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Experiments in projection lithography using soft x-rays
Microelectronic Engineering 13
(1991) 243-250
243
Elsevier
Experiments in Projection Lithography using Soft X-Rays
J . E . B j o r k h o l m * , J.Bokor*, L.Eichner*, R . R . F r e e m a n * , J . G r e g u s * , T . E . J e w e l l # , W.M.Mansfield*, A . A . M a c D o w e l l + , M . L . O ' M a l l e y * , E.L.Raab#, W.T.Silfvast%, L.H.Szeto*, D.M.Tennant*, W.K.Waskiewicz#, D.L.White#, D.L.Windt#, and O . R . W o o d II*
A T & T Bell L a b o r a t o r i e s
We have d e m o n s t r a t e d soft x-ray p r o j e c t i o n l i t h o g r a p h y using radiation at w a v e l e n g t h s of 14 nm and 37 nm with a c o m m e r c i a l l y a v a i l a b l e 20X r e d u c t i o n S c h w a r z s c h i l d camera. Line w i d t h s as small as 0.05 m i c r o n s have been printed. The resolution o b t a i n e d was e s s e n t i a l l y d i f f r a c t i o n limited. I r i d i u m coated m i r r o r s were used with 37 nm r a d i a t i o n and Mo/Si m u l t i l a y e r coated m i r r o r s with 14 nm radiation. A i:i m a g n i f i c a t i o n O f f n e r R i n g - f i e l d system with iridium coated m i r r o r s has been used with 42 nm radiation. This optic has imaged line widths as small as 0.2 microns, w h i c h is close to the d i f f r a c t i o n limit for this system. T r a n s m i s s i o n masks were used for all these e x p e r i m e n t s and the r a d i a t i o n was obtained from an u n d u l a t o r in the V a c u u m U l t r a v i o l e t S y n c h r o t r o n Storage Ring at B r o o k h a v e n National Laboratory. INTRODUCTION C u s t o m e r d e m a n d c o n t i n u e s to drive the m i n i m u m line w i d t h required by the i n t e g r a t e d circuit industry to smaller values. C u r r e n t l y the p r o d u c t i o n state of the art is about 0.7 micron. This is expected to reduce to about 0.25 m i c r o n by the late 1990's and line widths of 0.i m i c r o n are a p o s s i b l e r e q u i r e m e n t at the b e g i n n i n g of the next century. It is u n c l e a r w h e t h e r any of the c u r r e n t main lithographic technologies (Ultra V i o l e t (UV) Projection, X-Ray Proximity, e l e c t r o n beam direct write) can be d e v e l o p e d to meet the 0.i m i c r o n line w i d t h requirements. Our a p p r o a c h has been to look at the r e l a t i v e l y p o o r l y d e v e l o p e d soft X-ray region of the e l e c t r o m a g n e t i c s p e c t r u m (4-40 nm), and carry out some e x p l o r a t o r y e x p e r i m e n t s to i n v e s t i g a t e the p o s s i b i l i t y of a t t a i n i n g these line w i d t h goals u s i n g r e f l e c t i v e l i t h o g r a p h y cameras. Several a u t h o r s have r e c e n t l y s u g g e s t e d that soft x-ray p r o j e c t i o n techniques could find applications in a d v a n c e d lithographic systems. S i l f v a s t and Wood [i] and H a w r y l u k and Seppala [2] have described soft x-ray projection lithography systems using m u l t i l a y e r - c o a t e d optics and r e f l e c t i o n masks.
* # + %
C r a w f o r d s C o r n e r Road, Holmdel, New J e r s e y 07733 USA 600 M o u n t a i n A v . , M u r r a y Hill, New J e r s e y 07974 USA 510E B r o o k h a v e n Laboratory, Upton, New York 11973 USA P r e s e n t A d d r e s s : U n i v e r s i t y of Central Florida, O r l a n d o Florida 32826 USA
0167-9317/91/$3.50 © 1991 - Elsevier Science Publishers B.V.
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A l o o k at resolution useful:
t h e c l a s s i c a l f o r m u l a e u s e d in t h e a n d d e p t h of f o c u s in a d i f f r a c t i o n
Resolution=Kl
I NA
trade off between l i m i t e d s y s t e m is
D O F = ± K 2 k_~_ N A 2'
w h e r e ~ is t h e w a v e l e n g t h a n d N A is t h e n u m e r i c a l a p e r t u r e of t h e lens. T h e c o e f f i c i e n t s K1 a n d K2 d e p e n d on a v a r i e t y of f a c t o r s , s u c h as t h e c o n t r a s t of t h e r e s i s t a n d p r o c e s s i n g t e c h n i q u e s . In a p r o d u c t i o n e n v i r o n m e n t K1 = 0.8 is o f t e n used. T h e d e p t h of f o c u s a l s o d e p e n d s on s e v e r a l f a c t o r s , b u t p e o p l e o f t e n u s e K2 = 0.5. F r o m t h i s w e s e e t h a t at a w a v e l e n g t h of 13 nm, w e c a n u s e a c a m e r a w i t h an N A = 0.i a n d a c h i e v e a r e s o l u t i o n of 0.i m i c r o n w i t h a DOF +-0.65 micron. This DOF is already achievable with current t e c h n o l o g y . T h e N A is v e r y s m a l l c o m p a r e d to t h o s e c u r r e n t l y u s e d in U V l i t h o g r a p h y w h e r e N A > 0.4 is c o m m o n . A l o w n u m e r i c a l a p e r t u r e s i m p l i f i e s t h e d e s i g n of t h e c a m e r a t h a t c o u l d e v e n t u a l l y be made. F u r t h e r m o r e t h e c a m e r a s h o u l d h a v e a r e d u c t i o n factor, p e r h a p s 5X, a n d u s e r e f l e c t i o n r a t h e r t h a n t r a n s m i s s i o n m a s k s . A reduction camera can lessen mask making problems and a reflection m a s k w o u l d b e m o r e p h y s i c a l l y r o b u s t t h a n a t r a n s m i s s i o n mask. S o f t x - r a y p r o j e c t i o n p r i n t i n g h a s b e e n d e m o n s t r a t e d p r e v i o u s l y [3] but diffraction limited imagery was not obtained. We believed that the diffraction limit could be obtained by using high quality optics that are carefully aligned. This paper will describe our experimental work using a S c h w a r z s c h i l d c a m e r a i m a g i n g s o f t x - r a y s of w a v e l e n g t h 37 a n d 13.8 nm w h i c h d o e s i n d e e d s h o w t e n t h a n d e v e n 0.05 m i c r o n f e a t u r e s . We also describe an O f f n e r Ring-field camera using radiation of wavelength 42 nm which also shows near diffraction limited r e s o l u t i o n of 0.2 m i c r o n . EXPERIMENTAL
PROGRAM
O u r e x p e r i m e n t a l s o f t x - r a y l i t h o g r a p h y p r o g r a m i n c l u d e s s t u d i e s on s o f t x - r a y c a m e r a s u s i n g 37 n m [4] a n d 13.8 n m r a d i a t i o n [5,6] p l u s r e s i s t w o r k [7] a n d a m u l t i l a y e r f i l m p r o g r a m [8]. A s a s o f t x - r a y s o u r c e w e u s e t h e o u t p u t f r o m an u n d u l a t o r t h a t is i n s t a l l e d in t h e V a c u u m U V S y n c h r o t r o n S t o r a g e r i n g at B r o o k h a v e n National Laboratories [9]. The undulator provides a coherent collimated b e a m of n a r r o w b a n d p a s s r a d i a t i o n . The fundamental w a v e l e n g t h is a b o u t 40 n m a n d c a n b e v a r i e d b y a d j u s t i n g t h e g a p b e t w e e n t h e m a g n e t s . F o r t h e w o r k at s h o r t e r w a v e l e n g t h w e u s e d t h e t h i r d h a r m o n i c at a b o u t 13 nm. T h e i n t e n s e c o l l i m a t e d b e a m f r o m t h e undulator with the narrow wavelength b a n d p a s s g a v e us g r e a t e r control in a d j u s t i n g system parameters than would have been possible with the broad band distribution f r o m t h e 'fan' s h a p e d b e a m t h a t is o b t a i n e d f r o m b e n d i n g m a g n e t s . The experiments were c a r r i e d o u t in v a c u u m s i n c e m a t t e r is h i g h l y a b s o r b i n g of l i g h t in t h i s w a v e l e n g t h range. Instead of d e s i g n i n g and building new camera systems for our i n i t i a l e x p e r i m e n t s , w e c h o s e to t a k e t w o c o m m e r c i a l all m i r r o r s y s t e m s t h a t a r e c u r r e n t l y u s e d in t h e v i s i b l e a n d U V and, w i t h minimal modification convert them into soft x-ray cameras. These have been a Schwarzschild system with 20X reduction made by GCA T r o p e l a n d a IX O f f n e r R i n g - f i e l d s y s t e m m a d e b y P e r k i n Elmer. N e i t h e r c a m e r a is a c o n t e n d e r for a f i n a l d e s i g n . T h e S c h w a r z s c h i l d
J.E. Bjorkhohn et al. / Projection llithography using soft X-rays
245
has a v e r y s m a l l f i e l d (50 by I00 m i c r o n s ) , t h e O f f n e r R i n g - f i e l d has u n i t y m a g n i f i c a t i o n . We consider demagnification to be m a j o r a s s e t in p r o d u c i n g 0.i m i c r o n f e a t u r e s . T h e s e t w o c a m e r a s a r e to p r o v i d e us w i t h t h e e x p e r i e n c e n e e d e d for t h e n e x t p h a s e in the program. We h a v e o p t e d for a c o n s e r v a t i v e step-wise approach. We have s t a r t e d w i t h r a d i a t i o n of 40 nm, o p e n s t e n c i l t r a n s m i s s i o n masks and mirrors c o a t e d w i t h iridium. W e t h e n m o v e d on to u s e t h e s h o r t e r w a v e l e n g t h of 13 nm, m e m b r a n e m a s k s ( a b s o r b i n g p a t t e r n s on o n e side) a n d t h e m o r e r e f l e c t i n g m u l t i l a y e r e d c o a t e d optics. In f u t u r e e x p e r i m e n t s r e f l e c t i o n m a s k s w i l l be e m p l o y e d . SCHWARZSCHILD
CAMERA,
20X R E D U C T I O N
IMAGING
T h e f i r s t i m a g i n g e x p e r i m e n t s w e r e p e r f o r m e d w i t h 37 n m x-rays u s i n g a 1 4 . 2 6 5 m m focal l e n g t h S c h w a r z s c h i l d o b j e c t i v e w h i c h i m a g e s an o p e n s t e n c i l m a s k w i t h 20:1 r e d u c t i o n . T h e m i r r o r b l a n k s w e r e c o a t e d w i t h 17 n m of i r i d i u m o v e r 6 n m of c h r o m i u m , by Acton Research. The reflectivity was about 8% a n d the two mirror Schwarzschild camera had a transmission less than 1% of t h e i n c o m i n g r a d i a t i o n . T h e s c h e m a t i c e x p e r i m e n t a l l a y o u t is s h o w n in Fig. i. T h e m a s k w a s i l l u m i n a t e d w i t h a b e a m 0 . 8 2 5 d e g r e e off the o p t i c a x i s to a v o i d t h e c e n t r a l o b s c u r a t i o n c a u s e d b y t h e p r i m a r y mirror. A n o f f a x i s a p e r t u r e in t h e s y s t e m l i m i t e d t h e n u m e r i c a l a p e r t u r e to N A = 0.113. T h e o r y i n d i c a t e d t h a t t h e b e s t 0.2 m i c r o n l i n e s w o u l d be i m a g e d u s i n g t h i s NA. A larger NA would have p r o d u c e d l o w e r c o n t r a s t f e a t u r e s d u e to i n c r e a s e d a b e r r a t i o n s .
Off-axis Aperture
I Synchrotron Radiation
[ ~
\
~
\ Transmission Mask F i g u r e i. S c h e m a t i c with 20X reduction.
diagram
Image \ ~
.nmary
Mirror
coverea
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Secondary Mirror of
the
Off-axis
Schwarzschild
Camera
W e w e r e a b l e to o b t a i n g o o d 0.2 m i c r o n l i n e s a n d s p a c e s w h e r e a s the 0.i m i c r o n l i n e s a n d s p a c e s d i d n o t print. T h e c o h e r e n t cutoff f r e q u e n c y (NA/I) for t h i s s y s t e m c o r r e s p o n d e d to a b o u t 0.16 m i c r o n l i n e s a n d spaces. T h u s the 0.i m i c r o n l i n e s a n d s p a c e s d i d n o t p a s s through the system. The results were consistent with near d i f f r a c t i o n l i m i t e d imaging. A second s e t of m i r r o r s w a s c o a t e d w i t h 20 l a y e r p a i r s of a molybdenum-silicon multilayer that had a measured reflectance peak of 38% at 13.8 n m a n d a b a n d p a s s of 0.7 nm FWHM. C o m p u t a t i o n s s h o w e d t h a t w i t h N A = 0.083, 0.i m i c r o n l i n e s a n d s p a c e s h a v e the
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J.E. Bjorkhohn et aL / Projection Ilithography using soft X-rays
F i g u r e 2. S E M m i c r o g r a p h of 0.i m i c r o n l i n e s a n d s p a c e s in 60 n m of PMMA, p r i n t e d u s i n g t h e multilayer schwarzschild camera and 13.8 nm wavelength. The e x p o s u r e t i m e for t h i s s h o t w a s 23 sec.
Figure 3. SEM micrograph of 0 . 0 5 m i c r o n l i n e s a n d s p a c e s in 60 n m of PMMA, p r i n t e d u s i n g the multilayer coated schwarzschild c a m e r a a n d 13.8 nm wavelength.
b e s t c o n t r a s t . Fig. 2 s h o w s t e n t h m i c r o n l i n e s a n d s p a c e s in P M M A resist. We observed that the line e d g e s w e r e m o r e v e r t i c a l at 13.8 n m t h a n at 36 nm. W e b e l i e v e t h i s is b e c a u s e t h e r a d i a t i o n p e n e t r a t e d f u r t h e r i n t o t h e r e s i s t at 13.8 n m a n d p r o d u c e d a m o r e uniform exposure though the thickness. The coherent illumination c u t o f f f r e q u e n c y w i t h N A = 0.083 c o r r e s p o n d s to 0.08 m i c r o n l i n e s and spaces. Light from 0.05 micron lines and spaces did not pass through the aperture and were not printed. On opening up the a p e r t u r e to N A = 0.15 w e p r i n t e d 0.05 m i c r o n l i n e s a n d s p a c e s (Fig. 3). S i n c e t h i s l a r g e r n u m e r i c a l a p e r t u r e r e s u l t s in i n c r e a s e d a b e r r a t i o n s w e w e r e n o t s u r p r i s e d t h a t t h e q u a l i t y of t h e lines a n d s p a c e s w e r e n o t as h i g h as t h e 0.i m i c r o n l i n e s of Fig. 2. It is a l s o p o s s i b l e t h a t a d h e s i o n f a i l u r e of t h e r e s i s t c o n t r i b u t e d to the non-uniformities of t h e line. The resists we used were PMMA (polymethyl methacrylate), EBR-9 (2,2,2-trifluoroethyl ~-chloroacrylate) and PBS (polybutene-i s u l f o n e ) . T h e s e n s i t i v i t i e s of t h e s e t h r e e r e s i s t s , PMMA, E B R - 9 a n d PBS, were, r e s p e c t i v e l y , a b o u t 23, i, a n d 0.I m J / c m 2 at w a v e l e n g t h = 36 n m a n d a b o u t 55, 8, a n d 0.7 m J / c m 2 at 13 nm. Since these soft x-rays are highly absorbed by carbon compounds we used very thin l a y e r s of r e s i s t , a b o u t 60 nm, e i t h e r on b a r e s i l i c o n or on a t r i - l e v e l s y s t e m c o n s i s t i n g of t h e t h i n resist, 30 n m of g e r m a n i u m , a n d 300 n m h a r d - b a k e d S h i p l e y 1803 r e s i s t . Fig. 4 s h o w s 0.15 a n d 0.i m i c r o n l i n e s a n d s p a c e s w h e n i m a g e d a n d p r o c e s s e d u s i n g s u c h a tri-level resist scheme. Our best images were made with the high r e s o l u t i o n r e s i s t , PMMA. W e c o u l d u s e s u c h a l o w s e n s i t i v i t y r e s i s t because the camera's demagnification i n t e n s i f i e d t h e f l u x in t h e image plane by a factor of 400, i.e. the square of the demagnification factor.
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Figure 4. SEM m i c r o g r a p h of 0.i and 0.15 m i c r o n lines and spaces in silicon. A t r i - l e v e l resist was used as the imaging layer. The image was then t r a n s f e r r e d using reactive ion e t c h i n g into the s i l i c o n substrate.
RING-FIELD
EXPERIMENTS
-
NO
REDUCTION
The r i n g - f i e l d camera is a scanner with unity m a g n i f i c a t i o n sold by Perkin E l m e r as the M i c r o A l i g n PE300. This optic r e p r e s e n t s an e x p e r i m e n t a l a r r a n g e m e n t that is closer to a p r a c t i c a l l i t h o g r a p h i c tool than the S c h w a r z s c h i l d optic. We m o d i f i e d the s y s t e m by r e m o v i n g all the s c a n n i n g m e c h a n i s m and optics leaving only the two main spherical imaging mirrors. The s c h e m a t i c layout of the e x p e r i m e n t is shown in Fig. 5. The m i r r o r s were s e l e c t e d for good surface figure. To p r o d u c e r e f l e c t i v i t i e s of 10-15% in the 40-45 nm range they were c o a t e d w i t h 17 nm iridium on 6 nm chromium. The m i r r o r M2 was a p e r t u r e d to p r o d u c e a numerical a p e r t u r e of 0.135. This NA p r o d u c e s the best c o m p r o m i s e b e t w e e n d i f f r a c t i o n limited r e s o l u t i o n and low a b e r r a t i o n s [i0]. We also r e p o s i t i o n e d the m i r r o r s s l i g h t l y to o p t i m i z e the r e s o l u t i o n at this numerical aperture.
END VIEW
SIDE VIEW Transmission Mask
Curvafure
M2 [ ~ / ~
Mask ring of jgood correction
M2 )
M1
M1
Resisf ~.coveredl ; ~
Wafer L~ ~
Image
X . ~ ~
Figure 5. S c h e m a t i c d i a g r a m of the R i n g - f i e l d nm x-rays.
Wafer ring of good correction
camera used with 42
I.E. Bjorkholm et al. / Projection llithography using soft X-rays
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Fig. 6 s h o w s t h e m o d u l a t i o n t r a n s f e r f u n c t i o n p l o t for t h e o p t i c w h e n u s e d w i t h i n c o h e r e n t r a d i a t i o n of w a v e l e n g t h of 42 nm. T h e r i n g of g o o d focus is a b o u t 200 m i c r o n s w i d e w i t h a r a d i u s of 55 mm. We illuminated the transmission mask directly with the 2 mm d i a m e t e r x - r a y beam. T h e u s e f u l i l l u m i n a t e d a r e a is t h u s 0.2 x 2.0 mm. Both the mask and the resist coated wafer were mounted in a m e c h a n i s m t h a t a l l o w e d t h e m to b e b r o u g h t into focus, t i l t e d and transported laterally to a l l o w m u l t i p l e exposures. The chamber included a l o a d l o c k so w a f e r s could be inserted and removed w i t h o u t t h e w h o l e s y s t e m b e i n g v e n t e d to air. S i n c e t h i s is a one to one s y s t e m a n d t h e r e w e r e t h r e e m i r r o r s t h e x - r a y i n t e n s i t y on t h e r e s i s t w a s a b o u t o n e t h o u s a n d t i m e s l o w e r t h a n for t h e S c h w a r z s c h i l d . Thus our first experiments have been c a r r i e d o u t w i t h t h e v e r y s e n s i t i v e e - b e a m resist, PBS. The camera w a s b r o u g h t i n t o a p p r o x i m a t e focus w i t h v i s i b l e l i g h t b u t t h e final parameters (radial p o s i t i o n of t h e r i n g a n d focus) h a d to be determined by making x-ray exposures. These experiments emphasized a m a j o r p r o b l e m in s o f t x - r a y p r o j e c t i o n l i t h o g r a p h y w h e n w o r k i n g under medium vacuum ('i0 °6 torr). A n a b s o r b i n g f i l m of c a r b o n is d e p o s i t e d on s u r f a c e s s t r u c k by h i g h i n t e n s i t y x-rays. T h e r e s i d u a l p a r t i a l p r e s s u r e of l o w v o l a t i l e c a r b o n c o m p o u n d s t h a t a r e u s u a l l y present in v a c u u m systems are c r a c k e d by t h e x - r a y flux and d e p o s i t e d as carbon. T h i s is a p r o b l e m k n o w n to x - r a y s y n c h r o t r o n u s e r s w h o c o p e w i t h it b y u s i n g u l t r a - h i g h v a c u u m t e c h n i q u e s , s i n c e
1.0 DIFFRACTION ~ ~,.
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i
2ooo SPACIAL FREQUENCY
i
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a000
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,
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6000
(CYCLES/MM)
Figure 6 . Calculated square-wave modulation transfer function for the Ring-field camera with incoherent i l l u m i n a t i o n at a w a v e l e n g t h of 42 nm. In our experiments, nearly fully coherent i l l u m i n a t i o n at 42 n m w a s used; the cutoff frequency for t h i s c a s e is a b o u t 3000 i p / m m
F i g u r e 7. S E M m i c r o g r a p h of the l i n e a n d s p a c e m a s k i m a g e d into 60 nm of PBS r e s i s t w i t h t h e Ring-field c a m e r a u s i n g 42 nm x-rays. T h e 1.0, 0.5, 0.4, 0.3, 0.25 a n d 0.2 m i c r o n l i n e and spaces are printed. The 0.15 m i c r o n l i n e s a n d s p a c e s (to the u p p e r r i g h t of t h e 0.2 m i c r o n lines) are not printed. The e x p o s u r e t i m e for t h i s s h o t w a s i0 sec.
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the p h o t o r e s i s t is g e n e r a l l y a carbon b a s e d m a t e r i a l that could well be outgassing, the o p e r a t i o n of the optics in a clean carbon free s y s t e m p r e s e n t s a s i g n i f i c a n t p r o b l e m that needs to be a d d r e s s e d in future experiments. We found that a local partial p r e s s u r e of o x y g e n (about 10 .3 torr) in the r e g i o n w h e r e the soft x-rays strike any surface g r e a t l y r e d u c e d but did not e l i m i n a t e the d e p o s i t i o n of carbon. Fig. 7 shows a recent image from the r i n g - f i e l d camera u s i n g 42 nm r a d i a t i o n and the PBS resist. The 0.2 m i c r o n lines and spaces are the s m a l l e s t imaged. The 0.15 m i c r o n lines and spaces are not printed. The c o h e r e n t i l l u m i n a t i o n cutoff f r e q u e n c y for this system is 0.16 m i c r o n lines and spaces, so the 0.15 m i c r o n lines are not e x p e c t e d to be printed. The image shows s c u m m i n g and g r a n u l a r i t y a s s o c i a t e d w i t h this resist at these small line dimensions. The lowered c o n t r a s t of the 0.2 m i c r o n lines and spaces c o m p a r e d to other lines and spaces indicated that the optic is v e r y close to but p r o b a b l y not quite at the d i f f r a c t i o n limit. SUMMARY One of the m a i n q u e s t i o n s r e g a r d i n g the f e a s i b i l i t y of p r o j e c t i o n x-ray l i t h o g r a p h y is the p r a c t i c a l i t y of m a k i n g optics with a d e q u a t e figure accuracy. For d i f f r a c t i o n limited imaging the w a v e f r o n t error at the image plane has to be less than I/4. For the S c h w a r z s c h i l d e x p e r i m e n t at 13.8 nm with 2 r e f l e c t i o n s this t r a n s l a t e s to a RMS error of less than i/ll (13 angstroms) for each m i r r o r over the total active optical area of only 0.2 cm 2. This e x p e r i m e n t showed that d i f f r a c t i o n limited imaging is p o s s i b l e with m u l t i l a y e r e d m i r r o r s using 13.8 nm radiation. The active optical surface is, however, rather small. For the r i n g - f i e l d e x p e r i m e n t at 42 nm w i t h three reflections, d i f f r a c t i o n limited imaging t r a n s l a t e s to a RMS error of less than 1/14 (30 angstroms) for each m i r r o r over the total active optical area of ii0 cm 2. This large optical area d e m o n s t r a t e s that it is p o s s i b l e to make m i r r o r s that have close to the r e q u i r e d figure for large optical areas. A design study to d e t e r m i n e a m i r r o r layout for a p r a c t i c a l soft x-ray l i t h o g r a p h i c camera r e s u l t e d in the need for at least 4 m i r r o r s and a large ( > i00 cm 2) active area [ii]. W o r k i n g at 13.8 nm with such a camera w o u l d require a RMS error of 1/16 (8 angstroms) for each mirror. This is beyond, or right at the limit, of c u r r e n t optical f a b r i c a t i o n t e c h n i q u e s and needs to be a d d r e s s e d if p r o j e c t i o n xray l i t h o g r a p h y is to be developed.
REFERENCES [I]
W.T.Silfvast 8, 3 (1988).
and 0.R.Wood II, M i c r o e l e c t r o n i c
Engineering
[2]
A . M . H a w r y l u k and L.G.Seppala, J.Vac. Sci. Tech. B6, 2162 (1988).
[3]
H.Kinoshita, K.Kurihara, Y.Ishii and Y Torii, J.Vac. Sci. Tech. B7, 1648 (1989).
[4]
D.W.Berreman, J . E . B j o r k h o l m , L.Eichner, R.R.Freeman, T.E.Jewell, W . M . M a n s f i e l d , A . A . M a c D o w e l l , M . L . 0 ' M a l l e y , E.L.Raab, W.T.Silfvast, L.H.Szeto, D.M.Tennant, W . K . W a s k i e w i c z , D.L.White, D.L.Windt, 0 . R . W o o d II, and J.H.Bruning, Opt. Lett. 15, 529 (1990).
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[5]
T.E.Jewell, M.M.Becker, J.E.Bjorkholm, J.Boker, L.Eichner, R.R.Freeman, W.M.Mansfield, A.A.MacDowell, M.L.0'Malley, E.L.Raab, W.T.Silfvast, L.H.Szeto, D.M.Tennant, W.K.Waskiewicz, D.L.White, D.L.Windt and 0.R.Wood II, Proc. of SPIE Vol 1263 (1990)
[6]
J.E.Bjorkholm, L.Eichner, R.R. Freeman, J.Gregus, T.E.Jewell, W.M.Mansfield, A.A.MacDowell, E.L.Raab, W.T.Silfvast, L.H.Szeto, D.M.Tennant, W.K.Waskiewicz, D.L.White, D.L.Windt and 0.R.Wood II, to be publ. J.Vac. Sci.Technol. B Nov/Dec (1990)
[7]
D.W.Berreman, J.E.Bjorkholm, M.Becker, L.Eichner, R.R.Freeman, T.E.Jewell. W.M.Mansfield, A. A. MacDowell, M.L.O'Malley, E.L.Raab, W.T.Silfvast, L.H.Szeto, D.M.Tennant, W.K.Waskiewicz, D.L.White, D.L.Windt, 0.R.Wood II, Appl. Phys. Lets., 54 (2180), 1990.
[8]
D.L.Windt, SPIE 1990 Int. Symp. Optical and Optoelectronic Applied Science and Engineering. X-Ray and EUV Optics Conf. Voi.1243 (1990).
[9]
A.M.Fauchet, B.C.Craft, J.N.Galayda, H.Hsieh, A.Luccio, J.B.Murphy, C.Pellegrini, A.van Steenbergen, G.Vignola, L.H.Yu, R.R.Freeman and B.M.Kincaid, National Synchrotron Light Source Annual Report (National Technical Information Services, Springfield, Va., 1985) p.137
[i0] O.R.Wood II, W.T.Silfvast and T.E.Jewell, J.Vac. Sci. Technol. B7, 1613 (1989) [ii] T.E.Jewell, J.M.Rodgers and K.P.Thompson, in J.Vac. Sci.Technol. B, Nov/Dec (1990)
to be published
(1991) 243-250
243
Elsevier
Experiments in Projection Lithography using Soft X-Rays
J . E . B j o r k h o l m * , J.Bokor*, L.Eichner*, R . R . F r e e m a n * , J . G r e g u s * , T . E . J e w e l l # , W.M.Mansfield*, A . A . M a c D o w e l l + , M . L . O ' M a l l e y * , E.L.Raab#, W.T.Silfvast%, L.H.Szeto*, D.M.Tennant*, W.K.Waskiewicz#, D.L.White#, D.L.Windt#, and O . R . W o o d II*
A T & T Bell L a b o r a t o r i e s
We have d e m o n s t r a t e d soft x-ray p r o j e c t i o n l i t h o g r a p h y using radiation at w a v e l e n g t h s of 14 nm and 37 nm with a c o m m e r c i a l l y a v a i l a b l e 20X r e d u c t i o n S c h w a r z s c h i l d camera. Line w i d t h s as small as 0.05 m i c r o n s have been printed. The resolution o b t a i n e d was e s s e n t i a l l y d i f f r a c t i o n limited. I r i d i u m coated m i r r o r s were used with 37 nm r a d i a t i o n and Mo/Si m u l t i l a y e r coated m i r r o r s with 14 nm radiation. A i:i m a g n i f i c a t i o n O f f n e r R i n g - f i e l d system with iridium coated m i r r o r s has been used with 42 nm radiation. This optic has imaged line widths as small as 0.2 microns, w h i c h is close to the d i f f r a c t i o n limit for this system. T r a n s m i s s i o n masks were used for all these e x p e r i m e n t s and the r a d i a t i o n was obtained from an u n d u l a t o r in the V a c u u m U l t r a v i o l e t S y n c h r o t r o n Storage Ring at B r o o k h a v e n National Laboratory. INTRODUCTION C u s t o m e r d e m a n d c o n t i n u e s to drive the m i n i m u m line w i d t h required by the i n t e g r a t e d circuit industry to smaller values. C u r r e n t l y the p r o d u c t i o n state of the art is about 0.7 micron. This is expected to reduce to about 0.25 m i c r o n by the late 1990's and line widths of 0.i m i c r o n are a p o s s i b l e r e q u i r e m e n t at the b e g i n n i n g of the next century. It is u n c l e a r w h e t h e r any of the c u r r e n t main lithographic technologies (Ultra V i o l e t (UV) Projection, X-Ray Proximity, e l e c t r o n beam direct write) can be d e v e l o p e d to meet the 0.i m i c r o n line w i d t h requirements. Our a p p r o a c h has been to look at the r e l a t i v e l y p o o r l y d e v e l o p e d soft X-ray region of the e l e c t r o m a g n e t i c s p e c t r u m (4-40 nm), and carry out some e x p l o r a t o r y e x p e r i m e n t s to i n v e s t i g a t e the p o s s i b i l i t y of a t t a i n i n g these line w i d t h goals u s i n g r e f l e c t i v e l i t h o g r a p h y cameras. Several a u t h o r s have r e c e n t l y s u g g e s t e d that soft x-ray p r o j e c t i o n techniques could find applications in a d v a n c e d lithographic systems. S i l f v a s t and Wood [i] and H a w r y l u k and Seppala [2] have described soft x-ray projection lithography systems using m u l t i l a y e r - c o a t e d optics and r e f l e c t i o n masks.
* # + %
C r a w f o r d s C o r n e r Road, Holmdel, New J e r s e y 07733 USA 600 M o u n t a i n A v . , M u r r a y Hill, New J e r s e y 07974 USA 510E B r o o k h a v e n Laboratory, Upton, New York 11973 USA P r e s e n t A d d r e s s : U n i v e r s i t y of Central Florida, O r l a n d o Florida 32826 USA
0167-9317/91/$3.50 © 1991 - Elsevier Science Publishers B.V.
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A l o o k at resolution useful:
t h e c l a s s i c a l f o r m u l a e u s e d in t h e a n d d e p t h of f o c u s in a d i f f r a c t i o n
Resolution=Kl
I NA
trade off between l i m i t e d s y s t e m is
D O F = ± K 2 k_~_ N A 2'
w h e r e ~ is t h e w a v e l e n g t h a n d N A is t h e n u m e r i c a l a p e r t u r e of t h e lens. T h e c o e f f i c i e n t s K1 a n d K2 d e p e n d on a v a r i e t y of f a c t o r s , s u c h as t h e c o n t r a s t of t h e r e s i s t a n d p r o c e s s i n g t e c h n i q u e s . In a p r o d u c t i o n e n v i r o n m e n t K1 = 0.8 is o f t e n used. T h e d e p t h of f o c u s a l s o d e p e n d s on s e v e r a l f a c t o r s , b u t p e o p l e o f t e n u s e K2 = 0.5. F r o m t h i s w e s e e t h a t at a w a v e l e n g t h of 13 nm, w e c a n u s e a c a m e r a w i t h an N A = 0.i a n d a c h i e v e a r e s o l u t i o n of 0.i m i c r o n w i t h a DOF +-0.65 micron. This DOF is already achievable with current t e c h n o l o g y . T h e N A is v e r y s m a l l c o m p a r e d to t h o s e c u r r e n t l y u s e d in U V l i t h o g r a p h y w h e r e N A > 0.4 is c o m m o n . A l o w n u m e r i c a l a p e r t u r e s i m p l i f i e s t h e d e s i g n of t h e c a m e r a t h a t c o u l d e v e n t u a l l y be made. F u r t h e r m o r e t h e c a m e r a s h o u l d h a v e a r e d u c t i o n factor, p e r h a p s 5X, a n d u s e r e f l e c t i o n r a t h e r t h a n t r a n s m i s s i o n m a s k s . A reduction camera can lessen mask making problems and a reflection m a s k w o u l d b e m o r e p h y s i c a l l y r o b u s t t h a n a t r a n s m i s s i o n mask. S o f t x - r a y p r o j e c t i o n p r i n t i n g h a s b e e n d e m o n s t r a t e d p r e v i o u s l y [3] but diffraction limited imagery was not obtained. We believed that the diffraction limit could be obtained by using high quality optics that are carefully aligned. This paper will describe our experimental work using a S c h w a r z s c h i l d c a m e r a i m a g i n g s o f t x - r a y s of w a v e l e n g t h 37 a n d 13.8 nm w h i c h d o e s i n d e e d s h o w t e n t h a n d e v e n 0.05 m i c r o n f e a t u r e s . We also describe an O f f n e r Ring-field camera using radiation of wavelength 42 nm which also shows near diffraction limited r e s o l u t i o n of 0.2 m i c r o n . EXPERIMENTAL
PROGRAM
O u r e x p e r i m e n t a l s o f t x - r a y l i t h o g r a p h y p r o g r a m i n c l u d e s s t u d i e s on s o f t x - r a y c a m e r a s u s i n g 37 n m [4] a n d 13.8 n m r a d i a t i o n [5,6] p l u s r e s i s t w o r k [7] a n d a m u l t i l a y e r f i l m p r o g r a m [8]. A s a s o f t x - r a y s o u r c e w e u s e t h e o u t p u t f r o m an u n d u l a t o r t h a t is i n s t a l l e d in t h e V a c u u m U V S y n c h r o t r o n S t o r a g e r i n g at B r o o k h a v e n National Laboratories [9]. The undulator provides a coherent collimated b e a m of n a r r o w b a n d p a s s r a d i a t i o n . The fundamental w a v e l e n g t h is a b o u t 40 n m a n d c a n b e v a r i e d b y a d j u s t i n g t h e g a p b e t w e e n t h e m a g n e t s . F o r t h e w o r k at s h o r t e r w a v e l e n g t h w e u s e d t h e t h i r d h a r m o n i c at a b o u t 13 nm. T h e i n t e n s e c o l l i m a t e d b e a m f r o m t h e undulator with the narrow wavelength b a n d p a s s g a v e us g r e a t e r control in a d j u s t i n g system parameters than would have been possible with the broad band distribution f r o m t h e 'fan' s h a p e d b e a m t h a t is o b t a i n e d f r o m b e n d i n g m a g n e t s . The experiments were c a r r i e d o u t in v a c u u m s i n c e m a t t e r is h i g h l y a b s o r b i n g of l i g h t in t h i s w a v e l e n g t h range. Instead of d e s i g n i n g and building new camera systems for our i n i t i a l e x p e r i m e n t s , w e c h o s e to t a k e t w o c o m m e r c i a l all m i r r o r s y s t e m s t h a t a r e c u r r e n t l y u s e d in t h e v i s i b l e a n d U V and, w i t h minimal modification convert them into soft x-ray cameras. These have been a Schwarzschild system with 20X reduction made by GCA T r o p e l a n d a IX O f f n e r R i n g - f i e l d s y s t e m m a d e b y P e r k i n Elmer. N e i t h e r c a m e r a is a c o n t e n d e r for a f i n a l d e s i g n . T h e S c h w a r z s c h i l d
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has a v e r y s m a l l f i e l d (50 by I00 m i c r o n s ) , t h e O f f n e r R i n g - f i e l d has u n i t y m a g n i f i c a t i o n . We consider demagnification to be m a j o r a s s e t in p r o d u c i n g 0.i m i c r o n f e a t u r e s . T h e s e t w o c a m e r a s a r e to p r o v i d e us w i t h t h e e x p e r i e n c e n e e d e d for t h e n e x t p h a s e in the program. We h a v e o p t e d for a c o n s e r v a t i v e step-wise approach. We have s t a r t e d w i t h r a d i a t i o n of 40 nm, o p e n s t e n c i l t r a n s m i s s i o n masks and mirrors c o a t e d w i t h iridium. W e t h e n m o v e d on to u s e t h e s h o r t e r w a v e l e n g t h of 13 nm, m e m b r a n e m a s k s ( a b s o r b i n g p a t t e r n s on o n e side) a n d t h e m o r e r e f l e c t i n g m u l t i l a y e r e d c o a t e d optics. In f u t u r e e x p e r i m e n t s r e f l e c t i o n m a s k s w i l l be e m p l o y e d . SCHWARZSCHILD
CAMERA,
20X R E D U C T I O N
IMAGING
T h e f i r s t i m a g i n g e x p e r i m e n t s w e r e p e r f o r m e d w i t h 37 n m x-rays u s i n g a 1 4 . 2 6 5 m m focal l e n g t h S c h w a r z s c h i l d o b j e c t i v e w h i c h i m a g e s an o p e n s t e n c i l m a s k w i t h 20:1 r e d u c t i o n . T h e m i r r o r b l a n k s w e r e c o a t e d w i t h 17 n m of i r i d i u m o v e r 6 n m of c h r o m i u m , by Acton Research. The reflectivity was about 8% a n d the two mirror Schwarzschild camera had a transmission less than 1% of t h e i n c o m i n g r a d i a t i o n . T h e s c h e m a t i c e x p e r i m e n t a l l a y o u t is s h o w n in Fig. i. T h e m a s k w a s i l l u m i n a t e d w i t h a b e a m 0 . 8 2 5 d e g r e e off the o p t i c a x i s to a v o i d t h e c e n t r a l o b s c u r a t i o n c a u s e d b y t h e p r i m a r y mirror. A n o f f a x i s a p e r t u r e in t h e s y s t e m l i m i t e d t h e n u m e r i c a l a p e r t u r e to N A = 0.113. T h e o r y i n d i c a t e d t h a t t h e b e s t 0.2 m i c r o n l i n e s w o u l d be i m a g e d u s i n g t h i s NA. A larger NA would have p r o d u c e d l o w e r c o n t r a s t f e a t u r e s d u e to i n c r e a s e d a b e r r a t i o n s .
Off-axis Aperture
I Synchrotron Radiation
[ ~
\
~
\ Transmission Mask F i g u r e i. S c h e m a t i c with 20X reduction.
diagram
Image \ ~
.nmary
Mirror
coverea
Wafer
Secondary Mirror of
the
Off-axis
Schwarzschild
Camera
W e w e r e a b l e to o b t a i n g o o d 0.2 m i c r o n l i n e s a n d s p a c e s w h e r e a s the 0.i m i c r o n l i n e s a n d s p a c e s d i d n o t print. T h e c o h e r e n t cutoff f r e q u e n c y (NA/I) for t h i s s y s t e m c o r r e s p o n d e d to a b o u t 0.16 m i c r o n l i n e s a n d spaces. T h u s the 0.i m i c r o n l i n e s a n d s p a c e s d i d n o t p a s s through the system. The results were consistent with near d i f f r a c t i o n l i m i t e d imaging. A second s e t of m i r r o r s w a s c o a t e d w i t h 20 l a y e r p a i r s of a molybdenum-silicon multilayer that had a measured reflectance peak of 38% at 13.8 n m a n d a b a n d p a s s of 0.7 nm FWHM. C o m p u t a t i o n s s h o w e d t h a t w i t h N A = 0.083, 0.i m i c r o n l i n e s a n d s p a c e s h a v e the
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F i g u r e 2. S E M m i c r o g r a p h of 0.i m i c r o n l i n e s a n d s p a c e s in 60 n m of PMMA, p r i n t e d u s i n g t h e multilayer schwarzschild camera and 13.8 nm wavelength. The e x p o s u r e t i m e for t h i s s h o t w a s 23 sec.
Figure 3. SEM micrograph of 0 . 0 5 m i c r o n l i n e s a n d s p a c e s in 60 n m of PMMA, p r i n t e d u s i n g the multilayer coated schwarzschild c a m e r a a n d 13.8 nm wavelength.
b e s t c o n t r a s t . Fig. 2 s h o w s t e n t h m i c r o n l i n e s a n d s p a c e s in P M M A resist. We observed that the line e d g e s w e r e m o r e v e r t i c a l at 13.8 n m t h a n at 36 nm. W e b e l i e v e t h i s is b e c a u s e t h e r a d i a t i o n p e n e t r a t e d f u r t h e r i n t o t h e r e s i s t at 13.8 n m a n d p r o d u c e d a m o r e uniform exposure though the thickness. The coherent illumination c u t o f f f r e q u e n c y w i t h N A = 0.083 c o r r e s p o n d s to 0.08 m i c r o n l i n e s and spaces. Light from 0.05 micron lines and spaces did not pass through the aperture and were not printed. On opening up the a p e r t u r e to N A = 0.15 w e p r i n t e d 0.05 m i c r o n l i n e s a n d s p a c e s (Fig. 3). S i n c e t h i s l a r g e r n u m e r i c a l a p e r t u r e r e s u l t s in i n c r e a s e d a b e r r a t i o n s w e w e r e n o t s u r p r i s e d t h a t t h e q u a l i t y of t h e lines a n d s p a c e s w e r e n o t as h i g h as t h e 0.i m i c r o n l i n e s of Fig. 2. It is a l s o p o s s i b l e t h a t a d h e s i o n f a i l u r e of t h e r e s i s t c o n t r i b u t e d to the non-uniformities of t h e line. The resists we used were PMMA (polymethyl methacrylate), EBR-9 (2,2,2-trifluoroethyl ~-chloroacrylate) and PBS (polybutene-i s u l f o n e ) . T h e s e n s i t i v i t i e s of t h e s e t h r e e r e s i s t s , PMMA, E B R - 9 a n d PBS, were, r e s p e c t i v e l y , a b o u t 23, i, a n d 0.I m J / c m 2 at w a v e l e n g t h = 36 n m a n d a b o u t 55, 8, a n d 0.7 m J / c m 2 at 13 nm. Since these soft x-rays are highly absorbed by carbon compounds we used very thin l a y e r s of r e s i s t , a b o u t 60 nm, e i t h e r on b a r e s i l i c o n or on a t r i - l e v e l s y s t e m c o n s i s t i n g of t h e t h i n resist, 30 n m of g e r m a n i u m , a n d 300 n m h a r d - b a k e d S h i p l e y 1803 r e s i s t . Fig. 4 s h o w s 0.15 a n d 0.i m i c r o n l i n e s a n d s p a c e s w h e n i m a g e d a n d p r o c e s s e d u s i n g s u c h a tri-level resist scheme. Our best images were made with the high r e s o l u t i o n r e s i s t , PMMA. W e c o u l d u s e s u c h a l o w s e n s i t i v i t y r e s i s t because the camera's demagnification i n t e n s i f i e d t h e f l u x in t h e image plane by a factor of 400, i.e. the square of the demagnification factor.
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Figure 4. SEM m i c r o g r a p h of 0.i and 0.15 m i c r o n lines and spaces in silicon. A t r i - l e v e l resist was used as the imaging layer. The image was then t r a n s f e r r e d using reactive ion e t c h i n g into the s i l i c o n substrate.
RING-FIELD
EXPERIMENTS
-
NO
REDUCTION
The r i n g - f i e l d camera is a scanner with unity m a g n i f i c a t i o n sold by Perkin E l m e r as the M i c r o A l i g n PE300. This optic r e p r e s e n t s an e x p e r i m e n t a l a r r a n g e m e n t that is closer to a p r a c t i c a l l i t h o g r a p h i c tool than the S c h w a r z s c h i l d optic. We m o d i f i e d the s y s t e m by r e m o v i n g all the s c a n n i n g m e c h a n i s m and optics leaving only the two main spherical imaging mirrors. The s c h e m a t i c layout of the e x p e r i m e n t is shown in Fig. 5. The m i r r o r s were s e l e c t e d for good surface figure. To p r o d u c e r e f l e c t i v i t i e s of 10-15% in the 40-45 nm range they were c o a t e d w i t h 17 nm iridium on 6 nm chromium. The m i r r o r M2 was a p e r t u r e d to p r o d u c e a numerical a p e r t u r e of 0.135. This NA p r o d u c e s the best c o m p r o m i s e b e t w e e n d i f f r a c t i o n limited r e s o l u t i o n and low a b e r r a t i o n s [i0]. We also r e p o s i t i o n e d the m i r r o r s s l i g h t l y to o p t i m i z e the r e s o l u t i o n at this numerical aperture.
END VIEW
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Wafer ring of good correction
camera used with 42
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Fig. 6 s h o w s t h e m o d u l a t i o n t r a n s f e r f u n c t i o n p l o t for t h e o p t i c w h e n u s e d w i t h i n c o h e r e n t r a d i a t i o n of w a v e l e n g t h of 42 nm. T h e r i n g of g o o d focus is a b o u t 200 m i c r o n s w i d e w i t h a r a d i u s of 55 mm. We illuminated the transmission mask directly with the 2 mm d i a m e t e r x - r a y beam. T h e u s e f u l i l l u m i n a t e d a r e a is t h u s 0.2 x 2.0 mm. Both the mask and the resist coated wafer were mounted in a m e c h a n i s m t h a t a l l o w e d t h e m to b e b r o u g h t into focus, t i l t e d and transported laterally to a l l o w m u l t i p l e exposures. The chamber included a l o a d l o c k so w a f e r s could be inserted and removed w i t h o u t t h e w h o l e s y s t e m b e i n g v e n t e d to air. S i n c e t h i s is a one to one s y s t e m a n d t h e r e w e r e t h r e e m i r r o r s t h e x - r a y i n t e n s i t y on t h e r e s i s t w a s a b o u t o n e t h o u s a n d t i m e s l o w e r t h a n for t h e S c h w a r z s c h i l d . Thus our first experiments have been c a r r i e d o u t w i t h t h e v e r y s e n s i t i v e e - b e a m resist, PBS. The camera w a s b r o u g h t i n t o a p p r o x i m a t e focus w i t h v i s i b l e l i g h t b u t t h e final parameters (radial p o s i t i o n of t h e r i n g a n d focus) h a d to be determined by making x-ray exposures. These experiments emphasized a m a j o r p r o b l e m in s o f t x - r a y p r o j e c t i o n l i t h o g r a p h y w h e n w o r k i n g under medium vacuum ('i0 °6 torr). A n a b s o r b i n g f i l m of c a r b o n is d e p o s i t e d on s u r f a c e s s t r u c k by h i g h i n t e n s i t y x-rays. T h e r e s i d u a l p a r t i a l p r e s s u r e of l o w v o l a t i l e c a r b o n c o m p o u n d s t h a t a r e u s u a l l y present in v a c u u m systems are c r a c k e d by t h e x - r a y flux and d e p o s i t e d as carbon. T h i s is a p r o b l e m k n o w n to x - r a y s y n c h r o t r o n u s e r s w h o c o p e w i t h it b y u s i n g u l t r a - h i g h v a c u u m t e c h n i q u e s , s i n c e
1.0 DIFFRACTION ~ ~,.
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Figure 6 . Calculated square-wave modulation transfer function for the Ring-field camera with incoherent i l l u m i n a t i o n at a w a v e l e n g t h of 42 nm. In our experiments, nearly fully coherent i l l u m i n a t i o n at 42 n m w a s used; the cutoff frequency for t h i s c a s e is a b o u t 3000 i p / m m
F i g u r e 7. S E M m i c r o g r a p h of the l i n e a n d s p a c e m a s k i m a g e d into 60 nm of PBS r e s i s t w i t h t h e Ring-field c a m e r a u s i n g 42 nm x-rays. T h e 1.0, 0.5, 0.4, 0.3, 0.25 a n d 0.2 m i c r o n l i n e and spaces are printed. The 0.15 m i c r o n l i n e s a n d s p a c e s (to the u p p e r r i g h t of t h e 0.2 m i c r o n lines) are not printed. The e x p o s u r e t i m e for t h i s s h o t w a s i0 sec.
I.E. Bjorkhohn et aL / Projection llithography usbzg soft X-rays
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the p h o t o r e s i s t is g e n e r a l l y a carbon b a s e d m a t e r i a l that could well be outgassing, the o p e r a t i o n of the optics in a clean carbon free s y s t e m p r e s e n t s a s i g n i f i c a n t p r o b l e m that needs to be a d d r e s s e d in future experiments. We found that a local partial p r e s s u r e of o x y g e n (about 10 .3 torr) in the r e g i o n w h e r e the soft x-rays strike any surface g r e a t l y r e d u c e d but did not e l i m i n a t e the d e p o s i t i o n of carbon. Fig. 7 shows a recent image from the r i n g - f i e l d camera u s i n g 42 nm r a d i a t i o n and the PBS resist. The 0.2 m i c r o n lines and spaces are the s m a l l e s t imaged. The 0.15 m i c r o n lines and spaces are not printed. The c o h e r e n t i l l u m i n a t i o n cutoff f r e q u e n c y for this system is 0.16 m i c r o n lines and spaces, so the 0.15 m i c r o n lines are not e x p e c t e d to be printed. The image shows s c u m m i n g and g r a n u l a r i t y a s s o c i a t e d w i t h this resist at these small line dimensions. The lowered c o n t r a s t of the 0.2 m i c r o n lines and spaces c o m p a r e d to other lines and spaces indicated that the optic is v e r y close to but p r o b a b l y not quite at the d i f f r a c t i o n limit. SUMMARY One of the m a i n q u e s t i o n s r e g a r d i n g the f e a s i b i l i t y of p r o j e c t i o n x-ray l i t h o g r a p h y is the p r a c t i c a l i t y of m a k i n g optics with a d e q u a t e figure accuracy. For d i f f r a c t i o n limited imaging the w a v e f r o n t error at the image plane has to be less than I/4. For the S c h w a r z s c h i l d e x p e r i m e n t at 13.8 nm with 2 r e f l e c t i o n s this t r a n s l a t e s to a RMS error of less than i/ll (13 angstroms) for each m i r r o r over the total active optical area of only 0.2 cm 2. This e x p e r i m e n t showed that d i f f r a c t i o n limited imaging is p o s s i b l e with m u l t i l a y e r e d m i r r o r s using 13.8 nm radiation. The active optical surface is, however, rather small. For the r i n g - f i e l d e x p e r i m e n t at 42 nm w i t h three reflections, d i f f r a c t i o n limited imaging t r a n s l a t e s to a RMS error of less than 1/14 (30 angstroms) for each m i r r o r over the total active optical area of ii0 cm 2. This large optical area d e m o n s t r a t e s that it is p o s s i b l e to make m i r r o r s that have close to the r e q u i r e d figure for large optical areas. A design study to d e t e r m i n e a m i r r o r layout for a p r a c t i c a l soft x-ray l i t h o g r a p h i c camera r e s u l t e d in the need for at least 4 m i r r o r s and a large ( > i00 cm 2) active area [ii]. W o r k i n g at 13.8 nm with such a camera w o u l d require a RMS error of 1/16 (8 angstroms) for each mirror. This is beyond, or right at the limit, of c u r r e n t optical f a b r i c a t i o n t e c h n i q u e s and needs to be a d d r e s s e d if p r o j e c t i o n xray l i t h o g r a p h y is to be developed.
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