Poly electron-beam exposed

Interest in solution inhibition resist systems is not limited to photoresist technology. Systems that are sensitive to electron-beam irradiation have also been of active interest. While conventional positive photoresists may be used for e-beam applications (31,32), they exhibit poor sensitivity and alternatives are desirable. Bowden, et al, at AT T Bell Laboratories, developed a novel, novolac-poly(2-methyl-l-pentene sulfone) (PMPS) composite resist, NPR (Figure 9) (33,34). PMPS, which acts as a dissolution inhibitor for the novolac resin, undergoes spontaneous depolymerization upon irradiation (35). Subsequent vaporization facilitates aqueous base removal of the exposed regions. Resist systems based on this chemistry have also been reported by other workers (36,37). 

Development of Resist Patterns. Development was done in AZ2401 developer diluted with 2 to 5 times its volume of water AZ2401 is an aqueous solution of KOH with a surfactant. When the resist films were exposed to electron beam doses of 5 iC/cm2 at 25 keV, it usually took 1.5 to 2.0 min for complete development of the images using a diazo-naphthoquinone sensitizer with o-chloro-cresol-formaldehyde Novolak resin in (1 3) AZ2401/water developer. With poly(2-methyl-l-pentene sulfone) the chlorinated Novolak resin exposed to I juC/cm2, it took 2.0 min in (1 4) AZ2401 developer for complete image development. 

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 (/). 

Positive Electron-beam Resist of Poly (a-substituted Benzyl Methacrylate). The electron-beam resist behaviors of poly(a-substituted benzyl methacrylate)s are given in Table III. When the exposed resist films were developed with a mixture of MIBK and IPA, the sensitivities of these polymers were on the order of 10-4 C/cm2. When a dilute solution of sodium methoxide in methanol was used as a developer, the sensitivity was enhanced as compared with the former case, and increased with an increase in the bulkiness of the ester group of the polymer except for poly(a,a-diphenylethyl methacrylate). 

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...

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,... 

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. 

Figure 5. Difference infrared spectra between the atactic poly(a,a-dimethylbenzyl methacrylate)s unexposed and exposed to electron-beam of several doses. (A) 4x1 ( 5, (B) 1.6x1 0 4, (C) 7 x 1 ( 4 Clem1.

Then the decomposition is expected to be more favorable as the number of / -hydrogen atoms is larger. This is the case for the poly(a-substituted benzyl methacrylate)s as shown in Figure 7. However, when poly(t-butyl methacrylate) containing nine / -hydrogen atoms was exposed to an electron-beam, the amount of acid group formed was smaller than that for poly (a,a-... 

Figure 8. Difference infrared spectra between the polymers unexposed and exposed to electron-beam of 1 x 10 4 C/cm2. (A) Poly(a,a-diphenylethyl methacrylate-co-methyl methacrylate) containing 74.7 mol% of methyl methacrylate units (B) Poly (methyl methacrylate).

Fig. 6. Molecular weight of poly(p-vinyl phenol) in exposed MRS as a function of electron-beam dose.

Figure 12. Electron beam imaging of poly(di-n-pentylsilane) (0.14 ym) coated over 2.0 xm of a hard-baked AZ-type photoresist exposed at 20 iClcm and wet developed. Pattern transfer was by O2-RIE.

Figure 7.16 shows SEM images of one of the first commercial versions of the alicyclic polymer resist platform, based on poly(dinorbomene-aZr-maleic anhydride), and marketed under the brand name of ATOl by the JSR Corporation in 1998. Figures 7.17-7.19 show SEM images of other alicyclic platform resists, DHA-1001 and DHK-1000 series, exposed on ArF laser (193-nm) and electron-beam tools, respectively. These resists were produced by Dongjin Semichemical Co. Ltd. 

Polymers which undergo radiation degradation on exposure to radiation are also important commercially. The best-known example is the group of polymers used as positive resist materials in electron beam microlithography. These include aliphatic poly(sulfone)s and poly(methacrylate)s. Finally, an understanding of the radiation chemistry of polymers is essential for their application in environments where they are exposed to high doses of ionizing radiation, for example in the nuclear and space industries. 

Exposure to an electron beam source cleaves the polymer chains at the weak C-S bond with liberation of SOj, and in certain cases such as poly(2-methyl pentene sulfone), there is almost eomplete vaporization of the exposed regions when a 20-kV electron beam souree is used. The major limitation of this group of resists is... 

Acrolein was grafted onto poly(ethylene) which was exposed to electron beams. The remaining aldehyde groups could be transformed into hydrazone, oxime, and oxyacid units [107]. 

Fr chet and TuUy have used poly(aryl ether) dendrimers, terminated with carbonate end groups as photoresists [99]. The aqueous insoluble carbonate terminated dendrimers were applied as a layer to a substrate. This dendritic layer was exposed subsequently to deep UV radiation or electron beam radiation and then baked to convert the masked areas of the dendritic carbonate coating to the deprotected and water-soluble polyphenoxide dendrimers by decarboxylation of the peripheral groups. The resultant deprotected polyphenoxide dendrimers exhibited differential solubility to the carbonate terminated dendrimers, which was exploited by removal of either the carbonate terminated or hydroxyl terminated dendrimer by washing with protic organic or aqueous solvents, respectively (Figure 8.15). 

Magnus et al. [1015] described "nanopatteming" of CPs and production of nm-spaced electrodes achieved by exposing thin films of CPs such as poly(3-octyl-thiophene) with a 50 kV electron beam. This exposure reduced the solubility, following which chlorobenzene could be used as a "developer to dissolve unexposed CP. The work was done to demonstrate that patterning similar to that of inorganic semiconductors on a Si wafer could be achieved with organic CPs as well. 

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