The unimolecular decay of Aln± and Sin± sputtered clusters

The unimolecular decay of Aln± and Sin± sputtered clusters

Applied Surface Science 203±204 (2003) 118±121 The unimolecular decay of Aln and Sin sputtered clusters N.Kh. Dzhemilev*, A.D. Bekkerman, S.E. Maks...

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Applied Surface Science 203±204 (2003) 118±121

The unimolecular decay of Aln and Sin sputtered clusters N.Kh. Dzhemilev*, A.D. Bekkerman, S.E. Maksimov, V.I. Tugushev Arifov Institute of Electronics, Akademgorodok, Tashkent 700143, Uzbekistan

Abstract The decay processes of Aln  …n ˆ 2 24† and Sin  …n ˆ 2 12† clusters sputtered from Al, Si targets under Xe‡ and Cs‡ ion bombardment have been studied. We have determined the most probable fragmentation channels of positively and negatively charged clusters in the acceleration zone of tool in time region of 10 9 to 10 7 s from the emission moment and in the ®eld-free zones of the apparatus in the time interval of 5  10 7 to 5  10 5 s. The intensive fragmentation of Aln ‡ and Sin ‡ occurs with Al‡ and Si‡ formation. It permits to make a conclusion, that one part of Al‡ and Si‡ ions on the detector corresponds to ``true'' secondary ions, while another part is due to cluster decays and corresponds to the fragments. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Sputtering; Cluster; Fragmentation; Ions; Silicon; Aluminiun

1. Introduction The ¯ux of sputtering products under bombardment of solid surfaces by accelerated ions contain equally with atoms, the molecules and clusters too. These clusters are formed in vibrationally excited states and decay on their way from the sputtered target to the detector. Unimolecular decays in¯uence on the mass spectra, kinetic energy spectra and internal energy distributions of the clusters. In [1,2], cluster ions Aln ‡ , Sin, Gan ‡ , and Inn ‡ sputtered by ion bombardment were found to have along with the channel Mn ‡ ! Mn

1

‡

‡ M0

(1)

the second fragmentation channel: Mn ‡ ! M‡ ‡ Mn *

1

0

Corresponding author. Tel.: ‡998-1623973; fax: ‡998-1628767. E-mail address: [email protected] (N.Kh. Dzhemilev).

(2)

As far as theory and practice of quantitative analysis in secondary ions emission (SIE) are based on monoatomic ion yield, then a more detailed investigation of the second channel is needed to ®nd out the contribution of Al‡ and Si‡ of fragmentation origin into SIE monoatomic component. In the present paper, the fragmentation processes of Aln  …n ˆ 2 24† and Sin  …n ˆ 2 12† clusters sputtered from Al, Si targets under Xe‡ and Cs‡ ion bombardment with medium energies have been studied. 2. Experimental The investigations were performed using ion microanalyzer with reverse geometry double focusing, which has been described in detail elsewhere [3]. Primary ions after acceleration and following mass separation in a Wien ®lter bombarded the sample by an ion beam focused (30±50 mm) over 458 relative to the target normal. The measurements were performed on bombardment of Al and Si polycrystal samples by Xe‡ and

0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 7 1 2 - 2

N.Kh. Dzhemilev et al. / Applied Surface Science 203±204 (2003) 118±121

Cs‡ ions for a beam current density of 1 mA cm 2. The rastered area was 500 mm  500 mm on the sample surface. To eliminate the in¯uence of etching nonhomogeneity on the sample surface on measurement accuracy, the secondary ions were taken from the central region of etching raster with dimensions 200 mm  200 mm. For this purpose, the electron diaphragm (dynamic emittance matching) technique was used [3]. In the sample chamber and the whole ion ¯ight path from sample to detector a pressure was 10 7 Pa. The cluster ions, that did not undergo decay during time-of-¯ight from the target to detector were recorded as the peaks of the parent ions with mass Mn ‡ at electrostatic energy analyzer adjustment in the bandpass of ions with energy eU0. The ions that underwent a decay of Mn ‡ ! Mk ‡ ‡ Mn k 0 type in the ®eld-free zone L1 should be recorded at analyzer adjustment in the peak with the apparent mass m ˆ Mk 2 =Mn at simultaneous adjustment of the electrostatic energy analyzer for passing ions with medium energy …Mk =Mn †eU0 (eV). If the metastable ion survives to the area between the magnet and the electrostatic analyzer (zone L2) and is decomposed there, then in order to record two charged fragment ions the voltage in the electrostatic analyzer should be changed so that the fragment ions pass through it with the energy E ˆ …Mk =Mn †eU0. Besides, those ions with energies that differ from E by a magnitude larger than the window width of the electrostatic analyzer should be separated. In [3,4] it was shown, that most of the cluster ions decomposed in the zone of secondary ion acceleration, i.e. in close proximity to the target. This results in the appearance of protracted tails in the lowenergy part of the ion spectral lines. The apparatus described allows measurements to be made on the energy spectra by variation of the ion accelerating electrode.

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Fig. 1. The dependence of intensity of secondary Aln  ion yield on the number of atoms in the ion.

tion of the cluster component in positive-charged component is relatively low, i.e. the charge is carried out mainly by monoatomic Al‡ ions. Meanwhile the fraction of Aln ions in negative-charged component is extremely high, and the charge is carried out mainly by a cluster component. To clear up so considerable difference in the distribution I…Aln ‡ † ˆ f …n† and I…Aln † ˆ f …n† the experiments were performed on the determination of unimolecular decays of Aln ‡ ions in the ®rst ®eld-free zone of the apparatus. In Fig. 2, the dependence of the decay probability for Aln ‡ cluster ions in ®eld-free zone L1 on the number of atoms n in the parent ions is shown. The probability is given as P ˆ ‰I…Alk  †=I…Aln  † ‡ I…Alk  †Š  100%, where I…Aln ‡ † is the peak intensity of primary ions, I…Alk † is the intensity of fragment ions formed in the zone and k the number of atoms in the fragment ion. As it is seen from Fig. 2 for Aln ‡ ions two types of decay reactions (1) and (2) were observed, while for Aln ions only reaction of type (1) was observed, i.e. the cluster

3. Results and discussion Fig. 1 illustrates the dependencies I…Aln  † ˆ f …n† obtained at sputtering of Al by Xe‡ ions with energy E0 ˆ 8:5 keV and Cs‡ ions with E0 ˆ 13:5 keV. The yields of Aln ‡ and Aln ions were normalized to the yields of monoatomic Al‡ and Al ions, respectively. As it is seen from Fig. 1, the distributions I…Aln ‡ † ˆ f …n† and I…Aln † ˆ f …n† differ essentially. The frac-

Fig. 2. The dependence of cluster ion fragmentation probability P (%) on the number of atoms in the parent ion.

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decays with the formation of negative-charged Al ion fragments were not observed. Aln ‡ cluster ions with n < 7 decompose mostly via Al‡ ion ejection, since for n > 7 reactions of type (1) predominant. Fragmentation with Al‡ monomer ion ejection was observed up to the highest cluster numbers and the probability of this reaction has a noticeable correlation with the inverse processes probability, i.e. with an increase of probability of fragmentation via a reaction of type (1), the probability of fragmentation via a reaction of type (2) decreases and vice versa. It should be noted, that the total probability of the dissociation reaction increases with an increase of cluster number for n ˆ 6 and n ˆ 7. The comparison between the obtained data and the values of ionization potentials (IPs), known from the literature, has shown, that for n ˆ 2 14 IP…Al† < IP…Aln † [5]. It means, that for n ˆ 2 7 the dissociation energy of Aln ‡ by the channel (2) is always lower, than that by the channel (1), because Dn ‡ …Aln

1

‡

; Al†

ˆ IP…Aln 1 †

Dn ‡ …Al‡ ; Aln 1 † IP…Al† > 0

(3)

But at n > 8 the energy threshold of the reaction (1) becomes lower than the threshold of reaction (2), though IP(Aln) for n > 8 13 still exceeds IP(Al) calculated in [5]. The equation of energy balance for the negative clusters fragmentation gives: Dn …Mm ; Mn m † ˆ Dn …Mm ; Mn m †

‰EA…Mm †

EA…Mn †Š

to a time interval of 5  10 7 to 5  10 5 s. As was already mentioned, the most excited clusters would decompose with life-time of 5  10 9 to 5  10 7 s in the zone of cluster ions acceleration. This fact is testi®ed by the pronounce of long tails belonging to the studied ions in the negative energy region (Fig. 3). It is obvious that the number of ions of decay origin recorded with corresponding energy de®cit could be characterized by the height and slope of the left branch of the energy spectra. If one assumes, that the same processes take place in ®eld-free zone L1 and the most probable channels of fragmentation reactions are the same, then a more signi®cant rise and extent of Al7 ‡ ion distribution tail in comparison with Al6 ‡ would become apparent. This is due to the fact, that Al7 ‡ ions represent the product of the fragmentation reaction Al8 ‡ ! Al7 ‡ ‡ Al, the probability of which is about four orders higher than for the reaction Al7 ‡ ! Al6 ‡ ‡ Al. For Aln ‡ ions with n < 7 the preferential decay channel is the one with monomer ion ejection (Fig. 2). This leads to the appearance of a tail in the negative energy region in the energy spectrum of Al‡ monomer ions (Fig. 3). Si‡ monomer ions have similar energy spectra but with a less-de®ned left branch of the distribution caused by the considerably less probability of reaction (2) occurring for Sin ‡ ions. In Fig. 3, the energy distribution of Al ions is also

(4)

It follows from (4), that the generation of a fragment with higher electron af®nity is favorable. As the dependence EA…Aln † ˆ f …n† is the increasing one [6], the decays of Aln by the channel Aln ! Aln 1 ‡ Al are energetically more favorable. Thus, the decays of Aln ‡ clusters show that charge localization during the fragmentation is determined by the relation between IPs (electron af®nities) of generating fragments. When studying the fragmentation of Si‡ cluster ions the reactions with monomer ion ejection were recorded also, but only for clusters with 3, 4 and 6 atoms. The probability of this reactions was less than that for the corresponding Aln ‡ ions and amounted to 6  10 4 , 610 3, 2  10 3 %, respectively. The above reactions of cluster ion fragmentation occur at a distance far from the target and correspond

Fig. 3. Energy spectra of Al‡, Al , Si‡ (top) and Al6 ‡ , Al7 ‡ , Si5 ‡ (bottom) ions.

N.Kh. Dzhemilev et al. / Applied Surface Science 203±204 (2003) 118±121

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origin should be present in the ¯ux of particles emitted from the surface. This conclusion is based on the results of our estimation of the excitation energy of metastable Sin ‡ …n ˆ 3 10† clusters. The estimations have revealed that vibrational excitation energy acquired by Sin ‡ …n ˆ 3 10† clusters during the formation appeared to be insuf®cient for repeated fragmentation. This is corroborated by the lack of repeated fragmentation of Sin ‡ clusters in the time interval of 10 7 to 10 5 s. The intensities of ion fragments and neutral particles Aln 0 and Sin 0 measured in ®eld-free zone L1 of the mass-spectrometer are shown in Fig. 4. 4. Conclusion

Fig. 4. The dependence of intensity of fragment ions and neutral particles on the number of atoms in the fragment.

shown. As it is seen, in low energy spectrum region the tail characteristic for Si‡ and Al‡ spectra was not observed. This result testi®ed the absence of the decays of Aln ions with the formation of monoatomic Al ion fragments. Thus, it becomes clear, that the lines in SIE mass spectra of Al corresponding to monoatomic Al‡ ions present the lines of both monoatomic fragment ions and origin ones. Apparently, by this one could explain the fact, that the yield of Al‡ in comparison with the yield of Al has anomalously high intensity in relation to the cluster ion yield (Fig. 2). Since at the development of the model of atom ionization in SIE for comparison of the theory and experiment SIE coef®cients obtained experimentally negliging the contribution of fragment ion transmission into monoatomic ion ¯ux are often enlisted, then it is dif®cult to expect the agreement between theory and experiment, at least for aluminum and silicon. The above results allow to draw the following conclusion, namely: at sputtering of Al and Si by heavy ions as a result of decay reaction of type (2) stable neutral Aln 0 and Sin 0 clusters of fragmentation

The ¯ux of Al‡ ions registered by SIMS detector contains both Al‡ ions of initial origin and those resulting from fragmentation of Aln ‡ clusters …n ˆ 3 24† which underwent the decay nearby the surface according to the channel (2). The existence of the transitions in the zone of ion acceleration (targetimmersion objective lens) is testi®ed by extended ``tails'' in the energy spectra of Al‡ ions lying in low energy region. Both the amount and composition of Al‡ ions at SIMS detector exit strongly depend on the experimental conditions, namely: the life-time of clusters in the zone of acceleration, SIMS energy ``window'', type of objective lens for the secondary ions. Aln ‡ …n ˆ 3 7† cluster fragments contribute mainly into Al‡ ion ¯ux, but the contribution of Aln ‡ …n ˆ 8 24† decays is less signi®cant, since Aln ‡ ions decompose preferentially according to the channel (1) ranging from n ˆ 8. References [1] F.L. King, M.M. Ross, Chem. Phys. Lett. 164 (1989) 131. [2] A.D. Bekkerman, N.Kh. Dzemilev, V.M. Rotstein, Pisma v Zh. Tech. Fiz. 4 (1990) 58. [3] A.D. Bekkerman, N.Kh. Dzemilev, V.M. Rotstein, Surf. Interf. Anal. 15 (1990) 587. [4] N.Kh. Dzhemilev, S.V. Verkhoturov, Izv. Acad. Nauk SSSR, Ser. Fiz. 49 (1985) 1831. [5] W.A. Saunders, P. Fayet, L. Woeste, Phys. Rev. A 39 (1989) 4400. [6] G. Garterfoer, M. Gaussa, K.H. Meiwes-Broer, H.O. Lutz, Z. Phys. D 39 (1988) 1.