Silicon Photomultipliers (SiPM) are very promising devices for high energy physics (HEP) experiments due to their high photon detection effciency, miniaturized device size and insensitivity to high magnetic fields. Most often detectors are exposed to a high radiation dose for which reason the performance should degrade only minor under the applied radiation load. Decreasing the active depth of a SiPM microcell should help to strengthen the radiation hardness. Additionally for high energy particle physics experiments a large dynamic range is mandatory. This was a further driving reason at KETEK to scale down the microcell pitch and thereby losing only small amount in geometrical efficiency. With these large dynamic range SiPMs a photon detection efficiency in blue spectral range of 32% for 2500 microcells=mm<sup>2</sup> and 22% for 4400 microcells=<sup>mm2 </sup>was achieved. With an improved fabrication technology the dark noise level was decreased to about 250 kHz=mm<sup>2</sup> at 20% overvoltage, while the gain variation was still less than 1%=K. Further optimization of the depleted region increased the sensitivity in the output wavelength range of common scintillators (515 nm) by 20% compared to the standard devices. The performance of the KETEK SiPMs will be discussed in detail.
Silicon photomultipliers (SiPMs) are intensely evaluated as a potential replacement of photomultiplier vacuum tubes for several applications. Essential key features are the photon detection efficiency, the dark count rate, the optical crosstalk and the scalability of the active area and the microcell pitch.
In order to achieve considerable improvements of these parameters, KETEK has introduced a new manufacturing technology based on 200 mm wafers and 0.35 μ stepper lithography. Important aspects of the well-established and for many years optimized KETEK Silicon Drift Detector technology could be transferred to the SiPM process. Main items of the new technology are a narrow vertical trench around the individual microcells and an impurity getter: the first reduces the optical cross talk and the second the dark count rate by factor two, whereby the potential of this technology is still not maxed out.
A further aspect of the new technology is a low parasitic RC-value device concept: The reduction of parasitic RC-values is targeted on a scalability refinement of SiPM devices which is mandatory for active areas above 10 mm<sup>2</sup>. Finally the geometrical fill factor and the light entrance window of the KETEK device has been further improved for which reason a 50 μm cell pitch device with a photon detection efficiency of 60% in the blue range is presented. Beyond that the SiPM devices show an extremely low temperature coefficient of the gain. This is due to an operation at very high overvoltage along with a low temperature coefficient of the break down voltage.
The Silicon Photomultiplier (SiPM) is a novel device for low level light detection in various applications, for example scintillator- and fiber readout.<sup>1;2</sup> The SiPM is insensitive to magnetic fields and has a high photon detection effciency. Current devices have a high optical crosstalk probability, which causes a significant increase of the excess noise factor.<sup>3</sup> It may replace traditional Photo Multiplier Tubes (PMT) when the optical crosstalk is reduced to a lower level of below 10%. Depending on the quantity of hot electrons in the Geiger discharge approximately three to fifty secondary photons (in average three photons per 10<sup>5</sup> avalanche electrons<sup>4</sup>) with a wavelength range from 450nm to 1600nm are emitted from the excited cell in all directions.<sup>5</sup> Some of those secondary photons cause the discharge of the neighboring cell.<sup>6;7</sup> The different mechanism of optical crosstalk are categorized as direct and indirect crosstalk. To reduce direct crosstalk an optical barrier has to be implemented between the single micro cells.<sup>8</sup> Thus, we have investigated different technological concepts with regard to the trench shape, the trench etching process as well as the trench fill material.
Requirements like device miniaturization, insensitivity to magnetic field and cost aspects in the field of low level light detection will lead to a replacement of the conventional photomultiplier tube by Silicon Photomultiplier (SiPM) for several applications in case the photon detection efficiency will be comparably higher at the same price level. This novel solid-state sensor consists of an array of parallel connected avalanche photodiodes
operated in limited Geiger-mode. The triggered cells are recovered by an upstream connected quenching resistor.
The main characteristics are gain, noise, photon detection efficiency (PDE), dynamic range and time resolution.
To meet the requirements of various potential applications, SiPMs need to be available with several micro pixel
sizes and total active areas. For this reason KETEK produces devices with microcell pitches from 15μm up to
100μm and total active sensor areas from 1.0 x 1.0 mm<sup>2</sup> up to 6.0 mm x 6.0 mm<sup>2</sup>. The effects of this scaling on the SiPM device parameters are discussed.