Grazing Emission X-Ray Fluorescence (GEXRF) 

Principles - Applications - Publications

 

Principles

GEXRF is a complementary technique of the well-known TXRF (Total Reflection X-ray Fluorescence) method. Both techniques are based on reflection effects at the vacuum-sample interface. For electromagnetic radiation in the domain of x-rays the index of refraction is smaller than one and therefore the sample is optically less dense than the vacuum. This results in inaccessible regions for measurements: for TXRF the incoming beam can be externally totally reflected if it is incident below a certain critical angle, for GEXRF the emitted radiation cannot be detected below a certain critical angle. In both cases only a shallow surface layer contributes to the measured intensity if angles below the critical angle for total reflection are considered. Therefore both GEXRF and TXRF are extremely surface sensitive techniques.

 

Illustration of the differences between the GEXRF and TXRF setups. In the GEXRF setup, the emitted radiation is measured as a function of the exit angle with respect to the target surface whereas in TXRF the measurements are performed by considering the incident angle of the incoming beam with respect to the target surface.

 

In order to have a reasonable detection sensitivity, TXRF is only combined with energy-dispersive detectors since the incident radiation already needs to be well collimated. This restriction does not exist for GEXRF and therefore GEXRF is the technique of choice for us since it can be combined with the already existing von Hamos spectrometer at the University of Fribourg. Due to the high energy resolution of our experimental setup, the grazing emission geometry can be realized without using slits. Usual GEXRF setups use slits to define the grazing exit angle whereas in our case the selection is done by using the Bragg angle of the refracting crystal. In fact the Bragg angle defines an emission direction which x-rays have to follow in order to be detected. The target holding system of the von Hamos spectrometer was modified in order to adapt it to requirements of grazing emission conditions. Grazing emission conditions are realized by turning the flat target surface close to the emission direction defined by the Bragg angle of the refracting crystal.

 

Illustration of how grazing emission conditions are realized with the von Hamos geometry. The incident radiation can be the Bremsstrahlung from an x-ray tube, electrons form an electron gun or synchrotron radiation. The target is tilted close to the emission direction defined by the Bragg angle of the refracting crystal. A fixed target position corresponds to a fixed exit angle. A position-sensitive CCD is used for the collection of the x-rays diffracted by the crystal.
 
 
The presented GEXRF technique allows distinguishing between different types of surfaces and deposited impurities if the emitted x-ray intensity is recorded as a function of the exit angle.
 

Applications

  • Surface trace element analysis: Since GEXRF is a surface sensitive technique, it allows looking for and quantifying trace amounts on surfaces in a non-destructive way. This is especially useful in the semiconductor industry where metallic contaminants spoil the wafers but also needed in environmental research and even in the study of artists’ pigments. The measurements are performed at a single exit angle below the critical angle so that only a shallow surface layer contributes to the detected signal.

Detection sensitivity of the presented GEXRF setup for contaminants of interest in semiconductor applications. The direct detection limits are above the requirements of the International Technology Roadmap for Semiconductors (ITRS). However if a preconcentration technique like vapour phase decomposition (VPD), which collects the contaminants from the whole wafer surface in a single spot, is used the presented setup is sensitive enough.
  • Thin Layer Analysis: The dependence of the emitted x-ray intensity on the exit angle allows characterizing thin films deposited on a substrate. Exit angles below the critical angle give information on the surface morphology, exit angles above the critical angle give information on the amount of deposited material and the exit angle itself depends on the density. Thus it is possible to extract the thickness of the deposited layer.
 
Illustration of how data about the sample is extracted by scanning through grazing exit angles. The density of the thin layer is directly proportional to the density, the frequency of the oscillations depends on the nominal thickness of the layer and the intensity above the critical angle indicates the total amount of Cr atoms in the surface unit.

Illustration for thin layers of Cr deposited on top of Si substrates of how sensitive the presented GEXRF setup is. The part above the critical angle allows quantifying the amount of Cr and therefore one can precisely deduce how thick the deposited layer is if the critical angle and, thus, the density of the layer are known.

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  • Depth profiling: The extraction of the dopant concentration profile from Si wafers is in many aspects a problem of technological importance. Since modern integrated circuits are manufactured by using ion implanters, novel and better diagnostic tools for the depth profiling of group III and V doping elements in semiconductor materials are needed. By means of GEXRF it is possible to extract in a non-destructive way the depth distribution of dopant atoms and therefore it presents an alternative to the already established depth profiling methods.
 
For each implantation energy the parameters of the Gaussian distribution (assumed for the depth profile of the Al impurities) were determined by fitting the experimental angular scan. The experimental values for the center and the width of the Gaussian distribution agree quite well with the theoretically calculated values (lower part) and the fitted angular curves show a reasonable agreement with the experimental angular profiles (upper part).
  • Surface nanostructures’ morphology:  The investigation of surface nanostructures is of great importance for the semiconductor nanotechnology. Our high-resolution GEXRF method is a promising tool for investigating surface nanostructure assemblies as it can combine measurements of composition and distribution of atoms deposited on surfaces with measurements of the surface morphology. However the critical angle is well defined only for flat surfaces. Here the intensity observed in the vicinity of the critical angle is strongly dependent on the surface morphology.
GEXRF angular scans of Fe Kα lines and corresponding AFM pictures of the samples (1.5 μm scan size). For both samples the Fe layer deposited on a Si substrate has a nominal thickness of 5 nm. The difference in morphology observed on the AFM images is also strongly reflected in the GEXRF measurements. 

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Publications

 
This work is the result of a collaboration between the Universtiy of Fribourg, the Jan Kochanowski University and the European Sychrotron Radiation Facility where the measurements were performed at the ID21 beamline.
 

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IM ZUSAMMENARBEIT MIT