Fabrication of Different LGAD-Based Devices at BNL

Abstract

Low-Gain Avalanche Diodes (LGADs) are silicon sensors developed for the fast detection of minimum ionizing particles (mips). Characterized by an internal moderate gain that enhances the signal amplitude and built on thin silicon substrates of a few tens of microns, they exhibit excellent timing performance. However, to achieve a spatially uniform multiplication a large pixel pitch is needed, preventing a fine spatial resolution. To overcome this limitation and to create a 4D detector which can simultaneously provide good time and spatial resolution, a few options are under study at Brookhaven National Laboratory in collaboration with national and international partners and abroad. The AC-coupled LGAD approach is one of them, where metal electrodes are placed over an insulator at a fine pitch, and signals are capacitively induced on these electrodes. To enhance the radiation hardness, the gain layer can be buried under a few microns of high-resistivity epitaxial layer. In another device (the Deep-Junction LGAD), both an n and p layers are buried by an epitaxial layer or by a wafer-to-wafer bonding, while standard-implanted electrodes are patterned at the top of the device. The fabrication technology must be tailored for the specific LGAD family, and a fine tuning of a few process parameters needs to be carefully studied before being applied to the silicon wafers being fabricated in clean-room. Also, for the detection of low-penetrating particles, a new structure must be used where, in particular, the sign of the implants is switched. The structure, characteristics, TCAD simulations and laboratory tests carried on during the development of these devices will be presented.