An image sensor provides a grid of pixels, such as photosensitive diodes or photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, non-pinned layer photodiodes and/or transfer transistors, for recording an intensity or brightness of light. The pixel responds to the light by accumulating a charge—the more light, the higher the charge. The charge can be processed by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge-coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) image sensor.
Backside illuminated sensors are used for sensing a volume of exposed light projected towards the backside surface of a substrate. Backside illuminated sensors provide a high fill factor and reduced destructive interference, as compared to front-side illuminated sensors. The pixels are located on a front side of the substrate, and the substrate is thin enough so that light radiation projected towards the backside of the substrate can reach the pixels. Accordingly, the substrate is thinned to a reasonable thickness to provide a desired photo response and reduce cross-talk. However, the process of thinning the substrate may cause damage to the silicon crystal lattice which can lead to an increase in dark current for the image sensor device. That is, unwanted current generated by pixels in the absence of illumination. There may be other sources for dark current such as impurities in the silicon wafer and heat build up in the pixel area. Excessive dark current may lead to image degradation and poor device performance.
A need exists for a device and method that provides a backside illuminated image sensor with reduced dark current without adversely affecting the performance of the device.