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Cluster-ion emission from an aluminium surface under Cd+ ion bombardment
Znternationnf Journal
of Mass
S’pectrometry
and Zon Physics,
50 (1983)
349-351
349
Elsevier Science Publishers B-V., Amsterdam - Printed in The Netherlands
Short communication CLUSTER-ION EMISSION FROM UNDER Cd+ ION BOMBARDMENT
P. CHAKRABORTY
and SD.
AN ALUMINIUM
SURFACE
DEY
Saka Znstitute of Nuclear Physics, Bidhan Nagar, Calcutta 700 064 (India)
(Received 4 October 1982)
A special feature of secondary-ion mass spectrometry (SIMS) is the liberation of polyatomic particles (clusters), either in the neutral or ionic state, from ion-bombarded solid surfaces by a sputtering process. The cluster yield is much smaller than that of single atoms or ions. Nevertheless, cluster-ion emission has recently been the subject of extreme interest for the wealth of information it contains about the surface. At present, two contrasting mechanisms are generally used to explain the secondary emission of ionic clusters. One concerns vacuum recombination of individually emitted atoms or ions. This idea is supported by classical energy-transfer considerations [I], computer simulations [2,3] and recent experimental evidence [4], which indicate that secondary-ion clusters are not representative of the surface structure. They are formed in the space immediately above the surface by recombination of atoms which are emitted independently in the collision cascade created when the primary ions collide with the target lattice_ The other physical interpretation is that clusters are emitted “as such” directly from the surface and are thus composed only of atoms which are nearest neighbours on the surface. Some experimental measurements supporting this concept have been reported [5-71. Therefore, the interpretation of cluster yields is difficult because of the lack of agreement about a cluster formation mechanism. In the present work, some experimental data are reported for secondary Al +, Alf and All intensities from a more or less clean polycrystalline aluminium surface as a function of bombarding Cd+ ion energy in the range 3- 10 keV for various values of target current density in the range loo-250 E_LAcmm2. The apparatus has been described previously [Xl. Secondary ions are mass-analyzed by a quadrupole mass spectrometer [9] and then detected by a l&stage Cu-Be Allen-type electron multiplier set off-axis with respect to the quadruple, in conjunction with an electrometer amplifier. Prior to ion bombardment the target region is pumped differentially to a pressure of 5 X lo-’ torr. The target chamber, made of stainless steel, is connected to an 0020-7381/83/$03.00
0 1983 Elsevier Science Publishers B-V.
350
ultra-high vacuum sputter-ion pump by means of suitably designed stainless-steel tubes. The chamber is provided with two small openings, 2.5 and 1.5 mm in diameter, for primary-beam entrance and secondary-ion exit, respectively. The entire ultrahigh-vacuum chamber is coupled to an existing high-vacuum system in which the pressure ‘is 5 X 10m6 torr. The two openings in the chamber serve the basic requirement for maintaining the desired pressure difference between the two vacuum systems by a differential pumping arrangement. The experimental results of this study are presented in Fig. 1. The curves showing the variation of secondary-ion intensity for monomers, dimers and trimers of aluminium with projectile ion energy, for a given value of target current density (100 PA cmm2), are found to be similar in nature. In all cases, secondary-ion emission increases almost linearly with bombarding-ion energy, and then attains a broad maximum followed by a subsequent steady decline. The location of this maximum in all three cases is found to be - 5 keV. A similar phenomenon was observed [8] earlier in the case of Cu’ and cu; ion-emission intensities for copper bombarded with Zn+ ions, where the region of the maximum was found to be at - 7.5 keV for both ions. The locations of these maxima depend strongly on the projectile-target combina-
Fig. 1. Variation of secondary Al+ , All energy.
and Al;
ion intensities with bombarding Cd+ ion
351
tion. This finding is in agreement with the work of Wittmaack [lo], where ion-yield curves for all silicon clusters up to Sig were found to be almost identical, all having a broad maximum at - 10 keV, for Xe+ projectile ions. The appearance of the maxima followed by a subsequent steady dechne in secondary-ion emission in our observations may be attributed to the onset of a change in texture of the polycrystalline surface under ion bombardment [8]. The occurrence of the maxima for all the cluster species in about the same region and the similarity of the data shown in Fig. 1 suggest the idea that the secondary ionic species (atomic or clusters) are emitted simultaneously “as such” as a result of the same physical process; i.e., the constituents of the clusters are located at neightbouring sites before emission, rather than at next-nearest sites as suggested by computer simulations. Had the process of vacuum recombination of individually emitted atoms been reponsible for cluster formation, the secondary-ion yield behaviour would be expected to differ for differing cluster sizes because in that case surface structure would play practically no role in the process of cluster formation. ACKNOWLEDGEMENT
We thank Professor S.B. Karmohapatro and kind interest in this work.
for his continuous encouragement
REFERENCES 1 2 3 4 5 6 7 8 9 10
W. Gerhard, Z. Phys. B, 22 (1975) 31. N. Winograd, D.E. Harrison and B.J. Garrison, Surf. Sci., 78 (1978) 467. B.J. Garrison, N. Winograd and D.E. Harrison, J. Chem. Phys., 69 (1979) 1440. G.J. Slusser and N. Winograd, Surf. Sci., 95 (1980) 53. A. Benninghoven and A. Muller, Surf. Sci., 39 (1973) 416. A. Benninghoven, Surf. Sci., 53 (1975) 596. A. Buhl and A. Preisinger, Surf. Sci., 47 (1975) 344. P. Chakraborty and S.D. Dey, J. Appl. Phys., 52 (1981) 7002. S.D. Dey, S.B. Karmohapatro and B.M. Banejee, Indian J. Phys., 49 (1975) 797. K. Wittmaack, Phys. Lett. A, 69 (1979) 322.
of Mass
S’pectrometry
and Zon Physics,
50 (1983)
349-351
349
Elsevier Science Publishers B-V., Amsterdam - Printed in The Netherlands
Short communication CLUSTER-ION EMISSION FROM UNDER Cd+ ION BOMBARDMENT
P. CHAKRABORTY
and SD.
AN ALUMINIUM
SURFACE
DEY
Saka Znstitute of Nuclear Physics, Bidhan Nagar, Calcutta 700 064 (India)
(Received 4 October 1982)
A special feature of secondary-ion mass spectrometry (SIMS) is the liberation of polyatomic particles (clusters), either in the neutral or ionic state, from ion-bombarded solid surfaces by a sputtering process. The cluster yield is much smaller than that of single atoms or ions. Nevertheless, cluster-ion emission has recently been the subject of extreme interest for the wealth of information it contains about the surface. At present, two contrasting mechanisms are generally used to explain the secondary emission of ionic clusters. One concerns vacuum recombination of individually emitted atoms or ions. This idea is supported by classical energy-transfer considerations [I], computer simulations [2,3] and recent experimental evidence [4], which indicate that secondary-ion clusters are not representative of the surface structure. They are formed in the space immediately above the surface by recombination of atoms which are emitted independently in the collision cascade created when the primary ions collide with the target lattice_ The other physical interpretation is that clusters are emitted “as such” directly from the surface and are thus composed only of atoms which are nearest neighbours on the surface. Some experimental measurements supporting this concept have been reported [5-71. Therefore, the interpretation of cluster yields is difficult because of the lack of agreement about a cluster formation mechanism. In the present work, some experimental data are reported for secondary Al +, Alf and All intensities from a more or less clean polycrystalline aluminium surface as a function of bombarding Cd+ ion energy in the range 3- 10 keV for various values of target current density in the range loo-250 E_LAcmm2. The apparatus has been described previously [Xl. Secondary ions are mass-analyzed by a quadrupole mass spectrometer [9] and then detected by a l&stage Cu-Be Allen-type electron multiplier set off-axis with respect to the quadruple, in conjunction with an electrometer amplifier. Prior to ion bombardment the target region is pumped differentially to a pressure of 5 X lo-’ torr. The target chamber, made of stainless steel, is connected to an 0020-7381/83/$03.00
0 1983 Elsevier Science Publishers B-V.
350
ultra-high vacuum sputter-ion pump by means of suitably designed stainless-steel tubes. The chamber is provided with two small openings, 2.5 and 1.5 mm in diameter, for primary-beam entrance and secondary-ion exit, respectively. The entire ultrahigh-vacuum chamber is coupled to an existing high-vacuum system in which the pressure ‘is 5 X 10m6 torr. The two openings in the chamber serve the basic requirement for maintaining the desired pressure difference between the two vacuum systems by a differential pumping arrangement. The experimental results of this study are presented in Fig. 1. The curves showing the variation of secondary-ion intensity for monomers, dimers and trimers of aluminium with projectile ion energy, for a given value of target current density (100 PA cmm2), are found to be similar in nature. In all cases, secondary-ion emission increases almost linearly with bombarding-ion energy, and then attains a broad maximum followed by a subsequent steady decline. The location of this maximum in all three cases is found to be - 5 keV. A similar phenomenon was observed [8] earlier in the case of Cu’ and cu; ion-emission intensities for copper bombarded with Zn+ ions, where the region of the maximum was found to be at - 7.5 keV for both ions. The locations of these maxima depend strongly on the projectile-target combina-
Fig. 1. Variation of secondary Al+ , All energy.
and Al;
ion intensities with bombarding Cd+ ion
351
tion. This finding is in agreement with the work of Wittmaack [lo], where ion-yield curves for all silicon clusters up to Sig were found to be almost identical, all having a broad maximum at - 10 keV, for Xe+ projectile ions. The appearance of the maxima followed by a subsequent steady dechne in secondary-ion emission in our observations may be attributed to the onset of a change in texture of the polycrystalline surface under ion bombardment [8]. The occurrence of the maxima for all the cluster species in about the same region and the similarity of the data shown in Fig. 1 suggest the idea that the secondary ionic species (atomic or clusters) are emitted simultaneously “as such” as a result of the same physical process; i.e., the constituents of the clusters are located at neightbouring sites before emission, rather than at next-nearest sites as suggested by computer simulations. Had the process of vacuum recombination of individually emitted atoms been reponsible for cluster formation, the secondary-ion yield behaviour would be expected to differ for differing cluster sizes because in that case surface structure would play practically no role in the process of cluster formation. ACKNOWLEDGEMENT
We thank Professor S.B. Karmohapatro and kind interest in this work.
for his continuous encouragement
REFERENCES 1 2 3 4 5 6 7 8 9 10
W. Gerhard, Z. Phys. B, 22 (1975) 31. N. Winograd, D.E. Harrison and B.J. Garrison, Surf. Sci., 78 (1978) 467. B.J. Garrison, N. Winograd and D.E. Harrison, J. Chem. Phys., 69 (1979) 1440. G.J. Slusser and N. Winograd, Surf. Sci., 95 (1980) 53. A. Benninghoven and A. Muller, Surf. Sci., 39 (1973) 416. A. Benninghoven, Surf. Sci., 53 (1975) 596. A. Buhl and A. Preisinger, Surf. Sci., 47 (1975) 344. P. Chakraborty and S.D. Dey, J. Appl. Phys., 52 (1981) 7002. S.D. Dey, S.B. Karmohapatro and B.M. Banejee, Indian J. Phys., 49 (1975) 797. K. Wittmaack, Phys. Lett. A, 69 (1979) 322.