Dark current mechanisms in a 4-Transistor CMOS imager pixel with a negative gate bias on a transfer gate have been investigated. The increase of dark current with the negative gate bias has been attributed to the Gate-Induced-Leak (GIL) Trap Assisted Tunneling (TAT) by examining dark current dependency both on negative gate bias and on temperature. The negative gate bias on a transfer gate efficiently reduces Shockley -Read-Hall (SRH) dark current but generates GIL-TAT dark current when large negative gate bias is applied.
A negative bias operation of transfer gate has revealed a major origin of dark defects of CISs. It has been observed a strong visible light causes a damage of increasing dark current on operating condition and anneals the damage in power-off mode, which should be attributed to photon-assisted tunneling or emission and photon-stimulated annealing mechanisms.
Modern trends in camera module designs for both mobile and DSC applications are driving ever smaller pixels. At the same time, higher demands on the quality of the output image (DSC-like quality for mobile applications) requires maintaining pixel capacity, quantum efficiency (QE), and sensitivity, which becomes extremely difficult as the pixel size shrinks. This paper discusses pixel designs and process enhancements that enable a new generation of Image sensors with 1.75um, 1.4um, and smaller pixels with superior performance. The paper presents simulation data and experimental optical-electrical characteristics of our latest generation of 1.4um and 1.75um pixels, as well as improvements in pixel performance made through new pixel architectures and process enhancements. The paper presents performance comparisons of image sensors with different pixel sizes and array formats for the popular mobile 1/4 inch optical format including Aptina's newest 5Mpix sensor with 1.4um pixel size.
Noise reduction effects of column-parallel multiple sampling (CMS) for CMOS image sensors are investigated. The readout circuit called column-parallel multiple sampling circuits with a switched capacitor integrator reduces effectively not only for thermal noise but also for RTS noise. An experimental result using a 1Mpixel CMOS image sensor shows a low noise level of less than 2 electrons.
Image sensors for high-speed cameras and high-resolution video image sensors have similar technical issues in terms of the high speed pixel readout rate. Recent trend of implementing a video function to high-resolution digital still cameras and/or more pixels to video cameras requires technologies that have been utilized in high-speed image sensors. In this paper, recent technology developments on high-resolution, high-speed CMOS image sensors are presented with an example of our 8M-pixel UDTV sensors.
A high speed IMPACTRON^<TM> CCD line sensor with color sensing capability was developed by incorporation the technology of IMPACTRON^<TM> to the line sensor, which achieves both of high speed readout and high sensitivity. The charge splitter that divides charge from the fast clocked CCD resisters into two slower clocked registers has incorporated in the charge multiplication section. This provides a higher readout speed as well as the color separation directly on chip. The line sensor, which has the effective pixels of 3600(H)x2(V) and the pixel size of 10μmx10μm, realizes over 1000 times of the charge domain electron multiplication and less than one electron of the low noise performance. The line rate is 13kHz.
This paper describes the device performance of a 1.55-μm-square pixel interline transfer CCD (IT-CCD) having 12M pixels in a 1/2.3-inch optical format. The device has a novel metal wiring structure designed to increase the saturation signal that is determined by the effective supply voltage at the read-out gate, and employs a lateral overflow drain architecture to assist the low anti-blooming capability of the vertical overflow drain in the photodiode. As a result, we achieve a 14% higher saturation signal, and an anti-blooming performance that is 30-times superior to a 1.75-μm-square pixel CCD, despite the 27% smaller pixel area. These novel structures do not influence other performances of the device..