SPAD Image Sensor Isolation Stack for Low Dark Current
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Solution Overview
Problem
Existing image sensors using single-photon avalanche diodes face challenges in effectively separating impurity regions while maintaining optimal dark current characteristics and breakdown voltage, leading to deteriorated performance.
Innovation Solution
The image sensor incorporates an element isolation film with a stacked structure of insulating layers, where a second insulating layer of different material is formed only in specific areas between impurity regions to enhance insulation and reduce dark current, while maintaining the avalanche effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single-layer insulating film is used between impurity regions, then the structure is simple and manufacturing is easy, but dark current characteristics deteriorate and insulation is insufficient
Solution Approach 1:
The element isolation film uses a composite structure with multiple insulating layers (first insulating layer 231, second insulating layer 232, third insulating layer 233) made of different materials. This composite structure provides superior insulation performance and dark current suppression compared to single-layer films, while the layers are formed using standard semiconductor manufacturing processes.
Solution Approach 2:
The element isolation film is segmented into multiple distinct insulating layers, each potentially serving different functional purposes. The first insulating layer 231 contacts the substrate, the second insulating layer 232 is formed on a first portion of the first insulating layer, and the third insulating layer 233 completes the stacked structure, providing graduated insulation protection.
2Productivity
If impurity regions are placed close together to increase pixel density, then productivity and resolution improve, but electrical interference increases and breakdown voltage decreases
Solution Approach 1:
The element isolation film acts as an intermediary barrier between the first impurity region 210 and second impurity region 220. This intermediate insulating structure prevents direct electrical interaction between adjacent impurity regions, maintaining breakdown voltage and reducing electrical interference even when pixels are densely packed.
Solution Approach 2:
The insulating structure provides localized electrical isolation precisely where needed between impurity regions, while allowing other areas to maintain high conductivity for signal transmission. This localized quality control enables dense pixel packing without compromising overall device reliability.
3Reliability
If a thick insulating film is used to improve insulation between impurity regions, then breakdown voltage improves, but manufacturing complexity and device volume increase
Solution Approach 1:
Instead of increasing insulation thickness in a single dimension, the solution adds insulation capacity in the vertical dimension through a stacked multi-layer structure. This approach achieves superior breakdown voltage characteristics while maintaining a compact footprint and avoiding excessive single-layer thickness that would complicate manufacturing.
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 configuration improves dark current characteristics and breakdown voltage, enhancing the overall performance of the image sensor by effectively reducing noise and maintaining photon avalanche efficiency.
Implementation Method 1
an element isolation film between the first impurity region and the second impurity region in the first direction, the element isolation film including, a first insulating layer in contact with the substrate, a second insulating layer on a first portion of the first insulating layer and formed of a material different from a material of the first insulating layer
Implementation Method 2
research into image sensors using single photon avalanche diodes (SPADs) has been actively undertaken to accurately and quickly measure the distance from the image sensor to a subject
Data Source
AI summary
An image sensor includes a substrate including a plurality of pixel areas, a pixel isolation film disposed in the substrate, the pixel isolation film separating the plurality of pixel areas from each other, and each pixel area of the plurality of pixel areas including, a first impurity region, a second impurity region around the first impurity region in a first direction, and an element isolation film between the first impurity region and the second impurity region in the first direction, the element isolation film including, a first insulating layer, a second insulating layer on a first portion of the first insulating layer and formed of a material different from a material of the first insulating layer, and a third insulating layer, a portion of the third insulating layer on the second insulating layer, and a portion of the third insulating layer directly in contact with the first insulating layer.