Projects Measurement Methods and Calibrations International Comparisons and Standards Optical Metrology for the Semiconductor Industry Measurement and Calibration of Nano-Structured Optical Elements
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Manufacturing Metrology Division Program Advanced Optics Metrology Program Annual FTEs: 2.0 NIST staff 3.0 guest researchers 5.0 total FTEs Challenge: Provide methods, measurement services, and standards for SI-traceable metrology of optical figure and wavefront that contribute to innovations in the application and manufacture of optical elements and precision surfaces. Overview Optical technology is an enabling technology in many areas. Communication, bio-medical imaging, defense, astronomy, nano-scale manufacturing, semiconductor manufacturing, and many other applications depend crucially on optical systems. Manufacturing these optical systems depends in turn on the ability to measure their performance, which requires traceable metrology for optical figure and wavefront. In high-impact applications such as semiconductor lithography, uncertainties are at the demanding sub-nanometer level. Few standards exist in this area. Traceability requires standards-compliant uncertainty statements (i.e., based on the GUM – Guide to Uncertainty in Measurement) that are rare in the U.S. optics industry, but increasingly mandated for ISO-certified quality systems and export. Only a small number of very specialized optical companies practice the metrology of aspheric and nano-structured optical elements. No general, widely-recognized, validated way exists to calibrate these optical elements, and the application range and uncertainty of existing methods are poorly understood. This has hindered the manufacture and adoption of these optical elements, despite their ability for game-changing product innovations. The program addresses infrastructural metrology challenges faced by U.S. industry and other federal agencies through:
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Future Directions and Plans: Our measurement capability for flatness, sphericity, radius of curvature, and wafer thickness variation is mature. Future work will focus on the uncertainty analysis and comparison of measurement techniques for aspheres and nano-structured optical elements, and will address needs of collaborators such as NASA that are currently not met by industry. Awards and Recognition
Projects Advanced Optics Metrology Program Measurement Methods and Calibrations Challenge/Problem Addressed: Flat and spherical reference surfaces are key components in the traceability chain of interferometric measurements of optical figure and wavefront. The ability to perform absolute calibrations of these surfaces with uncertainties at the nanometer-level or below powerfully enhances the ability of the optics industry to manufacture and measure high-performance optical systems. To achieve high optical performance, low system weight, and low cost, modern optical systems increasingly use aspheric surfaces. However, measuring aspheric surfaces poses formidable metrology problems because of the difficulty of obtaining a reference wavefront that closely matches the desired form of the asphere. No single, widely recognized, general, validated way exists for calibrating or measuring complex surfaces with nm-level uncertainties. Common techniques are general interferometry using “stitching” methodology or refractive or diffractive “null” optics (e.g., computer generated holograms (CGH)). The application range and uncertainties of these techniques are poorly understood. Objective Provide full-aperture measurement methods and calibration services for optical figure of flat, spherical, and aspherical precision surfaces with state-of-the-art accuracies, focusing on calibration services for flatness, sphericity, and radius of curvature of optical surfaces with an aperture up to 300 mm, and specifications for the application range and achievable uncertainties of various measurement methods, including stitching, computer generated holograms, shearing interferometry, and measurements of local curvature. Accomplishments:
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Advanced Optics Metrology Program International Comparisons and Standards Challenge/Problem Addressed: Controversies on measurement procedures, specification compliance, and traceability continue to hamper international commerce in advanced optical elements. Standards-compliant uncertainty assessments (i.e., based on the GUM – Guide to Uncertainty in Measurement) are rare in the optics industry. Companies increasingly need to adopt ISO-certified quality procedures, especially for goods that are exported. Having traceable in-house metrology with well-documented uncertainty statements is thus becoming essential to optics and precision engineering companies. Objective Ensure effective access to international markets by the U.S. optics industry through international comparisons and harmonized standards for optical figure and wavefront. Accomplishments:
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Customers and Collaborators: Comparison Participants
Advanced Optics Metrology Program Optical Metrology for the Semiconductor Industry Challenge/Problem Addressed: As exposure tools for optical lithography evolve toward larger numerical apertures, the semiconductor industry expects continued demand for improved wafer flatness at the exposure site. The allowable site flatness error for 300 mm wafers is expected to be less than 50 nm by 2010 and may be as low as 25 nm by 2015, according to the International Technology Roadmap for Semiconductors (ITRS 2006). These goals present a challenge for both wafer polishing and metrology tools. Advances in lithography are crucial to continued growth in the semiconductor industry. The introduction of extreme ultraviolet lithography (EUVL) as the manufacturing process for next-generation semiconductors has been delayed by the low lifetime of EUVL reflective optics (more than a factor of 100 below what is required). Since optimal resolution requires the figure of the optics to be within a fraction of the operational wavelength, EUVL optical components must be fabricated to sub-nanometer tolerances and retain stability over several years of exposure. The effects of EUV exposure on long-term dimensional stability and phase change characteristics of EUV-reflective surfaces are largely unknown. Objective Provide measurement methods and reference artifacts for silicon wafer thickness variation and EUVL optical elements that address requirements of the International Technology Roadmap for the Semiconductor Industry (ITRS). Accomplishments:
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Advanced Optics Metrology Program Measurement and Calibration of Nano-Structured Optical Elements Challenge/Problem Addressed: Modern micro-fabrication technology has made possible the precise fabrication of three-dimensional structures that are small compared to the wavelength of light. This capacity opens up new ways to engineer the phase and amplitude of light waves to create light waves with unique properties, leading to the emergence of a new class of optics commonly referred to as “diffractive optics” to emphasize that device function is achieved through the scattering of light by microscopic structures with dimensions comparable to the wavelength of light. Diffractive optical elements (DOEs) have properties that traditionally polished optics cannot achieve. For example, a diffractive lens can focus a light wave at several points in space simultaneously. DOEs are therefore important components in many novel optics products and they have become a vibrant field of research. They can also improve the calibration of optical elements with aspheric surfaces. The NIST NanoFabrication facility provides unique resources for the fabrication of DOEs. Objectives Provide methods and capability for traceable measurement of wavefronts using customized diffractive optical elements. Provide documented methods to calibrate wavefronts generated by nano-structured optical elements that support the manufacture and adoption of nanostructured elements by U.S. industry. Accomplishments:
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