A prototype SOI pixel sensor for CEPC vertex

A small prototype SOI pixel sensor with in-pixel discriminator and binary readout has been developed as a part of R&D for the Circular Electron Positron Collider (CEPC). The sensor featuring 16 μm pitch was designed to achieve a single point resolution better than 3 μm. The design was submitted to a Multi Project Wafer and wafers were thinned to 75 μm before dicing for delivery. A fully-depleted substrate resulting in a large charge signal is important for the sensor performance, because such small pixel size can only accommodate simple amplifier and discriminator logic. 55Fe source tests have proven that the thinned chips are functional and their substrates are fully depleted at about −35 V bias voltage. Even though a large threshold dispersion of 114 e− was observed, the temporal noise is as small as 6 e−. Furthermore, a single point resolution of 2.3 μm has been demonstrated in the lab test with an infrared laser beam. Lessons learned from this prototype are important feedback to improve the next design.