Segregation of copper to (100) and (111) silicon surfaces from internal Cu{sub 3}Si precipitates

Segregation of transition metal impurities to surfaces or interfaces can have detrimental or beneficial effects in silicon-based microelectronic devices. Controlled segregation of impurities to regions remote from device structures, i.e. gettering is routinely used to prevent uncontrolled segregation to critical regions which may cause failure. Internal gettering is a widely used process in which oxide precipitates and associated lattice defects provide sites for precipitation of metal-silicide phases. Segregation of impurities onto surfaces of internal microcavities has also been examined as a potential gettering process. It was observed that gettering to cavities can dissolve pre-existing internal metal silicide precipitates of Cu, Au and Ni. The energetics of copper segregation to silicon surfaces were examined by measuring the Cu coverage after equilibration between Cu on the surface and internal Cu{sub 3}Si, for which the Cu chemical potential is known. For oxide-free surfaces the Cu coverage was close to one monolayer on (111) surfaces but was much smaller on (100) surfaces. The Cu coverage was greatly reduced by oxide passivation of the surface. LEED showed the 7 x 7 structure of the clean (111) silicon surface converted to a quasiperiodic 5 x 5 structure after equilibrating with Cu{sub 3}Si. The 2 x 1 LEED patterns for (100) surfaces indicated no change in surface structure due to the Cu{sub 3}Si. These results show that the free energy of copper in Cu{sub 3}Si is higher than that of copper on (111) surfaces but lower than that of copper on (100) surfaces

Release of H and He from TiC, stainless steel and graphite by pulsed electron and furnace heating

The release of implanted D and /sup 3/He from TiC coatings, SS 304 and graphite by pulsed electron beam (e-beam) heating and furnace heating has been investigated. Low fluence implants of D or /sup 3/He and saturation fluence D implants have been studied for 0.5 - 1.5 keV D and 3 keV /sup 3/He. The retained D or /sup 3/He was monitored by ion beam analysis. The 50 ns e-beam pulsing resulted in the release of D in all materials and was compared with release during isochronal annealing in a furnace. A substantial enhancement in the fractional D release was found for D saturated TiC (0.25 D to host atom ratio) compared with low fluence implants. In contrast no enhancement of D release was observed for D saturated graphite and SS 304 compared with low fluence implants. Release of /sup 3/He from TiC was also obtained by e-beam pulsed heating and this release was not affected by the presence of saturation concentrations of D. Comparison to furnace anneals and the calculated time evolution of the temperature profiles suggests a simple model for the D release based on diffusion-limited release in the case of pulsed e-beam treatments and trap-limited release in the case of furnace bulk heating. These processes are closely related to hydrogen recycle in tokamaks and have implications for T inventory control and He ash removal

Tritium retention in tungsten exposed to intense fluxes of 100 eV tritons

Tungsten is a candidate material for ITER as well as other future magnetic fusion energy devices. Tungsten is well suited for certain fusion applications in that it has a high threshold for sputtering as well as a very high melting point. As with all materials to be used on the inside of a tokamak or similar device, there is a need to know the behavior of hydrogen isotopes embedded in the material. With this need in mind, the Tritium Plasma Experiment (TPE) has been used to examine the retention of tritium in tungsten exposed to very high fluxes of 100 eV tritons. Both tungsten and tungsten containing 1% lanthanum oxide were used in these experiments. Measurements were performed over the temperature range of 423 to 973 K. After exposure to the tritium plasma, the samples were transferred to an outgassing system containing an ionization chamber for detection of the released tritium. The samples were outgassed using linear ramps from room temperature up to 1,473 K. Unlike most other materials exposed to energetic tritium, the tritium retention in tungsten reaches a maximum at intermediate temperatures with low retention at both high and low temperatures. For the very high triton fluences used (>10{sup 25} T/m{sup 2}), the fractional retention of the tritium was below 0.02% of the incident particles. This report presents not only the results of the tritium retention, but also includes the modeling of the results and the implication for ITER and other future fusion devices where tungsten is used

Time-resolved measurements of hydrogen and deuterium fluxes in the ASDEX plasma boundary

Hydrogen and deuterium fluxes parallel to the toroidal magnetic field were measured in the plasma boundary of ASDEX using graphite collector probes. Time resolution of the order of 100 ms can be obtained by rotating the cylindrical probes behind an aperture during the discharge. The trapped amount of hydrogen was determined by subsequent thermal desorption; in the analyses of deuterium the D(/sup 3/He,p)/sup 4/He nuclear reaction was used. Both methods yield quantitative results. Measurements were done for limiter and divertor discharges in the range of 4 to 20 cm outside the limiter or separatrix. The time distributions show a maximum flux at the beginning and the end of the discharge. The relatively lower flux during the plateau phase of the discharge is in the range 10/sup 15/ to 2 x 10/sup 17/ cm/sup -2/ sec/sup -1/, depending on the radial probe position; the maximum values are higher by a factor of 5 to 50. During neutral hydrogen injection, an additional maximum can be observed. The radial l/e-decay length is about 0.9 cm in front and 0.4 cm behind the fixed limiter. The results are compared with independent measurements in ASDEX and other plasma machines

Precipitation in ion-implanted Al during electron beam pulsed annealing

TEM and ion channeling were used to examine the microstructure of Al implanted with Zn or Sb following pulsed electron beam annealing with deposited energies of 0.7 to 1.6 J/cm/sup 2/. The Zn-implanted samples show a high density of dislocations in the near surface region. Zn precipitation is not seen in the electron diffraction patterns. For Sb, randomly oriented AlSb precipitates are observed, and precipitation is inferred to have occurred in molten Al. This is accounted for with the Al-Sb binary phase diagram

Erosion and deposition of metals and carbon in the DIII-D divertor

Net erosion rates at the outer strike point of the DIII-D divertor plasma were measured for several materials during quiescent H-mode operation with deuterium plasmas. Materials examined include graphite, beryllium, tungsten, vanadium and molybdenum. For graphite, net erosion rates up to 4 nm/sec were found. Erosion rates for the metals were much smaller than for carbon. Ion fluxes from Langmuir probe measurements were used to predict gross erosion by sputtering. Measured net erosion was much smaller than predicted gross erosion. Transport of metal atoms by the plasma across the divertor surface was also examined. Light atoms were transported farther than heavy atoms as predicted by impurity transport models

Depth profiling of oxidized a-C:D Layers on Be -- A comparison of {sup 4}He RBS and {sup 28}Si ERD analysis

In applications dealing with the deposition of amorphous hydrogenated carbon layers or in the determination of the composition of deposited layers on the walls of nuclear fusion plasma experiments, the analysis of mixtures of light elements on heavy substrates is necessary. Depth profiling by means of RBS is often difficult due to the overlap of the backscattering intensities of different constituents from different depths. The erosion and reaction of deposited amorphous deuterated carbon (a-C:D) films with a Be substrate due to annealing in air poses an analytical challenge especially if simultaneously the exchange of hydrogen isotopes should be monitored. The analysis of the different recoiling atoms from collisions with heavy ions in Elastic Recoil Detection (ERD) can provide a tool which resolves all constituents in a single analysis. In the present study the composition of intermixed layers on Be containing H, D, Be, C and O has been analyzed using conventional {sup 4}He RBS at 2.2 MeV together with 2.5 MeV {sup 4}He ERD for hydrogen isotope analysis. At these energies, an overlap of signals from different constituents could be avoided in most cases. As alternative method heavy ion ERD using Si{sup 7+} ions extracted from a 5 MeV Tandem Van de Graff accelerator was investigated. At a scattering angle of 30{degree} Si ions could not be scattered into the detector and a solid state detector without protecting foil could be used. Even in the intermixed layers at terminal energies of 5 MeV the heavy constituents could be separated while signals from recoiling hydrogen and deuterium atoms could be resolved on top of the signal from the Be substrate. For the analysis of the RBS and ERD data the newly developed spectra simulation program SIMNRA has been used which includes a large data bank for scattering and nuclear reaction cross sections. The depth profiles of all constituents extracted from the simulation are compared for both methods