Nanomanufacturing Metrology Optical Patterned Defect Inspection to Extend Manufacturing Yield Rick Silver National Institute of Standards and Technology Surface and Microform Metrology Group B. BarnesTool Design and Data Acquisition H. ZhouSimulation and Analysis Y. Sohn193 nm Scatterfield Microscope R. Quintanilha193 nm Tool Fabrication R. AttotaModeling and Analysis E. MarxSimulation Development

Nanomanufacturing MetrologyOutline Defect inspection challenges Scatterfield optical imaging f undamentals Simulation techniques for intentional defect arrays Polarization, scan axis, and wavelength sensitivity Reduced dimensionality structures Future directions

Nanomanufacturing Metrology The Defect Inspection Challenge Measuring sub-40 nm sized pattern defects throughout the entire chip. –22 nm node has 11 nm defect requirements Optical imaging methods used for full wafer defect inspection and SEM used for defect review. There is a fundamental challenge in having nanometer scale resolution and wafer scale throughput. The major semiconductor chip manufacturers have tagged this as a major obstacle to continued scaling and reducing cost per transistor.

Nanomanufacturing Metrology Scatterfield Optical Configuration: Source Optimization Condenser Back focal plane of condenser lens Field Lens A B C A B C Even illumination at the object focal plane Field Diaphragm CCD Camera relay lens beamsplitter objective aperture (conjugate to BFP) scanned to select illumination angle standard fiber illuminator Band pass filter (546 nm) polarizer fiber from lamp lens Scanning or fixed aperture allows selection of incident angles at the sample. Polarization at sample can be set Source optimization and aperture designs include static and dynamic elements.

Nanomanufacturing Metrology Industry Provided Intentional Defect Array (IDA) Wafers Polysilicon gate IDA stack on the left and Metal-1 trench IDA stack to the right. Silicon Polysilicon 35nm(min) 2 nm gate ox 100nm 130nm(pitch) TEOS 65nm(min) 400 nm 150nm Defect A Center Defect D Corner Extension Defect H Corner Extension Example defect cells for a logic circuit are shown above. The defects can be caused by patterning errors, particulate contamination or materials inhomogeniety.

Nanomanufacturing Metrology Direct images for three different defects. Most current metrology tools are highly automated dark field or bright field optical microscopes. The left figure shows the difference image from the 50 nm defect and right side shows the 30 nm defect differential image Simulation-Based Image Analysis: Differential Imaging

Nanomanufacturing Metrology Differential image simulations where the axes are scanned in increments of 15 o. The images are shown for each given illumination angle for plane wave illumination. The data shown are for simulations performed near best focus. The lack of symmetry in the IDA layout and in the defect position results in asymmetric effects about the optical axis. The left side shows the horizontal scan axis and the right figure shows the vertical scan direction. Angle Resolved Simulations on the Orthogonal Axes

Nanomanufacturing Metrology Graphical comparison of scanning aperture techniques and fixed higher NA methods. The high NA illumination configuration is modeled by summing the resulting intensity images calculated at every given angle. In the center is a simulation in blue for a single line imaged at high illumination NA and in red for a very low on axis illumination NA. When the intensities at each angle are summed, they result in the “blurred” or averaged signal in blue. This is the result of valleys and hills in the profiles adding together and suppressing much of the optical image content. Physical Comparison of High NA imaging and Angle Resolved Imaging

Nanomanufacturing Metrology Image maps showing polarization sensitivity and scan axis sensitivity for the Cy 60nm defects on the left and H50 on the right at the threshold. Color images are 2D intensity maps and grey scale images are 3D rendering of the color images. Polarization Sensitivity and Scan Direction

Nanomanufacturing Metrology Figures on the left are differential images at the wavelengths as labeled. The upper figure shows optical constants, n and k, for polysilicon and teos. Wavelength Effects NIST made an important contribution to understanding the wavelength dependent effects on contrast. Early assumptions of shortest wavelength imaging being optimum were shown to be invalid.

Nanomanufacturing Metrology Wavelength Comparisons: Cx nm 266 nm 365 nm 546 nm The horizontal data are as a function of illumination angle, from –45 o to 45 o. Figure demonstrates gains at shorter wavelengths but here at 193 nm. NIST modeling expertise is leveraged in producing these data.

Nanomanufacturing Metrology Experimental Demonstration: Defect Cx 070a SEM defect size = 45 nm (Image acquired using linearly polarized light) These data were acquired at 450 nm illumination and for defects near the industry limits. Experimental demonstration of the angle resolved scatterfield imaging method. The data show the defect clearly present at some angles of illumination and not resolvable at others. This is an important demonstration of the advantages of angle resolved illumination. -30° 30° -30°

Nanomanufacturing Metrology 0° 40° (+y) Defect Cx Detectability as a Function of Design Rule The design values are shown in the top row and SEM measurements are shown in the second row. The data show the defect is detected at the 32 nm printed size with a 450 nm source. Strong defect detectability dependence on illumination angle. 40° (-x)

Nanomanufacturing Metrology ND DefectND Litho Techniques with Reduced Two-dimensional Attributes FDTD modeling of 13 nm defect types: –A, Bx, By, Cx, Cy, D, K “Unit cell” for modeling is composed of 4 x 3 logic cells, 11 with no defect (ND), one with defect

Nanomanufacturing Metrology As  increases, the ripples in the difference signal are shifting. Defects A (Center) were modeled at several oblique angles. Defect A, Low Directionality IDA designs info is IMSI property Fixed  = 90°, TE Polarization Fixed  = 0°, TE Polarization

Nanomanufacturing Metrology Defect By, High Directionality IDA designs info is IMSI property

Nanomanufacturing Metrology Methodology: –Determine defect location without noise –Simulate noise based upon mean intensity at normal incidence. –After adding noise, determine if defect is detectable and where No defect detected Noise causes false positive Initial detectionContinued detection No noiseRMS noise = 0.5% RMS noise = 1.25%RMS noise = 2.0% Introducing Noise to Simulation Analysis NIST expertise in modeling line edge roughness and sample variation has led to more realistic simulation data.

Nanomanufacturing Metrology Defect A – All Angles - Specific high angles, such as  = 25 °,  = 90 ° and  = 30 °,  = 45 ° enhance the detection of defects. Defect size is 13 nm The green boxes show illumination configurations yielding acceptable defect detection. NIST is working closely with industry metrology tool manufacturers to implement improved modeling and experimental techniques.

Nanomanufacturing Metrology Complex control and optimization of illumination and viewing conditions for specific targets or samples. Dynamically-controlled structured illumination and source optimization using apertures and spatial light modulators (polarization, amplitude, phase)

Nanomanufacturing Metrology SYSTEM OVERVIEW Air table Upper table Optics box Stages DUV fiber Align laser (visible) Stage controllers (7-Axes) CCD PC CCD Monitor Frame grabber Excimer laser Motorized stage Beam combiner 193 nm Excimer Laser Optical Metrology System Developing hardware strategies enabling sophisticated angular scanning modes at 193 nm. Source optimization using a full field modification mode is an essential tool. This is expensive to develop for individual metrology tool companies and has much in common with state of the art lithography stepper design.

Nanomanufacturing Metrology Conclusions The simulation results demonstrate a range of defect types which can be measured better using polarization control, angle resolved illumination and wavelength optimization. The scatterfield optical microscopy data and analysis compare favorably to conventional higher illumination numerical aperture imaging. Spatial frequency modulation and source optimization of the illumination fields can be tailored to enhance specific content in the reflected fields. The highly directional aspects of future lithography are well suited to modulated illumination fields. MEL is playing an important role in developing and transferring measurement technology to targeted semiconductor metrology needs. Industry funding and leadership in defining defect metrology challenges motivate a well directed, strong research effort to assist industry in maintaining device scaling goals necessary for future profitability.