PIXAM: A full scale building block prototype sensor with in-pixel discrimination and improved rolling shutter architecture for the ALICE MFT

Digital CMOS monolithic pixels sensors have been chosen for the upgrades of internal trackers of the ALICE experiment at CERN, namely MFT (Muon Forward Tracker) and ITS (Inner Tracking System). The existing mature rolling shutter mode CMOS sensor architecture needs several improvements in terms of readout speed and power dissipation in order to fulfill the requirements of ALICE upgrades. In the framework of the current ALICE-MFT project, the proposed architecture consists in one hand to integrate the discriminator inside the pixel and on the other hand to activate several rows at the same time. The prototype sensor chip presented in this paper represents a FSBB (Full Scale Building Block) ASIC which gathers all validated architecture concepts in a large size chip of 10 mm × 6.9 mm (~1/3 of final sensor size). This chip, named PIXAM (PIXel sensor for Alice Mft), includes in each pixel of 25 μm × 25 μm: charge collection diode, amplification, CDS (Correlated Double Sampling) circuitry, and auto-zeroed discrimination. A digital sequencer generates and distributes the timing sequence needed for the pixels. In addition, zero suppression block has been designed on this chip to allow the data compression by keeping only addresses and data of hit pixels. The digital part has been hardened against SEU effects. A SPI slow control unit permits to program reference and bias values, configuration registers and the digital timing sequence.