The downscaling of features in the semiconductor industry has continuously placed pressure on optics-based measurement methods to yield new solutions for measuring ever-smaller devices. Such measurements are desirable as optics is unique in its combination of high throughput, sensitivity, and non-destructivity. Rigorous electromagnetic modeling has already extended the utility of optical methods such as scatterometry to the measurements of dimensions well-below the conventional diffraction limit. As tolerances decrease with feature size, greater emphasis has been placed upon reducing parametric uncertainties, which can be negatively affected by parametric correlations in the theory-to-experiment fitting process. Parametric uncertainty reductions can be realized though hybrid metrology, the proper statistical treatment of additional quantitative information.
As device feature sizes are now pushing towards the sub-5 nm domain, optics-based metrology faces a daunting new challenge. Consider that a 1 nm3 volume of crystalline silicon has just 50 atoms. Although the precise number of atoms across a 5 nm-wide line depends upon the lattice orientation, at these length scales dimensions can be expressed as few as ten atoms in width. At these near-atomic scales, quantized or atomistic effects must be considered especially with respect to the existing framework of electromagnetic scattering simulations and modeling that undergirds quantitative optical measurements. This challenge affects both conventional CMOS devices and also the variety of prospective new structures, such as “gate all around” transistors, nanowire based devices, and tunnel field effect transistors.
Accurate determination of the real and imaginary dielectric constants e_1 and e_2 prove to be key to the extensibility of Maxwell’s Equations to these low dimensional structures. Several potential solutions to aspects of this measurement challenge are found in the literature, ranging from empirical determinations assuming an effective media  to density functional theory calculations of the electronic properties and the bulk dielectric tensors . We will build upon such solutions from the literature and discuss additional alternatives such as the hybridization of multiple measurements or techniques, shorter measurement wavelengths, and enhanced hardware platforms.
Figure 1 (see attachment) shows preliminary work underway in this effort. A Tauc-Lorentz fitting of scatterometry parameters (Delta) and (Psi) yields the thickness-dependent e_1 and e_2 for our initial sample set. This work will address not just the implications for thin films (i.e. two-dimensional structures) but also outline the challenges for one-dimensional and zero-dimensional structures as well.
Figure 1. (a) Schematic of an initial sample set for experimentally demonstrating changes in the dielectric constant as a function of layer thickness. Samples were prepared using atomic layer deposition.
(b) Dielectric constants e_1, e_2 for the sample set as function of photon energy. Permittivity decreases as a function of decreasing HfO2 thickness.
 P. Ebersbach, et al., “Monitoring of ion implantation in microelectronics production environment using multi-channel reflectometry,” Proc. SPIE 9778, 977812 (2016).
 P. Pusching and C. Anbrosch-Draxl, “Atomistic Modeling of Optical Properties of Thin Films,” Adv. Eng. Mat. 8, 1151-1155 (2006).
A critical challenge in optical critical dimension metrology, that requires high measurement sensitivity as well as high throughput, is the dimensional measurements of features sized below the optical resolution limit. This paper investigates the relationships among dimensional sensitivity and key illumination beam conditions (e.g., angular illumination, partial coherence) for photomask feature characterization. Scatterfield images at the edge areas of multiple line structures on a Molybdenum Silicide (MoSi) photomask are analyzed to establish sensitivity to dimensional changes. Actinic scatterfield imaging experiments for these features are performed using the NIST 193 nm Scatterfield Microscope, designed to enable engineered illumination beams at the target. Illumination configurations that improve sensitivity are identified from imaging edges of multiple line targets having linewidths and spaces of about 1/3 wavelength.
The sizes of non-negligible defects in the patterning of a semiconductor device continue to decrease as the dimensions for
these devices are reduced. These “killer defects” disrupt the performance of the device and must be adequately controlled
during manufacturing, and new solutions are required to improve optics-based defect inspection. To this end, our group
has reported [Barnes et al., Proc. SPIE 1014516 (2017)] our initial five-wavelength simulation study, evaluating the
extensibility of defect inspection by reducing the inspection wavelength from a deep-ultraviolet wavelength to wavelengths
in the vacuum ultraviolet and the extreme ultraviolet. In that study, a 47 nm wavelength yielded enhancements in the
signal to noise (SNR) by a factor of five compared to longer wavelengths and in the differential intensities by as much as
three orders-of-magnitude compared to 13 nm. This paper briefly reviews these recent findings and investigates the
possible sources for these disparities between results at 13 nm and 47 nm wavelengths. Our in-house finite-difference
time-domain code (FDTD) is tested in both two and three dimensions to determine how computational conditions
contributed to the results. A modified geometry and materials stack is presented that offers a second viewpoint of defect
detectability as functions of wavelength, polarization, and defect type. Reapplication of the initial SNR-based defect
metric again yields no detection of a defect at λ = 13 nm, but additional image preprocessing now enables the computation
of the SNR for λ = 13 nm simulated images and has led to a revised defect metric that allows comparisons at all five
Qualitative comparisons have been made in the literature between the scattering off deep-subwavelength-sized defects and the scattering off spheres in free space to illustrate the challenges of optical defect inspection with decreasing patterning sizes. The intensity scattered by such a sphere (for diameters sized well below the wavelength) is proportional to its diameter to the sixth power, but also scales inversely to the fourth power of the wavelength. This paper addresses through simulation the potential advantages of applying shorter wavelengths for improved patterned defect inspection. Rigorous finite-difference time-domain 3-D electromagnetic modeling of the scattering from patterned defect layouts has been performed at five wavelengths which span the deep ultraviolet (193 nm), the vacuum ultraviolet (157 nm and 122 nm), and the extreme ultraviolet (47 nm and 13 nm). These patterned structures and defects are based upon publicly disclosed geometrical cross-sectional information from recent manufacturing processes, which then have been scaled down to an 8 nm Si linewidth. Simulations are performed under an assumption that these wavelengths have the same source intensity, noise sources, and optical configuration, but wavelengthdependent optical constants are considered, thus yielding a more fundamental comparison of the potential gains from wavelength scaling. To make these results more practical, future work should include simulations with more process stacks and with more materials as well as the incorporation of available source strengths, known microscope configurations, and detector quantum efficiencies. In this study, a 47 nm wavelength yielded enhancements in the signal-to-noise by a factor of five compared to longer wavelengths and in the differential intensities by as much as three orders-of-magnitude compared to 13 nm, the actinic wavelength for EUV semiconductor manufacturing.
Dimensional scaling trends will eventually bring semiconductor critical dimensions (CDs) down to only a few atoms in width. New optical techniques are required to address the measurement and variability for these CDs using sufficiently small in-die metrology targets. Recently, Qin et al. [Light Sci Appl, 5, e16038 (2016)] demonstrated quantitative modelbased measurements of finite sets of lines with features as small as 16 nm using 450 nm wavelength light. This paper uses simulation studies, augmented with experiments at 193 nm wavelength, to adapt and optimize the finite sets of features that work as in-die-capable metrology targets with minimal increases in parametric uncertainty. A finite element based solver for time-harmonic Maxwell’s equations yields two- and three-dimensional simulations of the electromagnetic scattering for optimizing the design of such targets as functions of reduced line lengths, fewer number of lines, fewer focal positions, smaller critical dimensions, and shorter illumination wavelength. Metrology targets that exceeded performance requirements are as short as 3 μm for 193 nm light, feature as few as eight lines, and are extensible to sub-10 nm CDs. Target areas measured at 193 nm can be fifteen times smaller in area than current state-of-the-art scatterometry targets described in the literature. This new methodology is demonstrated to be a promising alternative for optical model-based in-die CD metrology.
Evaluation of uncertainties is a critical element in quantitative nano-scale measurements using optical microscopy techniques. Instrument characterization underlies the quantitative evaluation of measurements of deep sub-wavelength features. The scatterfield microscopy technique, which articulates the illumination at the sample plane, is an efficient method for angle- and polarization-resolved microscope characterization to enable the uncertainty evaluations. The tool characterization results are used for the Fourier space normalization of electromagnetic simulation to permit comparisons with experimental imaging data. For this purpose the NIST 193 nm scatterfield microscope operating with an ArF Excimer laser was characterized. The illumination and collection optics were scanned angularly, utilizing a small aperture at the conjugate back focal plane of the objective lens so that the optics train was characterized with respect to angularly discrete cones of the illumination beam. Each cone beam can be approximated as a plane wave by using Köhler configuration, simplifying the analysis of the scattered light induced by the discrete illumination beam at the sample plane. Under this approximation, the illumination and entire tool function sets were measured at sample and imaging CCD planes, respectively, producing the collection tool function set numerically. We report tool imperfection effects upon these tool functions, specifically, comparing to the optical simulations of the designed optical paths varied with misalignments and aberrations that lead to changes in the tool functions. Through these comparisons, we investigate the relationship between the microscope tool imperfection factors to the deviations in the illumination as well as in the collected scattered light.
Patterning imperfections in semiconductor device fabrication may either be noncritical [e.g., line edge roughness (LER)] or critical, such as defects that impact manufacturing yield. As the sizes of the pitches and linewidths decrease in lithography, detection of the optical scattering from killer defects may be obscured by the scattering from other variations, called wafer noise. Understanding and separating these optical signals are critical to reduce false positives and overlooked defects. The effects of wafer noise on defect detection are assessed using volumetric processing on both measurements and simulations with the SEMATECH 9-nm gate intentional defect array. Increases in LER in simulation lead to decreases in signal-to-noise ratios due to wafer noise. Measurement procedures illustrate the potential uses in manufacturing while illustrating challenges to be overcome for full implementation. Highly geometry-dependent, the ratio of wafer noise to defect signal should continue to be evaluated for new process architectures and production nodes.
A scatterfield microscope for deep sub-wavelength semiconductor metrology using 193 nm light has been designed. In addition to accommodating the fixed numerical aperture and size of its commercial catadioptric objective lens, the illumination optics are formed to implement essential parameters necessary for angular illumination control at the sample plane. This angle-resolved scatterfield microscope requires access to a relatively large (> 10 mm) conjugate back focal plane as well as increased fluence from the ArF excimer laser source. The parametric optimization process yielded a telecentric conjugate back focal plane with appropriate numerical aperture and diameter by adjustment of the parameters of two interrelated lens groups.
We have previously introduced a new data analysis method that more thoroughly utilizes scattered optical intensity data collected during defect inspection using bright-field microscopy. This volumetric approach allows conversion of focus resolved 2-D collected images into 3-D volumes of intensity information and also permits the use of multi-dimensional processing and thresholding techniques to enhance defect detectability. In this paper, the effects of wafer noise upon detectability using volumetric processing are assessed with both simulations and experiments using the SEMATECH 9 nm node intentional defect array. The potential extensibility and industrial application of this technique are evaluated.
To measure the new SEMATECH 9 nm node Intentional Defect Array (IDA) and subsequent small, complex defects, a methodology has been used to exploit the rich information content generated when simulating or acquiring several images of sub-wavelength-sized defects through best focus. These images, which are xy planes, collected using polarized illumination are stacked according to focus position, z, and through interpolation, volumetric pixels (“voxels”) are formed sized approximately 40 nm per side. From the image data, an intensity can be assigned to each (x,y,z) position. These four-dimensional matrices are extensively filtered for defect detection using multi-dimensional intensity thresholding, nearest-neighbor criteria, continuity requirements, and other techniques standard to optical defect inspection. A simulation example with oblique angles of illumination is presented. Experimental results are shown from the NIST λ=193 nm Microscope using full-field illumination. Volumetric data analysis is compared against the processing of single 2-D images. Defect metrics for comparing planar and volumetric data are developed with the potential shown for a five-fold increase in defect sensitivity using volumetric data versus conventional imaging.
Smaller patterning dimensions and novel architectures are fostering research into improved methods of defect detection
in semiconductor device manufacturing. This experimental study, augmented with simulation, evaluates scatterfield
microscopy to enhance defect detectability on two separate 22 nm node intentional defect array wafers. Reducing the
illumination wavelength nominally delivers direct improvements to detectability. Precise control of the focus position is
also critical for maximizing the defect signal. Engineering of the illumination linear polarization and incident angle are
shown to optimize the detection of certain highly directional defects. Scanning electron microscopy verifies that sub
15 nm defects can be measured experimentally using 193 nm wavelength light. Techniques are discussed for taking
advantage of the complexities inherent in the scattering of highly directional defects within unidirectional patterning.
Although no one single set of parameters can be optimized to detect all defects equally, source optimization is shown to
be a realistic path towards improved sensitivity.
Rapidly decreasing critical dimensions (CD) for semiconductor devices drive the study of improved methods for the
detection of defects within patterned areas. As reduced CDs are being achieved through directional patterning,
additional constraints and opportunities present themselves in defect metrology. This simulation and experimental study
assesses potential improvements in patterned defect inspection that may be achieved by engineering the light incident to
the sample within a high-magnification imaging platform. Simulation variables include the incident angle, polarization,
and wavelength for defect types common to directional device layouts. Detectability is determined through differential
images between no-defect- and defect-containing images. Alternative metrologies such as interference microscopy are
also investigated through modeling. The measurement of a 20 nm defect is demonstrated experimentally using 193 nm
light. The complex interplay of unidirectional patterning and highly directional defects is explored using structured off-axis
illumination and polarization.
Resist-on-silicon sub-50-nm critical dimension targets have been investigated using a 193 nm angle-resolved
scatterfield microscope (ARSM). The illumination path of this microscope allows customization of the conjugate
back focal plane (CBFP) while separate collection paths permit both high-magnification and Fourier-plane
imaging. Aspects of the calibration of this microscope are presented. Full-field, Fourier-plane images are collected
as individual targets are illuminated using a field-of-view smaller than the target size; the range of incident polar
angles corresponds to the numerical aperture (NA) of the objective, NA = 0.08 to 0.74. Next, angle-resolved
scatterfield high-magnification imaging of these same targets are acquired in a conical mounting configuration
by scanning the 12 mm diameter CBFP with a 1 mm diameter aperture. The results of these measurements and
the prospects for quantitative, simultaneous measurement of multiple targets are discussed.
New techniques recently developed at the National Institute of Standards and Technology using bright-field optical tools
are applied to signal-based defect analysis of features with dimensions well below the measurement wavelength. A key
to this approach is engineering the illumination as a function of angle and analysis of the entire scattered field. In this
paper we demonstrate advantages using this approach for die-to-die defect detection metrology. This methodology,
scatterfield optical microscopy (SOM), is evaluated for defect inspection of several defect types defined by Sematech on
the Defect Metrology Advisory Group (DMAG) intentional defect array (IDA) wafers. We also report the systematic
evaluation of defect sensitivity as a function of illumination wavelength.
Theoretical simulations are reported that were carried out using a fully three-dimensional finite difference time domain
(FDTD) electromagnetic simulation package. Comprehensive modeling was completed investigating angle-resolved
illumination to enhance the detection of several defect types from the IDA wafer designs. The defect types covered a
variety of defects from the IDA designs. The simulations evaluate the SOM technique on defect sizes ranging from
those currently measurable to those the industry considers difficult to measure. The simulations evaluated both the 65
nm IDA metal-1 M1 trench and the polysilicon stack and more recent 13 nm linewidth logic cells.
The current photomask linewidth Standard Reference Material (SRM) supplied by the National Institute of
Standards and Technology (NIST), SRM 2059, is the fifth generation of such standards for mask metrology.
The calibration of this mask has been usually done using an in house NIST ultra-violet transmission microscope
and an Atomic Force Microscope (AFM). Recently, a new optical reflection scatterfield microscope has been
developed at NIST for wafer inspection, Critical Dimension (CD) and overlay metrology purposes.
Scatterfield microscopy relies on illumination engineering at a sufficiently large Conjugate Back Focal Plane
(CBFP) of the microscope.1 Our new scatterfield reflection microscope uses 193 nm excimer laser light as well as
sophisticated configurations to allow measurement of both the image plane and the Fourier plane using full-field
and angle-resolved illumination. By reducing the wavelength compared to many current metrology tools that
work in the visible light and near ultra-violet range, we have made substantial improvements in image resolution2
and commensurate gains in sensitivity to geometrical parameters.
We present a preliminary study on the use of this new microscope to calibrate and measure features of this SRM
photomask. The 193 nm scatterfield microscope is used in full-field mode with a NA range from 0.12 to 0.74
using our scatterfield imaging method. Experimental results obtained on isolated lines for different polarization
states of the illumination are presented and discussed. Pitch measurements are compared to the measurements
done on our NIST Ultra-Violet (UV) transmission microscope.
An angle-resolved scatterfield microscope (ARSM) featuring 193 nm excimer laser light was developed for measuring
critical dimension (CD) and overlay of nanoscale targets as used in semiconductor metrology. The microscope is
designed to have a wide and telecentric conjugate back focal plane (CBFP) and a scan module for resolving Köhler
illumination in the sample plane. Angular scanning of the sample plane was achieved by linearly scanning an aperture
across the 12 mm diameter CBFP, with aperture size as small as 0.4 mm for some scans. For each aperture, the sample
was illuminated over a range of angles from 12° to 48°, corresponding to a numerical aperture of 0.2 to 0.74. Angleresolved
measurement results are presented for grating targets with nominal linewidths down to 50 nm.
We have developed a set of techniques, referred to as scatterfield microscopy, in which the illumination is
engineered at a sufficiently large Conjugate Back Focal Plane (CBFP) of the microscope. A primary advance of
our new scatterfield microscope is the use of 193 nm excimer laser light. Sophisticated configurations have been
implemented to allow measurement of both the image plane and the Fourier plane using full-field and angleresolved
illumination. Here, the microscope is primarily used in an angular mode by engineering the CBFP to
enable angle-resolved scatterometer measurements with a numerical aperture (NA) range from 0.08 to 0.74.
Electromagnetic models - the Finite Element Method (FEM) and the Modal Method of Fourier Expansion
(MMFE) were used to model the experimental light scattering and evaluate the sensitivity to the geometrical
parameters and correlations.
In addition, experimental results obtained on line gratings for unpolarized illumination will be presented and
A scatterfield microscope using 193 nm laser light was developed that utilizes angle-resolved illumination for high
resolution optical metrology. An angle scan module was implemented that scans the illumination beam in angle space at
the sample by linearly scanning a fiber aperture at a conjugate back focal plane. The illumination light is delivered
directly from a source laser via an optical fiber in order to achieve homogeneous angular illumination. A unique design
element is that the conjugate back focal plane (CBFP) is telecentric allowing the optical axis of the fiber to be scanned
linearly. Initial results from full field and angle-resolved illumination are presented and potential applications in
semiconductor metrology are described.
Dependence of Köhler factor 2 (KF 2: angular homogeneity) and Köhler factor 3 (KF 3: wavefront homogeneity) on the
intensity profile of line target was investigated for an optical system designed for high-resolution optical metrology using
ArF excimer laser of a wavelength of 193 nm. The intensity profiles for the isolated and multiple lines of 60 nm
linewidth were simulated based on the diffraction propagation by introducing the changes of NA (KF 2) and aberrations
such as defocus and coma (KF 3) to the illumination beam. From the results it was demonstrated that the intensity
profiles for the line targets were influenced by the change of the illumination condition, being distorted in shape and
Accurate preparation of illumination is critical for high-resolution optical metrology applications such as linewidth and
overlay measurements. To improve the detailed evaluation and alignment of the illumination optics, we have separated
Koehler illumination into three components. The three Koehler illumination components are defined as full field spatial
intensity variation (Koehler factor 1), angular intensity homogeneity (Koehler factor 2), and wavefront phase/intensity
homogeneity (Koehler factor 3). We have also proposed a field aperture pattern transfer method to analyze the
illumination properties with respect to systematic variations, such as the shape of the source, the intensity distribution at
the back focal plane, and the displacements of elements along and off the optical axis. These factors were investigated in
both ideal and practical illumination systems. In particular, any angular asymmetry in the illumination proves to have a
detrimental effect upon the distribution of light that illuminates the target. Wavefront asymmetry is also studied in the
context of an optical system with a coherent or partially coherent light source.