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a. Silicon L, nitrogen and oxygen K edges
In Fig. 4.56 the solid line shows the transmission of the thin SiO2
A transmission curve for the SiO2+Si3N4+SiO2 sandwich (which is an exact
copy of the CCD gate isolator) is shown on the
Fig. 4.57. In addition to the silicon and oxygen edges this plot
shows a prominent nitrogen K edge, also shifted from the tabulated atomic
value. The transmission of this sandwich was modeled as a combination of
the Henke-derived Si3N4 transmission and the experimentally measured
transmission of SiO2. The best fit (thicknesses of both materials
being free parameters) is shown in Fig. 4.57 as the dotted line.
b. Silicon K edge
Fig. 4.58 contains absorption curves (not transmission!) of polysilicon, SiO2, and
the SiO2+Si3N4+SiO2 sandwich in the close vicinity of the silicon K edge.
Each sample shows a sharp resonant peak right above the edge.
Due to chemical
shifts each of the three materials exhibits the silicon K-edge at slightly
For polysilicon the peak is at 1841 eV; for SiO2 it is at 1847.3 eV.
The SiO2-Si3N4-SiO2 sandwich shows two distinct peaks which, although are not
well-resolved, can be determined to be at 1847.3 and 1844.7 eV. The first one
can obviously be attributed to SiO2, while the second one originates
Polysilicon absorption shows a lot of structure due to its crystalline and
ordered nature, whereas silicon dioxide has fewer peaks because
it is amorphous and uncorrelated interference from remote atoms smears out
The difference between the edge and resonant peak energies of polysilicon and SiO2 can be seen very nicely in the oxidized polysilicon sample transmission in Fig. 4.59. A dashed line in this plot represents the result of fitting to this data the product of the transmissions of the separate films of polysilicon and SiO2. The quality of the fit is so good that the dashed line can hardly be seen under the solid line.
In order to fill a gap in the data (no measurements were made in the range from 900 to 1300 eV), and also to extend the results to higher energies, we used Henke data (which should be adequate at energies far enough from the edges).
For each of the materials standard Henke data were used to fit the transmission at energies below the edge and far above the edge, where the near edge oscillations become weak. This procedure allows us to define the thickness of each film. The results are slightly different from the nominal values and are reflected in the labels in the above Figures. The densities of the materials used for those calculations were 2.19, 2.33 and 3.44 g/cm3 for SiO2, polysilicon and Si3N4, respectively. For both polysilicon and SiO2 near-edge measurements were combined with Henke atomic scattering factors for silicon and oxygen, and mass absorption coefficient were produced in the range from 60 eV to 15 keV.
We encountered some difficulties in our attempt to separate optical constants of the Si3N4 film from the other components of the sandwich, most likely due to deviations from stoichiometry in one or more constituent layers. Because of that, for further applications we used a transmission of the sandwich as a whole unit, instead of deriving a mass absorption coefficient for each component.