A 0.9 µm pixel size image sensor realized by introducing organic photoconductive film into the BEOL process

Abstract

A stacked image sensor with a 0.9 μm pixel size fabricated by using organic photoconductive film (OPF) was realized. It is the first trial to introduce an active material, that is, an organic semiconductor into the BEOL process. This pixel structure is fabricated by using a standard 45 nm BEOL process. However, after OPF deposition, it is essential to restrict the thermal budget and to avoid oxygen, moisture, and plasma irradiation. By controlling the above conditions, a demonstration of a stacked image sensor with OPF, which has high sensitivity, high saturation charge, and a wide incident light angle, was successfully performed.Introduction As the pixel size of image sensors shrinks, new technologies dependant on BEOL, such as lightpipes [1], the separation walls of on-chip color filters (OCFs) [2], and backside illumination (BSI) [3], have been widely developed and introduced into mass production recently. Pixel size has shrunk to nearly 1.0 μm.However, as silicon has a low absorption coefficient, the depth of photodiodes should be larger than 3 μm to ensure sensitivity regardless of pixel size, i.e., as pixel size becomes small, the aspect ratio of photodiode must be large. As a result, the leakage of light to the neighboring pixels in a photodiode increases. This leads to narrow incident light angles.To suppress the leakage of light and achieve wide incident light angles at a small pixel size, it is necessary to develop a stacked image sensor with another photoconductive film that works effectively even if it becomes thinner.Recently, organic semiconductors have been applied to photoelectric conversion devices, such as light emitting diodes, illumination, and solar cells [4, 5]. Some organic materials have a higher absorption coefficient than does silicon, as shown in Fig. 1. Therefore, in image sensors, the thickness of photoelectric conversion film (photodiode) can be decreased to under 0.5 μm by using organic materials, as shown in Fig. 2. This makes it possible to have both a high sensitivity and a wide incident light angle [6] at a pixel size under 1.0 μm.Because organic photoconductive film (OPF) is not so robust, there are some problems that must be overcome in order to introduce it into the BEOL process. It is necessary to restrict the thermal budget and to avoid oxygen, moisture, and plasma irradiation, which includes ultraviolet light.In addition to the above problems, in stacked image sensors, which are different from normal CMOS image sensors, the capacitance of lower pixel electrodes as well as plugs affects the electronic crosstalk of adjacent pixels, so that deciding both the pixel structure and the position of the lower pixel electrodes becomes important problem.In this work, to solve the above problems, we considered where the OPF, lower pixel electrodes, and global layer should be placed and proposed two structures: an "OPF over global layer" and an "OPF under global layer."Pixel Structure using OPF This stacked image sensor used 45 nm CMOS technologies on a 300 mm wafer. Three Cu fine laye...

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