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Plasma deposition of tribological and optical thin film materials with a filtered cathodic arc source
Surface and Coatings Technology 112 (1999) 257–260
Plasma deposition of tribological and optical thin film materials with a filtered cathodic arc source P.J. Martin a,*, A. Bendavid a, R.P. Netterfield a, T.J. Kinder a, F. Jahan b, G. Smith b a CSIRO Division of Telecommunications and Industrial Physics PO Box 218 Lindfield, NSW 2070, Australia b University of Technology Sydney, Broadway, NSW 2000, Australia
Abstract The recent development of the filtered arc deposition method (FAD) has shown that hard, wear-resistant materials can be deposited free of cathode microdroplets and with exceptional smoothness and reproducible properties. The microhardness and stress of TiN films are determined by the bias applied to the substrate during growth. Microhardness values have been measured over the range of 2000–3000 Hv. The absence of particulates in the deposited films renders the technique suitable for the preparation of high-quality optical dielectric oxide and metallic films on to ambient temperature substrates. Films of amorphous TiO with refractive indices of 2.45 at a wavelength of 600 nm can be easily prepared with a very low absorption. Optical-quality 2 films of Nb O and Al O have also been prepared similarly by reactive deposition from pure Nb and Al cathodes. Smooth Au 2 5 2 3 films with a high reflectivity are also deposited by the FAD process. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Filtered arc; Oxides; Titanium nitride; Stress
1. Introduction The application of the cathodic arc to the deposition of thin film materials has primarily arisen through the characteristics of the emission products [1]. From the point of view of film deposition, the important properties of the particles emitted from the arc are: the charge state, degree of ionization and energy. The ionized fraction of the emitted particles is a strong function of the cathode material and is dependent upon the residual vacuum [1]. The average ion energy for Ti is around 50 eV, which can be increased further by applying a negative bias to the substrate to accelerate the ions. The conventional cathodic arc evaporator produces copious amounts of microdroplets or macroparticles. These macroparticles range in size up to a few micrometres in diameter and lead to a severe degradation in film quality, resulting in film porosity. A surface that is covered with micrometre-sized macroparticles precludes its use from the production of high-quality films suitable for electrical and optical applications. Various schemes and devices have been employed to reduce macroparticles, but the most successful are based on the use of the curved plasma duct filter [2]. * Corresponding author. Tel: +61 2 94137126; Fax: +61 2 94137200; e-mail: [email protected]
The plasma duct filter is a quarter torus with a magnetic field parallel to the walls of the torus. A stream of low-density plasma is guided around the duct by a toroidal magnetic field of the order of 0.015 T. The heavy macroparticles and any neutral particles are trapped in the anode region, where they condense in the form of a thin film. The exiting filtered plasma beam then comprises 100% charged Ti ions and electrons and is directed into the vacuum chamber and deposited on to the substrate surface. Macroparticle-free plasma beams of metals, alloys and also carbon may be produced with this device.
2. Experimental The FAD deposition systems used in the study have been described in detail previously [3,4]. Forty-five and 90° plasma duct FAD systems were used. The 45° system was used for optical film deposition, and the 90° system was used for ion-assisted arc deposition ( IAAD) of TiN films [4]. In IAAD, the depositing Ti film is bombarded by 1200-eV nitrogen ions (N+ ) and the arrival ratio of 2 N+:Ti+ ( ji/jv), varied [4]. Substrates were glass micro2 slides, polished silicon wafers and polished steel (for TiN ). The temperature of deposition was approximately
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400 °C for TiN deposition and ambient for all other materials. The optical properties were measured by spectroscopic ellipsometry and spectrophotometry (Cary 5). Selected film microhardnesses were assessed using a UMIS 2000 ultra-microindentation system with a Berkovich indentor and surface roughness with a Topometrix AFM.
3. Results and discussion 3.1. Optical properties of TiO , All O , AlN and Nb O 2 2 3 2 5 deposited by the FAD method In general, it was found that transparent films of all materials could be deposited using a 100–200-mA ion current and 0.2-Pa reactive gas pressure. These conditions corresponded to a deposition rate of 1–2 mm h−1. The area of deposition was typically 10 cm in diameter. The films were deposited on to unheated substrates, unless stated otherwise, and no bias was used. 3.1.1. TiO results 2 The refractive index and extinction coefficient for a 1.2-mm-thick TiO film deposited on to an unheated 2 glass substrate at the rate of 2 mm h−1 are shown in Fig. 1. The refractive index at a wavelength of 600 nm is 2.46, and the extinction coefficient is <10−4. The index was slightly higher (2.51) for films deposited at higher deposition rates and lower oxygen partial pressures, but this also results in an increase in the extinction coefficient to 0.003. A test sample deposited on to a substrate heated to 300 °C had slightly higher refractive indices (2.55 at 600 nm). The structure of the TiO films 2 deposited on unheated substrates was examined by X-ray diffraction and was found to be amorphous. Peaks associated with the anatase phase were evident for the heated sample. The optical properties of the FAD deposited TiO films compare favourably with 2
Fig. 1. Refractive index and extinction coefficient as a function of wavelength for TiO films deposited by reactive FAD deposition. 2
those of materials prepared by other techniques, including r.f. magnetron sputtering, PICVD, ion plating and IBAD [5–9]. The rms roughness for a 220-nm-thick film deposited on to polished semiconductor-grade silicon was found to be 0.22 nm. The roughness increased to 0.32 nm for 1200-nm-thick films. The hardness of the 1.0-mm-thick films prepared on glass substrates was assessed using a UMIS indentation system. The maximum loading was 5 mN, and the indentation depths were 170–200 nm. The glass substrate hardness was found to be 6.26 GPa, and the film prepared on cold and heated glass substrates, 7.77 and 11.21 GPa, respectively. 3.1.2. Al O and AlN 2 3 In the case of aluminium oxide and nitride materials, both types of films were found to have an acceptably low absorption in the visible region and refractive indices comparable with those of films prepared by conventional ion-assisted techniques [10]. The refractive index of Al O was 1.666 at 600 nm, and the extinction coefficient 2 3 was 4×10−4. Randhawa [11] has previously demonstrated the use of the filtered arc technique in the deposition of high-quality Al O by arc evaporation of 2 3 an aluminium cathode and obtained a refractive index of 1.67–1.70 and extinction coefficient of 6×10−4. The present results for AlN are shown in Fig. 2, where it is seen that the index decreases linearly with wavelength over the range studied and that the extinction coefficient is less than 3×10−4 over the entire range. 3.1.3. Nb O 2 5 The optical data for niobium oxide deposited at 2 mm h−1 are shown in Fig. 3. The refractive index at 600 nm is 2.38, and the extinction coefficient is 3×10−4. The film was found to be amorphous by X-ray diffraction. High-refractive-index niobium oxide is difficult to prepare by conventional electron beam evaporation, and even postannealing produces layers with
Fig. 2. Refractive index and extinction coefficient as a function of wavelength for AlN films deposited by reactive FAD deposition.
P.J. Martin et al. / Surface and Coatings Technology 112 (1999) 257–260
259
enhanced optical properties of FAD deposited films may be explained as a result of the high deposition rate and, therefore, a relative decrease in the contamination rate. In addition, the higher incident energy of the condensing particles may increase the nucleation density and surface mobility of the adatoms on the surface, resulting in dense microstructures. The rms roughness of the films was measured at 1.1–1.4 nm for applied bias values of 0 to −40 V. This is compared with 1.8 and 2.2 nm for thermally evaporated and magnetron-sputtered films, respectively. 3.3. TiN Fig. 3. Refractive index and extinction coefficient as a function of wavelength for Nb O films deposited by reactive FAD deposition. 2 5
low indices ~2.17 [12]. These difficulties are usually attributed to changes in the stoichiometry of the melt and film porosity and water sorption into the film. Higher indices (2.24–2.28) have been reported for sputter-deposited films also with low extinction coefficients (k~0.004 at 530 nm) [13]. Sol–gel deposited layers [14] are generally of a lower index (n~1.82, k~0.003 at 530 nm). 3.2. Au Fig. 4 shows the reflectance of Au films over the wavelength region 350–1500 nm. The steep plasma edge that occurs in the visible region with a reflectance minimum at around 460 nm is characteristic for gold film and gold-like surfaces such as TiN. The data show the higher reflectance of FAD films over a reference sample deposited by magnetron sputtering, with a maximum reflectance of about 0.98 for FAD and 0.96 for the magnetron film at a wavelength of 800 nm. The
Fig. 4. Optical reflectivity of Au films deposited by FAD (broken line) and magnetron sputtering (solid line).
Titanium nitride films were prepared in both deposition systems using the FAD method and the ion-assisted arc deposition ( IAAD) technique. The substrates used were polished Si wafers, polished steels (SUS 316 and tool steel ) and carbon disks for RBS analysis. The FAD TiN was prepared under a range of bias conditions and on substrates heated to 400 °C. The IAAD TiN was deposited on to unbiased and unheated substrates. 3.3.1. Microhardness of TiN The microhardness of FAD TiN is strongly correlated with the applied substrate bias [3] and therefore also the induced compressive stress. An increase in the applied negative bias has the effect of reducing the film compressive stress through near-surface collision cascades, which leads to a rearrangement of recoil implanted atoms [4]. The film microhardness can be as high as 3000 Hv but at the expense of higher compressive stresses of the order of 10 GPa. The increased compressive stress also results in a poorer adhesion of the film to the substrate. In the case of IAAD TiN, the stress is controlled by the j+ /j+ arrival ratio and also the bomN Ti barding energy of the assisting N+ ion beam, but the 2
Fig. 5. Relationship between TiN microhardness and compressive stress for films deposited by FAD and IAAD.
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compressive stress–microhardness relationship is similar to FAD TiN. The data are shown in Fig. 5 for 1-mmthick TiN films deposited on to silicon [3,4].
4. Summary The filtered arc deposition technique has been shown to be a practical method in the synthesis of both opticalquality dielectric materials and conventional wear-resistant TiN films. Smooth gold films are also produced with a high reflectivity. The method shows great promise for the deposition of a range of technologically important materials.
References [1] P.J. Martin, R. Boxman, D. Sanders (Eds.), Handbook of Vacuum Arc Science and Technology, Noyes, New York, 1996.
[2] I.I. Aksenov, S.I. Vakula, V.G. Padalka, V.E. Strelnitskii, V.M. Khoroshikh, Sov. Phys. Tech. Phys. 25 (1980) 1164. [3] A. Bendavid, P.J. Martin, R.P. Netterfield, T.J. Kinder, Surf. Coat. Technol. 70 (1994) 97. [4] P.J. Martin, A. Bendavid, T.J. Kinder, L. Wielunski, Surf. Coat. Technol. 86 (1996) 271. [5] K. Okimura, N. Maeda, A. Shibata, Thin Solid Films 281 (1996) 427. [6 ] C.R. Ottermann, M. Heming, K. Bange, Mater. Res. Soc. Symp. Proc. 356 (1995) 187. [7] J. Otto, V. Paquet, Rh.Th. Kersten, H.-W. Etzkorn, R.M. Brusasco, J. Britten, J.H. Campbell, J.B. Thorsness, SPIE Vol. 1323, Optical Thin Films III: New Developments, 1990, p. 39. [8] M. Gilo, N. Croitoru, Thin Solid Films 283 (1996) 84. [9] X. Tang, Z. Fan, SPIE Vol. 1323, Optical Thin Films III: New Developments, 1990, p. 67. [10] P.J. Martin, R. Netterfield, T. Kinder, A. Bendavid, Appl. Opt. 31 (1992) 6734. [11] H. Randhawa, J. Vac. Sci. Technol. A 7 (3) (1989) 2346. [12] J.J. Van Glabbeek, R.E. Van De Leest, Thin Solid Films 201 (1991) 137. [13] R.L. Lee, C.R. Aita, J. Appl. Phys. 70 (1991) 2094. ¨ zer, M.D. Rubin, C.M. Lampert, Sol. Energy Mater. Sol. [14] N. O Cells 40 (1996) 285.
Plasma deposition of tribological and optical thin film materials with a filtered cathodic arc source P.J. Martin a,*, A. Bendavid a, R.P. Netterfield a, T.J. Kinder a, F. Jahan b, G. Smith b a CSIRO Division of Telecommunications and Industrial Physics PO Box 218 Lindfield, NSW 2070, Australia b University of Technology Sydney, Broadway, NSW 2000, Australia
Abstract The recent development of the filtered arc deposition method (FAD) has shown that hard, wear-resistant materials can be deposited free of cathode microdroplets and with exceptional smoothness and reproducible properties. The microhardness and stress of TiN films are determined by the bias applied to the substrate during growth. Microhardness values have been measured over the range of 2000–3000 Hv. The absence of particulates in the deposited films renders the technique suitable for the preparation of high-quality optical dielectric oxide and metallic films on to ambient temperature substrates. Films of amorphous TiO with refractive indices of 2.45 at a wavelength of 600 nm can be easily prepared with a very low absorption. Optical-quality 2 films of Nb O and Al O have also been prepared similarly by reactive deposition from pure Nb and Al cathodes. Smooth Au 2 5 2 3 films with a high reflectivity are also deposited by the FAD process. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Filtered arc; Oxides; Titanium nitride; Stress
1. Introduction The application of the cathodic arc to the deposition of thin film materials has primarily arisen through the characteristics of the emission products [1]. From the point of view of film deposition, the important properties of the particles emitted from the arc are: the charge state, degree of ionization and energy. The ionized fraction of the emitted particles is a strong function of the cathode material and is dependent upon the residual vacuum [1]. The average ion energy for Ti is around 50 eV, which can be increased further by applying a negative bias to the substrate to accelerate the ions. The conventional cathodic arc evaporator produces copious amounts of microdroplets or macroparticles. These macroparticles range in size up to a few micrometres in diameter and lead to a severe degradation in film quality, resulting in film porosity. A surface that is covered with micrometre-sized macroparticles precludes its use from the production of high-quality films suitable for electrical and optical applications. Various schemes and devices have been employed to reduce macroparticles, but the most successful are based on the use of the curved plasma duct filter [2]. * Corresponding author. Tel: +61 2 94137126; Fax: +61 2 94137200; e-mail: [email protected]
The plasma duct filter is a quarter torus with a magnetic field parallel to the walls of the torus. A stream of low-density plasma is guided around the duct by a toroidal magnetic field of the order of 0.015 T. The heavy macroparticles and any neutral particles are trapped in the anode region, where they condense in the form of a thin film. The exiting filtered plasma beam then comprises 100% charged Ti ions and electrons and is directed into the vacuum chamber and deposited on to the substrate surface. Macroparticle-free plasma beams of metals, alloys and also carbon may be produced with this device.
2. Experimental The FAD deposition systems used in the study have been described in detail previously [3,4]. Forty-five and 90° plasma duct FAD systems were used. The 45° system was used for optical film deposition, and the 90° system was used for ion-assisted arc deposition ( IAAD) of TiN films [4]. In IAAD, the depositing Ti film is bombarded by 1200-eV nitrogen ions (N+ ) and the arrival ratio of 2 N+:Ti+ ( ji/jv), varied [4]. Substrates were glass micro2 slides, polished silicon wafers and polished steel (for TiN ). The temperature of deposition was approximately
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 78 4 - 1
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P.J. Martin et al. / Surface and Coatings Technology 112 (1999) 257–260
400 °C for TiN deposition and ambient for all other materials. The optical properties were measured by spectroscopic ellipsometry and spectrophotometry (Cary 5). Selected film microhardnesses were assessed using a UMIS 2000 ultra-microindentation system with a Berkovich indentor and surface roughness with a Topometrix AFM.
3. Results and discussion 3.1. Optical properties of TiO , All O , AlN and Nb O 2 2 3 2 5 deposited by the FAD method In general, it was found that transparent films of all materials could be deposited using a 100–200-mA ion current and 0.2-Pa reactive gas pressure. These conditions corresponded to a deposition rate of 1–2 mm h−1. The area of deposition was typically 10 cm in diameter. The films were deposited on to unheated substrates, unless stated otherwise, and no bias was used. 3.1.1. TiO results 2 The refractive index and extinction coefficient for a 1.2-mm-thick TiO film deposited on to an unheated 2 glass substrate at the rate of 2 mm h−1 are shown in Fig. 1. The refractive index at a wavelength of 600 nm is 2.46, and the extinction coefficient is <10−4. The index was slightly higher (2.51) for films deposited at higher deposition rates and lower oxygen partial pressures, but this also results in an increase in the extinction coefficient to 0.003. A test sample deposited on to a substrate heated to 300 °C had slightly higher refractive indices (2.55 at 600 nm). The structure of the TiO films 2 deposited on unheated substrates was examined by X-ray diffraction and was found to be amorphous. Peaks associated with the anatase phase were evident for the heated sample. The optical properties of the FAD deposited TiO films compare favourably with 2
Fig. 1. Refractive index and extinction coefficient as a function of wavelength for TiO films deposited by reactive FAD deposition. 2
those of materials prepared by other techniques, including r.f. magnetron sputtering, PICVD, ion plating and IBAD [5–9]. The rms roughness for a 220-nm-thick film deposited on to polished semiconductor-grade silicon was found to be 0.22 nm. The roughness increased to 0.32 nm for 1200-nm-thick films. The hardness of the 1.0-mm-thick films prepared on glass substrates was assessed using a UMIS indentation system. The maximum loading was 5 mN, and the indentation depths were 170–200 nm. The glass substrate hardness was found to be 6.26 GPa, and the film prepared on cold and heated glass substrates, 7.77 and 11.21 GPa, respectively. 3.1.2. Al O and AlN 2 3 In the case of aluminium oxide and nitride materials, both types of films were found to have an acceptably low absorption in the visible region and refractive indices comparable with those of films prepared by conventional ion-assisted techniques [10]. The refractive index of Al O was 1.666 at 600 nm, and the extinction coefficient 2 3 was 4×10−4. Randhawa [11] has previously demonstrated the use of the filtered arc technique in the deposition of high-quality Al O by arc evaporation of 2 3 an aluminium cathode and obtained a refractive index of 1.67–1.70 and extinction coefficient of 6×10−4. The present results for AlN are shown in Fig. 2, where it is seen that the index decreases linearly with wavelength over the range studied and that the extinction coefficient is less than 3×10−4 over the entire range. 3.1.3. Nb O 2 5 The optical data for niobium oxide deposited at 2 mm h−1 are shown in Fig. 3. The refractive index at 600 nm is 2.38, and the extinction coefficient is 3×10−4. The film was found to be amorphous by X-ray diffraction. High-refractive-index niobium oxide is difficult to prepare by conventional electron beam evaporation, and even postannealing produces layers with
Fig. 2. Refractive index and extinction coefficient as a function of wavelength for AlN films deposited by reactive FAD deposition.
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enhanced optical properties of FAD deposited films may be explained as a result of the high deposition rate and, therefore, a relative decrease in the contamination rate. In addition, the higher incident energy of the condensing particles may increase the nucleation density and surface mobility of the adatoms on the surface, resulting in dense microstructures. The rms roughness of the films was measured at 1.1–1.4 nm for applied bias values of 0 to −40 V. This is compared with 1.8 and 2.2 nm for thermally evaporated and magnetron-sputtered films, respectively. 3.3. TiN Fig. 3. Refractive index and extinction coefficient as a function of wavelength for Nb O films deposited by reactive FAD deposition. 2 5
low indices ~2.17 [12]. These difficulties are usually attributed to changes in the stoichiometry of the melt and film porosity and water sorption into the film. Higher indices (2.24–2.28) have been reported for sputter-deposited films also with low extinction coefficients (k~0.004 at 530 nm) [13]. Sol–gel deposited layers [14] are generally of a lower index (n~1.82, k~0.003 at 530 nm). 3.2. Au Fig. 4 shows the reflectance of Au films over the wavelength region 350–1500 nm. The steep plasma edge that occurs in the visible region with a reflectance minimum at around 460 nm is characteristic for gold film and gold-like surfaces such as TiN. The data show the higher reflectance of FAD films over a reference sample deposited by magnetron sputtering, with a maximum reflectance of about 0.98 for FAD and 0.96 for the magnetron film at a wavelength of 800 nm. The
Fig. 4. Optical reflectivity of Au films deposited by FAD (broken line) and magnetron sputtering (solid line).
Titanium nitride films were prepared in both deposition systems using the FAD method and the ion-assisted arc deposition ( IAAD) technique. The substrates used were polished Si wafers, polished steels (SUS 316 and tool steel ) and carbon disks for RBS analysis. The FAD TiN was prepared under a range of bias conditions and on substrates heated to 400 °C. The IAAD TiN was deposited on to unbiased and unheated substrates. 3.3.1. Microhardness of TiN The microhardness of FAD TiN is strongly correlated with the applied substrate bias [3] and therefore also the induced compressive stress. An increase in the applied negative bias has the effect of reducing the film compressive stress through near-surface collision cascades, which leads to a rearrangement of recoil implanted atoms [4]. The film microhardness can be as high as 3000 Hv but at the expense of higher compressive stresses of the order of 10 GPa. The increased compressive stress also results in a poorer adhesion of the film to the substrate. In the case of IAAD TiN, the stress is controlled by the j+ /j+ arrival ratio and also the bomN Ti barding energy of the assisting N+ ion beam, but the 2
Fig. 5. Relationship between TiN microhardness and compressive stress for films deposited by FAD and IAAD.
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compressive stress–microhardness relationship is similar to FAD TiN. The data are shown in Fig. 5 for 1-mmthick TiN films deposited on to silicon [3,4].
4. Summary The filtered arc deposition technique has been shown to be a practical method in the synthesis of both opticalquality dielectric materials and conventional wear-resistant TiN films. Smooth gold films are also produced with a high reflectivity. The method shows great promise for the deposition of a range of technologically important materials.
References [1] P.J. Martin, R. Boxman, D. Sanders (Eds.), Handbook of Vacuum Arc Science and Technology, Noyes, New York, 1996.
[2] I.I. Aksenov, S.I. Vakula, V.G. Padalka, V.E. Strelnitskii, V.M. Khoroshikh, Sov. Phys. Tech. Phys. 25 (1980) 1164. [3] A. Bendavid, P.J. Martin, R.P. Netterfield, T.J. Kinder, Surf. Coat. Technol. 70 (1994) 97. [4] P.J. Martin, A. Bendavid, T.J. Kinder, L. Wielunski, Surf. Coat. Technol. 86 (1996) 271. [5] K. Okimura, N. Maeda, A. Shibata, Thin Solid Films 281 (1996) 427. [6 ] C.R. Ottermann, M. Heming, K. Bange, Mater. Res. Soc. Symp. Proc. 356 (1995) 187. [7] J. Otto, V. Paquet, Rh.Th. Kersten, H.-W. Etzkorn, R.M. Brusasco, J. Britten, J.H. Campbell, J.B. Thorsness, SPIE Vol. 1323, Optical Thin Films III: New Developments, 1990, p. 39. [8] M. Gilo, N. Croitoru, Thin Solid Films 283 (1996) 84. [9] X. Tang, Z. Fan, SPIE Vol. 1323, Optical Thin Films III: New Developments, 1990, p. 67. [10] P.J. Martin, R. Netterfield, T. Kinder, A. Bendavid, Appl. Opt. 31 (1992) 6734. [11] H. Randhawa, J. Vac. Sci. Technol. A 7 (3) (1989) 2346. [12] J.J. Van Glabbeek, R.E. Van De Leest, Thin Solid Films 201 (1991) 137. [13] R.L. Lee, C.R. Aita, J. Appl. Phys. 70 (1991) 2094. ¨ zer, M.D. Rubin, C.M. Lampert, Sol. Energy Mater. Sol. [14] N. O Cells 40 (1996) 285.