Characterization of Silicon Wafer Surfaces with Sr-txrf

Total reflection X-ray fluorescence in conjunction with synchrotron radiation (SR-TXRF) has demonstrated sensitivities for transition metals that are approximately 50 times better than what is obtained with conventional X-ray sources [1]. With the high flux, low divergence, and linear polarization of a SR source, fluorescence signals are enhanced while background contributions from elastic and inelastic scattering are reduced. Detection limits of 8 10 atoms/cm for transition metals on silicon surfaces have been routinely achieved at the SR-TXRF facility at the Stanford Synchrotron Radiation Laboratory (SSRL) [1]. This has enabled the semiconductor industry to perform high sensitivity surface analyses for process development. The spectrum in Fig. 1 illustrates the best detection limit obtained for Ni of 1.2 10 atoms/cm at SSRL using extended counting times. Given the relatively small footprint of the X-ray beam on the silicon wafer (1 mm 7 mm), this detection limit represents a total detected mass of approximately 0.1 fg. In addition to these “standard” TXRF measurements of metals directly on silicon surfaces, new processes involving Cu interconnect technologies have created requirements for measuring trace Cu impurities on barrier metals such as Ta. The addition of the heavy metal substrate layer results in a strong Ta L fluorescence signal that overlaps with the Cu K emission line and can even saturate the detector. This problem can be overcome by using the tunability of the SR source to choose an excitation energy below the K edge of Ta yet above that of the Cu. This effectively eliminates the Ta K fluorescence line. However, as we have shown for the analogous case of Al on Si, a significant background is still present due to inelastic X-ray Raman scattering. Generally, the cross-section for X-ray Raman scattering is low compared to the elastic Rayleigh or inelastic Compton scattering, but does become significant due to resonant enhancement if the incident photon energy is close to a major absorption threshold of the sample matrix as is the case here [2, 3]. In spite of this background that can reduce the ultimate sensitivity of the measurement, we will demonstrate below that it is still possible to choose an appropriate photon energy that minimizes these effects and results in a detection limit in the 10 atom/cm range. While TXRF alone can determine the type and THE RIGAKU JOURNAL VOL. 19 / NO. 2 & VOL. 20 / NO. 1 / 2003