Journal Articles

Measurement of Hydrogen Absorption in Liquid Lithium in the Flowing Lithium retention Experiment (FLIRE)

March 1, 2005

J. Nucl. Mater., 337-339, 1033-1037 (2005).

Stubbers, R., Olczak, W., Nieto, M., Ruzic, D. N.

Flowing metal plasma facing components (PFCs) have the ability to withstand the extreme conditions of future tokamaks. The FLIRE facility at the University of Illinois measures the retention properties, both for helium (ash pumping) and hydrogen (recycling regime, tritium inventory), of candidate liquid PFCs, such as lithium. Results of hydrogen absorption measurements in flowing liquid lithium are presented. Absorption experiments with a flowing lithium stream passing through a low-pressure neutral deuterium gas show 0.1–0.2% D concentration retained long-term in the sample. A similar experiment with a high deuterium pressure of approximately 1 Torr shows the same long-term deuterium retention. These values indicate that the deuterium absorption rate is limited by the dissociation rate of the molecular deuterium gas on the Li surface.

Illinois Debris-Mitigation EUV Applications Labratory (IDEAL)

February 1, 2005

Microelectronics Engineering, 77, 103-109 (2005).

Jurczyk, B. E., Vargas-Lopez, E., Neumann, M. N., Ruzic, D. N.

Gaseous discharge light sources are leading candidates for generating 13.5 nm wavelengths needed for next-generation optical lithography. Electrode debris reaching the first collector optic is a serious concern for device lifetime and cost of ownership. This paper describes the experimental setup and initial data obtained for testing secondary-plasma-based debris mitigation for EUV gas discharge light sources. Operation of a dense plasma focus, secondary RF debris mitigation system, and several in situ diagnostics were successfully tested, achieving first measurements for debris attenuation. It was also found that fast ion and fast neutral particle erosion processes at the optical mirror location dominate over deposition of sputtered metal if a collimator or “foil trap” is positioned between the hot pinch plasma and the first collector optic.

Origins of Debris and Mitigation through a Secondary RF Plasma system for Discharge-Produced EUV Sources

February 1, 2005

Microelectronics Engineering, 77, 95-102 (2005).

Vargas-Lopez, E., Jurczyk, B. E., Jaworski, M. A., Neumann, M. J., Ruzic, D. N.

RF plasma based mitigation has been studied as an improved debris mitigation scheme for extreme ultraviolet (EUV) sources. The RF plasma ionizes sputtered neutral debris and, when used in conjunction with a collimator (also known as a foil trap), inhibits that debris from reaching the collector optics. An ionization fraction of 61 ± 3% has been measured. In addition, increased scattering of the ion component of the debris has led to a decrease in erosive flux reaching the diagnostics. Results from in situ high-precision quartz crystal oscillators, ex situ surface characterization (Auger, XPS), and secondary plasma characterization is presented for a series of mitigation schemes, including a foil trap in conjunction with the RF plasma.

Erosion, Transport, and Tritium Codeposition Issues of a Beryllium Wall Tokamak

January 1, 2005

Fusion Engineering and Design, 72, 363-375 (2005).

Brooks, J. N., Allain, J. P., Alman, D. A., Ruzic, D. N.

We analyzed beryllium first wall sputtering erosion, sputtered material transport, and T/Be codeposition for a typical next-generation tokamak design—the fusion ignition research experiment (FIRE). The results should be broadly applicable to any future tokamak with a beryllium first wall. Starting with a fluid code scrapeoff layer attached plasma solution, plasma D0 neutral fluxes to the wall and divertor are obtained from the DEGAS2 neutral transport code. The D+ ion flux to the wall is computed using both a diffusive term and a simple convective transport model. Sputtering coefficients for the beryllium wall are given by the VFTRIM-3D binary-collision code. Transport of beryllium to the divertor, plasma, and back to the wall is calculated with the WBC+ code, which tracks sputtered atom ionization and subsequent ion transport along the SOL magnetic field lines. Then, using results from a study of Be/W mixing/sputtering on the divertor, and using REDEP/WBC impurity transport code results, we estimate the divertor surface response. Finally, we compute tritium codeposition rates in Be growth regions on the wall and divertor for D–T plasma shots using surface temperature dependent D–T/Be rates and with different assumed oxygen contents. Key results are: (1) peak wall net erosion rates vary from about 0.3 nm s−1 for diffusion-only transport to 3 nm s−1 for diffusion plus convection, (2) T/Be codeposition rates vary from about 0.1 to 10.0 mg T s−1 depending on the model, and (3) core plasma contamination from wall-sputtered beryllium is low in all cases (< 0.02%). Thus, based on the erosion and codeposition results, the performance of a beryllium first wall is very dependent on the plasma response, and varies from acceptable to unacceptable.

Chemically Enhanced Physical Vapor Deposition (CEPVD) of TaN Diffusion Barrier Layers for ULSI Devices

November 15, 2004

J. Vac. Sci. Technol., B 22(6), 2734-2742, Nov/Dec 2004.

Li, N., Ruzic, D. N., Powell, R. A.

Physical vapor deposition (PVD) using ionized metal plasmas (ionized PVD or IPVD) is widely used to deposit conducting diffusion barriers and liners such as Ta and TaN for use in ultra-large-scale integrated (ULSI) interconnect stacks. Ionized PVD films exhibit the low resistivity, high density, and good adhesion to underlying dielectric desired for this application. On the other hand, extending PVD beyond the 45 nm technology node is problematic since IPVD may not provide sufficient step coverage to reliably coat features having high aspect ratio and sub-100 nm dimensions. Alternatively, chemical vapor deposition (CVD) and atomic layer deposition (ALD) can be used to deposit highly conformal metal films, but the electrical performance and interfacial quality may not equal that of PVD. To address future ULSI barrier/liner deposition needs, a method providing PVD-like film quality and CVD-like step coverage would be highly attractive. We have recently reported a hybrid approach to film deposition, referred to as chemically enhanced physical vapor deposition (CEPVD), in which a chemical precursor is introduced at the substrate during IPVD to provide a CVD component to the overall deposition process. The isotropic precursor flux is intended to provide film deposition on surfaces that are not impacted by the directional ions, such as the lower sidewall of a narrow via or trench. Conversely, the kinetic energy delivered to the surface by the flux of ionized metal may serve to enhance the desorption of CVD byproducts, reduce incorporation of impurities, and increase film density. In order to investigate the potential of CEPVD to deposit barrier/liner films, we have focused on the Ta-N material system since Ta/TaN is widely used as a diffusion barrier in Cu damascene processing. IPVD TaN films were deposited by reactive sputtering of a Ta target in Ar/N2 using a planar magnetron and internal rf coils to provide a secondary ionization plasma for the sputtered neutrals. CEPVD was carried out by introducing a Ta-containing, organometallic precursor [tert-butylimino tris(diethylamino) tantalum] in the vicinity of the substrate surface during IPVD. Film thickness and step coverage were determined by cross-sectional scanning electron microscopy (SEM). Film composition, chemical state, and crystal structure were characterized using Auger electron spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction, respectively. Resistivity was measured by four-point probe. Cross-sectional SEM showed improved step coverage over IPVD TaN. CEPVD film properties were highly process dependent; however, unlike IPVD TaNx films that vary in stoichiometry but not purity, CEPVD “TaN” films contained relatively large amounts of carbon (~30%–60%) and could best be described as TaCxNy. Resistivity as low as ~370  µOmega cm was obtained for planar films of approximately 90 nm in thickness.

Studies of Liquid-Metal Erosion and Free Surface Flowing Liquid Lithium Retention of Helium at the University of Illinois

November 1, 2004

Fusion Engineering and Design, 72, 93-110 (2004).

Allain, J. P., Nieto, M., Coventry, M. D., Stubbers, R., Ruzic, D. N.

The erosion of liquid-metals from low-energy particle bombardment at 45° incidence has been measured for a combination of species and target materials in the ion-surface interaction experiment (IIAX) at the University of Illinois Urbana-Champaign. Measurements include bombardment of liquid Li, Sn–Li and Sn by H+, D+, He+, and Li+ particles at energies from 100 to 1000 eV and temperatures from 20 to 420 °C. Lithium sputtering near and just above the melting point shows little change compared to room temperature, solid-Li yields. When lithium is sputtered, about 2/3 of the sputtered flux is in the charged state. Temperature-dependent sputtering results show enhanced (up to an order-of-magnitude increase) sputter yields as the temperature of the sample is increased about a factor of two of the melting point for all liquid-metals studied (e.g., Li, Sn–Li, and Sn). The enhancement is explained by two mechanisms: near-surface binding of eroded atoms and the nature of the near-surface recoil energy and angular distribution as a function of temperature.

The Flowing Liquid Retention Experiment (FLIRE) measured particle transport by flowing liquid films when exposed to energetic particles. Measurements of retention coefficient were performed for helium ions implanted by an ion beam into flowing liquid lithium at 230 °C in the FLIRE facility. A linear dependence of the retention coefficient with implanted particle energy is found, given by the expression R = (5.3 ± 0.2) × 10−3 keV−1. The ion flux level did not have an effect for the flux level used in this work (not, vert, similar1013 cm−2 s−1) and square root dependence with velocity is also observed, which is in agreement with existing particle transport models.

Temperature Dependance of Liquid Sn Sputtering by Low Energy He and D Bombardment

October 1, 2004

J. Nucl. Mater., 335, 115-120 (2004).

Coventry, M. D., Allain, J. P., Ruzic, D. N.

Absolute sputtering yields of liquid tin from 240 to 420 °C due to irradiation by low-energy helium and deuterium have been measured. For ion energies ranging from 300 to 1000 eV, temperature enhancement of liquid tin sputtering was noted. These measurements were obtained by IIAX (the Ion-surface InterAction eXperiment) using a velocity-filtered ion beam at 45° incidence to sputter material from a liquid tin target onto deposition monitors. Sputtering yields from 500 eV ion bombardment at 45° incidence increase from 0.1 ± 0.03 and 0.019 ± 0.008 Sn particles/ion at room temperature, for He+ and D+ ions respectively, to 0.30 ± 0.12 and 0.125 ± 0.05 Sn particles/ion for 380 °C. Temperature enhanced sputtering has been seen in other liquid metals (namely lithium, tin–lithium, and gallium) using both ion beam and plasma irradiation.

Molecular Dynamics Simulation of Hydrocarbon Reflection and Dissociation Coefficients from Fusion-Relevant Carbon Surfaces

January 7, 2004

Physica Scripta, T111, 145-151 (2004).

Alman, D. A., Ruzic, D. N.

Reflection coefficients for carbon and hydrocarbon atoms/molecules on carbon-based surfaces are critically needed for plasma-surface interaction analysis in fusion devices, as carbon will continue to be used in next step devices like ITER. These have been calculated at different energies and angles with a molecular dynamics code using the Brenner hydrocarbon potential. Hydrogen saturated graphite was prepared by bombarding a graphite lattice with hydrogen, until a saturation at ~0.42 H:C. Carbon at 45° has a reflection coefficient (R) of 0.64 ± 0.01 at thermal energy, decreasing to 0.19 ± 0.01 at 10 eV. Carbon dimers (Rthermal = 0.51, R>1 eV ~ 0.10) tend to stick more readily than carbon trimers (Rthermal = 0.63, R10 eV = 0.16). Hydrocarbons reflect as molecules at thermal energies and break up at higher energies. The total reflection via these fragments decreases with energy, the number of unpaired electrons, and changing hybridization from sp3 to sp2 to sp. The results compare reasonably well with binary collision modeling for higher energies and experimental sticking data at thermal energies. A second surface, representing a “soft” redeposited carbon layer formed by the deposition of hydrocarbons onto a graphite surface, is also analyzed. In general, reflection is lower from the “soft” surface by 0.1–0.2. This reflection data can and has been incorporated in erosion/redeposition codes to allow improved modeling of chemically eroded carbon transport in fusion devices.

D+, H+ and He+ Sputtering of Solid and Liquid Phase Tin

March 1, 2003

J. Nucl. Mater., 313-316, 636-640 (2003).

Coventry, M. D., Allain, J. P., Ruzic, D. N.

Absolute sputtering yields of Sn in the solid and liquid phases for incident H+, D+, and He+ ions with energies of 300 to 1000 eV were determined via a series of experiments using the ion-surface interaction experiment (IIAX), a facility designed to measure such ion-surface interactions. IIAX incorporates an ion source with an array of ion beam filters and optics to bring a near mono-energetic beam of ions to the target, in this case at 45° to the surface normal. A pair of quartz crystal microbalances record the amount of mass collected from both sputtering and evaporation from the target. VFTRIM-3D modeling results of tin sputtering best fit the experimental data when the bulk of pure tin was modeled with 60% SnO coverage in the top three monolayers most likely indicating the presence of a very thin oxide layer during experimentation.

Advances in the modeling of Chemical Erosion/Redeposition of Carbon Divertors and Application to the JET Tritium Codeposition Problem

March 1, 2003

J. Nucl. Mater., 313-316, 424-428 (2003).

Brooks, J. N., Kirschner, A., Whyte, D. G., Ruzic, D. N., Alman, D. A.

We have improved the modeling of chemically eroded carbon transport and applied this to JET and ITER. New features are: (1) coupled REDEP and ERO-JET impurity transport code calculations for sputtered wall/divertor carbon, (2) MolDyn molecular dynamics calculations of carbon/hydrocarbon particle reflection at hydrogen-saturated carbon surfaces, (3) ADAS full collisional radiative carbon ion recombination rate coefficients. At low incident particle energies relevant to chemical-erosion (not, vert, similar0.1–15 eV), we predict high reflection coefficients (not, vert, similar20–100%), implying more net erosion and T/C codeposition than for full-sticking models. Calculated tritium codeposition rates for the JET MkIIA divertor, using ‘reference’ chemical erosion yields of order 1% – while higher than previously estimated are well short (Xnot, vert, similar1/40) of published data. Possible explanations include much higher chemical erosion yields.