Fine-Pitch 30 μm Cu-Cu Bonding by Using Low Temperature Microfluidic Electroless Interconnection

The development of direct Cu-to-Cu bonding technologies has attracted increasing attention due to the increased requirement for advanced high-density packaging. Among these technologies, the microfluidic electroless interconnection (MELI) process [1] can directly fabricate interconnection between Cu pillars at low temperatures without applying any pressure on the chips. The previous study [1] revealed that the uniform bonding of MELI-Cu can be achieved with the intermittent flow for chip-to-chip with 150 mu m pitch and 50 mu m standoff height. This study further improved the electroless Cu chip-to-chip bonding on Cu pillars with 28 mu m standoff height and pillar pitch down to 30 mu m. To meet the requirement of real applications, in this study, the Cu pillar array of 17,600 pillars were divided into three equal regions, and each region with a different pitch, i.e., 30 mu m, 35 mu m and 45 mu m, respectively. As a result, the control of flow patterns is more critical due to the complicated Cu pillar pattern and the higher amount of hydrogen gas bubbles from the electroless Cu plating process. The chips were subjected to electroless plating chemical in a microfluidic system under an ambient condition of 50 degrees C and the intermittent oscillatory flow was implemented into the electroless bonding system. The microstructure and electroless plating behavior of the electroless Cu was investigated using ion-milling, focused ion beam (FIB), and scanning electron microscope (SEM). The results showed that the primary cause of skip-plating and extraneous plating in the microfluidic system could result from loading factor and hydrogen gas bubbles. To enlarge the process window of the MELI-Cu for fine pitch, the additional pretreatment process was added before Cu plating to improve the deposition rate and deposition uniformity. Tensile test verified the bond strength of Cu-to-Cu bond can approach 80 MPa. To eliminate the defects within the Cu interconnection, the low-temperature post-annealing treatment also included in this study, and preliminary results showed that a high level of bonding quality without voids or seams can be obtained. Furthermore, a potential horizontal side-by-side MELI-Cu was proposed by using electroless Cu deposition on the surface around the Cu pillar. The horizontal Cu-to-Cu interconnection has a higher tolerance to the non-uniform Cu height and the misalignment of Cu pillars, regardless of the top-shaped of Cu pillar, which provides a competitive application compared to vertical interconnection.

Automated summary

This study focused on the MELI process for direct Cu-to-Cu bonding, investigating the bonding behavior and microstructure of the electroless Cu. The researchers also proposed a horizontal side-by-side MELI-Cu method that showed higher tolerance and competitiveness compared to vertical interconnections.