Development of Cu nanoparticle-ink and its flash light sintering process for various substrate

Abstract

The printed electronics (e.g. functional inks formulation, printing, and sintering) using polymer substrates have attracted substantial interest as an advanced alternative to conventional photolithography which is expensive and time-consuming technique since it is expected to introduce the electronic devices such as radio frequency identification tags, organic light emitting diodes and organic solar cells. In the point of view of the conductive nanoparticle-ink (NP-ink) fabrication, there are some requirements including excellent conductivity, thermal stability, mechanical and electrical durability and sintering efficiency. Therefore, novel NP-ink made with gold (Au) or silver (Ag) nanoparticles have been widely employed because of their excellent conductivity, stability and low melting point. However, these noble metals are too expensive to be widely commercialized. For this reason, copper (Cu) NP-ink have been proposed and developed with advantages of conductivity and cost for printed electronics. However, most Cu nanoparticles (NPs) can be easily oxidized in the air, so that they cannot be sintered via conventional thermal sintering method under ambient conditions. For these reasons, the flash light sintering process combined with PVP functionalized Cu NP-ink was proposed by Kim et al. to overcome these limitations. The flash light sintering method can immediately reduce the Cu oxide shell and form the pure Cu film at room temperature in ambient conditions during a few milliseconds. Therefore, the flash light sintering process was widely developed because of its many advantages. However, an in-depth study for flash light sintering of Cu NP-ink on various substrate has not been conducted yet. Because the composition of Cu NP-ink and flash light sintering condition is depending on the substrate type, it is important to analyze the sintering characteristics with respect to the substrate type. For applying the flash light sintering of Cu NP-ink to polymer substrate, the Cu NP-ink was fabricated with various amounts of organic dispersant. The effect of rheological properties of the fabricated Cu NP-ink such as viscosity, viscoelasticity and thixotropic index on the print-ability on polyimide (PI) substrate and its flash light sinter-ability were investigated. Also, substrate heating assisted flash light sintering process was employed to improve the conductivity and adhesion strength of the sintered Cu NP-ink films for warpage free Cu electrode. In order to investigate the flash light sintering process with substrate heating, in-situ resistance and temperature monitoring of Cu NP-ink were conducted. In addition, a transient heat transfer analysis was performed using finite-element analysis software to predict temperature gradients of Cu NP-ink and polymer substrate during flash light sintering process. The Cu NP-ink was fabricated using oxidized Cu NPs at various temperature to form a Cu electrode for applying to silicon (Si) substrate, which has high thermal conductivity. The effect of Cu oxide shell type on flash light sintering on Si wafer was investigated. In order to sinter the Cu NP-ink on Si wafer, multiple pulsed flash light sintering process was employed. The mechanism of multiple pulsed flash light sintering was investigated using in-situ resistance monitoring. In conclusion, the Cu NP-ink was successfully flash light sintered on PI substrate and Si wafer substrate with high electrical conductivity and adhesion strength. Moreover, the flash light sintered Cu NP-inks on PI substrate and Si wafer substrate were employed in digitizer and solar cell, respectively. Therefore, it is expected that the flash light sintering of Cu NP-ink would be widely contributed to electronics industry such as digitizer and solar cell application.