SPAD Image Sensor DBR Light Reflection

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Solution Overview

Problem

Existing SPAD image sensors suffer from light loss due to absorption by light-absorbing substrates and inefficient light directionality, particularly when using metal reflectors that risk electrical shorts.

Innovation Solution

A distributed Bragg reflector (DBR) composed of alternating insulating layers with different refractive indices is introduced between the substrate and interconnect structure, enhancing light reflection and resonance efficiency without electrical short risks.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If a metal reflector is used to reflect light, then light reflection efficiency is improved, but the risk of electrical shorts increases

Engineering Contradiction:

Improvelight reflection efficiencyVSAvoidelectrical short risk

Core Design Contradiction:

Loss of energyVSReliability

Solution Approach 1:

The patent introduces a dielectric reflector layer (insulating material) as an intermediary between the metal interconnect structure and the substrate. This dielectric layer acts as a mediator that provides both optical reflection functionality and electrical insulation, thereby maintaining light reflection efficiency while eliminating the risk of electrical shorts between metal layers and the substrate.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Strength

If the substrate is made light-absorbing, then mechanical strength is improved, but light loss increases

Engineering Contradiction:

Improvesubstrate mechanical strengthVSAvoidlight absorption loss

Core Design Contradiction:

StrengthVSLoss of energy

Solution Approach 1:

The patent converts the harmful light-absorbing property of the substrate into a beneficial feature by using the substrate's natural absorption to suppress unwanted background signals and noise. Meanwhile, the desired signal light is reflected by the dielectric reflector layer before it can be absorbed, thus converting the substrate's absorption characteristic from a disadvantage into an advantage for signal-to-noise ratio improvement.

Inventive Principle:

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

3Ease of manufacture

If conventional fabrication processes are used, then manufacturing simplicity is maintained, but fabrication complexity increases due to additional DBR layers

Engineering Contradiction:

Improvefabrication process simplicityVSAvoidDBR layer structure

Core Design Contradiction:

Ease of manufactureVSDevice complexity

Solution Approach 1:

The patent designs the dielectric reflector layer to serve multiple functions simultaneously: it acts as an optical reflector, an electrical insulator, and an integral part of the interconnect structure. By combining these functions into a single layer structure that can be formed using existing fabrication processes, the patent avoids adding significant complexity while achieving multiple benefits.

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

The DBR increases the sensitivity and resonance efficiency of SPAD image sensors by effectively reflecting light without electrical shorts, improving light utilization and directionality.

Implementation Method 1

Methodology Applied

Scientific EffectBragg reflection: Bragg Diffraction

Implementation Method 2

Methodology Applied

Scientific EffectOptical interference: Interference

Implementation Method 3

An image sensor operated in this mode is known as a single photon avalanche diode (SPAD) image sensor, or a Geiger-mode avalanche photodiodes or G-APD

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 4

An avalanche process can be triggered when a reverse biased p-n junction receives additional carriers, such as carriers generated by incident radiation. For example, in order to detect radiations with low intensities, the p-n junction is biased above its breakdown voltage, thereby allowing a single photon-generated carrier to trigger an avalanche current that can be detected

Methodology Applied

Scientific EffectAvalanche breakdown: Avalanche Breakdown

Data Source

PatentUS11127774B2Image sensor and associated fabricating method

Publication Date: 2021.09.21 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

An image sensor is disclosed. The image sensor includes: a common node heavily doped with dopants of a first conductivity type, the common node being within the substrate and abutting the front surface of the substrate; and a sensing node heavily doped with dopants of a second conductivity type opposite to the first conductivity type, the sensing node being within the substrate and abutting the front surface of the substrate; an interconnect structure, wherein the front surface of the substrate faces the interconnect structure; a distributed Bragg reflector (DBR) between the front surface of the substrate and the interconnect structure; a first contact plug passing through the DBR and coupling the common node to the interconnect structure; and a second contact plug passing through the DBR and coupling the sensing node to the interconnect structure.