Synthesis and characterization of copper ink and direct printing of copper patterns by inkjet printing for electronic devices
Graphical abstract
Introduction
Recently, the increased demand for various electronic devices and polymer-based printed circuit boards has led to a significant development in various printing technologies [1,2] such as screen printing [3], inkjet printing [4], gravure printing [5], and roll-to-roll printing [6]. In comparison with conventional vacuum deposition and photolithography processes, the above mentioned printing technologies have numerous advantages such as cost-effectiveness, process-time reduction, and decreased waste generation during the manufacturing process [[7], [8], [9]]. Among these methods, the inkjet printing technology is an economical and highly functional alternative for microscale patterning of metallic traces in microelectronic devices. This is a technique of printing the pattern with low viscosity conductive ink directly through a nozzle and then drying it [[10], [11], [12], [13], [14]].
Conductive ink has been developed with materials like metal precursors [15], molten metals [16], conductive polymers [17], and metallic nanoparticle suspensions [18]. In metal precursors, the conductivity might decrease due to the organic residues generated in the manufacturing process of conductive films. The disadvantage of using molten metals is that they have very high operating temperatures. Moreover, the conductivity of conductive polymers is lower as compared to other materials. Considering these issues, the method of inkjet printing with metallic nanoparticle suspensions is considered as the most promising solution [[19], [20], [21]].
Studies conducted to date have focused on inks made of silver or gold nanoparticles as they have high conductivity and better stability [7,15,[22], [23], [24]]. However, as they are expensive, they cannot be mass produced. To overcome these limitations, many studies are being performed to develop inks that can replace silver and gold nanoparticles [25,26]. Copper is a competitive alternative in this case, as copper-based nanoparticle ink has good conductivity and low cost. However, formation of an oxide film on the surface of the copper nanoparticles under ambient atmospheric conditions is a drawback [27,28]. The oxide film decreases the conductivity of the copper nanoparticles [29,30]. Therefore, the development of a copper nanoparticle ink that prevents the formation of an oxide layer on the copper nanoparticles is required. This ink must also be suitable for printing; hence, developing a relatively simple technique for the production process is necessary.
In this study, copper nanoparticles were synthesized by a modified polyol process. A copper oxide-based solution was first prepared by melting a copper-based precursor powder in an ethylene glycol-based solvent. Later, a polymer binder and a reducing agent were added to the prepared solution at normal room temperature [[31], [32], [33], [34]]. The partial reduction of the copper precursor by the reducing agent in the solution resulted in the formation of copper nanoparticles. In addition, copper complexes were synthesized to minimize the oxidation of the copper nanoparticles. Copper complexes were prepared by continuously stirring the copper precursor solution while adding a ligand to it. The principle that copper complex prevents oxidation of copper nanoparticles is presumed as follows. As the copper nanoparticles in the printed pattern are thermally treated, the contact surface between particles is increased and as the polymer binder and solvent, which negatively affect conductivity are removed at annealing temperature, the conductivity is increased. At the same time, the copper complex located on the outer surface of the pattern thermally decomposes and consequently prevents oxidation of the copper nanoparticles by blocking contact between them and the air. The printed patterns exhibit high conductivity even in the air.
The copper nanoparticle ink was finally produced by mixing the copper nanoparticles with copper complexes in a solution of ethylene glycol, isopropyl alcohol (IPA), and distilled water mixed in specific proportions. Line patterns as well as square patterns were printed on Si/SiO2 substrates using this copper nanoparticle ink. The substrates used in the printing were treated by four different processing methods. Based on these printed patterns, the evaluation of the electrical and physicochemical characteristics was carried out while considering the annealing conditions.
Section snippets
Synthesis of copper nanoparticles
Copper nanoparticles were synthesized at room temperature by a modified polyol process [[31], [32], [33], [34]]. The polymer binder and the reducing agent were added to the copper oxide solution, which resulted in the reduction of copper oxide after thermal treatment, thereby synthesizing copper nanoparticles. The solution was prepared by dissolving 10 g of copper(II) nitrate trihydrate in a mixture of 300 ml of distilled water and 100 ml of ethylene glycol, which acted as the solvent.
Characteristics of copper nanoparticle and copper complex
The properties of the copper nanoparticles and copper complexes used as raw materials in the ink production were characterized in this study. To obtain high conductivity at low temperatures for inkjet printing with conductive inks, metal nanoparticles of 100 nm or less are desirable [28].
The crystal structure and crystallographic orientation of the copper nanoparticles and copper complexes synthesized in this study were determined by analyzing the XRD spectra in comparison with the JCPDS
Conclusion
Currently, the demand for inkjet printing technology and metallic inks is increasing. Silver nanoparticle ink, being one of the more efficient metallic inks, has high conductivity and thermal stability. However, due to the high cost of silver, it cannot be abundantly used. Copper-based nanoparticle ink is a promising candidate material for the replacement of silver, because of the low cost and good conductivity. However, a drawback is the decrease in copper conductivity due to its oxidation in
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the 2017 Yeungnam University Research Grant (No.217A380135 and No.217A061024), and the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No.20174030201760).
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