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

Si wafers – key resource in semiconductor production Contaminants: o 3d transition metals (Fe, Ni, Zn, Cu) o Light metals (Al, Na) Max. contamination: 10 9 atoms/cm 2 ~ 1fg/g The only nondestructive analytical tehnique with required sensitivity is X-ray fluorescence spectroscopy

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

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

Characteristic x-ray emission line energies – element identification

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

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

X-ray – Si interaction cross-sections:

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

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

Detection limits: o 3d metals <10 9 atoms/cm 2 TXRF spectrum of “clean” Si wafer measured at SSRL Signal from Fe and Cu surface contamination is clearly observed

Measurement of Al contamination E Al < E excitation < E si Low energy Si tail despite fine energy tuning – RRS Detection limit: o Al slightly below atoms/cm 2

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

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

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

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: atoms/cm 2

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

Thank you for your attention.