The cost of silver tends to increase over the years and there is concern that it is a biocide. Indeed, it is far less abundant than the indium that printed electronics experts are trying to replace because of its cost escalation. The spot price of silver is up over 50% in the past 12 months and it continues to trend upward because silver supplies are falling fast while demand from China, India, Russia and Eastern Europe is climbing. Yet silver is printed in a huge number of products today from UHF antennas in approaching billions of RFID tags yearly, membrane keyboards, battery testers on batteries and other printed electronics and electrics.
Ironically, silver has replaced the more abundant copper in many of these applications because the copper, while having excellent electrical conductivity, had to be slowly electrodeposited and wastefully etched afterwards. In the past eight years, several organisations 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.
Which copper precursor ink?
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.
Emanuel Sachs of Massachusetts Institute of Technology has been working to reduce the extent to which conductive patterns needed to carry away the electricity from solar cells can be less of a nuisance in blocking the sunlight. His company 1366 Technologies derives its name from the flux of sunlight that hits the earth's outer atmosphere - 1366 watts per square meter and an enormous amount of energy available to harness. One goal is to replace expensive silver conductor bus patterns with copper ones but he 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 Pyrrhic victory when one is then faced with the same problems as those encountered with aluminum deposition, which proves practicable but brings big challenges of interconnection in its wake, because of its insulating oxide on the surface.
High annealing temperature
High annealing temperature can preclude use of low coast substrates in reel to reel printing processes such as those employing PET and PEN films though Novacentrix now offers a breakthrough in localised annealing to overcome this. See Breakthrough in metal ink cure: room temperature on cheap substrates. Additionally, 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 200ºC. 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 in 2008, we do not yet see this happening.
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 preferential metal choice for printed conductors. Given the rising cost of silver, IDTechEx sees increased work on copper inks to overcome its current limitations, some of which will be announced for the first time at forthcoming IDTechEx Printed Electronics events this year. Vendors are also seeing increased interest in silver coated copper inks. Speaking to IDTechEx at the SEMICON West exhibition in San Francisco last week, NanoDynamics confirmed the increase in sales of their silver coated copper flake, which has similar conductivity to silver flake and does not have the oxidation issues found with copper flake.
Typical SEM images of CU flake C1 6000F. Copper flake - Source NanoDynamics.
Which printing technology?
Ink jet printing
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 so many institutions have been trying to improve on this approach in subsequent 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 as 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 40-50 nm 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. The resistivity of the film reached to 17.2 μΩ cm at 325 ºC for 1 h in vacuum.
Microcontact printing - soft lithography
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. The surface was subsequently activated by PdCl2, allowing the selective activation via the formation of Pd-N complex. Patterning of copper structures with feature sizes of 50-200 μm was achieved by immersing the activated surface into an electroless copper plating bath. Surface chemistry was investigated after each surface modification and deposition step using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). This study demonstrated that a soft lithography approach for the fabrication of patterned copper and PPy structures on flexible PTFE films, offered new opportunities for the creation of polymer-based electronic devices and sensor arrays, said the authors.
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 lowering the cost of the capital for manufacturing equipment.
David Ramahi, President and CEO of Optomec, stated: "We are very pleased that ANI has committed to optimize its Copper for use with our M3D Aerosol Jet printing technology. Our customers have a rapidly growing appetite for Aerosol Jet proven materials as they deploy their systems in production applications for solar cells, displays, and printed electronics manufacturing. ANI's copper ink offers significant cost benefits to our mutual customers and we look forward to facilitating its adoption as an Aerosol Jet standard for next generation electronic products."
Dr. Yaniv, President and CEO of Applied Nanotech, stated: "Today the metallic conductive ink technology is based on expensive silver inks. The electronic printing industry is actively looking to replace silver inks with copper inks that can be deposited at low temperature in air. These proprietary copper inks based on copper nanoparticles are available today at Applied Nanotech and the collaboration with Optomec will provide an excellent total solution to the flexible electronics industry needs."
IDTechEx notes that relative newcomer Optomec has demonstrated superior performance to inkjet in certain performance parameters such as definition and ability to print on vertical surfaces such as those within etched patterns. It now has to prove scalability and cost effectiveness for an ever increasing number of potential applications. For more see Optomec and Applied Nanotech Holdings announce strategic co-operation.
IDTechEx has provided forecasts of markets, technology, capability and emerging competition in conductive materials and patterning to many of the largest companies in Japan, Europe and the USA. We can assist you with your materials and process development in this aspect of printed electronics with consultancy, events and publications. CEO Raghu Das, Dr Peter Harrop, Dr Harry Zervos, Susann Reuter and other IDTechEx staff based in three continents travel intensively to analyse the latest developments. What problems should you tackle to create a profitable business? How much will you sell? To whom? For what devices? What partnerships should you establish? Do you need a reality check before licensing in, buying a company, selling your company? We have the answers. Contact firstname.lastname@example.org for the USA, email@example.com for elsewhere.
At Printed Electronics Asia 2008 a multi billion dollar Japanese chemical company will be launching a new copper ink, followed by further similar announcements at Printed Electronics USA 2008 in San Jose in December. IDTechEx events provide new speakers, new topics and more new exclusives.
Also read Inorganic Printed and Thin Film Electronics.