THE FIM100 - PERFORMANCE OF A COMMERCIAL ATOM PROBE SYSTEM

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THE FIM100 - PERFORMANCE OF A COMMERCIAL ATOM PROBE SYSTEM

A. Cerezo, G. Smith, A. Waugh

To cite this version:

A. Cerezo, G. Smith, A. Waugh. THE FIM100 - PERFORMANCE OF A COMMERCIAL ATOM PROBE SYSTEM. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-329-C9-335.

�10.1051/jphyscol:1984955�. �jpa-00224441�

THE FIMlOO - PERFORMANCE OF A COMMERCIAL ATOM PROBE SYSTEM

A. C e r e z o , G.D.W. Smith and A.R. Waugh*

Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, Oxford 0X1 3PH, U.K.

*VG Soientifio Ltd., The Birches Industrial Estate, Imberhorne Lane, East Grinstead, Sussex, RH19 1UB, U.K.

Résumé - La sonde à atomes VG Scientific FIM100 offre la possibilité de puiser l'échantillon soit par la tension soit par laser. La compensation en énergie par un analyseur à secteur électrostatique conduit à des résolutions en masse de 2000 mesurées à mi-hauteur en mode d'impulsions de tension. Les propriétés de transmission et de résolution en masse du système sont décrites et des résultats préliminaires sur Alnico "5 et sur la martensite Fe—Ni-C sont reportés. Le "pile-up" des ions est considéré et une procédure de correction statistique des analyses est proposée.

Abstract - The VG Scientific FIM100 Atom Probe system has facilities for both voltage and laser pulsing of the specimen. Energy compensation by an electrostatic sector analyser yields FWHM mass resolutions of 2000 when using voltage pulses. The transmission and mass resolution properties of the system are described and preliminary results on Alnico5 and Fe-Ni-C martensite are reported. 'Pile-up' of ions is considered and a procedure for statistical correction of analyses is proposed.

The prototype of the VG Scientific FIM100 was delivered to Oxford in January 1984.

This is the first commercial atom probe system and was developed in collaboration with the Department of Metallurgy, Oxford University [l]. The vacuum system of the FIM100 consists of two main parts as shown in figure 1. Specimens are loaded via a fast-entry airlock into the storage/preparation chamber where up to six can be kept and where various treatments (e.g. heating, metal deposition, ion-beam thinning) can be carried out. A specimen can then be transferred to the analysis chamber which has full cooling and manipulation facilities and allows analysis by both atom probe and imaging atom probe (time-gated desorption probe) techniques.

Main chamber vacuum is typically lxlO-10 mbar and reaches 3 x 1 0 " ^ mbar when the cryostat is cooled. The instrument is equipped with timing and control

electronics and full on-line computing facilities as described elsewhere in these Proceedings [2J. In atom probe analysis, times-of-flight are measured to an accuracy of Ins and stored for future reference, while a variety of real-time displays are maintained. Pulsing of the specimen can be carried out with either high-voltage or nanosecond laser pulses.

For greatest mass resolution when using high-voltage pulses, the FIM100 is equipped with an energy compensator of the Poschenrieder type [3]. The main electrodes of the compensator are precision machined from an aluminium alloy forging to a tolerance of l/400mm (l/1000in.) and the sector has annular fringe- field electrodes and Herzog terminations. With a principal radius of 30cm, the system has a total flight path of 226cm. Since this system yields a large number of conditions giving spatial focussing, the voltage conditions are set to give highest mass resolution possible with greatest transmission. This corresponds to a ratio between the electrode voltages of 1.08, i.e. V1=-0.200V, V2=*+0.185V where

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984955

JOURNAL DE PHYSIQUE

V is the ion energy. Under typical operating conditions (largest acceptance angle, 15% pulse fraction) this yields a FWHM mass resolution of 2000; when measured at 10% of peak maxima the resolution is better than 800 (fig.2).

Decreasing the acceptance angle by increasing the tip to channel--plate distance (variable in the range 60-150mm, corresponding to an input angle of 32-14mrad) dramatically improves the mass resolution (figs.2 & 3).

Figure 4 shows the transmission characteristics of the FIMlOO energy compensator;

ions are passed without significant loss for a variation in their energy of -2.

Ions with energies less than 90% of the centre of the passband are totally stopped, thus allowing analysis to be carried out in the presence of image gas.

The corresponding variation of mass resolution at 10% of peak height is shown in figure 5. The departure of the transmission versus channel-plate distance (d) curve from a l/d2 relation (fig.6) indicates some limiting of the acceptance angle other than at the channel-plate aperture, but this effect is small. Pulse

fraction has only a minor effect on mass resolution (fig.71, yielding high resolution even at values of 25%.

One early use of the FIMlOO at Oxford has been in the study of magnetic materials of the Alnico type, where the high mass resolution available allows accurate quantitative analyses of constituents which have similar mass-to-charge ratios.

Figure 8 is a typical spectrum from Alnico5 showing cobalt and the isotopes of nickel and iron fully resolved. From the measured isotopic ratios of the iron peaks the hydride concentration is determined as being less than I%, most of the hydrogen probably being due to out-gassing of the channel-plate during imaging.

The increased mass resolution at greater tip to channel-plate distances gives practically unambiguous identification of species in, for example, studies of ordering. We have found partial ordering in the Alnico5 system (fig.9), this work being reported in full elsewhere in these Proceedings [4] The FIMlOO has also been used to examine Fe-Ni-C martensites for evidence of modulation in the carbon concentration following low temperature ageing [5 ] (fig

.

With the time-focussing obtained on the FIMlOO and similar energy compensated systems, the spread in flight times of a given species is approximately equal to the pulse pair resolution of the detection system (fig.2). In our case, peak width at 10% of maximum and pulse pair resolution are both about 511s. Thus, two ions of the same mass-to-charge ratio evaporated on a single pulse will not be distinguished and will be detected as a single ion. This 'pile-up' can produce errors in atom probe analysis, the error being greater for higher evaporation rates. A similar effect will occur in pulsed laser atom probes due to the small time spread of ions in this type of system [6]. The problem was first

investigated by T.T.Tsong et al. [7] who assumed that the number of ions per pulse follows a Poisson distribution. This presumes that the evaporation behaviour is that of random events occurring at a certain average rate which is often not the case. While the Poisson distribution (fig.11) provides a good approximation to the case of small voltage blocks or frequent voltage ramping, it does not match the experimental distribution for large voltage blocks or the case of analyses taken on a low-index pole (fig.12). However a fairly simple statistical correction can be applied to analyses to partially compensate for the effect of 'pile-up' without making any prior assumption as to the distribution of ions per pulse. By the multinomial distribution, if mutually exclusive events i occur with probability pi, the probability that ni of such events will occur in n trials is

If we assume that the probability of detecting two ions of the same species (taking different isotopes as separate species) on a given pulse to be small, then for m ions being generated on a pulse, the probability of detecting all m ions is

Thus g i v e n a c e r t a i n number of e v e n t s w i t h m i o n s d e t e c t e d we can c a l c u l a t e approximately how many i o n s were l o s t by ' p i l e u p ' . Such e v e n t s would a p p e a r a s ones of fewer i o n s p e r p u l s e , i . e . m-1, m-2 i o n e v e n t s e t c ; t h e number of each of s u c h o c c u r r e n c e s can a l s o be c a l c u l a t e d . The a l g o r i t h m f o r a c o r r e c t i o n p r o c e d u r e would be :-

For m i o n e v e n t s , r e p e a t e d from t h e l a r g e s t v a l u e of m t o m=2:

( 1 ) c a l c u l a t e t h e a p p a r e n t p r o b a b i l i t y f o r each s p e c i e s from t h e a n a l y s i s . ( 2 ) c a l c u l a t e t h e sum of p ( l , l , . . ) terms t o o b t a i n t h e number of e v e n t s where m i o n s were g e n e r a t e d .

( 3 ) c a l c u l a t e t h e p r o b a b i l i t y of e v e r y e v e n t t y p e where ' p i l e - u p ' would occur and i n each c a s e c a l c u l a t e t h e number of i o n s l o s t by ' p i l e - u p ' of 2,3, e t c . and c o r r e c t f o r t h e appearance of t h i s e v e n t a s one of (m-x) i o n s and ( 4 ) c o r r e c t t h e a n a l y s i s by a p p o r t i o n i n g t h e ' l o s t ' i o n s a c c o r d i n g t o t h e p r o b a b i l i t y of t h e i r l o s s from a g i v e n peak. Thus, i o n s l o s t by 'pile-up' of 2 s h o u l d be d i s t r i b u t e d amongst a l l t h e peaks a s pi2 where p i i s t h e p r o b a b i l i t y f o r t h e s p e c i e s , i.e. i t s atomic f r a c t i o n .

As an added r e f i n e m e n t , t h e a l g o r i t h m could a l s o t a k e account of t h e p r o b a b i l i t y of two i o n s w i t h i n t h e same peak being d e t e c t e d i n a s i n g l e p u l s e . This w i l l depend on t h e o p e r a t i n g c o n d i t i o n s and mass r e s o l u t i o n of t h e i n s t r u m e n t concerned. I n a high r e s o l u t i o n system t h e p r o b a b i l i t y f o r t h i s w i l l be s m a l l and s o g e n e r a t e l i t t l e e r r o r i n t h e c o r r e c t i o n . The procedure above may a l s o be i t e r a t e d i f necessary. Applying t h i s c o r r e c t i o n t o a spectrum o b t a i n e d from t h e Fe-Ni-C m a r t e n s i t e , we o b t a i n t h e f o l l o w i n g r e s u l t s :

T o t a l no of i o n s = 14300

Average i o n r a t e = 0.02 i o n s p e r p u l s e

I

I

I o n s c o l l e c t e d

I o n s l o s t ( c a l c u l a t e d )

C o n s i d e r i n g t h e i s o t o p i c r a t i o s of i r o n we o b t a i n : I o n e v e n t s (m=)

Apparent C o r r e c t e d

12073 1290

( E r r o r s a r e one s t a n d a r d d e v i a t i o n ) . 1 12515 11313

i s o t o p e

N a t u r a l Abundance Apparent

C o r r e c t e d

Even f o r t h e r e l a t i v e l y low e v a p o r a t i o n r a t e of 0.02 i o n s per p u l s e t h e r e i s a s i g n i f i c a n t l o s s of i o n s by 'pile-up'. It should be noted t h a t t h e measured abundance of 5 7 ~ e a f t e r c o r r e c t i o n of t h e a n a l y s i s i s s t i l l t o o h i g h ; t h i s is p r o b a b l y due' t o t h e p r e s e n c e of t h e 5 6 ~ e ~ 2 + s p e c i e s but i n d i c a t e s a l e v e l of h y d r i d e f o r m a t i o n of only f % , r e f l e c t i n g t h e e x c e l l e n t vacuum i n t h e system.

1816 17

288 -

2 775 1870

5 4 ~ e 5.8%

6.3 t 0.2%

5.7 c 0.2%

I

71 178

56Fe 91.7%

91 + 1%

92 + 1%

57Fe

1

2.2%

2.9 1 0.2%

2.7 2-0.2%

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Fig.1 - Schematic of FIMlOO vacuum system.

v."'"'

C H A M B E R C H A M B E L

"..,.",.TO.

Tungsten time spectrum -large input aperture.

Tungsten time spectrum -small input aperture.

Fig.3 - Mass r e s o l u t i o n s ( f u l l widths a t half maximum, a t tenth maximum and a t one hundredth of peak maximum) as a f u n c t i o n of acceptance angle.

IONS

50.

10.00 10.05 10.10 10.15 9.70 9.75 9.80 9.85

TIME 9 s ) TIME (ps)

Fig. 2 - Tungsten time spectra a t two acceptance angles

(0 = 32 mad and 0 = 20 m a d ) . - Ins timing channels.

r sews.

t a z w -

1

"T ,

1

'a2W=' 184W3' ISSW3.

lSIW3'

IONS

d h . . , I . 1 1 ,

50 -

Lh

Fig.4 - Transmission characteristics for the FIMIOO energy compensator system.

1

m'Am

1

Fig. 5

Analyser transmission vs. tip to screen distance.

Fig.6

-

5001

Fig. 7

1000-

500.

Pulse fraction (%) .

10 20 30

C9-334 JOURNAL DE PHYSIQUE

Mass spectrum from Alnico5 bright (a') phase.

IONS seFe*

1000-

I

from Alnico5.

- 2

' 3 9

z Fig.9 - Ladder diagram from 2

Alnico5 superlattice pole 0

(shown inset).

Ladder diawam for Alnico5 superlattice pole.

Fe ions

Carbon composition profile for RT aged Fe-Ni-C.

( S A W 9(E 100 ow>

Total number of ions collected ( ~ 1 0 0 0 )

Autocorrelation function for carbon in RT aged Fe-Ni-C.

(SAYRE W. 200 ICU81

Fig.10 - Micrograph, composition profile

f and autocorrelation analysis of room temperature aged Fe-Ni-C martensite.

Samples

-IJ 25 5 0 7 5 100

Fig.11

A family of Poisson distribution curves (theoretical).

x b) Fe-Ni-C o C ) Alnico5

Fig.12

Some experimental distributions:

a) W, voltage ramped,

b) Fe-Ni-C, constant voltage, c) Alnico5, superlattice pole.

Acknowledgements

Financial support for this work was provided by the Paul Instrument Fund of the Royal Society, the Science and Engineering Research Council (SERC), the

E.P.Abrahams Trust and VG Scientific Limited. A.Cerezo also wishes to thank VG Scientific for the provision of a CASE Award.

References

[l] Waugh A.R. and Smith G.D.W., Proc. 30th IFES, Philadelphia (1938).

[ 2 ] Cerezo A., Godfrey T.J., Moore A.J. and Smith G.D.W., these Proceedings.

[3] Poschenrieder W.P., Int .J.Mass Spec.Ion Phys. 9, (1972) 357.

[4] Hetherington M.G., Cerezo A., Jakubovics J.P. and Smith G.D.W., these Proceedings.

[5] Chang L. et al, these Proceedings.

[ 6 ] Tsong T.T. and Kinkus T.J., Phys.Rev.B. 9 (1984), 529.

[7] Tsong T.T., Ng Y.S. and Krishnaswamy S.V., Appl.Phys.Lett. 2 (1978), 778.