A novel position and time sensing active pixel sensor with field-assisted electron collection for charged particle tracking and electron microscopy

A new type of active pixel sensors (APSs) to track charged particles for particle physics experiments or to count number of electrons that cross any pixel at the focal plane of electron microscopes is described. The electric field of desirable shape is created inside the active volume of the pixel introducing the drift component in the movement of the signal electrons towards charge collecting electrodes. The electric field results from the flow of ∼100 mA/cm2 hole currents within individual pixels of the sensor. The proposed sensor is produced using a standard industrially available complementary metal oxide silicon (CMOS) process. There are two main advantages of the proposed detectors when compared to the present (February 2005) state-of-the-art, i.e. field-free APS sensors. The first advantage of a field-assisted transport mechanism is the reduction of the charge collection time and of the sharing of the signal electrons between adjacent pixels by diffusion. The second advantage is the freedom to use both kinds of MOS transistors within each pixel of the sensor. Thus, the full functional power of CMOS circuits can be embedded in situ. As an example, 16-bit scalers will be implemented in each pixel of the sensor for electron microscopy. The reduced collection time combined with the state-of-the-art electronics within each pixel provides the most complete information about the position and the timing of incident charged particles for particle physics experiments. Position resolution of new sensors was computationally simulated to be a few microns, that is, the same as the resolution of standard APSs. Moreover, the active depth of the sensor and the associate electronics is less than about 20 μm and a thinned down sensor together with its beryllium backing can have a total thickness of less than 0.1% of one radiation length. The reduction of the thickness of the detector reduces the amount of multiple scattering within the detector. The determination of the momentum and of the origin of a particle can be improved in particle physics experiments and images from electron microscopes can be sharper.