Pixel for CMOS image sensor having a select shape for low pixel crosstalk

Abstract

A novel CMOS image unit pixel layout having a photodiode including an optically optimized square image sensing region. The square image sensing layout provides for reduced electrical and color crosstalk and improved modulation transfer function (MTF) between neighboring pixels of an array of pixels.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates generally to image sensing technology. More specifically, the present invention relates to a CMOS image sensor having a novel layout that uses specially-matched microlenses. [0002] Traditionally, imaging technology has been centered around charge coupled device (CCD) image sensors. However, recently, CMOS imaging technology has become increasingly the technology of choice. There are a variety of reasons for this progression. First, CCD imagers require specialized facilities, which are dedicated exclusively to the fabrication of CCDs. Second, CCD imagers consume a substantial amount of power, since they are essentially capacitive devices, which require external control signals and large clock swings to achieve acceptable charge transfer efficiencies. Third, CCD imagers require several support chips to operate the device, condition the image signal, perform post processing and generate standard video output. This need fo...

AI Technical Summary

Benefits of technology

[0014] Use of a square image sensing area increases the distance between image sensing areas of adjacent pixels. This leads to a reduction in electrical and color crosstalk and improves the modulation transfer function (MTF).

[0015] In a second aspect of the invention, hemispherically-shaped microlenses are positioned over the image sensing areas of each CMOS pixel, to increase the effective fill factor of each pixel.

[0016] In one embodiment of the present invention, an array of CMOS pixels is provided, each pixel comprising a square image sensing region such that the distance between image sensing regions of neighboring pixels is optimized to reduce crosstalk between the neighboring pixels.

Problems solved by technology

First, CCD imagers require specialized facilities, which are dedicated exclusively to the fabrication of CCDs.

Second, CCD imagers consume a substantial amount of power, since they are essentially capacitive devices, which require external control signals and large clock swings to achieve acceptable charge transfer efficiencies.

Third, CCD imagers require several support chips to operate the device, condition the image signal, perform post processing and generate standard video output.

This need for additional support circuitry makes CCD systems complex.

Finally, CCD systems require numerous power supplies, clock drivers and voltage regulators, which not only further increase the complexity of the design, but also demands the consumption of additional significant amounts of power.

Despite the improvement in effective fill factor using hemicylindrically-shaped microlenses, there are negative performance factors, which are attributable to use of rectangular-shaped image sensing areas and hemicylindrically-shaped microlenses.

This ineffectiveness further involves light that is scattered and / or reflected and which ultimately arrives at the lenses 203 at oblique angles.

This second type of crosstalk occurs between pixels of different colors and is referred to in the art as “color crosstalk.” Color crosstalk leads to color distortion and is caused by the fact that silicon-based photodiodes have a wavelength-dependent photon absorption response.

In particular, color distortion can be significant for image arrays that use rectangular-shaped image sensing areas together with an RGB Bayer pattern color filter.

This difference in green pixel responsivity results in color distortion.

Finally, yet another problem that is observed when neighboring image sensing areas are too closely positioned, is a decrease in spatial resolution.

The lower the MTF, the less capable an imaging device is at picking up the fine detail and contrast in the object that is being imaged.

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