Seminar Author: Bojan Hiti Mentor: doc. dr. Matja Kavi
Determination of trace impurities on Si wafers with x-ray
fluorescence
Slide 2
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
Slide 3
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.
Slide 4
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
Slide 5
Characteristic x-ray emission line energies element
identification
Slide 6
Detector energy resolution should be high enough to distinguish
between characteristic x-ray energies of neighboring elements.
Semiconductor x-ray detector
Slide 7
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
Slide 8
X-ray Si interaction cross-sections:
Slide 9
Total reflection XRF For x-rays: n matter
Slide 10
Total reflection XRF setup at SSRL synchrotron Highest
sensitivity is obtained using excitation with synchrotron radiation
high photon flux ~ 10 12 photons/sec monochromatic ( E/E ~ 10 -4 )
and polarized light Stanford Synchrotron Radiation Lightsource
Slide 11
Detection limits: o 3d metals
Slide 12
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 10 10 atoms/cm 2
Slide 13
Grazing emission XRF Surface sensitivity achieved by grazing
detection angle Smaller yield than Total-reflection tehnique
Slide 14
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
Slide 15
Experimental Grazing-emission setup at ESRF synchrotron in
Grenoble Bragg crystal spectrometer installed at the ID21 beamline.
European Synchrotron Radiation Facility
Slide 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: 10 12 atoms/cm 2
Slide 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