The invention relates to techniques for local analysis of surfaces; more particularly, it is directed to techniques for mass spectral analysis of species of matter on, or forming a part of, a surface.
In surface analysis by mass spectroscopic techniques the specimen to be examined is placed in a high-vacuum chamber and bombarded by a probe beam of one kind or another. The probe beam removes a sample of matter from the surface, at least a portion of which is ionized in the process of removal or subsequent thereto. The ionized sample is then subjected to mass spectral analysis.
Examples of known techniques for surface mass spectral analysis include laser microprobe methods and secondary ion mass spectrometry. Both of these methods contain undesirable limitations. Laser microprobe mass spectrometry, for example, uses a focused high-intensity laser to irradiate a surface directly and blow off large amounts of material, only a small fraction of which is ionized as it departs from the surface. The high-intensity laser results in highly destructive sampling of the surface; the intense laser pulse used in typical instruments forms craters of 0.1 to 1.0 micrometers in depth and is therefore not truly surface-sensitive. The laser microprobe technique is also difficult to model because ionization efficiencies depend sensitively on the various collisional processes occurring in the laser-generated plasma at the surface.
In secondary ion mass spectrometry (SIMS) the surface under examination is bombarded with an ion beam or fast neutral beam, which sputters a sample of ionized and neutral matter from the surface. In general, quantitative analytical information is difficult to derive from the SIMS method because the physical processes that determine the ionization probability of the sputtered matter are not, as a rule, well understood and because ionization probabilities depend sensitively on surface composition (so-called matrix effects) and cleanliness (so-called chemical enhancement effects). As a practical matter, quantitative analytical information can be extracted from the SIMS method only by using especially prepared standards for comparison.
In many of the commonly practiced techniques, such as the laser microprobe or SIMS methods, only a small portion of the sample removed from the surface is ionized. One attempt to provide a highly ionized sample is disclosed in U.S. Pat. No. 4,001,582 in the name of Castaing, et al. In the Castaing method, particles sputtered from the surface are introduced into a chamber at high temperatures and are subjected to successive adsorptions and desorptions on the walls of the chamber. This process efficiently ionizes atoms of suitably low ionization potential with a high probability. However, molecules (as opposed to atoms) are generally dissociated in the hot ionizing chamber, so that the Castaing method yields information only on atomic species. Additionally, many atoms have ionization potentials too large to be ionized and detected by this method. Contamination and material degradation problems can also be severe in the extreme environment of the high temperature detector.
Another approach presenting enhanced ionization efficiency is disclosed by N. Winograd, J. P. Baxter and F. M. Kimock, Chemical Physics Letters, Vol. 88, No. 6, 1982 pp. 581-84. In this approach a laser is directed to a sample of neutral atoms, which have been sputtered from the surface under examination. The laser is tuned to a predetermined wavelength corresponding to an excited state of a preselected atom of interest known or expected to be present in the sample. The laser has sufficient intensity to induce resonance multiphoton ionization of the preselected atom. This method has the obvious drawback that it is necessary to tune the laser to a predetermined wavelength. Thus, the method is applicable only to certain species of matter which have known excitation spectra with excitation wavelengths accessible to the available laser and which are already known or strongly suspected to be present on the surface.