Fused silica diffusers, made by forming scattering centers inside fused silica glass, can exhibit desirable optical properties,
such as reflectance or transmittance independent of viewing angle, spectrally flat response into the ultraviolet wavelength
range, and good spatial uniformity. The diffusers are of interest for terrestrial and space borne remote sensing instruments,
which use light diffusers in reflective and transmissive applications. In this work, we report exploratory measurements of
two samples of fused silica diffusers. We will present goniometric bidirectional scattering distribution function (BSDF)
measurements under normal illumination provided by the National Institute of Standards and Technology (NIST)’s
Goniometric Optical Scatter Instrument (GOSI), by NIST’s Infrared reference integrating sphere (IRIS) and by the
National Aeronautics and Space Administration (NASA)’s Diffuser Calibration Laboratory. We also present
hemispherical diffuse transmittance and reflectance measurements provided by NIST’s Double integrating sphere Optical
Scattering Instrument (DOSI). The data from the DOSI is analyzed by Prahl’s inverse adding-doubling algorithm to obtain
the absorption and reduced scattering coefficient of the samples. Implications of fused silica diffusers for remote sensing
applications are discussed.
We describe the robotic optical scatter instrument (ROSI), a new robotic arm-based goniometer for in-plane and outof- plane reflectance and bidirectional reflectance distribution function (BRDF) measurements of surfaces. The goniometer enables BRDF measurements to be made at nearly any combination of incident and scattering angles, without obstruction from frames or cradles that occur in traditional goniometers made of nested rotation stages. We present exploratory measurements of in-plane and hemispherically-scanned out-of-plane BRDF on a sintered white polytetrafluoroethylene (PTFE) sample using a supercontinuum fiber laser-based tunable light source operated at a wavelength of 550 nm, in order to demonstrate the capabilities of the system. An initial assessment of uncertainties is presented.
STARR II is a planned NIST facility for spectral measurements of specular reflectance and diffuse bidirectional
reflectance distribution function (BRDF) that is the follow-on to the current NIST STARR (Spectral Tri-function Automated Reference Reflectometer) facility. STARR II will improve and expand on the measurement capabilities of STARR, increasing spectral coverage in the short-wave infrared spectral region and adding out-of-plane BRDF measurement capability. We present characteristics of a tunable light source constructed for STARR II, which is based upon a supercontinuum fiber laser and has a tuning range from 500 nm to 2450 nm, alongside those of the current lampbased system in STARR. We then discuss considerations for measuring BRDF using this source. The STARR II goniometer will employ an articulated robotic arm to center and orient the sample, and we calculate the transformations from orientation of the sample and receiver to incident and scattering angles for measurements of in-plane and out-ofplane BRDF. We also present, for the first time, an analytic expression for the correction of measured BRDF to true BRDF due to the finite illumination area and collection aperture, and to the effects of uniformity of illumination. Finally, we present exploratory BRDF measurements on a diffuse sample made using the tunable supercontinnum source.
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.
The Optical Technology Division of the National Institute of Standards and Technology (NIST) provides reference
measurements of specular and diffuse reflectance of materials, including measurements that provide traceability for
diffuser plaques that are used as onboard calibration standards in remote sensing. We are developing new
instrumentation that will enable angle-resolved Bidirectional Reflectance Distribution Function (BRDF) measurements
using a supercontinuum fiber laser-based source and a tunable monochromator. A significant improvement in optical
power density at the specimen over that of lamp-based sources is expected. We present an overview of the source design
and evaluation, including the predicted impact of supercontinuum sources on our next generation of BRDF measurement
instrumentation.
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 Wood's 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 Wood's 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.
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.
Thermal embossing nanoimprint lithography (NIL) is an area of continuing interest because it allows direct patterning of nanoscale structures into a wide variety of functional polymer materials. Measuring the shape evolution of nanoimprinted lines during thermal annealing can provide insights into mechanisms of polymer stability and the dynamics of polymer flow. Recently, we have used optical scatterometry to extract the profile of nanoimprinted lines in low- and high-molecular mass polymer gratings during annealing of the gratings at the glass transition temperature. The data are obtained in situ using a spectroscopic ellipsometer and analyzed using a rigorous-coupled-wave scatterometry model. The results obtained from scatterometry are in very good agreement with those measured ex situ by atomic force microscopy and specular x-ray reflectivity, revealing very different decay mechanisms for gratings in low- and high-molecular mass polymers. The role of the selection of grating model in determining the uncertainties the grating line profile extracted from scatterometry is also discussed.
The extraction of nanoscale dimensions and feature geometry of grating targets using signature-based optical
techniques is an area of continued interest in semiconductor manufacturing. In the current work, we have
performed angle-resolved scatterometry measurements on grating targets of 180 nm pitch fabricated by electron
beam lithography and anisotropic wet etching of (110)-oriented silicon. The use of oriented silicon results
in grating lines with nominally vertical sidewalls, with linewidths estimated by scanning electron microscopy
(SEM) to be in the sub-50 nm range. The targets were designed to be suitable for both optical scatterometry
and small-angle x-ray scattering (SAXS) measurement. As a consequence of the lattice-plane selective etch
used for fabrication, the target trenches do not have a flat bottom, but rather have a wide vee shape. We demonstrate
extraction of linewidth, line height, and trench profile using scatterometry, with an emphasis on modeling
the trench angle, which is well decoupled from other grating parameters in the scatterometry model and
is driven by the crystalline orientation of the Si lattice planes. Issues such as the cross-correlation of grating
height and linewidth in the scatterometry model, the limits of resolution for angle-resolved scatterometry at the
wavelength used in this study (532 nm), and prospects for improving the height and linewidth resolution obtained
from scatterometry of the targets, are discussed.
The interpretation of scatterometry measurements generally assumes that the grating extends over an area
large enough to intercept all the illumination provided by an incident beam. However, in practice, the gratings
used in scatterometry are relatively small. Thus, the detected light also includes both that scattered by
the grating as well as that from a region surrounding the grating because, generally, the incident beam illuminates
both the grating and the surrounding region. To model the effects of such real structures, simulations of
the effective reflectance were performed whereby the reflection from the grating was considered to be the
sum of the diffraction by the grating and the diffraction of the surrounding region, taking into account the
beam profile. To demonstrate the model, the illumination field was assumed to be Gaussian. Results are
shown for a specific target design consisting of a 50 μm square measured by normal incidence reflectometry.
Significant errors occur when the incident profile has wings that fall outside of the profile and when the scattered
light is partially apertured.
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.
Non-imaging optical critical dimension (OCD) techniques have rapidly become a preferred method for
measuring nanoscale features in semiconductors. OCD relies upon the measurement of an optical reflectance
signature from a grating target as a function of angle, wavelength and/or polarization. By comparing the signature
with theoretical simulations, parameters of the grating lines such as critical dimension (CD) linewidth,
sidewall angle, and line height can be obtained. Although the method is sensitive and highly repeatable, there
are many issues to be addressed before OCD can be considered a traceable metrology. We report on progress
towards accurate, traceable measurement, modeling, and analysis of OCD signatures collected on the NIST
goniometric optical scatter instrument (GOSI), focusing on recent results from grating targets fabricated using
the single-crystal critical dimension reference materials (SCCDRM) process. While we demonstrate good correlation
between linewidth extracted from OCD and that measured by scanning electron microscopy (SEM),
we also find systematic deviations between the experimentally obtained optical signatures and best fit theoretical
signatures that limit our ability to determine uncertainty in OCD linewidth. We then use the SCCDRM line
profile model and a χ2 goodness-of-fit analysis on simulated signatures to demonstrate the theoretical confidence
limits for the grating line parameters in the case of normally distributed noise. This analysis shows that
for the current SCCDRM implementation, line height and oxide layer undercut are highly correlated parameters,
and that the 3-σ confidence limits in extracted linewidth depend on the target pitch. Prospects for traceable
OCD metrology will be discussed.
We have implemented back focal plane (conoscopic) imaging in an optical microscope that has also been modified to allow selection of the illumination angles and polarization at the sample, and collected back focal plane images of silicon on silicon grating scatterometry targets with varying line widths. Using a slit illumination mask, the zero-order diffraction versus angle for −60° to +60° incident angles at a given polarization was obtained from a single image. By using reference images taken on a flat silicon background, we correct the raw target images for illumination source inhomogeneities and polarization-dependent transmission of the optics, and convert them to reflectance versus angle data for s- and p-polarizations, similar to that obtained from angle-resolved grating scatterometry. As with conventional scatterometry, the target lines need not be resolved for the reflectance signature to show sensitivity to small changes in the grating parameters. For a series of 300 nm pitch targets with line widths from 150 nm to 157 nm, we demonstrate nanometer-level sensitivity to line width with good repeatability, using 546 nm illumination. Additionally, we demonstrate a technique for separating the zero order from higher order diffraction on targets with multiple diffraction orders, allowing collection of both zero and higher order diffraction versus angle from the back focal plane image. As conventional images can be easily collected on the same microscope, the method provides a powerful tool for combining imaging metrology with scatterometry for optical critical dimension measurements in semiconductors.
An overview of the challenges encountered in imaging device-sized features using optical techniques recently developed in
our laboratories is presented in this paper. We have developed a set of techniques we refer to as scatterfield microscopy
which allows us to engineer the illumination in combination with appropriately designed metrology targets. The techniques
have previously been applied to samples with sub-50 nm sized features having pitches larger than the conventional Rayleigh
resolution criterion which results 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 which uses a combination of zero order optical response and edge-based
imaging. The approach is, however, more general and a series of analyses based on theoretical methods is presented. This
analysis gives a direct measure of the ultimate size and density of features which can be measured with these techniques and
addresses what measurement resolution can be obtained. We present several experimental results, optical simulations using
different electromagnetic scattering packages, and statistical analyses to evaluate the ultimate sensitivity and extensibility of
these techniques.
A long period grating-based bend sensor featuring direct bend sensitivity along with linear, bipolar curvature response is demonstrated. The sensor can be aligned for optimum sensitivity using the slight intrinsic curvature of the eccentric core fiber.
Fiber cavity etalon (FCE) sensors have demonstrated ultrahigh static strain sensitivity (~1 nε) when they are either surface-mounted to, or embedded, in graphite reinforced resin composites. Although a significant amount of data has been acquired at very low strain, little is known about their performance and durability in typical installations. Graphite/epoxy composite test specimens were fabricated to address practical concerns and to evaluate the reliability of embedded FCE sensors. Two different specimen configurations using two different composite fabrication methods were selected for sample installations: thin flat laminates and cylindrical struts. After fabrication, the FCE sensors were interrogated to ensure that they were still intact, to record a baseline response, and to determine any changes in response that might have occurred during manufacturing. Next, to determine the survivable strain limits of the embedded sensor, the specimens were loaded in tension to a predetermined strain level, unloaded, and then interrogated. Once these limits were found, the specimens were subjected to cyclic loading and periodically interrogated until sensor failure. The results from these tests provide practical strain limits for the embedded FCE sensor and show that the response does not change as a result of tensile cyclic loading.
E. Friebele, Heather Patrick, Barbara Wright, A. Greenblatt, E. Bolden, W. Simon, Daron Giles, M. Stringfield, G. Hidalgo, Brian Catanzaro, M. Maher, Kevin Uleck, M. Fox, Anthony Vizzini
Ultrahigh sensitivity fiber cavity etalon (FCE) sensors have been embedded in graphite-reinforced polymer tubes fabricated by two different methods: resin transfer molding (RTM) and standard autoclave curing, and FCEs have been embedded in autoclave-cured unidirectional flat laminates. Significant issues encountered in embedding the sensors include protecting the fiber egress during layup, curing, and breakout, survival of the butt-coupled splice between the cavity and lead fiber during composite cure, maintaining sensor location, and sensor reliability and response. Methods were successfully devised to overcome these obstacles.
Long period fiber grating devices are finding a broad range of applications as wavelength selective filters and spectrally selective sensors. In this paper I describe the theory and fabrication of long period fiber gratings, including mask considerations for UV fabrication, and give an overview of applications. Two applications are described in detail: an embeddable point sensor for strain and temperature in a structure, and an all-fiber, coupler-free Mach-Zehnder interferometer for WDM filtering applications.
We demonstrate repeatable, directional curvature determination to ± 0.05 m-1 using two long period fiber gratings bonded to either side of a metal plate. One grating was utilized for negative, and the other for positive curvature measurements by exploiting an unusual orientation dependence of the grating response.
Multiplexed FBG strain sensors are currently deployed for the monitoring and analysis of structural loading worldwide. One remaining issue is the separation of the strain-induced wavelength shift from that induced by temperature changes.
We demonstrate a passive fiber Bragg grating sensor demodulator based on the wavelength-dependent transmission of long period grating filters. Strain resolution of the system was 1 (mu) (epsilon) for dc strain in a 3.3 Hz bandwidth. Quasi-static and dynamic operation of the system were investigated.
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