Bart Vereecke

WITCH: a recoil spectrometer for weak interaction and nuclear physics studies

An experimental set-up is described for the precise measurement of the recoil energy spectrum of the daughter ions from nuclear beta decay. The experiment is called WITCH, short for Weak Interaction Trap for CHarged particles, and is set up at the ISOLDE facility at CERN. The principle of the experiment and its realization are explained as well as the main physics goal. A cloud of radioactive ions stored in a Penning trap serves as the source for the WITCH experiment, leading to the minimization of scattering and energy loss of the decay products. The energy spectrum of the recoiling daughter ions from the β-decays in this ion cloud will be measured with a retardation spectrometer. The principal aim of the WITCH experiment is to study the electroweak interaction by determining the beta-neutrino angular correlation in nuclear β-decay from the shape of this recoil energy spectrum. This will be the first time that the recoil energy spectrum of the daughter ions from β-decay can be measured for a wide variety of isotopes, independent of their specific properties.

Electrical improvement of CNT contacts with Cu damascene top metallization

We discuss the improvement in the electrical characterization and the performance of 150 nm diameter contacts filled with carbon nanotubes (CNT) and a Cu damascene top metal on 200mm wafers. The excellent agreement between the yield curves for the parallel and single contacts shows that a reliable electrical characterization is obtained. We demonstrate that integration changes improved the resistivity of the CNT contact significantly by reducing it from 11.8·10³ μΩ·cm down to 5.1·10³ μΩ·cm. Finally, a length scaling of the CNT contacts was used to find the individual contributors to the lowering of the single CNT contact resistance.

Fundamental weak interaction studies using polarised nuclei and ion traps

Two experiments to search for new physics beyond the standard model for electroweak interactions by measuring correlations between different spin and momentum vectors in nuclear β-decay are discussed. In the first experiment the correlation between the emission asymmetry and the longitudinal polarisation of positrons emitted by polarised nuclei is determined. This type of measurement is sensitive to the presence of right-handed currents but also to possible scalar and tensor-type currents in the weak interaction. The aim of the second experiment is to determine the βν-correlation in β-decay by measuring the energy spectrum of the recoil ions, using a Penning trap and a retardation spectrometer. In this case the focus is on the search for scalar currents in the weak interaction. The results of the experiments presented here are complementary to results from experiments in muon decay and at high-energy colliders.

Carbon nanotube interconnects: Electrical characterization of 150 nm CNT contacts with Cu damascene top contact

The integration of Carbon Nanotubes (CNT) in contact holes with TiN underlayer using CMOS compatible processes is discussed. Each process step was optimized by evaluating the electrical results obtained with contact test structures. Subsequently, this process was transferred to 150 nm diameter contact holes. We present the first electrical data obtained from automated probing of 150 nm diameter contacts filled with CNT connected by a Cu damascene top contact module. This constitutes a significant step forward towards the realization of CMOS contact modules with CNT interconnects.

WITCH: a recoil spectrometer for β-decay

The WITCH experiment will measure the recoil energy spectrum of the daughter ions in beta-decay. The main parts of the experiment are two Penning traps and a subsequent retardation spectrometer. The beta-decays take place in the ion cloud in the decay trap. Since the ion cloud is in vacuum and due to the cylindrical structure of the trap, the recoiling daughter ions can leave the cloud and the trap without any significant energy loss and can be energy analyzed in the retardation spectrometer. The WITCH experiment is set up foremost to study the electroweak interaction by measuring the beta-v angular correlation in nuclear beta-decay which can be inferred from the shape of the energy spectrum of the recoil ions. In the beginning the experiment will focus on pure Fermi decays which will allow to search for additional scalar coupling in the weak interaction. Since Penning traps have no restrictions regarding the element to be trapped the most suitable isotope can be picked for this purpose. The WITCH experiment is presently being set up at ISOLDE. Status and perspectives of the experiment will be presented in the following

Isospin mixing in the ground state of 52Mn

Abstract The presence of isospin mixing into the ground state of 52 Mn was studied via anisotropic positron emission from nuclei. With this method the isospin forbidden Fermi-component in the Gamow–Teller dominated β decay was determined. It is shown that sample purity and the control of positron scattering is of vital importance. Comparison between theory and experiment shows that shell model calculations of the isospin mixing probability deviate by a factor three to seven from experiment. For more recent Hartree–Fock-RPA based calculations the difference is over two orders of magnitude.

Digital pattern generator: an electron-optical MEMS for massively parallel reflective electron beam lithography

Abstract. The digital pattern generator (DPG) is a complex electron-optical MEMS that pixelates the electron beam in the reflective electron beam lithography (REBL) e-beam column. It potentially enables massively parallel printing, which could make REBL competitive with optical lithography. The development of the REBL DPG, from the CMOS architecture, through the lenslet modeling and design, to the fabrication of the MEMS device, is described in detail. The imaging and printing results are also shown, which validate the pentode lenslet concept and the fabrication process.

Electromigration and stress-induced-voiding in dual damascene Cu/low-k interconnects: a complex balance between vacancy and stress gradients

The influence of residual vacancy concentrations and stress gradients on electromigration both in the metal layer below and above copper vias with a diameter of 90nm integrated in low-k materials has been investigated. Variations in stress gradients and vacancy concentrations were created by applying different post-plating anneal conditions. The impact of these variations was quantified based on high temperature storage tests both at the optimum stress-induced-voiding temperature and around the copper stress free temperature. By linking the results from these high temperature storage tests to electromigration data, we observe residual vacancy concentrations contribute more to upstream electromigration, while downstream electromigration is more vulnerable to residual stress gradients.

Helicity structure in low- and intermediate-energy weak interaction

Abstract We report on the status of the measurements of the longitudinal polarization of positrons emitted by polarized 107In and 12N nuclei. Present results yield a lower limit of 306 GeV/c2 (90 % CL) for the mass of a possible W gauge boson with predominantly right-handed couplings, if interpreted in the framework of the manifest left-right symmetric model. In addition, the helicity structure of the weak interaction is discussed on the basis of experimental results at low and intermediate energies in the leptonic, semileptonic and hadronic sectors. Whereas in the minimal manifest left-right symmetric extension of the Standard Model the results from collider experiments are superior to results from other experiments, experiments in the three sectors of the weak interaction yield complementary information on the helicity structure of the interaction if interpreted in general left-right symmetric models.

Wafer-Level Electrical Evaluation of Vertical Carbon Nanotube Bundles as a Function of Growth Temperature

We have evaluated the resistance of carbon nanotubes (CNTs) grown at a CMOS-compatible temperature using a realistic integration scheme. The structural analysis of the CNTs by transmission electron microscopy (TEM) showed that the degree of graphitization decreased significantly when the growth temperature was decreased from 540 to 400 °C. The CNTs were integrated to form 150-nm-diameter vertical interconnects between a TiN layer and Cu metal trenches on 200 mm full wafers. Wafers with CNTs grown at low temperature were found to have a lower single-contact resistance than those produced at high temperatures. Thickness measurements showed that the low contact resistance is a result of small contact height. This height dependence is masking the impact of CNT graphitization quality on resistance. When benchmarking our results with data from the literature, a relationship between resistivity and growth temperature cannot be found for CNT-based vertical interconnects.