Ultra Thin SiO2 on Si: II, Issues in Quantification of the Oxide Thickness
M P Seah and S J Spencer, Surf. Interface Anal. 33 (2002) 640
National Physical Laboratory, Teddington, Middlesex, UK
email: [email protected]
This work is a precursor to an extensive analysis of the quantification of ultra-thin layers of
SiO2 on Si being conducted under the auspices of the Consultative Committee on Amount of
Substance (CCQM). An analysis is made of various quantification issues concerning the
analysis of ultra-thin layers of SiO2 on (100) and (111) polished Si surfaces by XPS. For the
analysis of the oxide thickness, doxide, a simple equation is generally used involving the ratio
of the measured 2p intensities for the oxide and elemental states, Rexpt, and two further
parameters; the attenuation length of photoelectrons in the oxide, LSiO2 and the ratio, Ro, of
the intensities of the Si 2p peak from bulk thermal SiO2 and from pure Si.
d oxide = LSiO 2 cos θ ln (1 + Rexpt / Ro )
This is valid for q £ 58o. An analysis of previously reported measurements of the attenuation
length gives an average value of only 6% less than the theoretical value. However, careful
measurements of Ro, via two routes, indicate consistently that a value closer to 0.88 ± 0.03
should be used rather than the calculated value of 0.53 ± 0.05. This difference may arise
through systematic uncertainties in the values for the relevant inelastic mean free paths, the
silicon dioxide density and the shake-up contributions. Previously reported experimental
values of Ro range from 0.67 to 0.87.
Uncertainties also arise from intensity variations caused by the crystal structure of the
substrate. These are mapped and a position, “A”, is found where further work is best
conducted. For the (100) surface, A is at 34o
from the surface normal in an azimuth mid-way
at 22½o between the [010] and [011] azimuths.
For the (111) surface A is at 25½o from the
surface normal in the [10 1 ] azimuth. This is
shown in the polar diagram of Fig 1. Data for
much of the present work is for the (100)
surface at a position “B” at 27o to 28½o from
the surface normal in the [110] azimuth, which
is equivalently good but which may degrade for
spectrometers with high angular resolution. If
the same equation is used for calculating the
thickness, position B leads to a calculated
thickness that is 4% less than that measured for
an average orientation, whereas data acquired
for normal emission lead to a value 18% lower,
and those measured at A are 2% higher. Data
measured for normal emission are not
recommended since the data are very sensitive
Fig 1 Polar
plot
showing
orientations A and B for analysis
to the exact angular acceptance of the analyser.
Measurements of the carbonaceous contamination confirm earlier conclusions that the
contamination is better described using data for an average polymer than for glassy carbon.
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