SPAD Image Sensor Hybrid Stacking for 3D Depth Sensing
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Solution Overview
Problem
Conventional back side illuminated image sensors suffer from limited functionality and lower than desired image quality and resolution, particularly in determining distance and achieving high-quality 3D imaging.
Innovation Solution
The implementation of a photon counting CMOS image sensor array with single-photon avalanche diodes (SPADs) using hybrid chip stacking and silicon on insulator (SOI) technology, where SPAD pixels are used for both low light level photon counting and time-of-flight measurements to generate high-resolution 3D images, with pixel circuits distributed between a top light sensing chip and an underlying ASIC chip for reduced power consumption and increased sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional back side illuminated image sensors are used, then manufacturing is simplified, but functionality is limited and image quality is lower
Solution Approach 1:
The patent implements a dual-mode pixel design where the same back side illuminated pixel structure can operate in both photon counting mode (for low-light imaging) and time-of-flight mode (for depth sensing). This multi-functionality resolves the contradiction by enabling the sensor to perform multiple tasks without requiring separate dedicated hardware for each function, thus improving adaptability while maintaining manufacturing simplicity.
Solution Approach 2:
The patent employs dynamic mode switching capability where pixels can be configured to operate in different modes (photon counting or time-of-flight) based on operational requirements. This dynamic reconfigurability allows the sensor to adapt its functionality in real-time, resolving the limitation of conventional sensors with fixed functionality while preserving the simplified back side illuminated manufacturing process.
2Device complexity
If conventional image sensors are used, then device complexity is reduced, but measurement precision for distance and image quality deteriorate
Solution Approach 1:
The patent introduces a temporal dimension to conventional spatial imaging by implementing time-of-flight measurement capability alongside photon counting. This adds the time dimension to the sensing process, enabling precise distance measurement and 3D depth mapping without significantly increasing structural complexity. The same pixel hardware performs both spatial detection and temporal measurement, resolving the contradiction between simplicity and precision.
Solution Approach 2:
The patent utilizes parameter changes in the SPAD operation mode to achieve different measurement modes. By adjusting bias voltage and timing parameters, the same hardware can switch between photon counting (high precision for light detection) and time-of-flight (high precision for distance measurement) modes, thereby achieving high measurement precision across different applications without increasing device complexity.
3Device complexity
If pixel circuits are integrated on the same chip, then device complexity is reduced, but power consumption increases and sensitivity decreases
Solution Approach 1:
The patent implements a segmented architecture where the sensor chip is divided into distinct functional regions: photon counting circuits on one substrate and time-of-flight circuits on another substrate, connected through inter-substrate wiring. This segmentation allows each circuit type to be optimized independently for power efficiency and performance, resolving the contradiction by reducing overall power consumption and enhancing sensitivity while maintaining manageable device complexity through modular design.
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 enables the creation of high-performance image sensors with improved resolution and 3D imaging capabilities, achieving small size and low power consumption while enhancing pixel sensitivity and reducing parasitic capacitance.
Implementation Method 1
Each pixel includes a photosensitive element that receives incident photons (light) and converts the photons into electrical signals
Implementation Method 2
single-photon avalanche diodes (SPADs) for single photon detection
Implementation Method 3
time-of-flight detection capabilities
Data Source
AI summary
A back side illuminated image sensor may operate using the single-photon avalanche diode (SPAD) concept in a Geiger mode of operation for single photon detection. The image sensor may be implemented using two layer stacking with a silicon on insulator (SOI) chip. The chip-to-chip electrical connections between the top level image sensing chip and the second level ASIC circuit chip may be realized at each pixel with a single bump connection per pixel. A light level signal may be obtained from pixels that have photon counting capabilities while a distance measurement signal for 3-dimensional imaging may be obtained from pixels that have time-of-flight (ToF) detection capabilities. Both types of pixels may be integrated within the same array and use the same SPAD structure placed on the top chip.