Metrology for Advanced Optics

From projecting computer chip designs onto silicon wafers to imaging remote galaxies, advanced optics are crucial to modern technology and science. As the demand for sophisticated optical components increases, so does the need for versatile and accurate inspection methods. NIST works with industry, universities, and government agencies to improve measurement methods for precision optical surfaces, to develop metrology for emerging nano-structured optics, to improve international measurement standards, and to calibrate optical reference artifacts. Program results contribute to innovations in the application and manufacture of advanced optical elements and precision surfaces.

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

Whether it’s for speedier Internet connections, better medical tools, or a higher-resolution satellite camera, mirrors and lenses are a crucial -- and often hidden -- component of modern life. These optical components are critical for telecommunications (fiber optics), for medicine (microscopy and endoscopy), for homeland security (surveillance and guidance systems), for manufacturing machines (laser cutting systems and camera sensors that let robots “see”), for telescopes, for semiconductor chip manufacturing, even for a quiet night at home with the DVD player, which relies on a laser and lens to read the disk. As the technologies grow more sophisticated, so do the properties of the required optics, with less tolerance for even small flaws. The Metrology for Advanced Optics program at NIST provides methods and standards for measuring the performance of these ever more sophisticated optical components.

The program tackles measurement problems for a variety of specific, intriguing optical challenges. Program researchers collaborate with NASA, for example, on measurement methods for X-ray telescopes that require hundreds of extremely thin mirrors that are nested like the layers of an onion. They also work with the semiconductor industry, helping to ensure precision in the lenses and mirrors used to project the microscopic image of a circuit pattern onto a chip. The image can become unfocused by minute unevenness in the semiconductor wafer, so the program develops optical methods to measure wafer flatness and thickness.

The depicted form error of the sphere was obtained from 138 overlapping images.

Next-generation optical components with nanoscale surface patterns have exciting properties not found in conventional optics, so new ways are needed to measure their effectiveness. Program researchers are also devising techniques to use these properties in their efforts to solve challenging measurement problems in conventional optics.

In addition to such specialized measurements, NIST continues to maintain the accuracy of optical reference artifacts needed by industry. Well-calibrated reference artifacts are critical for the manufacturing and inspection of optical elements, and manufacturers maintain their own versions. One of the program’s jobs is to calibrate these artifacts when requested. The program also develops techniques that allow manufacturers to self-check their own reference artifacts.

Since new product capabilities demand mirrors and lenses that are more accurate, smaller, and more complex, the program focuses on improving inspection techniques. For example, no general method exists to precisely measure a lens or mirror with a complex shape -- so the researchers are developing methods that enable manufacturers to still use flat or spherical reference objects to inspect optical surfaces with complex shapes.

The program also works with international organizations to ensure consistency between optics standards around the world. An American company wishing to sell a product to Australia, for example, needs to know that their inspection procedures will be accepted.

As the demand for complex, extremely accurate optics increases, so does the demand for versatile and accurate inspection methods. By developing and characterizing new measurement techniques, the Metrology for Advanced Optics program enables continuing progress and cost reduction in the multi-billion dollar U.S. optics and photonics industry.

• 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 is difficult to apply 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 twin-Fresnel zone plate, a nano-structured optical element 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 height transfer function. Analytical and experimental studies were completed that characterize the height transfer function for several interferometers and operating conditions.
• Completed, in collaboration with NASA, the design and theoretical analysis of a new measurement technique that uses two Computer Generated Holograms (CGHs) to address the challenge of measuring mandrels for the fabrication of x-ray telescope mirrors.
• Developed a toolbox for designing and analyzing phase shifting algorithms for interferometry, making it possible to measure a wider range of test parts while lowering uncertainties. For example, the toolbox made it possible to address the challenge of measuring the flatness error of optical windows with near-parallel surfaces by generating a phase-shifting algorithm that is insensitive to parasitic interference from the “back”-surface. Photomask substrates are an important example of this type of optical surface.