- Email: [email protected]
Study of Ar cluster ion bombardment of a sapphire surface
m
Nuclear Instruments and Methods in Physics Research B 121 (I 997) 493-497
__ k!lOMl
__
B
Beam Interactions with Material6 & Atoms
If!! ELSEVIER
Study of Ar cluster ion bombardment of a sapphire surface D. Takeuchi
*,
K. Fukushima,
J. Matsuo, I. Yamada
Ion Beam Engineering Experimeniul Luborurory, Kyoro University. Sukyo. Kyoto 606-01, Jupan
Abstract Sapphire is one of the most useful materials for applications requiring high mechanical strength with chemical and thermal durability. In order to modify the mechanical characteristics of the surface of sapphire, we have considered the use of cluster ion implantation techniques. Cluster ions can provide a low-energy and high-current beam, because each constituent atom in the cluster, with sizes of a few thousands, has an energy of only a few eV. Impact processes of energetic cluster ions on solid surfaces are quite different from those of monomer ions because of multiple-collisions and high-density energy deposition within a local surface region. In order to make clear the bombarding effects, a study of surface modification of sapphire by Ar cluster ion beams has been performed. Cluster ion traces on solid surfaces were observed by STM and AFM to investigate the bombarding effects. It was found that the thickness of the damaged layer produced on sapphire depends strongly on cluster ion energy.
1. Introduction Sapphire (single crystal Al,O,) is one of the most useful materials for applications requiring high mechanical strength, with chemical and thermal durability [I]. For instance, it is used as the premier midwave (3-5 km) IR (InfraRedI window material. Although it is more resistant to thermal shock than other oxide IR window materials, such as spinel, yttria and aluminum oxynitride [2], sapphire is stressed to its limit in some environments. Sapphire loses strength at temperatures above 5OO”C, because compressing the crystallographic c-axis causes twinning in rhombohedral planes [3-S]. The intersection of different twin systems, or the interaction of twins with pre-existing flaws, appears to initiate mechanical failure. In order to modify the mechanical characteristics of sapphire, ion implantation techniques have been considered [6-81. Internal stress effects, due to implantation, can influence the surface hardness. With a low ion dose, internal stress can be introduced into the crystalline substrate and the surface hardness increased. With a high ion dose, however, the near surface region can be amorphized and the internal stress relaxes in the modified surface region. Then the surface hardness decreases. Cluster ions provide a low-energy and high-current ion beam. This is because each constituent atom in a cluster with sizes of a few thousands has an energy of only several eV [9,10]. The impact processes of energetic cluster ions on solid surfaces are quite different from those of
* Corresponding author. [email protected]
Fax:
0168-583X/97/$17.00 Copyright PII SOl68-583X(96)00448-X
+ 81-7.57-5 16-774;
email:
monomer ions, because of multiple collisions and highdensity energy deposition only within a local surface region [ 1 l- 141. Therefore, gas cluster ion bombardment can modify an outermost surface region effectively. Various kinds of interesting phenomena resulting from the cascade interactions, and the extremely high-density energy deposition, have been reported [9- 161. We have investigated the effects of cluster ion bombardment on a sapphire surface [ 171. In this paper, the effects of cluster ion bombardment upon the characteristics of a sapphire surfaces have been studied, not only from a macroscopic, but also from a microscopic point of view.
2. Experiment The 200 keV gas cluster implantation system developed at Kyoto University is shown in Fig. I. It consists of a cluster generator, an electron bombardment ionizer, an E X B mass-filter (Wien-filter), an acceleration tube and a target chamber. Adiabatic expansion of high-pressure gas through a nozzle is utilized for the formation of a high-intensity gas cluster beam. The size distributions of Ar cluster ions are analyzed by the E X B mass-filter over a range of source pressures. The size distribution can be observed within the region of 95 to 110 in the case of 100 atom, of 400 to 600 in the case of 500 atom and of 1000 to 5000 in the case of 3000 atom clusters at a source pressure of 4 atm. With increasing source pressure, the size distribution shifted to larger sizes and the cluster ion beam intensity increased. The bombarding effects of Ar cluster ions on sapphire were investigated by AFM. RBS channeling measure-
0 1997 Elsevier Science B.V. All rights reserved VI. NANOSCALE
PROCESS/ETCHING
494
D. Tukeuchi et ul./Nucl.
Instr. und Meth. in Phys. Rex B I21 (1997) 493-497 Ground Potential
High Voltage Zone
Acceleration /Tube
Fig. I. Schematic diagram of 200 keV gas cluster implantation system.
ments, and dynamic Vickers microhardness measurements. Sapphire(0001) surfaces were irradiated with size-selected Ar cluster ions at energies of up to 150 keV. Sufficient monomer ion suppression could be achieved by using an E X B mass-filter. Since secondary electrons and ions disturb accurate measurements of ion beam current [18], a special Faraday cup was installed in the irradiation chamber for these experiments.
3. Results and discussion
The bombarding effect of Ar cluster ions on solid surface was investigated by STM and AFM observations. Large grain gold on mica and sapphire(OO01) substrates were used for targets. Large grain gold was prepared by vapor deposition and annealing in UHV. The sapphire substrate was annealed in air by a furnace in order to prepare a wide atomic-flat terrace [ 191. Fig. 2a shows a STM image of a trace on a gold grain due to the bombarding by 150 keV Ar cluster ions, with a size of 3000 atoms. Fig. 2b shows an AFM image of a trace on a sapphire(OOO1) terrace, due also to one with the size of 100. The former has a crater shape, while the latter a hill one. All STM images of Ar cluster ion traces on Au grain or those on HOPG surfaces, however, had a crater shape. It is likely that the AFM resolution is one order less than STM one. Therefore, the Ar cluster ion trace on sapphire probably also has a crater shape. The diameter of these traces were about 200 A when the energy of cluster ions were 150 keV. The diameter proved to be proportional to the cubic root of the acceleration voltage of the cluster ions as a result of STM observations. This indicates that the total energy of cluster ions is isotropically distributed into the substrate. Molecular dynamics simulation shows that hemisphere damage regions are formed at the moment of impact of an energetic ion cluster beam.
RBS channeling measurements have been used to study damage on sapphire surfaces irradiated by Ar cluster ions at various energies and ion doses. Fig. 3 shows typical RBS channeling spectra from unirradiated sapphire and sapphire irradiated by I50 keV Ar cluster ions at a dose of 1.25 x lOI ions/cm’. The average cluster size was 3000 atoms. Irradiating Ar atoms did not remain in the sapphire. The damage was obviously concentrated in a very shallow region. In comparing RBS channeling peak heights of aluminum and oxygen before and after irradiation, it was found that oxygen vacancies were formed on the surface after cluster ion irradiation. Fig. 4 shows the number of disordered atoms for a variety of ion doses with three different cluster sizes at 150 keV. The total number of disordered atoms was obtained by an integration over the surface-peak in the channeling spectra [ 181. The density of disordered atoms on the unirradiated surface, due to surface distortion and intrinsic vacancies, is shown as a background in Fig. 4 [20]. The number of disordered atoms due to cluster bombardment increased with ion dose and finally reached saturation. This suggests that the modified layer was totally amorphized. A slight cluster size dependence was observed. In the case of clusters with the sizes of 500 and 3000, the saturation numbers of disordered atoms were about 1.8 X lOI and 2.0 X lOI aluminum atoms/cm2, respectively. This corresponds to a depth of about 40 A. In order to compare with the monomer Ar ion irradiation case, the numbers of disordered atoms were predicted using TRIM (TRansport of Ions in Matter) calculations [21]. TRIM can be used to estimate the probability of vacancy production per ion. It can also estimate vacancy distribution versus depth. From the concentration of aluminum in sapphire, the number of aluminum atoms per I A layer depth is 4.7 X lOI atoms/cm2/A. The probability of vacancy production given by the TRIM calculation multiplied by the ion dose predicts the number of vacancies in the layer. We assume that the layer was completely
D. Tukeuchi
et uI./Nucl.
495
Instr. und Meth. in Phys. Rrs. B 121 (1997) 493-497
>
86
I
1
Sapphire
70 - boo+-
1
.
Sub.
. --...-.- Unirradiated 60 --Vacc=l50kV
I .25x1013~ons/cm2]
Al
1
450
600
550
500
J
650
CHANNEL Fig. 3. RBS channeling spectra from unirradiatcd sapphire(0001) and sapphire(OO0 of
1.25X
lOi
I)
irradiated by 1SO keV Ar cluster ions at a dose
ions/cm*.
The
average cluster size was 3000
atoms
can be considered relative to the TRIM prediction for the case of 300 eV Ar ions implanted to dose of 5 X 10” ions/cm*. The case of 100 atom Ar clusters accelerated through 150 kV, and bombarded with I x IO” ions/cm2, can be considered relative to the prediction for the case of 1.5 keV Ar ions implanted with I X lOi ions/cm’. The case of 3000 atom Ar clusters accelerated through 150 kV, and bombarded with I X lOI ions/cm2, should be also considered, relative to the case of 50 eV Ar ions implanted with 3 X lOI ions/cm’, but the TRIM simulation is not
sappnler
Fig. 2. (a) STM image of Au surface bombarded by 3000 atoms Ar cluster ions at V, = IS0 kV. Several doughnut-like traces were found. (b) AFM
image of sapphire surface bombarded
atoms Ar cluster ions at V, = i50
keV.
Several circular
by 100 traces
size
were found. amorphized when the calculated number of vacancies exceeded 4.7 x lOI atoms/cm’/,& The number of disordered atoms in each I A layer no longer increases once the layer becomes completely amorphized. The total number of disordered aluminum atoms was given by integration over the number calculated with this assumption. The results of the calculations are also depicted in Fig. 4. The disordered atoms in this calculation showed a tendency of saturation. The case of 500 atom Ar clusters accelerated through 150 kV, and bombarded with a dose of I X lOI ions/cm2,
l n +
3000 500
loo: Bare.
0.0. . ’ I ’ 0.0 0.2 0.4
3
0.6
8
0.8
I -
1.0
DOSE [xIO’~ ions/cm*] Fig. 4. Cluster ion dose dependence of the number of disordered atoms for various cluster sizes. The values were calculated from the surface peaks of RBS channeling spectra. VI. NANOSCALE
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D. Tukeuchi et al./ Nucl. Instr. and Meth. in Phys. Res. B I21
suitable for such a low energy regime. All the results in Fig. 4 include the background disordered value. In the case of TRIM calculations, the number of disordered atoms drastically decreases with Ar ion energy, because of the decrease of projected range. In comparing TRIM calculations with the experimental results, the energy of constituent atoms had a larger influence upon the TRIM calculations than it had upon the experimental results for the cluster ions. In the case of 50 eV, it is impossible for Ar monomer ions to produce large damage on a substrate. Even with such a low velocity in the cluster irradiation case, the thickness of the damaged layer was about 40 A, which was higher than the value calculated by TRIM. Therefore, cluster ion bombardment has a quite different mechanism of damage formation than that due to monomer ion bombardment. The process of damage by cluster ion is different from that by a monomer one according to these results. A monomer ion forms defects in the substrate by cascade collisions. On the other hand, a cluster ion forms a completely amorphized local area in the surface region as indicated by the results of STM and AFM observation and molecular dynamics simulations. In order to study the influence of cluster ion bombardment upon surface hardness, sapphire(OOO1) surfaces were bombarded by Ar cluster ions at various doses, sizes and energies. The surface hardness was determined by dynamic Vickers microhardness measurement. The measurement was carried out on the surfaces of irradiated samples by applying a maximum load of 0.1 g which produced indentations to depth of about 400 A from the surface. Since the penetration depth reached through to the crystalline material under the modified regions, the values of hardness in Fig. 5 and Fig. 6 include information concerning, not only the modified layers, but also of the crystal beneath. Fig. 5 shows the relative hardness of sapphire(OOO1) surfaces for a variety of ion doses, for three different cluster sizes. Each value of surface hardness is normalized to the value for the unirradiated sapphire surface. The surface hardness decreased with ion dose. In the case of monomer N, ion implantation at room temperature, the surface hardness of sapphire increases with ion dose until above about 1 X IO” ions/cm* [22]. In the case of Ar cluster ion bombardment, however, the surface hardness decreased even at a dose of 1 X 10” ionf/cm’. Though only 10 cluster ions impact in each 1000 A square area at this dose the bombardment influences the surface hardness of sapphire. This result strongly suggests that one cluster ion can modify a large surface area. A cluster size dependence was scarcely observed. Fig. 6 shows the surface hardness of sapphire(OOO1) surfaces for a variety of energies. The ion dose was fixed The surface hardness decreased at 1X10 l3 ions/cm2. with ion energy, because the thickness of the damaged layer depends on the energy of the cluster ions [23]. Due to cluster ion bombardment, the surface region of the sap-
f 1997) 493-497
1.4 -
Ar3000
--A--
Ar,,,
-m-
Ar,,,
1.2
unirradiated ;
1.0
x 2 I
0.8
$!
0.6
5 p!
0.4
---%____
----_____ --___
‘I-------
Am+-- Sapphire 0.2 150kV 0.0
J
._ IO"
IO"
10' 3
DOSE [ ions/cm’] Fig. 5. The relative hardness of sapphire(OO01) surface for a variety of ion doses for three different cluster sizes. Each value of surface hardness is normalized by the value of unirradiated sapphire surface.
phire was completely damaged, which resulted in the decrease of surface hardness in the Ar cluster case. Ar clusters damaged the very near surface regions so completely that the hardness decreased with cluster ion bombardment. Even with quite low ion doses such as 1 X 10” ions/cm2, the hardness decreased as shown in Fig. 5. Therefore, cluster ion bombardment can effectively modify the very near surface region of the sapphire.
‘.,~ 4 -7
1.
to
-:
3 Z
0.8 -
9 =
0.6 -
9 i= 4
0.4 -
2
Unirradiated
r Ar3000+ -
0.2
Sapphier
E 1 xl 013 ions/cm*
o.ot....~....~....~....~....~... 0
20
40
60
80
100 120 140
160
ENERGY [kV] Fig. 6. The surface hardness of sapphirefOWl) surfaces for a variety of energies. Ion dose was fixed at 1 X lOI ions/cm*.
D. Tukeuchi et ul./Nucl.
Instr. mul Meth. in Phys. Res. B I21 (1997) 493-497
4. Conclusions
121J.S. Lin and L.B. Wechesser.
Damage formed by cluster ion bombardment has been studied using a new high energy gas cluster ion accelerator. The damage produced on sapphire(OO01) substrate surfaces by Ar cluster ion irradiation is concentrated in a very shallow region. The depth of the damaged layer produced by 150 keV cluster ion irradiation is approximately 40 .&. There is no size dependence on the thickness of damaged layers. In the case of 3000 atom-cluster bombardment, the depth was much deeper than the depths calculated from energy of the individual cluster constituent atoms. This is the non-linear effect of a cluster. The damage increased with cluster ion dose, and showed a saturation effect at quite a low ion dose. This corresponds to complete amorphization. The sapphire surface hardness, however, decreased from as low a dose as 1 X IO” ions/cm’. STM observation revealed that cluster ion bombardment gave a crater-like trace, whose diameter was from tens to hundreds Wngstrijm. A cluster ion could form a big damaged area whose diameter was about 200 A. This size of damaged area can cover almost all the surface area at I X 10” ions/cm*. These effects indicate that cluster ion beam bombardment is an effective technique for surface modification of sapphire.
Acknowledgements The authors would like to express their thanks to the Radiation Laboratory Department of Nuclear Engineering in Kyoto University for their help in measurement by RBS. They also thank Mr. K. Fukushima for measuring surface hardness of sapphire substrate.
References [I] DC.
Harris, Infrared Window Press, Bellingham, Washington,
497
and Dome Materials 1992).
(SPIE
Johns Hopkins APL Techn. Digest 13 (1992) 379. [31 W.D. Scott and K.K. Orr, J. Am. Ceram. Sot. 66 ( 1983) 27. [41 SM. Widerhom, B.J. Hockey and DE. Roberts, Philos. Mag. 28 (1973) 783. I51 A.H. Heuer, Philos. Mag. 13 (1966) 379. b1 C.W. White, C.J. McHargue, P.S. Sklad. L.A. Boatner and G.C. Farlow, Mater. Sci. Rep. 4 (1989) 41. [71 C.J. Mchargue, M.E. O’Hem, C.W. White and M.B. Lewis, Mater. Sci. Eng. A 1 IS ( 1989) 36 I. [81 M.E. O’Hem. C.J. McHargue. C.W. White and G.C. Farlow. Nucl. Instr. and Meth. B 46 (1990) 171. [91 I. Yamada, W.L. Brown, J.A. Northby and M. Sosnowski. Nucl. Instr. and Meth. B 79 (1993) 223. I101 J. Matuso, M. Akizuki. J.A. Northby, G.H. Takaoka and I.Yamada, Proc. ICSSPIC-7, in: Surf. Rev. Lett.. in press. [I II H. Hsieh, R.S. Averback, H. Sellers and C.P. Flynn, Phys. Rev. B 45 (19921 4417. 1121 2. Insepov, M. Sosnowski and I. Yamada, in: Laser and Ion Beam Modification of Materials, ed. I. Yamada (Elsevier. Amsterdam, 1994) p. 1 I 1. [131 M. Akizuki, M. Harada, Y. Miyai, A. Doi, T. Yamaguchi, J. Matsuo, G.H. Takaoka, C.E. Ascheron and 1. Yamada, Nucl. Ins&. and Meth. B 99 (1995) 229. [141 I. Yamada. J. Matsuo, Z. lnsepov and M. Akizuki. Proc. IBMM-9, 1995, Nucl. Instr. and Meth. B 106 (1995) 165. [151 J. Matsuo, H. Abe, G.H. Takaoka and I. Yamada. Nucl. lnstr. and Meth. B 99 (I 995) 244. I161 M. Akizuki. M. Harada, Y. Miyai, A. Doi, T. Yamaguchi, J. Matsuo, G.H. Takaoka, C.E. Ascheron and I. Yamada. Proc. ICSSPIC-7, in: Surf. Rev. Lett., in press. [171 D. Takeuchi, J. Matsuo, A. Kitai and 1. Yamada, Mater. Sci. Eng., A. in press. [I81 M. Yoshimoto, T. Maeda, T. Ohnishi and H. Koinuma. Appl. Phys. Lctt. 67 (1995) 2615. [191 L.C. Feldman, R.L. Kaufman and P.J. Silverman, Phys. Rev. Lett. 39 (1977) 38. H. Amir, AI-Bayati, G. Kevin, Orrman-Rossiter, J.A. van den Berg and D.G. Armour, Surface Sci. 241 (1991) 91. Nucl. Instr. and Meth. 174 1211 J.P.Biersack and L.G.Haggmark, ( 19801 257. 1221 T. Hioki, A. Itoh, M. Okubo, S. Noda, H. Doi, J. Kawamoto and 0. Kamigaito, J. Mater. Sci. 21 (1986) 1321. [231 J. Matsuo, D. Takeuchi, A. Kitai and I. Yamada, Mater, Res. Sci., in press. Da
VI. NANOSCALE
PROCESS/ETCHING
Nuclear Instruments and Methods in Physics Research B 121 (I 997) 493-497
__ k!lOMl
__
B
Beam Interactions with Material6 & Atoms
If!! ELSEVIER
Study of Ar cluster ion bombardment of a sapphire surface D. Takeuchi
*,
K. Fukushima,
J. Matsuo, I. Yamada
Ion Beam Engineering Experimeniul Luborurory, Kyoro University. Sukyo. Kyoto 606-01, Jupan
Abstract Sapphire is one of the most useful materials for applications requiring high mechanical strength with chemical and thermal durability. In order to modify the mechanical characteristics of the surface of sapphire, we have considered the use of cluster ion implantation techniques. Cluster ions can provide a low-energy and high-current beam, because each constituent atom in the cluster, with sizes of a few thousands, has an energy of only a few eV. Impact processes of energetic cluster ions on solid surfaces are quite different from those of monomer ions because of multiple-collisions and high-density energy deposition within a local surface region. In order to make clear the bombarding effects, a study of surface modification of sapphire by Ar cluster ion beams has been performed. Cluster ion traces on solid surfaces were observed by STM and AFM to investigate the bombarding effects. It was found that the thickness of the damaged layer produced on sapphire depends strongly on cluster ion energy.
1. Introduction Sapphire (single crystal Al,O,) is one of the most useful materials for applications requiring high mechanical strength, with chemical and thermal durability [I]. For instance, it is used as the premier midwave (3-5 km) IR (InfraRedI window material. Although it is more resistant to thermal shock than other oxide IR window materials, such as spinel, yttria and aluminum oxynitride [2], sapphire is stressed to its limit in some environments. Sapphire loses strength at temperatures above 5OO”C, because compressing the crystallographic c-axis causes twinning in rhombohedral planes [3-S]. The intersection of different twin systems, or the interaction of twins with pre-existing flaws, appears to initiate mechanical failure. In order to modify the mechanical characteristics of sapphire, ion implantation techniques have been considered [6-81. Internal stress effects, due to implantation, can influence the surface hardness. With a low ion dose, internal stress can be introduced into the crystalline substrate and the surface hardness increased. With a high ion dose, however, the near surface region can be amorphized and the internal stress relaxes in the modified surface region. Then the surface hardness decreases. Cluster ions provide a low-energy and high-current ion beam. This is because each constituent atom in a cluster with sizes of a few thousands has an energy of only several eV [9,10]. The impact processes of energetic cluster ions on solid surfaces are quite different from those of
* Corresponding author. [email protected]
Fax:
0168-583X/97/$17.00 Copyright PII SOl68-583X(96)00448-X
+ 81-7.57-5 16-774;
email:
monomer ions, because of multiple collisions and highdensity energy deposition only within a local surface region [ 1 l- 141. Therefore, gas cluster ion bombardment can modify an outermost surface region effectively. Various kinds of interesting phenomena resulting from the cascade interactions, and the extremely high-density energy deposition, have been reported [9- 161. We have investigated the effects of cluster ion bombardment on a sapphire surface [ 171. In this paper, the effects of cluster ion bombardment upon the characteristics of a sapphire surfaces have been studied, not only from a macroscopic, but also from a microscopic point of view.
2. Experiment The 200 keV gas cluster implantation system developed at Kyoto University is shown in Fig. I. It consists of a cluster generator, an electron bombardment ionizer, an E X B mass-filter (Wien-filter), an acceleration tube and a target chamber. Adiabatic expansion of high-pressure gas through a nozzle is utilized for the formation of a high-intensity gas cluster beam. The size distributions of Ar cluster ions are analyzed by the E X B mass-filter over a range of source pressures. The size distribution can be observed within the region of 95 to 110 in the case of 100 atom, of 400 to 600 in the case of 500 atom and of 1000 to 5000 in the case of 3000 atom clusters at a source pressure of 4 atm. With increasing source pressure, the size distribution shifted to larger sizes and the cluster ion beam intensity increased. The bombarding effects of Ar cluster ions on sapphire were investigated by AFM. RBS channeling measure-
0 1997 Elsevier Science B.V. All rights reserved VI. NANOSCALE
PROCESS/ETCHING
494
D. Tukeuchi et ul./Nucl.
Instr. und Meth. in Phys. Rex B I21 (1997) 493-497 Ground Potential
High Voltage Zone
Acceleration /Tube
Fig. I. Schematic diagram of 200 keV gas cluster implantation system.
ments, and dynamic Vickers microhardness measurements. Sapphire(0001) surfaces were irradiated with size-selected Ar cluster ions at energies of up to 150 keV. Sufficient monomer ion suppression could be achieved by using an E X B mass-filter. Since secondary electrons and ions disturb accurate measurements of ion beam current [18], a special Faraday cup was installed in the irradiation chamber for these experiments.
3. Results and discussion
The bombarding effect of Ar cluster ions on solid surface was investigated by STM and AFM observations. Large grain gold on mica and sapphire(OO01) substrates were used for targets. Large grain gold was prepared by vapor deposition and annealing in UHV. The sapphire substrate was annealed in air by a furnace in order to prepare a wide atomic-flat terrace [ 191. Fig. 2a shows a STM image of a trace on a gold grain due to the bombarding by 150 keV Ar cluster ions, with a size of 3000 atoms. Fig. 2b shows an AFM image of a trace on a sapphire(OOO1) terrace, due also to one with the size of 100. The former has a crater shape, while the latter a hill one. All STM images of Ar cluster ion traces on Au grain or those on HOPG surfaces, however, had a crater shape. It is likely that the AFM resolution is one order less than STM one. Therefore, the Ar cluster ion trace on sapphire probably also has a crater shape. The diameter of these traces were about 200 A when the energy of cluster ions were 150 keV. The diameter proved to be proportional to the cubic root of the acceleration voltage of the cluster ions as a result of STM observations. This indicates that the total energy of cluster ions is isotropically distributed into the substrate. Molecular dynamics simulation shows that hemisphere damage regions are formed at the moment of impact of an energetic ion cluster beam.
RBS channeling measurements have been used to study damage on sapphire surfaces irradiated by Ar cluster ions at various energies and ion doses. Fig. 3 shows typical RBS channeling spectra from unirradiated sapphire and sapphire irradiated by I50 keV Ar cluster ions at a dose of 1.25 x lOI ions/cm’. The average cluster size was 3000 atoms. Irradiating Ar atoms did not remain in the sapphire. The damage was obviously concentrated in a very shallow region. In comparing RBS channeling peak heights of aluminum and oxygen before and after irradiation, it was found that oxygen vacancies were formed on the surface after cluster ion irradiation. Fig. 4 shows the number of disordered atoms for a variety of ion doses with three different cluster sizes at 150 keV. The total number of disordered atoms was obtained by an integration over the surface-peak in the channeling spectra [ 181. The density of disordered atoms on the unirradiated surface, due to surface distortion and intrinsic vacancies, is shown as a background in Fig. 4 [20]. The number of disordered atoms due to cluster bombardment increased with ion dose and finally reached saturation. This suggests that the modified layer was totally amorphized. A slight cluster size dependence was observed. In the case of clusters with the sizes of 500 and 3000, the saturation numbers of disordered atoms were about 1.8 X lOI and 2.0 X lOI aluminum atoms/cm2, respectively. This corresponds to a depth of about 40 A. In order to compare with the monomer Ar ion irradiation case, the numbers of disordered atoms were predicted using TRIM (TRansport of Ions in Matter) calculations [21]. TRIM can be used to estimate the probability of vacancy production per ion. It can also estimate vacancy distribution versus depth. From the concentration of aluminum in sapphire, the number of aluminum atoms per I A layer depth is 4.7 X lOI atoms/cm2/A. The probability of vacancy production given by the TRIM calculation multiplied by the ion dose predicts the number of vacancies in the layer. We assume that the layer was completely
D. Tukeuchi
et uI./Nucl.
495
Instr. und Meth. in Phys. Rrs. B 121 (1997) 493-497
>
86
I
1
Sapphire
70 - boo+-
1
.
Sub.
. --...-.- Unirradiated 60 --Vacc=l50kV
I .25x1013~ons/cm2]
Al
1
450
600
550
500
J
650
CHANNEL Fig. 3. RBS channeling spectra from unirradiatcd sapphire(0001) and sapphire(OO0 of
1.25X
lOi
I)
irradiated by 1SO keV Ar cluster ions at a dose
ions/cm*.
The
average cluster size was 3000
atoms
can be considered relative to the TRIM prediction for the case of 300 eV Ar ions implanted to dose of 5 X 10” ions/cm*. The case of 100 atom Ar clusters accelerated through 150 kV, and bombarded with I x IO” ions/cm2, can be considered relative to the prediction for the case of 1.5 keV Ar ions implanted with I X lOi ions/cm’. The case of 3000 atom Ar clusters accelerated through 150 kV, and bombarded with I X lOI ions/cm2, should be also considered, relative to the case of 50 eV Ar ions implanted with 3 X lOI ions/cm’, but the TRIM simulation is not
sappnler
Fig. 2. (a) STM image of Au surface bombarded by 3000 atoms Ar cluster ions at V, = IS0 kV. Several doughnut-like traces were found. (b) AFM
image of sapphire surface bombarded
atoms Ar cluster ions at V, = i50
keV.
Several circular
by 100 traces
size
were found. amorphized when the calculated number of vacancies exceeded 4.7 x lOI atoms/cm’/,& The number of disordered atoms in each I A layer no longer increases once the layer becomes completely amorphized. The total number of disordered aluminum atoms was given by integration over the number calculated with this assumption. The results of the calculations are also depicted in Fig. 4. The disordered atoms in this calculation showed a tendency of saturation. The case of 500 atom Ar clusters accelerated through 150 kV, and bombarded with a dose of I X lOI ions/cm2,
l n +
3000 500
loo: Bare.
0.0. . ’ I ’ 0.0 0.2 0.4
3
0.6
8
0.8
I -
1.0
DOSE [xIO’~ ions/cm*] Fig. 4. Cluster ion dose dependence of the number of disordered atoms for various cluster sizes. The values were calculated from the surface peaks of RBS channeling spectra. VI. NANOSCALE
PROCESS/ETCHING
496
D. Tukeuchi et al./ Nucl. Instr. and Meth. in Phys. Res. B I21
suitable for such a low energy regime. All the results in Fig. 4 include the background disordered value. In the case of TRIM calculations, the number of disordered atoms drastically decreases with Ar ion energy, because of the decrease of projected range. In comparing TRIM calculations with the experimental results, the energy of constituent atoms had a larger influence upon the TRIM calculations than it had upon the experimental results for the cluster ions. In the case of 50 eV, it is impossible for Ar monomer ions to produce large damage on a substrate. Even with such a low velocity in the cluster irradiation case, the thickness of the damaged layer was about 40 A, which was higher than the value calculated by TRIM. Therefore, cluster ion bombardment has a quite different mechanism of damage formation than that due to monomer ion bombardment. The process of damage by cluster ion is different from that by a monomer one according to these results. A monomer ion forms defects in the substrate by cascade collisions. On the other hand, a cluster ion forms a completely amorphized local area in the surface region as indicated by the results of STM and AFM observation and molecular dynamics simulations. In order to study the influence of cluster ion bombardment upon surface hardness, sapphire(OOO1) surfaces were bombarded by Ar cluster ions at various doses, sizes and energies. The surface hardness was determined by dynamic Vickers microhardness measurement. The measurement was carried out on the surfaces of irradiated samples by applying a maximum load of 0.1 g which produced indentations to depth of about 400 A from the surface. Since the penetration depth reached through to the crystalline material under the modified regions, the values of hardness in Fig. 5 and Fig. 6 include information concerning, not only the modified layers, but also of the crystal beneath. Fig. 5 shows the relative hardness of sapphire(OOO1) surfaces for a variety of ion doses, for three different cluster sizes. Each value of surface hardness is normalized to the value for the unirradiated sapphire surface. The surface hardness decreased with ion dose. In the case of monomer N, ion implantation at room temperature, the surface hardness of sapphire increases with ion dose until above about 1 X IO” ions/cm* [22]. In the case of Ar cluster ion bombardment, however, the surface hardness decreased even at a dose of 1 X 10” ionf/cm’. Though only 10 cluster ions impact in each 1000 A square area at this dose the bombardment influences the surface hardness of sapphire. This result strongly suggests that one cluster ion can modify a large surface area. A cluster size dependence was scarcely observed. Fig. 6 shows the surface hardness of sapphire(OOO1) surfaces for a variety of energies. The ion dose was fixed The surface hardness decreased at 1X10 l3 ions/cm2. with ion energy, because the thickness of the damaged layer depends on the energy of the cluster ions [23]. Due to cluster ion bombardment, the surface region of the sap-
f 1997) 493-497
1.4 -
Ar3000
--A--
Ar,,,
-m-
Ar,,,
1.2
unirradiated ;
1.0
x 2 I
0.8
$!
0.6
5 p!
0.4
---%____
----_____ --___
‘I-------
Am+-- Sapphire 0.2 150kV 0.0
J
._ IO"
IO"
10' 3
DOSE [ ions/cm’] Fig. 5. The relative hardness of sapphire(OO01) surface for a variety of ion doses for three different cluster sizes. Each value of surface hardness is normalized by the value of unirradiated sapphire surface.
phire was completely damaged, which resulted in the decrease of surface hardness in the Ar cluster case. Ar clusters damaged the very near surface regions so completely that the hardness decreased with cluster ion bombardment. Even with quite low ion doses such as 1 X 10” ions/cm2, the hardness decreased as shown in Fig. 5. Therefore, cluster ion bombardment can effectively modify the very near surface region of the sapphire.
‘.,~ 4 -7
1.
to
-:
3 Z
0.8 -
9 =
0.6 -
9 i= 4
0.4 -
2
Unirradiated
r Ar3000+ -
0.2
Sapphier
E 1 xl 013 ions/cm*
o.ot....~....~....~....~....~... 0
20
40
60
80
100 120 140
160
ENERGY [kV] Fig. 6. The surface hardness of sapphirefOWl) surfaces for a variety of energies. Ion dose was fixed at 1 X lOI ions/cm*.
D. Tukeuchi et ul./Nucl.
Instr. mul Meth. in Phys. Res. B I21 (1997) 493-497
4. Conclusions
121J.S. Lin and L.B. Wechesser.
Damage formed by cluster ion bombardment has been studied using a new high energy gas cluster ion accelerator. The damage produced on sapphire(OO01) substrate surfaces by Ar cluster ion irradiation is concentrated in a very shallow region. The depth of the damaged layer produced by 150 keV cluster ion irradiation is approximately 40 .&. There is no size dependence on the thickness of damaged layers. In the case of 3000 atom-cluster bombardment, the depth was much deeper than the depths calculated from energy of the individual cluster constituent atoms. This is the non-linear effect of a cluster. The damage increased with cluster ion dose, and showed a saturation effect at quite a low ion dose. This corresponds to complete amorphization. The sapphire surface hardness, however, decreased from as low a dose as 1 X IO” ions/cm’. STM observation revealed that cluster ion bombardment gave a crater-like trace, whose diameter was from tens to hundreds Wngstrijm. A cluster ion could form a big damaged area whose diameter was about 200 A. This size of damaged area can cover almost all the surface area at I X 10” ions/cm*. These effects indicate that cluster ion beam bombardment is an effective technique for surface modification of sapphire.
Acknowledgements The authors would like to express their thanks to the Radiation Laboratory Department of Nuclear Engineering in Kyoto University for their help in measurement by RBS. They also thank Mr. K. Fukushima for measuring surface hardness of sapphire substrate.
References [I] DC.
Harris, Infrared Window Press, Bellingham, Washington,
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(SPIE
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VI. NANOSCALE
PROCESS/ETCHING