The modelling and simulation of nonlinear photobleaching materials

Microelectronic Engineering 13 (1991) 493-496 Elsevier

THE MODELLING MATERIALS

Wen-an

Loong,

AND

Hong-tsz

493

SIMULATION

OF NONLINEAR

~HOTOBLEACHING

Pan

Institute of Applied Chemistry National Chiao Tung University Hsinchu 30050, Taiwan, Republic

of China

We found that the Dill's p a r a m e t e r s of n o n l i n e a r l y p h o t o b l e a c h i n g m a t e r i a l s of p-diazo-N,N-dimethylaniline chloride zinc chloride(DZC) and poly(di-n-hexylsilane)(PDHS) are irregularly d e p e n d e n t on their film thickness, and also their refractive indexes change during exposure. These difficulties make Dill's model inadequate for the simulation of nonlinear photobleaching m a t e r i a l s used in contrast e n h a n c e d lithography. A direct approach was p r o p o s e d to solve these difficulties. E q u a t i o n s with four p a r a m e t e r s were derived to simulate the bleaching curves of DZC, PDHS and reported b l e a c h i n g curve of CEM-2. Equations correlated very well with these curves by using best fitted parameters. Linearity was found with p a r a m e t e r s in these equations as a function of film thickness of DZC and PDHS. Based on this linearity, simulations of these nonlinear b l e a c h i n g curves are more accurate than those using Dill's model.

1.

INTRODUCTION

In our study of nonlinear photobleaching materials of p - d i a z o - N , N - d i m e thylaniline-chloride zinc chloride (DZC) and poly(di-n-hexylsilane) (PDHS), it was found that the measured values of Dill's parameters [I], A, B, and C of DZC and PDHS thin film bleaching curves were irregularly dependent on their thickness. This is contrary to the diazo-novolac based positive p h o t o r e s i s t s whose A, B, C values are independent of resist thickness. In addition, the r e f r a c t i v e index of DZC thin film was found to increase, PDHS to decrease, during exposure, respectively. Therefore, difficulties result when attempting to model the behavior of these nonlinear photobleaching m a t e r i a l s by using Dill's p a r a m e t e r s which are generally used for the m o d e l l i n g and simulation of contrast e n h a n c e d lithography [2-4]. Although Babu and B a r o u c h [5,6] have reported the exact solutions for exposure bleaching of n o n l i n e a r resist material, it is still very difficult to apply to m o d e l l i n g and simulation. A direct approach was used in this paper to solve these difficulties. Equations w i t h four parameters were derived to do the modelling of DZC and PDHS. The PROLITH p r o g r a m [3] was modified to perform the sidewall p r o f i l e simulations along w i t h these equations. The reported nonlinear p h o t o b l e a c h i n g curve of CEM-2 [4] (other name of CEM-388, from General Electric) can also be fitted very well.

2.

EXPERIMENTAL

DZC is obtained from Tokyo Kasei and Poly(N-vinylpyrrolidone) (PVP) is obtained from Janssen. The chemical structures of DZC and PVP are shown in Fig. i. The PDHS was prepared as follows. The Grignard reagent of n-hexylmagnesiumbromide was formed by the reaction of n - h e x y l b r o m i d e and m a g n e s i u m powder in dry ether, then, by titration, 2 moles of this freshly prepared G r i g n a r d reagent was reacted with I mole of t e t r a c h l o r o s i l a n e in n-heptane to

0167-9317/91/$3.50 © 1991 - Elsevier Science Publishers B.V.

W-A. Loo.b H-T. Pan

494

/ Nonlinear photobleaching

materials

form t h e d i - n - h e x y l d i c h l o r o s i l a n e . PDHS was p r e p a r e d by s o d i u m m e d i a t e d W u r t z c o u p l i n g of d i - n - h e x y l - d i c h l o r o s i l a n e . PDHS has an a v e r a g e M w of 6 6 , 0 0 0 (by GPC, r e l a t i v e to p o l y s t y r e n e ) . B l e a c h i n g m e a s u r e m e n t s w e r e p e r f o r m e d by u s i n g an O r i e l 500 W d e e p UV i l l u m i n a t o r e q u i p p e d w i t h an O r i e l 365 nm n a r r o w b a n d interference filter. The a p p a r a t u s is s i m i l a r to ref. 4. T h e s a m p l e s for b l e a c h i n g s t u d i e s w e r e p r e p a r e d as follows. The m i x t u r e of D Z C / P V P = 2:1 (w/w) in g l a c i a l a c e t i c acid [7] and PDHS in h e x a n e w e r e s e p a r a t e l y s p i n c o a t e d on t h i n q u a r t z p l a t e s w h i c h w e r e not t r e a t e d to m a t c h t h e i r r e f r a c t i v e i n d e x e s and t h e n s o f t b a k e d at 8 0 ° C for 30 min. Film t h i c k n e s s m e a s u r e m e n t s w e r e m a d e w i t h a D e k t a k IIA p r o f i l o m e t e r . The r e f r a c t i v e index of t h i n film w a s t a k e n u s i n g a Rudolph EL-III automatic ellipsometer.

3.

RESULTS

AND

DISCUSSION

The nonlinear photobleaching c u r v e s of D Z C + P V P w i t h d i f f e r e n t t h i c k n e s s are s h o w n in Fig. 2. F o r the p u r p o s e of c l a r i t y a n d e a s e of c o m p a r i s o n w i t h m o d e l s i m u l a t e d curves, o n l y a few data p o i n t s are shown by o p e n c i r c l e s for e a c h of t h e 6 b l e a c h i n g curves. From Fig. 2, the v a l u e s of D i l l ' s p a r a m e t e r s , A, B, a n d C, a r e m e a s u r e d as shown in Fig. 3 and i n d i c a t e t h a t t h e i r v a l u e s are i r r e g u l a r l y d e p e n d e n t on film thickness. The r e f r a c t i v e index of D Z C + P V P f i l m i n c r e a s e s f r o m a b o u t 1.6 up to 2.3 w i t h e x p o s u r e d o s e s of a b o u t 150 m J / c m 2. T h e a u t o m a t i c m o d e l l i n g and p r o f i l e s i m u l a t i o n are i n a c c u r a t e and i m p r a c t i c a l for contrast enhancement s t u d i e s in such a case. The f o l l o w i n g e x p r e s s i o n was d e r i v e d to fit t h e s e n o n l i n e a r p h o t o b l e a c h i n g c u r v e s of DZC.

T(D i)

=

P[1

1

]5

EXP ( Di s ~ Q

+

R

)+1 2

Dt =

0

T(Di)

dD i

=

S P I I n ( l + e u)

+

O i -Q

u -

2 In(e u) - 2 u

lu

1

eU+l

2(eU+l)

+

2

SRUluo[

-O

uo-

$

+

S

D i is d e f i n e d as incident dose, T(Di) the n o r m a l i z e d film t r a n s m i t t a n c e at Di, D t t h e t r a n s m i t t e d dose. P is d e f i n e d as a t r a n s m i t t a n c e change related parameter b e f o r e and a f t e r bleaching, and is s i m i l a r to t h e p a r a m e t e r A of D i l l ' s model. Q is a film t h i c k n e s s r e l a t e d p a r a m e t e r , R the t r a n s m i t t a n c e of t h e u n b l e a c h e d f i l m and S the slope r e l a t e d p a r a m e t e r at t h e i n f l e c t i o n point.

I

/ ~ CFI5/N-{~ )~-N2C1'~1

CH3\

ZnCl2

p-Diazo-N,N-dimethylanilinechloride DZC

.s .

.s 1

zinc chloride

z ~

.l . .6., .s .j

film Lhickness:

I--

"~CH2- ~CH~'n

2: 0.~

~

/

z

=0

3:0.87

.2 I

Poly(vinylpyrrofidone)

o oq

PVP Fig. 1 T h e c h e m i c a l D Z C and PVP.

6:2.71

,~

zoo

~

INCIDENT BOSE structures

of

'

5:2.0

,~o

soo

s~e

j

(mT/cm 2 )

Fig. 2 E x p e r i m e n t a l (open circle) a n d model s i m u l a t e d (solid line) b l e a c h i n g curves of D Z C w i t h v a r y i n g t h i c k n e s s .

'

W-A. Loong~ H-T. Pan / Nonlinear photobleaching materials

10-

495

I

:•,,® .7 6---

5-

~

.4

4urn

g 3"

I"

ol,

0

Diazonlum Sail Film Thickness (pm)

Fig. 3 M e a s u r e d Dill's parameters A, B a n d C are irregularly dependent on DZC film thickness.

"~

' " ' ,'.~ v~ ..... ~',~"~iazoniumI ~ l t Film Thickness (pro)

Fig. 4 Linear r e l a t i o n s h i p s of simulation parameters as a function of DZC film thickness. 1

.o

.9 .S

-.2

~-~ I,-

g

'

.7 .6

-.4 ~

-s S

~

.4

z

.3

-.6 .E o

-,7 ~, >

/ / f

,"

.

..,/

I i J

I /" _/,

~

I: o.2o~ ~: o.2, 3: o.~ 4: o.~ 5:

o.~

-.9

"o

0

~ Positlon Fra~center of

,

Fig. 5 S i m u l a t e d c o n c e n t r a t i o n of AZ1350J at its surface after exposure and sidewall p r o f i l e after wet development w i t h and w i t h o u t D Z C as CEL.

300 6oo INCIOENT DOSE

sou (ml/cm2 )

~200

Fig. 6 E x p e r i m e n t a l (circle) and model simulated (solid line) bleaching curves of PDHS with various thickness.

Both t h i n film t r a n s m i t t a n c e at specific incident doses and t r a n s m i t t e d doses can t h e n be calculated. The fitting results are shown as solid lines in Fig. 2. L i n e a r r e l a t i o n s h i p s were found for these four parameters as a function of DZC thin film t h i c k n e s s up to 2.71 pm as shown in Fig. 4. This e q u a t i o n can apply to any thickness of D Z C film within this range based on this linearity w h i c h Dill's model does not take into account. For the study of contrast enhancement, the D Z C + P V P system is used as contrast e n h a n c e m e n t layer (CEL), in other words, as top layer; the A Z - 1 3 5 0 J p o s i t i v e resist (photoactive compound, PAC) is u s e d as the b o t t o m layer. The c o n c e n t r a t i o n at the surface of AZ-1350J after exposure, and sidewall p r o f i l e of A Z - 1 3 5 0 J after exposure and wet development are simulated. The PROLITH p r o g r a m (v. 1.4) was modified and re-compiled to do the simulations along with the use of this equation. Fig. 5 shows that the use of DZC as a contrast e n h a n c e m e n t layer does improve the sidewall angle a n d reduces the standing wave effect. C h a n g i n g the e x p o n e n t i a l of c u b i c to I/2 in above equation, it c o r r e l a t e s very well with nonlinear photobleaching curves of PDHS films using best fitted parameters. The results are shown in Fig. 6. Again, linearity was found with these four p a r a m e t e r s as a function of PDHS thickness up to 0.54 pm.

496

W-A. Loong~ H-T. Pan

/ Nonfinear photobleaching

materials

I-

Circle: CEW-2 data from ref. .9-

Line: ~ l

.8-

P : 0.9l Q=45

(al "

R =-0.01

S:7

•

"

. AZI35~j, r~ CEL o ~ ~ CEL o DZC ~ EEL

o

u~ ~

~pQca u.b ~ , ~'itc~ 1.2 (~.]:

Thi~ AZI350j:1,0

3-

.5-

DZC

(d}

.6-

~=

. ^Z1350], ~ EEL, Bottol 2,1

o

1,8~-~ 1.5 c 1.2

.4.3 ~

/~

m

7o

: O.92

5o

POHSas EEL, Bott~ lap Bottom TQp

.2.6.1

I0

20

~0 40 ~ ;0 ~0 Incident Dose (mJlcm2)

;0

~0

1100 1110120

Fig. 7 R e p o r t e d (circle, from ref. 4) and m o d e l s i m u l a t e d (solid line) curves of CEM-2: t r a n s m i t t e d flux at m a x i m u m (a), at m i n i m u m (b); i n s t a n t a n e o u s cont r a s t (c), i n t e g r a t e d c o n t r a s t d).

omo

1'so z'oo Jso

3'oo ~o

Exposure Dose

4'oo 4'5o 5'oo

~5o ~oo

(mf/cm2)

Fig. 8 S i m u l a t e d s p a c e w i d t h and sidewall angle of A Z - 1 3 5 0 J with and w i t h o u t PDHS or D Z C as CEL.

T h e s t u d y of gain, w h e r e g a i n is d e f i n e d as the r a t i o b e t w e e n e n h a n c e d c o n t r a s t a n d a e r i a l image c o n t r a s t , i n d i c a t e s t h a t the g a i n is n e a r l y i n d e p e n d e n t on the t h i c k n e s s of t h e s e m a t e r i a l s . Fig. 7 shows t h a t the r e p o r t e d n o n l i n e a r b l e a c h i n g c u r v e of CEM-2 can be m o d e l l e d q u i t e w e l l by t h e use of the same e q u a t i o n for PDHS. T h e v a l u e s of fitted parameters are also shown. The s p a c e w i d t h and s i d e w a l l a n g l e of A Z 1350J w i t h a n d w i t h o u t D Z C or PDHS as CEL are s i m u l a t e d and c o m p a r e d as shown in Fig. 8. B o t h the s p a c e w i d t h a n d s i d e w a l l a n g l e are v e r y m u c h i m p r o v e d w i t h t h e D Z C or PDHS as CEL. By c o n s i d e r i n g the d i f f e r e n c e s of t h i c k n e s s , d e v e l o p e r etc. b e t w e e n D Z C and PDHS for simulations, the c o m p a r i s o n m a y not be t o t a l l y proper. However, it still p r o v i d e s a d i r e c t and u s e f u l c o m p a r i s o n to the p e r f o r m a n c e of d i f f e r e n t m a t e r i a l s in the study of c o n t r a s t e n h a n c e m e n t . T h e p a r a m e t e r s u s e d and d e t a i l s of the m o d e l l i n g and s i m u l a t i o n w i l l be r e p o r t e d in a n o t h e r p a p e r soon.

4.

CONCLUSIONS

To m o d e l the b l e a c h i n g b e h a v i o r of C E L m a t e r i a l s a f o u r - p a r a m e t e r e q u a t i o n is p r o p o s e d . The p a r a m e t e r s of the d e r i v e d s i m u l a t i o n e q u a t i o n s are l i n e a r w i t h f i l m t h i c k n e s s of DZC a n d PDHS. B a s e d on this l i n e a r i t y , s i m u l a t i o n of C E L b e h a v i o r is m o r e a c c u r a t e a n d c o n v e n i e n t than those u s i n g D i l l ' s model. T h e e n h a n c e m e n t of c o n t r a s t seems to be i n d e p e n d e n t of f i l m t h i c k n e s s of D Z C a n d PDHS u s e d in o u r studies. T h e r e p o r t e d n o n l i n e a r b l e a c h i n g c u r v e of C E M - 2 can a l s o be f i t t e d q u i t e well. T h e s e e q u a t i o n s are p o s s i b l e to a p p l y to the study of b l e a c h i n g a n d c o n t r a s t e n h a n c e d b e h a v i o r s of o t h e r n o n l i n e a r p h o t o b l e a c h i n g materials.

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F . H . D i l l , IEEE T r a n s a c t i o n s on E l e c t r o n Devices, ED-22, NO.7, 440 (1975). M.M.O'Toole, IEEE E l e c t r o n D e v i c e Letters, EDL-6, No.6, 282 (1985). C . A . M a c k , J.Vac. S c i . T e c h n o l . , A5(4), 1428 (1987). B . F . G r i f f i n g and P.R.West, Polym. Eng. & Sci., vol 23, No.17, 947 (1983). S . V . B a b u and E . B a r o u c h , IEEE E l e c t r o n D e v i c e Letters, EDL-7, 252 (1986). S . V . B a b u and E . B a r o u c h , IEEE E l e c t r o n D e v i c e Letters, EDL-8, 401 (1987). S.I.Uchino,T.Iwayanagi and M . H a s h i m o t o , Proc. SPIE, 920, i00 (1988).