Evaluation of Fly Cutting for Planarization in Cu/Polymer Hybrid Bonding and Its Effects on Polymer Materials and Processing Conditions

This study investigates the applicability of fly cutting as a CMP-free planarization technique for Cu/polymer hybrid bonding. Two polymer dielectrics - polyimide (PI) and polydimethylsiloxane (PDMS) - were selected to represent rigid and soft materials, respectively. Surface roughness and global planarization behavior were evaluated under two spindle speed conditions using a single-crystal diamond tool. Fly cutting successfully achieved sub-2 nm Cu surface roughness for both polymers and <25 nm roughness on the polymer surfaces. The degree of planarization (DoP) was maintained above 90% for PI and improved from 52% to 66.7% for PDMS when the spindle speed was reduced, indicating that planarization is influenced by polymer properties and cutting parameters.However, bonding evaluation revealed that Cu-Cu bonding was not achieved in either structure. In Cu/PDMS samples, thermal expansion of the soft polymer during bonding pushed the Cu pads apart despite an initial Cu height advantage. In Cu/PI samples, partial PI-PI bonding was achieved, but Cu-Cu delamination occurred at all temperatures due to insufficient Cu surface activation and expansion. Compared to PI-PI interface, PDMS-PDMS bonding was successfully achieved without voids, suggesting effective surface activation via O₂ plasma. These results demonstrate that while fly cutting is effective for planarizing both soft and rigid polymers, optimization of the CTE mismatch between Cu and polymer and surface activation are essential to realize stable Cu/polymer hybrid bonds.