CMOS Image Sensor Depletion Region Engineering
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Solution Overview
Problem
CMOS image sensors face challenges in maintaining high quantum efficiency and reducing electrical crosstalk while minimizing pixel size, as reducing pixel size decreases photodiode area and full well capacity, leading to degraded device characteristics.
Innovation Solution
An image sensor is fabricated with a first doping region of a second conductivity type formed under the photodiode to increase the depletion region, separated from the photodiode, which enhances quantum efficiency and crosstalk characteristics without affecting charge transfer characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pixel size is reduced to implement high density pixels, then the pixel density is improved, but the photodiode area and full well capacity are reduced leading to degraded device characteristics
Solution Approach 1:
The patent extends the depletion region in the depth direction (vertical dimension) by forming a first doping region beneath the photodiode, rather than increasing the photodiode area in the horizontal plane. This dimensional transition allows maintaining high pixel density while improving light detection capability through increased depletion region volume.
Solution Approach 2:
The patent changes the doping concentration parameter by forming a first doping region with a first doping concentration and a second doping region with a second doping concentration, where the ratio of the first doping concentration to the second doping concentration is between 0.01 and 1. This parameter optimization enables extended depletion region while maintaining charge transfer characteristics.
2Area of moving object
If the interval of photodiodes is reduced to increase photodiode area, then the photodiode area is improved, but quantum efficiency and crosstalk characteristics are degraded
Solution Approach 1:
Instead of increasing photodiode area horizontally (which reduces inter-pixel spacing and causes crosstalk), the patent increases the depletion region volume vertically by forming doping regions beneath the photodiode. This maintains adequate horizontal spacing for crosstalk prevention while improving light detection through extended depth.
3Quantity of substance
If ion implantation energy is reduced to increase maximum well capacity, then the full well capacity is improved, but the depletion region is reduced causing degradation in quantum efficiency and crosstalk characteristic
Solution Approach 1:
The patent optimizes doping concentration parameters by forming multiple doping regions with different concentrations (first doping concentration and second doping concentration with ratio between 0.01 and 1). This parameter optimization enables achieving both adequate depletion region for quantum efficiency and sufficient well capacity without relying solely on low energy ion implantation.
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
The solution improves quantum efficiency and crosstalk characteristics by expanding the depletion region of the photodiode, maintaining charge transfer efficiency and increasing photocurrent, especially in high wavelength bands, thus enhancing the signal-to-noise ratio and dynamic range.
Implementation Method 1
a first doping region of a second conductivity type formed under the photodiode to increase the depletion region
Data Source
AI summary
An image sensor includes an epi-layer of a first conductivity type formed in a substrate, a photodiode formed in the epi-layer, and a first doping region of a second conductivity type formed under the photodiode to separate the first doping region from the photodiode.