A few words about SPAD technology…
SPADs (Single Photon Avalanche Diodes) or photon-counting Avalanche Photo-Diodes (APDs) are at the core of the VL6180X architecture. An APD is a photo-detector which is operated with a relatively high reverse voltage (>13V in that case), slightly above the breakdown threshold voltage of the diode. In this mode, the absorption of a photon (i.e. a single element of light) can trigger a strong avalanche of electron-hole pairs in the diode channel, creating a peak of current that can then be used as an event relative to the arrival time of the photon.
Of course the avalanche must be controlled (to avoid overconsumption or device breakdown). To stop the avalanche an additional device (a quench circuitry acting as a resistor) is required to reduce the diode bias below the threshold voltage in a short time so that the avalanche is stopped and the detector is ready for detection of further photons after some recovery time (dead time~100ns). This is called the quench effect.
Each avalanche event results in a voltage pulse. This voltage pulse has a very fast raising edge which is used to accurately capture the photon arrival time.
SPADs used in ST IMG175 process consist of a detector diode, an inverter and NMOS pull-down which acts as a passive quench. The output is a digital pulse as described in the schematic below.
The distance to an object is determined by time measurement between emitted and received light pulse (infrared laser source). It is referred to as the direct Time-of-Flight (ToF) measurement of optical pulses sent by a light source.
The distance is retrieved thanks to the speed of light knowledge. For instance, an object at 1cm from the system would result in 66ps time shift between the emitter and the receiver.
This approach enables rapid, accurate distance measurements independent of the characteristics of the target object. Being immune to ambient illumination and optical path variations, the system consistently measures the same distance.
The first embedded SPAD generation has now been engaged in high volume production. R&D is working on next generation improving intrinsic performances (fill factor, Quantum efficiency,…) and digital densities (power consumption, chip size).
A few words about FLIM technology…
Fluorescence-lifetime imaging microscopy or FLIM is an imaging technique for producing an image based on the differences in the exponential decay rate of the fluorescence from a fluorescent sample. It can be used as an imaging technique in confocal microscopy, two-photon excitation microscopy, and multiphoton tomography.
The lifetime of the fluorophore signal, rather than its intensity, is used to create the image in FLIM. This has the advantage of minimizing the effect of photon scattering in thick layers of sample.
FLIM has been demonstrated using ToF cameras based on frequency domain principles. Time correlated single photon counting (TCSPC) is the most precise photon efficient technique for FLIM but has until recently been restricted to relatively slow scanning cameras. The advent of highly parallel CMOS SPAD arrays and on-chip time to digital converters have been demonstrated separately in fast FLIM and video rate ToF applications. However, there is significant commonality between these two applications in terms of (1) the required spectral range of the SPADs in the red/NIR (2) the time resolution and dynamic range of the TDCs in 50-100ps at 10-14bits (3) the pixel resolution of the arrays at around 10k-100kpixels. TCSPC requires significant numbers of digital gates to perform histogram processing to extract either ToF reflection peaks or fluorescence lifetime decay constants. In either case the processing operates on a single dataset produced by a common set of time-resolved pixel electronics which may conveniently be placed in the processor chip below the SPAD array. A high fill factor sensor in which an array of SPADs can be stacked over electronics would benefit both applications.
A few words about PET technology…
Positron emission tomography (PET) is a nuclear medicine, functional imaging technique that is used to observe metabolic processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule. Three-dimensional images of tracer concentration within the body are then constructed by computer analysis.
Few-photon detectors are still surprisingly dominated by 20th century photomultiplier tube (PMT) technology because of their best overall performance in respect of large area compatibility with macro-optics, spectral range and temporal consistency. SPAD arrays (often referred to as Silicon PhotoMultipliers or SiPMs) are a viable semiconductor based technology to replace PMTs in photon counting applications. SiPMs consume less power, are smaller, more robust and are not sensitive to magnetic fields. Arrays can be scalable and configurable for application specific performance.
The advantages of SiPMs over PMTs are further enhanced by implementing the SPADs in CMOS technology. Integrated electronics provides high dynamic range photon counting, together with picosecond photon timing. In PET applications, on-chip gamma event filtering and processing greatly reduces the computational demands on the coincidence processing unit. The result is lower power consumption, bill-of-materials and system cost.