Simulation of Resist Profiles for 0.5-Î¼m Photolithography at 248 nm

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Chapter 25 Simulation of Resist Profiles for 0 . 5 - μ m Photolithography at 248 n m R. K. Witts, T. M. Wolf, L. E. Stillwagon, and M. Y. Hellman AT&T Bell Laboratories, Murray Hill, NJ 07974 One of the methods under development at AT&T Bell Laboratories for submicron lithography is deep ultraviolet projection photolithogra phy. (1) Fine line definition is obtained by use of 248 nm light and a lens of large numerical aperture. Because of the large chro matic aberration of the quartz lens a spectrally line-narrowed krypton fluoride excimer laser is used as a light source. Microposit 2400 resist, manufactured by Shipley Co., has been shown to be sensitive at this short wavelength (2) and is being employed with the deep UV stepper. We report here the results of resist profile modeling for submicron photolithography at 248 nm. Various model parameters needed as input data were measured to characterize exposure and development of the resist. Determination of Resist Parameters. Most resists are designed for exposure at wavelengths longer than the 248.4 nm radiation provided by a KrF laser source. Wolf and coworkers (3) have found that the choice of a positive resist for use at this wavelength is limited. They evaluated a number of positive resists. Only Microposit 2415, and its newer analog, Microposit 2400-17, were compatible with the anticipated exposure time of 0.5 to 1.0 seconds for resist sensitivity of 100 to 200 mJ/cm needed with the new exposure tool developed by Pol and co workers. (1) The resist consists of three components: a resin, a photoactive compound or PAC (which acts as a dissolution inhibitor), and a solvent. Upon exposure, the PAC is destroyed, and this allows the resist film to dissolve in the aqueous basic developer. The SAMPLE program (4) is used to simulate exposure and development of features in the MP2400-17 positive resist. The three resist parameters A, Β and C defined by Dill and coworkers 05) and the solubility rates of the resist films as a function of exposure dose (or PAC content) must be determined to perform the simulation. 2

0097-6156/87/0346-0292506.00/0 © 1987 American Chemical Society Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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O p t i c a l Absorbance

293

of Resist Profiles

Measurements.

The r e s i s t parameters A, Β and C a r e n o r m a l l y determined from optical absorbance measurements o f exposed and unexposed resist f i l m s . ( 5 ) The Β parameter i s o b t a i n e d from t h e absorbance o f a f i l m t h a t i s g i v e n a s u f f i c i e n t exposure dose t o d e s t r o y a l l t h e p h o t o a c t i v e compound. The q u a n t i t y A+B i s o b t a i n e d from t h e absorbance o f an unexposed r e s i s t f i l m and C i s d e t e r m i n e d from t h e i n i t i a l s l o p e o f an absorbance v e r s u s exposure time p l o t . F i g u r e 1 shows the u l t r a v i o l e t (UV) s p e c t r a o f an unexposed MP2400-17 f i l m and a f i l m exposed t o 470 mJ/cm . The f i l m s were s p i n c o a t e d onto q u a r t z s u b s t r a t e s and baked at 90°C f o r 30 minutes i n a f o r c e d - a i r oven. The f i l m t h i c k n e s s a f t e r b a k i n g was 0.465 ym. Exposure was a c c o m p l i s h e d by d i r e c t i r r a d i a t i o n o f the f i l m t o t h e narrowed output o f a Math S c i e n c e s EXL-100 K r F excimer laser. T h i r t y p u l s e s were d e l i v e r e d at an average dose o f 15.7 mJ/cm p e r pulse. The spectrum o f the q u a r t z s u b s t r a t e was a u t o m a t i c a l l y s u b t r a c t e d from t h e s p e c t r a o f the f i l m s on t h e s u b s t r a t e s t o o b t a i n F i g u r e 1. The absorbance o f t h e two f i l m s i s the same at wavelengths below 310 nm and t h e s o l i d c u r v e i n F i g u r e 1 r e p r e s e n t s the absorbance o f b o t h f i l m s . The absorbance at 248 nm i s 0.49 f o r both f i l m s . D e s t r u c t i o n o f t h e PAC by the 248 nm r a d i a t i o n , as e v i d e n c e d by the d i s a p p e a r a n c e o f the l o n g e r wavelength a b s o r p t i o n i n t h e i r r a d i a t e d f i l m , presumably o c c u r s a f t e r n o n r a d i a t i v e energy transfer. A v a l u e o f 2.4 μπΓ i s o b t a i n e d f o r Β u s i n g v a l u e s o f 0.49 f o r the absorbance and 0.465 ym f o r t h e f i l m t h i c k n e s s (see r e f . 5 f o r details of calculation). S i n c e t h e f i l m does not b l e a c h at 248 nm, the A parameter i s 0 and t h e C parameter cannot be determined i n the normal way (5) from t h e absorbance d a t a . We e s t i m a t e d t h e C parameter u s i n g a chromatographic t e c h n i q u e t h a t w i l l be d e s c r i b e d later. 2

2

1

P r e p a r a t i o n and Exposure o f F i l m s . T h i s s e c t i o n d e s c r i b e s the p r e p a r a t i o n and exposure o f t h e f i l m s used i n t h e c h r o m a t o g r a p h i c and d i s s o l u t i o n r a t e measurements. MP2400-17 f i l m s , 0.465 Mm t h i c k , on t o p o f a 1.5 Pm t h i c k h a r d baked (HB) HPR206 f i l m were i r r a d i a t e d w i t h a K r F excimer l a s e r t h a t w i l l be d e s c r i b e d below. The HB206 f i l m was p r e p a r e d by s p i n c o a t i n g a HPR206 f i l m onto a 3 i n c h d i a m e t e r s i l i c o n wafer and b a k i n g the f i l m at 210°C f o r 1 hour. The MP2400-17 f i l m was s p i n c o a t e d on t o p o f the HB206 f i l m and t h e b i l e v e l f i l m was baked at 90°C f o r 30 m i n u t e s . The b a k i n g was done i n a f o r c e d - a i r oven. The bottom HB206 f i l m t o t a l l y absorbed any l i g h t that passed t h r o u g h t h e MP2400-17 f i l m , eliminating reflections from t h e substrate. F i g u r e 2 shows a schematic o f the system t h a t was used t o f l o o d expose the r e s i s t f i l m s . A Quanta Ray EXC-1 KrF excimer l a s e r was used f o r a l l e x p o s u r e s . The l a s e r output at 30 keV was 30-55 mJ/pulse/cm w i t h a 0.33% f l u o r i n e , 5% k r y p t o n , and 94.67% neon gas m i x t u r e at 45 p s i g . The beam, at 100 p u l s e s / s e c , was reduced t o a 3 mm d i a m e t e r spot u s i n g a p e r a t u r e s . A 5 mm d i a m e t e r l i g h t p i p e was used t o scramble t h e beam and improve u n i f o r m i t y . 2

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

294

POLYMERS FOR HIGH T E C H N O L O G Y

1.0 CONTROL.NO IRRADIATION

470mj/cm •

2

AT 248.4 nm

248 nm

g0.5h o m
Z>

ELUTION VOLUME —MOLECULAR WEIGHT F i g u r e 3 . G e l p e r m e a t i o n chromatograms o f (A) unexposed M i c r o p o s i t 2 4 0 0 - 1 7 and ( B ) M i c r o p o s i t 2 4 0 0 - 1 7 e x p o s e d t o 418 m J / c m The UV d e t e c t o r o u t p u t a t 2 5 0 nm i s s h o w n . 2

ELUTION VOLUME MOLECULAR WEIGHT F i g u r e 4 . The D R I d e t e c t o r o u t p u t 2400-17 and (B) M i c r o p o s i t 2400-17

•

f o r (A) unexposed M i c r o p o s i t e x p o s e d t o 300 m J / c m . 2


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2

300 mJ/cm i r r a d i a t i o n and no major new peaks appear i n t h e exposed f i l m chromatogram. T h i s f u r t h e r s u p p o r t s our h y p o t h e s i s t h a t t h e s m a l l peak i n chromatogram Β o f F i g u r e 3 i s due t o t h e n o v a l a c r e s i n and not t o t h e PAC. Values f o r M were c a l c u l a t e d u s i n g t h e PAC peak areas determined f o r s e v e r a l i r r a d i a t e d f i l m s as shown i n Table I . The a r e a o f t h e PAC peak i n each chromatogram was n o r m a l i z e d t o t h e a r e a o f the i n t e r n a l s t a n d a r d benzene peak and c o r r e c t e d by s u b t r a c t i n g t h e n o r m a l i z e d area o f t h e background peak t h a t was determined from t h e chromatogram o f t h e f i l m exposed t o 418 mJ/cm . The PAC c o n c e n t r a t i o n M i s p r o p o r t i o n a l t o t h e peak area and was n o r m a l i z e d by s e t t i n g t h e area o f t h e peak f o r t h e unexposed f i l m t o 1.0. F i g u r e 5 shows a p l o t o f l o g M v e r s u s dose. The s o l i d l i n e i s a l e a s t - s q u a r e s f i t t o t h e d a t a and C was determined from the s l o p e o f t h i s l i n e t o be 0.013 (mJ/cm )"" . 2

2

1

Table I . C a l c u l a t i o n o f M from t h e GPC peak a r e a d a t a

2

Dose (mJ/cm )

0 11.7 14.0 23.3 30.0 46.5 52.2 60.0 69.8 78.3 90.0 120.0 417.6

Peak A r e a *

Peak Area - Background**

2.65 2.32 2.26 1.69 1.63 1.44 1.23 1.21 0.95 0.71 0.78 0.57

3.19 2.86 2.80 2.23 2.17 1.98 1.77 1.75 1.49 1.25 1.32 1.11 0.54

-

M

1.00 0.88 0.85 0.64 0.62 0.54 0.46 0.46 0.36 0.27 0.29 0.22

—

Areas a r e n o r m a l i z e d t o benzene i n t e r n a l s t a n d a r d peak a r e a . ** Background a r e a e q u a l t o 0.54.

Measurements o f D i s s o l u t i o n R a t e . The d i s s o l u t i o n r a t e s , R, o f t h e r e s i s t f i l m s i n t h e d e v e l o p e r were measured u s i n g a l a s e r end-point d e t e c t i o n system. (6) F i g u r e 6 shows t h e output o f t h e d e v i c e f o r an i d e a l case. The change i n f i l m t h i c k n e s s between maxima ( o r minima) At i s g i v e n by At = λ/2η

(1)

where λ i s t h e wavelength o f l i g h t and η i s t h e r e f r a c t i v e i n d e x o f the f i l m . R can be measured by c a l c u l a t i n g At and measuring t h e time span between maxima. F o r t h i s s t u d y t h e d e v e l o p e r was a 3.5 : 1 m i x t u r e o f water and AZ 400K c o n c e n t r a t e at 27.1°C.


298


0.2 50

100

150

200

DOSE (mJ/cm ) 2

Figure 5. Log of the normalized of exposure dose.

PAC c o n c e n t r a t i o n

(M)

as a

function

UJ

ο >

TIME—^ Figure 6. Response of the f i l m f i l m of poly(methylmethacrylate) a t 2 1 . 8 °C.

t h i c k n e s s m o n i t o r f o r a 1.4 d i s s o l v i n g i n methyl ethyl


μιη-thick ketone

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F i g u r e 7 shows t h e t y p i c a l o u t p u t o f t h e e n d - p o i n t detection d e v i c e f o r the d i s s o l u t i o n , at 27.1°C, o f a p h o t o l y z e d MP2400-17 f i l m o n a 1.5 y m t h i c k HB206 f i l m . The i n i t i a l MP2400-17 film t h i c k n e s s was 0 . 4 6 5 y m . V a l u e s o f 1.55 a n d 6328A w e r e u s e d f o r η and λ (He-Ne l a s e r ) , r e s p e c t i v e l y . T h e v a l u e o f η was d e t e r m i n e d using a s i m i l a r end-point detection d e v i c e d u r i n g oxygen plasma etching of MP2400-17 films, and m e a s u r i n g At directly with a profilometer. T h e v a l u e f o r η was c a l c u l a t e d u s i n g E q u a t i o n 1. T h e o u t p u t f o r t h e d i s s o l u t i o n o f t h e b i l e v e l f i l m a p p e a r e d t o be i d e a l , b u t t h e HB206 f i l m a p p a r e n t l y s l o w l y d i s s o l v e d o r s w e l l e d i n t h e d e v e l o p e r and t h e o u t p u t c o n t i n u e d t o o s c i l l a t e a f t e r t h e t o p M i c r o p o s i t r e s i s t l a y e r h a d d i s s o l v e d , as shown i n F i g u r e 7 . The d i s s o l u t i o n r a t e was m e a s u r e d a s a f u n c t i o n o f d e p t h i n t o t h e f i l m f o r s e v e r a l p h o t o l y z e d M P 2 4 0 0 - 1 7 f i l m s on a HB206 f i l m . The d o s e was e s t i m a t e d a s a f u n c t i o n o f d e p t h i n t o t h e r e s i s t a c c o r d i n g t o l(z) = l(0)exp(-Bz). Thus, several pieces of data (dissolution r a t e as a f u n c t i o n o f dose) were o b t a i n e d from each d i s s o l u t i o n rate curve. T h e v a l u e o f t h e PAC c o n t e n t c o r r e s p o n d i n g to each d o s e was o b t a i n e d f r o m F i g u r e 5 a n d t h e r e s u l t i n g d a t a ( d i s s o l u t i o n r a t e R v s . M) w e r e p l o t t e d i n F i g u r e 8 . The s o l i d l i n e i n F i g u r e 8 i s a f i t of the d a t a to E q u a t i o n 3. Modeling. F o r p o s i t i v e p h o t o r e s i s t s t h e m o d e l o f D i l l 05) i s u s e d t o s i m u l a t e e x p o s u r e and d e v e l o p m e n t . In t h i s model the i l l u m i n a t i o n i n t e n s i t y I at 248 nm a n d the normalized concentration of photoactive compound a r e g i v e n b y t h e t w o c o u p l e d e q u a t i o n s , 31

l(x,z,t)[AM(x,z,t)

?z 3M

+

B]

l(x,z,t)M(x,z,t)C

Ht

(2)

i n t e r m s o f t h e e x p o s u r e p a r a m e t e r s A , B , and C . The v a l u e s o f A , Β and C used i n t h e s i m u l a t i o n were 0 , 2.4 y m " and 0 . 0 1 3 ( m j / c m ) * " , respectively. T h e r a t e o f d i s s o l u t i o n i n t h e d e v e l o p e r s o l u t i o n was m o d e l e d by t h e e x p r e s s i o n 1

R = exp(E

2

+ E M + E3M )

(3)

2

x

1

2

T h e t h r e e p a r a m e t e r s E , E2, and E were found by f i t t i n g E q u a t i o n 3 t o t h e m e a s u r e d v a l u e s o f R a s a f u n c t i o n o f PAC c o n c e n t r a t i o n , a s shown i n F i g u r e 8 . The v a l u e s o b t a i n e d a r e = 5.25, E • 0.45, E = -4.5. T h e SAMPLE p r o g r a m ( 4 ) was u s e d t o o b t a i n r e s i s t profiles. E x p o s u r e a t 248 nm w i t h a l e n s o f n u m e r i c a l a p e r t u r e NA = 0 . 3 8 a n d p a r t i a l c o h e r e n c e σ = 0 . 7 was a s s u m e d . T h e s u b s t r a t e was 1200 A o f Si0 on t h i c k HB206 p h o t o r e s i s t on s i l i c o n . That i s , the sensitive r e s i s t i s part of a t r i l e v e l s t r u c t u r e . E x p o s u r e d o s e was s e t to 120 m J / c m , a n d d e v e l o p m e n t t i m e s w e r e a b o u t 2 m i n . 1

3

2

3

2

2



300

START

BOTTOM LAYER REMOVAL

UJ

< ο >

TIME

—

F i g u r e 7. Response o f t h e f i l m t h i c k n e s s m o n i t o r d u r i n g d i s s o l u t i o n o f a 0 . 4 6 5 p m - t h i c k f i l m o f M i c r o p o s i t 2 4 0 0 - 1 7 o n a 1 . 5 μπιt h i c k HB206 f i l m . The s o l v e n t i s a 1 : 3 . 5 m i x t u r e o f AZ400K d e v e l o p e r and w a t e r a t 2 7 . 1 °C.

0.4

0.6

0.8

1.0

M F i g u r e 8 . P l o t o f I n R ( i n Â / s e c ) v s . n o r m a l i z e d PAC c o n c e n t r a t i o n , M . The s o l i d l i n e i s a f i t o f t h e d a t a t o E q u a t i o n 3 .


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F i g u r e s 9, 10, 11 and 12 show the r e s u l t s f o r 1.0, 0.5, 0.4 and 0.35 ym equal l i n e / s p a c e p a t t e r n s , r e s p e c t i v e l y . Curves are shown f o r p e r f e c t focus and f o r defocus of 1 ym and 2 μτη. These p r o f i l e s i l l u s t r a t e the best r e s u l t s that can be o b t a i n e d w i t h the system, s i n c e a l e n s f r e e from a b e r r a t i o n s i s assumed i n the calculation. The f i g u r e s show that we cannot expect t o a c h i e v e v e r t i c a l r e s i s t p r o f i l e s at t h e s e r e s o l u t i o n s . The s l o p e d p r o f i l e s u n d e r s c o r e the importance of o p t i c a l d e n s i t y and photobleaching w i t h exposures at 248 nm. A 0.465 ym t h i c k f i l m of M i c r o p o s i t 2400-17 has an o p t i c a l d e n s i t y of about 0.5. As a consequence the upper r e g i o n s of the r e s i s t r e c e i v e a l a r g e r dose than the bottom. T h i s broadens the top of the exposed r e g i o n s . An improvement i n r e s i s t p r o f i l e should be o b t a i n e d from a r e d u c t i o n of o p t i c a l d e n s i t y which would a l l o w a more u n i f o r m exposure throughout the resist film. P h o t o b l e a c h i n g at the e x p o s i n g wavelength would a l s o improve r e s i s t c o n t o u r s because of a c o n t r a s t enhancement e f f e c t . Depth of focus i s somewhat l e s s f o r 0.5 ym l i n e s and spaces than f o r 1 ym l i n e s and spaces, although i n p r a c t i c e a depth of focus of ±1 ym c o u l d be assumed f o r both p a t t e r n s . The presence of the s h a l l o w s t a n d i n g wave p a t t e r n i s due t o r e f l e c t i o n s from the S i 0 i n t e r l a y e r . F i g u r e 13 shows the e x p e r i m e n t a l r e s u l t s c o r r e s p o n d i n g t o the s i m u l a t i o n s . There i s an asymmetry i n the l i n e p r o f i l e p r o b a b l y because of a t i l t of the l e n s . The p a t t e r n s are s l i g h t l y overexposed, c a u s i n g the spaces between l i n e s t o be w i d e r than the lines. These f i g u r e s show the n o n - v e r t i c a l p r o f i l e s p r e d i c t e d by the s i m u l a t i o n . 2

Χ

(μνη)

Figure 9. S i m u l a t i o n of a l i n e p r o f i l e f o r a 1.0 μιη l i n e / s p a c e p a t t e r n . The edge of the s l i t i s a t χ = 0. P r o f i l e s are shown f o r defocus v a l u e s of 0, 1, and 2 μιη.


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POLYMERS FOR HIGH TECHNOLOGY

-0.25

0.0 0.25 Um) F i g u r e 10. S i m u l a t i o n o f a l i n e p r o f i l e f o r a 0.5 μπι l i n e / s p a c e p a t t e r n . The edge of the s l i t i s a t χ » 0. P r o f i l e s a r e shown f o r d e f o c u s v a l u e s o f 0, 1 , and 2 μπι. X

0.4 μτη i/S PATTERN 0.4

1°

50.3 co CO UJ

V

\

0.2

0Λ

_

°?b.2

2

^

λ X 0.0

Χ [μνη)

\ 0.2

F i g u r e 11. S i m u l a t i o n o f a l i n e p r o f i l e f o r a 0.4 μιη l i n e / s p a c e p a t t e r n . The edge o f the s l i t i s a t χ - 0. P r o f i l e s a r e shown f o r def ocus v a l u e s o f 0, 1, and 2 μπι. Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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0.35 μπ\ i/S PATTERNS

X

(/im)

F i g u r e 12. S i m u l a t i o n o f a l i n e p r o f i l e f o r a 0.35 μπι l i n e / s p a c e p a t t e r n . The edge o f the s l i t i s a t χ = 0. P r o f i l e s a r e shown f o r defocus v a l u e s o f 0, 1, and 2 μιη.


304

POLYMERS FOR HIGH TECHNOLOGY

F i g u r e 13. SEM photographs of 1.0, 0.8, 0.5, 0.4, and 0.35 line/space patterns.


μπι

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Acknowledgment s The authors thank J. Bennewitz for the deep UV stepper exposures. We also thank Andrea Pas tuck for the SEM photographs and Tanya Jewell for providing the value for the refractive index of the MP2400-17 resist film. References 1. Pol, V.; Bennewitz, J. H.; Escher, G. C.; Feldman, M.; Firtion, V. Α.; Jewell, T. E.; Wilcomb, Β. Ε.; Clemens, J. T.; "Excimer Laser-Based Lithography: A Deep Ultraviolet Wafer Stepper" SPIE Conf. on Microlithography, March 13, 1986. 2. Lin, B. J.; Proc. Electrochem. Soc., Electron and Ion Beam Science and Technology, International Conf., 1979, 78-5, 320. 3. Wolf, T. M.; Hartless, R. L.; Shugard, Α.; Taylor, G. N.; "Evaluation of Resists for Lithography at 248 nm. I. Positive Tone Resists", Int. Symp. on Electron, Ion, and Photon Beams, May, 1986. 4. Oldham, W. G.; Nandgaonkar, S. N.; Neurether, A. R.; O'Toole, M IEEE Trans. Electron Dev. 1979, ED26, 717. 5. D i l l , F. H.; Hornberger, W. P.; Hauge, P. S.; Shaw, J. M.; IEEE Trans. Electron Dev. 1975, ED22, 445. 6. Introduction to Microlithography, Thompson, L. F.; Wilson, C.B. Bowden, M. J. eds; Ch. 3. ACS Symposium Series No. 219, 1983. RECEIVED May 13, 1987


Simulation of Resist Profiles for 0.5-Î¼m Photolithography at 248 nm

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