Performance Optimization Of Pixel In TDI CMOS Image Sensor

The market of CMOS image sensor (CIS) is now larger than that ofcharge-coupled devices (CCD) image sensor, principally because CISs have variousadvantages over CCDs, such as lower power consumption, lower cost, random accessof image data, high-speed imaging and so on. Time-Delay Integration (TDI) imagesensor is a special kind of linear array image sensor. Since it implement long exposurein the form of two-dimensional pixel array structure and dynamic scanning mode, it isbetter than the ordinary linear array image sensor. Therefore, the sensitivity andsignal-to-noise ratio are both improved. They are widely used in the object detectionfield of low illumination environment, so characteristics like large photosensitive area,low dark current and zero tolerance for image lag are needed.This paper analyzes and improves the performance of the large size pinnedfour-transistor active pixel applied in TDI CIS in terms of charge transfer and darkcurrent. The performance of charge transfer of large size pixels at N buried layer andtransmission gate are optimized respectively. The performance of charge transfer isoptimized in N buried layer includes two aspects. One is insert a P-type implant layerduring the process of forming the non-uniform doped N buried layer. The other is tooptimize the photodiode layout as U-shaped to improve the charge transfer efficiencyof pixel. Compared with the traditional pixel, the charge transfer efficiency of theabove two optimized methods were increased by2times and3.3times respectively.And the transfer time is also shortened by26%and30%respectively. The chargetransfer efficiency was increased by9.5times both in the process and layoutoptimization. The structure of the transmission gate is optimized. During the period ofcharge transfer and integration time, applying different voltages on the TX1toachieve the purpose of improving the charge transfer efficiency. Compared with thetraditional pixel, the transfer time of optimized transmission gate structure shortened88ns by means of simulation. For the improvement of the performance of the dar kcurrent, the optimization mainly aims at the STI and p-type injection layer of pixel.The generation of dark current is due to the existence of interface state in the SI/SIO2interface. And the reduction of dark current effect is comparable between differentdesign adjustments by the chip measurement analysis. In conclusion, this paper proposed the optimization of the charge transfer anddark current of the pixel in the TDI CMOS image sensor. And it verifies thecorrectness and effectiveness of the optimization program through the theoreticalanalysis, software simulation and chip measurement results. This optimization methodprovides a reference value to improve the performance of pixel in TDI CMOS imagesensor.