Laboratory and synchrotron radiation total-reflection X-ray fluorescence: new perspectives in detection limits and data analysis

Abstract

Having established detection limits for transition elements exceeding current requirements of the semiconductor industry, our recent efforts at the Stanford Synchrotron Radiation Laboratory (SSRL) have focused on the improvement of the detection sensitivity for light elements such as Al. Data analysis is particularly challenging for Al, due to the presence of the neighboring Si signal from the substrate. Detection limits can be significantly improved by tuning the excitation energy below the Si-K absorption edge. For conventional TXRF systems this can be done by using a W-Mα fluorescence line (1.78 keV) for excitation.At a synchrotron radiation facility energy tunability is available. However, in both cases this results in a substantial increase in background due to resonant x-ray Raman scattering. This scattering dominates the background under the Al Kα fluorescence line, and consequently limits the achievable sensitivity for the detection of Al surface contaminants. In particular, we find that for a precise determination of the achievable sensitivity, the specific shape of the continuous Raman background must be taken into account in the data analysis. The data deconvolution presented here opens a new perspective for conventional TXRF systems to mitigate this background limitation. This results in a minimum detection limit of 2.4 x 10 9 atoms /cm 2 for Al. 1Based on these results it will also be demonstrated that by improving the detector resolution, the minimum detection limit can be improved significantly. For a detector resolution of 15 eV as predicted for novel superconducting tunnel junction detectors, an improvement in minimum detection limit of about a factor of 3 can be estimated. Keywords:Total reflection X-ray fluorescence, low Z elements, Stanford Synchrotron Radiation Laboratory IntroductionExploiting the unique capabilities of synchrotron radiation, total reflection x-ray fluorescence spectroscopy (TXRF) has been successfully applied to detect very low concentrations of impurities on Si wafer surfaces [1,2,3,4]. Synchrotron radiation offers several advantages over conventional x-ray tubes: It provides a high incident flux, especially from insertion devices, it is both low-divergence and linearly polarized, which leads to an increased fluorescence signal while reducing the elastically scattered background. In addition, tuning the x-ray energy to a characteristic absorption threshold of a typical element present in the wafer surface increases the excitation cross-section for this element which leads to higher fluorescence intensities. With these combined capabilities we routinely achieve an minimum detection limit of < 9 x 10 7 atoms/cm 2 for transition metals on Si wafer surfaces for a 1000 second spectrum.On the other hand, the detection of low Z elements such as Al is challenging because of the lower fluorescence yield as compared to transition metals. In addition, the detection of Al on Si wafers is difficult because of the presence of the much stronger Si substrate signal which tends to dominat...

Application of the high-resolution grazing-emission x-ray fluorescence method for impurities control in semiconductor nanotechnology

“…In this case, as reported earlier, 6,14,15 the DL for Al is affected by the presence of the x-ray resonant Raman scattering ͑RRS͒ process in Si. Indeed, for photon beam energies tuned below the Si K-edge in order to avoid the strong K␣ fluorescence line, the x-ray RRS profile overlaps with the weak fluorescence line from Al impurities.…”

Sample Preparation for Evaluation of Detection Limits in X-ray Fluorescence Spectrometry

“…Synchrotron radiation based X-ray fluorescence (SR-XRF) and total reflection X-ray fluorescence (SR-TXRF) techniques are the examples of such trends, where parts per trillion (ppt) level or, in absolute mass, femto gram detection sensitivities have been achieved. [22][23][24] Normally, XRF techniques are comparative in nature and type-standards are generally used to calibrate the spectrometers. Similarly, synthetic standards are employed to determine the DLs.…”

Resonant X-ray Raman scattering for Al, Si and their oxides

“…3(b) between the measured and fitted Ka 1,2 lines are probably due to this effect as a consequence of the finite beam energy resolution. The intensity excess found in the low-energy tails of the Ka 1,2 lines corresponds to the excitation of K-shell electrons into the M-shell and quasi-simultaneous filling of the 1s holes by L 2,3 electrons.…”

“…Several experiments [2,3] have shown that the detection limit of Al impurities on Si surface is limited by the presence of the X-ray resonant Raman scattering (RRS). For photon excitation energies tuned below the Si K-absorption edge to avoid the intense Si K fluorescence X-ray line, the RRS structure of Si is indeed overlapping with the Al K X-ray peak.…”