In x-ray fluorescence analysis an incoming x-ray beam is creating a photo-electron. The hole of this electron in the inner shell is filled with an electron from an outer shell. In this decay process a photon is emitted: the so-called X-Ray Fluorecence radiation. The XRF signal has a characteristic energy (‘line’) for each element. The atom is excited by an incoming x-ray and a photo-electron is emitted. An outer shell electron fills the hole and a photon is emitted. The energy of this photon is characteristic for the atom, each chemical element has a different line spectrum.

The x-ray fluorescence signal can be measured with an energy dispersive detector. The sensor is sensitive for the whole energy range of interest and measures all lines simultaneously. Example of a TXRF spectrum on the left: the sample was a multi element standard with 11 elements. The big peak on the left (Si) originates from the sample carrier, the excitation energy (primary beam) was 9.7keV.

Unlike other XRF-techniques the incidence angle of the primary x-ray beam is very shallow in TXRF. A typical angle is in the range of 0.1°. Therefore the x-ray is reflected on the surface and the penetration depth is a couple of nm only. TXRF is a purely surface sensitive analytical method.

Schematic diagram of TXRF spectrometer.

The quantification of TXRF spectra can be done directly, using first principle calculations only. However, the most commonly way is the use of a single element standard. The relative strength of the TXRF-signal from different elements does not depend on the matrix material, only on the primary x-ray beam settings.

TXRF offers direct measurements on wafer surfaces without sample preparation.  Main field of application in semiconductor industry is monitoring of surface contamination, down to less than 1E10 atoms/ cm² in direct measurement. The CAMECA TXRF 8300W Fab Tool is designed for this application. Surface preparation (VPD) can improve this by almost three orders of magnitude.

Wafer Mapping: Since TXRF is a non destructive and fast method, it is also possible to scan the entire wafer for contamination control. Depending on the wafer size, the machine takes up to several hundred x-ray spectra and generates maps for the different elements.
In contrast to direct TXRF, where a single spot is measured with typically 1000s, wafer mapping is done with 5-30s per spot. Putting all spectra together, the overall sensitivity is similar to direct TXRF. In addition wafer mapping TXRF provides information about the contamination distribution on the wafer. The risk to miss a localized particle contamination vanishes. The CAMECA’s TXRF8300W features a high precision wafer stage with 5 degrees of freedom which is essential for this functionality.

TXRF map of copper contamination on silicon.

Find more detail by checking our publications page.