Device simulation of Monolithic Active Pixel Sensors: Radiation damage effects

Vertexing for the future International Linear Collider represents a challenging goal because of the high spatial resolution required. CMOS Monolithic Active Pixel Sensors (MAPS) represent a good potential solution for this purpose. Up to now many MAPS sensors have been developed. They are based on various architectures and manufactured in different processes. However, up so far, the diode sensor has not been the subject of deep investigation. This is a cause for concern because the physical basis of such sensor is the partially depleted diode hit by minimum ionizing particles, in which the physical mechanisms for signal formation are not rigorously established. This is partly due to the presence of an important diffusion component in the charge transport. We present here simulations mainly based on the S-PISCES code, in which physical mechanisms affecting transport are taken into account. Diffusion, influence of residual carrier concentration due to the background doping level, and more importantly charge trapping due to deep levels in the active (detecting) layer are studied together with geometric aspects. The effect of neutron irradiation is studied to assess the effects of deep traps. A comparison with available experimental data, obtained on processed MAPS before or after neutron irradiation will be introduced. Simulated reconstruction of the Minimum Ionizing Particle (MIP) point of impact in two dimensions was also investigated. For further steps, guidelines for process choices of next Monolithic Active Pixel Sensors are introduced.