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Titanium nitride thin films deposited by reactive pulsed-laser ablation in RF plasma
Surface and Coatings Technology 151 – 152 (2002) 316–319
Titanium nitride thin films deposited by reactive pulsed-laser ablation in RF plasma A. Giardini, V. Marotta, S. Orlando*, G.P. Parisi CNR-IMS, Zona Industriale di Tito Scalo, I – 85050 Tito Scalo (PZ), Italy
Abstract Titanium nitride thin films were deposited on Si (100) substrates by pulsed laser ablation of a titanium target in a N2 atmosphere (gas pressure approx. 10 Pa) using a doubled frequency Nd:YAG laser (532 nm) also assisted by a 13.56-MHz radio frequency (RF) plasma. Deposition was carried out at various substrate temperatures ranging from 373 up to 873 K and films were analyzed by X-ray diffractometry, scanning electron microscopy and optical emission spectroscopy. A comparison between the ‘normal’ pulsed laser deposition (PLD) and the RF plasma-assisted PLD showed the influence of the plasma on the structural characteristics of the thin films. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Reactive pulsed laser deposition; Titanium nitride; Radio frequency (RF) plasma
1. Introduction The interest in thin film technology has grown enormously during the last few decades. In the field of metal cutting, 75% of all turning tools and approximately 40% of milling tools have coatings prepared either by chemical (CVD) or physical vapor deposition (PVD). Nitrides and carbides have attracted attention due to their high hardness and chemical inertness, and hence high wear resistance. Titanium nitride (TiN), also called osbornite, is one of the rarest minerals on earth. It is not found in terrestrial rocks, but is only found in enstatite chondrite meteorites. In spite of this, titanium nitride is the most common PVD hard coating in use today. It plays an important role in many industrial applications because of its hardness, high evaporation temperature (2950 8C), good chemical stability and metallic brightness w1x. TiN has an excellent combination of performance properties, attractive appearance and safety (meets the USA FDA requirements for surgical tools and implants, as well as food contact applications). It is a dense, refractory material with unusually high electrical conductivity. Titanium nitride can be used as an extremely hard, thin-film coating that is mostly applied to precision metal parts (cutting tools, * Corresponding author. Tel.: q39-0971-427259; fax: q39-0971427222. E-mail address: [email protected] (S. Orlando).
turbine blades, slitters, knives, sliding or rotating components, precision gears, etc.) w2x. It is widely employed in semiconductor manufacturing as a ‘diffusion barrier’ layer w2–4x, gate electrodes in field-effect transistors, and in very large-scale integrated (VLSI) microelectronics. TiN has peculiar optical properties, including an attractive gold-tinged appearance when pure, and high infrared transmission. It is used as an inorganic antireflective coating for lithography on top of aluminum metal, and as a coating for window glass and for decorative applications. Other applications include solar cells w5x, selective transparent films w2x, high-temperature photothermal conversion w6,7x and Schottky contacts w8x. TiN film deposition can be obtained by sputtering w2x, often from a titanium metal target in a nitrogen-containing atmosphere, reactive evaporation w2,9x, thermal nitridation of pure Ti w2,3x, ion implantation w4x, CVD w3x and pulsed laser ablation w10x. 2. Experimental The laser ablation experiments were carried out in a multiport stainless steel vacuum chamber equipped with a gas inlet, a rotating multi-target and a heatable substrate holder (Fig. 1). The base vacuum pressure of the deposition chamber was below10y3 Pa and the pressure during film deposition was 10 Pa. The deposition temperature of the substrate holder can be varied from room
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 5 6 3 - 8
A. Giardini et al. / Surface and Coatings Technology 151 – 152 (2002) 316–319
317
Fig. 1. Schematic layout of the heatable sample holder 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, into which is inserted the output of the matching network coming from the RF power generator.
temperature up to 1000 K. The fluence of a Quantel Nd:YAG 581 laser (wavelength 532 nm, pulse duration 7 ns, repetition rate 10 Hz) was maintained at 8 Jycm2. The laser impinges on the target, at 458 with respect to the normal, in a static atmosphere of N2. The PLD has been improved by an RF plasma just above the substrate holder, maintaining this last one grounded (Fig. 1). The reason why we chose this configuration, in contrast to that usually used in RF assisted depositions, is that in this manner ionic bombardment of the sample is minimized, without losing the possibility of having a reactive plasma just above the substrate. This choice was originally used in order to deposit optical coatings, for which uniformity of the surface is a very important characteristic. The grounded substrate holder is surrounded by an isolated stainless steel top hat, into which the RF contact cable is inserted. This top hat is provided with a 3-cmdiameter hole on the top to allow 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) connected to the vacuum chamber through a customized matching unit. The target material, a pressed (15 t) disk of titanium crystals 99.99q% (Aldrich 30 581-2), was rotated at 2 rev.y min during deposition. The gaseous species were collected on Si (100) substrates positioned at 5 cm in front of the target on a holder, heatable up to 1000 K. A Philips 500 scanning electron microscope (SEM) was utilized to evaluate the surface roughness of the thin films deposited. X-Ray spectra were recorded with a Siemens D5000 diffractometer using the Ka line (ls 0.154056 nm) of a Cu target as an X-ray source. Emission spectra of the plume were acquired with an ICCD Princeton Instruments detector (EEV 576=384
Fig. 2. SEM photographs of titanium nitride on Si (100) deposited: (a) without RF discharge; and (b) in the presence of RF nitrogen plasma. Both depositions were carried out in an atmosphere of 10 Pa N2 at a substrate temperature of 773 K. Both photographs are at 800= magnification with real dimensions of 100=70 mm2.
Fig. 3. XRD spectrum of titanium nitride deposited on Si (100) in the presence of 10 Pa RF nitrogen plasma at a substrate temperature of 773 K.
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A. Giardini et al. / Surface and Coatings Technology 151 – 152 (2002) 316–319
Fig. 4. Sections of optical emission spectra of a plume produced by pulsed laser ablation of (a) titanium target and (b) nitrogen plasma. Exposure time, 1000 ns.
CCD, 18-mm image intensifier, controller model ST-138 S, pulse Generator model FG-100) equipped with an Acton Research Corporation monochromator model Spectra-Pro 500i utilizing Winspec32 acquisition software. A UV-enhanced bundle of fiber optics transfers the emission of the plume, extracted by a lens system, onto the entrance slit of the monochromator.
presents a few small holes, which could be evidence of either incomplete growth or small damage induced by particulates, probably showing a certain degree of weakness in adhesion to the substrate. On the other hand, the RF plasma-deposited samples are characterized by very good adhesion, but they present a lot of small particulates, which perhaps could be minimized by varying the power of the RF discharge.
3. Results and discussion 3.2. X-Ray diffraction 3.1. Scanning electron microscopy The surface morphology of thin films deposited was studied by SEM. The smoothness of the surface is quite good, even if particulates are present in samples produced by both PLD processes, with and without the RF plasma (Fig. 2). The thin films deposited in an RF nitrogen plasma show a more uniform distribution of particulates, with smaller mean dimensions than for samples produced in nitrogen without the RF discharge. The surface of films produced by conventional PLD
The XRD spectra of titanium nitride did not show changes in the structure as a function of the deposition temperature. The structural growth of cubic TiN becomes appreciable starting at a substrate temperature of approximately 873 K (Fig. 3) for samples deposited in an RF nitrogen plasma. Comparing samples deposited by PLD and those produced by plasma-assisted PLD, the presence of the RF discharge contributes to enhance the mean energy per particle helping the growth of cubic TiN. The same effect could be obtained without the RF
A. Giardini et al. / Surface and Coatings Technology 151 – 152 (2002) 316–319
plasma, but it is necessary to heat the substrate holder to a higher temperature. 3.3. Optical multichannel analyzer (OMA) The emission spectra collected, with an exposure time of 1000 ns, during ablation of the titanium target clearly show all the excited states of Ti neutrals and ions travelling from the target towards the heated substrate. A small part of the emission spectrum is reported in Fig. 4a. In addition, in the spectrum of the plasma produced by the RF discharge, it is easy to recognize the emission of neutral and ionized molecular nitrogen. For instance, a small part of the nitrogen plasma spectrum is reported in Fig. 4b. Even if the OMA spectra confirm the high reactivity of the species involved in the deposition process, unfortunately, the strongest spectral emissions of TiN (301.4, 304.3, 613,86 and 619.95 nm) were, however, very weak in comparison to all the other lines present around them. Thus, the experimental data collected do not allow quantification of the formation of titanium nitride in the gas phase. 4. Conclusion We deposited titanium nitride thin films on Si (100) substrates by pulsed laser ablation of a titanium target in an atmosphere of 10 Pa N2 using a doubled frequency Nd:YAG laser (532 nm), also assisted by an RF (13.56 MHz) plasma. Deposition was carried out at various substrate temperatures, ranging from 373 up to 873 K. The surface morphology of thin films deposited has been approximately evaluated by SEM. The smoothness of the surface is quite good in both PLD processes, with and without the RF plasma. The RF nitrogen-plasma thin films show a more uniform distribution of particulates, with smaller mean dimensions then for films produced during ‘normal’ PLD. The XRD spectra of titanium nitride did not show changes in structure as a function of the deposition temperature. The structural growth of cubic TiN becomes appreciable starting at a substrate temperature
319
of approximately 873 K for samples deposited in an RF nitrogen plasma. The presence of the RF discharge contributes to heating of the substrate and seems to help the growth of cubic TiN. The OMA spectra confirm the high reactivity of the species involved in the deposition process, but the labile signals detected do not allow quantification of the formation of titanium nitride in the gas phase. Work is still in progress to evaluate the best value of applied power to generate the RF plasma in the present layout and then to compare the other possible configuration, in which the substrate is on the RF anode and the top hat is grounded, with the aim of minimizing the negative effects on the surface during deposition of the thin films. Clearly, ionic bombardment of the substrate in the latter configuration is more effective, with substantial evidence available. Acknowledgements This work was partially supported by Progetto Finalizzato MSTA II of Consiglio Nazionale delle Ricerche (CNR), Italy. The authors wish to thank Dr Antonio Morone and Prof Roberto Teghil for their stimulating discussions. References w1x J. Bonse, P. Rudolph, J. Kruger, S. Baudach, W. Kautek, Appl. Surf. Sci. 154y155 (2000) 659. w2x J.-E. Sundgren, Thin Solid Films 128 (1985) 45. w3x J. Hems, Semicond. Int. November (1990) 100. w4x A. Armigliato, M. Finetti, J. Garrido, S. Guerri, P. Ostoja, A. Scorzoni, J. Vac. Sci. Technol. A 3 (1985) 2237. w5x M.A. Nicolet, Thin Solid Films 52 (1978) 415. w6x B. Karlsson, J.-E. Sundgren, B.-O. Johansson, SPIE Proc. 401 (1983) 323. w7x M. Wittner, B. Studer, H. Melchior, J. Appl. Phys. 52 (1981) 5722. w8x L.C. Zang, S.K. Cheung, C.L. Liang, N.W. Cheung, Appl. Phys. Lett. 50 (1987) 445. w9x A.J. Aronson, D. Chen, W.H. Class, Thin Solid Films 72 (1980) 535. w10x J. Narajan, P. Tiwari, X. Chen, J. Singh, R. Chowdhurty, T. Zheleva, Appl. Phys. Lett. 61 (1992) 1290.
Titanium nitride thin films deposited by reactive pulsed-laser ablation in RF plasma A. Giardini, V. Marotta, S. Orlando*, G.P. Parisi CNR-IMS, Zona Industriale di Tito Scalo, I – 85050 Tito Scalo (PZ), Italy
Abstract Titanium nitride thin films were deposited on Si (100) substrates by pulsed laser ablation of a titanium target in a N2 atmosphere (gas pressure approx. 10 Pa) using a doubled frequency Nd:YAG laser (532 nm) also assisted by a 13.56-MHz radio frequency (RF) plasma. Deposition was carried out at various substrate temperatures ranging from 373 up to 873 K and films were analyzed by X-ray diffractometry, scanning electron microscopy and optical emission spectroscopy. A comparison between the ‘normal’ pulsed laser deposition (PLD) and the RF plasma-assisted PLD showed the influence of the plasma on the structural characteristics of the thin films. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Reactive pulsed laser deposition; Titanium nitride; Radio frequency (RF) plasma
1. Introduction The interest in thin film technology has grown enormously during the last few decades. In the field of metal cutting, 75% of all turning tools and approximately 40% of milling tools have coatings prepared either by chemical (CVD) or physical vapor deposition (PVD). Nitrides and carbides have attracted attention due to their high hardness and chemical inertness, and hence high wear resistance. Titanium nitride (TiN), also called osbornite, is one of the rarest minerals on earth. It is not found in terrestrial rocks, but is only found in enstatite chondrite meteorites. In spite of this, titanium nitride is the most common PVD hard coating in use today. It plays an important role in many industrial applications because of its hardness, high evaporation temperature (2950 8C), good chemical stability and metallic brightness w1x. TiN has an excellent combination of performance properties, attractive appearance and safety (meets the USA FDA requirements for surgical tools and implants, as well as food contact applications). It is a dense, refractory material with unusually high electrical conductivity. Titanium nitride can be used as an extremely hard, thin-film coating that is mostly applied to precision metal parts (cutting tools, * Corresponding author. Tel.: q39-0971-427259; fax: q39-0971427222. E-mail address: [email protected] (S. Orlando).
turbine blades, slitters, knives, sliding or rotating components, precision gears, etc.) w2x. It is widely employed in semiconductor manufacturing as a ‘diffusion barrier’ layer w2–4x, gate electrodes in field-effect transistors, and in very large-scale integrated (VLSI) microelectronics. TiN has peculiar optical properties, including an attractive gold-tinged appearance when pure, and high infrared transmission. It is used as an inorganic antireflective coating for lithography on top of aluminum metal, and as a coating for window glass and for decorative applications. Other applications include solar cells w5x, selective transparent films w2x, high-temperature photothermal conversion w6,7x and Schottky contacts w8x. TiN film deposition can be obtained by sputtering w2x, often from a titanium metal target in a nitrogen-containing atmosphere, reactive evaporation w2,9x, thermal nitridation of pure Ti w2,3x, ion implantation w4x, CVD w3x and pulsed laser ablation w10x. 2. Experimental The laser ablation experiments were carried out in a multiport stainless steel vacuum chamber equipped with a gas inlet, a rotating multi-target and a heatable substrate holder (Fig. 1). The base vacuum pressure of the deposition chamber was below10y3 Pa and the pressure during film deposition was 10 Pa. The deposition temperature of the substrate holder can be varied from room
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 5 6 3 - 8
A. Giardini et al. / Surface and Coatings Technology 151 – 152 (2002) 316–319
317
Fig. 1. Schematic layout of the heatable sample holder 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, into which is inserted the output of the matching network coming from the RF power generator.
temperature up to 1000 K. The fluence of a Quantel Nd:YAG 581 laser (wavelength 532 nm, pulse duration 7 ns, repetition rate 10 Hz) was maintained at 8 Jycm2. The laser impinges on the target, at 458 with respect to the normal, in a static atmosphere of N2. The PLD has been improved by an RF plasma just above the substrate holder, maintaining this last one grounded (Fig. 1). The reason why we chose this configuration, in contrast to that usually used in RF assisted depositions, is that in this manner ionic bombardment of the sample is minimized, without losing the possibility of having a reactive plasma just above the substrate. This choice was originally used in order to deposit optical coatings, for which uniformity of the surface is a very important characteristic. The grounded substrate holder is surrounded by an isolated stainless steel top hat, into which the RF contact cable is inserted. This top hat is provided with a 3-cmdiameter hole on the top to allow 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) connected to the vacuum chamber through a customized matching unit. The target material, a pressed (15 t) disk of titanium crystals 99.99q% (Aldrich 30 581-2), was rotated at 2 rev.y min during deposition. The gaseous species were collected on Si (100) substrates positioned at 5 cm in front of the target on a holder, heatable up to 1000 K. A Philips 500 scanning electron microscope (SEM) was utilized to evaluate the surface roughness of the thin films deposited. X-Ray spectra were recorded with a Siemens D5000 diffractometer using the Ka line (ls 0.154056 nm) of a Cu target as an X-ray source. Emission spectra of the plume were acquired with an ICCD Princeton Instruments detector (EEV 576=384
Fig. 2. SEM photographs of titanium nitride on Si (100) deposited: (a) without RF discharge; and (b) in the presence of RF nitrogen plasma. Both depositions were carried out in an atmosphere of 10 Pa N2 at a substrate temperature of 773 K. Both photographs are at 800= magnification with real dimensions of 100=70 mm2.
Fig. 3. XRD spectrum of titanium nitride deposited on Si (100) in the presence of 10 Pa RF nitrogen plasma at a substrate temperature of 773 K.
318
A. Giardini et al. / Surface and Coatings Technology 151 – 152 (2002) 316–319
Fig. 4. Sections of optical emission spectra of a plume produced by pulsed laser ablation of (a) titanium target and (b) nitrogen plasma. Exposure time, 1000 ns.
CCD, 18-mm image intensifier, controller model ST-138 S, pulse Generator model FG-100) equipped with an Acton Research Corporation monochromator model Spectra-Pro 500i utilizing Winspec32 acquisition software. A UV-enhanced bundle of fiber optics transfers the emission of the plume, extracted by a lens system, onto the entrance slit of the monochromator.
presents a few small holes, which could be evidence of either incomplete growth or small damage induced by particulates, probably showing a certain degree of weakness in adhesion to the substrate. On the other hand, the RF plasma-deposited samples are characterized by very good adhesion, but they present a lot of small particulates, which perhaps could be minimized by varying the power of the RF discharge.
3. Results and discussion 3.2. X-Ray diffraction 3.1. Scanning electron microscopy The surface morphology of thin films deposited was studied by SEM. The smoothness of the surface is quite good, even if particulates are present in samples produced by both PLD processes, with and without the RF plasma (Fig. 2). The thin films deposited in an RF nitrogen plasma show a more uniform distribution of particulates, with smaller mean dimensions than for samples produced in nitrogen without the RF discharge. The surface of films produced by conventional PLD
The XRD spectra of titanium nitride did not show changes in the structure as a function of the deposition temperature. The structural growth of cubic TiN becomes appreciable starting at a substrate temperature of approximately 873 K (Fig. 3) for samples deposited in an RF nitrogen plasma. Comparing samples deposited by PLD and those produced by plasma-assisted PLD, the presence of the RF discharge contributes to enhance the mean energy per particle helping the growth of cubic TiN. The same effect could be obtained without the RF
A. Giardini et al. / Surface and Coatings Technology 151 – 152 (2002) 316–319
plasma, but it is necessary to heat the substrate holder to a higher temperature. 3.3. Optical multichannel analyzer (OMA) The emission spectra collected, with an exposure time of 1000 ns, during ablation of the titanium target clearly show all the excited states of Ti neutrals and ions travelling from the target towards the heated substrate. A small part of the emission spectrum is reported in Fig. 4a. In addition, in the spectrum of the plasma produced by the RF discharge, it is easy to recognize the emission of neutral and ionized molecular nitrogen. For instance, a small part of the nitrogen plasma spectrum is reported in Fig. 4b. Even if the OMA spectra confirm the high reactivity of the species involved in the deposition process, unfortunately, the strongest spectral emissions of TiN (301.4, 304.3, 613,86 and 619.95 nm) were, however, very weak in comparison to all the other lines present around them. Thus, the experimental data collected do not allow quantification of the formation of titanium nitride in the gas phase. 4. Conclusion We deposited titanium nitride thin films on Si (100) substrates by pulsed laser ablation of a titanium target in an atmosphere of 10 Pa N2 using a doubled frequency Nd:YAG laser (532 nm), also assisted by an RF (13.56 MHz) plasma. Deposition was carried out at various substrate temperatures, ranging from 373 up to 873 K. The surface morphology of thin films deposited has been approximately evaluated by SEM. The smoothness of the surface is quite good in both PLD processes, with and without the RF plasma. The RF nitrogen-plasma thin films show a more uniform distribution of particulates, with smaller mean dimensions then for films produced during ‘normal’ PLD. The XRD spectra of titanium nitride did not show changes in structure as a function of the deposition temperature. The structural growth of cubic TiN becomes appreciable starting at a substrate temperature
319
of approximately 873 K for samples deposited in an RF nitrogen plasma. The presence of the RF discharge contributes to heating of the substrate and seems to help the growth of cubic TiN. The OMA spectra confirm the high reactivity of the species involved in the deposition process, but the labile signals detected do not allow quantification of the formation of titanium nitride in the gas phase. Work is still in progress to evaluate the best value of applied power to generate the RF plasma in the present layout and then to compare the other possible configuration, in which the substrate is on the RF anode and the top hat is grounded, with the aim of minimizing the negative effects on the surface during deposition of the thin films. Clearly, ionic bombardment of the substrate in the latter configuration is more effective, with substantial evidence available. Acknowledgements This work was partially supported by Progetto Finalizzato MSTA II of Consiglio Nazionale delle Ricerche (CNR), Italy. The authors wish to thank Dr Antonio Morone and Prof Roberto Teghil for their stimulating discussions. References w1x J. Bonse, P. Rudolph, J. Kruger, S. Baudach, W. Kautek, Appl. Surf. Sci. 154y155 (2000) 659. w2x J.-E. Sundgren, Thin Solid Films 128 (1985) 45. w3x J. Hems, Semicond. Int. November (1990) 100. w4x A. Armigliato, M. Finetti, J. Garrido, S. Guerri, P. Ostoja, A. Scorzoni, J. Vac. Sci. Technol. A 3 (1985) 2237. w5x M.A. Nicolet, Thin Solid Films 52 (1978) 415. w6x B. Karlsson, J.-E. Sundgren, B.-O. Johansson, SPIE Proc. 401 (1983) 323. w7x M. Wittner, B. Studer, H. Melchior, J. Appl. Phys. 52 (1981) 5722. w8x L.C. Zang, S.K. Cheung, C.L. Liang, N.W. Cheung, Appl. Phys. Lett. 50 (1987) 445. w9x A.J. Aronson, D. Chen, W.H. Class, Thin Solid Films 72 (1980) 535. w10x J. Narajan, P. Tiwari, X. Chen, J. Singh, R. Chowdhurty, T. Zheleva, Appl. Phys. Lett. 61 (1992) 1290.