Exploring the Interpad Gap Region in Ultra-Fast Silicon Detectors: Insights into Isolation Structure and Electric Field Effects on Charge Multiplication

We present an in-depth investigation of the interpad (IP) gap region in the ultra-fast silicon detector (UFSD) Type 10, utilizing a femtosecond laser beam and the transient current technique (TCT) as probing instruments. The sensor, fabricated in the trench-isolated TI-LGAD RD50 production batch at the FBK Foundry, enables a direct comparison between TI-LGAD and standard UFSD structures. This research aims to elucidate the isolation structure in the IP region and measure the IP distance between pads, comparing it to the nominal value provided by the vendor. Our findings reveal an unexpectedly strong signal induced near p-stops. This effect is amplified with increasing laser power, suggesting significant avalanche multiplication, and is also observed at moderate laser intensity and high HV bias. This investigation contributes valuable insights into the IP region's isolation structure and electric field effects on charge collection, providing critical data for the development of advanced sensor technology for the Compact Muon Selenoid (CMS) experiment and other high-precision applications.

Automated summary

We conducted an intensive investigation of the interpad (IP) gap region in the ultra-fast silicon detector (UFSD) Type 10 using a femtosecond laser beam and the transient current technique (TCT). By comparing TI-LGAD and standard UFSD structures, we aimed to clarify the IP region's isolation structure and measure the IP distance between pads, subsequently comparing it with the nominal value provided by the vendor. Our findings unexpectedly revealed a strong signal near p-stops, which exhibited significant avalanche multiplication with increasing laser power. This study provides valuable insights into the IP region's isolation structure and electric field effects on charge collection, offering critical data for the development of advanced sensor technology for the Compact Muon Selenoid (CMS) experiment and other high-precision applications.