Imaging Microanalysis of Materials with a Finely Focused Heavy Ion Probe

Abstract

Liquid metal ion sources (LMIS) in view of the quasi-point-like geometry of their emitting region and confined emission cone possess brightness (0I6 A cm 'sr-') which is adequate for the realization of high current density (I A cm-'), finely focused (>20 rm) probes. A 40 keV scanning ion microprobe, which makes use of a Ga LMIS (UCHRL SIM) is currently employed in our laboratory to obtain chemical maps of materials in a variety of interdisciplinary applications [1]. The instrument is composed of a two-lens focusing column, a hightransmission secondary ion energy analyzer and transport system, and an RF quadrupole mass filter for secondary ion mass spectrometry (SIMS). in addition, two-channel electron multiplier detectors, overlooking the target region, collect secondary electrons or ions for imaging of the surface topography and material contrast of a sample. Although a lateral resolution of 20 nm has been attained, sensitivity considerations favor operation at somewhat larger (50-70 nmt) probe size. The analytical image resolution is in fact critically dependent on the statistics of the mass analyzed signal [2], which in turn is determined by the rate of material removal by sputtering from the sample surface. Such a rate is proportional to the probe current, which decreases with the square of the probe diameter for chromatic-aberration-limited probes such as those extracted from LMIS. Thus at, e.g., 20 nm probe diameter, only 1-2 pA of probe current are available, the erosion rate is of the order 10-' rnonolayers/s and probe-size resolution can only be attained for elements of high ionization probability which will provide the highest signal statistics over an acceptable recording time (>I count/pixel in a 1024 x 1024 pixel scan for a 512 s acquisition time for the UC-HRL SIM). In view of the well known range of ionization probabilities (ion fractions) among sputtered atomic species, as well as of sputtering yields, the attainable analytical lateral resolution of finely focused probes becomes target-species dependent due to the above considerations. It also follows that for species difficult to ionize, high resolution SIMS imaging microanalysis is altogether precluded, unless by recourse to postionization techniques [3]. Two examples of applications of the UC-HRL SIM to the study of materials will be illustrated in the present context.