Thermally stable, low-temperature Si–Si bonding with suppressed void formation for μLHP-integrated 3D ICs

Abstract A direct Si–Si bonding process incorporating plasma surface activation and vacuum pre-heating prior to bonding has been developed for micro loop heat pipes (μLHPs), which are two-phase heat transport devices that circulate a working fluid by capillary force to achieve highly efficient heat removal, enabling effective cooling of three-dimensional integrated circuits (3D ICs). The vacuum pre-heating step at 400 °C, which is higher than the bonding temperature of 200 °C reduces hydrogen-bonded H₂O and surface Si–OH groups, thereby suppressing void formation caused by interfacial outgassing. The developed bonding process provides thermal stability up to 300 °C, and the sample subjected to post-annealing at 200 °C exhibits a bonding energy of 2.5 J m⁻² while maintaining a void-free interface.