Photogate with improved short wavelength response for a CMOS imager

Abstract

A photogate-based photosensor for use in a CMOS imager exhibiting improved short wavelength light response. The photogate is formed of a thin conductive layer about 50 to 3000 Angstroms thick. The conductive layer may be a silicon layer, a layer of indium and / or tin oxide, or may be a stack having an indium and / or tin oxide layer over a silicon layer. The thin conductive layer of the photogate permits a greater amount of short wavelength light to pass through the photogate to reach the photosite in the substrate, and thereby increases the quantum efficiency of the photosensor for short wavelengths of light.

Description

FIELD OF THE INVENTION [0001] The present invention relates generally to CMOS imagers and in particular to a CMOS imager having improved responsiveness to short wavelengths of light. BACKGROUND OF THE INVENTION [0002] There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices and hybrid focal plane arrays. CCD technology is often employed for image acquisition and enjoys a number of advantages which makes it the incumbent technology, particularly for small size imaging applications. CCDs are capable of large formats with small pixel size and they employ low noise charge domain processing techniques. [0003] However, CCD imagers also suffer from a number of disadvantages. For example, they are susceptible to radiation damage, they exhibit destructive read-out over time, they require good light shielding to avoid image smear and they have a high power dissipation for large arrays. Additional...

AI Technical Summary

Benefits of technology

"The present invention provides a photogate with improved response to short wavelength light. This is achieved by forming a gate oxide over a doped semiconductor substrate and a gate conductor on top of the gate oxide. The gate conductor has a specific thickness range. Additionally, a stacked photogate with two layers is also provided, where the upper layer is made of indium tin oxide or other material and the lower layer is made of silicon or other material. The very thin photogate allows more short wavelength light to pass through and reach the photosite in the substrate, thereby increasing the quantum efficiency of the photosensor."

Problems solved by technology

However, CCD imagers also suffer from a number of disadvantages.

For example, they are susceptible to radiation damage, they exhibit destructive read-out over time, they require good light shielding to avoid image smear and they have a high power dissipation for large arrays.

Additionally, while offering high performance, CCD arrays are difficult to integrate with CMOS processing in part due to a different processing technology and to their high capacitances, complicating the integration of on-chip drive sued signal processing electronics with the CCD array.

While there have been some attempts to integrate on-chip signal processing with CCD arrays, these attempts have not been entirely successful.

CCDs may also suffer from incomplete charge transfer from pixel to pixel which results in image smear.

A disadvantage of conventional photogates, however, is poor quantum efficiency for short wavelength light, i.e., wavelengths less than 500 nm, such as green, blue, or violet light.

Most prior art methods to solve the poor short wavelength efficiency of imagers have utilized photodiodes, and relatively few methods have been proposed for photogate-based imagers.

These methods add complexity to the manufacturing process of the imager, however, and have not succeeded in making photogates competitive with photodiodes when increased short wavelength sensitivity is desired.

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