Selective Generation of Cu Electrodes on Ultra-Thin Glass by Femtosecond Laser Reductive Sintering of CuO

Copper(II)-oxide-based femtosecond reductive laser sintering is applied to selectively produce high quality and flexible copper electrodes on ultra-thin glass with thicknesses between 30 and 100 µm.To increase the precursor wettability and the later copper adhesion, the ultra-thin glass substrates are plasma activated.An amplified near infrared femtosecond laser combined with a galvanometric scanner is used to produce two-dimensional copper layers.Different laser and process parameters such as scan speed, laser power and repetition rate are varied to generate these structures with unrestricted design.The metallization rate can be significantly increased by adjusting the repetition rate in a new holistic parameter variation approach.This acceleration is accomplished by increasing the heat accumulation at higher repetition rates to reach the precursor temperature required for chemical reduction and sintering more rapidly.The copper structures are analyzed by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and 4-tip resistance measurement, respectively.In an optimized process regime, low electrical sheet resistances down to 117 mΩ/sq are achieved.A hybrid approach combining reductive laser sintering using infrared and laser drilling using ultraviolet femtosecond laser radiation is demonstrated, to produce a functional electrical circuit by employing the fundamental wavelength and the third harmonic of the same laser system.