Testbeam Studies on Pick-Up in Sensors with Embedded Pitch Adapters

For silicon strip sensors, the tracking information specifications can lead to challenging requirements for wire bonding. A common strategy is to use external pitch adapters to facilitate this step in the production of detector modules. A novel approach previously discussed in [1], is to implement the pitch adapters in the sensor, by embedding a second layer of metal tracks. The use of these embedded pitch adapters (EPAs) decouples the bond pad layout of the sensor from its implant layout by moving the adaption to the sensor production step. This solution, however, can yield the risk of performance losses due to the increase of inter-strip capacitance, or unwanted capacitive coupling between the metal layers (cross-talk) or the silicon bulk and the second metal layer (pick-up). In the prototyping stage of the ATLAS tracker end-cap upgrade, where different bond-pad layouts on sensor and readout chip lead to extremely challenging wire-bonding conditions, sensors with different geometries of EPA implementations have been produced at Centro Nacional de Microelectronica (IMB-CNM, CSIC), Barcelona, Spain. In order to study the influence of the EPA on the sensor performance, these sensors, built into a prototype detector module, were investigated in an x-ray beam. In this contribution, results of a study on the pick-up phenomenon in sensor regions with a high density of second-metal-layer tracks are presented. The performed measurements were taken in 15~$\mu$m by 10~$\mu$m steps with a micro-focussed 15~keV photon beam which allows resolution of the details of the EPA geometry. The amount of signal degradation in primary strips in the presence of second-metal tracks as well as the size of the pick-up-induced signal in the deployed test setup are quantified. Recommendations regarding the geometry of the embedded pitch adapter are given, resulting in limits and opportunities for future applications. [1] M. Ullan et al., Embedded Pitch Adapters: a High-Yield Interconnection Solution for Strip Sensors, NIM A, 2016