H. De Witte

Studies of pear-shaped nuclei using accelerated radioactive beams

There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are ‘octupole deformed’, that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on 220Rn and 224Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.

Measurement of the first ionization potential of astatine by laser ionization spectroscopy

The radioactive element astatine exists only in trace amounts in nature. Its properties can therefore only be explored by study of the minute quantities of artificially produced isotopes or by performing theoretical calculations. One of the most important properties influencing the chemical behaviour is the energy required to remove one electron from the valence shell, referred to as the ionization potential. Here we use laser spectroscopy to probe the optical spectrum of astatine near the ionization threshold. The observed series of Rydberg states enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV. New ab initio calculations are performed to support the experimental result. The measured value serves as a benchmark for quantum chemistry calculations of the properties of astatine as well as for the theoretical prediction of the ionization potential of superheavy element 117, the heaviest homologue of astatine.

On-line yields obtained with the ISOLDE RILIS

The ISOLDE resonance ionization laser ion source (RILIS) allows to ionize efficiently and selectively many metallic elements. In recent yield surveys and on-line experiments with the ISOLDE RILIS we observed 23–34 Mg, 26–34 Al, 98–132 Cd, 149 Tb, 155–177 Yb, 179–200 Tl, 183–215 Pb and 188–218 Bi. The obtained yields are presented together with measured release parameters which allow to extrapolate the release efficiency towards more exotic (short-lived) nuclides of the same elements. 2002 Elsevier Science B.V. All rights reserved.

Evaluation of Time‐of‐Flight Secondary Ion Mass Spectrometry for Metal Contamination Monitoring on Si Wafer Surfaces

Due to stringent IC‐production requirements, the metallic contamination of silicon surfaces decreases to very low concentration levels (below ). Therefore new alternatives to measure the presence of these contaminants and to quantify their amounts are investigated. This study focuses on the capabilities of time of flight secondary ion mass spectrometry (TOF‐SIMS). It compares and combines TOF‐SIMS results with those of the standard technique direct total reflectance X‐ray fluorescence spectroscopy (TXRF). Next, it investigates the influence of the surface oxidation state (i.e., bare silicon, native and thermal oxide) on the TOF‐SIMS signal response. Then, TOF‐SIMS surface spectra are quantified using relative sensitivity factors, according to two different approaches (experimentally vs. theoretically derived). Finally, the influence of oxidation ambient on Fe redistribution in the thermal oxide layer is studied using the TOF‐SIMS dual beam option to perform depth profiling. A good general agreement is obtained between the quantified TOF‐SIMS surface spectra and the TXRF values. For the Fe distribution in the oxide layer, TOFSIMS reaches qualitatively the same conclusions as vapor‐phase decomposition‐droplet collection TXRF. For 5 nm oxide samples no difference between both furnace ambients can be observed. For 15 nm oxides, however, more Fe is trapped in the oxide when oxygen is added to the ambient.

Modeling of bombardment induced oxidation of silicon

Secondary ion mass spectrometry has become the preferred tool for impurity profiling primarily due to its excellent depth resolution and high detection sensitivity. Prerequisite in obtaining high detection sensitivity for positive secondary ions is the use of oxygen as primary ions. This leads to a high degree of oxidation of the sample surface, which is essential for a high secondary ion ionization efficiency. Unfortunately, this oxygen bombardment not only leads to the transformation of the original target surface into an oxidized layer but, as the latter requires a certain fluence before stationary state is reached, inherently causes some nonlinearities and transients in the secondary ion signal and the fluence-eroded depth relation. In this work a computer code implantation, sputtering, replacement/relocation, and diffusion (ISRD) has been optimized to predict the compositional changes of the sample surface (or altered layer formation), the sputter yields and the surface regression as a result of the ...

Characterization of ultra thin oxynitrides: A general approach

The determination of nitrogen depth profiles in thin oxynitride layers (1.5‐3 nm) becomes more and more important in microelectronics. The goal of this paper is to investigate a methodology for the characterization of thin oxynitride layers with the aim to establish in a quantitative manner the layer thickness, N-content and detailed N-depth profile. For this study ultra thin oxynitride films of 2.5 nm on Si were grown by oxygen O2 annealing of Si followed by a NO annealing. The global film characteristics were measured using spectroscopic ellipsometry (SE) (thickness), atomic force microscopy (AFM) for roughness and X-ray photoelectron spectroscopy (XPS) for total O- and N-content. Depth profiles of oxygen, silicon and nitrogen were obtained using (low energy) secondary ion mass spectroscopy (SIMS) and time of flight (TOF)-SIMS, high resolution-Rutherford backscattering (H-RBS) (magnetic sector and TOF) and high resolution-elastic recoil detection (H-ERD). A comparison of the results obtained with the diAerent techniques is presented and discussed. ” 2000 Elsevier Science B.V. All rights reserved.

Spectroscopic Quadrupole Moments in {96,98}Sr: Evidence for Shape Coexistence in Neutron-Rich Strontium Isotopes at N=60.

Neutron-rich {96,98}Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N=60.

In-gas-cell laser ionization spectroscopy in the vicinity of 100Sn: Magnetic moments and mean-square charge radii of N=50–54 Ag

Abstract In-gas-cell laser ionization spectroscopy studies on the neutron deficient 97–101Ag isotopes have been performed with the LISOL setup. Magnetic dipole moments and mean-square charge radii have been determined for the first time with the exception of 101Ag, which was found in good agreement with previous experimental values. The reported results allow tentatively assigning the spin of 97,99Ag to 9 2 and confirming the presence of an isomeric state in these two isotopes, whose collapsed hyperfine structure suggests a spin of 1 2 . The effect of the N = 50 shell closure is not only manifested in the magnetic moments but also in the evolution of the mean-square charge radii of the isotopes investigated, in accordance with the spherical droplet model predictions.

Evaluation of Atomic Layer Deposited NbN and NbSiN as Metal Gate Materials

Nb(Si)N films deposited by thermal atomic layer deposition (ALD) have been investigated as potential gate electrode materials in advanced complementary metal oxide semiconductor devices. The NbN films were deposited at 400°C using NbCl 5 and NH 3 as precursors. For NbSiN, SiCl 4 was added as a precursor, yielding an average silicon content of 3 atom % [presented at The Electrochemical Society Meeting, Oct 16-21, 2005, Los Angeles, CA]. The work function of these films was determined by high-frequency capacitance-voltage measurements on two thickness series, one of thermal oxides (6.5-20 nm) and one of ALD HfO 2 (2-6 nm) followed by an O 2 postdeposition anneal. It was found that NbN has a work function of ∼4.7 eV on SiO 2 and ∼4.9 eV on HfO 2 . However, thermal stability is limited to ∼950°C. The thermal stability on SiO 2 is increased by incorporation of silicon into the NbN. NbSiN has a work function of ∼4.7 eV on SiO 2 and ∼4.8 eV on HfO 2 .

In-line electrical metrology for high-k gate dielectrics deposited by atomic layer CVD

The use of an in-line electrical metrology tool for evaluation and characterization of high-k dielectrics has been investigated. The equivalent oxide thickness obtained from the KLA-Tencor Quantox agrees well with high-frequency capacitance-voltage (C-V) results. The ACTIV Jg index parameter shows good correlation with the gate current J gate obtained from C-V measurements. Therefore, this tool can be used as a fast and much less expensive evaluation tool for optimization of high-k process conditions and process stability monitoring.