ABSTRACT
Stacking wafers by means of a permanent bonding process and then thinning the device wafer already in the permanent stack to expose the previously formed via or creating the via in the thinned wafer already on the permanent device stack has been proposed and is in use for production of 3D stacked packages.3 Upon exposure of the TSV, or creation of the TSV, several BEOL process steps are still necessary to create the backside redistribution and interconnect, including passivation, photolithography, metallization, and so on. The stacked device, already of significant value because of multiple layers of circuitry, must therefore be exposed to the thermal and chemical processes necessary to create the backside redistribution and interconnects. Alternatively, several technologies use a temporary carrier, such as a carrier wafer, to allow the thinning and processing of a device wafer. The temporary carrier plus the device wafer may be processed just as if the stack were a wafer of standard thickness. The pair may be handled and processed without risk to the device through completion of the backside redistribution and interconnects. Upon completion of the interconnects, the thinned wafer may be permanently attached to the front side of another wafer and then separated from the carrier wafer for wafer-to-wafer processing (W2W), or separated from the carrier wafer and diced, and then dies may be attached one at a time to the devices. This flexibility allows for W2W bonding, die-to-wafer bonding (D2W), or die-to-die (D2D) bonding and therefore permits many alternatives in processing TSV-containing wafers.In the following pages, we will explore temporary bonding and TSV creation using several bonding and release (debonding) technologies currently in production on thinned wafers. Although backside processes will vary significantly from company to company, the handling and alteration of a thinned wafer on a carrier, including not only a thin device wafer and a thick carrier wafer but also the bonding material, must remain intact through high-temperature, high-energy-flow, and highly corrosive environments during backside processing. Each bonding and release technology has been developed to address these challenges in a slightly different way. The release of the completed “two-sided device” from the carrier is a major consideration and will involve significant review.
