ASTM E1217-11

Auger electron spectroscopy and X-ray photoelectron spectroscopy are used extensively for the surface analysis of materials. This practice summarizes methods for determining the specimen area contributing to the detected signal (a) for instruments in which a focused electron beam can be scanned over a region with dimensions greater than the dimensions of the specimen area viewed by the analyzer, and (b) by employing a sample with a sharp edge.

This practice is intended as a means for determining the observed specimen area for selected conditions of operation of the electron energy analyzer. The observed specimen area depends on whether or not the electrons are retarded before energy analysis, the analyzer pass energy or retarding ratio if the electrons are retarded before energy analysis, the size of selected slits or apertures, and the value of the electron energy to be measured. The observed specimen area depends on these selected conditions of operation and also can depend on the adequacy of alignment of the specimen with respect to the electron energy analyzer.

Any changes in the observed specimen area as a function of measurement conditions, for example, electron energy or analyzer pass energy, may need to be known if the specimen materials in regular use have lateral inhomogeneities with dimensions comparable to the dimensions of the specimen area viewed by the analyzer.

This practice can give useful information on the imaging properties of the electron energy analyzer for particular conditions of operation. This information can be helpful in comparing analyzer performance with manufacturer's specifications.

Information about the shape and size of the area viewed by the analyzer can also be employed to predict the signal intensity in XPS experiments when the sample is rotated and to assess the axis of rotation of the sample manipulator.

Examples of the application of the methods described in this practice have been published (1-7).

There are different ways to define the spectrometer analysis area. An ISO Technical Report provides guidance on determinations of lateral resolution, analysis area, and sample area viewed by the analyzer in AES and XPS (8), and ISO 18516:2006 describes three methods for determination of lateral resolution in AES and XPS. Baer and Engelhard have used well-defined ‘dots’ of a material on a substrate to determine the area of a specimen contributing to the measured signal of a ‘small-area’ XPS measurement (9). This area could be as much as ten times the area estimated simply from the lateral resolution of the instrument. The amount of intensity in ‘fringe’ or ‘tail’ regions could also be highly dependent on lens operation and the adequacy of specimen alignment. Scheithauer described an alternative technique in which Pt apertures of varying diameters were utilized to determine the fraction of ‘long-tail’ X-ray contributions outside each aperture on the measured Pt photoelectron signal compared to that on a Pt foil (10). In test measurements on a commercial XPS instrument with a focused X-ray beam and a nominal lateral resolution of 10 μm (as determined from the distance between the positions for 20% and 80 % of maximum signal when scans were made across an edge), it was found that aperture diameters of about 100 μm and 450 μm were required to reduce the photoelectron signals to 10 % and 1 %, respectively, of the maximum value (10). Knowledge of the effective analysis area is important when making tradeoffs between lateral resolution and detectability. In scanning Auger microscopy, the area of analysis is determined more by the radial extent of backscattered electrons than by the radius of the primary beam (11, 12, 13).

1.1 This practice describes methods for determining the specimen area contributing to the detected signal in Auger electron spectrometers and some types of X-ray photoelectron spectrometers (spectrometer analysis area) when this area is defined by the electron collection lens and aperture system of the electron energy analyzer. The practice is applicable only to those X-ray photoelectron spectrometers in which the specimen area excited by the incident X-ray beam is larger than the specimen area viewed by the analyzer, in which the photoelectrons travel in a field-free region from the specimen to the analyzer entrance. Some of the methods described here require an auxiliary electron gun mounted to produce an electron beam of variable energy on the specimen (“electron-gun method”). Other experiments require a sample with a sharp edge, such as a wafer covered with a uniform clean layer (for example, gold (Au) or silver (Ag)) and cleaved to obtain a long side (“sharp-edge method”).

1.2 This practice is recommended as a useful means for determining the specimen area viewed by the analyzer for different conditions of spectrometer operation, for verifying adequate specimen and beam alignment, and for characterizing the imaging properties of the electron energy analyzer.

1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.