Norio Toyosugi

Estimation of soft X-ray and EUV transition radiation power emitted from the MIRRORCLE-type tabletop synchrotron

The tabletop synchrotron light sources MIRRORCLE-6X and MIRRORCLE-20SX, operating at electron energies E(el) = 6 MeV and E(el) = 20 MeV, respectively, can emit powerful transition radiation (TR) in the extreme ultraviolet (EUV) and the soft X-ray regions. To clarify the applicability of these soft X-ray and EUV sources, the total TR power has been determined. A TR experiment was performed using a 385 nm-thick Al foil target in MIRRORCLE-6X. The angular distribution of the emitted power was measured using a detector assembly based on an NE102 scintillator, an optical bundle and a photomultiplier. The maximal measured total TR power for MIRRORCLE-6X is P(max) approximately equal 2.95 mW at full power operation. Introduction of an analytical expression for the lifetime of the electron beam allows calculation of the emitted TR power by a tabletop synchrotron light source. Using the above measurement result, and the theoretically determined ratio between the TR power for MIRRORCLE-6X and MIRRORCLE-20SX, the total TR power for MIRRORCLE-20SX can be obtained. The one-foil TR target thickness is optimized for the 20 MeV electron energy. P(max) approximately equal 810 mW for MIRRORCLE-20SX is obtained with a single foil of 240 nm-thick Be target. The emitted bremsstrahlung is negligible with respect to the emitted TR for optimized TR targets. From a theoretically known TR spectrum it is concluded that MIRRORCLE-20SX can emit 150 mW of photons with E > 500 eV, which makes it applicable as a source for performing X-ray lithography. The average wavelength, \overline\lambda = 13.6 nm, of the TR emission of MIRRORCLE-20SX, with a 200 nm Al target, could provide of the order of 1 W EUV.

Theory and characteristics of transition radiation emitted by low-energy storage-ring synchrotrons for use in X-ray lithography

Existing theory is developed further for description of transition radiation (TR) emitted by low-energy storage-ring synchrotrons. It takes into account the fact that the dielectric constant of the TR target material is a complex function, introduces an expression for the number of passes of an injected electron through the target, and accounts more precisely for the absorption of TR. It is shown that the consideration of the complexity of the dielectric constant results in notable changes of the TR spectrum for emitted photons with energies close to the ionization energies of the target material. Since such TR is used mostly for performing X-ray lithography (XRL), the sensitivity of the photoresist used in XRL is formulated. Maximization of this resist sensitivity can be used for designing optimum targets for XRL. Study of the transmission of TR through a commonly used XRL mask, and its partial absorption in a common photoresist, illustrates that TR emission with E = [490, 1860] eV is most useful for performing such XRL, while E approximately equal to 1 keV is best. It is shown that, for a particular target material, a target consisting of only one foil emits the most TR energy. Optimization of an Al target, based on maximization of the resist sensitivity, indicates that a target containing one Al foil with a thickness of about 200 nm would be best for performing XRL by our low-energy storage-ring synchrotron MIRRORCLE-20SX.

Source for extreme ultraviolet lithography by the tabletop storage ring MIRRORCLE

Advances of electron storage rings to beam currents of above 1 A and tabletop sizes make possible the development of a synchrotron-based source for EUV lithography (EUVL) at ~13.5-nm wavelength. The MIRRORCLE storage rings can provide on average 3-A electron beam current, 1-min lifetime, 15-ms radiation damping, and beam size ~3*3 mm2. MIRRORCLE-20SX, MIRRORCLE-6X, and MIRRORCLE-CV4 store electrons with energies of 20 MeV, 6 MeV, and 4 MeV, respectively. These machines can emit EUV from a tiny target, hit by the circulating beam, via transition radiation or diffusive radiation. Using a multilayer microelectromechanical system (MEMS) target allows enhancement and spectral purification of the emitted EUV. Aligning many such MEMS along the electron beam orbit and radiation collection by only one quasi-elliptical EUV mirror can provide EUV satisfying the joint requirements for an EUVL source.

Novel Edge‐Enhanced X‐ray Imaging by MIRRORCLE

Brilliant hard x‐rays are produced by the portable synchrotron named MIRRORCLE. The x‐ray images taken by MIRRORCLE show the enhanced edge effect even with contact imaging. Bodies composed of light elements are distinguished in spite of its dominated hard x‐ray components. The imaging mechanism involved, which is different from the phase contrast, is studied in this paper. It is found that the edge effect is partly due to the micron order x‐ray source spot size, and MeV region hard x‐rays.

The Portable Synchrotron MIRRORCLE‐6X

The construction of the tabletop synchrotron MIRRORCLE‐6X is completed. The electron beam current extracted from the 6‐MeV Microtron injector is reached 100 mA with 500 ns pulse duration and 400 Hz repetition rate. The measured main magnetic field distribution of the synchrotron showed good agreement with calculation. We found that the 6‐MeV electron beam can be injected from the designed injection port to the synchrotron with 20 mm⋅mrad acceptance according to the computer simulation using measured magnetic fields.

Development of the Portable Synchrotron MIRRORCLE‐CV for High Precision Non‐Destructive Testing

We started the development of the portable synchrotron MIRRORCLE‐CV series, which provides a high quality x‐ray beam for high precision non‐destructive testing (NDT). Computer simulations for the magnetic field design and electron dynamics reveal that the outer diameter of the synchrotron magnet can be as small as 30 cm. This synchrotron size approaches that of a conventional x‐ray tube.

Application of a Theory for Generation of Soft X-Ray by Storage Rings and Its Use For X-Ray Lithography

A theory has been developed for generation of soft X‐ray transition radiation (TR) by storage ring synchrotrons. It takes into consideration that the dielectric constant of the TR target material is a complex number, utilizes an explicit expression for the number of passes of an injected electron through the target, and describes more precisely the absorption of TR in the target. Such TR can be used for performing X‐ray lithography (XRL), and therefore a formula is included for the sensitivity of the photoresist used in XRL. TR targets for XRL can be optimized, based on finding a maximum of the resist sensitivity. Application of this theory to optimization of Mg target shows that a target containing only one Mg foil, with a thickness of about 245 nm is the best Mg target, for performing XRL by our storage ring synchrotron MIRRORCLE‐20SX.

Progress In Design And Preparation Of Targets For Lithography Sources Based on MIRRORCLE‐20SX

The power of transition radiation (TR), emitted from our storage ring synchrotron (SRS) MIRRORCLE‐20SX, for performing X‐ray lithography (XRL), and EUV lithography (EUVL) is calculated. Results are presented for a variety of elemental one‐foil TR emitting targets, as well as for multi‐foil non‐coherent TR targets, and multi‐foil coherent TR targets. One foil targets with optimum thickness for XRL were prepared by Al, collodion, and C foils.

EUV and soft X‐ray transition radiation from MIRRORCLE‐type tabletop synchrotrons

MIRRORCLE‐20SX tabletop synchrotron is under development for EUV lithography. We measured the soft X ray power from the existing synchrotron MIRRORCLE‐6X (6MeV machine). Transition radiation power from one thin foil target was detected by a plastic scintillator, which is calibrated by using conventional synchrotron radiation source at AIST. The output EUV power was 2.6 mW. From the experimental result of MIRRORCLE‐6X we can extrapolate the EUV power for the 20MeV machine as large as one watt per foil. We expect 100 watt EUV by 100 of foils.

Development of low energy and high brilliance x‐ray source using a portable synchrotron MIRRORCLE

We have developed the computer code “RTR1.0” for intensity calculations of the Resonance Transition Radiation produced by high‐energy electrons crossing the multi‐layer‐films target. By using the program “RTR1.0”, we have designed the optimum structure of the multi‐layer‐films target for portable electron synchrotrons “MIRRORCLE”.