Method to determine the analysis area of x-ray photoelectron spectrometers—illustrated by a Perkin–Elmer PHI 550 ESCA/SAM

Abstract

The area analyzed during routine x-ray photoelectron spectroscopy (XPS) measurements in a Perkin–Elmer PHI 550 spectrometer is determined by monitoring the intensity detected by the spectrometer from points on the sample surface addressed by a focused and rastered electron beam. The reference position for sample analysis is obtained using the manufacturer’s preferred procedure, originally established for Auger electron spectroscopy (AES), of moving the sample along the spectrometer axis until the correct energy is measured for a 2-keV electron beam with the analyzer operated in the AES mode. Using a sample set normal to the electron gun of the spectrometer and with the spectrometer operated in the XPS mode, the area analyzed is then determined as a function of the sample axial position. This area minimizes at the XPS focal position which appears to be 1.5 mm further from the spectrometer than the above reference position, in the particular instrument studied in Lausanne. This is attributed to inadequate manufacturing tolerances in this instrument. The analytical area has a diameter of 4 mm at the reference position, reducing to below 3 mm at the XPS focal position. This latter dimension is not sensitive to the pass energy but may be reduced to below 2 mm by use of the smallest apertures in the spectrometer. Theoretical expressions incorporating the mesh aberrations are derived which predict the above behaviors. Angle-dependent measurements, using the rotating azimuthal slit are shown to analyze an area which moves round a rather irregular path as the azimuth varies. Thus, angle-dependent studies to separate surface and bulk spectra in regular analysis are only meaningful for homogeneous samples set at the reference position. Inhomogeneities may lead to the spectra from one azimuth having no relation to those from another. The spectrometer analytical volume thus may be fully evaluated using this rastered electron beam method.