Gr Gb difference in 3M CMOS Image Sensor with 1.75μm pixel

Samsung Electronics Co., LTD., Nongseo-Dong, Giheung-Gu, Yongin-City, Gyeonggi-Do, Korea 446-711 Bumsuk Kim, Yoonho Jang, Jongjin Lee, Kyoungsik Moon, Eun-Gyu Lee, Alexander Getman, JungChak Ahn, and YongHee Lee.

Keywords: CMOS image sensor, Gr Gb difference, Bayer pattern, Maze-like noise, 4-shared structure, 2-shared structure

Abstract For CMOS image sensors with smaller pixel size, the pixel structure in which several pixels share

floating diffusion or transistors tends to be adopted to enhance photodiode capacity and sensitivity. We have observed that in spite of aforementioned benefits, application of this structure may result in sensitivity difference between the shared pixels when light is obliquely incident. On captured images it appears as a difference between Gr and Gb channels. In this paper we compare structures of initial and improved 2.25um pixels. And the new sensor with 1.75um pixel size is compared. The improved 1.75um pixel was designed much more symmetrically than the 2.25um pixel due to advantage of 90nm Cu process. Gr Gb difference according to microlens shift and sensor factors (relative sensitivity at oblique incidence) were measured and compared for both sensors with 2.25um and 1.75um pixel sizes. It was proved that tolerance of Gr Gb difference to microlens shifts was greatly improved and that sensor factor of 1.75um pixel is superior to that of 2.25um.

I. Introduction Typical color filter array which is used for CMOS image sensor (hereafter CIS) is Bayer pattern, which consists

of Red, Blue, and two Greens (Fig. 1a). Therefore green pixels are able to show more information than red and blue pixels. But as pixel size gets smaller, problems such as low sensitivity and small photodiode capacity become more and more serious. Smaller photodiode capacity causes larger shot noise, and lower sensitivity increases integration time to gather same amount of electrons. All of these lead to degradation of signal to noise ratio. Therefore most recent pixels in CIS tend to adopt shared structure to achieve larger photodiode capacity and sensitivity [1]. If several transistors and active area are shared, geometry and structure of sharing pixels are inevitably asymmetric. All these things related with smaller pixel size can lead to signal difference between sharing pixels. If the signals of Gr and Gb pixels are different from each other, a kind of noise is induced. After an advanced color interpolation is applied, the Gr Gb difference can be seen to be a spatial noise which looks like maze.(Fig. 1b) Gr Gb difference is usually defined as (Gr-Gb)/<G> where <G> is average of Gr and Gb. The maze-like spatial noise is well seen when Gr Gb difference is larger than 3% from experience.(Fig. 2) It is desirable to remove the difference by optimized design of pixels without any help of digital compensation block.

II. Gr Gb difference of 2.25μm pixel 2.25μm pixel was developed using 0.13μm copper process. It adopted 4-shared structure which maximizes area

of photodiode [2]. Four pixels share one floating diffusion and transistors such as reset and source follower. While the array of sharing pixels is 1x4, the structures of pixels in even and odd rows are different from each other just like 1x2 2-shared structure. But large Gr Gb difference was observed especially at oblique incidence of light. Finally metals were adjusted such that the Gr Gb difference at oblique incidence could be removed only by controlling microlens shift.

Fig. 3(a) shows the initial structure of 2.25μm pixel. Photodiodes are arranged asymmetrically due to the shared pixel structure. Photodiodes in even rows are moved upward from center of pixel, and those in odd rows downward. Electron may be generated outside of depletion region in odd rows, such that sensitivity could be lowered. They

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might contribute to crosstalk between pixels. If sensitivities of even and odd rows are different from each other when light is incident obliquely, Gr Gb difference and color tint will occur which were already shown in Fig. 1. In order to solve this problem the pixel structure was re-designed so as to be able to cancel out the Gr Gb difference due to the asymmetry. Metal shields were adjusted like Fig. 3(b). In this structure sensitivity difference between even and odd rows can be successfully removed only by optimizing microlens shift. It was confirmed that modified metal shields play a role of reducing the sensitivity difference between even and odd rows. Fig. 4 and 5 show the experimental result of Gr Gb difference according to microlens shift.

III. Gr Gb difference of 1.75μm pixel 1.75μm pixel was developed and designed using advanced 90nm Copper process. Both high fill factor as much

as 46% and good symmetry could be achieved even though the pixel adopted 2-shared structure. Therefore metal apertures are much more symmetric than those of 2.25μm pixel. Moreover pixel height was greatly reduced by using only two metal layers, which contributed to the improvement of tolerance to microlens shift and sensitivity at oblique incidence.

Gr Gb differences were measured according to microlens shift using 1/4inch 3M CIS with 1.75μm pixels. As a result, it is shown in Fig. 6 and Fig. 7 that the Gr Gb difference is much more insensitive to micolens shift compared with 2.25μm pixel.

The sensor factors of 1.75μm and 2.25μm pixels are compared in Fig. 8. Sensor factor is defined to be shading factor of pixel itself. Therefore it excludes shading of camera lens, and it is usually normalized at normal incidence. The sensor factors were measured using the camera lens of which the relative illumination and the angle of incidence according to image height are well known. The comparison in Fig. 8 shows that the sensor factor of 1.75μm pixel is superior to that of 2.25μm due to the much lower pixel height in spite of larger pixel size.

IV. Conclusion 2.25μm pixel was developed using 0.13μm copper process, and it adopted 1x4 4-shared structure. Gr Gb

difference was observed in the initial pixel structure at oblique incidence. But metal shields were adjusted to remove Gr Gb difference only by optimizing microlens shift. Gr Gb difference can be removed without help of digital logic.

1.75μm pixel was designed using the advanced 90nm Cu process. Though 1x2 2-shared structure was adopted, the fill factor as much as 46% could be achieved while the photodiodes and metal shields of even and odd rows are much more symmetric than those of 2.25μm. Since the pixel height was lowered by reducing number of metal layers, the sensor factor of 1.75μm is superior to that of 2.25μm in spite of smaller pixel size. Gr Gb difference was proved to be much more insensitive to microlens shift compared with 2.25μm.

References [1] M. Mori, et al., “A 1/4 inch 2M Pixel CMOS Image Sensor with 1.75 Transistor/Pixel,” ISSCC Dig. Tech.

Papers, pp. 110-111, Feb., 2004. [2] Young Chan Kim, et al., “1/2-inch 7.2MPixel CMOS Image Sensor with 2.25μm Pixels Using 4-Shared

Pixel Structure for Pixel-Level Summation”, 2006 IEEE International Solid-State Circuits Conference, pp. 1994-2003, Feb., 2006.

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Figures

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(a) (b)Figure1. (a) Bayer color filter pattern (b) Example of maze-like noise.

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Figure2. Spatial noise according to Gr Gb difference.

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Figure3. (a) Initial structure of 2.25μm pixel (b) Modified structure to remove Gr Gb difference only by optimizing microlens shift.

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Figure4. Contour of Gr Gb difference of 2.25μm pixel according to microlens shift.

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Figure5. Gr Gb difference of 2.25μm pixel according to microlens shift

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Figure6. Contour of Gr Gb difference of 1.75μm pixel.

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Figure7. Gr Gb difference of 1.75μm pixel according to microlens shift.

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Figure8. Comparison of sensor factors between 2.25μm and 1.75μm pixels.

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