Precision measurement of pixel sensor capacitance

The capacitance of the charge collection node of a sensor system is an important parameter for the design of the analog front-end electronics. The analog front-end of high-granularity sensors like for example hybrid pixel detectors need to be optimized for timing resolution, power consumption, and electronics noise - parameters which all depend on the pixel capacitance. Current pixel detector developments for the HL-LHC upgrade typically use silicon sensors with a pixel size in the order of 50 x 50 mu m(2) which have a pixel capacitance of several tens of fF, depending on the sensor geometry. We have developed a dedicated integrated circuit to be bump-bonded to a pixel sensor, which enables a precise pixel capacitance measurement by using the charge-based capacitance measurement method. In this paper, we will describe the measurement method and the implementation of the capacitance measurement chip (Pixcap65) and show measurement results of a planar pixel sensor whose pixel capacitance is influenced by variations of the implant geometry.