New device measures curved mirrors’ absolute shape to within 2 nanometers without touching them

Researchers at AIST have developed a device that measures the absolute surface profile of curved optical elements with high precision without touching them.

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. The accuracy of their surface profiles significantly affects the performance of these devices.

During manufacturing, it is crucial to determine not only fine-scale surface topography but also the absolute surface profile, including information such as the radius of curvature. These measurements are then used to correct the surface profile. However, measuring the surface profile of curved mirrors with accuracy on the order of a few nanometers without damaging the surface has been difficult.

The research is published in the journal Precision Engineering.

Reflected light reveals surface shape

We have developed a system that measures the surface slope (local angle) at each position on a curved surface over a wide angular range by analyzing the direction of reflected light. Because the local angle corresponds to changes in the surface profile, we can calculate the absolute surface profile by determining how those angles vary along the surface.

To measure the direction of the reflected light with high precision, we incorporated a high-precision angle measurement device (SelfA) that automatically corrects errors in the angle scale.

The system enables noncontact measurement of the absolute surface profile of curved mirrors with an accuracy of 2 nanometers. This technology is expected to support the manufacturing, development and evaluation of advanced optical components for high-performance optical systems.

Short wavelengths demand exacting mirrors

Short-wavelength light, such as extreme ultraviolet (EUV) and X-rays, is used to fabricate microscopic features in semiconductor devices and observe nanometer-scale structures in battery materials and biomolecules. In recent years, EUV lithography systems and state-of-the-art synchrotron radiation facilities that use this light have received significant attention.

Because short-wavelength light is difficult to transmit and lenses cannot be used to focus it or control its wavefront, large curved mirrors measuring several hundred millimeters across are used. A mirror’s surface profile significantly affects the accuracy of microscopic features produced during manufacturing and the precision of observations.

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 match design specifications with extremely high precision.

Measuring shape without touching the surface

During manufacturing, it is crucial to measure how much the finished surface profile deviates from the design and to correct it based on those results. This requires precise measurement of the absolute surface profile of curved mirrors, including geometric information such as the radius of curvature. Because functional coatings, such as multilayer coatings, are sometimes applied to high-performance optical elements, measurement methods must not touch or damage the surface.

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. There has therefore been strong demand for technology that can measure the absolute surface profile of curved mirrors without contact, with accuracy on the order of a few nanometers.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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