―Non-contact measurement of the shapes of optical elements used in EUV lithography systems and other equipment, with an accuracy of 2 nanometers―
Researchers) MASUDA Shusei Researcher, KONDO Yohan Senior Researcher, HORI Yasuaki Research Group Leader, HIRAI Akiko Associated with Research Group, BITOU Youichi Research Institute Deputy Director, Research Institute for Engineering Measurement, Length Standards Group
Short-wavelength light, such as extreme ultraviolet (EUV) and X-rays, is used for the microfabrication of semiconductor devices and observing nanometer-scale structures in battery materials and biomolecules. In recent years, significant attention has been given to EUV lithography systems that use this light, as well as to state-of-the-art synchrotron radiation facilities.
Since short-wavelength light is difficult to transmit and lenses cannot be used to focus or control its wavefront, large mirrors with curved surfaces, measuring several hundred millimeters in size, are used. The surface profile of the mirror has a significant effect on the machining accuracy of microstructures and the precision of observation. In particular, the surfaces of high-precision optical elements used in EUV lithography systems and synchrotron facilities must be extremely smooth, and their overall surface profile must conform to the design specifications with extremely high precision. Therefore, in the manufacturing process of optical elements, it is crucial to precisely measure the degree of deviation between the post-processing surface profile and the design surface profile, and to correct the surface profile based on those results. It is necessary to measure the absolute surface profile of curved mirrors with high precision, including geometric information such as the radius of curvature. Furthermore, since functional coatings, such as multilayer coatings, are sometimes formed on the surfaces of high-performance optical elements, non-contact measurement methods that do not damage the surface are required.
On the other hand, interferometry, a noncontact measurement method widely used in the past, involves comparisons with a reference surface or wavefront. This makes it difficult to evaluate the absolute surface profile. Therefore, there has been a strong demand for technologies that enable noncontact, ultra-high-precision measurement of the absolute surface profile of curved mirrors with accuracy on the order of a few nanometers.
Researchers at AIST have developed a new surface profile measurement device capable of measuring the absolute surface profile of curved optical elements with high precision using a non-contact method.
High-precision curved mirrors are used for light collection and wavefront control in extreme ultraviolet (EUV) lithography systems, synchrotron facilities, astronomical telescopes, gravitational wave detectors, and more. Their geometric accuracy significantly affects the performance of these devices. In the manufacturing process for such optical elements, accurately determining not only fine-scale surface topography but also the absolute surface profile, including parameters/information such as the radius of curvature, is crucial. These measurements are then used to correct the surface profile. However, it has been difficult to measure the surface profile of such curved mirrors at the level of a few nanometers without damaging the surface. We have now developed a measurement system that can accurately measure the surface slope (local angle) at each position on the curved surface over a wide angular range by analyzing the direction of reflected light. Since the local angle corresponds to changes in the surface profile, we can calculate the absolute surface profile by determining its distribution along the surface. To measure the direction of this reflected light with high precision, we incorporated a high-precision angle measurement device (SelfA) which is capable of automatically correcting errors in the angle scale. This has enabled non-contact, high-precision measurement of the absolute surface profile of curved mirrors, a task that was previously difficult, and achieved a measurement accuracy of 2 nanometers. This technology is expected to contribute to the advancement of high-performance optical systems as a foundational technology supporting the manufacturing, development, and evaluation of advanced optical components.
Journal: Precision Engineering
Title of paper: Non-contact Absolute Measurement of Curved Surface Profiles Using a Scanning Deflectometric Profiler
Authors: Shusei Masuda, Yohan Kondo, Yasuaki Hori, Akiko Hirai and Youichi Bitou
DOI: https://doi.org/10.1016/j.precisioneng.2026.05.011