Image Artifacts by Charge Pocket in Floating Diffusion Region on CMOS Image Sensors

Abstract

Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) are widely used in applications such as mobile equipment, PC cameras, and portable digital cameras due to their low power consumption and low cost. Today, CIS pixel sizes are being continuously reduced, down to the 1.0 μm level in 90-nm CMOS, as industrial applications require higher pixel density. The industry requests, however, continued good performance even when the pixel dimensions are further reduced. In particular, dark current in CIS is an important parameter that determines the image performance in low light. The dark current component can change the charge capacity in photo diodes (PD) and hence the output signal as a function of location and time (J. P. Albert, 2001 and H.-S. Philip, 1998). An increase in dark current can also affect the dynamic range due to an increase in shot noise (H.Y.Cheng, 2003). By suppressing the dark current, the fixed pattern noise of the imager and the white pixel-defects can be reduced (K. A. Parulski, 1985). Photo diodes and pixel architecture of image sensors have been optimized to reduce image artifacts and hence dark current. (N. V. Loukianova, 2003; H.I.Kwon, 2004). The key technology feature for high image-quality CMOS image sensor is the formation of a low-leakage buried photodiode with a transfer gate (TG). The buried PD is a promising concept to reduce the dark current. Many researchers reported that the vicinity of the shallow-trench isolation (STI) to the PD is the main source of dark leakage. By designing and characterizing special diode test structures, H. I. Kwon et al. (2004) demonstrated that the dark leakage decreases as the distance between STI and PD increases. Takashi Watanabe et al. (2010) showed that dark current can be improvefd by avoiding charge diffusion injection from the PD to the substrate using an optimized well structure under the PD. The effects of area, perimeter, and corner on lekage were, however, not separately investigated. The buried floating diffusion (FD), on the other side of the pass transistor, was also studied by H.I.Kwon et al. (2004) and K. Mabuchi, et al. (2004). H.I.Kwon et al. (2004) explained that