Negatively Biased Isolation Structures for Backside Illuminated Sensors
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Image sensors face increased dark current issues due to Shallow Trench Isolations (STIs), which can be mitigated but result in reduced photodiode area as passivation implant doping diffuses into the silicon substrate, compromising performance.
Innovation Solution
The implementation of C-shaped isolation structures surrounding the row of transistors, which are negatively biased to reduce dark current, eliminating the need for shallow trench isolations and thereby preserving the photodiode area by avoiding dopant diffusion into the photodiode region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If passivation implant doping is used to reduce dark current from STIs, then dark current is reduced, but photodiode area is reduced due to dopant diffusion into the photodiode region
Solution Approach 1:
The patent removes the STI structure entirely from the pixel device design, extracting the source of dark current generation. By replacing STI with C-shaped isolation structures that do not require passivation implants, the harmful dopant diffusion into the photodiode region is eliminated, preserving full photodiode area while still achieving dark current reduction through the isolation structure's inherent design
Solution Approach 2:
The patent changes the electrical parameter of the isolation structure by applying negative bias voltage to the C-shaped isolation structure. This parameter change creates a potential barrier that repels minority carriers (electrons in p-type substrate) from the isolation structure interface, reducing dark current without requiring passivation implants that would diffuse into the photodiode region
2Productivity
If STI structures are used for isolation, then pixel density is increased, but dark current increases due to defects and trap sites on STI sidewalls and bottom
Solution Approach 1:
The patent converts the potentially harmful interface between isolation structure and silicon substrate into a beneficial structure by using a C-shaped isolation design that maintains electrical isolation while minimizing interface defects. The negative bias applied to the isolation structure further converts the interface from a dark current source into a carrier-blocking barrier, turning what would normally be a harmful effect into a beneficial dark current reduction mechanism
Solution Approach 2:
Instead of using conventional STI structures that create interfaces prone to defect formation, the patent inverts the approach by using C-shaped isolation structures with negative bias. This inversion changes the mechanism from passive physical isolation (which creates defect-prone interfaces) to active electrical isolation (which creates carrier-blocking barriers), fundamentally reversing how isolation is achieved
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 approach effectively reduces dark current while maintaining the photodiode area, enhancing the sensitivity and performance of image sensors by eliminating defects and trap sites associated with STIs.
Implementation Method 1
The isolation structures are negatively biased. In response, minority carriers are repelled from the isolation structures, which reduces dark current.
Data Source
AI summary
Backside illuminated sensor pixel structure. In one embodiment, and image sensor includes a plurality of pixels arranged in rows and columns of a pixel array that are disposed in a semiconductor substrate. Individual photodiodes of the pixel array are configured to receive an incoming light through a backside of the semiconductor substrate. A front side of the semiconductor substrate is opposite from the backside. A plurality of transistors disposed proximate to the front side of the semiconductor substrate, are arranged in a row along an outer perimeter of the photodiodes of the respective pixel; and a plurality of isolation structures arranged to bracket the row of transistors along the outer perimeter of the photodiodes. A plurality of contacts electrically contacting the plurality of isolation structures, and the contacts are configured to voltage-bias the plurality of isolation structures.