Sputtering induced surface composition changes in copper-palladium alloys

Nuclear Instruments and Methods 191 (1981) 289-292 North-Holland Pubhshlng Company

289

SPUTTERING INDUCED SURFACE COMPOSITION CHANGES IN COPPER-PALLADIUM ALLOYS M SUNDARARAMAN, S K SHARMA, Laht KUMAR and R KRISHNAN Metallurgy Dn, tston, Bhabha A tomtc Research Centre, Trombay, Bombay-85, India

It has been observed that, m general, surface composltmn Is different from bulk composmon m multlcomponent materials as a result of 1on beam sputtering This compositional difference arises from factors hke preferential sputtering, radiation reduced concentration gra&ents and the knock-m effect In the present work, changes in the surface composmon of copper-palladium alloys, brought about by argon ion sputtering, have been studied using Auger electron spectroscopy Argonmn energy has been vaned from 500 eV to 5 keV Enrichment of palladmm has been observed in the sputter-altered layer The palladium enrichment at the surface has been found to be higher for 500 eV argon mn sputtering compared with argon mn sputtering at higher energies Above 500 eV, the surface composmon has been observed to remain the same Irrespective of the sputter Ion energy for each alloy composltmn The bulk composition ratm of palladmm to copper has been found to be hnearly related to the sputter altered surface composition ratm of palladmm to copper These results are discussed on the basis of recent theories of alloy sputtermg

1 Introduction Sputtering in combination with surface sensitive analytical techniques like Auger electron spectroscopy, X-ray photoelectron spectroscopy and secondary ion mass spectrometry, is used extensively for depth profihng [1 ] However, it has been found that sputtering often leads to changes in composition of the surface being analysed Therefore, for a reliable quantitative analysis, a thorough understanding of the sputtering process itself IS necessary Factors which can cause mhomogenelty in the sputter etching process have been reviewed by Wehner [1], ShImIZU [2] and Hoffmann [3] These factors include (1) preferential sputtering, (2) radiation Induced composl tIon gradients in the target, and (3) the knock-m eftect Preferential sputtering is said to occur wheneve~ the average composition of the sputtered particles differs from that of the outermost layers of the target Due to the different partial sputtering yields of the different components (or elements) an altered surface layer IS estabhshed When steady state conditions are reached, the target will be sputtered stoIchlometrlcally, simply from the conservation of matter However, the composition of the altered layer, m general, will be different from that of the bulk Moreover, radiation enhanced diffusion can give rise to concentration gradients In the sputter altered layer, different from the bulk This effect becomes more pronounced at higher temperatures when the sputter ion produced point defects become mobile 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 75 © 1981 North-Holland

Recoil Implantation from the surface will deplete it of its higher components and this effect becomes more pronounced at higher sputter Ion energ¢es All these processes operating singly or in combination might play an nnportant role in producing compositional changes on the surface as compared with the bulk Experimental results on binary alloys obtained by different workers [ 4 - 7 ] have shown that for components with medium and heavy atormc weight, the sputter yield ratios are in qualitative agreement with the sputtering yields of pure elements In this paper, we report our results on the sputtering of copper palladium alloys and attempt to Interpret these in terms of the recent theory of alloy sputtering In the binary copper-palladium system, a continuous series of solid solutions form over the entire composition range

2. Experimental Six polycrystalhne, cold-rolled copper--palladium alloys with known but different bulk compositions were studied The bulk compositions of the samples, determined using electron probe mlcroanalysls are shown in table 1 All the samples were polished to a rrurror finish using 0 1/am diamond paste and cleaned thoroughly in methanol and then In water A commercial Auger electron spectrometer from Physical Electronics Industries (Model Number 551) with a double pass cylindrical mirror analyser was used to Vl SURFACESCIENCE

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M Sundararaman et al / Sputtering mduced surjace composttton changes

Table 1 Bulk composltmn of copper-paUadlum alloys as determined by electron probe mlcroanalysls Sample label

A

B

C

D

E

F

Bulk composltmn (at %) paUadmm

8 96

17 10

25 6

43 80

64 14

86 40

carry out the sputtering studies Ion bombardment of the samples was performed with argon ions, their energy varying from 500 eV to 5 keV The partial pressure of argon was of the order of 6 6 X 10 -3 Pa Auger electron spectroscopy was used to observe the changes in the surface composition The primary electron beam energy and current were 3 keV and 20 #A, respectively

3. Results and discussion The samples were Initially sputtered for about 15 to 20 nun with argon ions having an energy of 500 eV so that a steady state condition could be attained in the sputter altered layer before measuring the peak to peak height of the differentiated Auger spectrum for copper (920eV) and palladium (326/330 eV) peaks The projected range of argon ions In copper palladium alloys has been determined to be of the order of 5 A for 500 eV Ion energy and of the order of 40 A for 5 keV Ion energy Escape depths of electrons having kinetic energies of 330 and 920 eV are of the order of 8 and 15 A respectively [8] Thus the escape depth of Auger electrons of copper and palladium used in the analysis was smaller than the projected range of argon ions (for argon ion energy above 1 keV) which was the same as the thickness ol the sputter altered layer So the measured Auger peak height represented the composition within the sputter altered layer However, in the case of 500 eV argon 1on sputtering, the thickness ot the sputter altered layer IS much smaller than the escape depth of copper Auger elect, ons Hence the calculated atonuc concentration of copper is an average value Iron: a layer which also includes a small portion of bulk of the sample Peak to peak height was converted into atomic concentration using the formula Q~: = (Ix/Sx)/2(IojSc~) , Ix being the Auger peak to peak height and S x the

relative eleniental sensitivity factor Values of relative elemental sensitivity factors were taken from the

Handbook of Auger Electron Spectroscopy by Physical Electronics [9] tn our calculations we assumed that the relative elemental sensltwity factor was independent of the escape depth of electrons and the backscattenng factor Hall and Morablto [10] and Penn [11] have calculated the effect of these factors on the elemental sensitivity factor and have found the error In the calculation of surface concentration to be less than 5% Argon Ion energy was varied in steps, the maximum energy used being 5 keV Sputtering was carried out for about 15 to 20 nnn at each ion energy before recording the differentiated Auger spectrunr of the sample Peak to peak height ratio was used in the analysis as it ehminated the effect of the variation ot the primary electron beam on the Auger peak height The calculated surface atomic composition ratio ol Pd(326/330 eV)/Cu(920 eV) at various argon ton energies is tabulated against the bulkatonuc composItmn ratio, determined using mtcroprobe analysis, m table 2 It is evident from the taole that enrichnlent of palladium occurred at the surface during sputterIng The sputtm "altered layer surtace composition ratio was tound to be larger at 500 eV Ion energy sputtering than at higher energy ion sputtering This value was slightly smaller than the actual surface composition ratio of the altered layer for the reasons already discussed Tlus high preferential sputtering of copper at 500 eV sputter Ion enelgy (winch was healer to the threshold energy for sputtering) could be explained on the basis ot the plojectile to target mass ratio Equal masses favour an efficmnt energy transfer and hence the component having a mass nearer to that of argon (in our case copper) would have a higher recoil energy Moreover, a light target elenrent rmght have Its moinentum Inverted by colliding with a h e a w component and not wce versa Hence the sputtering of hghl component copper was enhanced According to Slgnmnd and Anderson [12] at higher energies spulte:Ing occurs through a collision cascade mechanism Tire preferential sputtering of one component It: a multicomponent system depends on (a) the mass difference ratio of the constituent components, and (b) their surface binding energies

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M Sundararaman et al / Sputtering mduced surface composttton changes

Table 2 Surface atom]c composition ratio of Pd (326/330 eV)/Cu (920 eV) at varmus argon mn bombardment energaesm copper-palladium alloys Sample label

Bulk composition ratio

of (Pd/Cu) by EPMA

Surface atomic concentratmn ratm of (Pd/Cu) at argon 1on bombardment energy of (keV) 05

1

2

3

4

5 018 0 35 0 56 1 26 2 46 7 79

A

010

024

018

018

017

017

B

0 21

0 39

0 35

0 35

0 35

0 35

C D E 1~

0 34 0 78 1 79 6 34

0 60 1 30 2 53 8 17

0 57 1.23 2 46 7 78

056 1 22 2 45 7 77

0.56 1 23 2 46 7 77

0 57 1.24 2.45 7 77

For a system with a large mass difference iatlo of constituents the recod energy spectrum of the heavier component goes to zero faster than for lighter components and this gives rise to preferential sputtering ot the latter Moreover, a component having a lower surface binding energy has a higher probabdity of escaping from the surface even If all the components have the same recoil energy In particular reference to our system, copper has a surface binding energy of 355 kJ/mol whereas palladium has a surface binding energy of 375 kJ/mol [13] The atomic weight of palladium is larger than that of copper This suggests that enrichment of palladium should take place on sputtering which is confirmed by our data in table 2

~oo

f

-o

o

o10

It

O5

u

No change in the surface atormc composition ratio was observed as the ion energy was varied from 1 to 5 keV This was not in agreement with the result obtained by Goretzkl et al [14] on C u - N I alloys where they found that enrichment of nickel Increases with increasing sputter Ion energy. They attributed this increase to the preferential knocking-in of copper atoms into the bulk A plot of the bulk composition ratio versus the sputter-altered layer surface composition ratio is shown in fig 1 A linear relatlonslup was found to hold between these two parameters Accordlng to Shimlzu et al [16], the slope of the plot should be equal to the component sputter-yield ratio of copper to palladium Many workers [ 4 - 7 ] have observed that the sputter yield ratio is Independent of alloy composition as predicted by Shlrmzu's theory Our result also agreed with this general trend The calculated value of component sputter yield ratio from sputter yield values of pure copper and palladium [15] for 1 keV argon Ion sputtering was found to be 1 2 The component sputter yield ratio of Cu/Pd of 1 5 obtained from our plot agrees quahtatively with the value obtained from sputter yields of pure elements Betz et al [6] obtained a component sputter yield ratio of about 1 6 for copper-palladium alloy whereas West [17] found the sputter yield ratio of copper to palladium to be 1 7 for an alloy having 46 at % palladium

4 Conclusion 01

0[5 110 5~0 BULK ATOMIC COMPOSITION RATIO {Pd/C~)

100

t lg 1 Plot of surface atomic composition ratio Pd(326/330 eV)/Cu (920 eV) calculated from Auger peak height versus bulk composition ratio

(1) Palladium enrichment was observed on the sputter altered surface layer In a manner qualitatively consistent with theoretical predictions (2) The sputter-altered layer surface composition Vl SURFACE SCIENCE

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M Sundalaraman et a l / Sputtelmg reduced 7urjac c comt)osttton changes

was t o u n d to b e i n d e p e n d e n t o f s p u t t e r ion energy 1tl the range 1 to 5 k e V (3) The c o m p o n e n t s p u t t e r yzeld ratio o* c o p p e l to p a l l a d m m was f o u n d to be 1 5 and was i n d e p e n d e n t o f alloy c o m p o s i t i o n (4) At 500 eV 1on s p u t t e r i n g , hlghel pletelentzal sputtering of copper occurred The a u t h o l s wish to express t h e u g r a t i t u d e to Shrl C V S u n d a r a m and Dr M K A s u n d l for t h e i r s u p p o r t and c o n t i n u o u s interest

References [ 1] G K Wehner, an Methods of surface analysis, ed, A W Czanderna (Elsevier, New York, 1975) [2] R ShImlZU, an Proc 7th Int Vacuum Congress and 3rd Int Conf Sohd Surfaces, Vienna (1977) p 1417 [3] S Hofmann, m Proc 7th Int Vacuunr Congress and 3rd Int Conf Sohd Surfaces, Vienna (1977) p 2613

[4] P S Ho, J E Lewis and J K Howard, J Vac Scl Fechnol 14 (1977) 322 [5] G Betz, Mlcrochean Acta Suppl 8 (1979) 97 [ 6 ] G Betz, Surt Scl 92 (1980) 283 [7] H J Matlueu and I) Landolt, Surf SCl 53 (1975) 228 [8] A Josh1, L E Davis and PW Palmberg, m Methods ot surface analysis, ed, A W Czandcrna (Elsevier, New York, 1975) p 164 [9] Handbook of Auger electron spectroscopy, (Physlc,d Electronics Industries, Inc Eden Pr,urle, Mlnncsota, 1976) p 13 [10] PM Hall and J M Morablto, CRC Cnt rev Sol St Mat Scl (December, 1978) 53 [11] D R Penn, J Electron Spectroscopy 9 (1976) 29 [12] N Anderson and P SlgnIund, Mat ['ys Medd Dan Vld Selsk 39 (1974) No 3 [13} Hultgren ed, Selected values of the thermodynamic properties of the elements (ASM, Oluo, 1973) [14] H Goretzkl, A Muhlnatzer and J Nlckl m Proc 7th Int Vacuum Congress and 3rd Int Conf Sohd Surlaces, Vienna (1977) p 2387 [15] H Oechsner, Z Pbys 261 (1973)37 [16] R Shnnlzu and N Sackl, Surf Scl 62 (1977)751 [17] L A West, J Vacuum ScJ Technol 13 (1976) 198