Stacked Image Sensor Pixel Cell for Global Shutter Noise Reduction
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Solution Overview
Problem
Existing image sensors face distortion issues due to rolling shutter operations, especially in augmented reality/mixed reality/virtual reality applications, and suffer from noise charges that degrade the accuracy of light intensity representation in global shutter operations.
Innovation Solution
A pixel cell design incorporating a first semiconductor die with a photodiode and charge sensing device, and a second semiconductor die with an interface circuit, sampling capacitor, and AC capacitor, which enables simultaneous charge accumulation, sample-and-hold operations, and digital output generation to represent light intensity, while minimizing noise and footprint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If rolling shutter operation is used, then device complexity is reduced, but image distortion occurs
Solution Approach 1:
The pixel cell is divided into multiple functional regions: photodiode for charge generation, charge sensing device for charge storage, sampling capacitor for voltage sampling, and interface circuit for control. This segmentation allows simultaneous charge accumulation across all pixels (global shutter) while keeping each pixel's circuit complexity manageable.
Solution Approach 2:
The sampling capacitor acts as an intermediary between the charge sensing device and the output circuit. It converts the charge signal to a voltage signal and holds it steady, enabling accurate representation of light intensity without requiring complex real-time conversion circuits in each pixel.
2Manufacturing precision
If global shutter operation is used, then image distortion is reduced, but noise charges increase
Solution Approach 1:
The charge sensing device is extracted as a separate functional unit from the photodiode. This allows the photodiode to accumulate charge during the integration period while the charge sensing device remains isolated and ready to receive the charge signal, preventing noise accumulation in the sensing device during the global shutter operation.
Solution Approach 2:
The charge sensing device is reset to a known state before the integration period begins. This preliminary reset action removes any residual charge or noise from the previous frame, ensuring that only the charge generated during the current integration period is measured, thereby reducing noise charges.
3Measurement precision
If pixel cell size is increased, then light sensing accuracy is improved, but device footprint increases
Solution Approach 1:
The patent transitions from a planar layout to a three-dimensional stacked architecture. The photodiode, charge sensing device, sampling capacitor, and interface circuit are arranged in vertical layers rather than spreading out horizontally. This allows high-density integration of all necessary components for accurate light sensing within a small footprint suitable for wearable devices.
Solution Approach 2:
Multiple functional components are merged into a single integrated pixel cell structure. The photodiode, charge sensing device, sampling capacitor, and interface circuit are combined in a compact stacked configuration, achieving high light sensing accuracy without increasing the overall sensor footprint.
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 design reduces image distortion, improves light sensing accuracy, and allows for high-resolution image sensors with a small footprint, suitable for wearable devices by reducing noise and enabling efficient digital output transmission.
Implementation Method 1
A typical image sensor includes an array of photodiodes to sense incident light by converting photons into charge (e.g., electrons or holes)
Data Source
AI summary
Examples of image sensors are provided. In one example, a pixel cell comprises a first semiconductor die, a sampling capacitor, and a second semiconductor die which may include the sampling capacitor. The first semiconductor die includes a photodiode and a charge sensing device. The second semiconductor die forms a stack with the first semiconductor die, the second semiconductor die including an interface circuit coupled with the photodiode, the charge sensing device, and the sampling capacitor. The interface circuit is configured to: enable the photodiode to accumulate charge responsive to incident light within a integration period; transfer the charge from the photodiode to the charge sensing device; perform, using the sampling capacitor, a sample-and-hold operation to convert the charge in the charge sensing device into a voltage; and generate a digital output based on the voltage to represent an intensity of the incident light received by the photodiode.