Resist self-developing

Films of polyphthalaldehyde, sensitized by cationic photoinitiators, have been imaged at 2-5-mJ/cm in the deep ultraviolet (DUV) (see Section 3.10), at 1 pC/cm (20 kV) electron beam radiation and at an unspecified dose of Al-A x-ray radiation. The ultimate utility of this "self-developing" resist system will depend upon its efficacy as an etch barrier. It seems clear that such materials would not serve as adequate etch masks for... 

The simplest dry-developing scheme is that provided by the self-developing resists. The first example of this was reported by Bowden et al. 

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. 

Aldehyde Copolymer Self Developing Electron-beam Resists. The ceiling temperature for the copolymerization of aliphatic aldehydes is usually below 0°C and the copolymers are easily depolymerized into monomeric aldehydes above 150°C under vacuum. This depolymerization into monomers also occurs on electron-beam or X-ray exposure as evidenced by combined gas-liquid partition chromatography-mass spectrometry. As a result, the copolymers of aldehydes behaved as self-developing positive resists and almost complete development was accomplished without any solvent treatment. Electron-beam exposure characteristics of the aliphatic aldehyde copolymers studied here are... 

Recently it was disclosed in a Japanese patent that the copolymers of hexanal with propanal, butanal and isobutanal could be used as self-developing X-ray resists of 200 — 400 mJ/cm2 sensitivity (32). Our poly(ethanal-co-butanal) showed the sensitivity of 30 mJ/cm2 on the exposure to X-ray radiation without requiring a wet development process (Table VIII). Other copolymers also functioned as a positive self developing X-ray resist. 

Fijgure 3.39. Optical micrographs of self-developed images of a polyphthal-aldehyde-onium salt resist. Reproduced with permission from reference 105. Copyright 1986 Electrochemical Society.)... 

The third class of dry-developable resists involves heating the exposed resist films in a development step. This development method does not require expensive etching tools, is therefore economical, and could alleviate the potential problem of exposure tool contamination associated with the self-developing resist systems. Many of the plasma-developable resist systems involving a relief-bake step, as discussed in Section 3.2.4.1, have the thermal development characteristics to a certain extent. In the thermally developable resist scheme, development is minimal during irradiation but completed to the substrate upon postbaking. 

The PMMA polymer is the best known resist, due to the fact that it has the best resolution down to at least 5 nm [23]. It has been used exclusively in the LIGA process since the thick resist layer of the order of a few 100 jum could be made readily [14]. Many previous studies, including those on the self-development phenomena [2, 3] and laser ablation [5, 10, 12, 17], have been carried out on this polymer. Therefore, there are relatively more data... 

Following exposure, poly(olefin sulfones) can be developed by two main methods by solvent development or by thermal development. The exposed areas of the resist simply evaporate on heating, or in some cases during exposure, in a phenomenon termed self-development, which negatively impacts the vacuum of the exposure tool s electron column. The liquid development method is not without its drawbacks, as it requires a careful choice of solvent, since the development contrast depends only on molecular weight. ... 

When resist films coated with formulations comprising this endcapped polymer with 10% by weight of various onium salts are exposed to DUV photons or an electron beam, acids are generated from the onium salts that go on to catalyze the hydrolytic scission process of the endcap moieties, as shown in Scheme 7.45, with the net result being the evaporation of the irradiated areas, with rather catastrophic implications for the contamination of the optical elements of exposure tools. In this mechanism, the acid attacks the lone electron pair on oxygen and brings about the depolymerization of the entire polymer. This, in essence, was the first resist system that self-developed reliably at room temperature without any further processing or special conditions. ... 

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