Investigation of low temperature bonding process using indium bumps

Abstract

Hybrid pixel detector modules are state of the art components of X-ray cameras in synchrotron radiation experiments. Temperature-sensitive, high-Z sensor materials like Germanium, Cadmium-Telluride, Cadmium-Zinc-Telluride and MCT-sensors require an especially low temperature bonding process. The investigation presented in this paper focuses on a low temperature compression bonding process using a ductile bump material, like indium, and a dedicated UBM surface that is able to form intermetallic compounds with this bump metal. The test samples are based on a test chip design with the size of the MEDIPIX3 readout chip and an identical number of bumps, arranged in a 256 × 256 bump matrix with 55 μm pitch in X- and Y-directions. In addition to the indium bumped test chip, two typical UBM-pad configurations were investigated on the corresponding interconnect side: a pad-defined electroplated Ni-Au-UBM and a passivation opening-defined, PVD-deposited NiV-Au pad. Daisy chain structures, 4-Point-probe resistance measurement structures and isolation test structures were implemented to characterize the influence of bonding pressure, bonding temperature and dwell time of bonding pressure on the electrical interconnection resistance. Good bonding results and a corresponding minimum electrical resistance of daisy chains and individual interconnects were achieved at temperatures of 80̂C and 100̂C. A minimum bonding pressure of 3.2 MPa was necessary to achieve reliable bonding results. Daisy chain failures occurred especially in the corner of the chips. After electrical characterization, pull tests were performed to investigate the corresponding bonding area. Pull test results showed a larger bonding area for the pad-defined, electroplated UBM pad configuration in comparison to the passivation opening defined bonding pad.