Indium-Doped Lateral Gradient for Image Sensor Charge Transfer
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Solution Overview
Problem
Image sensors face issues with potential barriers and wells between the photosensitive element and the transfer gate, leading to incomplete charge transfer and image artifacts, such as image lag and ghost images, due to the use of boron doping which increases the barrier at the photodiode/transfer gate interface.
Innovation Solution
The use of indium doping with controlled implant energies and depths to create a lateral doping gradient that reduces the barrier and well potential, allowing for complete signal transfer without the straggle issues associated with boron implants, thereby enhancing the performance of CMOS and CCD image sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If boron doping is used to create a lateral doping gradient, then charge transfer across the transfer gate is improved, but the barrier at the photodiode/transfer gate interface increases
Solution Approach 1:
The patent changes the dopant type from boron to indium, altering the chemical and electrical parameters of the doping process. Indium has different atomic properties than boron, resulting in a lateral doping gradient that provides the necessary electric field for charge transfer without creating excessive potential barriers at the photodiode/transfer gate interface
Solution Approach 2:
The patent applies different doping characteristics to different regions: indium doping is used specifically at the photodiode/transfer gate interface region to minimize barriers, while the lateral doping gradient is maintained in the transfer gate region to facilitate charge transfer. This localized optimization of doping quality resolves the contradiction between barrier reduction and charge transfer enhancement
2Productivity
If lateral doping gradient is increased to drive electrons across transfer gate, then charge transfer is improved, but image lag increases
Solution Approach 1:
The patent changes the dopant parameter from boron to indium, which alters the electrical field distribution characteristics. Indium doping provides sufficient lateral field for complete charge transfer while avoiding the excessive field strength that causes image lag, thus improving both charge transfer efficiency and reducing image lag simultaneously
3Manufacturing precision
If boron implant energy is reduced to minimize straggle, then doping precision is improved, but unwanted compensation of photodiode N- implant occurs
Solution Approach 1:
The patent changes the dopant element from boron to indium, which has different implantation characteristics. Indium doping at appropriate energies provides precise lateral doping gradient formation without the straggle and compensation issues associated with boron, as indium atoms have different mass and diffusion properties that allow for cleaner, more controlled doping profiles
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 indium doping solution effectively minimizes image lag and improves charge transfer efficiency, reducing unwanted image artifacts and ensuring accurate image capture by lowering the barrier at the photodiode/transfer gate interface.
Implementation Method 1
indium doping with controlled implant energies and depths to create a lateral doping gradient
Implementation Method 2
provide a lateral electrical field that drives electrons from the photosensitive side, across the transfer gate, to the drain side
Data Source
AI summary
An active pixel using a transfer gate that has a polysilicon gate doped with indium. The pixel includes a photosensitive element formed in a semiconductor substrate and an n-type floating node formed in the semiconductor substrate. An n-channel transfer transistor having a transfer gate is formed between the floating node and the photosensitive element. The pixel substrate has a laterally doping gradient doped with an indium dopant.