APD Focal Plane Arrays with Backside Vias for Crosstalk Reduction
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Solution Overview
Problem
Avalanche photodiodes (APDs) face challenges in achieving single-photon sensitivity across a wide spectral range, particularly in detecting both visible and short-wave infrared (SWIR) wavelengths, due to limitations in spectral response and high sensitivity to thermal generation and electroluminescence-induced dark counts, which reduce image fidelity.
Innovation Solution
The introduction of etched vias through the backside of APDs reduces optical crosstalk by promoting reflection and absorption of photons, allowing visible-wavelength photons to reach the active region, thereby extending the spectral response from SWIR to visible wavelengths and minimizing spurious dark counts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the APD uses a solid substrate structure, then mechanical support is maintained, but optical crosstalk and dark counts increase due to photon reflection and transmission through the substrate
Solution Approach 1:
The continuous substrate is segmented by etching vias that extend through or partially through the substrate, creating discrete regions that optically isolate adjacent pixels while maintaining mechanical support through the remaining substrate material
Solution Approach 2:
Material is extracted from the substrate to form vias that remove the harmful optical path for reflected and transmitted photons, thereby reducing crosstalk while preserving the mechanical integrity of the remaining substrate structure
2Object-affected harmful factors
If the APD substrate is fully removed to eliminate crosstalk, then optical crosstalk is reduced, but mechanical support and structural integrity are lost
Solution Approach 1:
Instead of removing the entire substrate, the substrate is segmented by etching vias that create optical isolation between pixels while leaving the majority of the substrate intact to provide mechanical support
Solution Approach 2:
The substrate is modified locally at via locations to provide optical isolation, while the rest of the substrate maintains its original mechanical support function, creating different local properties for different functions
3Adaptability or versatility
If alternative materials with smaller band gaps are used for SWIR detection, then spectral response extends to SWIR wavelengths, but visible and NIR photons are absorbed before reaching the active region
Solution Approach 1:
The device is configured for backside illumination where photons enter through the substrate, and vias are etched from the backside to facilitate photon transmission to the active region while the substrate material itself becomes part of the optical path management solution
Solution Approach 2:
Via structures are extracted from the substrate to create optical pathways that guide photons from the backside entry point through the substrate to the active region, preventing absorption in unwanted areas while enabling detection across multiple spectral ranges
4Adaptability or versatility
If etched vias are introduced through the backside of APDs, then optical crosstalk is reduced and spectral response is extended, but device complexity and fabrication difficulty increase
Solution Approach 1:
Via etching is performed as a preliminary step before final device assembly and testing, allowing the optical isolation structure to be in place before photons are introduced into the system, simplifying the overall manufacturing workflow
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 significantly reduces crosstalk and dark counts, enhancing the spatial and temporal fidelity of images by allowing unimpeded access to visible photons and maintaining mechanical support for wafer-level processing, resulting in improved single-photon sensitivity across a broader spectral range.
Implementation Method 1
the vias are etched fully through the substrate of a GmAPD... the vias substantially reduce backside-reflected cross talk
Implementation Method 2
the vias substantially reduce backside-reflected cross talk by causing generated photons to impinge upon surfaces that promote reflection and/or absorption
Implementation Method 3
each incident photon creates only a single photo-excited electron by the photoelectric effect
Implementation Method 4
When the injected charge reaches a sufficiently high kinetic energy, it can generate an electron-hole pair through an inelastic collision with lattice atoms in a process referred to as 'impact ionization'
Implementation Method 5
At a sufficiently large electric-field intensity known as the 'avalanche breakdown field,' there is a finite probability that the avalanche multiplication process can lead to a self-sustaining avalanche
Implementation Method 6
the use of a full via that is aligned with the active region of the device provides unimpeded access to visible-wavelength photons... InP substrate absorbs over 99% of visible light
Data Source
AI summary
An avalanche photodiode (APD) array with reduced cross talk comprises, in the illustrative embodiment, a 2D array of Geiger-mode APDs, wherein a via is formed partially through the backside (substrate) of each APD in the array, wherein the via is offset from the active region of each said APD.