Back-Side Illuminated Pixel Sensor Light Shielding

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Solution Overview

Problem

Back side illuminated global shutter image sensors face challenges in creating an effective light shield to prevent spurious light from adding to the charge stored on the capacitor during readout, making it difficult to combine this technology with global shutter mode.

Innovation Solution

A light blocking layer is introduced between semiconductor layers in the pixel sensor cell, shielding the second transistor and dielectric isolated metallization stack, including a thin film transistor and metal-insulator-metal capacitor, to prevent spurious light effects.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If a light shield is created using a metal layer above the diffusion connected to the capacitor in a back side illuminated sensor, then spurious light immunity is improved, but the difficulty of manufacture increases due to the challenge of creating an effective light shield that prevents back side light from adding to stored charge

Engineering Contradiction:

Improvespurious light immunityVSAvoiddifficulty of creating effective light shield

Core Design Contradiction:

Object-affected harmful factorsVSEase of manufacture

Solution Approach 1:

The patent introduces a vertically stacked architecture where the light shield is positioned in a different spatial dimension (above the pixel array in a third dimension) rather than trying to block light from the back side directly. This dimensional change allows the shield to effectively block spurious light paths without interfering with the back side illumination of the photodiodes, resolving the manufacturing difficulty while maintaining spurious light immunity.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Solution Approach 2:

The patent introduces an intermediary readout structure (such as a transfer gate and storage region separated from the photodiode) that mediates between the photodiode and the readout circuitry. This intermediary structure allows charge to be transferred and stored away from the photodiode region, preventing spurious light from directly affecting the stored charge while maintaining the simplicity of back side illumination.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If global shutter mode is implemented in back side illuminated sensors, then image motion artifacts are eliminated, but spurious light immunity deteriorates because incidental light adds to stored charge during readout time

Engineering Contradiction:

Improveelimination of image motion artifactsVSAvoidspurious light adding to stored charge

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent segments the pixel structure into distinct functional regions: a photodiode region for charge generation, a transfer region for charge movement, and a storage region for charge holding during readout. This segmentation allows the stored charge to be physically separated from the photodiode, preventing spurious light from adding to the stored charge while maintaining global shutter operation that eliminates image motion artifacts.

Inventive Principle:

Principle #1Segmentation

3Object-affected harmful factors

If a light blocking layer is introduced between semiconductor layers to shield transistors and capacitors, then spurious light effects are reduced, but device complexity increases

Engineering Contradiction:

Improvespurious light effectsVSAvoidcomplexity of pixel sensor cell structure

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The patent designs the light blocking layer to serve multiple functions simultaneously: it blocks spurious light from reaching sensitive regions, provides structural support between layers, and facilitates charge transfer between photodiodes and storage regions. This multi-functionality reduces the need for additional specialized components, thereby limiting the increase in device complexity while effectively reducing spurious light effects.

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 enables the creation of a global shutter back side illuminated imager with enhanced light shielding, improving charge transfer performance and reducing image artifacts by effectively blocking spurious light.

Implementation Method 1

Methodology Applied

Scientific EffectLight blocking: Absorption (EM radiation)

Data Source

PatentUS9543356B2Pixel sensor cell including light shield

Publication Date: 2017.01.10 GLOBALFOUNDRIES US INC

AI summary

CMOS image sensor pixel sensor cells, methods for fabricating the pixel sensor cells and design structures for fabricating the pixel sensor cells are designed to allow for back side illumination in global shutter mode by providing light shielding from back side illumination of at least one transistor within the pixel sensor cells. In a first particular generalized embodiment, a light shielding layer is located and formed interposed between a first semiconductor layer that includes a photoactive region and a second semiconductor layer that includes the at least a second transistor, or a floating diffusion, that is shielded by the light blocking layer. In a second generalized embodiment, a thin film transistor and a metal-insulator-metal capacitor are used in place of a floating diffusion, and located shielded in a dielectric isolated metallization stack over a carrier substrate.