Development of a B-factory monolithic active pixel detector - the continuous acquisition pixel prototypes

Future vertex detection at an upgraded KEK-B Factory, currently the highest luminosity collider in the world, will require a detector technology capable of withstanding the increased track density and the larger radiation exposure. Near the beam pipe the current silicon strip detectors have projected occupancies in excess of 100%. Advances in monolithic active pixel sensors (MAPS) look very promising to address this problem. These devices are also quite attractive due to the possibility of making them very thin - essential for improved tracking and vertexing in the low momenta environment of a B-factory. In the context of the Belle vertex detector upgrade, the major obstacles to realizing such a device have been concerns about radiation hardness and readout speed. Two prototypes implemented in the TSMC 0.35 μm process have been developed to address these issues. Denoted the continuous acquisition pixel, or CAP, the two variants of this architecture are distinguished in that CAP2 includes an 8-deep sampling pipeline within each 22.5 μm² pixel. Experience with this deep sub-micron process indicates tolerable threshold voltage shifts for ionizing radiation in excess of 20 Mrad. In order to maintain low occupancy and insensitivity to radiation-induced increased leakage current, correlated double sampling with a 10 μs frame period is needed. Device description, hit resolution and irradiation results are presented.