Development of n-in-p silicon planar pixel sensors and flip-chip modules for very high radiation environments

In this paper we present R&D of n-in-p pixel sensors, aiming for a very high radiation environment up to a fluence of 1016 neq/cm2. To fabricate these sensors, two batches with different mask sets were employed: the first resulted in pixel sensors compatible with the ATLAS pixel readout frontend chip called FE-I3, and the second in FE-I3 and a new frontend chip, FE-I4, compatible sensors; small diodes were employed to investigate the width from the active diode to the dicing edge and the guard rings. Tests involving the diodes showed that the strong increase of leakage current was attributed to the edge current when the lateral depletion zone reaches the dicing edge and the lateral depletion along the silicon surface was correlated with the ‘field’ width. The onset was observed at a voltage of 1000 V when the width was equal to ∼400 μm. The pixel sensors that were diced at a width of 450 μm could successfully maintain a bias voltage of 1000 V. Hybrid flip-chip pixel modules with dummy and real chips were also fabricated. Lead (PbSn) solder bump bonding proved to be successful. However, lead-free (SnAg) solder bump bonding requires further optimization.