Characterization of the first full-size production for ePIC TOF layers

TL;DR

The paper investigates AC-LGADs as a solution to the high granularity and power challenges in fast-timing detectors for ePIC TOF and future e+e− facilities. It reports the first full-size HPK production for the ePIC TOF layer, characterizes both strip and pixel AC-LGAD devices via IV/CV and laser TCT, and quantifies yield, gain-layer uniformity, N+ resistivity, and dynamic charge-sharing behavior. The results show ~80% yield for 30 μm strips and ~90% for pixels, with the best geometry (50 μm thick, 500 μm pitch, 50 μm strips) achieving timing jitter below 20 ps and position jitter around 30 μm; S/N degrades with pitch. These findings demonstrate the viability of AC-LGADs for TOF applications and provide concrete design guidance for balancing fill factor, charge sharing, and readout complexity in future collider detectors.

Abstract

Low-Gain Avalanche Detectors (LGADs) are characterized by a fast rise time (500 ps) and extremely good time resolution (down to 17 ps). The intrinsic low granularity of LGADs and the large power consumption of readout chips for precise timing are problematic in near-future experiments such as e+e- Higgs factories (FCC-ee) and the ePIC detector at the Electron-Ion Collider. AC-coupled LGADs, where the readout metal is AC-coupled through an insulating oxide layer, could solve both issues at the same time thanks to the 100% fill factor and charge-sharing capabilities. Charge sharing between electrodes allows a hit position resolution well below the pitch/$\sqrt12$ of standard segmented detectors. At the same time, it relaxes the channel density and power consumption requirements of readout chips. Extensive laboratory characterization of AC-LGAD devices from the first full-size (up to 3x4 cm) production from HPK for ePIC will be shown in this contribution. Both pixel and strip geometry was produced and tested. This study was conducted within the scope of the ePIC detector time of flight (TOF) layer R&D program at the EIC.

Figures (9)

• Figure 1: Picture of a full-size strip sensor (Left), full-size pixel sensor (center) and test structures (Right).
• Figure 2: Current over voltage (IV) of one 30 $\mu$m wafer of strip detectors.
• Figure 3: Left: Linear fittings of $\mathrm{1/C^2}$ to find the gain layer depletion $\mathrm{V_{GL}}$. Right: variation of $\mathrm{V_{GL}}$ across a wafer (X-Y) for 50 $\mu$m strip detectors.
• Figure 4: Left: Pmax as a function of laser injection position perpendicular to the strip. Right: rise time of the pulse vs laser position.
• Figure 5: Left: time of 50% of the Pmax vs laser position perpendicular to the strip. Right: time of 50% of the Pmax vs laser position parallel to the strip, strip is connected on the left side.