Planarization of Cu/Polymer Surfaces via Fly Cutting: Influence of Polymer Properties and Processing Parameters

As the demand for fine-pitch and high-density interconnects continues to increase, hybrid bonding has emerged as a promising technology to replace conventional 3-D integration techniques. While chemical mechanical polishing (CMP) is widely used for Cu/dielectric structure planarization, it has limitations when applied to polymer materials due to surface damage and material loss caused by polymers' low hardness and high ductility. In this study, we propose fly cutting as an alternative to CMP - a mechanical, slurry-free planarization process - and evaluate its applicability to Cu/polymer structures. We selected rigid polyimide (PI) and soft polydimethylsiloxane (PDMS) as representative polymers to compare the degree of planarization (DoP), surface roughness, and bonding performance. Cu/PI structures achieved a high DoP up to 99% and low Cu surface roughness below 2 nm, regardless of spindle conditions. In contrast, Cu/PDMS structures achieved only 67% planarization due to PDMS deformation during fly cutting, leaving the PDMS surface lower than the Cu layer. This facilitates earlier Cu-Cu contact during bonding. ATR-Fourier transform infrared spectroscopy (ATR-FTIR) analysis elucidated the surface activation mechanism through O₂ plasma treatment, clarifying the polymer-polymer bonding process through functional group changes. Postbonding FE-SEM evaluation confirmed polymer squeeze behavior and changes in upper and lower Cu pad spacing according to temperature and spindle conditions, along with successful polymer bonding. Fly cutting showed promise as a low-cost polymer planarization method, although further optimization of the polymer and process conditions is needed.