# Electroless Copper Patterning on TiO\({}_{2}\)-Functionalized Mica for Flexible Electronics
## 3 Results and Discussion
### Thickening of Cu-ELD Layers by Copper Electroplating  
While the wet-chemical Cu-patterning of mica substrates yields layers with excellent conductivity compared to typical thin Cu-ELD layers, the relatively high resistivity of electroless copper layers may present challenges for practical applications requiring higher current-handling capacities. This is particularly important for the fabrication of functional mica-based PCBs. As discussed earlier, extending Cu-ELD durations beyond 3 min often led to the bubbling and delamination of the copper layer in this study. An industrially favored solution for enhancing the electrical conductivity of electrolessly deposited layers on dielectric surfaces is to use electrochemical plating to thicken the layers to the desired thickness. In this study, we also explored this approach by thickening the Cu-ELD layer through electrochemical copper deposition. Ag-activated TiO\({}_{2}\)/Mica substrates with a 2 min Cu-ELD coating were further plated in a two-electrode electrochemical copper plating bath. The anode was a 99.9% copper plate, and the electrolyte contained 200 g L\({}^{-1}\) CuSO\({}_{4}\).5H\({}_{2}\)O, 50 g L\({}^{-1}\) concentrated H\({}_{2}\)SO\({}_{4}\), and 50 mg L\({}^{-1}\) NaCl. The current density at the Cu-ELD cathode was set to 10 mA cm\({}^{-2}\), and the plating duration was 10 min, resulting in an additional 1 \(\upmu\)m of copper being deposited. As shown in Figure 9(a), the electroplated copper layer exhibited a dense, highly uniform morphology. Correspondingly, the EDX spectrum (Figure 9(b)) revealed that copper was the predominant element detected, and the Ti-K, O-K, and Ag-K peaks coming from the substrate are highly attenuated by the thicker copper layer, while the other elements K-peaks associated with the substrate (Al, Si, K, and Fe) become unobservable. The sheet resistance of the electrochemically thickened copper layer was measured to be an order of magnitude lower than the best-case Cu-ELD scenario, at 27 \(\pm\) 3 m\(\Omega\)/sq. \begin{table}
\begin{tabular}{c c c} \hline \hline \(\upmu\)**[Cu-ELD]\(\upmu\) min** & **Thickness (d), nm** & **R\({}_{s}\), \(\upmu\Omega\)/sq.** \\ \hline
1.0 & 71 & 0.595 \(\pm\) 0.022 \\
1.5 & 106 & 0.430 \(\pm\) 0.022 \\
2.0 & 132 & 0.316 \(\pm\) 0.015 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of thickness (d) and sheet resistance (R\({}_{\text{s}}\)) for Cu-ELD layers formed over 1–2 min of deposition. Figure 10: SEM-EDX analysis of a Cu layer (2 min Cu-ELD), thickened to 1 \(\upmu\)m via electrochemical Cu plating: (a) SEM surface morphology; (b) EDX spectrum. Functional Resistance-Temperature Detector Sensors on Cu-ELD Patterned TiO\({}_{2}\)/Mica Substrates  
As demonstrated in the previous subsections, the Cu-ELD patterning route on TiO\({}_{2}\)-functionalized mica allows for a relatively straightforward control of the Cu-layer thickness by adjusting the electroless deposition time. Given the relatively high resistivity of the \(\approx\)100 nm thick Cu-ELD layers, combined with the possibility of spatially selective patterning via photolithography (see Figure 1), one of the main objectives of this study was to explore the applicability of the Cu-ELD patterns as resistance temperature detectors (RTDs). The experiments were conducted on Cu-ELD layers with a serpentine RTD geometry (see Figure 2). Devices were fabricated with varied Cu-ELD durations in the 1-2 min range, and the temperature-dependent direct current resistance (DCR) was measured. Figure 11a shows a typical result, illustrating the resistance change in a Cu-ELD serpentine pattern deposited for 2 min in the electroless deposition bath. The temperature response was measured in 5 \({}^{\circ}\)C increments between 30 and 65 \({}^{\circ}\)C, with each temperature step held for 5 min. The results show a strong correlation between the substrate's temperature and the measured resistance across the serpentine, which is the fundamental operating principle of RTD-type temperature sensors.