Ultra-trace analysis of light elements and speciation of minute organic contaminants on silicon wafer surfaces by means of TXRF in combination with NEXAFS

Total reflection X-ray fluorescence analysis (TXRF) using monochromatized undulator radiation from the synchrotron radiation facility BESSY 11 has been employed to investigate light element impurities on silicon wafer surfaces. Dedicated droplet samples on silicon wafer surfaces were prepared and for the TXRF arrangements employed lower limits of detection (LOD) for C, N, Na, Mg and Al ranging from 0.3 pg to 1.3 pg. Calculated vapor phase decomposition (VPD) detection limits, derived from the assumption that the sample droplets were collected from a 200 mm wafer, ranged from 2 10(7) atoms/cm(2) to about 10(8) atoms/cm(2) for Na, Mg and Al. Using a thin window Si(Li) detector, the resonant Raman scattering effect limiting the LOD of Al was studied, and an initial LOD value for B was found to be 7 ng. In addition, a TXRF investigation of only a few nm thick C-Ni-C structure deposited on a wafer could be performed. In near edge X-ray absorption fine structure (NEXAFS) investigations in conjunction with TXRF analysis, several organic contaminants on silicon wafer surfaces were studied with respect to their speciation. A novel instrumentation for analyzing contamination on 200 mm and 300 mm silicon wafer surfaces by synchrotron based total reflection X-ray fluorescence (TXRF) has been designed by the Physikalisch-Technische Bundesanstalt, This instrumentation is also suited for energy-dispersive X-ray fluorescence (EDXRF) analysis of thin structures deposited on silicon wafers. The most prominent features are a high vacuum load-lock combined with an equipment front end module (EFEM) and a UHV irradiation chamber with an electrostatic chuck (ESC) mounted on an 8-axis manipulator. The whole surface of a 200 mm or a 300 mm wafer can be scanned by monochromatized undulator radiation. The commissioning of the instrumentation was initiated in November 2002 by a TXRF investigation of droplets containing 50 pg to 500 pg Na deposited on a 200 rum wafer, in which promising LOD values for Na were found. Off-line reference TXRF and EDXRF analyses will start in the spring of 2003. For this accumulation time of 100 s, a conservative estimate of the LOD value for Na ranged from 1.0 pg to 1.7 pg depending both on the absolute Na deposition in the droplet and the pile-up probability of O-Kalpha, overlapping with the Na-Kalpha fluorescence radiation. The 500 pg Na spectrum is also plotted in a logarithmic scale in the upper right part of figure 6, clearly showing the actual pile-up events of the O-Kalpha and Na-Kalpha at about 1.5 keV. Taking into account the ratio of the respective Na deposition present in the lateral maximum position to the total Na deposition in the droplets, the LOD values for Na range from about 170 fg to 250 fg.