Printable electronics and photonic curing: high performance in a flash
By Steve Leach, NovaCentrix
The silicon industry has achieved stunning advances over its nearly 50 year history. Every new generation of technology has offered increased computing power, faster operating speeds, and lower costs, which have enabled electronics to penetrate every facet of our lives. There remain, however, potential applications for which conventional silicon technology is not viable, either due to cost, fragility, or time to market considerations. For such applications, printable electronics is a new disruptive technology.
Printable electronics represents the merger of electronics and printing. The concept is to use high speed printing equipment to build electronic devices using specialized inks that when cured provide the basic building blocks of circuits: conductors, resistors, semiconductors, and dielectrics. Compared to standard equipment for manufacturing semiconductors and electronics, printing equipment is fast, inexpensive, and large area (i.e. “wide-web”), factors which together enable the promise of low cost and large area printable electronics.
The range of applications for printable electronics is quite large, encompassing RFID tags, flexible displays, sensors, photovoltaics, batteries, lighting, and logic and memory. The ability to print on common packaging materials will further extend applications into product branding and value-added packaging. The recent formation of industry initiatives around printable electronics is validation of the potential impact of printable electronics. For example, iNEMI’s “2007 International Electronics Manufacturing Initiative Roadmap” will include a section on organic and printable electronics. That document is scheduled to be available to the public in February 2007.
The chart shows an optimized cure routine for silver film on Mylar based on photonic cure resistivity data for a 2.5 micron thick silver film. Data courtesy of NovaCentrix.
Printable electronics is still an emerging technology, and there are technology gaps to be met before widespread adoption can occur. One such gap is the need for low temperature curing (or sintering or annealing) methods. Conductive inks, for example, are typically metallic-based, and must be sintered to realize high conductivities, which requires time and temperature. Lower temperatures generally mean longer processing times. A cure time of one minute on a 2,000 FPM (feet per minute) web necessitates a curing oven with nearly a half mile heated web path. Higher temperatures, on the other hand, can reduce the sintering time, but at the cost of requiring expensive substrates that can withstand the high temperatures. What is needed is a low temperature and rapid process for curing or sintering.
In response to this need, NovaCentrix has developed “photonic curing” technology, a low temperature, rapid sintering process. NovaCentrix Photonic Curing Systems instantly cure metal nanoparticle-based inks by exposing them to a brief, intense pulse of light from a xenon flash lamp. The system rapidly and selectively heats and fuses nanoscale metallic ink particles, forming highly conductive traces without heating the base substrate material. The technology operates at room temperature and is very fast. The energy is broadcast over the whole substrate with no need for shadow masks or expensive alignment techniques, and since it is in the form of light rather than heat, the energy will not damage thermally sensitive components or materials.
Photonic curing technology relies on the physical properties of nanoparticles. Metal nanoparticles are generally black and light absorbing. They have a high surface area to mass ratio, requiring very little light to heat them. A continuous source of radiation will heat the particles and within a few milliseconds they will transfer heat to the substrate. If the source of radiation is pulsed, with a duration that is shorter than the thermal equilibration time of the particles and substrate, then the nanoparticles will quickly heat and sinter before they can transfer much energy to the substrate.
The ideal radiant energy appears to be on the order of 1 J/cm2 delivered in about 1 ms for most systems of interest. Longer pulses require more energy to cure the particles and transfer too much heat to the substrate. Shorter pulses can vaporize smaller particles in the film, build up thermal gradients in the substrate, and explosively blow apart the film. Ideal curing conditions are determined by the particle type, size, film thickness, substrate type, substrate thickness, and particle binder system.
Photonic curing utilizes xenon strobes as a light source rather than lasers. Xenon strobes are preferable for several reasons. For example, lasers are far more complex and expensive devices. In addition, xenon strobes are far more efficient in converting electrical energy to light, with conversion efficiencies typically in the 50 percent range, which is 10 times greater than what is possible with the type of laser needed to cure metal particle-based films.
The technology behind photonic curing is closely related to NovaCentrix’s proprietary nanoparticle synthesis process, which begins with two metal rods in an enclosed tank. A high-power pulsed arc discharge (50 to 100 kA over 1ms duration) is drawn between the rods in an atmospheric pressure gas. The material at the end of the rods is ablated and heated to form a high-pressure (10-100 Atm) metal plasma. The plasma supersonically expands and quenches, yielding nanometer sized, single crystal, unaggregated particles in a gas suspension. These particles are conveyed out, collected, the rods are indexed in toward each other, and the process is repeated. The pulse frequency controls the production rate. By changing the composition of the quench gas from inert to reactive, either metals or metal compounds can be made.
Photonic curing uses a similar process to generate the intense light needed to cure nanometal-based films. The arc discharge is lower, so as to remain below the threshold that would normally ablate the electrodes and make nanoparticles. Even with the lower power discharge, the radiation is still dramatically more intense than that from a typical camera strobe. Just as with the nanoparticle synthesis process, the pulse duration and intensity can be varied, providing independent control over both the power and energy delivered to a surface.
Besides low temperature curing, there are additional benefits of photonic curing that make it suitable for printable electronics.
- By reducing the time to cure to less than a millisecond, photonic curing can be compatible with high-speed printing processes such as gravure and flexography without a large amount of dedicated floor space. In essence, the time to cure becomes matched to the time to print.
- The process is suited to nanoparticle-based materials, which also makes it well-suited to high resolution deposition methods and applications.
- The speed with which sintering occurs makes it possible to cure copper in air, which normally must be cured in an inert or reducing environment. NovaCentrix has demonstrated curing nano-copper based conductive inks in air, achieving resistivities below 40X bulk. This benefit opens the door to the use of inks that use lower cost materials.
- Once a material has been sintered, it will typically no longer absorb light. Thus there is the potential for building multilayer circuitry that does not thermally stress the underlying layers.
NovaCentrix has built a research and development system to scan curing conditions to optimize the conductivity of several films and substrates. The cure area, areal energy density, and the pulse length of the arc discharge are all adjustable.
When customers develop applications taking advantage of the benefits of photonic curing, there will be a need for high volume, continuous feed curing systems. In anticipation of this need, NovaCentrix is developing such systems, and has recently delivered a pilot-scale unit designed to cure Metalon branded inks on continuous films at speeds up to 50 feet per minute. NovaCentrix will continue developing high-speed commercial systems for integration with ink-jet, flexographic and gravure printing systems. These are intended to enable or accelerate the commercialization of printable electronics in RFID tags, displays, photovoltaics and other applications.
Steve Leach is chief executive officer of NovaCentrix (www.novacentrix.com) in Austin, Texas.