Roll-to-roll nanofabrication process for flexible Cu metal mesh transparent conducting electrodes

Ghaznavi, Ziam
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Transparent conducting electrodes (TCEs) are essential components in many optoelectronic and display technologies including light-emitting diodes, photovoltaics, and display touch screen panels. Transparent conducting oxides, specifically indium-doped tin oxide (ITO), are currently the industry standard TCE material due to their high electrical conductance and optical transparency. However, conventional ITO electrodes are intrinsically brittle, require high-temperature vacuum processing and suffer from fluctuating material costs making them undesirable for future generation optoelectronic and display devices and incompatible for flexible devices. As a result, many alternative TCEs have garnered significant research interest over the past few decades. Metal mesh-based electrodes have recently appeared as the most pragmatic solution to replace ITO for future flexible devices due to their highly tunable electrical and optical properties, low material cost and inherent mechanical robustness. However, metal mesh TCEs require high-resolution, ultra-large area nanoscale patterning on flexible polymer substrates which is beyond the capabilities of optical lithography. Moreover, commercial metal mesh electrodes will depend upon high throughput, scalable roll-to-roll (R2R) processing in order to meet the cost needs of the projected markets. In this work, nanoscale Cu metal mesh electrodes on flexible polycarbonate substrates and rigid quartz substrates are demonstrated using a novel R2R compatible fabrication process employing jet-and-flash nanoimprint lithography (J-FIL), linear ion source etching (LIS) and selective electroless Cu metallization (ECu) using a Pd seed layer. Process step verification details are provided including a morphological study of ECu deposition. Cu grain size is found to be independent of Pd seed layer thickness and plating time in solution, and resistivity of ECu deposited thin films was found to be about 8 times higher than bulk Cu at 13 μΩ cm. Rectangular cross-section trench patterns arranged in a square grid geometry embedded in UV cured imprint resist define the fabricated Cu metal meshes. Two trench dimensions were explored in this work: (i) height of 100 nm, linewidths of 300 nm and pitch of 3 μm, and (ii) height of 100 nm, linewidths of 500 nm and pitch of 5 μm. The most conductive flexible metal mesh sample achieved sheet resistance as low as 3.4 Ω/sq. and average transmittances of all samples was roughly 50% in the visible spectrum with significant potential for optimization. Comparison of measured spectral transmittance and simulations showed a reduction in broadband transmittance up to 40% due to the sputter-coated 3 nm thick Pd seed layer required to reliably catalyze the ECu reaction. The focus of the experimental work was to use existing J-FIL templates to establish the R2R compatible nanofabrication process. The optimum mesh design to maximize transmission while minimizing sheet resistance was not the focus of experiments. In order to further improve metal mesh TCE performance, a preliminary optimization strategy has been proposed to identify optimum mesh geometry given a target application. Optimization results tend towards a pitch to linewidth ratio of about 100 and maximize grid line aspect ratio for high transmittance while minimizing for sheet resistance. Simulation and process development insights incorporated in this thesis provide guidance for future research to further refine the proposed R2R compatible fabrication process for and design of flexible metal mesh TCEs