Plasma-free KOH-based surface activation for enhanced hybrid bonding strength

Abstract

This work demonstrates a plasma-free, short-time potassium hydroxide (KOH) wet pretreatment for a hybrid bonding process, targeting high SiO₂–SiO₂ bonding strength while suppressing Cu surface oxidation. KOH treatment at 50 °C for 15 s significantly improved SiO₂ wettability, reducing the deionized water contact angle, and enabled a maximum shear strength of 29.6 MPa after 300 °C annealing. The oxide etching behavior was quantified using a TiO₂ cap structure, confirming time-dependent selective SiO₂ thinning in 5 M KOH at 50 °C. To evaluate the Cu surface stability, 400 nm Cu films were analyzed after exposure to citric acid and subsequent KOH treatment. While atomic force microscope (AFM) roughness showed no pronounced change, the mean conductive AFM current indicated progressively reduced conduction with longer KOH treatment. These results identify short-time KOH as a key process window that balances surface activation and Cu stability for plasma-free hybrid bonding.