Study of shallow backside junctions for backside illumination of CMOS image sensors

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.