Photonic curing is the high-temperature thermal processing of a

thin film A thin film is a layer of materials ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...

using pulsed light from a flashlamp.K. A. Schroder, Technical Proceedings of the 2011 NSTI Nanotechnology Conference and Trade Show, 2, 220-223, 2011. When this transient processing is done on a low-temperature substrate such as plastic or paper, it is possible to attain a significantly higher temperature than the substrate can ordinarily withstand under an equilibrium heating source such as an

oven upA double oven A ceramic oven An oven is a tool that is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been use ...

. Since the rate of most thermal curing processes (drying,

sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing process used with metals, ceramics, plas ...

, reacting, annealing, etc.) generally increase exponentially with temperature (i.e. they obey the

Arrhenius equation In physical chemistry, the Arrhenius equation is a formula for the temperature dependence of reaction rates. The equation was proposed by Svante Arrhenius in 1889, based on the work of Dutch chemist Jacobus Henricus van 't Hoff who had noted in 188 ...

), this process allows materials to be cured much more rapidly than with an oven.K. A. Schroder, S. C. McCool, W. R. Furlan, Technical Proceedings of the 2006 NSTI Nanotechnology Conference and Trade Show, 3, 198-201, 2006. It has become a transformative process used in the manufacture of printed electronics as it allows inexpensive and flexible substrates to be substituted for traditional glass or ceramic substrates. Additionally, the higher temperature processing afforded by photonic curing reduces the processing time exponentially, often from minutes down to milliseconds, which increases throughput all while maintaining a small machine footprint.

Heat Transfer Dynamics

Photonic curing primarily relies on radiative heat transfer from the lamp to the object of interest during the time that the flashlamp is on, usually between 100 μs and 100 ms. After radiative heat impinges on this object,

thermal conduction Thermal conduction is the diffusion of thermal energy (heat) within one material or between materials in contact. The higher temperature object has molecules with more kinetic energy; collisions between molecules distributes this kinetic energy ...

through the object and convective loss to the atmosphere in contact with the material will occur until the object nears

thermal equilibrium Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in t ...

. Because of the intensity and short duration of the flashlamp pulse, extreme thermal gradients can occur in the object of interest. Those extreme gradients can be useful in exposing only certain parts of an object to high temperatures. For most applications of photonic curing, designers consider a layered stack of materials. The goal of a curing profile design is to reach sufficient temperature to cause

and metalization of a top layer or print, while avoiding exceeding the

glass transition temperature The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rub ...

, melting temperature, or

flash point The flash point of a material is the "lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture". The flash point is somet ...

of the layers beneath. The transient thermal process of dissipating the heat delivered by the flashlamp depends, again, on the convective thermal losses from the top and bottom layers of the material of interest, and on the thickness of each layer. For thick layers or layers with low thermal conductivity, heat can be dissipated before the temperature of lower layers in the stack can exceed a glass transition or melting temperature. This is the key feature of photonic curing that allows for the curing of metals and conductive inks and paste on low temperature materials.

Uses

Photonic curing is used as a thermal processing technique in the manufacturing of

printed electronics Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment suitable for defining patterns on material, such as screen printing, flexography, gravure ...

as it allows the substitution of glass or ceramic substrate materials with inexpensive and flexible substrate materials such as polymers or paper. The effect can be demonstrated with an ordinary camera flash.US Pat. #7,820,097. Industrial photonic curing systems are typically water cooled and have controls and features similar to industrial

lasers A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

. The pulse rate can be fast enough to allow curing on the fly at speeds beyond 100 m/min making it suitable as a curing process for roll-to-roll processing. Material processing rates can exceed 1 m²/s. The maturing complexity of modern printed electronics for customer applications demands high throughput manufacturing and improved device function. The functionality of the printed electronics is critically important as customers demand more out of each device. Multiple layers are designed into each device, requiring ever more versatile processing techniques. Photonic curing is uniquely suited to complement the processing needs in the manufacture of modern printed electronics by providing a fast, reliable and transformative processing step. Photonic curing enables a lower thermal processing budget with current materials, and it can provide a path to incorporate more advanced materials and functionality into future printed electronics.

Development

Photonic curing is similar to Pulse Thermal Processing, developed at

Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a federally funded research and development centers, federally funded research and development center in Oak Ridge, Tennessee, United States. Founded in 1943, the laboratory is sponsored by the United Sta ...

, in which a plasma arc lamp is used. In the case of photonic curing, the radiant power is higher and the pulse length is shorter. The total radiant exposure per pulse is less with photonic curing, but the pulse rate is much faster.

References

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