Stability Bounds for Micron Scale Ag Conductor Lines Produced by Electrohydrodynamic Inkjet Printing

Continuous conducting lines of width 5-20 mu m have been printed with a Ag nanoparticle ink using drop-on-demand (DOD) electrohydrodynamic (EHD) inkjet printing on Si and PDMS substrates, with advancing contact angles of 11 degrees and 35 degrees, respectively, and a zero receding contact angle. It is only possible to achieve stable parallel sided lines within a limited range of drop spacings, and this limiting range for stable line printing decreases as the contact angle of the ink on the substrate increases. The upper bound drop spacing for stable line formation is determined by a minimum drop overlap required to prevent contact line retraction, and the lower bound is governed by competing flows for drop spreading onto an unwetted substrate and a return flow driven by a Laplace pressure difference between the newly deposited drops and the fluid some distance from the growing tip. The upper and lower bounds are shown to be consistent with those predicted using existing models for the stability of inkjet printed lines produced using piezoelectric droplet generators. A comparison with literature data for EHD printed lines finds that these limiting bounds apply with printed line widths as small as 200 nm using subfemtoliter drop volumes. When a fine grid pattern is printed, local differences in Laplace pressure lead to the line width retracting to the minimum stable width and excess ink being transported to the nodes of the grid. After printing and sintering, the printed tracks have a conductivity of about 15%-20% of bulk Ag on the Si substrate, which correlates with a porosity of about 60%.

Automated summary

Continuous conducting lines with a width of 5-20 μm have been successfully printed using Ag nanoparticle ink and drop-on-demand (DOD) electrohydrodynamic (EHD) inkjet printing. The stability of the printed lines is affected by the contact angle and the spacing between drops. The upper and lower bounds of the drop spacing for stable line formation are consistent with existing models. After printing and sintering, the conductivity of the printed tracks is about 15%-20% of bulk Ag, indicating a porosity of about 60%.