Total Reflection X-Ray Fluorescence Analysis (TXRF)
Basically TXRF is an energy dispersive analysis with a special excitation geometry. In the standard case of EDXRF the angle between incident primary radiation and the sample is 45°. In addition the detector is placed normally to the incident beam thus the angle between sample and detector is also 45°. A narrow almost parallel beam is impinging at angles below the critical angle on the surface of the reflector which carries the sample as randomly distributed microcrystals in the center part of its surface. The principle setup is shown in Fig 1. The x-rays scarcely penetrate the reflector and thus the contribution from scattered primary radiation from the substrate is minimized. The sample itself is excited by both, the primary and the reflected beam therefore the fluorescent signal is practically twice as intense as in the standard EDXRF excitation mode.
One of the differences between EDXRF and TXRF is the excitation geometry which is changed in such a way that the primary radiation impinges at an angle below the critical angle of total reflection on the surface of the reflector carrying the sample. The critical angles are in the range of a few mrad for the typical reflector materials like quartz or Si and the primary radiation of 9.4 keV from a W-L or 17.5 keV from a Mo-anode x-ray tube.
Today TXRF is mainly used for non-destructive surface contamination analysis in the semiconductor industry and for chemical trace analysis as it offers detection limits in the pg range for excitation with X-Ray tubes and even in the fg region if synchrotron radiation is used. The effect of the angular dependence of the fluorescence intensity is used to non-destructively investigate surface impurities, thin surface layers and multilayer structures and is called “Grazing Incidence” or “Glancing Incidence” (GI) XRF
The main advantages of the TXRF setup are the following:
- Due to the total reflection of the incident photons only a very small part of the primary beam penetrates into the sample carrier. This leads to a drastically reduced spectral background contribution which originates from scattering on the substrate.
- As the incident beam is totally reflected from the sample carrier and the sample is excited by both the incident and the reflected beam resulting in a doubled fluorescence intensity.
- The extreme grazing incidence geometry allows placing the detector very close to the sample surface. This results in a large solid angle for the detection of the fluorescence radiation.
The advantage of measuring the surface contamination with TXRF is the fact that the wafer can be mapped by moving the wafer in the plane below the detector to obtain contamination information from each point of the wafer. The inspected area is determined by the detector aperture and detector crystal size. The spatially resolved analysis is done completely nondestructive and can be performed ”in-fab”, close to the point of fabrication.
To measure surface contaminations on wafers a special set-up is required, allowing to measure the wafer without any surface contact and with the possibility of an angle scan. Measuring the angle dependence of the fluorescence signal allows to distinguish the form of the contamination, film-type or particulate-type. From this kind of measurement it is also possible to check, if signal comes from a surface contamination or if the atoms are below the surface. Pursuing goals of higher sensitivity, synchrotron radiation (SR-) TXRF and other novel X-ray sources have already penetrated the frontiers of classical TXRF.
Table : Applications of TXRF (from. P.Wobrauschek, X-Ray Spectrom. 2007; 36: 289–300)
Industrial / Technical applications
Fine Arts / Archeological / Forensic:
Water: sea17, rain18, pore water, river19, mineral20, spring water, drinking water21, chemicals and deionized water
Body fluids: : blood22, serum,23 urine24, amniotic fluid25, cerebrospinal fluid26
surface analysis: Si-wafer surfaces, gaAs-wafer surfaces
ores, rocks, minerals, rare earth elements, quartz, mineral sands27, diamond, crystals,
Pigments28, paintings29, varnish30
Air: aerosols31, vapor, air dust32, airborn particles, fresh air
Tissue: hair, kidney, lung33, liver, stomach, nails, colon34
implanted ions: depth and profile variations
geological materials, bio-mineralisation,
bronzes, pottery, jewelry, manuscripts, egyptian masks
Soil: sewage sludges35, sediments36
Various: enzymes, polysaccharides37, glucose, proteins, cosmetics, bio-films, human bones
thin films: single layers, multi layers
textile fibers38, glass, cognac39, dollar bills39, gunshot residue39, drugs39, tapes39, sperm39, finger prints39
Plant material: Algae40, fine roots, cucumber plants, pollen41
oil: crude oil42, fuel oil, grease, pure fuel oil, waste oil, petroleum, oil-shale ash, Diesel
Foodstuff: fish, flour, fruts, crab, mussel, mushrooms, nuts, vegetables, wine43, tea, soft drinks, onion
chemicals: acids44, bases, salts, solvents,
Drinks: coffee45, spirituous beverages46, honey47,
fusion/fission research: transmutational elements in Al + Cu, lodine in water 48
Various: coal, peat49