Novel SiPM development

Participation of SiPM developers in the Project framework presents an opportunity to produce small pilot detector batches of SiPMs with the unique properties required for a given application. These abilities of targeted development, field tests, corrections to the previous generation’s design, and implementation of new design cycles are almost impossible to organize by other means.

E.g., TOF-PET requires SiPMs with advanced timing properties. The SiPM size should be of about 3×3 mm2 in accordance with the size of scintillator crystals.

It is known that SiPM time resolution depends on the size of SiPM’s active area – the smaller SiPM, the better time resolution. For single pixels with sizes of one dozen microns one can obtain time resolution at the level of 40–50 ps WFHM, whereas SiPM with a 1×1 mm2 active area, consisting of, for example, 100 pixels has time resolution of about 100 ps. SiPM’s of dimensions 3×3 mm2 have single-photon timing resolutions of approximately 300–500 ps.

There are different reasons for the degradation of timing resolution with increasing area. First, the total capacity of a SiPM becomes large, leading to slower rise time of SiPM signals due to greater integration over the resistivity load. The next reason is the influence of the metallization grid for interpixel connections. Due to distributed parameters of bus lines (capacity and inductivity), these act as analog delay lines for fired pixel signals. Consequently, different pixel position signal delays are different. It can be thus shown that the configuration of the metalization grid directly influences the final time resolution of an SiPM.

One of the aims of the proposed Project is to develop and produce SiPM’s with advanced timing properties resulting from pixel interconnection grid optimization and minimization of the SiPM total parasitic capacity. In order to achieve these goals, careful simulation using the SPICE and SYNOPSYS programs will be carried out. On the basis of simulation results, the technological plan and corresponding layout will be developed and a test batch of SiPMs with advanced timing properties will be produced.

Modern physics experiments require exploitation of detectors with very large number of active channels (from hundreds of thousands to several millions). To provide the required parameters of the detector systems with adequate numbers of photo detectors, the latter must have highly homogeneous characteristics and low cost. Foreign manufacturers have already shown that industrially-manufactured SiPMs meet these requirements.

Quite often SiPMs used in physics experiments or nuclear medicine devices are produced using monolithic multi-element matrix systems, ensuring a high packing density of active detecting elements. Such a matrix should contain from a few dozens to hundreds of channels with typical individual element size as large as 3×3 mm2. The presence of dead elements in the matrix can severely compromise performance. This condition dictates stringent quality control during SiPM production.

To provide SiPM production with reproducible parameters in different batches and especially to produce multi-element monolithic SiPM matrix structures, it is necessary to enforce strict organization in the manufacture of SiPM production. Although some samples of Russian SiPMs produced in various research projects demonstrate world-record performance, due to handmade production their basic characteristics (breakdown voltage, gain, efficiency of light detection, etc.) can show significant variation from sample to sample, and the output of the finished product remains low (~30—50%). This has led, thus far, to an inability to mass-produce monolithic SiPM matrix.

For the organization of industrial SiPM production in Russia, it is necessary to undertake a complex of comprehensive development work and production of relevant technical specifications for SiPM. This will require a certain number of technological production runs to determine the stability of performance on variations in the manufacturing processes, and the allowed tolerances on variations in these processes. As a result, the SiPM manufacturing technology will be optimized and the output rate increased, permitting the creation of cost-effective industrial SiPM production, including multi-element monolithic matrix devices.