SPAD Guard Ring Segmentation for Field Confinement
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Solution Overview
Problem
Single photon avalanche diodes (SPADs) face challenges in achieving low dark count rates (DCR) and high photon detection probabilities (PDP), which affect their sensitivity and accuracy due to thermal and tunneling-induced carrier events and electric field distribution.
Innovation Solution
A guard ring structure with a first and second well of opposite conductivity types is used to surround the multiplication zone, where the second well defines the zone laterally and has a steeper dopant concentration gradient, reducing the transition zone and preventing electric fields from exceeding the critical avalanche multiplication field outside the multiplication zone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single doping process is used to create the guard ring structure, then the manufacturing process is simpler, but the transition zone at the inner edge of the guard ring structure is larger, reducing photon detection probability
Solution Approach 1:
The guard ring structure is segmented into two separate doped regions: a first doped region and a second doped region. This segmentation allows each region to be optimized independently, with the second doped region having a steeper dopant concentration gradient to reduce the transition zone size, while the first doped region provides the necessary electrical isolation. The segmentation resolves the contradiction by enabling precise control of the transition zone without requiring the entire guard ring to be manufactured in a single complex process step.
Solution Approach 2:
Different dopant concentration gradients are applied to different parts of the guard ring structure. The second doped region has a steeper dopant concentration gradient specifically at its inner edge to minimize the transition zone, while the first doped region has a more gradual gradient suitable for electrical isolation. This local differentiation of doping characteristics allows the structure to simultaneously achieve small transition zones for high PDP and adequate isolation for low DCR.
2Reliability
If the guard ring structure is placed closer to the multiplication zone, then electrical isolation is improved, but the multiplication zone size is reduced, decreasing photon detection probability
Solution Approach 1:
The guard ring structure is divided into two separate doped regions positioned at different lateral distances from the multiplication zone. The first doped region is positioned closer to provide strong electrical isolation, while the second doped region is positioned farther away with a steep concentration gradient to minimize the transition zone encroachment into the multiplication zone. This segmentation allows both isolation effectiveness and multiplication zone area to be optimized.
Solution Approach 2:
The dopant concentration gradient parameter is changed between the two doped regions. The second doped region has a steeper dopant concentration gradient that causes the dopant concentration to drop off more rapidly with distance from the guard ring, thereby reducing the extent of the transition zone and allowing the multiplication zone to extend closer to the guard ring structure without being contaminated by excessive transition zone material.
3Manufacturing precision
If the dopant concentration gradient is reduced, then the transition zone size increases, but dark count rate increases due to electric field exceeding critical field outside multiplication zone
Solution Approach 1:
The guard ring structure is segmented into two doped regions with different dopant concentration gradients. The second doped region has a steeper gradient to minimize transition zone size, while the first doped region has a more gradual gradient that extends deeper into the substrate to provide strong electrical isolation and prevent electric field breakdown outside the multiplication zone. This segmentation allows both small transition zones and low dark count rates to be achieved simultaneously.
Solution Approach 2:
Different dopant concentration gradients are applied locally to different regions of the guard ring structure. The second doped region has a steep gradient localized at its inner edge to reduce the transition zone, while the first doped region has a more gradual gradient distributed over a larger depth to provide comprehensive electrical isolation. This local quality differentiation resolves the contradiction between transition zone size and dark count rate suppression.
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 increases the photon detection probability and decreases the dark count rate by effectively restricting the electric field to the multiplication zone, enhancing the sensitivity and accuracy of the SPAD.
Implementation Method 1
the second well defines the zone laterally and has a steeper dopant concentration gradient, reducing the transition zone
Implementation Method 2
effectively restricting the electric field to the multiplication zone
Implementation Method 3
preventing electric fields from exceeding the critical avalanche multiplication field outside the multiplication zone
Data Source
Figure 1A~3
Figure 4A~4C
AI summary
A method for manufacturing a semiconductor element comprising a single photon avalanche diode having a multiplication zone (AR) a guard ring structure with a second type of electrical conductivity comprises providing a semiconductor wafer with a first region (R) comprising a semiconductor material with the first type of conductivity. The method further comprises generating by a first doping process a first well (W1) of the guard ring structure having a first vertical depth, the first well (W1) laterally surrounding the multiplication zone (AR) and having a lateral distance (A) from the multiplication zone (AR). The method further comprises generating by a second doping process a second well (W2) of the guard ring structure having a second vertical depth, the second well (W2) laterally surrounding and adjoining a part of the first region for laterally defining the multiplication zone (AR).