A Back-Illuminated High-Sensitivity Small-Pixel Color CMOS Image Sensor with Flexible Layout of Metal Wiring

Abstract

The number of image sensors being produced, such as those used in video camcorders, digital still cameras, and cellular phone cameras is increasing every year. CCDs still dominate the market, especially in digital still cameras (DSC), which constitute the largest single application and for which there is pressure to reduce the pixel size. CCDs are currently ahead of CMOS image sensors (CIS) in terms of pixel size reduction [1]. The main reason that CCDs dominate the market is the lower image quality output of CIS devices, despite the fact that they offer many advantages such as high-speed operation and lower power consumption. One of the main reasons for this lower image quality is the lower sensitivity of the CIS device. The lower sensitivity is a result of the complicated structure above the photo-diode itself. Ordinary CIS devices use two or more metal layers, which act as an obstacle above the photo-diode. These metal layers obscure the light and reduce the photon gathering efficiency.We set out to solve these problems and, in this study, developed a CIS device with metal wiring under the photo-diode, which is called a back-illuminated CIS. The structure above the photodiode of this CIS is simple, consisting only of a color filter and an on-chip lens. There are no obstacles in the photon gathering path. We have incorporated this design into a 1/3.2inch, 1.3M-pixel CIS with 3.45µm square, three-transistor pixels. Figure 16.8.1 compares the cross-sections of our CIS with a conventional CIS. We confirmed that a back-illuminated structure offers advantages in terms of sensitivity and optical angle response. With our CIS, we can make the pixel size smaller and capture color images because we use an on-chip color filter, and do not use 3D interconnect in the pixels. Furthermore, the dark current is reduced with our device because we form the photo-diode and all of the circuits on the same silicon wafer [2,3,4].In addition to the improvement in the photon gathering, the silicon thickness also affects the sensitivity. Figure 16.8.2 shows that our CIS with 4µm thick silicon has 38% higher sensitivity than a conventional CIS for parallel light at a wavelength of 540nm. The dependence of the sensitivity on silicon thickness at a wavelength of 540nm indicates that the sensitivity is saturated for 5µm-thick Silicon (i.e., there is almost 100% photon absorption). However, the photo-diode with a silicon thickness of 4µm has sufficient sensitivity for many applications. We went on to find that the degradation in the sensitivity is only slightly dependent on the angle at which the light enters the back-illuminated CIS. Figure 16.8.3 shows that, for our CIS, the degradation is only 20% at a 20å ngle of incidence while the degradation is 85% for a conventional CIS at the same angle. Figure 16.8.4 provides a table comparing the performance of our CIS to a conventional structure. The back-illuminated CIS offers improved sensitivity and F-number dependence. In addition to these improvements, the image lag and dark output are simil...

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