Charles F. Musante

Interconnect and Packaging Technology for CMOS Image Sensors (Invited)

2. Basis Operation The CMOS imager consists of an array of pixels that detect the incident light. A commonly used pixel is the four transistor or “4T” cell (Fig. 1) [2]. To capture an image, the photodiode is first reversed biased (set to Vdd) using the reset gate and transfer gate (Fig. 2). When the shutter of the camera is opened, light that is incident on the photodiode will generate electron-hole pairs. To sense the charge in the photodiode, the transfer gate is turned on and the charge is moved to the floating diffusion. This changes the potential on the gate of the the source-follower circuit, and the resulting signal is detected at the output of the pixel by turning on the row select transistor. The photo-diode area is much smaller than the total area of the pixel. To overcome this loss of sensitivity, microlenses are used to focus light on the photo-diodes [3]. The quantum efficiency (i.e., the percentage of photogenerated carriers that are detected for each incoming photon) is greatly improved by using microlenses . In order to capture color information from the broad bandwidth incident light, color filters are used so that each pixel captures mainly one color of light (i.e., red, green, or blue). The color filters consist of dyed photoresist that is arranged in alternating rows or either green and blue or green and red [4]..