Time-of-Flight Sensor Transfer Gate Charge Separation

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Solution Overview

Problem

Time-of-flight optical sensors suffer from a poor signal-to-noise ratio due to the limited redirection of photo-generated charge carriers, which affects the precision of depth imaging measurements.

Innovation Solution

The integration of transfer gates in the optical sensor structure allows for the separation of read-out and storage regions, enabling a global shutter action and reducing noise by controlling the transfer of charge carriers, thereby improving signal quality and reducing the area required for read-out nodes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a separate read-out node is provided for each pixel, then the read-out precision is improved, but the area of the semiconductor substrate is increased

Engineering Contradiction:

Improveread-out precisionVSAvoidsemiconductor substrate area

Core Design Contradiction:

Measurement precisionVSArea of stationary object

Solution Approach 1:

Multiple pixels share a common read-out node, merging the read-out function for multiple pixels into a single node. This reduces the total area required for read-out nodes while maintaining the ability to read out charge carriers from multiple pixels sequentially through the shared node.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

Charge carriers are collected and stored in a storage region before being transferred to the read-out node. This preliminary collection and storage allows the read-out operation to be performed separately from the charge generation process, enabling time-multiplexed read-out of multiple pixels through a shared node without losing measurement precision.

Inventive Principle:

Principle #10Preliminary action

2Area of moving object

If the read-out node area is reduced, then the optically active area is increased, but the noise from the read-out circuit increases

Engineering Contradiction:

Improveoptically active areaVSAvoidread-out circuit noise

Core Design Contradiction:

Area of moving objectVSObject-affected harmful factors

Solution Approach 1:

The pixel structure is segmented into separate functional regions: a storage region for charge carrier collection and a read-out node for signal read-out. This spatial segmentation allows the read-out node to be small (maximizing optically active area) while the storage region provides sufficient capacity. The separation also allows the read-out circuit to operate on a smaller node with controlled noise through timing and gating mechanisms.

Inventive Principle:

Principle #1Segmentation

3Speed

If charge carriers are collected continuously, then the measurement speed is improved, but the signal-to-noise ratio deteriorates

Engineering Contradiction:

Improvemeasurement speedVSAvoidsignal-to-noise ratio

Core Design Contradiction:

SpeedVSMeasurement precision

Solution Approach 1:

The read-out operation is performed periodically in a time-multiplexed manner for different pixels sharing the same read-out node. During each pixel's turn, the transfer gate is controlled to transfer charge carriers from the storage region to the read-out node at specific time intervals. This periodic read-out allows continuous measurement capability while integrating charge over an extended period, improving the signal-to-noise ratio through temporal integration.

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

Charge carriers are collected and accumulated in the storage region during the integration period before being transferred to the read-out node. This preliminary accumulation of charge over time increases the signal strength before read-out, improving the signal-to-noise ratio while maintaining high measurement speed through the continuous cycle of collection and periodic read-out operations.

Inventive Principle:

Principle #10Preliminary action

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 the signal quality and precision of time-of-flight measurements by increasing the optically active area while minimizing noise and the size of the semiconductor substrate, allowing for more efficient charge carrier collection and read-out.

Implementation Method 1

The transfer gate may shift or reduce a potential barrier between the read-out node and the storage region during read-out of the charge carriers

Methodology Applied

Scientific EffectPotential barrier control: Electric Field

Implementation Method 2

Optical sensors and specifically time-of-flight optical sensors for depth imaging

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Data Source

PatentUS10455178B2Optical sensor device and method for operating a time-of-flight sensor

Publication Date: 2019.10.22 IFM ELECTRONIC GMBH

AI summary

An optical sensor device, which may be a time-of-flight sensor, comprises a pixel array having a plurality of pixels. Moreover, the optical sensor device comprises a read-out node configured to provide photo-generated charge carriers from a first pixel and a second pixel for read-out and a first transfer gate configured to enable a read-out of the first pixel using the read-out node and a second transfer gate to disable a read-out of the second pixel during read-out of the first pixel.