Annealing and Radiation Hardness Studies of p-type Silicon Diodes for CMS HGCAL under HL-LHC Fluence and Annealing Conditions

Sammanfattning

To handle the higher levels of radiation expected from the 10-fold increase in integrated luminosity during the High Luminosity LHC, the endcap calorimeters of the CMS detector will be replaced by the High Granularity Calorimeter (HGCAL). Hexagonal silicon pad sensor, fabricated on 8-inch p-type silicon wafers with active thicknesses of 300µm, 200µm, and 120µm, will cover 620m2 of the HGCAL, resulting in more than 6M silicon readout channels. The remaining wafer space from the hexagonal cut hosts small test structures for quality assurance and radiation hardness studies. A previous annealing study investigated silicon diodes exposed to fluences corresponding to the end of the HL-LHC of 2·1015 to 1.5·1016 neq/cm2. In a more realistic operational scenario, the detector accumulates fluence over 10 years with a two-year maintenance break in between, corresponding to Long Shutdown 4, which allows the sensors to anneal. In this study, diodes were initially exposed to one quarter of the final fluence, followed by in between annealing at 20.5◦C, 40◦C, and 60◦C, before further irradiation to reach fluences comparable to the previous study. Charge collection efficiency, saturation voltage, and leakage current damage rate are compared between the two studies to investigate the effect of the in between annealing. Results show that in between annealing of the first quarter of the fluence generally improves the electrical characteristics of all diodes across all temperatures. Thickness and fluence dependencies are observed in the charge collection efficiency and saturation voltage. Thicker diodes exhibit a more substantial reduction of the beneficial effect with increasing fluence, whereas thinner diodes show no significant impact from the in between annealing. As a result, this study improves the understanding of the expected behaviour of silicon sensors in the HGCAL during the HL-LHC phase.