Atsushi Tokuoka

Fabrication of high aspect ratio X-ray grating using silicon dry etching method

We have fabricated the high aspect ratio X-ray gratings for X-ray phase imaging. Silicon dry etching technology makes it possible to fabricate rectangular structures by repeating two steps of etching process and protection process. Then, we introduce the ability of Si dry etching technology in order to fabricate un-tapered, high precision Si microstructures containing rectangular patterns for X-ray grating. Au electroforming was realized from the bottom of the Si microstructure groove using sidewall protection method. In these technologies, we succeeded in fabricating about 40 μm thick, void-free Au structures in a space as narrow as 2.6 μm in large effective area of 60 mm squares on 4 inch Si wafer. Therefore, it is expected to be used in the production of a narrow pitch and higher aspect ratio microstructures such as X-ray gratings.

Fabrication of Si Mold Using ICP Etching for X-Ray Diffraction Grating

We have fabricated X-ray diffraction gratings for X-ray phase imaging using X-ray Talbot interferometer. In this paper, we propose the new low cost fabrication process using Si mold of Si dry etching and nano-imprint techniques. Si dry etching makes it possible to fabricate high aspect ratio rectangular microstructures. Therefore, this technique is expected to fabricate high precision grating pattern. In this paper, we propose the new low cost fabrication processes using Si mold of ICP-RIE and nano-imprint techniques. And, in order to form transparence imprint mold, we used thermal oxidation of Si mold. These demonstrations of thermal oxidation are promising method for high precision transparence imprint mold with low cost, and realized low cost optical device such as diffraction gratings.

Fabrication of large area X-ray diffraction grating for X-ray phase imaging

X-ray lithography, which uses highly directional synchrotron radiation, is one of the technologies that can be used for fabricating micrometer-sized structures. In X-ray lithography, the accuracy of the fabricated structure depends largely on the accuracy of the X-ray mask. Since X-ray radiation is highly directional, a micro-fabrication technology that produces un-tapered and high aspect ratio highly absorbent structures on a low absorbent membrane is required. Conventionally, a resin material is used as the support membrane for large area X-ray masks. However, resin membranes have the disadvantage that they can sag after several cycles of X-ray exposure due to the heat generated by the X-rays. Therefore, we proposed and used thin carbon wafers for the membrane material because carbon has an extremely small thermal expansion coefficient. We fabricated new carbon membrane X-ray masks, and these results of X-ray lithography demonstrate the superior performance.

Fabrication of Au structure using direct electroplating on Si structure

Lithography is generally used when fabricating microstructures. If the substrate is conducting, it is possible to form high aspect ratio metallic microstructures by electroplating. However, it is difficult to fabricate high aspect ratio microstructures using just resist as the masking material because the high aspect ratio required is difficult to achieve with resist. On the other hand, of high aspect ratio microstructures can be fabricated using silicon dry etching technology. Then again, good quality metallic structures can only be obtained by plating up from the bottoms of the etched grooves. Because it is difficult to form a seed layer just on the bottoms of the grooves, it is difficult to fabricate metallic microstructures in high aspect ratio grooves etched using silicon dry etching technology. To solve this problem, we developed a new fabrication method in which metallic microstructures are fabricated by electroplating directly into grooves etched in the Si after the sidewalls of the grooves have been coated with an insulating film. Au electroforming was accomplished from the bottoms of grooves etched into silicon. We were able to fill the etched grooves with Au over an area of 60mm squares. The depth of the grooves, and hence the thickness of the Au was 4µm, and the pitch was 5.3µm. Moreover, fabricating Au microstructures in deeper higher aspect ratio grooves was also attempted. As a result, 20 µm deep grooves were filled to a depth of 18µm with Au. It is expected that this technology can be used in the production of a wide variety of devices.

Fabrication of Carbon Membrane X-Ray Mask for X-Ray Lithography

X-ray radiographic imaging techniques have been applied in many fields. Previously we proposed a method for X-ray phase imaging using X-ray Talbot interferometry which requires the use of X-ray gratings. In this work, we fabricated the X-ray gratings needed for X-ray Talbot interferometry using an X-ray lithography technique. For X-ray lithography the accuracy of the fabricated structure depends largely on the accuracy of the X-ray mask. Conventionally a resin material is used for the support membrane for large area X-ray masks. However, resin membranes have the disadvantage that they can sag after several cycles of X-ray exposure due to the heat generated by the X-rays. For our new proposal we used thin carbon wafers for the membrane material because carbon has an extremely small thermal expansion coefficient. This new type of X-ray mask is very easy to process, and it is expected that it will lead to more precise X-ray masks. We fabricated carbon membrane X-ray masks on 6 inch wafers with a 1:1 line-to-space ratio and a pitch of 5.3 μm, covering a large effective area of 100 × 100 mm2 .Copyright