# Electroless Copper Patterning on TiO\({}_{2}\)-Functionalized Mica for Flexible Electronics
###### Abstract  
The formation of conductive copper patterns on mica holds promise for developing cost-effective flexible electronics and sensing devices, though it is challenging due to the low adhesion of mica's atomically flat surface. Herein, we present a wet-chemical method for copper patterning on flexible mica substrates via electroless copper deposition (Cu-ELD). The process involves pre-functionalizing 50 um thick muscovitc mica with a titanium dioxide (TiO\({}_{2}\)) layer, via a sol-gel dip-coating method with a titanium acetylacetonate-based sol. Photolithography is employed to selectively activate the TiO\({}_{2}\)-coated mica substrates for Cu-ELD, utilizing in situ photodeposited silver (Ag) nanoclusters as a catalyst. Copper is subsequently plated using a formaldehyde-based Cu-ELD bath, with the duration of deposition primarily determining the thickness and electrical properties of the copper layer. Conductive Cu layers with thicknesses in the 70-130 nm range were formed within 1-2 min of deposition, exhibiting an inverse relationship between plating time and sheet resistance, which ranged from 600 to 300 m\(\Omega\)/sq. The electrochemical thickening of these layers to 1 um further reduced the sheet resistance to 27 m\(\Omega\)/sq. Finally, the potential of Cu-ELD patterning on TiO\({}_{2}\)-functionalized mica for creating functional sensing devices was demonstrated by fabricating a functional resistance temperature detector (RTD) on the titania surface. mica; TiO\({}_{2}\); photodeposition; electroless deposition; flexible PCB substrate Article  
Electroless Copper Patterning on TiO\({}_{2}\)-Functionalized Mica for Flexible Electronics  
1  
[PERSON], [PERSON], B.  
include glass, silicon wafers, perovskite materials, and thin metal foils. These substrates, when thinned, offer bendability, thermal resistance, dimensional stability, and, in some cases, transparency, making them ideal for flexible solar powered devices [1; 7]. Mica is another example of a temperature-resistant, transparent, and flexible substrate. It provides excellent insulation, chemical stability, and thermal resistance [8; 9] and has been explored as a substrate for growing functional oxides [8; 9; 10; 11; 12]. However, as a silicate material, mica faces challenges in adhesion with conductive layers, which can be improved by surface roughening or using adhesion-promoting sublayers [13; 14; 15; 16]. Conductive layers on mica are generally deposited as conducting oxides [8; 17], metallic transition metal dichalcogenides [18], and, less frequently, metals [14]. Various techniques, such as sol-gel spin-coating [10], magnetron sputtering [11], pulsed laser deposition (PLD) [12; 17], molecular beam epitaxy [19], PVD, and CVD [18; 20], have been used to deposit such layers on mica. Despite its advantages, such as room temperature processing and cost-effectiveness, electroless metal deposition on mica is rarely reported [14]. Electroless metal plating is an autocatalytic process where conductive layers may be formed on any dielectric surface, preactivated with a catalyst, from an electrolyte containing metal ions, a reducing agent, and stabilizers. This method is commonly used for forming conductive copper layers in the electronics industry and the production of printed circuit boards, in particular. Electroless copper deposition technology can produce high-purity Cu layers suitable for forming vias and interconnects in the ultra-large-scale integration (ULSI) of integrated circuits [21; 22; 23]. One of the key advantages of electroless Cu deposition is its room temperature process, which helps avoid impurities like the undesirable oxide phases associated with Cu\({}_{2}\)O. These phases can poison the copper, leading to poor ductility and reduced electrical properties in smelled copper [24]. Electroless Cu deposition can achieve extremely high-purity Cu layers with an oxygen content as low as 74 ppm, meeting the standards for oxygen-free high-conductivity (OFHC) copper [25].