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Novel Sandwich X-Ray Mask Structures

IP.com Disclosure Number: IPCOM000045534D
Original Publication Date: 1983-Apr-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 55K

Publishing Venue

IBM

Related People

Gal, LV: AUTHOR [+2]

Abstract

The optical transparency, heat dissipation and mechanical stability of a boron-doped silicon substrate for an X-ray mask can be improved by depositing one or more suitable coatings on one or both sides of the substrate prior to or after deposition of the X-ray absorber pattern. It has been found, for example, that a coating of silicon carbide (SiC) followed by a coating of polymethyl methacrylate (PMMA) gives particularly good results.

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Novel Sandwich X-Ray Mask Structures

The optical transparency, heat dissipation and mechanical stability of a boron-doped silicon substrate for an X-ray mask can be improved by depositing one or more suitable coatings on one or both sides of the substrate prior to or after deposition of the X-ray absorber pattern. It has been found, for example, that a coating of silicon carbide (SiC) followed by a coating of polymethyl methacrylate (PMMA) gives particularly good results.

X-ray mask substrates consist of a thin membrane usually made of a thin (a few microns thick) inorganic material. This membrane is used to support a pattern of a metal which selectively absorbs X-rays and allows the formation of an image in an X-ray sensitive resist-coated wafer suspended beneath it.

In order to align the resist coated wafer to the X-ray mask, areas of the X-ray substrate membrane are set aside to allow radiation of an appropriate wavelength to be transmitted and subsequently reflected from alignment marks placed on the resist-covered wafer. In many cases, the alignment wavelength lies in the visible region of the optical spectrum, which requires that the substrate membrane be transparent to optical wavelengths, since any absorption of the alignment pave length will degrade the signal-to-noise ratio of the alignment signal.

There are two properties of this membrane which must be optimized in order to achieve good mask-wafer alignment and to preserve the fidelity of the pattern to be transferred into the resist. One property is the optical transparency, and the other property is the mechanical stability of the material. Sandwich structures can be fabricated to improve both of these properties.

There are three characteristics which determine the overall transparency of the X-ray mask: (1) Absorption losses intrinsic to the film. (2) Reflection losses. (3) Surface roughness.

In general, the optical constants which determine the absorption losses of the membrane cannot be altered by external means, such as the conditions of the deposition, etc. Once the membrane material is selected, the absorption losses are solely determined by the extinction coefficient of the material and its thickness. However, reflection losses can be reduced considerably by coating the membrane with materials of appropriate refractive indices. The table above show...