Method and apparatus for curing thin films on low-temperature substrates at high speeds

Abstract

A composite film is disclosed. The composite film includes a substrate, a thermal barrier layer located on top of the substrate, wherein the thermal barrier layer has a higher decomposition temperature than the substrate, and a thin film located on top of the thermal barrier layer, wherein the thin film is to be thermally processed by a pulsed light from a flashlamp.

Claims

1. A composite film comprising: a substrate; a thermal barrier layer located on top of said substrate, wherein said thermal barrier layer has a higher decomposition temperature than said substrate; and a thin film located on top of said thermal barrier layer, wherein said thin film is to be thermally processed by pulsed light from a flash lamp.

2. The composite film of claim 1, wherein said substrate is a polymer.

3. The composite film of claim 1, wherein said thermal barrier layer is a silicon dioxide layer.

4. The composite film of claim 1, wherein said thermal barrier layer is spin on glass.

5. The composite film of claim 1, wherein said thermal barrier contains silica particles.

6. The composite film of claim 1, wherein said thermal barrier contains ceramic particles.

7. The composite film of claim 1, wherein the thermal barrier contains silane derivatives as binders.

8. The composite film of claim 1, wherein said thin film has a lower decomposition temperature than said thermal barrier layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 is a diagram of a curing apparatus, in accordance with a preferred embodiment of the present invention;

(3) FIG. 2 is a diagram of a thermal barrier layer on a low-temperature substrate, in accordance with a preferred embodiment of the present invention;

(4) FIG. 3 is a diagram of an air knife within the curing apparatus from FIG. 1, in accordance with a preferred embodiment of the present invention; and

(5) FIG. 4 is a diagram of a cooling roller within the curing apparatus from FIG. 1, in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(6) For the present invention, curing is defined as thermal processing, which includes drying (driving off solvent), particle sintering, densification, chemical reaction initiation, phase transformation, grain growth, annealing, heat treating, etc. When curing materials on a low-temperature substrate, such as polymer or paper, one limiting factor in attaining a good cure is the decomposition of the substrate because a thin film (which is defined as a layer of material of less than 100 microns thick) often needs to be processed at temperatures close to or even beyond the decomposition temperature of the substrate. Furthermore, even if the thin films can be cured at a low temperature, the low decomposition temperature of the substrate increases the amount of time to thermally cure the material on the substrate. The above-mentioned problems can be overcome by the curing apparatus of the present invention.

(7) Referring now to the drawings and in particular to FIG. 1, there is depicted a diagram of a curing apparatus, in accordance with a preferred embodiment of the present invention. As shown, a curing apparatus 100 includes a conveyor belt system 110, a strobe head 120, a relay rack 130 and a reel-to-reel feeding system 140. Curing apparatus 100 is capable of curing a thin film 102 mounted on a low-temperature substrate 103 situated on a web or individual sheets being moved across a conveyor belt at a relatively high speed. Conveyer belt system 110 can operate at speeds from 2 to 1000 ft/min, for example, to more substrate 103. Curing apparatus 100 can accommodate a web of any width in 6-inch increments. Thin film 102 can be added on substrate 103 by one or combinations of existing technologies such as screen printing, inkjet printing, gravure, laser printing, xerography, pad printing, painting, dip-pen, syringe, airbrush, flexographic, chemical vapor deposition (CVD), PECVD, evaporation, sputtering, etc.

(8) Strobe head 120, which is preferably water cooled, includes a high-intensity pulsed xenon flash lamp 121 for curing thin film 102 located on substrate 103. Pulsed xenon flash lamp 121 can provide light pulses of different intensity, pulse length and pulse repetition frequency. For example, pulsed xenon flash lamp 121 can provide 10 s to 10 ms pulses with a 3 by 6 wide beam pattern at a pulse repetition rate of up to 1 kHz. The spectral content of the emissions from pulsed xenon flash lamp 121 ranges from 200 nm to 2,500 nm. The spectrum can be adjusted by replacing the quartz lamp with a cerium doped quartz lamp to remove most of the emission below 350 nm. The quartz lamp can also be replaced with a sapphire lamp to extend the emission from approximately 140 nm to approximately 4,500 nm. Filters may also be added to remove other portions of the spectrum. Flash lamp 121 can also be a water wall flash lamp that is sometimes referred to as a Directed Plasma Arc (DPA) arc lamp.

(9) Relay rack 130 includes an adjustable power supply 131, a conveyor control module 132, and a strobe control module 134. Adjustable power supply 131 can produce pulses with an energy of up to 4 kiloJoules per pulse. Adjustable power supply 131 is connected to pulsed xenon flash lamp 121, and the intensity of the emission from pulsed xenon flash lamp 121 can be varied by controlling the amount of current passing through pulsed xenon flash lamp 121.

(10) Adjustable power supply 131 controls the emission intensity of pulsed xenon flash lamp 121. The power, pulse duration and pulse repetition frequency of the emission from pulsed xenon flash lamp 121 are electronically adjusted and synchronized to the web speed to allow optimum curing of thin film 102 without damaging substrate 103, depending on the optical, thermal and geometric properties of thin film 102 and substrate 103.

(11) During curing operation, substrate 103 as well as thin film 102 are being moved onto conveyor belt system 110. Conveyor belt system 110 moves thin film 102 under strobe head 120 where thin film 102 is cured by rapid pulses from pulsed xenon flash lamp 121. The power, duration and repetition rate of the emissions from pulsed xenon flash lamp 121 are controlled by strobe control module 134, and the speed at which substrate 103 is being moved past strobe head 120 is determined by conveyor control module 132.

(12) A sensor 150, which can be a mechanical, electrical or optical sensor, is utilized to sense the speed of the conveyor belt of conveyor belt system 110. For example, the conveyor belt speed of conveyor belt system 110 can be sensed by detecting a signal from a shaft encoder connected to a wheel that made contact with the moving conveyor belt. In turn, the pulse repetition rate can be synchronized with the conveyor belt speed of conveyor belt system 110 accordingly. The synchronization of the strobe pulse rate f is given by:

(13) $f=\frac{0.2*S*O}{W}$
where f=strobe pulse rate [Hz] S=web speed [fit min] O=overlap factor (i.e., the average number of strobe pulses that are received by the substrate) W=curing head width [in]
For example, with a web speed of 200 ft/min, an overlap factor of 5, and a curing head width of 2.75 inches, the pulse rate of the strobe lamp is 72.7 Hz.

(14) By combining a rapid pulse train with moving substrate 103, a uniform cure can be attained over an arbitrarily large area as each section of thin film 102 is exposed to multiple pulses, which approximates a continuous curing system such as an oven.

(15) When thin film 102 is in direct contact with substrate 103, its heating is limited by the decomposition temperature of substrate 103 at the interface of thin film 102. This effect can be alleviated and better curing can be attained by placing a layer of thermal barrier:material with a higher temperature of decomposition than substrate 103 between thin film 102 and substrate 103.

(16) With reference now to FIG. 2, there is depicted a diagram of a thermal barrier layer added onto a low-temperature substrate, in accordance with a preferred embodiment of the present invention. As shown, a thermal barrier layer 201 is inserted between thin film 102 and substrate 103. Thermal barrier layer 201 enables a higher power radiation pulse to more deeply cure thin film 102 on substrate 103 that is thermally fragile. The usage of thermal barrier layer 201 enables a higher power irradiation and a slightly higher total energy, which results in a pulse having a shorter pulse length. When multiple rapid pulses are used, the time scale of curing is increased to a level that allows heat to be removed from substrate 103 during the curing process.

(17) Thermal barrier layer 201 is preferably a higher temperature material than substrate 103 yet with a lower thermal conductivity than substrate 103. Thermal barrier layer 201 can be made of, for example, a layer of silicon dioxide (SiO.sub.2). Other materials include silica particles or ceramic particles. Silane derivatives make excellent high temperature binders for these particles. A particularly convenient barrier layer is spin-on-glass (SOG), which is widely used in the semiconductor industry for wafer planarization as it can easily be applied to a large area with standard coating techniques. SOG allows thermal barrier layer 201 to be applied in-line in a reel-to-reel process at a high processing rate.

(18) Referring now to FIG. 3, there is depicted a diagram of an air knife within curing apparatus 100 from FIG. 1, in accordance with a preferred embodiment of the present invention. As shown, an air knife 301 is utilized to cool substrate 103 before, during, and/or after curing of thin film 102. Air knife 301 is applied from the top or bottom of substrate 103. When applied from the top, air knife 301 may also aid in removing additional solvent from thin film 102 during the curing process. Although there can be little convective cooling during a single pulse (1 ms), this technique can provide substantive cooling during a rapid pulse train, that may be greater than 100 ms.

(19) With reference now to FIG. 4, there is depicted a diagram of a cooling roller within curing apparatus 100 from FIG. 1, in accordance with a preferred embodiment of the present invention. As shown, a cooling roller 401 is utilized to cool substrate 103. Substrate 103 is drawn over roller 401 before, during, or after the curing process. Roller 401 functions to remove heat via conduction from substrate 103 after the curing process. Active cooling may be applied to roller 401 in order to maintain roller 401 at a constant temperature. Aside from pre-cooling substrate 103, though there can'be little external conductive cooling during a single pulse (1 ms), this technique can provide additional substantive cooling during a rapid pulse train that may be greater than 100 ms.

(20) As has been described, the present invention provides a curing apparatus and method for thermally processing thin films on low-temperature substrates at relatively high speeds.

(21) The following is an example of curing using the curing apparatus of the present invention with a sheet fed conveyor. A silver nanoparticle, aqueous-based ink, which is available commercially from Novacentrix Corporation, was loaded into an ink jet cartridge and printed onto a photopaper at approximately 300 nm thick. After printing, the is ink layer had a sheet resistance of approximately 20,000 ohm/square. The photopaper (i.e., substrate) was clamped onto a thick aluminum plate maintained at 27 C. and placed on a conveyor belt moving at 100 feet per minute. The curing region of the curing lamp was 2.75 wide in the web conveyance direction and 6 wide perpendicular to the web conveyance direction resulting in a beam area of 106 cm.sup.2. The strobe lamp was activated to provide multiple pulses at a frequency of 14.6 Hz with a pulse width of 450 microseconds, delivering 1.0 J/cm.sup.2 per pulse and an average radiant power of 2.2 KW/cm.sup.2. Each portion of the substrate received 2 overlapping pulses for a total of 2.0 J/cm.sup.2 of total energy. The total time of curing was approximately 0.15 seconds. After curing, the sheet resistance of the ink layer was reduced to 0.25 ohms per square. This corresponded to a resistivity of 8 micro-ohm-cm or five times the resistivity of bulk silver. The area of the ink layer was larger than the curing head, but the overlapping pulses resulting from the combination of rapid pulsing and a moving substrate allowed a uniform cure for an arbitrarily long pattern. In contrast, with conventional oven curing, an identical film/substrate can be placed in an oven at 100 C. (which is the highest working temperature of the substrate). After 30 minutes of curing, the resulting sheet resistance reached only 1.8 ohms/square.

(22) While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.