Engineering topics
Publications and source records attributed to Tombrello, T. A..
We report the use fo delta-doped charge-coupled devices (CCDs) for direct detection of electrons in the 50-1500 eV energy range. These are the first measurements with a solid state device to detect electrons in this energy range.
In this paper, we will briefly discuss delta-doped CCDs and their application of low-energy electron detection. We show that modification of the surface this way can greatly improve sensitivity to low energy electrons.
A pulsed DT neutron generator system, similar to that used in commercial well logging, offers the possibility of performing accurate elemental analyses to depths of tens of centimeters in a few seconds with the probe on the body's surface.
A new model is proposed to explain both localized damage regions and preferential etching of damage tracks caused by the passage of energetic ions in insulators. The formation of each region of extended defects is initiated by the Auger decay of a vacancy produced in an inner electronic shell of an atom of the insulator by an incident ion. This decay produces an intense source of ionization within a small volume around the decaying atom, which causes decomposition of the material in a manner similar to that observed in pulsed laser irradiation. The resulting chemical or crystalline modification of the material is the latent track, which can be preferentially etched due to its changed structure.
The use of thin, self-supporting carbon catcher foils allows one to measure sputtering yields in a broad range of materials with high sensitivity. Analyzing the foils with Rutherford forward scattering, sputtered Al, Si and P surface densities down to 5 x 10 to the 13th per sq cm with uncertainties of about 20 percent have been measured.
Track damage and the associated ejection of atoms and molecules from insulators, which occur as a result of ion-induced electronic excitation, are of interest both in their own right and because of the mechanisms through which the energy in the excited electrons is transformed into atomic motion. In this paper an overview is given of the phenomena that are observed. We show that there is a remarkable similarity between the damage profile along the ion's track in the solid and the yield of ejected atoms at the energy that corresponds to each point on the track. It is also seen that the density of extended defects (or, correspondingly, the ejected particle yield) appears to have a 'universal' form that is weakly dependent on the type of material. In the model presented this is a consequence of the inner-shell ionization of light elements in the solid by the incident ion; the resulting Auger decay produces an intense ionization spike that locally triggers the track formation/erosion process. This model allows the estimation of erosion yields/damage profiles for different ions and materials.
The diffusion of Xe in olivine, a major mineral in both meteorites and linear samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500 C. The fraction of implanted XE retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100 percent occurred for an implanting dose of 3 x 10 to the 15th power Xe ions/sq cm. Retention was less at lower doses, with approximately more than or = 50 percent loss at one hundred trillion Xe ions/sq cm. Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 micrometer layer for ten million years was calculated as a function of metamorphic temperature. Previously announced in STAR as N83-18528
Phenomena that involve accelerated ions in stellar processes that can be simulated with laboratory accelerators are described. Stellar evolutionary phases, such as the CNO cycle, have been partially explored with accelerators, up to the consumption of He by alpha particle radiative capture reactions. Further experimentation is indicated on reactions featuring N-13(p,gamma)O-14, O-15(alpha, gamma)Ne-19, and O-14(alpha,p)F-17. Accelerated beams interacting with thin foils produce reaction products that permit a determination of possible elemental abundances in stellar objects. Additionally, isotopic ratios observed in chondrites can be duplicated with accelerator beam interactions and thus constraints can be set on the conditions producing the meteorites. Data from isotopic fractionation from sputtering, i.e., blasting surface atoms from a material using a low energy ion beam, leads to possible models for processes occurring in supernova explosions. Finally, molecules can be synthesized with accelerators and compared with spectroscopic observations of stellar winds.
The sputtering of UF4 targets by energetic beams of O-16, F-19, and Cl-35 ions has been investigated for beam energies in the range 0.12 to 1.5 MeV/amu. The sputtering yields, which follow the same trend as the electronic part of the projectile energy loss in the material, are observed to have a strong dependence on the charge state of the incident ions. Data have been taken both in transmission and reflection (0 and 180 deg to the incident beam direction, respectively). Energy spectra of the neutral sputtered particles have been obtained for 5 MeV F-19 ions and for 13 MeV Cl-35 ions; in both cases the spectrum has a Maxwellian form. The data obtained are compared with several models of the high energy sputtering process.
Silicon, silicon dioxide, and silicon nitride have been sputtered with chlorine ions at 5 MeV and 20 MeV. While the yield from the silicon target was unmeasurably low, the insulating compounds exhibited the enhanced yields observed in other insulating targets. The yield follows the electronic stopping power and seems to be independent of the target's thermal properties. Some of the data suggest that the enhanced sputtering mechanism may be active in extremely thin films (not less than 3 monolayers).
Sputtering yields of solid SO2 by high energy ions were measured in order to study the mechanism for sputtering dielectrics with ions in the electronic stopping power region. The incident ions were helium and fluorine with energies ranging from 1.5 MeV to 25 MeV. Yields as high as 7000 SO2 molecules/incident F ion were measured; the 1.5 MeV He-4 beam had a sputtering yield of 50. The data are compared to yield measurements made on UF4 and H2O targets. There is a striking similarity in the yield as a function of the incident F energy for all three targets. The data compare favorably with theoretical yield curves based on a new model for the sputtering which considers the electronic excitations induced in the target by the incident beam. Measurements and calculations of this sort are also useful in understanding processes which occur on the surface of Jupiter's satellite Io, which is covered with SO2 frost and bombarded by energetic ions trapped in the Jovian magnetosphere.
It has been found that the adhesion of thin metal films on insulators, semiconductors, and metals could be improved by subjecting the material to a high-energy ion bombardment. Griffith et al. (1982) have first suggested a use of this technique with insulators. The present investigation has the objective to determine the mechanism for the adhesion enhancement. A description is presented of a preliminary transmission electron microscopy (TEM) study of thinned bonded samples of silver on silicon using electron diffraction. It is found that irradiation of a variety of thin film-substrate combinations by heavy ion beams will provide a remarkable improvement in the adherence of the film. The evidence for the mechanism involved in the enhancement of adhesion is discussed.
A common aspect of the models of such processes as track formation and induced desorption of molecular ions is the requirement that the electronic excitation have a sufficiently long lifetime (of the order of picoseconds) so that energy can be transferred to atomic motion. Attention is called to several phenomena recently observed indicating that atomic motion/chemical rearrangement can occur even when the electronic excitation is shorter lived (of the order of femtoseconds). Two such processes, which are also induced by MeV ion bombardment, are track damage in heavily doped compound semiconductors and greatly enhanced adhesion of metal films to metallic and semiconducting substrates. Since neither of these effects can easily be accommodated within the existing theoretical models, it is thought possible that an even richer variety of ion-induced effects will be discovered.
Watson and Haff (1980) have discussed a theory which is designed to explain quantitatively isotopic fractionation effects observed during sputtering of simple or complex targets. This theory is based on the assumption that most of the atoms sputtered from a surface originate in the top monolayer. The present investigation is mainly concerned with a direct experimental test of that assumption. The sputtering of both solid and liquid phases of gallium, indium, and the gallium-indium eutectic alloy is studied. Results obtained with the aid of ion scattering and Auger spectroscopy show that, in agreement with rough theoretical expectations, the surface monolayer of a gallium-indium alloy with 16.5 percent indium in bulk contains more than 94 percent indium, while the next layer can be only slightly enriched.
Electronic excitation induced by MeV/amu ion beams in a variety of materials has been employed successfully for a number of applications. The examples that will be presented are: modification of the surface reflectivities of optical materials; sputter-erosion of dielectrics; and enhancement of the adhesion of thin film coatings. All of these effects arise from the loss of energy by the ion beam to electrons in the target material; the mechanisms involved are at best qualitatively understood. This paper will stress not only the exploitation of such high energy bombardment techniques but will also briefly review attempts to expose the underlying causes.
The erosion of frozen SO2 due to bombardment by both light and heavy ions (He and F) was measured for bombarding energies of 0.08 to 1.3 MeV/amu. The number of SO2 molecules ejected from the target per incident ion (i.e., the sputtering yield) was 50 for 1.5 MeV He ions and 7300 for 6 MeV F ions. Ion bombardment followed by heating produced an oxygen/sulfur residue which was much more stable against subsequent ion bombardment than the initial frozen SO2. The erosion rate of SO2 frost on Jupiter's moon Io depends strongly on the elemental composition and energy spectra of the magnetospheric ion flux which bombards the surface. The combined effects of ion bombardment and heating which produced residues on the target substrates may also occur on Io from magnetospheric ion bombardment and heating by volcanism. The experimental results compare favorably with a new model of the sputtering process which considers the energy loss of the incident ion to electronic excitation in the target.
Sputtering yields of solid SO2 by high energy ions were measured in order to study the mechanism for sputtering dielectrics with ions in the electronic stopping power region. The incident ions were helium and fluorine with energies ranging from 1.5 MeV to 25 MeV. Yields as high as 7000 SO2 molecules/incident F ion were measured; the 1.5 MeV He4 beam had a sputtering yield of 50. The data are compared to yield measurements made on UF4 and H2O targets. There is a striking similarity in the yield as a function of the Energy for all three targets. The data compare favorably with theoretical curves based on a model for the sputtering which considers the electronic excitations induced the target by the incident beam. Measurements and calculations of the sort are also useful in understanding processes which occur on the surface of Jupiter's satellite Io, which is covered with SO2 frost and bombarded by energetic ions trapped in the Jovian magnetosphere.
The diffusion properties of xenon implanted into feldspar, a major mineral in meteorites and lunar samples, are investigated in light of the importance of xenon diffusion in the interpretation of early solar system chronologies and the retention time of solar-wind-implanted Xe. Known doses of Xe ions were implanted at an energy of 200 keV into single-crystal plagioclase targets, and depth profiles were measured by alpha particle backscattering before and after annealing for one hour at 900 or 1000 C. The fraction of Xe retained following annealing is found to be strongly dependent on implantation dose, being greatest at a dose of 3 x 10 to the 15th ions/sq cm and decreasing at higher and lower doses. Xe retention is also observed to be unaffected by two-step anneals, or by implantation with He or Ar. Three models of the dose-dependent diffusion properties are considered, including epitaxial crystal regrowth during annealing controlled by the extent of radiation damage, the creation of trapping sites by radiation damage, and the inhibition of recrystallization by Xe during annealing