Pulse Radiolysis Studies on the Mechanism of the High Sensitivity of

5 Pulse Radiolysis Studies on the Mechanism of the High Sensitivity of Chloromethylated Polystyrene as an Electron Negative Resist

Downloaded by NANYANG TECHNOLOGICAL UNIV on October 12, 2017 | http://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch005

Y . T A B A T A * , S. T A G A W A * , * * , and M. W A S H I O * *Nuclear Engineering Research Laboratory Faculty of Engineering University of Tokyo 22-2 Shirane Shirakata Tokai-mura, Naka-gun Ibaraki-ken, Japan **Research Center for Nuclear Science and Technology University of Tokyo 22-2 Shirane Shirakata Tokai-mura, Naka-gun Ibaraki-ken, Japan

The progress of technology for the high-resolution fabrication of semiconductor and magnetic bubble devices has required sub-micron exposure techniques such as electron beam x-ray and deep U V . Although a number of papers have been published on electron beam resists, reaction mechanisms of electron resists are still largely unknown since few studies on reactive intermediates by means of direct measurements have been done in order to elucidate the reaction mechanisms. Recently chloromethylated polystyrene ( C M S ) , a highly sensitive, high resolution electron resist with excellent dry etching durability, was developed. Very recently reactive intermediates in irradiated polystyrene, which is a starting material of C M S , have been studied and the transient absorption spectra of excimer (2-4), triplet states (2,5), charge-transfer complexes, and radical cations (6) of polystyrene have been measured. The present paper describes the cross-linking mechanism of the high sensitivity C M S resist and compares it to that of polystyrene on the basis of data on reactive intermediates of polystyrene and C M S . Experimental Details of the picosecond pulse radiolysis system for emission (7) and absorption (8) spectroscopies with response time of 20 and 60 ps, respectively, including a specially designed linear accelerator (9) and very fast response optical detection system have been reported previously. The typical pulse radiolysis systems are shown in Figures 1 and 2. The detection system for emission spectroscopy is composed of a streak camera (C979, H T V ) , a SIT 0097-6156/ 84/ 0266-0151 $06.00/0 © 1984 American Chemical Society

Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

152

MATERIALS FOR MICROLITHOGRAPHY

BEAM

GUIDE CELL

2

π c£p

BEAM

CATCHER

LENS

^H=> L E N S


MONOCHROMATOR

STREAK

CAMERA

COMPUTER

T.V.

GRAPHIC

MONITOR

DISPLAY

Figure 1. The

schematic diagram of the picosecond pulse system for emission spectroscopy.

radiolysis

LAMP

BEAM U

MIRROR

CATCHER

•LENS MONOCHROMATOR

PHOTO

DIODE

y TRANSIENT OR

DIGITIZER

GRAPHIC DISPLAY

DPO SYSTEM COMPUTER

Figure 2. The schematic diagram of the picosecond pulse radiolysis system for absorption spectroscopy.


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Chloromethylated Polystyrene

camera (Cl000-12, H T V ) , an analyzer (Cl098, H T V ) , a computer (PDP 11/34), a display system ( H T V and Tektronix) (see Figure 1). The detection system for absorption spectroscopy is composed of a very fast response photodiode (R1328U, H T V ) , transient digitizer (R7912, Tektronix) or D P O system (R7704, Tektronix), a computer (PDP 11/34), and a display unit (Tektronix) (see Figure 2). C M S was prepared from polystyrene and chloromethyl-methylether using S n C l as the catalyst (7,70,77). The structure of C M S is shown in Figure 3.


4

CH Cl 2

Figure 3. The structure of CMS: n/(m+n) chloromethylation ratio. Results and Discussions Solid Films. The excimer fluorescence of solid films of polystyrene was observed using pulse radiolysis. The decay curves of the excimer fluorescence observed at 340 nm for solid films of polystyrene and C M S are shown in Figures 4(a) and (b), respectively. The lifetime of the excimer fluorescence of polystyrene agree with the reference data (72). In C M S , the initial yield decreases and the decay rate of excimer fluorescence increases with increasing chloromethylation ratio of C M S . These experimental results indicate that the chloromethyl part of C M S quenches the excimer of C M S and scavenges the precursors of the excimer as described below. precursors of the excimer scavenged by -CH C1 2

the excimer quenched by -CH C1 2

The absorption spectrum observed in the pulse radiolysis of solid films of polystyrene is shown in Figure 5. The absorption spectrum around 540 nm is also very similar to the absorption spectrum of polystyrene excimer observed in irradiated polystyrene solutions in cyclohexane as reported previously (2,3). The absorption with the maximum at 410 nm was reported previously and was assigned to anionic species (13,14). The longer life absorptions, attributed to triplet excited polystyrene repeat units and nonidentifiable free radicals, were observed in a wave length region < 400 nm. The absorption spectrum of C M S films obtained in pulse radiolysis showed a peak around 320 nm and a very broad absorption around 500 nm as shown in Figure 6.



10 ns

I

CMS ( s o l i d ) 3 4 0 n m I

I

I

I

I

I

I

Figure 4. Typical oscilloscope trace of emission behavior obtained from pulse radiolysis of (a) polystyrene solid and (b) CMS solid at 340 nm.



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0.015

α 6

0.010 h

0.005

WAVELENGH ( n m ) I 0 300 400 500 600 Figure 6. The absorption spectrum obtained from pulse radiolysis of CMS solid. Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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MATERIALS FOR MICROLITHOGRAPHY


CMS and Polystyrene Solutions in Cyclohexane. Both monomer and excimer fluorescences were observed in the pulse radiolysis of polystyrene solution in cyclohexane. The decay curves of monomer and excimer fluorescences at 287 and 360 nm are shown in Figures 7(a) and (b), respectively. Energy migration on the polymer chain has been discussed elsewhere (75). The dependences of the decay of monomer fluorescence and the rise of excimer fluorescence on the

Ο

0.27

0.81

1.35

189

τι ME (ns) Figure 7. The decay curves obtained from pulse radiolysis of polystyrene solution in cyclohexane; (a) monomer and (b) excimer fluorescence monitored at 287 nm and 360 nm, respectively.



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chain length of oligomers have also been reported elsewhere (15). The lifetime of monomer fluorescence and the formation rate of excimer of polystyrene solution in cyclohexane are almost constant regardless of the chain length of the polymer. Figure 8 shows the transient absorption spectrum obtained in the pulse radiolysis of polystyrene solution in cyclohexane. The absorption band around 520 nm is very similar to that of polystyrene excimer (2,3,5). The decay follows first order kinetics with a lifetime of 20 ns. The decay rate agrees with that of the excimer fluorescence and excimer absorption. The longer life absorptions, attributed to the triplet states and free radicals (2,5), were observed at wave lengths

Pulse Radiolysis Studies on the Mechanism of the High Sensitivity of

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