Francois Goasmat

Fourier domain optical tool normalization for quantitative parametric image reconstruction

There has been much recent work in developing advanced optical metrology methods that use imaging optics for critical dimension measurements and defect detection. Sensitivity to nanometer-scale changes has been observed when measuring critical dimensions of subwavelength 20 nm features or when imaging defects below 15 nm using angle-resolved and focus-resolved optical data. However, these methods inherently involve complex imaging optics and analysis of complicated three-dimensional electromagnetic fields. This paper develops a new approach to enable the rigorous analysis of three-dimensional, through-focus, or angle-resolved optical images. We use rigorous electromagnetic simulation with enhanced Fourier optical techniques, an approach to optical tool normalization, and statistical methods to evaluate sensitivities and uncertainties in the measurement of subwavelength three-dimensional structures.

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.

Phase sensitive parametric optical metrology: exploring the limits of three-dimensional optical metrology

There has been much recent work in developing advanced optical metrology applications that use imaging optics for critical dimension measurements, defect detection and for potential use with in-die metrology. Sensitivity to nanometer scale changes has been observed when measuring critical dimensions of sub-wavelength features or when imaging defects below 20 nm using angle-resolved and focus-resolved optical data. However, these methods inherently involve complex imaging optics and analysis of complicated three-dimensional electromagnetic fields. This paper will develop a new approach to enable the rigorous analysis of three-dimensional through-focus optical images. We use rigorous electromagnetic simulation tools and statistical methods to evaluate sensitivities and uncertainties in the measurement of three dimensional layouts encountered in critical dimension, contour metrology and defect inspection.

Scatterfield microscopy of 22-nm node patterned defects using visible and DUV light

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.

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.

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.

Multiple-order Imaging for Optical Critical Dimension Metrology using Microscope Characterization

There has been much recent work in developing advanced optical metrology applications that use imaging optics for optical critical dimension (OCD) measurements, defect detection, and for potential use with in-die metrology applications. We have previously reported quantitative measurements for sub-50 nm CD dense arrays which scatter only the 0th-order specular diffraction component using angle-resolved scatterfield microscopy. Through angle-resolved and focus-resolved imaging, we now measure OCD targets with three-dimensional scattered fields that contain multiple Fourier frequencies. Experimental sensitivity to nanometer scale linewidth changes is presented, supported by simulation studies. A new, more advanced approach to tool normalization is coupled with rigorous electromagnetic simulations and library based regression fitting that potentially enables OCD measurements with sub-nanometer uncertainties for targets that scatter multiple Fourier frequencies.