A method was proposed for the high-performance laser direct writing of metal circuits on flexible polymer films and its selective removal of metal circuits, under the premise of non-destructive to polymers. Laser direct writing and selective removal of copper films on polyimide (PI) was considered as the example. For the aspect of laser direct writing, precursor ink with low decomposition temperature was prepared using copper acetate, n-octylamine, 2-amino-2-methyl-1-propanol (AMP), formic acid, and methanol. A linearly polarized, continuous-waved (CW) laser, with a power of only 1 W level and a central wavelength of 532 nm, was used to irradiate the precursor ink to form a copper film on the PI substrate via laser-thermal induced reduction and deposition. For the aspect of laser selective removal of the metal film from polymer dielectrics, two types of ultrafast lasers, namely green picosecond laser (12 ps and 532 nm) and near-infrared femtosecond laser (less than 1ps and 1 030 nm), were considered. Meanwhile, a Cu-PI composite film, made from the either magnetron sputtering technique by depositing a 1 μ m-thick copper film on the PI film, or the above-mentioned laser direct writing technique for generating a copper film on the PI substrate, was considered. For the aspect of laser direct writing, it was shown that a material deposition rate of 2.0 × 106 μm3/s (or 0.12 mm3/min) onto the PI film, as well as a deposition efficiency of 0.067 mm3/min/W, could be achieved for the copper film, with the electrical resistivity 11.8 ×10-8 Ω·m for the laser written copper. For the aspect of laser selective removal of metal circuits from polymer dielectrics, the underlying physical mechanism for using ultrafast laser with a suitable combination of wavelength and pulse was analyzed, aiming at a thorough removal of the metal film on the polymer film surface without damaging the PI film during laser ablating of the metal film. The key was to highly control the ablation threshold for polymers and metals, respectively, by considering the evolution law of linear light-absorption characteristics of metals and the nonlinear light-absorption characteristics of polymers at the wavelength. At the same time, the effect of using green picosecond laser to selectively erase the copper film on the PI surface was demonstrated, while near-infrared femtosecond laser was prone to damage the PI. Based on the light absorption characteristics corresponding to the reflection spectrum of copper, it was analyzed that green femtosecond laser could significantly promote the ablation threshold difference between the PI and copper, thus creating a wide range of processing parameter selection for pulse energy fluence, and facilitating an effect of a thorough removal of the metal film from the PI film without damaging the polymer during laser ablating of the metal film. In contrast, near-infrared femtosecond laser acted oppositely to the green picosecond laser mentioned above, namely damage the PI substrate once the laser etching reached the Cu-PI interface. A blackening of the PI substrate after finishing the selective removal of Cu was demonstrated. In conclusion, The CW green laser direct writing technique, together with the precursor ink proposed in this study, balances the manufacturing needs of low cost, high conductivity, and high manufacturing efficiency. By controlling both the pulse width and wavelength of ultrafast laser, ablation threshold difference between polymer and metal can be greatly promoted, thereby robustly removal of metal circuit materials on flexible polymer. Light sources such as green picosecond lasers can be applied to highly selective removal of various metal circuits.
Key words
laser direct writing /
polymer /
surface circuit /
selective removal /
precursor ink /
ultrafast laser /
ablation threshold
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Funding
The National Key Research and Development Program of China (2022YFB4601302);R&D Program of China Aerospace Science and Technology Corporation; Defense Industrial Technology Development Program (JCKY2023405C003)
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