3D TSVs are finally being implemented in DRAM stacks, with promises from leading manufacturers that they will be in production this year. However, 3D TSVs are still too costly to be implemented in consumer mobile products. Industry suppliers continue to work on optimizing critical materials and processes to bring down the cost of fabricating TSVs by improving yields – Dow Electronic Materials included.
Perhaps most notably, thin wafer handling has been a major sticking point for stacking die using TSVs. Temporary wafer bonding (TWB) has emerged as the method of choice for handling silicon wafers during the thinning and high-temperature backside processing required for the manufacture of 3D device structures. In particular, the debond step of the temporary bond/debond process has been challenging, with issues surrounding the adhesive materials used for this process, but why? Material suppliers need to combat issues with voiding, total thickness variation (TTV), high viscosity, adhesive conformity to topography, adhesive strength variations with topography, mechanical stability of adhesives for backside processing, particularly grinding, thermal stability, and particle contamination. For example, adhesives that are too strong can result in cracking the thin wafer during debond, while those that are too soft can cause delamination.
Out of these issues, today’s requirements for temporary wafer bonding (TWB) materials to be used in high volume manufacturing have been established. They include simple device and carrier wafer preparation, high-throughput wafer bonding, thermal stability to 300°C or higher, and clean room-temperature release directly from the device wafer using either mechanical or laser-assisted debonding.
Two years ago at the 2013 IMAPS Device Packaging Conference, in Scottsdale Arizona, Dow first demonstrated its temporary wafer bond material based on its benzocyclobutene (BCB)-permanent bonding adhesive due to its ability to meet these requirements. The highlight of this material was its clean-debonding characteristics due to an adhesion promoter spin-coated on the carrier to ensure that the adhesive would stay on the carrier during the mechanical debond.
This year, Dow demonstrated improvements in this TWB material that included the capability for use with either a mechanical lift-off without needing to first apply an adhesion promoter, or a laser-assisted debonding method that uses pulsed laser light to ablate the interface between the carrier and the TWB film, allowing the carrier to be cleanly and easily removed while the TWB film remains on the device-side surface. In both the mechanical and laser debond processes, the remaining TWB film can be cleanly tape-peeled from the device surface.
The details of this work are now available in the extended abstract and accompanying presentation that was presented at 2015 IMAPS Device Packaging.