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Dual ion-beam deposition of metallic thin films
ELSEVIER
Surface and Coatings
Technology
84 (1996)
485-490
Dual ion-beam deposition of metallic thin films Tom% Sikola a, JiEi Spousta a, Libuge Dittrichova a, Alois Nebojsa a, Vratislav PeEina b, Radek cegka a, Petr Dub a a Department of Physical Engineering, Technical University of Bmo, CZ-616 69 Brno, Czech Republic b Institute of Nuclear Physics, Academy of Sciences of the Czech Republic, CZ-250 68 Rez”, Czech Republic
Abstract
The influenceof argon ion-beambombardmentof growing Al, MO and Ti thin films depositedby ion-beamsputteringon their compositionand optical propertieswas studied.The Ar ion energy and ion-to-atom arrival ratio were 100-600eV and 0.15-1.75 respectively.The concentration of MO (Al) in the thin films decreased(increased)with ion energy as the Ar content increased (decreased).The Ti content wasbelow 35% for all ion energies.The ratio O/Ti was closeto the stoichiometricvalue of two for all ion energiesup to 400eV. Higher ion-beamenergiesand dosesled to higher valuesof the index of refraction for Al and Ti thin films. Furthermore, an increasein the energy of the ions causeda decreasein the deposition rates of all films due to resputteringof the thin film atomsand, in the caseof Al thin films, intensifiedthe amorphizationprocessin the Al/Si structure. Keywords: Ion-beam-assisted deposition;Sputtering; Thin film growth; Thin film properties
1. Introduction The deposition of thin films by the sputtering of targets with low energy ion beams (hundreds to thousands of electronvolts) is well known. In comparison with vacuum evaporation, this technique offers lower deposition rates. However, since the energy of the sputtered particles [ 1,2] is comparable with the binding energiesof the atoms, this can lead to compositional and structural changesin the deposited films. This method is more convenient for the sputtering of refractory materials (e.g. W, MO) than thermal vacuum evaporation. Comparison of this technique with magnetron sputtering shows that the deposition rate of the ion-beam method is usually lower, but a pressure of one order of magnitude lower can be used (approximately lo-’ Pa). The ion-beam deposition method can be modified to give the so-called dual ion-beam deposition technique [2], which is an ion-beam-assisted deposition (IBAD) method. In this case,the growing thin films are simultaneously bombarded by ions with energies of tens to hundreds of electronvolts generated in the secondary ion source. These ions transfer their energy to the neighbouring atoms of the surface layers, creating a localized thermodynamically unbalanced area. From the general point of view, this makes it possible to generate metastable compounds and structures which are not attainable at balanced conditions [ 3-51. 0257-8972/96/$15.00
0 1996 Elsevier
SSDZO257-8972(95)02823-4
Science S.A. All rights reserved
Experiments using the IBAD technique based on both vacuum evaporation and ion-beam sputtering have been reported. In Refs. [6] and [7], the role of ions on the structure of boron nitride has been reported. This has also been shown in Refs. [S-lo] for aluminium, metal carbides and nitrides respectively. Compositional changes in titanium nitride and hydrocarbon films due to simultaneous ion-beam bombardment were reported in Refs. [ 111 and [ 121 respectively. Improvements in the mechanical properties of C:H thin films (hardness, adhesion), TiO (friction) and TiN (stress modification, wear and hardness) were reported in Refs. [ 13-153 respectively. Modification of the electrical properties of alumina [9] and the optical parameters of oxides [ 161 by ion beams has also been performed. In our experiments, we have studied the influence of the simultaneous bombardment by an argon ion beam on the composition and optical properties of metallic thin films (MO, Ti, Al).
2. Experimental
details
The apparatus used for IBAD of thin films consists of two broad-beam Kaufman ion sources(Fig. 1) designed in our group [ 17,181. The primary ion-beam source with a grid diameter of 150 mm was applied to the
T. sikola
486
et al.ISurface
and Coatings
Gas inlet
Diffusion
Fig. 1. Schematic view of the apparatus ion-beam sources.
for IBAD
with two Kaufman
sputtering of the metallic targets. The secondary ion source with a grid diameter of 75 mm was used to bombard simultaneously the growing thin films in order to modify their properties. Both grids were made of molybdenum. The discharge chambers of the ion sources have a multi-pole magnetic field configuration. In the experiments, we used the concave grid arrangement for the primary ion source (sputtering), and thus its beam was focused onto a spot with a diameter of 5 cm on the target surface. The energy of the argon primary ion beam used for sputtering was 600 eV; its current density measured at the target was 0.5 mA cme2. The secondary ion source was run with an ion energy of 100-600 eV; average ion current densities at the substrate holder from 8 x lop4 to 7 x 10m3 mA cmm2 were used. This corresponds approximately to a variation in the ion-to-atom arrival ratio from 0.15 to 1.75. As our experimental facilities did not allow us to keep this ratio constant for all ion energies, the ion “energy” dependence of the thin film characteristics discussed below also includes a dependence on this ratio. The vacuum chamber was pumped by a diffusion pump (2000 1 s-l), equipped with a liquid nitrogen batTie to reduce the backstreaming of oil vapour from the pump into the chamber, and thus to decrease contamination of the treated samples by hydrocarbons. The minimal background pressure measured by a Penning vacuum gauge (Balzers) was about 1 x 10m4 Pa; the operational pressure of argon was kept within the interval (7-9) x 10m3 Pa. The targets sputtered were MO (99.6%), Ti (99.95%) and Al (99.995%). Thin films were deposited on different substrates, such as mirror-polished single crystal Si( loo), polycrystalline MO, Al,O,-based ceramic, glass and quartz. Before deposition, the substrates, except Si, were cleaned by acetone and ethyl alcohol and then presputtered for 10 min by 600 eV argon ions from the secondary source. The deposition time for all thin films was 1 h. The composition of the films deposited was studied
Technology
84 (1996)
485-490
by Rutherford backscattering spectrometry (RBS); the probe beam consisted of 2.18 MeV He ions and the detector was set at a 160” scattering angle. The data were analysed Using GISA~ analysis software. Structural analysis was performed using a Siemens D 500 X-ray diffractometer. The X-ray wavelength (Co KU) used for analysis was selected by a crystal monochromator placed in front of the detector. The optical constants n and k of the films deposited on silicon substrates were determined by a Gaertner ellipsometer at the wavelength jl = 632.8 nm. The morphology of the surface of the deposited thin films was studied by scanning electron microscopy (SEM) (JEOL, type JXA - 840 A) and scanning tunnelling microscopy (STM) (Tescan). The thickness of the thin films was measured by profilometry (Talystep, Taylor and Hobson).
3. Results and discussion 3.1. Film thickness and morphology The thicknesses of the individual thin films grown for 1 h at different energies of the bombarding ions are shown in Fig. 2. The thicknesses of all films decrease with the ion-beam energy due to the increasing resputtering effect of the ions. The thicknesses of Al, MO and Ti thin films, grown without simultaneous ion-beam bombardment, are 440, 280 and 100 nm respectively. The values of the sputtering yields calculated by the TRIM simulation program are 0.34, 1.20 and 0.61 for Al, MO and Ti respectively, which do not fully correspond to the above-mentioned thicknesses. This is caused by the fact that the growth rate is controlled not only by the sputtering yield of the target material, but also by other parameters such as the sticking coefficients, type of
400
I 4 -
I
E 300 2 Y2 200.o E 100 c
Energy
Fig. 2. Thickness of thin films grown function of the energy of the bombarding (measured by a profilometer).
[eV]
on silicon substrates as a Ar ions. Targets: MO, Ti, Al
T. &kola
et al./Surface
and Coatings
nucleation, microstructure and macrostructure of the layers. The surface morphology of the films was investigated by SEM. However, no significant changes in the quality of the surface were observed due to ion-beam bombardment of the films during deposition. In addition, the surface morphology of molybdenum thin films grown on an Si substrate without ion-beam bombardment (Fig. 3(a)) and with bombardment by Ar ions of 400 eV (Fig. 3(b)) was studied by STM. Comparing the figures, it is obvious that the roughness of the surface bombarded by the ions is higher. The statistical characteristics of the surface roughness (rootmean-square deviation) found by STM were 1.7 and 5.2 nm for the non-bombarded and bombarded surfaces respectively.
Technology
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84 (1996)
Ti
40
a . .. 20
487
485-490
-.--
.. .
I 100
I 200
.-
1 0
. 400
300
Energy
..I.. 500
T 600
[eV]
Fig. 4. Concentration of the target elements in the thin films grown on silicon substrates as a function of the energy of the bombarding Ar ions. Targets: MO, Ti, Al (determined from RBS).
3.2. Film composition
The contents of the sputtered target elements in the thin films deposited on silicon substrates as a function of the energy of the bombarding Ar ions are shown in Fig. 4. The Rutherford backscattering spectra of molybdenum thin films show only molybdenum in the films at lower energies. At higher energies (400 and 600 eV), the MO content decreases due to Ar atoms which have penetrated into the MO layers.
In contrast, the concentration of argon atoms, which form the major impurity in aluminium thin films, surprisingly decreases with increasing energy of the ions. Depending on the ion energy, the aluminium content in the Al thin film increases from about 85% to 99%. In the case of titanium films, prepared by sputtering of a titanium target, Ti is not a major element and its concentration is below 35% for all ion energies. This is caused by the strong ability of titanium to bind chemi-
(4 Fig. 3. Surface morphology (taken by STM) ion-beam bombardment with Ei = 400 eV.
@I of a molybdenum
thin film
grown
on an Si substrate:
(a) without
ion-beam
bombardment;
(b) with
488
T. sikola
et al./Surface
and Coatings
Technology
84 (1996)
485-490
J
MO
>’
Ti .. .
.
‘,. ,’ ‘., I\
Ar ”
400
L-
600
800
Channel
Fig. 5. Ar ion energy dependence of the concentration of four major elements contained in the thin films deposited on silicon substrates by sputtering of a titanium target (determined from RBS).
Fig. 6. Comparison of the Rutherford backscattering spectra of molybdenum thin films grown on alumina with bombardment by Ar ions of different energy.
3.3. Structure of thinjlms tally to oxygen and other reactive elements. For all ion energies up to 400 eV, oxygen is the major element accommodated in the films (Fig. 5). This most probably enters the vacuum chamber from the gas inlet system, as polyethylene hoses rather than stainless steel pipes were used. The O/Ti ratio is in the range 1.8-3. The latter value, obtained for 400 eV ion energy, is probably higher than that obtained at lower energies due to the increased concentration of molybdenum, as this element reduces the titanium content in the film. It is assumed that the O/Ti ratio is close to the stoichiometric value of two for all ion energies up to 400 eV, and thus the film can be considered to be titanium oxide (TiO,). At an ion energy of 600 eV, the oxygen content suddenly decreases to zero, its role as the major element in the film being completely taken over by molybdenum which has a concentration of 55 at.%! This significant amount of molybdenum can only be explained by a misalignment of the molybdenum grids of the secondary ion source which is significantly sputtered at higher ion energies. In addition to titanium (23%) the film contains an enhanced amount of argon (15%). The other prevailing impurities observable in the spectrum are Fe (6%) and W (1%) originating from the vacuum chamber walls (target holder) and filaments respectively. In Fig. 6, Rutherford backscattering spectra of molybdenum thin films grown on alumina with bombardment by Ar ions of different energy are compared. The descent of the molybdenum peak tail and the steepness of the edge of the aluminium plateau increase with increasing energy. This indicates that the penetration of molybdenum atoms into the ceramic substrate decreases and the interface MO ceramic becomes thinner with increasing energy. This surprising effect (the opposite from that which is expected) may be caused by an increasing penetration of argon ions into the ceramic during deposition which may block the diffusion of MO atoms into the substrate.
The films deposited were too thin for X-ray diffraction analysis to reveal their detailed structures. However, partial structural information on MO and Al films was obtained. In Fig. 7, the diffraction patterns for molybdenum films deposited without ion-beam bombardment (reference diffraction patterns) and with ion-beam bombardment (Ei= 200 eV) are compared, In the reference patterns, silicon peaks and one molybdenum peak ( 111) are visible. The most dominant peak corresponds to Si(400). Broadening of this peak occurs with increasing energy of the ions. This is caused by an ion-beaminduced increase in the deviation of the lattice parameters from their mean values. The intensity of the MO peak is reduced with increasing ion energy, which is most probably caused by a decreasing thickness of the films (Fig. 2). The interplanar separation beiween Mo( 111) planes found from X-ray analysis is 2.20A and differs slightly from the value of Mo(b.c.c.) (2.3 A). No dependence of this value on the energy is found. In the diffraction patterns of aluminium films, a small
‘;“o
0
20
40
60
80 20
Fig. 7. X-Ray by IBAD.
diffraction
patterns
100
120
[deg]
of MO thin
films
deposited
on Si
T. sikola
et al./Surface
and Coatings
peak of Al( 111) is apparent. The intensity of this peak decreases with increasing ion energy (decreasing film thickness) and almost disappears in the diffraction pattern corresponding to an ion energy of 600 eV. In addition, at higher energies of the ion beam, an increasing background at lower Bragg angles 28, caused by amorphization of the AljSi system, is observed. 3.4. Optical properties
The dependence of the index of refraction and absorption coefficient of the thin films grown on Si substrates on the energy of Ar ions is given in Fig. 8. The index of refraction of molybdenum thin films does not reveal any significant dependence on the ion energy. However, the index of refraction of Al and Ti thin films increases with increasing energy. The increase in the index of refraction with increasing ion-beam energy can be explained by a densification of the films [ 191. The index of refraction of titanium for ion energies up to 400 eV corresponds to that reported in Ref. [ 191. For an ion energy of 600 eV, values approaching that of the index of refraction of molybdenum are obtained, because it is the predominant element in the film deposited at this energy (Fig. 5). The absorption coefficient, which provides information on optical losses in materials, undergoes a significant decrease for Al at ion energies above 200 eV.
Technology
84 (1996)
485-490
489
4. Conclusions
Experiments on the IBAD of thin films were performed within the energy range 100-600 eV and for ion-to-atom arrival ratios of 0.15-1.75. For the upper energy range, the average energy transferred from the ion beam to one surface target atom is high enough to induce intensive sputtering of the growing layers. This corresponds to a decrease in the thin film thickness with increasing energy as demonstrated above. This high energy transfer can also initiate amorphization processes in the film and substrate and increase the surface roughness. The concentration of MO (Al) in the thin films decreases (increases) with ion energy as the Ar content increases (decreases). The content of Ti is below 35% for all ion energies. The O/Ti ratio is close to the stoichiometric value of two for all ion energies up to 400 eV. Higher ion-beam energies and doses lead to higher values of the index of refraction of Al and Ti thin films, most probably due to densification of the films.
Acknowledgements
The authors wish to express their thanks to Dr. FrantiSek Matiijka for profilometric measurements, Dr. Antonin Buchal for X-ray diffraction analysis, Dr. Lubomir Tfima (Tescan Ltd.) for STM experiments and Ivica KuStera and other students without whose help this study could not have been realized. This work was supported by GACR under Grant No. 202-94-0565.
References [l] [Z]
Al
Fig. 8. Ar ion energy dependence (Al, MO, Ti) deposited on silicon
of the optical constants substrates by IBAD.
of thin
P. Sigmund, Phys. Rev., 184 (1969) 283. J.M.E. Harper and J.J. Cuomo, J. Vat. Sci. Technol., 21 (1982) 737. 34th AVS National Symposium, c31 C. Schewebel and G. Gautherin, Anaheim, 1987. Nucl. Instrum. Methods B, 67 c41 J.W. Rabalais and D. Marton, (1992) 287. CSI J.S. Colligon, Mater. Sci. Eng. A, 139 (1991) 199-206. C61 Y. Andoh, K. Ogata and E. Kamijo, Nucl. Instrum. Methods B, 33 (1988) 678. and R.F. Davis, c71 D.J. Kester, KS. Ailey, D.J. Lichtenwalner J. Vat. Sci. Technol. A, 12 (1994) 3074. C81 V. Dietz, P. Erhart, D. Guggi, H.-G. Haubold, W. Jager, M. Prieler and W. Schilling, Nucl. Instrum. Methods B, 59/60 (1991) 284. H. Kobayashi and H. Morisaki, Nucl. Instrum. c91 S. Masaki, Methods B, 59160 (1991) 292. Cl01 Z. Min, W.-Z. Li and H.-D. Li, Nucl. Instrum. Methods B, 59160 (1991) 1358; H. Kheyrandish, J.S. Colligon and J.-K. Kim, J. Vat. Sci. Technol. A, 12 (1994) 2723. and J. Colligon, Thin Solid Films, Cl11 A.V. Bieli, H. Kheyrandish 200 (1991) 283. Cl21 P. Reinke, W. Jacob and W. Miiller, J. Appl. Phys., 74 (1993) 1354.
490 [13] [14] [15]
T. sikola
et al/Surface
and Coatings
S. Scaglione and G. Emiliani, J. Vat. Sci. Technol. A, 7 (1989) 2303. A.K. Rai and R.S. Bhattacharya, Nucl. Instrum. Methods B, 59/60 (1991) 280. M. Barth, W. Ensinger, V. Hoffmann and G.K. Wolf, Nucl. Instrum. Methods E, 59/60 (1991) 254; W. Li, X. He and H. Li, J. Appl. Phys., 75 (1994) 2002.
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[ 161 S.M. Rossnagel and J.J. Cuomo, Vacuum, 38 (1988) 73. [ 171 T. Sikola, Design, construction and application of the equipment based on the broad beam Kaufman ion source, Postgraduate Thesis, Technical University of Brno, 1993. [ 181 T. Sikola, J. Spousta and L. Dittrichova, Ser. Fat. Sci. Nat. Univ. Masaryk. Brun., 22 (1992) 119. [ 191 S. Mohan and M. Ghanashyam Krishna, Vacuum, 46 (1995) 645.
Surface and Coatings
Technology
84 (1996)
485-490
Dual ion-beam deposition of metallic thin films Tom% Sikola a, JiEi Spousta a, Libuge Dittrichova a, Alois Nebojsa a, Vratislav PeEina b, Radek cegka a, Petr Dub a a Department of Physical Engineering, Technical University of Bmo, CZ-616 69 Brno, Czech Republic b Institute of Nuclear Physics, Academy of Sciences of the Czech Republic, CZ-250 68 Rez”, Czech Republic
Abstract
The influenceof argon ion-beambombardmentof growing Al, MO and Ti thin films depositedby ion-beamsputteringon their compositionand optical propertieswas studied.The Ar ion energy and ion-to-atom arrival ratio were 100-600eV and 0.15-1.75 respectively.The concentration of MO (Al) in the thin films decreased(increased)with ion energy as the Ar content increased (decreased).The Ti content wasbelow 35% for all ion energies.The ratio O/Ti was closeto the stoichiometricvalue of two for all ion energiesup to 400eV. Higher ion-beamenergiesand dosesled to higher valuesof the index of refraction for Al and Ti thin films. Furthermore, an increasein the energy of the ions causeda decreasein the deposition rates of all films due to resputteringof the thin film atomsand, in the caseof Al thin films, intensifiedthe amorphizationprocessin the Al/Si structure. Keywords: Ion-beam-assisted deposition;Sputtering; Thin film growth; Thin film properties
1. Introduction The deposition of thin films by the sputtering of targets with low energy ion beams (hundreds to thousands of electronvolts) is well known. In comparison with vacuum evaporation, this technique offers lower deposition rates. However, since the energy of the sputtered particles [ 1,2] is comparable with the binding energiesof the atoms, this can lead to compositional and structural changesin the deposited films. This method is more convenient for the sputtering of refractory materials (e.g. W, MO) than thermal vacuum evaporation. Comparison of this technique with magnetron sputtering shows that the deposition rate of the ion-beam method is usually lower, but a pressure of one order of magnitude lower can be used (approximately lo-’ Pa). The ion-beam deposition method can be modified to give the so-called dual ion-beam deposition technique [2], which is an ion-beam-assisted deposition (IBAD) method. In this case,the growing thin films are simultaneously bombarded by ions with energies of tens to hundreds of electronvolts generated in the secondary ion source. These ions transfer their energy to the neighbouring atoms of the surface layers, creating a localized thermodynamically unbalanced area. From the general point of view, this makes it possible to generate metastable compounds and structures which are not attainable at balanced conditions [ 3-51. 0257-8972/96/$15.00
0 1996 Elsevier
SSDZO257-8972(95)02823-4
Science S.A. All rights reserved
Experiments using the IBAD technique based on both vacuum evaporation and ion-beam sputtering have been reported. In Refs. [6] and [7], the role of ions on the structure of boron nitride has been reported. This has also been shown in Refs. [S-lo] for aluminium, metal carbides and nitrides respectively. Compositional changes in titanium nitride and hydrocarbon films due to simultaneous ion-beam bombardment were reported in Refs. [ 111 and [ 121 respectively. Improvements in the mechanical properties of C:H thin films (hardness, adhesion), TiO (friction) and TiN (stress modification, wear and hardness) were reported in Refs. [ 13-153 respectively. Modification of the electrical properties of alumina [9] and the optical parameters of oxides [ 161 by ion beams has also been performed. In our experiments, we have studied the influence of the simultaneous bombardment by an argon ion beam on the composition and optical properties of metallic thin films (MO, Ti, Al).
2. Experimental
details
The apparatus used for IBAD of thin films consists of two broad-beam Kaufman ion sources(Fig. 1) designed in our group [ 17,181. The primary ion-beam source with a grid diameter of 150 mm was applied to the
T. sikola
486
et al.ISurface
and Coatings
Gas inlet
Diffusion
Fig. 1. Schematic view of the apparatus ion-beam sources.
for IBAD
with two Kaufman
sputtering of the metallic targets. The secondary ion source with a grid diameter of 75 mm was used to bombard simultaneously the growing thin films in order to modify their properties. Both grids were made of molybdenum. The discharge chambers of the ion sources have a multi-pole magnetic field configuration. In the experiments, we used the concave grid arrangement for the primary ion source (sputtering), and thus its beam was focused onto a spot with a diameter of 5 cm on the target surface. The energy of the argon primary ion beam used for sputtering was 600 eV; its current density measured at the target was 0.5 mA cme2. The secondary ion source was run with an ion energy of 100-600 eV; average ion current densities at the substrate holder from 8 x lop4 to 7 x 10m3 mA cmm2 were used. This corresponds approximately to a variation in the ion-to-atom arrival ratio from 0.15 to 1.75. As our experimental facilities did not allow us to keep this ratio constant for all ion energies, the ion “energy” dependence of the thin film characteristics discussed below also includes a dependence on this ratio. The vacuum chamber was pumped by a diffusion pump (2000 1 s-l), equipped with a liquid nitrogen batTie to reduce the backstreaming of oil vapour from the pump into the chamber, and thus to decrease contamination of the treated samples by hydrocarbons. The minimal background pressure measured by a Penning vacuum gauge (Balzers) was about 1 x 10m4 Pa; the operational pressure of argon was kept within the interval (7-9) x 10m3 Pa. The targets sputtered were MO (99.6%), Ti (99.95%) and Al (99.995%). Thin films were deposited on different substrates, such as mirror-polished single crystal Si( loo), polycrystalline MO, Al,O,-based ceramic, glass and quartz. Before deposition, the substrates, except Si, were cleaned by acetone and ethyl alcohol and then presputtered for 10 min by 600 eV argon ions from the secondary source. The deposition time for all thin films was 1 h. The composition of the films deposited was studied
Technology
84 (1996)
485-490
by Rutherford backscattering spectrometry (RBS); the probe beam consisted of 2.18 MeV He ions and the detector was set at a 160” scattering angle. The data were analysed Using GISA~ analysis software. Structural analysis was performed using a Siemens D 500 X-ray diffractometer. The X-ray wavelength (Co KU) used for analysis was selected by a crystal monochromator placed in front of the detector. The optical constants n and k of the films deposited on silicon substrates were determined by a Gaertner ellipsometer at the wavelength jl = 632.8 nm. The morphology of the surface of the deposited thin films was studied by scanning electron microscopy (SEM) (JEOL, type JXA - 840 A) and scanning tunnelling microscopy (STM) (Tescan). The thickness of the thin films was measured by profilometry (Talystep, Taylor and Hobson).
3. Results and discussion 3.1. Film thickness and morphology The thicknesses of the individual thin films grown for 1 h at different energies of the bombarding ions are shown in Fig. 2. The thicknesses of all films decrease with the ion-beam energy due to the increasing resputtering effect of the ions. The thicknesses of Al, MO and Ti thin films, grown without simultaneous ion-beam bombardment, are 440, 280 and 100 nm respectively. The values of the sputtering yields calculated by the TRIM simulation program are 0.34, 1.20 and 0.61 for Al, MO and Ti respectively, which do not fully correspond to the above-mentioned thicknesses. This is caused by the fact that the growth rate is controlled not only by the sputtering yield of the target material, but also by other parameters such as the sticking coefficients, type of
400
I 4 -
I
E 300 2 Y2 200.o E 100 c
Energy
Fig. 2. Thickness of thin films grown function of the energy of the bombarding (measured by a profilometer).
[eV]
on silicon substrates as a Ar ions. Targets: MO, Ti, Al
T. &kola
et al./Surface
and Coatings
nucleation, microstructure and macrostructure of the layers. The surface morphology of the films was investigated by SEM. However, no significant changes in the quality of the surface were observed due to ion-beam bombardment of the films during deposition. In addition, the surface morphology of molybdenum thin films grown on an Si substrate without ion-beam bombardment (Fig. 3(a)) and with bombardment by Ar ions of 400 eV (Fig. 3(b)) was studied by STM. Comparing the figures, it is obvious that the roughness of the surface bombarded by the ions is higher. The statistical characteristics of the surface roughness (rootmean-square deviation) found by STM were 1.7 and 5.2 nm for the non-bombarded and bombarded surfaces respectively.
Technology
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84 (1996)
Ti
40
a . .. 20
487
485-490
-.--
.. .
I 100
I 200
.-
1 0
. 400
300
Energy
..I.. 500
T 600
[eV]
Fig. 4. Concentration of the target elements in the thin films grown on silicon substrates as a function of the energy of the bombarding Ar ions. Targets: MO, Ti, Al (determined from RBS).
3.2. Film composition
The contents of the sputtered target elements in the thin films deposited on silicon substrates as a function of the energy of the bombarding Ar ions are shown in Fig. 4. The Rutherford backscattering spectra of molybdenum thin films show only molybdenum in the films at lower energies. At higher energies (400 and 600 eV), the MO content decreases due to Ar atoms which have penetrated into the MO layers.
In contrast, the concentration of argon atoms, which form the major impurity in aluminium thin films, surprisingly decreases with increasing energy of the ions. Depending on the ion energy, the aluminium content in the Al thin film increases from about 85% to 99%. In the case of titanium films, prepared by sputtering of a titanium target, Ti is not a major element and its concentration is below 35% for all ion energies. This is caused by the strong ability of titanium to bind chemi-
(4 Fig. 3. Surface morphology (taken by STM) ion-beam bombardment with Ei = 400 eV.
@I of a molybdenum
thin film
grown
on an Si substrate:
(a) without
ion-beam
bombardment;
(b) with
488
T. sikola
et al./Surface
and Coatings
Technology
84 (1996)
485-490
J
MO
>’
Ti .. .
.
‘,. ,’ ‘., I\
Ar ”
400
L-
600
800
Channel
Fig. 5. Ar ion energy dependence of the concentration of four major elements contained in the thin films deposited on silicon substrates by sputtering of a titanium target (determined from RBS).
Fig. 6. Comparison of the Rutherford backscattering spectra of molybdenum thin films grown on alumina with bombardment by Ar ions of different energy.
3.3. Structure of thinjlms tally to oxygen and other reactive elements. For all ion energies up to 400 eV, oxygen is the major element accommodated in the films (Fig. 5). This most probably enters the vacuum chamber from the gas inlet system, as polyethylene hoses rather than stainless steel pipes were used. The O/Ti ratio is in the range 1.8-3. The latter value, obtained for 400 eV ion energy, is probably higher than that obtained at lower energies due to the increased concentration of molybdenum, as this element reduces the titanium content in the film. It is assumed that the O/Ti ratio is close to the stoichiometric value of two for all ion energies up to 400 eV, and thus the film can be considered to be titanium oxide (TiO,). At an ion energy of 600 eV, the oxygen content suddenly decreases to zero, its role as the major element in the film being completely taken over by molybdenum which has a concentration of 55 at.%! This significant amount of molybdenum can only be explained by a misalignment of the molybdenum grids of the secondary ion source which is significantly sputtered at higher ion energies. In addition to titanium (23%) the film contains an enhanced amount of argon (15%). The other prevailing impurities observable in the spectrum are Fe (6%) and W (1%) originating from the vacuum chamber walls (target holder) and filaments respectively. In Fig. 6, Rutherford backscattering spectra of molybdenum thin films grown on alumina with bombardment by Ar ions of different energy are compared. The descent of the molybdenum peak tail and the steepness of the edge of the aluminium plateau increase with increasing energy. This indicates that the penetration of molybdenum atoms into the ceramic substrate decreases and the interface MO ceramic becomes thinner with increasing energy. This surprising effect (the opposite from that which is expected) may be caused by an increasing penetration of argon ions into the ceramic during deposition which may block the diffusion of MO atoms into the substrate.
The films deposited were too thin for X-ray diffraction analysis to reveal their detailed structures. However, partial structural information on MO and Al films was obtained. In Fig. 7, the diffraction patterns for molybdenum films deposited without ion-beam bombardment (reference diffraction patterns) and with ion-beam bombardment (Ei= 200 eV) are compared, In the reference patterns, silicon peaks and one molybdenum peak ( 111) are visible. The most dominant peak corresponds to Si(400). Broadening of this peak occurs with increasing energy of the ions. This is caused by an ion-beaminduced increase in the deviation of the lattice parameters from their mean values. The intensity of the MO peak is reduced with increasing ion energy, which is most probably caused by a decreasing thickness of the films (Fig. 2). The interplanar separation beiween Mo( 111) planes found from X-ray analysis is 2.20A and differs slightly from the value of Mo(b.c.c.) (2.3 A). No dependence of this value on the energy is found. In the diffraction patterns of aluminium films, a small
‘;“o
0
20
40
60
80 20
Fig. 7. X-Ray by IBAD.
diffraction
patterns
100
120
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T. sikola
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and Coatings
peak of Al( 111) is apparent. The intensity of this peak decreases with increasing ion energy (decreasing film thickness) and almost disappears in the diffraction pattern corresponding to an ion energy of 600 eV. In addition, at higher energies of the ion beam, an increasing background at lower Bragg angles 28, caused by amorphization of the AljSi system, is observed. 3.4. Optical properties
The dependence of the index of refraction and absorption coefficient of the thin films grown on Si substrates on the energy of Ar ions is given in Fig. 8. The index of refraction of molybdenum thin films does not reveal any significant dependence on the ion energy. However, the index of refraction of Al and Ti thin films increases with increasing energy. The increase in the index of refraction with increasing ion-beam energy can be explained by a densification of the films [ 191. The index of refraction of titanium for ion energies up to 400 eV corresponds to that reported in Ref. [ 191. For an ion energy of 600 eV, values approaching that of the index of refraction of molybdenum are obtained, because it is the predominant element in the film deposited at this energy (Fig. 5). The absorption coefficient, which provides information on optical losses in materials, undergoes a significant decrease for Al at ion energies above 200 eV.
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4. Conclusions
Experiments on the IBAD of thin films were performed within the energy range 100-600 eV and for ion-to-atom arrival ratios of 0.15-1.75. For the upper energy range, the average energy transferred from the ion beam to one surface target atom is high enough to induce intensive sputtering of the growing layers. This corresponds to a decrease in the thin film thickness with increasing energy as demonstrated above. This high energy transfer can also initiate amorphization processes in the film and substrate and increase the surface roughness. The concentration of MO (Al) in the thin films decreases (increases) with ion energy as the Ar content increases (decreases). The content of Ti is below 35% for all ion energies. The O/Ti ratio is close to the stoichiometric value of two for all ion energies up to 400 eV. Higher ion-beam energies and doses lead to higher values of the index of refraction of Al and Ti thin films, most probably due to densification of the films.
Acknowledgements
The authors wish to express their thanks to Dr. FrantiSek Matiijka for profilometric measurements, Dr. Antonin Buchal for X-ray diffraction analysis, Dr. Lubomir Tfima (Tescan Ltd.) for STM experiments and Ivica KuStera and other students without whose help this study could not have been realized. This work was supported by GACR under Grant No. 202-94-0565.
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Al
Fig. 8. Ar ion energy dependence (Al, MO, Ti) deposited on silicon
of the optical constants substrates by IBAD.
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