CMOS Image Sensor Local Impurity Region Dark Current
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Solution Overview
Problem
CMOS image devices suffer from parasitic dark currents due to dangling bonds at interfacial surfaces, causing ghost images and image lag, which existing solutions like p-type impurity regions do not adequately address.
Innovation Solution
The CMOS image sensor unit cells are designed with non-uniformly doped channel regions and strategically placed impurity regions to inhibit dark current parasitics, where a p-type impurity region is partially overlapped with the transfer gate to direct electrons away from the photodiode and a n-type impurity region is used to lower the potential barrier, facilitating charge carrier removal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a p-type impurity region is formed on the entire surface of the substrate to reduce dark current, then dark current is reduced, but the potential barrier becomes abrupt and electrons are forced back towards the photodiode causing image lag
Solution Approach 1:
The patent applies local quality by forming the p-type impurity region only in specific locations: at the interface between the substrate and gate oxide layer, and locally underneath the transfer gate where it overlaps with the channel region. This localized approach reduces dark current at critical interfaces without creating abrupt potential barriers across the entire substrate, thereby preventing electron backflow to the photodiode while maintaining effective dark current suppression.
Solution Approach 2:
The p-type impurity region is segmented into distinct areas: one at the substrate-gate oxide interface and another locally underneath the transfer gate. This segmentation allows each region to perform its specific function independently - the interface region suppresses dark current while the localized region under the gate controls potential barrier formation, avoiding the adverse effects of a continuous p-type region across the entire substrate.
2Ease of operation
If the n-type photodiode region extends underneath the transfer gate to inhibit image lag, then image lag is reduced, but a potential barrier is induced that forces electrons back towards the photodiode causing dark current
Solution Approach 1:
The patent applies local quality by positioning the p-type impurity region specifically at the substrate-gate oxide interface and locally underneath the transfer gate. This creates a non-uniform doping profile that suppresses dark current at critical interfaces while allowing the n-type photodiode region to extend underneath the transfer gate for image lag reduction, as the localized p-type region does not create a continuous potential barrier.
Solution Approach 2:
The p-type impurity region acts as an intermediary element that mediates between the conflicting requirements of image lag reduction and dark current suppression. By positioning it locally at the interface and underneath the gate, it provides a transition zone that allows electron flow for image lag reduction while suppressing dark current generation through interface state passivation.
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 significantly reduces dark current levels, preventing ghost images and image lag by effectively managing electron flow and potential barriers within the image sensor.
Implementation Method 1
a p-type impurity region is formed in the interfacial surface between layers of different materials (i.e., at the surface of the substrate, where the CMOS image sensor is formed)
Implementation Method 2
the impurities in the p-type impurity region inhibit the dark source current
Implementation Method 3
A unit pixel of the CMOS image device includes a photodiode for sensing images
Data Source
AI summary
According to a CMOS image device and a method of manufacturing same, dark current is decreased by a local impurity region. The image device includes a semiconductor substrate, and a transfer gate formed on a predetermined portion of the semiconductor substrate and electrically insulated from the semiconductor substrate. A photodiode is formed in the semiconductor substrate on one side of the transfer gate, and a floating diffusion region is formed on the semiconductor substrate in the other side of the transfer gate. A local impurity region of a first conductivity type is formed to be partially overlapped the transfer gate between the photodiode and the floating diffusion region.