Jacopo Bronuzzi
This thesis aims at demonstrating a novel technique for the characterization of interfaces obtained by a CMOS-compatible Surface Activated Bonding (SAB) process between silicon wafers. This enables the optimization of the two main components of monolithic silicon detectors, the CMOS circuitry for the read-out and the sensing layer, by fabricating them in different substrates and then by bonding them together. Therefore, to be collected by the read-out circuitry, charges generated by radiation in the bulk have to traverse the bonding interface, whose electrical properties need to be characterized.
The first part of this thesis is focused on the evaluation of Transient Current Technique (TCT) for this purpose. TCT is largely used for the study of radiation damage in silicon detectors, and consists in the injection of a localized cloud of electrons inside a detector based on a reverse-biased diode, that is drifted by the electric field. A transient current signal is generated, whose shape is related to the electric field profile that may be affected by lattice defects generated by radiation. In this context, the bonding process is expected to generate a thin amorphous silicon interface between the two bonded substrates. This layer can be seen as full of defects and therefore it is expected to influence the electric field, and the TCT current signal. This is demonstrated by means of Sentaurus TCAD numerical simulations and an analytical model, using a diode with the bonding interface in the middle of the bulk as test structure.
The second part of the thesis describes the characterization of the interface, generated by bonding high resistivity wafers at CEA-Leti in Grenoble. For this purpose, Schottky diodes are fabricated on these stacks at EPFL, and then characterized with CV, IV and TCT techniques. The results obtained are compared to simulation data, to show that the electric field did not extend to the bulk, preventing charges to be collected. This is an issue for the fabrication of radiation detectors, since there would not be collection of charges generated at the sensing bulk. Following these conclusions, two solutions are proposed. First, the optimization of the bonding process to reduce the number of traps. Second, a modification of the detector design in such a way that the bonding interface is located at the PN junction, since the electric field is maximum at this position, and therefore the influence of traps is less important.
The last part of this thesis is devoted to the description of a new charge injection technique for TCT measurements. Instead of using a laser, charges are injected by means of nanosecond voltage pulses, applied to dedicated wells fabricated on the PN junction contact. Injection occurs by thermionic emission, while the charges drift, as in standard TCT measurements. This novel method of charge injection is called electrical injection TCT (el-TCT). It could allow to perform on-line TCT measurements during experiments.
EPFL2018
