Solid-State Image Sensor Dark Current Reduction

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Solution Overview

Problem

In solid-state image sensors with global shutter function, the noise in image signals is increased due to dark current caused by interface states on the charge retention region, leading to unwanted signal components.

Innovation Solution

A solid-state image sensor design that includes a photoelectric converter, a generated-charge retention portion, a generated-charge transfer portion, an output charge retention portion, and a generated-charge retention gate portion, which applies control and bias voltages to control potential and facilitate charge transfer, reducing dark current influence by using a semiconductor region with a different conductivity type and applying intermediate voltages.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If charges are retained in the charge retention region to achieve global shutter function, then the global shutter function is achieved, but dark current increases causing noise in image signals

Engineering Contradiction:

Improveglobal shutter functionVSAvoiddark current

Core Design Contradiction:

Adaptability or versatilityVSObject-generated harmful factors

Solution Approach 1:

The patent changes the electrical parameters (voltage levels and timing) applied to the charge retention region. By applying a first voltage during exposure and a second voltage (different from the first) during charge transfer, the patent optimizes charge retention while minimizing dark current effects. This parameter change allows the same region to serve dual purposes: retaining charges for global shutter and reducing dark current noise.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If interface states exist on the semiconductor surface in the charge retention region, then charge retention is achieved, but interface states trap charges causing dark current and noise

Engineering Contradiction:

Improvecharge retention capabilityVSAvoiddark current noise

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The patent converts the harmful effect of interface states into a beneficial one. Instead of trying to eliminate interface states, the patent uses the voltage change strategy to exploit the interface state behavior: during the voltage transition period, interface states release trapped charges, creating a potential gradient that actually aids in complete charge transfer to the floating diffusion region, thereby reducing residual charges and dark current noise.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Productivity

If charges are completely transferred from the charge retention region to the floating diffusion region, then charge transfer efficiency is improved, but residual charges remain due to interface states causing noise

Engineering Contradiction:

Improvecharge transfer efficiencyVSAvoidimage signal quality

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The patent employs periodic voltage application to the charge retention region. By repeatedly applying voltage changes (first voltage during exposure, second voltage during transfer), the patent ensures complete charge extraction. The periodic nature of this voltage switching allows multiple opportunities for charge transfer, ensuring that even charges trapped by interface states are eventually released and transferred, thereby improving both transfer efficiency and signal quality.

Inventive Principle:

Principle #19Periodic 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 design effectively transfers all charges while reducing or eliminating the dark current influence, improving image signal quality by minimizing noise.

Implementation Method 1

a photoelectric conversion device for generating an electric charge corresponding to incident light

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

a generated-charge transfer portion configured to transfer the charge from the photoelectric converter to the generated-charge retention portion to perform generated-charge transfer by allowing electrical conduction

Methodology Applied

Scientific EffectElectrical conduction: Conduction (electrical)

Implementation Method 3

a generated-charge retention gate portion configured to apply a control voltage for controlling potential of the generated-charge retention portion

Methodology Applied

Scientific EffectElectrical conduction control: Conduction (electrical)

Data Source

PatentUS11032499B2Solid-state image sensor and imaging apparatus

Publication Date: 2021.06.08 SONY GROUP CORP

AI summary

A photoelectric converter generates a charge corresponding to the exposure amount during an exposure period. The generated-charge retention portion and the output charge retention portion retain the charge. The generated-charge transfer portion transfers the charge from the photoelectric converter to the generated-charge retention portion to perform the transfer after the elapse of the exposure period. The retained-charge transfer portion transfers the charge retained in the generated-charge retention portion to the output charge retention portion to perform the transfer. The generated-charge retention gate portion applies a control voltage that is a voltage for controlling potential of the generated-charge retention portion to the generated-charge retention portion during a period of the transfer and the retained-charge transfer, applies a bias voltage that is a voltage having a polarity different from the control voltage to the generated-charge retention portion during a period different from the period of the generated-charge transfer and the retained-charge transfer.