Vertical Gate Transistor Pixel Structure for BSI Sensitivity
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Solution Overview
Problem
The sensitivity of Back-Side Illumination (BSI) image sensor pixels is limited by the saturation of photodiodes, which affects image quality, and existing methods to increase sensitivity often require increasing the substrate surface area or adding more components.
Innovation Solution
The design includes a photodiode with a P-conductivity type substrate region, an N-conductivity type electric charge collecting region, and isolated transfer gates formed in deep trenches, allowing efficient charge transfer without increasing the pixel's surface area, using a silicon-on-insulator substrate and epitaxial formation of read and bias regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the photodiode size is increased to improve charge storage capacity, then sensitivity is improved, but pixel area increases
Solution Approach 1:
The transfer gate is configured vertically extending from the front surface through the substrate to the rear surface, utilizing the third dimension (depth) to provide an efficient charge transfer path. This vertical configuration allows complete charge evacuation from the photodiode without requiring larger lateral dimensions, thus maintaining high sensitivity within a compact pixel area.
Solution Approach 2:
The transfer gate is segmented into multiple portions: a first portion extending from the front surface to a first depth, a second portion from the first depth to a second depth, and a third portion from the second depth to the rear surface. This segmentation allows optimized doping concentrations and geometries at different depths, improving charge transfer efficiency without increasing overall pixel area.
2Productivity
If more transfer gates are added to improve charge transfer efficiency, then charge evacuation is improved, but device complexity increases
Solution Approach 1:
The single transfer gate is segmented into multiple portions along its vertical extension, with each portion having optimized characteristics for its specific depth range. This segmentation improves charge transfer efficiency at different stages of the transfer process while maintaining a single continuous gate structure, avoiding the complexity of multiple separate gates.
Solution Approach 2:
Instead of adding multiple lateral transfer gates, the solution extends a single gate vertically through the substrate thickness, utilizing the depth dimension to achieve complete charge evacuation. This approach provides efficient charge transfer without the structural complexity of multiple gates arranged in different spatial configurations.
3Reliability
If the substrate thickness is increased to improve light capture, then sensitivity is improved, but manufacturing complexity increases
Solution Approach 1:
The transfer gate extends vertically through the entire substrate thickness, providing a continuous charge transfer path from the light-capturing region at the rear surface to the readout region at the front surface. This vertical configuration efficiently handles charges regardless of substrate thickness variations, maintaining manufacturing flexibility while ensuring complete charge evacuation.
Solution Approach 2:
Different portions of the transfer gate have different doping concentrations optimized for their specific functions: the first portion has a first doping concentration for initial charge collection, the second portion has a second doping concentration for intermediate transfer, and the third portion has a third doping concentration for final charge evacuation. This local optimization improves overall performance without requiring uniform changes throughout the substrate.
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 configuration enhances the sensitivity of the photodiodes by efficiently transferring charges to the read region, maintaining a compact pixel structure and improving image quality without adding components or increasing substrate area.
Implementation Method 1
an electric charge collecting region for collecting electric charges appearing when a rear face of the substrate region receives light
Data Source
AI summary
The present disclosure relates to a photodiode comprising: a P-conductivity type substrate region, an electric charge collecting region for collecting electric charges appearing when a rear face of the substrate region receives light, the collecting region comprising an N-conductivity type region formed deep in the substrate region, an N-conductivity type read region formed in the substrate region, and an isolated transfer gate, formed in the substrate region in a deep isolating trench extending opposite a lateral face of the N-conductivity type region, next to the read region, and arranged for receiving a gate voltage to transfer electric charges stored in the collecting region toward the read region.