Photodetector n-layer structure for quantum efficiency
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Solution Overview
Problem
Active CMOS image sensors face challenges in maintaining high quantum efficiency and low cross-talk as pixel sizes decrease, with existing methods either increasing cross-talk or reducing quantum efficiency, and manufacturing limitations restrict the depletion depth of photodetectors.
Innovation Solution
The introduction of a deep, low-concentration n-type layer between a p-type substrate and a p-type layer in the photodetector structure, which extends the depletion depth and reduces cross-talk without compromising charge capacity or dynamic range, achieved through multiple high-energy implants or epitaxial growth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the depletion depth of the photodetector is increased to improve quantum efficiency and reduce cross talk, then collection efficiency improves, but charge capacity decreases and dark-current generation increases
Solution Approach 1:
The patent applies local quality by creating a deep, lightly-doped n-type layer specifically in the collection region of the photodetector, while maintaining higher doping concentrations in other regions. This localized modification extends the depletion depth to improve quantum efficiency and reduce cross talk without reducing the overall charge capacity of the device, as the lightly-doped layer is confined to a specific depth range (e.g., 2-5 micrometers) rather than affecting the entire detector volume.
Solution Approach 2:
The patent changes the doping concentration parameter by introducing a lightly-doped n-type layer with doping concentration of 10^15 to 10^17 atoms/cm³, which is significantly lower than conventional doping levels. This parameter change extends the depletion depth by reducing the doping concentration in the bulk material, thereby improving quantum efficiency while the overall charge capacity is maintained through the specific depth and concentration profile of the lightly-doped layer.
2Object-affected harmful factors
If the depletion depth is increased by reducing bulk doping concentration, then cross talk reduces, but dark-current generation increases
Solution Approach 1:
The patent applies local quality by confining the lightly-doped region to a specific depth range (e.g., 2-5 micrometers) within the photodetector structure. This localized lightly-doped layer extends the depletion depth to reduce cross talk between adjacent pixels, while the limited depth and controlled doping concentration (10^15 to 10^17 atoms/cm³) prevent excessive dark-current generation that would occur with uniform bulk doping reduction.
Solution Approach 2:
The patent changes the doping concentration parameter to 10^15 to 10^17 atoms/cm³ in the deep layer, which is low enough to extend depletion depth and reduce cross talk, but the specific concentration and depth profile are optimized to minimize dark-current generation. The parameter change is applied locally rather than uniformly throughout the bulk material, allowing cross talk reduction without proportionally increasing dark current.
3Length of stationary object
If high-energy implantation is used to achieve deeper n-diode to p-epi junction, then depletion depth increases, but manufacturing complexity increases due to masking layer limitations
Solution Approach 1:
The patent applies segmentation by dividing the doping process into multiple steps: first forming a shallow n-type layer, then performing a second high-energy implantation to create the deep lightly-doped layer. This segmentation allows the deep junction to be formed while using standard masking techniques, as each implantation step can be controlled independently and the masking layer thickness requirements are reduced compared to attempting to form the entire deep structure in a single step.
Solution Approach 2:
The patent applies preliminary action by first forming a shallow n-type layer before performing the high-energy implantation to create the deep lightly-doped layer. This preliminary layer serves as a foundation that facilitates the subsequent deep implantation process, allowing the formation of the deep junction without requiring excessively thick masking layers, as the preliminary structure guides the implantation process.
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 enhances both quantum efficiency and cross-talk performance simultaneously, maintaining imaging performance characteristics while overcoming manufacturing limitations by extending the depletion depth and directing stray electrons back into the photodetector.
Implementation Method 1
each pixel consists of a photodetector element and one or more transistors to read out a voltage representing the light sensed in the photodetector
Implementation Method 2
a carrier generated greater than this depletion depth can diffuse laterally into adjacent photosites which contributes to cross talk
Data Source
AI summary
An image sensor with an image area having a plurality of photodetectors of a first conductivity type includes a substrate of the second conductivity type; a first layer of the first conductivity type spanning the image area; a second layer of the second conductivity type; wherein the first layer is between the substrate and the second layer, and the plurality of photodetectors is disposed in the second layer and abut the first layer.