Effect of impurities on SIMS

Effect of impurities

on SIMS

received 3 May 1979 J S Colligon

and G Kiriakidis,

Department

of Electrical Engineering,

University of Salford, Salford M5 4WT, UK

Results showing the effect of implantation of various species into copper on the secondary Cu+ ion signal are presented. Preliminary analysis indicates that both a physical change in the sample induced by the implantation and a chemical effect due to the presence of the implant species are responsible for the observed changes.

Introduction In the last decade considerable effort has been directed towards a better understanding of the mechanisms responsible for emission of secondary ions from ion bombarded solids. However, to the present time no complete explanation which will allow the conversion of a secondary ion signal to an equivalent concentration level of that species in the bombarded matrix has been given although some empirical models have been shown to produce reasonable answers under certain bombardment conditions1-4. One of the principal problems of quantitative surface analysis by SIMS is the sensitive dependence of the secondary ion signal of a given species on the nature of adjacent atoms in the lattice, for example, the presence of oxygen can cause a ten to onehundredfold increase in Al+ signals. Even the probing beam species can contribute to this effect6. In an attempt to understand this basic problem of SIMS analysis techniques the present programme of work has been concerned with the measurement of the secondary ion signal Cu+ from a polycrystalline OFHC copper sample during bombardment by 10 keV Ar+ ions. The copper sample was identical in all cases except that, prior to analysis, a small quantity of another species was ion implanted into it. Thus, in principle, a Gaussian distribution (approximately) of implant concentration level was present in known magnitude (usually a total dose of 1016 ions cmm2) and the effect of this ‘controlled’ contamination on the Cu+ signal could be seen layer by layer as the 10 keV Ar+ probe sputtered through the sample. Of course, secondary processes such as recoil implantation, radiation enhanced diffusion and nonuniformity of probe beam could lead to distortion in results. However, where the observed effects manifest themselves in the form of a ‘Gaussian’ distribution it is reasonable to assume that these secondary effects are insignificant. Experimental

Oxygen free high conductivity copper was used as the reference target material for this series of investigations. Implantation was carried out in the standard target chamber of the University of Salford Isotope Separator. Samples were then transferred to a uhv target chamber also on the separator which housed the SIMS apparatus. The uhv chamber was not baked, however, as this may have changed the impurity distributions, so the SIMS was carried out in a vacuum of 10-s torr. Ten or 20 keV Ar+ Vacuum/Volume 29/number 10. 0042-207X/79/1 101-0357$02.00/O @ Pergamon Press LtdlPrinted in Great Britain

ions produced in the same Isotope Separator were steered and focused into this uhv chamber producing a probe current of density 2-8 ,uA cme2. A mirror-field SIMS assembly, already described elsewhere’, was used for analysis. The probe beam from the isotope separator passed through apertures in the pair of mirror plates which are angled at 45” to the beam axis. T’he lower energy secondary ions returning to the field between these plates cannot surmount the potential barrier and are turned through 90” into a quadrupole mass spectrometer. This system combines the advantage of a high collection efficiency with that of screening the sensitive dynode in the quadrupole detector from the sample surface, hence, reducing noise level to a minimum. A control sample, implanted with 20 keV Ar+ and sputtered with 10 keV Ar+ showed that the secondary ion Cu+ signal remained almost constant apart from a small initial high value, probably associated with the development of a surface oxide in the interval between implantation in one chamber and transfer via air to the other. The following implants were also carried out: K+, P+, Cs+, Tl+, Pb+, 63Cu+, ‘Yu+. The K+, P+, Cs+, Tl + implants were all at 20 keV to total doses of 1Ol6 ions cmT2 and subsequent SIMS analysis of the Cu+ signal is shown in Figure 1. The Pb+ implant dose was 10” ions cmm2 at 20 keV (Figure 2) the 63Cu+/65Cu+ implants were doses of 1016 ions cmm2 at 20 keV (Figure 3).

Discussion

Analysis of the data shows that the subsequent Cu+ signal is always changed, usually by a significant amount, as a result of the previous bombardment. There are, however, one or two serious anomalies. First, the K+ implant appears to create surface conditions which suppress Cu+ ion emission and, even after many hours of further sputtering, this state of affairs remains. K+, thus, stays on the surface indefinitely, in the same way as seen by Wehner*. Similarly the Cs+ implant shows evidence of a non-Gaussian tail in the subsequent Cu+ signal, indicative of retention of Cs by a knock-on or diffusion process and P+ shows this to a greater extent. One of the most difficult results to explain is the enhancement in Cu+ after a prebombardment by 63Cu+ or 65Cu+. Similar effects have been seen in measurements of neutral sputtering yields when the bombarding species is changed9, i.e. from Ar+ --f Cu to Cu+ + Cu, and a logical explanation of the 357

J S Colligon and G Kiriakidis: Effect of impurities on SIMS x

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Figure 1. The secondary ion emission current 63Cu + as a function of bombardment time after various preimplants. 160 2Ok•V, x x Xx

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Figure 2. The secondary ion signal 6aCu+ following implantation with 10t7 ions cm 2 of 20 keV Pb ÷

present results is that a similar adjustment of the lattice to the new implant species is also responsible for the change in secondary ion emission. Naturally the magnitude of change is larger for secondary ion emission since it is not only sensitive to a change in binding energy of lattice atoms but also to the subtle electronic relationships between atoms which dictate whether ionization is promoted and whether an excited electronic state survives or not as the atom escapes. 358

The present work, thus, indicates, for the first time, that it is not only the chemical nature of the implant species which changes the ionization probability of escaping atoms but also the effect on the lattice structure of the implant species. There is no obvious experimental method for the separation of these two processes. However, it is possible to calculate a ratio R which is the change in Cu + yield per 1016 implant ions per square centimetre, the 'change' being referred to the value normally

J S Colligon and G Kiriakidis: Effect o f impurities on S I M S 4 0 (6 ions/cm 2 ~ 2 0 k e Y 63Cu ~ C u

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Figure 3. The 63Cu + and 6SCu + signals following implantation with 10t 6 ions c m - 2 of 20 keV. (a) 6 aCu + and (b) 65Cu ÷ ions.

o b t a i n e d w h e n the previous a n d p r o b e b e a m s are A r +. O n this basis T a b l e 1 h a s been derived using the leading edge o f the G a u s s i a n in e a c h case (up to its m a x i m u m ) where d i s t o r t i o n is

Table 1. Coefficients R and Rc (defined in text) for copper in counts per 1016 ions cm -2 (all values listed are to be multiplied by 104) Ion

R

R~

Potassium Cesium Phosphorus Thallium Lead Copper 63 Copper 65 Copper (average)

--0.75 + 1.24 +99.80 +73.84 + 0.92 + 8.74 +7.27 + 8.00

--8.75 -6.76 +91.80 +65.84 -- 7.08 ----

m i n i m a l . Thus, if Icu ÷ is the c o p p e r ion signal at t i m e t a n d n t h e total i m p l a n t fluence in units o f 1016 c m -2

j

,t"

R=

A l t h o u g h lead shows a negative effect it is c o n s i d e r e d t h a t this m a y be partly due to r e t e n t i o n o f the lead in the c o p p e r lattice, d u r i n g s p u t t e r i n g by a r g o n ions, a factor w h i c h ' h a s been seen to occur for lead in a l u m i n i u m 1°. T h e decrease in P b + signal s h o w n in F i g u r e 2 does n o t necessarily imply t h a t the lead c o n c e n t r a t i o n is decreasing but could indicate t h a t P b is n o t being sputtered. In such a situation the local c o n c e n t r a t i o n o f lead in the surface layers could d u r i n g b o m b a r d m e n t b e c o m e particularly high in the same way as alloys c o n t a i n i n g a light a n d heavy species b e c o m e enriched in the heavier element 11. A p a r t f r o m the drastic r e d u c t i o n in surface c o n c e n t r a t i o n of copper, the whole a t o m i c collision cascade will, therefore, develop in a different m a n n e r a n d the p r o d u c t i o n a n d survival probabilities o f excited a n d ionized i m p l a n t e d states m u s t change. Thus, with lead, the r e a c t i o n is a result of m a n y complex m e c h a n i s m s a n d the simple chemical relationships c o u l d well be s w a m p e d . Clearly, the above, o f necessity, a p p r o x i m a t e analysis c a n n o t s h o w m o r e t h a n a general t r e n d a n d m a n y m o r e species s h o u l d be used to establish the extent o f the v a r i a t i o n of chemical effect o n ion emission over the whole periodic table.

Icu d t

o

½xn

'

w h e r e t ' is the time at w h i c h the s e c o n d a r y C u ÷ signal reaches its m a x i m u m value. As p o i n t e d out, such a table of values gives a n overall picture o f at least two effects: chemical a n d lattice change. T h e only way to a t t e m p t to extract the chemical c o n t r i b u t i o n to this effect is to m a k e the u n r e a s o n a b l e a s s u m p t i o n t h a t the lattice c h a n g e is always the same as for the C u ÷ ---* C u case. T h u s , by s u b t r a c t i n g o u t a t e r m o f m a g n i t u d e equal to the C u + - 4 C u value of R a ' c h e m i c a l ' r a t i o Rc h a s also been derived. N o w the position is consistent with the degree o f electropositivity of the chemical species; K + h a v i n g t h e m o s t negative effect, Cs ÷ s o m e w h a t less a n d all o t h e r species so far investig a t e d except P b ÷ h a v i n g a positive effect (Re > 0).

Conclusion T h e present results have s h o w n t h a t the s e c o n d a r y ion emission of Cu + f r o m a c o p p e r lattice is d e p e n d e n t o n the previous i m p l a n t a t i o n experience o f the target. Since a c h a n g e o f p r o b e ion f r o m Cu + to A r + also shows a n effect this m u s t imply t h a t the lattice reorders d u r i n g the second b o m b a r d m e n t and, d u r i n g this a d j u s t m e n t , the s e c o n d a r y C u + signal is d r a m a t i c a l l y increased. In a d d i t i o n to this there is seem to be a chemical effect o n the m a g n i t u d e o f Cu + signal d e p e n d e n t o n the chemical n a t u r e of the i m p l a n t species. F u r t h e r e x p e r i m e n t s with a wide variety o f different i m p l a n t species are being u n d e r t a k e n to establish the p a t t e r n o f these effects across the w h o l e periodic table o f elements. 359

J S Colligon and G Kiriakidis: Effect of impurities on SIMS

References C A Andersen and J R Hinthorne, Analyt Chem, 45, 1973, 1421. 2 C A Andersen, Int J Mass Spec Ion Phys, 2, 1969, 61. 3 Z Jurela, Radiat Eft, 13, 1972, 167. a j M Schroeer, T N Rodin and R C Bradley, SurfSci, 34, 1973, 171. 5 j Maul and K Wittmaack, SurfSci, 47, 1975, 358. C A Andersen and J R Hinthorne, Science, 175, 1972, 853. 7 G Kiriakidis, J S Colligon and S P Chenakin, Radiat Eft, 41, 1979, 119.

360

s G K Wehner, Proc 3 Soviet Conf on Interaction of Atomic Particles with Solids, Kiev USSR, 28-31 October (1974). 9 H H Andersen and H L Bay, Int Conf on Ion Surface Interaction, Sputtering and Related Phenomena, Garching September 1972 Edited by R Behrisch, W Poschenreider, P Staib and H Vvrbeek, Gordon and Breach, London, p 63 (1973). 11 G Kiriakidis, C E Christodoulides, G Carter and J S Colligon, ApplPhys, 19, 1979, 191. I W L Brown, Proc VII Int Conf on Atomic Collisions in Solids, Moscow, September (1977).