# Electroless Copper Patterning on TiO\({}_{2}\)-Functionalized Mica for Flexible Electronics
## 4 Conclusions  
In this work, we demonstrated the use of electroless copper deposition (Cu-ELD) for forming conductive copper patterns on muscovite mica using a wet-chemical process. The pre-functionalization of mica with a TiO\({}_{2}\) layer allowed the selective activation of the mica surface with a photodeposited Ag catalyst for Cu-ELD patterning, which is also adaptable to other inorganic substrates.  
At a fixed photodeposition-activation UV dose, the Cu-ELD deposition time was the primary factor influencing the thickness, morphology, and electrical properties of the conductive copper patterns formed. Conductive layers with a thickness of 70-130 nm formed within 1-2 min, with the sheet resistance decreasing from 600 to 300 m\(\Omega\)/sq. as the plating time increased. Thickening these layers to 1 \(\upmu\)m by electrochemical plating further reduced sheet resistance to 27 m\(\Omega\)/sq. These tunable-thickness Cu-ELD layers, formed in a serpentine sensor topology, were employed as functional resistance temperature detector (RTD) elements, with the deposition time reducing the intrinsic resistance from 670 \(\Omega\) to  
\begin{table}
\begin{tabular}{c c c c} \hline \hline _t[Cu-ELD]_, **min** & \(\mathbf{R^{30}}\), \(\mathbf{\Omega}\) & \(\Delta\mathbf{R}\) (30–65 \({}^{\circ}\)C), \(\mathbf{\Omega}\) & **TCR, ppm \({}^{\circ}\)C–1** \\ \hline
1.0 & 673.12 \(\pm\) 4.80 & 23.41 \(\pm\) 0.57 & 995 \(\pm\) 8 \\
1.25 & 158.83 \(\pm\) 0.23 & 7.08 \(\pm\) 0.21 & 1260 \(\pm\) 26 \\
1.5 & 132.23 \(\pm\) 0.67 & 6.71 \(\pm\) 0.06 & 1447 \(\pm\) 29 \\
1.75 & 104.10 \(\pm\) 0.44 & 5.26 \(\pm\) 0.23 & 1435 \(\pm\) 64 \\
2.0 & 50.04 \(\pm\) 0.45 & 3.15 \(\pm\) 0.12 & 1807 \(\pm\) 37 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Results from the measurements used to determine the temperature coefficient of resistance.  
Figure 12: Summary of the effects of Cu-ELD plating time on the RTD functionality of Cu patterns: (**a**) initial resistance at 30 \({}^{\circ}\)C (R30) as a function of deposition time; (**b**) absolute change in resistance from 30 to 65 \({}^{\circ}\)C and TCR as a function of R30.  
50 \(\Omega\). The temperature coefficient of resistance (TCR) for the as-fabricated patterns ranged from 995 to 1800 ppm \({}^{\circ}\)C\({}^{-1}\), increasing with layer thickness.  
The flexibility of Cu-ELD as a wet-chemical approach, when combined with TiO\({}_{2}\)-functionalization and Ag photodeposition activation, highlights its potential for fabricating electronic and sensing devices on flexible substrates like mica. The RTD sensors fabricated in this study may also be fine-tuned for use as low-cost, disposable sensors in biomedical applications, given their sensitivity within the body temperature range. Future work should explore alternative catalysts to minimize the impact on the TCR and investigate mica's substrate flexibility effects on Cu-ELD layers for tensiometric sensor applications.  
Conceptualization, B.I.S., B.R.T. and I.T.I.; methodology, B.I.S. and B.R.T.; investigation, B.I.S., B.R.T. and V.M.M.; data curation, B.I.S., B.R.T. and V.M.M.; formal analysis, B.I.S., B.R.T., V.M.M. and I.T.I; writing--original draft preparation, B.I.S. and B.R.T.; writing--review and editing, B.I.S., B.R.T., V.M.M. and I.T.I.; visualization, B.I.S., B.R.T. and V.M.M.; supervision, funding acquisition, and project administration, I.T.I. All authors have read and agreed to the published version of the manuscript.  
This study is financed by the European Union NextGenerationEU through the National Recovery and Resilience Plan of the Republic of Bulgaria, project No. BG-RRP-2.004-0005 \"Improving the research capacity and quality for international recognition and sustainability of TU-Sofia\".  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.