[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The detection of low z atoms presents instead

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The detection of low Z atoms presents instead additional problems that have been addressed with the development of Synchrotron Radiation TXRF [210] and that establish new, intrinsic limits to the lowest detectable concen- tration of impurities. When detecting elements lighter than silicon, in fact, it is necessary to utilize excitation radiation below the Si K-edge threshold in order to cut down the intense silicon fluorescence that with its low energy tail would bury the emission from aluminum. However, when exciting below the silicon threshold, the emission spectra is dominated by a resonant Raman background. This spectral distribution of inelastically scattered photons is produced by incoming photons that lose the energies to excite 2s electrons in the continuum. This process is responsible for a triangular-shaped background peaked at the exciting energy decrease of 100 eV and far extending in the low energy tail. This background, at variance with the elastic scattering, does not present a pronounced variation with the detection geometry and limits the LLD for Al to a value of 7 · 10 9 atoms/cm 2 . Figure 7.78 shows the spectra from two reference wafers with 10 11 and 10 12 Al atoms/cm 2 as compared to the spectrum from a clean wafer. The implication of the Raman effect in limiting the LLD for Al is quite evident.
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Methodological Developments and Applications 545 10 7 10 6 10 5 10 4 2 4 6 8 10 12 TXRF spectrum from an ultra-clean wafer Intensity (counts/200000s) Energy (keV) Fe Ni Cr Si Cl S Elastic scatter peak h n =10.93keV Zn Fig. 7.77. SR-TXRF spectrum from an ultra-clean wafer from Motorola used as cross calibration test between SSRL and ESRF TXRF facilities. The Cr, Fe, and Ni contaminants are in the order of few 10 8 atoms/cm 2 and the lower limit of detection in 10 7 atoms/cm 2 scale 8000 3.5e12 atoms/cm 2 (AI) 1.6e11 atoms/cm 2 (AI) 7000 6000 5000 4000 3000 2000 1000 0 Blank 1200 1300 1400 1500 1600 Energy (eV) Intensity (counts/1000s) 1700 1800 1900 2000 Fig. 7.78. Spectra of Al contaminated (1 . 6 × · 10 11 at/cm 2 and 3 . 5 × · 10 12 at/cm 2 ) and blank wafers (energy 1730 eV, incidence angle 8 mrad). The rightmost peak at 1740 eV is residual Si fluorescence excited by the high energy tail of the multilayer pair transmission. The central peak at 1600 eV is the Raman background that ex- tends at lower energies interfering considerably with the Al peak at 1490 eV
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546 C. Streli et al. In the case that the required mapping detection limits for impurities of low Z elements would be lower than 7 × 10 9 atoms/cm 2 it would be necessary to move from energy dispersive detection toward wavelength dispersive solu- tions, that because of the higher energy resolution of optical elements such as multilayers would increase the relevance of the low energy fluorescence peaks with respect to the Raman background.
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