DEVELOPMENT AND INITIAL APPLICATIONS OF A POSITION

JOURNAL DE P H Y S I Q U E Colloque C6, suppl6ment au n0ll, Tome 49, novembre 1988

DEVELOPMENT AND INITIAL APPLICATIONS OF A POSITION-SENSITIVE ATOM PROBE A.

CEREZO, T.J.

G O D F R E Y and G.D.W. S M I T H

Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, GB-Oxford OX1 3PH, Great-Britain

Abstract - A position-sensitive detector system has been combined with a short flightpath atom probe to yield an instrument of large collection angle which identifies both the chemical nature and position of field evaporated ions. This allows the distribution of all elements to be mapped out simultaneously and, with depth profiling, permits threedimensional reconstruction of the original compositional variations. Applications of the technique to the study of ordering, fine-scale precipitation and spinodal decomposition in metallic specimens are described, together with examples of the analysis of surface films in ceramic oxide superconductors, and interfaces in semiconductor materials.

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INTRODUCTION

One of the limitations of existing atom probe designs is the amount of potential information which is lost in the destructive analysis. Currently it is possible to obtain either a quantitative analysis from a small selected area of the specimen (as in the probe-hole atom probe) or a map of the distribution of a single species over a more extended area (the imaging atom probe). In order to gain the largest volume of data, an ideal system would map both position and chemical information over the whole specimen surface. This requires a combination of a field-ion microscope, time-of-flight spectrometry and a position-sensitive detector (PSD). Substantial literature exists on the design and construction of position-sensitive detectors e . . 4 These systems are typically used for low light level imaging, such as in astrophysical applications. In the area of field-ion microscopy a PSD has previously been used to accumulate field-ion images and time-gated photo-induced desorption images [5]. The present work uses a wedge-and-strip type PSD in a short flight-path atom probe configuration to detect ions field evaporated from a region of 10-20nm in diameter on the specimen surface [681. The resultant system yields both flight-time and spatial information for detected species, although due to the serial nature of the PSD, the position data can only be obtained for one ion per desorption pulse. A low evaporation rate must therefore be used to reduce the probability of multiple ion events. With the data available from the detector, the distributions of all the elements present on the specimen surface can be obtained simultaneously (allowing for possible Subsequently, compositions may be calculated trajectory aberrations of the evaporated ions [9]). from selected regions of the analysed area by considering only ions which lie with a given range of co-ordinates. As evaporation proceeds, and successive atomic layers are analysed, a map of the microchemical variations originally present in the sample is built up in three dimensions. 2

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DETECTOR DESIGN

Figure 1 shows a schematic diagram of the position-sensitive atom probe (POSAP) design, full details of which are given elsewhere [ 7 ] The double channel-plate assembly provides detection of single ions incident on the front face, and the resultant charge cloud is divided by the wedge-and-strip anode (WSA) [ 3 , 4 ] in such a way as to encode the position of impact of the ion. A timing signal derived from the channel plate output is used both for the time-of-flight determination and to synchronise the measurement of the WSA output charges. The flight times are measured to an accuracy of Ins, and PSD output charges are encoded via a 12-bit ADC, with the co-ordinate calculation being carried out in software.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988605

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flexible thermal /connection

Figure 1. Schematic diagram of the position-sensitive atom probe system.

vacuum chamber

anode The prototype POSAP detector developed at Oxford is based around a 30mm double channel plate assembly. It has been mounted on the VG FIMlOO instrument at 900 relative to the imaging channel plate, allowing specimens to be imaged conventionally, and a selected region brought onto the detector for analysis. The distance from specimen tip to detector is 12Omm, yielding a detection angle of about 14O (250mrad) which corresponds to an angular field of view of 21° (370mrad). For a specimen imaged in neon at 8kV, this is equivalent to a region approximately 15nm in diameter. Analysis can be carried out using either voltage pulses, or with 5ns laser pulses from a Nd-YAG system. The mass resolution obtained in either case is found to be about 1/30 full-width at half maximum, as would be expected given the short flight path of the instrument. This includes a mass spread due to the range of flight paths to the flat detector (expected to generate an error of om/m = 1.5%) which could be compensated for via a simple correction in the mass-to-charge calculation. However, this mass resolution is wholly adequate in many systems, as shown in the mass spectrum from an yttrium barium cuprate (YBaCuO) ceramic superconductor, figure 2. Despite the spread in each peak, the species are easily identified, and useful information is obtained (see below). Only the overlap between 02+ and Cu2+ at around m/n=32 causes any serious difficulty for quantitative analysis. The spatial resolution obtained with the PSD is indicated by the two images of figure 3, showing a comparison between a field-ion image obtained on a conventional channel-plate/screen assembly (figure 3a) and the digitised image produced after accumulation of 90,000 field-ionised gas atoms Although the quality of the ,computer image is limited by counting on the PSD (figure 3b). statistics and the method of display, the resolution is comparable to that photographed from the imaging screen. Figure 4 shows the position resolution which is achieved during POSAP analysis,

Figure 2.

POSAP mass spectrum obtained from a YBaCuO ceramic superconductor.

Figure 3. Comparison of the field-ion micrograph (a) and a digitised field-ion image obtained with the positionsensitive detector (b) from che same region of a cungsten specimen. Conditions were: specimen voltage 7kV, temperature 60K, He imaging gas pressures 10-5mbar (a) and 10-7mbar ( b ) .

Figure 4. a) Digitised fieldion image of the (110) pole of a tungsten imaged in 5~10-~mbar He, at 12.5kV. b) Distribution of fieldevaporated tungsten ions ( ~ 3 + and w4+) showing resolution of individual (110) planes.

where individual atomic planes evaporated from the (110) pole of a tungsten specimen are easily resolved. The 2000 ions displayed in the image of figure 4b) represent the removal of about 0.3 monolayers of material. This image is similar to that observed with the field desorption of tungsten in the imaging atom probe, showing that the position-sensing system is introducing no significant errors. The spatial resolution of the POSAP will thus be similar to that obtained with current atom probe techniques (e.g. imaging atom probe), being limited by possible trajectory aberrations close to the specimen surface [ 9 ] .

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RESULTS

The POSAP has been used to study microstructure in a number of metallfc alloys, as well as superconductor surfaces and interfaces in multiple quantum-well layers [8,10]. This instrument has proved to be remarkably easy to use, since the lower fields during POSAP analysis greatly reduces the likelyhood of specimen fracture, even while operating at reduced specimen temperatures (30-40K) in order to minimise pulse fraction and trajectory aberration effects (see above). Up to a million ions can be obtained routinely from a single specimen, posing serious difficulties in the storage and analysis of such quantities of data. In many cases, the display of data also provides some challenge as we have the requirement to illustrate three-dimensional spatial distributions for several elements. The use of colour has become almost essential, since this allows the display of a number of elements simultaneously. However, the full visualisation of the three-dimensional variations is very difficult with typical computer graphics, let alone standard publication forms. It is normally only possible to display a section, or a series of sections through the data (as in the examples below), with a typical section having a thickness of 1-4nm. This is likely to be the only solution available short of computer-aided holography [ll], although various forms of 3-D enhancement, such as shading, might provide some benefits.

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Figure 5 shows a POSAP a n a l y s i s from t h e permanent magnet a l l o y Alnico 2 . T h i s m a t e r i a l c o n s i s t s p r i m a r i l y of an i s o t r o p i c i n t e r c o n n e c t e d m i c r o s t r u c t u r e of alpha-1 (Fe-Co r i c h ) and a l p h a - 2 (AX-Ni r i c h ) phases, a s seen i n t h e POSAP image o f f i g u r e 4a. S i m i l a r phases a r e observed i n t h e a n i s o t r o p i c Alnico 5 [12], a s w e l l a s Alnico 7 [13]. However i n t h e Alnico 2 a l l o y , a t h i r d The Fe, phase was found i n t h e form o f d a r k l y imaging p a r t i c l e s w i t h i n t h e alpha-2 r e g i o n [14]. A l , and Cu d i s t r i b u t i o n s obtained by t h e POSAP ( f i g u r e 4b) show such a p a r t i c l e , i n d i c a t i n g t h a t F u r t h e r a n a l y s i s shows t h e phase i s i n f a c t almost pure Cu, t h e phase i s h i g h l y copper-rich. This composition i s c o n s i s t e n t with t h e Al-Ni-Cu phase diagram with o n l y 6atX A 1 i n s o l u t i o n . [151, and thermodynamic c a l c u l a t i o n s i n d i c a t e t h a t t h e copper phase p r e c i p i t a t e s o u t o f t h e alpha-2 phase a t low ageing temperatures. The formation of t h e s e Cu p a r t i c l e s provides one mechanism f o r t h e i n c r e a s e o f c o e r c i v i t y w i t h copper c o n t e n t i n Alnicos c o n t a i n i n g o v e r 2.5wtgCu. I n a number o f previous s t u d i e s of copper-bearing i r o n a l l o y s , t h e conventional atom probe was used t o measure t h e composition of copper p a r t i c l e s p r e s e n t a t t h e e a r l i e s t s t a g e s of ageing [16-201. However, a n a l y s i n g s m a l l dark p a r t i c l e s such a s t h e s e i n t h e atom probe i s made d i f f i c u l t by t h e need t o a c c u r a t e l y a l i g n a region of u n c e r t a i n e x t e n t on t h e probe h o l e , and any misalignment can l e a d t o an i n c o r r e c t composition d e t e r m i n a t i o n . I n t h e POSAP, t h i s i n i t i a l s e l e c t i o n is n o t r e q u i r e d , and t h e s i z e o f t h e p a r t i c l e is determined d i r e c t l y from t h e a n a l y s i s . A POSAP image from Fe-l.3wtXCu-l.4wt%Ni aged t o peak hardness i s shown i n f i g u r e 5. T h i s Cu p a r t i c l e was t h e s m a l l e s t one so f a r observed i n t h e POSAP, although t h e dark a r e a i n t h e f i e l d i o n image was f a r l a r g e r than t h e 1-2nm diameter p r e c i p i t a t e s i z e a c t u a l l y m e a s u r e d . A composition c a l c u l a t e d from t h e region of t h e p a r t i c l e shows almost' pure copper, w i t h o n l y a maximum of S a t % Fe. T h i s i s c o n t r a r y t o r e s u l t s o b t a i n e d i n conventional atom probe a n a l y s i s o f model a l l o y s [16,19,20], which i n d i c a t e d a i r o n c o n t e n t o f approximately 50atX a t t h i s h e a t t r e a t m e n t , b u t i s much c l o s e r t o t h e v a l u e expected from thermodynamic c a l c u l a t i o n s [20].

It has a l r e a d y been shown t h a t t h e depth r e s o l u t i o n of t h e POSAP matches t h a t expected from t h e c o n v e n t i o n a l atom probe, and t h a t i t i s p o s s i b l e t o monitor t h e c o l l a p s e o f (110) p l a n e s from t h i s r e g i o n o f a t u n g s t e n specimen d u r i n g POSAP a n a l y s i s . This generates the p o s s i b i l i t y of o b t a i n i n g a d i r e c t , a b s o l u t e depth s c a l e w i t h i n t h e a n a l y s i s of a low index r e g i o n o f a f i e l d - i o n specimen. The technique a l s o provides a method of s t u d y i n g o r d e r with i n c r e a s e d compositional accuracy, s i n c e more i o n s can be c o l l e c t e d p e r l a y e r i n t h e POSAP than i n t h e conventional AP. F i g u r e 7 i l l u s t r a t e s t h i s , showing t h e a n a l y s i s from a (100) r e g i o n o f a Ni3A1 specimen corresponding t o t h e removal of a f r a c t i o n of a monolayer. Within t h e c e n t r a l a r e a t h e o r d e r is c l e a r l y seen w i t h a l t e r n a t i n g N i ( b l u e ) and N i - A 1 ( b l u e and g r e e n ) p l a n e s . Due t o t h e l a r g e c o l l e c t i o n a r e a of t h e POSAP, t h i s type o f experiment does n o t s u f f e r from t h e l a r g e f l u c t u a t i o n s i n i o n r a t e encountered i n o r d e r i n g s t u d i e s using t h e probe-hole atom probe. The POSAP a l s o provides a powerful technique f o r t h e study of s u r f a c e s , s i n c e i t s l a r g e r c o l l e c t i o n a n g l e allows a more a c c u r a t e composition determination from very t h i n l a y e r s , w h i l s t t h e s p a t i a l r e s o l u t i o n can show t h e presence of any t h i c k n e s s v a r i a t i o n s o r i n t e r f a c e roughness. Work i s c u r r e n t l y i n p r o g r e s s a t t h e a u t h o r s ' l a b o r a t o r y t o s t u d y s u r f a c e and i n t e r f a c e s on ceramic oxide superconductors by both POSAP and c o n v e n t i o n a l atom p r o b e t e c h n i q u e s [21]. Figure 8 shows a c r o s s - s e c t i o n through t h e POSAP a n a l y s i s o f an y t t r i u m barium c u p r a t e (YBaCuO) superconductor vacuum annealed f o r 2 h o u r s a t 500%. The s u r f a c e i s found t o be e n r i c h e d i n barium (shown i n g r e e n ) with a monolayer t h i c k n e s s of composition Ba-15at%O i n t h i s case.

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CONCLUSIONS

The p o s i t i o n - s e n s i t i t e atom probe provides a powerful new t o o l i n t h e s t u d y o f s u r f a c e s and i n t e r f a c e s , a s w e l l a s a l l o y phase m i c r o s t r u c t u r e . Given i t s a b i l i t y t o combine t h r e e d i m e n s i o n a l r e c o n s t r u c t i o n o f c o m p o s i t i o n a l v a r i a t i o n s , with sub-nanometre r e s o l u t i o n , and a c c u r a t e a n a l y s i s o f s e l e c t e d r e g i o n s from w i t h i n t h e d a t a volume, t h i s new s y s t e m i s a n important e x t e n s i o n t o c u r r e n t atom probe i n s t r u m e n t a t i o n .

ACKNOWLEDGEMENTS The a u t h o r s thank D r . M. G. Hetherington and M s . R . M. Hoyle f o r t h e i r h e l p i n o b t a i n i n g r e s u l t s used i n t h i s paper, and P r o f e s s o r S i r P e t e r H i r s c h FRS f o r t h e p r o v i s i o n o f l a b o r a t o r y facilities. We a l s o acknowledge t h e h e l p o f W. B. Burton and t h e SERC Rutherford Appleton Laboratory i n t h e production of t h e anode. AC i s g r a t e f u l t o t h e Science and E n g i n e e r i n g Research Council and Wolfson College, Oxford f o r support i n t h e form o f Research Fellowships during t h e course of t h i s work. S t u d i e s of iron-copper a l l o y s were c a r r i e d o u t with support from CEGB Berkeley and UKAEA Harwell L a b o r a t o r i e s , and t h e Alnico 2 m a t e r i a l was s u p p l i e d c o u r t e s y o f M. S a t u r , Swift-Levick L t d . , S h e f f i e l d .

F i g u r e 5 ( l e f t ) . Pe, A 1 and Cu e l e m e n t d i s t r i b u t i o n s f r o m two a r e a s a b o u t 10nm diameter i n an A l n i c o 2 permanent a) shows a l p h a - 1 ( P e - C o magnet a l l o y . r i c h ) and a l p h a - 2 ( A1-Ni rich) areas, w h i l e b) a l s o a 5nm p a r t i c l e o f a t h i r d C u - r i c h phase.

F i g u r e 6. POSAP a n a l y s i s f r o m a 1-2nm copper p r e c i p i t a t e i n Fe-1. 3wtXCu 1 . 4wt$Ni s o l u t i o n t r e a t e d and a g e d t o peak h a r d n e s s ( 5 5 0 ' ~f o r 2 h o u r s ) . T h i s image r e p r e s e n t s a s e c t i o n t h r o u g h t h e a l l o y o f a b o u t lnm d e p t h .

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F i g u r e 7. N i and A 1 d i s t r i b u t i o n s f r o m t h e ( 1 0 0 ) r e g i o n of an ordered N i 3 A 1 s p e c i m e n , s h o w i n g t h e a l t e r n a t e N i and mixed p l a n e s .

F i g u r e 8. POSAP c r o s s - s e c t i o n t h r o u g h t h e s u r f a c e o f a n YBaCuO s u p e r c o n d u c t o r a f t e r vacuum a n n e a l i n g f o r 2 h o u r s a t 500' C.

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[7] [8]

[9] [lo] [ll] 1121 [I31 [I41 [I51 [I61 [IT] [I81 [I91 [20] [21]

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