Part 1: Wavelength dependent nanosecond laser patterning of very thin indium tin oxide films on glass substrates

Selective laser patterning of very thin (20 nm) indium tin oxide (ITO) films on glass substrates using 266, 355, 532 and 1064 nm nanosecond (ns) pulses is investigated. An ablative mechanism is observed at a laser wavelength of 266 nm, where inter-band absorption enables direct vaporisation of the ITO material. Atomic force microscopy confirms selective film removal at longer wavelengths where the photon energies are less than that associated with the material<>s electronic band gap. Selective patterning at these longer wavelengths, close to the threshold fluence values, is partly attributed to a non-ablative thermally driven melt flow; the material flow is radial in direction, directed from the centre of the laser spot towards the crater edge. This molten flow leads to re-solidified material at the edge of the crater and can cause unwanted glass damage when subsequent overlapped pulses are used to selectively pattern these ultra-thin films. The re-solidified material can be minimised by increasing the spatial overlap of pulses. The experimental results are interpreted using a simple finite element laser heating model. The results are discussed in terms of how low fluence laser pulses can be applied to selectively pattern thin transparent conductive oxides for touch panel displays, and other large area electronic applications, with good electrical isolation and minimal damage to thin glass substrates.