Silicon Surface Metal Contamination Measurements using Grazing-Emission XRF Spectrometry
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[4]. This has led to
the development of Grazing-Emission X-ray
Fluorescence
Spectrometry (GEXRF) [5, 6], which is roughly an inversion of the traditional TXRF set-up. GEXRF is a technique whereby the sample is irradiated with an uncollimated, polychromatic incident X-ray beam roughly perpendicular to the substrate and detection of surface sensitive Xrays is done at grazing angles using wavelength dispersive detection systems. The determination of low Z elements on silicon wafers is amongst the potential applications of this technique, as has been demonstrated earlier [7]. This paper focuses on two main topics, i.e. firstly, evaluation of straight GEXRF measurements of Na, Mg, Al, K and Ca contaminated silicon wafers in comparison with established analysis techniques such as TXRF (K and Ca), and Atomic Absorption Spectroscopy (Na, Mg, Al), and secondly, initial results will be presented on Vapor Phase Decomposition - Droplet Collection (VPD-DC) pre concentration procedures, to improve GE-XRF limits of detection for the above mentioned elements. EXPERIMENT Sample preparation Wafers (150 mm, p-Cz [100]) received an initial clean consisting of SPM, dHF, SC-1, and dHC1 prior to controlled metal contamination. Multi-element (Na, Mg, Al, K, and Ca) synthetic calibration samples were prepared by means of spin coating. Wafers were contaminated at 5 different concentration levels covering roughly two orders of magnitude (spin solution 0. 1, 0.5, 1, 5 and 10 Jtg/g of respective metals in dilute HNO3 solution). Suitable blanks were included in the analysis procedure. All prepared samples were cross calibrated with alternative analytical techniques (if applicable for element of interest) such as VPD-DC-AAS, VPD-DC-TXRF and 397 1997 Materials Research Society Mat. Res. Soc. Symp. Proc. Vol. 477 ©
straight TXRF. All TXRF measurements were done on a Atomika TXRF8010 using a Mo X-ray tube at an angle of 1.3 mrad. Typical wafer surface metal contamination levels were in the lx 1011 to 5x10 13 at/cm 2 range depending on the element. For straight GEXRF measurements, wafers were cut to squares of 5x5 cm (due to geometry requirements of the current prototype). For VPD-DC-GEXRF measurements, wafers were exposed to HF vapor for 5 till 8 minutes (VPD-step) to dissolve any metal contamination in or on the Si02 layer, and subsequently scanned automatically with a Programmable Automatic Droplet SCANner (PADSCAN [9]). This automated tool allows to scan (and thus collect metal residues from the wafer surface) in a controlled manner either full or partial segments (1/4 or 1/2) of wafers ranging from 100 till 300mm. Metal contamination is collected with 50gtl droplets of a mixture containing HF/H202/H20 (1/3/96 ratio by volume, Ashland GB quality); droplets are subsequently dried under IR radiation. Instrumentation A research GEXRF spectrometer, based on a conventional PHILIPS PW 2400 XRF spectrometer, has been used. It should be noted that the current prototype design can be further optimized with respect to excitation and detection of trace
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