Solid-State Photomultiplier Optical Isolation Structure
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Solution Overview
Problem
Conventional solid-state photomultiplier (SSPM) designs suffer from optical crosstalk between adjacent avalanche photodiode (APD) cells and high dark count rates due to the lack of isolation structures, leading to reduced active area and increased noise in radiographic imaging systems.
Innovation Solution
Incorporating an optical isolation structure with a light absorbing or reflecting material into the SSPM to separate each APD cell from adjacent cells, formed by etching trenches in the epitaxial layer and filling them with the material, which reduces optical crosstalk and dark count rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If no isolation structures are positioned between APD cells, then manufacturing complexity is reduced, but optical crosstalk between adjacent APD cells increases
Solution Approach 1:
The patent introduces an optical isolation structure as an intermediary element positioned between adjacent APD cells. This structure contains a light absorbing material that acts as a mediator to intercept and absorb optical photons before they can travel between APD cells and trigger false avalanche events, thereby eliminating optical crosstalk without requiring complex active control mechanisms
Solution Approach 2:
The optical isolation structure is selectively positioned only in the regions between adjacent APD cells where optical crosstalk occurs, rather than uniformly across the entire device. The light absorbing material is deposited specifically in the isolation trenches at locations where photons could potentially travel between cells, providing localized suppression of the harmful effect while maintaining the overall simplicity of the device structure
2Object-generated harmful factors
If minimum distance is maintained between adjacent APD cells to control optical crosstalk, then optical crosstalk is reduced, but active area of the SSPM is reduced
Solution Approach 1:
By introducing the optical isolation structure as an intermediary, the patent enables APD cells to be positioned closer together than the traditional minimum distance required for passive geometric isolation. The light absorbing material in the isolation trenches actively intercepts photons, allowing the spacing between cells to be reduced while maintaining low crosstalk levels, thereby increasing the fill factor and active area of the SSPM
Solution Approach 2:
The patent changes the parameter of cell spacing by introducing the optical isolation structure, which allows the distance between adjacent APD cells to be reduced below the conventional minimum. This parameter change enables higher cell density and increased active area while maintaining optical isolation through the light absorbing material rather than relying solely on geometric separation
3Ease of manufacture
If single layer silicon wafer is used without isolation structures, then manufacturing process is simplified, but dark count rate increases
Solution Approach 1:
The optical isolation structure serves as an intermediary that simultaneously addresses both optical crosstalk and dark count rate issues. The light absorbing material in the isolation trenches intercepts photons that would otherwise travel between cells and trigger dark count events, thereby reducing the dark count rate without adding complex active control mechanisms or multiple wafer layers
Solution Approach 2:
The isolation structure is implemented locally at the interfaces between APD cells where the harmful effects of optical crosstalk and dark count generation occur, rather than requiring uniform modification across the entire wafer structure. This localized approach maintains the simplicity of the single layer silicon wafer manufacturing process while effectively suppressing dark count rates at the critical regions
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 solution effectively minimizes optical crosstalk and dark count rates, allowing for a higher fill factor and improved photon counting and energy discrimination capabilities in radiographic detectors, enhancing the performance of imaging systems like CT and PET.
Implementation Method 1
the optical isolation structure contains a light absorbing material deposited therein
Implementation Method 2
The SSPM is comprised of a plurality of Geiger-mode avalanche photodiodes (APDs) or 'microcells' that amplify each single optical photon from the scintillator into a large and fast signal current pulse
Implementation Method 3
the scintillator component illuminates upon reception of radiographic energy
Data Source
AI summary
A solid-state photomultiplier (SSPM) includes an optical isolation structure therein. The SSPM includes a substrate and an epitaxial diode layer positioned on the substrate. A plurality of avalanche photodiodes (APDs) are fabricated on the epitaxial diode layer and the optical isolation structure is positioned about the plurality of APDs to separate each of the plurality of APDs from adjacent APDs. The optical isolation structure contains at least one of a light absorbing material and a light reflecting material deposited therein to reduce optical crosstalk and dark count rate in the SSPM.