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Heavy Metal Salts of Acidic Copolymers for use as a Radiation Sensitive Resists

IP.com Disclosure Number: IPCOM000084719D
Original Publication Date: 1975-Dec-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Feder, R: AUTHOR [+5]

Abstract

Electron beam and X-ray lithographic techniques for the production of microelectronic circuitry generally use polymethyl methacrylate (PMMA) as resist material. The major advantage of PMMA over other proposed materials is its capability for high resolution. The major shortcoming of the process using PMMA is its low throughput, which is limited by the radiation dose required to change the solubility characteristics of PMMA sufficiently to allow selective development of the exposed areas.

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Heavy Metal Salts of Acidic Copolymers for use as a Radiation Sensitive Resists

Electron beam and X-ray lithographic techniques for the production of microelectronic circuitry generally use polymethyl methacrylate (PMMA) as resist material. The major advantage of PMMA over other proposed materials is its capability for high resolution. The major shortcoming of the process using PMMA is its low throughput, which is limited by the radiation dose required to change the solubility characteristics of PMMA sufficiently to allow selective development of the exposed areas.

One source of inefficiency of PMMA, and of other polymeric resist materials containing only Row 2 elements of the periodic chart, is their low-absorption coefficient for ionizing radiation. A 5000 angstroms thick film absorbs only about 5% of the incident energy of X-rays with lambda = 8.3 angstroms or electrons with E = 20kV.

Here, chemical compounds are described which, by virtue of heavy metals ionically bound to polymer chains, have higher absorption coefficients for ionizing radiation than conventional resists and, therefore, are more sensitive resists in radiation processing. The specific materials prepared and tested thus far are the thallous and the cesium salts of a copolymer of methyl methacrylate and methacrylic acid (they will henceforth be abbreviated as TLP(MM-MA) and CsP(MM-MA), respectively). Salts of other monovalent cations, such as Ag/+/, CH(3)Hg/+/, etc., or with other acidic polymers can be prepared in an analogous manner and should have similar properties.

Synthesis of TLP(MM-MA), Method A. A copolymer prepared by bulk free radical polymerization of methyl methacrylate containing 10% methacrylic acid (Polysciences Inc.), is dissolved as received in dioxane (conc. 2%). To a freshly prepared solution of thallous ethoxide in dry dioxane (0.9% W/v) is added under heavy stirring, an equal volume of the polymer solution which results in the precipitation of a highly swelled gel. The stirring is continued for 24 hours; then the precipitate is separated, dried and ground. The material is found to be incompletely soluble in cyclopentanone; the residue and the turbidity are removed by centrifugation. The solution is coated on Si wafers...