Lanfeng Guo,Renlong Liu,Zhaobo He,Shaoping Li,Tong Tan,Changyuan Tao
The copper connectivity technique is essential for achieving electrical interconnection in wafer level packaging (WLP), system in packaging (SiP), and 3D packaging. The essential processing material for copper connectivity is a copper sulfate electroplating solution in which organic additives play a crucial role in the regularity of copper electrodeposition. In this study, electrochemical tests, X-ray diffraction, 3D profiling, and scanning electron microscopy were used to investigate the leveling effect and mechanism of polyquaternary ammonium urea-containing polymer (PUB2) in the process of copper electrodeposition on-chip copper connections. PUB2 has excellent polarization ability on the target surface, remains unaffected by the sulfur additive SPS and poly(ethylene glycol), and displays a strong ability to regulate the copper deposition rate of through-holes and surface wiring. The waviness of the wafer surface wiring was reduced from 130 to approximately 70 nm after optimizing the PUB2 concentration, and the surface roughness was reduced from 10 to approximately 7 nm. The coating was dispersed evenly, and the rate of through-hole filling was improved by 57%. This study not only examined PUB2 leveling performance and mechanisms but also devised a research method and system for electroplating additives to facilitate the development and application of new electroplating additives.
The development of WBG (wide bandgap) semiconductors has put forward higher requirements for packaging and interconnection technology. Cu sintering is widely considered as an advanced interconnection technology which can be used in high temperature and high power density service. In this paper, a new type of self-reducible Cu nanoparticle paste was proposed to solve the problems of easy oxidation and high sintering temperature of normal Cu nanoparticles. The proposed Cu paste was developed by mixing formic acid treated Cu nanoparticles, reducing solvents and MOD (metal organic decomposition) solutions. Based on the new type of self-reducible Cu nanoparticle paste, Cu-Cu bonding experiments were carried out with different bonding temperature and time, and a reliable Cu-Cu joint with a high shear strength of 52.01 MPa can be achieved at 250 °C under ambient condition, without any assistance of reducing or inert atmosphere. In addition, the MOD assisted self-reduction and sintering mechanism was proposed after shear strength testing, element composition analyzing, cross-sectional morphology and fracture structure observation, which can provide an effective theoretical support for the practical application of Cu-Cu bonding.
Copper paste sintering has great potential for WBG (wide bandgap) semiconductor packaging, but the problems of copper susceptibility to oxidation and high sintering temperatures have hindered its development. A micron copper paste suitable for low-temperature sintering was successfully formulated using the synergistic effect of metal organic decomposition (MOD) ink and ascorbic acid (AA) to achieve high shear strength sintered joints. During the sintering process, AA acted as a reducing agent to effectively remove oxide on the surface of the Cu microparticles (MPs), while the decomposition process of MOD was facilitated by the reduction products. The diffusion channels formed by the decomposed nanoparticles connect the Cu-MPs, forming dense sintered joints. At a sintering temperature of 250 °C, the combination of MOD and AA in the paste resulted in a significant improvement in the shear strength of the sintering joints, reaching 102.2 MPa. This is in contrast to the addition of only MOD or AA, which resulted in a shear strength of less than 5 MPa. Furthermore, the strength of the sintered joints was increased to 128.1 MPa after storage at 250 °C for 1000 h. At a sintering temperature of 210 °C, the strength could still reach 47.6 MPa, which was approximately double the strength of traditional Sn-based solder joints. This has greater practical significance for the fabrication of high-strength bonded joints and the expansion of the sintering process window.
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