ADVANCES IN X-RAY OPTICS AT THE NATIONAL PHYSICAL LABORATORY

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ADVANCES IN X-RAY OPTICS AT THE NATIONAL PHYSICAL LABORATORY

A. Franks

To cite this version:

A. Franks. ADVANCES IN X-RAY OPTICS AT THE NATIONAL PHYSICAL LABORATORY.

Journal de Physique Colloques, 1984, 45 (C2), pp.C2-69-C2-72. �10.1051/jphyscol:1984217�. �jpa-

00223843�

ADVANCES I N X-RAY O P T I C S A T THE N A T I O N A L P H Y S I C A L LABORATORY

A . Franks

Division of Mechanical and Optical Metrology, National Physical Laboratory, Teddington, Middlesex TWll OLW, U.K.

Resume - Les p r o g r b s d e l a technologic d e f a b r i c a t i o n a u s s i b i e n que d e l a m e t r o l o g i e donnent l a p o s s i b i l i t 6 d e d6velopper d e s composants o p t i q u e s

r a y o n s X d e h a u t e q u a l i t d pour l 8 u s a g e d e l l a s t r o n o m i e , d e s

r e c h e r c h e s e n r a d i a t i o n s y n c h r o t r o n i q u e , e t de l a microscopic.

A b s t r a c t - Advances i n manufacturing technology and metrology have made p o s s i b l e t h e development o f h i g h q u a l i t y X-ray o p t i c a l components f o r u s e i n astronomy, s y n c h r o t r o n r a d i a t i o n r e s e a r c h and microscopy.

I

- INTRODUCTION

Renewed i n t e r e s t i n X-ray r e f l e c t i o n o p t i c s i n t h e e a r l y 1950s came a b o u t a s t h e r e s u l t o f t h e p i o n e e r i n g e f f o r t s o f , f o r example Ehrenberg [ I ] , K i r k p a t r i c k and Baez [ 2 ] and Wolter [3,4]. E f f o r t s were f i r s t d i r e c t e d towards X-ray microscopy, b u t t h e i n i t i a l enthusiasm faded a f t e r i t became e v i d e n t t h a t t h e achievement o f h i g h r e s o l u t i o n would r e q u i r e a p r e c i s i o n o f s u r f a c e f i g u r e and f i n i s h o f t h e m i r r o r s u r f a c e s which was beyond c u r r e n t c a p a b i l i t i e s . X-ray astronomy provided a new s t i m u l u s i n t h e 1960s t o d e v e l o p g r a z i n g i n c i d e n c e m i r r o r s . Synchrotron r a d i a t i o n r e s e a r c h p r o v i d e d t h e n e x t a p p l i c a t i o n and t h i s was followed by t h e development o f X-ray microscopes f o r l a s e r f u s i o n r e s e a r c h [51. The technology t o produce r e f l e c t i o n X-ray microscopes t o complement o p t i c a l and e l e c t r o n microscopes o n l y now a p p e a r s t o b e w i t h i n o u r g r a s p .

I1 - SPECIFICATION FOR X-RAY OPTICAL COMPONENTS

I t i s n o t p o s s i b l e t o make g e n e r a l i z e d s t a t e m e n t s a b o u t t h e s p e c i f i c a t i o n s f o r o p t i c a l components e x c e p t t o s t a t e t h a t t h e t r e n d i s towards a c h i e v i n g a r c second o r b e t t e r r e s o l u t i o n i n X-ray t e l e s c o p e s and t o sub-micrometre r e s o l u t i o n i n microscopes, which t r a n s l a t e s t o sub-arc second r e s o l u t i o n i n a n g u l a r terms.

The i m p l i c a t i o n s i n t e r m s o f manufacturing t o l e r a n c e s t o meet t h e s e r e q u i r e m e n t s h a v e p r e v i o u s l y been d i s c u s s e d [61 and may b e summarized a s f o l l o w s .

The most demanding t o l e r a n c e s are t h o s e a s s o c i a t e d w i t h a x i a l s l o p e and s u r f a c e roughness. It i s c o a v e n i e n t and r e v e a l i n g t o s t a t e t h e s l o p e e r r o r i n t e r m s of t h e p e r m i t t e d a m p l i t u d e v a r i a t i o n s o f t h e s u r f a c e f o r t h e r a n g e o f s u r f a c e wavelengths (waviness) which may r e s u l t from t h e manufacturing p r o c e s s e s . For t h e X-ray t e l e s c o p e q u o t e d i n C61, t h e t o l e r a n c e s o n s u r f a c e a m p l i t u d e c o r r e s p o n d i n g t o t h e r e q u i r e d 0.05 a r c s e c s l o p e t o l e r a n c e a r e 0.12 v m ,

0.012 m, a n d 2.5 nm f o r s u r f a c e wavelengths o f 500 mm, 50 mm, and 10 mm r e s p e c t i v e l y . The t o l e r a n c e s f o r s u r f a c e f i n i s h a r e e q u a l l y s e v e r e . I n t h e example q u o t e d , t h e p r o p o r t i o n o f e n e r g y which f a l l s w i t h i n a 1 a r c s e c d i a m e t e r a p e r t u r e f o r a n on-axis p o i n t s o u r c e i s no more t h a n 50% f o r a roughness o f 1 nm

(rms). S i m i l a r c a l c u l a t i o n s have been made f o r a n X-ray microscope [ 7 1 which show t h a t t h e s u r f a c e p r o f i l e may n o t d e p a r t by more t h a n 2 nm from i t s t h e o r e t i c a l form and t h a t t h e s u r f a c e roughness s h o u l d n o t exceed 0 . 2 rim (rms).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984217

C2-70 JOURNAL DE _PHYSIQUE

I11 - PRODUCTION OF X-RAY OPTICAL COMPONENTS

Developments h a v e t a k e n p l a c e i n t h r e e main a r e a s i n o r d e r t o a c h i e v e t h e t o l e r a n c e s d i s c u s s e d i n S e c t i o n 11.

1. M a n u f a c t u r i n g Technology. - The r o u t e t o w a r d s e f f i c i e n t p r o d u c t i o n i s by way o f v e r y p r e c i s e machining o f m i r r o r s u b s t r a t e s , p a r t i c u l a r l y by diamond t u r n i n g o f m e t a l s a n d p r e c i s i o n g r i n d i n g o f g l a s s e s , f o l l o w e d by l a p p i n g and p o l i s h i n g . Lapping and p o l i s h i n g machines f o r components r a n g i n g i n s i z e s a p p r o a c h i n g 1 m down t o 10 mm have been s p e c i a l l y developed f o r X-ray components [8,91. The s u r f a c e q u a l i t i e s which have been a c h i e v e d by p o l i s h i n g f l a t d i s c s o f some m a t e r i a l s i s shown i n F i g . 1. F o r S p e c t r o s i l , f o r example, t h e s u r f a c e r o u g h n e s s was a b o u t 0.02 nm ( r m s ) , measured w i t h a l a t e r a l r e s o l u t i o n o f 50 nm, on t h e NPL-upgraded T a l y s t e p machine [ l o ] .

2. Metrology. - A r a n g e o f i n s t r u m e n t s h a s been developed a t NPL t o c a t e r f o r t h e measurement o f a x i a l p r o f i l e , and s u r f a c e r o u g h n e s s , t o nanometre and sub-nanometre l e v e l s o f a c c u r a c y r e s p e c t i v e l y , a s w e l l a s d i a m e t e r and c i r c u l a r i t y [6,11,123.

Measure e n t o f s u r f a c e f i g u r e t o a n u n c e r t a i n t y o f 2 nm i s i l l u s t r a t e d i n F i g . 2 , t t i h i c h shows t h e a x i a l p r o f i l e and t h e d e p a r t u r e from t h e t h e o r e t i c a l p r o f i l e - t h e e r r o r c u r v e - o f t h e X-ray microscope, r e f e r r e d t o i n S e c t i o n I V , m e a ~ u r e d ~ w i t h t h e NPL l a s e r a u t o c o l l i m a t o r C121.

3. M a t e r i a l s E v a l u a t i o n . - The f u n d a m e n t a l c r i t e r i o n i n s e l e c t i n g a m i r r o r m a t e r i a l i s t h a t i t must b e p o l i s h a b l e . The l i m i t s i n s u r f a c e f i g u r e and f i n i s h which c a n b e a c h i e v e d depend on t h e m e c h a n i c a l , c h e m i c a l a n d p h y s i c a l s t a b i l i t i e s o f t h e s e l e c t e d m a t e r i a l s a s w e l l a s o n t h e i r m i c r o s t r u c t u r a l i n h o m o g e n e i t i e s . Some o f t h e more f a v o u r e d m a t e r i a l s a r e ( i ) t h e v e r y low t h e r m a l y p a n s i o n c o e f f i c i e n t m a t e r i a l s , C o r n i n g 7971 ULE s i l i c a and t h e g l a s s c e r a m i c F e r o d u r , a l t h o u g h t h e r e is some d o u b t a b o u t t h e i r l o n g term s t a b i l i t i e s , ( i i ) t h e ( v e r y homogeneous s i l i c a g l a s s e s s u c h a s s y n t h e t i c v i t r e o u s s i l i c a ( e g

~ p e c t r o s i ' l ) and r e m e l t e d q u a r t z ( e g Homosil), ( i i i ) some o f t h e r e f r a c t o r y

I

m a t e r i a l s s u c h a s s i l i c o n c a r b i d e and ( i v ) where t h e a p p l i c a t i o n demands a m e t a l s u b s t r a t e , e l e c t P o l e s s n i c k e l c o a t e d m a t e r i a l s , s u c h a s b e r y l l i u m o r aluminium [10,131.

I V - EXAMPLES OF G R A Z I N G INCIDENCE OPTICS

F i g u r e 3 i l l u s t r a t e s e x t r e m e s i n t h e s i z e s o f W o l t e r o p t i c s : t h e A r i e s X-ray T e l e s c o p e [ I 4 1 which was s u c c e s s f u l l y flown i n 1980 and a n X-ray microscope. An X-ray microscope j o i n t l y d e v e l o p e d by t h e Lawrence Livermore N a t i o n a l L a b o r a t o r y , AWRE a n d NPL f o r l a s e r f u s i o n r e s e a r c h i s shown i n F i g . U. It h a s a m a g n i f i c a t i o n of 2 2 X b u t i t s geometry i s e x t r e m e l y u n f a v o u r a b l e f o r m i c r o s t r u c t u r a l s t u d i e s b e c a u s e o f t h e v e r y l a r g e m i r r o r t o image d i s t a n c e o f o v e r 6 m. T h i s r e n d e r s i t v e r y s u s c e p t i b l e t o image d e g r a d a t i o n , p a r t i c u l a r l y by s c a t t e r , b u t n e v e r t h e l e s s i t i s c a p a b l e o f r e s o l v i n g somewhat b e t t e r t h a n 1 urn, a s c a n be s e e n i n F i g . 5, which i s a n X-ray micrograph o f a n h e x a g o n a l g o l d g r i d . Although t h e peak t o background r a t i o i s p o o r , t h e development work on t h i s m i c r o s c o p e h a s shown t h a t r e s o l u t i o n s o f 100 nm o r b e t t e r s h o u l d now be a c h i e v a b l e .

To t a k e f u l l a d v a n t a g e o f t h e p o t e n t i a l r e s o l u t i o n o f m u l t i l a y e r m i r r o r s u s e d a t

normal i n c i d e n c e , t h e y w i l l r e q u i r e t o b e made t o t h e same t o l e r a n c e s a s Wolter

m i c r o s c o p e s . A t p r e s e n t , m u l t i l a y e r s u b s t r a t e s a r e g e n e r a l l y , e i t h e r p l a n e o r

s p h e r i c a l , a n d f o r t h e s e s i m p l e f o r m s , t h e n e c e s s a r y s u r f a c e r o u g h n e s s

t o l e r a n c e s h a v e a l r e a d y been a c h i e v e d , a s i l l u s t r a t e d i n F i g . 1.

REFEL S i c 1 2 n m

" T

SPECTROSIL

l&p-p*/

0 20vm 40vm 60vm 80vm

Fig. 1 - T a l y s t e p t r a c e s of p o l i s h e d f l a t s showing t h a t peak-to-valley roughnesses of 0.1 nm a r e now a c h i e v a b l e .

-

-120

-

-160 ^{I I I}

,

0 2 4 6 8 1 0 1 2 1 4

D~stance along hyperbolotd (mm)

Fig. 2 - A x i a l p r o f i l e and e r r o r curve of a n X-ray microscope.

Measurements can be made t o an u n c e r t a i n t y of 1 nm.

Fig. 4 - Wolter microscope.

F i g . 3 - Wolter t e l e s c o p e and microscope.

F i g . 5 - X-ray micrograph of an

hexagonal g o l d g r i d t a k e n with

A 1 K r a d i a t i o n ( A

0.83 nrn).

C2-72 JOURNAL DE PHYSIQUE

Another i m p o r t a n t c l a s s o f f o c u s i n g m i r r o r developed a t NPL i s one where t h e o p t i c a x i s i s n o t a n a x i s o f symmetry. These may b e made by d i r e c t l y machining t h e component t o t h e r e q u i r e d s h a p e , o r by e l a s t i c a l l y bending p l a n e m i r r o r s i n t o c i r c u l a r , p a r a b o l i c o r e l l i p t i c a l c y l i n d e r s [15,161. The a d v a n t a g e s o f t h e b e n d i n g t e c h n i q u e a r e t h e s i m p l i c i t y o f m a n u f a c t u r e c o u p l e d w i t h g r e a t e r v e r s a t i l i t y . F o r example, a p a r a b o l i c c o l l i m a t o r h a s been c o n s t r u c t e d i n which t h e a n g l e i s v a r i a b l e t o c a t e r f o r a r a n g e o f w a v e l e n g t h s , w h i l e t h e f o c a l l e n g t h r e m a i n s c o n s t a n t .

V - CONCLUSIONS

A s a r e s u l t o f a d v a n c e s i n m a n u f a c t u r i n g t e c h n i q u e s , m e t r o l o g y and t h e u n d e r s t a n d i n g o f m a t e r i a l s , i t i s now p o s s i b l e t o produce h i g h q u a l i t y X-ray o p t i c a l components by w e l l - d e f i n e d m a n u f a c t u r i n g p r o c e d u r e s , r a t h e r t h a n by t r a d i t i o n a l , e m p i r i c a l o p t i c a l c r a f t s m a n s h i p .

ACKNOWLEDGEMENTS - I m p o r t a n t c o n t r i b u t i o n s t o t h e work d e s c r i b e d i n t h i s p a p e r were made by M r T.H.English, M r A.E.Ennos, M r K.Lindsey, M r C.J.Robbie, D r M.Stedman and Mr M.S.Virdee.

R e f e r e n c e s

[ I ] K i r k p a t r i c k P. and Baez A . V . , J.Opt.Soc.Am. 3, (1948) 766 [21 E h r e n b e r g W . , J . 0 p t . S o c . A ~ ~ . 2 (1949) 741

[ 3 ] W o l t e r H . , Ann.Phys. 10 (1952) 94 [ 4 ] W o l t e r H . , Ann. Phys. 10

1952) 286

[ 5 ] Seward F.D. and P a l m i e r i T.M., R e v . S c i . I n s t . 46 (1975) 204

171 F r a n k s A . and G a l e B . , t o b e p u b l i s h e d i n ttX-Ray Microscopytf, S p r i n g e r S e r i e s i n O p t i c a l S c i e n c e s (1983)

[81 F r a n k s A . , Proc.SPIE 184 (1979) 110

[ g ] F r a n k s A . , G a l e B . , L i n d s e y K . , Pugh D.J., Robbie C.J. a n d Stedman M . , Ann.N.Y.Acad.Sci. 342 (1980) 167

[ l o ] L i n d s e y K . , Nucl.Inst.Meth. (1983) i n t h e p r e s s [ I 1 1 Stedman M . , Proc.SPIE 136 (1982) 2

[ I 2 1 Ennos A.E. a n d V i r d e e M.S., Proc.SPIE = i n t h e p r e s s [ I 3 1 L i n d s e y K. a n d F r a n k s A . , P r o c . S P I E = (1979) 46

[ I 4 1 C a t u r a R.C., Acton L.W., B e r t h e l s d o r f R . , Culhane J . L . , S a n f o r d P.W. and F r a n k s A . , Proc.SPIE 184 (1981) 2 3

[ I 5 1 F r a n k s A., P r o c . Workshop on X-ray I n s t r u m e n t a t i o n f o r S y n c h r o t r o n R a d i a t i o n R e s e a r c h , SSRL R e p o r t No. 78/04 (1978) V I I -101

[ I 6 1 Stedman M . , P r o c . Workshop on X-ray I n s t r u m e n t a t i o n f o r . S y n c h r o t r o n

R a d i a t i o n R e s e a r c h , SSRL R e p o r t No. 78/04 (1978) V I I - ¹⁴²