Evidence for the Auger neutralization mechanism in secondary ion emission

Surface Science 115 (1982) LI41-L146 North-Holland Publishing Company

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S U R F A C E SCIENCE LETTERS EVIDENCE FOR T H E AUGER N E U T R A L I Z A T I O N M E C H A N I S M IN S E C O N D A R Y ION E M I S S I O N Michael J. VASILE Bell Laboratories. Murray Hill, New Jersey 07974, USA

Received 30 November 1981; accepted for publication 12 January 1982

Secondary ion yields, S+(E), for Cu ÷ ejected from Cu(lll) and Cu(ll0) have been determined for the secondary ion kinetic energy range from I to 50 eV. A plot of log S ÷(E) versus E-t/2 falls on a single straight line over three orders of magnitude of S +(E). The slope, A/a is 2x106 cm/s, in good agreement with values reported for the related process, the survival probability of excited neutral copper atoms ejected from a copper surface by ion bombardment. This provides strong evidence that Auger neutralization is the dominant factor governing secondary ion survival.

This study deals with the mass and energy selected secondary ions ejected from single crystal copper surfaces by 2 kV Ar + primary ions. Secondary ion yields are obtained as a function of the secondary ion kinetic energy, and these results are used to test the Auger neutralization mechanism. Several distinct mechanisms have been proposed to account for secondary ion emission, and experimental verification of any of these mechanisms has proven to be elusive. The present situation with regard to these secondary ion production mechanisms is adequately discussed in ref. [1]. Most mechanisms consider excitation processes that produce the positive ion at the surface, but neutralization of a departing positive ion also affects the secondary ion yield. Assuming that excitation processes c a n . b e decoupled from neutralization processes, then the latter aspect of this rather complicated physical phenomen o n m a y be tested by examining the ion survival probability. The probability that an ion will escape to infinity from the near-surface region of a solid [2,3] is P(oo, v+), given by P(oo, v±) - e x p ( - - A / a v ± ) .

(1)

A is the Auger neutralization rate at the surface, a-~ is the distance at which the neutralization rate drops to l / e of its surface value, and v± is the component of the ion velocity normal to the surface. In an experimental situation, we observe the ion yield at essentially an infinite distance from the surface and express this yield as S + ( E ) = No+ ( E ) / N o ( E ) . Here, No+ ( E ) is the number of ions per second, and N0(E) is the number of neutral particles 0039-6028/82/0000-0000/$02.75 © 1982 North-Holland

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M.J. Vasile / Evidence for Auger neutralization mechanism

per second, each at kinetic energy E. Assuming that a large fraction of the secondary particles ejected from the solid begin their trajectories as ions, then S + ( E ) - P ( ~ , vx). Restricting the latter assumption still requires S + ( E ) P ( ~ , eL) from the definition of S + ( E ) . A measurement of the ion yield as a function of kinetic energy therefore provides a test for the Auger neutralization mechanism, since In S + ( E ) oc - ( A / a ) v~ 1

(2)

The instrument used to collect the secondary ions is described in a recent publication [4]. The primary ion beam is normal to the target, and current densities are in the "static" regime, meaning that the time necessary for an analysis is much less than the time for removal of one monolayer. Energy selection is achieved with a novel, high transmission "resistive disk" energy filter, which performs energy selection in the same way as a set of concentric hemispheres. The instrument geometry is arranged to optimize collection of secondary ions ejected within an angular cone of - 12 ° around the surface normal. An analytically derived transmission function, T(E), is used to convert the mass and energy selected secondary ion count rate I , , ( E ) to the ion ejection rate, No+ (E),

No+ ( E ) = I,,( E ) / T ( E ) .

(3)

Precise verification of the transmission function of the energy filter-mass spectrometer combination has not been carried out as yet, but the signal intensity versus energy bandwidth, primary ion beam radius, and quadrupole acceptance are in excellent agreement with the derived function. An independent check [4] of T ( E ) was also made using published values [5] of absolute secondary ion yields for Cu, AI, Ni and Si targets. The function, T ( E ) , gave values within a factor of four of the experimental measurements. The secondary ion emission coefficient is expressed as 3' + ( E ) ,

y + ( E ) = Ira( E ) / i o T ( E ),

(4)

where 10 is the primary ion count rate. Results have been obtained for Cu(ll0) and C u ( l l l ) crystals, which were spark cut, spark planed, and electropolished prior to mounting. The analyzer chamber was pumped to 5 × 10 - l0 Torr, and residual gas analysis showed that the partial pressure of oxygen was !.5 × 10- t l Torr. Oxygen partial pressures in the range of 2 - 3 × 10 - I ° Torr were measured with the differentially pumped primary ion gun in operation. Kinetic energy distributions of 63Cu+ ions were measured at various stages of surface cleanliness. The positive secondary ion mass spectrum was used to determine the chemical state of the surface. Column A in table 1 shows the mass spectrum obtained from a C u ( l l l ) surface after evacuation, bake out, and sputtering with an ion dose equivalent to 2 monolayers removed. The kinetic energy distribution of the 63Cu+ ion

M.J. Vasile / Evidencefor Auger neutralization mechanism

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Table I Secondary ion mass spectra from Cu(I 1 I) surfaces count rate, c p s / n A of primary ions, at the peak in the kinetic energy distribution of the ion listed Mass

Ion

A

B

C

63 126 142 158 189 205 221

Cu + Cu ~Cu20 + Cu 2S + Cu~ Cu30 + Cu3S +

8.6× 104 464 68 250 90 57 430

2 × 104 320 I1 7 I00 4 6

4)< 104 475 237 7 150 200 ND

Spectra A and C taken at resolution A M = 1.0 amu. Spectrum B taken at resolution A M = 1.3 a m u to improve signal intensity. Background count rate ~ 1 cps. N D = not detected above background.

associated with the presence of surface oxide and sulfur is plotted as "t + ( E ) in fig. 1, curve 1. The oxide was sputtered away readily at low temperature (15 monolayer ion dose at 80°C), but removal of the sulfur required heating to temperatures of 350-400°C with sputtering and oxidation-reduction cycles [6] (700 L 02 followed by 104 L CO). The mass spectrum in column B, table 1, was obtained after the cleaning procedure, and the corresponding kinetic energy distribution of 63Cu+ is curve 2 in fig. 1. The secondary ion coefficient y + ( E ) decreases as expected with the removal of surface oxygen [5], and the decrease is pronounced in the low energy segment of the distribution. Exposure to

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Fig. I. Kinetic energy distributions of 63Cu ÷ ions ejected from the C u ( l l l ) single crystal. The vertical axis is -/+(E), the secondary ion emission coefficient. Surface conditions fol: the curves are described in the text.

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M.J. Vasile / Evidencefor Auger neutralization mechanism

molecular oxygen at 160°C in stages to a total dose of 600 L produced curve 3 in fig. 1 and the mass spectrum in column C, table 1. Again, the low energy ions are strongly affected by the adsorption of oxygen. The (Cu z + 32) + and (Cu 3 + 32) + ainu ions were assigned as Cu2S + and Cu3S +, since there was no increase in their count rate after exposure of the surface to oxygen. CuO + and CuO2+ ionic species were also not observed in the spectra. Determination of the ion yield S + ( E ) requires knowledge of the neutral particle ejection rate, No(E). A first approximation to No(E) for the secondary particle energy range in this study is given by

No(E) = loU P( E ),

(5)

with

P( E ) = A cosgp E / ( E + Eb) 3,

(6)

where N is the total sputtering yield, E is the kinetic energy of the sputtered atom, and E b is the surface binding energy. P(E) is the probability that a particle is ejected with energy E at an angle q~ with respect to the normal. The constant A = 2Eb/~r, is evaluated from the restraint that the integral probability over all directions and energies is unity. The energy dependence in eq. (6) was arrived at from a study using 43 kV Ar + primary ions on copper [7], but the general behavior holds well for primary ion energies in the 1 kV range. The form of eq. (6) was also derived in the isotropic limit of the linear collision cascade theory of sputtering [8]. Eq. (6) predicts a maximum in the neutral kinetic energy distribution a t Eb//2, and a 1/E 2 dependence as E >> E b, both of which are observed for 900 eV Ar ~ on Cu [9]. Anisotropy in the direction of sputtered atoms should be considered since single crystals are being bombarded. Experiments have shown that - 7 5 % of the particles sputtered from Cu(100) by Cs + bombardment (90 ° incidence, 1 to 10 kV energy) followed the cosine distribution [10], thus retention of the cos,~ term is adequate for the purposes of this study. An expression-for S +(E), the ion yield of particles ejected in the normal direction, may then be written

No+(E) I,,(E)~r(E+Eb) 3 S + ( E ) - No(E) = T(E) Io2EbNE

(7)

Eq. (7) should give the absolute ion yield within the framework of the approximations used in deriving eq. (6) and the transmission function, T(E). It is expected that these ion yields will be at least of the correct order of magnitude. Absolute accuracy in the ion yield is not essential to test for neutralization effects in the ion emission process, but it is of interest to calculate these yields as closely as possible. The sputtering yield for 2 kV Ar + on Cu is taken as two atoms per incident ion [11], and E b is taken as the cohesive energy (3.5 eV) when computing S + (E). The ion yields S + ( E ) are plotted semi-logarithmically versus E -I/2 in

M.J. Vasile / Evidence for Auger neutralization mechanism

1.0

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Fig. 2. Plots of the ion yields S ÷(E) from Cu(ll I) surfaces for 2 keV primary ions versus E L/2 Surface conditions corresponding to the curves are described in the text.

fig. 2, for the measured energy range. Curve I corresponds to a surface with oxygen and sulfur present (column A, table l), and kinetic energy distribution 1 in fig. 1. Curve2 is for an intermediate stage of cleaning, with minimal oxygen but sulfur still evident; curve 3 corresponds to column B, table l, and kinetic energy distribution 2 in fig. l, i.e. an essentially clean surface; curve 4 was obtained after the 600 L oxygen exposure. A good straight line fit of eq. (2) over three orders of magnitude is obtained for the clean copper surface, with a slope of A/a = 2.0 × l 0 6 c m / s . Such a straight line fit was reproducible; it was obtained twice on the Cu(l I l) crystal after cleaning and oxidation-reduction cycles. The Cu(110) crystal also gave similar results with an A/a value of 2.0 × l 0 6 c m / / s . This value is somewhat higher than the upper bound of the range estimated by Wittmaack [12] (2.5 × 105 c m / s < A/a < 1.7 × 10 6 c m / s ) based upon E b -----5 eV. Van der Weg and Bierman [13] report the same value, A/a = 2 × 10 6 c m / s for the radiationless transitions which govern the survival probability of CuI excited atoms ejected from Cu by 80 keV argon ions, and White and Tolk [14] also obtain this value for CuI ejected by 3 keV argon ions. The results in this study are the first experimental observation that positive secondary ion yields exhibit a functional dependence in accord with Auger neutralization rates at the surface. Very recently, Yu has shown [15] that the

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M.J. Vasile / Evidence for Auger neutralization mechanism

p r o b a b i l i t y of O - ejection from c h e m i s o r b e d oxygen on v a n a d i u m a n d n i o b i u m is d e p e n d e n t u p o n v ± . K r a u s s a n d G r u e n [16] a t t e m p t e d such an analysis for b e r y l l i u m ion ejection over a limited energy range but a linear fit was not o b t a i n e d . T h e y used an a p p r o x i m a t e l l / E 2 d e p e n d e n c e for the neutral kinetic energy d i s t r i b u t i o n , a n d did not r e p o r t m a k i n g a c o r r e c t i o n for ion transmission through the instrument. T r e a t i n g the d a t a in this s t u d y in the s a m e way, i.e. neglecting c o r r e c t i o n s for i n s t r u m e n t transmission a n d using E - t / 2 instead o f E / ( E + E b ) 3 yields plots of log S + ( E ) versus E - 1/2 very similar to those o b t a i n e d b y K r a u s s a n d G r u e n [16]. T h e results shown in fig. 2 also d e m o n s t r a t e d r a m a t i c a l l y that even a small surface c o n c e n t r a t i o n of oxygen (or sulfur) p r e f e r e n t i a l l y enhances the yield of the low energy ions. This p h e n o m e n o n is in qualitative a g r e e m e n t with the results f o u n d for CuI ejecti9 n [13,14], and m a y c o n t r i b u t e to the c u r v a t u r e o b s e r v e d b y K r a u s s a n d G r u e n , since they r e p o r t e x t r e m e difficulty in maintaining an oxygen-free b e r y l l i u m surface. T h e a g r e e m e n t between the findings of this study for Cu + a n d those of the p h o t o n emission studies of excited Cu is s t r o n g evidence that A u g e r neutralization governs the survival p r o b a b i l i t y of s e c o n d a r y ions. The c o i n c i d e n c e of the A / a value is surprising, a n d implies that the transition rates for the neutralization of ions a n d d e e x c i t a t i o n o f CuI are virtually the same. T h e a u t h o r w o u l d like to a c k n o w l e d g e Messrs. J.C. T u l l y and N . H . Tolk for m a n y s t i m u l a t i n g discussions d u r i n g the course of this work.

References [1] K. Wittmaack, in: Inelastic Ion-Surface Collisions, Eds. N. Tolk, J.C. Tully, W. Heiland and C.W. White (Academic Press, 1977) p. 153. [2] H. Hagstrum, ref. [I], p. 1. [3] H. Hagstrum, Phys. Rev. 96 (1954) 336. [4] M.W. Siegel and M.J. Vasile, Rev. Sci. Instr. 52 (1981) 1603. [5] A. Benninghoven, Surface Sci. 53 (1975) 596. [6] F.H.P.B. Habraken, E.Ph. Kieffer and G.A. Bootsma, Surface Sci. 83 (1979) 45. [7] M.W. Thompson, Phil. Mag. 18 (1968) 377. [8] P. Sigmund, in: Inelastic Ion-Surface Collision, Eds. N. Tolk, J.C. Tully, W. Heiland and C.W. White (Academic Press, 1977) p. 121. [9] H. Oechsner, Z. Physik 288 (1970) 433. [10] N.T. Olson and H.P. Smith, Phys. Rev. 157 (1967) 241. [1 I] P. Sigmund, Phys. Rev. 184 (1969) 383. [ 12] K. Wittmaack, ref. [ 1], p. 190. [13] W.F. van der Weg and D.J. Bierman, Physica 44 (1969) 206. [14] C.W. White and N.H. Toik, Phys. Rev. Letters 26 (1971) 486. [15] M.Yu, Phys. Rev. Letters 47 (1981) 1325. [16] A.R. Krauss and D.M. Gruen, Surface Sci. 92 (1980) 14.