Neil A. Beaudry

Effects of mask roughness and condenser scattering in EUVL systems

The wavefront reflected from extreme UV lithography mirror and mask surfaces can contain a non-negligible amount of phase variation due to roughness of the mirror surface and variations in multilayer thin-film coatings. We examine the characteristics of image and pattern formation as a function of phase variations originating at the mask surface and at condenser mirrors. A theoretical development and a Monte- Carlo simulation are used to show relationships between statistics of the phase variations and the mask pattern, coherence factor, and numerical aperture of the projection camera. Results indicate that it is possible to produce nearly 1 percent line-edge roughness in a photoresist pattern from moderate values of phase variations.

Dielectric tensor measurement from a single Mueller matrix image

A technique for measuring dielectric tensors in anisotropic layered structures, such as thin films of biaxial materials, is demonstrated. The ellipsometric data are collected in a quasi-monochromatic Mueller matrix image acquired over a large range of incident and azimuthal angles by illuminating a very small area on the sample with a focused beam from a modulating polarization state generator. After the beam interacts with the sample, the reflected and/or transmitted light is collected using an imaging polarization state analyzer. An image of the exit pupil of a collection objective lens is formed across a CCD such that each pixel collects light from a different angle incident on the sample, thus acquiring ellipsometric data at numerous incident angles simultaneously. The large range of angles and orientations is necessary to accurately determine dielectric tensors. The small but significant polarization aberrations of the low-polarization objective lenses used to create and collect the focused beams provide a significant challenge to accurate measurement. Measurements are presented of a thin-film E-type polarizer and a stretched plastic biaxial film.

Effects of object roughness on partially coherent image formation.

Phase perturbations in the object plane of a partially coherent imaging system are found to produce artifacts in the aerial image. It is demonstrated that phase perturbations of as little as lambda/30 rms can produce visible deformation in the final image for modest coherence factors, such as sigma(c) = 0.4. A combination of simulation and experiment is used to demonstrate the effects. Application to line-edge roughness in lithography is described.

Evanescent-imaging-ellipsometry-based microarray reader

We describe the development of a label-less ellipsometric imaging microarray reader. The ability of the ellipsometric microarray reader to measure binding of sample to microarray surface is verified using oligonucleotide complementary DNA (cDNA) microarrays. Polarized light illuminates the microarray surface through a glass substrate at an angle beyond the critical angle and changes in the polarization of totally internally reflected light resulting from binding events on the microarray surface are measured. This polarization change is used to measure the thickness of biomolecules bound to the microarray. A prototype ellipsometric imaging microarray reader is constructed and calibrated, and the performance is evaluated with cDNA microarrays. The microarray reader measures changes in refractive index changes as small as 0.0024 and thickness changes as small as 0.28 nm. The optimization of angle of incidence and substrate refractive index necessary to achieve high sensitivity is also described. This ellipsometric technique offers an attractive alternative to fluorescence-microarray readers in some genomic, proteomic, diagnostic, and sensing applications.

Scattering and coherence in EUVL

We illustrate the importance of considering scattering from the illuminator in extreme UV lithography systems. Our results indicate that a significant amount of amplitude modulation noise is present in the aerial image if scatter is present in a Koehler illuminator. The effect depends on the spatial frequency of the pattern on the mask, the numerical aperture of the projection camera, the coherence factor, and placement of the plane in the illuminator where the scattering occurs.

Retinal oximetry using intravitreal illumination.

Purpose: To demonstrate spectroscopic retinal oximetry measurements on arteries and veins in swine using intravitreal illumination. Retinal arterial and venous saturations are measured for a range of inspired O2 levels after pars plana vitrectomy. Methods: Pars plana vitrectomy and intravitreal manipulations were performed on two female American Yorkshire domestic swine. Light from a scanning monochromator was coupled into a fiberoptic intraocular illuminator inserted into the vitreous. The retinal vessels were illuminated obliquely, minimizing vessel glints. Multispectral images of the retinal vasculature were obtained as the swines arterial blood oxygen saturation was decreased from 100% to 67% in decrements of approximately 10%. Retinal vessel spectra were used to calculate oxygen saturation in selected arteries and veins. Arterial oxygen saturations were calibrated using blood gas analysis on blood drawn from a Swan-Ganz catheter placed in the femoral artery. Results: Oblique illumination of retinal vessels using an intravitreal fiberoptic illuminator provided a substantial reduction in the central vessel glint usually seen in fundus images, thus simplifying the analysis of spectral data. The vessel shadows were displaced from the vessel image simplifying the light paths in the eye. Using a full spectral analysis simplified by the light path reductions, we calculated retinal vessel saturations. The reduction of glint allowed for increased accuracy in measuring retinal vessel spectral optical density. Abnormally low retinal venous oxygen saturations were observed shortly after pars plana vitrectomy. Conclusions: Retinal oximetry using intravitreal illumination has been demonstrated. As a research tool, intravitreal illumination addresses several difficulties encountered when performing retinal oximetry with transcorneal illumination.

Imaging properties of a patterned rough surface: effects of roughness correlation and partial coherence

An investigation of partially coherent speckle is performed, including the effects of a pattern structure in the object plane of an imaging system. It is demonstrated that, when forming an image of a patterned rough surface into a binary photoresist, the width variations produced in the resultant pattern (also called line edge roughness) can increase as the imaging system becomes more incoherent. This unusual behavior occurs because of an interaction between the speckle produced by a rough surface and the partially coherent imaging properties of the pattern structure. It is shown that the increase in line edge roughness for a more incoherent system requires that the surface roughness of the object structure have a correlation length that is substantially smaller than the resolution of the imaging system.

Characterization of a bit-wise volumetric storage medium for a space environment.

We report playback performance results of volumetric optical data storage disks that are made from a class of light-absorbing (photo-chromic) compounds. The disks are exposed to a simulated space environment with respect to temperature and radiation. To test for temperature sensitivity, a vacuum oven bakes the disks for certain amount of time at a designated temperature. Radiation exposure includes heavy ions and high energy protons. Disks fail in high temperature and large proton-dose conditions. Heavy ions do not cause significant disk failure. The prevention of disk failure due to harsh space environments is also discussed.

Diffuse spectral fundus reflectance measured using subretinally placed spectralon

The diffuse fundus reflectance and the spectral transmittance of the swine sensory retina was measured in vivo using intravitreal illumination. Pars plana vitrectomy and intravitreal manipulations were performed on a female American Yorkshire domestic swine. Light from a scanning monochromator was coupled into a fiber optic intraocular illuminator inserted into the vitreous. A 1.93-mm(2) region of the illuminated fundus was imaged from an oblique illumination angle. Multispectral retinal images were acquired for four experimental conditions: the eye (1) prior to vitrectomy, (2) after vitrectomy, (3) after insertion of a Spectralon disk super-retinally, and (4) after subretinal insertion of the disk. The absorption of melanin and hemoglobin in the red wavelengths was used to convert relative spectral reflectance to absolute reflectance. The flux scattered from the super-retinal Spectralon was used to correct for scattering in the globe. The transmittance of the sensory retina was measured in vivo using the scatter corrected subretinal Spectralon disk reflectance. The hemoglobin and melanin components of the spectrum due to scattered light were removed from the retinal transmission spectrum. The in vivo spectral transmittance of the sensory retina in this swine was essentially flat across the visible spectrum, with an average transmittance >90%.

Wavelength dependence of the apparent diameter of retinal blood vessels

Imaging of retinal blood vessels may assist in the diagnosis and monitoring of diseases such as glaucoma, diabetic retinopathy, and hypertension. However, close examination reveals that the contrast and apparent diameter of vessels are dependent on the wavelength of the illuminating light. In this study multispectral images of large arteries and veins within enucleated swine eyes are obtained with a modified fundus camera by use of intravitreal illumination. The diameters of selected vessels are measured as a function of wavelength by cross-sectional analysis. A fixed scale with spectrally independent dimension is placed above the retina to isolate the chromatic effects of the imaging system and eye. Significant apparent differences between arterial and venous diameters are found, with larger diameters observed at shorter wavelengths. These differences are due primarily to spectral absorption in the cylindrical blood column.