C. E. Sosolik

A technique for accurate measurements of ion beam current density using a Faraday cup

We have performed measurements of the spatial distribution of current in various alkali and reactive ion beams over the energy range 5–600 eV using a Faraday cup. Ion beam current densities have been extracted from these measurements using a simple deconvolution procedure. Our results reveal that the beams are Gaussian in shape with a constant width, σ, for energies greater than approximately 75 eV and for all ion species investigated. This width is consistent with that determined from the distribution of oxygen on a Cu(001) crystal after an O+ ion beam deposition, measured using Auger electron spectroscopy. Using the measurement technique outlined in this article, together with the linear relationship between current density and Faraday cup current, it is possible to determine the beam current density using a single current measurement.

A hyperthermal energy ion beamline for probing hot electron chemistry at surfaces

An ultrahigh vacuum ion beamline and chamber have been assembled to produce hyperthermal (<400 eV) energy ions for studying hot electron chemistry at surfaces. The specific design requirements for this modified instrument were chosen to enable the exposure of a metal-oxide-semiconductor (MOS) device to monoenergtic, well-collimated beams of alkali ions while monitoring both the scattered beam flux and the device characteristics. Our goal is to explore the role that hot electrons injected toward the MOS device surface play in the neutralization of scattered ions. To illustrate the functionality of our system, we present energy-resolved spectra for Na+, K+, and Cs+ ions scattered from the surface of a Ag(001) single crystal for a range of incident energies. In addition, we show MOS device current-voltage characteristics measured in situ in a new rapid-turnaround load lock and sample translation stage.

Thermally enhanced neutralization in hyperthermal energy ion scattering

Neutralization probabilities are presented for hyperthermal energy Na+ ions scattered from a Cu(001) crystal as a function of surface temperature and scattered velocity. A large enhancement in neutralization is observed as the temperature is increased. Velocity-dependent charge transfer regimes are probed by varying the incident energy, with the most prominent surface temperature effects occurring at the lowest energies. The data agree well with results obtained from a model based on the Newns-Anderson Hamiltonian, where the effects of both temperature and velocity are incorporated.

First multicharged ion irradiation results from the CUEBIT facility at Clemson University

A new electron beam ion trap (EBIT) based ion source and beamline were recently commissioned at Clemson University to produce decelerated beams of multi- to highly-charged ions for surface and materials physics research. This user facility is the first installation of a DREEBIT-designed superconducting trap and ion source (EBIS-SC) in the U.S. and includes custom-designed target preparation and irradiation setups. An overview of the source, beamline, and other facilities as well as results from first measurements on irradiated targets are discussed here. Results include extracted charge state distributions and first data on a series of irradiated metal-oxide-semiconductor (MOS) device targets. For the MOS devices, we show that voltage-dependent capacitance can serve as a record of the electronic component of ion stopping power for an irradiated, encapsulated oxide target.

Towards hot electron mediated charge exchange in hyperthermal energy ion–surface interactions

We have made Na (+) and He (+) ions incident on the surface of solid state tunnel junctions and measured the energy loss due to atomic displacement and electronic excitations. Each tunnel junction consists of an ultrathin film metal-oxide-semiconductor device which can be biased to create a band of hot electrons useful for driving chemical reactions at surfaces. Using the binary collision approximation and a nonadiabatic model that takes into account the time-varying nature of the ion-surface interaction, the energy loss of the ions is reproduced. The energy loss for Na (+) ions incident on the devices shows that the primary energy loss mechanism is the atomic displacement of Au atoms in the thin film of the metal-oxide-semiconductor device. We propose that neutral particle detection of the scattered flux from a biased device could be a route to hot electron mediated charge exchange.

Kinetic energy offsets for multicharged ions from an electron beam ion source

Using a retarding field analyzer, we have measured offsets between the nominal and measured kinetic energy of multicharged ions extracted from an electron beam ion source (EBIS). By varying source parameters, a shift in ion kinetic energy was attributed to the trapping potential produced by the space charge of the electron beam within the EBIS. The space charge of the electron beam depends on its charge density, which in turn depends on the amount of negative charge (electron beam current) and its velocity (electron beam energy). The electron beam current and electron beam energy were both varied to obtain electron beams of varying space charge and these were related to the observed kinetic energy offsets for Ar4+ and Ar8+ ion beams. Knowledge of these offsets is important for studies that seek to utilize slow, i.e., low kinetic energy, multicharged ions to exploit their high potential energies for processes such as surface modification. In addition, we show that these offsets can be utilized to estimate the effective radius of the electron beam inside the trap.

Encapsulating Ion-Solid Interactions in Metal-Oxide-Semiconductor (MOS) Devices

We report on a measurement of low energy ion irradiation effects on as-grown films of SiO2 on a Si substrate. Beams of normally incident Na+ ions with kinetic energies of 2 keV to 5 keV were focused onto ~ 1900 Å SiO2 films. Aluminum top metal contacts were subsequently deposited onto these targets such that irradiated regions and unexposed (pristine) regions of the target could be compared using capacitance-voltage (C-V) measurements of individual metal-oxide-semiconductor (MOS) devices. The C-V data reveal an energy-dependent shift in the flatband voltage ( VFB) that can be returned to its near-pristine value by a low temperature anneal. An increase in the density of interface states ( Dit) inferred from the C-V curves is found to have a superlinear dependence on the incident kinetic energy. These data are consistent with previously observed UV radiation effects on MOS oxides, where transferred energy leads to electron-hole pair production and the diffusion and trapping of holes throughout the oxide. Our measured trapped hole densities are compared with calculated densities, which are based on the incident ion dose and the predicted ion implantation range, to arrive at a fractional yield for hole survival and measurement within an encapsulated MOS device.

Energy dissipation of highly charged ions on Al oxide films

Slow highly charged ions (HCIs) carry a large amount of potential energy that can be dissipated within femtoseconds upon interaction with a surface. HCI-insulator collisions result in high sputter yields and surface nanofeature creation due to strong coupling between the solids electronic system and lattice. For HCIs interacting with Al oxide, combined experiments and theory indicate that defect mediated desorption can explain reasonably well preferential O atom removal and an observed threshold for sputtering due to potential energy. These studies have relied on measuring mass loss on the target substrate or probing craters left after desorption. Our approach is to extract highly charged ions onto the Al oxide barriers of metal-insulator-metal tunnel junctions and measure the increased conductance in a finished device after the irradiated interface is buried under the top metal layer. Such transport measurements constrain dynamic surface processes and provide large sets of statistics concerning the way individual HCI projectiles dissipate their potential energy. Results for Xe(q +) for q = 32, 40, 44 extracted onto Al oxide films are discussed in terms of postirradiation electrical device characteristics. Future work will elucidate the relationship between potential energy dissipation and tunneling phenomena through HCI modified oxides.

ENERGY- AND ANGLE-DEPENDENT TRENDS IN THE TRAPPING PROBABILITY OF O+ INCIDENT ON CU(001)

Abstract We have measured the trends in the trapping probability of 5–600 eV O + ions incident on Cu (0 0 1) at 45 ∘ and along the sample normal. The trapping probabilities, measured using Auger Electron Spectroscopy (AES), depend strongly on the incident ion energy, but not on the incident angle. Significant differences are observed between these trends and those previously measured for Na + incident on Cu (0 0 1) over a similar range of energies and angles, particularly for the 45 ∘ angle of incidence. We conclude that these observations reflect differences in the ion-surface scattering potentials.

A gas cell apparatus for measuring charge exchange cross sections with multicharged ions

A gas cell apparatus to measure charge exchange cross sections for charge state- and energy-resolved ion beams with neutrals is described. The design features a short well-defined interaction region required for beams of multicharged ions with high cross sections. Our method includes measuring the beam transmission at four different neutral pressures and extracting the cross section from the slope of a beam loss vs pressure plot. The design and procedure were tested for Ar+ interacting with neutral Ar gas over the incident ion energy range of 1.0-5.0 keV. The charge exchange cross sections agree well with previous complementary measurement techniques.