Grazing   Incidence X-Ray Fluorescence Analysis  (GI-XRF)

Grazing Incidence X-ray Fluorescence Analysis (GIXRF) is a powerful technique for depth-profiling and characterization of thin layers in depths up to a few hundred nanometers. By measurement of fluorescence signals at various incidence angles GIXRF provides information on depth distribution and total dose of the elements in the layers. The technique is very sensitive even in depths of a few nanometers.

In X-ray fluorescence analysis (XRF) the intensity of the wave field is generally assumed to be locally constant in vacuo and to be exponentially decreasing in solids. However, in grazing incidence XRF (GIXRF) the primary beam appears as an evanescent wave field or as a standing wave field with locally dependent electric field fluctuations. The intensity of the fluorescence radiation emitted by atoms which are excited by these fields is direct proportional to the wave field intensity. Therefore the fluorescent signal emitted by a sample refers to the varying field intensity of the standing or evanescent wave field within the sample. Moreover it additionally provides information on the elemental composition of the sample. As the distribution of nodes and antinodes of the standing wave field within the sample is a function of the incident angle of the primary radiation the (consequently angle dependent) fluorescence signal can be used for nondestructive depth profiling of concentration profiles as well as analysis of layered structures and of nanoparticles on the surface of the reflector.

 

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Fig.1: X-Ray intensities above (within the standing wave field) and below a thick Si-flat calculated for different angles of incidence. It can be seen that the dependence of the distance D between nodes and antinodes is a function of the incident angle. Inside the medium, the intensity decreases as a function of the refraction angle. Calculated for Mo-Ka radiation; the critical angle is at 1.8mrad.

For the analysis of thin-film-like samples (“nanofilms”) on a reflecting substrate the analyte has to be positioned within the standing wave field in front of the substrate.

Finally for determination of the layers present in a given sample the measurement data, obtained by varying the incident angle of primary radiation across the critical angle of total reflection of the substrate and - in case of reflecting layers - the layer material, are evaluated by an iterative fitting procedure on the basis of modeling calculations

As GIXRF does not provide unambigous depth profile information and needs a realistic input depth profile for fitting, in the context of the EC funded European Integrated Activity of Excellence and Networking for Nano and Micro-Electronics Analysis (ANNA) GIXRF is used as a complementary technique to Secondary Ion Mass Spectrometry (SIMS) for the characterization of Ultra Shallow Junctions (USJ).

 

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Fig.2: As fluorescence data from a Si-wafer with As-implant.
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Fig.3: SIMS profile, TRIM simulated profile and GIXRF profile resulting from the change of the SIMS profile by means of the developed fitting procedure for the GIXRF data.