Pixel-Array Isolation Using Guard Rings and Overlapping Trenches

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Solution Overview

Problem

In image sensors, electrical leakage current between the pixel array and the periphery region causes noise, reducing image quality and resolution, especially in deep photodetectors with small pixels, where maintaining electrical isolation is challenging due to difficulties in forming guard rings that extend deep enough into the semiconductor substrate.

Innovation Solution

Combining guard rings with deep trench isolation on the backside of the pixel-array substrate, where deep trench isolation prevents current flow at depths the guard ring does not reach, and doping regions to form guard rings that extend from the front surface, effectively resisting leakage current between the pixel array and the periphery region.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If guard rings are formed from the front side to reduce leakage current, then electrical isolation is improved, but in deep photodetectors the guard rings cannot extend deep enough into the substrate

Engineering Contradiction:

Improveelectrical isolationVSAvoidguard ring depth

Core Design Contradiction:

ReliabilityVSLength of stationary object

Solution Approach 1:

The patent introduces a vertical dimension solution by forming trenches from the back side of the substrate that extend deep into the semiconductor material. These trenches provide the necessary depth for electrical isolation that cannot be achieved by front-side guard rings alone, effectively solving the depth limitation problem by utilizing the substrate thickness dimension.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Solution Approach 2:

The patent combines two isolation mechanisms: front-side doped guard rings and back-side deep trenches. This hybrid approach merges the advantages of both methods - the guard rings provide lateral isolation at shallow depths while the trenches provide vertical isolation at greater depths, achieving comprehensive electrical isolation throughout the substrate volume.

Inventive Principle:

Principle #5Merging (Combining)

2Quantity of substance

If high doping concentration is used to achieve full well capacity in deep photodetectors, then pixel performance is improved, but electrical isolation between regions becomes more difficult to maintain

Engineering Contradiction:

Improvedoping concentrationVSAvoidelectrical isolation

Core Design Contradiction:

Quantity of substanceVSReliability

Solution Approach 1:

The patent segments the isolation function into two distinct components: doped guard rings for lateral isolation and physical trenches for vertical isolation. This segmentation allows the high doping concentration to be used for achieving full well capacity while the trenches provide the necessary vertical barrier to maintain electrical isolation, preventing the leakage current that would otherwise occur in heavily doped deep photodetectors.

Inventive Principle:

Principle #1Segmentation

3Reliability

If deep trench isolation is used to prevent current flow at depths, then leakage current is reduced, but the process requires removing and replacing substrate material

Engineering Contradiction:

Improveleakage current preventionVSAvoidtrench formation process

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent accesses the back side of the substrate to form trenches vertically downward, utilizing the thickness dimension to achieve deep isolation without requiring complex lateral lithographic processes. This approach simplifies manufacturing by using standard etching and filling techniques from the back side, avoiding the need for costly deep-lithography equipment while still achieving the necessary isolation depth.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

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 combined isolation method effectively reduces leakage current, maintaining voltage differences between regions and enhancing image quality by preventing unwanted electrical flow across the pixel-array substrate.

Implementation Method 1

a guard ring formed of a doped semiconductor, enclosing the pixel array, and extending into the semiconductor substrate from the front surface

Methodology Applied

Scientific EffectElectrical resistance: Electrical Resistance

Implementation Method 2

use of deep trench isolation prevents leakage current by removing material from the substrate and replacing it with electrical insolation

Methodology Applied

Scientific EffectElectrical insulation: Dielectric

Data Source

PatentUS11862678B2Electrical isolation in pixel-array substrates using combination of doped semiconductor guard rings and overlapping isolation trenches

Publication Date: 2024.01.02 OMNIVISION TECHNOLOGIES INC

AI summary

A pixel-array substrate includes a semiconductor substrate with a pixel array, a back surface, and a front surface, and a guard ring formed of a doped semiconductor, enclosing the pixel array, and extending into the semiconductor substrate from the front surface, the back surface forming a trench extending into the semiconductor substrate, the trench overlapping the guard ring. A method for reducing leakage current into a pixel-array includes doping a semiconductor substrate to form a guard ring that extends into the semiconductor substrate from a front surface, encloses a pixel array, excludes a periphery region, and resists a flow of electric current, and forming, into a back surface of the semiconductor substrate, a trench that penetrates into the back surface and overlaps the guard ring, the guard ring and the trench configured to resist the flow of electric current between the pixel array and the periphery region.