Patent Application
20080001271
The IC component 126 is disposed over the IC component 112 in a flipped configuration, with the operative surfaces 120, 132 of the two IC components 112, 126 facing, and the respective electrical contacts 118, 130 aligned. The conductive bumps 140 form conductive paths between respective electrical contacts 118, 130 on the two IC components. After the conductive bumps are activated—such as by heat, curing, or the like—the interstitial spaces between the conductive bumps 140, between the IC components 112, 126, may be filled with a dielectric adhesive 141. The adhesive 141 mechanically bonds the IC components 112, 126, and prevents differential thermal expansion. Both IC components 112, 126 and the wire bonds 116 may be encapsulated in a dielectric material 136, such as epoxy, plastic, or the like.
In one embodiment, the IC component 112 provides a conductive path 142 for one or more electrical contacts 130 on the flipped IC component 126 to an interposer 114 pin contact 122. The path is through a conductive bump 140, a corresponding electrical contact 118, the conductive path 142 and a second electrical contact 118 that is wire bonded 116 to the pin contact 122. This provides a path for electrical signals of the flipped IC component 126 to external package contacts 124.
A recent development in integrated circuit fabrication technology is the provision of conductive vias extending through the substrate to the back, or non-operative, side. This allows for wire bonding to either an electrical contact, or pad, formed in a backside metalization process, or in some cases directly to the metal filling the via. In one embodiment, the flipped IC component 126 may include one or more vias 144, each connecting an electrical contact 130 on the operative surface 132 to a corresponding electrical contact 146 on the non-operative surface 148. In another embodiment, the non-operative surface 148 does not include any electrical contacts 146. In either case, a wire bond 150 may electrically connect the via 144 to either an electrical contact 118 on the IC component 112 or a pin contact 122 on the interposer 114.
In the flipped, stacked-chip IC package 100, the entire, two-dimensional surface area of the smaller of the operative surfaces 120, 132 is available to form a data transfer bus between the IC components 112, 126, via corresponding electrical contacts 118, 130 connected by conductive bumps 140. Furthermore, the conductive bumps 140 are short, thus avoiding many of the transmission line effects (e.g., ringing, parasitic capacitance, and the like) of electrical connectors having a greater length, such as wire bonds 116. Accordingly, very wide data transfer buses may easily be formed between the two IC components 112, 126, and may be operated at high frequencies. For example, data transfer buses may be configured as x64, x128, x256, x512, x1024, x2048, and so on. As well known in the art, all other parameters being equal, a wider parallel data transfer bus increases bus bandwidth. In many applications, data transfer bus bandwidth—in particular, bandwidth to memory devices—is a significant performance bottleneck.
As one non-limiting example, the IC component 112 may comprise a processor, and the IC component 126 may comprise a memory device (or vice versa). When the IC components 112, 126 are packaged in the flipped, stacked-chip configuration depicted in
The interconnection between the IC components 112, 126 may be implemented in a wide variety of ways, as known in the traditional flip chip packaging art. In one embodiment, the conductive bumps 140 comprise solder. Solder bump flipped, stacked-chip packaging comprises, in general, preparing a wafer for solder bumping; forming or placing the solder bumps 140; attaching the bumped IC component 112, 126 to the non-bumped IC component 112, 126; filling the interstitial spaces between solder bumps 140 with a dielectric adhesive; attaching one of the IC components 112, 126 to the interposer 114; and wire bonding I/O signals to the interposer pin contacts. These are steps are described in some detail herein, to enable one of ordinary skill in the art to make and use the present invention. Processing for other forms of conductive bumps 140 is similar, and may be readily derived by those of ordinary skill in the art without undue experimentation.
The wafer is prepared for solder bumping—a process referred to as Under Bump Metalization (UBM)—by forming a plurality of special-purpose layers on selected electrical contacts 118, 130. The UBM process may include cleaning, removing one or more insulating aluminum oxide layers, and providing a pad metallurgy that will protect the IC component 112, 126 while making a good mechanical and electrical connection to the solder bump 140. An adhesion layer provides bonding to both the electrical contact 118, 130 metal and the surrounding passivation, providing a strong, low-stress mechanical and electrical connection. A diffusion barrier layer limits the diffusion of solder into the underlying material. A solder wettable layer offers an easily wettable surface to the molten solder during assembly, for good bonding of the solder to the underlying metal. A protective layer may be required to prevent oxidation of the underlying layer.
Solder bumps 140 may be formed or placed on the UBM in many ways, including but not limited to evaporation, electroplating, printing, jetting, stud bumping, and direct placement. Each of these methods may yield different bump size and spacing; require different solder composition; impose different equipment, assembly temperature, and UBM requirements; and have different costs and manufacturing time. Assembly of the IC components 112, 126 may include handling, placing, fluxing, and solder joining operations. Those of skill in the art may adjust the assembly process to account for the bumped IC component 112, 126, the solder bump 140, the assembly equipment, costs, and other factors. Bumped IC components 112, 126 may be placed by fine-pitch surface-mount equipment or by high-accuracy flip chip placement equipment. In either case, the bumped IC component 112, 126 must be aligned with the corresponding, mirror-image patterned electrical contacts 118, 130 on the non-bumped IC component 112, 126 before placement.
Once the IC components 112, 126 are aligned, the solder bumps 140 are activated by heating the solder sufficiently that it flows between the respective electrical contact 118, 130, and upon solidifying, forms a mechanical and electrical connection. A variety of fluxes, with differing application and cleaning requirements, are known in the art to assist the solder flow process. Solder bump 140 activation may be in a belt furnace, by hot gas, or by other means.
One function of the solder bump 140 is to provide a space between the two IC components 112, 126. In one embodiment, this inter-chip space is filled with a dielectric adhesive joining the entire operative surfaces 120, 132 of the two IC components 112, 126. The dielectric adhesive protects the bumps from moisture or other environmental hazards, and provides additional mechanical strength to the assembly. However, its most important purpose, particularly with solder bumps connections on operative surfaces 120, 132 having a large surface area, is to compensate for thermal expansion differences between the two IC components 112, 126. The dielectric adhesive mechanically locks the two IC components 112, 126 together so that differences in thermal expansion do not break or damage the electrical connection of the solder bumps 140. The dielectric adhesive must bond well to the chip passivation on the operative surfaces 120, 132 of both IC components 112, 126. It must also be compatible with the flux. A cleaning step to remove flux residues may be required before applying the dielectric adhesive. The dielectric adhesive may be needle-dispensed along one or two edges of the solder-joined IC components 112, 126. It is drawn into the inter-chip space by capillary action, and may be heat-cured to form a permanent bond.
In one embodiment, the conductive bumps 140 comprise nickel, with a protective layer of gold. The nickel bumps 140 are built up through an electroless plating technique. Most integrated circuit fabrication processes produce an aluminum oxide layer on the electrical contacts 118, 130. This is removed through zinc displacement plating, using a zincate solution, and the conductive bump 140 is then formed by selective electroless plating of nickel in a wet chemical, maskless process. The wafer is cleaned and all exposed metal other than the electrical contacts 118, 130 is passivated or covered with resist. The zincation process removes the native aluminum oxide, and replaces it with a thin layer of zinc.
After zincation, nickel is deposited from a hypophosphate-based nickel bath, with the bump 140 thickness determined by the plating time. Once nickel bumps 140 of a predetermined height have been formed, a thin layer of immersion gold is plated over the zinc to protect the surface from oxidizing. The bumped IC component 112, 126 may be joined to the non-bumped IC component 112, 126 using solder or conductive adhesives. The conductive adhesive may be stenciled onto the non-bumped IC component 112, 126 electrical contacts 118, 130, or the bumped IC component 112, 126 may be dipped into a thin layer of conductive adhesive to coat the conductive bump 140. The electroless Ni—Au bumping process has cost advantages resulting from eliminating the masking and metal sputtering required by some other methods, and from allowing parallel batch processing of multiple wafers, which increases throughput and reduces costs.
In one embodiment, the conductive bumps 140 comprise gold stud bumps 140 formed by a modified wire bonding process. As known in the art, in generating wire bonds 116, the tip of a gold bond wire is melted to form a small sphere. A wire bonding tool presses this sphere against an aluminum electrical contact 118, 130, applying mechanical force, heat, and ultrasonic energy to create a metallic connection. The wire bonding tool then extends the gold wire to a pin pad 122 on the interposer 114 (or electrical contact 18, 30 of another IC component in traditional stacked-chip packaging, as depicted in
In one embodiment, isotropically conductive, silver filled polymers are stencil printed through metal stencils to form conductive, adhesive polymer bumps 140 on the UBM deposited over the electrical contacts 118, 130. The UBM process may comprise zincation to removes the native aluminum oxide and replace it with a thin layer of zinc, followed by plating one or more layers of nickel and a protective immersion layer of gold, as described above. The conductive, adhesive polymer is then stenciled over the electrical contacts 118, 130 of the bumped IC component 112, 126. The polymers may be either thermoset, which cure with heat, or thermoplastic, which soften with heat. Thermoset bumps 140 are activated by heat curing. Thermoplastic bump 140 connections activate in a few seconds. The interstitial spaces between the conductive adhesive bumps 140 may then be filled with a dielectric adhesive.
Various embodiments of the present invention have been described herein to enable those of ordinary skill in the art to make and use the invention. Integrated circuit manufacturing technology, and in particular flip chip packaging technology, advances rapidly, and any technology that advantageously flip mounts an IC component to an interposer may be utilized in the flipped, stacked-chip IC packaging system and method of the present invention. Accordingly, although the present invention has been described herein with respect to particular features, aspects and embodiments thereof, it will be apparent that numerous variations, modifications, and other embodiments are possible within the broad scope of the present invention, and accordingly, all variations, modifications and embodiments, hereby known or not yet invented, are to be regarded as being within the scope of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
