1. Field of the Invention
The present invention relates to a Z-axis conductive film and a method for fabricating the same, and more particularly, the present invention relates to a composite conductive film including a polymer matrix and conductive nanowires and a method for fabricating the same.
2. Description of the Related Art
Interconnection technology of a flip-chip package for the I/O pitch less than 50 μm (micrometer) is accomplished by Z-axis conductive films. However, Z-axis conductive film cannot be used in a flip-chip package with pitch smaller than 30 μm because the size of the conductive particles in Z-axis conductive film is approximately 3 μm, and this size cannot be reduced any further. As a result, Z-axis conductive film cannot be used in the flip-chip package of the pitch less than 30 μm. The electrical conduction of Z-axis conductive film is realized by contact between metallic films chemically electroplated on surfaces of polymer particles and electrodes of a chip and a substrate. This contact is a kind of physical contact, and has a larger joint resistance relative to the chemical joint of soldering. Hence, Z-axis conductive film is not suitable for integrated circuit devices driven by current.
In addition, joint resistance is related to the density of the conductive particles in conductive film. But the density of the conductive particles in conventional Z-axis conductive film is not very high for the purpose of maintaining insulation in X and Y directions (i.e. avoiding lateral short circuits). As the pitches of the packaged devices become smaller in the future, the electrodes' areas decrease. Joint resistance will be increased as the density of the conductive particles is decreased.
In addition to Z-axis conductive film, solder bumps are used to electrically connect the electrodes of the chip and substrate. Since the coefficients of thermal expansion (CTE) of the chip and substrate are mismatched, the stress there between adversely influences the reliability of the connection of the chip and the substrate. It is necessary to use underfill between the chip and the substrate after packaging. However, when the jointing pitch is reduced to a size of less than 100 μm, the underfill does not easily enter the space between the chip and the substrate. The current methods to resolve this drawback include: (i) replacing the ball-shaped solder bump with a copper stud having a high height-to-width ratio to increase the gap between the chip and the substrate; and (ii) adapting conductive polymer bumps with low Young's modulus to serve as stress buffers. However, the above methods have disadvantages. The Young's modulus of the copper stud is larger than that of the solder bump, and is a poor stress buffer. The resistance of the conductive polymer is at least ten times greater than that of metal. Therefore, the conductive polymer is not suitable for electrical connection of the flip-chip package with fine pitches and small electrode areas.
Accordingly, a Z-axis conductive film for electrical connection of a fine-pitched flip-chip package was developed. For example, U.S. Pat. No. 5,805,426, entitled “Microelectronic Assembles Including Z-Axis Conductive Films”, provides a Z-axis conductive film, as shown in FIG. 1, which uses a nanoporous polymer film as a template. By filling pores of the polymer film, a composite conductive film formed of nanowires (31, 34, 37) and polymer is provided. The chip and substrate can be directly press jointed together by this composite conductive film. Electrical connection there between is established by the metallic nanowires (31, 34, 37) and pads (32, 33, 35, 36) of the chip and substrate. The CTE of the composite conductive film can be varied or its thermal conductivity can be increased by selectively filling different metals in the pores of different positions. The nanoporous polymer film is made by exposing a nonporous resin film to accelerated ion beam having sufficient energy or a light beam to pass through the entire thickness of the film. The above method is costly and time-consuming. Moreover, the uniformity of the pore diameters is not easily controlled. The differences of the pore diameters can be as great as hundreds of nanometers or more. Since the pores of the polymer film are previously formed, the polymer film cannot be a B-stage polymer. Thus, the polymer film cannot provide sufficient adhesion during a subsequent jointing step by thermal press to maintain contact between the electrodes of the chip and the substrate and metal nanowires. Thus, the reliability of electrical connection of the composite polymer film is degraded.
Additionally, U.S. Pat. No. 5,262,226 provides an Z-axis conductive film, as shown in FIG. 2, which includes an alumina substrate 2 having a plurality of metal nanowires 3 formed therein. U.S. Pat. No. 5,262,226 thus provides a conductive film 1 made of an alumina substrate 2 and metal nanowires 3, which is made by two methods. One method involves selectively undergoing an anodic oxidation process to form a conductive film 1 composed of aluminum (Al) 3/alumina (Al2O3) substrate 2. The conductive aluminum 3 can be replaced by solder ball/gold/solder ball. However, this manufacturing method is limited to the capability of a photolithographic process, and can merely manufacture metal wires with a diameter of 20 μm or more. The other method is firstly to manufacture a porous template of alumina, and then selectively electroplate metal in some of the pores to form a conductive film having a plurality of metal wires. Thereafter, one electrode is respectively formed at the upper and lower ends of each metal wire to joint a substrate-level chip. Alumina (Al2O3) is used as a substrate of the conductive film 1 of U.S. Pat. No. 5,262,226. Alumina has good heat-dissipation and insulating properties, but its Young's modulus is too large and too fragile to release stress generated during packaging. Moreover, the adhesion between alumina and the substrate, as well as between alumina and the chip is insufficient to maintain electrical connection of the electrodes and the conductive film.
Accordingly, the intention is to provide a Z-axis conductive film with fine pitches, low resistance and high jointing strength, which can overcome the drawbacks of the prior art.