Buried Optical Isolation for Image Sensor Stray Light
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Solution Overview
Problem
In image sensors, especially those with small geometry pixels and near-infrared detection applications, stray light reaches the storage region due to ineffective optical isolation, leading to noise and performance degradation.
Innovation Solution
An optical isolation element is buried in the substrate between the light receiving side and the storage region, extending parallel to the light receiving side, and can be an air cavity or filled with optically absorbing materials, optimized for near-infrared wavelengths to minimize stray light capture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a backside optical mask is used to block stray light from reaching the storage region, then light blocking effectiveness is improved, but the solution becomes ineffective for small pixel pitch and thick substrates due to strong diffraction and increased crosstalk
Solution Approach 1:
The patent transitions from a two-dimensional backside mask approach to a three-dimensional buried optical isolation element extending through the substrate thickness. This vertical dimension allows effective stray light blocking even in thick substrates without the diffraction problems affecting backside masks. The isolation element is positioned at a specific depth (350 nm to 1 μm from the front surface) to optimally intercept stray light paths while maintaining effectiveness across varying substrate thicknesses and pixel pitches.
Solution Approach 2:
The buried optical isolation element acts as an intermediary structure between the light-receiving surface and the storage region. Rather than relying on surface-level masking, this intermediate element embedded within the substrate provides optical isolation by absorbing or reflecting stray light before it reaches the storage region, effectively mediating the light interaction in a way that works for both thin and thick substrates.
2Use of energy by moving object
If substrate thickness is increased to improve near-infrared detection quantum efficiency, then absorption efficiency is improved, but stray light isolation becomes more difficult and crosstalk increases
Solution Approach 1:
The buried optical isolation element performs preliminary action by intercepting and blocking stray light before it can reach the storage region and cause crosstalk. This preventive measure is built into the substrate structure itself, allowing thick substrates to be used for improved near-infrared absorption without suffering from the stray light problems that would otherwise increase with thickness.
Solution Approach 2:
The optical isolation element is positioned at a specific local region within the substrate (350 nm to 1 μm from the front surface) rather than uniformly distributing isolation throughout. This localized placement optimally intercepts stray light paths while minimizing impact on the overall substrate thickness needed for near-infrared detection, creating a targeted solution that addresses stray light without compromising absorption efficiency.
3Measurement precision
If pixel pitch is reduced to increase sensor resolution, then imaging quality is improved, but optical isolation becomes more difficult to integrate and stray light protection decreases
Solution Approach 1:
The patent moves optical isolation from a two-dimensional surface mask to a three-dimensional structure extending through the substrate. This vertical dimension provides effective isolation even when horizontal pixel dimensions are reduced, allowing high-resolution sensors with small pixel pitch to maintain stray light protection that would be impossible with conventional surface masks.
Solution Approach 2:
The optical isolation function is segmented into discrete buried elements positioned at specific depths within the substrate. This segmentation allows each isolation element to be optimized for its specific location and function, providing effective stray light blocking for each pixel region independently, which is crucial for maintaining isolation in high-resolution sensors with many closely-spaced pixels.
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 stray light transmission to the storage region by 40-60%, improving signal-to-noise ratio and absorption efficiency, even in small pixel geometries and thick substrates, while eliminating the need for backside masks and lateral trenches.
Implementation Method 1
an optical isolation element of the storage region which is buried in the substrate between a light receiving side of the substrate and the storage region and extends in parallel to the light receiving side
Implementation Method 2
said optical isolation element being an air cavity or being a cavity filled with an optically absorbing material in a wavelength range of interest
Data Source
AI summary
The invention provides an image sensor comprising a light receiving side (Fa) and, in a substrate (100), a photoelectric conversion region (PD) capable of converting light received from the light receiving side (Fa) into a charge, a storage region (SN) capable of storing a charge transferred from the photoelectric conversion region and an optical isolation element of the storage region. The optical isolation element (60) is buried in the substrate between the light receiving side and the storage region.