1. Lecture 18. Chemical: XPS

2. X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate the chemical composition of surfaces.

3. XPS is used to measure:
elemental composition of the surface (top 1–10 nm usually)
empirical formula of pure materials
elements that contaminate a surface
chemical or electronic state of each element in the surface
uniformity of elemental composition across the top surface (aka, line profiling or mapping)
uniformity of elemental composition as a function of ion beam etching (aka, depth profiling)

4. Components of an XPS system
The main components of a commercially made XPS system include:
A source of X-rays
An ultra-high vacuum (UHV) stainless steel chamber with UHV pumps
An electron collection lens
An electron energy analyzer
Mu-metal magnetic field shielding
An electron detector system
A moderate vacuum sample introduction chamber
Sample mounts
A sample stage
A set of stage manipulators

5. XPS Energy Scale
The XPS instrument measures the kinetic energy of all collected electrons. The electron signal includes contributions from both photoelectron and Auger electron lines.

6. Quantitative Analysis by XPS
For a Homogeneous sample:
I = NsDJLlAT
where: N = atoms/cm3
s = photoelectric cross-section, cm2
D = detector efficiency
J = X-ray flux, photon/cm2-sec
L = orbital symmetry factor
l = inelastic electron mean-free path, cm
A = analysis area, cm2
T = analyzer transmission efficiency

7. Quantitative Analysis by XPS
N = I/sDJLlAT
Let denominator = elemental sensitivity factor, S
N = I / S
Can describe Relative Concentration of observed elements as a number fraction by:
Cx = Nx / SNi
Cx = Ix/Sx / S Ii/Si
The values of S are based on empirical data.

8. Instrumentation for XPS
Surface analysis by XPS requires irradiating a solid in an Ultra-high Vacuum (UHV) chamber with mono-energetic soft X-rays and analyzing the energies of the emitted electrons.

9. X-ray Photoelectron Spectrometer

10. XPS Spectrum The XPS peaks are sharp.
In a XPS graph it is possible to see Auger electron peaks.
The Auger peaks are usually wider peaks in a XPS spectrum.
Aluminum foil is used as an example on the next slide.

11. XPS Spectrum