Construction of a windowless Si-anode X-ray tube for a more efficient excitation of low Z elements on Si-wafer surfaces in total reflection fluorescence analysis

To improve the achievable detection limits of low Z element with TXRF, a commercially available 2 kW X-ray tube (SEIFERT Type SF 60, Ahrensburg) with a 40 μm×8 mm fine focus has been modified. A windowless X-ray tube has been realized by removing the Be window out of the tube. The original Cu anode block has been changed to Al, because of sputtering reasons. A 4–6 μm thick pure silicon layer has been sputtered on the Al substrate. The geometry of the anode has been constructed in a specific way in order to optimize the photon flux of the X-ray beam concerning self-absorption and brilliance. Direct vacuum tight coupling to the measuring chamber and operation at 10⁻⁶ mbar vacuum was successfully shown. First measurements have been perfomed with a detector suitable for the detection of low energy photons in total reflection XRF geometry. Sodium has been analyzed on a Si-wafer surface and detection limits of 36 pg (corresponds to 3E9 atoms/cm²) have been achieved and are 10 times better than the detection limits for Na excited with a 1.3 kW Cr standard tube of 330 pg. With this developed X-ray tube the detection limits required by the Semiconductor industry for Si wafer surface contamination quality control are fulfilled.