CMOS Image Sensor Red Pixel Quantum Efficiency via Deep Implant
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Solution Overview
Problem
CMOS image sensors face challenges with dark current generation, reduced fill factor, and floating body effects, which affect quantum efficiency and signal-to-noise ratio, particularly in red pixel collection and wavelength-dependent photon absorption.
Innovation Solution
The implementation of a p-type epitaxial region with a deep n-type region (N-Tub) and p-type ground contacts under red pixel locations, along with n-type substrate structures, to enhance photon collection depth and reduce floating body effects while maintaining fill factor and quantum efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a deep n-type region (N-Tub) is formed in a p-type substrate to reduce dark current, then dark current is reduced, but red quantum efficiency is reduced due to thin collection depth
Solution Approach 1:
The patent applies local quality by forming a p-type epitaxial region specifically beneath red pixel locations within the N-Tub structure. This creates a localized p-type collection region that is optimized for red photon detection while the surrounding n-type substrate continues to provide dark current reduction. The selective formation of p-type regions only where red pixels are located allows simultaneous achievement of low dark current and high red quantum efficiency.
2Productivity
If pixel size is decreased to increase pixel density, then more pixels fit in the sensor area, but fill factor is reduced leading to lower sensitivity
Solution Approach 1:
The patent extends the photon collection capability into the vertical dimension by forming deep n-type and p-type regions that penetrate through the substrate. This allows photons to be collected from greater depths below the pixel surface, effectively increasing the photosensitive area without increasing the planar pixel footprint. This dimensional extension maintains sensitivity even as pixel sizes are reduced to increase density.
3Object-affected harmful factors
If n-type substrate is used to reduce cross-talk, then cross-talk is reduced, but floating body effects are generated affecting pixel stability
Solution Approach 1:
The patent introduces a p-type epitaxial region as an intermediary layer between the n-type substrate and the red pixels. This p-type region acts as a mediator that provides stable electrical potential for the red pixels (reducing floating body effects) while the underlying n-type substrate continues to provide cross-talk reduction. The p-type region serves as an intermediate structure that reconciles the conflicting requirements of using n-type substrate for cross-talk reduction while maintaining pixel stability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach improves quantum efficiency and reduces dark current and floating body effects, enhancing the sensitivity and dynamic range of CMOS image sensors without increasing fill factor or pixel density.
Implementation Method 1
a significant portion of the pixel area is dedicated to the support transistors (amplifier, reset, and row select), which are relatively opaque to visible light photons and cannot be utilized for photon detection
Implementation Method 2
Each pixel includes a photo-conversion device, e.g., a photogate, photoconductor, or photodiode having an associated charge accumulation region within a substrate for accumulating photo-generated charge
Implementation Method 3
dark current is a result of current generated from trap sites inside or near the photodiode depletion region, surface leakage at silicon/surface interface; band-to-band tunneling induced carrier generation as a result of high fields in the depletion region; junction leakage coming from the lateral sidewall of the photodiode
Implementation Method 4
band-to-band tunneling induced carrier generation as a result of high fields in the depletion region
Implementation Method 5
the desired reduction of floating body effects when n-type substrates (such as an n-tub as disclosed in U.S. Patent Application Publication US 2005/0133825 A1) are implemented under the image sensor pixel's photosensor
Data Source
AI summary
A method and structure for providing a high energy implant in only the red pixel location of a CMOS image sensor. The implant increases the photon collection depth for the red pixels, which in turn increases the quantum efficiency for the red pixels. In one embodiment, a CMOS image sensor is formed on an p-type substrate and the high energy implant is a p-type implant that creates a p-type ground contact under the red pixel, thus reducing dark non-uniformity effects. In another embodiment, a CMOS image sensor is formed on an n-type substrate and a high energy p-type implant creates a p-type region under only the red pixel to increase photon collection depth, which in turn increases the quantum efficiency for the red pixels.