- Email: [email protected]
Applications of gas cluster ion beams for materials processing
MATERIALS SCIENCE & ENGINEERING
ELSEVIER
A
Materials Scienceand EngineeringA217/218 (1996) 82-88
Applications
of gas cluster ion beams for materials processing Isao Yamada
Ion Beam Engineering Experimental LuboFatoty, Kyoto University Sakyo, Kyoto 606, Japan
Abstract Irradiation effectsof gasclusterion beamshave beenstudiedexperimentallyand simulatedtheoretically for various ion species, energiesand materials combinations. It is shown that cluster ion-surface interactions offer new opportunities for surface processingsuchasvery shallowimplantation (lessthan 0.1 pm in Si), very high rate sputtering (two ordersof magnitudehigher than for monomerions), lateral sputtering effects, atomically smooth surface formation (averagesurfaceroughnesslessthan 1 nm), thin surfacelayer formation at low substratetemperatureetc. Keywords: Applications;Materialsprocessing; Gasclusterion beams
1. Introduction
Ion beams have been used for over thirty years to modify materials in manufacturing of integrated circuits and also for hardening and improving the tribological properties of surfaces [I]. Ion beams used for these applications range in energy from keV to MeV and, depending on their mass, penetrate into the target material to depths ranging from tens of nanometers to microns. However, recent trends in material and surface modifications require ranges in the order of nanometers [2]. For these processes, the required ion energies are in the low-energy range of a few eV to a few hundreds of eV. These low-energy ion beams are very difficult to produce at high intensity because of space charge limitations. The development of gas cluster ion beam technology is our current challenging task to overcome these limitations. The kinetic energy of ionized clusters is shared by their constituent atoms so that in a 10 keV cluster of 1000 atoms, each atom carries only 10 eV. While a collective motion of the cluster atoms plays an important role during the impact, cluster ion beams cause direct bombarding effects only within the first few monolayers of the target material. Gas cluster ion beams offer many new and unique opportunities for surface processing including chemically activated reactions, shallow implantation, very high rate sputtering, surface smoothing and surface cleaning [3,4]. We have developed 10 kV, 30 kV and 200 kV accelerators which allow the production of mass-selectedcluster ion beams
from gas sources. Irradiation effects of gas cluster ion beams have been studied experimentalIy and simulated theoretically for various ion energies and ion-material combinations. The results show that irradiation effects, such as sputtering, atomic penetration, damage accumulation and dynamic annealing of cluster ion beams, are significantly different from those of monomer ion beams. The typical surface interactions and application areas are shown in Fig. 1. In this paper, some new aspects of cluster ion-surface interactions which deviate significantly from monomer ion and molecular ion-surface interactions will be discussed. Arising from these results, new application fields of ionized cluster beams for surface modifiLow charge
to mass
High
ratio
sputtering
yield
Multiple collision at near-surface reoion
High density energy deposition
* Shallow
implantation Low energy
effect
Fig. 1. Typical surfaceinteractionsand applicationareasof gas clusterion beamtechnique. 0921-5093/96/$15.00 0 1996- ElsevierScience S.A. All rightsreserved PII s0921-5093(96)10358-9
I. Yamada
Skimmer
M.B.P
/ Materials
Science
Neutral beam detector l-i h
and Engineering
A2171218
Ionizer and mass filter
(1996)
83
82-85
I
I
D.P.
T.i.P
----I
I a T.M.P
Fig. 2. Schematic diagram of 30kV gas cluster ion beam equipment.
cation are proposed with respect to technologically important aspects, such as shallow implantation (less than 0.1 pm in Si), very high yield sputtering (two orders of magnitude higher than for monomer ions [3]), lateral sputtering phenomena [4], atomically smooth surface formation (average surface roughness less than 1 nm), thin surface layer formation (very thin oxide layers) at room temperature [5]. 2. Equipment
Three different cluster ion beam systems which can accelerate cluster ions up to 10 kV, 30 kV and 200 kV, respectively, have been constructed in our laboratory [6]. Fig. 2 shows a schematic diagram of the 30 kV gas-cluster ion beam equipment. Cluster ion beams from gaseous source materials, such as Ar and CO,, are generated by expanding the corresponding gases through a Lava1 nozzle at room temperature into a high-vacuum chamber after collimating by a skimmer. Subsequent ionization is performed by electron bombardment. Mass analysis is accomplished by an electrostatic retarding potential method using the different kinetic energies of clusters of different masses. The analysis of cluster sizes has also been performed by electron diffraction. Mass selected cluster ions with sizes of up to 5000 atoms were electrostatically accelerated with voltages in the range of 30 kV. Typical retarding spectra and cluster size distributions are shown in Fig. 3. The size of the cluster ions ranges up to a few thousand atoms, and the average size of the clusters is about 3000. Monomer ions are sufficiently suppressed by applying a low extraction voltage and adjusting electrostatic lenses. The ion current of mass-separated Ar clusters was measured as a function of source pressure. Below 1000 Torr, cluster ions are not observed and only monomer ions are found in the ion beam. The cluster ion current increases linearly with the source gas pressure. The threshold pressure for cluster formation is found to be around
1000 Torr [6]. The highest cluster ion beam current obtained is about 100 nA. This current corresponds to an Ar atom flux of 1.9 x 1Or5 atoms s-l cm-‘. 3. Molecular dynamic simulation
Three snapshots of an Ar,,, cluster impact with an energy of 10 eV per constituent atom on an Si(100) surface modeled by 32768 atoms at room temperature are shown in Fig. 4. According to MD simulations, the huge local energy deposition by collisions of cluster ions with surfaces is expected to cause new sputtering effects [7]. These effects concern the angular distribution of sputtered atoms and the sputter yield per constituent atom. The angular distribution deviates significantly from the cosine distribution at larger angles in case of cluster ion sputtering. While in the case of sputtering by monomer ions more atoms leave the surface into normal direction [8], in the case of sputtering by cluster ions significantly strong lateral sputtering is predicted. The angular distribution of sputtered target atoms which is shown in Fig. 5 indicates the occurrence of lateral sputtering. Cluster I”““”
0
1000 I
Size
2000
3000 ..,.
4000
5000 I.
/,,,,,,.
t
-50
4
0
50
100 Retarding
150
200 Voltage
250
300
350
(V)
Fig. 3. Typical retarding spectra and cluster size distributions.
84
I. Yunzada / Materials Science and Engiizeering A21 7/218 (1996) 82-88
Monomer 150keV
OPS
0.5ps
3.ops
Fig. 4. A snapshot of an impact of a 349 Ar atom cluster v/it11an energy of 1OeV per cluster atom on an Si (100) surface. OO
Simulations of the influence of the cluster size N on the sputter yield Y show that the sputter yield per cluster ion increases stronger than the number of constituent atoms. From the calculations, an increase of sputter yield per constituent atom proportional to N2.4 is derived [9]. This increase of sputter yield in the case of identical particle energies indicates cluster-typical effects, such as very high local densities of energy deposition. 4. Characteristics of gas cluster bombardment 4.1. Shnliow imphtntion
It is expected that cluster ion implantation is connected with shallow penetration into solid material. However, no experimental results have been published with regard to this subject up until now. Fig. 6 shows the channeling spectra of Si (100) surfaces bombarded with 150 keV Ar cluster and 150, 100 and 50 keV Ar monomer ions. The size distribution of the A.r cluster ion beam ranged from 1000 to 5000. The peak size was
I\ Clusterioal15OkeV
2000
3000
Depth (ii) Fig. 6. Channeling spectra of an Si (100) surface bombarded with Ar cluster and monomer ions.
approximately 3000 atoms per cluster. The doses of cluster and monomer ions were 2.5 x 1013 and 1 x 10” ions per cm2, respectively. The Ar cluster ion beam current was 5 nA. The thickness of the damaged layer caused by cluster ion irradiation with cluster ions is less than 25 run which is much smaller than in the case of monomer ion implantation with the same total energy. At this implantation energy, the average energy per constituent atom of the clusters is estimated to be approximately 50 eV. The thickness of the damaged layer of 25 nm is much larger than the expected one. To damage a layer of this thickness by Ar monomer ions, an energy of 10 keV is necessary. According to the MD simulation, this discrepancy may be explained by a shock wave effect produced by cluster ion bombardment. The shock wave could propagate into a deeper region of the substrate and produce an amorphous state in a region much deeper than the penetration depth of the ions.
'Z
01 ?’ IQ 1
! 250 MINIMUM
CLUSTER
500 SIZE
(molecules/cluster)
Fig. 5. Angular distribution of sputtered target atoms by Ar cluster ion.
Fig. 7. Influence of the size of CO, clusters on the sputtering yield per cluster for identical acceleration voltages of 10 kV for the given targets.
1. Yamada
/ Materials
Science
and Engineering
A2171218
(1996)
52-58
8.5
Aug 27 94 s37z poly-Si HFTaCwerPSZ Pa=36 PV-329
fug 15 94 64% polySi 1OkV Nt50SE15 As=7.1 PV-67
PS3
Fig. 8(a, b).
4.2. High yield sputtering
The sputtering effects have been studied on metal and SiO, surfaces irradiated with CO, and Ar cluster ions. The results shown in Fig. 7 were obtained at the cluster energy of 10 keV, i.e. the energy per constituent atom varies inversely proportional to cluster size. The sputtering yield per cluster ion is by a factor of more than 100 higher than for monomer ions. The sputtering yield has been observed to have a strong dependence on the target species for Ar clusters with an energy of 20 keV [lo]. However, the sputtering yield is much higher than that of monomer ions, by about one to two orders of magnitude. With the same
equipment, Ar monomer ions have also been produced, and the same targets have been sputtered. The yields of monomer ion beam sputtering agreed well with the predicted and measured values [ll]. The dependencies of the sputtering yield on the target species may be explained by influences of the chemical binding energy and surface binding energy of the target atoms. 4.3. Surface smoothing
Changes in surface roughness after cluster ion bombardment have been measured using an atomic force microscope. Fig. 8 shows the comparison of the average surface roughness (Ra) of poly-Si grown on Si substrate
I. Yumada 1 Materials Sciexe and Engineering A217218 {1996) 82-88
Aug 10 94 ~372 poly.Si IOkV N=l5E15
Ra=34 Pv=m
PS?
Fig. 8. AFM images of unirradiated, cluster and monomer ion irradiated surfaces. Acceleration voltage is 10 kV. Dose is 5 x 10” ions per cm’. (a) Original surface (b) after gas cluster bombardment (c) after molecular ion bombardment.
and sputtered surfaces which have been irradiated with CO, cluster ions and monomer ions accelerated with 10 kV. The minimum cluster size was approximately 250 molecules. In the case of cluster bombardment, Ra decreased from an initial value of 3.7 nm monotonically with sputtering time and saturated at a value of 0.7 nm. In the case of monomer ion bombardment, the minimum value of Ra was 3.4 nm. For many other materials, such as Pt, Cu, SiOp, S&N, films and glass substrates, similar results have been obtained [12]. This smoothing effect could be explained by lateral sputtering which is suggested by MD results [7]. 4.4. Surfflce cleaning
A high removal rate of surface impurities by cluster ion bombardment has been confirmed in connection with low damage effects [13]. Fig. 9 shows the dose dependence of normalized impurity concentrations on silicon substrate surfaces irradiated with cluster and monomer ions. Silicon wafers were intentionally contaminated with 1 x 1Ol5 Cu atoms per cm2 and 6 x 1Or2 Ni atoms per cm2 and subsequently irradiated with 20 keV Ar and 10 keV CO, ions, respectively. Compared with the smaller reduction of impurity concentration by about 4% in the case of monomer irradiation, 80% of the impurity atoms were removed from the surface after irradiation with Ar and CO, cluster ions at doses of 6 x 1014 and 2 x lOI ions per cm’, respectively. The impurity concentration exponentially decreased with increasing dose. The removal rates of impurities by Ar and CO, cluster ion irradiation were 100 and 40 times higher than by monomer ion irradiation, respectively.
The difference corresponds approximately to the difference in sputtering yield. In addition, the energies per constituent atom were 7 eV and 40 eV in the case of Ar and CO2 cluster ions, respectively. Thus, low irradiation damage can be expected. 4.5. Thin film formation
High density bombardment with low energy cluster ions can modify thin substrate surface layers. Si, Ti and Be substrates have been bombarded with 20 keV CO, cluster ions. The average cluster size was 3000 molecules. In the case of Si substrate, approximately a 7 nm thick SiO, layer was formed on the substrate surface at room temperature at a dose of 2 x 1015 ions
Ar monomer
CN-3000.20kW 0
1
2
3
1 4
Dose (1 OYons/ctn2) Fig. 9. Dose dependence of normalized impurity concentration.
I. Yamada 1 Materials Science and Engineering A217/218 (1996) 82-85
0470 b Binding
87
460 450 Energy (ev)
Fig. 12. XPS spectrum of a Ti film surface after bombardment 20 keV CO, cluster ions. Native
oxide
with
5. Conclusions
:A 0 2000
1500
WAVENUMBER
1000
500
(cm-‘)
Fig. 10. FT-IR spectra of SiO, cluster ion irradiation.
per cm’ [13]. Fig. 10 shows Fourier transform infrared FT-IR spectra of silicon surfaces irradiated with CO, gas cluster ions at room temperature. The results of XPS and TEM analysis of film thickness were consistent [5]. By conventional thermal oxidation, high temperature treatment ( > 700 “C) is necessary to obtain such a thick SiO, layer. The influence of cluster size at a given constant cluster energy and ion dose on the thickness of the oxide layer formed is depicted in Fig. 11. It is evident that a certain minimum cluster size is necessary for irradiation-induced surface oxidation at the given dose, A Ti film surface was irradiated with CO, cluster ions at 20 kV with a dose of 1.3 x IOr ions per cm2. An XPS spectrum of a stoichiometric TiO, layer formed at room temperature is shown in Fig. 12.
160,
I
1 co2 Q, I40 _ CLUSTER g 120Nz.250 IOO-
Va=lOkV
-
DOSE (Id5 ions/cm2) Fig. 1I. Ion dose dependence of oxide thickness for various cluster ion energies.
A gas cluster beam system has been developed, and the fundamental irradiation effects of accelerated gas cluster ions have been studied. The results show that a few hundred atoms cluster beam of an energy of a few hundred keV creates a thinner damaged layer on Si substrate surfaces than a monomer ion beam of the same energy. Surface smoothing, surface cleaning and very high yield sputtering by cluster ion bombardment were also observed on various kinds of materials resulting from lateral sputtering which was predicted by MD calculations. It has been demonstrated that cluster beams exhibit unique ion-surface interactions. They can be applied to modify a wide variety of materials as a promising tool for future surface processing. References [l] D.F. Downey, M. Farley, KS. Jones and G. Ryding (eds.), Proc. Int. Conf. Ion Imp/. Technol.-92, Gainsville, FL, 1992, North Holland, Amsterdam, 1993. [2] I. Yamada, H. Ishihara, E. Kamijo, T. Kawai, C.W. Allen and C.W. White (eds.); Proc. 3rd ZUMRS and Int. ConJ on Adu. Mater., Tokyo, 1993, Elsevier, 1994. [3] I. Yamada, G. Takaoka, M.I. Current, Y. Yamashita and M. Ishii, Nucl. Instr. atzd Methods, B74 (1993) 341. [4] I. Yamada, G. Takaoka, rM. Akizuki, C.E. Ascheron and J. Matsuo, in S. Coffa, G. Ferla, F. Priolo and E. Rimini (eds.) Proc. Int. CosJ Ion Impi. Technol.-94, Catania, Italy, 1994, North Holland, Amsterdam, 1995. [5] M. Akizuki, M. Harada, Y. Miyai, A. Doi, T. Yamaguchi, J. Matsuo, G.H. Takaoka, C.E. Ascheron and I. Yamada, Proc. ICSSPIC-7, Kobe, 1994, SurJ Rev. Lett., 3 (1996) 591. [6] J. Matsuo, M. Akizuki, J.A. Northby, G.H. Takaoka and I. Yamada, Proc. ICSSPIC-7, Kobe, 1993, SurJ Rev. Lett., 3 (1996) 1017. [73 Z. Insepov, M. Sosnowski and I. Yamada, in I. Yamada, H. Ishihara, E. Kamijo, T. Kawai, C.W. Allen and C.W. White (eds.), Proc. 3rd IUMRS and Znt. Cot3f. Adv. Mater., Tokyo, 1993, Elsevier, 1994. [S] C. Ascheron, J.P. Biersack, P. Goppelt and J. Erxmeyer, n+cc. Inst. Methods, BSOj81 (1993) 3. [9] 2. Insepov and I. Yamada, Proc. 13th Int. Conf. Appl. Accel. Res. Ind., Denton, 1994, Nucl. Insty. Methods, B99 (1995) 248.
88
I. Yurnada
/ Mmriais
Science
and Engineeritzg
[lo] I. Yamada, J. Matsuo, Z. Insepov and M. Akizuki, NE&. Instr. Methods, 8106 (1995) 165. [ll] J. Bohdanksky, G.L. Chen, W. :Eckstein, J. Roth, B.M.U. Scherzer and R. Behrisch, J. Nucl. Mater., Ill,!112 (1987) 717. 1121 T. Yamaguchi, J. Matsuo, M. Akizuki, C.E. Ascheron G.H.
A217/218
(1996)
82-88
Takaoka and I. Yamada, Proc. 13th Conf. Appl. of Accel., Res. Ind., Denton, 1994, Nxl. Ivst~. Methods, B99 (1995) 237. [i3] M. Akizuki, J. Matsuo, M. Harada, S. Ogasawara, A. Doi, K. Yoneda, T. Yamaguchi, G.H.Takaoka, C.E. Ascheron and I. Yamada, Proc. 13th Conf. Appl. Accel., Res. Ind., Denton, 1994, Nucl. Instr. Methods, B99 (1995) 229.
ELSEVIER
A
Materials Scienceand EngineeringA217/218 (1996) 82-88
Applications
of gas cluster ion beams for materials processing Isao Yamada
Ion Beam Engineering Experimental LuboFatoty, Kyoto University Sakyo, Kyoto 606, Japan
Abstract Irradiation effectsof gasclusterion beamshave beenstudiedexperimentallyand simulatedtheoretically for various ion species, energiesand materials combinations. It is shown that cluster ion-surface interactions offer new opportunities for surface processingsuchasvery shallowimplantation (lessthan 0.1 pm in Si), very high rate sputtering (two ordersof magnitudehigher than for monomerions), lateral sputtering effects, atomically smooth surface formation (averagesurfaceroughnesslessthan 1 nm), thin surfacelayer formation at low substratetemperatureetc. Keywords: Applications;Materialsprocessing; Gasclusterion beams
1. Introduction
Ion beams have been used for over thirty years to modify materials in manufacturing of integrated circuits and also for hardening and improving the tribological properties of surfaces [I]. Ion beams used for these applications range in energy from keV to MeV and, depending on their mass, penetrate into the target material to depths ranging from tens of nanometers to microns. However, recent trends in material and surface modifications require ranges in the order of nanometers [2]. For these processes, the required ion energies are in the low-energy range of a few eV to a few hundreds of eV. These low-energy ion beams are very difficult to produce at high intensity because of space charge limitations. The development of gas cluster ion beam technology is our current challenging task to overcome these limitations. The kinetic energy of ionized clusters is shared by their constituent atoms so that in a 10 keV cluster of 1000 atoms, each atom carries only 10 eV. While a collective motion of the cluster atoms plays an important role during the impact, cluster ion beams cause direct bombarding effects only within the first few monolayers of the target material. Gas cluster ion beams offer many new and unique opportunities for surface processing including chemically activated reactions, shallow implantation, very high rate sputtering, surface smoothing and surface cleaning [3,4]. We have developed 10 kV, 30 kV and 200 kV accelerators which allow the production of mass-selectedcluster ion beams
from gas sources. Irradiation effects of gas cluster ion beams have been studied experimentalIy and simulated theoretically for various ion energies and ion-material combinations. The results show that irradiation effects, such as sputtering, atomic penetration, damage accumulation and dynamic annealing of cluster ion beams, are significantly different from those of monomer ion beams. The typical surface interactions and application areas are shown in Fig. 1. In this paper, some new aspects of cluster ion-surface interactions which deviate significantly from monomer ion and molecular ion-surface interactions will be discussed. Arising from these results, new application fields of ionized cluster beams for surface modifiLow charge
to mass
High
ratio
sputtering
yield
Multiple collision at near-surface reoion
High density energy deposition
* Shallow
implantation Low energy
effect
Fig. 1. Typical surfaceinteractionsand applicationareasof gas clusterion beamtechnique. 0921-5093/96/$15.00 0 1996- ElsevierScience S.A. All rightsreserved PII s0921-5093(96)10358-9
I. Yamada
Skimmer
M.B.P
/ Materials
Science
Neutral beam detector l-i h
and Engineering
A2171218
Ionizer and mass filter
(1996)
83
82-85
I
I
D.P.
T.i.P
----I
I a T.M.P
Fig. 2. Schematic diagram of 30kV gas cluster ion beam equipment.
cation are proposed with respect to technologically important aspects, such as shallow implantation (less than 0.1 pm in Si), very high yield sputtering (two orders of magnitude higher than for monomer ions [3]), lateral sputtering phenomena [4], atomically smooth surface formation (average surface roughness less than 1 nm), thin surface layer formation (very thin oxide layers) at room temperature [5]. 2. Equipment
Three different cluster ion beam systems which can accelerate cluster ions up to 10 kV, 30 kV and 200 kV, respectively, have been constructed in our laboratory [6]. Fig. 2 shows a schematic diagram of the 30 kV gas-cluster ion beam equipment. Cluster ion beams from gaseous source materials, such as Ar and CO,, are generated by expanding the corresponding gases through a Lava1 nozzle at room temperature into a high-vacuum chamber after collimating by a skimmer. Subsequent ionization is performed by electron bombardment. Mass analysis is accomplished by an electrostatic retarding potential method using the different kinetic energies of clusters of different masses. The analysis of cluster sizes has also been performed by electron diffraction. Mass selected cluster ions with sizes of up to 5000 atoms were electrostatically accelerated with voltages in the range of 30 kV. Typical retarding spectra and cluster size distributions are shown in Fig. 3. The size of the cluster ions ranges up to a few thousand atoms, and the average size of the clusters is about 3000. Monomer ions are sufficiently suppressed by applying a low extraction voltage and adjusting electrostatic lenses. The ion current of mass-separated Ar clusters was measured as a function of source pressure. Below 1000 Torr, cluster ions are not observed and only monomer ions are found in the ion beam. The cluster ion current increases linearly with the source gas pressure. The threshold pressure for cluster formation is found to be around
1000 Torr [6]. The highest cluster ion beam current obtained is about 100 nA. This current corresponds to an Ar atom flux of 1.9 x 1Or5 atoms s-l cm-‘. 3. Molecular dynamic simulation
Three snapshots of an Ar,,, cluster impact with an energy of 10 eV per constituent atom on an Si(100) surface modeled by 32768 atoms at room temperature are shown in Fig. 4. According to MD simulations, the huge local energy deposition by collisions of cluster ions with surfaces is expected to cause new sputtering effects [7]. These effects concern the angular distribution of sputtered atoms and the sputter yield per constituent atom. The angular distribution deviates significantly from the cosine distribution at larger angles in case of cluster ion sputtering. While in the case of sputtering by monomer ions more atoms leave the surface into normal direction [8], in the case of sputtering by cluster ions significantly strong lateral sputtering is predicted. The angular distribution of sputtered target atoms which is shown in Fig. 5 indicates the occurrence of lateral sputtering. Cluster I”““”
0
1000 I
Size
2000
3000 ..,.
4000
5000 I.
/,,,,,,.
t
-50
4
0
50
100 Retarding
150
200 Voltage
250
300
350
(V)
Fig. 3. Typical retarding spectra and cluster size distributions.
84
I. Yunzada / Materials Science and Engiizeering A21 7/218 (1996) 82-88
Monomer 150keV
OPS
0.5ps
3.ops
Fig. 4. A snapshot of an impact of a 349 Ar atom cluster v/it11an energy of 1OeV per cluster atom on an Si (100) surface. OO
Simulations of the influence of the cluster size N on the sputter yield Y show that the sputter yield per cluster ion increases stronger than the number of constituent atoms. From the calculations, an increase of sputter yield per constituent atom proportional to N2.4 is derived [9]. This increase of sputter yield in the case of identical particle energies indicates cluster-typical effects, such as very high local densities of energy deposition. 4. Characteristics of gas cluster bombardment 4.1. Shnliow imphtntion
It is expected that cluster ion implantation is connected with shallow penetration into solid material. However, no experimental results have been published with regard to this subject up until now. Fig. 6 shows the channeling spectra of Si (100) surfaces bombarded with 150 keV Ar cluster and 150, 100 and 50 keV Ar monomer ions. The size distribution of the A.r cluster ion beam ranged from 1000 to 5000. The peak size was
I\ Clusterioal15OkeV
2000
3000
Depth (ii) Fig. 6. Channeling spectra of an Si (100) surface bombarded with Ar cluster and monomer ions.
approximately 3000 atoms per cluster. The doses of cluster and monomer ions were 2.5 x 1013 and 1 x 10” ions per cm2, respectively. The Ar cluster ion beam current was 5 nA. The thickness of the damaged layer caused by cluster ion irradiation with cluster ions is less than 25 run which is much smaller than in the case of monomer ion implantation with the same total energy. At this implantation energy, the average energy per constituent atom of the clusters is estimated to be approximately 50 eV. The thickness of the damaged layer of 25 nm is much larger than the expected one. To damage a layer of this thickness by Ar monomer ions, an energy of 10 keV is necessary. According to the MD simulation, this discrepancy may be explained by a shock wave effect produced by cluster ion bombardment. The shock wave could propagate into a deeper region of the substrate and produce an amorphous state in a region much deeper than the penetration depth of the ions.
'Z
01 ?’ IQ 1
! 250 MINIMUM
CLUSTER
500 SIZE
(molecules/cluster)
Fig. 5. Angular distribution of sputtered target atoms by Ar cluster ion.
Fig. 7. Influence of the size of CO, clusters on the sputtering yield per cluster for identical acceleration voltages of 10 kV for the given targets.
1. Yamada
/ Materials
Science
and Engineering
A2171218
(1996)
52-58
8.5
Aug 27 94 s37z poly-Si HFTaCwerPSZ Pa=36 PV-329
fug 15 94 64% polySi 1OkV Nt50SE15 As=7.1 PV-67
PS3
Fig. 8(a, b).
4.2. High yield sputtering
The sputtering effects have been studied on metal and SiO, surfaces irradiated with CO, and Ar cluster ions. The results shown in Fig. 7 were obtained at the cluster energy of 10 keV, i.e. the energy per constituent atom varies inversely proportional to cluster size. The sputtering yield per cluster ion is by a factor of more than 100 higher than for monomer ions. The sputtering yield has been observed to have a strong dependence on the target species for Ar clusters with an energy of 20 keV [lo]. However, the sputtering yield is much higher than that of monomer ions, by about one to two orders of magnitude. With the same
equipment, Ar monomer ions have also been produced, and the same targets have been sputtered. The yields of monomer ion beam sputtering agreed well with the predicted and measured values [ll]. The dependencies of the sputtering yield on the target species may be explained by influences of the chemical binding energy and surface binding energy of the target atoms. 4.3. Surface smoothing
Changes in surface roughness after cluster ion bombardment have been measured using an atomic force microscope. Fig. 8 shows the comparison of the average surface roughness (Ra) of poly-Si grown on Si substrate
I. Yumada 1 Materials Sciexe and Engineering A217218 {1996) 82-88
Aug 10 94 ~372 poly.Si IOkV N=l5E15
Ra=34 Pv=m
PS?
Fig. 8. AFM images of unirradiated, cluster and monomer ion irradiated surfaces. Acceleration voltage is 10 kV. Dose is 5 x 10” ions per cm’. (a) Original surface (b) after gas cluster bombardment (c) after molecular ion bombardment.
and sputtered surfaces which have been irradiated with CO, cluster ions and monomer ions accelerated with 10 kV. The minimum cluster size was approximately 250 molecules. In the case of cluster bombardment, Ra decreased from an initial value of 3.7 nm monotonically with sputtering time and saturated at a value of 0.7 nm. In the case of monomer ion bombardment, the minimum value of Ra was 3.4 nm. For many other materials, such as Pt, Cu, SiOp, S&N, films and glass substrates, similar results have been obtained [12]. This smoothing effect could be explained by lateral sputtering which is suggested by MD results [7]. 4.4. Surfflce cleaning
A high removal rate of surface impurities by cluster ion bombardment has been confirmed in connection with low damage effects [13]. Fig. 9 shows the dose dependence of normalized impurity concentrations on silicon substrate surfaces irradiated with cluster and monomer ions. Silicon wafers were intentionally contaminated with 1 x 1Ol5 Cu atoms per cm2 and 6 x 1Or2 Ni atoms per cm2 and subsequently irradiated with 20 keV Ar and 10 keV CO, ions, respectively. Compared with the smaller reduction of impurity concentration by about 4% in the case of monomer irradiation, 80% of the impurity atoms were removed from the surface after irradiation with Ar and CO, cluster ions at doses of 6 x 1014 and 2 x lOI ions per cm’, respectively. The impurity concentration exponentially decreased with increasing dose. The removal rates of impurities by Ar and CO, cluster ion irradiation were 100 and 40 times higher than by monomer ion irradiation, respectively.
The difference corresponds approximately to the difference in sputtering yield. In addition, the energies per constituent atom were 7 eV and 40 eV in the case of Ar and CO2 cluster ions, respectively. Thus, low irradiation damage can be expected. 4.5. Thin film formation
High density bombardment with low energy cluster ions can modify thin substrate surface layers. Si, Ti and Be substrates have been bombarded with 20 keV CO, cluster ions. The average cluster size was 3000 molecules. In the case of Si substrate, approximately a 7 nm thick SiO, layer was formed on the substrate surface at room temperature at a dose of 2 x 1015 ions
Ar monomer
CN-3000.20kW 0
1
2
3
1 4
Dose (1 OYons/ctn2) Fig. 9. Dose dependence of normalized impurity concentration.
I. Yamada 1 Materials Science and Engineering A217/218 (1996) 82-85
0470 b Binding
87
460 450 Energy (ev)
Fig. 12. XPS spectrum of a Ti film surface after bombardment 20 keV CO, cluster ions. Native
oxide
with
5. Conclusions
:A 0 2000
1500
WAVENUMBER
1000
500
(cm-‘)
Fig. 10. FT-IR spectra of SiO, cluster ion irradiation.
per cm’ [13]. Fig. 10 shows Fourier transform infrared FT-IR spectra of silicon surfaces irradiated with CO, gas cluster ions at room temperature. The results of XPS and TEM analysis of film thickness were consistent [5]. By conventional thermal oxidation, high temperature treatment ( > 700 “C) is necessary to obtain such a thick SiO, layer. The influence of cluster size at a given constant cluster energy and ion dose on the thickness of the oxide layer formed is depicted in Fig. 11. It is evident that a certain minimum cluster size is necessary for irradiation-induced surface oxidation at the given dose, A Ti film surface was irradiated with CO, cluster ions at 20 kV with a dose of 1.3 x IOr ions per cm2. An XPS spectrum of a stoichiometric TiO, layer formed at room temperature is shown in Fig. 12.
160,
I
1 co2 Q, I40 _ CLUSTER g 120Nz.250 IOO-
Va=lOkV
-
DOSE (Id5 ions/cm2) Fig. 1I. Ion dose dependence of oxide thickness for various cluster ion energies.
A gas cluster beam system has been developed, and the fundamental irradiation effects of accelerated gas cluster ions have been studied. The results show that a few hundred atoms cluster beam of an energy of a few hundred keV creates a thinner damaged layer on Si substrate surfaces than a monomer ion beam of the same energy. Surface smoothing, surface cleaning and very high yield sputtering by cluster ion bombardment were also observed on various kinds of materials resulting from lateral sputtering which was predicted by MD calculations. It has been demonstrated that cluster beams exhibit unique ion-surface interactions. They can be applied to modify a wide variety of materials as a promising tool for future surface processing. References [l] D.F. Downey, M. Farley, KS. Jones and G. Ryding (eds.), Proc. Int. Conf. Ion Imp/. Technol.-92, Gainsville, FL, 1992, North Holland, Amsterdam, 1993. [2] I. Yamada, H. Ishihara, E. Kamijo, T. Kawai, C.W. Allen and C.W. White (eds.); Proc. 3rd ZUMRS and Int. ConJ on Adu. Mater., Tokyo, 1993, Elsevier, 1994. [3] I. Yamada, G. Takaoka, M.I. Current, Y. Yamashita and M. Ishii, Nucl. Instr. atzd Methods, B74 (1993) 341. [4] I. Yamada, G. Takaoka, rM. Akizuki, C.E. Ascheron and J. Matsuo, in S. Coffa, G. Ferla, F. Priolo and E. Rimini (eds.) Proc. Int. CosJ Ion Impi. Technol.-94, Catania, Italy, 1994, North Holland, Amsterdam, 1995. [5] M. Akizuki, M. Harada, Y. Miyai, A. Doi, T. Yamaguchi, J. Matsuo, G.H. Takaoka, C.E. Ascheron and I. Yamada, Proc. ICSSPIC-7, Kobe, 1994, SurJ Rev. Lett., 3 (1996) 591. [6] J. Matsuo, M. Akizuki, J.A. Northby, G.H. Takaoka and I. Yamada, Proc. ICSSPIC-7, Kobe, 1993, SurJ Rev. Lett., 3 (1996) 1017. [73 Z. Insepov, M. Sosnowski and I. Yamada, in I. Yamada, H. Ishihara, E. Kamijo, T. Kawai, C.W. Allen and C.W. White (eds.), Proc. 3rd IUMRS and Znt. Cot3f. Adv. Mater., Tokyo, 1993, Elsevier, 1994. [S] C. Ascheron, J.P. Biersack, P. Goppelt and J. Erxmeyer, n+cc. Inst. Methods, BSOj81 (1993) 3. [9] 2. Insepov and I. Yamada, Proc. 13th Int. Conf. Appl. Accel. Res. Ind., Denton, 1994, Nucl. Insty. Methods, B99 (1995) 248.
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