Monolithic Active Pixel Sensors for high resolution vertex detectors

A novel Monolithic Active Pixel Sensor (MAPS) for charged particle tracking is presented. The partially depleted thin epitaxial layer of a low-resistivity silicon wafer is used as a sensitive detector volume from which the charge liberated by ionising particles is collected by diffusion. The sensor is a photodiode within a special structure allowing the high detection efficiency required for tracking applications. Two prototypes have been designed and fabricated using standard 0.6 and 0.35-micron CMOS processes. Results of the first prototype are presented, which is made of four arrays, each containing 64x64 pixels with a readout pitch of 20 microns in both directions. Extensive tests made with a soft X-ray source (Fe-55) and beams of minimum ionising particles (pions of 15 and 120 GeV/c) at CERN have demonstrated the predicted performance. The individual pixel noise of around 12 ENC leads to an extremely favourable signal to noise ratio for minimum ionising particles for which over 1000 electrons can be collected at the peak of the Landau distribution. The new circuit in the 0.35-micron process on a thinner epitaxial layer seems to have similar good performance. These new devices are extremely promising for future high precision vertex detectors as they provide solutions with a very low material budget and with high resolution at relatively low cost due to their fabrication in a standard submicron CMOS technology.