Murari Kartikeya, Etienne-Cummings Ralph, Thakor Nitish, Cauwenberghs Gert
Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, MD 21205 USA.
IEEE Sens J. 2009 Jul 1;9(7):752-760. doi: 10.1109/JSEN.2009.2021805.
While great advances have been made in optimizing fabrication process technologies for solid state image sensors, the need remains to be able to fabricate high quality photosensors in standard CMOS processes. The quality metrics depend on both the pixel architecture and the photosensitive structure. This paper presents a comparison of three photodiode structures in terms of spectral sensitivity, noise and dark current. The three structures are n(+)/p-sub, n-well/p-sub and p(+)/n-well/p-sub. All structures were fabricated in a 0.5 mum 3-metal, 2-poly, n-well process and shared the same pixel and readout architectures. Two pixel structures were fabricated-the standard three transistor active pixel sensor, where the output depends on the photodiode capacitance, and one incorporating an in-pixel capacitive transimpedance amplifier where the output is dependent only on a designed feedback capacitor. The n-well/p-sub diode performed best in terms of sensitivity (an improvement of 3.5 x and 1.6 x over the n(+)/p-sub and p(+)/n-well/p-sub diodes, respectively) and signal-to-noise ratio (1.5 x and 1.2 x improvement over the n(+)/p-sub and p(+)/n-well/p-sub diodes, respectively) while the p(+)/n-well/p-sub diode had the minimum (33% compared to other two structures) dark current for a given sensitivity.
虽然在优化固态图像传感器的制造工艺技术方面已经取得了巨大进展，但仍然需要能够在标准CMOS工艺中制造高质量的光电传感器。质量指标取决于像素架构和光敏结构。本文从光谱灵敏度、噪声和暗电流方面对三种光电二极管结构进行了比较。这三种结构分别是n(+)/p-衬底、n阱/p-衬底和p(+)/n阱/p-衬底。所有结构均采用0.5微米3金属、2多晶硅、n阱工艺制造，并共享相同的像素和读出架构。制造了两种像素结构——标准的三晶体管有源像素传感器，其输出取决于光电二极管电容；另一种包含像素内电容跨阻放大器，其输出仅取决于设计的反馈电容。n阱/p-衬底二极管在灵敏度（分别比n(+)/p-衬底和p(+)/n阱/p-衬底二极管提高3.5倍和1.6倍）和信噪比（分别比n(+)/p-衬底和p(+)/n阱/p-衬底二极管提高1.5倍和1.2倍）方面表现最佳，而在给定灵敏度下，p(+)/n阱/p-衬底二极管的暗电流最小（与其他两种结构相比为33%）。
