Double Cathode Configuration for the Nb Coating of HIE-ISOLDE Cavities

The Quarter Wave Resonator (QWR) cavities for HIE-ISOLDE project at CERN have entered their ending phase of production. Some R&D; is still required to improve the uniformity of the Nb layer thickness on the cavity surface. In order to improve this behaviour one approach which has been proposed is to replace the single cathode with a double cathode and test the suitability of different deposition techniques. With this change it is possible to control the plasma and power distribution separately for the inner and outer part of cavity and thereby potentially improve film uniformity throughout the cavity and coating duration. In this study a comparison between the deposition rates obtained using a single cathode and a double cathode using Direct Current (DC)-bias diode sputtering, DC-magnetron sputtering (DCMS) and Pulsed DC-magnetron sputtering (PDCMS) is presented. The morphology of the thin film samples were compared using Focused Ion Beam (FIB) cross section milling and Scanning Electron Microscopy (SEM) analysis

Growth and characterization of gold catalyzed SiGe nanowires and alternative metal-catalyzed Si nanowires

The growth of semiconductor (SC) nanowires (NW) by CVD using Au-catalyzed VLS process has been widely studied over the past few years. Among others SC, it is possible to grow pure Si or SiGe NW thanks to these techniques. Nevertheless, Au could deteriorate the electric properties of SC and the use of other metal catalysts will be mandatory if NW are to be designed for innovating electronic. First, this article's focus will be on SiGe NW's growth using Au catalyst. The authors managed to grow SiGe NW between 350 and 400°C. Ge concentration (x) in Si1-xGex NW has been successfully varied by modifying the gas flow ratio: R = GeH4/(SiH4 + GeH4). Characterization (by Raman spectroscopy and XRD) revealed concentrations varying from 0.2 to 0.46 on NW grown at 375°C, with R varying from 0.05 to 0.15. Second, the results of Si NW growths by CVD using alternatives catalysts such as platinum-, palladium- and nickel-silicides are presented. This study, carried out on a LPCVD furnace, aimed at defining Si NW growth conditions when using such catalysts. Since the growth temperatures investigated are lower than the eutectic temperatures of these Si-metal alloys, VSS growth is expected and observed. Different temperatures and HCl flow rates have been tested with the aim of minimizing 2D growth which induces an important tapering of the NW. Finally, mechanical characterization of single NW has been carried out using an AFM method developed at the LTM. It consists in measuring the deflection of an AFM tip while performing approach-retract curves at various positions along the length of a cantilevered NW. This approach allows the measurement of as-grown single NW's Young modulus and spring constant, and alleviates uncertainties inherent in single point measurement

Thin Film (High Temperature) Superconducting Radiofrequency Cavities for the Search of Axion Dark Matter

5 pages, 6 figures. v2: minor updates after referee comments, matches published version in IEEEThe axion is a hypothetical particle which is a candidate for cold dark matter. Haloscope experiments directly search for these particles in strong magnetic fields with RF cavities as detectors. The Relic Axion Detector Exploratory Setup (RADES) at CERN in particular is searching for axion dark matter in a mass range above 30 $\mu$eV. The figure of merit of our detector depends linearly on the quality factor of the cavity and therefore we are researching the possibility of coating our cavities with different superconducting materials to increase the quality factor. Since the experiment operates in strong magnetic fields of 11 T and more, superconductors with high critical magnetic fields are necessary. Suitable materials for this application are for example REBa$_2$Cu$_3$O$_{7-x}$, Nb$_3$Sn or NbN. We designed a microwave cavity which resonates at around 9~GHz, with a geometry optimized to facilitate superconducting coating and designed to fit in the bore of available high-field accelerator magnets at CERN. Several prototypes of this cavity were coated with different superconducting materials, employing different coating techniques. These prototypes were characterized in strong magnetic fields at 4.2 K.This project has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 730871 (ARIES-TNA). BD and JG acknowledge funding through the European Research Council under grant ERC-2018-StG-802836 (AxScale). We also acknowledge funding via the Spanish Agencia Estatal de Investigacion (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) under project PID2019- 108122GB-C33, and the grant FPI BES-2017-079787 (under project FPA2016-76978-C3-2-P). Furthermore we acknowledge support from SuMaTe RTI2018-095853-B-C21 from MICINN co-financed by the European Regional Development Fund, Center of Excellence award Severo Ochoa CEX2019- 000917-S and CERN under Grant FCCGOV-CC-0208 (KE4947/ATS).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Intégration 3D de nanofils Si-SiGe pour la réalisation de transistors verticaux 3D à canal nanofil

The goal of this thesis is to build and characterize nanowire based field-effect-transistors. These FET will have either back or wrapping gate using standard CMOS process. Theses transistors will allow us to increase the integration density in back end stages of IC's fabrication and add new functionnalities suc as reconfigurable interconnections. The thesis will be done in collaboration between LTM/CNRS and CEA/INAC/SP2M/SiNaPS laboratories using the PTA facilities located in MINATEC.Le but de cette thèse est de réaliser et d’étudier les propriétés électroniques d’un transistor à canal nanofil monocristallin à base de Si/SiGe (voir figure), élaboré par croissance CVD-VLS, à grille enrobante ou semi-enrobante en exploitant une filière technologique compatible CMOS. Ces transistors vont nous permettre d’augmenter la densité d’intégration et de réaliser de nouvelles fonctionnalités (par exemple : des interconnections reconfigurables) dans les zones froides d’un circuit intégré. La thèse proposée se déroulera dans le cadre d'une collaboration entre le laboratoire LTM-CNRS et le laboratoire SiNaPS du CEA/INAC/SP2M et utilisera la Plateforme Technologique Amont (PTA) au sein du pôle MINATEC

3D Integration of Si/SiGe heterostructured nanowires for nanowire transistors.

Le but de cette thèse est de réaliser et d’étudier les propriétés électroniques d’un transistor à canal nanofil monocristallin à base de Si/SiGe (voir figure), élaboré par croissance CVD-VLS, à grille enrobante ou semi-enrobante en exploitant une filière technologique compatible CMOS. Ces transistors vont nous permettre d’augmenter la densité d’intégration et de réaliser de nouvelles fonctionnalités (par exemple : des interconnections reconfigurables) dans les zones froides d’un circuit intégré. La thèse proposée se déroulera dans le cadre d'une collaboration entre le laboratoire LTM-CNRS et le laboratoire SiNaPS du CEA/INAC/SP2M et utilisera la Plateforme Technologique Amont (PTA) au sein du pôle MINATEC.The goal of this thesis is to build and characterize nanowire based field-effect-transistors. These FET will have either back or wrapping gate using standard CMOS process. Theses transistors will allow us to increase the integration density in back end stages of IC's fabrication and add new functionnalities suc as reconfigurable interconnections. The thesis will be done in collaboration between LTM/CNRS and CEA/INAC/SP2M/SiNaPS laboratories using the PTA facilities located in MINATEC

Unbalanced Cylindrical Magnetron for Accelerating Cavities Coating

We report in this paper the design and qualification of a cylindrical unbalanced magnetron source. The dedicated magnetic assemblies were simulated using a finite element model. A hall-effect magnetic probe was then used to characterize those assemblies and compared to the theoretical magnet profiles. These show a good agreement between the expected and actual values. the qualification of the different magnetic assemblies was then performed by measuring the ion flux density reaching the surface of the sample to be coated using a commercial retarding field energy analyzer. The strongest unbalanced configuration shows an increase from 0.016A.cm^-2 to 0.074A.cm^-2 of the ion flux density reaching the sample surface compared to the standard balanced configuration for a pressure 5.10^-3 mbar and a plasma source power of 300W

The influence of cooldown conditions at transition temperature on the quality factor of niobium sputtered quarter-wave resonators for HIE-ISOLDE

Superconducting quarter-wave resonators (QWRs) are to be used in the ongoing linac upgrade of the ISOLDE facility at CERN. The cavities are made of niobium sputtered on copper substrates. They will be operated at 101.28 MHz at 4.5 K providing 6 MV/m accelerating gradient with 10 W power dissipation. In recent measurements, we found the thermal gradient along the cavity during the niobium superconducting transition has an impact on the cavity quality factor. On the other hand, the speed of the cooling down through the superconducting transition turned out to have no influence on the cavity Q-factor

Microscopic Examination of SRF-quality Nb Films through Local Nonlinear Microwave Response

The performance of superconducting radio-frequency (SRF) cavities is sometimes limited by local defects. To investigate the RF properties of these local defects, a near-field magnetic microwave microscope is employed. Local third harmonic response (P3f) and its temperature-dependence and RF power-dependence are measured for one Nb/Cu film grown by Direct Current Magnetron Sputtering (DCMS) and six Nb/Cu films grown by High Power Impulse Magnetron Sputtering (HiPIMS) with systematic variation of deposition conditions. Five out of the six HiPIMS Nb/Cu films show a strong third harmonic response that is likely coming from a low-Tc surface defect with a transition temperature between 6.3 K and 6.8 K, suggesting that this defect is a generic feature of air-exposed HiPIMS Nb/Cu films. One possible origin of such a defect is grain boundaries hosting a low-Tc impurity such as oxidized Nb. Time-Dependent Ginzburg-Landau (TDGL) simulations are performed to better understand the measured third harmonic response. The simulation results show that the third harmonic response of RF vortex nucleation caused by surface defects can qualitatively explain the experimental data. Moreover, the density of surface defects that nucleate RF vortices, and how deep an RF vortex travels through these surface defects, can be extracted qualitatively from third harmonic response measurements. From the point of view of these two properties, the best Nb/Cu film for SRF applications can be identified.Comment: 19 pages, 19 figure

Niobium thin film thickness profile tailoring on complex shape substrates using unbalanced biased High Power Impulse Magnetron Sputtering

The authors report in this paper the possibility to control the thickness profile of a thin film deposited by High Power Impulse Magnetron Sputtering (HiPIMS). It is shown that the combination between a HiPIMS discharge, an unbalanced magnetic configuration and the application of a negative bias onto the surface to coat enables tailoring on demand the coating thickness profile. This effect is hereafter used to coat complex shapes such as low-beta accelerating cavities with a niobium layer. The authors first present the magnetic design proposed to obtain an unbalanced cylindrical sputtering source. Numerical simulations are then used to predict the electron density and energy spatial distributions that can subsequently be correlated to the ionization region shape. Finally, the authors present the effect of such technique comparing Direct Current Magnetron Sputtering (DCMS), HiPIMS and biased HiPIMS using, respectively, a balanced and an unbalanced magnetic configuration, as well as detailing the effect of modifying either the magnetic field lines distribution or the magnetic strength

CERN Based TcT_{c} Measurement Station for Thin-Film Coated Copper Samples and Results on Related Studies

In the framework of the Future Circular Collider (FCC) Study, the development of thin-film coated superconducting radio-frequency (SRF) cavities capable of providing higher accelerating fields (10 to 20 MV/m against 5 MV/m of LHC) represents a major challenge. The development of a test stand commissioned at CERN for the inductive measurement of the critical temperature (T$_{c}$c) of SC thin-film deposited on copper samples for SRF application is presented in this work. Based on new studies for the production of Non Evaporable Getters (NEG) coated chambers, the first results of an alternative forming method for seamless copper cavities with niobium layer integrated in the production process are also presented