# Electroless Copper Patterning on TiO\({}_{2}\)-Functionalized Mica for Flexible Electronics
## 3 Results and Discussion
### Electroless Copper Deposition on Ag-Activated TiO2/Mica Substrates  
The morphology and structure of copper layers formed via electroless deposition (Cu-ELD) on Ag-activated TiO2/Mica substrates were examined using SEM and AFM imaging. In all cases, the activation dose (DUV) for the silver catalyst was 2.5 J cm\({}^{-2}\), and the only parameter varied was the duration of the Cu-ELD process, with plating times of 1, 1.5, and 2 min. Figure 6 shows SEM images of the TiO2/mica substrate before (Figure 6a) and after Ag activation and 2 min of Cu-ELD (Figure 6b). The composition results, obtained via EDX measurements, are displayed in Figure 6c,d, respectively. Initially, the pristine TiO2/Mica substrate exhibited a uniform, flat morphology, and the EDX analysis aligned well with the expected chemical composition of Muscovite 2M1 (KAl2(Si,Al)4O10(OH)2). However, trace amounts of Mg and Fe (\(\approx\)1 at.%) were detected, which is not unexpected for naturally sourced, technical-grade mica [36]. The surface-deposited TiO2 was also detectable at 0.8 at.%, although the thinness of the layer and the high penetrability of the 20 kV electron beam likely contributed to this low value. Figure 5: AFM images of the substrates, at a 10 \(\times\) 10 \(\upmu\)m field of view, at different stages of surface treatment: (**a**) pristine mica surface; (**b**) TiO2-functionalized mica surface; (**c**) TiO2/Mica surface after photodeposition of Ag at DUV of 2.5 J cm\({}^{-2}\). Figure 6: _Cont._  
The Cu-ELD layer formed after 2 min of plating (Figure 6b) comprised flat, roughly spherical copper domains which were 50-100 nm in diameter. After Cu-ELD, copper became the dominant element in the EDX analysis (Figure 6d). The presence of both Ti and Ag from the Ag/TiO\({}_{2}\)/Mica underlayer also became more apparent due to the surface sensitivity of the measurement, enhanced by copper's electron stopping power. Additional AFM analysis was conducted to investigate the effects of the Cu-ELD duration on copper layer morphology. As shown in Figure 7a,b, which present AFM images of copper layers formed after 1 and 2 min of electroless deposition, there were no significant differences in morphology, with both layers exhibiting relatively flat copper domains of roughly spherical symmetry. However, increased coarseness was observed in the copper layer deposited for a longer time, corroborated by the root-mean-square (RMS) roughness measurements, where Rq increased from 13.64 nm to 23.23 nm. The phase composition of the Cu-ELD layer was further studied using XRD. Despite the dominance of mica reflection peaks in the Ag/TiO\({}_{2}\)/mica patterns (viz. Figure 4a), metallic copper was identifiable through the distinct Cu (111) reflection at 43.4\({}^{\circ}\) 20 (PDF#03-065-9026). Figure 8 presents a close-up of this region in the XRD pattern for Cu-ELD layers formed on TiO\({}_{2}\)/Mica for 1, 1.5, and 2 min. The results clearly show that longer Cu-ELD durations led to an increase in the intensity of the Cu (111) peak, as the copper layer thickness increased. The mean crystallite sizes, determined from the FWHM of the Cu (111) peak using the Scherrer equation, were 38.3 nm, 39.1 nm, and 39.4 nm, respectively. This indicates that the layers are polycrystalline, with the domain-like features observed in SEM and AFM images consisting of multiple Cu grains, and that the plating duration had little effect on the crystallite size. Figure 6: SEM images and EDX spectra of the samples: (**a,c**) TiO\({}_{2}\)/Mica substrate; (**b,d**) Cu-ELD layer after 2 min on Ag/TiO\({}_{2}\)/Mica substrate. Figure 7: AFM topography images, at a 10 \(\times\) 10 μm field of view, for the Cu-ELD layer formed after: (**a**) 1 min and (**b**) 2 min of Cu-ELD plating. Both layers were deposited on Ag/TiO\({}_{2}\)/Mica substrates activated with D\({}^{\text{UV}}\) of 2.5 J cm\({}^{-2}\). The substantial growth in Cu layer thickness during the 1-2 min of Cu-ELD was further confirmed by AFM microscopy, performed on the serpentine track topology (see Figure 2).