We adopted (111)-oriented Cu with high surface diffusivity to achieve low-temperature and low-pressure Cu/SiO₂ hybrid bonding. Electroplating was employed to fabricate arrays of Cu vias with 78% (111) surface grains. The bonding temperature can be lowered to 200 °C, and the pressure is as low as 1.06 MPa. The bonding process can be accomplished by a 12-inch wafer-to-wafer scheme. The measured specific contact resistance is 1.2 × 10⁻⁹ Ω·cm², which is the lowest value reported in related literature for Cu-Cu joints bonded below 300 °C. The joints possess excellent thermal stability up to 375 °C. The bonding mechanism is also presented to provide more understanding on hybrid bonding.

X-ray nanodiffraction was used to measure the thermal stress of 10 µm nanotwinned Cu bumps in Cu/SiO₂ hybrid structures at −55 °C, 27 °C, 100 °C, 150 °C, and 200 °C. Bonding can be achieved without externally applied compression. The X-ray beam size is about 100 nm in diameter. The Cu bump is dominated by (111) oriented nano-twins. Before the hybrid bonding, the thermal stress in Cu bumps is compressive and remains compressive after bonding. The average stress in the bonded Cu joint at 200 °C is as large as −169.1 MPa. In addition, using the strain data measured at various temperatures, one can calculate the effective thermal expansion coefficient (CTE) for the 10 µm Cu bumps confined by the SiO₂ dielectrics. This study reports a useful approach on measuring the strain and stress in oriented metal bumps confined by SiO₂ dielectrics. The results also provide a deeper understanding on the mechanism of hybrid bonding without externally applied compression.

A thorough evaluation on selecting a bondable laser release material for redistribution layer (RDL)-first and die-first fan-out wafer-level packaging (FOWLP) is presented in this article. Four laser release materials were identified based on their absorption coefficient at 355 nm. In addition, all four of these materials possess thermal stability above 350 °C and pull-off adhesion on a Ti/Cu layer greater than 8 psi, further illustrating their compatibility in FOWLP. To further evaluate these materials, focus ion beam (FIB) inspection was used to examine the profile of a single-pulse laser ablation region, ensuring free penetration of 355-nm laser energy. With a designated die bonding test vehicle, die shift less than 1.5 $\mu \text{m}$ and rotation less than 0.02° before and after molding was achieved with the material bondable at temperatures lower than 200 °C. Furthermore, this same material required a laser energy of less than 3 W for laser release. A bondable laser release material that eliminates the requirement for a die attach material from the traditional process flow facilitates the development of cost-effective FOWLP.

This paper presents an evaluation of a negative-tone i-line photosensitive polymeric bonding material for achieving pre-bonding in Cu/polymer hybrid bonding at low temperatures via Cu damascene process. The polymeric material has a Tg ≤100°C, Young’s modulus of 99 MPa, dielectric constant of 2.6, and dissipation factor of 0.0016 at 10 GHz. Shear tests revealed a bond shear strength of over 10 MPa when pre-bonded at 30°C. Subsequent thermal compression bonding (TCB) at 300°C for 1 hour increased the bond shear strength to over 60 MPa. The material is compatible with the typical Cu damascene process involving lithography, Ti barrier layer deposition, Cu seed layer deposition, Cu electroplating, and Cu CMP. The photosensitive nature of the low-Tg polymeric bonding material, along with its compatibility with the Cu damascene process, opens up possibilities for low-temperature Cu/polymer hybrid bonding.