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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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