LiDAR has become a critical requirement for Advanved Driver Assistance Systems (ADAS) as the automotive industry moves towards improved driver safety and autonomous cars. Silicon Photomultipliers (SiPM) and Single Photon Avalanche Diodes (SPAD) sensors are emerging as the most promising sensor technology for long range, >100m, direct time-of-flight LiDAR that needs to function in bright daylight and with low reflectance targets.
SensL is developing a new range of SiPM, the R-Series, that have improved detection efficiency at longer wavelengths used in LiDAR. In parallel to the sensor development, SensL is working to understand the fundamental advantages SiPM and SPAD sensor arrays provide long-range, ADAS LiDAR systems.
It will be shown that to achieve long range LiDAR with eye-safe lasers, a sensor with single photon sensitivity is required. This is due to the low number of returned photons from distances greater than 100m. When ambient daylight conditions are taken into account, the small returned signal at these distances can be easily lost in the noise and histogramming multiple laser pulses will be shown to provide the only method which allows for accurate time of flight ranging operation.
The histogramming technique and the architecture used to implement it will be described. A portable long-range LiDAR demonstrator using SiPM sensors has been developed and will be presented including range accuracy versus distance and low reflective targets. This will be compared to a detailed Monte Carlo model which will be shown to accurately describes SiPM and SPAD array operation in LiDAR ranging.
Packaging can have a significant impact on the performance characteristics of Silicon Photomultipliers (SiPM) sensors as well as having an impact on reliability and yield. To provide the highest performance possible, SensL have recently developed and tested a surface mount, through silicon via (TSV) package that provides high array fill factor, high photon detection efficiency (PDE) and magnetic resonance imaging (MRI) system compatibility. The PDE of TSV packaged sensors will be shown to be the highest when compared to traditional SiPM package types. In addition the PDE in the UV and blue region will be shown to approach that of unprotected bare die. Additionally, the TSV package has minimal deadspace outside of the active area which will be shown to allow close packing when used in a sensor array. It will be shown that arrays of TSV sensors have the highest fill factor currently possible when creating arrays from singulated die. Additionally, it will be shown that TSV parts are non-magnetic and results of images taken with the TSV SiPM in a 3 Tesla magnetic resonance imaging (MRI) system will be shown to have no impact on the MRI system.
SensL C-Series Silicon Photomultiplier (SiPM) sensors are fabricated in a high-volume CMOS foundry to a custom SensL process, and packaged as a reflow solderable surface mount device. Advances in SiPM production have resulted in significant improvement in PDE, dark current as well as tighter breakdown voltage uniformity for the C-Series SiPM sensors. The SiPM are fabricated with a shallow P-on-N junction optimized for the detection of shorter wavelength photons, with a peak PDE of 41% at 420nm and excellent sensitivity extending to wavelengths <300nm. The dark currents have been reduced through the reduction of damage during semiconductor processing and an order of magnitude reduction has been achieved. The breakdown voltage variation has been improved through process optimization to minimize variations. With these process improvements typical dark count rates of ~30kHz/mm2 are achieved simultaneously with breakdown voltage uniformity of ±213mV demonstrated. In addition, application specific measurements of CRT (Coincidence Resolving Time) that are relevant to PET (positron emission tomography) will be shown to be 210ps at 7.5V overvoltage. In addition to device characterization work, this paper will address the wafer-level fabrication and testing, package level testing required by high volume SiPM sensor applications.
Large area optical detection systems are required for applications including cell imaging, spectroscopy, nuclear
medicine, bio diagnostics, radiation detection and high energy physics. Each of these applications requires that a detector
or detector arrays be closely coupled with light sources or optical couplers such as fibres or light couplers. In this paper,
the scaling of novel Silicon Photomultiplier detectors to tile across a large area is presented. In particular, a novel method
is discussed for compact packaging of SPM detectors into a tiled 2D detector array for large area imaging and 2D spatial
detection. The SPM detector has performance characteristics comparable to vacuum photon multiplier tubes used in
these applications today but offers several performance and system design advantages including spatial resolution,
optical over exposure, small form factor, weight, magnetic insensitivity and low bias operation.
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