Various research fields of CASTECH are listed and introduced in details.
Multi-Layer Optics

During the past two decades, tremendous efforts have been made to develop more brilliant, stronger light source in the extreme ultraviolet(EUV) ( 100 - 0.3 nm) and x-ray ( 0.3 - 0.01 nm) spectral region. The development and construction of the 3rd generation of synchrotron light facility and the development of X-ray lasers around the world are the fruits of such efforts. Nowadays many areas of science and technology benefits from such development. For the better usage and manipulation of such light sources, the high quality of optics in the spectral region must be provided. In the extreme ultraviolet ( 100 - 0.3 nm) and x-ray ( 0.3 - 0.01 nm) spectral region, the index of refraction of most of material is very close to unity. Most of materials are good absorbers of the EUV light. Hence the lenses can not be used at all. Traditionally focusing has been done by mirrors coated with high Z elements.

Mo/Si Multilayer x-ray mirror
fabricated for 18.2 nm light


However since the index of refraction is very close to unity, the reflection off a well-polished metal surface at normal incidence is also very small ( order of 10-5-10-4). The reflection optics have been used at grazing incidence angle, despite of the distortion of image quality. The improvement of image quality requires optics with high reflectivity at normal incidence.

How do we achieve this? People has adopted the idea of quarter wave plate from optics or equivalently the idea of Bragg reflection off atomic planes in a crystal. Consider an artificially-made multilayer structure as shown in the above figure where the thickness of each layer is of the order of the wavelength of light under interest (since we are dealing with EUV lights, the thicknesses of layers are of a few nanometers ). The lights are reflected off each interface, even though small, and the reflected lights interfere. But if they are in phase at a certain direction, the intensity at that particular direction will be significant (This is basically Bragg condition of X-ray scattering by atomic planes in a crystal). In fact, the intensity can be increased a few hundreds to thousand times. Hence the performance of multilayer x-ray optics is strongly dependent on the wavelength under usage, the structure of the multilayer system (layer thickness, materials used, and the number of layers, etc.) and the anlge of incidence. The multilayer x-ray optics has to be designed for the specific purpose in mind.

further readings and related works

during the past years, Dong Eon Kim's group has done the following investigations with respect to multilayer x-ray optics:

"First stage in the development of a reflection imaging x-ray microscope in schwarzchild configuration using a soft x-ray laser at 18.22 nm", Opt. Lett. 17, 157 (1992).
"Mo-Si multilayer as soft x-ray mirrors for the wavelengths around 20 nm region", J. Vac. Sci. Technol. A 12, 148 (1994).
"structural analysis of Mo-Si multilayered x-ray mirror by x-ray diffraction," J. Korean Phys. Soc. 29 (1), 74 (1996).
"Structural characterization of a Mo/Si multilayer reflector by means of x-ray diffraction measurement," J. Vac. Sci. Technol. A 154, 2291 (1997).
"Optimized structures of multilayer soft x-ray reflectors in the spectral range of 30 to 300 A," Japanese Journal of Applied Physics 37, 2728 (1998).
"Nonspecular x-ray reflectivity of partially correlated interface roughness of a Mo/Si multilayer," Phys. Rev. B 57 (15), 8786 (1998).
"Characterization of a multilayer soft x-ray reflector fabricated by pulsed laser deposition," Appl. Suf. Sci. 127-129, 531 (1998).
"transmission characteristics of multilayer structure in the soft x-ray spectral region and its application to the design of quarter-wave plates at 13 and 4.4nm," J. Vac. Sci. Tech. A. 17, 398 (1999).
"Normal-incidence pulsed-laser deposition: better method for fabrication of multilayer structures," Appl physics A 70, 235 (2000)

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