Direct-write formation of integrated bottom contacts to laser-induced graphene-like carbon

We report a simple, scalable two-step method for direct-write laser fabrication of 3D, porous graphene-like carbon electrodes from polyimide films with integrated contact plugs to underlying metal layers (Au or Ni). Irradiation at high average CO2 laser power (30 W) and low scan speed (similar to 18 mm s)(-1) leads to formation of 'keyhole' contact plugs through local ablation of polyimide (initial thickness 17 mu m) and graphitization of the plug perimeter wall. Top-surface laser-induced graphene (LIG) electrodes are then formed and connected to the plug by raster patterning at lower laser power (3.7 W) and higher scan speed (200 mm s)(-1). Sheet resistance data (71 +/- 15 omega sq.)(-1) indicates formation of high-quality surface LIG, consistent with Raman data which yield sharp first- and second-order peaks. We have also demonstrated that high-quality LIG requires a minimum initial polyimide thickness. Capacitance data measured between surface LIG electrodes and the buried metal film indicate a polyimide layer of thickness similar to 7 mu m remaining following laser processing. By contrast, laser graphitization of polyimide of initial thickness similar to 8 mu m yielded devices with large sheet resistance (>1 k omega sq.)(-1). Raman data also indicated significant disorder. Plug contact resistance values were calculated from analysis of transfer line measurement data for single- and multi-plug test structures. Contacts to buried nickel layers yielded lower plug resistances (1-plug: 158 +/- 7 omega , 4-plug: 31 +/- 14 omega) compared to contacts to buried gold (1-plug: 346 +/- 37 omega , 4-plug: 52 +/- 3 omega). Further reductions are expected for multi-plug structures with increased areal density. Proof-of-concept mm-scale LIG electrochemical devices with local contact plugs yielded rapid electron transfer kinetics (rate constant k (0) similar to 0.017 cm s(-1)), comparable to values measured for exposed Au films (k (0) similar to 0.023 cm s)(-1). Our results highlight the potential for integration of LIG-based sensor electrodes with semiconductor or roll-to-roll manufacturing.