Martin Y. Sohn

Three-dimensional deep sub-wavelength defect detection using λ = 193 nm optical microscopy

Optical microscopy is sensitive both to arrays of nanoscale features and to their imperfections. Optimizing scattered electromagnetic field intensities from deep sub-wavelength nanometer scale structures represents an important element of optical metrology. Current, well-established optical methods used to identify defects in semiconductor patterning are in jeopardy by upcoming sub-20 nm device dimensions. A novel volumetric analysis for processing focus-resolved images of defects is presented using simulated and experimental examples. This new method allows defects as narrow as (16 ± 2) nm (k = 1) to be revealed using 193 nm light with focus and illumination conditions optimized for three-dimensional data analysis. Quantitative metrics to compare two-dimensional and three-dimensional imaging indicate possible fourfold improvements in sensitivity using these methods.

Enhancing 9 nm Node Dense Patterned Defect Optical Inspection using Polarization, Angle, and Focus

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.

Enabling quantitative optical imaging for in-die-capable critical dimension targets

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.

Quantitative tool characterization of 193nm scatterfield microscope

An investigation of an optical microscope tool characterization was presented for the quantitative measurements of deep sub-wavelength features using Fourier plane normalization method. The NIST 193 nm scatterfield microscope operating with an ArF Excimer laser, which has a capability of articulating the angular incident beam at the sample plane using an aperture scanning at the conjugate back focal plane (CBFP), was characterized through the illumination and collection optical paths. Each incident cone beam at the sample plane can be approximated as a plane wave as in 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 the sample and imaging CCD planes, respectively, producing the collection tool function set numerically. The two sets of optical tool functions will be used to normalize scattering simulations in the Fourier space domain of the CCD image in the collection path. We investigated some aspects of the beam distributions of the illumination beam at the sample plane with respect to the change of the optical components and report the illumination and collection tool function distributions that were obtained by angular scanning of an aperture at conjugate back focal plane.

Effects of wafer noise on the detection of 20-nm defects using optical volumetric inspection

Abstract. 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.

Optimizing the nanoscale quantitative optical imaging of subfield scattering targets.

The full 3-D scattered field above finite sets of features has been shown to contain a continuum of spatial frequency information and, with novel optical microscopy techniques and electromagnetic modeling, deep-subwavelength geometrical parameters can be determined. Similarly, by using simulations, scattering geometries and experimental conditions can be established to tailor scattered fields that yield lower parametric uncertainties while decreasing the number of measurements and the area of such finite sets of features. Such optimized conditions are reported through quantitative optical imaging in 193 nm scatterfield microscopy using feature sets up to four times smaller in area than state-of-the-art critical dimension targets.

Optical volumetric inspection of sub-20nm patterned defects with wafer noise

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.

193 nm scatterfield microscope illumination optics

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.

Design of angle-resolved illumination optics using nonimaging bi-telecentricity for 193 nm scatterfield microscopy

Accurate optics-based dimensional measurements of features sized well-below the diffraction limit require a thorough understanding of the illumination within the optical column and of the three-dimensional scattered fields that contain the information required for quantitative metrology. Scatterfield microscopy can pair simulations with angle-resolved tool characterization to improve agreement between the experiment and calculated libraries, yielding sub-nanometer parametric uncertainties. Optimized angle-resolved illumination requires bi-telecentric optics in which a telecentric sample plane defined by a Köhler illumination configuration and a telecentric conjugate back focal plane (CBFP) of the objective lens; scanning an aperture or an aperture source at the CBFP allows control of the illumination beam angle at the sample plane with minimal distortion. A bi-telecentric illumination optics have been designed enabling angle-resolved illumination for both aperture and source scanning modes while yielding low distortion and chief ray parallelism. The optimized design features a maximum chief ray angle at the CBFP of 0.002° and maximum wavefront deviations of less than 0.06 λ for angle-resolved illumination beams at the sample plane, holding promise for high quality angle-resolved illumination for improved measurements of deep-subwavelength structures using deep-ultraviolet light.

Extensibility of optics-based metrology for sub-5nm technology (Conference Presentation)

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 [1] to density functional theory calculations of the electronic properties and the bulk dielectric tensors [2]. 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. [1] P. Ebersbach, et al., “Monitoring of ion implantation in microelectronics production environment using multi-channel reflectometry,” Proc. SPIE 9778, 977812 (2016). [2] P. Pusching and C. Anbrosch-Draxl, “Atomistic Modeling of Optical Properties of Thin Films,” Adv. Eng. Mat. 8, 1151-1155 (2006).