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

3 n ee baur methodological developments and

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3 ee Baur
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Methodological Developments and Applications 499 Spot-wise TXRF data cannot be generalized for the whole surface without performing statistically valid measurements that permit generalization. How- ever, whole-surface results can be provided after a vapor phase decomposition (VPD) preparation of the native silicon oxide that has become a standard preparation procedure of wafer surface analysis. Combining TXRF with VPD leads to improved detection limits (LOD) of dissolved surface contamination and provides reproducible data on overall surface cleanliness, particularly, when the VPD preparation is automated. In order to increase the reproducibil- ity of the VPD method, an automated surface preparation system has been recently developed by our group 4 and accepted by the international semicon- ductor community. Pursuing goals of higher sensitivity, synchrotron radiation (SR) TXRF and other novel X-ray sources have already penetrated the fron- tiers of classical TXRF [86]. Comparative methods of surface analysis such as inductively coupled plasma mass spectrometry after VPD preparation (VPD-ICP-MS), secondary ion mass spectrometry (SIMS), scanning Auger microprobe (SAM), scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX), high resolution electron energy loss spectroscopy (HR-EELS), Rutherford back scattering (RBS), etc. require equipment of the same or higher complexity and are troubled by more tedious calibration, data management, and quality control procedures. Data management of VPD-ICP-MS requires, e.g., a memory capacity 60% larger than required for VPD-TXRF. However, under optimized plasma and nebulizer conditions, ICP-MS, an ideal tool for ultra pure water, can be read- ily applied to monitoring a limited number of light metal surface contaminants after VPD preparation [87]. The main difference between hyphenated VPD and SIMS, SAM, EDX, EELS, or RBS is that the VPD methods are integrat- ing and thus average the surface contamination whereas the other methods are probing a spot of the surface. As a result, VPD-ICP-MS is incapable of providing mapping or spot-wise information [88]. Here we provide a review of TXRF as a paramount part of an industrial ultra trace analytical inspection system for process media and novel microana- lytical methods for monitoring the cleanliness of silicon wafer. The methodical design had to balance affordable costs and severe requirements of robust, ultra trace analytical routine procedures. The validation of mono-isotopic elements such as Na, K, and Al has been an issue to be solved because the complementary graphite furnace atomic absorption spectrometry (GFAAS) is not sensitive enough to be used as an independent method and statistics sets the limits of interlaboratory corre- lation at the lower range of ng/ml (ppb) [89]. Consequently, interlaboratory evaluation of ultra trace analytical methods cannot be applied to validation in the range of lower ppts (ng/l) either. In the sub-ppb range, we had to go back
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