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IMAGING ATOM PROBE COMPUTER EXPERIMENTS
M. Miller
To cite this version:
M. Miller. IMAGING ATOM PROBE COMPUTER EXPERIMENTS. Journal de Physique Collo-
ques, 1984, 45 (C9), pp.C9-337-C9-342. �10.1051/jphyscol:1984956�. �jpa-00224442�
JOURNAL DE PHYSIQUE
Colloque C9, supplément au n°12, Tome 45, décembre 1984 page C9-337
IMAGING ATOM PROBE COMPUTER EXPERIMENTS
M.K. Miller
Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
Résumé - Des représentations de cartes élémentaires par sonde à atomes ont été générées par simulations numériques sur ordinateur pour évaluer les performances et déterminer les paramètres expérimentaux appropriés.
Les paramètres variables étaient la géométrie du spécimen, la structure du cristal, l'orientation, l'efficacité de détection, l'étendue et la morphologie des structures, la composition et.la quantité de matériaux extraite du spécimen. Une ségrégation aux joints a pu être distinguée lorsque le coefficient de partition entre les niveaux d'élément soluble aux joints et dans la matrice était approximativement 10 divisé par le niveau d'élément soluble dans la matrice (en %) avec un seuil à 1 % pour une monocouche de surface. La présence et la forme de petites particules isolées ont pu être distinguées lorsque la différence en concentration entre les particules et la matrice était à peu près de 5 %. Cette diffé- rence de composition était aussi utilisable pour révéler la morphologie des microstructures modulées.
Abstract - Computer simulations of imaging atom probe elemental maps were generated to evaluate the limits of performance of the IAP and to determine suitable experimental parameters. The parameters varied were specimen geometry, crystal structure, orientation, detection efficiency, extent and morphology of features, composition, and amount of material removed from the specimen. Segregation to boundaries could be distinguished when the partitioning coefficient b e t w e e n the l e v e l s of s o l u t e in the b o u n d a r y and m a t r i x was approximately 10 divided by the matrix solute level (in %) with a cutoff at 1 % of a monolayer coverage. The presence and shape of small isolated particles could be distinguished when the difference in solute concentration between the particles and the matrix was ~5 %.
This composition difference was also required in order to reveal the morphology of modulated microstructures.
I -INTRODUCTION.
Imaging atom probe (IAP) elemental maps are used to determine the spatial distribution of a single chosen species ( element ) with near atomic resolution. In this paper computer experiments are presented to outline suitable experimental parameters for producing elemental maps and to evaluate the limits performance of the IAP in three metallurgical ly interesting cases — precipitates, modulated micro- structures and boundaries. The experimental parameters cannot easily be determined in the IAP because of the practical difficulties of tailoring the microchemistry and microstructure of a given system over an extended range. A previous study by Delargy [1] using random dot maps has indicated the limitations of visual interpretation of these patterns. Since the contrast in the elemental maps is determined by the precise details of the microstructure and their interpretation is subjective, the values presented here should only be taken as a general guide to the performance of the IAP.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984956
JOURNAL DE PHYSIQUE
I1 - EXPERIMENTAL.
T h e m o d e l c h o s e n t o s i m u l a t e IAP g a t e d e l e m e n t a l m a p s w a s a t h i c k s h e l l v a r i a n t o f t h e t h i n s h e l l m o d e l d e v e l o p e d b y M o o r e [ 2 ] f o r s i m u l a t i n g f i e l d - i o n m i c r o g r a p h s . The i m a g e s show t h e c o m p l e t e f i e l d o f v i e w o f t h e i m a g i n g a t o m p r o b e f o r a s i n g l e c h o s e n s p e c i e s . E a c h d o t i n t h e i m a g e c o r r e s p o n d s t o t h e i m p a c t o f a s i n g l e i o n o n t h e d e t e c t o r . T h i s m o d e l d o e s n o t t a k e i n t o a c c o u n t t h e a d d i t i o n a l c o m p l i c a t i o n s d u e t o t h e e f f e c t s o f l o c a l n e i g h b o u r s , l o c a l m a g n i f i c a t i o n d i f f e r e n c e s b e t w e e n p h a s e s , o r t r a j e c t o r y a b e r a t i o n s . I t s h o u l d a l s o b e n o t e d t h a t m o s t e l e m e n t s e v a p o r a t e w i t h m o r e t h a n o n e c h a r g e s t a t e o r s p e c i e s s o t h a t t h e c o m p o s i t i o n s q u o t e d s h o u l d b e a d j u s t e d a c c o r d i n g l y .
The g e o m e t r y o f t h e m i c r o s c o p e s i m u l a t e d w a s b a s e d o n t h e s t a n d a r d P a n i t z c o n f i g u r a t i o n [ 3 ] c o n s i s t i n g o f a s p e c i m e n t o d e t e c t o r d i s t a n c e o f 1 1 8 mm a n d a f i e l d o f v i e w o f 36O. The e x p e r i m e n t a l l y a d j u s t a b l e p a r a m e t e r s w e r e t h e s p e c i m e n r a d i u s , c r y s t a l s t r u c t u r e , o r i e n t a t i o n , d e t e c t i o n e f f i c e n c y , e x t e n t a n d m o r p h o l o g y o f f e a t u r e s , c o m p o s i t i o n , a n d t h e amount o f m a t e r i a l r e m o v e d f r o m s p e c i m e n . The s p e c i m e n r a d i u s a n d s h e 1 1 t h i c k n e s s o f t h e s i m u l a t i o n s w e r e g e n e r a l i s e d i n t e r m s o f t h e l a t t i c e p a r a m e t e r , a,, o f t h e m a t e r i a l .
111 - RESULTS
AND
DISCUSSION.The amount o f m a t e r i a l r e m o v e d o r s h e l l t h i c k n e s s s h o u l d b e s u f f i c i e n t t h a t a s i g n i f i c a n t n u m b e r o f a t o m s a r e d e t e c t e d w h i l e r e m o v i n g t h e minimum amount o f m a t e r i a l . L i m i t i n g t h e s h e l l t h i c k n e s s , i n a d d i t i o n t o p r e s e r v i n g t h e s p e c i m e n , a l l o w s s u c c e s s i v e m a p s o f d i f f e r e n t e l e m e n t s t o b e c o r r e l a t e d w i t h o u t a s i g n i f i c a n t l o s s o f s p a t i a l r e s o l u t i o n . The number o f a t o m s d e t e c t e d d e p e n d s o n s p e c i m e n r a d i u s , c r y s t a l s t r u c t u r e , c o m p o s i t i o n , amount o f m a t e r i a l e v a p o r a t e d f r o m t h e s p e c i m e n a n d d e t e c t i o n e f f i c i e n c y . The t o t a l number o f i o n s d e t e c t e d f o r a b c c i r o n s p e c i m e n a s a f u n c t i o n o f s p e c i m e n r a d i u s f o r s h e l l t h i c k n e s s o f a, i s shown i n F i g u r e 1. The s p e c i m e n r a d i u s i s e x p r e s s e d i n t e r m s o f t h e e x p e r i m e n t a 1 1 y m e a s u r e d B e s t I m a g i n g V o l t a g e o f t h e s p e c i m e n . A 1 0 kV n e o n i m a g e i s e q u i v a l e n t t o a s p e c i m e n r a d i u s o f a p p r o x i m a t e l y 50 nm. I f t h i n s h e l l s o f m a t e r i a l , t y p i c a l l y l e s s t h a n 0.25 a,, a r e r e m o v e d t h e p o l e s t r u c t u r e w i l l b e p r o n o u n c e d t h u s m a k i n g a n y m o r p h o l o g i c a l i n t e r p r e t a t i o n d i f f i c u l t .
The l o w e r l i m i t a t w h i c h a s o l u t e c o n c e n t r a t i o n may b e d e t e c t e d f o r a s i n g l e s p e c l e s d e p e n d s o n t h e s p e c i m e n r a d i u s , t h e amount o f m a t e r i a l e v a p o r a t e d a n d t h e n o i s e l e v e l o f t h e d e t e c t o r . S i n c e t h e e l e m e n t a l maps a r e a c c u m u l a t e d u s i n g p u l s e d f i e l d e v a p o r a t i o n , a s o p p o s e d t o a s i n g l e f i e l d e v a p o r a t i o n p u l s e , t h e t i m e d e p e n d e n t n o i s e l e v e l ( t y p i c a l l y a f e w c o u n t s p e r s e c o n d ) a l s o i n c r e a s e s w i t h i n c r e a s i n g a m o u n t o f m a t e r i a l r e m o v e d . T h i s e f f e c t i v e l y m a k e s t h e l e v e l o f d e t e c t i o n r e l a t i v e l y i n s e n s i t i v e t o s h e l l t h i c k n e s s a n d s p e c i m e n r a d i u s a n d i s o f t h e o r d e r o f 0 . 0 1 a t % a s s h o w n i n F i g u r e 2 f o r s o l u t e c o n c e n t r a t i o n s b e t w e e n 0.001 a n d 100 %.
The a b i l t y t o d e t e r m i n e t h e s h a p e o f p a r t i c l e s i s u s e f u l i n t h e r a r e c a s e s w h e r e t h e r e i s n o c o n t r a s t i n t h e f i e l d - i o n i m a g e o r i n c a s e s w h e r e t h e c o n t r a s t i s m a s k e d b y o t h e r m o r e p r o m i n e n t f e a t u r e s . T h e p r e s e n c e a n d a p p r o x i m a t e s h a p e o f sma 1 1 p a r t i c l e s may b e d e t e r m i n e d f r o m t h e e l e m e n t a l m a p s a s s h o w n i n F i g u r e 3 f o r a m a c r o l a t t i c e o f c u b i c p a r t i c l e s i n a s o l u t e f r e e m a t r i x . T h e s h a p e o f i s o l a t e d p a r t i c l e s becomes d i s c e r n i b l e when t h e s o l u t e l e v e l i n t h e p a r t i c l e s i s g r e a t e r t h a n a p p r o x i m a t e l y 5 % w i t h a s h e l l t h i c k n e s s o f a o . , The c o m p o s i t i o n d i f f e r e n c e s t h a t a r e r e q u i r e d t o d i s t i n g u i s h t h e micro-
s t r u c t u r e a l s o d e p e n d s o n t h e amount o f m a t e r i a l removed a s shown i n
-
DETECTION EFFICIENCY
-
-
-
4 8 12 16 X) 24 28
TIP RADIUS (kV)
0 4 8 12 16 20 24 28
TIP RADIUS IkVI
F i g 1. Number o f a t o m s e v a p o r a t e d F i g 2. Number o f a t o m s d e t e c t e d i n a, t h i c k s h e l l f o r a b c c i n a a, t h i c k s h e l l f o r a b c c i r o n s p e c i m e n a s a f u n c t i o n o f i r o n s p e c i m e n a s a f u n c t i o n o f s p e c i m e n r a d i u s ( a, = 0.286 nm ). s p e c i m e n r a d i u s a n d s o l u t e
c o n c e n t r a t i o n .
10 % 2223 i o n s
15 % 3310 ions
2 5 a 5 4 1 2 ions F i g u r e 3 . E l e m e n t a l maps o f a c u b i c m a c r o l a t t i c e o f p a r t i c l e s
c o n t a i n i n g 0 t o 25 % s o l u t e i n a s o l u t e f r e e m a t r i x . S h e l l t h i c k n e s s = 1 a. : S p e c i m e n r a d i u s = 1 7 5 a.
65 % d e t e c t i o n e f f i c i e n c y : b c c c r y s t a l s t r u c t u r e .
t h e t w o s e r i e s o f maps f o r a m o d u l a t e d m i c r o s t r u c t u r e t h a t g r a d u a l l y p h a s e s e p a r a t e s t o w a r d s p h a s e b o u n d a r i e s o f 0 a n d 50 % s o l u t e f r o m a n i n i t i a l s o l u t e c o n t e n t o f 25 %, F i g u r e 4. The m i c r o s t r u c t u r e c o u l d b e r e s o l v e d w h e n t h e d i f f e r e n c e i n c o m p o s i t i o n w a s a p p r o x i m a t e l y 1 0 %
C9-340 JOURNAL DE PHYSIQUE
Figure 4. Elemental maps of a modulated microstructure with 0.1 ( A ) and 1 a. ( B ) shell thickness. Numbers below micrographs indicate solute content in both phases.
Specimen radius = 250 a, : bcc crystal structure
w i t h a s h e l l t h i c k n e s s o f a, a n d i n c r e a s e d t o a p p r o x i m a t e l y 20 % i n t h e 0.1 a,, s h e l l t h i c k n e s s c a s e . A u t o c o r r e l a t i o n a n a l y s i s o f i n t e n s i t y l l n e s c a n s i n f a v o r a b l e d i r e c t i o n s a c r o s s t h e e l e m e n t a l maps i n F i g u r e 4b was a b l e t o d e t e c t p h a s e s e p a r a t i o n when t h e c o m p o s i t i o n d i f f e r e n c e s w e r e g r e a t e r t h a n 10 %.
I t s h o u l d b e n o t e d t h a t t h e s i z e r a n g e o f p a r t i c l e s t h a t may b e d e t e r m i n e d i s l i m i t e d t o t h o s e t h a t a r e l e s s t h a n t h e f i e l d o f v i e w o f t h e i m a g e w h i c h i s t y p i c a l l y b e t w e e n 1 5 a n d 80 nm i n d i a m e t e r . When t h e p a r t i c l e s a r e i n c l o s e p r o x i m i t y o r when c o m p o s i t i o n g r a d i e n t s a r e p r e s e n t a l a r g e r c o m p o s i t i o n d i f f e r e n c e i s r e q u i r e d .
An e s t i m a t e o f t h e c o m p o s i t i o n d i f f e r e n c e s may b e made f r o m t h e e l e m e n t a l maps b y d e t e r m i n i n g t h e number o f a t o m s p e r u n i t a r e a [ 4 ] . H o w e v e r , a n a b s o l u t e v a l u e o f t h e c o m p o s i t i o n c a n n o t b e o b t a i n e d s o l e l y f r o m e l e m e n t a l maps a s t h e t o t a l a m o u n t o f m a t e r i a l r e m o v e d f r o m t h e s p e c i m e n i s n o t known. An e s t i m a t e o f t h e t o t a l a m o u n t o f m a t e r i a l removed may b e o b t a i n e d , a s p r o p o s e d by B r e n n e r and M i l l e r , b y t h e a d d i t i o n o f a n u n g a t e d d e t e c t o r o u t s i d e t h e f i e l d o f v i e w o f t h e IAP d e t e c t o r b u t i n s i d e t h e l a r g e r f i e l d of v i e w of t h e c o m p l e t e image.
O b s e r v a t i o n of s e g r e g a t i o n t o i n t e r n a l b o u n d a r i e s was found t o depend o n t h e p a r t i t i o n i n g c o e f f i c i e n t a n d t h e l e v e l s o f s o l u t e i n t h e m a t r i x . The p a r t i t i o n i n g c o e f f i c i e n t , K , i s t h e r a t i o of t h e l e v e l s o f s o l u t e i n t h e b o u n d a r y a n d i n t h e m a t r i x . E x a m p l e s o f d e c o r a t e d b o u n d a r i e s e a c h c o n t a i n i n g 50 % o f a m o n o l a y e r f o r m a t r i x s o l u t e l e v e l s o f 1 , 5 , a n d 1 0 % a r e shown i n F i g u r e 5. T h e b o u n d a r y was c l e a r l y d i s t i n g u i s h e d when t h e p a r t i t i o n i n g c o e f f i c i e n t was 10 o r 50.
I n c a s e s s u c h a s t r a c e e l e m e n t e m b r i t t l e m e n t , where t h e m a t r i x s o l u t e l e v e l i s 0.1 % o r l o w e r , l a r g e r p a r t i t i o n i n g c o e f f i c i e n t s a r e r e q u i r e d a s shown i n F i g u r e 6 . An e m p i r i c a l r e l a t i o n s h i p f o r t h e mimimum o b s e r v a b l e p a r t i t i o n i n g c o e f f i c i e n t , KObs, f o r a s h e 1 1 t h i c k n e s s of a. was f o u n d i n t e r m s of t h e m a t r i x s o l u t e l e v e l a s f o l l o w s
Kobs = 10 / m a t r i x s o l u t e l e v e l ( i n % ).
1 % Matrix K = 5 0
5 % Matrix K = 1 0
1 0 % Matrix K = 5
F i g u r e 5. E l e m e n t a l maps of a d e c o r a t e d boundary a s a f u n c t i o n of m a t r i x s o l u t e c o n t e n t . Specimen r a d i u s = 250 a, : b c c c r y s t a l s t r u c t u r e : S h e l l t h i c k n e s s = 1 a.
d e t e c t i o n e f f i e c i e n c y = 100 % : Boundary c o v e r a g e = 50 % of a m o n o l a y e r .
C9-342 JOURNAL DE PHYSIQUE
T h i s r e l a t i o n s h i p i s o n l y a p p l i c a b l e when t h e c o v e r a g e o f t h e b o u n d a r y l a y e r was g r e a t e r t h a n 1% o f a m o n o l a y e r b e c a u s e o f t h e s m a l l number o f a t o m s d e t e c t e d . F o r e x a m p l e , o n l y a p p r o x i m a t e l y 4 0 s o l u t e a t o m s o r i g i n a t e f r o m t h e b o u n d a r y when a 1 0 a. t h i c k s h e l l i s e v a p o r a t e d a n d t h e c o v e r a g e i s 1 %.
0 . 1 % Matrix 0 . 1 % Matrix
F i g u r e 6 . E l e m e n t a l maps o f a d e c o r a t e d b o u n d a r y a t l o w m a t r i x s o l u t e c o n t e n t . S p e c i m e n r a d i u s = 250 a. : b c c c r y s t a l s t r u c t u r e d e t e c t i o n e f f i c i e n c y = 1 0 0 %.
A s i g n i f i c a n t i n c r e a s e i n t h e a p p a r e n t w i d t h o f t h e b o u n d a r y i s o n l y o b s e r v e d w h e n t h e b o u n d a r y i s i n c l i n e d a t a l a r g e a n g l e t o t h e s p e c i m e n a x i s a n d when m o r e t h a n a p p r o x i m a t e 1 y 0 . 5 nm o f m a t e r i a l i s f i e l d - e v a p o r a t e d f r o m t h e s p e c i m e n .
A c k n o w l e d g m e n t s
R e s e a r c h s p o n s o r e d b y t h e D i v i s i o n o f M a t e r i a l s S c i e n c e s , U.S.
D e p a r t m e n t o f E n e r g y , u n d e r c o n t r a c t DE-AC05-840R21400 w i t h M a r t i n M a r i e t t a E n e r g y S y s t e m s , I n c . T h e a u t h o r t h a n k s D r . S.S. B r e n n e r f o r h e l p f u l d i s c u s s i o n s .
R e f e r e n c e s
[ I ] K.M. D e l a r g y , P r o c 2 8 t h . I n t . F i e l d E m i s s i o n Symposium, P o r t l a n d ( 1 9 8 1 ) , p 1 3 0 .
[2] A.J.W. Moore, J P h y s Chem S o l i d s , 2 3 ( 1 9 7 1 ) 9 0 7 . [ 3 ] J . A . P a n i t z , Rev S c i I n s t r u m , 44 ( 1 9 7 3 ) 1 0 3 4 .
[ 4 ] M . K . M i l l e r , T . J . G o d f r e y , P.A. B e a v e n , P.R. W i l l i a m s , K.M. D e l a r g y a n d G.D.W. S m i t h , P r o c 2 5 t h . I n t F i e l d E m i s s i o n Symposium, A l b u q u e r q u e , ( 1 9 7 8 ) , p 5 0 .
