Study of Shallow Backside Junctions for Backside Illumination of CMOS Image Sensors

Abstract

Backside illumination complementary metal oxide semiconductor image sensors (BSI CISs) represent an advanced technology that produces high-quality image sensors. However, BSI CISs are limited by high dark signals and noise signals on the backside. To address these problems, backside junctions are commonly used. High-dose backside junctions effectively reduce dark signals and noise signals. The depth of the implantation profile is a key factor in determining the junction depth. A laser thermal annealing process is conducted only near the surface to the activation, and thus broader doping profiles are limitations to be activation of dopants. Changing the dopant from B to BF2 can decrease the implant projected range. However, there are abnormal activation rates for BF2 in applications involving laser thermal annealing processes for shallow junctions. Although the need for BF2 is increasing, a mechanism for its slow activation and low activation rates has not yet been confirmed. Here, we identify the mechanism by which BF2 undergoes low activation after a melting threshold temperature and explain why this phenomenon occurs. In addition, we confirm a condition that provides high activation rates of BF2 and show the reduction of dark signals and noise signals at the high density BSI CISs.