Using angle scan txrf stratified microstructures can

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angle characteristics are a function of the layer thickness [139, 140]. Using angle-scan TXRF, stratified microstructures can be accurately analyzed for element composition, layer thickness, and density [135]. Shallow doping profiles, particularly those of As, require reliable nanoscale information on the dopant distribution. Particularly at high concentration, As forms clusters in monocrystalline Si and, therefore, not all the As atoms are electrically active donors. We have evaluated various analytical techniques and found that optimized secondary ion mass spectrometry (SIMS) methods can be reliably applied to layers at 5 nm and below, but for the upper subsurface, the technique of choice is angle-dependent TXRF, as shown in Fig. 7.44 [141]. The angle-scan TXRF is a nondestructive method, whereas the combination of TXRF with layer-by-layer chemical etching provides reproducible results by a destructive type of stratigraphy [104, 142–144]. Characterization of Layers on Si Wafers and Implants in Si Wafers by TXRF TXRF allows the nondestructive element analysis of particulate spot size sam- ples but also of near surface layers, layers on top of a reflecting surface, as well as so-called buried layers and depth profiles below a reflecting surface. This is a consequence of the variation of the primary intensity above and below the surface with the angle of incidence. The variation results from an interference caused by the superposition of incoming and reflected beam as can be seen in Fig. 7.45 for Si and Mo-K α radiation. Above the surface a standing wave is formed in the intersection of incident and reflected beam. For the critical angle 1.8 mrad above the surface for Mo- K α on Si nodes and antinodes follow with a distance of about d = 18 nm and the first antinode coincides with the surface. Assuming a reflectivity of 90%, the antinodes have a 3.6-fold intensity of the primary beam. Below the surface the intensity is damped exponentially within a depth of some 10 nm
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Methodological Developments and Applications 511 Depth (nm) Depth (nm) Depth (nm) Concentration (10 20 atoms/cm 3 ) Concentration (10 20 atoms/cm 3 ) Concentration (10 20 atoms/cm 3 ) 0 5 10 15 20 25 0 5 10 15 20 5 (a) (b) (c) 4 3 2 1 0 0 4 8 12 16 SIMS SIMS TXRF TXRF both both SIMS TXRF both 1 0.75 0.5 0.25 0 0 5 10 15 20 25 30 Fig. 7.44. Implantation profiles of shallow As doping with implantation energy ( a ) 500 eV, ( b ) 1 keV and ( c ) 5 keV using SIMS, TXRF, and SIMS in combination with TXRF (penetration depth). For angles smaller than the critical angle, the distance d is stretched and the first antinode moves away from the surface. Inside the substrate, the intensity is damped within a few nanometers. For angles larger than the critical angle d is compressed, the oscillations vanish, and intensity approaches unity; also inside the substrate to some micrometers depth, the penetration depth increases. The big advantage of excitation close to the critical angle for the investigations of depth profiles and buried layers close to the surface is the fact that the intensity of the exciting radiation due to the standing wave phenomenon is up to a factor of 4 higher than excited at an angle higher than the critical angle.
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  • Spring '14
  • MichaelDudley

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