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Depth profiling

The photoelectrons have a very short inelastic mean free path - this means that although generated at roughly equal numbers from the surface to deep inside the sample, only the photoelectrons emerging from the topmost layers will survive the passage to the surface unscattered. Photoelectrons that have undergone inelastic collisions will have lost a fraction of their kinetic energy - and hence no longer contribute to the intensity of the peak but will find itself in the rise in background signal on the left side of the peak in the spectrum (corresponding to lower kinetic energy).

This means that only atoms sitting in the topmost nanometers of the sample contribute to the intensity of the peak making the technique very surface sensitive. To access deeper layers of the sample one can use the argon ion gun to sputter off the surface layers of the sample - and then record a new set of spectra. A series with cycles of measurement with subsequent sputtering can be set up in this way creating a depth profile of the sample.

Example: Si wafer thin layers of oxide, nitride and polysilicon

Below is an example of a depth profile. The sample is a sandwich of:

Silicon wafer
Silicon oxide
Silicon nitride
Polysilicon

In the spectra required to make such a depth profile, one can distinguish the various chemical environments of silicon depending on whether it is bonded to:

Silicon as in the bulk (oxidation state 0).
Oxygen, either fully oxidized (oxidation state IV) in SiO₂ or in intermediate states with a mixture of silicon and oxygen bonds (oxidation state I, II and III)
Nitrogen, also fully oxidized (oxidation state IV)

and plot their atomic percentages along with the percentages of oxygen and nitrogen.

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-=Depth profiling=
+'''Feedback to this page''': '''[mailto:labadviser@nanolab.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.nanolab.dtu.dk/index.php?title=Specific_Process_Knowledge/Characterization/XPS/XPS_Depth_profiling click here]'''
+<!--Checked for updates on 24/8-2021. ok/ jmli-->
-[[Image:Stochiometry 20110510.JPG|500x500px|left|thumb|The composition of a NiCr as a function of film depth (etch time). The relationship between Cr and Ni is quite constant through the 70nm thick film. Measurements done with XPS-ThermoScientific. ]]
+<!-- Page reviewed 8/5-2023 jmli -->
+{{Author-jmli1}}
-The analysis is made on a chosen spot on the sample surface (chosen with the system camera). The technique is, as written above, very surface sensitive and probes only the top nanometers of the sample.
+=Depth profiling=
-With the ion beam gun on the system an etch of the sample can be done. The system measures the desired spectrum, does an etch step and measures again. A series of etch cycles can be set up, measuring the composition of the sample at different depths (for example at different depth of a film).
+The photoelectrons have a very short inelastic mean free path - this means that although generated at roughly equal numbers from the surface to deep inside the sample, only the photoelectrons emerging from the topmost layers will survive the passage to the surface unscattered. Photoelectrons that have undergone inelastic collisions will have lost a fraction of their kinetic energy - and hence no longer contribute to the intensity of the peak but will find itself in the rise in background signal on the left side of the peak in the spectrum (corresponding to lower kinetic energy).
+This means that only atoms sitting in the topmost nanometers of the sample contribute to the intensity of the peak making the technique very surface sensitive. To access deeper layers of the sample one can use the argon ion gun to sputter off the surface layers of the sample - and then record a new set of spectra. A series with cycles of measurement with subsequent sputtering can be set up in this way creating a depth profile of the sample.
+== Example: Si wafer thin layers of oxide, nitride and polysilicon ==
-'''Example: NiCr film'''
+Below is an example of a depth profile. The sample is a sandwich of:
+* Silicon wafer
+* Silicon oxide
+* Silicon nitride
+* Polysilicon
-As an illustration, a figure to the left, shows an elemental analysis through a metallic film consisting of Ni and Cr. The metallic layer was about 70 nm thick, and the atomic percentage of Ni and Cr was measured through the layer.
+[[File:profile sandwich 3.PNG|700px|{{photo1}}]]
-In the graph, you see the atomic % as a function of etch depth, and it is possible to detect that the relationship between Ni and Cr is fairly constant through the metallic film.
+In the spectra required to make such a depth profile, one can distinguish the various chemical environments of silicon depending on whether it is bonded to:
+* Silicon as in the bulk (oxidation state 0).
+* Oxygen, either fully oxidized (oxidation state IV) in SiO<sub>2</sub> or in intermediate states with a mixture of silicon and oxygen bonds (oxidation state I, II and III)
+* Nitrogen, also fully oxidized (oxidation state IV)
+and plot their atomic percentages along with the percentages of oxygen and nitrogen.