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Energy spectra of sputtered positive ions under Cs+ bombardment
Nuclear Instruments and Methods in Physics Research B 269 (2011) 990–994
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Energy spectra of sputtered positive ions under Cs+ bombardment Hubert Gnaser Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, D-67663 Kaiserslautern, Germany Institute for Surface and Thin-Film Analysis IFOS, Trippstadter Str. 120, D-67663 Kaiserslautern, Germany
a r t i c l e
i n f o
Article history: Received 2 August 2010 Received in revised form 10 November 2010 Available online 17 December 2010 Keywords: Energy spectra Sputtered ions Cs+ bombardment
a b s t r a c t The emission-energy spectra of atomic and molecular secondary ions sputtered from various metals and semiconductors (Al, Cu, In, Si, InP, and InSb) under 5.5-keV Cs+ irradiation were investigated. The emitted ions were detected in a high-sensitivity double-focusing secondary-ion mass spectrometer. Specifically, + + the energy distributions of Cs+, Csþ 2 , MCs , and M ions (where M designates one of the target elements) were recorded for emission energies E 6 125 eV. All ion species exhibit a peak at low energy (E < 5 eV), but differ significantly in the respective fall-off to high emission energies. The influence of the oxygen partial pressure in the vicinity of the sputtered surface on the energy spectra was examined for Cs+ ions emitted from Si. With an increase of the ratio r of the O2 flux to the Cs+ flux, the spectra shift to higher emission-energy values, with the total shift amounting to 0.45 eV at a value of r 3.3. Concurrently, the intensity of Cs+ increases by 30%. The measured emission distributions of Cs+ ions from different samples were compared with the predictions of the electron-tunneling model of secondary-ion formation. It is found that the experimental spectra can be reproduced quite well when employing specific sets of parameters in that theoretical concept. The possible limitations of such a comparison are discussed. Ó 2011 Published by Elsevier B.V.
1. Introduction The irradiation of solids by energetic particles gives rise to the emission of atoms and molecules from the surface, a process usually termed sputtering [1]. The spectral distributions in terms of the (kinetic) emission energy and the emission angle of these sputtered species reflect, to some degree, the atomistic processes occurring during the dissipation of the projectile’s energy in the solid and in the sputtering event proper. Hence, from the energy and angular distributions of sputtered species information about the collision processes might be derived [2]. Although the sputtered flux is composed primarily of neutral species, positively and negatively ionized atoms and molecules are observed; their number (i.e., the ionization probability) can be strongly enhanced by using + reactive bombarding ions, such as Oþ 2 and Cs . This feature is extensively used in secondary-ion mass spectrometry (SIMS) [3] for the sensitive analysis of surfaces and thin films. Unfortunately, the mechanisms leading to the ionization of sputtered atoms and molecules are only poorly understood [4,5]. Using, for example, Cs+ primary bombarding ions, the near-surface region of a sample will be loaded with Cs which, in turn, causes a lowering of the surface work function U within the irradiated area [6–8]. This reduction in work function increases negative ion yields (sometimes by several orders of magnitude) while it decrease the yields of positive
E-mail address: [email protected] 0168-583X/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.nimb.2010.12.050
ions, but the theoretical description of these phenomena is still rather incomplete [9]. In an attempt to improve this situation, in the present work the energy spectra of positive atomic and molecular ions sputtered from a variety of metal and semiconductors under Cs+ bombardment were measured. There were several reasons to do this: (i) as mentioned above, Cs+ ions are widely used in SIMS analyses and the ion-enhancement effected in this way is of utmost importance in that technique. (ii) The loading of the surface with Cs leads to a very high yield of emitted Cs+ (because of its low ionization potential) and, in addition, to the emission of a substantial amount of molecular ions carrying one or more Cs-atoms; some of these are also of analytical relevance [10,11]. (iii) Energy spectra of such ions may provide (direct) information on ionization processes as most theoretical concepts predict some kind of energy- (velocity-) dependence of the ionization probability of sputtered species. + + Therefore, energy spectra of Cs+, Csþ 2 , MCs , and M ions (where M designates one of the target elements) were recorded for emission energies E 6 125 eV, using 5.5 keV Cs+ bombarding ions. For a number of samples the useful yields (i.e., the product of ionization probability and instrument transmission) of some of those ionic species have been reported in previous publications [12–14]. 2. Experimental The experiments were performed in a secondary-ion mass spectrometer (Cameca IMS-4f [15]). Sputtering was done with Cs+ ions
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H. Gnaser / Nuclear Instruments and Methods in Physics Research B 269 (2011) 990–994
10
6
Si 10
5
Cs
intensity (counts/s)
10
10
10
10
10
Cs 2
4
SiCs + Si
2
1
0
50
3.1. Energy spectra of Cs ,
100
emission energy (eV) 10
6
InP 10
5
Cs 10
10
10
10
+ Cs 2 + PCs +
+
InCs + In
4
+
P
3
2
1
0
0
50
3. Results and discussion Csþ 2,
+
3
0
10
+
+ +
intensity (counts/s)
produced in a surface-ionization source. With the source at a potential of +10 kV with respect to ground and the sample at +4.5 kV, the primary impact energy was 5.5 keV, at an incidence angle of 40° relative to the surface normal. The ion current was 20 nA and the beam was raster-scanned across areas between (125 lm 125 lm) and (500 lm 500 lm). Sputtered positive ions were collected from a circular area (centered within the raster) with a diameter ranging from 8 lm to 60 lm. The instrument incorporates a double-focusing mass spectrometer consisting of an electrostatic sector followed by a magnetic sector field, both with a 90° deflection angle. The electrostatic sector is a spherical condenser with an energy dispersion DE = 17.3 cm, whereas the magnetic sector features a homogeneous magnetic field with inclined field boundaries. At the crossover of the secondary beam in a plane intermediate between the two sectors an energy-selecting slit is located; by reducing the width of this slit, the secondary ions’ energy bandwidth DE is reduced and the smallest value observed experimentally amounted to DE 2 eV. Detection of secondary ions was done by a discrete-dynode electron multiplier and count rates were limited to <1 MHz to avoid saturation of the detector. As in the author’s previous studies on the energy distributions of negative sputtered ions [7,16–18], energy spectra in the present work were recorded by scanning the target potential by ±125 V around its nominal value of 4.5 kV (with a minimal step width of 1 V), while keeping the other optical elements of the secondaryion beam line unchanged. With the width of energy slit being reduced, only ions whose original emission energy and the kinetic energy gained by the acceleration from the actual sample potential add up to a constant energy E0 = 4500 eV can pass the slit and be detected. To improve the resolution in the peak region of the spectra, a reduced scan of ±12.5 V (minimal step width of 0.1 V) was used in that energy regime. The specimens used were polycrystalline metal samples of Al, Cu and In, and single-crystal wafers of Si, InP and InSb; the latter can be expected to be amorphized in the near-surface region due to ion irradiation. Upon insertion into the instrument their surfaces were cleaned by extensive sputtering. Hence, the measurements reported in the following refer to steady-state sputtering: under these conditions the surface region is loaded with cesium due to implantation and concurrent erosion and this presence of Cs results in a pronounced lowering of the work function within the irradiated area.
100
emission energy (eV) +
+
MCs , and M ions
+ + Representative energy spectra of Cs+, Csþ 2 , MCs , and M ions from two samples (Si and InP) are depicted in Fig. 1. These refer to the data as obtained, i.e., without any correction for a possible energy-dependent transmission of the instrument (see below). Although the spectra of all ion species exhibit a peak at low energy (E < 5 eV), the respective fall-off to high emission energies show clear variations. The differences between atomic and molecular ions can be ascribed to the fact that the latter may not survive the emission event and undergo fragmentation if their kinetic energy is too high. The specific shape (peak position and width) of the spectrum for a given ion emitted from different specimens may also vary considerably. This observation is illustrated more clearly in Fig. 2 which shows normalized spectra of Cs+ ions sputtered from various targets. Principally, the variations in the peak position and the spectral width could be caused by several distinct processes: (i) the binding energies of Cs ions may vary for these materials and result in shifts of the peak of the energy spectra. (ii) The variations might
+
Csþ 2,
Fig. 1. Energy spectra of Cs , MCs+, and M+ secondary ions sputtered from Si (upper panel) and InP (lower panel) under 5.5 keV Cs+ ion bombardment. All data refer to steady-state conditions.
be ascribed to differences in the surface work function U of those materials under Cs+ irradiation. It will be discussed below that the actual magnitude of U can change the energy spectra of ions in a dramatic way [19,20]. Only in a few cases the values (or changes) of U due to dynamic Cs+ sputtering have been determined experimentally: Cs+ bombardment under conditions essentially identical to the present ones leads to a work-function reduction of DU = 1.3 eV in Si [8]. For somewhat different bombarding conditions, DU-reductions of 2.5 eV, 2.2 eV and 1.3 eV were reported in Al, Si and Ni, respectively [21]. For much lower Cs+ bombarding energies (250–750 eV), the lowering of the work function of Si was more pronounced, DU 2.5–3.5 eV [22,23]. (iii) Finally, velocity(or energy-) dependent ionization processes could produce the variance of the Cs+ energy distributions shown in Fig. 2; this aspect will be discussed below in more quantitative terms.
H. Gnaser / Nuclear Instruments and Methods in Physics Research B 269 (2011) 990–994
1.0 Si Cu InP InSb
0.8
+ normalized Cs intensity
+ normalized Cs intensity
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0.4
Si
normalized Cs+ intensity
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0.6
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-1
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emission energy (eV)
j(O ) / j(Cs+ ) 2 < 0.0003 0.01 0.034 0.17
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emission energy (eV)
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emission energy (eV)
Si
A recent investigation [24] of the influence of oxygen on the ion emission under low-energy Cs+ irradiation of Si indicated that oxygen adsorbed onto the silicon surface may provide an attachment site for impinging cesium ions, thereby increasing the retention capacity of cesium considerably and changing secondary ion yields. In fact, an enhancement of the Cs+ yield from Si bombarded at an elevated O2 pressure was observed already some time ago [13]. To examine these processes in more detail, energy spectra of Cs+ ions emitted from Si were recorded at different oxygen partial pressures. Fig. 3 displays these spectra; the parameter is the ratio of the nominal O2 flux impinging onto the surface to the flux of Cs+ primary ions, r = j(O2)/j(Cs+). The data show that with increasing r the spectra tend to shift to higher energies (see inset). Fig. 3 depicts this shift as a function of r: a total shift of 0.45 eV for r 3.3 is found. Also shown is the variation of the Cs+ intensity which is found to increase by about 30% in the range of O2 pressure investigated. (The values of j(O2) at the sample surface might actually been higher than indicated because O2 is delivered to the analysis chamber via a capillary that ends in close vicinity to sample.) When enquiring into possible reasons for the observed shift of the energy spectra, two effects can be envisaged: (i) the shift with increasing j(O2) is due to a lowering of the surface work function; however, a reduction of U can be expected to decrease the yield of positive ions, as found previously[8], while the Cs+ intensity increases in the present experiment (cf. Fig. 3). (ii) The addition of oxygen may increase the surface binding energy because the CsAO bond strength is much stronger than that of CsACs bonding [25]; a higher surface binding energy will shift the energy distributions of all sputtered species to higher energies, irrespective of their charge state [2]. It is noted that a very similar change in work function (0.44 eV) has been observed in 5 keV Cs+ bombardment of Ag under an elevated oxygen pressure [26]; however, in that case the Cs+ ion intensity was decreasing with increasing O2 flux. 3.3. Comparison of energy spectra with theoretical ion-formation models Several distinct ionization schemes have been proposed [4,27] to describe the ionization process of sputtered ions. The so-called
normalized Cs+ intensity
3.2. Variation of Cs+ energy spectra with O2 partial pressure
1.0
1.0
Cs+ intensity shift of spectra
0.5
0.5
0.0
0.0
10
shift of energy spectra (eV)
Fig. 2. Normalized energy spectra of Cs+ secondary ions sputtered from different samples under 5.5 keV Cs+ ion bombardment. All data refer to steady-state conditions.
-4
10
-3
10
-2
10
-1
10
0
10
1
+ j(O2) / j(Cs ) Fig. 3. Normalized Cs+ energy spectra for different ratios of the O2 and Cs+ arrival rates, j(O2)/j(Cs+), at the surface (upper panel) and the onset region of the spectra (inset). The lower panel displays the measured intensity of Cs+ and the shift of the energy spectra as a function of j(O2)/j(Cs+).
electron-tunneling model [28,29] envisages the electronic transition as a resonant electron transfer process between a sputtered atom and the valence band of the solid. For positively charged sputtered ions, P+ can be approximated as
Pþ / exp½ðI UÞ=0 ;
ð1Þ
I being the ionization potential of the sputtered ion, U the work function of the surface, and e0 is proposed to be proportional to the component of the ion’s emission velocity perpendicular to the surface [30]: 0 / avn = avcosh, with a being a parameter that depends on the specific sample material and h the ions’ (polar) emission angle. The validity and limitations of this theoretical approach have been critically discussed in a recent review [9]. In the following, a comparison of the measured energy spectra with the predictions of the electron-tunneling model will be attempted. In order to do this, two additional aspects have to be addressed, namely (i) the energy distribution of the total sputtered flux (mostly neutrals), a part of which may eventually be ionized, and (ii) the energy-dependent instrumental transmission, required for an appropriate correction of the measured spectra.
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YðEÞ / E=ðE þ UÞ3 :
ð2Þ
In fact, this equation has been widely used in comparisons with experimental energy spectra of neutral atoms and a rather good agreement has been found in many cases[2]. The emission-energy and angle dependence of the transmission T(E, h) of the SIMS instrument used in this work is largely determined by the extraction field and the lenses which transfer the sputtered ions from the surface into the spectrometer. All ions with an energy smaller than a critical energy Ec will pass the spectrometer (i.e., T < 1); for ions with E > Ec, T < 1 and only those ions that are emitted from the surface with an angle smaller than a cut-off angle hc are transmitted. The magnitude of Ec and hc depends on specific parameters of the instrument and, for hc, also on E [35]. The critical energy is
Ec ¼ g2 E0 ;
ð3Þ
with g ¼ d=8Dg and the pass energy of the ions E0 = 4500 eV. Here, d is the diameter of the ‘‘contrast aperture’’, located in the beam’s crossover plane in front of the energy sector, D is the accelerating gap in front of the sample and g is the cross-over magnification. In the present work, the numerical values of these parameters were d = 150 lm, D = 5 mm, and g = 0.28 [36]. Hence, g = 1.34 102 and Ec = 0.8 eV. The cut-off angle depends on the ion’s emission energy [35].
1=2 E0 sin hc ¼ g : 1 E
ð4Þ
Eq. (4) implies that the higher the ion energy the smaller is the acceptance cone for ions to be transmitted through the spectrometer. For example, for values of E = 3, 5, 10, and 50 eV the cut-off angle decreases as hc = 31.2°, 23.7°, 16.5°, and 7.3°, respectively. The magnitude of hc is of relevance for the determination of the normal emission velocity vn = vcosh of sputtered ions which, in turn, influences the ionization probability P+ (see Eq. (1)); for the hc values given above, cosh is reasonably close to unity (cosh = 0.86, 0.92, 0.96, and 0.99) and it appears, therefore, feasible that, at least in the range E P 5 eV, the emission energy E can used to compute vn, v n ¼ ð2E=MÞ1=2 cos h / E1=2 . For a typical angular emission distribution of the sputtering yield, Y(h) / cos h [32], it was noted [20,35] that for E > Ec the transmission of the present instrument scales as T / Ec/E E1. Its transmission has recently been computed also utilizing an ion optics simulation program [37] and T(E) / E1 was found, largely invariant for a considerable range of operating parameters. Hence, that relation was employed to correct the measured spectra for the energy-dependence of the transmission. With the approximation discussed above, the energy-dependent yield of positive ions Y+ would then be given by the product of Eqs. (1) and (2), with 0 / avn.
IU exp av n ðE þ UÞ E IU / exp pffiffiffi : a E ðE þ UÞ3
Y þ ðEÞ ¼ YðEÞPþ ðEÞ /
E
3
ð5Þ
1.0
normalized Cs+ yield
The energy distribution of the sputtered species can be described quite satisfactorily by the predictions of the analytical sputtering theory of Sigmund [31]. This and a closely related approach [32,33] result in an approximate energy-dependence of the yield Y(E) of sputtered atoms that scales with the surface binding energy of the released atoms U [32], YðEÞ / E=ðE þ UÞ32m , where the parameter m in the exponent characterizes the interatomic potential employed in the analytical theory [32]. The value of m has been shown [34] to be close to unity for the low energies involved in sputtered atoms and m = 0 is chosen here for further computations. Then, the energy-dependence of the sputtering yield is given by
Theory Y+=E/(E+U)3 exp[–(I–φ)/aE1/2] U = 2 eV, a = 0.1 eV1/2 I–φ (eV) 0.75 1.2 1 1.5
0.8
0.6
Experiment Si Cu InP InSb
0.4
0.2
0.0 0
10
20
30
40
emission energy (eV) Fig. 4. Comparison of experimental and theoretical Cs+ energy spectra. The experimental data (open symbols) are corrected for the instrumental transmission T (T / E1). The theoretical distributions (solid lines) are computed with Eq. (5), using different values of I–U, and U = 2 eV and a = 0.1 eV1/2.
This expression can be compared with the transmission-corrected experimental energy spectra. Fig. 4 shows such a comparison displaying the measured spectra of Cs+ sputtered from different targets (see Fig. 2) corrected for the transmission and spectra computed via Eq. (5); for the latter U = 2 eV, a = 0.1 eV1/2 and different values of I–U (0.75, 1, 1.2, and 1.5 eV) were used. Since the ionization potential of Cs is ICs = 3.89 eV, those would correspond to an actual surface work function of 2.39, 2.69, 2.89, and 3.14 eV, respectively. It can be seen that with this range of values for U the experimental spectra of the different samples can be reproduced quite reasonably. Furthermore, it was found that such an agreement can be produced also by a variation of the parameter a in Eq. (5) while keeping I–U constant. An uncertainty relates to the surface binding energy U which is not known, but may be different for different materials loaded with Cs. It is envisaged that the application of this approach to other ion species (e.g., the atomic ions sputtered for the various samples) could corroborate the selection of a distinct set of parameters in Eq. (5) (U, a, and U). Then, more detailed information might be extracted from the comparison of the measured spectra with the outcome of Eq. (5); eventually, this could indicate if the latter constitutes a suitable theoretical representation of the energy distributions of sputtered ions. 4. Conclusions The energy spectra of atomic and molecular ions sputtered from a variety of metals and semiconductors under Cs+ bombardment illustrate the wide variance of emission distributions for Cs+, Csþ 2, MCs+, and M+ ions. Although all spectra peak at low energies (a few eV), their high-energy behavior is distinctly different. In particular the atomic M+ secondary ions exhibit a very gradual fall-off towards high emission energies. An increased oxygen partial pressure tends to shift the spectra of Cs+ emitted from Si to higher energies and slightly enhances the Cs+ intensity. To establish the significance of this finding, similar measurements should be carried out for other ion species and additional specimens. The measured Cs+ spectra from various samples can be reproduced very reasonably by means of the predictions of a theoretical ion-forma-
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tion model; an extension of this approach to other secondary-ion species will be useful for the specification of the parameters entering that model. References [1] P. Sigmund (Ed.), Fundamental Processes in Sputtering of Atoms and Molecules, K. Dan. Vidensk. Selsk. Mat. Fys. Medd., vol. 43, 1993. [2] H. Gnaser, in: R. Behrisch, W. Eckstein (Eds.), Sputtering by Particle Bombardment, Springer, Berlin, 2007, p. 231. [3] A. Benninghoven, F.G. Rüdenauer, H.W. Werner, Secondary Ion Mass Spectrometry, Wiley, New York, 1987. [4] M.L. Yu, in: R. Behrisch, K. Wittmaack (Eds.), Sputtering by Particle Bombardment, vol. III, Springer, Berlin, 1991, p. 91. [5] A. Wucher, Appl. Surf. Sci. 255 (2008) 1194. [6] M. Bernheim, F. Le Bourse, Nucl. Instrum. Methods B 27 (1987) 94. [7] H. Gnaser, Phys. Rev. B 54 (1996) 16456. [8] H. Gnaser, Phys. Rev. B 54 (1996) 17141. [9] K. Wittmaack, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 52 (2006) 465. [10] Y. Gao, J. Appl. Phys. 64 (1988) 3760. [11] H. Gnaser, H. Oechsner, Surf. Interface Anal. 17 (1991) 646. [12] H. Gnaser, H. Oechsner, Surf. Interface Anal. 21 (1994) 257. [13] M. Haag, H. Gnaser, H. Oechsner, Fresenius J. Anal. Chem. 353 (1995) 565. [14] H. Gnaser, in: G. Gillen, R. Lareau, J. Bennett, F. Stevie (Eds.), Secondary Ion Mass Spectrometry SIMS XI, Wiley, Chichester, 1998, p. 915. [15] H.N. Migeon, C. Le Pipec, J.J. Le Goux, in: A. Benninghoven, R.J. Colton, D.S. Simons, H.W. Werner (Eds.), Secondary Ion Mass Spectrometry SIMS V, Springer, Berlin, 1986, p. 155.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
H. Gnaser, Nucl. Instrum. Methods B 149 (1999) 38. H. Gnaser, Nucl. Instrum. Methods B 164–165 (2000) 705. H. Gnaser, Phys. Rev. B 63 (2001) 045415. K. Wittmaack, Phys. Scr. T6 (1983) 71. K. Wittmaack, Surf. Sci. 429 (1999) 84. T. Wirtz, H.-N. Migeon, H. Scherrer, Int. J. Mass Spectrom. 225 (2003) 135. J. Brison, N. Mine, S. Poisseroux, B. Douhard, R.G. Vitchev, L. Houssiau, Surf. Sci. 601 (2007) 1467. J. Brison, J. Guillot, B. Douhard, R.G. Vitchev, H.-N. Migeon, L. Houssiau, Nucl. Instrum. Methods B 267 (2009) 519. B. Berghmans, W. Vandervorst, J. Appl. Phys. 106 (2009) 033509. B. Berghmans, B. Van Daele, L. Geenen, T. Conard, A. Franquet, W. Vandervorst, Appl. Surf. Sci. 255 (2008) 1316. B. Saha, S. Sarkar, P. Chakraborty, H. Gnaser, Surf. Sci. 602 (2008) 1061. P. Williams, Surf. Sci. 90 (1979) 588. J.K. Nørskov, B.I. Lundqvist, Phys. Rev. B 19 (1979) 5661. M.L. Yu, N.D. Lang, Phys. Rev. Lett. 50 (1983) 127. N.D. Lang, Phys. Rev. B 27 (1983) 2019. P. Sigmund, Phys. Rev. 184 (1969) 383. P. Sigmund, in: R. Behrisch (Ed.), Sputtering by Particle Bombardment, vol. I, Springer, Berlin, 1981, p. 9. M.W. Thompson, Phys. Rep. 69 (1981) 335. M. Vicanek, J.J. Jimenéz-Rodríguez, P. Sigmund, Nucl. Instrum. Methods B 36 (1989) 124. G. Slodzian, in: A. Septier (Ed.), Applied Charge Particle Optics, Academic, New York, 1980, p. 1. M. Lepareur, Rev. Technol. Thomson-CSF 12 (1980) 225. J. Lörincik, K. Franzreb, P. Williams, Appl. Surf. Sci. 231–232 (2004) 921.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Energy spectra of sputtered positive ions under Cs+ bombardment Hubert Gnaser Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, D-67663 Kaiserslautern, Germany Institute for Surface and Thin-Film Analysis IFOS, Trippstadter Str. 120, D-67663 Kaiserslautern, Germany
a r t i c l e
i n f o
Article history: Received 2 August 2010 Received in revised form 10 November 2010 Available online 17 December 2010 Keywords: Energy spectra Sputtered ions Cs+ bombardment
a b s t r a c t The emission-energy spectra of atomic and molecular secondary ions sputtered from various metals and semiconductors (Al, Cu, In, Si, InP, and InSb) under 5.5-keV Cs+ irradiation were investigated. The emitted ions were detected in a high-sensitivity double-focusing secondary-ion mass spectrometer. Specifically, + + the energy distributions of Cs+, Csþ 2 , MCs , and M ions (where M designates one of the target elements) were recorded for emission energies E 6 125 eV. All ion species exhibit a peak at low energy (E < 5 eV), but differ significantly in the respective fall-off to high emission energies. The influence of the oxygen partial pressure in the vicinity of the sputtered surface on the energy spectra was examined for Cs+ ions emitted from Si. With an increase of the ratio r of the O2 flux to the Cs+ flux, the spectra shift to higher emission-energy values, with the total shift amounting to 0.45 eV at a value of r 3.3. Concurrently, the intensity of Cs+ increases by 30%. The measured emission distributions of Cs+ ions from different samples were compared with the predictions of the electron-tunneling model of secondary-ion formation. It is found that the experimental spectra can be reproduced quite well when employing specific sets of parameters in that theoretical concept. The possible limitations of such a comparison are discussed. Ó 2011 Published by Elsevier B.V.
1. Introduction The irradiation of solids by energetic particles gives rise to the emission of atoms and molecules from the surface, a process usually termed sputtering [1]. The spectral distributions in terms of the (kinetic) emission energy and the emission angle of these sputtered species reflect, to some degree, the atomistic processes occurring during the dissipation of the projectile’s energy in the solid and in the sputtering event proper. Hence, from the energy and angular distributions of sputtered species information about the collision processes might be derived [2]. Although the sputtered flux is composed primarily of neutral species, positively and negatively ionized atoms and molecules are observed; their number (i.e., the ionization probability) can be strongly enhanced by using + reactive bombarding ions, such as Oþ 2 and Cs . This feature is extensively used in secondary-ion mass spectrometry (SIMS) [3] for the sensitive analysis of surfaces and thin films. Unfortunately, the mechanisms leading to the ionization of sputtered atoms and molecules are only poorly understood [4,5]. Using, for example, Cs+ primary bombarding ions, the near-surface region of a sample will be loaded with Cs which, in turn, causes a lowering of the surface work function U within the irradiated area [6–8]. This reduction in work function increases negative ion yields (sometimes by several orders of magnitude) while it decrease the yields of positive
E-mail address: [email protected] 0168-583X/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.nimb.2010.12.050
ions, but the theoretical description of these phenomena is still rather incomplete [9]. In an attempt to improve this situation, in the present work the energy spectra of positive atomic and molecular ions sputtered from a variety of metal and semiconductors under Cs+ bombardment were measured. There were several reasons to do this: (i) as mentioned above, Cs+ ions are widely used in SIMS analyses and the ion-enhancement effected in this way is of utmost importance in that technique. (ii) The loading of the surface with Cs leads to a very high yield of emitted Cs+ (because of its low ionization potential) and, in addition, to the emission of a substantial amount of molecular ions carrying one or more Cs-atoms; some of these are also of analytical relevance [10,11]. (iii) Energy spectra of such ions may provide (direct) information on ionization processes as most theoretical concepts predict some kind of energy- (velocity-) dependence of the ionization probability of sputtered species. + + Therefore, energy spectra of Cs+, Csþ 2 , MCs , and M ions (where M designates one of the target elements) were recorded for emission energies E 6 125 eV, using 5.5 keV Cs+ bombarding ions. For a number of samples the useful yields (i.e., the product of ionization probability and instrument transmission) of some of those ionic species have been reported in previous publications [12–14]. 2. Experimental The experiments were performed in a secondary-ion mass spectrometer (Cameca IMS-4f [15]). Sputtering was done with Cs+ ions
991
H. Gnaser / Nuclear Instruments and Methods in Physics Research B 269 (2011) 990–994
10
6
Si 10
5
Cs
intensity (counts/s)
10
10
10
10
10
Cs 2
4
SiCs + Si
2
1
0
50
3.1. Energy spectra of Cs ,
100
emission energy (eV) 10
6
InP 10
5
Cs 10
10
10
10
+ Cs 2 + PCs +
+
InCs + In
4
+
P
3
2
1
0
0
50
3. Results and discussion Csþ 2,
+
3
0
10
+
+ +
intensity (counts/s)
produced in a surface-ionization source. With the source at a potential of +10 kV with respect to ground and the sample at +4.5 kV, the primary impact energy was 5.5 keV, at an incidence angle of 40° relative to the surface normal. The ion current was 20 nA and the beam was raster-scanned across areas between (125 lm 125 lm) and (500 lm 500 lm). Sputtered positive ions were collected from a circular area (centered within the raster) with a diameter ranging from 8 lm to 60 lm. The instrument incorporates a double-focusing mass spectrometer consisting of an electrostatic sector followed by a magnetic sector field, both with a 90° deflection angle. The electrostatic sector is a spherical condenser with an energy dispersion DE = 17.3 cm, whereas the magnetic sector features a homogeneous magnetic field with inclined field boundaries. At the crossover of the secondary beam in a plane intermediate between the two sectors an energy-selecting slit is located; by reducing the width of this slit, the secondary ions’ energy bandwidth DE is reduced and the smallest value observed experimentally amounted to DE 2 eV. Detection of secondary ions was done by a discrete-dynode electron multiplier and count rates were limited to <1 MHz to avoid saturation of the detector. As in the author’s previous studies on the energy distributions of negative sputtered ions [7,16–18], energy spectra in the present work were recorded by scanning the target potential by ±125 V around its nominal value of 4.5 kV (with a minimal step width of 1 V), while keeping the other optical elements of the secondaryion beam line unchanged. With the width of energy slit being reduced, only ions whose original emission energy and the kinetic energy gained by the acceleration from the actual sample potential add up to a constant energy E0 = 4500 eV can pass the slit and be detected. To improve the resolution in the peak region of the spectra, a reduced scan of ±12.5 V (minimal step width of 0.1 V) was used in that energy regime. The specimens used were polycrystalline metal samples of Al, Cu and In, and single-crystal wafers of Si, InP and InSb; the latter can be expected to be amorphized in the near-surface region due to ion irradiation. Upon insertion into the instrument their surfaces were cleaned by extensive sputtering. Hence, the measurements reported in the following refer to steady-state sputtering: under these conditions the surface region is loaded with cesium due to implantation and concurrent erosion and this presence of Cs results in a pronounced lowering of the work function within the irradiated area.
100
emission energy (eV) +
+
MCs , and M ions
+ + Representative energy spectra of Cs+, Csþ 2 , MCs , and M ions from two samples (Si and InP) are depicted in Fig. 1. These refer to the data as obtained, i.e., without any correction for a possible energy-dependent transmission of the instrument (see below). Although the spectra of all ion species exhibit a peak at low energy (E < 5 eV), the respective fall-off to high emission energies show clear variations. The differences between atomic and molecular ions can be ascribed to the fact that the latter may not survive the emission event and undergo fragmentation if their kinetic energy is too high. The specific shape (peak position and width) of the spectrum for a given ion emitted from different specimens may also vary considerably. This observation is illustrated more clearly in Fig. 2 which shows normalized spectra of Cs+ ions sputtered from various targets. Principally, the variations in the peak position and the spectral width could be caused by several distinct processes: (i) the binding energies of Cs ions may vary for these materials and result in shifts of the peak of the energy spectra. (ii) The variations might
+
Csþ 2,
Fig. 1. Energy spectra of Cs , MCs+, and M+ secondary ions sputtered from Si (upper panel) and InP (lower panel) under 5.5 keV Cs+ ion bombardment. All data refer to steady-state conditions.
be ascribed to differences in the surface work function U of those materials under Cs+ irradiation. It will be discussed below that the actual magnitude of U can change the energy spectra of ions in a dramatic way [19,20]. Only in a few cases the values (or changes) of U due to dynamic Cs+ sputtering have been determined experimentally: Cs+ bombardment under conditions essentially identical to the present ones leads to a work-function reduction of DU = 1.3 eV in Si [8]. For somewhat different bombarding conditions, DU-reductions of 2.5 eV, 2.2 eV and 1.3 eV were reported in Al, Si and Ni, respectively [21]. For much lower Cs+ bombarding energies (250–750 eV), the lowering of the work function of Si was more pronounced, DU 2.5–3.5 eV [22,23]. (iii) Finally, velocity(or energy-) dependent ionization processes could produce the variance of the Cs+ energy distributions shown in Fig. 2; this aspect will be discussed below in more quantitative terms.
H. Gnaser / Nuclear Instruments and Methods in Physics Research B 269 (2011) 990–994
1.0 Si Cu InP InSb
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+ normalized Cs intensity
+ normalized Cs intensity
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Si
normalized Cs+ intensity
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j(O ) / j(Cs+ ) 2 < 0.0003 0.01 0.034 0.17
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Si
A recent investigation [24] of the influence of oxygen on the ion emission under low-energy Cs+ irradiation of Si indicated that oxygen adsorbed onto the silicon surface may provide an attachment site for impinging cesium ions, thereby increasing the retention capacity of cesium considerably and changing secondary ion yields. In fact, an enhancement of the Cs+ yield from Si bombarded at an elevated O2 pressure was observed already some time ago [13]. To examine these processes in more detail, energy spectra of Cs+ ions emitted from Si were recorded at different oxygen partial pressures. Fig. 3 displays these spectra; the parameter is the ratio of the nominal O2 flux impinging onto the surface to the flux of Cs+ primary ions, r = j(O2)/j(Cs+). The data show that with increasing r the spectra tend to shift to higher energies (see inset). Fig. 3 depicts this shift as a function of r: a total shift of 0.45 eV for r 3.3 is found. Also shown is the variation of the Cs+ intensity which is found to increase by about 30% in the range of O2 pressure investigated. (The values of j(O2) at the sample surface might actually been higher than indicated because O2 is delivered to the analysis chamber via a capillary that ends in close vicinity to sample.) When enquiring into possible reasons for the observed shift of the energy spectra, two effects can be envisaged: (i) the shift with increasing j(O2) is due to a lowering of the surface work function; however, a reduction of U can be expected to decrease the yield of positive ions, as found previously[8], while the Cs+ intensity increases in the present experiment (cf. Fig. 3). (ii) The addition of oxygen may increase the surface binding energy because the CsAO bond strength is much stronger than that of CsACs bonding [25]; a higher surface binding energy will shift the energy distributions of all sputtered species to higher energies, irrespective of their charge state [2]. It is noted that a very similar change in work function (0.44 eV) has been observed in 5 keV Cs+ bombardment of Ag under an elevated oxygen pressure [26]; however, in that case the Cs+ ion intensity was decreasing with increasing O2 flux. 3.3. Comparison of energy spectra with theoretical ion-formation models Several distinct ionization schemes have been proposed [4,27] to describe the ionization process of sputtered ions. The so-called
normalized Cs+ intensity
3.2. Variation of Cs+ energy spectra with O2 partial pressure
1.0
1.0
Cs+ intensity shift of spectra
0.5
0.5
0.0
0.0
10
shift of energy spectra (eV)
Fig. 2. Normalized energy spectra of Cs+ secondary ions sputtered from different samples under 5.5 keV Cs+ ion bombardment. All data refer to steady-state conditions.
-4
10
-3
10
-2
10
-1
10
0
10
1
+ j(O2) / j(Cs ) Fig. 3. Normalized Cs+ energy spectra for different ratios of the O2 and Cs+ arrival rates, j(O2)/j(Cs+), at the surface (upper panel) and the onset region of the spectra (inset). The lower panel displays the measured intensity of Cs+ and the shift of the energy spectra as a function of j(O2)/j(Cs+).
electron-tunneling model [28,29] envisages the electronic transition as a resonant electron transfer process between a sputtered atom and the valence band of the solid. For positively charged sputtered ions, P+ can be approximated as
Pþ / exp½ðI UÞ=0 ;
ð1Þ
I being the ionization potential of the sputtered ion, U the work function of the surface, and e0 is proposed to be proportional to the component of the ion’s emission velocity perpendicular to the surface [30]: 0 / avn = avcosh, with a being a parameter that depends on the specific sample material and h the ions’ (polar) emission angle. The validity and limitations of this theoretical approach have been critically discussed in a recent review [9]. In the following, a comparison of the measured energy spectra with the predictions of the electron-tunneling model will be attempted. In order to do this, two additional aspects have to be addressed, namely (i) the energy distribution of the total sputtered flux (mostly neutrals), a part of which may eventually be ionized, and (ii) the energy-dependent instrumental transmission, required for an appropriate correction of the measured spectra.
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YðEÞ / E=ðE þ UÞ3 :
ð2Þ
In fact, this equation has been widely used in comparisons with experimental energy spectra of neutral atoms and a rather good agreement has been found in many cases[2]. The emission-energy and angle dependence of the transmission T(E, h) of the SIMS instrument used in this work is largely determined by the extraction field and the lenses which transfer the sputtered ions from the surface into the spectrometer. All ions with an energy smaller than a critical energy Ec will pass the spectrometer (i.e., T < 1); for ions with E > Ec, T < 1 and only those ions that are emitted from the surface with an angle smaller than a cut-off angle hc are transmitted. The magnitude of Ec and hc depends on specific parameters of the instrument and, for hc, also on E [35]. The critical energy is
Ec ¼ g2 E0 ;
ð3Þ
with g ¼ d=8Dg and the pass energy of the ions E0 = 4500 eV. Here, d is the diameter of the ‘‘contrast aperture’’, located in the beam’s crossover plane in front of the energy sector, D is the accelerating gap in front of the sample and g is the cross-over magnification. In the present work, the numerical values of these parameters were d = 150 lm, D = 5 mm, and g = 0.28 [36]. Hence, g = 1.34 102 and Ec = 0.8 eV. The cut-off angle depends on the ion’s emission energy [35].
1=2 E0 sin hc ¼ g : 1 E
ð4Þ
Eq. (4) implies that the higher the ion energy the smaller is the acceptance cone for ions to be transmitted through the spectrometer. For example, for values of E = 3, 5, 10, and 50 eV the cut-off angle decreases as hc = 31.2°, 23.7°, 16.5°, and 7.3°, respectively. The magnitude of hc is of relevance for the determination of the normal emission velocity vn = vcosh of sputtered ions which, in turn, influences the ionization probability P+ (see Eq. (1)); for the hc values given above, cosh is reasonably close to unity (cosh = 0.86, 0.92, 0.96, and 0.99) and it appears, therefore, feasible that, at least in the range E P 5 eV, the emission energy E can used to compute vn, v n ¼ ð2E=MÞ1=2 cos h / E1=2 . For a typical angular emission distribution of the sputtering yield, Y(h) / cos h [32], it was noted [20,35] that for E > Ec the transmission of the present instrument scales as T / Ec/E E1. Its transmission has recently been computed also utilizing an ion optics simulation program [37] and T(E) / E1 was found, largely invariant for a considerable range of operating parameters. Hence, that relation was employed to correct the measured spectra for the energy-dependence of the transmission. With the approximation discussed above, the energy-dependent yield of positive ions Y+ would then be given by the product of Eqs. (1) and (2), with 0 / avn.
IU exp av n ðE þ UÞ E IU / exp pffiffiffi : a E ðE þ UÞ3
Y þ ðEÞ ¼ YðEÞPþ ðEÞ /
E
3
ð5Þ
1.0
normalized Cs+ yield
The energy distribution of the sputtered species can be described quite satisfactorily by the predictions of the analytical sputtering theory of Sigmund [31]. This and a closely related approach [32,33] result in an approximate energy-dependence of the yield Y(E) of sputtered atoms that scales with the surface binding energy of the released atoms U [32], YðEÞ / E=ðE þ UÞ32m , where the parameter m in the exponent characterizes the interatomic potential employed in the analytical theory [32]. The value of m has been shown [34] to be close to unity for the low energies involved in sputtered atoms and m = 0 is chosen here for further computations. Then, the energy-dependence of the sputtering yield is given by
Theory Y+=E/(E+U)3 exp[–(I–φ)/aE1/2] U = 2 eV, a = 0.1 eV1/2 I–φ (eV) 0.75 1.2 1 1.5
0.8
0.6
Experiment Si Cu InP InSb
0.4
0.2
0.0 0
10
20
30
40
emission energy (eV) Fig. 4. Comparison of experimental and theoretical Cs+ energy spectra. The experimental data (open symbols) are corrected for the instrumental transmission T (T / E1). The theoretical distributions (solid lines) are computed with Eq. (5), using different values of I–U, and U = 2 eV and a = 0.1 eV1/2.
This expression can be compared with the transmission-corrected experimental energy spectra. Fig. 4 shows such a comparison displaying the measured spectra of Cs+ sputtered from different targets (see Fig. 2) corrected for the transmission and spectra computed via Eq. (5); for the latter U = 2 eV, a = 0.1 eV1/2 and different values of I–U (0.75, 1, 1.2, and 1.5 eV) were used. Since the ionization potential of Cs is ICs = 3.89 eV, those would correspond to an actual surface work function of 2.39, 2.69, 2.89, and 3.14 eV, respectively. It can be seen that with this range of values for U the experimental spectra of the different samples can be reproduced quite reasonably. Furthermore, it was found that such an agreement can be produced also by a variation of the parameter a in Eq. (5) while keeping I–U constant. An uncertainty relates to the surface binding energy U which is not known, but may be different for different materials loaded with Cs. It is envisaged that the application of this approach to other ion species (e.g., the atomic ions sputtered for the various samples) could corroborate the selection of a distinct set of parameters in Eq. (5) (U, a, and U). Then, more detailed information might be extracted from the comparison of the measured spectra with the outcome of Eq. (5); eventually, this could indicate if the latter constitutes a suitable theoretical representation of the energy distributions of sputtered ions. 4. Conclusions The energy spectra of atomic and molecular ions sputtered from a variety of metals and semiconductors under Cs+ bombardment illustrate the wide variance of emission distributions for Cs+, Csþ 2, MCs+, and M+ ions. Although all spectra peak at low energies (a few eV), their high-energy behavior is distinctly different. In particular the atomic M+ secondary ions exhibit a very gradual fall-off towards high emission energies. An increased oxygen partial pressure tends to shift the spectra of Cs+ emitted from Si to higher energies and slightly enhances the Cs+ intensity. To establish the significance of this finding, similar measurements should be carried out for other ion species and additional specimens. The measured Cs+ spectra from various samples can be reproduced very reasonably by means of the predictions of a theoretical ion-forma-
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tion model; an extension of this approach to other secondary-ion species will be useful for the specification of the parameters entering that model. References [1] P. Sigmund (Ed.), Fundamental Processes in Sputtering of Atoms and Molecules, K. Dan. Vidensk. Selsk. Mat. Fys. Medd., vol. 43, 1993. [2] H. Gnaser, in: R. Behrisch, W. Eckstein (Eds.), Sputtering by Particle Bombardment, Springer, Berlin, 2007, p. 231. [3] A. Benninghoven, F.G. Rüdenauer, H.W. Werner, Secondary Ion Mass Spectrometry, Wiley, New York, 1987. [4] M.L. Yu, in: R. Behrisch, K. Wittmaack (Eds.), Sputtering by Particle Bombardment, vol. III, Springer, Berlin, 1991, p. 91. [5] A. Wucher, Appl. Surf. Sci. 255 (2008) 1194. [6] M. Bernheim, F. Le Bourse, Nucl. Instrum. Methods B 27 (1987) 94. [7] H. Gnaser, Phys. Rev. B 54 (1996) 16456. [8] H. Gnaser, Phys. Rev. B 54 (1996) 17141. [9] K. Wittmaack, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 52 (2006) 465. [10] Y. Gao, J. Appl. Phys. 64 (1988) 3760. [11] H. Gnaser, H. Oechsner, Surf. Interface Anal. 17 (1991) 646. [12] H. Gnaser, H. Oechsner, Surf. Interface Anal. 21 (1994) 257. [13] M. Haag, H. Gnaser, H. Oechsner, Fresenius J. Anal. Chem. 353 (1995) 565. [14] H. Gnaser, in: G. Gillen, R. Lareau, J. Bennett, F. Stevie (Eds.), Secondary Ion Mass Spectrometry SIMS XI, Wiley, Chichester, 1998, p. 915. [15] H.N. Migeon, C. Le Pipec, J.J. Le Goux, in: A. Benninghoven, R.J. Colton, D.S. Simons, H.W. Werner (Eds.), Secondary Ion Mass Spectrometry SIMS V, Springer, Berlin, 1986, p. 155.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
H. Gnaser, Nucl. Instrum. Methods B 149 (1999) 38. H. Gnaser, Nucl. Instrum. Methods B 164–165 (2000) 705. H. Gnaser, Phys. Rev. B 63 (2001) 045415. K. Wittmaack, Phys. Scr. T6 (1983) 71. K. Wittmaack, Surf. Sci. 429 (1999) 84. T. Wirtz, H.-N. Migeon, H. Scherrer, Int. J. Mass Spectrom. 225 (2003) 135. J. Brison, N. Mine, S. Poisseroux, B. Douhard, R.G. Vitchev, L. Houssiau, Surf. Sci. 601 (2007) 1467. J. Brison, J. Guillot, B. Douhard, R.G. Vitchev, H.-N. Migeon, L. Houssiau, Nucl. Instrum. Methods B 267 (2009) 519. B. Berghmans, W. Vandervorst, J. Appl. Phys. 106 (2009) 033509. B. Berghmans, B. Van Daele, L. Geenen, T. Conard, A. Franquet, W. Vandervorst, Appl. Surf. Sci. 255 (2008) 1316. B. Saha, S. Sarkar, P. Chakraborty, H. Gnaser, Surf. Sci. 602 (2008) 1061. P. Williams, Surf. Sci. 90 (1979) 588. J.K. Nørskov, B.I. Lundqvist, Phys. Rev. B 19 (1979) 5661. M.L. Yu, N.D. Lang, Phys. Rev. Lett. 50 (1983) 127. N.D. Lang, Phys. Rev. B 27 (1983) 2019. P. Sigmund, Phys. Rev. 184 (1969) 383. P. Sigmund, in: R. Behrisch (Ed.), Sputtering by Particle Bombardment, vol. I, Springer, Berlin, 1981, p. 9. M.W. Thompson, Phys. Rep. 69 (1981) 335. M. Vicanek, J.J. Jimenéz-Rodríguez, P. Sigmund, Nucl. Instrum. Methods B 36 (1989) 124. G. Slodzian, in: A. Septier (Ed.), Applied Charge Particle Optics, Academic, New York, 1980, p. 1. M. Lepareur, Rev. Technol. Thomson-CSF 12 (1980) 225. J. Lörincik, K. Franzreb, P. Williams, Appl. Surf. Sci. 231–232 (2004) 921.