Image Sensor Pixel Cell Non-Destructive Readout via Deep Trench Isolation
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Solution Overview
Problem
Conventional CMOS image sensor pixel cells have destructive readouts, which reduce light sensitivity as the image charge in photodiodes is lost after each readout, making it difficult to achieve high dynamic range across varying lighting conditions.
Innovation Solution
The implementation of deep trench isolation structures for capacitive coupling in pixel cells allows for non-destructive readouts, enabling the accumulation of light charge over an entire frame time and improving light sensitivity through automatic exposure control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional destructive readout is used, then circuit complexity is reduced, but light sensitivity deteriorates due to charge loss after each readout
Solution Approach 1:
The pixel cell is segmented into multiple independent readout paths: a first readout path for non-destructive sampling that preserves accumulated charge, and a second readout path for standard signal output. This segmentation allows simultaneous destructive and non-destructive readout operations, resolving the contradiction between maintaining light sensitivity and managing circuit complexity.
Solution Approach 2:
A capacitive coupling structure is introduced as an intermediary between the photodiode and the readout circuitry. This capacitor enables voltage sampling without direct charge transfer, allowing non-destructive readout while isolating the photodiode's accumulated charge from the readout circuit's destructive effect.
2Adaptability or versatility
If multiple successive samples with long and short integration times are used, then high dynamic range is achieved, but productivity decreases due to multiple exposures
Solution Approach 1:
The pixel cell performs preliminary non-destructive sampling of the accumulated charge at intermediate time points during the integration period. This preliminary action provides exposure information without destroying the accumulated charge, enabling automatic exposure control to adjust integration time dynamically and achieve high dynamic range while maintaining high frame rates.
Solution Approach 2:
The non-destructive readout path provides real-time feedback on the accumulated charge level to the automatic exposure control circuit. This feedback mechanism allows dynamic adjustment of integration time based on actual lighting conditions, achieving high dynamic range adaptability while optimizing productivity through single-exposure operation.
3Reliability
If photodiode charge is read out destructively, then manufacturing precision requirements are reduced, but light sensitivity is reduced due to charge disappearance
Solution Approach 1:
The capacitive coupling structure serves as an intermediary that enables voltage sensing without direct electrical connection to the photodiode's charge storage node. This isolation protects the accumulated charge from being affected by manufacturing variations in the readout circuit, maintaining light sensitivity while reducing sensitivity to manufacturing tolerances.
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 enhances light sensitivity and supports high dynamic range by allowing non-destructive sampling of image charges, enabling improved performance across a wide range of lighting conditions without losing accumulated charge.
Implementation Method 1
a deep trench isolation structure disposed proximate to the photodiode and providing a capacitive coupling to the photodiode
Data Source
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Figure 5
AI summary
A pixel cell includes a photodiode coupled to photogenerate image charge in response to incident light. A deep trench isolation structure is disposed proximate to the photodiode to provide a capacitive coupling to the photodiode through the deep trench isolation structure. An amplifier transistor is coupled to the deep trench isolation structure to generate amplified image data in response to the image charge read out from the photodiode through the capacitive coupling provided by the deep trench isolation structure. A row select transistor is coupled to an output of the amplifier transistor to selectively output the amplified image data to a column bitline coupled to the row select transistor.