Selective copper generation on polymers by reductive laser sintering represents an advantageous alternative to conventional mask-based approaches such as lithography or processes based on noble metals. A variety of applications in sensing, thermoelectronics and optoelectronics have already been demonstrated. For laser sintering, ultrashort-pulsed lasers enable precise control of the introduced energy and a reduced heat-affected zone. In turn, this type of laser can also be used to implement other processes such as ablation, modification, or the integration of embedded microfluidics and micro-optical systems. In previous studies, copper patterns with high spatial resolution have been achieved by using microscope objectives. However, design of microelectronic circuits also consists of broad conductive paths and contact areas, which result in time-consuming processing when hatching fine lines. The use of galvanometric scanners with long focal length lenses results in larger focal diameters that enable faster processing. However, this is also accompanied by higher laser powers, loading the heat-sensitive substrates and promoting reoxidation of the resulting copper structures, which can dramatically increase the achieved resistivities.
Against this background, we report on the possibility to accelerate femtosecond reductive laser sintering of copper(II) oxide on the surface of transparent cyclic olefin copolymers by varying the focus size. By using different focusing conditions, different process speeds are achieved and the structural accuracies are controlled. The generated copper layers are investigated using scanning electron microscopy, energy-dispersive X-ray spectroscopy and 4-tip resistivity measurements to achieve high copper content at low resistivities in the resulting copper films at increased metallization rate.