Overview of Technique
The Axis Ultra is an X-ray Photoelectron Spectrometer (XPS/ESCA), produced by Kratos Analytical. This instrument provides elemental and chemical information about the surface region (first 1- 30 monolayers) of nearly any solid material. This, with addition to its ion etching capabilities, also allows detection of elemental composition as a function of depth. The instrument has exceptional small spot capabilities (<15 microns), and thus high spatial resolution is achieved. In addition to scanning capabilities, the instrument is capable of real time photoelectron imaging of the sample surface (showing only electrons of a chosen energy). The spectrometer also includes a scanning Auger electron gun, providing higher spatial resolution down to 0.1 micrometer. The X-ray source is supplied through a monochromator. Analysis of nonconducting samples is possible, due to the Charge Neutralization System. XPS is useful for chemically sensitive materials since damage from the x-ray beam is minimal. It is particularly useful for insulating materials such as polymers, oxides, and powders where charging effects limit other surface techniques. The system includes the following capabilities:
- Magnetic Immersion Lens
- Charge Neutralization system
- 165 mm radius Spherical Mirror Analyzer with eight channel detector for spectroscopy measurements
- Outer hemisphere and multi-channel plate for imaging mode.
- Auger electron gun
- Ion gun for depth analysis
- Advanced software for background subtraction and curve fitting.
- Automatic, software managed, measurement sets.
Basics and Tutorials
XPS is a powerful technique for the detection of variations in chemical composition and oxidation state. In XPS, soft X-rays (about 1500 eV for our instrument) illuminate a region of the sample being analyzed, and photolelectrons emanating from it are energy analyzed. From the energies detected, the elements present and their chemical state (valence) can be determined. In these energies, only electrons emitted from atoms near the surface of the sample escape without losing energy, thus, the technique is very surface sensitive. Subtle changes in peak positions and shape can yield important information on changes in surface chemistry, giving the XPS its ability to determine the elemental composition on the surface of all non-volatile materials semi-quantitatively (It is sensitive to all elements except H). Using advanced peak fitting, very rich information can be found regarding the local oxidation and chemical states of the composing elements. Depth analysis: XPS is sensitive to the top 5 nm of a sample, thus it can be used to provide elemental composition as a function of depth by analyzing a sample while removing surface layers by ion etching.
Basics of X-ray photoelectron spectroscopy / ESCA: In the future, we plan to organize lecture courses for those who will be interested in deeper understanding of analytical techniques and instrumentation offered by our center. For a while, for basic education and principle understanding of the XPS / ESCA, we kindly ask you to use following web resources:
The AXIS ULTRA integrates the Kratos patented Magnetic Immersion Lens and Charge Neutralization System with the new Spherical Mirror Analyser. The Spherical Mirror Analyser provides real time chemical state and elemental imaging using the full range of pass energies and multi-point analysis from either real time or scanned images without the need for sample translation. A microchannel plate and phosphor detection system are incorporated into the system to provide a parallel imaging capability with high spatial resolution and high sensitivity. Advanced zoom optics enable a variable viewing area for the identification of macro or micro features. The instrument's exceptional small spot capabilities (<15 microns) are achieved via a combination of the magnetic lens and selected area apertures. The AXIS Ultra provides high energy resolution and high spatial resolution, high sensitivity, allowing time efficiency and ease of use.
When the AXIS Ultra is used in spectroscopy mode, the electron are dispersed between the inner and outer hemispheres as in a conventional x-ray photoelectron spectrometer. A large 165 mm radius and an eight channel detector ensure high spectral resolution and a high sensitivity. Spot size apertures combined with electrostatic scan plates enable small area spectra to be recorded from different positions on the sample without sample translation.
Switching to imaging mode turns off the conventional hemispherical and electrons now pass through the outer hemisphere and into the spherical mirror analyser. The parallel image of the sample surface is maintained and projected on the multichannel plate (MCP) detector. The great advantage of the spherical mirror analyser is that real time photoelectron images of the sample surface can be recorded allowing easy identification of features invisible to optical microscopy. As the energy resolution of the detector is not compromised the chemical distribution of surface species is easy to determine.