Evaluation of UBM oxidation through air exposure and heating and effectiveness of wet and plasma cleaning on solder joint formation during TCB

Abstract

In 3D stacking chips, interconnections play an important role and selection of right materials is very significant. In this paper, a systematic study is done to characterize metal oxide growth rate with time and temperature for three most used UBM (under bump metallization) materials i.e. copper, cobalt, and nickel for the interconnections. Furthermore, effectiveness of two cleaning techniques such as wet cleaning and plasma cleaning before TCB (thermo compression bonding) process is investigated. As the metal oxidation, depending on air exposure through time, needs a quick and accurate evaluation, SE (spectroscopic ellipsometry) is employed to measure optical phase quantity (Delta) and thickness. To monitor grain size evolution and measure roughness of the sample surface, AFM (Atomic Force Microscopy) is used. Moreover, wettability of the materials surface, as a key factor in bonding, is measured by Contact Angle tool. As copper is considered a main material for UBM, TOF-SIMS (time of flight second ion mass spectroscopy) as well as XRD (X-ray diffraction) techniques were selected to investigate oxidation concentration along depth profile and oxide phase of the surfaces for the samples respectively and cross section of the oxide layer is observed through SEM.