Lateral Overflow Image Sensor Dark Current Suppression
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Solution Overview
Problem
CMOS image sensors face challenges with dark currents and white pixels due to miniaturization and integration demands, leading to susceptibility in low light conditions and saturation issues.
Innovation Solution
The solution involves connecting a floating diffusion to a high voltage during integration to increase capacity and using low leakage transistors to suppress dark current and white pixel issues, while adjusting voltages and timing operations to align reset levels and reduce dark current and white pixel effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel cells are miniaturized and integrated to meet higher resolution demands, then image sensor resolution is improved, but dark current and white pixel susceptibility increase
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the voltage at the floating diffusion node during different operational phases. During integration, the floating diffusion is connected to a high voltage to increase capacity and reduce dark current. During readout, the voltage is adjusted to align reset levels and minimize white pixel effects. This voltage parameter adjustment directly addresses the contradiction by adapting the circuit state to operational requirements.
Solution Approach 2:
The patent implements dynamics by making the floating diffusion connection state changeable between different voltage levels. The circuit transitions from a high voltage connection during integration to a different voltage configuration during readout. This dynamic reconfiguration allows the system to optimize performance for each phase, resolving the contradiction between resolution and dark current susceptibility.
2Quantity of substance
If floating diffusion is connected to high voltage during integration, then capacity is increased and dark current is reduced, but voltage difference management becomes more complex
Solution Approach 1:
The patent applies preliminary action by pre-configuring the floating diffusion connection to high voltage before the integration phase begins. This preliminary setup ensures that the floating diffusion is already in the optimal state for maximum capacity and minimal dark current during the entire integration period. The voltage adjustment is performed in advance, simplifying the overall voltage management strategy.
Solution Approach 2:
The patent uses an intermediary approach by introducing a controlled voltage connection mechanism that mediates between the floating diffusion and the high voltage source. This intermediary control allows the system to achieve high capacity while managing voltage differences through a regulated connection, rather than directly exposing the floating diffusion to high voltage without control.
3Object-affected harmful factors
If low leakage transistors are used to suppress dark current, then dark current is reduced, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies parameter changes by adjusting the voltage at the floating diffusion node to compensate for transistor leakage effects. By dynamically changing the voltage parameter during integration and readout phases, the system can suppress dark current effects without requiring extremely precise transistor manufacturing. The voltage adjustment provides a post-manufacturing compensation mechanism.
Solution Approach 2:
The patent implements feedback by using the voltage at the floating diffusion node as a control parameter that responds to operational conditions. The system monitors and adjusts the voltage based on the integration and readout phase, providing feedback control that compensates for transistor leakage variations and reduces the impact of manufacturing precision limitations.
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 effectively reduces dark current and white pixel issues by managing voltage differences and capacitance, enhancing the accuracy and performance of image sensors in low light conditions.
Implementation Method 1
pixels having photosensitive elements (e.g., photodiodes) that absorb a portion of the incident image light and in response generate corresponding electrical charge
Data Source
AI summary
Image sensors having reduced dark current and white pixel are disclosed herein. In one embodiment, each pixel of the image sensor includes a photodiode (PD), a first floating diffusion (FD1) coupled to the photodiode through a transfer (TX) transistor, a second floating diffusion (FD2) coupled to the FD1 through a dual floating diffusion (DFD) transistor, and a lateral overflow integrating capacitor (LOFIC) coupled between the FD2 and a variable reference voltage (VCAP). A method for a correlated double sampling (CDS) readout includes: exposing a photodiode (PD) to light during an exposure period and increasing a capacitance of the LOFIC by setting the VCAP to a high voltage (H) level during an integration period of the exposure period.