XPS Surface Analysis

X-ray photoelectron spectroscopy (XPS) uses x-rays to remove an electron from a core level to the Fermi (1s) level. Knowing the photon energy and measuring the kinetic energy with the electron spectrometer permit direct determination of the binding energy of electrons, which is unique and can be used to identify specific elements. Because the mean free path of the electrons is very small, only electrons that originate in the top atomic layers are detected.

X-ray photoelectron spectroscopy is a surface-sensitive technique, and an inert environment such as an ultrahigh vacuum system--for example, 10-10 torr--is required for measurements. A typical x-ray photoelectron spectroscopy analytical chamber consists of an x-ray source, an electron energy analyzer, an electron multiplier, a pulse-counting detector, and a computer system. An ion gun is employed for sputtering and compositional depth profiling. For good resolution during profiling, the ion beam--for example, argon--must be defocused over a surface area of approximately 1 cm2 (0.15 in.2), because photoelectrons are collected from a surface area of several square millimeters during analysis.

Surface sensitivity of XPS is comparable to other surface analysis techniques. Data collection is slow compared with other surface analysis techniques, but analysis time can be decreased substantially when high resolution or chemical state identification is not needed. The requirement for ultrahigh vacuum conditions may necessitate an overnight vacuum pumpdown before analysis. Qualitative analysis can be performed in 5 to 10 min. Quantitative analysis requires 1 h to several hours, depending on information desired.

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