Patent Application
20090286383
This application claims priority to U.S. Provisional Application Serial Nos. 61/099,027 and 61/053,574, which are hereby incorporated by reference herein.
The present invention relates in general to nano-sized wires and whiskers.
For decades Sn/Pb components and circuit assemblies have been used for high reliability, high performance military, space and commercial applications. With government legislation intended to eliminate lead from electronics manufacturing and packaging, the next generation of high performance devices will likely rely on lead-free technology. Unfortunately, lead-free surfaces readily form nanowires or whiskers that can grow in length may cause electrical short circuits between adjacent components, circuit elements, or devices. The tendency of tin surfaces to form nanowires or whiskers is prevalent, in particular, during the electro-deposition processes utilized in the electronic industry. One way of mitigating the nanowire or whisker formation is to realize a more robust tin-based electro-deposition process with very low potential for whisker growth, which eventually can be used to fabricate electronic circuits in high reliability weapon systems. One approach to improve electro-deposition may require control of a large number of parameters such as dopants, impurities, additives, and wetting agents. An alternative approach is to develop post-deposition or even post-assembly treatments to the circuit board assemblies to eliminate the nanowires or whiskers and/or limit their formation.
The assignee of this application has been successful in developing a copper conductive ink using copper metallic nanoparticles for inkjet printed electronic applications on various substrates (see U.S. Provisional Applications Ser. Nos. 61/077,711, 61/081,539 and 61/053,574, all of which are hereby incorporated by reference herein). The inkjet deposition process for this copper ink is compatible with ambient atmospheric environments (no inert environments are required), and a drying process may be performed at temperatures under 100° C. This achievement results from a process of photosintering the printed copper ink traces, whereby a very short flash of photonic energy transforms the dried copper ink to metallic copper traces with resistivities as low as 3×10−6 ohm-cm.
The aforementioned photosintering process may also be utilized to flatten (melt down or evaporate away) copper nanowires and copper whiskers to the level of the base copper surface, basically causing them to disappear. This process can also be applied to reduce or eliminate tin whiskers or nanowires that are developing during the electro-deposition process by utilizing the photosintering processes and systems as post deposition and/or post assembly processes. This process will also work to eliminate or at least diminish tin whiskers or nanowires as well as or better than as demonstrated on copper whiskers due to tin's lower melting temperature. Furthermore, embodiments of the present invention can be used to diminish or eliminate nanowires or whiskers of other materials. The photosintering systems are inexpensive, are commercially available, and require less than 1 second process time. Furthermore, they can be comfortably integrated with current laboratory and industrial electro-deposition systems.
Treatment, elimination, diminishment, reduction of nanowires and whiskers:
Referring to
In step 101, provided is a copper substrate, such as a substrate with copper electroplated thereon, a copper wire on a substrate, or copper powders on a substrate. In step 102, the copper oxide nanowires/whiskers grow as a result of thermal oxidation. The growth parameters may be 200-700° C. for 0.5-3 hours in air or ambient oxygen environment. The nanowires/whiskers are then diminished, reduced in number and/or length, or eliminated with photosintering in step 103. Photosintering may be performed with an Xe lamp at an energy density of 0.1-15 J/cm2 for 100-900 μs. In step 104, the sample may then be inspected by microscopy.
The digital photo of
CuO nanowires are a promising p-type semiconductor material (1.2-1.4 eV band gap). CuO nanowires have potential applications as gas sensors and as counter electrodes of dye solar cells. Combined with the photosintering technique, the CuO nanowires may become much more useful. For example, photosintering can bond the CuO nanowires on polymer substrates for producing flexible devices. Photosintering may also modify the CuO nanowires (or other metal or metal oxide nanowires or whiskers) to meet application requirements.
The mechanism accounting for the huge change of tin whiskers after pulse photosintering is the fast thermal effects. The heat generated in tin whiskers during the intense photo flashing causes the tin whiskers to plastically distort, melt, or even evaporate, since the tin has relatively low melting and boiling points. This is strongly supported by SEM observation of the samples.
The digital image in
The digital images in
The SEM digital images in
The thermal effect nature causes the treatment efficiency to depend on the heat dissipative ways. The tin whiskers running out of or normal to a substrate surface are more likely to be destroyed than those lying on the substrate surface, because the latter more easily dissipate the heat flow to the substrate, as is show in
Energy flash curing involves a short, high intensity light pulse. This light is absorbed by the metal nanoparticles and converted to heat. This heat then causes the metal nanowires or whiskers to deform and melt. A high intensity lamp may be used to supply enough energy. Energy flash curing is most effective when it is used in the high intensity—short time flash mode, because under these conditions the thermal energy that is produced remains within the nanowires or whiskers, and there is minimal or no thermal damage to the substrate surface. A shorter flash time provides sufficient energy to penetrate the metal plating surface and relieve the stress and strain of the plated material without compromising the surface film.
The optical energy that is absorbed by whiskers or nanowires will convert into thermal energy (DE). This thermal energy will increase the temperature of the whiskers or nanowires. Some thermal energy from the nanomaterials will dissipate into the substrate and the surrounding air. For bulk metal, the increased temperature (DT) can be given by the equation: ΔE=mC, ΔT, where m is the mass of bulk metal, and Cp is the specific heat of the metal. Because the energy flash occurs in the microsecond time scale, the tin does not have the time to oxidize, thus maximizing the electronic conductivity. In oven curing, for example, the curing process can take hours, which allows the sample to oxidize if air is allowed to be present.
The maximum temperature reached by the substrate or other parts on the substrate is less than 100° C. This low curing temperature prevents the substrate or electronic components from thermal breakdown that would otherwise occur at higher temperatures demanded by other deposition processes. This low temperature process is non-destructive. The broad spectrum source is non-damaging even to photo-sensitive materials such as Kevlar or optical top coats.
Embodiments of the present invention will successfully operate to diminish or eliminate metal whiskers (e.g., Sn or Ni whiskers), oxide whiskers (e.g., CuO, or ZnO), or compound and element semiconductor whiskers (e.g., GaN or Si whiskers).
Referring to
A flow chart of a photo-curing process is illustrated in
In some embodiments, a thermal curing step can be introduced subsequent to dispensing the film 803 and prior to the photo-curing step 816. The substrate 804 and deposited film 803 may be cured using an oven or by placing the substrate 804 on the surface of a heater, such as a hot plate. For example, in some implementations, the film 803 is pre-cured in air at 100° C. for 30 minutes before photo-curing. Alternatively, the thermal curing may be performed by directing a laser onto the surface of the film 803. Following the drying and/or thermal curing step 814, a laser beam or focused light from the light source 806 may be directed in step 816 onto the surface of the film 803. The light source may serve to photo-cure the film 803 such that it has low resistivity. Generally, the metal films are insulating after the printing/dispensing 812 and drying 814 steps. After the photo-curing process, however, the insulating film becomes a conductive film 809 (see
In some implementations, the dispenser 802 is used to deposit a blanket film or a coarse outline of the pattern. Typically, printing techniques can achieve feature sizes on the order of 25-50 microns or greater. If finer features are necessary, the pattern/blanket film can be refined or reduced using a focused beam of light or laser, in which case the features are defined by the spot size of the laser or by the focus of the light beam. Typically, light can be focused to 1 micron or less. Thus, submicron features may be possible. Ultimately, the feature size may be limited by the size of the nanoparticles used in the conductive film. Metal particles may be formed to have features on the order of 1-5 nm.
| Number | Date | Country | |
|---|---|---|---|
| 61099027 | Sep 2008 | US | |
| 61053574 | May 2008 | US |
