Precision Measurement of Pixel Sensor Capacitance

TL;DR

This paper presents a new integrated circuit for precise pixel capacitance measurement in high-granularity sensors, aiding the optimization of front-end electronics for applications like the HL-LHC upgrade.

Contribution

Development of a dedicated capacitance measurement chip (Pixcap65) and a charge-based method for accurate pixel capacitance evaluation.

Findings

Measurement of pixel capacitance influenced by implant geometry variations.

Implementation of a charge-based capacitance measurement method.

Successful demonstration on a planar pixel sensor.

Abstract

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 $50 \times 50 um^2$ 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 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.