Unraveling diffusion behavior in Cu-to-Cu direct bonding with metal passivation layers.

Cu/SiO2 hybrid bonding presents a promising avenue for achieving high-density interconnects by obviating the need for microbumps and underfills. Traditional copper bonding methods often demand temperatures exceeding 400 °C, prompting recent endeavors to mitigate bonding temperatures through investigations into metal passivation bonding. In this study, we scrutinized the diffusion behavior associated with various metal passivation layers (Platinum, Titanium, Tantalum, and Chromium) in the context of low-temperature direct copper bonding and delved into the essential bonding mechanisms. We observed a deviation from conventional metal-metal bonding factors, such as surface roughness and grain size, in the diffusion behavior. Remarkably, our analysis revealed a pronounced correlation between the crystallinity of the metal passivation layers and diffusion behavior, surpassing the influence of other experimental factors. Subsequent post-bonding examinations corroborated consistent diffusion behavior in Pt and Cr passivation samples with disparate crystallinities, reinforcing the significance of crystallinity in the bonding process. Our findings underscore crystallinity as a pivotal factor governing diffusion behavior, even under varied bonding conditions. These insights are instrumental in achieving exceptional bonding characteristics at lower temperatures in Cu/SiO2 hybrid bonding. Implications of this study extend to the prospect of advancing highly integrated systems through die-to-wafer bonding, marking a substantial stride toward future applications.

Fabrication of advanced bump layer for IC power delivery.

Metal interconnection in the IC technologies is more important than ever for a device performance. A robust power delivery is one of scaling challenges due to increasing operating frequencies, increasing power density, and decreasing supply voltages. Especially, the on-chip power delivery problem becomes much harder as a device scales down due to the lower voltage, higher current density, thinner metal layer, and smaller pad size. The power delivery is in general controlled by minimizing IR drop and controlling circuit noise through circuit designs. However, in this study the newly designed bumps called advanced bump layer (ABL) were evaluated to improve power delivery. The two types of ABL bumps were designed and fabricated by Cu electroplating. Bump height uniformity, surface roughness, plated structure, and sheet resistance were characterized.

Anti-oxidant copper layer by remote mode N2 plasma for low temperature copper-copper bonding.

An anti-oxidant Cu layer was achieved by remote mode N2 plasma. Remote mode plasma treatment offers the advantages of having no defect formation, such as pinholes, by energetic ions. In this study, an activated Cu surface by Ar plasma chemically reacted with N free radicals to evenly form Cu nitride passivation over the entire Cu surface. According to chemical state analysis using XPS, Cu oxidation was effectively prevented in air, and the thickness of the Cu nitride passivation was within 3 nm. Based on statistical analysis using the DOE technique with N2 plasma variables, namely, RF power, working pressure, and plasma treatment time, we experimentally demonstrated that a lower RF power is the most effective for forming uniform Cu nitride passivation because of a lower plasma density. When the N2 plasma density reached approximately 109 cm-3 in which the remote mode was generated, high energy electrons in the plasma were significantly reduced and the amount of oxygen detected on the Cu surface was minimized. Finally, low temperature (300 °C) Cu-Cu bonding was performed with a pair of the anti-oxidant Cu layers formed by the remote mode N2 plasma. Cu atomic diffusion with new grains was observed across the bonded interface indicating significantly improved bonding quality over bare Cu-Cu bonding.