Qualification of Bump Bonding in CMS Inner Tracker Pixel Modules for the Phase-2 Upgrade

Abstract: To fully exploit the increased luminosity of the HL-LHC, the CMS Inner Tracker is undergoing a major upgrade to withstand extreme radiation levels and data rates, while improving granularity and reducing material budget. The upgraded modules employ thin planar and 3D silicon pixel sensors, bump bonded to a new radiation-hard readout chip, the CROC, designed in 65 nm CMOS technology and powered via a serial scheme. Ensuring the quality of bump bonding between sensors and readout chips is critical for efficient detector operation. This work presents the qualification procedures developed to identify missing or defective bumps using multiple test methods, including crosstalk analysis, reverse/forward bias testing, and X-ray or beta source imaging. Results from prototype modules are presented, and advantages and limitations of each method are discussed.