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Optical Surface Metrology and Nano-Structured Optics

Summary:

Aspheric surfaces are indispensable in high-performance optical systems. The ability to accurately manufacture these surfaces to the required shape depends on the ability to measure them. In this project we develop and characterize procedures that address this measurement challenge through the application of Computer Generated Holograms (CGHs). The project focuses on an innovative application of CGHs to measure the mandrels used to form mirrors for X-ray telescopes.

Description:

The project addresses metrology needs of U.S. industry and other agencies for the manufacture and application of ultra-precision surfaces and optical elements possessing high added-value. Ultra-precision surfaces and optical elements are essential to product innovations in many high-tech areas, such as semiconductor manufacturing, medical technology, defense, homeland security, consumer products, office automation, information technology, and science. Product examples range from an enormous variety of imaging systems to hard-drive platters, automotive lighting, laser beam shapers, and x-ray focusing optics. Advanced optical elements boost competitiveness and technological leadership in a broad range of manufacturing sectors. “Light is the tool of the future”, as evidenced by the growth in optical inspection, machine vision, and laser systems for welding and additive manufacturing. Advances in nano-scale and semiconductor manufacturing fundamentally rely on advances in ultra-precision surfaces and high-performance optical elements, such wafers, photomasks, and the lenses and mirrors used in optical projection lithography.

Both the manufacture and development of ultra-precision surfaces and optical elements critically depends on the ability to measure their performance. Despite advances in deterministic manufacturing techniques, only optical surfaces that can be measured can be made. This is particularly important for imaging systems performing at the limit imposed by the wave nature of light (“diffraction limit”), be they very large optical telescopes with apertures of many meters, or small cameras in mobile phones which must achieve good imaging performance in a tiny space.

Advanced optical elements incorporate features that yield vastly improved performance but pose significant measurement challenges. Examples of such features are complex surfaces, i.e., surfaces that are neither flat nor spherical, micro- and nano-scale surface structures, extreme accuracies, special materials and coatings, and adaptive technologies. The development and manufacture of these advanced features depend strongly on advances in traceable metrology for optical figure and wavefront. In high-impact applications, such as semiconductor lithography, the required form accuracies are at the (sub-) nanometer level. Traceability requires standards-compliant uncertainty statements that are often difficult to develop, but are increasingly demanded for ISO-certified quality systems and export. No general, widely-recognized, validated way exists to calibrate complex and nano-structured (optical) surfaces, and the application range and uncertainty of existing methods are poorly understood. This is a persistent measurement barrier to the widespread manufacture and adoption of these elements, despite their high potential for product innovations.

The project addresses these measurement challenges through the following objectives:

  • Development and characterization of traceable, large-scale, metrology holograms for measurement of advanced aspheric and freeform surfaces.
  • Innovative calibration methods and measurement services with sub-nanometer uncertainty for optical reference artifacts.
  • Procedures and standards for characterizing the uncertainty of interferometric inspection methods.
  • Measurement methods and reference artifacts that address future requirements of the semiconductor industry for the characterization of precision surfaces.

Major Accomplishments:

  • Established the Nanostructured Optics Laboratory at NIST for research and development of advanced diffractive and nanostructured optics and their application to the metrology of complex surfaces.
  • Developed a new method for deformation-free flatness measurements of thin optics, such as photomask blanks for extreme ultraviolet (EUV) lithography. In the method, the optic is floated on a liquid with a high specific gravity. This eliminates mounting induced deformations, and the measured flatness error results only from fabrication errors and coating stresses. At the request of SEMATECH, flatness measurements were performed on photomask blanks and substrates for EUV lithography. The measurement results are used to model photomask blank behavior on chucks.
  • Developed a new approach to the challenge of measuring the radius of curvature of surfaces with a large radius of curvature. Examples of such surfaces are mirrors in beamlines and imaging systems, and test plates for evaluating lenses. The traditional radius bench measurement method cannot be applied to these surfaces due to the required large displacement of the test artifact and the large cavity length. The new NIST approach eliminates these requirements through the application of a diffractive element, such as a twin-Fresnel zone plate, that incorporates two different focal lengths.
  • Developed, in collaboration with NASA, a method that employs a mirror with a special height relief pattern to assess the spatial height transfer function of an interferometer and its variation over the interferometer aperture. The increasing need for measurements of complex structures with high spatial frequency content requires consideration of the interferometer height transfer function, i.e., the dependence of the measured height (or phase) on the spatial frequency content of the measured surface. Analytical and experimental studies were completed that characterize the height transfer function for several interferometers and operating conditions.
  • Developed, in collaboration with NASA, a new measurement technique that uses two Computer Generated Holograms (CGHs) to measure the geometry of mandrels for the fabrication of the next generation of x-ray telescope mirrors.
Figure 1. Form error of a 1 kg silicon sphere. The form error was obtained from 138 overlapping images.
Figure 1. Form error of a 1 kg silicon sphere. The form error was obtained from 138 overlapping images.

Lead Organizational Unit:

pml

Facilities/Tools Used:

  • XCALIBIR, a multi-configuration phase-shifting interferometry system with an aperture of 300 mm for demanding measurements of form and radius of curvature. The instrument is located in a class 1000 cleanroom controlled to ± 0.02 °C.
  • Commercial phase-shifting interferometers with apertures up to 150 mm.
  • Nanostructured Optics Laboratory with a massively-parallel, maskless lithography tool for the fabrication of metrology holograms and other advanced diffractive optics on optical quality substrates.
  • The NanoFab at the NIST Center for Nanoscale Science and Technology, a world-class facility for the fabrication and characterization of micro- and nano-scale features (http://www.cnst.nist.gov/nanofab/nanofab.html)

Figure 2. Application of the NIST XCALIBIR interferometer to measure the form of a single-crystal silicon sphere with a mass of 1 kg.
Figure 2. Application of the NIST XCALIBIR interferometer to measure the form of a single-crystal silicon sphere with a mass of 1 kg.

Figure 3. Zone Plate Array Lithography (ZPAL) tool for the fabrication of diffractive optics on optical quality substrates.
Figure 3. Zone Plate Array Lithography (ZPAL) tool for the fabrication of diffractive optics on optical quality substrates.

Staff:

Ulf Griesmann, Project Co-Leader
Johannes A. Soons, Project Co-Leader
Quandou Wang

Related Programs and Projects:

  • Dimensional Measurement Services (in 683.01)
Contact

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

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

100 Bureau Drive, M/S 8220
Gaithersburg, Maryland 20899-8220