A Comprehensive Study of Interface Damage at Cu/Polyimide Interface in Redistribution Layers of Fan-out Wafer Level Packaging

In recent years, fan out wafer level packaging (FOWLP) plays an important role in advanced packaging technology, because of the advantages such as small package size, excellent electric properties and high I/O density and applicable to heterogeneous integration. It can be applied to many devices such as baseband chip, power management IC, radio frequency (RF) switch/RF transceiver, radar, audio codec, micro controller unit, etc. In FOWLP, the redistribution layer (RDL), generally composed of polyimide (PI) and Cu, may rearrange the I/O location of the chip and become an important component of FOWLP structure. During the construction of RDL, it is usually necessary to increase the whole structure of FOWLP to the temperature above 350°C to realize the thermal curing of PI, and then cool it to room temperature for Cu plating. However, due to the large difference in coefficient of thermal expansion (CTE) between PI and Cu, the thermal mismatch during cooling process will lead to obvious stress concentration at the Cu/PI interface, which has a great risk of interface delamination and seriously affects the reliability of FOWLP. Therefore, it is very important to study the risk of crack initiation and propagation at Cu/PI interface.In this study, CZM technology was used to simulate the cooling process of the three-layer RDL structure from 350 °C to 25 °C, and the Cu/PI interface in the RDL structure was analyzed. The Cu/PI interface at the corner of all Cu vias in the RDL meets the damage initiation standard, resulting in interface damage. Among them, the Cu/PI interface near the corner of the Cu via in the second layer of RDL has the most serious damage. In the position where the risk of disbonding failure at Cu/PI interface is greatest, the damage factor is always 0 when the temperature is 350~145 °C, indicating that when the cooling does not exceed 205 °C, the interface damage will not occur. The preparation process of RDL can be improved by reducing the PI curing temperature. Increasing the thickness of the Cu trace can slightly reduce the damage degree of Cu/PI interface, but it is not a key factor affecting Cu/PI interface. The change of Cu via height will significantly affect the degree of Cu/PI interface damage, and the Cu/PI interface damage degree of Cu/PI interface with lower height Cu via sidewall is lighter.