Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) imagers, are commonly used in photo-imaging applications.
A CMOS imager circuit includes a pixel array, each one of the pixels including a photosensor which may be one of a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. A readout circuit is connected to each pixel and includes at least an output field effect transistor formed in the substrate and a charge transfer section formed on the substrate adjacent the photogate, photoconductor or photodiode having a sensing node, typically a floating diffusion node, connected to the gate of an output transistor. The imager may include at least one electronic device such as a transistor for transferring charge from the underlying portion of the substrate to the floating diffusion node and one device, also typically a transistor, for resetting the node to a predetermined level prior to charge transference.
In a CMOS imager, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. CMOS imagers of the type discussed above are generally known as discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524 and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc., which are hereby incorporated by reference in their entirety.
FIG. 1 illustrates a block diagram for a CMOS imager 10. The imager 10 includes a pixel array 20. The pixel array 20 comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array 20 are all turned on at the same time by a row select line and the pixels of each column are selected for output by a column select line. A plurality of row and column lines are provided for the entire array 20.
The row lines are selectively activated by the row driver 32 in response to row address decoder 30 and the column select lines are selectively activated by the column driver 36 in response to column address decoder 34. Thus, a row and column address is provided for each pixel. The CMOS imager 10 is operated by the control circuit 41, which controls address decoders 30, 34 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 32, 36, which apply driving voltage to the drive transistors of the selected row and column lines.
Each column contains sample and hold circuitry (S/H) 38 comprising sampling and holding capacitors and switches associated with the column driver 36 that read and store pixel reset signal Vrst and a pixel image signal Vsig for selected pixels. A differential signal (Vrst−Vsig) is produced by differential amplifier 40 for each pixel, which is digitized by analog-to-digital converter 45 (ADC). The analog-to-digital converter 45 supplies the digitized pixel signals to an image processor 50, which forms and outputs a digital image output.
As previously noted, a typical CMOS image sensor includes an array of pixels 20, arranged in a predetermined number of columns and rows. Generally, each pixel photosensor generates charge for a same amount of time and has an associated dynamic range. The dynamic range for an image sensor is commonly defined as the ratio of its largest non-saturating signal to the standard deviation of the noise under dark conditions. Dynamic range refers to the range of incident light that can be accurately sensed by an image sensor in a single frame of pixel data. It is desirable to have an image sensor with a high dynamic range in order to image scenes that generate high dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others.
Dynamic range is limited on an upper end by the charge saturation level of the photosensor, and on a lower end by noise imposed limitations and/or quantization limits of the analog-to-digital converter used to convert the analog pixel output signals to digital signals. When the dynamic range of an image sensor is too small to accommodate the variations in light intensities of the imaged scene, image distortion occurs.
What is needed, therefore, is an image sensor, and methods of fabricating and operating the image sensor to achieve an improved dynamic range, which may be implemented using conventional CMOS processing techniques.