Study of high- and low-work-function surfaces for hyperthermal surface ionization using an absolute Kelvin probe

We have performed a study of high- (>6 eV) and low- (<3 eV) work-function surfaces in order to identify suitable target materials as an ion source for a new type of mass spectrometer technique based on hyperthermal surface ionization (HSI). In this application a molecular beam of neutral gas molecules is ionized by supersonic collision on a target surface. High-work-function surfaces produce positive ions (pHSI), and low-work-function surfaces negative ions (nHSI). Analytical merits of HSI include very high sensitivity, atmospheric pressure inlet, and informative mass spectra. As this technique does not use electron-impact filaments, the amount of cracking products is substantially reduced. The ultra-high-vacuum (UHV) scanning Kelvin probe is a technique producing relative work-function topographies between a scanning reference tip and the sample in a truly noninvasive fashion with high accuracy (1-2 meV). We demonstrate a novel extension of this technique, using photoelectric determination, which produces absolute work-function data even if the tip work function is not known. Using this hybrid probe, together with scanning electron microscopy and Auger electron spectroscopy, we have followed (a) work-function topographies of clean surfaces in UHV, (b) changes in work function with oxidation that are related to surface cleaning processes, (c) the temperature-dependent oxidation kinetics of polycrystalline metal surfaces (Re, Pt, Mo, W, and Pd) for pHSI, and (d) the stability of Ca, Gd, and LaB6 under residual gases for nHSI. We will report the optimum parameters for target stability and performance under both pHSI and nHSI operating conditions. We will also illustrate informative mass spectra produced by time-of-flight HSI. (C) 2001 American Vacuum Society.