Back-Illuminated Image Sensor Frontside Backside Photodetector Pairs
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Solution Overview
Problem
Back-illuminated image sensors face challenges with low photodetector charge capacity and significant pixel-to-pixel color crosstalk due to grid distortions caused by the thinning process, which affects quantum efficiency and increases costs.
Innovation Solution
The implementation of frontside and backside photodetector pairs with a specific configuration that includes n-type and p-type regions, along with a global alignment technique to reduce grid distortions and improve charge transfer, enhancing photodetector capacity and color crosstalk performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If back-illuminated image sensors are used to improve quantum efficiency, then QE performance is improved, but grid distortions occur during thinning process causing color crosstalk
Solution Approach 1:
The patent divides the photodetector into frontside and backside components, with the backside photodetector positioned closer to the color filter array. This segmentation allows the backside photodetector to capture light more efficiently while the frontside photodetector handles charge transfer, resolving the contradiction between QE improvement and alignment precision.
Solution Approach 2:
The patent introduces an intermediary charge transfer mechanism between the backside photodetector and frontside photodetector. This intermediary system allows the backside photodetector to receive light efficiently while transferring charges through controlled pathways, mitigating the color crosstalk issue caused by grid distortions.
2Measurement precision
If photodetector area is reduced to increase resolution, then resolution is improved, but photodetector charge capacity decreases
Solution Approach 1:
The patent utilizes the third dimension (depth) by implementing a stacked architecture with frontside and backside photodetectors. This dimensional transition allows smaller pixel footprints to maintain adequate charge capacity by extending the photodetector volume in the depth direction, resolving the contradiction between resolution and charge capacity.
Solution Approach 2:
The patent creates a composite photodetector structure combining frontside and backside photodetector layers with different functional optimizations. The backside photodetector is optimized for light reception in smaller pixels, while the frontside photodetector handles charge transfer, together providing both high resolution and adequate charge capacity.
3Adaptability or versatility
If wafer thinning process is performed to create back-illuminated sensor, then backside light reception is enabled, but stress accumulation causes grid distortions
Solution Approach 1:
The patent performs preliminary actions during the wafer fabrication process to compensate for expected stress-induced distortions. Alignment marks are pre-positioned and the color filter array is designed with compensation patterns that account for anticipated grid distortions during subsequent thinning, enabling backside light reception while maintaining structural stability.
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 increases photodetector charge capacity by 25-75% and significantly reduces pixel-to-pixel color crosstalk, improving overall image sensor performance and reducing manufacturing costs.
Implementation Method 1
light-sensitive photodetectors that convert incident light into electrical signals
Data Source
Figure 1(a)~1(b)
Figure 1(c)~1(d)
Figure 2
AI summary
A back-illuminated image sensor includes a sensor layer (702) of a first conductivity type having a frontside (704) and a backside (706) opposite the front side. An insulating layer (708) is disposed over the backside. A circuit layer (710) is formed adjacent to the front side such that the sensor layer is positioned between the circuit layer and the insulating layer. One or more front side regions (728) of a second conductivity type are formed in at least a portion of the front side of the sensor layer. A backside region (740) of the second conductivity type is formed in the backside of the sensor layer. A plurality of front side photodetectors (718f, 720f, 722f) of the first conductivity type is disposed in the sensor layer. A distinct plurality of backside photodetectors (718b, 720b, 722b) of the first conductivity type separate from the plurality of front side photodetectors is formed in the sensor layer contiguous to portions of the backside region of the second conductivity type.