Mitsunori Toyoda

Frustration of direct photoionization of Ar clusters in intense extreme ultraviolet pulses from a free electron laser

We have measured the kinetic energies of fragment ions from Ar clusters (average cluster size N~ 10?600) exposed to intense extreme ultraviolet free electron laser pulses (? ~ 61 nm, I~ 1.3? 1011 W cm?2). For small clusters (N 200), the average kinetic energy of ions strongly increases with increasing the cluster size, indicating a promotion of the multiple ionization, whereas the average kinetic energy is observed to be saturated for N 200. Considering how many photoelectrons can escape from the cluster, it was found that the size dependence of the ion kinetic energy exhibited the frustration of direct photoionization, which resulted from the strong Coulomb potential of the highly ionized cluster.

A Soft-X-Ray Imaging Microscope with a Multilayer-Coated Schwarzschild Objective: Imaging Tests

We constructed a soft-X-ray imaging microscope based on a multilayer-coated Schwarzschild objective. It provides an element-sensitive image in terms of the characteristic soft X-rays selectively reflected by the multilayer coating. The Schwarzschild objective was designed to have a 50× magnification and a numerical aperture of 0.25. The mirrors of the objective were coated with Mo/Si multilayers to reflect the Si L emission. The overall throughput of the objective was 14% at a peak wavelength of 13.3 nm. The 5-µm-wide stripe of lithographically patterned SiO2 was observed under irradiation with an electron beam of 1 µA accelerated to 2.5 kV.

At-Wavelength Extreme Ultraviolet Lithography Mask Observation Using a High-Magnification Objective with Three Multilayer Mirrors

Motivated by the need for at-wavelength observation of extreme ultraviolet (EUV) lithography masks, we developed a full-field EUV microscope that has a multilayer-mirror objective. This objective is based on an innovative optical design that gives a magnification of over ×1400, enabling us to use a conventional charge-coupled device (CCD) camera as the detector. In addition, when the objective is corrected for off-axis aberrations, it has a large field of view of a few hundred micrometers, permitting rapid inspection of a whole mask. We demonstrate this novel design by presenting at-wavelength images of a mask.

High throughput and wide field of view EUV microscope for blur-free one-shot imaging of living organisms

We present and demonstrate the use of an extreme ultraviolet (EUV) microscope that was developed in-house. Images are acquired using Bragg reflection multilayer optics and a laser-produced plasma light source. The upper-limit spatial resolution of the EUV microscope is 130 nm with a 10 ns exposure time and 250 x 250 microm(2) field of view. Resolution is superior to that of visible microscopes with the same size of field of view, and the exposure time is short enough to observe fine structures in-vivo. Observation of the cerebral cortex of a mouse is demonstrated.

Simulation Analysis of the Characteristics of a High Magnification Imaging Optics for the Observation of Extreme Ultraviolet Lithography Mask to Predict Phase Defect Printability

By employing simulation, we analyzed the characteristic of the optics of high-magnification multilayer-coated mirror employed for the examination of extreme ultraviolet lithography (EUVL) mask, and we also examined the performance of phase defect printability prediction. The imaging optics comprises Schwarzschild optics and a concave mirror; and it is modeled as an imaging means with an annular-shaped pupil. In this simulation, tilted coherent illumination that was successfully applied in an EUV microscope constructed at a beamline of the NewSUBARU, was assumed. Observation images of mask patterns affected by phase defects were simulated assuming EUVL masks representing half pitches of 16 and 11 nm generations; and those simulated results were compared with the simulated reduction-projection images on wafer formed by an exposure tool. Although the high-magnification observation optics does not completely emulate the printed pattern images on wafer it predicts the existence of phase defects and predict the value of their impacts.

Demonstrating 30-nm spatial resolution of three-multilayer-mirror objective for extreme ultraviolet microscopy: Imaging test by observing lithography mask

To confirm the high spatial resolution expected in extreme ultraviolet (EUV) microscopy, fine grating patterns with a half-pitch of less than 100 nm on a lithography mask were imaged using a full-field microscope based on a multilayer-mirror objective. When the tilted illumination technique is applied to this novel imaging system, a spatial resolution better than 20 nm can be expected at a wavelength of 13.5 nm. We demonstrated high resolution via EUV reflection images of test patterns with a half-pitch between 30 and 80 nm.

Observation of Residual-Type Thin Absorber Defect on Extreme Ultraviolet Lithography Mask Using an Extreme Ultraviolet Microscope

A local etching technique using electron beams has been proposed to repair defects on an absorber pattern. In this paper, the effect of pattern contrast over a thin absorber film left on a multilayer was examined. Various residual absorber films on the multilayer were fabricated and examined using an extreme ultraviolet (EUV) microscope employed as an at-wavelength mask pattern inspection tool. As a result, the contrast decreased by 30% in the remaining thickness of 2.9 nm for half-pitch 225 nm features. The EUV microscope proved to be an effective tool for examining the post repair effects.

Observation of phase defect on extreme ultraviolet mask using an extreme ultraviolet microscope

Abstract. Influences of phase defect structures on extreme ultraviolet (EUV) microscope images were examined. Phase defects on the bottom of a multilayer (ML) do not always propagate vertically upward to the ML’s top surface. For this study, two types of masks were prepared. One was an EUV blank with programmed phase defects made of lines in order to analyze the inclination angle of the phase defects. The other was an EUV mask that consists of programmed dot type phase defects 80 nm wide and 2.4 nm high with absorber patterns of half-pitch 88-nm lines-and-spaces. The positions of the phase defects relative to the absorber lines were designed to be shifted accordingly. Transmission electron microscope observations revealed that the line type phase defects starting from the bottom surface of the ML propagated toward the ML’s top surface, while inclined toward the center of the EUV blank. At the distances of 0 and 66 mm from the center of the EUV blank, the inclination angles varied from 0 to 4 deg. The impacts of the inclination angles on EUV microscope images were significant even though the positions of the phase defect relative to the absorber line, as measured by a scanning probe microscope, were the same.

Theoretical investigation of the meaning of odd-order aspherical surface and numerical confirmation of effectiveness in rotational-symmetric but off-axis optics

Even though odd-order aspherical surfaces have sometimes been used in optics, their meaning and effectiveness have not been discussed enough to be fully understood. However, we have already discussed and derived mathematically that odd-order aspherical surfaces cannot be represented in the form of a power series of even-order even when rotationally symmetric. We have also explained that this result does not contradict the fact that the set of Zernikes circle polynominals forms a complete system and that their rotational symmetric terms consist only of even-order terms of radial coordinates. First, we reconsider these mathematical discussions. Second, we reveal that the first- and third-order aspherical surfaces are valuable in practical lens designing for catoptoric projection optics of extreme ultraviolet lithography.

ZERO-ORDER-FREE, MULTILAYER-COATED LAMINAR GRATING IN THE 31 NM REGION

A laminar holographic grating was coated with a Mo/Si multilayer and the grating efficiency was measured between 27 and 35 nm for an incidence angle of 10°. The zero order was almost suppressed by the negative interference due to the groove profile. The maximum efficiency of first order was 4.8% at 31 nm. It was also found that the spectral wavelength provided by the grating at 10° incidence ranges from 28 nm to 32 nm.