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Speckle Reduction by Fiber Optic Beam Retardation

IP.com Disclosure Number: IPCOM000041108D
Original Publication Date: 1987-Aug-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Hibbs, MS: AUTHOR

Abstract

The application of optical fibers to coherence reduction in narrow bandwidth laser illumination of lithographic exposure systems is reported.

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Speckle Reduction by Fiber Optic Beam Retardation

The application of optical fibers to coherence reduction in narrow bandwidth laser illumination of lithographic exposure systems is reported.

Ultraviolet excimer lasers can be used as a bright light source for optical microlithography; however, the refractive optics are typically highly dispersive. To control the dispersion, the laser bandwidth is reduced in order to hold resolution tolerances. A reduction of laser bandwidth will increase the temporal coherence of the light until speckle becomes a serious problem. "Speckle" is a term used to describe bright light spots caused by highly coherent light scattering from random imperfections in an optical system and coherently interfering with the image, producing large intensity fluctuations. Speckle induced by highly coherent lasers has prevented their effective use as a light source for optical lithography. Temporally incoherent illumination does not produce this problem because the scattered light has a rapidly fluctuating phase relationship with the image.

A prior solution to speckle induced by highly coherent light is to make long exposures while rapidly varying some part of the illumination system. Rotating ground glass elements and moving mirrors are methods used to vary speckle patterns during an exposure. A technique used in a pulsed system, i.e, an excimer laser, is to vary the illumination optics between pulses. After a long averaging time, the light intensity fluctuations due to speckle may be reduced to an arbitrarily l...