Characterization of the impact of dry etch removal of the passivation oxide on top of a backside-illuminated CMOS image sensor

We have performed a gradual removal of the passivation films inside a window on top of a finished backsideilluminated CMOS image sensor (CIS) using a dry etch and then characterized its impact on the key performance parameters of the image sensor. The characterization included both top-down and cross-section SEM analysis as well as opto-electronic measurements of the key image sensor parameters. Our results show that as the passivation thickness drops below a threshold value, the dark current increases significantly while the optical response degrades gradually. The physical analysis shows this corresponds to the loss of electrical passivation of the backside sensor interface. With the complete removal of passivation, the back surface electron generation with subsequent diffusion to the photodiode depletion region pathway dominates the dark current. As such the activation energy of dark current is still close to the silicon bandgap. The read noise of the sensor when corrected for the dark current shot noise is unchanged after the window etch. The pixel-level dark current for all splits follows the Meyer-Neldel rule with the same characteristic temperature.