Methyl methacrylate, electron-beam sensitivities

Figure 42. A plot of proton beam vs electron beam sensitivity for several resist systems. COP is a copolymer of glycidyl methacrylate and ethyl acrylate, PVC is poly (vinylcinnamate), PCS is poly (chlorostyrene), PTBMA is polyO-butyl methacrylate), PVA is poly (vinyl acetate), PMMA is poly (methyl methacrylate). (Reproduced with permission from Ref. 57 J...

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.

The incorporation of small percentages (<10%) of 3-oximino-2-butanone methacrylate (4) into poly(methyl methacrylate) (PMMA) (Scheme I) results in a four fold increase in polymer sensitivity in the range of 230-260 nm flO.l 11. Presumably, the moderately labile N-O bond is induced to cleave, leading to decarboxylation and main chain scission (Scheme II). The sensitivity is further enhanced by the addition of external sensitizers. Also, preliminary results indicated that terpolymerization with methacrylonitrile would effect an additional increase. These results complement those of Stillwagon (12) who had previously shown that copolymerization of methyl methacrylate with methacrylonitrile increased the polymer s sensitivity to electron beam irradiation. The mole fraction of the comonomers was kept low in order to insure retention of the high resolution properties of PMMA (3.41. 

In this article we will describe two different types of positive electron-beam resists, which were briefly reported in our previous communications (2,3). One is the homopolymer or copolymer with methyl methacrylate and a-substituted benzyl methacrylate, which forms methacrylic acid units in the polymer chain on exposure to an electron-beam and can be developed by using an alkaline solution developer. In this case, the structural change in the side group of the polymer effectively alters the solubility properties of the exposed polymer, and excellent contrast between the exposed and unexposed areas is obtained. The other is a self developing polyaldehyde resist, which is depolymerized into a volatile monomer upon electron-beam exposure. The sensitivity was extremely high without using any sensitizer. 

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,... 

Poly(methyl methacrylate), PMMA, has remained the standard by which to judge positive-working electron-beam resists for over a dozen years (1>.2 .3 4) Hundreds of rival polymers have been disclosed. Most of them exceed PMMA in sensitivity. However, the combination of properties which include stability, sensitivity, contrast, adhesion, and solubility have kept PMMA in the limelight. 

The structure of itaconic acid bears some resemblance to methacryllc acid. While It Is know that methacryllc acid and methacrylic anhydride do enhance sensitivity when copolymerized with methyl methacrylate, some of the Increase In speed may be due to porosity (7,12). The gas formation may cause a higher rate of dissolution then can be attributed to chain scission alone. Pittman and co-workers have reported G(s) values for poly(methacryllc anhydride) of 0.4 (10) and 1.8 and 2.9 (14) based on gamma radiation experiments. Hiroaka (12) measured G(s) by gas evolution on Irradiation of films with electron beams and established values In the ration 1 2 6 for the methyl methacrylate. A terpolymer with the three components In the molar ratio of 70 15 15 (same three monomers) was selected by Moreau et al (13) on the basis of complete lithographic evaluation. The speed Is 4 to lOx that of PMMA. As aforementioned, an Increase In G(s) may be only partially responsible. 

Gipstein, W. Moreau, and O. Need, Poly(methyl methacrylate isobutylene) copolymers as highly sensitive electron beam resists, J. Electrochem. Soc. 123, 1105 (1976). 

Most polymers that are positive resists tend to depolymerize via a monomer-unzipping action when degraded, and poly(methyl methacrylate) (PMMA) is typical of this type. Unfortunately, the sensitivity of PMMA to electron beam radiation is low, and in an attempt to improve this feature, PMMA derivatives have been prepared by replacing the a-methyl group with more polar electron-withdrawing substituents, e.g.. Cl, CN, and CF3, to assist electron capture (Figure 16.5). Modification of the... 

Acrylate polymers slowly undergo chain scission upon irradiation with uv light and electron beams. While this property has been used to advantage [acrylic polymers such as poly(methyl methacrylate) have seen use as high resolution but low sensitivity electron beam and DUV resists], in this instance it is undesirable as it compromises plasma etch resistance, and complicates the metrology of acrylate resist patterns when using scanning electron microscopy (112). 

The interaction volume of 20 kV electrons in poly(methyl methacrylate) (PMMA) has been shown directly by using its radiation sensitivity [39]. After exposure to a beam of electrons, the material was cross sectioned, polished, and etched. Pear-shaped holes up to 10/rm deep appear (Fig. 2.5) showing where the beam interacted with the PMMA (and reduced its molecular weight) [39]. Calculation of the interaction... 

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