UCLA

With the slow-down of Moore’s law of scaling transistors, the industry is looking for 3D IC technology to extend the Moore’s law by stacking chips vertically. In the 3D IC technology, Joule heating is the most serious reliability concern because of increased power density. Moreover, there are new interconnects in the package to support vertical stacking, including the Through Silicon Via (TSV) inside silicon die, μ-bumps between different dies, and redistribution layer (RDL) to fan out the current between μ-bumps and TSVs, and between TSV and flip chip solder joints. Thus, how does joule heating affect the reliability of the new interconnects is of most interest. In this thesis, we applied the electromigration test to induce joule heating in our packaging system, and studied the weak link of the 3D IC system first. It is found that the redistribution layer is the weak link and failed by burn-out voids. The failure mode is evaluated by finite element analysis and found to be joule heating enhanced electromigration. Then, we optimized the redistribution design in the power delivery system to reduce the thermal effect and minimize the IR drop (covered in Chapter 3). It is found that an optimal power distribution system requires larger TSVs integrated at the super fat capture levels (RDL), rather than small TSVs captured at the lower levels. We further evaluated the electromigration resistance of optimized RDL and found that fat wire can pump more current but the surface treatment of Cu RDL limits the maximized current. In Chapter 4, we found that the lateral joule heating transfer through Si interposer will induce high enough temperature gradient inside the un-powered microbumps next to the powered microbumps, and fail the un-powered microbumps by thermomigration. The heat transfer creates a large temperature gradient, in the order of 1000 ᵒC/cm, through the un-powered microbumps in the neighboring chip, so the microbumps failed by thermomigration. In our test structure, we have found other microbumps which were failed by electromigraion as well as by constant temperature annealing. We used synchrotron radiation tomography to compare the failure in these three kinds of microbumps: microbumps under electromigration, microbumps under thermomigration, and microbumps under a constant temperature thermal annealing. The results show that the microbumps under thermomigration have the largest damage. Our calculations showed that indeed the electromigration and thermomigration driving force are in the same order. The latter induced atomic flux of Sn to go from the cold end to the hot end, resulting in depletion and void formation at the cold end. Furthermore, the temperature gradient tends to enhance sidewall surface diffusion of Sn to react with Ni and Cu. This sidewall surface diffusion of Sn can cause significant void formation in the solder layer in the microbumps.