Average ellipsometric thicknesses

Figure 2. Average ellipsometric thickness of (a) SAMs from Series 1, (b) SAMs from Series 2, and (c) SAMs from Series 3. All measured thicknesses were reproducible within 2 A of the reported average value.

Ellipsometric Thicknesses. The thicknesses of the monolayers were measured with a Rudolph Research Auto EL III ellipsometer equipped with a He-Ne laser (632.8 nm) at an incident angle of 70° from the surface normal. A refractive index of 1.45 was assumed for all films. For a given sample, the data were averaged over three separate slides using three spots per slide. 

More precisely, the average deposited thickness is measured at short times using ellipsometry, before "phase separation," and then the ratio of the areas covered by each film at late times allows calculating their thickness, if mass conservation is assumed. The calculated values agree with the values of the UB and LB provided by ellipsometric profiles of microdroplets. Precise thickness measure-... 

The confrontation of this elegant prediction by experiment has taken many years to complete because all experimental techniques available at that time did not have the proper spatial resolution. Spectroscopic methods such as EPR, NMR, IR are only sensitive to the fraction of monomers attached to the wall. Ellipsometry measures the first moment of the monomer distribution and vields an average length called the ellipsometric thickness Hydrodynamic methods, based on flow restric-tion in capillaries covered by the adsorbed layer, yield another average length related to the maximum extension of the adsorbed layer To study concentration profiles at the solid-liquid interface, one has thus to devise special tools. 

The results of the ellipsometric study are presented in Table 9. As is clear from the table, the resultant average thickness of the semiconductor layer, obtained from one bilayer precursor, is about 0.8 nm. This value can be considered the thickness resolution of this technique. It is worth mentioning that among the available techniques, only molecular beam epitaxy allows one to reach such resolution. However, the proposed technique is much simpler and does not require complicated or expensive equipment. 

Figure 2. (a) Ellipsometric data for multilayer composites of 1-1-1-. .. grown on a planar Si wafer, (b) Ellipsometric data for ZrP-2 multilayer films deposited with (circles) and without (squares) ad d electrolyte. The average thicknesses are 38 and 7 A, respectively. 

The process of formation of a multilayer film on the ITO coated glass from sequential addition of PABA/RNA bilayers was observed with UV-Vis Spectroscopy as shown in Figure 3.34. The film growth observed with the deposition of additional bilayers suggests that the multilayer formation of PABA/RNA is reproducible with sequential deposition. All spectra exhibit an intense and sharp peak attributed to the w-tt and bipolaron band transitions. The bipolaron absorption band at 800 nm, associated with complexation of RNA with PABA, increases linearly with the number of PABA/RNA bilayers (Figure 3.34 inset). The linear relationship between absorbance and the number of deposited bilayers indicates that the deposition was reproducible from layer to layer, i.e., the amount of PABA adsorbed in each bilayer was the same. In addition to these results, multilayer formation was observed with ellipsometric and X-ray photoelectronic spectroscopy. The linear increase in film thickness with number of PABA/RNA bilayers was observed using ellipsometry. The average thickness of the PABA/RNA bilayer built up on a silicon substrate was approximately 10 nm. Additionally, X-ray photoelectron... 

The ellipsometric method has been developed by Yukioka and Inoue (1991, 1994). The principles of the technique and the model used for calculating the thickness of the interphase are schematically illustrated in Figs. 4.13 and 4.14, respectively. The retardation (A) and reflection ratio (tan(i/<)) can be determined from the ellipsometric readings. The adopted model assumes the existence of four layers air, thin polymer-1, interphase, and thick polymer-2 (see Fig. 4.14). In the interphase the refractive index is assumed to be an average n = iti+ ni)H. Thus, one can compute the best value of the interfacial thickness, do, to fit the observed values of A and tan( ). The following relations were derived for the computation of d ... 

Since an additional ellipsometric measurement would be needed to determine the carbon-overcoat thickness, the ellipsometric measurement of PFOM thickness directly on non-carbon-overcoated silicon is more straightforward. Silicon strips and wafers were dip coated with PFOM. The PFOM thickness measnred by ellipsometry and the dIX from XPS are listed in table 4.8. The thickness measured by ellipsometry was divided by the dIX from XPS for each sample (last two columns in table 4.8). The experimentally determined average electron mean free path for PFOM film is X = 2.66 nm. Sliders were dip coated with PFOM at the same conditions as the silicon wafers and strips, and dIX was measured on the air bearing surface of each slider by XPS. These dIX were multiplied by A, = 2.66 nm, as determined above, to estimate the PFOM thickness on the air bearing surface. These results are listed in table 4.9. The concentration of the PFOM solution was 650 ppm, and the withdrawal rate was 1.6 mm/s. 

The reflectivity curve obtained for the film was shifted to higher angle as the number of layer increases around 42-45°. The thickness of the films was calculated from the resonance angle shift by a modified Fresnel equation. The calculated average thickness of self-assembled Zr EPPI film was 13.1 A and this value was similar to ellipsometric results. 

Any real surface contains a layer whose optical properties differ from those in the bulk crystal. That may be a thin film on the surface, in particular an oxide film, contamination, relaxed or reconstructed layer, or surface roughness. Therefore with the help of Eq. (5.1) an effective dielectric function, (e), is determined, which corresponds to an average over the region penetrated by the incident light. In order to extract the optical properties of a transition layer, the substrate contribution to (e) must be evaluated. This is usually performed by applying a three-phase model (see Section 3.1.3). Then the ellipsometric ratio, p, can be written using Eq. (3.40). The complex dielectric function (its real and imaginary parts) and the thickness of the transition layer (phase 2) are considered as the three unknown parameters. However, the measurements of the complex quantity p provide only two equations for them. To obtain the third one, it is necessary to invoke additional, physically reasonable restrictions. 

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