Research on BEOL Failures of the Chip-Package Interaction by Shear Tests of the Bumps

Abstract

The continuous demand for high-speed, low-power consumption and multi-functionality of new generation electronic products has led to an exponential increase for high-yield and high-reliability packaging technologies, which has promoted the application of the 7nm node process technology to promote higher I/Os and smaller bump pitches. The advantages of copper pillar bump, which was widely used in high-density packaging, include fine pitch, high strength, low interconnection resistance, excellent electromigration performance, and lead-free packaging solutions. The low-k/ultra-low-k (LK/ULK) material medium used in back-end of the line (BEOL) can effectively reduce the parasitic capacitance without reducing the wiring density. Therefore, to improve thermomechanical and electrical performance, chips with a 42nm node and beyond usually integrate LK/ULK structure and metal line in BEOL. However, the encapsulation of the chip exceeding the 28nm node would lead to a sharp increment in the number of BEOL interconnection layers (vertical) and high-density copper traces (horizontal). The thermomechanical stress caused by the mismatch of the CTE of wafer and packaging material may cause LK/ULK delamination, bump cracks, and UBM peeling failures. Therefore, the accurate analysis and calculation of the thermomechanical stress in the film structure has always been the focus of engineering and academia. The integrity of the bumps and microstructure of BEOL can be evaluated effectively by shear test method, which was suitable for structural inspection. In this work, the shear test simulation was applied for single bump to characterize the failures of BEOL structures, e.g., fractures of ULK/LK. Owing to the fragile characteristics of LK/ULK, BEOL was susceptible to external loads. When the copper pillar bumps above BEOL were subjected to shearing forces caused by thermal mismatch, the failure would occur in some microstructures within BEOL, especially in ULK/LK interfaces. Because the bumps near die corner suffered a critical shear load, so the single bump of the aforementioned area was investigated by a shear simulation model. Studies had shown that the shear rate had less effect on the maximum shear stress, but the increment of the shear height brought about more fractures of LK, which showed that reducing the bump height helps to reduce the risk of BEOL damage under thermomechanical loads. Generally, the shear test of copper pillar bumps could effectively evaluate the strength and adhesion of the BEOL film interfaces. As an early evaluation under extreme conditions, this simulation was designed to check the integrity of ULK/LK stack film and the strength of the bump structure through the chip package interaction without underfill protection and finally find the optimal packaging solution.