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Electron spectroscopy laboratory (P. Jiříček)

The properties of the solid surfaces are investigated by electron spectroscopy methods at Electron spectroscopy laboratory (ESL). The laboratory is equipped with two spectrometers. The ADES 400 (Angular distribution electron spectrometer) photoelectron spectrometer (VG Scientific) was bought in 1980 (Fig. 1). The second one, AXIS Supra (Fig. 2), was delivered by Kratos Analytical Ltd. at the end of 2014.

ADES 400

In the ADES 400 a 150° spherical sector analyzer with a 50 mm mean radius is moved in horizontal plane around a specimen. Using this property of the analyzer the angle between impinging of x-ray radiation (or ultraviolet radiation, or primary electrons) and direction of detection can be set independently.

Besides the energy analyzer the other component are attached to the chamber of the spectrometer. These instruments allow to apply following spectroscopic method:
XPS – X-ray photoelectron spectroscopy,
UPS – Ultraviolet photoelectron spectroscopy,
PhD – Photoelectron diffraction,
AES – Auger electron spectroscopy,
EPES – Elastic peak electron spectroscopy,
EELS – Electron elastic loss spectroscopy,
LEED – Low energy electron diffraction.

Some parts of the spectrometer have changed during last few years. These equipment are installed (or can be attached) to thet ADES 400 now:
X-ray source (XR50, SPECS GmbH)
source of ultraviolet radiation (UVL - Li, VG Microtech),
electron source (EGG-3101, Kimball Physics, Inc.),
optics for low energy electron diffraction (ErLEED 3000, SPECS GmbH),
mass spectrometer (Prisma, Pfeiffer Vacuum),
ion source (AG2, VG Scientific),
ion source (IGE 12/38, SPECS GmbH),
manipulator (UMD20, VG Scientific),
manipulator (HPT Translator, VG Scienta),
evaporation source (EFM 3, Omicron nanoTechnology).
Recording of the electron spectra is performed by COLLECT software. Measured spectra are evaluated by PRESENT and UNIFIT programs.

AXIS Supra – high performance, automated XPS spectrometer

AXIS Supra is photoelectron spectrometer which allows to collect spectra from any material that is stable under ultrahigh vacuum irrespective of the material properties. The Axis Supra is based on the proven Axis technology comprising: magnetic and electrostatic transfer lenses; co-axial electron only charge neutralisation; spherical mirror and hemispherical electron energy analyzers. Kratos developed innovations such as the delay-line detector for spectroscopy and imaging modes and high energy, motorised, X-ray excitation sources. Main features are:

  • Automated X-ray source with motorized x-ray mirror and improved small spot performance.
  • Improved XPS imaging capability (1µm).
  • New ESCApe data system.
  • New flexi-lock and sample analysis chamber optics.
  • Automated transfer between the flexi –lock and sample analysis chamber.
  • Samples mounted on large (90 x 32 mm) sample platens – automated handling with 3 platens in the instrument.

Sample analysis chamber
The Sample analysis chamber (SAC) is manufactured from mu-metal and has fifteen ports with line of sight to the sample analysis position. The chamber has been designed to accommodate the large 500 mm Rowland circle monochromatic AlKα X-ray source. Ultrahigh vacuum is achieved in the SAC by an oil-free scroll backed turbomolecular pump with a titanium sublimation pump providing secondary pumping.

Flexi-lock/sample loading
Samples are introduced into the sample analysis chamber through the Flexi-lock. This turbomolecular pumped chamber contains the automated sample magazine with storage for up to 3 large sample holders. The Flexi-lock also houses the large field of view optical camera for acquiring an image of the full sample holder which is the first step in the sample identification and analysis work flow.

Large area, high sensitivity
Efficient collection of photoelectrons by the magnetic and electrostatic lenses combined with the large 165 mm mean radius hemispherical analyzer ensure that the Axis Supra boosts high sensitivity in large area analysis mode. The 500 mm Rowland circle X-ray monochromator produces high energy resolution spectra which may be collected in a matter of minutes.

Outstanding performance is not limited to conducting samples. The low energy electron only charge neutralization system allows photoelectron spectra to be collected from insulating samples.

Selected area spectroscopy
Selected area spectroscopy is achieved by inserting an aperture into the electrostatic lens column and forming a virtual probe at the sample surface. Selected area spectra can be acquired from 15 μm diameter spots from any point within the field of view. A simple mouse click is used to define the analysis position from an image deflecting the virtual probe using electrostatic scan plates. This approach removes the need to move the sample and hence uncertainties introduced with repeated sample translation.

Ion sputter depth profiling
The Axis Supra can be configured with gas cluster ion source (GCIS) depending on the type of sample to be profiled.

The multi-mode Ar GCIS (Minibeam6) is capable of generating Arn +clusters consisting of hundreds or even thousands of Ar atoms. As the energy of the ion shared by all atoms in the cluster the energy per projectile atom, or partition energy, can be as low electron volts. At these energies cluster ions only sputter material from the near-surface region leaving the subsurface layer undamaged.

The use of cluster ions for sputter depth profiling organic materials has led to successful depth profiles from many different types of multi-layer materials The GCIS can also be operated in monoatomic Ar+ mode which a better suited to conventional depth profiling inorganic material.

All control and status read-backs are displayed in the ESCApe acquisition software with pre-defined operating conditions provided in look-up table. Similarly the argon gas supply for the ion source can be tuned on and off as required during unattended operation with pressure in monoatomic mode controlled by an automatically regulated piezoelectric valve.

Parallel imaging, spectra from images and multivariate analysis
The Axis Supra`s fast parellel imaging allows the lateral distribution on surface chemistry to be investigates. Photoelectron images are projected onto the 2-dimentional delay-line detector collecting XPS images much faster, and at higher resolution, than the more traditional sequential rastered beam approach. The unique spherical mirror analyzer (SMA) operates in fixed analyser transmission mode ensuring that the energy resolution of photoelectron images is constant for all kinetic energies. This is of particular importance for quantitative imaging applications. Parallel images may also be acquired at lower pass energies to improve the energy resolution, analogous to spectroscopy mode, making chemical state imaging routine.

ISS – ion scattering spectroscopy
Information obtained from the photoelectron measurement can be complemented by the findings from ISS spectra. The primary ions (He+, Ar+)for ISS experiment are generated at the GCIS.