Study of Hydrophilic Si Direct Bonding with Ultraviolet Ozone Activation for 3D Integration

Wafer-level hydrophilic silicon direct bonding is demonstrated using ultraviolet ozone (UVO) activation. The activation is performed at room temperature in atmospheric ambient for 3, 6, 9, and 12 min under UVO exposure, respectively. The activation process helps to form a hydrophilic surface (contact angle < 7 degrees) for hydrophilic bonding and to improve the surface roughness to <0.25 nm possibly by removing the surface hydrocarbon contaminants. After annealing at 200 degrees C for 3 hr, excellent bonding strength is achieved. High bonding uniformity is obtained in the bonded wafers which are activated for 3 min exposure in UVO. Helium over-pressure in a bombing chamber and helium leak rate detection by a mass spectrometer are used for hermeticity measurement of Si cavities sealed and bonded after UVO activation. The detected helium leak rate indicates that the best hermeticity can be obtained with an activation time of 3 min. The C-V curves of the metaloxide-semiconductor (MOS) capacitors which are fabricated using UVO clean on the Si surface prove that prolonged exposure to UVO can degrade the bonding quality.