Vijaya Surla

Sputtering Studies of Multi-Component Materials by Weight Loss and Cavity Ring-Down Spectroscopy (Postprint)

Abstract : We report sputtering studies of multi-component spacecraft materials. We employ two complementary diagnostic methods: weight loss measurements and cavity ring-down spectroscopy (CRDS). The weight loss measurements provide total sputter yields as a function of ion energy and incidence angle. We present sputter yields from weight loss measurements for xenon ion sputtering of molybdenum, quartz, boron nitride, and kapton. The CRDS provides species-specific sputtering data (number density and velocity) as well as information on the differential (angular) sputtering distributions. We present CRDS results for the sputtering of molybdenum (from a molybdenum sample), and demonstrate measurements of multi-component materials by measuring the sputtering of chromium, iron, and molybdenum from Inconel 718.

Absolute UV and VUV emission in the 110-400 nm region from 13.56 MHz driven hollow slot microplasmas operating in open air

We present absolute optical emission spectra in the 110–400 nm regions from radio-frequency-driven (13.56 MHz) hollow slot microplasmas operating in open air at atmospheric pressure. The term microplasma in our research refers to inter-electrode separation (100–600 µm) only, as electrode lengths are scalable from 1 to 30 cm. This creates an extended slot plasma and an associated afterglow plume as described herein. Spectra are presented for gas flows through the microelectrodes of argon and helium with small admixtures of hydrogen and nitrogen into open air. The spectra are discussed in terms of species origin and magnitude of the dominant emission lines. Atomic O and N lines dominate the 110–200 nm region, whereas, in the 200–400 nm region, NO, N2, and NH molecular lines are strongest. The role of the state in the open air microplasmas is discussed and the second positive system of molecular nitrogen (N2(C 3Πg–B 3Πg)), is used to measure the rotational (gas) temperature. Finally, we compare the efficiency and magnitude of light emission from the open air microplasmas with values attainable from commercial sealed mercury lamps in the UVB and UVC regions.

Differential sputter yield profiles of molybdenum due to bombardment by low energy xenon ions at normal and oblique incidence

We report differential sputter yield measurements for Xe+ ions incident on molybdenum for ion incidence angles of 0°, 15°, 30° and 45°, and ion energies of 250, 350, 500 and 750 eV. The angularly resolved profiles are obtained with a quartz crystal microbalance that is moved to a series of locations above the target. Total sputter yields found by integrating the differential sputter yields are in accord with the literature values. For normally incident ions the profiles are under-cosine, while for obliquely incident ions the profiles show azimuthal variation with pronounced forward sputtering lobes. We discuss the use of published expressions from Zhang and Zhang (2004 Radiat. Eff. Defects Solids 159 301–7) for describing the measured profiles. We find that the profiles can be reasonably described with these expressions by using two free parameters: the total sputter yield (Y) and a characteristic energy (E*). We compare changes in angular sputtering profiles from lightly versus heavily sputtered targets.

Detection of sputtered metals with cavity ring-down spectroscopy

We report on use of cavity ring-down spectroscopy (CRDS) as a means to detect and quantify ion sputtering of refractory metal species. CRDS measurements are made with a neodymium:YAG-pumped optical parametric oscillator laser system in the 375-400 nm region. CRDS sputtering measurements are presented for argon ions incident on iron, aluminum, molybdenum, and titanium. The measurements are based on absorption from fine-structure levels of the electronic ground-state multiplets. For each species, characteristic spectra are provided, the dependence of sputtered particle number density on the beam current is examined, measured densities are compared with a sputter model, and detection limits are determined. For iron, aluminum, and titanium we probe multiple fine-structure levels within the ground-state multiplet and obtain information on their relative populations.

Sputter erosion measurements of titanium and molybdenum by cavity ring-down spectroscopy

We report cavity ring-down spectroscopy measurements of the gas-phase number density of titanium and molybdenum sputtered by argon ions. A neodymium: yttrium–aluminum–garnet pumped optical parametric oscillator laser system is used to probe optical absorption features of titanium and molybdenum in the 375–400 nm region. For an 18 mA ion beam current, and 750 eV ions, we find that the spatially averaged number density of sputtered atoms is 6.4±0.6×108 and 5.1±0.5×108 cm−3, for titanium and molybdenum, respectively. The measured number densities exhibit the expected linear behavior versus beam current, and are in reasonable agreement with values found from a simple sputtering model. Our current configuration yields number density detection limits of ∼9×106 and ∼7×105 cm−3, for titanium and molybdenum, respectively. The technique may ultimately provide a means to measure sputter erosion rates, which are of particular importance for the electric propulsion field.

Electrical and optical emission characteristics of radio-frequency-driven hollow slot microplasmas operating in open air

We employ hollow slot electrodes, with pd values of ∼10 Torr cm and average E/N values of ∼70 Td, to create plasmas in open air. We measure the 13.56 MHz Irf–Vrf electrical characteristics of the plasma. Stable discharges, with sinusoidal currents, are obtained up to power densities of 14 kW/cm3, and root-mean-square radio-frequency (rf) currents of 1.5 A/cm of slot length, before nonsinusoidal currents and rf glow-to-arc transitions occur. We report the absolute optical emission in the vacuum ultraviolet region located between 110 and 155 nm, with a focus on the 149 nm atomic nitrogen line. For this atomic N line alone, we find an emitter efficiency of 0.0024.

AZIMUTHAL DIFFERENTIAL SPUTTER YIELDS OF MOLYBDENUM BY LOW ENERGY Xe + BOMBARDMENT

We present the development of a sputtering diagnostic based on a quartz crystal microbalance (QCM). The QCM system allows angularly resolved (differential) sputter yield measurements as a function of both polar angle and azimuthal angle. We discuss development of the QCM diagnostic and present sputtering data for molybdenum by xenon ions in the energy range 250-750 eV for bombardment at 0o, 30o, and 45o angles of incidence. Total sputter yields, found from integrating the differential sputtering profiles, are found to be in reasonable agreement with those in the literature. We fit the differential sputtering profiles with polynomials and theoretical expressions. For the energy range studied, we find under-cosine profiles for normally incident ions, and profiles dominated by forward scattering for the non-normally (obliquely) incident ions. Nomenclature y = differential sputtering yield (atoms/ion/steradian) Py = polynomial fit to differential sputtering yield (atoms/ion/steradian) Zy = theoretical fit to differential sputtering yield (atoms/ion/steradian) Y = total sputter yield (atoms/ion) YP = total sputter yield based on polynomial fit (atoms/ion) YZ = total sputter yield based on theoretical fit (atoms/ion) α = polar angle measured from surface normal β = ion incidence angle measured from surface normal φ = azimuthal angle in target plane measured from forward scattering direction

Species-Specific Sputtering Measurements with Cavity Ring-Down Spectroscopy

Abstract : We report sputtering studies using cavity ring-down spectroscopy (CRDS). The high sensitivity of the technique and its non-intrusive nature make it amenable to both in situ device studies as well as basic characterization studies. We provide demonstrative measurements of sputtered particles showing the ability to determine species-specific number density and velocity. We summarize a spatial-scanning approach for differential sputter yield measurements and give a measurement example based on a tantalum target. We discuss the use of CRDS for measurement of multi-component materials and provide experimental results for detection of a Fe-Mn target as well as a proposed detection scheme for boron nitride.

Absolute UV and VUV emission in the 100-400 nm region from 13.56 MHz driven hollow slot microplasmas operating in open air

Summary form only given. We present absolute optical emission spectra in the 110-400 nm region from radio-frequency driven (13.56 MHz) hollow slot microplasmas operating in open air at atmospheric pressure. Spectra are presented for gas flows through the micro-electrodes of argon and helium with small admixtures of hydrogen and nitrogen into open air. The spectra are discussed in terms of species origin and magnitude of the dominant emission lines. Atomic O and N lines dominate the 110-200 nm region whereas in the 200-400 nm region, NO, N/sub 2/, N/sub 2//sup +/ and NH molecular lines are strongest. The role of the N/sub 2/ (A 3/spl square/u+) state in the open air micro-plasmas is discussed and the 2nd positive system of molecular nitrogen, (N/sub 2/ (C 3/spl square/g-B 3/spl square/g)), is used to measure the rotational (gas) temperature. Finally, we compare the efficiency and magnitude of light emission from the open air microplasmas with values attainable from commercial sealed mercury lamps in the UVB and UVC regions.