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Optical Methods for 3-D Nanostructure Metrology


We develop new approaches to optical microscopy and electromagnetic modeling to enable improved metrology of nanoscale structures with dimensions more than an order of magnitude below traditional resolution limits. New applications and standards produced from these methods include patterned semiconductor defect metrology, energy-related materials inspection, lithographic patterning overlay metrology, and critical dimension metrology.


Measurement speed and accuracy are both required for the effective control of high-volume manufacturing processes that incorporate billions of nanoscale objects and features. Optical microscopy (OM) is a high-throughput metrology methodology that provides a unique advantage since it is a high-bandwidth measurement method that is inherently parallel. However, OM techniques have not traditionally been considered useful for nanometrology applications because their resolution is conventionally thought to be limited by the Rayleigh limit to one half the illumination wavelength, or at best roughly 200 nm for visible or near ultra-violet illumination.

This project is a leader in the research and development of new approaches to high magnification optical microscopy that surpass these conventional limits. Quantitative measurements with sensitivity to features less than 1/20th the wavelength can be made by analysis of scattered light profiles and the use of physics-based modeling. We have developed scatterfield optical microscopy, a technique that uses engineered illumination within an optical system with conventional high numerical aperture (NA) collection optics. This illumination is tailored to the target of interest, and the combination of the structured illumination and target results in a three-dimensional interference field above the sample. The resultant three-dimensional scattered electromagnetic field is probed full using various combinations of incident linear polarization and angle-resolved scanning as well as focus-resolved scanning. With significant accomplishments already achieved, the nascent technique of Scatterfield microscopy is already being transferred to industry. Scatterfield microscopy will have a major impact to help enable the cost-effective mass-production of nanotechnology products.

Figure 2. Schematic of the 3-D electromagnetic scattering field above a periodic array.
Figure 2. Schematic of the 3-D electromagnetic scattering field above a periodic array.

Current efforts are focused on patterned semiconductor defect metrology, energy-related materials inspection, and lithographic patterning overlay and critical dimension metrology. Common to all these applications is the leveraging of ongoing development and refinement of theoretical modeling, optical column design, and experimental methods. Latest advancements in electromagnetic scattering models include in-house development of three-dimensional finite-difference time domain (FDTD) and rigorous coupled-wave analysis (RCWA) codes for performing comparisons to measurement. The custom-built NIST 193 nm Scatterfield Microscope further extends the resolution limits and enables accurate metrology of next generation of semiconductor lithography processes. Its customized, open architecture permits new experimental methodologies including experimental illumination control using structured illumination, optical field control, advanced optical train characterization, and normalization methods. A critical part of this project is the development and transfer to industry of these methods which are now realized to be essential for accurate optical microscopy.

Figure 3. Comparison between experimental and simulated differential images of a bridge defect.
Figure 3. Comparison between experimental and simulated differential images of a bridge defect.

Advances in semiconductor defect metrology are made by controlling the illumination and collection path spatial frequency and polarization. Semiconductor manufacturing productivity is directly impacted by the quality of current methods for defect inspection. We have reported the observation of defects as small as 15 nm ± 2 nm, or around λ/12 using our 193 nm Microscope. In-house FDTD simulations permit a full analysis of detectability as a function of wavelength, focus, polarization, and angle. Project success is accelerated through successful, ongoing relationships with industrial partners. Feedback from this research is used for decisions on next-generation inspection tools.

Figure 4. Constructing a model for simulating the reflectivity from fuel cell membranes.
Figure 4. Constructing a model for simulating the reflectivity from fuel cell membranes.

Research with industrial partners is being conducted to impact the inspection of energy-related materials, specifically fuel cell membranes. Research is being conducted to understand how Scatterfield optical techniques can be used to solve critical fuel cell manufacturing metrology barriers, including precious metal catalyst measurement and defect detection. Experimentally, Scatterfield angle-resolved and wavelength-resolved scans are performed on various platinum-based Catalyst Coated Membranes (CCM). Enabling optical platinum loading measurements will facilitate high volume manufacturing of these CCMs while minimizing waste of this precious metal. In addition, modeling of CCM layers is pursued to achieve quantitative theory to experiment agreement.

Figure 5. Loading a 200 mm wafer onto the world-class NIST 193 nm Microscope.
Figure 5. Loading a 200 mm wafer onto the world-class NIST 193 nm Microscope.

These methods have impacted lithographic overlay and critical dimension (CD) metrology. Sub-nanometer shifts in overlay offset, the offset between subsequent photolithographic layers, have been characterized extensively and a Standard Reference Material (SRM 5000) produced. Significant dimensional information with sensitivity to features 1/20th the measurement wavelength can be extracted from the analysis of scattered light profiles through the use of structured illumination, specifically engineered targets, and physics-based image process modeling. CD metrology of two- and three-dimensional arrayed objects is performed by combining angle-resolved optical microscopy with rigorous electromagnetic simulations. Parametric fits and uncertainties are determined by comparing simulations to experiment. The high-magnification platform allows very small areas (<1 µm2) to be measured, permitting in-die measurements of CD and overlay. The emerging field of “hybrid metrology” stemmed directly from this activity and provides added extensibility to optical measurements. Our developments in hybrid metrology and the use of Bayesian statistics are now being adopted by industry. Application of Scatterfield Microscopy has extended the resolution limits of current technology by at least a factor of ten, and there is no known barrier to the measurement of features above 5 nm.

Additional Technical Details:

Challenge/Problem Addressed: Both the semiconductor industry and the evolving nanomanufacturing sector are facing enormous challenges measuring nanometer scale features over large areas, needed for effective manufacturing process control. Both sectors require innovative high-throughput, non-contact metrology methods to minimize defects, quantify critical dimensions, and sample large areas effectively for maximum yield. This presents a tremendous challenge, to quantify features and identify defects at the nanometer scale over areas ranging from 1000 µm2 to 1 m2 in dimension.

Major Accomplishments:

  • Achieved quantitative agreement between rigorous modeling and experimental data for finite arrays of sub-20 nm lines, validating the utility of optical metrology methods for sub-wavelength features.
  • Constructed and operating a world-class 193 nm optical metrology platform.
  • Funded by SEMATECH to investigate optical methods for defect inspection of advanced low dimensionality structures.
  • Awarded two patents, for our overlay “supertarget” (US 7,772,710) and for scatterfield scanning methods (US 7,812,943).
  • Published first quantitative demonstration of new “hybrid metrology” using Bayesian approach on sub-40 nm CD lines.
  • First demonstration of superb, robust sensitivity with optical scatterfield for fuel cell MEAs.
  • Developed in-house FDTD and RCWA codes fully operational with custom flexible Fourier imaging capabilities.
  • Multiple invited presentations given at major conferences on structured illumination for defects as recently adopted by industry.
  • Implemented a new 3-D optical field measurement with full polarization-dependent angle-resolved and focus-resolved tool normalization.
Figure 1. The NIST 193 nm Microscope.
Figure 1. The NIST 193 nm Microscope.

Lead Organizational Unit:


Source of Extramural Funding:

  • U.S. Department of Energy


  • Corning Specialty Materials
  • GlobalFoundaries
  • JCMwave
  • IBM
  • Intel
  • Nanometrics
  • University of Maryland
  • Several additional partners in the U.S. Semiconductor Industry

Physical Measurement Laboratory (PML)
Semiconductor & Dimensional Metrology Division (683)

General Information:
301-975-5609 Telephone
301-869-0822 Facsimile

100 Bureau Drive, M/S 8212
Gaithersburg, Maryland 20899-8212