James A. Whitby

High Spatial Resolution Time-of-Flight Secondary Ion Mass Spectrometry for the Masses: A Novel Orthogonal ToF FIB-SIMS Instrument with In Situ AFM

We describe the design and performance of an orthogonal time-of-flight (TOF) secondary ion mass spectrometer that can be retrofitted to existing focused ion beam (FIB) instruments. In particular, a simple interface has been developed for FIB/SEM instruments from the manufacturer Tescan. Orthogonal extraction to the mass analyser obviates the need to pulse the primary ion beam and does not require the use of monoisotopic gallium to preserve mass resolution. The high-duty cycle and reasonable collection efficiency of the new instrument combined with the high spatial resolution of a gallium liquid metal ion source allow chemical observation of features smaller than 50u2009nm. We have also demonstrated the integration of a scanning probe microscope (SPM) operated as an atomic force microscope (AFM) within the FIB/SEM-SIMS chamber. This provides roughness information, and will also allow true three dimensional chemical images to be reconstructed from SIMS measurements.

Potential analytical applications of negative ions from a pulsed radiofrequency glow discharge in argon

Detection of negative ions from a conventional analytical glow discharge source using argon as the working gas is reported. The negative ions are recorded using a pulsed discharge source coupled with a time-of-flight mass spectrometer. A considerable enhancement in the “afterglow” region of the negative ion signal for halogens and halogenated molecules and reduction in background is observed. This is the first time when negative ions have been reported for halogen containing materials and we illustrate our findings with results from the polytetrafluoroethene polymer (PTFE).

Detection of negative ions in glow discharge mass spectrometry for analysis of solid specimens.

A new method is presented for elemental and molecular analysis of halogen-containing samples by glow discharge time-of-flight mass spectrometry, consisting of detection of negative ions from a pulsed RF glow discharge in argon. Analyte signals are mainly extracted from the afterglow regime of the discharge, where the cross section for electron attachment increases. The formation of negative ions from sputtering of metals and metal oxides is compared with that for positive ions. It is shown that the negative ion signals of F− and TaO2F− are enhanced relative to positive ion signals and can be used to study the distribution of a tantalum fluoride layer within the anodized tantala layer. Further, comparison is made with data obtained using glow-discharge optical emission spectroscopy, where elemental fluorine can only be detected using a neon plasma. The ionization mechanisms responsible for the formation of negative ions in glow discharge time-of-flight mass spectrometry are briefly discussed.

The role of oxygen in analytical glow discharges: GD-OES and GD-ToF-MS studies

The influence of up to 0.8% volume fraction of oxygen on the argon plasma in a direct current analytical glow discharge was studied for iron, titanium, copper and gold samples using both time-of-flight mass spectrometry and high resolution optical UV-Vis Fourier transform spectrometry. A sudden decrease in sputtering rates, optical emission intensities and ion signals is observed at a current and material dependent threshold fraction of oxygen, and is attributed in part to the formation of an oxide layer on the surface. Beyond the threshold oxygen fraction, a further large continuing decrease in ion signals is observed as the oxygen fraction is increased. This smooth decrease of the observed ion signals is attributed to quenching of ions and argon metastable atoms by oxygen in the flowing afterglow in the flow tube used to promote ion transport to the mass spectrometer sampling orifice. At the threshold oxygen fraction, signals from the mass spectrometer decrease by orders of magnitude more than the optical emission observed from analyte and gas ions. This disproportionate decrease is also likely to be due to reactions in the flow tube. The negative mass spectra were also measured for iron, titanium and copper samples and a significant increase in ion signals is observed at material dependent critical oxygen concentration. Results of studies using calamine (a sample with oxide layer) in pure argon are reported and compared with argon/oxygen mixtures.

DESIGN AND PERFORMANCE OF TWO ORTHOGONAL EXTRACTION TIME-OF-FLIGHT SECONDARY ION MASS SPECTROMETERS FOR FOCUSED ION BEAM INSTRUMENTS

The design and performance of two orthogonal extraction time-of-flight mass spectrometers are reported that were adapted to existing focused ion beam microscopes for secondary ion mass spectrometry. The performances of these designs were compared to that of a prototype previously described by our group. The differences include newly designed transfer ion optics and in the use of a larger microscope chamber. The two new prototypes allow a mass resolving power of either 600 Th/Th (compact design) or 3000 Th/Th (high resolution design) while simultaneously achieving a lateral spatial resolution of less than 50 nm. The spectrometers and their performance (effective ion yield, mass resolving power, lateral, and depth resolution) are described and compared. Additionally, example applications are presented with multivariate statistical methods to visualize the data sets. Both time-of-flight mass analyzers use orthogonal extraction which avoids the need to pulse the primary ion beam, and the of use monoisotopic gallium to preserve the mass resolution. The goal of the design was a cost-effective accessory to augment typical focused ion beam-scanning electron microscopy applications as an alternative to the cost of a dedicated secondary ion mass spectrometer. The modified instrument allows excellent non destructive imaging and easy sample access, and benefits from the presence of complementary non destructive analytical and imaging techniques that exploit the presence of an electron microscope.

A glow discharge time-of-flight mass spectrometry (GD-TOFMS) study of the ‘hydrogen effect’ using copper, iron and titanium cathodes

We report TOFMS investigations on the effects of hydrogen added to a dc GD argon plasma, in detail over the most likely concentration range occurring in analytical work (0–0.10% v/v), together with an overview to 0.8% v/v; typical GD-OES discharge conditions were used. Many previous studies on the effects of hydrogen in analytical GD-MS used much higher hydrogen concentrations. In this paper, we report for the first time the ‘hydrogen effect’ on relative ion signal intensities of matrices, plasma and added gas, for very low but analytically important hydrogen concentrations. We investigate the behavior of doubly charged argon ions (Ar++), dimer ions (Cu2+, Fe2+, Ti2+, Ar2+) and polyatomic ions (ArH+, CuH+, FeH+, TiH+), which can play important roles in plasma processes. We discuss the probable discharge mechanisms in order to gain a greater understanding of the fundamental processes involved in the ionization of the species observed in this study and their likely effects on analytical results. We propose a mechanism to explain the increase in the matrix signal which occurs if an argon/hydrogen mixture is used as the plasma gas.