Imaging SIMS at 20 nm Lateral Resolution: Exploratory Research Applications

Abstract

Notwithstanding the high brightness of liquid metal ion sources (LMIS), ion probes based on their use still encounter fundamental current limitations, as their size is reduced. Due to the intrinsically large energy spread (5–10 eV) of the ions extracted from LMIS, in fact conventional electrostatic round-lens focusing columns are predominantly chromatic-aberration limited, yielding probes of size-independent current density. The probe current then decreases with the square of the probe size [1]. Thus, the recently developed University of Chicago (UC)-Hughes Research Laboratories (HRL) scanning ion microprobe (SIM) yields currents of 40 keV Ga⁺ or In⁺ ions typically in the range 1.5 – 30 pA, for probe sizes in the range 20 – 90 nm [2],[3]. Consequently, in addition to a number of intrinsic and instrumental limitations [4], the SIMS sensitivity, proportional to the amount of target material which can be sputtered and analyzed within acceptable scan times, also deteriorates rapidly as the probe size is reduced. Even with the use of a high-transmission SIMS system, such as that implemented in the UC-HRL SIM, useful applications of probes of a few pA, at the limits of spatial resolution presently attained, become restricted to selected favorable problems. These will involve primarily the mapping of segregated components of elements of low ionization potential or high electron affinity. Nevertheless, such minuscule probe currents can be exploited to unique advantage when it is of the essence to limit sputter etching of the sample to a few surface atomic monolayers. Several examples of high spatial resolution SIMS imaging, involving the above concepts, will be shown here.