Back-Side Illuminated CMOS Sensor Dark Current Suppression
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Solution Overview
Problem
Back-illuminated CMOS image sensors face challenges in reducing dark current due to interface trapping, which degrades image accuracy, and the formation of an n+ layer at the back side surface is difficult using existing methods, also affecting sensitivity.
Innovation Solution
A pinned photodiode structure with a positive-charged silicon nitride anti-reflecting layer is introduced on the back side to suppress dark current, and a fabrication process involving silicon nitride layers is used to reduce interface states and enhance sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a back-illuminated CMOS image sensor structure is used to avoid light blocking by metal interconnects, then light sensitivity is improved, but dark current increases due to interface trapping at the back side interface
Solution Approach 1:
An insulator layer is introduced as an intermediary between the silicon substrate and the external environment at the back side interface. This insulator layer passivates interface states that would otherwise trap charges and generate dark current, thereby reducing the harmful dark current effect while preserving the back-illuminated structure's light sensitivity advantage
Solution Approach 2:
The patent employs a simple oxide layer (such as silicon dioxide) formed through standard oxidation processes to passivate the back side interface. This inexpensive and easily manufacturable layer effectively suppresses interface trapping without requiring complex additional structures, achieving dark current reduction in a cost-effective manner
2Object-generated harmful factors
If an n+ layer is formed at the back side surface to reduce interface trapping, then dark current is reduced, but the fabrication process becomes more difficult and sensitivity is affected
Solution Approach 1:
The patent replaces the difficult-to-form n+ layer with a simple oxide layer that can be formed through standard oxidation processes. This oxide layer achieves effective interface passivation and dark current reduction without requiring complex ion implantation or high-temperature thermal activation steps, significantly simplifying the fabrication process
Solution Approach 2:
Instead of changing the doping concentration parameter (as would be required for n+ layer formation), the patent uses an insulator layer to change the electrical properties at the interface through passivation. This alternative approach achieves the same dark current reduction goal without the manufacturing complexity of high-concentration doping
3Object-generated harmful factors
If three transistors or four transistors are used in the pixel with CDS technique, then dark current and kTC-reset noise are managed, but pixel size increases
Solution Approach 1:
The patent extracts the dark current suppression function from the pixel circuit transistors and implements it separately through a back side interface passivation layer. This allows the pixel circuit to use fewer transistors (3T or 4T with shared terminals) while the insulator layer independently handles dark current reduction, enabling pixel size reduction without sacrificing dark current management capability
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 effectively reduces dark current generation at the back side interface, improves image sensor accuracy, and simplifies the fabrication process, leading to higher sensitivity and cost-effective production.
Implementation Method 1
a first layer with positive charges disposed on the second surface, the first layer being configured to form an electron accumulation region at the light-receiving surface of the photodetector for suppressing a dark current at a back side interface of the image sensor
Implementation Method 2
A typical image sensor senses light by converting photons into electrons or holes that are integrated (collected) in sensor pixels
Data Source
AI summary
An image sensor including at least one pixel for collecting charge in its photodiode is provided. The image sensor comprises: a substrate having a first surface on a front side and a second surface on a back side, a photodetector formed in the silicon substrate and having a light-receiving surface on the second surface, and a first layer with positive charges disposed on the second surface, the first layer being configured to form an electron accumulation region at the light-receiving surface of the photodetector for suppressing a dark current at a back side interface of the image sensor. A method for fabricating an image sensor including a first layer with positive charges is also provided.