Advanced Back-End Processing Techniques For Infrared Focal Plane Arrays

Abstract

With the continued scaling of infrared focal plane array (FPA) pixel pitch down to the diffraction limit, current backend processing techniques are becoming less viable. For one class of detectors, FPAs are formed when pixels of a detector array are electrically connected to analogous elements on a separate readout integrated circuit (ROIC) chip with malleable In bumps. Currently, these In bumps are formed by a standard thick photoresist liftoff process. Maintaining high connectivity with this process becomes difficult or impossible as pitch is reduced because of liftoff failure due to the increased aspect ratio required of bumps. Another class of infrared FPAs epoxies the detector array to the ROIC. A via is etched through the detector material of each pixel down to ROIC contact pads. Interconnects are currently formed by evaporating metal into the via, linking the detector array to the ROIC. As pixel pitch is reduced, obtaining proper interconnect becomes increasingly difficult due to the line of sight requirement of evaporation.

In this work, two novel techniques to realize reduced pitch interconnects were developed and demonstrated that do not have the limitations of current techniques. For In-based FPAs, a template transfer process was developed that does not require a thick liftoff process. In this technique, In bumps are formed by electroplating on a separate, patterned template wafer and then transferred to the detector array or ROIC using a flip-chip bonder. A low-friction, amorphous fluoropolymer was used to shape the bumps and encourage transfer from the template wafer. A proof of principle for this process was obtained, demonstrating the transfer of 5.5 micron thick, 10 micron pitch In bumps to a mechanical ROIC. For the via-based FPAs, interconnection was achieved by electrochemical deposition of Ni films. Both electroplated and electroless Ni processes were developed for this purpose. After confirming the compatibility of these processes with detector and ROIC materials, Ni was plated into the vias of active HgCdTe photodetectors. This resulted in diffusion limited I-V characteristics that were stable through thermal cycling. Electroless Ni via contacts formed on an active 5 micron pitch FPA resulted in 99.94% connectivity.