In recent years, X-ray telescopes have been shrinking in both size and weight to reduce cost and volume on
space flight missions. Current designs focus on the use of MEMS technologies to fabricate ultra-lightweight and
high-resolution X-ray optics. In 2006, Ezoe et al. introduced micro-pore X-ray optics fabricated using anisotropic
wet etching of silicon (110) wafers. These optics, though extremely lightweight (completed telescope weight 1
kg or less for an effective area of 1000 cm2), had limited angular resolution, as the reflecting surfaces were flat
crystal planes. To achieve higher angular resolution, curved reflecting surfaces should be used.
Both silicon dry etching and X-ray LIGA were used to create X-ray optics with curvilinear micro-pores;
however, the resulting surface roughness of the curved micro-pore sidewalls did not meet X-ray reflection criteria
of 10 nm rms in a 10 μm2 area. This indicated the need for a precision polishing process. This paper describes
the development of an ultra-precision polishing process employing an alternating magnetic field assisted finishing
process to polish the micro-pore side walls to a mirror finish (< 4 nmrms). The processing principle is presented,
and a polishing machine is designed and fabricated to explore the feasibility of this polishing process as a possible
method for processing MEMS X-ray optics to meet X-ray reflection specifications.
We have been developing ultra light-weight X-ray optics using MEMS (Micro Electro Mechanical Systems)
technologies.We utilized crystal planes after anisotropic wet etching of silicon (110) wafers as X-ray mirrors and
succeeded in X-ray reflection and imaging. Since we can etch tiny pores in thin wafers, this type of optics can
be the lightest X-ray telescope. However, because the crystal planes are alinged in certain directions, we must
approximate ideal optical surfaces with flat planes, which limits angular resolution of the optics on the order of
arcmin. In order to overcome this issue, we propose novel X-ray optics based on a combination of five recently
developed MEMS technologies, namely silicon dry etching, X-ray LIGA, silicon hydrogen anneal, magnetic fluid
assisted polishing and hot plastic deformation of silicon. In this paper, we describe this new method and report
on our development of X-ray mirrors fabricated by these technologies and X-ray reflection experiments of two
types of MEMS X-ray mirrors made of silicon and nickel. For the first time, X-ray reflections on these mirrors
were detected in the angular response measurements. Compared to model calculations, surface roughness of the
silicon and nickel mirrors were estimated to be 5 nm and 3 nm, respectively.
In this paper, influences of Fresnel diffraction for the advance accuracy in sub-micron resolution of the PCT (Planepattern to Cross-section Transfer) technique are discussed. Some analytical simulations were performed for a prediction of X-ray intensity distribution. The X-ray mask pattern employed in this work was a set of right triangles placed in double rows facing each other which was designed by the fact that when mask slit becomes narrower while approaching the corner, the influence of the diffraction gradually becomes more significant. In X-ray lithography, especially for optical applications, it has been realized that the Fresnel diffraction is most effective factor for designing the shape of slits in submicron. The group of triangle mask patterns has 1.48 μm-pitch and 20 μm-height with 0.5 μm-thick Ta absorber. The submicron structure was successfully fabricated by PCT with a proximity gap of 300 μm. The fabricated structure exposed by 1.84kJ/cm3 X-ray dose has 190 nm in height. The analysis was summarized by comparing the PCT simulations and the data from experimental results.
The 3D structural shape-control using Synchrotron Radiation (SR) lithography for the configurations of less than a micron-size has been realized. The fabrication process will be described in details. Moreover, the structure with aspect ratio as high as 4 was achieved. The briefly introduced fabrication process is to deposit a PMMA (polymethylmethacrylate) layer to a silicon substrate by spin coating. The layer is used as the X-ray resist. Subsequently, to expose SR onto the resist through an X-ray mask, then to develop the exposed resist. The principal shape-control is accomplished by optimizing each parameter influencing the resist formation, the exposed SR dosage, and development time. All mentioned above are the parameters determined from the fabrication of an arbitrary shape which is the main purpose in this paper. The targeted evaluation of the fabricated structures is to provide the line and space of 1μm pitch, 1.9μm line-height, and aspect ratio of 4. The technique for optimization of the experimental condition and each parameter for the fabrication process will be explained in the paper. This research is expected to be useful for other related work on manufactures of sub-micron structure. The suggested applications are; a variety of optical elements such as the polarized light beam splitters, diffraction optical elements, and a number of applications in device or system which requires nanoscale structures will find this work employable. The fabrication technique of higher aspect ratio and narrower line-width will be investigated in the future research.
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