Speaker
Description
During the High Luminosity phase of LHC, up to 140 - 200 proton-proton collisions per bunch crossing will bring severe challenges for event reconstruction. To mitigate pileup effects, an extended upgrade program of the CMS experiment is expected. Among these, a new timing layer, the MIP Timing Detector (MTD), will be integrated between the tracker and the calorimeters. With a time resolution of 30-60 ps, the MTD will enable 4D vertexing, bringing significant improvements in track-to-vertex association and object identification. The MTD is composed of two subsystems based on different technologies: the Barrel Timing Layer (BTL) consists of LYSO:Ce scintillating crystals readout by SiPMs, and the Endcap Timing Layer (ETL) is made of Low-Gain Avalanche Diodes.
The LGADs are an important development in silicon detector technology. They boast advantages such as excellent time resolution and signal-to-noise ratio, rendering them crucial for next-generation detectors. LGADs can be quickly evaluated for key electrical parameters—including breakdown voltage and depletion voltage—through I-V and C-V tests. These tests enable us to assess the quality of LGADs.
Measurements were conducted on a probe station, where single probes were used to connect the pad of the LGAD test structure. A Keithley 2470 high-voltage source meter supplied voltages ranging from 0 to 320 V in 5 V increments, with a protection current set to 5 microamperes. A Keithley 6482 picoammeter recorded current values at different voltages, generating I-V curves from which breakdown voltages were derived. Similarly, using the same probe platform, C-V curves of the pads can be measured, and 1/C²-V curves can be analyzed to determine the depletion voltage of the LGADs. A comparison between our measured breakdown voltages and the factory-measured breakdown voltages distribution revealed good consistency, confirming the accuracy and reliability of our test setup. For the sensor array, a 16×16 probe card was used to connect to the sensor, with a switch matrix board developed and employed to select each LGAD channel being measured while grounding the rest of the channels and the guard ring. The switch matrix board also reads and processes data from the probe card. The Keithley 2470 high-voltage source meter supplied voltages from 0 to 200 V with non-uniform step sizes, and the total protection current was set to 500 microamperes. Current measurements were taken using the Keithley 6482 picoammeter.
In summary, this talk presents the characterization in the laboratory of the extensive arrays of LGAD prototypes of 16x16 channels and test structures for the CMS ETL, including I-V and C-V measurement experimental setup and preliminary measured results.
