Total Reflection X-Ray Fluorescence (TXRF)
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ABSTRACT Total reflection X-Ray fluorescence (TXRF) is a glancing x-ray analytical technique which is used primarily to measure surface metal contamination on semiconductor substrates. This is a review of Total reflection X-Ray Fluorescence (TXRF) applications for silicon semiconductor processing. In addition, some comments are made about the future of TXRF, and in particular, synchrotron radiation TXRF (SR-TXRF)
INTRODUCTION This is a review of Total reflection X-Ray Fluorescence (TXRF) 1t 0 applications for semiconductor processing, particularly silicon. The application of TXRF to other semiconductor materials is discussed 2 6 27 101 25 elsewhere: GaAs , ,59; GaP , and diamond . TXRF for surface analysis is a relatively new technology. One of the first publications occurred in 1986 using synchrotron radiationi. Publications using commercially available TXRF instruments for semiconductor applications began in 1988. Today there are on the order of 100 TXRF instruments worldwide in the semiconductor 110 industry, and an ASTM method has now been approved . Since 1988 there have been about 100 publications in this field, but this number does not include numerous abstracts and publications in Japan where the majority of the commercial instruments are found today. The commercial instruments were developed for the primary application of characterizing the cleaning of planar silicon wafers, however, numerous unforeseen applications were developed by users and many of those applications are reported here. In essence TXRF has much broader application today in the semiconductor industry than supporting the cleaning of silicon wafers. In TXRF an X-ray beam of low divergence impinges the surface of the sample below the critical angle for total external reflection, thereby exciting only a shallow depth of a few nanometers by the evanescent wave decay into the sample surface. If the impinging angle approaches the critical angle, then the excitation depth can be much greater than a few nanometers. X-ray sources in commercial instruments are W L-beta, Mo Kalpha, and Cu K-alpha from either rotating anodes or tubes, and monochromators are commonly used. The excited fluorescent X-rays are emitted from the surface and detected by an energy dispersive detector, typically a Si(Li) detector of diameter 8 to 10 mm. Quantification is completed using reference samples. Detection limits in the late 1980's 2 were on the order of 1011 to 1012 atoms/cm and are today on the order of l09 2 to 1010 atoms/cm for most of the critical elements in the semiconductor applications. An important method to improve the detection limits by two orders of magnitude (reaching 107 to 108 atoms/cmz using today's commercial instruments) is to preconcentrate the impurities from the entire surface of the wafer (typically 150 to 200 mm in diameter) into a residue of diameter less than 5 mm on the surface of the wafer, and to analyze the residue using the TXRF. One of the more common preconcentration techniques is 377
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