Fabrication and Testing of AC-coupled Low-Gain Avalanche Diodes at BNL

We present the fabrication and the first functional tests on a new class of silicon devices: the AC-coupled Low-Gain Avalanche Diodes (AC-LGAD). Because of its good timing performance, the LGAD was originally developed to support the silicon tracker at High-Energy Physics experiments for the reconstruction of pile-up events originating from a same bunch crossing. Pile-up events happen in fact at slightly different times and adding a fourth dimension (the time) is crucial to resolve them. On the other hand, LGADs suffers from poor spatial resolution. AC-LGADs offer a solution to this drawback, while retaining the good timing performance. Test structures have been fabricated at BNL, using the internal silicon processing facility, and have been tested with fast electronics and radioactive sources. Large gains and fast signal have been measured, while the noise stays low and comparable to the standard LGAD case. Other wafer fabrications are ongoing to further improving performances and correct pitfalls in the very first fabrication.