LOOKING AT TRACE IMPURITIES ON SILICON WAFERS WITH SYNCHROTRON RADIATION Because even very low levels of metal impurities can seriously degrade silicon integrated circuits, extremely sensitive methods must be developed to improve wafer cleaning.
PETER GINTER/ WWW.PETERGINTER.DE
ecause of the invention of the transistor ~50 years ago, progress in silicon technology has been phenomenal. Today’s very sophisticated integrated circuits contain >100 million transistors. Their fabrication involves a complex series of steps, among them silicon dioxide growth, plasma etching, metal deposition, high-energy dopant implantation, and wet chemical cleaning. All these processes are potential sources of surface contamination. Even very low levels of metal impurities can cause serious degradation of silicon integrated circuits by diminishing carrier lifetime, reducing the dielectric breakdown of gate oxides (which are often 7 keV, the tail of the inelastic scattering feature and the escape peak dom-
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Ag Cl
Fe
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FIGURE 2. TXRF spectrum of a clean silicon wafer surface. The spectrum shows the silicon substrate peak, chlorine from the wafer surface, silver from the collimator of the detector, and iron and copper contamination at very low levels, representing a minimum detection limit for copper of 3.4 107 atoms/cm2 after 5000 s of integration time ( = 0.085°; h = 11,200 eV). In the high-energy region, the spectrum shows the elastically and inelastically scattered SR.
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h = 1730 eV
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FIGURE 3. Fluorescence spectrum (dots) from a wafer containing 3 1011 atoms/cm2 aluminum. In addition, the modeled profile of the X-ray Raman scattering is shown (green), as well as the Gaussian fits for the aluminum signal (red) and the elastically scattered SR (red). The sum of the Gaussian fit and the Raman profile representing the simulated spectrum is shown in blue ( = 0.1°; h = 1730 eV).
X-ray Raman scattering is generally low compared to the elastic Rayleigh or inelastic Compton scattering, but it can become significant due to resonant enhancement if the incident photon energy is close to a major absorption edge of the sample matrix. Figure 3 shows a typical fluorescence spectrum (dots) from a wafer intentionally contaminated with 3 1011 atoms/cm2 aluminum obtained for an excitation energy of 1730 eV and an angle of incidence of 0.1° (critical angle = 0.9°) for 10,000 s. The figure also contains the deconvolved contribution from the elastically scattered signal at 1730 eV, the aluminum K line, and the calculated continuous Raman background (18). This figure demonstrates that it is the inelastic Raman scattering that ultimately determines the minimum detection limit for aluminum, which has been derived for a bending magnet beamline to be 2.4 109 atoms/cm2 for a 10,000-s count time, corresponding to 7.6 109 atoms/cm2 for a standard 1000-s count time. With a brighter undulator X-ray source, this result can be further improved to 2.7 109 atoms/cm2 after a 1000-s integration time (19).
Extending TXRF to compound semiconductors
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Another area in which TXRF is increasingly applied is monitoring transition-metal contamination in photonic devices such as solid-state lasers and photon detectors. Such compound semi-
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h = 12.7 keV Zn Kα
Intensity (cts /1000 s)
the first spectrum of this series with the last one showed the same overall features revealing identical concentrations, which demonstrates that there is no photon-beam-enhanced desorption of trace impurities even for extended counting times. In addition, this indicates that our vacuum system does not introduce any trace contaminants onto the wafer surface. Apart from the detection of transition metals, TXRF can also be extended to detecting elements with Z
