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| Projects Calibration of aspheric optics with Computer Generated Holograms Application of nano-structured optics to radius measurements of spherical surfaces with large radii Comparison and uncertainty evaluation of generic methods for measuring aspheres Measurement of wafer flatness and wafer thickness variation International comparison of flatness measurements Development of a service for full-area calibration of optical reference surfaces Method to measure the phase transfer function of phase-shifting interferometers
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Manufacturing Metrology Division Program Metrology for Advanced Optics Program Manager: Hans Soons Annual FTEs: 3.0 NIST staff 3.5 guest researchers 6.5 total FTEs Challenge: Enable innovations in the application and manufacture of advanced optical elements by providing methods, services, and standards for SI-traceable metrology of optical figure and wavefront. Overview The program addresses infrastructural metrology needs of the U.S. optics and photonics industry for the application and manufacture of precision optical elements possessing high added-value.The optics and photonics industry is a vibrant part of the U.S. economy. In 2005, revenues amounted to $43B, or 15% of the $284B worldwide optics and photonics market. The optics and photonics market is larger than the semiconductor market and is growing faster1 . Optical and photonic technology is a driver for innovation, whose economic and scientific impacts are expected to increase in the 21st century2 ,3. Advanced optical and photonic components enable product innovations in many high-tech areas, such as communication, medical technology, information technology, defense, machine vision, consumer electronics, office automation, and astronomy. Optical and photonic technologies furthermore boost competitiveness and technological leadership in a broad range of manufacturing sectors. “Light is the tool of the future3”, as evidenced by the growth in machine vision, optical inspection, and laser systems for material processing (macro- and microscopic laser assisted machining, joining, and growing). Advances in nano-scale and semiconductor manufacturing continue to depend on advances in optical technologies, such as optical projection lithography4 . Advanced optical components incorporate new, improved, or multiple functionalities through the use of aspheric surface shapes (rotationally symmetric and freeform), extreme accuracies, special materials and coatings, adaptive technologies, smart technologies, or micro- and nanoscale surface structures. Both the manufacture and application of these advanced optical components critically depends on the ability to measure their performance. This requires metrology for optical figure and wavefront with uncertainties at the sub-nanometer level for high-impact applications such as high-end imaging and optical projection lithography4. The approach to the measurement of surface figure and wavefront is to use phase measuring interferometry. A major challenge is the lack of a generic method to measure aspheric or nano-structured optical elements with nanometer-level uncertainties. The program addresses the following current and future fundamental metrology needs:
The program concentrates on infrastructural measurement methods and services that: 1) enable innovations in a wide range of optical and photonic applications; 2) have a high impact on the U.S. metrology chain; 3) provide a generic approach to characterize and reduce measurement uncertainties; and 4) remove controversy in commerce over measurement results and specification compliance. The program has a strategic focus on techniques and protocols that allow users to make traceable measurements in their own facilities. Standards-compliant uncertainty assessments are rare in the optics industry, but increasingly required for ISO-certified quality systems and export. The program develops uncertainty statements, compliant with the Guide to the Expression of Uncertainty in Measurement (GUM)5 , for typical measurement set-ups that act as templates which industry can easily adopt. The program contributes to the ANSI/OEOSC Optics and Electro-Optics Standards Council Technical Advisory Group to ensure that standards developed by the ISO/TC172 Technical Committee on Optics and Photonics reflect the state-of-the-art. Where possible, the program seeks to eliminate the requirement of calibrated transfer artifacts through the development of absolute calibration methods. Absolute calibration methods enable the separation of instrument errors from errors of the part by exploiting the invariance of some systematic errors during reversals and shifts. This approach is well suited to phase-measuring interferometry due to the high repeatability of the measurement process. Absolute calibration techniques reduce cost while addressing the challenge of ever increasing stability requirements for reference artifacts. Our research is guided by customer requirements, standardization developments, and industry roadmaps, such as the International Technology Roadmap for Semiconductors (ITRS)4 . Research in new areas, such as nano-structured optical surfaces, is conducted in collaboration with customers working on leading-edge applications. 1 European technology platform Photonics21, “Photonics in Europe: Economic impact,” 2007. 2 U.S. National Research Council, “Harnessing Light: Optical Science and Engineering for the 21st Century,” 1998. 3 European technology platform Photonics21, “Towards a bright future for Europe: Strategic research agenda in photonics,” 2006. 4 International Technology Roadmap for Semiconductors, 2007. 5 International Organization for Standardization (ISO), Guide to the Expression of Uncertainty in Measurement, 1995 Why NIST The program promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology for precision optical elements. It does this by leveraging the NIST core competencies in measurement science, rigorous traceability, and development and use of standards. The focus of the program is on results that support enabling infrastructural metrology that is not attractive to commercial investment, yet offers significant leverage in a broad range of applications. The program has unique features that distinguish it from related research in industry and academia: 1) emphasis on development of rigorous and generic procedures to characterize measurement uncertainty that comply with international standards, 2) provision of measurement services, accessible to any U.S. company, at the top of the U.S. traceability chain, 3) emphasis on generic procedures that can be applied to a broad class of measurement challenges in optics, 4) long-term commitment, expertise, and neutrality essential for the development of harmonized and unbiased National and International standards, and 5) access to unique measurement and fabrication facilities with overlapping capabilities enabling rigorous quality control. Major facilities used by the program are:
6 International Organization for Standardization (ISO), Guide to the Expression of Uncertainty in Measurement, 1995 Measurement Services: The program provides NIST Special Tests for flatness, sphericity, and radius of curvature of optical reference surfaces with an aperture up to 300 mm. Optical flats and spheres are critical components in the traceability chain of measurements performed using phase-shifting interferometry. Phase-shifting interferometry with computer-aided data analysis is the leading method for measurement of ultra-precision surfaces and optical elements. The measurement uncertainty is to a large extent determined by the form errors of the spherical and flat reference surfaces. The provided services address the top of the respective traceability chain. Program Objectives Objective 1: Promote innovation in the manufacture and application of nano-structured optical elements by developing methods for their performance evaluation. Modern nano-fabrication technology has made possible the realization of complex three-dimensional structures with dimensions comparable to, or smaller than, the wavelength of light. This capacity opens up many new ways to engineer the phase and amplitude of light waves, leading to the emergence of the new class of nano-structured optics. Nano-structured optics have properties that polished optics cannot achieve. For example, a nano-structured lens can be designed to focus a light-wave at several points in space simultaneously. Nano-structured elements offer many opportunities for innovations in optics and photonics and have become a vibrant field of research. Nano-structured optics also help solve problems in the measurement of polished optical elements. A special class of nano-structured optics, computer generated holograms (CGHs), are used as auxiliary optics for the form measurement of aspheric surfaces. Our goals are to determine the relation between the performance of nano-structured optics and fabrication errors, develop performance metrics and performance measurement methods, and improve methods and uncertainty statements for the application of nano-structured optics to calibrate aspheric optics. Research in this area is conducted in collaboration with customers working on leading-edge applications, such as the manufacturing of aspheric segmented mirrors for X-ray telescopes. The NIST CNST NanoFab facility provides a wide range of resources that can be applied to fabricate nano-structured optical elements and characterize their structure Projects Metrology for Advanced Optics Project 1.1: Calibration of aspheric optics with Computer Generated Holograms Project Overview: The goals of this project are the development and characterization of CGHs for the calibration of aspheric optics. The project focuses on an innovative application of CGHs to measure the shape of non-focusing polished optics. An example of this type of optical surface is the mandrel used in forming segmented mirrors for X-ray telescopes. The new measurement approach addresses the shortcomings of other methods for this very challenging measurement problem. The CGHs incorporate an innovative fiducialization scheme to establish an absolute scaling of the measurement by precisely fixing the geometry independent of the part under test. This absolute geometry is critical for applications where multiple optics upstream feed into a common optical train downstream (e.g., X-ray telescopes and other segmented optical systems). The project includes development of methods to validate the wavefronts generated by the CGHs, assessment of the relation between fabrication errors and CGH performance, and development of standards-compliant measurement uncertainty statements that address the effects of CGH performance and alignment errors. Deliverables and Intermediate Milestones:
Customer and Collaborator:
Metrology for Advanced Optics Project 1.2: Application of nano-structured optics to radius measurements of spherical surfaces with large radii.
Project Overview: The goal of this project is the application of nano-structured surfaces to solve the difficult problem of measuring the form error and radius of curvature of spherical optical surfaces with a large radius of curvature. Examples of such optical surfaces are mirrors in beamlines and imaging systems, and test plates for evaluating lenses. Interferometric measurements of spherical surfaces are typically made using transmission spheres. The radius of curvature is obtained by measuring the displacement of the test artifact between the confocal and cat’s eye positions (nodal bench method). The method requires displacement of the test artifact over a length equal to the radius of curvature, and is difficult to apply to artifacts with radii exceeding a few meters. The large cavity between reference surface and artifact furthermore increases uncertainties in the measurement of the radius and sphericity. In the new method proposed by NIST, the transmission sphere is replaced with a twin-Fresnel zone plate, a nano-structured optical element. The zone plate generates beams with two different primary focal lengths, one for the confocal position and one for the cat’s eye position. The two focal lengths are chosen to enable radius of curvature measurements that only require a small displacement of the artifact, and allow for a small cavity between artifact and reference. Deliverables and Intermediate Milestones:
Metrology for Advanced Optics Program Objectives Objective 2: Promote innovation in the manufacture and application of aspheric optics through development and characterization of generic methods for asphere measurement. Aspheric surfaces are indispensable in modern optical systems through their combination of high optical performance, low system weight, and low cost. However, measuring aspheric surfaces poses formidable metrology problems because of the difficulty of obtaining a reference wave-front that closely matches the desired form of the asphere. No single, widely-recognized, general, validated way exists for measuring aspheric surfaces with nanometer-level uncertainties. This is a measurement barrier to the development, manufacture, and subsequent application of aspheric elements. Project 2.1: Comparison and uncertainty evaluation of generic methods for measuring aspheres Project Overview: Several methods have been proposed and applied to measure aspheres. The application range and achievable uncertainties of these methods are poorly understood. The goal of this project is a rigorous standards-compliant uncertainty assessment of generic methods for measuring aspheres, with a focus on zonal and annular stitching, local curvature measurement, and computer generated holograms (CGHs). The uncertainty assessment will be validated through comparison of the methods using measurement equipment at NIST and through collaboration with partners outside of NIST. Deliverables and Intermediate Milestones:
Customers and Collaborators:
Metrology for Advanced Optics Program Objectives Objective 3:Provide measurement methods and reference artifacts that address future requirements of the semiconductor industry in the areas of silicon wafer flatness and extreme ultraviolet lithography (EUVL) optics. The semiconductor industry expects continued demand for improved wafer flatness at the exposure site to avoid blurring of ever smaller circuit features due to out-of-focus exposures. This is a challenge for both wafer polishing and wafer metrology tools. We address this challenge by providing interferometric measurement methods and reference wafers with calibrated thickness and thickness variation. We also provide measurements of wafers for customers working on the improvement of wafer polishing processes (e.g., QED Technologies and MEMC Electronic Materials Inc). The introduction of EUVL for next-generation semiconductor manufacturing has been delayed by the short lifetime of the multilayer reflective optics. EUVL optical components must be fabricated to sub-nanometer tolerances and retain stability over several years of exposure. The effects of EUV radiation exposure on the stability of the reflected wave-front are largely unknown. We are pursuing a joint project with the NIST Physics Laboratory with potential funding from OMP or SEMATECH to obtain data on the long-term stability of EUVL optics Project 3.1: Measurement of wafer flatness and wafer thickness variation Project Overview The goals of this project are the development, characterization, and application of interferometric measurement methods for silicon wafer flatness and thickness variation. With the current introduction of immersion lithography, the wafer flatness requirement at the exposure site has become very stringent due to the high numerical aperture of the immersion exposure optics. Next-generation lithography methods, such as EUVL, also have very demanding flatness requirements. The International Roadmap for the Semiconductor Industry (ITRS) predicts an allowed site flatness error for 300 mm wafers of less than 45 nm by 2010 and 25 nm for 450 mm wafers by 20154. This remains a challenge for both wafer fabrication and wafer measurement. In this project we develop and apply interferometric methods to measure the flatness and thickness variation of silicon wafers. The infrared interferometer developed by NIST yields a detailed map of the thickness variation of a 300 mm wafer. Current research is focused on 1) extending the range of the instrument to enable measurements of thin wafers (250 µm), 2) reducing measurement errors due to ghost reflections and non-linearities in phase-shifting, and 3) feasibility study on the measurement of next-generation 450 mm wafers using sub-aperture stitching. Deliverables and Intermediate Milestones:
Customers and Collaborators:
Metrology for Advanced Optics Program Objectives Objective 4: Promote innovation and competitiveness in the manufacture and application of ultra-precision surfaces and optics through the development and improvement of harmonized procedures for the calibration of reference artifacts and phase measuring interferometers critical for precision optical metrology. Phase-shifting interferometry with computer-aided data analysis is the leading method for measurement of ultra-precision surfaces and optical elements. Interferometric form measurements of flat, spheric, and aspheric surfaces with uncertainties at the nm-level have become the state of the art. The measurement uncertainty is to a large extent determined by the form errors of the reference surfaces. Optical flats and spheres are used for this purpose, and are critical components in the traceability chain for measurements of optical elements. The impact of other error sources, including the phase transfer characteristics of the interferometer, must also be characterized. Project 4.1: International comparison of flatness measurements Project Overview Several National Measurement Institutes (NMIs) provide calibration services for flat reference surfaces. The approaches used to calibrate the master flats vary. NIST initiated an international comparison of flatness measurement capability of 300 mm diameter optical flats. NIST will act as the pilot laboratory, and the comparison will be executed in a star pattern. The NMIs planning to participate are: CSIRO (Australia), NPL (UK), NMIJ (Japan), KRISS (Republic of Korea), and PTB (Germany). Deliverables and Intermediate Milestones:
Collaborators:
Metrology for Advanced Optics Project 4.2: Development of a service for full-area calibration of optical reference surfaces
Project Overview The goal of this project is to bridge the gap between recent advances achieved at NIST on the full-area calibration of optical reference surfaces and the efficient delivery of an economically viable measurement service to our customers. NIST offers Special Tests for the measurement of optical reference flats based on concepts developed in the early 1970s. These tests no longer serve the needs of many customers because they do not provide flatness error data for the entire area of the flat, but only straightness data along selected diameters, and their uncertainty lags requirements of state-of-the-art applications. Over the last decade, we have developed and improved absolute methods for full-area flatness and sphericity calibration of optical surfaces up to 305 mm in diameter, yielding a significant reduction in uncertainties (typically below 2 nm rms). We also improved methods for measuring the radius of curvature of optical surfaces. These methods were developed in response to requests for full-area calibrations with low uncertainties from major organizations such as NASA, ITT, Goodrich, Zeiss, Zygo, and Stanford University (Laser Interferometer Gravitation Observatory). Full-area calibration with low uncertainties is important for the characterization of aspheric surfaces, because most methods for the characterization of such surfaces use a reference sphere or a reference plane. We are not able to respond to requests for full-area calibrations in a timely and cost-effective manner because: 1) Measurements are currently performed on XCALIBIR, a unique phase-measuring interferometer facility with a heavy research load. Performing the tests requires a reconfiguration of the experimental setup, resulting in scheduling delays. 2) Each test currently requires an absolute calibration of our reference surfaces, which is a time-consuming procedure. 3) Methods for calibration of spherical surfaces with low uncertainty are not available for parts with a large radius of curvature. 4) Our absolute full-area calibration techniques and instrumentation are limited to surfaces in a vertical orientation (horizontal optical axis). Deliverables and Intermediate Milestones:
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Metrology for Advanced Optics Project 4.3: Method to measure the phase transfer function of phase-shifting interferometers
Project Overview The goal of this project is a method to characterize the phase transfer function of phase-shifting interferometers. In characterizing the uncertainty of a phase-shifting interferometer, its phase transfer function, the dependence of the measured phase (or height) on the spatial frequency, is rarely considered. Most conventional applications of large aperture interferometers are measurements of smoothly polished lenses or mirror surfaces, for which often only relatively low spatial frequencies are of interest. However, the need for measurements of complex structures with high spatial frequency content is increasing, which requires consideration of the phase transfer characteristics of the interferometer. In this project, a mirror with a special phase relief will be fabricated using a lithography-based process. The mirror can be used to measure the phase transfer function in a fashion analogous to the measurement of the modulation transfer function using a line target. The 150 mm diameter mirror has several patterns (reminiscent of moth antennae) with variable spacing in the radial direction. The mirror will be applied to evaluate the phase transfer characteristics of several types of interferometers under various conditions (e.g., amount of defocus). Deliverables and Intermediate Milestones:
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