Patent Application
20100000762
This document relates to metallic pastes and inks that can be used to form metallic conductors with improved conductivity.
Metallic pastes, like metallic inks, can be formulated with metal nanoparticles, liquid vehicles, dispersants, and other additives. Additives can be included to alter physical properties such as viscosity, wetting, and contact angle on chosen substrates. The higher viscosity of pastes (e.g., about 10,000 cP to about 60,000 cP) compared to inks (e.g., less than about 5000 cP) facilitates persistent dispersion of metal nanoparticles. Use of a metallic ink or paste can be based on a number of factors including printing method and substrate. Inks with a low viscosity (e.g., less than about 20 cP, or between about 10 cP and about 20 cP) may be ink-jet printed or aerosol printed. Pastes are too viscous to be ink-jet printed and may be applied by screen printing or other methods that are suitable for higher viscosities.
Some metallic pastes or inks, however, may require heating at elevated temperatures in an inert atmosphere-conditions which can be unsuitable for certain applications, such as flexible electronics (e.g., with polymeric substrates). Additionally, some metallic pastes or inks include one or more liquid components with a high boiling point. When a metallic paste or ink with a high boiling point component is cured by a slow thermal sintering process in air, the high boiling point components can form non-volatile products that remain in the cured conductor. Thermal decomposition to non-volatile products can also during thermal sintering in an inert atmosphere, yielding contaminated conductors and relatively high resistivity. In some cases, organic residues from the liquid vehicle degrade the adhesion between the conductor and the substrate, reducing the quality of the metallic conductor.
Metallic compositions (e.g., inks and pastes) formulated for low temperature processing are suitable for use in the manufacturing of printed electronics, as conductive adhesives, or in other applications including the manufacture and assembly of various electrical components and circuits, such as electrodes and interconnects. The metallic compositions include metal nanoparticles (e.g., copper, nickel, silver, gold, aluminum, cobalt, molybdenum, zinc, and the like) in optically transparent vehicles suitable for photosintering. The nanoparticles in these composition may be selected according to size and passivation coating, and the composition may be formulated to allow precision printing. The nanoparticles in the printed composition may be cured (e.g., photosintered, thermally sintered, or both) into bulk metallic films or lines at temperatures compatible with plastic substrates.
Metallic compositions described herein are formulated to yield cured conductors with reduced amounts of organic residue from the liquid vehicle. The metallic compositions may be sintered (e.g., in less than about 5 msec, less than about 2 msec, or less than about 1 msec) to produce metallic conductors using a photosintering process. In this photosintering process, a high-intensity light pulse (e.g., about 50,000, 100,000, or 150,000 lux or higher) is absorbed by the metal nanoparticles in the composition and then converted into heat. As a result, the metallic composition may be subjected to a short, high thermal pulse that rapidly evaporates organic components before these components undergo thermal oxidation or decomposition. This photosintering of metallic compositions (e.g., pastes and inks) yields conductors with high conductivity that may be formed at lower temperatures, and lower resistivities than some thermal sintering processes alone.
In some embodiments, the metallic compositions described herein may be formulated such that photosintering and/or thermal sintering of the composition on a polymer substrate (e.g., in air or in a forming gas) provides conductors that have a lowered resistivity. For example, copper conductors made from copper pastes described herein may have a resistivity between about 1×10−3 Ω·cm and about 1×10−6 Ω·cm. That is, the copper conductors may have a resistivity of less than about 1×10−3 Ω·cm, less than about 1×10−4 Ω·cm, less than about 1×10−5 Ω·cm, or greater than about 1×10−6 Ω·cm.
An embodiment of preparation of metallic compositions used to form conductors with high conductivity (low resistivity) is illustrated in
A metallic paste prepared as described in
The dried paste may be cured in a forming gas or in air. For example, the dried paste may be thermally sintered for about 60 min at about 350° C. in a mixture of up to about 10 vol % hydrogen in nitrogen (e.g., about 3-5 vol % hydrogen in nitrogen). For a copper paste with nanoparticles in a range of about 20 nm to about 200 nm, a resistivity of the thermally sintered film may be about 3×10−4 Ω·cm. The forming gas may reduce copper oxides in the dried paste to copper. For example, the hydrogen component in the forming gas reacts with the copper oxides to form copper and water as shown below:
CuO+H2→Cu+H2O
and
Cu2O+H2→2Cu+H2O
The water vapor can be carried off in the forming gas.
The thermally sintered metallic composition may be photosintered in a forming gas or in air to reduce its resistivity. Photosintering includes subjecting the metallic composition to a flash of light. The intensity (as measured by the voltage) and duration (as measured by the pulse width) of the light flash may be selected to reduce blow off of the metal particles from the substrate, to reduce resistivity of the resulting conductor, and to increase adhesion of the resulting conductor to the substrate. In an example, after photosintering of a thermally sintered copper conductor in air, a thickness of the conductor may be about 1 μm, and a resistivity of the conductor may be about 2×10−5 Ω·cm.
A metallic paste may be dried in air or in an inert atmosphere. A metallic paste, or a dried metallic paste, may be thermally cured to form a metallic conductor. The thermally sintered conductor may be photosintered to reduce the resistivity of the conductor. In some cases, a dried metallic paste can be photosintered without undergoing thermal sintering.
In an example, thermal sintering may include the following steps. A substrate with a dried metallic paste is loaded into a quartz tube at room temperature. The quartz tube is evacuated (e.g., to about 100 mTorr). The quartz tube may be heated (e.g., to about 350° C.) and purged with a forming gas (e.g., about 4 vol % hydrogen mixed with nitrogen) until the temperature is stabilized. The coated substrate may be heated for about 60 min at 350° C. After the forming gas and heater are turned off, and the tube may be purged with an inert gas (e.g., nitrogen) to cool the substrate (e.g., to below 100° C.). The substrate with the thermally sintered conductor may be removed from the quartz tube.
In step 208, the dried or thermally sintered metallic paste is photosintered. A high voltage flash xenon lamp may be used for photosintering. Photosintering may be achieved at temperatures of less than about 100° C. (e.g., ambient temperature, or about 20° C.), to yield a conductor with reduced electrical resistivity and increased adhesion to the substrate. U.S. Patent Application Publication No. 2008/0286488, which is incorporated by reference herein, describes a photosintering process.
A comparison between photosintering and thermal sintering is shown in Table 1.
The formulations for copper pastes ANI-1 and ANI-2 are shown in Table 2.
13-methoxyl-3-methyl-1-butanol
2Mean diameter 50 nm
3Daejoo organic vehicle from Daejoo Fine Chemical Co., LTD. (Korea)
4Mean diameter 200 nm
Points 304, 306, and 308 indicate a resistivity of about 4.5×103 Ω·cm for conductive films formed by thermally sintering copper paste ANI-1 in air at about 100° C., about 200° C., and about 300° C. Point 310 indicates a resistivity of about 2×10−4 Ω·cm for a conductive film formed by thermally sintering copper paste ANI-1 in air (points 304, 306, and 308) followed by photosintering in air at about 20° C. (1.2 msec, 1200 V). The photosintering step thus reduces the resistivity by over seven orders of magnitude.
Points 312, 314, and 316 indicate resistivities of about 4×102 Ω·cm, about 2×102 Ω·cm, and about 2×104 Ω·cm for conductive films formed by thermally sintering ANI-2 paste (nanoparticle size 200 nm, no additives or dispersants) in a forming gas (4 vol % H2 in N2) at 300° C., 350° C. 500° C., respectively. Thus,
In some embodiments, a dried metallic paste may be photosintered without an intermediate thermal sintering step. This may be advantageous for substrates damaged by higher thermal sintering temperatures. Because of the low temperatures involved in the photosintering process (e.g., below 100° C.), photosintering can be used to form conductors (with or without the use of a forming gas environment) with a resistivity in the order of 10−5 Ω·cm or less from metallic pastes and inks on substrates including polymers such as polyethylene, polyester, Flame Retardant 4, and the like, without damaging the substrate.
Metallic compositions (e.g., pastes or inks) may be used to make interconnects on printed circuit boards.
High viscosity copper pastes (e.g., about 10,000 cP to about 60,000 cP) may be prepared with high copper loadings (e.g., about 50 wt % to about 80 wt %) to allow for printing of thick lines. Thick lines with a low resistivity can carry high current density that may be required for many electronic devices. The copper pastes may be printed into the desired form of the interconnect features 402 in
The metallic pastes described herein may be used to make multi-layer interconnections to reduce interconnect length and electrical resistance. As a result, a high density interconnect with lighter weight, smaller real estate, less noise and less loss on electrical signals can be formed to yield improved chip to chip connects. The metallic pastes described herein may also be used to eliminate wire bonding processes so as to increase performance and reliability of circuit design for chip to board interconnects, as well as removing the risk of tin whisker growth risk (and subsequent short circuiting) that can be caused by the use of lead-free solder. By using a metallic paste that can be directly deposited (e.g., screen printed) and photosintered, manufacturing of multi-layer boards is simpler and less costly.
In some cases, a conductive bump formed from a metallic paste or ink may be used to create an interface between an integrated circuit and other electronic circuitry.
In some cases, as illustrated in
Manufacturing costs can be reduced when metallic bumps are formed of copper, for example, rather than gold, for several reasons. First, copper droplets can be positioned precisely with selected printing techniques (e.g., copper inks can be inkjet printed). Second, since the copper bumps can be cured (e.g., photosintered) at less than 100° C., a wider array of substrate materials may be used without being damaged by a high-temperature process. Third, since the copper bumps can be cured at low temperatures, the sintering can occur in an air environment (rather than an inert environment), with minimal oxidation of the metal occurring during bonding between the chip pads and the conductive bumps.
It is to be understood that while the invention has been described in conjunction with the detailed description thereof; the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.
This application claims priority to U.S. Provisional Application Ser. Nos. 61/077,711 to Roundhill et al. and 61/081,539 to Roundhill et al., the entire disclosures of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61077711 | Jul 2008 | US | |
| 61081539 | Jul 2008 | US |
