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 /spl mu/m 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 /spl mu/m/sup 2/ 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 /spl mu/s frame period is needed. Device description, hit resolution and irradiation results are presented.