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

Abstract

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 CO₂laser power (30 W) and low scan speed (∼18 mm s)^-1leads to formation of 'keyhole' contact plugs through local ablation of polyimide (initial thickness 17μm) 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 Ω sq.)^-1indicates 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 ∼7μm remaining following laser processing. By contrast, laser graphitization of polyimide of initial thickness ∼8μm yielded devices with large sheet resistance (>1 kΩ 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 Ω , 4-plug: 31 ± 14 Ω) compared to contacts to buried gold (1-plug: 346 ± 37 Ω , 4-plug: 52 ± 3 Ω). 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 constantk⁰ ∼ 0.017 cm s^-1), comparable to values measured for exposed Au films (k⁰ ∼0.023 cm s)^-1. Our results highlight the potential for integration of LIG-based sensor electrodes with semiconductor or roll-to-roll manufacturing.

Keywords: additive manufacture; contact resistance; electrochemistry; laser-induced graphene.