Low-light detectors and multidetector imagers enable a wide range of applications, including cell imaging, biodiagnostic instrumentation, semiconductor wafer inspection, particle counting, nuclear medicine, radiation detection and quantum cryptography, as well as low-light imaging applications such as security cameras and ranging lidar systems. These applications rely on the ability of the sensor to detect light levels that range from a single photon (less than an attowatt) to millions of photons (picowatts to nanowatts) per second, incident on the detector surface. Traditionally, photomultiplier tubes have been used for these applications.1 A photomultiplier tube is a vacuum tube detector that uses a coated photocathode to convert incident photons into photoelectrons, which then are accelerated by the high electric field. Their interaction with multiple metal dynodes causes a shower of electrons, which are collected at the anode of the detector. The electrons are read out as a current that indicates the level of light incident on the photocathode surface. Depending on the configuration of the photomultiplier tubes and the voltages that are applied, the detector can operate in single-photon-counting mode as well as in analog or linear mode.In single-photon-counting mode, the detector must amplify the signal to a level where a single photon — incident on the detector surface — causes a measurable signal at the output of the detector. A photomultiplier tube uses high-voltage power supplies on the order of 1000 to 2000 V and external discriminator circuits to convert the electric charge signal into a digital signal. When in photon-counting mode, the signal is binary, allowing the number of incident photons to be counted and the arrival time of the photon to be determined from the timing of the detector’s output pulse. Single-photon-counting mode allows a wide range of measurement techniques — such as fluorescence lifetime imaging and laser rangefinding — that rely on knowing precisely the arrival time of the photon. Single-photon-counting techniques are ideal for signal fluxes of less than 5 million photons per second. Above that rate, photon arrival time is hard to discriminate because the possibility that two or more can arrive at the same time is increased. Higher signal levels require analog or linear mode of operation to accurately determine the signal level.Analog or linear operationIn analog or linear operation mode, the photon flux is determined by measuring the amount of current flowing through the detector. The detector must operate with a high gain (typically on the order of 106) to allow the electrical signal to be measured by external electronics.Over the past 50 years, photomultiplier tubes have been developed that allow single-photon counting and the linear mode detection of photons. However, the devices suffer from several problems, including high-kilovolt voltage power supplies fragility, magnetic field sensitivity and photocathode damage from exposure to high photon flux. A novel detector known as a silicon photomultiplier has been developed that replaces the photomultiplier tube in many applications.2,3 It uses a large parallel array of silicon photon-counting diodes and integrated quenching elements to provide single-photon analog mode sensitivity for low-light sensing (Figure 1). In this detector, incident photons are detected by the single-photon-counting diodes in the array, which avalanche and release a fixed quantity of charge onto the output node. The quenching element — a resistor in this configuration — quenches the avalanche and resets the diode. The output detector therefore is made up of the charge contributions from all single-photon-counting diodes in the array. Figure 1. Silicon photomultipliers use a parallel array of silicon photon-counting diodes and integrated quenching elements to provide analog mode sensitivity for low-light sensing.An exciting feature of these detectors is that they operate on low voltages of less than 100 V (30 to 50 V is typical). They also have a high quantum efficiency in visible and ultraviolet light, are immune to magnetic fields, provide uniform gain across the array, are robust with no damage from high light levels and are small in size. In addition, multichannel detectors can be made with fine-pitch resolutions, which are not possible in vacuum tube photomultiplier tube detectors.5 To illustrate the high degree of uniformity of a silicon photomultiplier, an output histogram of the detector pulsed with a low photon flux signal is shown in Figure 2.Figure 2. In this histogram showing the detector pulsed with a low-photon-flux signal, the photon number can be resolved easily because each peak in the output spectrum represents a different number of photons being detected. This spectrum is possible only by using a detector in which the output across the array of silicon photon-counting diodes is uniform and matched throughout the detector.High-volume productionCommercial products based on the detector have been slow to evolve mainly because of the complex process and system knowledge required to successfully manufacture and operate thousands of photon-counting detectors in parallel. However, significant research and development in the technology over the past five years has produced processing techniques that allow the detectors to be manufactured in high-volume silicon semiconductor production facilities. Figure 3. These 100- and 150-mm silicon wafers contain detectors with active areas from 1 to 9 mm2. Figure 3 shows 100- and 150-mm silicon wafers containing detectors with active areas from 1 to 9 mm2. Die can be automatically tested at the wafer level and then diced and packaged as a back-end, postprocess step. The detectors’ production takes advantage of the inherent high production quality, automation and batch processing techniques found in modern semiconductor manufacturing. Another benefit of the technology is the ability to form large-area arrays. Although the compact form factor of silicon photomultipliers clearly has an advantage over that of existing photomultiplier tubes — a key feature in point-of-care and point-of-use applications — the technology also must compete with much larger-area photomultiplier tube detectors. Packaging techniques are being developed that will provide detector areas that can be scaled to much larger sizes.4 These include flex, glass and ceramic packaging methods. Figure 4 shows an array packaging solution that uses glass technology to enable the manufacture of pixelated and nonpixelated arrays of silicon photomultiplier detectors. These technologies are scalable to large detection areas for applications requiring meters of active area detection, such as nuclear medicine, radiation detection and security applications.Figure 4. Glass technology can allow the manufacture of pixellated and nonpixellated arrays of silicon photomultiplier detectors.The applications for the silicon technology broadly fall under two categories: high-performance single detectors and large-area arrays. For single high-performance detectors, a key application is microscopy in which the driving factor behind the adoption of the technology is its high quantum efficiency and low excess noise. This combination is required to represent high-contrast scenes accurately — for example in fluorescence microscopes — and it improves images overall.In large-area detection, a key driver of the technology is nuclear medicine; for example, positron emission tomography (PET), which can be used for cancer screening. In a typical PET scanner, the entire body is surrounded by a ring of detectors that detect the light emission of coincident high-energy (511 keV) photons, which result from positrons emitted from radioisotopes that interact with electrons inside the body. These high-energy photons are converted to visible photons by scintillating crystals placed in a ring around the patient. The silicon photomultiplier detectors distinguish visible photon pulses, on the order of 50 ns long, and they must determine both the number of photons arriving on the detector and the photon time stamp to create an image of the cancerous tumor.These and other application areas are driving silicon photomultiplier technology to higher performance and to compatibility with existing photomultiplier tube systems. The technology allows high-volume and high-performance silicon detectors to be manufactured with performance comparable to that of photomultiplier tubes but without many of the negative aspects of the vacuum tube.Just as the silicon transistor replaced the vacuum tube in electronics, silicon photomultiplier technology is replacing photomultiplier tubes in existing applications and is a key enabler for next-generation low-light detection systems.Meet the authorCarl Jackson is the chief technology officer at SensL in Cork, Ireland.References1. E. Flyckt and C. Marmonier, eds. (2002). Photomultiplier Tubes: Principles & Applications.2. V. Saveliev (2004). The recent development and study of silicon photomultiplier. NUCL INSTRUM METHODS PHYS RES A, pp. 528-532.3. A.G. Stewart et al (2007). Performance of 1 mm2 silicon photomultiplier, IEEE J QUANTUM ELECTRON, in press.4. P.J. Hughes et al (Feb. 14, 2007). Tiled silicon photomultipliers for large-area low-light sensing applications. PROC SPIE, Vol. 6471, 2007.5. D.J. Herbert (2007). Performance optimization of silicon photomultipliers. Presented at IEEE Nuclear Science Symposium, Oct. 27 to Nov. 3, 2007, Honolulu.