Metrology for Advanced Optics

From herding light into fiber optic cables to projecting computer chip designs onto silicon wafers to the newest medical microscope, mirrors and lenses are crucial to modern technology. But the tiniest flaw in a mirror or lens can render the technology useless. Manufacturing optical surfaces to the required precise shape can only be done if one can measure such flaws.  Therefore, NIST works with industry to improve measurement techniques for flat and curved optical surfaces, to develop techniques to examine novel optical surfaces that have nanoscale surface patterns, to improve international measurement standards, and to calibrate reference artifacts. The work is essential to the multibillion dollar optics and photonics industry, which is larger than the semiconductor industry and growing faster, enabling improvements in communication, medical technology, defense systems, information technology, and consumer electronics.

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.  

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 multibillion dollar U.S. optics and photonics industry.

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• Application of nano-structured optics to radius measurements of spherical surfaces with large radii: 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 interferometric radius bench method is difficult to apply to these surfaces due to the required large displacement of the test artifact and large cavity length.  In the new NIST approach, the transmission sphere used in the radius bench method 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.  We are currently conducting experiments to evaluate the accuracy and application range of the new method.
• Development and characterization of generic methods for asphere measurement: We developed and characterized an experimental system for estimating the form errors of aspheric surfaces from measurements of local curvature.  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 problems because of the difficulty of obtaining a reference wave-front that closely matches the desired form of the asphere.  The curvature-based measurement approach does not require such a reference wavefront.  In collaboration with the Argonne National Laboratory, we compared the shape of an aspheric mirror measured with the system with measurement results obtained by other methods (coordinate measuring machine (CMM), long-trace profiler, and stitching interferometry).  The results obtained with the curvature approach were comparable to those obtained with a very good CMM, but not yet sufficient for measurements of precision optical surfaces.  We are currently working on an innovative approach to local curvature measurement that holds promise to significantly increase measurement accuracy.
• Measurement of wafer flatness and wafer thickness variation: We developed and characterized an infrared interferometer (1552 nm) for measuring the thickness variation of 300 mm silicon wafers with a standard uncertainty of 5 nm.  The measurement technique was developed to address the need of the semiconductor industry for improved wafer flatness to avoid blurring of ever smaller circuit features due to out-of-focus exposures.  Improving the wafer flatness at the exposure site is a challenge for both wafer polishing and wafer metrology tools.  The new measurement system will allow for the production of reference wafers with a calibrated thickness variation that enable users and manufacturers of wafer inspection tools to evaluate and improve measurement performance. Using our measurements, a U.S. company developed a process to manufacture ultra-flat wafers that meet the projected site flatness requirements of the semiconductor industry well into the next decade.
• Calibration of reference artifacts for precision metrology: We developed and characterized methods for the absolute calibration of optical flats, taking into account variability of mounting induced deformations.  The methods were developed in response to requests from leading U.S. optics manufacturers and research institutions for full-area flat calibrations with low uncertainties.  Optical flats are critical components in the traceability chain for measurements of ultra-precision surfaces and optical elements.  We realized the capability to calibrate 300 mm flats with a standard uncertainty of 0.2 nm rms, and executed NIST special tests for customers.  We initiated an international comparison of flatness metrology and developed the test artifact.
• Method to measure the phase transfer function of phase-shifting interferometers: In collaboration with NASA, we developed, fabricated, and characterized a mirror with an innovative pattern to test the spatial phase transfer function of a phase-shifting interferometer.  Phase-shifting interferometry with computer-aided data analysis is the leading method for measurement of ultra-precision surfaces and optical elements.  In characterizing the uncertainty of the obtained measurements, the 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.  We designed and fabricated a mirror with a special phase relief pattern to measure this phase transfer function.  The 150 mm diameter mirror is currently being applied to evaluate several types of interferometers under various conditions (e.g., amount of defocus).