Negative resist composition

Practical Negative Resist Compositions Arising from Photopolymerization of Monomers in the Presence of Polyfunctional Components 

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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 value in units of incident dose per unit area for either a positive or negative resist system is of little value unless accompanied by a detailed description of the conditions under which it was measured. This description should include, at the minimum, the initial film thickness, the characteristics of the substrate, the temperature and time of the post- and pre-bake, the characteristics of the exposing radiation, and the developer composition, time and temperature. The structure, copolymer ratio, sequence distribution, molecular weight, and dispersity of polymers included in the formulation should also be provided. 

The resolution capability of a resist is directly related to resist contrast (7) which, for a negative resist, is related to the rate of crosslinked network formation at a constant input dose. It is somewhat more complicated for a positive resist being related to the rate of chain scission and the rate of change of solubility with molecular weight with the latter being markedly solvent dependent. Contrast, like sensitivity, is governed by the type of chemical reactions that occur in the polymeric resist and is affected by molecular parameters such as molecular weight distribution and chemical composition. 

The lift-off process is usually employed to fabricate metal electrodes. This method, as opposed to the wet-etch process, allows the dual-composition electrode to be patterned in a single step [747]. In order to achieve well-defined metal electrodes in a channel recess using the lift-off technique, the metal (Pt/Ta) will not be deposited onto the sidewalls of the photoresist structure (see Figure 2.32). This discontinuity of the deposited metal layer around the sidewalls allows metal on the resist to be removed cleanly from the surface without tearing away from the metal on the surface. Thus negative resists were used because they can be easily processed to produce negatively inclined sidewalls. To achieve this, the photoresist is subjected to underexposure, followed by overdevelopment [141]. 

Eaton DF, unpublished results for a photopolymer composition containing 5wt% initiator, 35 wt% difunctional acrylate monomer, and a polymethacrylate binder. The composition was washed out after exposure to ultraviolet light through a step wedge that is, the photopolymer functioned as a negative resist... 

All lithographic imaging using photopolymerization negative resists relies on the difference between the solubility of the starting resin compositions and the cross-linked form. Following the methods of Delzenne and co-workers... 

Positive resists differ from their negative counterparts principally in their response to actinic radiation, despite the fact that the essential composition of the two resist types are similar in many ways each contains sensitizers or appropriate radiation-sensitive compounds, resins, solvents, and additives. Unlike some negative resists, positive resists do not swell in developer. Moreover, the use of aqueous stripping and developing solutions greatly simplifies the equipment selection for positive resists in process equipment tooling hy allowing low-cost readily available plastics to be used as containers. Problems from the use of flammable solvents are minimized with positive resists. ... 

Ito and C.G. Willson, Chemical amplification in the design of dry developing resist materials, Polym. Eng. Sci. 23, 1012 (1983) H. Ito, C.G. Willson, and J.M.J. Frechet, Positive and negative working resist compositions with acid generating photoinitiator and pol3mier with acid labile groups pendant from polymer backbone, U.S. Patent No. 4,491,628 (1985). 

Even minor loadings of layered clay in PNCs result in substantial reduction in flammabifity which has been attributed to the build-up of a protective char layer involving the clay [260]. Substantial improvement in the ablative properties of materials due to a. similar char formation has also been reported [261 ]. These properties could lead to the development of novel fire resistant composites without the use of halogenated compounds that have negative environmental impact and hydrated inorganic oxides that require substantial loading. 

Koyanagi, T. Takahashi, S. Kitaori, T. Nagasawa, K. Negative-working radiation-sensitive resist composition and pattern forming method. Jpn. Kokai Tokkyo Koho JP 08062833, 1996 Chem. Abstr. 1996, 725,45141. 

Ito, H., Willson, C.G., and Frechet, J.M.J. (1985) Positive and Negative Working Resist Compositions with Add Generating Photoinitiator and Polymer with Acid Labile Groups Pendant from Polymer Backbone. US Patent 4,491,628. 

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