Backside-Illuminated Image Sensor Grid Layout for Low Crosstalk
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Solution Overview
Problem
Backside-illuminated image sensors face optical crosstalk issues due to light leakage between adjacent photodiodes, which reduces dynamic range and obscures color contrast, especially when imaging fluorescent stimuli and emissions simultaneously.
Innovation Solution
The implementation of a metal grid with dielectric layer on a backside-illuminated image sensor, where the metal grid is aligned over isolation structures and defines apertures over photodiode regions, reducing optical crosstalk by absorption and reflection, and the dielectric layer ensures sensitivity is not affected.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a buffer oxide layer is used to protect the backside surface of the semiconductor substrate, then manufacturing reliability is improved, but optical crosstalk between adjacent photodiodes increases
Solution Approach 1:
The buffer oxide layer is segmented by removing portions between adjacent photodiodes on the backside surface, creating isolated oxide regions over each photodiode while eliminating the continuous oxide paths that cause light leakage and crosstalk between neighboring photodiodes
Solution Approach 2:
The harmful portions of the buffer oxide layer are extracted by selective removal between photodiodes, eliminating the light-blocking obstacles that cause crosstalk while preserving the protective oxide layer directly over each photodiode sensing region
2Measurement precision
If color filters are positioned between each photodiode and incident light to achieve color sensing, then color detection capability is improved, but optical crosstalk from adjacent photodiodes increases
Solution Approach 1:
The buffer oxide layer is segmented into isolated regions over each photodiode, preventing light from leaking laterally between adjacent photodiodes and ensuring that each photodiode only detects light intended for it, thereby preserving color detection accuracy
Solution Approach 2:
The segmented buffer oxide layer acts as an intermediary structure that blocks lateral light propagation between photodiodes while maintaining electrical isolation and mechanical stability, preventing crosstalk without interfering with the color filter-photodiode optical path
3Adaptability or versatility
If light-attenuating filters are positioned on low-sensitivity photodiodes to extend dynamic range, then dynamic range capability is improved, but optical crosstalk from adjacent photodiodes reduces the effectiveness
Solution Approach 1:
The buffer oxide layer is segmented to eliminate continuous oxide paths between photodiodes, preventing light from bypassing the light-attenuating filters on low-sensitivity photodiodes and ensuring that these filters effectively control the dynamic range 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
This configuration effectively reduces optical crosstalk between photodiodes, enhancing dynamic range and maintaining sensitivity, thereby improving image quality, especially in bright and dimly lit regions.
Implementation Method 1
a metal grid disposed on the backside of the semiconductor substrate, the metal grid defining a plurality of apertures aligning with respective light sensing regions of the photodiodes
Implementation Method 2
a protective dielectric layer disposed on the backside of the semiconductor substrate between the metal grid and the isolation structures
Implementation Method 3
an array of photodiodes formed in a semiconductor substrate, each individual photodiode is electrically isolated from other photodiodes
Data Source
AI summary
A backside-illuminated image sensor includes photodiodes in photodiode regions electrically isolated by filled trenches with openings in a dielectric layer over the photodiodes. The image sensor has a metal grid aligned over the trenches, the metal grid within 80 nanometers of the trenches. The image sensor is formed by: fabricating photodiodes in photodiode regions of a frontside of a silicon substrate with source-drain regions of transistors, the photodiodes electrically isolated by deep trenches, each photodiode within a photodiode region of the substrate; forming the filled trenches in a backside of the semiconductor substrate; forming protective oxide and process stop layers over the backside of the semiconductor substrate; depositing a metal grid over the deep trenches, removing the process stop layer from over photodiode regions; and depositing color filters over the photodiode regions.