Backside Trench Isolation for Image Sensor Crosstalk Reduction
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Solution Overview
Problem
Backside illumination image sensors face issues with pixel crosstalk and signal degradation due to stray light leakage into unintended regions of the substrate, which degrades global shutter efficiency and causes image ghosting and smear effects, especially in image sensors with thick silicon substrates.
Innovation Solution
The implementation of backside trench isolation structures filled with optically absorptive materials and a light shielding layer, which are biased to enhance electrical isolation and absorb or reflect stray light, preventing it from reaching dark regions of the image sensor pixels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If backside illumination image sensors are used to eliminate metal routing interference, then light absorption by metal routing lines and vias is reduced, but pixel crosstalk occurs due to stray light leaking into unintended substrate regions
Solution Approach 1:
The substrate back surface is segmented into isolated regions by trench structures that physically divide the continuous substrate into separate zones. These trenches act as barriers that segment the light paths, preventing stray light from one pixel region from reaching adjacent pixel regions, thereby reducing pixel crosstalk while maintaining the backside illumination advantage.
Solution Approach 2:
Optically absorptive material is introduced as an intermediary substance within the trench structures. This material serves as a mediator that actively absorbs stray light before it can propagate to unintended regions. The absorptive material converts the harmful stray light energy into other forms (typically heat), preventing it from causing pixel crosstalk.
2Measurement precision
If the silicon substrate thickness is increased to improve pixel sensitivity, then more light can be captured, but stray light can travel further and cause more crosstalk
Solution Approach 1:
The trench structures extract and remove the problematic light propagation paths from the substrate. By creating physical voids or filled trenches that extend through the substrate thickness, the design extracts the stray light problem from the system, preventing light from traveling across the entire substrate thickness to cause crosstalk, while still allowing sufficient light transmission to the photodiodes for high sensitivity.
3Adaptability or versatility
If global shutter operation is implemented to capture moving objects, then motion capture capability is improved, but image quality degrades due to optical leakage into floating diffusion regions
Solution Approach 1:
The trench isolation structures are positioned and configured to preemptively block stray light paths before the light can reach the floating diffusion regions during global shutter operation. This preliminary anti-action prevents the optical leakage problem from occurring in the first place, allowing global shutter functionality to operate effectively without degrading image quality through pre-positioned optical barriers.
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 solution effectively reduces pixel crosstalk and enhances global shutter efficiency by shielding dark regions from stray light, improving image quality even in image sensors with thick silicon substrates.
Implementation Method 1
backside trench isolation structures filled with optically absorptive materials
Implementation Method 2
a light shielding layer, which are biased to enhance electrical isolation and absorb or reflect stray light
Data Source
AI summary
A backside illumination image sensor with an array of image sensor pixels is provided. Each pixel may include a photodiode, a storage diode, and associated circuitry formed in a front side of a semiconductor substrate. In accordance with an embodiment, a trench isolation structure may be formed directly over the storage diode but not over the photodiode from a back side of the substrate. The backside trench isolation structure may be filled with absorptive material and can optionally be biased to a ground or negative voltage level. A light shielding layer may also be formed over the backside trench isolation structure on the back side of the substrate. The light shielding layer may be formed from absorptive material or reflective material, and may also be biased to a ground or negative voltage level.