Sharp Interface Simulations and Experimental Characterization of Surface Diffusion Driven Phase Evolution in Bonded Interconnects
Creators: name: Jois, Chetan Sudarshan
Publisher: name: Purdue University Graduate School
Year: 2025
Subjects: classificationCode: 401706
subject: Numerical modelling and mechanical characterisation
classificationCode: 401707
subject: Solid mechanics

description: Phase evolution mechanisms in micro and nanoscale devices present unique challenges due to distinct behaviors at reduced scales due to surface diffusion. In solder microbumps with size less than 100 micron, surface diffusion causes voiding in the solder bump accompanied by intermetallic formation at the copper sidewall. As the scale and pitch of the interconnects in electronic packages decrease, there is a need to fabricate test devices that can accurately characterize the phase evolution. The usual approaches to studying phase evolution either rely on large-scale test devices that overlook the influence of reduced device sizes or require prolonged testing that limits systematic comparisons across materials. Furthermore, modeling the phenomena at scale require simulation frameworks that can handle physics such as surface diffusion. This work aims to overcome these limitations by developing experimental and computational frameworks to study voiding and phase transformations within interconnects. To this end, test devices, termed inline microjoints, are developed for non-destructively tracking phase evolution in micro scale interconnects under thermal aging. The test devices are demonstrated on CuSn microbumps and SnBi solder joints. Results on the CuSn microbumps demonstrate that void growth in smaller sized bumps stop earlier due to the conversion of the entire solder region into Cu<sub>6</sub>Sn<sub>5</sub>. In the SnBi inline joints, the effective electromigration diffusivity was characterized using both thickness of Bi accumulation imaged under back scatter electron (BSE) imaging, and the rate of resistance growth. Both thickness and resistance was obtained on the same bumps due to the non-destructive nature of the test. The diffusivity values obtained were found to match closely with those reported in literature. During the interrupted testing, a nonlinear transient resistance growth was observed at the beginning of each test. The nonlinear growth could not be attributed to joule heating and the cause for it needs further investigation. To complement these experimental findings, a sharp interface simulation framework based on Enriched Isogeometric Analysis (EIGA) is developed in this thesis to model intermetallic and void growth in microbumps, which aligns well with experimental outcomes. An extension to the modeling methodology is developed to simulate surface diffusion with imposed flux boundary conditions on open surfaces. The simulations are applied to model surface diffusion in Cu surface during the anneal step in hybrid bonding. The results of the simulations are shown to agree with trends reported in literature. Finally, future applications of the experimental procedure developed in this thesis in the context of hybrid bonding is also discussed. By integrating experimental and simulation techniques, this work contributes tools to analyze phase evolution in interconnects in advanced electronic packages.