Determination of trace impurities on Si wafers with x-ray fluorescence

1. Determination of trace impurities on Si wafers with x-ray fluorescence Seminar Author: Bojan Hiti Mentor: doc. dr. Matjaž Kavčič

2. Si wafers – key resource in semiconductor production • Contaminants: • 3d transition metals (Fe, Ni, Zn, Cu) • Light metals (Al, Na) Max. contamination: 109 atoms/cm2 ~ 1fg/g • The only nondestructive analytical tehnique with required sensitivity is X-ray fluorescence spectroscopy

3. X-ray fluorescence spectroscopy • Excitation of inner electron shells with x-ray photons • X-ray tube • synchrotron radiation • Detection of the emitted • x-ray fluorescence. • Analytical tehnique used to determine elemental composition of the sample • Consists of:

4. X-ray interaction with matter • Photoeffect on atomic inner shells • A core-hole is filled by a higher-energy-level electron • Exceeding energy is taken over by • Secondary X-ray • (X-ray fluorescence - XRF) • Auger electron

5. Characteristic x-ray emission line energies – element identification

6. Detector energy resolution should be high enough to distinguish between characteristic x-ray energies of neighboring elements. Semiconductor x-ray detector

7. X-ray interaction with matter • Resonant x-ray Raman scattering (RRS) • Significant only at energies just below absorption edge • Intermediate virtual state • Secondary X-ray is emitted • Elastic scattering • Inelastic (Compton) scattering

8. X-ray – Siinteraction cross-sections:

9. Total reflection XRF • For x-rays: nmatter<1 • High yield (n°counts/s) due to detector proximity • Only first few nm of the sample are excited

10. Highest sensitivity is obtained using excitation with synchrotron radiation • high photon flux ~ 1012 photons/sec • monochromatic (DE/E ~ 10-4) and polarized light Stanford Synchrotron Radiation Lightsource Total reflection XRF setup at SSRL synchrotron

11. TXRF spectrum of “clean” Si wafer measured at SSRL • Detection limits: • 3d metals <109 atoms/cm2 Signal from Fe and Cu surface contamination is clearly observed

12. Measurement of Al contamination • EAl < Eexcitation < Esi • Low energy Si tail despite fine energy tuning – RRS • Detection limit: • Alslightly below 1010 atoms/cm2

13. Grazing emission XRF • Surface sensitivity achieved by grazing detection angle • Smaller yield than Total-reflection tehnique

14. Grazing emission detection can be combined with high-resolution x-ray spectroscopy : • High energy resolution crystal spectrometer employing Bragg diffraction on the analyzer crystal • Position • sensitive • detector Energy resolution ~1eV • Analyzing crystal

15. Experimental Grazing-emission setup at ESRF synchrotron in Grenoble European Synchrotron Radiation Facility Bragg crystalspectrometer installed at the ID21 beamline.

16. High resolution enables discerning between Al and Si Raman scattering signal • Theoretically lower detection limits can be achieved • Problem: small detection efficiency • Achieved detection limit: 1012 atoms/cm2

17. Conclusion • Can provide solution for the light metals measurement • Detection efficiency is substantialy smaller than in TXRF method • Main tehnique for trace impurity detection • Very good performance for transition metals measurement • Cannot lower detection limit for light metals

18. Thank you for your attention.