Science1 publisher2 min readPublished
AIST gauges curved mirrors' absolute shape to 2 nanometres without touching them
AIST researchers built a no-contact profiler that measures the absolute shape of curved mirrors to within 2 nanometres from reflected-light angles. Makers of EUV and synchrotron mirrors have found that measurement hard to take at a few nanometres without risking the surface.
The Scientist · Science desk
What happened
- Interferometry, the usual non-contact method, compares a mirror against a reference surface or wavefront, so it struggles to deliver the absolute profile.
- An angle-measurement device called SelfA, which automatically corrects errors in its own angle scale, reads the direction of the reflected light.
- EUV and X-ray light cannot be focused by lenses, so the systems that use it rely on curved mirrors several hundred millimetres across.
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Why it matters
- capability Polishers could check a finished mirror's departure from its design, radius of curvature included, without first making a reference optic to compare it against.
- decision Makers of multilayer-coated optics could move absolute-shape checks to after coating, the stage at which a contact probe is ruled out.
- constraint Across a 300 mm mirror, 2 nm is about 7 parts per billion of its width, so the figure cannot be assumed to hold on production-sized parts until tests at that scale are reported.
The design starts from a limit of the older tool. An interferometer measures a surface against a reference surface or wavefront, so what it reports is a comparison, and getting the absolute profile out of that is hard [5]. A slope measurement needs no reference. The AIST profiler reads the local angle of the mirror at each position from the direction of the reflected light, across a wide range of angles. The absolute profile then follows from how those angles vary along the surface [3]. The paper calls the instrument a scanning deflectometric profiler [12].
Every point of the profile is computed from angles, so the angle measurement sets the accuracy. To read the reflected light's direction precisely, the team built in SelfA, an angle-measurement device that automatically corrects errors in its own angle scale [4]. With it, AIST reports non-contact measurement of the absolute profile of curved mirrors to 2 nanometres [2].
The users are the instruments that cannot use lenses. Short-wavelength light such as EUV and X-rays is hard to transmit and cannot be focused by lenses, so these systems steer it with curved mirrors several hundred millimetres across [8]. The same kind of precision mirror is used in EUV lithography, synchrotron facilities, astronomical telescopes and gravitational-wave detectors [9]. Their makers measure how far a finished surface departs from the design, radius of curvature included, and then correct it [6]. Some of these optics carry multilayer coatings, and a measurement that touches the surface could damage them [7].
The thing the 2 nm figure doesn't tell you is the size of mirror it was measured on. Take 300 mm, the low end of "several hundred millimetres". Holding 2 nm across that width is about 7 parts per billion [1]. The release does not give the size or curvature of the test mirrors, the scan time, or the independent measurement the 2 nm result was checked against. A production line would need all of those before it replaced an interferometer in its correction loop.
I think the method meets a need the field has stated plainly. According to the AIST account, measuring curved mirrors to a few nanometres without damaging them has been difficult, and demand for such a tool has been strong [10]. The evidence so far supports a working instrument. AIST's claim for manufacturing is a forecast: the account says the technology "is expected to support the manufacturing, development and evaluation of advanced optical components for high-performance optical systems" [11].
What to watch
- Whether the Precision Engineering paper reports the test mirrors' size and curvature, and what independent measurement the 2 nm result was verified against.
- Whether an EUV lithography or synchrotron mirror maker adopts the profiler in a polishing-correction loop, and how its scan time compares with interferometry.