Electronegativity effects in chemical sputtering

L679

Surface Science 220 (1989) L679-L686 North-Holland, Amsterdam

SURFACE

SCIENCE

LETTERS

ELECTRONEGATIVITY E. SACHER,

J.F. CURRIE

EFFECTS IN CHEMICAL SPUTTERING and A. YELON

Groupe des Couches Minces and D$artement de GEnie Physique, Ecole Polytechnique C.P. 6079, Succursale “A”, MonirPal, Quebec, Canada, H3C 3A7 Received

13 January 1989; accepted for publication

de MontrPal,

19 June 1989

Relative yields for chemical sputtering are commonly correlated with combinations of work function, ionization potential and electron affinity values. Here, we show that they correlate equally well with electronegativity values, as is expected for the post-impact ionization of sputtered neutral molecular species. Further, such plots have slopes whose absolute values are independent of the sign of the sputtered ion and of experimental conditions, as required by theory.

The bombardment of a surface by a beam of primary ions of sufficient energy causes sputtering, including the emission of secondary ions from that surface. This process is presently poorly understood, particularly as to when the secondary ion is produced. The review literature on this subject [l-3] contains both the view that the secondary ion already exists on emission and that it is subsequently formed from neutral species. Theory is of little help here. This is because the difficulty in developing a general theory is so great that the models chosen are generally based on limited amounts of internally consistent data; thus, there are theories which deal with both points of view. These theories do, however, differ in the following respect: those which deal with the emission of ions [1,2] are largely phenomenological while those which deal with post-emission ionization [l-3] are microscopic, indicating that they are amenable to step-by-step physical and chemical development. The most complete theory dealing with post-emission ionization is that of Norskov and Lundqvist [4-61, which is unique in its ability to deal quantitatively with both positive and negative secondary ions. We show here that this theory, with an appropriately modified exponential term, is capable of correlating a vast amount of literature data from several sources, obtained using different techniques, accumulated over a long period of time. We will further show that the slopes obtained from semilogarithmic plots of the modified theory are linear, as found for the unmodified theory, but that their absolute values are now constant, as required by the theory. The theory of Narskov and Lundqvist is based on both ionization and ion escape probabilities; the latter is shown to be finite only outside the surface 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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potential barrier, meaning that those ions capable of escape and detection are produced there. The ion yield, Y, was taken to be exponentially related to the work function, $J, the authors assuming that the exponential takes the form ($J - Z) for positive ions and - (+ - A) for negative ions; Z is the ionization potential and A the electron affinity. This type of exponential relationship has been used by all authors, irrespective of their sputtering models [7-141. While there is no doubt that this exponential dependence is experimentally found for both positive and negative ions, the work function has no meaning outside a solid; it should be replaced by electronegativity, x, to which it is linearly related [15]. Indeed, we have recently shown [16] that both A and I are also linearly related to x, all having different slopes. Electronegativity is defined as the power of an atom to attract electrons. Although often used qualitatively, a more precise definition has recently been demonstrated: it is the negative of the chemical potential of the electrons in a molecular bond [17-191 and, therefore, directly applicable to the dissociation and ionization presently under discussion. In this paper, we show that literature data from many sources, for both positive and negative ions, are exponentially related to x. The plots so obtained have the same form and statistics as those plotted against (+ - Z) or - ( $J - A), as they must. However, the slopes of the plots against x all have approximately the same absolute values, making them independent of experimental conditions. The experimental quantity on which we shall concentrate is the relative yield, Yre,. This is the number of ions detectable for a given bombarding current, and is essentially the only quantity pertinent to our discussion which has been measured. Relative rates of positive ion yield, for chemical sputtering by 13.5 keV O- ions, were taken form the work of Storms, Brown and Stein [20]. This is, by far, the largest compilation available, covering 50 elements. These data are plotted against + - Z in fig. 1, where extensive scatter is observed, due to individual element matrix effects. This was convincingly demonstrated by Morgan and Werner [21], who showed that the scatter was reduced to nil for different elements sputtered from a common matrix. The ionization potentials used here were determined from electronegativity values for these elements, it having been recently shown [16] that both I and A are linearly related to the electronegativity, x. Despite the scatter, the data in fig. 1 fit the equation log Yre, + 1.06 = (7.33 + 1.46)( + - I) + 30.2.

(1)

The correlation coefficient for these 50 data points is 0.587562. This indicates a statistical significance of > 0.9995, an excellent fit of the data points to this semilogarithmic equation. Negative ion data from the same study [20], for chemical sputtering by 16.5 keV Cs+ ions, were found to fit the equation log Yre,f 1.23 = (-0.57

+ 0.14)( -$I + A) + 2.20.

(2)

E. Sacher et al. / Electronegaiivity

I

I -3.7

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effects in chemical sputtering

I

I

-3.5 q~ - I

I -3.3

(eV)

Fig. 1. A plot of the logarithm of the relative yield of positive ions sputtered by 13.5 keV O- ions versus ( I#I- I).

Here, too, the correlation coefficient, 0.501345, indicates a statistical significance of > 0.9995, despite similar data scatter. The derivation of Nsrskov and Lundqvist [4] indicates that, except for the appropriate electronic function of the ion in the exponential term, all the constant terms in the pre-exponential and exponential should be identical for positive and negative ions. Thas is, plots for both positive and negative ions should have the same absolute value for the slope. As seen from eqs. (1) and (2), the two slopes, 7.33 and -0.57, differ in absolute value by more than an order of magnitude. It is difficult to imagine any reasonable modification of the assumed dependence which could bring the two slopes into agreement. While it is clear that local work functions during sputtering will be modified by such effects as crystal orientation, amorphization and implantation of the sputtering species (virtually impossible to measure during such an experiment), we would not expect that taking account of these would remove such a large discrepancy. As noted earlier, the dissociation and ionization of a neutral ejected molecule would depend on electronegativity. In order to test this, we have replotted the data of fig. 1 as log Yre, versus x (fig. 2). This plot is similar to fig. 1, as both cp and I are proportional to x [15,16]. The equation describing the data for the positive ion yield from O-bombardment is logY,,*1.06=(-1.58+0.31)x+7.34

(Ia)

and the correlation coefficient of 0.587562 is, of course, identical to that of eq.

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11 ““““““‘I 0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

X Fig. 2. A plot of the logarithm of the relative yield of positive ions sputtered by 13.5 keV O- ions versus x.

(1). Similarly, becomes

the data for negative

log Y,, f 1.23 = (1.46 + 0.36)~

ion yield from bombardment

+ 1.65;

by Csf ions

(2a)

again, the correlation coefficient of 0.501345 is identical to that of eq. (2). Note that, within experimental error, the slopes are equal in magnitude and opposite in sign. To ensure that this correspondence is not accidental, it would be best to submit other sets of data to the same analysis. Until recently, the only other set of data over a wide enough range of x values which could be used to verify this constancy of slope magnitude was that of Gtinthershulze [22]. He obtained sputtering rates of metals in H, at 850 V. While such conditions should give physical rather than chemical sputtering, the large range of sputtering rates obtained makes it clear that chemical sputtering occurred; Roth [23] suggests that leakage permitted impurity sputtering. The data fit the equation log mmol/A/h

f 0.44 = (1.51 f 0.32)~

- 2.21;

(3)

the statistical significance was > 0.9995, based on a correlation coefficient of 0.716286 for 23 data points. The slope has the same magnitude as those found in eqs. (la) and (2a), and its sign indicates negative ion sputtering. Recently, Wilson [24] has obtained relative sensitivity factors (RSF) for ions implanted into HgCdTe and CdTe, using O- and Cs+ primary beams. Although there was significant scatter in the data, the logarithm of the positive ion RSF appeared linear when plotted against I, as did the logarithm of the

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negative ion RSF when plotted against A. The slope of the positive ion data, log1.7 X 1O22= 22.23 (see eq. (2) of ref. [24]) was twice as high as that for the negative ion data. We have recalculated these data when both are plotted against x. For the positive ions, log RSF f 1.18 = (2.11+ 0.20)~ + 16.78,

(4)

with a correlation coefficient of 0.772063; for 38 data points, this gives a statistical significance of > 0.9995. Similarly, for the negative ions, log RSF f 1.07 = ( - 1.40 zfr0.31)~ + 26.33,

(5)

with a correlation coefficient of 0.719818; for 21 data points, this gives a statistical significance of > 0.9995. In keeping with the model of Norskov and Lundqvist [4], the slopes in eqs. (4) and (5) are close in magnitude to each other and to the values in eqs. (la), (2a) and (3). Thus, all the literature data presently available over a wide enough range of x values indicate that the relationships to el~tronegati~ty are identical for both positive and negative chemical ion sputtering, in agreement with the theoretical model of Nerskov and Lindqvist [4]. This argues for the following qualitative model: an excited neutral secondary molecule is emitted on ion impact; this molecule may ultimately undergo separation and ionization. The probability of this happening is related to the ability of one of the nuclear charges making up the molecule to attract its bonding electrons; this latter definition is the basis of two methods of calculating electronegativity values [25-271. That is, the enhancement [20] of negative ion yield by O- and that of positive ion yield by Csf are due to el~tronegativity differences of the atoms making up the sputtered neutral molecule, one of which comes from the primary beam. Next, let us consider a clear case of physical sputtering. The data of Maissel [28], for Ar+ ion sputtering at 200 V, present much scatter, but fit the equation log Y f 0.30 = (- 0.30 & 0.21)x + 0.037.

(6)

The correlation coefficient of 0.289595, for 24 data points, gave a statistical significance of just under 0.9500. One may argue that this fit is fortuitous because of the scatter and the small range of el~tronegati~ty values. However, a compa~son with the data of Lagreid and Wehner 1291, for Art ion sputtering at 400 V, shows a remarkable similarity: many of the data points are identical and all the rest are within experimental error. These latter results fit the equation log Y + 0.27 = (-0.34

f 0.19)~ + 0.37;

(7)

here, the correlation coefficient for 28 points was 0.339874, giving a statistical significance of just above 0.9500. Further, similar data on other rare gas ions [29,30] gave similar results.

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Let us note that sputtering also induces Auger electron emission [31]. There are, in fact, two components to the electron spectrum: a broader component similar to what is found for electron-induced Auger emission and, superimposed, a narrower spectrum, usually referred to as “atomic-like”. These peaks have been shown [32-351 to arise from the Auger-like autoionization of sputtered neutral species. That is, collisionally excited neutrals, with a core electron promoted to a normally empty valence orbital, are sputtered off the surface; one of the decay channels open to such excited neutrals is for the excited electron to return to its core hole with a simultaneous Auger-like loss of a valence electron, a process called autoionization. Autoionization differs from Auger electron emission in that the former produces a singly charged ion while the latter produces a doubly charged ion. While no direct comparisons appear to have been made, it is likely that the ions so produced are those measured by the sputtering yield. Finally, let us consider the effect of highly electronegative (O-) or electropositive (Cs’) primary beams on the absolute (rather than the relative) secondary ion yield, I,,. This is normally written [13] in the form I,*cc P,*C,Y,

under constant beam conditions, where P,* is the ionization probability of element i, C, is its concentration in the matrix and Y is the matrix sputtering yield. The proportion~ity constant contains information on instrument sensitivity and beam current. Enhancements of the sputtering yield due to the use of O- or Cs+ primary beams are due to an increase in Pi+, and not in Y [7.8,11-14,20,36-401. As noted by Wittmaack 1411, the ionization probability is enhanced by the ionic character of the molecular bond between the primary beam species and the target atom. This, in turn, depends on the electronegativity of the primary beam species, That is, the enhanced secondary ion yields for highly electronegative or electropositive primary beams is due to the enhanced ionization probability caused by the increased ionic character of the bond. This is supported, in the case of oxygen, by autoionization data on oxidized and unoxidized metals: autoionization yields are higher for the oxidized metals, and their peak maxima exhibit a slight shift to lower energies [42-491. In summary, our findings support the view that ionization occurs subsequent to chemical sputtering, the sputtered molecule contains an atom of the primary beam, and that the yield is governed by electronegativity rather than by the work function. Our findings also strongly suggest that electronegativity plays a role in physical sputtering. The authors wish to thank the Natural Sciences and Engineering Research Council of Canada and the Fonds FCAR du Quebec for funding.

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