Printed Electronics World has often reported about the great need for low cost copper ink - copper is around 90% cheaper than silver yet it has 90% of its conductivity. A key challenge to achieve this is coping with the formation of copper oxide, which is an insulator. Copper readily oxides in air, and as a result the synthesis and processing of copper inks for conductors has been expensive.
As previously reported in Printed Electronics World, NovaCentrix's PulseForge Photonic Curing systems address the processing side by enabling the sintering of printed copper films in air. This technology has eliminated the need for a vacuum or reducing atmosphere for copper processing and furthermore allowed for curing at very high speed (>100 fpm). Since this process occurs rapidly, the copper does not have time to oxidize.
Now Novacentrix has also tackled the problem of making copper nanoparticle ink at low cost. The inks NovaCentrix have created are not copper but rather consist of copper oxide (CuO) and a reducer. When processed in a PulseForge system, the film undergoes a chemical reduction to render a highly conductive copper film. By making both the ink synthesis and the processing scalable, a good, inexpensive, printed conductive pattern is now a reality.
Reduction of copper oxide inks into copper thin film
A reducible metal oxide ink approach has several advantages over a pure metal ink. First, copper oxide is inherently cheap. Nanoparticles of copper oxide are an order of magnitude cheaper than current nanocopper sources and even more than an order of magnitude cheaper than nanosilver. Copper oxide particles have more surface charge than copper particles, so they are easier to disperse. The density of copper oxide is only 6.31 g/cc, which is 30% less dense than copper and 40% less dense than silver meaning it also doesn't settle in dispersion over time as readily. Furthermore, sintering in solution or plating of various printing components is minimized as the surface energy of copper oxide is much less than copper. Finally, working with an oxide means that one doesn't need to worry about the particles oxidizing.
For depositing the copper oxide inkjet inks, a commercial Epson C88+ Photo Stylus Inkjet Printer was used. This printer is readily available at low cost (~$100 USD) and is a good platform for basic printing of other Metalon inkjet inks from NovaCentrix. The printer settings were customized to achieve the desired print deposition characteristics. Typical substrates were photopaper or coated PET.
After printing, the wet films were placed in a PulseForge™ 3100 system and cured on a conveyor belt at 100 fpm in open air. In a single pass, the PulseForge dried the film, heated it to modulate the reduction reaction, and sintered the particles producing a highly conductive printed pattern. The figure below shows the film as printed and after PulseForge processing. Initially, the film is black and has a sheet resistance of at least 1 GOhm/sq. After processing, which takes about 1 ms, the film is a shiny copper color with sheet resistance 70 mOhm/sq. This corresponded to approximately 3X bulk Cu resistivity on a mass basis.
Figure SEM Image of the copper oxide ink as printed (left) followed by the same film (right) post-processing showing densification and conversion to copper with the PulseForge 3100.
Processing thicker films
The conversion from CuO to Cu, as well as the densification upon sintering, causes a contraction of the film, which is adhered to the substrate. This condition causes stress in the film, which means that this technology is limited to probably only a few microns for single pass processing. NovaCentrix has tested an experimental copper oxide flexographic ink. The coating was thicker (>1 micron) than the inkjet coating (~300 nm) and the resistivity was about 40 mOhm/sq.
NovaCentrix report, "In general, thicker films preferably have some metal content to decrease their contraction during processing. Since the PulseForge system can sinter oxygen-free copper particles in air as well as reduce copper oxide, it can also do the intermediate: reduce and sinter partially oxidized copper particles. We've made coatings using unprotected copper particles in the micron-size range and processed them with this reduction technology yielding films with sheet resistances in the 40 mOhm/sq range. Since partially-oxidized, micron-sized copper particles are also inexpensive the resulting film is still economical to produce."
The copper oxide ink material is currently available on the open market from NovaCentrix at $75/kg in bulk.
For more attend Printed Electronics USA 2009.