Advances in Small Spot ESCA
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Abstract The changes in Electron Spectroscopy for Chemical Analysis (ESCA) equipment since 1982 have significantly expanded the range of applications and general utility of the technique. Most of these changes center around improvements in spatial resolution although there have also been improvements in speed and energy resolution. The implications of these changes extend beyond the obvious ability to obtain information from small features on samples. The three major approaches to controlling spatial resolution are: (1) defined area x-ray sources, (2) limited field-of-view electron analyzers, and (3) imaging electron analyzers. Each of these will be reviewed briefly and their salient features contrasted. These advances in the instrumentation have brought about the following benefits to users of the ESCA technique: (1) ability to analyze small features, (2) rapid depth profiling, (3) multiple samples, and (4) improved charging control. Examples are included.
Introduction The purpose of this article is twofold. The first is to briefly review the changes in ESCA equipment, also called x-ray photoelectron spectroscopy (XPS), that have occurred in the last few years and to contrast the various approaches to obtaining small spot capability. The changes that have increased the speed and flexibility of the technique will also be pointed out. The second purpose is to describe and illustrate the benefits obtained from the improved instrumentation. The most obvious benefit is the ability to analyze small features and obtain maps and line scans. Four indirect benefits that have manifested themselves so far are the abilities to perform rapid depth profiles, analyze many small samples in a reasonable size chamber, improve charging control, and ease the preparation of special samples.1-4 The ESCA technique (detailed in "Fundamentals of X-Ray Photoemission Spectroscopy," by F.J. Grunthaner in this issue) has been recognized as offering significant utility in materials analysis because of the chemical information obtained, low sample damage, and the ability to work with insulating samples. These capabilities have been traditionally offset by the spatial resolution and speed of the technique, although it has been pointed out that the limits imposed by the equipment available are far from any fundamental limits.5 This fact has resulted in extensive commercial
and private activity in instrument development.6'8 Acquisition times have typically dropped an order of magnitude for comparable analysis area and signal-to-noise ratios. At the same time the analysis range has been extended down from a minimum area of approximately 5 mm2 to about 0.01 mm2.6"8
Three fundamentally different approaches have been taken to improve ESCA equipment performance, each with different advantages and disadvantages. We will describe each and attempt to delineate the attributes of each approach with the resulting advantages and disadvantages. This description is organized around the followcontinued
Thin Sample
X-Ray Anode Foil
Photoelectrons
Electron G
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