Capillary Filling of Nanoparticle Inks in OpenMicrochannels
Tina Mitteramskogler, Vito M. Di Pietro, Helene Außerhuber, MichaelMühlberger
PROFACTOR GmbH, Steyr, 4407, Austria
E-mail: tina.mitteramskogler@profactor.at

With the current trend of decreasing feature sizes of electrical interconnects, e.g. on printed
circuit boards, the digital fabrication of high-resolution conductive lines is of increasing
interest. While inkjet printing of nanoparticle inks allows for feature sizes down to several tensof micrometres, using prepatterned substrates and employing capillary forces even 5 µmconductive channels were achieved [1,2]. In this work, we show the fabrication of micrometre
sized nanoparticle tracks with lengths up to several millimetres by inkjet printing nanoparticle
silver inks onto nanoimprinted microchannels and subsequent drying and sintering.
The process we are aiming at is depicted in Figure 1. First, the open microchannels are createdon the substrate by UV-nanoimprint lithography. Then a liquid reservoir is inkjet printed onto
the structured area, which starts the capillary filling. After the microchannels are filled with theliquid, the solvent is evaporated, leaving only the nanoparticles behind. Thus, after sintering
the nanoparticles form a high-resolution conductive track inside the microchannels. We areworking towards further improving the filling process and increasing the filled length by trying
to better understand the behaviour of fluids in open microchannels through experiments,simulations and theoretical considerations. We present experiments using three different
patterns: rectangular, lenticular and V-shaped grooves, as shown in Figure 2.
First experiments on V-grooves and rectangular channels imprinted in OrmoComp (micro resisttechnology) verify the condition on the contact angle for channel filling arising from theminimization of the Gibbs free energy [3]. An example is shown in Figure 3, where twodifferent contact angle inks were deposited on V-shaped grooves and only the one fulfilling thecontact angle condition is filling the microstructures. Using these findings as starting point, wepresent simulation and experimental results on V-shaped, rectangular and lenticular channels
filled with various nanoparticle inks and demonstrate the fabrication of high-resolution tracks
down to 1 µm width and 5 mm length, as shown in Figure 4 and Figure 5.
Funding from the multiLINK project (funded by the Austrian ministry for transport, innovationand technology bmvit) and LINK project is gratefully acknowledged. This work was performed
within the NEAT project, which receives funding from the Austrian Research Promotion
Agency (FFG) under Grand Agreement No. 871438.
[1] Smith, P.J.; Shin, D.-Y.; Stringer, J.E.; Derby, B.; Reis, N.; "Direct ink-jet printing andlow temperature conversion of conductive silver patterns" Journal of Materials Science 2006
41,(13) 4153–4158.
[2] Horváth, B.; Křivová, B.; Schift, H.; "Nanoimprint meets microfluidics: Development ofmetal wires from nanoparticle ink filled capillaries" Micro and Nano Engineering 2019 3,22–30.
[3] Butt, H.-J.; Graf, K.; Kappl, M.; Physics and Chemistry of Interfaces. Wiley, 2006.
[4] Herminghaus, S.; Brinkmann, M.; Seemann, R.; "Wetting and Dewetting of Complex
Surface Geometries" Annual Review of Materials Research 2008 38,(1) 101–121.
[5] Brakke, K.A.; "The Surface Evolver" Experimental Mathematics 1992 1,(2) 141–165.

Figure 1. Fabrication process of high-resolution tracks. First, a master structure is pressed into a liquid
UV curable polymer, which is hardened by UV light. After separation, the inverse protrusions of thestamp are replicated into the polymer. Placing silver ink onto the structured substrate leads to capillary
filling of the channels and creation of high-resolution silver tracks after drying and sintering.

Figure 2. Cross section of the microstructures used for guiding the nanoparticle inks. From left to right:
V-shaped, rectangular and lenticular grooves.

Figure 3. Microscope image V-grooves of 31 µm periodicity imprinted in OrmoComp (micro resisttechnology) with nanoparticle inks (PV Nano Cell) manually deposited on top. The left image shows
the resulting conductive lines for an ink that has a low contact angle θ fulfilling the capillary filling
condition 𝜃 < (𝜋 − 𝛼)⁄2 [4]. The original deposited droplet vanishes completely and fills themicrochannels up to a length of 1 cm. Right: A nanoparticle ink with a higher contact angle depositedon the same structure stays within the deposited region.

Figure 4. Left: Close up of the microchannels shown in Figure 3 left. Scale bar 200 µm. Right: SEM
image of 1 µm wide and 5 mm long nanoparticle lines created by capillary filling of rectangularstructures of 1 µm half-pitch and 850 nm height imprinted in OrmoComp (micro resist technology).
Scale bar 2 µm.
Figure 5. Simulation results of the minimal energy
configuration of a liquid (blue) with contact angleof 50° (left) or 90° (right) on a V-shaped substratewith opening angle of 60°. Note that the 90° result
corresponds to a spherical wedge. Simulations
performed with “Surface Evolver” [5].