Methodology of selective metallic thin film ablation from susceptible polymer substrate using pulsed femtosecond laser

Electronic devices are progressively fabricated on flexible substrates allowing new fields of applications. A maskless and very flexible structuring process is offered by ablation using ultra-short pulse laser irradiation. Usually, certain areas of a functional thin film coating are locally removed to yield the needed device structures. Micro laser patterning quality is not only influenced by the beam properties (beam profile, fluence) but also by pulse overlap, scan repetitions and many other factors such as substrate and coating material. This makes process parameter optimization a challenging task. In this paper, we present a systematic approach to efficiently find suitable parameters for laser-induced ablation of thin films on susceptible polymer substrates. As an example, we use a sputtered NiCr coating with a thickness of 100 $nm$ on a polyimide film made by spin coating of PI-2611 precursor (@ 2000 $rpm$ > ca. 8 ${\mu}m$, HD microsystems). Irradiation is conducted using a fs-laser with infrared wavelength of 1030 $nm$ and a pulse length of 212 $fs$. The energy per pulse is varied in the range of 0,29 ${\mu}J$ to 4,83 ${\mu}J$, yielding fluence values between 70 $mJ/cm^2$ and 1,11 $J/cm^2$. Ablation threshold fluence for a 100 $nm$ layer of NiCr was found to be ${\phi}_{th}$ ($N_P$:1)=0,50 $J/cm^2$ and the incubation factor was evaluated to be ${\xi}$=0,53. A clear distinction must be made between the material ablation at the surface of a bulk material and the selective removal of a thin film. In the second case effects such as flaking occur in practice and influence the ablation characteristic. We then investigate different varieties of thin film removal including dot-, line- and areal-ablation. The methodology is presented using a practical example, but can be applied to selective ablation of a wide range of thin film systems.