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Temporary Wafer Bond Adhesives with High Thermal Stability and Enhanced Processability
Keywords: Temporary Adhesive, Wafer bonding, Thermoplastic polyimide
As semiconductor geometries continue to shrink, there is increasing effort to develop methods for packaging die that will bridge the scale gap between the die and the circuit board while maximizing part performance and minimizing part size. Stacked memory modules have recently become commercial and modules where memory and processor die are mounted on silicon interposers have been adopted for high performance applications. Both of these applications use through-silicon-vias (TSVs) for electrical connection through thinned silicon. Another approach is seen in various fan-out packaging schemes which employ multiple layers of interconnect and avoid the need for through-silicon vias. For both die stacks (3D packaging) and interposers (2.5 D packaging) wafer thinning is used to reveal the TSVs. During the thinning and subsequent back side passivation and circuitization, the front side of the wafer is held to a carrier substrate using an adhesive. When these process steps are complete, the front side must be released from the carrier and the temporary adhesive removed. A number of different materials have been developed as temporary adhesives for 2.5D and 3D packaging. HD-3007 and HD-3008 are two formulations that have been developed by HD MicroSystems. When cured, these two products form the same thermoplastic polyimide. This polyimide is characterized by high thermal stability, high adhesion to glass and silicon and solubility in organic solvents such as N-methyl pyrollidone. Wafer-to-wafer bonding temperatures for these materials range from 250 to 300°C depending on the thickness of the adhesive layer and the bonding time. The carrier wafer may be de-bonded at room temperature by irradiating with an excimer laser through the carrier. In this presentation we will discuss the properties and performance of a new polyimide adhesive designed to bond below 250°C while still maintaining high thermal stability. We will review the effects of coating and curing processes on film uniformity and properties. The impact of film thickness on bonding temperature will also be discussed. A case study will be presented where the adhesive was coated over copper pillars and then bonded to a carrier wafer. Lastly, the performance of the adhesive during subsequent backside processing to form a redistribution layer will be detailed.
Mel Zussman,
HD MicroSystems
Wilmington, DE

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