Transient Current Technique characterization of bonded interfaces for monolithic silicon radiation detectors

Jacopo Bronuzzi
EPFL, 2018
Thèse EPFL

Résumé

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.

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https://infoscience.epfl.ch/record/260458?ln=fr

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Jacopo Bronuzzi
EPFL, 2018
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/260458?ln=fr

À propos de ce résultat

Concepts associés (16)

Concepts associés

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Publications associées (12)

Publications associées

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When exposed to ionising radiation, living tissue can potentially suffer somatic and genetic damage - effects depending mainly on the radiation dose or energy absorbed, the type of radiation, and the type and mass of cells affected. It is well known that large doses of radiation lead to high damage of the cell nucleus and additional cell structures, which results in harmful somatic effects, and even rapid death of the individual exposed, while at low doses, cancer is by far the most important possible consequence. Understanding the mechanisms by which low doses of radiation cause cell damage is thus of great significance, not only from this viewpoint but also from that of practical medical physics applications such as radiotherapy treatment planning. Ionising radiation, such as electrons and positrons, begins to cause damage to the genome of a living cell by direct ionisation of atoms, thus depositing energy in the DNA double helix itself. The energy threshold for inducing strand-breaks by electrons, however, is around 7 eV, well below the energy levels required for direct ionization. The low-energy electrons that are set in motion around the tracks of energetic charged particles, for example, are responsible for a multitude of low-energy events (energy transfer of the order of 10 eV), which play a significant role in inducing molecular damage. Assessing the spatial configuration of energy transfer events and the deposited energy spectra, in regions of cellular and sub-cellular dimensions, can be aimed at via the application of appropriate Monte Carlo simulation tools. Such calculations depend primarily on an accurate knowledge of the production and subsequent slowing down of secondary electrons that form the basic structure of the charged particle track. In the above context, an important requirement is the provision of detailed quantitative information concerning the interaction cross sections of electrons over an energy range extending down to low energies, i.e. including the sub-excitation domain. In addition, developing fast particle-transport simulation algorithms to cover the entire slowing down process efficiently is a key aspect. Thus, the present doctoral research has, as global goal, the development and validation of new Monte Carlo calculational tools for electron and positron transport in biological materials, both at high and low energies. More specifically, it aims at providing (i) a comprehensive and accurate set of appropriate cross section data, and (ii) a fast and reliable algorithm for the simulation of charged particle transport. Thus, the first part of the thesis concerns the assessment, further development and validation of standard theoretical models for generating electron and positron cross sections to cover the main interactions of these particles with matter, in particular with the basic atomic components of biomaterials (water, bio-polymers, etc.). This has been done for bremsstrahlung, and both elastic and inelastic scattering, considering a wide range of atomic numbers and high up to thermal incident particle energies. In particular, the excitation cross sections for medium and low energies (down to 1 eV) have been derived by using a new formalism based on many-body field theory. The accuracy of the presently obtained data sets are assessed against other theoretical models, as also a large experimental database for each type of interaction, so that both a comprehensive coverage and adequate accuracies have been ensured for the cross section data sets generated. In the second part of the thesis, an extension of the Monte Carlo code system PENELOPE is first undertaken such that use can be made of elastic scattering differential cross sections which have been made available in numerical form. Thereby, new computational routines (incorporated into the new code PENELAST) prepare the cross sections, needed for a given energy and scattering angle, by applying a fast and accurate sampling technique to a provided data set. The present development will allow various electron and positron cross section data libraries, appropriately formatted, to be used with PENELOPE for benchmarking purposes. The development of a high calculation-speed (Class I) Monte Carlo tool for charged particle transport in biological materials has then been addressed. Thereby, a numerical algorithm for calculating the multiple-scattering angular distributions of high energy electrons and positrons is developed, based on the multiple-scattering theory of Lewis which accounts for energy losses within the continuous slowing down approximation. Partial-wave elastic scattering differential cross sections made available in numerical form, as indicated above, are used for the calculations, the inelastic scattering differential cross sections being obtained from the Sternheimer-Liljequist generalized oscillator strength model implemented in PENELOPE. The new code LEWIS has been used to calculate multiple-scattering angular distributions for given path lengths and can be readily adopted for Class I Monte Carlo simulations. The simultaneous generation of a large number of Legendre expansion coefficients is rendered possible, both rapidly and accurately. Results from LEWIS have been found to be in satisfactory agreement, both with detailed simulations carried out using PENELAST and with various sets of experimental data for high to medium energy electrons. In brief, the present research represents a significant improvement in the quality of Monte Carlo modelling of charged particle slowing down processes, thus contributing to understanding the. role of low-energy secondary electrons in radiation protection studies. It will also allow the further development of a complete Class I Monte Carlo code, which can then be reliably used in practical applications such as radiation treatment planning.

EPFL2005

Chargement

Analysis of Femtosecond Electron Bunches at the SwissFEL Injector Test Facility

Bennie Smit

The SwissFEL (Swiss Free Electron Laser) is a free electron laser which is being built at the Paul Scherrer Institute. The SwissFEL will be a user facility to study processes on unexplored scales of space and time. In order to achieve these aims, X-ray pulses on the order of femtoseconds must be generated. For such short X-ray pulses, the electron bunch length must be comparable to the final X-ray pulse length. In order to achieve such a short electron bunch length, it must be monitored at every compression stage. This work proposes two novel methods to measure such electron bunches, namely a novel spectral method that analyses the frequency content of electron bunches that becomes easier with shorter bunch length and a transverse deflecting cavity method which can conceptually measure a zero bunch length. Because the SwissFEL was not built at the time of the PhD, all experiments were performed at the SwissFEL Injector Test Facility (SITF). The SITF is a 200 MeV test bed linear electron accelerator. Theoretically the bunch length is limited due to the uncorrelated energy spread and the higher order components of the bunch compressor. However, due to the limited electron energy in combination with the short electron bunches, space charge effects dominate the beam in the transverse phase space as well as in the longitudinal phase space. A transverse deflecting cavity (TDC) measurement was used to measure a reference bunch length to which the spectral fluctuations could be correlated. In a TDC the electric fields are oriented perpendicular to the beam direction and streaks the head and tail in opposite directions onto a screen. In a conventional TDC measurement, the projected beam size onto the temporal axis is the current profile, provided that the intrinsic beam size is small. However, in the SITF the intrinsic beam size was large due to the aforementioned space charge effects. This led to the development of a novel bunch length analysis method in a space charge dominated beam. In the proposed analysis method, a dispersive section in combination with a TDC was used to visualise the longitudinal phase space. The longitudinal phase space was sliced in the momentum domain to reveal the difference between the intrinsic and the streaked beam in a momentum slice. The bunch length could be extracted by recording the times for all slices. Both the simulations and experiments suggest the method enables the measurement of bunch length in a space charge dominated beam in which the transverse intrinsic beam size is large. The measured minimal bunch length from simulations and experiments are of the same order, 19.5 fs to 24.5 fs, respectively. Since the TDC at PSI has a specified resolution of 20 fs at 200 pC, this method may be used to reduce the resolution. The spectral content of femtosecond electron bunches contains electron bunch length information. As an electron beam collides with an optical transition radiation (OTR) foil, radiation is emitted. The radiation is collected by means of a mirror setup into an optical fibre and into an optical spectrometer. The spectral range of the spectrometer is 316 THz (950 nm) to 1364 THz (220 nm). From OTR theory a flat OTR spectrum was expected and measured. As the electron bunch length was reduced below 70 fs, periodic fluctuations started to appear on the spectra. These fluctuations broaden with decreasing bunch length and show a good agreement with the bunch length measured with the TDC.

EPFL2015

Chargement

Carbon/Hybrid Perovskite Interfaces: From Light Emission to Radiation Detection

Pavao Roko Andricevic

In this dissertation, carbon is proposed as an alternative electrode material to interface with semiconducting organic-inorganic metal halide perovskites in various optoelectronic applications. Carbon electrodes may offer, good device performance, while providing improved stability due to their inert nature. The focus will be on a special architecture of carbon nanotubes, called the vertically aligned carbon nanotube forest (VACNTs), in which the CNTs are self-assembled into a vertically oriented array during growth on a substrate. I start discussing, the unexpected discovery, namely, that single crystals of methylammonium lead perovskites (MAPbBr3 and MAPbCl3) grow directly on these VACNT forests. The peculiarity is that fast-growing single crystals engulfed the individual CNTs, as protogenetic inclusions, resulting in a three-dimensionally enlarged photosensitive interface. Sensitive photodetector devices were fabricated utilizing this ¿exotic¿ junction, detecting low light intensities (< 20 nW) from the UV range to 550 nm. Moreover, a photocurrent was recorded at zero external bias voltage, which points to a plausible formation of a p-n junction of the MAPbBr3 single crystal and VACNT forest interface. While, attempting to push the I-V characteristics of these photodetectors to the limits, we were granted with another unexpected discovery, the bright green electroluminescence of these devices, observed at room temperature. Under an applied electric field, charged ions in the crystal drifted and accumulated near the electrodes, resulting in an in operando formed p¿i¿n heterojunction. The decreased interface energy barrier and the strong charge injection due to the CNT tip enhanced electric field, resulted in a bright green light emission up to 1800 cd m-2 at room temperature. Furthermore, the possibility of designing perovskite-based ultrasensitive, low cost detectors for high-energy radiation was demonstrated. MAPbBr3 single crystal ¿-ray detectors, equipped with carbon electrodes, were fabricated allowing radiation detection by photocurrent measurements at room temperature with record sensitivities (333.8 µC Gy-1 cm-2). Importantly, the devices operated at low bias voltages (< 1.0 V), which may enable, future low-power operation in energy-sparse environments, including space. The detector prototypes were exposed to radiation from a 60Co source at dose rates up to 2.3 Gy h-1 under ambient and operational conditions for over 100 hours, without any sign of degradation. We postulate that the excellent radiation tolerance stems from the intrinsic structural plasticity of the organic-inorganic halide perovskites, which can be attributed to a defect-healing process by fast ion migration at the nanoscale level. Since, the sensitivity of the ¿-ray detectors is proportional to the volume of the employed MAPbBr3 crystals, a unique crystal growth technique was introduced, baptized as the `oriented crystal-crystal intergrowth¿ or OC2G method, yielding crystal specimens with a volume and mass of over 1¿000 cm3 and 3.8 kg, respectively. Large volume specimens have a clear advantage for radiation detection, however, the demonstrated kilogram scale crystallogenesis coupled with future cutting and slicing technologies may have additional benefits, for instance, enable the development for the first time crystalline perovskite wafers, which may challenge the status quo of present and future performance limitations in all optoelectronic applications.

EPFL2019

Chargement

Analysis of Femtosecond Electron Bunches at the SwissFEL Injector Test Facility

Bennie Smit

EPFL2015

EPFL2005

Carbon/Hybrid Perovskite Interfaces: From Light Emission to Radiation Detection

Pavao Roko Andricevic

EPFL2019