Low-Energy Electron Attachment at Sub-meV Resolution
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electron attachment resonances negative ions↗
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Pulsed, gridded electron reversal ionizer
The design and performance of a new-type source of negative ions is presented. This device retains the idea of the so-called reversal electron attachment detector to produce low-energy electrons by reversing electron trajectories at an electrode. Electrons are produced in the present device by several ribbon filaments that surround a cylindrical wire grid. They are accelerated through the grid, then decelerated to zero or near-zero velocity at the surface of a tube. The tube (anode) is perforated with small holes through which effuse the target molecules. Electrons attach to form either parent and/or fragment negative ions. Attachment takes place at a location above the anode corresponding to a match between the electron energy and the attachment resonance energy. Negative ions are pulsed out of the attachment region and focused onto the entrance plane of a quadrupole mass selector. Electron and ion trajectory calculations are presented and the performance illustrated with six molecules having attachment resonances in the range 0.0-2.2 eV. The detection sensitivity is below 1 part-per-trillion, with a calculated conversion rate of 1.3 percent. Signal nonlinearity is discussed in terms of ion space-charge effects. The device also produces positive ions, and an analogous treatment may be carried through to characterize its performance in this mode.
Boumsellek, S.↗
Langmuir Probe Measurements of Inductively Coupled Plasma in CF4/AR/O2 Mixtures
Inductively coupled plasmas of CF4:Ar:O2, which have been of importance to material processing, were studied in the GEC cell at 80:10:10, 60:20:20, and 40:30:30 mixture ratios. Radial distributions of plasma potential (V(sub p)), electron and ion number densities (n(sub e) and n(sub i)), electron temperature (T(sub e)), and electron energy distribution functions (EEDFs) were measured in the mid-plane of plasma across the electrodes in the pressure range of 10-50 mTorr, and RF power of 200 and 300 W. V(sub p), n(sub e) and n(sub i), which peak in the center of the plasma, increase with decrease of pressure. T(sub e) also increases with pressure but peaks toward the electrode edge. Both V(sub p) and T(sub e) remain nearly independent of RF power, whereas n(sub e) and n(sub i) increase with power. In all conditions the EEDFs exhibit non-Maxwellian shape and are more like Druyvesteyn form at higher energies. They exhibit a broad lip in the energy range 0-10 eV suggesting an electron loss mechanism, which could be due to via resonance electron attachment processes producing negative ions in this rich electronegative gas mixture. This behavior is more prominent towards the electrode edge.
Rao, M. V. V. S.↗
Langmuir Probe Measurements of Inductively Coupled Plasmas in CF4/Ar/O2 Mixtures
Inductively coupled plasmas of CF4:Ar:O2, which have been of importance to material processing, were studied in the GEC cell at 80:10:10, 60:20:20, and 40:30:30 mixture ratios. Radial distributions of plasma potential (V(sub p)), electron and ion number densities (n(sub e) and n(sub i), electron temperature (T(sub e)), and electron energy distribution functions (EEDFs) were measured in the mid-plane of plasma across the electrodes in the pressure range of 10-50 mTorr, and RF (radio frequency) power of 200 and 300 W. V(sub p), n(sub e) and n(sub i), which peak in the center of the plasma, increase with decrease of pressure. T(sub e) also increases with pressure but peaks toward the electrode edge. Both V(sub p) and T(sub e) remain nearly independent of RF power, whereas n(sub e) and n(sub i) increase with power. In all conditions the EEDFs exhibit non-Maxwellian shape and are more like Druyvesteyn form at higher energies. They exhibit a broad dip in the energy range 0-10 eV suggesting an electron loss mechanism, which could be due to via resonance electron attachment processes producing negative ions in this rich electronegative gas mixture. This behavior is more prominent towards the electrode edge.
Rao, M. V. V. S.↗
Measuring Traces Of Oxygen By Resonant Electron Attachment
Method of detecting trace amounts of oxygen based on dissociative attachment of electrons to oxygen molecules followed by measurement of resulting flux of negative oxygen ions in mass spectrometer. High sensitivity achieved in method by exploiting resonance in dissociative attachment of electrons to oxygen molecules: electron-attachment cross section rises to high peak at incident electron kinetic energy of 6.2 eV. Relative concentrations below 1 ppb detected. Devised to increase sensitivity of detection of oxygen in processing chambers in which oxygen regarded as contaminant; for example, chambers used in making semiconductor devices and in growing high-purity crystals.
Man, Kin Fung↗
Ultralow Energy Electron Attachment at Sub-Millielectron Volt Resolution
The technique of rare-gas photoionization has been extended by use of direct laser ionization to electron energies epsilon in the range 0-100 meV, with a resolution Delta(epsilon) of 0.4-0.5 meV (FWHM). Tunable UV light at (Lambda)276 nm is produced using a pulsed Nd:YAG laser and nonlinear mixing techniques. The beam is frequency tripled in a pulsed jet of xenon. The VUV radiation, tunable at (Lambda)92 nm, is then used to photoionize Xe at its 2P(sub 1/2) threshold (single-photon ionization). The photoelectrons produced interact with admixed target gas to generate negative ions through the s-wave capture process. Recent results in electron attachment to SF(sub 6) will be reported which show resonance structure at the opening of the ground-state vibrational channels. This structure corresponds to the process of vibrational excitation + attachment, which is superimposed on the underlying s-wave (direct) capture process. It should be a general phenomenon, present in a wide variety of zero-energy electron attaching molecules.
Chutjian, A.↗
Sub-ppb Oxygen Contaminant Detection in Semi-Conductor Processing
Gaseous contaminants such as oxygen, water vapor, nitrogen and hydrocarbons are often present in the processing environment in semiconductor device fabrication and in containerless materials processing. The contaminants arise as a result of outgassing from hot surfaces or they may be part of the impurities in commercial ultra-high purity gases. Among these gaseous contaminants, oxygen is the most reactive and, therefore, has the most adverse effects on the end product. There has been an intense effort at the Jet Propulsion Laboratory to develop different types of oxygen sorbents to reduce oxygen concentration in a microgravity processing environment to sub-ppb (parts-per-billion) levels. Higher concentrations can lead to rapid surface oxide formation, hence reducing the quality of semiconductor devices. If the concentration of oxygen in a processing chamber at 1000oC is in the ppb level, it will only take approximately 10 seconds for an oxide layer to form on the surface of a sample. The interaction of oxygen with the water surface can lead to the formation of localized defects in semi-conductor devices, hence decreasing the manufacturing yield. For example, efficient production of 64 Mb RAM chips requires contaminations below ppb levels. This paper describes a technique for measuring trace quantities of oxygen contaminants by recording the monoatomic negative ions, O-, using mass spectrometry. The O- formation from the e--O2 interaction utilizes the electron dissociative attachment method that is greatly enhanced at the resonant energy (6.8 eV). The device combines a small gridded electron ionizer with a compact mass spectrometer. The concentrations of oxygen have been measured using the method of standard additions by diluting O2 in N2. The lowest detection limit obtained was 1.2 kHz (O- count rate) at a concentration of 10-10, corresponding to 0.1 ppb.
Sub-ppb Contaminant↗
Positronium ions and molecules
Recent theoretical studies on positronium ions and molecules are discussed. A positronium ion is a three particle system consisting of two electrons in singlet spin state, and a positron. Recent studies include calculations of its binding energy, positron annihilation rate, and investigations of its doubly excited resonant states. A positronium molecule is a four body system consisting of two positrons and two electrons in an overall singlet spin state. The recent calculations of its binding energy against the dissociation into two positronium atoms, and studies of auto-detaching states in positronium molecules are discussed. These auto-dissociating states, which are believed to be part of the Rydberg series as a result of a positron attaching to a negatively charged positronium ion, Ps-, would appear as resonances in Ps-Ps scattering.
Ho, Y. K.↗
Reversal ion source - A new source of negative ion beams
A new type of ion source utilizing beams of electrons and target molecules, rather than a diffuse, volume plasma, is described. The source utilizes an electrostatic electron 'mirror' which reverses trajectories in an electron beam, producing electrons at their turning point having a distribution of velocities centered at zero velocity. A gas which attaches zero-velocity electrons is introduced at this turning point. Negative ions are produced by an attachment or dissociative attachment process. For many of the thermal electron-attaching molecules the cross sections can be quite large, varying as the inverse square root of the electron energy or just the s-wave threshold law. The efficiency and current density of the ion source for production of Cl(-) through the large, thermal energy attachment process is estimated. It is argued that the source can be used for the production of negative ions through attachment resonances located at higher energies as well.
Orient, O. J.↗
Theoretical study of the homonuclear tetramers and pentamers of the group IB metals (Cu, Ag, and Au)
The optimal structures of the homonuclear tetramers and pentamers of Cu, Ag, and Au are found to be planar trapezoidal. However, on the basis of spin densities deduced from electron spin resonance experiments, it has been suggested that these pentamers have distorted trigonal bipyramidal structures. This apparent discrepancy is reconciled by the fact that Mulliken populational analyses indicate that the open-shell spin density distribution on the planar structures are also consistent with experiment. Since the vertical electron attachment energies for both the tetramers and pentamers are in good agreement with experiment, the negative ions probably also have planar trapezoidal structures. The trends in ionization potentials, atomization energies, and electron affinities with cluster size are discussed.
Bauschlicher, Charles W., Jr.↗