Low-energy sputtering of small neutral clusters from alloys

Nuclear Instruments and Methods m Physms Research B 82 (1993) 347-351 North-Holland

Beam Interactions with Materials & Atoms

Low-energy sputtering of small neutral clusters from alloys H. Gnaser and H. Oechsner Fachbereich Phystk, Umuersttat Katserslautern, W-6750 Kmserslautern, Germany Recewed 7 December 1992 and m rewsed form 25 March 1993

Emission-angle integrated ymlds of small neutral clusters ejected from binary alloys (NI 0 sW0 2' CU063Zno 37, and Cu 028W0 72 ) due to Ar + and Xe + impact in the energy range from 30 to 1000 eV are compared with the respectwe yields from pure Cu and Nl Detection is done by sputtered-neutral mass spectrometry and hemispherical specimens are used Compared to elemental samples the relative &mer and trlmer ymlds are enhanced for the alloys This enhancement is more pronounced for Xe + projectiles and for low bombarding energies At energies of about 1 keV the relative yield data from alloys and pure samples appear to merge This observation is ascribed to the presence of an element ( W m this case) whose mass matches more favorably that of the projectde and thus faclhtates the ejection of small clusters Whde for Ar + bombardment the yields of &mers and tnmers exhibit a dependence which is m accordance with a statistical formatmn mechamsm and which is valid down to very low energms (about 100 eV), such a correlation is not observed for Xe + ~on sputtering of the alloys

I. Introduction T h e flux of m a t e r i a l ejected from a surface due to ion b o m b a r d m e n t is c o m p o s e d of a wide variety of species: a p a r t from a t o m s molecules of all sizes are found. T h e s e s p u t t e r e d p a m c l e s can b e c h a r g e d or n e u t r a l a n d investigations of t h e i r emission a n d formation processes constitute a m a j o r part of this issue [1]. T h e p r e s e n t work studies the ejection of small n e u t r a l clusters (dlmers a n d trimers) from different binary alloys a n d thus extends t h e m o r e a b u n d a n t previous work [2] o n e l e m e n t a l samples. F o r this type of specim e n s several a u t h o r s [3-8] have r e p o r t e d t h e yield of an n - a t o m cluster X~ (for n < 3) to d e p e n d on the n t h power of the (average) yield of s p u t t e r e d atoms X: Y ( X . ) c( [ Y ( X ) ] "

(1)

A l t h o u g h r e c e n t m o l e c u l a r dynamics simulations [9] did not a g r e e with eq. (1) in t h a t they p r o d u c e d a decrease of the relative d i m e r yield with increasing total ymld, t h e validity of eq. (1) was nicely c o n f i r m e d for dimers a n d trimers s p u t t e r e d from Cu a n d Ni by low-energy A r + ions [10,11]. This was t h o u g h t to b e an indication t h a t small clusters are f o r m e d by a c o m b i n a tlve association of two or m o r e target atoms from i n d e p e n d e n t b u t closely c o r r e l a t e d recods Clearly, with increasing cluster size such c o r r e l a t i o n effects are of increasing i m p o r t a n c e for a cluster to leave with a

Correspondence to H Gnaser, Fachberelch Physlk, Umversltat Katserlautern, W-6750 Kalserslautern, Germany

m i n i m u m of internal energy In o r d e r to survive the ejection process In m u l t l c o m p o n e n t samples which are subjected to ion b o m b a r d m e n t substantial compositional gradients may develop [12,13], provided the c o n s t i t u e n t s differ in mass a n d / o r binding energy Conceivably, such a c h a n g e in near-surface stoichiometry will alter t h e evolution of the collision cascade (in particular at low ion impact energies) and, thereby, the emission of clusters In o r d e r to investigate this hypothesis, in the p r e s e n t work d l m e r a n d t r l m e r yields from different binary alloys (NIW, CuW, C u Z n ) were c o m p a r e d with the data o b t a i n e d u n d e r Identical conditions from p u r e Cu a n d Ni. To study the influence of projectile mass A r + a n d Xe ÷ ions were employed with energies ranging from 30 to 1000 eV. As In the previous work [10] neutral species were m o n i t o r e d by m e a n s of s p u t t e r e d n e u t r a l mass s p e c t r o m e t r y F u r t h e r m o r e , h e m i s p h e r i cal specimens were employed in o r d e r to obtain emission-angle i n t e g r a t e d yields; these are of utmost importance in the low-energy regime c o n s i d e r e d here w h e r e emission d l s m b u t i o n s are k n o w n [14] to vary drastically.

2. Experimental T h e e x p e r i m e n t s were carried out in a sputteredn e u t r a l mass s p e c t r o m e t e r (Leybold INA-3) described in detail elsewhere [15]. T h e e l e c t r o n c o m p o n e n t (density ~ 101° cm - 3 ) of a low-pressure (1.5 × 10 3 m b a r )

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H Gnaser, H Oechsner / Sputtermg of neutral clusters from alloys

348

rf plasma sustained by electron cyclotron wave resonance [16] is employed for post-iomzmg sputtered neutral species Ions are extracted from this plasma and accelerated onto the target which is biased negatively to effect sputtermg. From Langmmr-probe measurements the plasma potential (about + 30 V) was deter-, mined. Because of the moderately low temperature of the plasma electrons (6 eV), production of and bombardment by doubly charged ions is neghglble. Current densities amount to about 1 m A / c m 2. Leaving the plasma, post-ionized neutral species are guided into the quadrupole mass filter by means of two sets of electrostatic lenses and a broad-bandpass energy analyzer. Hemispherical samples with a radius of 2 5 mm were manufactured from high-purity, polycrystalhne alloys of Cuo63Zn037, Cuo28W072 and Nl0sW02 and pure nickel and copper They were mounted on the sample holder of the instrument in such a way [10] that their convex surfaces face the entrance aperture of the mass spectrometer, while the plane rear side was about 6 mm from the actual specimen holder The distance from the sample to the spectrometer entrance, i.e. the particles' flight length through thc plasma, amounted to 25 mm Contrary to the ordinary operation of the mstrument, in this work the samples were thus completely immersed in the plasma to achieve homogeneous, normal-incidence ion bombardment

3. Results and discussion In the following yields of atoms and clusters sputtered from the different tagets will be presented Those yields are the measured intensities corrected for the

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isotopic abundance of the species momtored and normahzed to the bombarding current. All data refer to steady-state conditions. No corrections, however, were made for the varying concentrations and the differences in ejection velocity, lomzatlon cross-sections and, for clusters, the possible influence of dlssooatlon While the latter effects are hard to quantify, the influence of the concentration is only clear-cut for atoms; under equilibrium conditions their yields scale with the bulk concentrations For molecules a correction can be made provided eq. (1) is valid Emphasis is therefore laid not so much on absolute yield data but rather on their variation with sample composition, impact energy and ion mass.

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H Gnaser, H Oechsner / Sputtering of neutral clusters from alloys •

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Fig. 1 shows the relative ymlds of Cu dlmers sputtered from Cu and two alloys, C u W and CuZn, for Ar b o m b a r d m e n t In all cases the dependences on impact energy are rather similar, although a small enhancement of Cu 2 at low energms is observed for C u W as compared to the other specimens. At an energy of ~ 1 keV the ymld ratios seem to merge towards a value Y ( C u 2 ) / Y ( C u ) ~ 0.15. Although this number does not necessarily constitute the absolute yield of sputtered Cu 2 dimers for the reasons outhned in the foregoing paragraph, it is comparable with previous experimental values in this energy regime [10,17-19]. Molecular dynamics calculations of Cu sputtering, on the other hand, produce somewhat conflicting results' while Y ( C u 2 ) / Y ( C u ) ~ 0.14 is reported for 1 keV Ar impact

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in [9], other simulations [20] give a dlmer-to-atom ratio of 0.014 for 5 keV Ar projectiles (both data refer to a Cu(100) surface). For Ar + bombardment of NiW and pure NI the relative dlmer and trlmer yields are plotted in fig. 2. Again, at low energies a m o d e r a t e yield enhancement is found for the alloy sample which amounts to a factor of 2 - 4 for nmpact energies of 100 eV Noteworthy ns the high detection sensitivity (see fig. 2b) which enables monitoring a NI 3 yield as low as 10 5 of the atom ymld. For the N1W sample, fig. 3 depicts the dlmer-toatom yield ratio as a function of Y(N1) and constitutes thus a test of the vahdity of eq (1). In fact, an excellent linear correlation in agreement with eq. (1) is observed; it is valid down to very low impact energies ( < 100 eV). A similar good agreement has also been demonstrated [21] for the trlmer yield Y(N13). This appears rather surprising m view of the fact that at these low bombarding energies the surface of the NiW alloy is strongly enriched in W (recent computer simulations [21] indicate that the Ni surface concentration amounts to only ~ 20% at 100 eV Ar impact). To form a dimer or even a trlmer under these condmons indicates that the atoms which are to constitute the cluster do not necessarily reside on contiguous surface sites. The enhancement observed for alloys is considerably more pronounced for heavy projectiles, namely xenon Fig. 4 depmts data obtained from the NIW alloy and pure N1 Relative NI 2 and N13 yields are both much higher for the alloy sample, in particular m the low-energy range. In this regime, the collision cascades will not be fully developed and sputtering can be expected to be dominated by Individual collisions (e.g., between the projectile or a primary knock-on and a target atom). While this will be largely valid for both kind of samples, the presence of a specimen con-

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H Gnaser, H Oechsner / Sputtermg of neutral clusters from alloys

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stltuent heavier than the projectile will facilitate mom e n t u m reversal and should thus Increase the total sputtering yield (in fact, atomic yields from NIW are enhanced compared to pure N1); apparently, this enhancement strongly promotes the ejection of clusters It is noteworthy that for atomic and cluster species the yields from both samples merge for impact energies of about 1 keV. Plotting the yield ratio Y(N12)/Y(NI) versus the atomic yield Y(N1) to check the validity of eq (1) it is noted that in the case of pure Ni (fig 5a) a hnear dependence is found in accordance with eq. (1) and in agreement with previous data [10,11] for Ar impact; by contrast, distinct deviations are observed for the NIW alloy (fig. 5b) In the example the relative dimer yield is strongly enhanced as compared to the

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elemental sample (see also fig. 4) While eq. (1) reflects essentially the stattsncal nature of the multlmer formation process, other factors which are determined by the details of the ejection events may be equally important For example, the size of the surface area from which atoms are sputtered can be expected to have a decisive influence on cluster yields. Conceivably, this quantity will depend on the impact energy and, perhaps, on the presence and amount of an alloying species Very similar observations are made in a comparison of a pure Cu and a CuW alloy specimen under Xe irradiation. Due to mass interferences no reliable data for Cu z dimers could be recorded; the atomic yields and the relative trimer yields are plotted m fig. 6 as a function of the bombarding energy Contrary to the

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H Gnaser, H Oechsner / Sputtermg of neutral clusters from alloys r a t h e r m o d e r a t e differences u n d e r A r + impact (cf. fig 1) a very p r o n o u n c e d t r l m e r yield e n h a n c e m e n t is f o u n d for t h e alloy u n d e r X e ÷ b o m b a r d m e n t . As for N l W it is most &stinct at low projectile energies Clearly, an ~mportant question in this context of alloy s p u t t e r i n g [12,13] is the existence of compositional gradients in the solid's near-surface region and their d e p e n d e n c e o n impact energy a n d projectile ion. F o r the W - c o n t a i n i n g alloys (NiW, C u W ) used here an enrtchment of W at the surface is expected since it is the heavier a n d m o r e tightly b o u n d species; this anticipation is c o r r o b o r a t e d by c o m p u t e r simulations a n d assocmted e x p e r i m e n t s [21]. T h e s e data also showed t h a t the W - e n r i c h m e n t mcreases with decreasmg impact energy O n t h e o t h e r h a n d , a posltwe h e a t of mixing [13] of the c o n s t i t u e n t s m i g h t p r o d u c e regions of essentially p u r e m a t e r m l even for the c o m p o n e n t with the lower (average) c o n c e n t r a t i o n It is also not clear to w h a t extent ( b o m b a r d m e n t - m d u c e d ) segregation may play a role in the a l t e r a t i o n of t h e surface composition

4. Summary T h e m a i n f e a t u r e s o b s e r v e d m the p r e s e n t work are the following: (l) T h e yields of small n e u t r a l clusters are h i g h e r from alloys t h a n from e l e m e n t a l specimens provided the mass of o n e of the alloy c o n s t i t u e n t s t e n d s to increase the s t o p p i n g p o w e r of the projectde (ii) This effect is m o r e p r o n o u n c e d at very low (neart h r e s h o l d ) energies, while it disappears for energies above 1 keV. 0ii) T h e o b s e r v e d yield e n h a n c e m e n t also a p p e a r s to invalidate, for certain b o m b a r d m e n t / s p e c i m e n c o m b l n a h o n s , the simple power c o r r e l a t i o n between m u l t l m e r yield a n d s p u t t e r i n g (atomic) yield which was c o n s i d e r e d p r o o f of t h e statistical f o r m a t i o n process of small clusters. A p p a r e n t l y , different or additional m e c h a n i s m s are operative u n d e r this s p u t t e r i n g conditions

351

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