Development of AC-LGADs for Large-Scale High-Precision Time and Position Measurements

Low Gain Avalanche Detectors (LGADs) are thin silicon detectors with moderate internal signal amplification and time resolution as good as 17 ps for minimum ionizing particles. However the current major limiting factor in granularity is due to protection structures preventing breakdown caused by high electric fields at the edge of the segmented implants. This structure, called Junction Termination Extension (JTE), causes a region of 50-100 μm of inactive space. Therefore, the granularity of LGAD sensors is currently limited to the mm scale. This challenge could be overcome by employing AC coupled LGADs (AC-LGADs) which can provide spatial resolution on the 10‘s of um scale. This is achieved by un-segmented gain layer and N-layer, then a di-electric layer separates the N+ and the metal readout pads. This design allows for 100% fill factor with no dead regions inside the sensor. The high spatial precision is achieved by using the information from multiple pads, exploiting the intrinsic charge sharing capabilities of the AC-LGAD provided by the common N-layer. Using a focused IR-Laser scans directed either at the read-out side on the front and the bias side on the back of the sensor, several detector parameters have been investigated with the goal of optimizing the sensor design: sheet resistance, thickness of the isolation di-electric, doping profile of the gain layer, and pitch and size of the readout pads. Furthermore we will show an interpolation technique for hit reconstruction and time-of-arrival measurement based on charge sharing. The data are used to recommend a base-line sensor for near-future large-scale application like the Electron-Ion Collider or the PIONEER experiment where simultaneous precision time and position resolution is required in the tracking detectors.