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Reactive sputtering by SF6 cluster ion beams
s
Nuclear Instruments and Methods in Physics Research B I21 (I 997) 484-488 __ Nl[lMl
mm
B
Beam Interactions with Materials 8 Atoms
@ ELSEVIER
Reactive sputtering by SF, cluster ion beams N. Toyoda * , H. Kitani, J. Matsuo, I. Yamada Ion Beam Engineering Experimentul Lrrborutory. Kyoto Uniuersity. Suhyo. Kyoto 606-01. Jupan
Abstract Reactive gas cluster ion beams were formed by an adiabatic expansion of SF, with He mixture through a Lava1 nozzle and their reactive sputtering effects with solid surfaces have been studied. Si, W and Au samples were irradiated with SF, cluster ion beams at an energy of 20 keV. Due to the chemical reaction of SF, clusters with Si and W, sputtering yields of Si (1300 atoms/ion) and W (320 atoms/ion) were dramatically enhanced compared with those by Ar cluster ions (Si: 24, W: 35 atoms/ion). Sputtering yields of SF, cluster ions increased exponentially with the increase of acceleration energy, on the contrary, those of Ar cluster ions were proportional to the energy. Chemical reaction is the predominant sputtering process at an energy of around 5 keV. A Si(lO0) surface irradiated with SF, cluster ions was quite smooth, because of the smoothing effect of cluster ions.
1. Introduction
ergy, and the morphology discussed.
Clusters consist of two to several thousands of atoms or molecules loosely combined with each other by attractive intermolecular forces. If a cluster ion, containing several thousands of atoms is accelerated to an energy of a few tens of keV, each constituent atoms has an energy of only a few eV. It is difficult to obtain such a low energy monomer ion beam because of dispersion due space-charge effect. In addition, a cluster ion beam can transport larger quantities of atoms than a monomer ion beam for the same ion current. In energetic cluster ion bombardment, several thousands of atoms collide with target atoms within a radius of a few tens of A in a few ps. Consequently, the impact processes of cluster ions are quite different from those of monomer ions. There are multiple collisions near to the surface and a high density of energy deposition. We have reported experimental results of the unique physical sputtering effects of gas cluster ion beams, such as surface smoothing, high yield sputtering, surface cleaning, thin film formation at low temperature, shallow implantation and so on [l-6]. However, in spite of many studies of physical sputtering by cluster ions, there have been few studies of reactive sputtering. In this work, we have chosen SF, as the reactive gas and formed a SF, cluster ion beam. Experimental results of reactive sputtering effects for several materials, the dependence of the reactive sputtering yield on acceleration en-
* Corresponding author. 0168-583X/97/$17.00 Copyright PII SOI 68-583X(96)00555-
surface
are
2. Experiment Fig. I shows a schematic diagram of the 30 keV gas cluster ion beam equipment. This apparatus consists of four parts: source chamber, differential pumping chamber, ionizing chamber and target chamber. Clusters are generated during an adiabatic expansion of high pressure gas through a nozzle into a vacuum. High pressure (2000-4000 Torr) gases expand through a Lava1 nozzle into a source chamber, pumped by a mechanical booster pump. The nozzle diameter is 0.1 mm and the gas temperature is 300 K. When the cluster beam collides with residual gas in the source chamber, a shock wave called a Mach disk appears in front of the beam and causes a decrease of the beam intensity. Therefore, a skimmer to collimate a cluster beam has to be upstream of the Mach disk, and the nozzle-to-skimmer distance (de) is optimized at the point where highest beam intensity is obtained (d, = 20 mm). The source chamber was kept below 1 X IO- ’ Torr during irradiation. Subsequently, neutral clusters are ionized by electron bombardment with an energy of I50 eV. In order to reduce the fraction of the monomer ions, a very low (< 20 V> extraction voltage (Vex,) is applied. The kinetic energy of clusters is higher than that of monomers, therefore, monomer ions in the cluster beam are dispersed by space charge effects. The fraction of the monomer ions in the cluster ion beam is less than 2%. Mass separation of clusters is performed in the ioniza-
0 1997 Elsevier Science B.V. All rights reserved
I
of the sputtered
N. Toyodu et ul./Nucl.
485
Instr. and Meth. in Phys. Rex B 121 (1997) 484-488
Neutral beam
ionizer and
Faraday
M.B.P. Source Differential punchamber ping chamber Fig. 1. A
schematic diagram of the 30
tion chamber using an electrostatic lens system which was designed to utilize inherent chromatic aberration. The mass distribution of the beam is measured by the retarding potential method. The kinetic energy of a neutral cluster (K,,,) is expressed as follows Y
Kc,, = -kT,N, Y-
1
where y. k, To and N are the specific heat ratio, Boltzmann constant, gas temperature and cluster size, respectively. Thus, the kinetic energy of a cluster is proportional to the cluster size N, and the cluster size distribution is obtained from an energy distribution of the beam measured by the retarding potential method. Mass selected cluster ions are accelerated up to 30 keV and scanned by deflectors in the target chamber to obtain uniform irradiation. The uniformity of the irradiated area is better than 5%, measured by ellipsometry. In order to measure the correct ion current, secondary electrons, generated by collisions of cluster ions with the target. are suppressed with an electrode, biased - 90 V below that of the target, and the ion dose is obtained from an integration of the ion current. The density of the cluster ion current is 2.X PA/cm’ at the target with an energy of 20 keV. The target chamber is kept below I X 10e6 Torr by a turbo molecular pump during irradiation. Si. W and Au were irradiated by Ar monomer, Ar cluster and SF, cluster ions at room temperature. The energy ranged from 5 to 25 keV and the dose was 5 X IO” ions/cm’. An average cluster size of the Ar and SF, cluster ion beams was approximately 3000 and 2000. respectively. W and Au were deposited on silicon substrates. A stainless grid with an interval of I mm was set above the target as a mask. The sputtered depth was measured with a contact surface profile meter scanned over the edge between the irradiated and not-irradiated area. The sputtering yields were obtained from the sputtered depth. the density of each material and the ion doses. Additionally, Si(lO0) substrates, irradiated with SF, cluster ions, were observed by Atomic Force Microscope (AFM) to study surface morphology.
ionization chamber keV gas
cE;;;r
T.M.P.
cluster ion beam equipment.
3. Formation
of SF, cluster beams
Cluster formation is a consequence of attractive intermolecular forces, therefore, it is easy to generate clusters from gases whose intermolecular forces are strong, such as Ar or CO,. However, because SF, molecules have many vibration modes, much cooling is required to make a SF, cluster beam. Thus, we had to use He as the carrier gas. He helps to dissipate the heat of condensation during cluster formations. Fig. 2 shows a dependence of the SF, mole fraction X on the beam intensity at P,,,,, of 4000 Torr (X = PSFb/P,otnl* where PsF6, ptotal are partial pressure of SF, and total pressure, respectively). There is a shutter on the beam axis in the differential pumping chamber, and beam intensities were obtained from a variation of the pressure of each chamber. In Fig. 2, the beam intensity increased with the increase of the fraction of condense gas to a maximum at X = 0.06, where the intensity of the SF, cluster beam is ten times higher than that of pure SF, gas. Subsequently, the beam intensity reduced for X > 0.06, because of insufficient cooling effects by He.
z1,,,/,,,, ,,,,,, _\i
;
00.00 0.05
0.10
0.15
0.25
0.20
SF6 mole fraction
X
Fig. 2. Neutral SF, beam intensity versus SF, mole fraction (X). X is defined as P,,,
/P,ora,. The beam intensity increased with X
to a maximum at X = 0.06.
VI. NANOSCALE
PROCESS/ETCHING
N. T~yodu et ul./NucI.
486
Insrr. und Meth. in Phys. Res. B 121 11997) 484-488
Valente et al. [7] generated an SF, cluster beam by tree jet expansion of SF, and an inert gas mixture through a Lava1 nozzle with a f,,,,, of 4.8 bar, and X was approximately 0.10. Torchet et al. [8] also generated SF, cluster beams from a SF,-Ne mixture with a P,,,,, of 20 bar, and X was 0.40. The difference from our results is that they applied much higher pressure and used Ne as carrier gas, which is heavier than He, and the carrier gas has a sufficient cooling effect, even when X was 0.40. Thus, our experimental results show good agreement with these results. Similarly, reactive cluster beam could also be generated from gases such as NO, NO, or 0, with He mixtures [9].
/
4. Reactive sputtering by SF, cluster ion beams
Ar monomer
ion
Reference (Kanaya
1oooj
r
et. al.)
m
Ar cluster ion
0
SF, cluster ion
FioD. 3. Sputtering yield of Si. W and Au by Ar monomer,
Ar
cluster and SF, cluster ions at an energy of 20 %eV. The predicted sputtering yields obtained from the collision cascade theory [I I] are also shown. The sputtering yield of Au and W with Ar cluster ion beams is 42, 35 atoms/ion,
respectively. However,
cluster ion beams, that of W (320
atoms/ion)
enhanced, while that of Au is 1 12 atoms/ion.
I
”
”
Energy [keV]
There is a strong dependence between the sublimation energy of a target and the physical sputtering yield by inert gas ion bombardment. However, if the incident ion reacts with the target, the sputtering yield rises. In this work, the targets were sputtered by Ar monomer ions, Ar cluster ions and SF, cluster ions, and sputtering yields were measured to study reactive sputtering effects. Fig. 3 shows the sputtering yields of Si, W and Au, at an energy of 20 keV. The sputtering yield is defined as the average number of atoms sputtered by one cluster ion. The temperature of the substrate was 300 K. In addition to these experimental results, the predicted sputtering yields
m
’
5x 1 0i5ionsIcm2 cluster size Ar : 3000, SF, : 2000
with SF,
is chemically
Fig.
4. The energy dependence of the sputtering yields of Cu
and Ag (0) ions
with Ar cluster ions, and those of Si with
(0). The
SF,
(n
)
cluster
sputtered depth of Cu and Ag is proportional to the
acceleration energy, and there is a threshold of physical sputtering at a total energy of 6.5 LeV. With SF, cluster ions, the sputtered depth of Si increases exponentially,
and Si was sputtered a 300 A
depth at an energy of 5 keV, which is less than the threshold of physical sputtering.
obtained from the collision cascade theory [IO] are shown to confirm the accuracy of the experimental technique. The sputtering yields of Ar cluster ions is about ten times higher than those of Ar monomer ions because of the high density energy deposition in a local area. There is a strong dependence of the sputtering yield on the sublimation energy of the targets. (Au: 3.9 eV, W: 8.8 eV [I I]) Fig. 3 also indicates the reactive sputtering yields of Si and W due to SF, cluster ion bombardments. Compared with W and Au, which are close in atomic number, the sputtering yield of Au (42 atoms/ion) is higher than that of W (35 atoms/ion) by Ar cluster ions. With SF, cluster ions, that of W is 320 atoms/ion, which is three times higher than that of Au (I I2 atoms/ion). Similarly, the sputtering yield of Si by SF, cluster ions is I300 atoms/ion, which is 5.5 times higher than that with Ar cluster ions (24 atoms/ion). SF, molecules don’t react with Si or W at room temperature. It was reported that chemical reactions of SF, molecules with Si were stimulated by energetic Ar monomer ion bombardments [ 121, and it was explained that SF, molecules dissociated by energetic bombardments and, as a result, reacted with Si atoms. We suppose that the dissociation of a SF, molecule occurs with collisions of SF, cluster ion with a Si or W surface in the same way, and consequently SiF, or WF,, which are volatile compounds, are produced, respectively. Thus, the sputtering yields increased as a result of production of volatile compounds of SiF, or WF,, which is promoted by SF, cluster ion bombardment.
N. Toyodu et ul./Nucl.
Instr. und Meth. in Phys. Res. B 121 (1997) 484-488
187
Fig. 5. AFM images of a Si(lOO) surface irradiated with SF, cluster ion beams at an energy of 20 LeV and a dose of I X IO” ions/cm’. The average roughness is 8.8 A and the scan area is I .O pm square. The Si surface is very flat because of the physical smoothing effect of cluster ion beams.
Fig. 4 shows the dependence on acceleration energy of the sputtered depth of Si by SF, cluster ions, and those of Cu and Ag by Ar cluster ions. The acceleration energy ranged from 5 to 25 keV. and the ion dose was fixed at 5 X IO” ions/cm’. In the case of Ar cluster ions, the sputtered depth was proportional to the acceleration energy and extrapolation of the line suggests a threshold of sputtering at 6.5 keV. It means a threshold of physical sputtering by cluster ions. However, in the case of the SF, cluster ions, the sputtered depth increased exponentially with the acceleration energy. Si was sputtered to a 300 A depth by SF, cluster ions even with an energy of 5 keV, which is less than the threshold of physical sputtering. The energy of each constituent molecules was 2.5 eV, derived from the average cluster size (N = 2000) and acceleration energy (5 keV). This value of energy is between the activation energy of chemical reaction with Si and the displacement energy of Si crystals. Chuang reported that vibrationally excited SF, molecules were very reactive to silicon [ 131. Each constituent SF, molecules could be vibrationally excited by energetic cluster ion bombardments. Thus. Si atoms are removed by this complex chemical reaction and. as a result, the threshold energy of reactive sputtering becomes obscure. From these results, it can be concluded that chemical reaction is the predominant process for the sputtering of Si at an energy of around 5 keV. Fig. 5 shows the morphology obtained from atomic force microscope (AFM) of a Si(lO0) surface irradiated with SF, cluster ions. The average surface roughness of I pm square area was 8.8 A, after sputtering of a 1.0 pm depth. When an energetic cluster ion collides with a target, sputtered atoms distribute laterally on the surface, even at
a normal incidence. These laterally distributed atoms cause redeposition or self-sputtering and, consequently. the surface roughness is reduced. We call this effect “lateral sputtering” and no roughening mechanism is observed in cluster irradiation. We suppose that the ‘. lateral sputtering” effect occurs with reactive cluster ions in the same way.
5. Summary SF, cluster beams were generated by an adiabatic expansion of SF, through a nozzle with mixtures of 94% He as carrier gas. It was found that reactive sputtering occurred in SF, cluster ion bombardments. However, SF, molecules don’t react with Si or W at room temperature. When a cluster ion collides with target atoms, dissociation of SF, molecules must occur at the same time. The sputtering yield of SF, cluster ions increased exponentially with the increase in acceleration energies. The threshold energy of sputtering was obscure, because chemical reactions were the predominant process at energies less than 5 keV. Also, the Si(lO0) surface irradiated with SF, clusters was quite flat because of the “lateral sputtering” peculiar to cluster ion bombardments.
References
[II I. Yamada, W.L. Brown, J.A. Northby and M. Sosnowski, Nucl. Instr. and Meth. B 79 (1993) 223. J. Matsuo, Z. Insepov and M. Akizuki. Nucl. Instr. and Meth. B 106 (1995) 16.5. [31 M. Akizuki, J. Matsuo, M. Harada, S. Ogasawara. A. Doi, K.
[21 I. Yamada,
VI. NANOSCALE
PROCESS/ETCHING
488
[4]
[5] [6] [7] [S]
N. Toyoda et al./Nucl.
Instr. andh4eth. in Phys. Res. B I21 (1997) 486488
Yoneda, T. Yamaguchi, G.H. Takaoka, C.E. Ascheron and I. Yamada, Nucl. Instr. and Meth. B 99 (1995) 229. T. Yamaguchi, J. Matsuo, M. Akizuki, C.E. Ascheron, G.H. Takaoka and I. Yamada, Nucl. Instr. and Meth. B 99 (1995) 237. M. Akizuki, J. Matsuo, I. Yamada, M. Harada, S. Ogasawara and A. Doi, Jpn. J. Appl. Phys. 35 (1996) Pt. I, No. 2B. J. Matsuo, D. Takeutchi, A. Kitai and I. Yamada, Nucl. Instr. and Meth. B 112 (1996) 89. E. Valente and L. Bartell, J. Chem. Phys. 79 (1983) 2683. Cl. Torchet, M.-F. de Feraudy and B. Raoult, J. Chem. Phys. 103 (1995) 3074.
[9] M. Akizuki, J. Matsuo, G.H. Takaoka, M. Harada, S. Ogasawara, A. Doi and I. Yamada. Mater. Sci. Eng. A, in press. [IO] K. Kanaya, K. Houjou, K. Koga and F. Toki, Jpn. J. Appl. Phys. 12 (1973) 1297. [I I] D.E. Harrison, Jr. and G.D. Magnuson, Phys. Rev. 122 (1961) 1421. 1121 J.W. Cobum and H.F. Winters, J. Appl. Phys. 50 (1979) 3189. [13] T.J. Chuang, J. Chem. Phys. 74(1981) 1453.
Nuclear Instruments and Methods in Physics Research B I21 (I 997) 484-488 __ Nl[lMl
mm
B
Beam Interactions with Materials 8 Atoms
@ ELSEVIER
Reactive sputtering by SF, cluster ion beams N. Toyoda * , H. Kitani, J. Matsuo, I. Yamada Ion Beam Engineering Experimentul Lrrborutory. Kyoto Uniuersity. Suhyo. Kyoto 606-01. Jupan
Abstract Reactive gas cluster ion beams were formed by an adiabatic expansion of SF, with He mixture through a Lava1 nozzle and their reactive sputtering effects with solid surfaces have been studied. Si, W and Au samples were irradiated with SF, cluster ion beams at an energy of 20 keV. Due to the chemical reaction of SF, clusters with Si and W, sputtering yields of Si (1300 atoms/ion) and W (320 atoms/ion) were dramatically enhanced compared with those by Ar cluster ions (Si: 24, W: 35 atoms/ion). Sputtering yields of SF, cluster ions increased exponentially with the increase of acceleration energy, on the contrary, those of Ar cluster ions were proportional to the energy. Chemical reaction is the predominant sputtering process at an energy of around 5 keV. A Si(lO0) surface irradiated with SF, cluster ions was quite smooth, because of the smoothing effect of cluster ions.
1. Introduction
ergy, and the morphology discussed.
Clusters consist of two to several thousands of atoms or molecules loosely combined with each other by attractive intermolecular forces. If a cluster ion, containing several thousands of atoms is accelerated to an energy of a few tens of keV, each constituent atoms has an energy of only a few eV. It is difficult to obtain such a low energy monomer ion beam because of dispersion due space-charge effect. In addition, a cluster ion beam can transport larger quantities of atoms than a monomer ion beam for the same ion current. In energetic cluster ion bombardment, several thousands of atoms collide with target atoms within a radius of a few tens of A in a few ps. Consequently, the impact processes of cluster ions are quite different from those of monomer ions. There are multiple collisions near to the surface and a high density of energy deposition. We have reported experimental results of the unique physical sputtering effects of gas cluster ion beams, such as surface smoothing, high yield sputtering, surface cleaning, thin film formation at low temperature, shallow implantation and so on [l-6]. However, in spite of many studies of physical sputtering by cluster ions, there have been few studies of reactive sputtering. In this work, we have chosen SF, as the reactive gas and formed a SF, cluster ion beam. Experimental results of reactive sputtering effects for several materials, the dependence of the reactive sputtering yield on acceleration en-
* Corresponding author. 0168-583X/97/$17.00 Copyright PII SOI 68-583X(96)00555-
surface
are
2. Experiment Fig. I shows a schematic diagram of the 30 keV gas cluster ion beam equipment. This apparatus consists of four parts: source chamber, differential pumping chamber, ionizing chamber and target chamber. Clusters are generated during an adiabatic expansion of high pressure gas through a nozzle into a vacuum. High pressure (2000-4000 Torr) gases expand through a Lava1 nozzle into a source chamber, pumped by a mechanical booster pump. The nozzle diameter is 0.1 mm and the gas temperature is 300 K. When the cluster beam collides with residual gas in the source chamber, a shock wave called a Mach disk appears in front of the beam and causes a decrease of the beam intensity. Therefore, a skimmer to collimate a cluster beam has to be upstream of the Mach disk, and the nozzle-to-skimmer distance (de) is optimized at the point where highest beam intensity is obtained (d, = 20 mm). The source chamber was kept below 1 X IO- ’ Torr during irradiation. Subsequently, neutral clusters are ionized by electron bombardment with an energy of I50 eV. In order to reduce the fraction of the monomer ions, a very low (< 20 V> extraction voltage (Vex,) is applied. The kinetic energy of clusters is higher than that of monomers, therefore, monomer ions in the cluster beam are dispersed by space charge effects. The fraction of the monomer ions in the cluster ion beam is less than 2%. Mass separation of clusters is performed in the ioniza-
0 1997 Elsevier Science B.V. All rights reserved
I
of the sputtered
N. Toyodu et ul./Nucl.
485
Instr. and Meth. in Phys. Rex B 121 (1997) 484-488
Neutral beam
ionizer and
Faraday
M.B.P. Source Differential punchamber ping chamber Fig. 1. A
schematic diagram of the 30
tion chamber using an electrostatic lens system which was designed to utilize inherent chromatic aberration. The mass distribution of the beam is measured by the retarding potential method. The kinetic energy of a neutral cluster (K,,,) is expressed as follows Y
Kc,, = -kT,N, Y-
1
where y. k, To and N are the specific heat ratio, Boltzmann constant, gas temperature and cluster size, respectively. Thus, the kinetic energy of a cluster is proportional to the cluster size N, and the cluster size distribution is obtained from an energy distribution of the beam measured by the retarding potential method. Mass selected cluster ions are accelerated up to 30 keV and scanned by deflectors in the target chamber to obtain uniform irradiation. The uniformity of the irradiated area is better than 5%, measured by ellipsometry. In order to measure the correct ion current, secondary electrons, generated by collisions of cluster ions with the target. are suppressed with an electrode, biased - 90 V below that of the target, and the ion dose is obtained from an integration of the ion current. The density of the cluster ion current is 2.X PA/cm’ at the target with an energy of 20 keV. The target chamber is kept below I X 10e6 Torr by a turbo molecular pump during irradiation. Si. W and Au were irradiated by Ar monomer, Ar cluster and SF, cluster ions at room temperature. The energy ranged from 5 to 25 keV and the dose was 5 X IO” ions/cm’. An average cluster size of the Ar and SF, cluster ion beams was approximately 3000 and 2000. respectively. W and Au were deposited on silicon substrates. A stainless grid with an interval of I mm was set above the target as a mask. The sputtered depth was measured with a contact surface profile meter scanned over the edge between the irradiated and not-irradiated area. The sputtering yields were obtained from the sputtered depth. the density of each material and the ion doses. Additionally, Si(lO0) substrates, irradiated with SF, cluster ions, were observed by Atomic Force Microscope (AFM) to study surface morphology.
ionization chamber keV gas
cE;;;r
T.M.P.
cluster ion beam equipment.
3. Formation
of SF, cluster beams
Cluster formation is a consequence of attractive intermolecular forces, therefore, it is easy to generate clusters from gases whose intermolecular forces are strong, such as Ar or CO,. However, because SF, molecules have many vibration modes, much cooling is required to make a SF, cluster beam. Thus, we had to use He as the carrier gas. He helps to dissipate the heat of condensation during cluster formations. Fig. 2 shows a dependence of the SF, mole fraction X on the beam intensity at P,,,,, of 4000 Torr (X = PSFb/P,otnl* where PsF6, ptotal are partial pressure of SF, and total pressure, respectively). There is a shutter on the beam axis in the differential pumping chamber, and beam intensities were obtained from a variation of the pressure of each chamber. In Fig. 2, the beam intensity increased with the increase of the fraction of condense gas to a maximum at X = 0.06, where the intensity of the SF, cluster beam is ten times higher than that of pure SF, gas. Subsequently, the beam intensity reduced for X > 0.06, because of insufficient cooling effects by He.
z1,,,/,,,, ,,,,,, _\i
;
00.00 0.05
0.10
0.15
0.25
0.20
SF6 mole fraction
X
Fig. 2. Neutral SF, beam intensity versus SF, mole fraction (X). X is defined as P,,,
/P,ora,. The beam intensity increased with X
to a maximum at X = 0.06.
VI. NANOSCALE
PROCESS/ETCHING
N. T~yodu et ul./NucI.
486
Insrr. und Meth. in Phys. Res. B 121 11997) 484-488
Valente et al. [7] generated an SF, cluster beam by tree jet expansion of SF, and an inert gas mixture through a Lava1 nozzle with a f,,,,, of 4.8 bar, and X was approximately 0.10. Torchet et al. [8] also generated SF, cluster beams from a SF,-Ne mixture with a P,,,,, of 20 bar, and X was 0.40. The difference from our results is that they applied much higher pressure and used Ne as carrier gas, which is heavier than He, and the carrier gas has a sufficient cooling effect, even when X was 0.40. Thus, our experimental results show good agreement with these results. Similarly, reactive cluster beam could also be generated from gases such as NO, NO, or 0, with He mixtures [9].
/
4. Reactive sputtering by SF, cluster ion beams
Ar monomer
ion
Reference (Kanaya
1oooj
r
et. al.)
m
Ar cluster ion
0
SF, cluster ion
FioD. 3. Sputtering yield of Si. W and Au by Ar monomer,
Ar
cluster and SF, cluster ions at an energy of 20 %eV. The predicted sputtering yields obtained from the collision cascade theory [I I] are also shown. The sputtering yield of Au and W with Ar cluster ion beams is 42, 35 atoms/ion,
respectively. However,
cluster ion beams, that of W (320
atoms/ion)
enhanced, while that of Au is 1 12 atoms/ion.
I
”
”
Energy [keV]
There is a strong dependence between the sublimation energy of a target and the physical sputtering yield by inert gas ion bombardment. However, if the incident ion reacts with the target, the sputtering yield rises. In this work, the targets were sputtered by Ar monomer ions, Ar cluster ions and SF, cluster ions, and sputtering yields were measured to study reactive sputtering effects. Fig. 3 shows the sputtering yields of Si, W and Au, at an energy of 20 keV. The sputtering yield is defined as the average number of atoms sputtered by one cluster ion. The temperature of the substrate was 300 K. In addition to these experimental results, the predicted sputtering yields
m
’
5x 1 0i5ionsIcm2 cluster size Ar : 3000, SF, : 2000
with SF,
is chemically
Fig.
4. The energy dependence of the sputtering yields of Cu
and Ag (0) ions
with Ar cluster ions, and those of Si with
(0). The
SF,
(n
)
cluster
sputtered depth of Cu and Ag is proportional to the
acceleration energy, and there is a threshold of physical sputtering at a total energy of 6.5 LeV. With SF, cluster ions, the sputtered depth of Si increases exponentially,
and Si was sputtered a 300 A
depth at an energy of 5 keV, which is less than the threshold of physical sputtering.
obtained from the collision cascade theory [IO] are shown to confirm the accuracy of the experimental technique. The sputtering yields of Ar cluster ions is about ten times higher than those of Ar monomer ions because of the high density energy deposition in a local area. There is a strong dependence of the sputtering yield on the sublimation energy of the targets. (Au: 3.9 eV, W: 8.8 eV [I I]) Fig. 3 also indicates the reactive sputtering yields of Si and W due to SF, cluster ion bombardments. Compared with W and Au, which are close in atomic number, the sputtering yield of Au (42 atoms/ion) is higher than that of W (35 atoms/ion) by Ar cluster ions. With SF, cluster ions, that of W is 320 atoms/ion, which is three times higher than that of Au (I I2 atoms/ion). Similarly, the sputtering yield of Si by SF, cluster ions is I300 atoms/ion, which is 5.5 times higher than that with Ar cluster ions (24 atoms/ion). SF, molecules don’t react with Si or W at room temperature. It was reported that chemical reactions of SF, molecules with Si were stimulated by energetic Ar monomer ion bombardments [ 121, and it was explained that SF, molecules dissociated by energetic bombardments and, as a result, reacted with Si atoms. We suppose that the dissociation of a SF, molecule occurs with collisions of SF, cluster ion with a Si or W surface in the same way, and consequently SiF, or WF,, which are volatile compounds, are produced, respectively. Thus, the sputtering yields increased as a result of production of volatile compounds of SiF, or WF,, which is promoted by SF, cluster ion bombardment.
N. Toyodu et ul./Nucl.
Instr. und Meth. in Phys. Res. B 121 (1997) 484-488
187
Fig. 5. AFM images of a Si(lOO) surface irradiated with SF, cluster ion beams at an energy of 20 LeV and a dose of I X IO” ions/cm’. The average roughness is 8.8 A and the scan area is I .O pm square. The Si surface is very flat because of the physical smoothing effect of cluster ion beams.
Fig. 4 shows the dependence on acceleration energy of the sputtered depth of Si by SF, cluster ions, and those of Cu and Ag by Ar cluster ions. The acceleration energy ranged from 5 to 25 keV. and the ion dose was fixed at 5 X IO” ions/cm’. In the case of Ar cluster ions, the sputtered depth was proportional to the acceleration energy and extrapolation of the line suggests a threshold of sputtering at 6.5 keV. It means a threshold of physical sputtering by cluster ions. However, in the case of the SF, cluster ions, the sputtered depth increased exponentially with the acceleration energy. Si was sputtered to a 300 A depth by SF, cluster ions even with an energy of 5 keV, which is less than the threshold of physical sputtering. The energy of each constituent molecules was 2.5 eV, derived from the average cluster size (N = 2000) and acceleration energy (5 keV). This value of energy is between the activation energy of chemical reaction with Si and the displacement energy of Si crystals. Chuang reported that vibrationally excited SF, molecules were very reactive to silicon [ 131. Each constituent SF, molecules could be vibrationally excited by energetic cluster ion bombardments. Thus. Si atoms are removed by this complex chemical reaction and. as a result, the threshold energy of reactive sputtering becomes obscure. From these results, it can be concluded that chemical reaction is the predominant process for the sputtering of Si at an energy of around 5 keV. Fig. 5 shows the morphology obtained from atomic force microscope (AFM) of a Si(lO0) surface irradiated with SF, cluster ions. The average surface roughness of I pm square area was 8.8 A, after sputtering of a 1.0 pm depth. When an energetic cluster ion collides with a target, sputtered atoms distribute laterally on the surface, even at
a normal incidence. These laterally distributed atoms cause redeposition or self-sputtering and, consequently. the surface roughness is reduced. We call this effect “lateral sputtering” and no roughening mechanism is observed in cluster irradiation. We suppose that the ‘. lateral sputtering” effect occurs with reactive cluster ions in the same way.
5. Summary SF, cluster beams were generated by an adiabatic expansion of SF, through a nozzle with mixtures of 94% He as carrier gas. It was found that reactive sputtering occurred in SF, cluster ion bombardments. However, SF, molecules don’t react with Si or W at room temperature. When a cluster ion collides with target atoms, dissociation of SF, molecules must occur at the same time. The sputtering yield of SF, cluster ions increased exponentially with the increase in acceleration energies. The threshold energy of sputtering was obscure, because chemical reactions were the predominant process at energies less than 5 keV. Also, the Si(lO0) surface irradiated with SF, clusters was quite flat because of the “lateral sputtering” peculiar to cluster ion bombardments.
References
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