Measurement of pixel sensor capacitances with sub-femtofarad precision

Pixel sensors play an essential role in contemporary experiments in high energy physics and they also have a diverse application potential in many related fields. Their development follows the evolution of technology, materials and processing techniques coming into play with time. One of the key parameters characterizing a pixel sensor is the pixel capacitance. The pixel capacitance determines the noise performance of a pixel detector and is an important input parameter for the design of pixel read-out electronics. Its precise knowledge is notoriously difficult. We present a dedicated integrated circuit - the PixCap chip - allowing precise pixel sensor capacitance measurements. The design of the PixCap chip as well as results of the capacitance measurements achieved with silicon planar sensors, diamond sensors and silicon 3D sensors are subject of this paper. This work is motivated by the development of sensors and read-out electronics for an upgrade of the ATLAS pixel detector.