Femtosecond Laser Direct Writing of Conductive Carbon Layers on Flexible Kapton Polyimide: A Parametric Study of Process, Microstructure, and Electrical Properties

Femtosecond laser direct writing on Kapton polyimide was investigated using ultraviolet (343 nm) and near-infrared (1030 nm) irradiation. Sixteen conditions were screened optically and electrically. Scanning electron microscopy (SEM) micrographs were obtained for four infrared conditions and one ultraviolet condition; van der Pauw measurements were performed on the ultraviolet- and infrared-processed samples, and X-ray diffraction (XRD) was performed on the infrared sample with the lowest reported sheet resistance (IR-O1). None of the twelve ultraviolet conditions produced a measurable electrical response, whereas all four infrared conditions yielded measurable sheet resistances. The lowest reported sheet resistance, 65.1 Ω/□, was obtained at a repetition rate of 200 kHz, a nominal power setting of 5%, and a scan speed of 20 mm s−1. XRD analysis of IR-O1 revealed an additional broad reflection in the graphite (002) region, supporting graphitic stacking in that sample. Bond scission and aromatization of polyimide, as described in the literature, provide a possible molecular pathway to graphitic stacking, although this pathway was not directly investigated. Quantitative SEM analysis did not resolve differences in the apparent open-porosity area fraction across the infrared conditions, despite variations in sheet resistance; the present data do not establish the mechanism responsible for these variations. Because the ultraviolet and infrared conditions differed in repetition rate and nominal power setting, wavelength cannot be isolated as the cause of their different electrical outcomes. These findings highlight the importance of combining electrical and microstructural characterization when assessing femtosecond-laser-modified Kapton.