Evidence of charge multiplication in thin 25 μm × 25 μm pitch 3D silicon sensors

Characterization measurements of 25 μm × 25 μm pitch 3D silicon sensors are performed, for devices with active thickness of 150 μm. Evidence of charge multiplication caused by impact ionization below the breakdown voltage is observed in sensors operated at −45 ◦C. Small-pitch 3D silicon sensors have potential as high precision 4D tracking detectors that are also able to withstand radiation fluences beyond 10^16 neq/cm^2. This is applicable for use at future facilities such as the High-Luminosity Large Hadron Collider and the Future Circular Collider. Characteristics of these devices are compared to those of similar sensors of pitch 50 μm × 50 μm, showing comparable charge collection at low voltage, and acceptable leakage current, depletion voltage, breakdown voltage, and capacitance despite the extremely small cell size. The unirradiated 25 μm × 25 μm sensors exhibit charge multiplication above about 90V reverse bias, while, as predicted, no multiplication is observed in the 50 μm × 50 μm sensors below their breakdown voltage. The maximum gain observed below breakdown is 1.33.