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Measurement of Optical Properties of Refractive Materials at Actinic Wavelength

IP.com Disclosure Number: IPCOM000009079D
Publication Date: 2002-Aug-06
Document File: 4 page(s) / 77K

Publishing Venue

The IP.com Prior Art Database

Related People

Paul Dewa: AUTHOR [+2]

Abstract

The optical properties of refractive material used for DUV lithography have been historically measured at visible wavelengths and scaled to calculate the property at the actinic wavelength. These properties have included index of refraction homogeneity, densification and stress induced birefringence. Recently, phenomena have been discovered that do not scale linearly with wavelength. Two examples are intrinsic birefringence in Calcium Fluoride and the photo refractive effect in irradiated fused silica. We describe here methods of measuring material for these sorts of effects at the actinic wavelength in order to test those scaling laws. With this data, we can better predict the performance of lithographic objectives at wavelength.

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Measurement of Optical Properties of Refractive Materials
at Actinic Wavelength

Paul Dewa, Michael Linder

Corning Inc.

Keywords:

homogeneity measurements, refractive materials, optical properties, fused silica, calcium fluoride, fluoride crystals, interferometry, DUV, 248, 193, 157, microlithography

Abstract:

The optical properties of refractive material used for DUV lithography have been historically measured at visible wavelengths and scaled to calculate the property at the actinic wavelength.  These properties have included index of refraction homogeneity, densification and stress induced birefringence.  Recently, phenomena have been discovered that do not scale linearly with wavelength.  Two examples are intrinsic birefringence in Calcium Fluoride and the photo refractive effect in irradiated fused silica.  We describe here methods of measuring material for these sorts of effects at the actinic wavelength in order to test those scaling laws.  With this data, we can better predict the performance of lithographic objectives at wavelength.

Motivation

The performance of an optical system is based on two key criteria: First, the optical design, which defines the theoretical performance of the system, and the sensitivity of the performance against variation of the theoretical parameters.  Secondly, the practical implementation of the system.  Key parameters that influence the performance are surface shape of the optical elements, absolute value, dispersion and homogeneity of the refractive index, distances between surfaces, orientation of optical elements, and reflection characteristics of optical surfaces.  Mechanical elements of the systems may cause secondary effects, for example by stray light or thermal expansion.

For high-performance applications such as DUV lithography, all those parameters are tightly specified and controlled.  Historically, measurements of the refractive index homogeneity have been made mainly at a wavelength that is convenient to use in the interferometric measurements used, namely mercury lines early on, and with the success of the HeNe laser this switched to the wavelength of 632.8 nm.  The assumption used in doing so is that the homogeneity pattern will remain the same when switching to other wavelengths and the numerical values could be easily scaled.

While the scaling is a reasonable first order approximation, recently some effects have been discovered that showed that this approximation might not be sufficient in all cases. It is easily appreciable that the inhomogeneity of the refractive material could be conceived as a localized variation of the material, either of the density, or of the microscopic composition.  In the first case, when we consider a local density variation, we would consider this a variation in the number of oscillators and therefore assume a scaling of the homogeneity in accordance to the dispersion of the material.  In the second case, we could assume that each contributing compound to have its own var...