Two-Wafer Active Pixel Sensor Architecture for CMOS Scaling
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Solution Overview
Problem
CMOS image sensors face challenges in scaling to smaller dimensions due to degradation in pixel performance caused by shallow trench isolation and heavily doped retrograde wells, leading to increased dark current and metallic contamination, which complicates the integration and optimization of photodetectors with CMOS circuits.
Innovation Solution
The implementation of an image sensor using two separate semiconductor wafers, where the sensor wafer includes photodetectors, transfer mechanisms, and charge-to-voltage conversion mechanisms, while the support circuit wafer has an interconnect layer and CMOS device layer, with direct connections between charge-to-voltage conversion mechanisms on the sensor wafer and amplifiers on the support circuit wafer, reducing capacitance and eliminating bright point defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If deeply scaled sub-micron CMOS processes are used to achieve small pixels, then pixel size is reduced, but pixel performance degrades due to increased dark current and metallic contamination
Solution Approach 1:
The patent divides the image sensor into two separate wafers: a sensor wafer containing photodetectors and a circuit wafer containing CMOS circuits. This segmentation allows each wafer to be optimized independently, enabling small pixel sizes on the sensor wafer without compromising pixel performance through the use of deeply scaled CMOS processes on the circuit wafer.
Solution Approach 2:
The patent introduces an intermediary connection structure between the sensor wafer and circuit wafer that avoids direct contact between photodetectors and highly doped ohmic contact regions. This intermediary approach prevents metallic contamination and reduces dark current while still enabling electrical connection between the photodetectors and CMOS circuits.
2Reliability
If photodetectors are directly connected to highly doped ohmic contact regions, then electrical connection is achieved, but dark current increases due to contact damage and metallic contamination
Solution Approach 1:
The patent introduces an intermediary connection structure between the sensor wafer and circuit wafer that avoids direct contact between photodetectors and highly doped ohmic contact regions. This intermediary approach prevents metallic contamination and reduces dark current while still enabling electrical connection between the photodetectors and CMOS circuits.
Solution Approach 2:
The patent extracts the highly doped ohmic contact regions from direct contact with photodetectors by placing them on a separate circuit wafer. This extraction eliminates the source of metallic contamination and contact damage to photodetectors while maintaining the necessary electrical connection through the inter-wafer interface.
3Adaptability or versatility
If two separate wafers are used with direct inter-wafer connections, then manufacturing flexibility is improved, but capacitance in connections increases
Solution Approach 1:
The patent transitions from planar connections to three-dimensional vertical stacking, with the sensor wafer positioned above the circuit wafer. This dimensional change allows for shorter connection paths and reduced parasitic capacitance while maintaining the manufacturing flexibility benefits of separate wafer processing.
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 high image quality and high fill factor with improved design flexibility and reduced manufacturing costs, allowing for easier scaling of transistors without increasing pixel defects.
Implementation Method 1
each pixel region including a photodetector
Data Source
AI summary
A vertically-integrated image sensor includes a sensor wafer connected to a support circuit wafer. Each pixel region on the sensor wafer includes a photodetector, a charge-to-voltage conversion mechanism, a transfer mechanism for transferring charge from the photodetector to the charge-to-voltage conversion mechanism, and a reset mechanism for discharging the charge-to-voltage conversion mechanism. The support circuit wafer includes an amplifier and other support circuitry for each pixel region on the sensor wafer. An inter-wafer connector directly connects each charge-to-voltage mechanism on the sensor wafer to a respective gate to an amplifier on the support circuit wafer.