High-Frequency Copper Interconnects and Epoxy Dielectrics for Future Multi-Chip Packages

Developing new processes and materials is essential for achieving multichip packages capable of chip-to-chip (C2C) data rates of 50 GB/s or higher; however, a fundamental issue arises due to the skin effect, which concentrates current at the surface of the conductor at high frequencies. The roughness of the copper (Cu)-epoxy interface presents a trade-off between performance and reliability. While a rough Cu-epoxy interface enhances mechanical adhesion, it negatively impacts signal integrity. This study investigates the extension of Cu interconnects and epoxy dielectrics to multi-GHz frequencies. The trade-offs between signal integrity and reliability factors are examined at high frequencies of up to 18 GHz. This study used wet etching to adjust the roughness at the Cu-epoxy interface. The morphology was measured using atomic force microscopy (AFM) and two-dimensional fast Fourier transform (2D FFT). Peel tests and vector network analysis were conducted to explore the effects of both the type and extent of roughness. The trade-offs between power efficiency and adhesion are presented and discussed. Given the trade-offs associated with rough Cu interconnects, enhancing chemical adhesion for smooth Cu interconnects is crucial. Our findings suggest that smooth Cu interconnects with thinner layers of cuprous oxides (Cu2O, CuI) and amine-functional silane adhesion promoters significantly enhance interfacial adhesion with epoxy dielectrics by nearly an order of magnitude. For the first time, we provide X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy evidence showing the formation of Cu(Ⅰ)-O-Si bonds at Cu interfaces coated with silane. Therefore, the use of relatively smooth interconnects can reduce skin losses while still preserving mechanical integrity and reliability. We investigate the failure mechanisms of Cu interconnects that incorporate cuprous and cupric oxides (CuO, CuⅡ) through Auger electron spectroscopy (AES). These findings demonstrate that both cupric oxides and thicker layers of cuprous oxide result in noticeably weaker interfaces than thin layers of cuprous oxides with silane adhesion promoters. While silane enhances the adhesion of the Cu-epoxy interface, there are long-term reliability concerns about silane due to its sensitivity to humidity. Given the complexity of the degradation process in the Cu and epoxy system, experimental results are crucial for assessing the long-term reliability of that system. We compare the stability of various Cu-epoxy adhesion promotion methods, such as amine functional silane, methylimidazole, and metal interlayers such as Ni, NiCo0.64P0.25, NiW0.48P0.10, and CoW0.66P0.17. The adhesion strength and sheet resistance are investigated after 1500 temperature cycles ranging from -40 to 125 ℃. The metal interlayers, particularly CoW0.66P0.17, enhance the long-term stability of the Cu-epoxy interface by mitigating oxidation. The oxidation process at the Cu-epoxy interface is investigated using energy-dispersive X-ray spectroscopy (EDS). Signal integrity and adhesion are assessed with a four-point probe and a peel tester.