Michael Pellin

Materials used or considered for plasma-side applications are not entirely satisfactory, particularly in the medium edge temperature (100 to 300 eV) regime. An approach to impurity control based on self-sustaining surface segregated low-Z layers with high secondary ion fractions was tested in laboratory experiments. A crucial requirement is that substrate sputtering yields be reduced about an order of magnitude by monolayer adsorbate coverages. Theoretical and experimental evidence is adduced to support the contention that overlayer coverages (a monolayer of Li on Cu) result in profound reductions of substrate (e.g., Cu) sputtering yields. The conclusion that a material such as a dilute alloy of Li in Cu could function as a limiter or a divertor plate material is, in part, based on the fact that more than 85% of the sputtered flux originates in the first atomic layer (e.g., Li) of the target.

Because of the practical importance of sputtering, numerous theories and computer simulations are used for predicting many aspects of the sputtering process. Unfortunately, many of the calculated sputtering results are untested by experiment. Until recently, most sputtering experiments required either very high ion fluences or the detection of only minor constituents of the sputtered flux, i.e., ions. These techniques may miss the subtleties involved in the sputtering process. High-detection-efficiency mass spectrometry, coupled with the laser ionization of neutral atoms, allows the detection of the major sputtered species with very low incident ion fluences. The depth-of-origin of sputtered atoms is one example of an important but poorly understood aspect of the sputtering process. By following the sputtering yield of a substrate atom with various coverages of an adsorbed overlayer, the depth of origin of sputtered atoms has been determined. Our results indicate that two-thirds of the sputtered flux originates in the topmost atomic layer. The ion-dose dependence of sputtering yields has long been assumed to be quite minor for low- to-moderate primary ion fluences. We have observed a two-fold decrease in the sputtering yield of the Ru(0001) surface for very low primary ion fluences. Data analysis results in a cross section for damage of 2.7 {plus minus} 1.0 à 10⁻¹⁵cm². 40 refs., 3 figs., 2 tabs.

Velocity distributions and relative populations in the fine structure levels of the a(5)D/sub J/ ground state of Fe atoms, produced by sputtering with a 3 keV argon ions, were investigated by Doppler shifted laser induced fluorescence. The laser system employs a single mode, scanning ring dye laser, amplified by a sequence of three excimer pumped flowing dye cells. Frequency doubling in a KD*P crystal was used to produce high energy ( .5 mJ) pulses of narrowband tunable UV output near 300 nm. Laser power influence on effective velocity bandwidth was investigated. Favorable light collection geometry minimized distortion of the velocity spectra from apparatus averaging effects. In impurity flux diagnostic applications in fusion devices, substantial spatial averaging may occur. In the latter case, the narrow velocity bandwidth (70 m/s, transform limit) of the present laser system is particularly useful.

Electron-stimulated desorption of neutral aluminum from the system CH3O/Al(111) has been directly monitored via quasiresonant photoionization with 193 nm excimer laser light and confirmed by two-step resonant ionization, utilizing the Al 3D (sup 2)D manifold. Velocity distribution measurements for the neutral Al peak at approximately 800 m/s for 1 keV incident electron energy. An absolute yield of 3.2 x 10(exp -6) Al atoms/electron was determined by comparison with sputtering measurements in the same apparatus. This is the first observation of electron-stimulated metal desorption from adsorbate-covered metallic surfaces.

ABSTRACT We report the use of atomic layer deposition (ALD) to synthesize thin superconducting films and multilayer superconductor-insulator (S-I) heterostructures. The ALD technique applied to superconducting films opens the way for a variety of applications, including improving the performance and decreasing the cost of high energy particle accelerators, superconducting wires for energy storage, and bolometers for radiation detection. Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and associated phenomena in homogeneous layers in the ultra-thin film limit. In this respect, we will present results of ALD-grown transition metal-based superconductors, including nitrides, carbides, and silicides of niobium, nitrides of molybdenum and titanium, and Nb1-xTixN/AlN-based S-I heterostructures. Transport measurement for various composition and film thicknesses will be presented.

We present evidence of extinct Tc in presolar SiC grains in the form of an anomalous Ru isotopic composition compared to the one expected from the AGB stars that produced the grains. We show that AGB stars do not produce enough Tc to leave a detectable Ru anomaly in early solar system materials.

The samples returned to Earth by the NASA's Genesis Mission contain a record of the elemental and isotopic abundances of the Solar Wind (SW). This record is formed by the SW ions implanted in the near-surface regions of the Genesis sample collectors, so that the SW material can be distinguished from a terrestrial contamination, which occurred due to the crash

A new secondary neutral mass spectrometry (SNMS) instrument implementing laser post ionization (LPI) of ion sputtered and laser desorbed neutral species has been developed and constructed for the specific purpose of quantitative analysis of metallic elements at ultra trace levels in solar wind collector samples returned to Earth by the Genesis Discovery mission. The first LPI SNMS measurements are focusing on determining Al, Ca, Cr, and Mg in these samples. These measurements provide the first concentration and isotopic abundances determinations for several key metallic elements and also elucidate possible fractionation effects between the photosphere and the solar wind compositions. It is now documented that Genesis samples suffered surface contamination both during flight and during the breach of the Sample Return Capsule when it crashed. Since accurate quantitative analysis is compromised by sample contamination, several features have been built into the new LPI SNMS instrument to mitigate this difficulty. A normally-incident, low-energy (<500 eV) ion beam combined with a keV energy ion beam and a desorbing laser beam (both microfocused) enables dual beam analyses. The low-energy ion beam can be used to remove surface contaminant by sputtering with minimum ion beam mixing. This low-energy beam also will be used to perform ion beam milling, while either the microfocused ion or laser beam probes the solar wind elemental compositions as a function of sample depth. Because of the high depth resolution of dual beam analyses, such depth profiles clearly distinguish between surface contaminants and solar wind implanted atoms. In addition, in-situ optical and electron beam imaging for observing and avoiding particulates and scratches on solar wind sample surfaces is incorporated in the new LPI SNMS instrument to further reduce quantification problems. The current status of instrument tests and analyses will be presented. This work is supported by the U. S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38, and by NASA under Work Orders W-19,895 and W-10,091.

Results from the development of a novel type of anode for electrowinning Mg are reported. A tailored alloy system based on the binary Cu-Al can be made to form a thin alumina layer on its surface that is relatively impervious to attack by the molten chloride melt at high temperature. This barrier is thin enough (5--50 nm) to conduct electrical

An ultrananocrystalline diamond (UNCD) element formed in a cantilever configuration is used in a highly sensitive, ultra-small sensor for measuring acceleration, shock, vibration and static pressure over a wide dynamic range. The cantilever UNCD element may be used in combination with a single anode, with measurements made either optically or by capacitance. In another embodiment, the cantilever UNCD element is disposed between two anodes, with DC voltages applied to the two anodes. With a small AC modulated voltage applied to the UNCD cantilever element and because of the symmetry of the applied voltage and the anode-cathode gap distance in the Fowler-Nordheim equation, any change in the anode voltage ratio V1/V2 required to maintain a specified current ratio precisely matches any displacement of the UNCD cantilever element from equilibrium. By measuring changes in the anode voltage ratio required to maintain a specified current ratio, the deflection of the UNCD cantilever can be precisely determined. By appropriately modulating the voltages applied between the UNCD cantilever and the two anodes, or limit electrodes, precise independent measurements of pressure, uniaxial acceleration, vibration and shock can be made. This invention also contemplates a method for fabricating the cantilever UNCD structure for the sensor.

A number of popular analytical techniques rely on ion sputtering or laser desorption to probe solid samples. The popularity of this class of techniques is derived from the fact that they produce information on elemental and molecular compositions at trace levels. These techniques are particularly amenable to small sample analysis, since both ion and photon beams can be focused to sub-micron dimensions. Because ion sputtering and laser desorption consume material, there exists a trade off between sample size and achievable detection limit. This trade off is quantified by an instruments useful yield, which is defined as the number of atoms detected per atoms consumed. Laser post-ionization secondary neutral mass spectrometry (LPI-SNMS) has useful yields significantly higher than competing techniques and is thus well suited for trace analysis of small samples. With LPI-SNMS, either a pulse of energetic ions or photons remove material from a solid surface into the gas phase. The desorbed material, predominantly ground state neutral atoms, is photo-ionized by one or more lasers and then extracted into a mass spectrometer for detection. At Argonne National Laboratory, we have developed a new reflectron time-of flight (TOF) mass spectrometer especially designed to optimize useful yield in LPI-SNMS measurements. Using ion optics simulations, an improved extraction design has been developed that allows photo ions from a large (4 × 4 × 3 mm^3) volume above a sample surface to be transmitted through a TOF mass spectrometer with > 98% efficiency. Efficient extraction from such a large ionization volume means that more than 40% of all desorbed species are available for detection, producing an overall useful yield of > 30%. Such a high sensitivity allows analysis of small samples at trace levels never before achievable, opening many new applications. For example, the new LPI-SNMS instrument will allow (1) part-per-trillion detections of solar wind elements implanted in the top 100 nm layer of the collectors of NASA's Genesis Discovery mission; (2) part-per-billion detection of 100 nm particles, such as interstellar dust; and (3) part-per-million detection in 100 nm surface features with monolayer depth resolution. Construction of the new instrument has recently been completed. Measurements to characterize its sensitivity have begun and will be presented.

Laser fluorescence spectroscopy (LFS) is a promising impurity atoms generated from surfaces exposed to fusion plasmas. Here the first continuous wave laser fluorescene measurement of impurities generated during a single tokamak pulse is reported. The results point out the promise of LFS as an essentially real time, in-situ diagnostic allowing a detailed comparison of impurity generation and transport with Tokamak parameters and operating conditions. Results presented will include the Zr-atom density and velocity distribution produced from a Zr-metal target during an Apex Tokamak discharge.

The ion nanoprobe is a new instrument designed for isotopic, chemical, and possibly molecular analysis at lateral resolutions of a few nanometers. This instrument, now under construction, will be applied to a broad range of problems in cosmochemistry.

Three different instruments using laser ionization techniques will be described. Results from the SARISA instrument with a demonstrated figure of merit of .05 (atoms detected/atoms sputtered) for resonance ionization; detection of Fe at the sub-part-per-billion level in ultrapure Si; and features of the instrument such as energy and angle refocusing time-of-flight (EARTOF) mass spectrometer and multiplexing for simultaneous detection of secondary ions and neutrals.

Secondary Neutral Mass Spectroscopy (SNMS) uses the secondary neutral sputtered fraction, and can increase both yield and quantitative analysis. In this paper, lasers are used to ionize and then detect the sputtered neutral particles. This method is used with oxygen ion implantation to study Fe on Si substrates. In this way, Fe could be detected at the 500 ppT level in a single monolayer.

Polycrystalline Ag, Ag 20Au 80, Ag 40Au 60, Ag 80Au 20 and Au samples were bombarded with 15 keV Ar + at 60° incidence and the resulting secondary neutral yield distribution was studied by non-resonant laser postionisation mass spectrometry. Neutral clusters containing up to 21 atoms were observed for the targets. The yield of neutral clusters, Ag mAu n- m, containing n atoms, Yn, was found to follow a power in n, i.e. Yn ∝ n- δ, where the exponent δ varied from 3.2 to 4.0. For a fixed n, the cluster yields showed a variation with number of gold atoms similar to that expected for a binomial distribution. In addition, the cluster compositions from the sputtered alloys were indicative of sputtering from a gold rich surface.

CHILI, a new RIMS instrument with ~10 nm lateral resolution and 40-50% useful yield, is under construction at the University of Chicago. It will be applied to the analysis of samples from the Stardust mission and may be able to date presolar dust.

Embedded gold and mechanical deformation in silica were used to investigate initiation of laser-induced damage at 3.55-nm (7.6 ns). The nanoparticle-covered surfaces were coated with between 0 and 500 nm of SiO by e-beam deposition. The threshold for observable damage and initiation site morphology for these &#39;&#39;engineered&#39;&#39; surfaces was determined. The gold nanoparticle coated surfaces with 500nm SiO coating exhibited

Atomic Layer Deposition (ALD) is a process that synthesizes materials in successive monolayers, at rates up to 1 micron/hour. We have been using this technique at Argonne as a possible way to improve superconducting radio frequency (SCRF) cavities performances. Initial experiments using tunneling spectroscopy and ALD have led to a new model for dissipation mechanisms occurring at the surface and

Atomic layer deposition was used to synthesize niobium silicide (NbSi) films with a 1:1 stoichiometry, using NbF5 and Si2H6 as precursors. The growth mechanism at 200oC was examined by in-situ quartz crystal microbalance (QCM) and quadrupole mass spectrometer (QMS). This study revealed a self-limiting reaction with a growth rate of 4.5 {\AA}/cycle. NbSi was found to grow only on oxide-free

Niobium, with its very high HC1, has been used in superconducting RF cavities for accelerator systems for 40 years with continuous improvement. The quality of cavities (Q) is governed by the surface impedance RBCS, which depends on the quasiparticle gap, delta, and the superfluid density, nS. Both of these parameters are seriously affected by surface imperfections (metallic phases, dissolved oxygen,