Inkjet printing of nano-silver conductive lines on polyimide substrates

Microfabrication of electronic and mechanical structure at the submillimeter scale is typically a time-consuming and expensive process. Lithographic techniques for silicon micromachining, used to fabricate integrated circuits and MEMS, take several weeks to go from drawings to completed chips, and require expensive facilities and extreme processing conditions. Therefore, for speeding up the fabrication process and saving the materials, inkjet printing technology which multiple small volumes of metallic, semiconducting, or insulating material are expelled at computer-defined positions on subtracts has been adopted. Piezoelectric inkjet printhead is adopted as the dispensing injector in the study due to the lower temperature operation compared to the thermal bubble inkjet printhead. From the literature review it could be found out that such processes to date have been limited in terms of line width of the microstructure (i.e. resolution), electrical conductivity, and material property. Therefore, this study will aim at constructing an inkjet-fabrication system with the combination of a commercial printhead and a computer-controlled moving gantry stage. The fabricating method is expelling silver nanoparticle droplets with fixed quantity and accurate registration on flexible Polyimide (PI) substrates with different roughness additively to produce conductive lines with different line widths and resistances. Microflow visualization (μ-FV) has been performed to study the silver nanoparticle droplet ejected from a drop-on-demand piezoelectric inkjet printhead and the equilibrium line characteristic of the nano-silver droplet deposition on a PI substrate. The unipolar waveform with a frequency of 1000 Hz and an amplitude of 60 V has been adopted for ejecting the silver nanoparticle droplet with a solid content of 30%, surface tension of 30 mN/m and viscosity of 15 cps. The back pressure is modulated to prevent the formation of satellite droplets experimentally. The PI substrate was placed onto a computer-controlled three-axis moving stage capable of a movement accuracy of 30 μm. Therefore multiple prints of the nano-silver conductive lines have also been carried out based on the moving accuracy. The ejected droplets under various back pressures from - 1.6 kPa to -0.8 kPa have been observed. The deposited silver nanoparticle conductive lines with the inter-dot spacings from 90 μm to 140 μm have also been investigated. After the thermal treatment (sinter temperature of 200°C and sinter duration of 1 hour), the optical microscopic images of the deposited silver nanoparticle conductive lines with the inter-dot spacing from 20 μm to 40 μm before and after sintering have been obtained. For the first time, the quadruple prints of the silver nanoparticle conductive lines on the PI films have been investigated to observe the line widths and electrical resistances. The silver nanoparticle conductive lines with quadruple prints on the PI substrates have a line width of 500 μm and a resistance of 1.4 Ω per centimeter. The present results not only provide the relevant information to the researchers for accurate analyses, but also the adequate fluid properties to the engineers for various purposes. In this way the inkjet fabrication could be adopted in many other applications for mass production.