Flexographic printing for electronics nears 1μm resolution

Technology News | December 9, 2016

By Rich Pell

Materials & processes Optoelectronics Energy Harvesting Displays & Interfaces Sensing / Conditioning Wearables Flexible Electronics

Where the resolution of flexographic printing using elastomeric stamps is typically limited to tens of micrometres, due to liquid instabilities and ink spreading issues during stamping, the researchers have developed a novel type of stamp material specifically engineered to work well with a number of commonly available electronic inks at near 1μm resolution.

The material had to have pores large enough to host colloidal ink particles when wetted with the electronic inks, yet not so large as to impact negatively the printed features. It had to be solvent resistant, mechanically compliant to establish uniform contact, yet durable.

The researchers found what they were looking for by growing vertically aligned carbon nanotubes (CNTs) on a lithographically patterned silicon substrate, creating repeatable microstructures, 99% porous. After various chemical surface treatments and a conformal pPFDA polymer coating, the stamps were fit for purpose, supporting the capillary-driven loading of ink and nanoscale contact-mediated ink transfer.

SEM images of a stamp microstructure, 25μm by 25μm (left), and (right) optical and atomic force microscopy images of an array of such structures and the resulting printed Ag ink patterns. (Source MIT)

As a demonstration, the researchers then printed a number of micrometre-scale patterns of a variety of functional nanoparticle inks, including Ag, ZnO, WO₃, and CdSe/ZnS, onto both rigid and compliant substrates. That was performed at a resolution and a printing speed (0.2 m/s) far superior to today’s industrial printing technologies, they wrote in their paper, hinting that their process could make cheap mass-produced electronics a reality if translated into roll-to-roll tool fabrication.

In one example, the MIT team printed conductive networks of transparent electrodes, a key layer to connect to LEDs, LCDs, touch-screen panels, or solar cells to name a few applications. In a single flexographic printing step using the an honeycomb-patterned nanoporous stamp, the researchers were able to print a thin Ag honeycomb with a minimum linewidth of 3μm between adjacent holes, with a transparency of 94% (from 200 to 800nm) and a sheet resistance of 3.6 ohm/sq.

(top left) Magnified SEM image of honeycomb-structured CNT stamp with a minimum internal linewidth of 3μm. The schematics paired with optical microscope images show the CNT stamp in various conditions, dry, inked, and the resulting printed pattern on a glass substrate (after solvent evaporation). (Source MIT)

The sheet resistance is half that of comparable 90% transparency indium tin oxide achieved today, and one-tenth the value reported for other Ag nanowire networks at 90% transparency. The printed patterns exhibited a rather uniform nanoscale thickness, from 5 to 50nm.

SEM image of the stamp feature (upper left) and
optical microscope image of the printed Ag NP ink
pattern (lower left) of a flower-like pattern with feature
widths varying from 20 to 150 μm.
On the right, fluorescencemicroscope image
(wavelength emission, 620 nm) of printed QD ink
(CdSe/ZnS, from 5 to 6nm) of a pattern with
minimum internal linewidth of 5 μm and hole size of 11 μm.
(Source MIT).

Another demonstration was the printing a colloidal quantum dot ink (CdSe/ZnS), in a honeycomb pattern with a minimum internal linewidth of 5μm and holes 11μm in diameter.

To ensure precise printing and to apply the right stamp pressure for a thin uniform result, the team developed a model taking into consideration the stamp and the substrate’s respective roughness as well as the concentration of nanoparticles in the ink.

Here the resolution of the printed patterns was somewhat restricted to the resolution of the lithographic equipment used and the researchers think that using single-walled CNTs of significantly smaller diameter (1 to 2nm) and spacing could push flexographic printing to submicrometer features.

The researchers expect that a multi-layered roll-to-roll printing approach could yield complete electronic circuits and energy harvesting solutions on the cheap, at industrial scale. This could range from pervasive sensors on food packaging to weather monitoring solutions printed on glass windows.