Evaluation of Water-Soluble Diazonium Salts as Contrast

Chapter 16 Evaluation of Water-Soluble Diazonium Salts as Contrast-Enhancement Materials Using a g-Line Stepper 2

S.-I. Uchino1,T. Ueno1,T. Iwayanagi1,H. Morishita1, S. Nonogaki1,S.-I. Shirai, and N. Moriuchi 1Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan Hitachi Device Development Center, Ohme, Tokyo 198, Japan Water soluble aromatic diazonium salts have been evaluated as photobleachable dyes for contrast enhancement lithography using a g-line stepper. The diazonium salts used in this experiment are zinc chloride salts of 4-diazo-2-ethoxy- Ν,Ν-dimethylaniline chloride 4-diazo-2,5-diisopropoxymorpholino-benzene chloride and 4-diazo-2,5-dimethyl- Ν,Ν-dimethylaniline chloride. An aqueous solution of diazonium salt and polyvinyl pyrrolidone is used as a contrast enhancement material. An improved resist profile is obtained with this CEL material when a g-line stepper is used. Resist contrast is discussed in terms of such optical characteristics as the quantum yield of bleaching and the molar absorption coefficient for the materials. The thermal stability of diazonium salts in an aqueous solution is also examined. 2

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2

1. Introduction In recent years, rapid progress has been made in lithographic technology along with a concomitant reduction in the minimum feature size. The object of microphotolithography is to form high resolution resist patterns. Various techniques have been evaluated for improving the resolution of photoresist. Griffing and West introduced the concept of contrast enhancement lithography (CEL) which improves the contrast of the resist process (1,2,3). Ihe CEL process involves coating a CEL layer containing photobleachable dye on a conventional resist. A CEL layer is opaque before exposure but becomes transparent during exposure. When the areal image of a mask is incident on such a layer, the regions of the layer that are exposed to the highest intensity bleach through first, while those parts of the layer that receive the lowest intensity bleach through at a later timed). Therefore, the degraded optical image caused by the lens system of the exposure apppratus can be improved by passing the exposure light through the CEL layer. The contrast enhancement materials reported so far can be classified into three categories, namely nitronesO,2,3), polysilanes(4), and a diazonium salt(5). However, the use of nitrone and polysilane in the CEL process presents a problem, because organic solvents are required in the film forming and 0097-6156/87/0346-0188$06.00/0 © 1987 American Chemical Society Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Diazonium

Salts

189


removal p r o c e s s e s . The use o f s u c h t o x i c s o l v e n t s i s n o t d e s i r a b l e i n the p r o d u c t i o n e n v i r o n m e n t . A l t h o u g h t h e use o f a d i a z o n i u m s a l t as a CEL dye has been r e p o r t e d by H a l l e ( 5 ) , t h e performance was n o t satisfactory because o f its low optical density at exposure wavelength. T h i s paper r e p o r t s on water s o l u b l e d i a z o n i u m s a l t s which o f f e r good o p t i c a l c h a r a c t e r i s t i c s f o r t h e CEL p r o c e s s . 2. Experimental 2.1 CEL M a t e r i a l s The d i a z o n i u m salts used in the experiment are listed in T a b l e 1. Two o f them, D2 and D4, were s y n t h e s i z e d through Ν,Ν-dimethylation, nitrosation, reduction, and diazotization of p-xylidine and o-phenetidine, respectively. The d e t a i l s of s y n t h e s i s f o r D4 a r e d e s c r i b e d h e r e . A solution of o - p h e n e t i d i n e (137g) i n water (250g) was mixed w i t h d i m e t h y l s u l f a t e (265g) at 5°C. A f t e r t h e s o l u t i o n was made a l k a l i n e by a d d i t i o n o f a 30% aqueous sodium h y d r o x i d e s o l u t i o n , t h e r e a c t i o n mixture was extracted with ether. The e x t r a c t was d i s t i l l e d t o y i e l d Ν,Ν-dimethyl amino compound, b . p . 110^/18mmHg. Ν,Ν-dimethylamino compound (133g) in acetic a c i d (242g) and water (300g) was mixed with a 30% aqueous sodium nitrate s o l u t i o n (280g) a t 5°C. The p r e c i p i t a t e d nitroso compound was f i l t e r e d out and washed w i t h c o l d w a t e r . To a s t i r r e d mixture of the nitroso compound ( 4 0 g ) , methanol ( 2 0 0 m l ) , and p a l l a d i u m carbon (0.1g) was added a 30% aqueous sodium b o r o h y d r i d e solution under n i t r o g e n b u b b l i n g . The r e a c t i o n m i x t u r e was s t i r e d u n t i l l i t became c o l o r l e s s . After the solvent and palladium were removed, the r e a c t i o n mixture was extracted with ether and distilled to give amino compound, b.p.105°C/1mmHg. A m i x t u r e o f amino compound (32g) and c o n c e n t r a t e d h y d r o c h l o r i c acid (58.5g) was c o o l e d t o below 0°C w i t h c r u s h e d i c e (40g) w h i l e a 30% aqueous sodium n i t r a t e (45g) was s l o w l y added. A 50% aqueous zinc chloride s o l u t i o n (54g) was added to t h e s o l u t i o n and t h e s o l u t i o n was k e p t below 10°C f o r 15 m i n u t e s . The p r e c i p i t a t e d D4 was f i l t e r e d o f f and the solid was dissolved in a 1% aqueous hydrochloric acid solution and mixed with a 50% aqueous z i n c c h l o r i d e s o l u t i o n (10g). The r e c r y s t a l l i z e d D4 was f i l t e r e d o f f and washed w i t h e t h e r and d r i e d under a vacuum. D2 was a l s o s y n t h e s i z e d i n t h e same manner. 2 . 2 E v a l u a t i o n o f CEL performance u s i n g d i a z o n i u m s a l t s CEL s o l u t i o n s were obtained by dissolving poly(N-vinyl p y r r o l i d o n e ) and a d i a z o n i u m s a l t (D2, D3 o r D4) i n aqueous a c e t i c acid. The s o l u t i o n s were s p i n - c o a t e d on a c o n v e n t i o n a l p h o t o r e s i s t l a y e r formed on a s i l i c o n w a f e r . CEL l a y e r s on q u a r t z s u b s t r a t e were used f o r o p t i c a l t r a n s m i t t a n c e measurements. Exposure was c a r r i e d out w i t h a H i t a c h i RA 501 g - l i n e r e d u c t i o n aligner. UV a b s o r p t i o n spectra were measured on a H i t a c h i 340 s p e c t r o p h o t o m e t e r and i n f r a r e d s p e c t r o p h o t o m e t e r . The e x p o s u r e c h a r a c t e r i s t i c curves were o b t a i n e d by p l o t t i n g normalized f i l m thickness versus the logarithm o f exposure t i m e . The improvement i n r e s i s t contrast was e v a l u a t e d by comparing the y-value (slope of the exposure characteristic curve) for

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

POLYMERS FOR HIGH

190

TECHNOLOGY


conventional photoresist with that o f the same system having a CEL layer on top of the photoresist. After exposure, the CEL layer was removed using a water r i n s e . OFPR800 (Tokyo Chka Co.) was used as the p o s i t i v e photoresist and developed with a 2.38? aqueous s o l u t i o n of tetramethylammonium hydroxide. 3» Results and discussion 3*1 Diazonium s a l t s and binder polymers Photobleachable dyes f o r water soluble CEL layers are required to have the following properties: ( D a strong absorption at the wavelength of the g - l i n e stepper (436 nm), (2) a high s o l u b i l i t y i n water, (3) a high thermal s t a b i l i t y i n the aqueous s o l u t i o n and i n the f i l m . Ihree diazonium salts, 4-diazo-2,5-dimethylΝ,Ν-dimethylaniline chloride zinc chloride (D2), 4-diazo-2,5-diisopropoxymorpholino-benzene chloride zinc chloride (D3), and 4-diazo-2-ethoxy- Ν,Ν-dimethylaniline chloride zinc chloride (D4) which met the above requirements, were selected as the photobleachable materials. The s o l u b i l i t y o f these compounds i n water i s dramatically improved by adding acetic acid. Since conventional novolak photoresist i s insoluble i n a mixture o f a c e t i c acid and water, the CEL layer can be coated without the need for a b a r r i e r l a y e r s . In a d d i t i o n , the CEL layer can be e a s i l y removed after exposure by r i n s i n g i t i n water I t i s neccessary that the binder polymers (1) be compatible with diazonium s a l t s , (2) be uniform i n f i l m thickness when the layer i s coated on the photoresist f i l m , (3) show a high gas permeability to allow nitrogen photogenerated during the exposure o f diazonaphtoquinone r e s i s t to d i f f u s e . Many water soluble polymers were examined, and i t has been found that poly(N-vinyl pyrrolidone) and i t s copolymers s a t i s f y the above requirements. The UV absorption spectra o f the CEL layers which consist o f these diazonium s a l t s (D2, D3, and D4) and poly(N-vinyl pyrrolidone) are shown i n Fig.1. 3.2 Bleaching reaction of diazonium s a l t s The UV transmittance o f the D4-CEL layer increases with exposure time as shown i n Fig.2. This increase i n transmittance i s caused by the decomposition of diazonium s a l t . The photochemical reaction of diazonium s a l t s has been thoroughly investigated(6). The main reaction of diazonium s a l t s i n the s o l i d phase can be described by the following fromula;

Infrared spectra o f the unbleached D4-CEL layer and the completely bleached layer are shown i n Fig.3. The sharp absorption band at 2150 cm- which i s assigned to N-N stretching v i b r a t i o n completely disappears a f t e r exposure (Fig.3). The broad bands at 3450 crrf and 3000 empare due to water and a dimethylamino group, respectively. Similar infrared spectra were observed for both the D2- and D3-CEL l a y e r s . 1

1


16.

UCHINO ET A L .

Water-Soluble

Diazonium

Salts

191

Table 1 Diazonium s a l t s evaluated as CEL dyes


" \ S U B . R

N A M E \

Ri

D1

-N(CH )

2

-H

D2

-N(CH )

2

-CH

D3

-N_0

OA

-N(CH )

3

3

R

2

3

-CH

3

3

-OCH2CH

2

*ε(χΐο )

380

3.6

39A

3. 1

A OA

2.7

A09

3.1

-H

3

•OCH(CH ) - O C H ( C H ^

3

^max(nm)

2

3

2

-H

4

GENERAL FORMULA R

2

* molar absorption coefficient in water ^ rnax a t

W A V E L E N G T H (nm)

Fig.1 UV absorption spectra o f the CEL layers containing diazonium s a l t s . Absorbance i s normalized at the maximum absorbance of the CEL l a y e r .



Fig.2 Infrared spectra of D4-CEL l a y e r , bleached layer

(a) unbleached layer (b)



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UCHINO ET A L .

Water-Soluble

Diazonium

193

Salts

3.3 Thermal s t a b i l i t y o f diazonium s a l t I t i s well known that most o f the diazonium compounds are thermally unstable and e a s i l y decompose i n an aqueous s o l u t i o n . Therefore, the thermal s t a b i l i t y o f diazonium s a l t s i n an aqueous s o l u t i o n and i n a f i l m was evaluated by a k i n e t i c analysis o f the thermal decomposition and d i f f e r e n t i a l scanning calorimetry (DSC) analysis. The thermal decomposition o f diazonium s a l t s i n an aqueous s o l u t i o n i s a f i r s t order reaction as shown i n Fig.4. Arrhenius p l o t s o f diazonium s a l t s i n an aqueous s o l u t i o n are shown i n Fig.5. The r e l a t i o n s h i p between the decomposition temperature (Td) obtained from DSC curves and the decomposition rate constant (k) o f the diazonium s a l t s at 5°C are shown i n Fig.6. A good l i n e a r r e l a t i o n s h i p i s observed between Td and In k f o r both the aqueous s o l u t i o n and the f i l m from t h i s f i g u r e . These l i n e a r r e l a t i o n s h i p s make i t possible to predict the s t a b i l i t y o f diazonium s a l t s i n an aqueous s o l u t i o n and i n f i l m from the decomposition temperature Td in the s o l i d s t a t e . For example, Td f o r D2 i s the highest i n the three diazonium s a l t s s u i t a b l e f o r g - l i n e exposure. The decomposition rate constant of an aqueous s o l u t i o n o f D2 at 5°C can be estimated to be 2.3x10" (hr') from the l i n e a r r e l a t i o n s h i p i n Fig.6. This value means that 5% o f the D2 i n an aqueous s o l u t i o n w i l l decompose a f t e r storage f o r 6 months at 5°C 3.4 Bleaching c h a r a c t e r i s t i c s The bleaching curves o f the CEL layers are shown i n Fig.7. Two parameters which express the o p t i c a l e f f i c i e n c y o f the CEL layers can be derived from the bleaching curves. Oie i s the W T b r a t i o where Έ> represents the i n i t i a l transmittace of the CEL layer and Τ represents the transmittance of the completely bleached CEL l a y e r . High contrast enhancement i s provided by a high WE> ratioO). Since the three diazonium s a l t s have a large WTb r a t i o , a good improvement i n r e s i s t contrast can be expected. The other parameter . r e l a t e s to bleaching speed which i s a function o f molar absorption c o e f f i c i e n t ε) and bleaching quantum y i e l d ( 0 ) . This parameter w i l l be discussed i n section 3.6. 3.5 Contrast and r e s i s t p r o f i l e s The performance of the CEL layer can be evaluated from the exposure characteristic curves for the conventional photoresist (0FPR800) and the CEL/OFPR800 layer systems shown i n Fig.8. The CEL layer thicknesses used i n these experiments were 0.8 um f o r D2, 1.0 um f o r D3, and 0.5 um f o r D4. A 1.0 um t h i c k OFPR800 layer was used as the imaging l a y e r . The contast r a t i o i s defined as C E l / where ν represents the contrast o f 0FPR800 without a CEL layer and *cEL stands f o r the contrast o f 0FPR800 with the CEL l a y e r . The contrast r a t i o s f o r the three diazonium salt-CEL systems were 1.74 f o r D2, 2.62 f o r D3, and 2.52 f o r D4 (Fig.8). A good improvement i n the r e s i s t image i s expected from these contrast r a t i o s . Scanning electron microphotographs o f a 1.0 um line/space Y

y


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


194

Time

(hr)

F i g . 4 F i r s t order k i n e t i c s for the decomposition of the diazonium salts.



16. UCHINO ET A L .

Water-Soluble

Diazonium

3.0

195

Salts

3.5 1000/Τ (°k)

Fig.5 Arrhenius p l o t o f the aqueous salts.

s o l u t i o n o f the diazonium

-6

soln.

·

film — ο —

-8 ^ D 3 q 2 >^

k«23xKT* ( h ? T \ ^

c-12 -14 -16

100

1

1 120

I

1

!

1

1

1

140 160 DECOMP. TEMP. C O

Fig.6 Relationship between decomposition rate and temperature i n s o l i d state f o r the diazonium s a l t s .



196


10)

0.5 1.0 EXPOSURE TIME(s)

1.5

Fig.7 Bleaching curves o f the CEL Layers. CEL layer thickness i s 0.8 um f o r D2, 1.0 um f o r D3, and 0.5 um f o r D4.

Fig8. Exposure c h a r a c t e r i s t i c curves f o r the 0FPR800 and CEL/OFPR800 systems. The f i l m thickness o f 0FPR800 was 1.0um. The thickness o f each CEL layer was the same as i n Fig.7.


16.

UCHINO ET AL.

Water-Soluble

Diazonium

197

Salts


r e s i s t pattern printed with and without a D4-CEL layer are shown i n Fig.9. The photographs show that the r e s i d u a l f i l m thickness and wall p r o f i l e s of the r e s i s t are improved with the diazonium salt-CEL system. 3.6 CEL curve According to G r i f f i n g and West, the main requirements f o r CEL materials are as f o l l o w s ( l ) : (1) the molar absorption c o e f f i c i e n t for the photobleachable dye must be as high as p o s s i b l e , (2) the Τφ/Tb r a t i o must be as large as p o s s i b l e , and (3) the quantum y i e l d of the bleaching reaction must be as high as p o s s i b l e . These requirments were examined from the viewpoint o f the r e l a t i o n s h i p between the r e s i s t contrast and such o p t i c a l properties of the photobleachable dyes as the quantum y i e l d and the molar absorption c o e f f i c i e n t . I t was found that the contrast r a t i o (?cEL/y) can be expressed by using two parameters ? and a. Parameter £ i s given by ε Φ I to , where ε i s the molar absorption c o e f f i c i e n t , Φ i s the quantum y i e l d of the bleachable m a t e r i a l , and lb to i s the s e n s i t i v i t y o f the photoresist. Parameter .a i s given by WTb-1, which represents the transmittance r a t i o of the bleached layer to the unbleached one. Similar parameters have been reported by Diamand and Sheats. (7) The values of parameters £ and a. can be obtained from the bleaching curves (Fig.7) and the resist sensitivity. The r e l a t i o n s h i p between the contrast r a t i o and the two parameters (P and a) i s shown i n Fig.10. This curve i s referred to as the CEL curve. I t can be seen from t h i s curve that the contrast increases with increasing â-values, and there i s an optimum P-value which maximizes the contrast. Contrast r a t i o s can be calculated from the bleaching curve (Fig.7) and the s e n s i t i v i t y o f the photoresist, while the. observed contrast r a t i o s can be d i r e c t r y obtained from the exposure c h a r a c t e r i s t i c curves shown i n Fig.8. The calculated contrast r a t i o s were compared with the observed r a t i o s i n the CEL curve. The calculated values are i n f a i r agreement with the observed values. The CEL curve i s thus thought to be a useful t o o l f o r designing CEL dyes because i t can predict the performance of the dyes used for CEL materials. A detailed discussion of t h i s curve w i l l be reported i n a subsequent paper. 0

4.

Conclusion Three kinds of water soluble diazonium s a l t s were evaluated as photobleachable dyes for the enhancement of contrast in photolithography. CEL l a y e r s formed from these deazonium s a l t s and polyvinyl pyrrolidone have good o p t i c a l c h a r a c t e r i s t i c s and they can improve the a b i l i t y o f the conventional photoresist. A more thermally stable diazonium s a l t or method of s t a b i l i z i n g the aqueous s o l u t i o n i s needed for the diazonium salt-CEL l a y e r s i n p r a c t i c a l use. A CEL curve was presented which can express the r e l a t i o n s h i p between r e s i s t contrast and o p t i c a l properties such as molar absorption c o e f f i c i e n t and quantum y i e l d . This curve i s thought to be a useful t o o l f o r designing CEL m a t e r i a l s .




198

Fig.9 Scanning microphotograph o f photoresist patterns. (a) without D4-CEL l a y e r , (b) with D4-CEL. Feature d e f i n i t i o n i s enhanced i n submicron structure.

Ρ (=€^Ioto)

0 CEL curve: r e l a t i o n s h i p between r e s i s t contrast r a t i o s p t i c a l c h a r a c t e r i s t i c parameters o f CEL materials(P and a ) .


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199


Acknowledgments The authors would like to thank Toyozi Ohshima and Ryou-ichi Narushima of Hitachi Chemicals Co., Ltd. for synthesizing the diazonium salts, and to express our gratitude to Dr. Hiroshi Yanazawa of Hitachi Central Research Laboratory for making the optical exposure experiments used in this study. References 1) B.F. Griffing, P.R. West, Poly. Eng. Sci., 23, 947(1983) 2) B.F. Griffing, P.R. West, IEEE Electron Device Lett., EDL-4, 14(1983) 3) P.R. West, B.F. Griffing, SPIE, 394, 33(1983) 4) D.C. Hofer,et al., SPIE,469, 108(1984) 5) L.F. Halle, J. Vac. Sci. Technol.,133, 323(1985) 6) Kosar: Light sensitive systems, Chapter 6, Jhon Wiley and sons, New York(1965) 7) J.J. Diamond, J.R. Sheasts, IEEE Electron Device Lett., EDL-7, 383(1986)

RECEIVED May 6, 1987


Evaluation of Water-Soluble Diazonium Salts as Contrast

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