Study Of Wide Dynamic Range CMOS Image Sensors With Multi-exposure Technologies

As the CMOS technology develops, CMOS image sensor is now meeting and even exceeding the performance of CCD image sensors. CMOS image sensors are widely used due to its low power, low cost and highly integration features. The dynamic range of CMOS image sensors is usually between 60 dB to 70 dB. In some applications such as automotive imaging and security surveillance, which demand over 100 dB dynamic range, usual image sensors cannot capture all the details in bright as well as dark areas. Multi-exposure technology is commonly used to extend dynamic range, but it also brings high data rate and large data volume. To overcome this problem, this thesis studies the pixel design, timing optimization and image fusion technologies for wide dynamic range CMOS image sensors with multi-exposure technology.This thesis gives a detailed description of the basis of CMOS image sensors with wide dynamic range, and based on the 0.18μm CMOS image sensor process platform, a double doped photodiode is adopted to build an electronic field inside the photodiode; non-uniform doped transfer channel and transfer gate are proposed to adjust the potential in the charge transfer route to gradient distribution. These two schemes eliminate barriers and pockets, and transfer efficiency is improved, which finally leads to the enhancement of the photo response and the extension of dynamic range. In light of the high data rate problem of multi-exposure technology, a compact readout scheme is proposed to lower the readout speed by alternatively read out long and short exposed signals of different line. The readout speed can be lower over 60% with this compact readout scheme. Based on the typical 4T pixel architecture, an in-pixel dual exposure synthesis technique is proposed. This technique adjusts the PD capacity during exposure, then long and short exposed are accumulated in pixel. The output from pixel is the wide dynamic range signal, which greatly alleviates the data rate and volume problem induced by multi-exposure.Major innovations in this thesis are as follows:1. We proposed a non-uniform doped transfer gate to improve the charge transfer efficiency. This scheme modifies the distribution of the potential under the transfer gate, and eliminates potential pocket or barrier. A ladder type potential is obtained in the channel from PD to FD, which could accelerate the charge transfer. It is verified by simulations and measurement, and complete charge transfer was achieved with zero image lag.2. A compact readout method is proposed which could be utilized with multiple exposure technology to extend the readout time. This method can relieve the high readout rate problem induced by traditional multi-exposure technology for dynamic range extension. The readout time can be extend 286% by using the compact readout scheme.3. An in-pixel dual exposure synthesis technique is proposed to extend the dynamic range of CMOS image sensors. Compared with multiple exposure technology, this approach do not need extra memories and digital processors, and the wide dynamic range data are read out once per frame cycle. A proto chip is fabricated, and the dynamic range of which is extended from 51.59 dB to 90.53 dB.