Patent Application
20120049384
The microelectronics and semiconductor packaging industries have begun to shift to printable electronics. Electronics circuits comprise a variety of components that are electrically connected to each other. Such electrical connections between different components may be made of conductive metal traces that can be printed on substrates with conductive inks The inks are processed and sintered after deposition on a substrate in order to become conductive. Thermal sintering uses a high temperature (e.g., ≧250° C.) to fuse the nanoparticles in the inks Photonic (photo) and laser sintering utilize a very high intensity lamp/laser to fuse the nanoparticles in a very short period of time (e.g., microseconds) with a low temperature and so as not to damage the underlying substrates. However, the photo/laser sintering process has limits that require low thermal conductivity material for substrates in order for the nanoparticles to effectively absorb energy and sinter before heat energy dissipates into the substrate. In other words, the substrates that can be used in these applications will be very limited for low thermal conductivity materials.
On the other hand, low thermal conductive substrates can be used for flexible printable electronics. Low temperature melting point materials such as polyethylene (PE), polyester (PET), etc., will prevent the nanoparticle inks from proper sintering, and the substrates will be damaged, with the result that the resistivity will be very high.
Embodiments of the present invention disclose a photo sintering process to effectively sinter metallic nanoparticles on a polyimide substrate, thus causing the film to be very conductive near the bulk material. On other hand, the photo sintering process does not perform well on nanoparticle inks coated on substrates possessing a high thermal conductivity, such as ceramics and silicon wafer. Table 1 shows the thermal conductivity for a variety of materials.
Low conductivity materials, such as polyimide, can be used as a coating material onto other high thermal conductivity substrates, such as ceramics and silicon wafer, in order to isolate heat energy dissipation from nanoparticles during a photo sintering process so that the nanoparticles are fused more effectively. How quickly heat dissipates depends on the thickness of the low thermal conductivity material (e.g., polyimide film).
The following experiment was conducted for showing how the present invention operates. Three wafers were spin coated with 1, 1.5, and 2.3 microns thick DuPont PI-2610 polyimide, respectively, and thermal cured at 350° C. for 30 minutes. One bare silicon wafer was used for a reference (wafer #1). All four wafers were coated with copper ink using a drawdown process. After a 60 minute drying process at 100° C., each wafer was divided into three zones that were individually sintered with three different energy levels. The resistance for each zone and each wafer was measured with a voltmeter, with the results shown in Table 2, which shows the electrical resistances of a copper film after photo sintering with various coating thicknesses of polyimide on silicon wafers.
Except for zone 3 of wafer 4, all zones from the four wafers did not experience a change in resistance after photo sintering. Zone 3 of wafer 4 experienced a change in its metallic color at the highest energy level, as shown in
Wafers 1, 2, and 3 all had high resistance (greater than 20 mega ohms). Wafer 4 at the center zone with 20 ohms resistance as shown in
In addition to the liquid polyimide disclosed above, a dry polyimide film was also utilized. The copper ink was coated on a 50 micron polyimide film (Kapton). The sample was placed on a silicon wafer and a carbAL high thermal conductive heat sink, as shown in
In addition, laser sintering was utilized on silicon wafers with the same setup as described above. The laser was a solid state diode with an 830 nm wavelength and an 800 mW power. The focus beam size was 15 microns in diameter and controlled by a collimator and an objective lens, as shown in
This laser had sufficient power to sinter and fuse the nanoparticles and turn the copper ink conductive. There were four silicon wafers coated with various polyimide thicknesses of 1, 1.5, 2, and 3 microns, respectively, along with a bare silicon wafer as a reference. The resistivity of each wafer is plotted with laser power in
Furthermore, a variety of polyimide thicknesses were coated on silicon wafers and cured at 350° C. for one hour. Then the standard copper ink was coated by drawdown, dried in an oven, and photo/laser sintered. Electrical measurements were performed and characterized the copper ink samples.
Three types of polyimide material made by DuPont were used to spin coat on silicon wafer at 1000, 2000, 3000, 4000 and 5000 rpm.
After samples were prepared, both photo and laser sintering were performed on the copper inks Different types of sintering were compared versus resistivity and adhesion, as well as line width for laser sintering. Table 3 shows samples photo sintered at the same energy level with various thicknesses of polyimide. Table 4 shows samples laser sintered at a fixed power level with various thicknesses of polyimide.
Referring to
1. The effectiveness of a photo sintering process depends on not only metallic nanoparticle size, but also the type of substances.
2. Effective photo sintering is achieved with nanoparticles below 300 nm.
3. The thermal conductivity of substrates will affect metallic ink photo sintering. The lower the thermal conductivity of the substrate, the better the electrical conductivity of the nanoparticle film.
4. High thermal conductive substrates can be tailored and isolated by coating low thermal conductivity material, such as polyimide or polymer, for an effective photo sintering process.
5. The thickness of coating of polyimide required to isolate thermal heat dissipation is approximately 1-50 microns.
6. The copper ink becoming a conductive film has been demonstrated on high thermal conductive material such as silicon wafer with both laser and photo sintering.
7. Heat dissipation on high thermal conductive silicon wafers has been shown with a variety of polyimide thicknesses coated on a wafer. A low thermal conductive material can be used as a buffer layer to slow down heat dissipation and enhance the photo or laser sintering.
8. Copper ink may be sintered well with polyimide coated on a silicon wafer with resistivity at 1×10−5 ohm-cm by photo sintering and 4×10−6 ohm-cm by laser sintering.
9. The polyimide material may be not only utilized as a heat insulator on high thermal conductive substrates and enhance copper ink photo and laser sintering effectiveness, but also applied to low melting temperature substrates as a heat insulator to protect from heat damage during a sintering process.
10. Polyimide layer and metal trace layer can be repeated several times as multilayer circuits.
11. Polyimide layer can be used as a dielectric material and incorporated as capacitors.
12. Nano-copper ink can be used at top layer conductor as a contact metal in two-dimensional and three-dimensional chip packaging applications.
This application claims priority to U.S. Provisional Patent Applications Ser. Nos. 61/174,758 and 61/163,894.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US10/28811 | 3/26/2010 | WO | 00 | 9/28/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61163894 | Mar 2009 | US | |
| 61174758 | May 2009 | US |
