Dielectric Relaxation Correction Circuitry for Image Sensors
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Solution Overview
Problem
Image sensors with capacitors are susceptible to dielectric relaxation, leading to lag signals that negatively impact image quality due to charge remaining on the capacitors after discharge, making it challenging to universally correct for the artifacts caused by these signals across various operating conditions.
Innovation Solution
The implementation of dielectric relaxation compensation circuitry, including shielded pixels and a frame buffer, allows for the measurement and correction of lag signals by leveraging the linear relationship between voltage stress and lag signal magnitude, enabling effective compensation for both steady-state and transient effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If capacitors are used in image pixels for signal storage and processing, then image sensor functionality and dynamic range are improved, but dielectric relaxation causes lag signals that degrade image quality
Solution Approach 1:
The patent introduces an intermediary correction circuit between the capacitor and the readout circuitry. This correction circuit includes a correction capacitor and associated switching circuitry that actively compensates for the lag signal by injecting corrective charge, thereby mediating the harmful effect of dielectric relaxation without removing the functional capacitor
Solution Approach 2:
The patent implements a feedback mechanism where the lag signal is measured during a measurement phase and then used to generate a correction signal during the next integration phase. The correction circuit adjusts the capacitor discharge based on the previously measured lag, creating a closed-loop feedback system that continuously compensates for dielectric relaxation effects
2Object-affected harmful factors
If correction circuitry is added to compensate for lag signals, then image quality is improved, but device complexity increases
Solution Approach 1:
The patent merges the lag measurement function and lag correction function into a single integrated correction circuit block. The same correction capacitor and switching circuitry are used both to measure the lag signal during the measurement phase and to apply correction during the imaging phase, thereby reducing overall circuit complexity compared to having separate measurement and correction systems
Solution Approach 2:
The correction circuit is designed with multi-functionality, where the correction capacitor and associated transistors serve dual purposes: measuring the lag signal in one phase and compensating for it in another phase. This universal design reduces the total component count and circuit complexity while maintaining effective lag compensation
3Measurement precision
If measurement and correction procedures are implemented, then lag signal accuracy is improved, but processing time and operational complexity increase
Solution Approach 1:
The patent employs periodic action by dedicating specific time phases for lag measurement and correction. During the measurement phase, the circuit measures the lag signal; during the integration phase, it applies correction. This periodic cycling allows accurate measurement without continuous processing, optimizing the balance between precision and time efficiency
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 significantly improves image quality by accurately correcting for lag signals, reducing artifacts such as ghost images and enhancing the dynamic range of image sensors, regardless of stress time, float time, and voltage stress variations.
Implementation Method 1
Some image sensors include imaging pixels with capacitors. The capacitors may be susceptible to dielectric relaxation (lag).
Implementation Method 2
The dielectric relaxation experienced by the capacitor may result in some charge remaining on the capacitor (e.g., a lag signal) after discharge.
Data Source
AI summary
Some image sensors include pixels with capacitors. The capacitor may be used to store charge in the imaging pixel before readout. The capacitor may be a metal-insulator-metal (MIM) capacitor that is susceptible to dielectric relaxation. Dielectric relaxation may cause lag in the signal on the capacitor that impacts the signal on the capacitor during sampling. The image sensor may include dielectric relaxation correction circuitry that leverages the linear relationship between voltage stress and lag signal to correct for dielectric relaxation. The image sensor may include shielded pixels that operate with a similar timing scheme as the imaging pixels in the active array. Measured lag signals from the shielded pixels may be used to correct imaging data.