Probing Diffusion Kinetics with Secondary Ion Mass Spectrometry

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Kinetics with Secondary Ion Mass Spectrometry

Roger A. De Souza and Manfred Martin Abstract Secondary ion mass spectrometry (SIMS) is a powerful analytical technique for determining elemental and isotopic distributions in solids. One of its main attractions to researchers in the field of solid-state ionics is its ability to distinguish between isotopes of the same chemical element as a function of position in a solid. With enriched stable isotopes as diffusion sources, this allows self-diffusion kinetics in solids to be studied. In this article, taking oxygen isotope diffusion in oxides as our main example, we present the standard experimental method, and, subsequently, we discuss several promising developments, in particular the opportunities offered by thin-film geometries, and the investigation of inhomogeneous systems, including possible fast diffusion along grain boundaries and making space-charge layers at interfaces “visible.” These examples demonstrate that SIMS is capable of probing mass transport processes over various length scales, ranging from some nanometers to hundreds of micrometers.

Introduction Driven by the semiconductor industry, secondary ion mass spectrometry (SIMS) has evolved over the past 40 years from a branch of atomic physics into a powerful analytical tool.1,2 Its ability to detect all elements with high spatial resolution, in most cases down to the parts-per-million to parts-per-billion level, has been, and continues to be, used extensively in monitoring fabrication processes and in performing failure studies of microelectronic devices. The commitment of the industry toward ever smaller dimensions has been matched by corresponding increases in the performance of SIMS technology. Why does this technique play such a prominent role in the field of solid-state ionics (SSI)? Few of the 92 naturally occurring elements are highly mobile as ions in solids; among the cations, only H+, Li+, Na+, Cu+, and Ag+ show high mobility, while mobile anions are limited to F − and O2 −. Furthermore, in solids that exhibit significant ionic conductivity, these highly mobile ions are present in high, not low, concentrations (i.e., of the order of

1023 cm−3 not 1015 cm−3). SIMS hardware, in addition, is complex, expensive, and far from widespread (while almost all universities have, for example, a nuclear magnetic resonance machine, few have a SIMS facility). Nevertheless, SIMS holds a unique position in the field of SSI, and in this article, we discuss various literature examples to demonstrate why this is the case and to demonstrate that application of SIMS within the field is set to grow. The principal attraction of SIMS for researchers in SSI is that, since it is a mass spectrometric technique with (high) spatial resolution, it is capable of distinguishing between isotopes as a function of position in a solid (the functioning principle of SIMS is illustrated in Figure 1). This capability permits tracer diffusion experiments to be carried out, in particular using stable isotopes. A tracer diffusion expe