The ALICE-ITS upgrade requires 50 µm thin CPS covering about 10 m² detection areas. These CPS should have higher read-out speeds and radiation tolerance than those already achieved in the Pixel Detector (PXL) equipping the STAR experiment at RHIC/BNL. These requirements are the main motivation to migrate from a 0.35 µm process used for the development of MIMOSA28 dedicated to the STAR-PXL detector to a smaller feature size process. A 0.18 µm process, based on a high resistivity epitaxial layer of sizeable thickness, has therefore been chosen for these new devices. [Ref. TWEPP 2013, Id: 62]
After validation of the process, two sensor architectures, called MISTRAL and ASTRAL, are being developed at IPHC. MISTRAL is derived from MIMOSA28 sensor. It is based on a column parallel read-out with amplification and correlated double sampling (CDS) inside each pixel. Each column is terminated with two high precision discriminators in order to read out 2 rows simultaneously. The matrix is read out in a rolling shutter mode (200ns/2-rows). The discriminator outputs are processed through an integrated zero suppression logic (SUZE02). With this mature architecture, MISTRAL, with its ~1.3x3 cm² sensitive area, is currently developed to suit both the inner and outer layers of the ITS. The MISTRAL sensor for the inner layers will provide a single point resolution of ~5 µm with a power consumption of ~185 mW/cm² and a readout frequency of ~25 k frame/s. In order to reduce the power density for the outer layers, the MISTRAL sensor for the outer layers will rely on larger pixels to provide a single point resolution of ~10 µm. The power consumption diminishes to ~100 mW/cm² while the readout frequency increases to 40-50 k frame/s.
ASTRAL provides a further improvement in terms of power consumption and readout speed. It integrates signal discrimination inside each pixel. As a consequence, the analogue signals driving over centimetre long traces are replaced by the digital signals. The ASTRAL sensor is proposed for the innermost layers with a single point resolution of ~5 µm. Its expected power consumption is ~85 mW/cm² with a read-out frequency of ~50 k frames/s.
The first prototypes of FSBB-M and FSBB-A were designed and fabricated. Their tests with X-ray and beta sources will start in June 2014.
This contribution will discuss in details the design of both sensors and summarise their laboratory test results.