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 s urfaces in order to identify suitable target materials as an ion source for a new type of mass spectrometer technique based on hyperthermal surface io nization (HSI). In this application a molecular beam of neutral gas molecul es is ionized by supersonic collision on a target surface. High-work-functi on surfaces produce positive ions (pHSI), and low-work-function surfaces ne gative ions (nHSI). Analytical merits of HSI include very high sensitivity, atmospheric pressure inlet, and informative mass spectra. As this techniqu e does not use electron-impact filaments, the amount of cracking products i s substantially reduced. The ultra-high-vacuum (UHV) scanning Kelvin probe is a technique producing relative work-function topographies between a scan ning reference tip and the sample in a truly noninvasive fashion with high accuracy (1-2 meV). We demonstrate a novel extension of this technique, usi ng photoelectric determination, which produces absolute work-function data even if the tip work function is not known. Using this hybrid probe, togeth er with scanning electron microscopy and Auger electron spectroscopy, we ha ve followed (a) work-function topographies of clean surfaces in UHV, (b) ch anges in work function with oxidation that are related to surface cleaning processes, (c) the temperature-dependent oxidation kinetics of polycrystall ine 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 optim um parameters for target stability and performance under both pHSI and nHSI operating conditions. We will also illustrate informative mass spectra pro duced by time-of-flight HSI. (C) 2001 American Vacuum Society.