Services

The expert staff of the NCF is available to assist you with your use of any of our instruments. If you are a new user, having problems with achieving your goals, or just do not have enough time to learn we can help. We offer assisted use service as well as a full analysis service depending on your needs. In addition we offer X-ray photoelectron spectroscopy.

XPS

Description

X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is a technique in which x-rays are employed to generate photoelectrons. Analysis of the energetics of these electrons may be used to elucidate and quantify the elemental composition and chemical state of the material being analyzed.

Background

First observed in 1887 by Hertz [1] and explained by Einstein for the 1921 Nobel Prize in Physics [2], the photoelectric effect is utilized in a variety of modern spectroscopic techniques. The key feature of the phenomenon is that photons behave as particles when interacting with materials. The current induced by ionizing radiation is proportional to the number of incident photons, assuming there is a sufficient amount of energy to eject the electron from the material. The energy required to free an electron is broken into two parts. The binding energy, EB, is the energy required to free the electron from its associated atom, i.e. the energy of its quantum state. The work function, ?, is the energy required to remove an electron from the surface of the material to infinity. This is a classical phenomenon due to the attractive force of an image charge as the electron departs [3]. The overall relationship is:

E_kin = hv - Φ - E_B

From conservation of energy, the kinetic energy of the emitted electron, Ekin, plus the binding energy of the electron and the work function must equal the energy of the incident photon, h?. Observation of the kinetic energy of the photoelectrons offers information on the chemical states of the atoms in the material.

Types of Testing

Survey Scans- A survey scan is typically used as a starting point for analysis, this examines binding energies from 0-1100 eV. This takes approximately 1 hour.
Multiplex Scans- After individual component of interest have been determined, higher resolution spectra of individual peaks can be obtained in a multiplex scan. The time required for a multiplex varies with the number of peaks and atomic percentage of the material under investigation. As a starting point, a multiplex scan will take at least 1.5 hours.
Depth profiling- This is the combination of a multiplex scan with sputtering. After sputtering off a layer of the material of the interest, a multiplex is performed, this can be repeated to investigate the chemical composition of a film at various depths within the film. This analysis typically takes several hours.

Sample Preparation

Samples will be analyzed in ultra high vacuum conditions, they should be as clean as possible, this means free from fingerprints etc.
Polymers cannot be sputtered, but they can be CO2 snow cleaned
Wafers: Sample size ~ 1 cm2
Powders: Enough powder to press a pellet is required, ~0.5 ml *Powders can not be sputtered