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Doped Resist for Electrons and X Rays with Enhanced Sensitivity

IP.com Disclosure Number: IPCOM000084718D
Original Publication Date: 1975-Dec-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 16K

Publishing Venue

IBM

Related People

Feder, R: AUTHOR [+5]

Abstract

Organic polymers such as polymethyl methacrylate (PMMA) are used as resists in X-rays and electron-beam lithography. Bonds in the polymer are broken by the incident radiation, reducing the molecular weight and thus making the resist more soluble in the exposed parts. These polymers have a low-absorption coefficient; 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 = 20 kV.

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Doped Resist for Electrons and X Rays with Enhanced Sensitivity

Organic polymers such as polymethyl methacrylate (PMMA) are used as resists in X-rays and electron-beam lithography. Bonds in the polymer are broken by the incident radiation, reducing the molecular weight and thus making the resist more soluble in the exposed parts. These polymers have a low- absorption coefficient; 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 = 20 kV.

An increase in the absorption of these resists can be obtained by doping with a heavy element like U, Er, Pt, Au, etc. These heavy elements will absorb a larger fraction of the incident radiation energy and transfer it to the polymer where a radiation chemical reaction (like main chain scission) occurs. The result is a resist with enhanced sensitivity. The dopants can be present as "monodispersed" atoms, organometallic compounds or colloidal size particles.

Generally, every dopant will also affect the chemical and mechanical properties of a polymer, and it is important to select a doping method which does not have any negative effect on these properties.

The doping of the polymers can be realized by:
1. Adding to the polymer solution an organometallic solution

which is mutually soluble with the polymer containing

solution.
2. Dissolving a salt, ion, metal or a complex of the desired

metal in an organic solvent, which is mutually soluble with

the polymer containing solution.
3. Preparing a colloidal suspension of the desired element, or

its compound, in an organic solvent which is mutually soluble

with the polymer containing solvent and which, upon mixing of

the colloid containing solvent and the polymer containing

solvent, does not cross-link or otherwise adversely affect

the film forming properties of the polymer.

In the following are described two specific methods for the preparation of doped PMMA. Both methods not only give enhanced absorption but also reduce the dissolution of the unexposed resist, such that larger development times can be used without the loss of resist in the unexposed part. Preparation of Uranium Doped PMMA.

Uranyl nitrate is readily dissolved in esters, alcohols, ketones and ethers. When the organic phase is brought in contact with an aqueous uranyl nitrate and there is an excess NO(3) present in the water phase (added as KNO(3), Ca(NO(3))(2), NH(4)NO(3) or HNO(3)), the UO(2)(NO(3))(2). 4H(2)O is extracted quantitively into the organic phase. Another important extractant which does not require an excess of NO(3) in the water phase is tributyl phosphate.

Advantage is taken of the fact that isoamyl acetate, cyclohexanon and methyl isobutyl ketone extract UO(2)(NO(3))(2) . 4H(2)O from aqueous solutions, and

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the above uranyl complex containing organic solvents are mutually soluble with a polymethyl methacrylate chlorobenzene solution.

Uranium metal is dissolved in nitric acid and the...