Polystyrene excimer

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 CMS films obtained in pulse radiolysis showed a peak around 320 nm and a very broad absorption around 500 nm as shown in Figure 6. 

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 <400 nm, although the anionic species of polystyrene with the absorption maximum at 410 nm as seen in solid films (cf. Figure 5) was not observed. Figure 9 shows the absorption spectrum observed in the pulse radiolysis of CMS solution in cyclohexane. 

J. P. and Moller, M. (2001) Excimer laser ablation of gold-loaded inverse polystyrene-block-poly (2-vinylpyridine) micelles. Appl. Phys. A, 72, 679-685. 

Figure 1. Ratio of excimer (le) to monomer (Im) fluorescence intensifies of spin cast polystyrene films as a function of (spin speed)...

For the distyrylbenzene carbon-centered tetramer 46b, the fluorescence spectrum in the solid him differs from the spectra in solution or in a polymer matrix due to excimer formation [93]. A concentration of 5% in a polystyrene matrix is sufficient for a distinct broadening of the emission. For the higher homologue 46c, a fluorescence maximum of 472 nm was measured in freshly prepared films. If the film is thermally annealed, the spectrum shifts to 511 nm, probably due to intermolecular arrangement that favors excimer formation. 

Following the initial observation113 of excimer fluorescence from dissolved polystyrene, Hirayama114 has reported a systematic survey of the fluorescence spectra of the di- and triphenylalkanes shown in Figure 14. In addition to the normal molecular spectrum of the phenyl group exhibited by all the molecules listed, an excimer band is exhibited by those systems in which the planar phenyl groups are separated by exactly three carbon atoms or a distance of 2.54 A in the trans propane chain. Since there is no evidence of a... 

Recently chloromethylated polystyrene (CMS), 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 CMS, 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 CMS resist and compares it to that of polystyrene on the basis of data on reactive intermediates of polystyrene and CMS. 

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 CMS are shown in Figures 4(a) and (b), respectively. The lifetime of the excimer fluorescence of polystyrene agree with the reference data (12). In CMS, the initial yield decreases and the decay rate of excimer fluorescence increases with increasing chloromethylation ratio of CMS. These experimental results indicate that the chloromethyl part of CMS quenches the excimer of CMS and scavenges the precursors of the excimer as described below. 

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 (15). The dependences of the decay of monomer fluorescence and the rise of excimer fluorescence on the... 

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.

The absorptions at both 500 nm and 320 nm follow first order kinetics with a lifetime of 420 ns. This absorption species is neither the excimer of polystyrene nor free cationic species of polystyrene. Although the excimer of polystyrene has an absorption band around 500 nm, the lifetime is only 20 ns. Further the free cationic species of polystyrene should live for a longer time in this solution, and the absorption band should exist in a longer wavelength region (6). These considerations of lifetime and absorption spectrum lead us to conclude that the absorption spectrum shown in Figure 12 is due to the charge transfer-radical complex between polystyrene and Cl radical (2,4,17). A very similar... 

The only head-to-head polymer which has been examined for excimer fluorescence is polystyrene 25). Unfortunately, the synthetic route to this polymer leaves a number of stilbene-based structures in the sample, which have a lower-energy singlet state than either PS monomer (285 nm) or excimer (330 nm). Thus, fluorescence from these intrinsic stilbene traps was seen in the spectra of head-to-head PS in pure films and, to a lesser extent, in fluid solution. In the latter, the fluorescence of PS monomer was predominant, and the small amount of stilbene fluorescence was increased when a nonsolvent (methanol or cyclohexane) was added to the 2-methyl-tetrahydrofuran solution. In films of the polymer, stilbene fluorescence was the major spectral band, although some PS excimer fluorescence was also present in the spectrum. No monomer fluorescence at 285 nm was detected from films. Given the impure nature of the head-to-head PS sample, no conclusions on excimer formation in these systems could be drawn. 

This value agrees with the lifetime of the excimer of polystyrene. The intensity of the excimer fluorescence decreased with increasing chloromethylation ratio. Electrons produced in cyclohexane, one of the precursors of the excimer, were scavenged by chloromethylated part of polystyrene. Absorption spectra of the excited states and the polymer radicals were measured by laser photolysis of the cyclohexane solutions. The results are summarized in Fig. 10 [67]. 

Very recently Kouchi et al. constructed an ion beam pulse radiolysis system and use it for the study of the LET effect in irradiated polystyrene thin films [106]. The nanosecond pulsed MeV ion beam with the variable repetition rate was obtained by chopping ion beams from a Van de Graaff. Time profiles of the excimer fluorescence from polystyrene thin films, excited by He+ impact, were... 

The investigation of the ion beam interaction with polystyrene by means of ion beam pulse radiolysis has the advantage that the reactive intermediates can be directly detected. Time profiles of the excimer fluroescence from ion irradiated polystyrene were measured using the polystyrene thin films. Thus, the transient phenomenon excited by the ion with a definite kinetic energy was observed. 

In ion-beam irradiated polystyrene, some kinds of reactive intermediates are produced. The excimer, which is one of the reactive intermediates, emits intense fluorescence. Thus, we measured the time profiles of the excimer fluorescence (328.5 nm) from ion irradiated polystyrene thin films. One of the results is shown in Fig. 6a and b for irradiation with 0.6 MeV He+ (several hundred pA beam current). In Fig. 6a, the irradiation time dependence of the excimer fluorescence intensity is shown. In Fig. 6b, the time profile (I) was recorded with an irradiation time of 0 s-139 s (low dose-time profile), and (II) in the irradiation time of 1839 s- 3839 s (high dose-time profile). The following experimental results were obtained. 

Fig. 7. Time profiles of the excimer from polystyrene resist films (0.5 pm thick) irradiated with ions. These time profiles were not influenced by the quenching seen in Fig. 6. The straight lines correspond to the fluorescence lifetime obtained by the electron pulse radiolysis study of polystyrene [38], From Ref. 35...

The main reactions taking place when chloromethylated polystyrene (CMS) and chloromethylated poly(diphenylsiloxane) (SNR) are irradiated with high energy electrons or deep UV (KrF excimer laser, 248 nm) radiation have been studied. The results are discussed in terms of short-lived reactive species generated using pulse radiolysis and laser (248 nm) photolysis techniques. 

Details of the laser photolysis system have been reported elsewhere (19). Polystyrene or CMS in cyclohexane solution in a 1x1x2 cm quartz cell was excited by a 248-nm pulse from a KrF excimer laser (lambda Physik EGM 500). The width of the pulse was 15 ns (full width at half-maximum (FWHM)). Transient spectra were monitored point by point by using a conventional nanosecond laser photolysis method. 

Deep UV Resist Reactions of CMS. The laser photolysis studies on CMS and polystyrene solutions in cyclohexane were carried out at 248 nm using a KrF excimer laser. The intensity of monomer and excimer fluorescence of CMS becomes weaker with increasing chloromethylation ratio, but the lifetime of the excimer is essentially independent of the chloromethylation ratio being almost the same as the excimer lifetime of polystyrene (20 ns). The lifetime of the monomer fluorescence has not been investigated by nanosecond laser photolysis because of the short lifetime (about 1 ns) (20,21) of the singlet. 

Fig. 1 shows the transient absorption spectra observed during photolysis of cyclohexane solutions of CMS. The absorption spectrum with the peak at 520 nm (spectrum a) is identical to that of the excimer of polystyrene (spectrum C) (22) and has a similar lifetime of 20 ns. The quenching rate of the absorption at 520 nm by O2 is comparable to what one would expect for the CMS excimer. 

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