Poly , electron-beam exposure

Formulation of Resist Solutions. Forty grams of a Novolak resin was mixed with 10 g of the photoactive compound, and dissolved in 100 g of bis-2-methoxy-ethylether. After wafers were spin-coated, the samples were immediately placed on a hot plate at 82 C for 14 min. The formulation procedure of a composite resist of poly (2-methyl-1-pentene sulfone) in the Novolak resin is as follows the polysulfone was mixed with the resin (13 wt% solid), and then dissolved in 2-methoxyethyl acetate the films were spin-coated onto silicon wafers, and then baked at 100°C for 20 min prior to electron beam exposure. 

Figure 3. SEM pictures of a composite resist made of poly (2-methyl-1 -pentene sulfone) and o-chloro-m-cresol-formaldehyde Novolak resin after electron beam exposures with doses of (a) 2 /xC/cm2, and (b) 1 fiC/cm2.

The results mentioned here clearly indicate that the enhancement in the sensitivity and 7-value of the poly (a,a-dimethyl benzyl methacrylate) resist over poly(methyl methacrylate) is mainly due to facilitated formation of methacrylic acid units on electron-beam exposure. The exposed area, which contains CH,... 

The reaction of poly (methyl methacrylate) on electron-beam exposure has been thoroughly studied and the elimination of methoxycarbonyl group is considered to be the primary mechanism, by which the main-chain scission is initiated (20,21). However, in this work the formation of acid carbonyl group... 

Table VII the electron-beam exposure characteristics are given for the soluble poly (triphenylmethyl methacrylate-co-methyl methacrylate)s. The sensitivity on alkaline development was strongly influenced by the copolymer composition. The highest sensitivity was obtained on the copolymer containing 93.7 mol% methyl methacrylate. The copolymer of highest sensitivity showed the 7-value of 6.3, which was nearly twice as large as that for poly(methyl methacrylate). Formation of methacrylic acid units on exposure is obvious from the infrared spectrum. However, the mechanism of the occurrence should be different from the case of the a,a-dimethylbenzyl methacrylate polymer since there are no /3-hydrogen atoms in the triphenylmethyl group, and may be similar to the case of poly (methyl methacrylate). This will be explored in the near future.

As mentioned before, it was reported that the electron-beam exposure of poly(phthalaldehyde) resulted in spontaneous formation of a relief image with... 

Table IX. Electron-beam Exposure Characteristics3 of Poly(ethanal-co-butanal)s of Various Molecular Weights Prepared in Toluene at —78°C with Et2AlNPh2...

Fig. 1. Molecular weight distributions of MRS before and after electron-beam exposure. Dose 50 MC/cm. Peaks 1, 2, 3 are azide, poly(p-vinylphenol), and primary amine, respectively.

Poly(glycidyl methacrylate) (PGMA), a well-known negative electron beam resist first reported by Hirai et al. (55), actually functions as a positive-tone resist upon DUV exposure (Table 3.1) (56). The epoxide functionality responsible for cross-linking under electron beam exposure does not absorb in the DUV region, and the response of PGMA to DUV radiation is determined by the absorption due to the n — tt transition of the carbonyl chromo-... 

Fig. 9 The change of the Cj, core level signal of poly(methacrylic acid) (a) before UV-exposure, (b) after 3 KeV proton beam, irradiation, (c) er 23 KeV electron beam exposure, (d) UV-exposure in air, (e) after UV-exposure in nitrogen.

Because of the above limitations of PBS and SNS resist systems, poly(chloro-acrylate-co-a-methylstyrene) (XXVIII) resist was developed at Nippon Zeon in the 1990s. Based on chain scission events, the resist has found wide acceptance in mask-making applications, particularly for device design rule <180 nm at doses of 8 p.C/cm on 10 kV electron-beam exposure tools. It is marketed under the brand name ZEP. In spite of the widespread acceptance... 

S.H.M. Persson, P. Dyreklev, and O. Inganas, Patterning of poly(3-octylthiophene) conducting polymer films by electron beam exposure, Adv. Mater., 8,405-408 (1996). 

Poly(methyl methacrylate) (PMMA) is the standard positive e-beam resist, usually purchased in two high molecular weight forms (495 or 950 kg moP ) in a casting solvent such as chlorobenzene or anisole. For example 950 kg mol PMMA, 4 % in anisole is a commonly used solution. Electron beam exposure breaks the polymer into fragments (as shown in Scheme 13.7) that can be dissolved in a 1 1 MIBK IPA developer (where MIBK is methyl isobutyl ketone and IPA is isopropyl alcohol). 

The problem in electron-beam lithography is that image distortion and pattern placement errors arise by resist charging during electron-beam exposure. 3-Alky-lated PTs, e.g. a copolymer made from 3-methyl- and 3-butylthiophene or water-soluble poly(3-thienyl-2-ethanesulfonate) as discharge interlayer and top layer eliminate the charging problem during the electron-beam exposure [144]. 

The first example of resist relief image formation in the absence of a wet developing step was reported by Bowden and co-workers in 1974 60). These workers reported that exposure of certain poly (olefin sulfones) to electron beam radiation resulted in spontaneous relief image formation. If the films were cast thin enough and the substrates were heated, it was possible to produce clean images in the resist films by exposure alone, thereby avoiding a wet development step. 

For further enhancement of electron beam sensitivity, the chlorinated Novolak resin was studied using poly (2-methyl-1-pentene sulfone) as a dissolution inhibitor. The chlorinated Novolak resin mixed well with the polysulfone, and there was no phase separation observed when the films were spin-coated. With 13 wt% of the polysulfone, the chlorinated Novolak resist cast from a cellosolve acetate solution yielded fully developed images with R/Ra = 9.2 after exposure to 2 / 2. It gave fully developed images with R/R0 = 3.2 at a dose of 1 / 2, as shown in Figure 3. There are some problems with this resist system some cracking of the developed resist images... 

Many papers have been published on positive electron-beam resists. These resists are mostly polymers which are degraded upon electron-beam irradiation. The resulting lower molecular weight polymer in the exposed area can be selectively removed by a solvent under certain developing conditions. The development is accomplished by the difference in the rate of dissolution between the exposed and unexposed areas, which is a function of the molecular weight of the polymer. Recently, Willson and his co-workers reported the new type of positive resist, poly(phthalaldehyde), the exposure of which in the presence of certain cationic photoinitiators resulted in the spontaneous formation of a relief image without any development step (/). 

Figure 2. Infrared spectra of atactic poly(a,a-dimethylbenzyl methacrylate)s unexposed (A) and exposed(B) to electron-beam, isotactic poly (a,a-dimethylbenzyl methacrylate) exposed(C) and poly(methacrylic acid)(D). Exposure charge density 1.6 x 10-4 C/cm2, film thickness 0.5 pm, prebake at 142° C. Reproduced with permission from Ref. 2. Copyright 1983, "Springer...

Spectral subtraction usually provides a sensitive method for detecting small changes in the sample. Figure 5 shows the difference spectra between the atactic poly(a,a-dimethylbenzyl methacrylate) s unexposed and exposed to electron-beam at several doses. The positive absorption at 1729 cm-1 is due to the ester carbonyl group consumed on the exposure and the negative ones at 1700 and 1760 cm-1 to the acid and acid anhydride carbonyl groups formed, respectively. The formation of methacrylic acid units was more easily detected using the difference spectrum However, these difference spectra could not be used for the quantitative determination because the absorptions overlap somewhat. 

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