Introduction to lateral resolution and analysis area measurements in XPS

Abstract

Imaging and small-spot (small area) XPS have become increasingly important components of surface chemical analysis during the last three decades, and its use is growing. Some ambiguity in the use of terminology, understanding of concepts, and lack of appropriate reference materials leads to confusing and not always reproducible data. In this paper, it is shown that by using existing knowledge, appropriate test specimens, and standardized approaches, problems of comparability and such reproducibility issues recently observed for XPS data reported in the scientific literature can be overcome. The standardized methods of ISO 18516:2019, (i) the straight-edge, (ii) the narrow-line, and (iii) the grating method, can be used to characterize and compare the lateral resolution achieved by imaging XPS instruments and are described by reporting examples. The respective measurements are made using new test specimens. When running an XPS instrument in the small-spot (small area) mode for a quantitative analysis of a feature of interest, the question arises as to what contribution to the intensity originates from outside the analysis area. A valid measurement approach to control the intensity from outside the nominal analysis area is also described. As always, the relevant resolution depends on the specific question that needs to be addressed. The strengths and limitations of methods defining resolution are indicated.

Summary of ISO/TC 201 International Standard ISO 18516:2019 Surface chemical analysis—Determination of lateral resolution and sharpness in beam‐based methods with a range from nanometres to micrometres and its implementation for imaging laboratory X‐ray photoelectron spectrometers (XPS)

“…Principal layout of the ETHZ sample developed for testing the measurement capabilities in the imaging mode achieved by recent XPS laboratory instruments (cf. references 22,23 ). The pattern comprises (a) a Ti box in Au for executing the straight edge method; (b) a series of consecutive square‐wave A–B–A gratings (A = Ti, B = Au) with bar widths of 30, 20, 10 and 5 μm, that is, the respective grating periods (‘pitches’) P = 60, 40, 20 and 10 μm (cf.…”

“…Recently, a test sample made by using an innovative technology 22 and displaying a layout (see Figure 1) inspired by the pattern on BAM L200 was developed by colleagues at ETH Zurich, Switzerland. 22,23 The Ti pattern in Au is planar on the deep nanometre scale. Edge effects will be negligible for XPS instruments.…”

“…Edge effects will be negligible for XPS instruments. This test sample was designed to enable a determination of the lateral resolution for recent imaging XPS laboratory instruments 23 . Typical lateral resolutions achieved by those instruments are on the deep micrometre scale, that is, somewhat below 10 μm.…”

“…This test sample was designed to enable a determination of the lateral resolution for recent imaging XPS laboratory instruments. 23 Green line: Fitted edge spread function (EFS). Detailed information on instrument settings, image treatments and analysis by using different CasaXPS software components 24 are given in the supporting information.…”

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“…Approaches to overcome this limitation exist and can be classified as bottom‐up or top‐down approaches. [ 2,7 ]…”