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This article was originally published in a journal published by Elsevier, and the attached copy is provided by Elsevier for the author s benefit and for the benefit of the author s institution, for

1 This article was originally published in a journal published by Elsevier, and the attached copy is provided by Elsevier for the author s benefit and for the benefit of the author s institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues that you know, and providing a copy to your institution s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier s permissions site at:

2 Applied Surface Science 252 (2006) Abstract Determination of energy dependent ionization probabilities of sputtered particles P. Mazarov, A.V. Samartsev, A. Wucher * Department of Physics, University of Duisburg-Essen, D Duisburg, Germany Available online 2 May 2006 We present a novel method to determine the spectral ionization probability of sputtered species as a function of their emission velocity or energy. The technique is based on detection of neutral and ionic species in a reflectron time-of-flight mass spectrometer under otherwise identical experimental conditions. Using a pulsed ion extraction scheme in combination with sufficiently short primary ion pulses, the spectral ionization probability a þ ðvþ can be determined without knowledge of possible energy discrimination effects in instrument transmission. Comparing the measured ionization probability with theoretical predictions, we find that none of the prevailing ionization models is capable of describing the experimental data over the whole velocity range studied. # 2006 Elsevier B.V. All rights reserved. Keywords: Kinetic energy distribution; Spectral ionization probability; Sputtering 1. Introduction The formation of secondary ions in sputtering is still not well understood. Various theoretical models describing the ionization of a sputtered particle have appeared in the literature (see [1]for a review). Experimental evidence needed to cross-examine the validity of the different models must concentrate on the velocity dependence of the ionization probability. A number of studies have appeared where the spectral ionization probability a þ ðvþ was determined from a measurement of the kinetic energy distribution of secondary ions alone, assuming that the energy distribution of the sputtered neutrals is known, for instance from linear cascade sputtering theory [2 4]. As pointed out several times in the literature [5,6], the latter assumption is questionable, rendering the results of such studies unreliable. Only few investigations have been conducted where secondary ions and their neutral counterparts were detected in the same experiment [7 10]. Close inspection of the procedures used in these studies (including our own previous work [5]) reveals that possible variations of instrumental parameters like sampled solid angle and velocity intervals, energy dependent ion optical transmission, etc., between secondary ions and postionized neutrals cannot safely be excluded. This is particularly true in the regime of low velocities, where problems may arise from the practically unavoidable influence of space charge in electron impact postionization methods employed to detect the sputtered neutrals. In this work, we have extended our recently developed method for determination of velocity integrated ionization probabilities [11] to the determination of emission velocity resolved data. The strength of the technique, which utilizes time-of-flight mass spectrometry in connection with laser postionization, is that neutral and ionic particles emitted from the investigated surface are detected under exactly the same experimental conditions. We present first results obtained for In atoms sputtered from a clean indium surface under bombardment with 5 kev Ar + ions. 2. Experimental * Corresponding author. Tel.: ; fax: address: wucher@uni-essen.de (A. Wucher). The experiments are performed in an ultrahigh vacuum reflectron time-of-flight mass spectrometer (TOF-MS) equipped with an excimer laser for postionization of sputtered neutrals. The methodology to obtain kinetic energy distributions of sputtered neutrals with this instrument has been described in much detail elsewhere [12]. A polycrystalline /$ see front matter # 2006 Elsevier B.V. All rights reserved. doi: /j.apsusc

3 P. Mazarov et al. / Applied Surface Science 252 (2006) indium sample is bombarded by a pulsed 5-keV Ar + ion beam. Neutral indium atoms released from the surface are postionized by single photon absorption from an intense UV laser beam (hn = 6.4 ev). The generated photoions are extracted into the TOF spectrometer by means of a pulsed HV potential which is applied to the sample about 10 ns after the firing of ionizing laser pulse. Secondary ions are detected without activating the ionization laser, leaving the remainder of the experiment unchanged. Particular care is taken that the sample potential is kept exactly at ground level before the extraction pulse is fired, ensuring field-free conditions in the extraction region. Due to the largely different signal levels of postionized neutrals and secondary ions, the former are recorded by direct digitization of the charge produced by the MCP detector, whereas the latter are detected in a single ion counting mode by digitizing and averaging the output signal of a discriminator. The absolute calibration between both detection modes is performed as described in detail elsewhere [12]. In this experiment, it is important to ensure that the postionization volume is precisely overlapped with the sensitive volume of the mass spectrometer. The latter is mapped by translating the focused laser beam in directions along and perpendicular to the surface normal. As seen in Fig. 1, the sensitive volume is centered at a distance of 7 mm above the surface and has a mean width of about 2 mm along the surface normal, determined by the time focusing properties of the reflectron [11]. Detected secondary ions are those which have evolved in the field-free space above the surface and are present in the sensitive volume at the firing time of the extraction pulse. Note that the mass spectrometer cannot distinguish between these ions and those produced by the laser. As shown in the inset of Fig. 1, the detected peaks of In + secondary ions and postionized In 0 neutrals in the TOF mass spectra look very similar but not exactly identical, since the SIMS spectrum is convoluted by the output pulse shape of the discriminator (TTL, 10 ns width). The detected emission velocity is selected by the temporal delay t between primary ion and extraction pulses, with the velocity resolution Dv=v ¼ Dr=r þ vt p =r ¼ 0:28 þ 0:014v ðkm=sþ beingdeterminedbytheappliedprimaryionpulse width (t p = 100 ns), the spatial extension of the sensitive volume (Dr = 2 mm) and the distance between that volume and the surface (r = 7 mm). Note that the probed velocity interval Dv strongly increases with increasing v, limiting the applicability of the technique to v 20 km=s. As shown in detail elsewhere [13], the measured delay time spectra S(t)are convertedintoemissionvelocityorenergydistributionsby SðtÞt f ðvþ/ Dr þðr=tþdt or f ðeþ/ SðtÞt 2 Dr þðr=tþdt with v ¼ r=t and E ¼ mv 2 =2. The first term in the denominator of Eq. (1) represents the number density of particles in the sensitive volume at the firing time of the ion extraction pulse, while the second term arises from the flux of particles into that volume during the rise time Dt of that pulse (25 ns). The spectral ionization probability a þ ðvþ is determined directly from the ratio between the signals measured with and without the ionizing laser. If the postionization probability is unity across the entire sensitive volume of the mass spectrometer, this determination yields the correct absolute value of a + regardless of the Jacobian transformation (Eq. (1)). We stress that the determination of a þ ðvþ is independent of (i) possible energy discrimination effects arising from velocity dependent instrument transmission and (ii) the variation of Dv across the investigated velocity interval. 3. Results and discussion Fig. 2 shows the emission energy spectra of sputtered neutral indium atoms and the respective singly positively charged secondary ions as evaluated from Eq. (1). In order to examine possible energy discrimination effects of our instrument, we compare the neutral spectrum with the well-known prediction of (1) Fig. 1. Signal of postionized sputtered neutral indium atoms vs. distance between ionization laser and surface. Inset: TOF spectrum of postionized neutrals and secondary ions. All data refer to a delay time of t =3ms corresponding to v ¼ 2:3km=s or E = 3.25 ev. Fig. 2. Emission energy distribution of neutral In atoms and In + secondary ions sputtered from a pure indium surface under bombardment with 5 kevar + ions. Solid line: least squares fit of Eq. (2) with U 0 = 2.2 ev.

4 6454 P. Mazarov et al. / Applied Surface Science 252 (2006) linear cascade theory [14]: E f ðeþ/ ðe þ U 0 Þ 3 (2) using U 0 as a fit parameter. The corresponding fit to the neutral spectrum (solid line) results in a value of U 0 = 2.1 ev, which is reasonably close to the sublimation energy of indium (2.5 ev) [8]. The deviations observed both at low and high energies might in principle be induced by instrumental artifacts, but also indicate that the magnitude of such effects is probably not very large in the present case. The resulting ionization probability a + In as a function of emission velocity is shown in Fig. 3. The horizontal error bars are derived from the calculation of Dv as described above, the vertical error bars denote the standard deviation within a set of several independent measurements with varying settings of the TOF spectrometer, leading to different instrument transmission functions and, hence, different shapes of the velocity distributions shown in Fig. 2. Three distinct velocity regimes can be discerned in Fig. 3.In the range 2 10 km/s, we find a relatively strong variation of a + In, whereas the dependence is much weaker both below and above that range. The measured data can be compared to published theories of secondary ion formation, reviews of which can be found in Refs. [1,15,16]. In many of the prevailing ionization models, the velocity dependence of a + is described by the relation [16]: a þ / exp v 0 v? where v? denotes the normal component of the particle emission velocity and v 0 contains microscopic parameters describing the electronic interaction between the outgoing atom and the solid surface. If we interpret our data in terms of Eq. (3), we find v 0 6km=s in the medium velocity range [straight line fit in (Fig. 3a)]. This value compares well with data obtained for a number of atoms in experiments similar to that performed here [5,8 10]. The deviation from Eq. (3) at low velocities has been (3) observed before [5,9,10] and can in part be interpreted in terms of the particle being slowed down by the image force after ionization [1]. A another published explanation assumes the superposition of a second, velocity independent ionization mechanism [17,18]. A deviation from the prediction of Eq. (3) is also observed for high velocities. This fact becomes even more obvious if the data are extrapolated to infinite velocity, yielding a þ ðv!1þ In contrast, all published theories predict the proportionality factor in Eq. (3) to be of the order of unity. A weaker velocity dependence than predicted by Eq. (3) can be rationalized in terms of a localized kinetic electronic excitation of the solid [15 19]. Parametrizing the latter by an effective electronic temperature T e, the velocity dependence of the ionization probability can be described as a þ / v n [15,16,19], the power n being essentially determined by the value of T e. The straight line fit in (Fig. 3b) reveals n 1.3 in the medium velocity range, which would be consistent with T e 2500 K. At present, there is no published explanation for the break towards a weaker velocity dependence in the limit of high velocities. 4. Conclusion A method to determine the spectral ionization probability of sputtered particles is described. The particular strength is that sputtered secondary ions and their neutral counterparts are detected under exactly the same experimental conditions with respect to instrumental parameters like the detected solid angle and velocity intervals, velocity dependent ion optical acceptance and transmission, etc. Hence, possible instrumental artifacts influencing the measurement of velocity distributions are identical for secondary ions and postionized neutrals and therefore cancel in the determination of the ionization probability. From our perspective, the results presented here comprise the first set of experimental ionization probability data which is reliable in the velocity range below approximately 2 km/s (corresponding to kinetic emission energies below about 6 ev). Comparing the measured data with theoretical predictions, we find that none of the prevailing ionization models is capable to describe the experimental data in the whole velocity range studied. Acknowledgment The authors would like to thank Dr. Klaus Wittmaack for valuable comments and discussions. References Fig. 3. Spectral ionization probability of In atoms sputtered from a clean, polycrystalline indium surface under bombardment with 5 kev Ar + ions under 458 incidence. [1] M.L. Yu, in: R. Behrisch, K. Wittmaack (Eds.), Sputtering by Particle Bombardment III, Springer, Berlin, 1991, p. 91. [2] M. Vasile, Nucl. Instrum. Meth. B 40/41 (1988) 282. [3] M.J. Vasile, Phys. Rev. B 29 (1984) [4] I.F. Urazgil din, Nucl. Instrum. Meth. B 78 (1993) 271. [5] A. Wucher, H. Oechsner, Surf. Sci. 199 (1988) 567. [6] K. Wittmaack, Surf. Sci. 429 (1999) 84. [7] A. Wucher, H. Oechsner, in: A.D. Romig, Jr., W.F. Chambers (Eds.), Proceedings of the 21st Annual Conference of the Microbeam Analysis Society 1986, San Francisco Press, 1986.

5 P. Mazarov et al. / Applied Surface Science 252 (2006) [8] T.R. Lundquist, Surf. Sci. 90 (1979) 548. [9] B.N. Makarenko, A.B. Popov, A.A. Shaporenko, A.P. Shergin, Rad. Eff. 113 (1990) 263. [10] A.B. Popov, S.B. Kablukov, B.N. Makarenko, A.P. Shergin, Nucl. Instrum. Meth. B 78 (1993) 290. [11] A. Wucher, R. Heinrich, C. Staudt, in: A. Benninghoven, P. Bertrand, H.N. Migeon, H.W. Werner (Eds.), Secondary Ion Mass Spectrometry (SIMS XII), Elsevier Science, 2000, p [12] M. Wahl, A. Wucher, Nucl. Instrum. Meth. B 94 (1994) 36. [13] A. Wucher, M. Wahl, H. Oechsner, Nucl. Instrum. Meth. B 82 (1993) 337. [14] M.W. Thompson, Philos. Mag. 18 (1968) 377. [15] Z. Sroubek, Spectrochim. Acta 44B (1989) 317. [16] Z. Sroubek, J. Lorincik, Surf. Rev. Lett. 6 (1999) 257. [17] D. Klushin, V.M.Y. Gusev, S.A. Lysenko, I.F. Urazgildin, Phys. Rev. B 54 (1996) [18] E.Yu. Usman, Yu.T. Matulevich, I.F. Urazgil din, Vacuum 56 (2000) 293. [19] Z. Sroubek, Nucl. Instrum. Meth. 194 (1982) 533.

Formation of large clusters during sputtering of silver

Nuclear Instruments and Methods in Physics Research B 164±165 (2000) 677±686 www.elsevier.nl/locate/nimb Formation of large clusters during sputtering of silver C. Staudt, R. Heinrich, A. Wucher * Fachbereich