Yoshinori Takei

Development of a numerically controlled elastic emission machining system for fabricating mandrels of ellipsoidal focusing mirrors used in soft x-ray microscopy

Ellipsoidal mirrors are one of the most promising types of X-ray mirror, because the mirror can focus X-rays to nanometer size with a large aperture and no chromatic aberration. However, so far ideal ellipsoidal mirrors cannot be realized by any manufacturing methods. One of the reasons is there is no fabrication method to process their inside surface with a diameter of several millimeters with nanometer-level accuracy. We propose and develop a manufacturing process of the ellipsoidal mirror. First, a master which has the reversed shape is prepared using grinding, polishing and Elastic Emission Machining (EEM). EEM can finish the surface shape to within 2nm (RMS). Then, the ellipsoidal mirror is produced by replicating the surface using an electroforming deposition method. By conducting the process without any stress at room temperature, replicating the surface roughness and shape with nanometer order accuracy is possible. In this paper, we report the current status of manufacturing of the ellipsoidal mirror.

Development of high-order harmonic focusing system based on ellipsoidal mirror

We have developed a focusing system for extreme ultraviolet light produced by high-order harmonic generation. An ellipsoidal mirror with a precise surface shape was fabricated and installed into the focusing system. A rigid mirror manipulator and a beam profiler were employed to perform precise and stable mirror alignment. As a demonstration of the focusing performance, high-order harmonics in the wavelength range of 13.5-19.5 nm were successfully focused into a 2.4 × 2.3 μm(2) spot.

Effect of focusing flow on stationary spot machining properties in elastic emission machining

Ultraprecise optical elements are applied in advanced optical apparatus. Elastic emission machining (EEM) is one of the ultraprecision machining methods used to fabricate shapes with 0.1-nm accuracy. In this study, we proposed and experimentally tested the control of the shape of a stationary spot profile by introducing a focusing-flow state between the nozzle outlet and the workpiece surface in EEM. The simulation results indicate that the focusing-flow nozzle sharpens the distribution of the velocity on the workpiece surface. The results of machining experiments verified those of the simulation. The obtained stationary spot conditions will be useful for surface processing with a high spatial resolution.

Development of Surface Profile Measurement Method for Ellipsoidal X-Ray Mirrors using Phase Retrieval

An ellipsoidal mirror is a promising type of X-ray mirror, because it can focus X-rays to nanometer size with a very large aperture and no chromatic aberration. However, ideal ellipsoidal mirrors have not yet been realized by any manufacturing method. This is partly because there is no evaluation method for its surface figure profile. In this paper, we propose and develop a method for measuring surface figure profile of ellipsoidal mirrors using phase retrieval. An optical design for soft X-ray focusing, the employed phase retrieval method and an experimental optical system specialized for wavefront measurement using a He-Ne laser are reported.

100 μm-size stationary spot machining in elastic emission machining

Recently, by using various ultraprecision machining methods, optical devices having both nanometer-level figure accuracy and nanometer-level smoothness have become obtainable. However, improvement of the spatial resolution in figuring is still strongly required, particularly when the shape of optical devices is complicated. We have been developing a figure correction system using elastic emission machining (EEM). In this study, an EEM nozzle head having a minute aperture with a diameter of less than 60 µm was developed. The minute hole was prepared by micro-electrical discharge machining (EDM). A stationary spot profile with a diameter of 100 µm was experimentally obtained. Using the developed nozzle, spatial resolution better than 100 µm is expected to be realised in the EEM figuring system.

Three-dimensional shape measurement for x-ray ellipsoidal mirror

An X-ray ellipsoidal mirror requires nanometer-level shape accuracy for its internal surface. Owing to the difficulty in processing the surface, electroforming using a high precision master mandrel has been applied to mirror fabrication. In order to investigate the replication accuracy of electroforming, a measurement method for the entire internal surface of the mirror must be developed. The purpose of this study is to evaluate the shape replication accuracy of electroforming. In this study, a three-dimensional shape measurement apparatus for an X-ray ellipsoidal mirror is developed. The apparatus is composed of laser probes, a contact probe, reference flats, a z-axis stage, and a rotation table. First, longitudinal profiles of a mandrel or mirror placed vertically on the rotation table are measured at several angular positions. Subsequently, without realignment of the measured sample, circularity at every height is measured at regular intervals of 0.1 mm. During each measurement, the effect of motion errors is calculated and subtracted from each profile by referring to the distances between the probes and reference flats. Combining the circularity data with the longitudinal profiles, a three-dimensional error distribution of the entire surface is obtained. Using a mandrel with nanometer-level shape accuracy and a replicated mirror, the performance of the measurement apparatus and the replication accuracy are evaluated. Measurement repeatability of single-nanometer order and replication accuracy of sub-100-nm order are confirmed.

Development of ellipsoidal focusing mirror for soft x-ray and extreme ultraviolet light

Mirrors are key devices for creating various systems in optics. Focusing X-ray and extreme ultraviolet (EUV) light requires mirror surfaces with an extremely high accuracy. The figure of an ellipsoidal mirror is obtained by rotating an elliptical profile, and using such a mirror, soft X-ray and EUV light can be focused to dimensions on the order of nanometers without chromatic aberration. Although the theoretical performance of ellipsoidal mirrors is extremely high, the fabrication of an ideal ellipsoidal mirror remains problematic. Based on this background, we have been working to develop a fabrication system for ellipsoidal mirrors. In this proceeding, we briefly introduce the fabrication process and the soft X-ray focusing performance of the ellipsoidal mirror fabricated using the proposed process.

Fabrication of a precise ellipsoidal mirror for soft X-ray nanofocusing

In X-ray focusing, grazing incidence mirrors offer advantages of no chromatic aberration and high focusing efficiency. Although nanofocusing mirrors have been developed for the hard X-ray region, there is no mirror with nanofocusing performance in the soft X-ray region. Designing a system with the ability to focus to a beam size smaller than 100 nm at an X-ray energy of less than 1 keV requires a numerical aperture larger than 0.01. This leads to difficulties in the fabrication of a soft X-ray focusing mirror with high accuracy. Ellipsoidal mirrors enable soft X-ray focusing with a high numerical aperture. In this study, we report a production process for ellipsoidal mirrors involving mandrel fabrication and replication processes. The fabricated ellipsoidal mirror was assessed under partial illumination conditions at the soft X-ray beamline (BL25SU) of SPring-8. A focal spot size of less than 250 nm was confirmed at 300 eV. The focusing tests indicated that the proposed fabrication process is promising for X-ray mirrors that have the form of a solid of revolution, including Wolter mirrors.

Development of surface profiler for master mandrel of x-ray ellipsoidal mirror

The performance of ellipsoidal mirrors, which can be used to focus soft X-rays to nanometer spots, has not yet been optimized. Development of the surface profiler used in the fabrication process is a key step toward improving the performance of such mirrors. Because ellipsoidal mirrors have a complex geometry, our group has developed the following two-step process for their fabrication. First, a master mandrel with the inverse shape is prepared, after which the ellipsoidal mirror is fabricated by replicating the surface using an electroforming method. In this study, we develop a surface profiler for the master mandrel using multiple displacement sensors and motorized stages. One displacement sensor is used to measure the surface profile and the others are used to measure the motion errors of the stages. The longitudinal surface profiles of the mandrel could be measured with a repeatability of 1.58 nm (RMS). Based on the measured shape error profile, shape correction processing was conducted using elastic emission machining (EEM), which is an ultra-precision technique. After performing EEM three times, the shape error of the mandrel improved from 20.5 nm (RMS) to 4.2 nm (RMS).

Evaluation of figure accuracy of Wolter mirror fabricated by electroforming

The Wolter mirror is a promising imaging device for soft x-ray microscopy owing to its excellent characteristics. Its annular aperture enables high-NA design while maintaining high photon transfer efficiency. However, its deep and narrow cylinder-like shape makes its fabrication difficult. Despite its long history, the Wolter mirror has not been practically used for high-resolution microscopy. We have been developing a fabrication process for grazing incidence mirrors with rotationally symmetric shapes. The mirrors are replicated from precisely machined mandrels. We employ electroforming as a replication method with high replication accuracy and reproductivity. Here, we report the first fabrication of a Wolter mirror and discuss the replication quality in electroforming. The imaging quality of Wolter mirror is also evaluated in an observation experiment using a visible-light microscope.