Pulsed laser micro-scribing of copper thin films on polyimide substrate in NaCl solution

Recently, there is an increasing interest to create micro-channels on metal thin films for diverse applications, such as biomedical, micro channel heat exchangers, chemical separation processes and microwave antenna. Nanosecond (ns) Nd³⁺:YAG laser has been studied for generating micro-channels on Cu thin film (35 μm) deposited on polyimide substrate (50 μm). A pulsed Nd³⁺:YAG laser (532 nm / 355 nm) based scribing was performed in air and water ambiancePlasma shielding phenomenon is observed to influence the depth of microchannel at higher energies. A novel pump-probe experiment has been conducted for verifying the plasma shielding effect in air. In underwater scribing the recast layer was reduced significantly as compared to that in air. Laser scribing of Cu thin film followed by chemical etching using FeCl₃ was studied. However, the approach of chemical etching resulted in undercut and thinning of Cu film. Alternatively, laser material processing in NaCl solution was studied. Cl⁻ ions present in the solution reacts with Cu which is removed from the sample via laser ablation and forms CuCl₂. Formation of CuCl2 in turn improved the surface morphology of the channel through localized etching. The surface roughness parameter Ra was less than 400 nm for NaCl solution based scribing which is smaller compared to air and underwater based methods which are typically around 800 nm or above. Preliminary studies using femtosecond (fs) laser based Cu scribing in air with the fluence of 0.5 J/cm² resulted in a crated depth of 3 μm without any recast layer.