Patent Application
20150189761
This invention relates in general to an apparatus and method for forming a pattern on a substrate by depositing a nanoparticle ink onto the substrate, and sintering the ink using concentrated light energy and more particularly to formulation and curing of a nanoparticle ink with favorable conductivity.
Fabrication of mass-produced electronic items typically involves temperature- and atmosphere-sensitive processing. Conventional material deposition systems for electronic fabrication, including plasma-enhanced chemical vapor deposition PECVD and other vacuum deposition processes, rely on high temperatures and rigidly controlled ambient conditions. Conventional processes for circuit board manufacture are typically subtractive, applying a conductive or other coating over a surface, treating the coating to form a pattern, then removing unwanted material. The conventional method for forming copper traces is one example of this process, requiring multiple processing steps, with the use of toxic chemicals for etching and the complications and cost of proper waste disposal.
Recent advances in printed electronics provide solutions that reduce the cost, complexity, and energy requirements of conventional deposition methods and expand the range of substrate materials that can be used. For printed electronics, materials can be deposited and cured at temperatures compatible with paper and plastic substrates and can be handled in air. In particular, advances with nanoparticle-based inks, such as silver, copper, and other metal and semiconductor nanoparticle-based inks, for example, make it feasible to print electronic circuit structures using standard additive printing systems such as inkjet and screen printing systems. Advantageously, due to their very small particle sizes, nanoparticle-based inks have lower curing temperatures than those typically needed for bulk curing where larger particles of the same material are used.
Commercially available systems for curing nanoparticles typically employ heat from convection ovens or Xenon flash illumination energy. In such illumination systems, the Xenon lamps emit pulsed light that is directed onto films of nanoparticles to be cured. High light energy levels are required for nanoparticle curing. Exemplary nanoparticle-based inks such as Intrinsiq Material Ltd. CI-002 ink, a copper nanoparticle based inkjet ink, or CP-001 paste, a copper nanoparticle-based screen print ink, can be sintered through the use of photonic energy from Xenon lamp or other illumination, provided that the illumination system delivers adequate energy to volatilize coatings used in the ink formulations and to sinter and cure the inks over large surface areas.
Conventional approaches for conditioning of the nanoparticle material, however, suffer from a number of deficiencies. Xenon lamp emission energy is characteristically distributed over a broad range of wavelengths and often includes wavelengths that can cause unwanted effects, even at non-peak energy levels. This inherent spectral spread in Xenon lamp emission can have effects that result in incomplete or uneven curing. One result can be limited penetration of light energy into thicker films or premature sealing of top surface layers, trapping unwanted organic species in the remaining structure. This type of problem can occur when higher frequency light, such as light energy from the tail of the spectral distribution curve, inadvertently sinters the film and renders its top layers opaque to other wavelengths of emitted Xenon light, delaying or preventing complete curing of the lower layers. When this happens, the binder or organic suspension in which nanoparticles are suspended is only partially removed, causing uneven sintering, which can limit the conductivity of the applied materials.
With Xenon light, the distribution of energy intensity is non-symmetrical; the co-lateral dispersive energy that is produced can reduce curing efficiency or may even cause overheating and damage to the substrate. Further, pulsing of the Xenon lamp or other light source tends to create high energy peaks that can ablate films rather than melt and reflow films. As a result, the cured product may not have the desired structure.
Conventional methods are also limited with respect to the number of substrates that can be used. With materials having high thermal conductivity, such as aluminum, silicon, and ceramics, the applied energy intended for curing may dissipate too quickly. With such materials, heat can be drawn away from the area of incident light before sintering occurs. Furthermore, particular wavelengths emitted from the Xenon lamps can damage some polymeric films and other substrates, making them less suitable for curing. Adhesion to various substrates is also a problem; relatively poor adhesion of existing methods and formulations limits the number of substrates that can be used.
Performance provided by printing methods is encouraging, but there is considered to be room for improvement. One goal is to approximate more closely the conductivity of copper traces that are applied when using conventional photolithographic/etching methods. Conventionally formed copper traces have a resistivity of about 1.7 μΩcm; this provides a reference value against which relative conductivity of a material is compared. The “bulk ratio” is a multiple of this resistivity and is used as a practical measure of how well a conductive trace performs. In conventional practice as of the date of this application, printed traces formed from nanoparticle copper have been shown to achieve bulk ratios no better than about 6× (that is, no lower than 6 times 1.7 μΩcm). Claims by researchers to achieving lower values of bulk resistivity have not been substantiated. Even improvements that achieve incremental improvements in bulk ratio would be considered to be extremely useful.
Barriers to improved performance include limitations on how much energy can be applied in sintering without degrading the porosity of the finished material. Higher levels of sintering energy would be advantageous, but conventional formulations limit the amount of energy that can be applied.
In conventional practice, all solvent materials must be dissipated prior to sintering, so that the applied traces are fully dry before heat energy is applied. If the applied ink is not fully dried, porosity increases and, consequently, resistivity increases as trapped solvent is vaporized by the photonic energy. Because trace solvents have been shown to cause problems with the sintering process, it is standard procedure to dry the applied ink as fully as possible prior to treatment with sintering energy. This extends the overall processing time that is needed for printed nanoparticle traces.
Thus, it can be seen that there is a need for improved methods for formulating and processing nanoparticulate inks and similar materials.
It is an object of the present invention to advance the art of formulating and curing nanoparticle-based inks. With this object in mind, embodiments of the present invention provide a method for forming a conductive pattern on a substrate, the method comprising:
According to an alternate aspect of the present disclosure, an embodiment of the present disclosure provides a method for forming a conductive pattern on a substrate, the method comprising:
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments, and by reference to the accompanying drawings.
The following is a detailed description of the preferred embodiments of the invention, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several figures. It is understood that the elements not shown specifically or described may take various forms well know to those skilled in the art.
Where they are used, the terms “first”, “second”, and so on, do not necessarily denote any ordinal or priority relation, but may be used for more clearly distinguishing one element or time interval from another.
In the context of the present disclosure, the term “ink” is a term of art that broadly applies to a material that is deposited in a pattern on a substrate in a viscous, generally fluid or paste form and is sintered and otherwise cured after deposition and drying by applying a curing energy such as heat or light energy. Sintering is a curing process by which curing energy effects a structural change in the composition and/or arrangement of particles in the ink.
In the context of the present disclosure, bulk ratios are expressed with an appended “×”, so that, for example, a material with about twice the resistivity of conventionally formed copper traces is expressed as having a bulk ratio of “2×”.
Curing may also have additional aspects for ink conditioning, such as sealing or removal of organic coatings or other materials that are provided in the ink formulation but not wanted in the final, printed product. In the context of the present invention, the term “curing” is used to include drying, sintering, and any post-sintering processing as well as other curing processes that apply light energy for conditioning the deposited ink.
The terms “nanoparticle-based material”, “nanoparticle-based ink”, “nanoparticle material”, “nanoparticle ink”, or “nanoparticulate material” refer to an ink or other applied viscous fluid that has an appreciable amount of nanoparticulate content, such as more than about 5% by weight or volume.
In the context of the present invention, the term “substrate” refers to any of a range of materials upon which the nanoparticle ink is deposited for curing. Exemplary substrates include polymers such as plastics; textiles; paper; sheet materials; metals; ceramics; and other materials that provide a suitable surface for depositing a pattern of nanoparticle-based ink. Substrates can be flexible or rigid.
As used herein, the term “energizable” relates to a device or set of components that perform an indicated function upon receiving power and, optionally, upon receiving an enabling signal.
The background section outlines a number of problems with conventional methods for sintering using Xenon light and other broadband light energy. Embodiments of the present invention address the problem of curing and sintering for nanoparticle-based materials using an array of one or more diode lasers, preferably arranged in series or in parallel. Each laser in the array emits a continuous or quasi-continuous beam for nanoparticle curing. According to embodiments of the present invention, the laser diode array is arranged so that the emitted laser energy extends across the width of a substrate, allowing single-pass curing of high volumes of material. Alternate embodiments enable curing in a swath that does not fully extend across the width of the substrate.
Embodiments of the present invention combine aspects of nanoparticle-based ink formulation with curing and sintering operations in order to achieve conductive traces of superior conductivity and adhesion. Results obtained using the formulation and processing described herein have achieved bulk ratios lower than 3×, approaching more closely the conductivity of copper traces than has been achieved with conventional methods.
Embodiments of the present invention can use copper nanoparticles coated with a polymer or, alternately, a combination of copper nanoparticles coated with polymer and copper/copper-oxide (Cu:CuO) nanoparticles that have a copper core with a CuO shell or coating and about 8% PVP (Polyvinylpyrrolidone) dispersant. In the context of the present disclosure, the phrase “copper oxide nanoparticles” is equivalently used for copper/copper-oxide (Cu:CuO) nanoparticles. Nanoparticles formed from other metal and semiconductor materials can alternately be used.
The ink is advantageously prepared in a solvent that contains a sintering enhancer, such as about 5 weight % of glycerol. Alternately, some other solvent having a high boiling point above about 200 degrees C. and a low vapor pressure below about 2 mm mercury at 20 degrees C. can be used, with a liquid or solid sintering enhancer in place of some or all of the glycerol content. Glycerol, with a vapor pressure of about 1 mm at 20 degrees C., appears to work well. Alternative sintering agents can include solid additives that may be non-volatile but are vaporized when sintering energy is applied.
In the context of the present disclosure, a “high boiling point” solvent is a solvent material having a boiling point (b.p.) in excess of about 200 degrees C. A “low boiling point” solvent has a boiling point below 200 degrees C. The high boiling point solvent can be glycerol (b.p. 290 degrees C.) or can be taken from the group consisting of 1,2-dodecanediol; 1,2-decanediol; N-methylpyrrolidone; diethylene glycol; diethylene glycol monoethylether; diethylene glycol monobutylether; diethylene glycol monoethylether acetate; diethylene glycol monobutylether acetate; dipropylene glycol; dipropylene glycol monobutylether; and 2-methyl-2,4-pentanediol. Low boiling-point solvents that can be used include ethanol; butanol; 2-methoxyethanol; 1-propanol, 2-propanol; and 1-methoxy-2-propanol, for example.
Table 1 lists formulations for the conventional comparative examples and for example embodiments of the present invention that are described herein. Percentages in Table 1 and in this disclosure are weight % unless specifically noted otherwise.
The glycerol or other sintering enhancer can be added at any stage in the ink formulation, such as during mixing before high shear or during ultrasonic processing. The added sintering enhancer does not appear to affect the stability of the formulation.
It can be appreciated that using a solvent of this type in the formulation extends the drying time over that conventionally used for conductive ink application. Thus, when following conventional sintering practices, solvent composition of this type would be considered an undesirable feature of an ink formulation. However, instead of requiring an extended drying time due to the added glycerol, embodiments of the present invention reduce the drying time when compared with conventional processing methods. Advantageously, given the formulations shown in D, E, F, and G of Table 1, the printed traces are ready for sintering when not yet fully dry. Subsequent examples describe particular dryness levels for the applied nanoparticle-based inks prior to sintering.
The printed electronic structures that can be formed by the present method are made of a metal or semi-metal, such as semiconductor material. Suitable metals for printing and curing in a pattern include, but are not limited to, copper, gold, silver, nickel, and other metals and alloys. Semi-metal materials including silicon can also be used. Furthermore, silicon particles that have been doped to provide semiconducting behavior (for example, doped with phosphorous or arsenic) are also suitable. Therefore, the present method can be used in production of both electronic structures, such as connecting traces between devices, and semiconducting devices themselves.
The nanoparticle ink used in embodiments of the present invention comprises the metal or semi-metal with a binder or coating (typically organic). The binder or coating in the ink helps to prevent agglomeration and to maintain the surface area, which confers many of the advantageous properties of nanoparticles. The nanoparticles used in the ink formulation can be between 0.5-500 nm diameter. Advantageously, therefore, the present invention can be implemented for a wide range of nanoparticle inks including those with larger particles which are often cheaper to produce.
The high surface area of the nanoparticles is advantageous, so that the energy required to transform the nanoparticles in the ink, such as by sintering or curing, is less than for bulk materials. Therefore, as the laser illumination not only removes the coating or binding materials in the ink formulation, it also causes a transformation of the material.
Upon receiving the illumination energy, the individual metal/semi-metal nanoparticles bond to form a metal/semi-metal structure, in the form of a densified metal or semi-metal film (depending on the material of the nanoparticle ink). As the laser illumination can be focused to a small spot size, the metal structure that is formed is localized to areas impacted by the laser. The high degree of accuracy with which the laser can be directed results in the formation of the high resolution printed structures.
The flow diagram of
Instead of requiring an extended drying time due to the glycerol, as would be expected, embodiments of the present invention shorten the drying time as compared with drying times typical of conventional processing methods. Shorter drying times are used because the applied traces are ready for sintering when not yet fully dry. The inventors have found that it is advantageous to reduce the time between printing and sintering so that a measurable amount of high boiling point solvent remains in the deposited ink when sintering energy is applied. Sintering energy is applied to a partially wet patterned substrate.
The graph of
The inventors have found that sintering once the solvent percentage is lower than about 8% and exceeds at least about 3% provides significantly improved conductivity over conventional approaches that require a fully dried ink for sintering. Thus, even though a high boiling point solvent is used in the ink formulation, the drying time for sintering with the inventive formulations is within the range of drying times conventionally used where fully dry ink is required. Moreover, the inventors have found that there is a range of acceptable drying conditions, including drying at room temperature under at least partial vacuum, drying under ambient atmospheric pressure conditions, drying under applied heat, etc. In a particular application, the rate of drying, hence the time interval between printing and sintering, is adjusted so that the deposited ink on the substrate has from about 3-8% solvent composition, forming a partially wet patterned substrate.
Laser energy is used for sintering the applied traces on the partially wet patterned substrate. By way of example, the schematic diagram of
Continuing with the
Transport apparatus 90 more generally provides relative motion for forming a pattern and can also operate wherein substrate 120 is stationary and one or more of energizable surface conditioning, printing, and curing components, such as apparatus 130, 140, 150, 164, 170, 240, and 250 are swept along the surface of substrate 120 to perform pattern deposition and curing operations.
It can be appreciated that the printing and sintering apparatus embodiments of
Washing or cleaning step 20 in the sequence of
According to an embodiment of the present invention, the substrate 120 material is transported by means of transportation apparatus 90 (
Optional temperature control element 100 (
Optional coating step 30 in the
Continuing with the
Auxiliary drying equipment, not explicitly shown in the
As shown in the perspective view of
The use of laser light allows for the selection of a light wavelength that is well suited for the sintering of the nanoparticles while eliminating or minimizing damage to the coating. By using lasers, embodiments of the present invention apply monochromatic light to the substrate at wavelengths most favorable to sintering and other curing functions, without contributions from other wavelengths, such as lower wavelength light that can be heavily absorbed in the upper layers of deposited material. As noted earlier, absorption of wavelengths in upper layers nearest the surface can cause these upper layers to be inadvertently sealed, trapping binder and other materials that must be removed from beneath the surface. Advantageously, laser illumination provides sufficient energy for the removal of component materials in the precursor nanomaterial. This includes materials useful for improving ink application but not wanted in the final product, such as organic binders and particle coatings. With laser light, the spectral content and intensity can be specified and controlled so that the laser delivers the proper energy to the applied material, at the proper depth. In this way, problems such as unwanted sealing of top layers can be avoided.
According to an embodiment of the present invention, diode laser arrays are formed of diodes having different emission wavelengths, wherein the emission wavelengths of at least two of the laser diodes in the array differ from each other by more than 25 nm. By spacing optical fibers 200 in the coupling block 210, the subsequent illumination lens 220 accommodates accurate spot placement for each wavelength. There can be some trade-off of spot size verses proximity between the different wavelengths. The focal spot size can also vary as a function of wavelength.
Advantageously, separate laser channels can be addressed simultaneously or sequentially. With simultaneous addressing, printing relies on the difference in required curing wavelengths for different materials. For example, ink having high copper (Cu) content tends to cure with applied energy in the near infrared, whereas some inks high in silicon content cure uniformly using shorter wavelengths.
When driving laser diodes, pulse width modulation (PWM) can be used for controlling power levels and for temporally interspersing the illumination wavelengths. This permits the use of different wavelengths, both to cure different materials and to provide curing energy at different depths. For example, a longer wavelength can be useful for curing material at a greater depth. Wavelengths that have been found to be suitable for curing include, but are not limited to: 193 nm, 248 nm, 308 nm, 355 nm, 488 nm, 532 nm, 808 nm, 860 nm, 975 nm, 1064 nm, and CO2 laser wavelengths.
Each deposited material or ink can have different curing properties, responding differently to light of various wavelengths and intensities. Where multiple materials are deposited, it may be suitable to cure the different materials under the same conditions or to vary wavelength and intensity levels appropriately. According to an embodiment using a single illumination apparatus 170 as in
With the addition of glycerol or other suitable high BP solvent, excess laser energy from sintering may be transferred to the task of decomposing the dispersant/binder, improving thermal transfer and leaving a highly conductive pattern.
Thermal characteristics of the substrate can complicate the task of sintering in a number of ways when conventional Xenon flash energy is used. Substrates having relatively high thermal conductivity, such as aluminum, silicon, and ceramic substrates, for example, can conduct the needed heat away from the area of incident light before sintering energy levels are reached. Polymer-based substrates, such as ITO coated plastic substrates, can be damaged due to the higher thermal conductivity of the ITO coating. Embodiments of the present invention help to address problems related to thermal response by using laser light that can be focused onto a small area.
It is found that the present method is particularly suitable for a number of substrates including PET (polyethylene terephthalate), PI (Polyimide), PE (polyethylene), PP (Polypropylene), PVA (poly-vinyl alcohol), SiN (silicon nitride), ITO (indium tin oxide) and glass. On such substrates, the present application provides an improved method for producing high resolution lines compared to other systems. In particular, the direct transformation (curing, sintering or otherwise) of the material by the laser allows for higher resolution features, reduces or avoids the need for adding further layers such as photoresist layers, and requires fewer stages to produce than do conventional methods. Printing and curing of electronic materials and components can be performed at low volumes as well as for large-scale, high volume production.
The following process steps describe using copper nanoparticles; it should be noted that formulations with other conductor and semiconductor materials could alternately be used.
Cu nanoparticles (25 weight %) with a dispersing agent were mixed with ethylene glycol and ethanol at a ratio of (40:60). The suspension was high shear mixed at 5000 rpm for 1 hour. An ultrasonic horn was then used to break up any aggregates for 1 hour with chilled water to minimize solvent evaporation from overheating. This ink was filtered through a 1.2 um filter.
Blend Cu Nanoparticle Ink with Glycerol—
0.1-50 weight % of Glycerol was added into the Cu nanoparticle ink. Agitation was used to mix the solvents for 1 min.
Deposit Ink onto Substrate—
The blended Cu nanoparticle ink is deposited onto the substrate, such as by inkjet printing or other printing technique. The coatings are then partially dried in an oven or other drying apparatus, leaving only the high boiling point solvent component in the deposited ink, so that the ink is still considered to be wet. (Refer to the preceding description on ink drying given with reference to
The partially dried blended Cu nanoparticle ink is laser-sintered and cured. Remaining uncured areas are washed off. Washing is done, for example, using ultra-sonication.
Although the above process has been described with examples that us copper nanoparticles, this process would be applicable to nanoparticles using other materials. For example, ink formulation, deposition, and sintering can employ other conductive and semiconductor materials such as silver (Ag), gold (Au), palladium (Pd), platinum (Pt), nickel (Ni), silicon (Si), including doped silicon, alumina (Al2O3) and their combinations thereof. For each of these materials, solvent mixture, ink deposition, and sintering processes would follow similar steps as those described for copper nanoparticles, with corresponding changes according to the conductive or semiconductor materials used.
Cu nanoparticles (25 weight %) with a dispersing agent were mixed with ethylene glycol and ethanol at a ratio of (40:60). The suspension was high shear mixed at 5000 rpm for 1 hour. An ultrasonic horn was then used to break up any aggregates for 1 hour with chilled water to minimise solvent evaporation from overheating. This ink was filtered through a 1.2 um filter.
Glycerol (5 weight %) was added into the above formulation. The mixture then agitated for about 1 min to allow sufficient mixing of the solvents.
The blended Cu nanoparticle ink was spin-coated onto a glass substrate. The coating was then partially dried in an oven at 25° C. in vacuum for 100 mins.
The wet blended Cu nanoparticle ink was laser sintered and cured using a continuous wave 808 nm diode laser. Remaining uncured areas were washed off by ultra-sonication using H2O/IPA (50:50) followed by a mixture of ethylene glycol/ethanol (40/60).
Table 2 in
Table 3 in
Addition of glycerol or other high boiling point solvent to existing Cu nanoparticle ink formulations provides a number of advantages, including the following:
Measured results have shown that embodiments of the present invention significantly increase the level of conductivity that can be achieved using printed conductive traces. The inventors have shown the capability to routinely achieve 2-2.5× bulk Cu conductivity. Previous methods for forming conductive traces were unable to achieve better than about 6.6× bulk Cu conductivity.
Low vapor pressure means glycerol will remain in the coating at standard temperature and pressure.
Embodiments of the present invention enable reel-to-reel processing with on board printing, and fast curing without totally removing the solvent.
Referring again to the sequence of
Embodiments of the present invention advantageously allow high resolution features to be produced in a single stage process. In particular, the invention avoids the need for an extra layer, such as a photoresist layer, and its subsequent processing. Furthermore, unlike photoresist methods, the method of the present invention does not require the use of etchants to remove the unprotected, uncured structure. This is advantageous as it simplifies the production process and greatly reduces costs related to waste handling. In addition, embodiments of the present invention allow a measure of accuracy with direct placement of electronic traces and structures. It is known, for example, that etchants used in conventional electronic patterning can result in excessively sloped tracks or undercut features, whereas the use of lasers to directly cure/transform the material allows for well-defined edges to be formed.
Some embodiments of the present invention provide a method for forming a conductive pattern on a substrate, the method comprising:
According to an embodiment of the present invention, the high boiling point solvent is glycerol. The laser light sources can be laser diodes. The copper oxide particles can have a polymer coating.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. For example, other high boiling point solvents can be used in place of glycerol.
This application claims the benefit of U.S. Provisional application U.S. Ser. No. 61/918,945, provisionally filed on Dec. 20, 2013, entitled “A METHOD FOR DEPOSITING AND CURING NANOPARTICLE-BASED INK”, in the names of Michael J. Carmody, Richard J. Dixon, Chu Wong Aaron Chan, Kai Man Kerry Yu, Hsin-Yi Sherry Tsai, Glenn Shackleford, and Janet Heyen, incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61918945 | Dec 2013 | US |
