Air Gap Reflection Structures for BSI Photodiode Quantum Efficiency
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Solution Overview
Problem
Back side illumination (BSI) image sensors face reduced quantum efficiency due to photons refracting, diffusing, or scattering through the bottom surface of photodiodes, which decreases the absorption of incident light.
Innovation Solution
Incorporating air gap reflection structures, such as holes or trenches, below the photodiodes to reflect photons back into the photodiode, increasing the absorption of incident light by utilizing the low refractive index of air to enhance total internal reflection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If photons are allowed to pass through the bottom surface of photodiodes, then the device structure is simple, but quantum efficiency is reduced due to refraction, diffusion, or scattering of photons
Solution Approach 1:
An air gap is introduced as an intermediary layer between the photodiode bottom surface and the underlying substrate. This air gap acts as a mediator that enables total internal reflection of photons by creating a refractive index difference, thereby preventing photon loss while maintaining structural simplicity
Solution Approach 2:
The refractive index parameter is changed by introducing an air gap (refractive index ≈1.0) between the photodiode silicon layer (refractive index ≈3.5) and the substrate. This parameter change creates the conditions for total internal reflection, improving quantum efficiency without significantly increasing device complexity
2Loss of energy
If air gap reflection structures are incorporated below photodiodes, then quantum efficiency is increased by reflecting photons back, but device complexity increases
Solution Approach 1:
The air gap serves as a simple intermediary structure that provides optical reflection functionality. Rather than using complex reflective coatings or additional optical components, the air gap leverages the inherent refractive index difference to achieve photon reflection, minimizing the increase in device complexity
3Ease of manufacture
If the bottom surface of photodiodes is left as is, then manufacturing is simple, but photons are lost through refraction and scattering
Solution Approach 1:
The air gap is formed as an intermediary structure during the BSI fabrication process. This can be achieved by selective removal of oxide layers or by controlling deposition processes to leave air gaps, which are relatively simple modifications to existing manufacturing processes compared to adding complex reflective structures
Solution Approach 2:
Material (such as oxide layers) is selectively removed or not deposited in specific regions to create air gaps. This extraction approach creates the reflection structures by removing material rather than adding complex components, maintaining ease of manufacture
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 increases the quantum efficiency of the pixel sensor by redirecting photons that would otherwise be lost, thereby improving the overall performance of the image sensor.
Implementation Method 1
The air gaps may be configured to reflect photons of incident light that would otherwise refract, diffuse, or scatter through a bottom surface of a photodiode... the air gaps... utilizing the low refractive index of air to enhance total internal reflection
Data Source
AI summary
A pixel array may include air gap reflection structures under a photodiode of a pixel sensor to reflect photons that would otherwise partially refract or scatter through a bottom surface of a photodiode. The air gap reflection structures may reflect photons upward toward the photodiode so that the photons may be absorbed by the photodiode. This may increase the quantity of photons absorbed by the photodiode, which may increase the quantum efficiency of the pixel sensor and the pixel array.