JP2016501431A - Method for forming a thin film conductor on a substrate - Google Patents

Abstract

A method of forming a thin film conductor is disclosed. A thin film precursor material is first deposited on a porous substrate. The thin film precursor material is then irradiated using a light pulse to convert the thin film precursor material into a thin film such that the thin film is more electrically conductive than the thin film precursor material. Finally, compressive stress is applied to the thin film and the porous substrate, further increasing the electrical conductivity of the thin film. In one embodiment, the depositing is done by printing. In one embodiment, the porous substrate is paper.

Description

  The present invention relates generally to thin films, and more specifically to a method of forming a thin film conductor on a substrate.

  Photocuring is a high temperature heat treatment of a thin film using light pulses from a flashlight. Photocuring allows thin films on low temperature substrates to be processed in a much shorter time (about 1 millisecond) than using a furnace (which takes seconds to minutes).

  According to a preferred embodiment of the present invention, a thin film precursor material is first deposited on a porous substrate. The thin film precursor material is then irradiated with a light pulse to convert the thin film precursor material into a thin film such that the thin film is more conductive than the thin film precursor material. Finally, compressive stress is applied to the thin film and the porous substrate, further increasing the electrical conductivity of the thin film.

  All features and advantages of the present invention will become apparent in the following detailed description.

The invention itself, as well as its preferred forms of use, further objects and advantages, are best understood by referring to the following detailed description of exemplary embodiments thereof, when read in conjunction with the accompanying drawings. Will be understood.
FIG. 1 is a high level process flow diagram of a method for forming a thin film conductor on a substrate, according to a preferred embodiment of the present invention. FIG. 2 is a schematic diagram of a photocuring apparatus according to a preferred embodiment of the present invention. FIG. 3 is a graph showing the height profile of the copper film on the paper substrate before and after being compressed using the method depicted in FIG.

  The relatively short processing time enabled by photocuring can cause problems. One of the photocuring artifacts is that rapid heating of the thin film can generate gas in the thin film. If the gas evolution is strong enough, the thin film will undergo full cohesive failure, i.e. it may explode. More generally, thin films develop some porosity. In many cases, porosity is not critical, but under certain conditions it can result in a membrane being mechanically more fragile than its denser counterpart. Furthermore, if the thin film has some electronic functionality such as electrical conductivity, its sheet resistance will be higher as a result of porosity. An increase in porosity within the membrane can also exhibit an increase in surface roughness as well. This can hinder the attachment of electrical components and reduce the aesthetic appearance of the thin film. In addition, the increase in porosity can cause increased degradation of the thin film over time if the treated thin film is generally sensitive to elements such as water or oxygen found in the environment. Accordingly, it would be desirable to provide an improved method for forming thin film conductors on a substrate.

  With reference now to the drawings, and in particular with reference to FIG. 1, a flowchart of a method of forming a thin film conductor on a substrate is depicted in accordance with a preferred embodiment of the present invention. Beginning at block 100, a thin film precursor material is first deposited on the substrate, as shown at block 110. The material is then thermally processed using a photocuring device so that the thin film precursor material becomes a thin film material, as depicted in block 120. The electrical conductivity of the thin film material is higher than that of the thin film precursor material. Finally, as shown in block 130, a compressive stress is applied to the thin film material located on the substrate, increasing the density of the thin film material so that its electrical conductivity of the thin film material can be further increased. Bring.

(A. Deposition of thin film precursor material)
The thin film precursor material can be in particulate form. The thin film precursor material can also be dispersed in a liquid. Thin film precursor materials can be used for printing methods such as scoon printing, inkjet, aerosol jet, flexographic printing, gravure printing, laser, pad, dipping pen, injector, or coating methods such as airbrush, paint, roll coating, slot die coating, etc. It can be deposited on the substrate by one or a combination thereof.

  Alternatively, the thin film precursor material can be deposited without a liquid, including vacuum deposition techniques such as chemical vapor deposition (CVD), PECVD, evaporation, sputtering, and the like. Other dry coating techniques on which thin film precursor materials can be deposited include electrostatic deposition, xerography, and the like.

The thin film precursor material preferably comprises a metal and / or a metal compound such as an oxide, salt, or organometallic. The thin film precursor material can be copper, nickel, cobalt, silver, carbon, aluminum, silicon, gold, tin, iron, zinc, titanium, and the like. Examples of oxides include Cu ₂ O, CuO, Co ₃ O ₄ , Co ₂ O ₃ , NiO, and the like. Examples of salts include copper (II) nitrate, copper (II) chloride, copper (II) acetate, copper (II) sulfide, and nitrates, chlorides, acetates, and sulfides of cobalt, nickel, silver, etc. including. If the thin film precursor material includes a metal compound, a reducing agent is also generally associated with it.

  The substrate, which can be porous, preferably has a maximum working temperature of less than 450 ° C. Examples include polymers and cellulose. Examples of porous substrates include calendered fiber-based membranes such as cellulose (eg, paper) or polyethylene (eg, Tyvek® from DuPont®). Alternatively, porosity can be induced in the substrate by foaming the substrate material.

(B. Photocuring of thin film precursor material)
When the thin film precursor material is printed in liquid, the heat treatment of the thin film precursor material evaporates the solvent. If the thin film precursor material is in the particulate form of the final thin film, photocuring additionally sinters the thin film precursor material. If the thin film precursor material consists of multiple species (such as metal compounds and reducing agents) that are designed to chemically react with each other, the heat treatment additionally causes the precursor thin film material to react and generally the metal A final thin film is formed.

  The thin film precursor material can be thermally processed using a photocuring apparatus. Referring now to FIG. 2, a schematic diagram of a photocuring apparatus according to a preferred embodiment of the present invention is illustrated. As shown, the light curing apparatus 200 includes a conveyor system 210, a strobe head 220, a relay rack 230, and a reel / reel delivery system 240. The photocuring apparatus 200 can irradiate a thin film precursor material 202 deposited on a substrate 203 located on a web that is conveyed past a strobe head 220 at a relatively high speed.

  The strobe head 220 includes a high intensity xenon flash lamp 221 for curing the thin film precursor material 202 located on the substrate 203. The xenon flash lamp 221 can provide pulses of different intensity, pulse length, and pulse repetition frequency. For example, the xenon flash lamp 221 can provide 10 μs to 10 ms pulses with a 3 inch × 6 inch wide footprint at a pulse repetition rate of up to 1 kHz. The spectral components of radiation from the xenon flash lamp 221 range from 200 nm to 2,500 nm. The spectrum can be tuned by replacing the quartz lamp with a cerium-doped quartz lamp and removing most of the radiation below 350 nm. Quartz lamps can also be replaced with sapphire lamps to spread radiation from about 140 nm to about 4,500 nm. The xenon flash lamp 221 can also be a water-cooled wall flash lamp, sometimes referred to as a directional plasma arc (DPA) lamp.

  Relay rack 230 includes an adjustable power supply, a conveyor control module, and a strobe control module. The adjustable power supply can provide pulses with up to 4 kJ energy per pulse. The adjustable power supply is connected to the xenon flash lamp 221 and the intensity of radiation from the xenon flash lamp 221 can be varied by controlling the amount of current through the xenon flash lamp 221.

  The adjustable power supply controls the radiation intensity of the xenon flash lamp 221. The power, pulse duration, and pulse repetition frequency of the radiation from the xenon flash lamp 221 are electronically adjusted in real time and synchronized to the web speed, and the optical, thermal, and geometry of the thin film precursor material 202 and the substrate 203. Depending on the specific characteristics, the thin film precursor material 202 can be optimally cured without damaging the substrate 203. Preferably, the duration of irradiation of each light pulse is less than the time for the thermal equilibration time of the stack comprising the thin film precursor material 202 on the substrate 203.

  During irradiation with the light pulse, the substrate 203 as well as the thin film precursor material 202 is moved by the conveyor system 210. The conveyor system 210 moves the thin film precursor material 202 under the strobe head 220 where the thin film precursor material 202 is cured by a high speed light pulse from a xenon flash lamp 221. The power, duration, and repetition rate of the radiation from the xenon flash lamp 221 is controlled by the strobe control module, and the speed at which the substrate 203 is moved past the strobe head 220 is determined by the conveyor control module.

  When the xenon flash lamp 221 is emitting a light pulse, the thin film precursor material 202 is instantaneously heated to provide energy to cure the thin film precursor material 202. When the fast pulse train is synchronized to the moving substrate 203, uniform curing can be achieved over any large area. This is because each section of the thin film precursor material 202 can be exposed to multiple light pulses close to a continuous curing system such as a furnace.

(C. Compression of processing material)
After the thin film precursor material 202 located on the substrate 203 is photocured using the flashlamp 221 to form the thin film material 202 ′, the compressive stress increases the density of the thin film material 202 and the substrate 203. The thin film material 202 ′ and the substrate 203 are applied. The thin film material 202 ′ on the substrate 203 can be compressed by one or a combination of existing techniques such as stamping, casting, rolling, calendering, pressing, embossing, lamination, and the like.

  Rolling is preferably used in a reel / reel manufacturing setting by a set of pinch rollers 260. The pinch roller 260 compresses the load so that the peak pressure applied to the thin film material 202 ′ and the substrate 203 exceeds 25% of the ultimate tensile strength (UTS) of the bulk thin film material after photocuring under standard conditions. It can be applied. For relatively soft and ductile metals such as copper, the preferred compression pressure range is 7,500 to 30,000 psi (ie, 25% to 100% of its ultimate tensile strength at standard conditions).

  Since the substrate 203 is porous, it is compressible and responds to compression by reducing the thickness while maintaining the same width as a fiber-based substrate such as paper. This single dimensional change ensures that the thin film material 202 ′ is not damaged by lateral deformation of the substrate 203. The peak pressure that can be applied by pinch roller 260 to a non-porous polymer substrate such as PET may be limited because PET is a low temperature polymer that tends to be relatively soft. PET will deform laterally at lower pressure thresholds than other substrates. It can cause damage to the thin film material 202 ′ and the substrate 203.

  The pinch roller 260 is driven at an angular velocity ω = v / r (where ω is the angular velocity of the pinch roller 260 and r is the radius of the pinch roller 260) and is adjusted and synchronized to the web speed v. Thus, depending on the mechanical and geometrical properties of the thin film material 202 ′ and the substrate 203, the thin film material 202 ′ can be optimally densified without degrading the substrate 203.

  In some situations, the thin film material 202 ′ on the substrate 203 is subjected to dynamic compressive stress (a magnitude that oscillates over time) using a pinch roller 260 driven at a certain frequency and lower on the pinch roller 260. It may be advantageous to use average force to achieve high peak pressure, extend tool life time, and / or increase maximum web speed.

  Heating the pinch roller 260 to a temperature between the standard temperature of the substrate 203 and the maximum working temperature results in similar results using the standard temperature pinch roller 260 due to softening of the thin film material 202 'during compression. The pressure required to achieve this can be reduced.

  The compressive stress applied to the thin film material 202 'deposited on the substrate 203 can increase the density of the thin film material 202'. Particle or solution based deposition materials have a lower density than bulk precursor materials due to residual pore structure in the deposited layers. In addition, the photocuring process may introduce additional porosity into the thin film material 202 '. The volume (volume ratio) of the pore space to the layer volume will vary depending on the material, process, and particle size. Decreasing the pore volume ratio increases the density of the material, improves its performance in terms of increased electrical conductivity (if conductive), improved mechanical stability and hardness, and improves surface roughness. Improve chemical resistance (if the material is susceptible to corrosion) by modifying surface properties such as reducing the degree and improving the soldering ability and illusion of the ratio of surface area to volume. Compressing the thin film material 202 'increases its density, which brings the deposited thin film material 202' closer to the properties of the bulk thin film material.

  The following examples illustrate various methods of applying compressive stress to a thin film material located on a substrate. The result of the compressive stress is a densification of the thin film material on the substrate, which can improve the conductivity, mechanical stability, and chemical resistance of the thin film material.

Example 1 Compressive stress imparted to a mesoporous copper thin film on a paper substrate A screen printable version of copper oxide reducing ink (product number ICI-021 available from NovaCentrix (Austin, Texas)) is 230 Printed on Wausau 110 lb precision index paper using a mesh flat screen. The print was then dried in a 140 ° C. oven for 5 minutes to remove excess solvent. Initially, the ink had a sheet resistance of about 1 GΩ / □. That is, the resistance usually measured by a multimeter was an open circuit.

  The ink was converted to a conductive mesoporous copper film using a photocuring device (such as a PulseForge® 3300 X2 photocuring system manufactured by NovaCentrix (Austin, Texas)). The machine settings used for curing were 430 V, 1,600 ms, overlap factor 5 and web speed 16 feet / minute. The sheet resistance after photocuring was 17.2 mΩ / □.

The mesoporous copper thin film was densified through the following process. When a pair of steel rollers (1.7 inches in diameter x 3.0 inches in length) were pulled through the rollers, a compressive force of 2,875 lbf was applied to the foamed copper film on the paper. The cross-sectional area of compression was 0.074 in ² , giving an average of 38,850 psi to the printed conductor. Densification through compression of mesoporous copper reduced the sheet resistance to 9.3 mΩ / □. Therefore, compression of mesoporous copper reduced its resistivity by 46%.

  In addition to improved electrical conductivity, additional benefits of the overall performance of the compressed copper film became apparent during surface mount device (SMD) mounting evaluation, mechanical stability testing, and environmental testing. .

  The compacted copper demonstrated a significantly improved success rate of mounting the SMD component on top of the converted mesoporous copper. The failure rate was 50% in the case of SMD silicon chips that were thermobonded to mesoporous copper. Compressed copper demonstrated a much higher success rate of 90% due to its low surface roughness. In addition, the reduction in surface roughness modifies the optical properties of the converted copper from matte to approximately specular at high pressure.

  Referring now to FIG. 3, a graph illustrating the height profile of the copper foam on the paper before and after compression is illustrated. Both total height and surface roughness are reduced, indicating increased copper film density and reduced surface roughness. Specifically, the surface roughness was reduced from 25 microns to 5 microns. The entire thin film stack was reduced by about 50 microns in thickness.

  For copper on paper, there is a saturation point below UTS where pressure improves the electrical conductivity of pure copper. The mesoporous copper after conversion was about 30 mΩ / □ in sheet resistance. The mesoporous copper film compressed at 8,300 psi (27% UTS of pure copper) had a sheet resistance of 22 mΩ / □. At a pressure of 12,000 psi (40% UTS of pure copper), the sheet resistance reached a minimum value of 20 mΩ / □ (saturation point). Further, when the applied pressure was increased to 25,000 psi (83% UTS of pure copper), no improvement was seen in the sheet resistance of the copper film. However, following conductivity over time, it was observed that copper films compressed at 12,000 psi increased sheet resistance by 20% over 40 days in air. Copper films compressed at 25,000 psi increased sheet resistance by only 5% over 40 days in air. Thus, pressures exceeding 40% UTS (12,000 psi) of pure copper are required for corrosion resistance and stability over time for copper thin films.

  Increasing the applied stress by a factor of 2 did not improve the electrical conductivity of the film, but the stability was significantly improved. This is because the pore space volume fraction decreases with increasing pressure (25,000 psi) and / or the copper material is “resurrected” like a fully yielded and compressed half pressure membrane It means not. The revival effect is generally seen in conventional sheet metal formation. In a manufacturing environment, the material must be compressed or rolled through multiple stages to reduce the size of the sheet metal part. Single stage dimensional reduction is not useful due to the tendency of the metal to expand in the reduced thickness due to the elastic deformation component of the process. In this case, the foamed copper compressed at 80% UTS yields completely and prevents residual elastic stress from degrading the overall performance and stability of the compressed film.

  After photocuring, the converted mesoporous copper has a high surface area to volume ratio and contributes to its poor natural corrosion resistance. Compression of mesoporous copper significantly reduces the copper surface area to volume ratio and improves the corrosion resistance of the material. Environmental testing was performed after conversion of the bare on the paper substrate and on the compressed copper film. Compressed copper demonstrates significantly improved corrosion resistance when tested in an environment at 85 ° C./100% relative humidity for 24 hours. Uncoated mesoporous copper on paper cannot withstand such environmental tests, but compressed copper is uncoated and can withstand no detectable conductivity change. The uncoated compressed copper film passed industry standards (1,000 hours at 85 ° C./85% relative humidity) with only a 20% increase in resistivity. An additional cost advantage for production becomes apparent because the required volume of material to encapsulate the compressed copper film is reduced relative to the converted copper.

  When it is desirable to reduce only the surface roughness of the thin film, a significantly lower pressure can be used. The cured film of mesoporous copper on paper substrate exhibiting an average surface roughness of 5 microns is compressed at 2,600 psi (9% UTS of pure copper) to reduce the average surface roughness to 2 microns I let you. At this pressure, the electrical conductivity of the mesoporous copper film did not change.

Example 2 Compressive stress imparted to a nickel porous thin film on a paper substrate A screen printable version of nickel flake ink (product number 79-89-16 available from NovaCentrix (Austin, Texas)) is 230 mesh. Printed on Wausau 110 lb precision index paper using a flat screen. The print was dried and dried in a 150 ° C. oven for 5 minutes to remove excess solvent. After oven drying, the sheet resistance was measured as 77Ω / □.

  The dried ink is photocured and a highly conductive porous nickel film is formed using a photocuring device (such as PulseForge® 3300 X2 photocuring system manufactured by NovaCentrix (Austin, Texas)). Formed. The settings on the light curing apparatus used for curing were 540V, 1,100 ms, overlap factor 4, web speed 14 feet / minute. Photocuring reduced the sheet resistance of the nickel film on the paper substrate to 550 mΩ / □.

The porous nickel thin film was densified through the following process. When a pair of steel rollers (1.7 inches in diameter x 3.0 inches in length) was pulled through the rollers, a compressive force of 2,464 lbf was applied to the porous nickel film on the paper. The cross-sectional area of compression was 0.074 in ² , giving an average of 33,300 psi to the printed conductor. Densification through compression of porous nickel reduced the sheet resistance to 60 mΩ / □. The compression of porous nickel reduced its resistivity by 89%.

Example 3 Compressive stress applied to a silver thin film on a paper substrate A screen printable version of silver flake ink (product number HPS-030LV available from NovaCentrix (Austin, Texas)) uses a 230 mesh flat screen. Printed on Wausau 110 lb precision index paper. The print was dried in a 170 ° C. oven for 5 minutes to remove excess solvent and cause sintering of the silver flakes. After oven drying, the sheet resistance was measured as 16.9 mΩ / □.

The 5 micron thick silver trace on the paper substrate was subjected to densification through the following process. A pair of steel rollers (1.7 inches in diameter x 3.0 inches in length) provided a compressive force of 1,848 lbf to the silver film on the paper when pulled through the rollers. The cross-sectional area of compression was 0.074 in ² , giving an average of 24,970 psi to the printed conductor. Densification via silver compression reduced the sheet resistance to 14.2 mΩ / □. The compression of the silver film reduced the resistivity by 16%.

Example 4 Compressive Stress Applied to Mesoporous Copper Thin Film on PET Substrate A screen printable version of copper oxide reducing ink (product number ICI-021 available from NovaCentrix (Austin, Texas)) is 230 It was printed on ST505 polyethylene terephthalate (PET) film using a mesh flat screen. The print was then dried in a 140 ° C. oven for 5 minutes to remove excess solvent. Initially, the ink had a sheet resistance of about 1 GΩ / □. That is, the resistance usually measured by a multimeter was an open circuit.

  The ink was converted to a conductive mesoporous copper film using a photocuring device (such as a PulseForge® 3300 X2 photocuring system manufactured by NovaCentrix (Austin, Texas)). The machine settings used for curing were 360 V, 2500 ms, overlap factor 1, and web speed 16 feet / minute. The sheet resistance after photocuring was 46 mΩ / □.

The mesoporous copper thin film was densified through the following process. When a pair of steel rollers (1.7 inches in diameter x 3.0 inches in length) was pulled through the rollers, a compressive force of 1027 lbf was applied to the foamed copper film on the paper. The cross-sectional area of compression was 0.074 in ² , giving an average of 13,873 psi to the printed conductor. Densification through compression of mesoporous copper reduced the average sheet resistance to 34 mΩ / □. Thus, compression of mesoporous copper reduced its resistivity by 26% and reduced its surface roughness.

  The pressure applied to the mesoporous copper thin film on PET was about half that used in Example 1. This was done to preserve the copper film due to the tendency of PET to deform laterally at a pressure above its yield pressure of 15,000 psi.

  When compressive stress is applied only to the printed areas of the thin film (ie, not the entire thin film and substrate), a significantly higher pressure (above the yield pressure of a substrate such as PET) is applied to the mesoporous copper film and Given to a non-porous PET substrate, it can increase the density and electrical conductivity of the thin film. The restriction of rolling compression at pressures above the yield pressure of the non-porous substrate is eliminated because local lateral deformations in the thin film conductor do not interfere with the adjoining of the thin film conductor, as do full area compression. This type of area specific compression of the printed circuit may be accomplished through the use of a stamping tool such as an embossing roller. The embossing roller may have a raised pattern that matches the printed circuit pattern, contacting only the printed area on the substrate, compressing it, and leaving most of the substrate uncompressed. In general, the technique is useful for printed deposition that covers less than 50% of a substrate. As the percentage of deposited area increases to 100%, the area specific compression tends to behave like a rolling compression, and the entire web of the substrate is compressed, thus losing the advantage.

  As described, the present invention provides a method of forming a thin film conductor on a substrate.

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

Claims (20)

A method of forming a thin film conductor on a substrate, the method comprising:
Depositing a thin film precursor material on a porous substrate;
Irradiating the thin film precursor material with a light pulse to convert the thin film precursor material into a thin film, the thin film being more electrically conductive than the thin film precursor material;
Applying compressive stress to the thin film and the porous substrate to further increase the electrical conductivity of the thin film.   The method of claim 1, wherein the depositing is performed by printing.   The method of claim 1, wherein the porous substrate is paper.   The method of claim 1, wherein the porous substrate is a polymer.   The method of claim 1, wherein applying the compressive stress is accomplished by rolling or calendaring.   The method of claim 1, wherein the compressive stress is greater than 25% of the ultimate tensile strength of the thin film at standard temperature and pressure.   The method of claim 1, wherein applying the compressive stress oscillates in magnitude over time.   The method of claim 1, wherein the thin film precursor material comprises a particulate metal.   9. The method of claim 8, wherein the particulate metal is a metal selected from the group consisting of copper, nickel, cobalt, silver, and combinations thereof.   The method of claim 1, wherein the thin film precursor material comprises a particulate metal oxide and a reducing agent.   The method of claim 1, wherein the thin film precursor material comprises a metal salt and a reducing agent. A method of forming a thin film conductor on a substrate, the method comprising:
Depositing a thin film precursor material in a pattern on a substrate;
Irradiating the thin film precursor material with a light pulse to convert the thin film precursor material into a thin film, the thin film being more electrically conductive than the thin film precursor material;
Applying compressive stress only to the patterned areas of the thin film and the substrate to further increase the electrical conductivity of the thin film.   The method of claim 12, wherein the depositing is performed by printing.   The method of claim 12, wherein the substrate is paper.   The method of claim 12, wherein the substrate is a polymer.   The method of claim 12, wherein applying the compressive stress is accomplished by rolling or calendaring.   The method of claim 12, wherein the compressive stress is greater than 25% of the ultimate tensile strength of the thin film at standard temperature and pressure.   13. The method of claim 12, wherein applying the compressive stress oscillates in magnitude over time.   The method of claim 12, wherein the thin film precursor material comprises a particulate metal.   The method of claim 12, wherein the pattern covers less than 50% of the total area of the substrate.

Images