Kohei Okuda

Development of a high-speed nanoprofiler using normal vector tracing method for high-accuracy mirrors

High-precision optical elements are used in various fields. Ultraprecise aspherical mirrors that offer nanofocusing and high coherence are used to concentrate high-brightness X-rays in developing third-generation synchrotron radiation facilities. In industry, extreme ultraviolet (wavelength: 13.5 nm) lithography, which is used to fabricate semiconductor devices, uses high-accuracy aspherical mirrors for its projection optical systems. The demand for rapid progress in nanomeasurement technologies is increasing because it is difficult to realize next-generation ultraprecise mirrors with the required precision by conventional processing. The measuring method itself requires superhigh precision. We developed an innovative nanoprofiler that can directly measure the figure of high-accuracy mirrors without using a reference surface. The principle of our measuring method is to determine the normal vectors by causing the optical paths of the incident and reflected light at the measurement point to coincide; it is based on the straightness of laser light and the accuracy of rotational goniometers. From the acquired normal vectors and their coordinates, the three-dimensional shape is calculated by a reconstruction algorithm. We measured concave spherical mirrors and compared the results with those using a Fizeau interferometer. The profiles of the mirrors were consistent within the range of error in their middle portions. In addition, we evaluated the performance of an airflow control unit by measuring a concave spherical mirror. This unit suppressed the influence of environmental change, and drastically improved the repeatability.

The measurement of an aspherical mirror by three-dimensional nanoprofiler

Aspherical optical elements with high accuracy are important in several fields such as third-generation synchrotron radiation and extreme-ultraviolet lithography. Then the demand of measurement method for aspherical or free-form surface with nanometer resolution is rising. Our purpose is to develop a non-contact profiler to measure free-form surfaces directly with repeatability of figure error of less than 1 nm PV. To achieve this purpose we have developed three-dimensional Nanoprofiler which traces normal vectors of sample surface. The measurement principle is based on the straightness of LASER light and the accuracy of a rotational goniometer. This machine consists of four rotational stages, one translational stage and optical head which has the quadrant photodiode (QPD) and LASER head at optically equal position. In this measurement method, we conform the incident light beam to reflect the beam by controlling five stages and determine the normal vectors and the coordinates of the surface from signal of goniometers, translational stage and QPD. We can obtain three-dimensional figure from the normal vectors and the coordinates by a reconstruction algorithm. To evaluate performance of this machine we measure a concave aspherical mirror ten times. From ten results we calculate measurement repeatability, and we evaluate measurement uncertainty to compare the result with that measured by an interferometer. In consequence, the repeatability of measurement was 2.90 nm (σ) and the difference between the two profiles was ±20 nm. We conclude that the two profiles was correspondent considering systematic errors of each machine.

A new optical head tracing reflected light for nanoprofiler

High accuracy optical elements are applied in various fields. For example, ultraprecise aspherical mirrors are necessary for developing third-generation synchrotron radiation and XFEL (X-ray Free Electron LASER) sources. In order to make such high accuracy optical elements, it is necessary to realize the measurement of aspherical mirrors with high accuracy. But there has been no measurement method which simultaneously achieves these demands yet. So, we develop the nanoprofiler that can directly measure the any surfaces figures with high accuracy. The nanoprofiler gets the normal vector and the coordinate of a measurement point with using LASER and the QPD (Quadrant Photo Diode) as a detector. And, from the normal vectors and their coordinates, the three-dimensional figure is calculated. In order to measure the figure, the nanoprofiler controls its five motion axis numerically to make the reflected light enter to the QPD’s center. The control is based on the samples design formula. We measured a concave spherical mirror with a radius of curvature of 400 mm by the deflection method which calculates the figure error from QPD’s output, and compared the results with those using a Fizeau interferometer. The profile was consistent within the range of system error. The deflection method can’t neglect the error caused from the QPD’s spatial irregularity of sensitivity. In order to improve it, we have contrived the zero method which moves the QPD by the piezoelectric motion stage and calculates the figure error from the displacement.

Measurement of high accurate mirror using nanoprofiler with self-calibratable rotary encoder

Surface profiler using some stages requires high accurate encoder, and self-calibratable rotary encoder can detect angle error. Therefore the nanoprofiler has been developed with the encoder. The profile results have high repeatability of 0.17 nm.