A New Method of Photopatterning with LB Films Based on a Chemically Amplified Mechanism1

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CHEM. EiES. CHINESE

U. 2006, 2 2 ( 4 ) , 543-546

A New Method of Photopatterning with LB Films Based on a Chemically Amplified Mechanism * LI Tie-shengl

*

, Masaya

Mitsuishi' and Tokuji Miyashta'

1. Department of Chemistry, Zhengzhou University, Zhengzhou 450052 , P. R. China ; 2. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577 , Japan Received Aug. 31, 2005 A new approach to introducing a photoacid generator( PAG) into Langmuir-Blodgett( LB) films to draw photopatterns as a lithogaphic process is described here. The chemically amplified positive-tone resist system used here consists of two components : a copolymer, p l y ( dodecrylacrylamide-co4-t-butyloxylvinyl-phenylc~bonate) [ P ( DDA-tBVPC53 ) ] and a PAG , tri ( 2,3-dibromopropyl) isocyanurate ( TDBPIC ) . In the two-component system, the acid generated by the PAG catalyzes the deprotection reaction of P ( DDA-t-BVPC53) , to remove the tert-butoxycarbonyl group( t-BOC) in the exposed region during the postexposure baking process, thus rendering the exposed region soluble to alkaline aqueous solvents to form a positive tone. Photolithographic properties of the LB films have been evaluated. The patterns can be resolved with a resolution of 1 pm line width by UV irradiation, followed by development with an alkaline solution. The LB films can be used to generate etched gold relief images on a glass substrate via an aqueous iodide, like ammonium iodide, in alcohoVwater as the etchant. The etch resistance of such LB films is s&iciently good, allowing patterning of a gold film suitable for photomask fabrication. Keywords LB film; Copolymer; Photopatterning; Chemically amplified resist Article ID 1005-9040( 2006) -04-543-04

Introduction Deep-UV lithography is generally recognized as the most promising lithographic technology, for the production of high-resolution devices. However, lithography requires high sensitivity, high resolution, and high resistance. To make this technology more applicable, new resist materials and processes must be developed. Chemically amplified mechanisms used in photopatterning can meet the necessary requirement^"^^, in which spin-coated films are used currently. However, there have been a few reports on chemically amplified mechanisms that are applied to LB films. The Langmuir-Blodgett ( LB ) technique makes it possible to prepare a thin film with controlled thickness with a molecular size and well-defined molecular orient a t i ~ n ' ~ ' ~Because ]. of this superior feature, polymer LB films have been recently investigated in the application to the high-resolution lithographic technology"*81. In a previous study, we have succeeded in producing fine patterns via the polymerization of alkylacrylamide monomer LB films, through a cross-linking reaction in

the polymer LB films under deep-UV and electron beam irradiation, which resulted in a negative tone. We have also obtained a new type of positive photoresist with high resolution without using any developmental process (which is called dry-development) [9--141. Because a tert-butoxycarbonyl protecting group ( t-BOC ) can be removed by a catalytic amount of strong acid, it becomes possible for new chemically amplified resist systems to be developed. A series of copolymers have been developed by means of the radical reaction of DDA with tert-butylvinylphenylcarbonate (t-BVPC) , in which DDA forms LB films, and their photolithographic properties have also been investigated[15-181 Here, we report a new approach, which is, to introduce a photoacid generator ( PAG) into P ( DDA-tBVPC53 ) LB films directly, aiming at increasing the sensitivity of the P ( DDA-t-BVPC53 ) system and the physical characteristics of P( DDA-t-BVPC53 ) LB films with the PAG [ tri ( 2 ,3-dibromopropyl isocyanurate

( TDBPIC ) ] introduced, as well as their lithographic

* Supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Sports, and Culture ( No. 14205130) , and Natural Science Foundation of Henan Province( No. 061 1020100). * * To whom correspondence should be addressed. Email: lts34@ zzu. edu. cn; miya@ mail. tagen. ac. j p

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CHEM. RES. CHINESE U.

properties.

Experimental The preparation of DDA was carried out according to the method in ref. [ 91. tea-Butyl4vinylphenyl carbonate( t-BVPC) was obtained from the Aldrich Chemical Company, Inc. It was distilled in vacuum prior to use. Tri ( 2,3-dibromopropyl) isocyanurate ( TDBPIC ) was synthesized via a general procedure described by Bruns et al. [19] The copolymer P( DDA-t-BVPC53) was synthesized by adopting the procedure in reference[lS1. The copolymerization of DDA with t-BVPC was performed through a free radical polymerization route in dry toluene at 60 “c: initiated with 2,2’-azobis ( isobutyronitrile) ( AIBN) that was purified in methanol. After polymerization, the copolymer was purified by reprecipitation in an excess of acetonitrile. The structures, the compositions, the molecular properties, and the spectra of the obtained copolymers were determined as follows. The measurement of surface pressure ( T ) -area ( A ) isotherms and the deposition of monolayers were performed on a Langmuirtrough system( FSD-50, 5 1 , USI) at 15 “c: at a compression speed of 14 cm2/min. The rate of deposition was 10 m d m i n for both up and down strokes. Filtered, deionized pure water ( 18. 2 M a cm, MilliQII , Millipore) was used as the subphase. The UV-Vis spectra of the LB films on a quartz substrate were recorded on a Hitachi U-3OOO UV-Vis spectrophotometer. The LB films containing the PAG were deposited on both sides of the CaC1, wafers. The IR spectra were recorded on a JASCOFVIR-230 Fourier transform infrared spectrometer. The molecular weight of the copolymer was determined by a Toyo Soda gel permeation chromatograph ( GPC ) with polystyrene as the standard. The optical exposures were performed with an Hg lamp ( UXN501MD, USHIO). The thickness of each LB film was determined with surface profilometry via a Sloan Dektak 3ST surface profilometer. The Au substrate used for etching was made with a Hitachi El01 Ion Sputter instrument. The molar fractions of t-BVPC in the copolymers were determined with I H NMR spectrometry. Patterns were observed via Olympus Vanox-T microscopy. All the measurements were carried out at mom temperature.

Results and Discussion 1 Monolayer Behavior on Water Surface and Film Formation The content of t-BVPC in the copolymer, P( DDAt-BVPC53), was determined to be 53% by means of H NMR. The number-average molecular weight ( M,)

’

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of P ( DDA-z-BVPC53) was determined to be 2.09 x lo4 and M , / M , = 1. 83. The solution used to form LB films was prepared by dissolving P ( DDA-t-BVPC53 ) and tri ( 2,3-dibromopropyl) isocyanurate ( TDBPIC ) in chloroform and filtering the mixture through a stack of 0.5-pm Teflon filter. The solution was then spread on the water subphase, and the solvent was.allowed to evaporate. All the pressure ( qr )-area ( A ) isotherms of P( DDA+BVPC53)/TDBPIC measured at 15 ‘t show a steep rise in surface pressure and have a relatively high collapse pressure ( Fig. 1 ) . Apparently, the isotherms change with the change in content of TDBPIC. The initial flat stage in the pressure( area( A ) isotherms becomes shorter with a decrease in content of TDBPIC , and the collapse pressure also decreases, although the P( DDA-t-BVPC53 ) /TDBPIC monolayer becomes unstable, which is attributed to the existence of TDBPIC. From the experimental results it can be seen that a copolymer with different contents of TDBPIC can form a condensed monolayer.

0.2 0.3 0.4 0. 5 Surface area/(nm* molecule-’)

Fig. 1 Surface pressure(n)-area(A ) isotherms of P( DDA-r-BVPC53)/(6%TDBPIC) (a), P( DDA-t-BVPC53)/( ll%TDBPIC) ( b ) , P( DDA-r-BVPC53)/( 19%TDBPIC)( c ) and P( DDA-t-BVPC53)/( 21%TDBPIC)( d ) on the surface of water at 15 T.

The IR spectrum of the P ( DDA-t-BVPC53 ) / ( 11% TDBPIC in mass fraction) LB film on a CaC1, crystal ( 108 layers ) was measured ( Fig. 2 ) . The 0.08

0. 06

5

<

r

7

1 1

0.04

T

0. 02

0. 00

3500

3000

2500

2000

1500

1000

i~ fcrn-’

Mg.2 IR Spectrum Of P( DDA+BVPC53)/( 11% TDBPIC) LB fUm deposited on CaCl,.

W Tie-shenget al.

No. 4

-'

absorption band at about 1690 cm was representative of carbonyl groups in TDBPIC molecules, indicating that TDBPIC could be transferred into P ( DDA-tBVPC53 ) LB films under the experimental conditions. Fig. 3 shows the UV spectra of P ( DDA-tBVPC53/( 11% TDBPIC in mass fraction ) LB films with different layers. It shows that excellent LB films can be fabricated on a quartz substrate.

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the sensitivity of the P ( DDA-t-BVPC53/( 11% TDBPIC) LB film is higher than that of the P ( DDA-tBVPC53 ) LB film under the same conditions, because of the addition of TDBPIC. .Y 0. a

1. 0 0.8

10 100 Exposure time/min

0.6 rd;

100 layers 80 layers

Fig. 4

o. 0

60 layers 40 layers

80 Number of layers 40

0.0

200

250

300

350

400

A/nm

Fig. 3

UV spectra of P ( DDA-f-BVPC53) /( 11 % TDBPIC) LB films deposited on quartz. Inset: plots of the absorbance at 193 nm versus the number of LB films deposited.

1000

Sensitive curves of P ( DDA-t-BVPC53 ) / ( 11 % TDBPIC) ( U ) and P ( DDA-t-BVPC53) ( b ) LB filmS.

P( DDA-t-BVPC53 ) / ( 11% TDBPIC) LB films of 40 layers were directly exposed to deep UV light through a photomask in air for 30 min, which was followed by baking at 90 "c for 30 s , and then they were developed with a 3% tetramethylaluminum hydroxide (TMAH) aqueous solution( Fig. 5 ) .

2 Properties of Photolithography The P ( DDA-t-BVPC53 )/TDBPIC system functions mechanistically in the manner shown in Scheme 1. The photolysis of TDBPIC generates a local concentration of strong Bronsted acid( HBr) . On postexposure baking for a few seconds, the t-BOC groups undergo acidolysis by hydrogen bromide to produce two volatile products, carbon dioxide and isobutylene , and then the acid ( H + ) catalyst is regenerated and phenolic hydroxyl is liberated. In this way, this system [ P ( DDA-tBVPC53 ) /TDBPlC ] is more sensitive than the corresponding P( DDA-t-BVPC53 ) system. NR

a

NR

(TDRPIC) R = CHzCHBrCHzRr

hv

.

+ HBr

R1= C H F H =CHBr

+ C O z f C P s + H+ I

OH

I

Scheme 1 Schematic diagram for the chemically amplified mechanism. Fig. 4 shows the sensitive curves of a 60-layer P ( DDA+BVPC53)/( 11% TDBPIC ) LB film and a P( DDA-t-BVPC53 ) LB film, measured via contacting method under deep-UV irradiation at a fixed light intensity for a certain exposure time in air. It is clear that

Fig. 5 Optical micrograph of positive-tones of P( DDA-CBWC53) / ( 11 % TDBPIC ) LB films on the silicon wafer.

As shown in Fig. 5 , a clear positive pattern was obtained with a resolution of 1 m , which is lower than that of the patterns obtained with P( DDA-tBVPC53)'"'. From the results we can conclude that the sensitivity of the P ( DDA-t-BVPC53 ) / ( 11% TDBPIC) LB films has been improved compared with that of P ( DDA-t-BVPC53) LB films, but the resolution of the P ( DDA+BVPC53)/( 11% TDBPIC) LB films is lowe r , because the uniformity of LB films becomes worse on account of the introduction of TDBPIC. Deep UV irradiation on spin-coated films of P ( DDA-t-BVPC53 ) / ( 11% TDBPIC ) on a silicon wafer have been carried out. However, a fine pattern cannot be obtained under identical conditions as those in the case of P ( DDA-tBVPC53 ) /( 11% TDBPIC ) LB films.

3

Etching Properties of P(DDA-t-BWC53)/

( 11 %TDBPIC) LB Films The substrates used in this study were glass wafem

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covered with a gold film of 50-nm thickness. The surface of the Au film was hydrophilic because of a very thin layer of native Au oxide. To give rise to hydrophilic interactions between the substrate and the film, the substrate was immersed into the subphase of water before a sample was spread on the surface of the water. Deposition occurred during both downstroke and upstroke at a speed of 10 m r d m i n , so that it was a Y type structure‘”]. Deposition ratios for all the upstrokes and downstrokes were about 1.0 and 0.95, respectivel y , indicating that excellent LB films could be transferred onto an Au substrate. LB films of P ( DDA-t-BVCP53 ) /( 1 1% TDBPIC ) of 1 0 , 15, 20, 2 5 , 3 0 , and 40 layers were irradiated with a deep UV lamp for different time lengths. After exposure, the LB films baked at 90 T for 30 s were developed by 1% TMAH aqueous solution. The patterned samples with different layers of P( DDA-tBVCP53 ) / ( 1 1% TDBPIC) LB films were immersed in an Au film etching solution for a suitable period of time to transfer the resist pattern to the Au film. After etching, the P( DDA-t-BVCP53)/( 1 l%TDBPIC) LB films were stripped with chloroform or toluene. The patterns on the Au filmwith 20-layer P( DDA-t-BVPC53 ) / ( 1 1 % TDBPIC) LB films are positive patterns as shown in Fig. 6.

Fig.6

Etched patterns on the gold film on a glass substrate.

The resolution of the patterns is 1.0 p m line width, which is the best resolution among those of the saniples used in the above experiments. These results demonstrate that ultra-thin resist films have satisfactory etch resistance for transfemng a pattern from the resist to an Au film with 50 nm thickness via wet etching.

Conclusions We have carried out a preliminary study on an approach to introduce TDBPIC into P ( DDA-t-BVPC53 )

LB films directly, which has not been reported before. The resulting chemically amplified system can be pro-

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cessed in a positive mode, which makes it much more versatile than the systems obtained with a conventional method. A line width of 1.0 prn has been obtained in a positive mode and the etch resistance of LB films of this system has also been investigated, which is sufficient for photomask fabrication. Although the resolution of this system is lower than those of the systems obtained with a conventional method, its sensitivity has been improved. There is a potential for this novel method of making patterns to be used to make new resist materials in the future.

Acknowledgment We would like to thank Prof: T. Miyazaki and Dr.

Y. Ando, Department of Applied Physics, Graduate School of Engineering, Tohoku University, for allowing us to use the s u f m e pro$lometer.

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