Philips digital silicon photomultiplier technology
Silicon photomultipliers (SiPMs) comprise a two-dimensional array of avalanche photodiodes (APDs) that are reverse biased to a voltage above their normal reverse breakdown voltage – typically to around 30V to 70V.
Operating in this so-called ‘Geiger mode’, the diodes are ultra-sensitive to single electron-hole pairs, which result in individual diodes undergoing avalanche breakdown. These electron-hole pairs can be generated either by the absorption of a photon (the desired ‘photon count’ signal), or by thermal energy or electron tunneling (unwanted noise signals). The unwanted background noise produced by thermally generated electron-hole pairs and/or electron tunneling, together with false counts due to defective diodes, are collectively referred to as the ‘dark count’.
In a conventional SiPM, each avalanche photodiode is connected in series with a polysilicon ‘quenching’ resistor that rapidly quenches the avalanche breakdown by limiting the diode current to a sufficiently low level. All of these diode/resistor ‘microcells’ are connected in parallel, which means that simultaneous (or near-simultaneous) avalanche breakdown in two or more photodiodes results in a stepped output current. On-chip capacitance and inductance effects ‘smear’ this output signal into an analog waveform that must be processed by complex ASICs in order to recover the photon count.
Philips’ Digital Photon Counting SiPM
The digital silicon photomultiplier technology developed by Philips equips each individual avalanche photodiode with its own 1-bit on-chip ADC (Analog to Digital Converter) in the form of a CMOS inverter. Each microcell that experiences avalanche breakdown therefore produces its own digital output that is captured, along with the digital outputs from all other triggered microcells, by an on-chip counter. The Philips digital SiPM therefore converts digital events (photon detections) directly into a digital photon count. As a result, it is capable of achieving significantly better photon-count resolution than conventional SiPMs.
Reduced dark count
To overcome the ‘dark count’ problem associated with conventional SiPMs, each microcell in a Philips digital SiPM is equipped with an addressable static memory cell that can be used to disable or enable the microcell. Microcells that show high dark count levels can therefore be prevented from contributing false counts to the SiPM’s output. This facility allows Philips’ digital SiPMs to achieve better signal-to-noise ratios than conventional devices. Because defective microcells in the array can be disabled, it also helps to improve production yield.
Circuitry is also added to each microcell to actively quench and recharge the microcell after triggering. This active quenching/recharging in Philips’ SiPMs improves their recovery time and reduces power consumption. As a result, detector modules constructed using Philips’ new SiPM technology typically only require air cooling. Cooling below ambient temperature is only required in applications that require ultra-low dark count levels.
In contrast to conventional analog SiPMs, in which parasitic capacitance and inductance degrade timing performance, all microcells in the Philips’ digital SiPM are connected via a low-skew balanced trigger network to an on-chip time-to-digital converter. The timing resolution of this converter is 20 ps, thereby preserving the excellent intrinsic timing performance of the Geiger-mode avalanche photodiodes.
Coupled with the advantages of a solid-state solution (ruggedness, smaller size, lighter weight, lower operating voltages and excellent immunity to magnetic fields), these features make Philips’ digital SiPMs the photomultiplier of choice for demanding photon-counting applications.