Take-off angle

Owing to the limited escape depth of photoelectrons, the surface sensitivity of XPS can be enlianced by placing the analyser at an angle to the surface nonnal (the so-called take-off angle of the photoelectrons). This can be used to detemiine the thickness of homogeneous overlayers on a substrate. 

This is demonstrated by the XPS spectra in figure B 1.25.5(a) which show the Si 2p spectra of a silicon crystal with a thin (native) oxide layer, measured under take-off angles of 0° and 60° [12]. When the take-off angle is... 

Figure Bl.25.5. (a) XPS spectra at take-off angles of 0° and 60° as measured from the surface nonnal from a silicon crystal with a thin layer of Si02 on top. The relative intensity of the oxide signal increases significantly at higher take-off angles, illustrating that the surface sensitivity of XPS increases, (b) Plot of...

Si /Si 2p peak areas as a fiinction of take-off angle. The solid line is a fit which corresponds to an oxide thickness of 2.0 mn (from [12]). 

XPS can be used to determine the composition of a solid as a function of distance away from the surface and into the bulk of the solid. Such a depth profile can be constructed in two ways. One way in which a depth profile can be constructed is by using a beam of inert gas ions to sputter away material from the surface of the sample and to then record the XPS spectrum. If this procedure is repeated several times, a profile showing the composition of the material as a function of sputtering time and thus of depth into the sample can be constructed. Another way to construct a depth profile involves tilting the sample with respect to the X-ray beam. In Fig. 17A, the take-off angle or the angle between the sample surface and the direction of propagation of the ejected photoelectrons is 90 . In... 

Fig. 17. The analysis depth in XPS varies as a function of the take-off angle or the angle between the sample surface and the direction in which the ejected electrons are propagating.

It can be seen that varying the take-off angle will change the depth of analysis. Thus, a depth profile can also be constructed by obtaining XPS spectra as a function of take-off angle. 

The X-rays leave the specimen at a take-off angle 4>, are collimated by two slits Si and S2 before falling on to a crystal (bent to a radius 2R, where R is the... 

In this expression, n/p] pec is the mass absorption coefficient of X-rays from element A in the specimen, a is the detector take-off angle, p is the density of the specimen... 

Fig. 31 Core-line XPS spectra for clean and oxidized Hf(Sio.5As0.5)As at take-off angles of 90° or 15°. Reprinted with permission from [113], Copyright Wiley...

In electron probe microanalysis the surface characteristics of the specimen are also of importance. Figure 8.34 illustrates how an uneven surface can lead to variable attenuation of the emitted X-rays and the importance of the angle between detector and specimen surface being as near 90° as possible. Such a high take off angle will minimize the surface effects. 

Figure 3.13b shows the Si4+/Si intensity ratio as a function of take-off angle, along with a fit based on Expression (3-10). The agreement up to take-off angles of... 

One can use (3-9) and (3-10) to evaluate the thickness from only one XPS spectrum at a known take-off angle 6. However, as Fadley has already pointed out in his 1976 review [36], several assumptions have to be made to derive (3-9). The most important are that the overlayer has a homogeneous composition, a uniform thickness, and a flat morphology. Particularly the last assumption appears critical. 

The tendencies seen are quite general. For small off-axis angles, i.e. nearly perpendicular to the macroscopic surface plane, shading is relatively unimportant and the main deviating effect is due to the variation of the local take-off angle over the surface, with the result that the /(//s ratio is larger than for a flat layer... 

We conclude that XPS is a great tool for determining layer thickness in the range of a few nanometer, provided one measures the full take-off angle dependence, to test the applicability of the uniform layer model implicitly assumed in (3-9) and (3-10). 

Figure 3.15 O Is / Ag 3d5/2 XPS intensity ratio as a function of take-off angle for two oxygen species on polycrystalline silver. The data corresponding to an O 1 s binding energy of 528.4 eV are attributed to subsurface oxygen in Ag, the other with a binding energy of 530.5 eV to oxygen atoms adsorbed on the Ag surface (data from Baschenko et al. (39J).

Figure 3.16 Experimental and theoretical azimuthal dependence of Ag 3d XPS signal from a Ag (110) surface at three different take-off angles (note that the latter are defined with respect to the surface, in contrast to all other figures in this book adapted from Takahashi et al. [41]).

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