Heather J. Patrick

Annealing of Bragg gratings in hydrogen‐loaded optical fiber

We have conducted a detailed study of the thermal stability of Bragg gratings written in hydrogen‐loaded and unloaded germanium‐doped optical fiber. Interference of either continuous‐wave or pulsed ultraviolet light was used to induce the index modulation gratings. Some gratings were kept at room temperature and others were annealed at fixed temperatures for 10–20 h. For temperatures between room temperature and 350 °C, gratings in the hydrogen‐loaded fiber showed significantly greater decay than those in the unloaded counterpart. The ultraviolet‐induced index modulation in hydrogen‐loaded fiber was reduced by 40% after 10 h at 176 °C, whereas it was reduced by only 5% in unloaded fiber under the same conditions. The annealing behavior of gratings written using the pulsed source was identical to that of gratings written with the continuous‐wave source, and the thermal stability of gratings in hydrogen‐loaded fiber did not depend on the magnitude of the index modulation. We also observed that the annealing...

Growth of Bragg gratings produced by continuous-wave ultraviolet light in optical fiber

We have written Bragg gratings of as much as 94% reflectance in germanium-doped optical fiber by two-beam interference of 244-nm continuous-wave UV light. We measured grating reflectance as a function of exposure time for UV light intensities ranging from 1.5 to 47 W/cm2. The observed dependence of index modulation on time and intensity does not agree with the predictions of a model based on depletion of a defect population by one-photon absorption.

Scatterfield microscopy for extending the limits of image-based optical metrology

We have developed a set of techniques, referred to as scatterfield microscopy, in which the illumination is engineered in combination with appropriately designed metrology targets to extend the limits of image-based optical metrology. Previously we reported results from samples with sub-50-nm-sized features having pitches larger than the conventional Rayleigh resolution criterion, which resulted in images having edge contrast and elements of conventional imaging. In this paper we extend these methods to targets composed of features much denser than the conventional Rayleigh resolution criterion. For these applications, a new approach is presented that uses a combination of zero-order optical response and edge-based imaging. The approach is, however, more general and a more comprehensive set of analyses using theoretical methods is presented. This analysis gives a direct measure of the ultimate size and density of features that can be measured with these optical techniques. We present both experimental results and optical simulations using different electromagnetic scattering packages to evaluate the ultimate sensitivity and extensibility of these techniques.

Developing an uncertainty analysis for optical scatterometry

This article describes how an uncertainty analysis may be performed on a scatterometry measurement. A method is outlined for propagating uncertainties through a least-squares regression. The method includes the propagation of the measurement noise as well as estimates of systematic effects in the measurement. Since there may be correlations between the various parameters determined by the measurement, a method is described for visualizing the uncertainty in the extracted profile. The analysis is performed for a 120 nm pitch grating, consisting of photoresist lines 120 nm high, 45 nm critical dimension, and 88° side wall angle, measured with a spectroscopic rotating compensator ellipsometer. The results suggest that, while scatterometry is very precise, there are a number of sources of systematic errors that limit its absolute accuracy. Addressing those systematic errors may significantly improve scatterometry measurements in the future.

Scatterometry for in situ measurement of pattern reflow in nanoimprinted polymers

We use optical scatterometry to extract the time evolution of the profile of nanoimprinted lines in low and high molecular mass polymer gratings during reflow at the glass transition temperature. The data are obtained continuously during the anneal using a spectroscopic ellipsometer and analyzed using a rigorous-coupled-wave model. We show excellent agreement of scatterometry results with ex situ measurements of line height by atomic force microscopy and specular x-ray reflectivity. The in situ scatterometry results reveal differences in the shape evolution of the grating lines indiscernible by other methods.

Optical critical dimension measurement of silicon grating targets using back focal plane scatterfield microscopy

We demonstrate optical critical dimension measurement of lines in silicon grating targets using back focal plane scatterfield icroscopy. In this technique, angle-resolved diffraction signatures are obtained from grating targets by imaging the back focal plane of a brightfield microscope that has been modified to allow selection of the angular distribution and polarization of the incident illumination. The target line profiles, including critical dimension linewidth and sidewall angle, are extracted using a scatterometry method that compares the diffraction signatures to a library of theoretical signatures. Because we use the zero-order component of the diffraction, the target features need not be resolved in order to obtain the line profile. We extracted line profiles from two series of targets with fixed pitch but varying linewidth: a subresolution 300-nm-pitch series, and a resolved 600-nm-pitch series. Linewidths of 131 nm to 139 nm were obtained, with nanometer-level sensitivity to linewidth, and a linear relationship of linewidth obtained from scatterfield microscopy to linewidth measured by scanning electron microscopy was demonstrated. Conventional images can be easily collected on the same microscope, providing a powerful tool for combining imaging metrology with scatterometry for optical critical dimension measurement.

Effect of Bandwidth and Numerical Aperture in Optical Scatterometry

We consider the effects of finite spectral bandwidth and numerical aperture in scatterometry measurements and discuss efficient integration methods based upon Gaussian quadrature in one dimension (for spectral bandwidth averaging) and two dimensions inside a circle (for numerical aperture averaging). Provided the wavelength is not near a Woods anomaly for the grating, we find that the resulting methods converge very quickly to a level suitable for most measurement applications. In the vicinity of a Woods anomaly, however, the methods provide rather poor behavior. We also describe a method that can be used to extract the effective spectral bandwidth and numerical aperture for a scatterometry tool. We find that accounting for spectral bandwidth and numerical aperture is necessary to obtain satisfactory results in scatterometry.

Decrease of fluorescence in optical fiber during exposure to pulsed or continuous-wave ultraviolet light

Abstract We exposed optical fibers to UV light and simultaneously measured the intensity of the blue fluorescence from the fiber core. Two silica glass fibers with different core dopants were investigated: a germanium-doped fiber and a germanium-boron-codoped fiber. The fibers were exposed transversely to pulsed or continuous-wave 244 nm light for times ranging from a few minutes to over an hour. For all UV intensities and exposure times used, the fluorescence decreased during UV exposure. For a given fiber, the fractional decrease in fluorescence seen from the side of the fiber was dependent only on the total UV fluence. The side-collected fluorescence from the germanium-doped fiber decreased to 60% of its initial value after exposure to 3600 J/cm 2 fluence, while the fluorescence from the germanium-boron-codoped fiber decreased to 40% of its initial value after the same fluence. We compared the temporal characteristics of the fluorescence radiated transversely to the exposed region with that which was collected from the end of the fiber. The temporal characteristics of the fluorescence guided down the core and collected from the end were masked by photodarkening that occurred on the same time scale. We relate the observed fluorescence decrease to competing theories of its origin and relationship to photoinduced refractive index changes.

BRDF measurements of graphite used in high-temperature fixed point blackbody radiators: a multi-angle study at 405 nm and 658 nm

We have measured the polarization-resolved bidirectional reflectance distribution function (BRDF) for two types of graphite used in the fabrication of high-temperature fixed point blackbody cavities in and out of the plane of incidence. Measurements were made at room temperature using 405 nm and 658 nm laser sources, and the samples were illuminated at angles of incidence varying from normal incidence to 70°. All of the samples exhibited non-Lambertian behaviour, including enhanced forward scatter at high incident angles, especially for s-polarized incident light. The directional-hemispherical reflectance for unpolarized incident light, obtained by integrating the BRDF measured at individual points in a hemisphere over the sample, ranged from 0.083 to 0.101, depending upon sample and incident angle. The potential impact of these measurements on emissivity models for graphite blackbody radiators and radiance temperature scale dissemination is discussed.

Mueller matrix bidirectional reflectance distribution functionmeasurements and modeling of diffuse reflectance standards

We measure the Mueller matrix bidirectional reflectance distribution function (BRDF) of pressed and sintered powdered polytetrafluoroethylene (PTFE) reflectance standards for an incident angle of 75°. Rotationallyaveraged Mueller matrices from the materials showed a small asymmetry M12 ≠ M21 and M34 ≠ -M43 in the in-plane geometry. This asymmetry, however, followed Helmholtz reciprocity rules. A significant anisotropy was observed in the sintered samples, which was manifested as non-zero off-block diagonal elements that depended upon rotation of the samples. Modeling using a Mueller matrix extension to the radiative transfer equation was performed. While there was not quantitative agreement, some aspects of the data were observed, including the asymmetry. Availability of an improved Mueller matrix phase function should improve the quality of the model-experiment agreement.