Switched Deep Trench Isolation for Image Sensor Crosstalk
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Solution Overview
Problem
As image sensors are miniaturized, pixel cell sizes decrease, leading to increased challenges with pixel cell crosstalk and unwanted signal transfer, as well as elevated dark current rates, which affect image quality and resolution.
Innovation Solution
The implementation of switched deep trench isolation structures in pixel cells, which are biased capacitive type isolation structures, eliminates the need for P-type doped regions in the photodiode area, allowing for higher fill factor and increased full well capacity, and reduces lag time by using a negative readout pulse voltage to transfer image charge efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If image sensors are miniaturized to reduce size, then the device becomes more compact and portable, but pixel cell crosstalk and unwanted signal transfer between pixel cells increase
Solution Approach 1:
The image sensor is divided into isolated pixel cells using deep trench isolation structures. These trenches physically segment the semiconductor substrate between adjacent pixel cells, preventing electrical crosstalk while allowing each pixel cell to function independently. The isolation structures create discrete regions that maintain signal integrity despite miniaturization.
Solution Approach 2:
Switched deep trench isolation structures act as intermediary elements between adjacent pixel cells. These isolation structures include conductive regions that can be selectively activated or deactivated to control signal isolation. The intermediary structures provide dynamic control over crosstalk prevention, allowing the system to adapt isolation levels based on operational requirements.
2Volume of moving object
If pixel cell size is decreased to maintain resolution in smaller sensors, then device miniaturization is achieved, but dark current rates increase
Solution Approach 1:
Deep trench isolation structures segment the semiconductor substrate to create electrically isolated regions. This segmentation prevents dark current generated in one pixel cell from affecting adjacent cells, effectively containing dark current to its local region of origin and preventing cumulative dark current effects across the sensor array.
Solution Approach 2:
The switched deep trench isolation structures modify electrical parameters dynamically by changing the conductivity state of isolation regions. By adjusting the electrical state of these structures, the system can optimize dark current suppression while maintaining operational performance, effectively controlling dark current levels through parameter modulation.
3Object-generated harmful factors
If P-type doped regions are used in the photodiode area for isolation, then pixel cell isolation is improved, but fill factor and full well capacity decrease
Solution Approach 1:
The invention extracts and removes P-type doped regions from the photodiode area, replacing them with switched deep trench isolation structures. This extraction eliminates the need for space-consuming doped regions while maintaining effective pixel cell isolation, thereby preserving fill factor and full well capacity in the photodiode region.
Solution Approach 2:
Switched deep trench isolation structures serve as intermediary isolation elements that replace traditional P-type doped regions. These intermediary structures provide the necessary electrical isolation between pixel cells without occupying photodiode area, allowing the photodiode to maintain maximum active area for light detection.
4Ease of manufacture
If traditional isolation structures are used, then manufacturing is simpler, but lag time increases and image charge transfer efficiency decreases
Solution Approach 1:
The isolation structures are designed to be dynamically switchable rather than statically fixed. The switched deep trench isolation structures can change their electrical state in response to operational signals, enabling rapid image charge transfer while maintaining isolation when needed. This dynamic capability reduces lag time by allowing quick state transitions during charge transfer operations.
Solution Approach 2:
The switched isolation structures operate through periodic switching cycles, alternating between isolation states and conductive states. This periodic action synchronizes with the image capture and transfer cycle, providing isolation during integration and enabling rapid charge transfer during readout, thereby reducing overall lag time while maintaining manufacturability.
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, enhances full well capacity, and decreases lag time by eliminating P-type doped regions and utilizing a negative readout pulse voltage to push image charge out of the photodiode, thereby improving image sensor performance and resolution.
Implementation Method 1
by using a negative readout pulse voltage to transfer image charge efficiently
Data Source
AI summary
A pixel cell includes a photodiode disposed in an epitaxial layer in a first region of semiconductor material to accumulate image charge. A floating diffusion is disposed in a well region disposed in the epitaxial layer in the first region. A transfer transistor is coupled to selectively transfer the image charge from the photodiode to the floating diffusion. A deep trench isolation (DTI) structure disposed in the semiconductor material. The DTI structure isolates the first region of the semiconductor material on one side of the DTI structure from a second region of the semiconductor material on an other side of the DTI structure. The DTI structure includes a doped semiconductor material disposed inside the DTI structure that is selectively coupled to a readout pulse voltage in response to the transfer transistor selectively transferring the image charge from the photodiode to the floating diffusion.