Tunable DBR Narrow-Band Filter for Stable Image Sensor Wavelengths
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Solution Overview
Problem
Current wavelength filters in image sensors, such as absorptive, microcavity, and plasmonic filters, face limitations in controlling narrow transmission bands, achieving high transmission efficiency, and maintaining spectral stability across different incident angles, leading to increased costs and reduced performance.
Innovation Solution
A wavelength-tunable narrow band filter is developed, comprising a substrate with neighboring photodetectors and a filter structure that includes a first and second distributed Bragg reflector (DBR) and a varying thickness interlayer, allowing for discrete or continuous thickness adjustments to tune transmission wavelengths, and optionally incorporating a second interlayer with different refractive indices to fine-tune the filter's performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional filters (absorptive, microcavity, or plasmonic) are used, then wavelength discrimination is achieved, but transmission efficiency is reduced and manufacturing complexity increases
Solution Approach 1:
The filter structure is segmented into multiple distinct layers: a first DBR stack with specific refractive index contrast, a second DBR stack with different refractive index contrast, and intermediate layers connecting them. Each layer segment contributes differently to the overall spectral response, enabling precise control of transmission characteristics while maintaining high efficiency through optimized segment design.
Solution Approach 2:
The filter employs composite material structures with different refractive indices arranged in alternating layers. The first DBR uses materials with one refractive index contrast ratio while the second DBR uses materials with a different refractive index contrast ratio, creating a composite structure that achieves both narrow bandwidth and high transmission efficiency simultaneously.
2Reliability
If conventional filters are used, then wavelength filtering is achieved, but spectral stability across incident angles deteriorates
Solution Approach 1:
The dual-DBR composite structure with different refractive index contrasts provides inherent angular stability. The first DBR stack with higher refractive index contrast handles the primary reflection, while the second DBR stack with lower refractive index contrast compensates for angle-dependent phase variations, maintaining spectral stability across different incident angles.
Solution Approach 2:
The filter design incorporates controlled variations in layer thicknesses and refractive index ratios between the two DBR stacks. By adjusting these parameters, the filter achieves broadband operation with maintained spectral stability, as the parameter variations compensate for angle-induced wavelength shifts.
3Measurement precision
If multiple masks and absorptive materials are used, then wavelength discrimination is improved, but manufacturing cost increases
Solution Approach 1:
The manufacturing process is segmented into sequential deposition steps for different layer stacks, allowing precise thickness control for each layer without requiring multiple photomasks. The first DBR stack is deposited with specific thicknesses, followed by the second DBR stack with different thicknesses, achieving precise wavelength discrimination through controlled deposition rather than mask-based patterning.
Solution Approach 2:
The invention replaces the mechanical mask-based patterning system with a controlled thin-film deposition system. Instead of using multiple photomasks to define wavelength-selective regions, the precise wavelength filtering is achieved through controlled deposition of alternating high and low refractive index layers with precisely controlled thicknesses, eliminating mask-related costs and complexity.
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
The solution enables high transmission efficiency with narrow transmission bands, allowing photodetectors to effectively discriminate between different wavelengths and colors, while reducing the need for multiple masks and absorptive materials, thus lowering costs and improving spectral stability.
Implementation Method 1
a first and second distributed Bragg reflector (DBR)
Implementation Method 2
a first and second distributed Bragg reflector (DBR) and a varying thickness interlayer
Implementation Method 3
a varying thickness interlayer, allowing for discrete or continuous thickness adjustments to tune transmission wavelengths
Implementation Method 4
a complementary metal-oxide-semiconductor (CMOS) image sensor may comprise an array of photodetectors to convert an optical image to digital data
Data Source
AI summary
Various embodiments of the present application are directed towards an image sensor including a wavelength tunable narrow band filter, as well as methods for forming the image sensor. In some embodiments, the image sensor includes a substrate, a first photodetector, a second photodetector, and a filter. The first and second photodetectors neighbor in the substrate. The filter overlies the first and second photodetectors and includes a first distributed Bragg reflector (DBR), a second DBR, and a first interlayer between the first and second DBRs. A thickness of the first interlayer has a first thickness value overlying the first photodetector and a second thickness value overlying the second photodetector. In some embodiments, the filter is limited to a single interlayer. In other embodiments the filter further includes a second interlayer defining columnar structures embedded in the first interlayer and having a different refractive index than the first interlayer.