Research On Surface Modification Of Flexible Substrates To Trigger Copper Deposition And Its Application In Flexible Electronics

Flexible conductive patterns which are mainly used to interconnect components are of great importance in flexible electronics.Even under bending,folding and stretching,the flexible conductive patterns have the ability to maintain the function of electronic devices.Owing to the good conductivity,flexibility and low price,copper film is the most commonly used flexible metal conductive materials.Copper conductive patterns could be deposited on flexible substrate surface by the processes of activating flexible substrates with soluble modification reagents and initiating copper deposition.Although the modification reagents can selectively activate flexible substrates and improve the adhesion strength between the deposited copper patterns and substrates,one modification reagent is not compatible with different flexible substrates.Consequently,the research emphases in this dissertation are to develop compatible modification reagents for different flexible substrates and optimize process parameters of electroless copper plating for depositing robust copper conductive patterns on flexible substrates.Polyimide?PI?film as a polymer material with excellent high temperature resistance is a common flexible electronic substrate.In this experiment,potassium hydroxide,glycerol and deionized water were selected as raw materials to develop a compatible KOH modification reagent for PI film.The activation mechanism of PI substrate in the modified region was analyzed by quantum chemical calculations and experiments.The performance of copper patterns deposited on PI film and their application in flexible electronics were investigated.The experimental results demonstrate that carboxyl groups are detected on the PI film modified by KOH solution.When the ration of water to glycerol is v?water?:v?glycerol?=3:1,continuous fine copper conductive patterns with about 10?m in width are fabricated on PI film.After 60 min deposition,the adhesion strength between copper layer and PI film is up to the 5B level according to the standard of ASTM D3359.Meanwhile,the surface of roughness of copper patterns is 20 nm without detectable pores and impurity.The resistivity of copper conductive patterns with dense grains is 2.82×10^-6?·cm.When the bending curvature radius is 2 mm,the resistivity increases only 16.0%after 1000 times 180°repetitive bending.Poly?ethylene terephtalate??PET?film is a colorless transparent organic polymer.It is a common flexible substrate for transparent conductive patterns and devices.In this experiment,silver nitrate as the catalyst source,thiourea as a complexing agent,bisphenol A diglycidyl ether?BADGE?and curing agent 593 as bridge materials,Propylene glycol monomethyl ether?PM?as a solvent were selected as raw materials to develop an Ag⁺/ER modification reagent which could covalently combine with PET film.The preparation principle of the Ag⁺/ER modification reagent and the covalent bonding mechanism between the modification reagent and PET film were mainly studied.The experimental results prove that thiourea molecules can improve the stability of Ag⁺in the modification reagent and prevent Ag⁺from being reduced to metallic silver by BADGE when the molar ratio of thiourea to silver nitrate is 3:1.The surface and through hole wall of PET film can be modified by Ag⁺/ER modification reagent to fabricate multilayer copper patterns.Silver ions in the Ag⁺/ER surface are reduced to metallic silver after air plasma treatment.The copper patterns deposited 30 min display a low resistivity of 2.68×10^-6?·cm and maintain considerable conductivity after 1000 times 180°cyclical bending even at a curvature radius of 2 mm.Owing to the wide source of raw materials,low price and environmental friendliness,paper is a commonly used flexible electronic substrate.In this experiment,silver nitrate as the catalyst source,mercaptosuccinic acid?MA?as a complexing agent,methyl methacrylate?MMA?and curing agent?XBH-30?as bridge materials,ethanol as a solvent were selected as raw materials to developed an Ag⁺/PMMA reagent with low Young's modulus of elasticity after curing.The effect of curing agent content on the performance of Ag⁺/PMMA layer was studied through comparative experiments.The interface combination modes of Ag⁺/PMMA as a bridge layer,paper substrate and copper patterns were analyzed.The experimental results verify that the complexing agent MA can improve the stability of Ag⁺in the modification reagent without being reduced to metallic silver.The pores in paper can absorb Ag⁺/PMMA reagent through capillary action to form mechanical lock.The cured layer?m?MMA?:m?cruing agent?=4:1?has good flexibility and small Young's modulus of elasticity,which can effectively reduce the interfacial stress between the cured modification layer and substrates.The resistivity of copper conductive patterns deposited 40 min is 2.74×10^-6?·cm and becomes 3.20×10^-6?·cm increasing only 16.7%after 100 times 180°cyclical folding.Polydimethlsiloxane?PDMS?film is a piezoelectric material and possesses good tensile properties.It is a common substrate for flexible pressure sensors and wearable electronics.In this experiment,2-mercaptobenzothiazole?MBT?as a complexing agent,PDMS prepolymer and curing agent as bridge materials,butanone as a solvent were selected as raw materials to developed a compatible MBT/PDMS reagent for PDMS film.Theoretically,the interfacial stress between the bridge layer?cured MBT/PDMS?and PDMS film is small.The selection mechanism of complexing agents and solvents in the MBT/PDMS reagent was analyzed by comparative experiments.The adsorption Ag⁺abilities of different functional groups in MBT molecules were simulated by quantum chemical calculations.The experimental results exhibit that the resistivity of copper patterns deposited 30 min is 2.72×10^-6?·cm.Furthermore,the resistivity of copper patterns becomes 3.01×10^-6?·cm increasing only 10.7%after 200 times cyclical stretching with 15%deformation.