Japan's National Institute of Advanced Industrial Science and Technology (AIST) has built an instrument that measures the absolute surface profile of curved optical elements to an accuracy of 2 nanometers without contacting the surface. For EUV lithography optics manufacturing, where multilayer-coated mirrors several hundred millimeters across must match design specifications at nanometer scale, the figure addresses a metrology gap that conventional interferometry has not closed.

The research team, led by Shusei Masuda, published its results in the journal Precision Engineering under the title "Non-contact absolute measurement of curved surface profiles using a scanning deflectometric profiler" (DOI: 10.1016/j.precisioneng.2026.05.011).

Why absolute profile matters

High-precision curved mirrors collect light and control wavefronts in extreme ultraviolet (EUV) lithography systems, synchrotron radiation facilities, astronomical telescopes and gravitational wave detectors. Their surface profile directly determines the performance of those instruments. In an EUV scanner's optics, profile errors propagate into the accuracy of the microscopic features printed on wafers.

Short-wavelength light — EUV and X-rays — barely transmits through materials, so lenses cannot focus or steer it. Instead, optical designers rely on large curved mirrors. AIST notes that surfaces of high-precision elements used in EUV lithography systems and synchrotron facilities must be extremely smooth and their overall shape must match design specifications with extreme precision.

Manufacturing requires two kinds of measurement. Fine-scale topography tells the polisher about local roughness. The absolute profile — including geometric information such as the radius of curvature — tells whether the mirror as a whole sits within tolerance. Fabricators use both to correct the figure. Measuring a curved mirror's absolute profile to a few nanometers without damaging the surface has been the hard part.

The constraint has tightened as coatings have become standard equipment. High-performance optical elements frequently carry functional coatings such as multilayer stacks — the Mo/Si multilayers of EUV mirrors being the canonical case — so any measurement method that touches the surface risks destroying an expensive, finished optic.

Deflectometry instead of interferometry

Interferometry, the noncontact method the optics industry has used for decades, compares the test surface against a reference surface or wavefront. That comparison encodes deviation from the reference, not the absolute shape. Distinguishing reference errors from surface errors becomes difficult, and extracting absolute quantities such as radius of curvature becomes unreliable at the nanometer level.

The AIST system takes a different route. It measures the surface slope — the local angle — at each position on the curved surface across a wide angular range by analyzing the direction of light reflected from the surface. Because the local angle corresponds to changes in the surface profile, the instrument can reconstruct the absolute profile by tracking how those angles vary along the surface.

The precision bottleneck in deflectometry is the angle scale itself. To measure reflected-light direction with high accuracy, the team integrated a high-precision angle measurement device called SelfA, which automatically corrects errors in the angle scale during operation. That self-correction is what pushes the reconstructed profile accuracy to 2 nanometers.

Commercial relevance

For semiconductor metrology and optics fabrication, the result lands at a useful point in the supply chain. ASML's EUV scanners and the mirror suppliers behind them — companies such as Zeiss SMT and Japanese coating and polishing specialists — depend on figure verification at intermediate manufacturing steps, before coatings and after figuring. A noncontact, absolute measurement with 2 nm accuracy gives fabricators a correction target grounded in the mirror's true geometry rather than in a comparison against another artifact that itself carries uncertainty.

The technique also reaches beyond lithography. Synchrotron and X-ray facilities, which AIST notes have drawn significant attention alongside EUV lithography in recent years, need the same class of mirror. Astronomical telescopes and gravitational wave detectors face equivalent figure requirements on large curved surfaces.

AIST positions the technology as support for manufacturing, development and evaluation of advanced optical components for high-performance optical systems. If the deflectometric profiler scales to full-size EUV mirror substrates and integrates into production lines, nanometer-accurate absolute metrology could shorten correction loops and raise yield on the most expensive optics in the semiconductor tool chain.