Electron spectroscopy (XPS, AES, UPS)

Introduction

X-ray Photoelectron Spectroscopy known as XPS or ESCA (Electron Spectroscopy for Chemical Analysis) was developed in the mid 1960s by K. Siegbahn and his research group. Siegbahn was awarded the Nobel Prize for Physics in 1981 for his work in XPS. The technique is based on the photoelectric effect, i.e. the sample is illuminated with X-ray photons that results in the ejection of photoelectrons from the surface.

Principle of operation

The photoemission process is illustrated below. The binding energy of the photoemitted electrons carries the information on the elements from which they originate as well as the chemical bonding of the elements.

In XPS (X-ray photoelectron spectroscopy) the sample is exposed to an X-ray beam and the energies of characteristically emitted photoelectrons are measured by the electron analyser as shown by the figure below.

For example, XPS can readily distinguish between the ionic and covalent forms of fluorine, or whether the metal is in its oxidised or metallic state. Therefore another name is often used for XPS, i.e. Electron Spectroscopy for Chemical Analysis (ESCA). Instead of X-rays, resonance radiation obtained from noble gas discharge lamps or even high energy electrons can be used to excite electrons from solid surfaces and these methods of electron spectroscopy are known as Ultraviolet photoelectron spectroscopy (UPS) and Auger Electron Spectroscopy (AES), respectively.

Applicability

Due to the short inelastic mean free paths of the photoemitted electrons, electron spectroscopy measurements only analyse the outermost two through ten atomic layers of the surface. This means that electron spectroscopy is truly a surface sensitive analysis method. However, if the analysed region is embedded below the surface, ion etching can be used to remove the topmost atomic layers. The principle of ion etching is shown below by the first figure below. High energy Ar ions remove atoms, ions and clusters from the topmost layers of the sample and the chemical composition of the uncovered material can again be determined by XPS. By repeated ion etching and XPS measurements (i.e. depth profiling), the depth distribution of the elements in the sample can be accurately determined. This is demonstrated by the second figure below.

Electron spectroscopy is sensitive to as low as 0.1 atom percent and detects elements except H and He. It is non-destructive and it can be applied to all solid materials, including insulators such as polymers and glasses.