Bilevel resists

There is a current drive in microlithography to define submicron features in bilevel resist structures. The introduction of organometallic components, most notably organosilicon substituents, into conventional resists is one promising approach. To this end, organosilicon moieties have been primarily utilized in starting monomers (1-4) or in post-polymerization functionalization reactions on the polymer (5,6). Little work has been done on the reaction of preformed reactive oligomers to synthesize block copolymer systems. 

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. 

Bilevel resist schemes utilizing the gas-solid reactions of metal compounds (MR, M = metal, R = reactive group) with polymer films. 

Figure 13 Schematic representation of the bilevel resist process employing an oxygen reactive ion etching pattern transfer technique.

High resolution negative resists are needed for masked ion beam lithography (MIBL) and for the fabrication of MIBL masks by E-beam lithography (EBL). The MOTSS copolymer resists were developed to obtain the resolution of fine features that a bilevel resist can best provide. The flexibility afforded by choosing the structure of the HS, the copolymer composition, and the molecular weight allows a resist to be tailored by simple synthesis adjustments to have the particular sensitivity and etch protection which best suits the application. 

The chemistry and physics of plasmas are extremely complex Chapter 8 of this book presents detailed information. The use of oxygen RIE has promoted the development of bilevel-resist schemes with silicon-containing top-layer resists (174-175). This subject will be deferred to a later section for in-depth discussion. 

Figure 8. Schematic of a bilevel-resist process using a silicon-containing top layer. Pattern transfer to the bottom planarizing layer is achieved by oxygen...

Brominated poly(l-trimethylsilylpropyne) is an example of a substituted polyacetylene that is suitable for bilevel-resist processes (34). Requiring both exposure and postexposure bake (PEB) steps, samples of the polypropyne having a mole fraction of bromine from 0.1 to 0.2 per monomer unit exhibit sensitivities in the order of 25 mj/cm. Submicrometer resolution has been demonstrated, and etching-rate ratios relative to hard-baked photoresist planarizing layers are —1 25. 

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]. 

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