X-Ray Photoelectron Spectroscopy
X-Ray Photoelectron Spectroscopy (sometimes called “ESCA”) measures the chemical composition of the top five nanometers of surface. Soft x-rays stimulate the ejection of photoelectrons whose kinetic energy is measured by an electrostatic electron energy analyzer. Small changes to the energy are caused by chemically-shifted valence states of the atoms from which the electrons are ejected; thus, the measurement provides chemical information about the sample surface.
Why is Photoelectron Spectroscopy (XPS) so useful? Because it follows what happens on surfaces, particularly the physical chemistry.
The Small Spot system, based on a Vacuum Generators ESCA-Lab 2, is composed of two gate valve-coupled stainless-steel ultra-high vacuum (UHV) chambers in which the pressure is in the low 10−10 Torr range in the measurement chamber and high 10−9 Torr range in the loadlock chamber. Samples, individually screwed to a carrier plate, are loaded first onto an aluminum transfer plate, placed into the loadlock chamber, evacuated to the low 10−8 Torr range, and then transferred into the measurement chamber.
After transfer, the sample to be measured is installed onto a special manipulator arm, Omniax™. A feature of this arm allows heating of the loaded sample, the temperature of which is recorded by type-C thermocouples. The back of the samples are heated by indirect electron bombardment. This is achieved by biasing a tungsten filament emitter negatively with respect to the sample. Other samples, up to four inch diameter and on kg in weight, may be loaded and transferred onto a robust bottom-mounted stage for analysis at room temperature only.
X-ray photoelectron spectroscopic (XPS) measurements are made using an aluminum K-α electron bombardment x-ray source and hemispherical electrostatic electron energy analyzer. The XPS data are collected with the analyzer operating in the constant analyzer pass energy mode, with a total instrument energy resolution 0.75 eV FWHM. Photoelectrons can be collected from a 4 x 4 mm2 down to a 150 µ-diameter sample area at 10o from the sample surface normal. Photoelectron binding energies (BE) are referenced to the analyzer Fermi level.
Other capabilities of the system include Auger electron spectroscopy (AES), energy-dispersive x-ray (EDX) bulk spectroscopy, and secondary electron yield (SEY) analysis in the measurement chamber, plus ion/ electron bombardment, quantum efficiency and residual gas analysis in both measurement and loadlock chambers.
1. Analysis chamber 2. Loadlock chamber(QE,process) 3. Sample plate entry 4. Sample transfer plate
5. Rack and pinion travel 6. Sample plate stage 7. XYZθ OmniaxTM manipulator 8. Sample on Omniax
9. Electrostatic energy analyzer 10. X-ray source 11. SEY/SEM electron gun 12. Microfocus ion gun
13. Sputter ion gun 14. To pressure gauges and RGA 15. To vacuum pumps 16. Gate valve
Click on the image below to view an animation (1.3M avi file) that demonstrates this ability to follow surface reactions and why this is so important to the understanding of how surfaces interact with their environment (usually vacuum, in the case of accelerators).
Evolution of Chemical States during Heating:
ILC Nb sample for Cavity Processing “as received” with Native Oxide
(AVI file requires Windows Media, Real Player or similar)
Hasan Padamsee of Cornell University posed the following question to us: Could ILC thermally process Nb cavities at 400oC instead of 1300oC (as had been the case at J-Lab and LEP.) The cost savings would be enormous because the lower T is done by air bake and the higher T with a vacuum furnace. The next slide shows, as a function of T, how the Nb2O5 native oxide dissolves into the Nb bulk, but not completely. Enough near-surface suboxide NbO remains to cause superconducting current losses and decreased Q. Undesirable result: Lower surface field, lower accelerating gradient. We showed this important result without expending budget on building (and destroying) test cavities.
