[Paper] A CMOS Image Sensor with 240 μV/e– Conversion Gain, 200 ke– Full Well Capacity, 190-1000 nm Spectral Response and High Robustness to UV light

Image sensors are utilized in various fields such as food analysis, biological science, remote sensing and so on1)~5). As for the image sensor in this field, a wide spectral sensitivity, a high robustness to ultraviolet (UV) light, a high input referred conversion gain (CG) and a wide dynamic range (DR) are important as basic performances. The image sensor that achieves these performances is utilized not only for image measurement, emission and absorption analyses, but also spectral imaging which obtain spectral information in two dimensional image. Light sensing for various wavelength such as a UV and a near-infrared (NIR) light and various light intensity conditions has following challenges. Regarding UV light, the penetration depth of UV light in Si is short. In order to increase the UV light sensitivity, the photo electrons generated within several nanometers from Si surface must be collected. Therefore it is required to form electric filed in the surface penetration region of UV light. However, if the interface states of the SiO2/Si are activated, it causes a loss of sensitivity and generation of the dark current6)7). Thus the dopant profile of a Si surface must be carefully adjusted to improve UV light sensitivity without degradation. When Si is irradiated by UV light, the photo carriers have sufficient energy to be injected to SiO2. Thus some of the excited photo carriers are trapped in SiO2 and become fixed charges. This charging of SiO2 causes a change of potential near the photodiode (PD) surface. In addition, interface states are generated by UV light irradiation. Due to these fixed charges and interface states, UV light sensitivity tends to be degraded6)7). To solve these issues, it has been reported that forming a high concentration p+ layer with steep dopant concentration profile on the flattened Si surface in the PD is effective to consistently achieve high UV light sensitivity and high robustness of a sensitivity and a dark current toward UV light exposure8)9). Regarding NIR light, in order to increase the NIR light sensitivity, the electrons generated over several micron meters from Si surface must be collected. Therefore it is required to form sensible region deeply in Si. In addition, for spectrophotometric analyses and spectral imaging, incident light levels for a wide Abstract The structure and performances of a CMOS image sensor fabricated by integrating technologies in a series of process flow that achieve wide spectral response, high robustness to ultraviolet (UV) light, high conversion gain (CG) and high full well capacity (FWC) is described. For PD junction formation, a high concentration p+ layer with steep dopant concentration profile was formed on a thick p-epitaxial layer. The concentration of the surface p+ layer was tuned sufficiently high in order to maintain the high UV light sensitivity and dark current against the deuterium lamp irradiation. For the FD structure, a lightly doped drain implantation before sidewall formation was omitted to reduce gate overlap capacitance. A CMOS image sensor using a 0.18 μm CMOS process technology achieved a high CG of 240 μV/e–, a high FWC of 200 ke–, a wide spectral response for 190-1000 nm and a high robustness to deuterium lamp irradiation stress.