Back-illuminated SPAD Sensor Wafer Segmentation
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Solution Overview
Problem
Monolithically-integrated SPAD image sensors face limitations in fill factor due to electrical circuitry consumption and contamination issues during fabrication, which affect photon detection efficiency and timing response, with a trade-off between detection efficiency and timing resolution.
Innovation Solution
A back-illuminated SPAD image sensor design featuring a sensor wafer with SPAD regions and a separate circuit wafer, including an anode gradient layer, cathode region, and guard ring layer with dopant concentration gradients, and deep trench isolation regions to reduce contamination and enhance photon detection efficiency without increasing wafer thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thicker semiconductor wafer is used to improve photon detection efficiency, then photon detection efficiency is improved, but timing resolution deteriorates because charge carriers must propagate through a thicker wafer
Solution Approach 1:
The patent divides the semiconductor wafer into multiple functional layers: a first semiconductor layer containing SPAD pixels for photon detection, and a second semiconductor layer containing circuitry. This segmentation allows the first layer to be optimized for photon detection efficiency while the second layer handles electronic functions, eliminating the need to increase overall wafer thickness for better detection efficiency.
Solution Approach 2:
The patent transitions from a planar integration approach to a three-dimensional stacked architecture where the sensor layer and circuit layer are vertically separated. This dimensional change enables independent optimization of each layer's thickness and properties, allowing the detection layer to achieve high photon detection efficiency without compromising timing resolution through excessive thickness.
2Device complexity
If electrical circuitry is integrated with SPAD regions on the same wafer, then device integration is improved, but fill factor deteriorates because circuitry consumes space on the wafer
Solution Approach 1:
The patent segments the device into two separate wafers: one dedicated to SPAD pixel arrays and another to circuitry. This physical separation allows the SPAD wafer to achieve high fill factor with minimal circuitry interference, while maintaining full integration functionality through wafer-level bonding that connects the two segmented parts.
3Ease of manufacture
If metals and contaminants are present during fabrication, then fabrication process is simplified, but performance deteriorates due to increased noise in the SPAD image sensor
Solution Approach 1:
The patent divides the fabrication process into separate stages for the sensor wafer and circuit wafer, allowing each to be fabricated under optimized conditions. The sensor wafer can be fabricated with stringent contamination control for high noise performance, while the circuit wafer handles metallization and complex interconnections, reducing the risk of contamination affecting SPAD performance.
4Reliability
If a thicker semiconductor wafer is used to improve photon detection efficiency, then photon detection efficiency is improved, but power consumption increases due to higher breakdown voltage
Solution Approach 1:
The patent segments the device into a thin sensor layer for photon detection and a separate circuit layer, eliminating the need for a thick wafer. This allows the sensor layer to maintain low breakdown voltage and power consumption while achieving high photon detection efficiency through optimized layer thickness and composition.
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
The design improves photon detection efficiency and maintains timing performance by optimizing the fabrication process for SPAD regions and electrical circuitry, reducing contamination, and minimizing electrical and optical crosstalk, while allowing for larger light-gathering areas.
Implementation Method 1
The anode gradient layer includes a back edge dopant concentration gradient that extends from a back surface of the anode gradient layer, a first side edge dopant concentration gradient that extends from an interior of the anode gradient layer to a first edge of the anode gradient layer, and a second side edge dopant concentration gradient that extends from an interior of the anode gradient layer to a second edge of the anode gradient layer
Implementation Method 2
An SPAD region is a photosensitive region that is configured to detect low levels of light (down to a single photon) and to signal the arrival times of the photons
Implementation Method 3
An SPAD region is a photosensitive region that is configured to detect low levels of light (down to a single photon)
Implementation Method 4
a deep trench isolation region adjacent to the SPAD region
Data Source
AI summary
A back-illuminated single-photon avalanche diode (SPAD) image sensor includes a sensor wafer stacked vertically over a circuit wafer. The sensor wafer includes one or more SPAD regions, with each SPAD region including an anode gradient layer, a cathode region positioned adjacent to a front surface of the SPAD region, and an anode avalanche layer positioned over the cathode region. Each SPAD region is connected to a voltage supply and an output circuit in the circuit wafer through inter-wafer connectors. Deep trench isolation elements are used to provide electrical and optical isolation between SPAD regions.