Simulation and measurements of charge collection in monolithic active pixel sensors

A preliminary study of the charge collection in the recently proposed Monolithic Active Pixel Sensor devices for minimum ionising particles tracking is presented. The baseline pixel architecture is similar to a visible light CMOS camera, emerged as a competitor to widespread CCDs. Free electrons created by an impinging particle are collected by a photodiode from a thin partially depleted epitaxial silicon layer allowing 100% of fill-factor. Such a structure is fabricated using standard CMOS process. The sensor and associated readout electronics are integrated onto the same wafer, resulting in a low cost, high resolution and possibly thin detector. The crucial points of a CMOS detector are the time and the efficiency of the charge collection. These factors, in spite of undeniable advantages of CMOS detectors, can limit their performances. The detailed 3-D simulations using commercially available ISE-TCAD package are carried out in order to study a charge collection process. Although it is dominated by thermal diffusion, more than 1000 electrons are collected in the 3×3 pixels cluster within a time of 100 ns in a 15 μm thick epitaxial layer. Simulation results are compared to measurements performed on the prototype APS CMOS MIMOSA using either a fast Infrared (IR) laser or a high-energy particle beam as an excitation source.