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Thin films deposition in RF generated plasma by reactive pulsed laser ablation
Applied Surface Science 197±198 (2002) 338±342
Thin ®lms deposition in RF generated plasma by reactive pulsed laser ablation A. Giardini, V. Marotta, A. Morone, S. Orlando*, G.P. Parisi CNR-IMS, Zona Industriale di Tito Scalo, I-85050 Tito Scalo, PZ, Italy
Abstract Metal oxide thin ®lms have been deposited on Si(1 0 0) substrates by reactive pulsed laser ablation of metallic targetÐtitanium, tungstenÐin the presence of a 13.56 MHz radio frequency (RF) plasma, 10 Pa static atmosphere of O2, using a doubled frequency Nd:YAG laser. The gaseous species were collected on Si(1 0 0) substrates positioned in front of the target on a heatable holder, up to 1000 K. The deposited thin ®lms were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The comparison between conventional pulsed laser deposition (PLD) and the RF plasma-assisted PLD showed the in¯uence of the plasma on the surface roughness, and a better adhesion to the substrates by the plasma-aided thin ®lms. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Reactive pulsed laser deposition; RF plasma; Oxides; Thin ®lms
1. Introduction Tungsten trioxide (WO3) is an important electrochromic material [1]. It is used for gas and humidity sensors, as well as low voltage varistors [2]. It has been recognized that WO3 can be colored through electro-, photo-, gas-, laser-induced and thermochromism processes [3]. It can be also used as a catalyst, as a window for solar cells, electrochromic devices, including ``smart'' windows, electronic information displays and color memory devices [4,5] high-density memories and photoelectric sensors. In its fully oxidized state, WO3 is transparent throughout the visible and infrared regions of the spectrum. When it is reduced by injection of electrons along with neutralizing counter-ions, usually H or Li, tungsten oxide
*
Corresponding author. Tel.: 39-971-427259; fax: 39-971-427222. E-mail address: [email protected] (S. Orlando).
absorbs and re¯ects light, taking on a blue-gray color in transmission. Thin ®lms of tungsten oxide have been made by a wide variety of methods. These include physical methods, such as vacuum evaporation [2,3], sputtering [4], pulsed laser deposition (PLD) [5], and chemical methods, such as chemical vapor deposition (CVD) [6], sol±gel and electrochemical methods. Titanium oxide thin ®lms are used in optical and protective coatings, microelectronic applications, and photochemical active layers. As examples, TiO2 shows superconductivity [7] and stoichiometric TiO2 acts as an insulator. Furthermore, TiO2 has excellent transparency in the visible and near-IR region, photoelectric properties, photocatalytic activities, and chemical stability. Accordingly, the titanium oxides are attractive for electric, optical, or catalytic applications. Thin ®lms of TiO2 have been deposited by physical vapor deposition (PVD) [8], sputtering [9], CVD [10], sol±gel processes [11], and pulsed molecular beam deposition [12].
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 3 9 5 - 1
A. Giardini et al. / Applied Surface Science 197±198 (2002) 338±342
In this work we present the deposition of titanium oxide and tungsten oxide thin ®lms performed by reactive pulsed laser ablation and deposition (RPLAD). This technique combines the several advantages of conventional PLD, such as deposition in relatively high partial pressure, crystallization of ®lms at lower temperature because of the higher energy of the ablated particles in the laser-produced plume, and relatively high deposition rates [13], with the enhancement of the reactivity in the gas phase due to the presence of a reactive buffer gas. In our experiments, the reactive gas is a radio frequency (RF) generated oxygen plasma. 2. Experimental The laser ablation experiments were carried out in a multi-port stainless steel vacuum chamber equipped with a gas inlet, a rotating multi-target and a heatable substrate holder (Fig. 1). The vacuum pressure of the deposition chamber was below 10 3 Pa whereas the O2 gas pressure during ®lm deposition, was 10 Pa. The substrate deposition temperature could be varied from room temperature up to 1000 K. The ¯uence of the laser employedÐQuantel Nd:YAG 581, l 532 nm, pulse duration 7 ns, repetition rate 10 HzÐhas been kept nearly constant at 8 J/cm2. The laser impinges on the target at 458, with respect to the normal, in a static O2 atmosphere.
Fig. 1. Schematic layout of the heatable sample holder (H) hooded by a stainless steel top-hat, with a 30 mm diameter hole to allow the plume coming from the rotating target to reach the substrate, in which the output of the matching network coming from the RF power generator is inserted (E.I.: electrical insulator, T.I.: thermal insulator).
339
The PLD setup has been improved by employing a RF plasma system placed just above the substrate holder (Fig. 1), maintaining this last one electrically connected to the ground. The substrate holder is surrounded by an isolated stainless steel top-hat connected to the RF generator, through a customized matching unit. A 3 cm diameter hole on the top-hat allows the deposition of the plume coming from the ablated target. The RF power generator is a 13.56 MHz ENI Model OEM-6, maximum power output 650 W. The target materials, 1 in. diameter pressed, at about 300 MPa, disk of titanium crystals >99.99% purity (Aldrich 30,581-2), and a tungsten foil, 0.5 mm thickness, >99.9% purity (Aldrich 35,718-9), were rotated at 2 rpm during deposition. The gaseous species were collected on Si(1 0 0) substrates positioned, with the on-axis con®guration, 5 cm far from the target. A Philips 500 scanning electron microscopy (SEM) was utilized to evaluate the surface roughness of the deposited thin ®lms. X-ray spectra were detected by a Siemens D5000 diffractometer using the Ka line l 0:154056 nm of a Cu target as an X-ray source. 3. Results and discussion 3.1. SEM The surface morphology of the thin ®lms has been studied by SEM. Titanium oxide thin ®lms present a lot of particulates on the surface of both types of produced samples, with and without the RF plasma. SEM photographs of titanium oxide deposited on Si(1 0 0) without RF discharge (a) and in the presence of RF oxygen plasma (b) are reported in Fig. 2; both the photographs are at 400 magni®cation. On the contrary, tungsten oxide thin ®lms show a smooth surface with only few particulates (not shown). The explanation of the abundance of particulate in the titanium-based thin ®lms could be connected to the type of target utilized in the deposition process. During the laser ablation, the pressed disk of titanium crystals releases much more droplets (``microcrystals''), easily detectable due to their light emitting, than the metal foil of tungsten. To evaluate the adhesion of the deposited thin ®lms, we performed tape
340
A. Giardini et al. / Applied Surface Science 197±198 (2002) 338±342
Fig. 2. SEM photographs of titanium oxide deposited on Si(1 0 0): (a) without RF discharge; (b) in the presence of RF oxygen plasma. Both the depositions have been performed in 10 Pa O2 atmosphere at 773 K substrate temperature. Both the photographs are at 400 magni®cation.
tear tests which con®rmed a slight difference between samples produced with and without RF plasma. The RF plasma deposited thin ®lms presented a very good adhesion.
3.2. X-ray diffraction (XRD) In Fig. 3 the XRD patterns of the tungsten oxide samples deposited on Si(1 0 0) in RF oxygen plasma
A. Giardini et al. / Applied Surface Science 197±198 (2002) 338±342
341
Fig. 4. XRD spectrum of TiO2-rutile deposited at 873 K on Si(1 0 0) without (top) and with (bottom) RF oxygen plasma.
Fig. 3. XRD patterns of the tungsten oxide samples deposited on Si(1 0 0) in RF oxygen plasma, as function of the growing temperature, from 373 K (top) to 973 K (bottom).
are reported as function of the substrate temperature, from 373 to 973 K. It is quite evident that at low temperature the growth of cubic W3O is preferred, but this phase decreases rapidly, and the formation of
cubic WO3 is favored in the thermal range of 500± 750 K. At 773 K it is possible to note the cubic phase of metallic tungsten, evidenced by the arrows on the shoulders of the main peaks of the corresponding frame in Fig. 3. At higher temperature only the cubic phase of metallic tungsten can be recognized in the spectra of the thin ®lms deposited. The X-ray spectra of titanium oxide thin ®lms do not show a clear relationship of the structure as function of the deposition temperature. The spectra of samples deposited at 873 K with (bottom) and without (top) the RF oxygen plasma are compared in Fig. 4. Only the spectra of plasma-assisted samples (Fig. 4, bottom) showed a very sharp peak at 27.48, corresponding to tetragonal TiO2-rutile(1 1 0). 4. Conclusion Deposition of titanium oxide and tungsten oxide thin ®lms has been performed on Si(1 0 0) substrates by RF plasma-assisted reactive pulsed laser ablation of
342
A. Giardini et al. / Applied Surface Science 197±198 (2002) 338±342
metallic target in 10 Pa O2 atmosphere using a doubled frequency Nd:YAG laser (532 nm). Depositions have been performed at various substrate temperatures ranging from 373 up to 873 K. The surface morphology of deposited thin ®lms has been evaluated by SEM analysis. XRD analyses have shown that tungsten oxide samples, produced in RF oxygen plasma, grow as cubic W3O and as cubic WO3 depending upon the deposition temperature. Above 800 K only the cubic phase of metallic tungsten can be recognized in the spectrum of the thin ®lms deposited. The XRD spectra of titanium oxide thin ®lms did not show a clear correlation of the structure as function of the deposition temperature. The spectra show the growth of tetragonal TiO2-rutile in samples deposited at 873 K, in the presence of RF O2 plasma. A comparison between the conventional PLD and the RF plasma-assisted PLD showed the in¯uence of the plasma on the surface roughness and a better adhesion to the substrates by the plasma-aided thin ®lms, as con®rmed also by adhesive tape tear tests. In conclusion, the data con®rm that RF plasma-enhanced reactive PLD is a suitable technique for depositing oxide thin ®lms. Work is still in progress to identify the best settings of the experimental parameters, such as oxygen pressure, RF applied power, target±substrate distance, laser ¯uence, with the aim to optimize the effects on the surface and structure, during deposition of the ®lms.
Acknowledgements This work was partially supported by Progetto Strategico MSTA II of Consiglio Nazionale delle Ricerche (CNR) of Italy. The authors wish to thank Prof. Roberto Teghil and Dr. Antonio Santagata for skilful discussions. References [1] G.J. Fang, Z.L. Liu, G.C. Sun, K.L. Yao, Phys. Stat. Sol. (a) 184 (2001) 129. [2] C. Cantalini, H.T. Sun, M. Faccio, M. Pelino, Sens. Actuators B 31 (1996) 81. [3] M. Green, Z. Hussain, J. Appl. Phys. 69 (1991) 7788. [4] Z. Xu, J.F. Vetelino, R. Lec, D.C. Parker, J. Vac. Sci. Technol. A 8 (1990) 3634. [5] E. Haro-Poniatowski, M. Jouanne, J.F. Morhange, Appl. Surf. Sci. 127±129 (1998) 674. [6] R.G. Gordon, S. Barry, J.T. Burton, R.N.R. BroomhallDillard, Thin Solid Films 392 (2001) 231. [7] T.B. Reed, M.D. Banus, M. Sjostrand, P.H. Keeson, J. Appl. Phys. 43 (1972) 2478. [8] K.N. Rao, M.A. Murthy, S. Mohan, Thin Solid Films 176 (1989) 181. [9] L.-J. Meng, M.P. dos Santos, Thin Solid Films 226 (1993) 22. [10] A. Shibata, Jpn. J. Appl. Phys. 30 (1991) L650. [11] Y. Takahashi, Y. Matsuoka, J. Mater. Sci. 23 (1988) 2259. [12] F. Imai, K. Kunimori, T. Manabe, T. Kumagai, H. Nozoye, Thin Solid Films 310 (1997) 184. [13] W.S. Hu, Z.G. Liu, J. Sun, S.N. Zhu, Q.Q. Xu, D. Feng, Z.M. Ji, J. Phys. Chem. Solids 58 (1997) 857.
Thin ®lms deposition in RF generated plasma by reactive pulsed laser ablation A. Giardini, V. Marotta, A. Morone, S. Orlando*, G.P. Parisi CNR-IMS, Zona Industriale di Tito Scalo, I-85050 Tito Scalo, PZ, Italy
Abstract Metal oxide thin ®lms have been deposited on Si(1 0 0) substrates by reactive pulsed laser ablation of metallic targetÐtitanium, tungstenÐin the presence of a 13.56 MHz radio frequency (RF) plasma, 10 Pa static atmosphere of O2, using a doubled frequency Nd:YAG laser. The gaseous species were collected on Si(1 0 0) substrates positioned in front of the target on a heatable holder, up to 1000 K. The deposited thin ®lms were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The comparison between conventional pulsed laser deposition (PLD) and the RF plasma-assisted PLD showed the in¯uence of the plasma on the surface roughness, and a better adhesion to the substrates by the plasma-aided thin ®lms. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Reactive pulsed laser deposition; RF plasma; Oxides; Thin ®lms
1. Introduction Tungsten trioxide (WO3) is an important electrochromic material [1]. It is used for gas and humidity sensors, as well as low voltage varistors [2]. It has been recognized that WO3 can be colored through electro-, photo-, gas-, laser-induced and thermochromism processes [3]. It can be also used as a catalyst, as a window for solar cells, electrochromic devices, including ``smart'' windows, electronic information displays and color memory devices [4,5] high-density memories and photoelectric sensors. In its fully oxidized state, WO3 is transparent throughout the visible and infrared regions of the spectrum. When it is reduced by injection of electrons along with neutralizing counter-ions, usually H or Li, tungsten oxide
*
Corresponding author. Tel.: 39-971-427259; fax: 39-971-427222. E-mail address: [email protected] (S. Orlando).
absorbs and re¯ects light, taking on a blue-gray color in transmission. Thin ®lms of tungsten oxide have been made by a wide variety of methods. These include physical methods, such as vacuum evaporation [2,3], sputtering [4], pulsed laser deposition (PLD) [5], and chemical methods, such as chemical vapor deposition (CVD) [6], sol±gel and electrochemical methods. Titanium oxide thin ®lms are used in optical and protective coatings, microelectronic applications, and photochemical active layers. As examples, TiO2 shows superconductivity [7] and stoichiometric TiO2 acts as an insulator. Furthermore, TiO2 has excellent transparency in the visible and near-IR region, photoelectric properties, photocatalytic activities, and chemical stability. Accordingly, the titanium oxides are attractive for electric, optical, or catalytic applications. Thin ®lms of TiO2 have been deposited by physical vapor deposition (PVD) [8], sputtering [9], CVD [10], sol±gel processes [11], and pulsed molecular beam deposition [12].
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 3 9 5 - 1
A. Giardini et al. / Applied Surface Science 197±198 (2002) 338±342
In this work we present the deposition of titanium oxide and tungsten oxide thin ®lms performed by reactive pulsed laser ablation and deposition (RPLAD). This technique combines the several advantages of conventional PLD, such as deposition in relatively high partial pressure, crystallization of ®lms at lower temperature because of the higher energy of the ablated particles in the laser-produced plume, and relatively high deposition rates [13], with the enhancement of the reactivity in the gas phase due to the presence of a reactive buffer gas. In our experiments, the reactive gas is a radio frequency (RF) generated oxygen plasma. 2. Experimental The laser ablation experiments were carried out in a multi-port stainless steel vacuum chamber equipped with a gas inlet, a rotating multi-target and a heatable substrate holder (Fig. 1). The vacuum pressure of the deposition chamber was below 10 3 Pa whereas the O2 gas pressure during ®lm deposition, was 10 Pa. The substrate deposition temperature could be varied from room temperature up to 1000 K. The ¯uence of the laser employedÐQuantel Nd:YAG 581, l 532 nm, pulse duration 7 ns, repetition rate 10 HzÐhas been kept nearly constant at 8 J/cm2. The laser impinges on the target at 458, with respect to the normal, in a static O2 atmosphere.
Fig. 1. Schematic layout of the heatable sample holder (H) hooded by a stainless steel top-hat, with a 30 mm diameter hole to allow the plume coming from the rotating target to reach the substrate, in which the output of the matching network coming from the RF power generator is inserted (E.I.: electrical insulator, T.I.: thermal insulator).
339
The PLD setup has been improved by employing a RF plasma system placed just above the substrate holder (Fig. 1), maintaining this last one electrically connected to the ground. The substrate holder is surrounded by an isolated stainless steel top-hat connected to the RF generator, through a customized matching unit. A 3 cm diameter hole on the top-hat allows the deposition of the plume coming from the ablated target. The RF power generator is a 13.56 MHz ENI Model OEM-6, maximum power output 650 W. The target materials, 1 in. diameter pressed, at about 300 MPa, disk of titanium crystals >99.99% purity (Aldrich 30,581-2), and a tungsten foil, 0.5 mm thickness, >99.9% purity (Aldrich 35,718-9), were rotated at 2 rpm during deposition. The gaseous species were collected on Si(1 0 0) substrates positioned, with the on-axis con®guration, 5 cm far from the target. A Philips 500 scanning electron microscopy (SEM) was utilized to evaluate the surface roughness of the deposited thin ®lms. X-ray spectra were detected by a Siemens D5000 diffractometer using the Ka line l 0:154056 nm of a Cu target as an X-ray source. 3. Results and discussion 3.1. SEM The surface morphology of the thin ®lms has been studied by SEM. Titanium oxide thin ®lms present a lot of particulates on the surface of both types of produced samples, with and without the RF plasma. SEM photographs of titanium oxide deposited on Si(1 0 0) without RF discharge (a) and in the presence of RF oxygen plasma (b) are reported in Fig. 2; both the photographs are at 400 magni®cation. On the contrary, tungsten oxide thin ®lms show a smooth surface with only few particulates (not shown). The explanation of the abundance of particulate in the titanium-based thin ®lms could be connected to the type of target utilized in the deposition process. During the laser ablation, the pressed disk of titanium crystals releases much more droplets (``microcrystals''), easily detectable due to their light emitting, than the metal foil of tungsten. To evaluate the adhesion of the deposited thin ®lms, we performed tape
340
A. Giardini et al. / Applied Surface Science 197±198 (2002) 338±342
Fig. 2. SEM photographs of titanium oxide deposited on Si(1 0 0): (a) without RF discharge; (b) in the presence of RF oxygen plasma. Both the depositions have been performed in 10 Pa O2 atmosphere at 773 K substrate temperature. Both the photographs are at 400 magni®cation.
tear tests which con®rmed a slight difference between samples produced with and without RF plasma. The RF plasma deposited thin ®lms presented a very good adhesion.
3.2. X-ray diffraction (XRD) In Fig. 3 the XRD patterns of the tungsten oxide samples deposited on Si(1 0 0) in RF oxygen plasma
A. Giardini et al. / Applied Surface Science 197±198 (2002) 338±342
341
Fig. 4. XRD spectrum of TiO2-rutile deposited at 873 K on Si(1 0 0) without (top) and with (bottom) RF oxygen plasma.
Fig. 3. XRD patterns of the tungsten oxide samples deposited on Si(1 0 0) in RF oxygen plasma, as function of the growing temperature, from 373 K (top) to 973 K (bottom).
are reported as function of the substrate temperature, from 373 to 973 K. It is quite evident that at low temperature the growth of cubic W3O is preferred, but this phase decreases rapidly, and the formation of
cubic WO3 is favored in the thermal range of 500± 750 K. At 773 K it is possible to note the cubic phase of metallic tungsten, evidenced by the arrows on the shoulders of the main peaks of the corresponding frame in Fig. 3. At higher temperature only the cubic phase of metallic tungsten can be recognized in the spectra of the thin ®lms deposited. The X-ray spectra of titanium oxide thin ®lms do not show a clear relationship of the structure as function of the deposition temperature. The spectra of samples deposited at 873 K with (bottom) and without (top) the RF oxygen plasma are compared in Fig. 4. Only the spectra of plasma-assisted samples (Fig. 4, bottom) showed a very sharp peak at 27.48, corresponding to tetragonal TiO2-rutile(1 1 0). 4. Conclusion Deposition of titanium oxide and tungsten oxide thin ®lms has been performed on Si(1 0 0) substrates by RF plasma-assisted reactive pulsed laser ablation of
342
A. Giardini et al. / Applied Surface Science 197±198 (2002) 338±342
metallic target in 10 Pa O2 atmosphere using a doubled frequency Nd:YAG laser (532 nm). Depositions have been performed at various substrate temperatures ranging from 373 up to 873 K. The surface morphology of deposited thin ®lms has been evaluated by SEM analysis. XRD analyses have shown that tungsten oxide samples, produced in RF oxygen plasma, grow as cubic W3O and as cubic WO3 depending upon the deposition temperature. Above 800 K only the cubic phase of metallic tungsten can be recognized in the spectrum of the thin ®lms deposited. The XRD spectra of titanium oxide thin ®lms did not show a clear correlation of the structure as function of the deposition temperature. The spectra show the growth of tetragonal TiO2-rutile in samples deposited at 873 K, in the presence of RF O2 plasma. A comparison between the conventional PLD and the RF plasma-assisted PLD showed the in¯uence of the plasma on the surface roughness and a better adhesion to the substrates by the plasma-aided thin ®lms, as con®rmed also by adhesive tape tear tests. In conclusion, the data con®rm that RF plasma-enhanced reactive PLD is a suitable technique for depositing oxide thin ®lms. Work is still in progress to identify the best settings of the experimental parameters, such as oxygen pressure, RF applied power, target±substrate distance, laser ¯uence, with the aim to optimize the effects on the surface and structure, during deposition of the ®lms.
Acknowledgements This work was partially supported by Progetto Strategico MSTA II of Consiglio Nazionale delle Ricerche (CNR) of Italy. The authors wish to thank Prof. Roberto Teghil and Dr. Antonio Santagata for skilful discussions. References [1] G.J. Fang, Z.L. Liu, G.C. Sun, K.L. Yao, Phys. Stat. Sol. (a) 184 (2001) 129. [2] C. Cantalini, H.T. Sun, M. Faccio, M. Pelino, Sens. Actuators B 31 (1996) 81. [3] M. Green, Z. Hussain, J. Appl. Phys. 69 (1991) 7788. [4] Z. Xu, J.F. Vetelino, R. Lec, D.C. Parker, J. Vac. Sci. Technol. A 8 (1990) 3634. [5] E. Haro-Poniatowski, M. Jouanne, J.F. Morhange, Appl. Surf. Sci. 127±129 (1998) 674. [6] R.G. Gordon, S. Barry, J.T. Burton, R.N.R. BroomhallDillard, Thin Solid Films 392 (2001) 231. [7] T.B. Reed, M.D. Banus, M. Sjostrand, P.H. Keeson, J. Appl. Phys. 43 (1972) 2478. [8] K.N. Rao, M.A. Murthy, S. Mohan, Thin Solid Films 176 (1989) 181. [9] L.-J. Meng, M.P. dos Santos, Thin Solid Films 226 (1993) 22. [10] A. Shibata, Jpn. J. Appl. Phys. 30 (1991) L650. [11] Y. Takahashi, Y. Matsuoka, J. Mater. Sci. 23 (1988) 2259. [12] F. Imai, K. Kunimori, T. Manabe, T. Kumagai, H. Nozoye, Thin Solid Films 310 (1997) 184. [13] W.S. Hu, Z.G. Liu, J. Sun, S.N. Zhu, Q.Q. Xu, D. Feng, Z.M. Ji, J. Phys. Chem. Solids 58 (1997) 857.