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.
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.
Polydimethylsiloxane (PDMS) is fundamental materials in the field of biotechnology. Because of its biocompatibility,
microfabricated PDMS sheets are applied to micro-reactors and microchips for cell culture. Conventionally, the microstructures
were fabricated by means of cast or imprint using molds, however it is difficult to fabricate the structures at
high aspect ratios such as through-holes/vertical channels. The fabrication of the high-aspect structures would enable us
to stack sheets to realize 3D fluidic circuits. In order to achieve the micromachining, direct photo-ablation by short
wavelength light is promising. In the previous works, we investigated ablation of transparent materials such as silica
glass and poly(methyl methacrylate) induced by irradiation with laser plasma EUV light. We achieved smooth and fine
nanomachining. In this work, we applied our technique to PDMS micromachining. We condensed the EUV light onto
PDMS surfaces at high power density up to 108 W/cm2 using a Au coated ellipsoidal mirror. We found that PDMS sheet
was ablated at a rate up to 440 nm/shot. It should be emphasized that through hole with a diameter of 1 μm was
fabricated in a PDMS sheet with a thickness of 4 μm. Thus we demonstrated the micromachining of PDMS sheets using
laser plasma EUV light.
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.
We have investigated nano-ablation of silica glass and ablation process using focused laser plasma soft Xrays.
Laser plasma soft X-rays were generated by irradiation of a Ta target with Nd:YAG laser light. The
soft X-rays were focused on silica glass plates using an ellipsoidal mirror at fluences up to 1 J/cm2. In order
to fabricate nano-trenches, a silica glass plate was irradiated with laser plasma soft X-rays through the
windows of a line and space mask. We demonstrated fabrication of nano-trenches with a width of 50 nm.
It should be noted that the feature size is more precise than that estimated from the thermal diffusion
length for the 10-ns X-rays (i.e. 80 nm). Furthermore, the ablated area has a depth of 470 nm and a
roughness of 1 nm after ten shots of irradiation. Thus, the X-ray irradiation technique have a significant
feature of direct nanomachining. The ablation occurs at fluences F beyond a ablation threshould Fth and
ablation depth per pulse D obeys the law D = 1/α ln(F/Fth), where α is an effective absorption coefficient.
These results suggest that absorbed energy is accumulated in the absorbed region without energy diffusion
until ablation occurs. In addition, time-resolved mass spectroscopy revealed that silica glass is broken
into atomic ions and atomic neutrals during ablation. Because Si+ and O+ ions have kinetic energies of
10-30 eV, non-thermal process such as Coulomb explosion may be driving force behind the ablation. Such
non-thermal process enables us to fabricate nano-structures on silica glass.
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