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Energy distributions of sputtered metal Al-clusters
Nuclear Instruments and Methods m Physics Research B 82 (1993) 323-328 North-Holland
[~ ~ ~]
B
Beam Interactions with Materials & Atoms
Energy distributions of sputtered metal Al-clusters W. Husinsky, G. Nicolussi and G. Betz lnstttut fur Allgememe Phystk, Techmsche Unwersttdt Wwn, Wtedner Hauptstrasse 8-10, A-1040 Wien, Austria Received 6 November 1992 and m revised form 8 January 1993
The energy d~strlbutlons of neutral clusters escaping from metal targets under ion bombardment have been measured by nonresonant laser post-ionization and a time-of-flight (TOF) technique Measurements of various Al- and Sn-atoms, dlmers and tnmers from pure Al and AlSn alloy targets show that the ionization process is crucial for the interpretation of the measured spectra. In particular, fragmentation of large clusters can easily mask and &sturb the measured energy distribution of clusters with fewer atoms and of monomers, ff the lomzation probabdlty is low, as it is the case for multlphoton lomzatlon. The results obtained for the energy distribution of Al-atoms, dlmers and tnmers show that simple theories for the cluster formation cannot predict the correct energy &stributions, but molecular dynamics slmulahons agree well with the experiments.
1. Introduction The formation process and the physics of clusters represents a fascinating field of modern physics [1]. The assembling of atoms into larger and larger clusters finally results m the formation of a solid. Many of the physical properties can be studied as they change from their atomic values to the characteristic solid state values. Among the secondary particles emitted during particle bombardment of surfaces a considerable amount of clusters can be found. This has been observed as early as 1959 [2] for carbon. Cluster emission has been investigated for the first time systematically by Homg [3]. Theories for the formation of secondary clusters as result of particle bombardment of surfaces have been originally proposed nearly twenty years ago [4] and have been complemented and modified up to the present [5-8], Experimental data, in particular for neutral clusters, have been obtained by various groups and by different post-ionization techmques [9-14]. But even recent experiments leave us with many open questions as far as cluster yields and the corresponding energy distributions are concerned. This will be shown in more detail in this work. Fragmentation of clusters, mainly due to the postionizing radiation, represents one of the largest unknowns m these kind of experiments. Different meth-
Correspondence to W Husmsky, Institut fiir Allgemelne Physlk, Technlsche Unwersltat Wien, Wledner Hauptstrasse 8-10, A-1040 Wlen, Austria
ods for post-ionization may contribute quite differently to fragmentation, thus complicating a comparison between different experimental results [15]. Fragmentation, first of all will severely influence the measurement of the cluster yields. Furthermore, the measured energy distribution of the various cluster species will be strongly influenced by fragmentation.
2. Experimental Post-ionizauon of sputtered neutral atoms and clusters has been performed by laser ionization. In all experiments reported here we have used an excimer laser operating at 193 nm For Al-atoms and also clusters containing Al-atoms only one photon is necessary to ~onize the particles. Thus a h~gh lonizahon probability and consequently saturation of the postlomzation process is ensured. Following the laser postionization the particles are accelerated and massanalyzed in a time-of-flight spectrometer (TOFS) as shown in fig. 1. The two flight paths in the TOFS m&cated m the figure by A and B differ by the additional use of an ion mirror (reflectron) in geometry B. This operating mode of the TOFS allows measurements with a mass resolution of 2000. Furthermore, it ensures the suppression of all the SIMS ions. For velocity measurements, as will be discussed in detail m the next secuon, geometry A seems to be favorable. This geometry ensures a better transmission uniformity essential for reliable energy measurements. However, contribuhons due to SIMS ions have to be subtracted and the mass resoluhon is considerably lower than for geometry B.
0168-583X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
324
W Husmsky et al / Energy distributions of sputtered metal Al-clusters R o t a t a b l e T a r g e t on
3. Time-of-flight spectra
x-y-z Manipulator
'1OF E×tract~on
TOF Lenseq
I
i "
Detector
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Reflectron Detector ( Channel Plate Geometry A ) Fig 1. Schematic of the laser post-ionization time-of-flight spectrometer Two modes of operation are possible, a straight hne geometry A, and a mode B employing an ion mirror (reflectron) for increases mass resolution and suppressing SIMS ions. The laser pulse can be set with a variable delay with respect to the ion pulse m order to measure the velocity distribution of sputtered neutral particles.
The velocity selection is achieved by delaying the post-ionizing laser pulse (15 ns long) by an amount At (typically 0-4000 ns) with respect to the ion pulse. The distance between the target and the laser beam was approximately 3 ram. The half-width of the primary ion pulse is approxamately 70 ns. We have used 8 keV A r + ions. In order to define the active laser volume and the distance between target and laser beam as accurately as possible, a small movable slit with 0.1 mm slit width was placed approximately 2 cm in front of the target. It should, however, be mentioned that for some measurements (Al-trtmers) the slit had to be moved out of the laser b e a m in order to prevent noise at the channel plate, possibly due to laser-ablated ions.
The omginal data obtained are time-of-flight spectra of the particular masses investigated. At this point it should be stressed once more, that the velocity distributions are obtained from the delay between the ion and the laser pulse. Thus the flight time of the neutral particles from the target to the laser beam is measured, The T O F spectrometer itself, then, is only used to determine the specific masses of the particles. In principle the T O F spectra (and thus the masses) of all secondary particles can be measured simultaneously. However, due to large signal Intensity differences in some cases the spectra shown in the following has been recorded one after the other. In particular, the signal of A1 atoms is extremely large. In the experimental setup used for our experiments an uncertainty for At of + 25 ns has to be considered. Therefore, particular care is necessary for the exact measurement of the T O F distribution of low masses in the high-energy tail of the distribution. In principle the T O F S using geometry B offers large advantages as far as the mass resolution and the suppression of SIMS ions are concerned. However, suppressing the SIMS ions in the reflectron can also reduce the amount of high-energy, post-ionized neutrals In addition, the reflectron geometry tends to slightly maximize the transmission of the spectrometer for one particular energy group depending on the entrance angle of the ions into the Ion mirror. Using the straight T O F geometry A, therefore, seems to be the better choice, even if one has to cope with the problem of additional SIMS ions. We have found experimentally by comparing the T O F spectra of A1 atoms obtained with geometry A and B respectively, that in geometry B, depending on the voltage settings of the TOFs, the high-energy or lowenergy part of the spectrum can be reduced G e o m e t r y A, on the contrary, yields A1 spectra which can be very well fitted by the theoretical distributions. At this point some words concerning the interpretation of the measured signals are necessary. The shape of the measured T O F spectrum will be influenced and possibly modified by several experimental parameters: the ionization probablhty, the velocity of the particles, the length of ion and laser pulses, the dimensions of the interaction volume. Different approaches are possible to account for this problem [16-18]. We have chosen a new method which is described in detail in a separate paper [19]. Basically all these methods yield the same results, but we have chosen the latter one, because the Incorporation of different effects seems to be very easy and straightforward in this case. Starting with a given flux distribution at the target, a Monte Carlo program "counts" the post-ionized particles as a function of the delay between ion and laser pulse. Thus assuming a given velocity distribution and
W Husmsky et al / Energy dtstnbutlons of sputtered metal Al-clusters e x p e r i m e n t a l p a r a m e t e r s the expected s p e c t r u m can b e s i m u l a t e d a n d c o m p a r e d directly with t h e m e a s u r e d data. F o r A1 a t o m s we have s t a r t e d with the well-known S i g m u n d - T h o m p s o n d i s t r i b u t i o n [20] f ( E ) d E = E / ( E + Eb )n+l with two free p a r a m e t e r s : n (chosen 2 for atoms) a n d E b. In m a n y e x p e r i m e n t s of the type described here n o n r e s o n a n t laser post-ionization is used for conven i e n t d e t e c t i o n of all clusters of interest. Even if several a u t h o r s have also e m p l o y e d r e s o n a n t ionization in selected cases [13] m a n y r e c e n t results have b e e n obt a i n e d with n o n r e s o n a n t ionization [11-13]. This, however, r e p r e s e n t s a c o n s i d e r a b l e source of errors, as will b e shown in the following. In fig 2 the T O F spectra for A1 a n d Sn atoms a n d AISn-clusters s p u t t e r e d from an A 1 - S n alloy are shown. A1 atoms a n d A l S n clusters can b e ionized with o n e 193 n m p h o t o n Two ( n o n r e s o n a n t ) p h o t o n s are n e e d e d to ionize Sn atoms. Following the a r g u m e n t s m e n t i o n e d above t h e A1 s p e c t r u m agrees well with the theoretical predictions. It is i m m e d i a t e l y obvious from fig. 2 t h a t the Sn s p e c t r u m c a n n o t b e u n d e r s t o o d in a straightforward m a n n e r from the A1 T O F spectrum, because its m a x i m u m a p p e a r s later t h a n for the heavier A1Sn clusters. Applying the fitting p r o c e d u r e described earlier, it t u r n s out t h a t t h e " S n - a t o m d i s t r i b u t i o n " in reality follows t h e c o r r e s p o n d i n g S n - d i m e r s p e c t r u m This i m p o r t a n t new finding has also b e e n o b s e r v e d by C o o n et al. [16]. T h e y have used a different e x p e r i m e n tal a p p r o a c h to verify t h a t the cluster distribution can mask t h e a t o m distribution. This can b e u n d e r s t o o d if o n e considers t h a t the probability of n o n r e s o n a n t t w o - p h o t o n ionization of (Sn) atoms c a n easily b e lower by two orders of magnitudes as c o m p a r e d to t h e q u a s i r e s o n a n t f r a g m e n t a t i o n I
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Flight Time [nsec] Fig. 2. Time-of-flight spectra of Al (circles) and Sn (triangles) atoms as well as AlSn-clusters (squares) ejected from an AlSn-alloy target under 8 keV Ar + bombardment The A1atom spectrum is taken as a reference standard The position of the expected Sn TOF distribution is indicated by a dashed line. All spectra are normalized to 1
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Flight Time [nsec] Fig. 3. Time-of-flight spectra of Al-atoms (triangles), -dimers (open squares) and -trimers (circles) sputtered from a pure Al-metal target under 8 keV Ar + bombardment The full hnes are the best fits obtained by the Monte Carlo program described m the text starting with a S~gmund-Thompson distribution for the fitting parameters as given in the figure The error bars in&cate the approxamate error on the time axis according to the _+25 ns uncertainty of determining At = 0 as mentioned m the text All spectra are normahzed to 1
of Sn-clusters. It a p p e a r s t h a t in f r e q u e n t cases fragm e n t a t i o n results in one of t h e c o n s t i t u e n t s left in an ionic state, thus feeding t h e Sn a t o m c h a n n e l in the T O F S . T h e cluster-to-atom ratios in ion sputtering typically are o n the o r d e r of a few p e r c e n t [9,11,15], which m a k e s the a r g u m e n t s above r e a s o n a b l e . It should not be ignored t h a t d e c o m p o s i t i o n of higher mass clusters in s p u t t e r i n g of metals is very substantial due to the high i n t e r n a l energy of s p u t t e r e d m u l t i m e r s [20]. W e have also m e a s u r e d the velocity distribution of A1 a t o m s a n d clusters using 308 n m e x o m e r r a & a t i o n for post-ionization. In this two p h o t o n s are n e e d e d to ionize Al-atoms. However, a close r e s o n a n c e m a k e s the first step of t h e ionization q u a s i r e s o n a n t , resulting in a large ionization cross section. T h e absolute value of the A1 a t o m signal is nearly equivalent a n d in s a t u r a t i o n for 193 a n d 308 nm. Consequently, also t h e velocity distributions are identical. T h e ionization for Al-clusters, however, is r e d u c e d for 308 n m as comp a r e d to 193 nm, resulting in a n o n s a t u r a t e d Al-cluster signal. T h e T O F distribution is slightly shifted to larger flight times. W e believe this to be a c o n s e q u e n c e of an i n t e r f e r e n c e with f r a g m e n t e d Al-dimers. T h e T O F - s p e c t r a of s p u t t e r e d Al-atoms, -dtmers a n d -trxmers are shown in fig. 3. T h e spectra are c o m p a r e d with the simulated spectra o b t a i n e d according to the p r o c e d u r e m e n t i o n e d above a n d described in m o r e detail in ref. [19]. T h e fits shown are o b t a i n e d for t h e E b a n d n values given in the figure E b a n d n c o r r e s p o n d to the values in t h e well k n o w n S l g m u n d T h o m p s o n distribution describing the flux of s p u t t e r e d
326
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Energy dlstnbuttons of sputtered metal Al-clusters
particles f ( E ) d E = E / ( E + Eb) n+l. D u e to the twoparameter problem the fitting procedure is always left with some ambiguities. For the measured spectra shown in fig. 3 it turns out that the measured signal is more or less identical to the density distribution of the sputtered particles, if described with the corresponding parameters E b and n. For the atoms, a falloff at higher energies with n = 2 is pretty well fulfilled. A steeper falioff with n = 3 is observed for dimers and trlmers Conversion of the T O F spectra of fig. 3 into the corresponding energy distributions is shown in fig. 4 For the atoms the fit is identical to the theoretical density distribution with E u = 3 eV and n = 2. W e can, therefore, assume that the measured signal represents the density distribution also for the dimers and trimers. Comparison with the first according to ref. [19] then yields the result that the dimer- as well as trimer-flux distribution behaves like E -3 for high energies, as compared to E -2 for atoms.
4. Discussion In order to obtain reliable information on the energy distributions of sputtered neutral metal multlmers we consider the m e a s u r e m e n t of the atom energy distribution as a standard. Laser Ionization with 193 nm exclmer radiation and time-of-flight spectroscopy of A1 atoms allows us to measure the velocity and energy distribution with high accuracy. Due to the efficient one-photon ionization the velocity distribution of A1 atoms can be measured without interference from frag-
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Energy [eV] Fig. 4 Energy spectra of AI-atoms (triangles), -dlmers (open squares) and -tnmers (orcles) sputtered from a pure Al-metal target under 8 keV Ar + bombardment The spectra are obtained from the measured spectra m fig 3 The full hnes are the best fits obtained by the Monte Carlo program described in the text starting with a Sigmund-Thompson distribution for the fitting parameters as given m the figure. Furthermore the distribution of the atoms is compared with the theoretical densay &stributmn, which in this case is identical with the experimental data All spectra are normalized to 1
mentlng dlmers, trimers or other clusters. The T O F spectrum (fig. 3) and the energy spectrum (fig. 4) obtained for A1 atoms agrees very well with the established theory for cascade sputtering. The conclusions which we can draw from the measurement of the dimer and !rimer energy distribution about the mechanisms rely heavily on the assumption that no interference from higher cluster is present. There is strong evidence that this assumption is reasonable. In general, the ionization energy decreases with increasing cluster size. Furthermore, the ionization cross section increases with increasing cluster size due to an enhanced probability of resonant excitations. The fraction of sputtered clusters containing n + 1 atoms decreases in general by one or two orders of magnitude as compared to the cluster w~th n constituents Therefore, interference effects should be relevant in those cases where the (n + 1) cluster experiences a dramatic increase in ionization probability as compared to the (n) cluster This situation is encountered for the Sn atoms and AlSn clusters. Simple theoretical models [4-8] predict an energy distribution different from the ones measured here. In particular, the high-energy tall of the distribution according to these theories should fall off with E - 2 [6] (direct emission of intact clusters, not probably for pure metal targets) or E -45 [6] (recombination model for clusters), respectively. Also recent measurements of Cu-clusters [16] indicate that the experimental evidence proofs that the models mentioned above are too simple. A more promising approach seems to check the experimental results with molecular dynamics simulation (MD) [21-23]. M D has been tried for explaining the sputtering process and the cluster formation hereby for quite some time [21,22] using pairwise potentials. Recently, the use of e m b e d d e d - a t o m potentials [23,24] has proven to be of great advantage for the calculation of cluster yields and cluster energy distributions. We have performed M D calculations for AI atoms and dimers sputtered from an Al single crystals A1(111) by 1 keV A r ions. These parameters should be also representative for our experimental conditions as described in a previous section. An e m b e d d e d - a t o m type potential [25] combined with a B o r n - M a y e r potential at higher energies for A1 has been used. In this way excellent agreement with the experimental dimer bond length was obtained. However, the dissociation energy obtained overestimates the experimental value A potential cutoff radius of 5 ,~ has been chosen in the calculations. The target consisted of 2331 atoms, and 3600 ion impacts have been calculated. For each ion impact the development of the collision cascade has been followed up to 1 ps. Sputtered atoms and dlmers have been identified using the criteria that the particle is further away from the surface than twice the cutoff radius after 1 ps.
IV.. Husmsky et al / Energy dtstnbuttons of sputtered metal Al-clusters
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Energy[eV] Fig 5 Comparison of measured and calculated energy spectra for AI atoms and dlmers Measurements are gwen by full symbols (squares for atoms and triangles for dlmers) Molecular dynamics results are shown by open symbols The full hnes correspond to the Slgmund-Thompson theory as m the prewous figures. T h e results o b t a i n e d are c o m p a r e d with the experim e n t a l d a t a in fig. 5 Basically the b e h a v i o r of the experimentally o b t a i n e d energy distributions is reproduced. T h e energy distribution of the dimers is in excellent a g r e e m e n t . T h e a t o m distribution also falls off s o m e w h a t s t r o n g e r at energies above 20 eV, which we ascribe tentatively to the influence of t h e lower primary ion energy in t h e simulation. F u r t h e r m o r e , from the calculation a total s p u t t e r i n g yield of 3.5 a n d an A l - d i m e r yield of 0.5 was o b t a i n e d . Finally, it m i g h t b e i n t e r e s t i n g to speculate a b o u t a possible physical m e a n i n g of the E b values o b t a i n e d from t h e spectra All the values d e d u c e d from the spectra are close to the t a b u l a t e d values for the h e a t of s u b h m a t l o n of A l - a t o m s from Al-metal. T h e Importance of the b i n d i n g of the a t o m In the sohd for this free p a r a m e t e r , t h e r e f o r e , seems to be established. However, speculations a b o u t o b t a i n m g i n f o r m a t i o n of binding of the " c l u s t e r " to the surface from the d l m e r or t r i m e r spectra has to b e r e g a r d e d r a t h e r cautiously, a n d we would not like to p u s h possible t h e o r i e s too far at this stage.
Acknowledgements This work has b e e n s u p p o r t e d by the " F o n d s zur F 6 r d e r u n g d e r w m s e n s c h a f t h c h e n F o r s c h u n g " of Aus-
trla, u n d e r project # P 8 7 0 6 - P H Y . T h e authors would also like to express t h e i r t h a n k s to S. Coon, M. Pelhn, W. Calaway, H. U r b a s s e k a n d A. W u c h e r for valuable discussions.
References [1] M L. Mandmh, W D Reents and V E Bondybey, m Atomic and Molecular Clusters, ed E.R Bernstem (Elsevier, Amsterdam, Oxford, New York, 1990) p 69. [2] E Dornenburg and H Hmterberger, Z. Naturforsch. 16A (1959) 765 [3] R E Honlg, m Advances m Mass Spectrometry, ed R.M Elhott, vol 2 (Pergamon, London, 1963) [4] W. Gerhard, Z. Phys B22 (1975) 31 [5] G P Konnen, A T l p a n d A E d e V n e s , Radlat Eft 21 (1974) 269 [6] H M Urbassek, Nucl Instr. and Meth. B18 (1987) 587. [7] K Snowdon, R Hentschke, W Hefland and P. Hertel, Z Phys A318 (1984) 261 [8] K Snowdon, Nucl. Instr and Meth B9 (1985) 132. [9] H Oechsner and W Gerhard, Surf Scl 44 (1974) 480 [10] R A Brlzzolara and C B Cooper, Nucl Instr and Meth B43 (1989) 136 [11] W Husmsky, P Wurz, A Traunfellner and G Betz, Fresenms J Anal Chem 341 (1991) 12 [12] P Wurz, W Husmsky and G. Betz, Appl Phys. A52 (1991) 213 [13] S R. Coon, W.F Calaway, J W Burnett, M J Pelhn, D M Gruen, D R. Spmgel and J.M White, Surf Scl. 259 (1991) 275
328
W Husmsky et al / Energy dtstrtbuttons of sputtered metal ALclusters
[14] H Gnaser and W O Hofer, Appl Phys A48 (1989) 261 [15] W Hofer, in: Sputtering by Particle Bombardment 1II, ed R. Behnsch (Springer, Berhn, Heidelberg, 1991) p. 15 [16] S R Coon, W F. Calaway, M.J Pelhn, G A Curlee and J.M White, this issue (SPUT'92), Nucl Instr and Meth B82 (1993) 329 [17] M Inoue, Jpn. J Appl Phys. 31 (1992) 1530 [18] S R Coon, S R Calaway, M J Pelhn and J M. White, to be submitted [19] G Betz, W Huslnsky and G Nlcolussl, to be pubhshed
[20] A Wucher and B J. Garrison, Phys. Rev. B46 (1992) 4855. [21] D E. Harrison, J Appl Phys. 47 (1976) 2252 [22] D E Harrison, P. Avourls and P Walkup, Nucl Instr and Meth. B18 (1986) 349 [23] B J Garrison, N Wmograd, D M Deaven, C T Reinmann, D Y Lo, T A Tombrello, D E Harrison and M H Shapiro, Phys Rev B37 (1988) 7197 [24] A, Wucher and B J Garrison, Surf Scl. 260 (1992) 257 [25] H. Urbassek, prwate communication (1992)
[~ ~ ~]
B
Beam Interactions with Materials & Atoms
Energy distributions of sputtered metal Al-clusters W. Husinsky, G. Nicolussi and G. Betz lnstttut fur Allgememe Phystk, Techmsche Unwersttdt Wwn, Wtedner Hauptstrasse 8-10, A-1040 Wien, Austria Received 6 November 1992 and m revised form 8 January 1993
The energy d~strlbutlons of neutral clusters escaping from metal targets under ion bombardment have been measured by nonresonant laser post-ionization and a time-of-flight (TOF) technique Measurements of various Al- and Sn-atoms, dlmers and tnmers from pure Al and AlSn alloy targets show that the ionization process is crucial for the interpretation of the measured spectra. In particular, fragmentation of large clusters can easily mask and &sturb the measured energy distribution of clusters with fewer atoms and of monomers, ff the lomzation probabdlty is low, as it is the case for multlphoton lomzatlon. The results obtained for the energy distribution of Al-atoms, dlmers and tnmers show that simple theories for the cluster formation cannot predict the correct energy &stributions, but molecular dynamics slmulahons agree well with the experiments.
1. Introduction The formation process and the physics of clusters represents a fascinating field of modern physics [1]. The assembling of atoms into larger and larger clusters finally results m the formation of a solid. Many of the physical properties can be studied as they change from their atomic values to the characteristic solid state values. Among the secondary particles emitted during particle bombardment of surfaces a considerable amount of clusters can be found. This has been observed as early as 1959 [2] for carbon. Cluster emission has been investigated for the first time systematically by Homg [3]. Theories for the formation of secondary clusters as result of particle bombardment of surfaces have been originally proposed nearly twenty years ago [4] and have been complemented and modified up to the present [5-8], Experimental data, in particular for neutral clusters, have been obtained by various groups and by different post-ionization techmques [9-14]. But even recent experiments leave us with many open questions as far as cluster yields and the corresponding energy distributions are concerned. This will be shown in more detail in this work. Fragmentation of clusters, mainly due to the postionizing radiation, represents one of the largest unknowns m these kind of experiments. Different meth-
Correspondence to W Husmsky, Institut fiir Allgemelne Physlk, Technlsche Unwersltat Wien, Wledner Hauptstrasse 8-10, A-1040 Wlen, Austria
ods for post-ionization may contribute quite differently to fragmentation, thus complicating a comparison between different experimental results [15]. Fragmentation, first of all will severely influence the measurement of the cluster yields. Furthermore, the measured energy distribution of the various cluster species will be strongly influenced by fragmentation.
2. Experimental Post-ionizauon of sputtered neutral atoms and clusters has been performed by laser ionization. In all experiments reported here we have used an excimer laser operating at 193 nm For Al-atoms and also clusters containing Al-atoms only one photon is necessary to ~onize the particles. Thus a h~gh lonizahon probability and consequently saturation of the postlomzation process is ensured. Following the laser postionization the particles are accelerated and massanalyzed in a time-of-flight spectrometer (TOFS) as shown in fig. 1. The two flight paths in the TOFS m&cated m the figure by A and B differ by the additional use of an ion mirror (reflectron) in geometry B. This operating mode of the TOFS allows measurements with a mass resolution of 2000. Furthermore, it ensures the suppression of all the SIMS ions. For velocity measurements, as will be discussed in detail m the next secuon, geometry A seems to be favorable. This geometry ensures a better transmission uniformity essential for reliable energy measurements. However, contribuhons due to SIMS ions have to be subtracted and the mass resoluhon is considerably lower than for geometry B.
0168-583X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
324
W Husmsky et al / Energy distributions of sputtered metal Al-clusters R o t a t a b l e T a r g e t on
3. Time-of-flight spectra
x-y-z Manipulator
'1OF E×tract~on
TOF Lenseq
I
i "
Detector
I Channel Plate [B[ Geometry B
Reflectron Detector ( Channel Plate Geometry A ) Fig 1. Schematic of the laser post-ionization time-of-flight spectrometer Two modes of operation are possible, a straight hne geometry A, and a mode B employing an ion mirror (reflectron) for increases mass resolution and suppressing SIMS ions. The laser pulse can be set with a variable delay with respect to the ion pulse m order to measure the velocity distribution of sputtered neutral particles.
The velocity selection is achieved by delaying the post-ionizing laser pulse (15 ns long) by an amount At (typically 0-4000 ns) with respect to the ion pulse. The distance between the target and the laser beam was approximately 3 ram. The half-width of the primary ion pulse is approxamately 70 ns. We have used 8 keV A r + ions. In order to define the active laser volume and the distance between target and laser beam as accurately as possible, a small movable slit with 0.1 mm slit width was placed approximately 2 cm in front of the target. It should, however, be mentioned that for some measurements (Al-trtmers) the slit had to be moved out of the laser b e a m in order to prevent noise at the channel plate, possibly due to laser-ablated ions.
The omginal data obtained are time-of-flight spectra of the particular masses investigated. At this point it should be stressed once more, that the velocity distributions are obtained from the delay between the ion and the laser pulse. Thus the flight time of the neutral particles from the target to the laser beam is measured, The T O F spectrometer itself, then, is only used to determine the specific masses of the particles. In principle the T O F spectra (and thus the masses) of all secondary particles can be measured simultaneously. However, due to large signal Intensity differences in some cases the spectra shown in the following has been recorded one after the other. In particular, the signal of A1 atoms is extremely large. In the experimental setup used for our experiments an uncertainty for At of + 25 ns has to be considered. Therefore, particular care is necessary for the exact measurement of the T O F distribution of low masses in the high-energy tail of the distribution. In principle the T O F S using geometry B offers large advantages as far as the mass resolution and the suppression of SIMS ions are concerned. However, suppressing the SIMS ions in the reflectron can also reduce the amount of high-energy, post-ionized neutrals In addition, the reflectron geometry tends to slightly maximize the transmission of the spectrometer for one particular energy group depending on the entrance angle of the ions into the Ion mirror. Using the straight T O F geometry A, therefore, seems to be the better choice, even if one has to cope with the problem of additional SIMS ions. We have found experimentally by comparing the T O F spectra of A1 atoms obtained with geometry A and B respectively, that in geometry B, depending on the voltage settings of the TOFs, the high-energy or lowenergy part of the spectrum can be reduced G e o m e t r y A, on the contrary, yields A1 spectra which can be very well fitted by the theoretical distributions. At this point some words concerning the interpretation of the measured signals are necessary. The shape of the measured T O F spectrum will be influenced and possibly modified by several experimental parameters: the ionization probablhty, the velocity of the particles, the length of ion and laser pulses, the dimensions of the interaction volume. Different approaches are possible to account for this problem [16-18]. We have chosen a new method which is described in detail in a separate paper [19]. Basically all these methods yield the same results, but we have chosen the latter one, because the Incorporation of different effects seems to be very easy and straightforward in this case. Starting with a given flux distribution at the target, a Monte Carlo program "counts" the post-ionized particles as a function of the delay between ion and laser pulse. Thus assuming a given velocity distribution and
W Husmsky et al / Energy dtstnbutlons of sputtered metal Al-clusters e x p e r i m e n t a l p a r a m e t e r s the expected s p e c t r u m can b e s i m u l a t e d a n d c o m p a r e d directly with t h e m e a s u r e d data. F o r A1 a t o m s we have s t a r t e d with the well-known S i g m u n d - T h o m p s o n d i s t r i b u t i o n [20] f ( E ) d E = E / ( E + Eb )n+l with two free p a r a m e t e r s : n (chosen 2 for atoms) a n d E b. In m a n y e x p e r i m e n t s of the type described here n o n r e s o n a n t laser post-ionization is used for conven i e n t d e t e c t i o n of all clusters of interest. Even if several a u t h o r s have also e m p l o y e d r e s o n a n t ionization in selected cases [13] m a n y r e c e n t results have b e e n obt a i n e d with n o n r e s o n a n t ionization [11-13]. This, however, r e p r e s e n t s a c o n s i d e r a b l e source of errors, as will b e shown in the following. In fig 2 the T O F spectra for A1 a n d Sn atoms a n d AISn-clusters s p u t t e r e d from an A 1 - S n alloy are shown. A1 atoms a n d A l S n clusters can b e ionized with o n e 193 n m p h o t o n Two ( n o n r e s o n a n t ) p h o t o n s are n e e d e d to ionize Sn atoms. Following the a r g u m e n t s m e n t i o n e d above t h e A1 s p e c t r u m agrees well with the theoretical predictions. It is i m m e d i a t e l y obvious from fig. 2 t h a t the Sn s p e c t r u m c a n n o t b e u n d e r s t o o d in a straightforward m a n n e r from the A1 T O F spectrum, because its m a x i m u m a p p e a r s later t h a n for the heavier A1Sn clusters. Applying the fitting p r o c e d u r e described earlier, it t u r n s out t h a t t h e " S n - a t o m d i s t r i b u t i o n " in reality follows t h e c o r r e s p o n d i n g S n - d i m e r s p e c t r u m This i m p o r t a n t new finding has also b e e n o b s e r v e d by C o o n et al. [16]. T h e y have used a different e x p e r i m e n tal a p p r o a c h to verify t h a t the cluster distribution can mask t h e a t o m distribution. This can b e u n d e r s t o o d if o n e considers t h a t the probability of n o n r e s o n a n t t w o - p h o t o n ionization of (Sn) atoms c a n easily b e lower by two orders of magnitudes as c o m p a r e d to t h e q u a s i r e s o n a n t f r a g m e n t a t i o n I
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of Sn-clusters. It a p p e a r s t h a t in f r e q u e n t cases fragm e n t a t i o n results in one of t h e c o n s t i t u e n t s left in an ionic state, thus feeding t h e Sn a t o m c h a n n e l in the T O F S . T h e cluster-to-atom ratios in ion sputtering typically are o n the o r d e r of a few p e r c e n t [9,11,15], which m a k e s the a r g u m e n t s above r e a s o n a b l e . It should not be ignored t h a t d e c o m p o s i t i o n of higher mass clusters in s p u t t e r i n g of metals is very substantial due to the high i n t e r n a l energy of s p u t t e r e d m u l t i m e r s [20]. W e have also m e a s u r e d the velocity distribution of A1 a t o m s a n d clusters using 308 n m e x o m e r r a & a t i o n for post-ionization. In this two p h o t o n s are n e e d e d to ionize Al-atoms. However, a close r e s o n a n c e m a k e s the first step of t h e ionization q u a s i r e s o n a n t , resulting in a large ionization cross section. T h e absolute value of the A1 a t o m signal is nearly equivalent a n d in s a t u r a t i o n for 193 a n d 308 nm. Consequently, also t h e velocity distributions are identical. T h e ionization for Al-clusters, however, is r e d u c e d for 308 n m as comp a r e d to 193 nm, resulting in a n o n s a t u r a t e d Al-cluster signal. T h e T O F distribution is slightly shifted to larger flight times. W e believe this to be a c o n s e q u e n c e of an i n t e r f e r e n c e with f r a g m e n t e d Al-dimers. T h e T O F - s p e c t r a of s p u t t e r e d Al-atoms, -dtmers a n d -trxmers are shown in fig. 3. T h e spectra are c o m p a r e d with the simulated spectra o b t a i n e d according to the p r o c e d u r e m e n t i o n e d above a n d described in m o r e detail in ref. [19]. T h e fits shown are o b t a i n e d for t h e E b a n d n values given in the figure E b a n d n c o r r e s p o n d to the values in t h e well k n o w n S l g m u n d T h o m p s o n distribution describing the flux of s p u t t e r e d
326
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particles f ( E ) d E = E / ( E + Eb) n+l. D u e to the twoparameter problem the fitting procedure is always left with some ambiguities. For the measured spectra shown in fig. 3 it turns out that the measured signal is more or less identical to the density distribution of the sputtered particles, if described with the corresponding parameters E b and n. For the atoms, a falloff at higher energies with n = 2 is pretty well fulfilled. A steeper falioff with n = 3 is observed for dimers and trlmers Conversion of the T O F spectra of fig. 3 into the corresponding energy distributions is shown in fig. 4 For the atoms the fit is identical to the theoretical density distribution with E u = 3 eV and n = 2. W e can, therefore, assume that the measured signal represents the density distribution also for the dimers and trimers. Comparison with the first according to ref. [19] then yields the result that the dimer- as well as trimer-flux distribution behaves like E -3 for high energies, as compared to E -2 for atoms.
4. Discussion In order to obtain reliable information on the energy distributions of sputtered neutral metal multlmers we consider the m e a s u r e m e n t of the atom energy distribution as a standard. Laser Ionization with 193 nm exclmer radiation and time-of-flight spectroscopy of A1 atoms allows us to measure the velocity and energy distribution with high accuracy. Due to the efficient one-photon ionization the velocity distribution of A1 atoms can be measured without interference from frag-
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mentlng dlmers, trimers or other clusters. The T O F spectrum (fig. 3) and the energy spectrum (fig. 4) obtained for A1 atoms agrees very well with the established theory for cascade sputtering. The conclusions which we can draw from the measurement of the dimer and !rimer energy distribution about the mechanisms rely heavily on the assumption that no interference from higher cluster is present. There is strong evidence that this assumption is reasonable. In general, the ionization energy decreases with increasing cluster size. Furthermore, the ionization cross section increases with increasing cluster size due to an enhanced probability of resonant excitations. The fraction of sputtered clusters containing n + 1 atoms decreases in general by one or two orders of magnitude as compared to the cluster w~th n constituents Therefore, interference effects should be relevant in those cases where the (n + 1) cluster experiences a dramatic increase in ionization probability as compared to the (n) cluster This situation is encountered for the Sn atoms and AlSn clusters. Simple theoretical models [4-8] predict an energy distribution different from the ones measured here. In particular, the high-energy tall of the distribution according to these theories should fall off with E - 2 [6] (direct emission of intact clusters, not probably for pure metal targets) or E -45 [6] (recombination model for clusters), respectively. Also recent measurements of Cu-clusters [16] indicate that the experimental evidence proofs that the models mentioned above are too simple. A more promising approach seems to check the experimental results with molecular dynamics simulation (MD) [21-23]. M D has been tried for explaining the sputtering process and the cluster formation hereby for quite some time [21,22] using pairwise potentials. Recently, the use of e m b e d d e d - a t o m potentials [23,24] has proven to be of great advantage for the calculation of cluster yields and cluster energy distributions. We have performed M D calculations for AI atoms and dimers sputtered from an Al single crystals A1(111) by 1 keV A r ions. These parameters should be also representative for our experimental conditions as described in a previous section. An e m b e d d e d - a t o m type potential [25] combined with a B o r n - M a y e r potential at higher energies for A1 has been used. In this way excellent agreement with the experimental dimer bond length was obtained. However, the dissociation energy obtained overestimates the experimental value A potential cutoff radius of 5 ,~ has been chosen in the calculations. The target consisted of 2331 atoms, and 3600 ion impacts have been calculated. For each ion impact the development of the collision cascade has been followed up to 1 ps. Sputtered atoms and dlmers have been identified using the criteria that the particle is further away from the surface than twice the cutoff radius after 1 ps.
IV.. Husmsky et al / Energy dtstnbuttons of sputtered metal Al-clusters
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Energy[eV] Fig 5 Comparison of measured and calculated energy spectra for AI atoms and dlmers Measurements are gwen by full symbols (squares for atoms and triangles for dlmers) Molecular dynamics results are shown by open symbols The full hnes correspond to the Slgmund-Thompson theory as m the prewous figures. T h e results o b t a i n e d are c o m p a r e d with the experim e n t a l d a t a in fig. 5 Basically the b e h a v i o r of the experimentally o b t a i n e d energy distributions is reproduced. T h e energy distribution of the dimers is in excellent a g r e e m e n t . T h e a t o m distribution also falls off s o m e w h a t s t r o n g e r at energies above 20 eV, which we ascribe tentatively to the influence of t h e lower primary ion energy in t h e simulation. F u r t h e r m o r e , from the calculation a total s p u t t e r i n g yield of 3.5 a n d an A l - d i m e r yield of 0.5 was o b t a i n e d . Finally, it m i g h t b e i n t e r e s t i n g to speculate a b o u t a possible physical m e a n i n g of the E b values o b t a i n e d from t h e spectra All the values d e d u c e d from the spectra are close to the t a b u l a t e d values for the h e a t of s u b h m a t l o n of A l - a t o m s from Al-metal. T h e Importance of the b i n d i n g of the a t o m In the sohd for this free p a r a m e t e r , t h e r e f o r e , seems to be established. However, speculations a b o u t o b t a i n m g i n f o r m a t i o n of binding of the " c l u s t e r " to the surface from the d l m e r or t r i m e r spectra has to b e r e g a r d e d r a t h e r cautiously, a n d we would not like to p u s h possible t h e o r i e s too far at this stage.
Acknowledgements This work has b e e n s u p p o r t e d by the " F o n d s zur F 6 r d e r u n g d e r w m s e n s c h a f t h c h e n F o r s c h u n g " of Aus-
trla, u n d e r project # P 8 7 0 6 - P H Y . T h e authors would also like to express t h e i r t h a n k s to S. Coon, M. Pelhn, W. Calaway, H. U r b a s s e k a n d A. W u c h e r for valuable discussions.
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[14] H Gnaser and W O Hofer, Appl Phys A48 (1989) 261 [15] W Hofer, in: Sputtering by Particle Bombardment 1II, ed R. Behnsch (Springer, Berhn, Heidelberg, 1991) p. 15 [16] S R Coon, W F. Calaway, M.J Pelhn, G A Curlee and J.M White, this issue (SPUT'92), Nucl Instr and Meth B82 (1993) 329 [17] M Inoue, Jpn. J Appl Phys. 31 (1992) 1530 [18] S R Coon, S R Calaway, M J Pelhn and J M. White, to be submitted [19] G Betz, W Huslnsky and G Nlcolussl, to be pubhshed
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