Onset and evolution of laser induced periodic surface structures on indium tin oxide thin films for clean ablation using a repetitively pulsed picosecond laser at low fluence

Abstract

The onset and evolution of laser induced periodic surface structures (LIPSS) is of key importance to obtain clean ablated features on indium tin oxide (ITO) thin films at low fluences. The evolution of subwavelength periodic nanostructures on a 175 nm thick ITO film, using 10 ps laser pulses at a wavelength of 1032 nm, operating at 400 kHz, is investigated. Initially nanoblisters are observed when a single pulse is applied below the damage threshold fluence (0.45 J cm−2); the size and distribution of nanoblisters are found to depend on fluence. Finite difference time domain (FDTD) simulations support the hypothesis that conductive nanoblisters can enhance the local intensity of the applied electromagnetic field. The LIPSS are observed to evolve from regions where the electric field enhancement has occurred; LIPSS has a perpendicular orientation relative to the laser polarization for a small number (<5) of applied pulses. The LIPSS periodicity depends on nanoblister size and distribution; a periodicity down to 100 nm is observed at the lower fluence periphery of the Gaussian irradiated area where nanoblisters are smallest and more closely arranged. Upon irradiation with successive (>5) pulses, the orientation of the periodic structures appears to rotate and evolve to become aligned in parallel with the laser polarization at approximately the same periodicity. These orientation effects are not observed at higher fluence—due to the absence of the nanoblister-like structures; this apparent rotation is interpreted to be due to stress-induced fragmentation of the LIPSS structure. The application of subsequent pulses leads to clean ablation. LIPSS are further modified into features of a shorter period when laser scanning is used. Results provide evidence that the formation of conductive nanoblisters leads to the enhancement of the applied electromagnetic field and thereby can be used to precisely control laser ablation on ITO thin films.

Anisotropic Resistivity Surfaces Produced in ITO Films by Laser‐Induced Nanoscale Self‐organization

“…Figure S2a, Supporting Information). The transition from LIPSS perpendicular to parallel to the laser electric field has been reported upon static irradiation of ITO films by Farid and coworkers, [ 23 ] and attributed to the formation of nanoblisters in the LIPSS initially perpendicular to the field generating an uneven fluence profile on the surface.…”

“…Figure S2c, Supporting Information). Such a splitting phenomenon has been reported in ITO films upon irradiation with both ps‐ and fs‐laser pulses, [ 22–24 ] and attributed to progressive compositional [ 22 ] or surface roughness changes [ 24,25 ] during the structure evolution. Yet, none of these works has reported the homogeneous formation of single period, LSF‐LIPSS, like those shown in Figure 1.…”

“…Similarly, Farid et al. [ 23 ] have proposed the formation of In‐rich nanoblisters as the trigger agent for LIPSS formation in ITO.…”

“…Farid et al. [ 23 ] have reported on the evolution of LIPSS morphologies in static and moving samples. In the former, they observed after a few pulses ⊥‐LIPSS to develop.…”

“…More recently, femtosecond-laser pulses at 1030 nm have been used to generate ablative ⊥-LIPSS with a very small period [16] (Λ ≈ λ/9) using a slow beam scanning speed. Farid et al [23] have reported on the evolution of LIPSS morphologies in static and moving samples. In the former, they observed after a few pulses ⊥-LIPSS to develop.…”