Backside-Image Sensor Metal Grid Dark Current Reduction
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Solution Overview
Problem
Conventional CMOS image sensors suffer from high dark current due to defects and surface dangling bonds in the substrate, which affects their performance, especially in low-light conditions.
Innovation Solution
The image sensor design includes a substrate with a photo-sensitive region and a connection region, featuring a buffer layer, a metal grid with staggered wires, and trenches for color filters, which reduces dark current by inducing charges to recombine with defects and improving light intensity through reduced optical crosstalk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CMOS image sensor structure is used, then manufacturing process is simple, but dark current is large due to defects and surface dangling bonds
Solution Approach 1:
A buffer layer is introduced as an intermediary between the substrate and the photodiode. This buffer layer serves as a mediator that prevents direct interaction between the substrate defects (dangling bonds) and the photodiode, thereby reducing dark current while maintaining overall structural simplicity
Solution Approach 2:
The sensor structure is segmented into distinct functional layers: substrate, buffer layer, and photodiode layer. This segmentation isolates the photodiode from substrate defects while allowing each layer to perform its specific function, reducing dark current without significantly complicating the manufacturing process
2Illumination intensity
If photodiodes are located behind circuit transistors (front-side illuminated), then manufacturing is easier, but light intensity is reduced due to light blocking effect
Solution Approach 1:
The patent inverts the conventional front-side illuminated architecture by adopting a back-side illuminated structure where photodiodes are positioned in front of the circuit transistors rather than behind them. This inversion allows light to directly reach the photodiodes without being blocked by transistor structures, significantly improving light intensity and low-light performance
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 and enhances image sensor performance by minimizing recombination between charges and defects, leading to improved light capture and signal conversion in low-light conditions.
Implementation Method 1
reduces dark current by inducing charges to recombine with defects
Implementation Method 2
the photodiodes are located behind the circuit transistors... allow the light to directly enter the photodiodes
Data Source
AI summary
An image sensor includes a substrate having a first surface and a second surface. The substrate includes a photo-sensitive region and a connection region. The image sensor also includes a buffer layer formed on the first surface of the substrate in the photo-sensitive region, and a metal grid formed on the buffer layer and including a plurality of staggered metal wires. The metal grid is connected to an operation voltage, and a plurality of trenches are formed in the metal grid with each trench surrounded by the metal wires. The image sensor further includes a plurality of color filters formed in the plurality of trenches of the metal grid. The metal grid induces charges in the substrate to prevent recombination between the charges generated by photo-sensitive components and the defects in the substrate. As such, the dark current is reduced, and the performance of the image sensor is improved.