Failure Mechanism And Reliability Analysis Of Cu Interconnect Structures Under Thermal Stress

With the development of integrated circuits into ultra-deep-submicron or nanometer era, copper has replaced aluminum and become the mainstream interconnection metal material. But copper interconnection and aluminum interconnection are different in material property, manufacturing technology and wiring structure, which leads to big differences between their reliability. Copper interconnect has its special failure mode and failure mechanism. Among them, stress migration (SM) under thermal stress, thermal cycle stability under temperature cyclic stress and interaction between the copper/low k chip and flip chip packages are all main factors to influence Cu interconnection reliability. This article carried out research on the failure mechanism and reliability of Cu interconnection structures under thermal stress and got the following results:First, based on the mechanism of stress induced voiding (SIV), three different copper interconnection structures were designed and tested under the high temperature storage conditions. The structures include different metal width, adding dummy via or narrow metal finger in the narrow-wide interconnection structures. Their failure phenomenon and effect on the reliability of copper were analyzed. The result showed that at the case of constant via size, the probility of SIV increased with the bottom metal width increasing. Adding dummy via or narrow metal finger can both effectively improve the SIV performance of copper interconnection. In addition, the influence of via microstructure variation and the via-line overlap length variation on the thermal stress character of via and bottom metal under via were discussed and analyzed. Via microstructure changes are impacted by the variation of the via formation process, including the via height variations, gouging via depth variations, via bottom barrier layer thickness variations, etc.Second, thermal cycle stability of the copper interconnection structure was studied under the temperature cycling conditions. Damage development procedure and its mechanism were analyzed. The results show that under the action of the alternating temperature cyclic stress, copper metal surface will enduce surface morphology damage phenomenon, such as hill, wrinkle and void. Then micro-crack appeared. At last, big crack formed. The reason for this is plastic deformation slips along the grain boundary under the action of cyclical temperature alternating load. Big slippage leads to hills and void. Finally, the accumulate damage in the copper results in crack formation. At the same time, the rigid etch stop layer (ESL) existent in the trench and Air-Gap existent in the trench and vias were discussed and analyzed, showing how the thermal stress and plastic strain in the interconnect are improved.Finally, under the thermal-mechanical stress due to high temperature reflow soldering, effect of the interaction between Cu/low k chip and flip chip packages on the chip integrity was studied. Results showed that the thermal-mechanical stress residued on the Cu/low k structures and affected its’integrity and reliability. The interaction combined with the outside thermal stress can cause potential crack damage in the device, which might not have immediately obvious influence on the performance of the devices but will threaten its reliability. Such damage will lead to two types of failure modes of the Cu/low k-FC package devices and affect their reliability.