J. Nucl. Mater., 313-316, 641-645 (2003).
Allain, J. P., Coventry, M. D., Ruzic, D. N.
The lithium-sputtering yield of liquid lithium as a function of sample temperature has been measured in the ion-surface interaction experiment (IIAX). Lithium sputtering is measured for D+, He+ and Li+ bombardment at energies between 100 and 1000 eV at 45° incidence. In this work VFTRIM-3D is used to provide a qualitative physical picture of mechanisms responsible for the temperature dependence of liquid-lithium sputtering. The present study is done for 700 eV He+ bombardment of liquid lithium at 45° incidence with respect to the target normal. The lithium-sputtering yield, after evaporation is taken into account, is found to increase almost an order of magnitude when the target temperature is increased from the melting point up to roughly 410 °C. The deposited energy distribution near the liquid-lithium surface is found to play a significant role in explaining the observed enhanced lithium sputtering as well as the temperature dependence of the surface binding energy.
J. Nucl. Mater., 313-316, 646-650 (2003).
Nieto, M., Ruzic, D. N., Allain, J. P., Coventry, M. D., Vargas-Lopez, E.
The flowing liquid surface retention experiment (FLIRE) has been designed to provide fundamental data on the retention and pumping of He, H and other species in flowing liquid surfaces. The FLIRE facility currently uses an ion beam source, which injects ions into a flowing stream of liquid lithium. Its design allows the liquid lithium to flow between two vacuum chambers that become isolated from each other when the lithium flows. Flow velocities between 0.5 and 3.0 m/s down two ramps inside the upper vacuum chamber can be achieved. The ramps and lines where the liquid lithium flows are heated to temperatures ranging from 250 to 500 °C to prevent any possible freezing. A dual residual gas analyzer system monitors the partial pressure of the implanted species in both vacuum chambers. The release rate of gas atoms in the second chamber is directly related to the mechanisms of transport within the metal bulk and also the process of desorption from the surface. For the case of helium, the diffusion coefficient was calculated to be 4.5×10−3 cm2/s at 250 °C, with an uncertainty of ±2×10−3 cm2/s. Helium retention coefficients on the order of 10−4 were obtained based on the experimental data.
Fusion Engineering and Design, 61-62, 245-250 (2002).
Allain, J. P., Nieto, M., COventry, M. D., Neumann, M. J., Vargas-Lopez, E., Ruzic, D. N.
The flowing liquid surface retention experiment (FLIRE) has been designed to provide fundamental data on the retention and pumping of He, H and other species in flowing liquid surfaces. The FLIRE concept uses an ion source with current densities near 0.5 mA/cm2 and a working distance of 30–40 mm. The ion source injects 300–5000 eV ions into a flowing stream of liquid lithium at nearly normal incidence. FLIRE is a dual chamber unit. The liquid lithium flows into one vacuum chamber isolating it from a bottom vacuum chamber. Two residual gas analyzers with a quadrupole mass spectrometer, monitor the partial pressure of the implanted species in each vacuum chamber measuring retention and diffusion coefficients. A liquid–metal (LM) injection system experiment has been carried out to verify the capability of transporting liquid lithium. Results show that liquid metal velocities of the order of 1 m/s can be achieved. Safety tests conclude that exposing 300 °C lithium to atmosphere result in benign chemical reactions. A test of the external and internal heating systems conclude that LM transfer lines can be heated to temperatures near 270 °C and ramp temperatures near 400 °C.
Nuclear Fusion, 42, 202-210 (2002).
Allain, J. P., Ruzic, D. N.
The absolute sputtering yields of D+, He+ and Li+ on deuterium saturated solid lithium have been measured and modelled at 45° incidence in the energy range 100-1000 eV. The Ion-surface InterAction Experiment (IIAX) was used to measure the absolute sputtering yield of lithium in the solid phase from bombardment with a Colutron ion source. The lithium sample was treated with a deuterium plasma from a hollow cathode source. Measurements also include bombardment of non-deuterium-saturated lithium surfaces. The results lead to the conclusion that the chemical state of the deuterium treated lithium surface plays a major role in the decrease of the lithium sputtering yield. Specifically, preferential sputtering of implanted deuterium atoms over lithium atoms in deuterium treated samples results in a decrease of at least 60% of the lithium sputtering yield, in the case of He+ bombardment. These results also demonstrate that lithium self-sputtering is well below unity and that the fraction of sputtered species in an ionic state ranges from 55 to 65% for incident particle energies between 100 and 1000 eV. Furthermore, correlation of Monte Carlo VFTRIM-3D simulations and IIAX experimental data demonstrate that the surface composition has a one to one ratio between deuterium and lithium components.
J. Appl. Phys., 91, 605-612 (2002).
Juliano, D. R., Ruzic, D. N., Allain, M. M. C., Hayden, D. B.
A computer simulation was created to model the transport of sputtered atoms through an ionized physical vapor deposition (IPVD) system. The simulation combines Monte Carlo and fluid methods to track the metal atoms that are emitted from the target, interact with the IPVD plasma, and are eventually deposited somewhere in the system. Ground-state neutral, excited, and ionized metal atoms are tracked. The simulation requires plasma conditions to be specified by the user. Langmuir probe measurements were used to determine these parameters in an experimental system in order to compare simulation results with experiment. The primary product of the simulation is a prediction of the ionization fraction of the sputtered atom flux at the substrate under various conditions. This quantity was experimentally measured and the results compared to the simulation. Experiment and simulation differ significantly. It is hypothesized that heating of the background gas due to the intense sputtered atom flux at the target is primarily responsible for this difference. Heating of the background gas is not accounted for in the simulation. Difficulties in accurately measuring plasma parameters, especially electron temperature, are also significant.
Surf. and Coatings Technol., 149, 161-170 (2002).
Li, Ning, Allain, J. P., Ruzic, D. N.
Reactive sputtering of aluminum oxide in a planar magnetron system is conducted with a mixture of O2 and Ar reacting with and bombarding an aluminum target. The aluminum target is powered by a pulsed directed current (DC) bias which functions to discharge the accumulated ions on the insulating AlOx film surface during the positive duty cycle and suppresses arc formation. A seven-turn helical antenna sits below the magnetron sputtering system in the vacuum system and delivers radio-frequency (RF) power to generate a secondary plasma in the chamber. This plasma can efficiently ionize the sputtered flux, achieving ionized physical vapor deposition (IPVD). A gridded energy analyzer (GEA) and a quartz crystal microbalance (QCM) are located in the substrate plane to allow the ion and neutral deposition rates to be determined. Electron temperature and electron density are measured by a RF compensated Langmuir probe. A RF power of 500 W significantly increases the deposition rate of AlOx up to half of the Al deposition rate in metallic mode at the total pressure of 1.33 Pa (10 mtorr). At 3.33 Pa (25 mtorr), the ionization fraction of Al atoms reaches 90%. In addition the RF power extends the range of O2 partial pressure in which the sputtering occurs in the metallic mode. SEM photos show that the secondary RF plasma makes the films smoother and denser due to a moderate level of ion bombardment. The deposition rates and ionization fractions fluctuate as a function of O2 partial pressure. These variations can be explained by the combined variation of sputtering at the target, electron temperature and electron density.
J. Vac. Sci. Technol., A, 19, 1004-1007 (2001).
Ranjan, R., Allain, J. P., Hendricks, M. R., Ruzic, D. N.
Ti and TiN films are used as diffusion barrier layers in Al and Cu metallization. They are often produced using physical-vapor-deposition techniques and are subject to energetic particle bombardment during subsequent processes. Therefore, the sputtering yield for ion-induced physical sputtering is important. The absolute sputtering yields of Ti and TiN target materials with 400–700 eV normally incident N and Ar ions are measured here. The experimental values are favorably compared to simulation results from TRIM.SP, which is a vectorized Monte Carlo code simulating ion–surface interaction using a binary collision mode. The phenomenon of reactive sputtering of Ti with incident N is also discussed.
J. Nucl. Materials, 290-293, 185-190 (2000).
Brooks, J. N., Rognlien, T., Ruzic, D. N., Allain, J. P.
A sputtering erosion/redeposition analysis was performed for three candidate tokamak fusion reactor liquid divertor surfaces–lithium, tin–lithium (Sn80Li20), and flibe (LiF+BeF2 salt). The analysis uses coupled edge-plasma, impurity-transport, and sputtering codes (UEDGE/WBC/VFTRIM), and available sputtering data. A pure-lithium surface strongly absorbs impinging D–T ions-this results in a high temperature, low density, (
200 eV, 1×1019 m−3) low-recycle plasma edge regime. Lithium appears to perform well in this regime. Although overall sputtering is high, self-sputtering is finite. Most (95%) of the sputtered lithium is confined to the near-surface region and redeposited on the divertor with the remainder (5%) also being redeposited after transport in the scrape-off layer. Lithium core plasma contamination is low (10−4 Li/D–T). Tin–lithium and flibe would likely operate in a high-recycle regime (e.g., 30 eV, 3×1020 m−3). Erosion/redeposition performance of these materials is also good, with finite self-sputtering and negligible core plasma contamination predicted, but with some concern about changing surface composition due to different constituent element redeposition distances.
J. Nucl. Materials, 290-293, 180-184 (2001).
Allain, J. P., Ruzic, D. N., Hendricks, M. R.
The absolute sputtering yields of D+, He+ and Li+ on solid lithium have been measured and modeled at low energies in the ion-surface interaction experiment (IIAX). The experiment has been extended to measure physical sputtering from liquid lithium surfaces bombarded by D+, He+ and Li+. A Colutron ion source is used to create and accelerate gaseous or metal ions onto the liquid metal target. A plasma cup removes any oxides and saturates the surface with deuterium. A small high-temperature, HV substrate heater is used to heat the 0.76 g lithium sample past its melting point to 200°C. Upon melting, a thin oxide layer is formed on the exposed lithium surface, which is cleaved by an in situ arm rotated in front of the target. Results suggest that the absolute sputtering yield of lithium is less than unity. In addition, the behavior of liquid lithium self-sputtering suggests stratification of the top liquid metal surface. This is consistent with VFTRIM-3D modeling, where D atoms migrate into the bulk while the first few monolayers remain mostly lithium.
J. Nucl. Materials, 290-293, 33-37 (2001).
Allain, J. P., Ruzic, D. N., Hendricks, M. R.
The absolute sputtering yields from bombardment of D+,He+ and Li+ on liquid tin–lithium eutectic have been measured and modeled at energies between 200 and 1000 eV. The Ion-surface InterAction Experiment (IIAX) has been optimized to reliably measure the absolute sputtering yield of many ion-target combinations including solid and liquid lithium. A Colutron ion source is used to create and accelerate gaseous or metal ions onto a liquid metal target. The bombarding ions are mass-selected through an E X B filter and decelerated near the target. The target can be rotated in order to provide variation in the angle of incidence. Deuterium plasma from a hollow cathode source is used to remove any remaining oxides. Upon melting of the sample a thin oxide layer forms and is cleaved by an in situ arm. Results show that sputtering yields from liquid tin–lithium are larger than pure lithium. In addition, modeling with VFTRIM-3D confirms that Li atoms segregate to the surface of liquid tin–lithium. This is consistent with results of ion fraction of sputtered atoms, which show a sputtered-atom ion fraction of 65% for liquid tin–lithium, equal to pure liquid lithium, and <10% for solid tin–lithium.
