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

Μ m teflon filter is placed in front of the

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µ m Teflon filter is placed in front of the detector to reduce the intensity of the Si substrate signal and prevent saturation of the detector.
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Methodological Developments and Applications 535 10 6 10 5 10 4 1000 100 2000 4000 6000 8000 1.2 10 4 Energy (eV) Escape peak Si Cl Intensity (3000s) a = 0.085 o hv = 11280eV 1 10 4 Fig. 7.69. TXRF spectrum of an ultraclean wafer taken for 3000 s at an excitation energy of 11.28 keV. The MDL for this spectrum is 4 . 4 × 10 7 atoms/cm 2 This results in a decrease in intensity between 2 keV and 2.5 keV, but does not significantly affect the fluorescence intensity of the transition elements of interest [202, 203, 206]. Although the MDL can be easily obtained from the standard spectrum of Fig. 7.68, the background which arises from the higher signals associated with the standard gives a higher apparent MDL than one measured using a sample with lower concentrations. Therefore the MDLs quoted here are typically obtained using samples with low overall levels whose concentration has been calibrated using a standard. The most advanced processes for wafer cleaning available in the industry are truly able to produce wafers whose levels of surface contamination are below what can be observed even with SR-TXRF. The spectrum of such an ultraclean wafer taken with 11.28 keV excitation energy is shown in Fig. 7.69. The only features observed are the Si, Cl, Ag, and the escape and scatter peaks. The Cl comes from the cleaning solution and is viewed as a benign component that does not pose problems for gate oxidation and the Ag is an artifact from the detector collimator. Ag was intentionally chosen as the colli- mator material because the Ag L fluorescence peaks were small and out of the region of interest. This spectrum was taken for a count time of 3000 s and cor- responds to an MDL of 4 . 4 × 10 7 atoms/cm 2 . Note that this concentration corresponds to a detected mass of approximately 0.1 fg when the 2 × 7 mm 2 sampling area is taken into account. The SSRL TXRF facility is sufficiently
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536 C. Streli et al. 10 6 10 7 10 5 10 4 2000 6000 4000 8000 1 10 4 1.2 10 4 Energy (eV) MDL (Ni): 1.2 E7 atoms/cm 2 Fe Ni hv = 11280 eV a = 0.009 o c(Fe) = 2.2 E8 atoms/cm 2 c(Ni) = 1.5 E8 atoms/cm 2 Si Cl Intinsity (counts/20.4h) Fig. 7.70. TXRF spectrum of an unintentionally contaminated wafer taken for 20.4 h of counting time at an excitation energy of 11.28 keV. The MDL for this spectrum is 1 . 2 × 10 7 atoms/cm 2 well behaved that the use of extended counting times to improve detection lim- its for ultra clean wafers can be routinely performed. In an effort to determine the extent to which this is true, a very lightly contaminated wafer was mea- sured for 1000 s, then repeatedly measured using a series of 5000 s spectra with a total counting time of approximately 48 h. Figure 7.70 shows the sum of the individual spectra after 20.4 of the 48 h.
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