions were the same as in Fig. 1.
4016
S. Petrovic´ et al. / Applied Surface Science 254 (2008) 4013–4017
3.1. OM analysis The first step in examination of the morphology of the irradiated targets was the analysis by an optical microscope, Fig. 1. The damage of the WTi thin film/Si system can be clearly recognized even after one laser pulse in Fig. 1b. This damage was attributed to the strongest laser peaks of 49 J/cm2 fluence. The surrounding peaks of lower fluence, 24 J/cm2 apparently did not produce any damage at this stage, but started doing so after about 10 pulses (visible in Fig. 1c, after 20 pulses). The damage threshold of the present target can be estimated at a value of laser fluence hitting the target between 24 J/cm2 and 49 J/cm2, since laser intensity peaks of the former fluence do not produce damage on the first pulse, while the latter fluence does. A more detailed surface characterization included in-depth observation of the modifications by SEM, AFM, and profilometry, as described in following section. 3.2. SEM analysis The action of 10, 50, and 100 accumulated laser pulses resulted in significant damage of the WTi/Si system, Fig. 2. The bright and dark regions (Fig. 2) typically correspond to the WTi thin film and Si substrate, respectively. Surface changes can be summarized as follows: (i) partial ablation of the WTi thin film (registered also by OM), Fig. 2a1, 2b1, and 2c1; (ii) appearance of hydrodynamic features like resolidified material at the periphery of the ablated area, Fig. 2a2, 2b2, and 2c2; and (iii) appearance of thin film cracks 2a3, 2b3, and 2c3. Nanostructures in the form of globules Fig. 2b3, and 2c3 and polygonal silicon grains Fig. 2a3 are visible in resolidified regions. Globules have an average diameter between 100 nm and 200 nm (Fig. 2a2, 2b2, and 2c2), whereas Si polygons are
Fig. 3. Profilometer 3D view of the WTi/Si system irradiated area: (a) after 5 pulses and (b) after 200 pulses. The maximum height of the heaped up material (in z direction) was 0.5 mm. Experimental conditions were the same as in Fig. 1.
smaller than 100 nm in any dimension, with redeposited WTi at their boundaries (Fig. 2a3). Increasing the number of cumulative laser pulses (from 10 to 100, Fig. 2a2, 2b2, 2c2) leads to appearance of new globules and some of those previously formed grow by coalescence. Nanostructures in the form of rectangular grains showed a tendency of disappearance with pulse count increase. The rectangular grains appear to have well-defined boundaries, with an orientation corresponding to the orientation of the silicon substrate (for comparison, non-irradiated thin film image is shown in Fig. 2d). Generally, generation of nanostructures (globules and rectangles) can be attributed to processes like recrystallization of the material, hydrodynamic effects, surface instabilities, condensation, etc. [1]. 3.3. Three-dimensional profilometer analysis A 3D analysis of the WTi thin film/silicon system after laser action is presented in Fig. 3. Generally, non-homogeneous material distribution upon laser irradiation with 5 and 200 pulses was confirmed. The WTi thin film material was heaped up over the surface. Aggregates of varying height, depending on the number of pulses, appear in each damage area. A complete profilometer 3D analysis yielded ablation depths and heights of
Fig. 4. Z-coordinates (a) and volume of the heaped up material (b) of the damage area as a function of the number of accumulated pulses.
S. Petrovic´ et al. / Applied Surface Science 254 (2008) 4013–4017
4017
regime, with a rectangular array of peaks of various fluences. It was estimated that only peaks with fluences above approximately 24 J/cm2 induced changes/damage at the target, after at least 10 pulses. Damage threshold for a single pulse can be estimated at a value between 24 J/cm2 and 49 J/cm2. All the laser-induced changes were attributed to the thermal energy generated in the target by absorption of the laser pulse. Since the present target is complex, i.e. combined of two materials with different thermophysical properties, exfoliation was evident on the surface. The effects can be summarized as follows: (i) partial ablation/exfoliation of the WTi film, the magnitude of which was related to the beam intensity; (ii) partial modification of the silicon substrate in the form of polygonal grains, with WTi redeposited at polygon boundaries; and (iii) appearance of hydrodynamic features including nanoglobules. The process of irradiation was also followed by appearance torch-like plume in front of the target, after at least 4 laser pulses. Acknowledgments
Fig. 5. AFM images of the irradiated area: (a) thin film before laser irradiation; (b) and (c) are images of the center and periphery of the damaged areas after 50 pulses, experimental conditions were the same as in Fig. 1.
This research was supported by Ministry of Sciences of the Republic Serbia, Contract No. 142065 and by Ministry of Science and Technology of the Republic of Slovenia, as a part of bilateral cooperation between Institute of Nuclear Science— Vincˇa and ‘‘Jozˇef Stefan’’ Institute, Ljubljana. References
the heaped up material (HH), Fig. 4. The ablation depths were in the range from 30 nm to 70 nm, whereas the maximal HH material was 790 nm (Fig. 4a). Also, the volume of the heaped up material on the surface increases with the number of pulses (Fig. 4b). 3.4. AFM analysis An AFM analysis of the laser modified WTi/Si system is presented in Fig. 5. The AFM analysis of the sample before laser irradiation (Fig. 5a) confirmed that the thin film had a polycrystalline form. The surface contained fine grains which were uniformly distributed over the substrate. The initial mean value of the surface roughness was 0.5 nm, which corresponded to the roughness of the silicon substrate. Surface roughness/ morphology was drastically changed after laser irradiation (Fig. 5b and c), so that the final roughness in the central and peripheral zones of the irradiated area after 50 pulses (Fig. 5b and c) were 270 nm and about 20 nm, respectively. 4. Conclusion A study of morphological and structural changes of a WTi thin film/silicon system induced by nanosecond TEA CO2 laser operating at 10.6 mm is presented. The film of WTi was 190 nm thick, deposited by dc sputtering on a silicone substrate of ntype (1 0 0) orientation. The laser was used in a multi-mode
[1] P. Stefanov, N. Minkovski, I. Balchev, I. Avramova, N. Sabotinov, Ts. Marinova, Appl. Surf. Sci. 253 (2006) 1046. [2] B. Gakovic, M. Trtica, P. Panjan, M. Cekada, Appl. Phys. A 79 (4–6) (2004) 1353. [3] L.L. Sartinski, S. Barchikovski, N. Wagenda, B.M. Rud, I.I. Timofeeva, Appl. Surf. Sci. 253 (2007) 4295. [4] M. Ullmann, S.K. Friedlander, A. Schmidt-Ott, J. Nanoparticle Res. 4 (2002) 499. [5] M.S.F. Lima, F.P. Ladario, R. Riva, Appl. Surf. Sci. 252 (2006) 4420. [6] S. Bhagat, H. Han, T.L. Alford, Thin Solid Films 515 (2006) 1998. [7] H. Ramarotafika, G. Lepperiere, Thin Solid Films 266 (1995) 267. [8] V. Guidi, D. Boscarino, E. Comini, G. Faglia, M. Ferroni, C. Malagu, G. Martinelli, V. Rigato, G. Sberveglieri, Sens. Actuators B: Chem. 65 (1–3) (2000) 264. [9] M. Giulio, D. Manno, G. Micocci, A. Serra, A. Tepore, J. Mater. Sci. 9 (1998) 1568. [10] L. Meda, R.C. Breitkopt, T.E. Haas, R.U. Kirss, Thin Solid Films 402 (2002) 126. [11] C. Louro, A. Cavaleiro, J. Electrochem. Soc. 144 (1) (1997) 259. [12] S. Petrovic, B. Gakovic, M. Trtica, T. Nenadovic, Laser Part. Beams 19 (2001) 195. [13] M.S. Trtica, S.V. Ribnikar, Appl. Phys. B 49 (1989) 379. [14] J. Hermann, C. Boulmer-Leborgne, I.N. Mihailescu, B. Dubreuil, J. Appl. Phys. 73 (1993) 1091. [15] S. Petrovic, B. Adnadjevic, D. Perusko, N. Popovic, N. Bundaleski, M. Radovic, B. Gakovic, Z. Rakocevic, J. Serb. Chem. Soc. 71 (8–9) (2006) 969. [16] S. Petrovic, N. Bundaleski, D. Perusko, M. Radovic, J. Kovac, M. Mitric, B. Gakovic, Z. Rakocevic, Appl. Surf. Sci. 253 (12) (2007) 5196. [17] S. Petrovic, M. Trtica, B. Gakovic, Z. Rakocevic, S. Miljanic, T. Nenadovic, 20th Symposium of the Physics of Ionized Gases–SPIG, Contributed Papers, (2000), p. 231.
Applied Surface Science 254 (2008) 4013–4017 www.elsevier.com/locate/apsusc
Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses S. Petrovic´ a,*, B. Gakovic´ a, D. Perusˇko a, M. Trtica a, B. Radak a, P. Panjan b, Sˇ. Miljanic´ c ˇ A, P.O. Box 522, 11001 Belgrade, Serbia Institute of Nuclear Sciences, VINC b ‘‘Jozˇef Stefan’’ Institute, Jamova 39, 1000 Ljubljana, Slovenia c Faculty of Physical Chemistry, Studenski trg 14-16, 11000 Belgrade, Serbia
a
Received 29 November 2007; received in revised form 18 December 2007; accepted 18 December 2007 Available online 4 January 2008
Abstract Interaction of a nanosecond transversely excited atmospheric (TEA) CO2 laser, operating at 10.6 mm, with tungsten–titanium thin film (190 nm) deposited on silicon of n-type (1 0 0) orientation, was studied. Multi-pulse irradiation was performed in air atmosphere with laser energy densities in the range 24–49 J/cm2. The energy absorbed from the laser beam was mainly converted to thermal energy, which generated a series of effects. The following morphological changes were observed: (i) partial ablation/exfoliation of the WTi thin film, (ii) partial modification of the silicon substrate with formation of polygonal grains, (iii) appearance of hydrodynamic features including nano-globules. Torch-like plumes started appearing in front of the target after several laser pulses. # 2007 Elsevier B.V. All rights reserved. PACS : 68.55.Jk; 52.38.Mf; 79.20.Ds Keywords: WTi thin films; Laser ablation; Nanosecond laser pulse
1. Introduction The research into laser-induced solid surface modifications is significant in many aspects. Understanding of physical and chemical mechanisms of interaction can be important both at the fundamental and technological levels. Some important aspects of the use of lasers in material processing, particularly those that involve material removal and heat driven processes, are: melting, vaporization, condensation of vapor in the gas phase, diffusion, segregation, resolidification, laser–plume interaction, etc. All of these factors are responsible for changes of the structure, composition and chemical state of the irradiated surface. Laser irradiation may modify surface composition and structure of metals and alloys, increasing their resistance to wear and corrosion [1,2]. With laser irradiation it is possible to process and modify localized surface areas [3]. Laser ablation has attracted much attention as a relatively new technique for material processing in forming
* Corresponding author. Tel.: +381 11 8066425; fax: +381 11 8066425. E-mail address: [email protected] (S. Petrovic´). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.12.041
stable nano-sized metal particles [4]. Nanoparticles have shown very promising applications in lubrication and magnetic recording media production [5]. Electrical contact and diffusion barrier of tungsten (W) and tungsten–titanium (WTi) thin films have been studied for microelectronic applications [6,7], development of gas sensors [8,9], functional coatings such as smart windows [10] and protective anticorrosive and oxidation resistant coatings [11]. Nanoparticles made of tungsten and titanium can enhance mechanical properties of metallic alloys, such as steel, by dispersion strengthening of the metallic matrix [5]. Earlier results with a WTi coating on a steel substrate [12] showed noticeable effects of the substrate on the morphology of the damage induced, even though the thickness of the coating was larger than in the present case. This indicates the importance of the substrate material in such systems. Laser-induced surface modifications of a WTi thin film with thicknesses in the nano-domain have not been sufficiently reported in literature so far. The main objective of the present work was to study the surface modification induced by a TEA CO2 laser (at 10 mm wavelength) of a WTi thin film deposited on a silicon substrate. Morphological changes at laser energy
4014
S. Petrovic´ et al. / Applied Surface Science 254 (2008) 4013–4017
densities (fluences) ranging from 24 J/cm2 to 49 J/cm2 were investigated as a function of the number of accumulated laser pulses. 2. Experimental The tungsten–titanium thin films were deposited by dc sputtering of the 90%W–10%Ti wt. target by Ar+ ions. The substrate used in the experiment was silicon of n-type (1 0 0) orientation. Silicon wafers (0.5 mm thick) were polished and cleaned before deposition. Additional cleaning was also performed in the vacuum chamber by electron heating up to a temperature of 130 8C. The deposition was carried out by a Balzers Sputtron II vacuum system. The conditions during the deposition process were: acceleration voltage and current 1.5 kV and 0.7 A, respectively, partial pressure of argon 1.33 10 1 Pa. Under these experimental conditions the constant deposition rate was 0.14 nm/s, which produced a WTi thin film of 190 nm thickness, as measured by a Talystep I profilometer. The laser used in the experiment was an ultraviolet preionized TEA CO2 system [12]. It operated with a nontypical CO2/X, X = H2, H2/N2 gas mixture, which increased the efficiency of the laser. The laser pulse shape can generally be controlled by adjusting the gas mixture content [13,14]. Characteristics of the
laser pulses were the following: working gas mixture CO2/N2/ H2 = 1/2.1/1.3 (1000 mbar total), output pulse energy up to 170 mJ, initial spike 120 ns at FWHM (with about 2 ms tail), transversally multi-mode (approx. 1 cm square array), wavelengths 10.5709 mm and 10.5909 mm, repetition rate 2 Hz. Irradiation of the WTi thin film/Si system was carried out in ambient air atmosphere at a pressure of 1013 mbar. The laser beam was focused through a KBr lens of 6 cm focal length in a direction perpendicular to the sample surface. Various analytical techniques, before and after laser irradiation, were used for target characterization. Optical microscopy (OM) was used for initial general analysis of the modifications obtained. Surface morphology was further observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Topographic changes were characterized by a profilometer and AFM. Reflectivity of the target was observed in the wavelength region from 6.5 mm to 14 mm, by a Specord 75 IR spectrophotometer. 3. Results and discussion An X-ray analysis of the initial non-irradiated WTi thin film showed its polycrystalline structure, composed of a bcc tungsten (W) phase with a presence of a hcp titanium (Ti)
Fig. 1. Optical microscopy analyses of the WTi thin film/Si system after irradiation with TEA CO2 laser. The laser operated in multi-mode regime, with fluences ranging from 24 J/cm2 to 49 J/cm2 in individual peaks: (a) WTi thin film on Si prior laser action; WTi/Si system after (b) 1, (c) 20, (d) 50, (e) 100, (f) 200 laser pulses and (g) schematic of the laser cross-section intensity.
S. Petrovic´ et al. / Applied Surface Science 254 (2008) 4013–4017
phase [15]. The presence of titanium in the WTi thin film resulted in an expanded crystal lattice of W, in which the lattice ˚ . The main grain size of the WTi thin parameter was 3.2237 A film was estimated at about 14 nm, according to the tungsten peak width [16]. The reflectivity measurements in the spectral region around 10.6 mm showed that the thin film of WTi had an initial reflectivity of 87%. It is well known that the reflectivity of a target surface depends on the state of its surface, number of previously accumulated laser pulses, radiation wavelength, etc. [17]. Initially absorbed laser pulses change surface morphology and can cause physico-chemical transformations. These changes additionally increase target absorptivity. Consequently, the level of the target damage after hundreds of accumulated laser pulses can be significant. In the present experiments, the laser beam had a whole range of fluences at each pulse, due to its multi-mode
4015
cross-section. It is observable in Fig. 1 that fluences of the central peaks (estimated at 49 J/cm2) induced damage even at the first pulse, but the surrounding peaks (estimated at fluences of 24 J/ cm2) induce noticeable damage only after about 10 pulses. Generally, laser radiation is converted into thermal energy at the target, which generates series of effects. In case of complex targets with different thermophysical properties of the materials, as in the present experiment, exfoliation is possible. Difference in the thermal expansion coefficients between the WTi thin film and silicon substrate generates additional stress at the interface. A direct consequence of this is the appearance of microcracks and exfoliation of the thin film. Appearance of torch-like plume in front of the target was observed, but only after four successive pulses. The surface morphological changes can be summarized as the following.
Fig. 2. SEM analyses of the WTi/Si system induced by the TEA CO2 laser. (a–c) The WTi/Si system after irradiation with 10, 50, and 100 laser pulses, respectively; (a1, b1, c1) partially ablated regions; (a2, b2, c2) central parts of ablations; (a3, b3, c3) details of the same (central) areas with a different magnification; (d) WTi thin film surface before irradiation. Experimental conditions were the same as in Fig. 1.
4016
S. Petrovic´ et al. / Applied Surface Science 254 (2008) 4013–4017
3.1. OM analysis The first step in examination of the morphology of the irradiated targets was the analysis by an optical microscope, Fig. 1. The damage of the WTi thin film/Si system can be clearly recognized even after one laser pulse in Fig. 1b. This damage was attributed to the strongest laser peaks of 49 J/cm2 fluence. The surrounding peaks of lower fluence, 24 J/cm2 apparently did not produce any damage at this stage, but started doing so after about 10 pulses (visible in Fig. 1c, after 20 pulses). The damage threshold of the present target can be estimated at a value of laser fluence hitting the target between 24 J/cm2 and 49 J/cm2, since laser intensity peaks of the former fluence do not produce damage on the first pulse, while the latter fluence does. A more detailed surface characterization included in-depth observation of the modifications by SEM, AFM, and profilometry, as described in following section. 3.2. SEM analysis The action of 10, 50, and 100 accumulated laser pulses resulted in significant damage of the WTi/Si system, Fig. 2. The bright and dark regions (Fig. 2) typically correspond to the WTi thin film and Si substrate, respectively. Surface changes can be summarized as follows: (i) partial ablation of the WTi thin film (registered also by OM), Fig. 2a1, 2b1, and 2c1; (ii) appearance of hydrodynamic features like resolidified material at the periphery of the ablated area, Fig. 2a2, 2b2, and 2c2; and (iii) appearance of thin film cracks 2a3, 2b3, and 2c3. Nanostructures in the form of globules Fig. 2b3, and 2c3 and polygonal silicon grains Fig. 2a3 are visible in resolidified regions. Globules have an average diameter between 100 nm and 200 nm (Fig. 2a2, 2b2, and 2c2), whereas Si polygons are
Fig. 3. Profilometer 3D view of the WTi/Si system irradiated area: (a) after 5 pulses and (b) after 200 pulses. The maximum height of the heaped up material (in z direction) was 0.5 mm. Experimental conditions were the same as in Fig. 1.
smaller than 100 nm in any dimension, with redeposited WTi at their boundaries (Fig. 2a3). Increasing the number of cumulative laser pulses (from 10 to 100, Fig. 2a2, 2b2, 2c2) leads to appearance of new globules and some of those previously formed grow by coalescence. Nanostructures in the form of rectangular grains showed a tendency of disappearance with pulse count increase. The rectangular grains appear to have well-defined boundaries, with an orientation corresponding to the orientation of the silicon substrate (for comparison, non-irradiated thin film image is shown in Fig. 2d). Generally, generation of nanostructures (globules and rectangles) can be attributed to processes like recrystallization of the material, hydrodynamic effects, surface instabilities, condensation, etc. [1]. 3.3. Three-dimensional profilometer analysis A 3D analysis of the WTi thin film/silicon system after laser action is presented in Fig. 3. Generally, non-homogeneous material distribution upon laser irradiation with 5 and 200 pulses was confirmed. The WTi thin film material was heaped up over the surface. Aggregates of varying height, depending on the number of pulses, appear in each damage area. A complete profilometer 3D analysis yielded ablation depths and heights of
Fig. 4. Z-coordinates (a) and volume of the heaped up material (b) of the damage area as a function of the number of accumulated pulses.
S. Petrovic´ et al. / Applied Surface Science 254 (2008) 4013–4017
4017
regime, with a rectangular array of peaks of various fluences. It was estimated that only peaks with fluences above approximately 24 J/cm2 induced changes/damage at the target, after at least 10 pulses. Damage threshold for a single pulse can be estimated at a value between 24 J/cm2 and 49 J/cm2. All the laser-induced changes were attributed to the thermal energy generated in the target by absorption of the laser pulse. Since the present target is complex, i.e. combined of two materials with different thermophysical properties, exfoliation was evident on the surface. The effects can be summarized as follows: (i) partial ablation/exfoliation of the WTi film, the magnitude of which was related to the beam intensity; (ii) partial modification of the silicon substrate in the form of polygonal grains, with WTi redeposited at polygon boundaries; and (iii) appearance of hydrodynamic features including nanoglobules. The process of irradiation was also followed by appearance torch-like plume in front of the target, after at least 4 laser pulses. Acknowledgments
Fig. 5. AFM images of the irradiated area: (a) thin film before laser irradiation; (b) and (c) are images of the center and periphery of the damaged areas after 50 pulses, experimental conditions were the same as in Fig. 1.
This research was supported by Ministry of Sciences of the Republic Serbia, Contract No. 142065 and by Ministry of Science and Technology of the Republic of Slovenia, as a part of bilateral cooperation between Institute of Nuclear Science— Vincˇa and ‘‘Jozˇef Stefan’’ Institute, Ljubljana. References
the heaped up material (HH), Fig. 4. The ablation depths were in the range from 30 nm to 70 nm, whereas the maximal HH material was 790 nm (Fig. 4a). Also, the volume of the heaped up material on the surface increases with the number of pulses (Fig. 4b). 3.4. AFM analysis An AFM analysis of the laser modified WTi/Si system is presented in Fig. 5. The AFM analysis of the sample before laser irradiation (Fig. 5a) confirmed that the thin film had a polycrystalline form. The surface contained fine grains which were uniformly distributed over the substrate. The initial mean value of the surface roughness was 0.5 nm, which corresponded to the roughness of the silicon substrate. Surface roughness/ morphology was drastically changed after laser irradiation (Fig. 5b and c), so that the final roughness in the central and peripheral zones of the irradiated area after 50 pulses (Fig. 5b and c) were 270 nm and about 20 nm, respectively. 4. Conclusion A study of morphological and structural changes of a WTi thin film/silicon system induced by nanosecond TEA CO2 laser operating at 10.6 mm is presented. The film of WTi was 190 nm thick, deposited by dc sputtering on a silicone substrate of ntype (1 0 0) orientation. The laser was used in a multi-mode
[1] P. Stefanov, N. Minkovski, I. Balchev, I. Avramova, N. Sabotinov, Ts. Marinova, Appl. Surf. Sci. 253 (2006) 1046. [2] B. Gakovic, M. Trtica, P. Panjan, M. Cekada, Appl. Phys. A 79 (4–6) (2004) 1353. [3] L.L. Sartinski, S. Barchikovski, N. Wagenda, B.M. Rud, I.I. Timofeeva, Appl. Surf. Sci. 253 (2007) 4295. [4] M. Ullmann, S.K. Friedlander, A. Schmidt-Ott, J. Nanoparticle Res. 4 (2002) 499. [5] M.S.F. Lima, F.P. Ladario, R. Riva, Appl. Surf. Sci. 252 (2006) 4420. [6] S. Bhagat, H. Han, T.L. Alford, Thin Solid Films 515 (2006) 1998. [7] H. Ramarotafika, G. Lepperiere, Thin Solid Films 266 (1995) 267. [8] V. Guidi, D. Boscarino, E. Comini, G. Faglia, M. Ferroni, C. Malagu, G. Martinelli, V. Rigato, G. Sberveglieri, Sens. Actuators B: Chem. 65 (1–3) (2000) 264. [9] M. Giulio, D. Manno, G. Micocci, A. Serra, A. Tepore, J. Mater. Sci. 9 (1998) 1568. [10] L. Meda, R.C. Breitkopt, T.E. Haas, R.U. Kirss, Thin Solid Films 402 (2002) 126. [11] C. Louro, A. Cavaleiro, J. Electrochem. Soc. 144 (1) (1997) 259. [12] S. Petrovic, B. Gakovic, M. Trtica, T. Nenadovic, Laser Part. Beams 19 (2001) 195. [13] M.S. Trtica, S.V. Ribnikar, Appl. Phys. B 49 (1989) 379. [14] J. Hermann, C. Boulmer-Leborgne, I.N. Mihailescu, B. Dubreuil, J. Appl. Phys. 73 (1993) 1091. [15] S. Petrovic, B. Adnadjevic, D. Perusko, N. Popovic, N. Bundaleski, M. Radovic, B. Gakovic, Z. Rakocevic, J. Serb. Chem. Soc. 71 (8–9) (2006) 969. [16] S. Petrovic, N. Bundaleski, D. Perusko, M. Radovic, J. Kovac, M. Mitric, B. Gakovic, Z. Rakocevic, Appl. Surf. Sci. 253 (12) (2007) 5196. [17] S. Petrovic, M. Trtica, B. Gakovic, Z. Rakocevic, S. Miljanic, T. Nenadovic, 20th Symposium of the Physics of Ionized Gases–SPIG, Contributed Papers, (2000), p. 231.