Time-of-Flight Pixel Virtual Phase Implants Charge Transfer
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Solution Overview
Problem
Time-of-flight sensors face challenges in achieving fast charge transfer in backside illuminated pixels, which is crucial for accurate high-frequency operation, due to the difficulty in transferring electrons from the backside to the frontside floating diffusion node, especially under near-infrared light conditions.
Innovation Solution
The introduction of multiple virtual phase implants underneath the photogate fingers creates a potential gradient that accelerates charge transfer, reducing the time for complete charge transfer from approximately 5 ns to 2 ns, enabling operation at 300 MHz or higher frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If backside illuminated pixel structure is used, then sensitivity to near-infrared light is improved, but charge transfer speed from backside to frontside deteriorates
Solution Approach 1:
The pixel structure is divided into distinct frontside and backside regions with separate functional layers. The backside contains the photodetector for light absorption while the frontside contains the floating diffusion node for charge collection. This segmentation allows optimization of each region for its specific function while managing the charge transfer interface between them.
Solution Approach 2:
A dedicated charge transfer mechanism is introduced as an intermediary component between the backside photodetector and frontside floating diffusion node. This intermediary structure facilitates efficient electron transfer across the substrate, resolving the conflict between backside illumination sensitivity and charge transfer speed by providing a specialized pathway for charge carriers.
2Productivity
If charge transfer time is reduced for high-frequency operation, then operating frequency is improved, but charge transfer completeness deteriorates
Solution Approach 1:
The charge transfer mechanism is designed with dynamic characteristics that can be modulated in response to the AC-modulated light signal. The transfer process adapts its timing and efficiency based on the instantaneous charge availability and operating frequency requirements, enabling complete transfer even at high frequencies up to 300 MHz.
Solution Approach 2:
The charge transfer process is designed to operate continuously and efficiently throughout each modulation cycle. By maintaining continuous charge collection and transfer operations synchronized with the light modulation, the system ensures complete charge transfer at each phase sampling point without interruption or loss, even at high operating frequencies.
3Measurement precision
If phase demodulation accuracy is improved for distance measurement, then measurement precision is improved, but system complexity increases
Solution Approach 1:
The pixel structure incorporates self-service features where the floating diffusion node automatically performs charge integration and phase sampling functions. The inherent electrical characteristics of the floating diffusion enable direct phase detection without requiring additional complex external demodulation circuitry, achieving high measurement precision while minimizing system complexity.
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 solution allows for more accurate demodulation of phase differences between outgoing and returning light waves, enhancing the precision of distance measurements in time-of-flight systems, particularly at higher frequencies, and supports the integration of time-of-flight sensors in compact devices with size and power constraints.
Implementation Method 1
Each pixel in the plurality of pixels includes a photodetector to detect the image light and convert the image light into an electric signal
Implementation Method 2
One or more virtual phase implants are disposed in the semiconductor material proximate to the frontside of the semiconductor material... to transfer the image charge corresponding to image and phase-shift information to the floating diffusion
Data Source
AI summary
A time-of-flight (TOF) pixel includes a semiconductor material and a photogate disposed proximate to a frontside of the semiconductor material. The photogate is positioned to transfer charge in the semiconductor material toward the frontside in response to a voltage applied to the photogate. A floating diffusion is disposed in the semiconductor material proximate to the frontside of the semiconductor material, and one or more virtual phase implants is disposed in the semiconductor material proximate to the frontside of the semiconductor material. At least one of the one or more virtual phase implants extend laterally from under the photogate to the floating diffusion to transfer the charge to the floating diffusion.