Silicon-Germanium Epitaxy Layer for Image Sensor Dark Current and Lag
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional CMOS image sensors face a trade-off between image lag and dark current or white pixel performance due to the limited area of the remaining portion of the n-type pinned photodiode, which affects electron transfer and image quality.
Innovation Solution
A silicon-germanium epitaxy layer with a lower bandgap energy is formed on a silicon-based substrate, allowing for a larger p-type pinned photodiode embedded in the n-type pinned photodiode, reducing the additional potential barrier and improving both dark current and image lag performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the remaining portion of the n-type pinned photodiode is made small to embed the p-type pinned photodiode, then the dark current and white pixel performance are improved, but the image lag increases due to additional potential barrier
Solution Approach 1:
The patent introduces a silicon-germanium epitaxy layer with different bandgap energy than conventional silicon, changing the fundamental electrical parameters of the photodiode structure. This material parameter change allows the remaining portion to have both small area (for dark current performance) and reduced potential barrier (for electron transfer), resolving the contradiction between dark current performance and image lag
Solution Approach 2:
The patent uses a composite structure combining silicon-based substrate with silicon-germanium epitaxy layer. This composite material approach leverages the beneficial properties of both materials: the mechanical stability of silicon substrate and the optimized electrical properties of silicon-germanium layer, enabling simultaneous improvement in dark current performance and reduction of image lag
2Object-generated harmful factors
If the remaining portion of the n-type pinned photodiode is made large to reduce potential barrier, then the image lag is reduced, but the dark current and white pixel performance deteriorate
Solution Approach 1:
By changing the material composition to silicon-germanium epitaxy layer, the patent alters the bandgap energy parameter, which fundamentally changes the relationship between area and potential barrier. This allows achieving low image lag with smaller remaining portion area while maintaining dark current performance through the modified material properties
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 silicon-germanium epitaxy layer decreases the total bandgap energy, enhancing dark current and white pixel performance while reducing image lag, thus optimizing both aspects simultaneously.
Implementation Method 1
the silicon-germanium epitaxy layer has a bandgap energy lower than that of a conventional pure silicon epitaxy layer
Data Source
AI summary
An image sensor device includes a silicon-based substrate, a silicon-germanium epitaxy layer, an isolation feature, an active pixel cell and a logic circuit. The silicon-germanium epitaxy layer is on the silicon-based substrate, in which the silicon-germanium epitaxy layer has a composition of Si1-xGex, where 0<x<1. The isolation feature is disposed in the silicon-germanium epitaxy layer to define a pixel region and a periphery region of the silicon-germanium epitaxy layer. The active pixel cell is disposed in the pixel region of the silicon-germanium epitaxy layer. The logic circuit is disposed in the periphery region.