Burst-mode fs-laser direct writing for full-thickness oxidation of Ta thin films

Direct fs-laser (1030nm/200fs) write of a throughout oxide Ta${2}$O${5}$ on a 200nm Ta film was achieved using a combined ps- and ns- burst mode (Burst-in-Burst or BiB) of fs-pulse exposure at a high 0.6MHz repetition rate. Few micrometers-wide lines were formed at the center of 12~$μ$m focal spot by controlled oxidation without ablation. The oxidized regions were flat and optically transparent. Wavelength-scale self-organized ripples of oxidized Ta${2}$O${5}$ sub-1~$μ$m gratings were recorded by rastering a $1\times 1$mm$^2$ area. The oxidized ripples with periodic pattern $\sim wavelength$ were aligned with the polarization of the writing beam. Energy deposition in the burst-mode oxidation is discussed by comparing 200fs and 20~ps BiB-mode writing modes. The presented strategy of self-guided oxidation with heat deposition by BiB fs-laser opens an opportunity for debris-free and annealing-free oxidation on a sub-wavelength scale.

💡 Research Summary

The paper reports a novel method for full‑thickness oxidation of a 200 nm tantalum (Ta) thin film using a femtosecond (fs) laser operating at 1030 nm wavelength and 200 fs pulse duration. The key innovation is the use of a “Burst‑in‑Burst” (BiB) exposure scheme in which a train of picosecond (ps) bursts (10 pulses, 2.5 GHz spacing) is immediately followed by a train of nanosecond (ns) bursts (10 pulses, 62.5 MHz spacing). The overall repetition rate is 0.6 MHz, delivering a cumulative energy per focal spot that is sufficient to raise the surface temperature above the oxidation threshold of Ta (~600 °C) while remaining well below the ablation threshold (~0.1 J cm⁻²).

In practice, a 12.6 µm focal spot (NA = 0.1) is scanned across the sample at speeds ranging from 10 to 50 µm s⁻¹. At the slowest speed, the accumulated heat produces a uniform, flat‑topped oxide line about 5 µm wide and 400 nm tall—exactly twice the original metal thickness—indicating complete conversion of the 200 nm Ta layer to Ta₂O₅. Faster scans reduce the line width according to the √t diffusion scaling, and at speeds above 50 µm s⁻¹ localized melting and slight ablation appear due to steeper thermal gradients.

When the BiB mode is replaced by single‑pulse exposure (either 200 fs or 20 ps pulses) and the beam is raster‑scanned over a 1 × 1 mm² area, self‑organized surface ripples (gratings) emerge with a period Λ ≈ 860 nm, essentially equal to the laser wavelength. These ripples are aligned with the linear polarization of the incident beam and are attributed to dipole scattering and surface plasmon‑mediated field enhancement. The ripple formation occurs only at low cumulative fluences (~0.3 mJ cm⁻²); higher fluences in BiB mode suppress rippling and yield a smooth, transparent oxide layer.

Energy analysis shows that a single fs pulse in BiB mode carries ~1 nJ, while the total energy delivered by one ps‑ns burst pair is ~96 nJ. The resulting dose per focal spot can reach 59 kJ cm⁻² for the slowest scan, far exceeding the energy required for oxidation but still an order of magnitude below the ablation threshold. When the per‑pulse energy is increased to ≥12 nJ, the surface temperature approaches the melting point of Ta₂O₅ (≈1870 °C), leading to a molten phase and a glossy surface finish.

The authors conclude that the BiB approach enables (i) controlled heat accumulation that drives oxidation without material removal, (ii) sub‑diffraction‑limited line writing (≈5 µm width) and wavelength‑scale self‑organized gratings, (iii) debris‑free and annealing‑free processing, and (iv) a clear delineation of the energy window separating oxidation from ablation. These capabilities open pathways for fabricating transparent oxide patterns for photonic, nonlinear optical, photocatalytic, and electronic applications without the need for post‑processing steps such as chemical etching or thermal annealing.