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Appendix B XPS X-ray photoelectron spectroscopy
501
Appendix B
XPS X-Ray Photoelectron Spectroscopy
1 Principles Photoelectron spectroscopy is a non- destructive surface analysis technique, based upon one of the fundamental interactions of photons with matter: the photoelectric effect. The sample, kept in vacuum, is irradiated by photons emitted from an X - ray tube, an UV discharge lamp or a synchrotron emitter. The photon energy can be absorbed by an electron in an atom of the solid sample. This results in the ejection of a so-called photoelectron with a kinetic energy Ek. This kinetic energy will be conserved by the electron until it leaves the solid sample, provided it does not suffer any further inelastic col!i3ion within the solid. The energy can then be measured to a high precision by an electrostatic analyzer. As a consequence, the photoelectron binding energy in the solid with respect to the Fermi level (EbF) can be extracted as the so-called Einstein conservation law:
E F b = h v -ek-d~spectromete r
where ER is the kinetic energy of the photoelectron measured outside the solid, and
~)spectrometeris the work function of the spectrometer, which is usually treated as a known constant. Therefore, a core level with a known binding energy must be used to calibrate the binding energy scale.
502 The photoelectrons ejected from the atom in the solid have different kinetic energies, according to the electronic level and the type of atom they come from. As the electronic structure of an atom is a unique fingerprint of this atom, photoelectron spectra allow an elemental analysis of the target. All the elements form the periodic table (except hydrogen) can be detected from their characteristic electronic energy levels. Furthermore, it was early discovered that the photoelectron kinetic energy also depends on the chemical environment of the ionized atom. Ek is a function of the electronic charge on the atom. If, in a chemical bonding with another atom, some charge is transferred to a more electronegative or from a more electropositive neighbouring atom, the electron kinetic energy will be consequently lower or higher, respectively. The study of this chemical shift then allows to complement the elemental analysis with a chemical analysis. When studying the different environments of a certain element (like for N: nitrides, silazanes and amines), one is more interested in the chemical shift than in the absolute binding energy. In these cases, the above mentioned energy reference problem becomes of less importance. Depending on the energy of the source, different electron energy levels can be studied. When X-ray photons are used, the electron core levels are excited and the technique is called XPS or ESCA (Electron Spectroscopy for Chemical Analysis). When UV photons are used, the available energy provides only the possibility of studying the outer electron shells. Therefore UPS (Ultraviolet Photoelectron Spectroscopy) studies the valence band structures of materials. Although the photon penetration depth is relatively large in solid materials (a few micrometres for kiloelectronvolt X - rays), the analyzed photoelectrons come from the superficial layers only. Electrons photoemitted from deeper layers suffer inelastic collisions in the material. The main free path of electrons whose kinetic energy ranges between 0 and 1500 eV is typically 0.3 to 3 nm.
503 2 Instrumentation Figure B. 1 presents the basic scheme of an XPS apparatus. X-ray source I
electron analysis I
I
I
monochromator
•
electronic ~
....
sample anoae power II supply
vacuum system
~fety ][interlock[
I
f
I computer control ]
II detection control ] I
electronics
Figure B.1 Schematic presentation of an X-ray Photoelectron Spectrometer.
As a source, A1 and Mg anodes are most currently used. Their photon energy is high enough (1487 and 1254 eV, resp.) to reach at least one core level of any element. Also, their natural line width is small enough to allow the recording of well-resolved photoelectron spectra. Most of the spectrometers are based on an electrostatic hemispherical analyzer, equipped with electrostatic lenses to collect, focus, retard or accelerate the photoelectron beam. Some special precautions are vital. XPS, being a surface analysis technique, is extremely sensitive towards surface contamination. Therefore, the sample has to be analyzed in a ultra high vacuum, which preserves its initial surface composition. When 7" is defined as the time necessary to cover the surface with one layer of adsorbate contamination, it can be inferred from table B.1 that the vacuum in the
504
spectrometer must be of the order of 10-9 or better, since one set of photoelectron spectra needs at least 15 minutes. It should be mentioned however, that the values of table B. 1 are rather pessimistic, since it is supposed that the sticking coefficient (o) is 1. Table B.1 Calculation of the surface contamination rate for a surface in different vacua
P (pressure in torr) r(seconds)
10.6 1
10s 10z
101~ 104
1012 10 6
Another problem is that measurements on insulating materials are always plagued with electrostatic charging problems: when exciting photoelectrons in the sample, its surface is left charged and therefore the energy reference of the spectrum changes with time. Therefore, a flood-gun charge compensation technique (emitting very low energy electrons) has to be used. Summarizing, XPS is a very powerful surface technique, analyzing specifically the upper layer (0.3 - 3 nm) of the substrate. It is a non-destructive analysis tool, detecting any element (except hydrogen) that is present above 0.1%. Quantitative as well as qualitative information can be obtained. An important limitation of the technique is its high cost and its bulky size. Also, due to the requirement of ultra high vacuum conditions, the analysis times are long and the instrument is not trivial to work with.
Bibliography D. Briggs, Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy, Heyden, 1977. D. Briggs and M.P. Seah, Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, John Wiley and Sons, 1983. J.J. Pireaux and R. Sporken, X-Ray Photoelectron Spectroscopy, in Analysis of Microelectronic Materials and Devices, eds. M. Grasserbauer and H.W. Werner, John Wiley and Sons, 1991.
Appendix B
XPS X-Ray Photoelectron Spectroscopy
1 Principles Photoelectron spectroscopy is a non- destructive surface analysis technique, based upon one of the fundamental interactions of photons with matter: the photoelectric effect. The sample, kept in vacuum, is irradiated by photons emitted from an X - ray tube, an UV discharge lamp or a synchrotron emitter. The photon energy can be absorbed by an electron in an atom of the solid sample. This results in the ejection of a so-called photoelectron with a kinetic energy Ek. This kinetic energy will be conserved by the electron until it leaves the solid sample, provided it does not suffer any further inelastic col!i3ion within the solid. The energy can then be measured to a high precision by an electrostatic analyzer. As a consequence, the photoelectron binding energy in the solid with respect to the Fermi level (EbF) can be extracted as the so-called Einstein conservation law:
E F b = h v -ek-d~spectromete r
where ER is the kinetic energy of the photoelectron measured outside the solid, and
~)spectrometeris the work function of the spectrometer, which is usually treated as a known constant. Therefore, a core level with a known binding energy must be used to calibrate the binding energy scale.
502 The photoelectrons ejected from the atom in the solid have different kinetic energies, according to the electronic level and the type of atom they come from. As the electronic structure of an atom is a unique fingerprint of this atom, photoelectron spectra allow an elemental analysis of the target. All the elements form the periodic table (except hydrogen) can be detected from their characteristic electronic energy levels. Furthermore, it was early discovered that the photoelectron kinetic energy also depends on the chemical environment of the ionized atom. Ek is a function of the electronic charge on the atom. If, in a chemical bonding with another atom, some charge is transferred to a more electronegative or from a more electropositive neighbouring atom, the electron kinetic energy will be consequently lower or higher, respectively. The study of this chemical shift then allows to complement the elemental analysis with a chemical analysis. When studying the different environments of a certain element (like for N: nitrides, silazanes and amines), one is more interested in the chemical shift than in the absolute binding energy. In these cases, the above mentioned energy reference problem becomes of less importance. Depending on the energy of the source, different electron energy levels can be studied. When X-ray photons are used, the electron core levels are excited and the technique is called XPS or ESCA (Electron Spectroscopy for Chemical Analysis). When UV photons are used, the available energy provides only the possibility of studying the outer electron shells. Therefore UPS (Ultraviolet Photoelectron Spectroscopy) studies the valence band structures of materials. Although the photon penetration depth is relatively large in solid materials (a few micrometres for kiloelectronvolt X - rays), the analyzed photoelectrons come from the superficial layers only. Electrons photoemitted from deeper layers suffer inelastic collisions in the material. The main free path of electrons whose kinetic energy ranges between 0 and 1500 eV is typically 0.3 to 3 nm.
503 2 Instrumentation Figure B. 1 presents the basic scheme of an XPS apparatus. X-ray source I
electron analysis I
I
I
monochromator
•
electronic ~
....
sample anoae power II supply
vacuum system
~fety ][interlock[
I
f
I computer control ]
II detection control ] I
electronics
Figure B.1 Schematic presentation of an X-ray Photoelectron Spectrometer.
As a source, A1 and Mg anodes are most currently used. Their photon energy is high enough (1487 and 1254 eV, resp.) to reach at least one core level of any element. Also, their natural line width is small enough to allow the recording of well-resolved photoelectron spectra. Most of the spectrometers are based on an electrostatic hemispherical analyzer, equipped with electrostatic lenses to collect, focus, retard or accelerate the photoelectron beam. Some special precautions are vital. XPS, being a surface analysis technique, is extremely sensitive towards surface contamination. Therefore, the sample has to be analyzed in a ultra high vacuum, which preserves its initial surface composition. When 7" is defined as the time necessary to cover the surface with one layer of adsorbate contamination, it can be inferred from table B.1 that the vacuum in the
504
spectrometer must be of the order of 10-9 or better, since one set of photoelectron spectra needs at least 15 minutes. It should be mentioned however, that the values of table B. 1 are rather pessimistic, since it is supposed that the sticking coefficient (o) is 1. Table B.1 Calculation of the surface contamination rate for a surface in different vacua
P (pressure in torr) r(seconds)
10.6 1
10s 10z
101~ 104
1012 10 6
Another problem is that measurements on insulating materials are always plagued with electrostatic charging problems: when exciting photoelectrons in the sample, its surface is left charged and therefore the energy reference of the spectrum changes with time. Therefore, a flood-gun charge compensation technique (emitting very low energy electrons) has to be used. Summarizing, XPS is a very powerful surface technique, analyzing specifically the upper layer (0.3 - 3 nm) of the substrate. It is a non-destructive analysis tool, detecting any element (except hydrogen) that is present above 0.1%. Quantitative as well as qualitative information can be obtained. An important limitation of the technique is its high cost and its bulky size. Also, due to the requirement of ultra high vacuum conditions, the analysis times are long and the instrument is not trivial to work with.
Bibliography D. Briggs, Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy, Heyden, 1977. D. Briggs and M.P. Seah, Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, John Wiley and Sons, 1983. J.J. Pireaux and R. Sporken, X-Ray Photoelectron Spectroscopy, in Analysis of Microelectronic Materials and Devices, eds. M. Grasserbauer and H.W. Werner, John Wiley and Sons, 1991.