Development of deep N-well monolithic active pixel sensors in a 0.13 mu m CMOS technology

By exploiting the triple-well option available in a deep-submicron CMOS process, we developed monolithic active pixel sensors (MAPS) with the unique features of full analog signal processing and digital functionality implemented at the pixel level. After briefly reviewing the results achieved with the first prototype chip, we report on the extensive measurements on the second prototype, containing both single channel sensors, with an improved noise figure, and an 8x8 pixel array. For the pixel having a collecting electrode area of 900um2 we measured an equivalent noise charge of about 40 electrons. Using the 55Fe 5.9 keV line, we obtained a Signal-to-noise (S/N) ratio of about 30. The pixel matrix (50x50um2) has been successfully readout up to 30 MHz. Through noise scans, an expected significant threshold dispersion has been measured. The measurements presented in this paper confirm the capability of our MAPS, based on the deep n-well concept, to be operated as ionizing radiation detectors and suggest a series of improvements we are already implementing in the design of the next prototype chip.