Shuichi Torii

Characteristics of extreme ultraviolet emission from mid-infrared laser-produced rare-earth Gd plasmas

We characterize extreme ultraviolet (EUV) emission from mid-infrared (mid-IR) laser-produced plasmas (LPPs) of the rare-earth element Gd. The energy conversion efficiency (CE) and the spectral purity in the mid-IR LPPs at λL = 10.6 μm were higher than for solid-state LPPs at λL = 1.06 μm, because the plasma produced is optically thin due to the lower critical density, resulting in a CE of 0.7%. The peak wavelength remained fixed at 6.76 nm for all laser intensities studied. Plasma parameters at a mid-IR laser intensity of 1.3×10(11) W/cm(2) was also evaluated by use of the hydrodynamic simulation code to produce the EUV emission at 6.76 nm.

Tuning extreme ultraviolet emission for optimum coupling with multilayer mirrors for future lithography through control of ionic charge states

We report on the identification of the optimum plasma conditions for a laser-produced plasma source for efficient coupling with multilayer mirrors at 6.x nm for beyond extreme ultraviolet lithography. A small shift to lower energies of the peak emission for Nd:YAG laser-produced gadolinium plasmas was observed with increasing laser power density. Charge-defined emission spectra were observed in electron beam ion trap (EBIT) studies and the charge states responsible identified by use of the flexible atomic code (FAC). The EBIT spectra displayed a larger systematic shift of the peak wavelength of intense emission at 6.x nm to longer wavelengths with increasing ionic charge. This combination of spectra enabled the key ion stage to be confirmed as Gd18+, over a range of laser power densities, with contributions from Gd17+ and Gd19+ responsible for the slight shift to longer wavelengths in the laser-plasma spectra. The FAC calculation also identified the origin of observed out-of-band emission and the charge s...

Direct etching of poly(methyl methacrylate) using laser plasma soft X-rays

We have investigated direct etching of poly(methyl methacrylate) (PMMA) using laser plasma soft X-rays. We focused soft X-rays of 6–25 nm on PMMA sheets efficiently using an Au coated ellipsoidal mirror. 7×10-2% of laser energy is estimated to be converted to soft X-rays that was incident to the PMMA sheets. We found that PMMA surfaces are etched beyond a threshold power density. The energy density of absorbed soft X-rays at the threshold is 7 kJ/cm3. The results suggest that etching is governed by decomposition into fragments.

Responses of polymers to laser plasma EUV light beyond ablation threshold and micromachining

We have investigated responses of PDMS, PMMA and acrylic block copolymers (BCP) to EUV light from laserproduced plasma beyond ablation thresholds and micromachining. We generated wide band EUV light around 100 eV by irradiation of Ta targets with Nd:YAG laser light. In addition, narrow band EUV light at 11 and 13 nm were generated by irradiation of solid Xe and Sn targets, respectively, with pulsed CO2 laser light. The generated EUV light was condensed onto samples, using an ellipsoidal mirror. The EUV light was incident through windows of contact masks on the samples. We found that through-holes with a diameter of 1 μm can be fabricated in PDMS sheets with thicknesses of 10 μm. PDMS sheets are ablated if they are irradiated with EUV light beyond a threshold power density, while PDMS surfaces were modified by irradiation with the narrow band EUV light at lower power densities. Effective ablation of PMMA sheets can be applied to a LIGA-like process for fabricating micro-structures of metals using the practical apparatus. Furthermore, BCP sheets were ablated to have micro-structures. Thus, we have developed a practical technique for microma chining of PMMA, PDMS and BCP sheets in a micrometer scale.

Synchrotron-radiation-stimulated etching of polydimethylsiloxane using XeF2 as a reaction gas

Synchrotron-radiation-stimulated etching of silicon elastomer polydimethylsiloxane using XeF2 as an etching gas is demonstrated.

Responses of organic and inorganic materials to intense EUV radiation from laser-produced plasmas

We have investigated responses of polymers to EUV radiation from laser-produced plasmas beyond ablation thresholds and micromachining. We concentrated on fabricate precise 3D micro-structures of PDMS, PMMA, acrylic block copolymers (BCP), and silica. The micromachining technique can be applied to three-dimensional micro-fluidic and bio-medical devices. The EUV processing is a promising to realize a practical micromachining technique. In the present work, we used two EUV radiation sources; (a) Wide band EUV light in a range of 10{300 eV was generated by irradiation of Ta targets with Nd:YAG laser light at 500 mJ/pulse. (b) Narrow band EUV light at 11 and 13 nm was generated by irradiation of solid Xe and Sn targets, respectively, with pulsed TEA CO2 laser light. The generated EUV light was condensed onto the materials at high power density beyond the ablation thresholds, using ellipsoidal mirrors. We found that through-holes with a diameter of one micrometer an be fabricated in PMMA and PDMS sheets with thicknesses of 4-10 micrometers, at 250 and 230 nm/shot, respectively. The effective ablation of PMMA sheets can be applied to a LIGA-like process for fabricating micro-structures of metals for micro- and nano-molds. PDMS sheets are ablated if it is irradiated with EUV light beyond a distinct threshold power density, while PDMS surfaces were modified at lower power densities. Furthermore, BCP sheets were ablated to have 1-micrometer structures. Thus, we have developed a practical technique for micromachining of PMMA, PDMS and BCP sheets in a micrometer scale.

Silica ablation process induced by focused laser plasma soft x-rays

We have investigated ablation process of silica glass induced by X-ray irradiation. X-rays around 100 eV were generated by irradiation of Ta targets with Nd:YAG laser light. The laser plasma soft X-rays have a pulse duration of 10 ns. The soft X-rays were focused on silica surfaces at up to 108 W/cm2. We found that silica glass can be ablated by X-ray irradiation. Typically, the ablated surface have a roughness of 1 nm after ablation by 500 nm in depth. Further, trenches with a width of 50 nm can be clearly fabricated on silica surface. Thus, high quality, practical micromachining can be achieved by the X-ray technique. It is remarkable that more precise features can be fabricated on silica surface than the thermal diffusion length. The results implies non-thermal ablation process. We observed ions ejected from silica surfaces during the irradiation and found that ions are almost atomic species such as Si+, O+, Si2+, O2+, SiO+. The results revealed that silica surfaces are broken into atomic species by X-ray irradiation. Among X-ray ablated species, 0.5-15 % are estimated to be ionized. Even though 0.5 % atoms are ionized in silica surface, the energy density of Coulomb repulsive force is higher than the energy density of binding energy of silica glass. Therefore, we can conclude that Coulomb repulsion between X-ray generated ions are essential for X-ray ablation of silica glass.

Commentary: Nano- and micromachining using laser plasma soft X-rays

Investigating interactions of intense soft X-rays with solid surfaces and resulting phenomena is still challenging. One could expect material removal from the surfaces (ablation) caused by soft X-ray irradiation at a precision as high as the diffraction limit of the soft X-rays. The X-ray ablation technique could enable us to fabricate nano- and micro-structures on sufaces of even transparent materials such as silica glass, polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS), which are highly valued for their use in the fields of nanometric chemical analysis and chemical reactions in medicine and biotechnology. For machining at high resolution, light with a short wavelength is preferable because of the diffraction limit. In the X-ray region, however, most of the materials have less optical absorption at shorter wavelengths, resulting in no energy transfer of the light to the materials, that is, no ablation. Therefore, photons in the range of 100 eV–1 keV are suitable for micromachining. So far, synchrotrons have been utilized for extensive studies on interaction of soft X-rays with inorganic materals such as silica glass. However, silica surfaces are etched at a low rate and the exposed surfaces are modified to Si-rich ones. In contrast, photo-etching of polymers has been realized. PMMA surfaces can be removed (etched) by X-ray irradiation without further chemical treatment. PMMA can also be etched by chemicals after X-ray irradiation, which is applied to fabricate micro-molds of PMMA and metals in the Lithographie Galvanoformung und Abformung (LIGA) process. In addition to synchrotrons radiation, soft X-rays from laserproduced plasma have been applied to PMMA ablation [1,2]. Barkusky et al. irradiated PMMA with focused laser plasma soft X-rays (LPSXs), using a Schwaltzschild objective that reflects X-rays at 13.5±1.0. Although they achieved high energy as high as 1 J/cm 2 , X-rays out of the

Micromachining of polydimethylsiloxane using EUV light

We studied the method to fabricate microstructures in PDMS sheet using laser-generated EUV light. In the high power density EUV light irradiation region, PDMS can be machined without chemical modification.

Ablation process of PMMA induced by irradiation with laser plasma EUV light

We have investigated ablation process of PMMA induced by irradiation with laser plasma EUV light. Appling the technique, micro-structured mold can be fabricated by transferring structures of the master PMMA plate.