Stacked CMOS Pixel Sensor Without STI for Dark Current Reduction
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Solution Overview
Problem
The scaling down of CMOS image sensors is limited by transistor structures, which cause crystalline damage and leakage/dark current due to shallow trench isolation (STI) structures, reducing photodetector performance and limiting the size of the photodetector.
Innovation Solution
A stacked CMOS image sensor design that omits STI structures at the first IC chip, using shallow and deep wells with dielectric trench isolation, allowing for enhanced scaling by maintaining photodetector size and improving electrical isolation without significant performance tradeoffs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If STI structures are used to provide electrical isolation, then electrical isolation is improved, but crystalline damage and leakage/dark current increase
Solution Approach 1:
The patent removes the STI structure from the photodetector region entirely, extracting the harmful element that causes crystalline damage and leakage current. Electrical isolation is then achieved through alternative means (p-n junction isolation) that do not involve trench isolation structures, thereby eliminating the source of crystalline damage while maintaining isolation functionality.
Solution Approach 2:
The patent introduces a p-n junction isolation structure as an intermediary mechanism to provide electrical isolation without the harmful effects of STI. This intermediate isolation approach uses doped regions rather than physical trenches, achieving the same electrical isolation goal through a different physical mechanism that doesn't cause crystalline damage.
2Reliability
If photodetector size is increased to improve charge transfer, then charge transfer is improved, but area consumption increases
Solution Approach 1:
The patent converts the previously harmful STI structure into a beneficial design choice by deliberately omitting it. This omission, which initially seems to remove a useful isolation mechanism, actually benefits the photodetector by eliminating crystalline damage and leakage current, thereby improving charge transfer efficiency without requiring larger photodetector areas.
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
Enables further scaling down of the pixel sensor with reduced leakage/dark current, enabling smaller pixel pitches and improved charge transfer efficiency, while maintaining performance.
Implementation Method 1
The photodetector is configured to accumulate charge in response to incident radiation
Data Source
AI summary
Various embodiments of the present disclosure are directed towards a stacked complementary metal-oxide semiconductor (CMOS) image sensor in which a pixel sensor spans multiple integrated circuit (IC) chips and is devoid of a shallow trench isolation (STI) structure at a photodetector of the pixel sensor. The photodetector and a first transistor form a first portion of the pixel sensor at a first IC chip. A plurality of second transistors forms a second portion of the pixel sensor at a second IC chip. By omitting the STI structure at the photodetector, a doped well surrounding and demarcating the pixel sensor may have a lesser width than it would otherwise have. Hence, the doped well may consume less area of the photodetector. This, in turn, allows enhanced scaling down of the pixel sensor.