Steel Metal

Abstract

In this paper, we investigated the photolithographic properties of poly(N-tetradecylmethacryl-amide-co-t-butyl 4-vinylphenyl carbonate) [p(TDMA-tBVPC)] thin films prepared by Langmuir-Blodgett (LB) technique.  The copolymer forms a stable monolayer on a water surface and LB films with any desired number of layers.  The copolymer has a structure being subject to the main chain scission and deprotection of t-butoxycarbonyloxy group by deep UV irradiation.  The positive-tone patterns of the p(TDMA-tBVPC56) LB film with 60 layers could be obtained by deep UV irradiation followed by development with alkaline aqueous solution.  The resolution of the pattern was 0.75 mm, which is the resolution limit of the photomask employed.  The etching resistance of p(TDMA-tBVPC56) LB film was also investigated permitting etching of the gold film.
Keywords

Langmuir-Blodgett films, Copolymer, Photolithography, Etching
Introduction

The continuing trend toward higher circuit density in microelectronic devices has motivated research efforts in varieties of high-resolution lithography techniques, including electron beam (EB), X-ray, and deep UV irradiation.  Use of ultra-thin films and new materials have been proposed as approaches to improve resolution in lithography.  The Langmuir-Blodgett (LB) technique is very effective method used to prepare well-defined ultra-thin film with controlled thickness and orientation at a molecular level.  Therefore, LB films are expected to realize ultra-high resolution photolithography [1-4].

In previous studies, [5-7] we have found that N-octadecylacrylamide forms a uniform LB film with a highly ordered structure, and yielded a fine negative pattern by photopolymerization.  Furthermore, we have also succeeded in the preparation of preformed polymer LB film that has a cross-linking group [8].  By the cross-linking reaction with deep UV and electron beam irradiation we obtained a fine negative pattern consisting of two-dimensional network.  All of these polymer LB films resulted in negative-tone photopatterns.  On the other hand, we also obtained positive type photopatterns using poly(N-tetradecylmethacrylamide)(p(TDMA)) LB films without any development process (self-development) [9, 10].  It was found that the higher sensitivity could be obtained by changing the alkyl side chain to the short-branched type [11].  In addition, the deprotection reaction of t-butoxycarbonyloxy group has also been used in positive patterning of polymer LB films [12-14].  Combining these interesting properties, the improvement of not only the sensitivity but also the imaging quality can be expected.  In this work, we prepared the copolymers of photodegradable N-tetradecylmethacrylamide (TDMA) with t-butyl 4-vinylphenyl carbonate (tBVPC) (Figure 1) aiming at the fabrication of a new type of positive resist taking place both main chain scission and polarity change caused by t-butoxycarbonyloxy group deprotection.

AZojomo - The "AZo Journal of Materials Online" Chemical structure of p(TDMA-tBVPC)

Figure 1. Chemical structure of p(TDMA-tBVPC).
Experimental

The copolymer (p(TDMA-tBVPC)) was prepared by free-radical copolymerization of N-tetradcylmethacrylamide (TDMA) with t-butyl 4-vinylphenyl carbonate (tBVPC) in toluene at 60˚C.  Measurement of surface pressure ( p) - area (A) isotherm and deposition of monolayers were carried out at 15˚C with a Langmuir trough system (FSD-50 and 51, USI) with a compression speed of 14 cm2/min.  The rate of deposition was set at 10 mm/min at both up- and down-strokes.  Deionized pure water (Milli-QII, MILLIPORE) was used as the subphase.  The copolymer was dissolved in chloroform at a concentration of ca. 1 mM and the solution was spread on the water surface.  The glass, quartz, and silicon slides on which LB film was deposited were initially cleaned by a UV-O3 cleaner (NL-UV253, Nippon Laser Electronic); then they were made hydrophobic with n-octyltrichlorosilane.  UV absorption measurements were recorded with a Hitachi U-3000 UV-Vis spectrophotometer.  The molar ratio of the tBCPV in the copolymer were determined from 1H NMR of p(TDMA-tBVPC).  Molecular weight was determined with a Toyo Soda gel permeation chromatography (GPC) using a polystyrene standard.  IR spectra were measured with a JASCO-IR 230 spectrometer.  Deep UV irradiation was carried out with a deep UV lamp (UXM-501MA, USHIO) through an IR-cut filter.  The thickness of copolymer LB film was determined with surface profilometry using a Sloan Dektak 3ST.  Gold film was deposited onto a glass substrate with a vacuum evaporator (V-KS200, Osaka Vacuum, Ltd).
Results and Discussion
Formation of Copolymer LB Films

The molecular weight of p(TDMA-tBVPC) are summarized in Table 1.  The copolymer(p(TDMA-tBVPC)) was spread on the water subphase from chloroform solution (ca. 1mM) to measure surface pressure (p) - area(A) isotherms (Figure 2).  The copolymer p(TDMA-tBVPC) monomers have collapse pressure.  Their curves stand sharply.  We can conclude that they can form a condensed monomer on the water subphase.  The p(TDMA-tBVPC) monolayer could be transferred onto a solid substrate with a transfer ratio of almost unity.  UV absorption spectra of p(TDMA-tBVPC56) LB film on quartz were measured as a function of the number of layers (Figure 3).  The absorbance at 193 nm apparently increases linearly with the number of layers deposited, indicating the regular deposition of the copolymer monolayer onto the solid substrate.