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

TL;DR

This study demonstrates direct-write oxidation of a 200 nm Ta film to Ta2O5 using burst-in-burst femtosecond laser exposure at a high repetition rate, achieving both sub-diffraction through-thickness oxidation and self-organized sub-wavelength ripple patterning without ablation. The authors combine ps- and ns-bursts to sustain surface heating and drive oxidation while avoiding debris, enabling oxide lines with widths near the focal spot and wavelength-scale ripples aligned to the writing polarization. Through AFM, SEM, optical imaging, and FDTD modelling, they show two regimes: (i) sub-diffraction line oxidation across the film and (ii) large-area ripple formation up to 1×1 mm^2, with ripple periods Λ around 800–860 nm. Energy considerations indicate oxidation is energetically favorable under BiB-mode conditions compared with ablation, highlighting the potential for annealing-free, debris-free nanoscale oxide patterning with applications in photonics and surface chemistry.

Abstract

Direct fs-laser (1030~nm/200~fs) write of a throughout oxide Ta$_{2}$O$_{5}$ on a 200~nm Ta film was achieved using a combined ps- and ns- burst mode (Burst-in-Burst or BiB) of fs-pulse exposure at a high 0.6~MHz 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 200~fs 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.

Figures (14)

• Figure 1: Concept: (a) fs-laser driven oxidation of metal in the Burst-in-Burst mode via direct write at (b) sub-diffraction limit on micrometer scale and self-organization of ripples at sub-wavelength scale. The Transparent-Opaque patterning is achieved under ambient conditions. Diffraction on a laser-inscribed ripples' grating of $\Lambda = 860$ nm period, where transparency was achieved as a result of complete tantalum oxidation.
• Figure 2: Burst oxidation of Ta (Burst-in-Burst or BiB-mode). (a) Schematics of sample's cross section and an actual profile (Bruker, Germany) of the oxidized Ta2O5 sub-diffraction-limited micro-lines formed by scan of a $1.22\lambda/NA =12.6~\mu$m diameter focal spot (the wavelength $\lambda = 1030$ nm, numerical aperture of objective lens $NA = 0.1$); single fs-pulse energy for BiB was $E_f\simeq 0.97$ nJ (on the sample) and the conditions of exposure are discussed in the text. Oxide thickness is twice the original Ta 200 nm film (see Fig. \ref{['f-side']} for cross section). (b) The used BiB mode (Carbide, Light Conversion) of 1030 nm/200 fs pulses: number of pulses in ps-burst-1 $N_p = 10$ (2.5 GHz burst) and in ns-burst-2 $N_n = 10$ (62.5 MHz burst) at an overall 602.7 kHz repetition rate.
• Figure 3: (a) Optical images in reflection and transmission of BiB-mode oxidized Ta film on SiO2. Pulse energy of a single fs-pulslet in BiB was $E_f = 1.35$ nJ. Line exposure was carried out in the constant frequency mode, and the starting location received larger exposure, which caused ablation. (b) Wider than focal spot oxidation of Ta with apparent molten phase thermally quenched; full power corresponding to fs-pulslets $E_f = 14.51$ nJ.
• Figure 4: Direct write oxidation using $NA = 0.9$ objective lens. Transmission and reflection optical images of the laser oxidised lines at different scan speeds and pulse energies for linear (a), and circular (b-d) polarizations. Focal diameter $1.22\lambda/NA = 1.4~\mu$m; scan speed was scaled as $2.5\times 2^n~\mu$m/s with $n = 0-7$. The number of BiB combined pulses per diameter of focal spot at the laser repetition rate $f_l$ is $N = t_{dw}f_l =337.5$ to 2.6 pulses for the lines (from left to right).
• Figure 5: Optical images in reflection of surface patterns. Single-pulse oxidation of Ta at 602.7 kHz by self-organization of ripples. Focal spot of $1.22\lambda/NA =12.6~\mu$m diameter (the wavelength $\lambda = 1030$ nm, numerical aperture of objective lens $NA = 0.1$); single fs-pulse energy was changed from $E_f\simeq 0.38$ nJ to 0.32 nJ (on the sample) in 40 steps with energy reduction by $\Delta E_f = 1.5$ pJ during line-by-line scan with $\delta x = 2.5~\mu$m step between the lines. Scanning was one directional (arrow marker). Right-inset shows corner region of $1\times 1$ mm$^2$ area recorded in $\sim 10$ hours scan (see Fig. \ref{['f-nice']}(b)).