CMOS Image Sensor Fabrication for Charge Transfer Optimization
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Solution Overview
Problem
CMOS image sensors with a 4-Tr structure face issues with image lag and poor photo-response characteristics due to the use of a floating diffusion node, which can be exacerbated by the deterioration of charge transfer ability when attempting to improve these traits.
Innovation Solution
A method for fabricating a CMOS image sensor that involves forming a photodiode region and a second conductive type ion region on a substrate, with a thermal process to diffuse the ions, enhancing the charge transfer ability without compromising the charge storage ability of the photodiode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the doping concentration at the surface of the silicon substrate is increased to improve charge transfer ability, then the transfer characteristics are improved, but the charge storage ability and photoelectron collecting capability of the photodiode are reduced
Solution Approach 1:
The patent applies local quality by creating different doping concentration zones within the photodiode region. A first doping concentration is applied to a first region (deeper in the substrate) while a second, higher doping concentration is applied to a second region (near the surface). This spatial differentiation allows the deeper region to maintain charge storage capability while the surface region provides enhanced charge transfer ability to the transfer transistor channel.
Solution Approach 2:
The patent segments the photodiode region into multiple zones with different doping characteristics. By dividing the photodiode region into a first region with lower doping concentration and a second region with higher doping concentration, the patent enables each segment to perform its specific function - the first region for charge storage and the second region for charge transfer - thereby resolving the contradiction between storage and transfer capabilities.
2Reliability
If a photogate electrode is formed on the photodiode to transfer accumulated signal to output node, then image lagging problems are solved, but photo-response characteristics are deteriorated
Solution Approach 1:
The patent applies parameter changes by modifying the doping concentration parameter at the photodiode surface rather than changing the structural configuration. By increasing the doping concentration in the second region near the surface, the patent improves charge transfer efficiency and eliminates image lagging without introducing a photogate electrode that would block light and deteriorate photo-response characteristics.
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 approach improves the transfer characteristics of the CMOS image sensor by increasing the doping concentration at the surface of the silicon substrate, thereby enhancing the charge transfer ability without reducing the photodiode's unique charge storage and photoelectron collecting capabilities.
Implementation Method 1
diffusing the second conductive type lightly doped ion region into the second conductive type ion region by a thermal process
Implementation Method 2
implanting a second conductive type impurity ion into an entire surface of the substrate where the transistor is to be formed
Data Source
AI summary
A CMOS image sensor and a method for fabricating the same are disclosed, in which transfer characteristics are improved. The method includes forming a photodiode region and a second conductive type ion region on a surface of a first conductive type substrate by implanting a second conductive type impurity ion into an entire surface of the substrate where a transistor is to be formed, forming a second conductive type lightly doped ion region in the substrate corresponding to the photodiode region by lightly implanting a second conductive type impurity ion only in an area where the photodiode region is opened, and diffusing the second conductive type lightly doped ion region into the second conductive type ion region by a thermal process.