Microstructure and orientation control of electrodeposited copper and its application in advanced electronic packaging

Abstract

In advanced IC packaging, chip stacking has become a promising technology to overcome the physical limitations imposed by Moore's Law while still achieving higher interconnect density and compact form factors. The electroplated copper used for these planar and vertical connections must meet various structural stability requirements across different processes. For advanced thin RDL applications (width below 5 μm), a highly stable copper structure is necessary to resist electromigration problems caused by high current density. For the direct Cu-Cu bonding process, an active copper structure can help reduce bonding temperature and thermal budget, minimizing high-temperature-induced warpages and voids, and decreasing potential damage to temperature-sensitive devices. To make electroplated copper more adaptable for advanced packaging, microstructure control was utilized by changing the additives content in the electroplating solution to tunnelling the stability of plated copper for RDL and Cu-Cu bonding application. Firstly, we successfully developed a composite copper structure that combines fine grains and nanotwins, which exhibits enhanced super-filling capabilities and improved structural stability. This structure can effectively resist surface damage, even after prolonged periods. Additionally, it demonstrates an electromigration (EM) lifespan that is 2.9 times longer than its coarse-grain RDLs counterpart. Secondly, a unique metastable composite copper structure is created by integrating a nanograin structure into the (111)-oriented coherent nanotwinned copper. This metastable composite remains stable at room temperature and nanograin-rich area experiences significant grain growth across the bonding interface at higher temperatures. Successful direct copper-to-copper bonding can be achieved at 170 °C for 30 minutes, resulting in enhanced mechanical and electrical performance, as assessed by mechanical shear tests, electromigration tests, and thermal cycling tests. To further reduced the thermal budget of bonding process, then a super unstable nanograin copper structure specifically aims for Cu-Cu bonding at temperatures below 100 °C, or even at room temperature. This super unstable nanograin copper structure exhibits rapid self-annealing behavior, undergoing complete recrystallization at room temperature within 250 minutes or at 60 °C within just 5 minutes. The driving force for grain growth can be attributed to the large excess energy stored in the high-angle grain boundaries (HAGBs) and dislocations. The calculated activation energy for the coarsening of super unstable copper grains is approximately 0.57 eV/atom, which is significantly lower than those reported for nanograin coppers using the same differential scanning calorimetry method.