Bilevel resists materials

The structures of the dimethylsiloxane block copolymers and respective parent homopolymers prepared for use as positive, bilevel resist materials are shown in Figure 1. Most copolymers were synthesized with >10 wt % silicon. The selection of PDMSX block length and novolac chemical composition proved to be the two most critical variables in achieving adequate resolution. 

The incorporation of PDMSX into conventional novolac resins has produced potential bilevel resist materials. Adequate silicon contents necessary for O2 RIE resistance can be achieved without sacrificing aqueous TMAH solubility. Positive resist formulations using an o-cresol novolac-PDMSX (510 g/mole) copolymer with a diazonaphthoquinone dissolution inhibitor have demonstrated a resolution of coded 0.5 pm L/S patterns at a dose of 156 mJ/cm2 upon deep-UV irradiation. A 1 18 O2 etching selectivity versus hard-baked photoresist allows dry pattern transfer into the bilevel structure. 

Silylated methacrylates are also useful in positive, chain scission, bilevel deep-UV resists (30, 31). However, the silyl substituent must be carefully selected to avoid an excessive decrease in Tg. For example, pentamethyl-disiloxypropyl methacrylate has limited utility in resist applications, because it decreases the Tg (30). Copolymerization with other monomers may yield glassy, oxygen-resistant materials, but syntheses requiring controlled polymerization of three or more constituents may be unduly complicated. 

Resist materials consist of electrically conductive and UV-absorbing polymers or of organic materials containing polymers such as 3-methoxyPT. When such a resist is used as the lower layer of bilevel resists, deformation of pattern and decrease of resolution are suppressed during the UV or electron-beam exposure [145]. 

All future alternatives will require new resists and processes, and for the first time, manufacturing lines will be using at least two different resists. These new materials must have satisfactory sensitivity, resolution, and process latitude. In addition, the deep-UV tools will have limited depth of focus (1-2 (xm) and will be useful only with relatively planar surfaces. Multilayer-resist schemes have been proposed to overcome these limitations, and the simplest is the bilevel scheme that requires a resist that can be converted, after development, to a mask resistant to O2 reactive ion etching (RIE). Resistance to O2 RIE can be achieved by incorporating an element into the resist structure that easily forms a refractory oxide. Silicon performs this function very well and is relatively easy to include in a wide variety of polymer structures. 

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