CMOS SPAD Deep Well Multiplication Junction for Red Light Sensitivity
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Solution Overview
Problem
CMOS single photon avalanche diodes (SPADs) suffer from high noise due to band-to-band tunnelling and have peak detection efficiency at blue light wavelengths, limiting their effectiveness in applications requiring red and near-infrared sensitivity, such as range detection and biological experiments, due to shallow active regions and high energy blue light phototoxicity.
Innovation Solution
A SPAD design with a deep well structure formed in a CMOS fabrication process, where the doping is controlled to create a breakdown voltage smaller at the junction between the deep well and the substrate than elsewhere, forming a multiplication region, and a guard ring is used to raise the peripheral breakdown voltage, enhancing red and near-infrared sensitivity by locating the multiplication junction deeper in the substrate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the active region is made shallow to improve response time, then bandwidth is improved, but sensitivity to red and near-infrared light deteriorates
Solution Approach 1:
The patent moves the multiplication region from a shallow surface location to a deep subsurface location (approximately 10 micrometers below the surface), utilizing the depth dimension to resolve the contradiction. This deep multiplication region maintains fast response time while being positioned where red and near-infrared photons are absorbed, thereby improving sensitivity to these wavelengths without sacrificing bandwidth.
2Reliability
If high doping concentrations are used to improve carrier generation, then detection efficiency is improved, but noise due to band-to-band tunnelling increases
Solution Approach 1:
The patent applies different doping concentrations to different spatial locations: high doping concentrations are used specifically in the multiplication region to ensure efficient carrier generation and avalanche breakdown, while lower doping concentrations are used in surrounding guard ring regions to minimize band-to-band tunnelling noise. This localized differentiation of doping quality resolves the contradiction between detection efficiency and noise reduction.
3Reliability
If blue light is used for illumination, then detection efficiency is improved, but phototoxicity in biological experiments increases
Solution Approach 1:
The patent changes the operational parameter of the detector to be sensitive to longer wavelengths (red and near-infrared) instead of blue light. By positioning the multiplication region deep in the substrate where red and near-infrared photons are absorbed, the system achieves high detection efficiency with these wavelengths, thereby eliminating phototoxicity concerns associated with blue light illumination in biological applications.
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 the sensitivity of SPADs to red and near-infrared light, allowing for more effective use in applications like digital communication and biological experiments, with reduced noise and phototoxicity, and enables eye-safe range detection with increased illumination efficiency.
Implementation Method 1
the breakdown voltage is smaller at the junction between the bottom of the deep well and the second region than elsewhere around the first region, whereby the junction forms a SPAD multiplication region
Implementation Method 2
Photodiodes convert light into electrical signals
Data Source
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AI summary
A single photon avalanche diode (400) is disclosed, for use in a CMOS integrated circuit, the single photon avalanche diode, SPAD, comprising: a deep n-well region (406) formed above a p-type substrate (402); an n-well region (408) formed above and in contact with the deep n-well region (406); a cathode contact (412) connected to the n-well region (408) via a heavily doped n-type implant (410); a lightly doped region (428) forming a guard ring around the n-well and deep n-well regions; a p-well region (416, 422) adjacent to the lightly doped region; and an anode contact (420, 426) connected to the p-well region via a heavily doped p-type implant (418, 424); the junction (414) between the bottom of the deep n-well region and the substrate forming a SPAD multiplication region when an appropriate bias voltage is applied between the anode and cathode and the guard ring breakdown voltage being controlled with appropriate control of the lateral doping concentration gradient such that the breakdown voltage is higher than that of the planar SPAD multiplication region.