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Replacing Printed Silver with Copper
By Raghu Das
Silver has replaced copper in many applications that include UHF antennas in billions of RFID tags every year, membrane keyboards, battery testers on batteries and other printed electronics and electrics. This has happened because the copper, while having excellent electrical conductivity, had to be slowly electrodeposited and wastefully etched afterwards. In the past eight years, several organizations have declared that they can print copper.
But there are problems including which copper precursor ink to use, poisoning of circuits, oxidation making interconnects troublesome, high temperature needed to anneal copper precursor inks, their toxicity and resulting conductivity being much less than bulk.
There are now many patents on copper precursor inks, including organometallic compounds, some liquid and others solid but usable in solution. As these compounds are not very volatile, they can be jetted onto a surface with minimal evaporative losses. Some have been designed to be "self-reducing", i.e. they decompose cleanly on gentle heating to copper metal and a volatile organic compound (such as an alkoxy amine) that is removed by evaporation, leaving a conductive copper film. Unfortunately, deposition of these compounds in an inert atmosphere may be necessary. A reducing agent in the atmosphere may be needed to facilitate clean removal of the organic ligand — a further complication.
One goal is to replace expensive silver conductor bus patterns with copper ones but Emanuel Sachs of MIT notes, "Unlike silver, copper poisons the performance of silicon PVs, so it will be crucial to include a low cost diffusion barrier that stops direct contact between the copper and the silicon."
Another problem is oxidation. Copper all too readily creates insulating oxides on its surface making printing of copper conductor patterns a hollow victory when faced with challenges of interconnection because of this insulating oxide on the surface.
High Annealing Temperature
High annealing temperature can preclude use of low cost substrates in reel-to-reel printing processes such as those using PET and PEN films though Novacentrix now offers a breakthrough in localised annealing to overcome this. The annealing occurs so quickly using the Novacentrix process that it helps to overcome the problem of copper oxidation, because the oxide layer has hardly any time to form unlike in an oven where annealing occurs over several minutes or longer.
In 2000, Cheong Min Hong and S. Wagner reported on source/drain metallization for amorphous silicon thin-film transistors made by inkjet printing of copper. Contact pads of a metal organic copper precursor were inkjet printed, and converted to copper metal but at a maximum process temperature of 200°C. The copper contacts were used as the mask for back-channel etch. Laser printed toner was used for all other mask levels in a photoresist-free fabrication process. They declared that the inkjet printing of copper contacts represents a further step toward an all-printed thin-film transistor technology but so far this year, we have not yet seen this happen.
Silver Coated Copper
Several companies supply copper flake or copper flake ink such as Parelec and NanoDynamics, and others such as Cabot supply Nickel flake ink, but silver has been the preferred metal choice for printed conductors. Given the rising cost of silver, IDTechEx sees increased work on copper inks to overcome its current limitations. Vendors are also seeing increased interest in silver coated copper inks. At the recent SEMICON West in San Francisco, NanoDynamics confirmed the increase in sales of its silver coated copper flake, which has similar conductivity to silver flake and does not have the oxidation issues found with copper flake.
In 2001, C. J. Curtis et al of the National Renewable Energy Laboratory USA, reported that they had evaluated metal-organic and hybrid metal-organic/metal nanoparticle inks for use in the inkjet printing of copper conducting lines. Pure, smooth, dense, highly conductive coatings were produced by spray printing with (hexafluoroacetylacetonato)copper(I)-vinyltrimethylsilane (Cu(hfa) VTMS) and (hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene) (Ag(hfa)COD) metal-organic precursors on heated substrates. Good adhesion to the substrates tested — glass, Kapton tape and Si — was achieved without use of adhesion promoters.
The silver metal-organic ink was also used to print metal lines and patterns with a commercial inkjet printer. Hybrid inks comprised of metal nanoparticles mixed with the metal-organic complexes were also used to deposit copper films by spray printing. This approach gave dense, adherent films that were much thicker than those obtained using the metal-organic inks alone. However, the copper coatings had conductivities at least an order of magnitude less than bulk. The result: many institutions have been trying to improve on this approach for several years.
In 2002, G.G. Rozenberg, E. Bresler, S.P. Speakman, C. Jeynes, and J.H.G. Steinke reported on patterned low temperature copper-rich deposits using inkjet printing in Applied Physics Letters, Vol. 81, No. 27, pp. 5249-5251. An overview of metal deposition using ink-jet printing techniques was given in US patent 5,132,248 (Drummond et al.). A review of copper deposition techniques, including plating, chemical vapor deposition (CVD) and ink-jet printing, has been given by Rickerby and Steinke.
In 2007, researchers at Yonsei University in Korea demonstrated the ink-jet printing of metal nanoparticles as an attractive method for direct patterning of conductive metal lines, "owing to low-cost, low-waste, and simple process". They developed a conductive ink containing copper nanoparticles. Copper particles with a size of 40 to 50nm were synthesized by polyol process, from which the well-dispersed conductive ink with low viscosity was prepared. They demonstrated a direct writing of the conductive lines using Cu conductive ink. The ink-jet printed copper patterns exhibited metal-like appearance and became highly conductive upon heat treatments.
There is no agreement as to which printing technology is best for printing copper patterns for use in electronic and electrical circuits. In 2005 Dr Prissanaroon et al of the Centre for Materials and Surface Science, Department of Physics, La Trobe University, Victoria 3086, Australia, reported on the use of microcontact printing (iCP) to form micrometer-scale patterns of copper on poly(tetrafluoroethylene) (PTFE). They illustrated the use of these patterned surfaces as substrates for electrodeposition of polypyrrole (PPy) sensor structures. A patterned elastomeric stamp was used to deliver a nitrogen-containing silane coupling agent to the argon plasma-pretreated PTFE surface.
Aerosol Jet Printing
On July 9 2008, Applied Nanotech Holdings, Inc. announced that its subsidiary, Applied Nanotech, Inc. ("ANI"), had established a strategic development program with Optomec, Inc. a global leader in the emerging field of printed electronics for solar, display, electronic packaging and flexible electronics applications. As a part of the commitment, ANI will install a dedicated Optomec M3D Aerosol Jet printer at its facilities in order to adapt its revolutionary copper ink to Optomec's patented ultra high resolution printing technology. By utilizing ANI's copper ink, the Optomec printer will offer the solar, display, flexible circuit and PCB manufacturers contact-free deposition of high quality, low cost metal lines.
The Optomec printing solution is able to produce much finer lines than is currently possible with traditional screen printing and inkjet printing equipment. The combined ANI/Optomec copper ink printing solution will provide an alternative to silver inks facilitating lower cost, coupled with the promise of higher reliability. Furthermore, ANI's copper inks do not require expensive vacuum installation or inert gas environment, thus lowering the cost of the capital needed for manufacturing equipment.
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