CERN Accelerating science

Theses
Title	Double-sided 3D silicon detectors for the high-luminosity LHC
Author(s)	Köhler, Michael (Freiburg U.)
Publication	207.
Thesis note	PhD : Freiburg U. : 2011
Thesis supervisor(s)	Jakobs, Karl
Note	Presented 12 Sep 2011
Subject category	Detectors and Experimental Techniques
Accelerator/Facility, Experiment	RD50 CERN SPS
Abstract	To extend the physics potential of the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research, a luminosity upgrade is planned. The HL-LHC (High-Luminosity LHC) is foreseen to start operation approximately in the year 2024. The peak instantaneous luminosity will be increased by a factor of five compared to the design specification of the LHC. The increased track density requires a finer segmentation of the detectors employed to investigate the particle collisions. Over the projected lifetime of the HL-LHC, the tracking detectors will have to withstand a five to ten times higher radiation dose than that at the LHC. In silicon detectors, radiation damage increases the leakage current, the effective doping concentration and the charge carrier trapping probability. These effects lead to a decrease of the signal-to-noise ratio after high radiation fluences. As the silicon detectors currently installed in the LHC experiments are not expected to be sufficiently radiation tolerant for the HL-LHC, novel detector technologies are under study. In the current ATLAS detector at the LHC, planar n-in-n silicon pixel detectors and planar p-in-n silicon strip detectors are used. For the ATLAS upgrade, planar silicon n-in-p detectors are foreseen for the region to be equipped with strip detectors. In the inner pixel layer, which is closest to the interaction point, the detectors will have to withstand an unprecedentedly high radiation fluence of 2x10^16 n_eq/cm^2 (1 MeV neutron equivalent particles per square centimetre). An option for extremely radiation hard detectors are 3D detectors with columnar electrodes etched into the substrate perpendicular to the surface. In contrast to traditional planar detectors, where the electrodes are limited to the detector surface, the electrodes of 3D detectors extend into the third dimension, i.e. into the detector depth. In 3D detectors, the distance for drift of generated charge carriers and for depletion is given by the spacing between columnar electrodes of opposite doping types rather than by the detector thickness as in planar detectors. Therefore, enhanced radiation hardness is expected due to reduced trapping and a reduced depletion voltage, while the total ionised charge is determined by the substrate thickness. As a simplification of the original 3D detector design, double-sided 3D detectors have been developed. The electrodes pass through the substrate only partially, which increases the mechanical stability and simplifies the fabrication technology. In this thesis, the performance of double-sided 3D detectors is investigated in detail for the first time. The measurements were performed with strip detectors: on one side of the sensors, the columnar electrodes are connected to 4-8 mm long strips. The response of the detectors to high-energy pions, electrons emitted by a beta source and an infrared laser is studied. Special emphasis is put on signal measurements as a function of the particle's point of incidence. Also, detailed noise measurements were conducted. In order to investigate the radiation hardness of the detectors, they were irradiated with protons up to fluences that are expected for the HL-LHC inner pixel layers. The measurements were performed before any radiation-induced modification of the detector properties and after irradiation to different fluences. The dependence of the detector performance on the radiation fluence was measured separately with 3D detectors in n-in-p and in p-in-n layout. A comparison of the radiation hardness of the two designs is presented. Furthermore, the radiation hardness of planar n-in-p detectors is studied and compared to double-sided 3D detectors. A focus of this thesis is the investigation of charge multiplication effects, which can occur in the presence of high electric fields and which enhances the measured signal. While multiplication of the liberated charge carriers does not occur in conventional silicon tracking detectors before any radiation-induced modification of the detectors, it was recently observed in highly irradiated detectors. The high electric fields present in 3D detectors lead to an enhanced charge multiplication probability. Implications of charge multiplication on the detectors' signal and noise are studied.

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