UV Enhanced low temperature wafer direct bonding and interface quality test

Ultraviolet (UV) exposure process, as an additional process following traditional wet chemical activation processes, was applied to low temperature hydrophilic silicon wafer direct bonding. The UV source emitted mixed spectrum of light with 254 nm and 185 nm wavelength. The comparative trial was performed to investigate the effects of additional UV exposure and to optimize the exposure time. Infrared inspection, tensile strength test and FSEM scan were performed to verify the qualification with detailed results and discussion. It is found that bond rate of wafers treated with 5 minutes UV exposure increased after annealing at 350degC while several voids would appear for exposure time of 10 minutes. Besides, the relationship of annealing temperature and bond rate was also studied. It is found that as annealing temperature raised, the formed gases would be easy to accumulate in the existing cavities or form new voids in the interface to decrease the bond rate. The relatively higher tensile strength up to 15.1 MPa was obtained when exposure to UV for 5 minutes. Finally, FSEM images were given with explanation of different thicknesses of interface layers due to UV exposure. It is concluded that as an additional cleaning and modification treatment, appropriate UV exposure is demonstrated as an effective technique to enhanced bond quality. High bond rate, tensile strength as well as thinner interface layer can be achieved by exposure for 5 minutes. Prolonging the exposure time may decrease the bond-ability as well as the bond quality.