Ellipsometric measurements

Ellipsometry in the vacuum UV (< 190 nm) enables the analysis of materials for the next generation lithography (photoresist, AR coatings) at the latest exposure wavelengths (157 nm and 193 nm). The short wavelengths increase the sensitivity of ellipsometric measurements of ultra thin films (<10 nm). New prospects are expected for the analysis of thin metallic and dielectric layers. 

Ellipsometry is concerned with the measurement of the changes in polarisation state, as well as light intensity, on reflection since these parameters are highly sensitive probes of the thickness and refractive index, rtf, of a surface film. A full treatment of the principles involved in ellipsometric measurements can be found in any one of several excellent reviews (see references). 

As noted above, the deposits made with the H-cell design were thin, only about 0.4 ML/cycle. For a 200 cycle deposit, they appeared deep blue, the result of interference effects in the 30 nm thick film. With the new cycle program and large thin-layer flow-cell, the best deposits appeared gold in color, and ellipsometric measurement indicate they correspond to the deposition of very close to 1 ML/cycle. A study of the thickness as a function of the number of cycles is shown in Figure 18, where... 

For homopolyelectrolyte, we first studied the ellipsometric measurement of the adsorption of sodium poly(acrylate) onto a platinum plate as a function of added sodium bromide concentration (5). We measured the effect of electrolyte on the thickness of the adsorbed layer and the adsorbances of the polyelectrolyte. It was assumed that the Donnan equilibrium existed between the adsorbed layer and the bulk phase. The thickness was larger and the adsorbance of the polyelectrolyte was lower for the lower salt concentration. However, the data on the molecular weight dependence of both the adsorbance and the thickness of the adsorbed polyelectrolyte have been lacking compared with the studies of adsorption of nonionic polymers onto metal surfaces (6-9). 

Some Understanding as to How Ellipsometiy Works, in consideration of what happens in ellipsometric measurements of a film on an electrode... 

Fig. 7.58. Ellipsometrically measured thickness of an adsorbed layer of 1-octyne-3-ol on iron.

Caylor et al. [134] performed ellipsometric measurements at the liquid-liquid interface of Pitzer s system. The experiments yielded a decrease in ellipticity, when approaching the critical point. This is in clear contrast to a (/ — v) divergence predicted theoretically [135] and observed experimentally [136] for nonionic Ising mixtures. The results point toward an anisotropic interface caused by an orientation of ion pairs perpendicular to the interface. A rough model for such an interface captured some of the observed features. 

The silanized wafers 1-5 were suspended in 10 ml of a 10 3 M guest solution in methanol for 15 min, washed extensively with chloroform, and finally rinsed with acetone. The wafers were then dried in a stream of argon for 30 min prior to ellipsometric measurements. 

Peyser and Stromberg63 used the ATR method to measure the thickness of a polystyrene layer adsorbed on a quartz surface from cyclohexane solution at 35 °C and compared it with the thickness obtained by ellipsometry. Good agreement was observed although the ellipsometric measurements were made for the chrome plate. 

The first ellipsometric measurement of the thickness of the adsorbed layer and the adsorbance of a polyelectrolyte and a negative adsorbance of salt onto a solid surface was reported by Takahashi et al.U4) They measured the adsorption of sodium poly(acrylate) (M = 950 x 103) onto a platinum plate as a function of the concentration of added sodium bromide. In an aqueous polyelectrolyte solution with an added simple salt, the bulk phase is a three-component system which consists of a polyelectrolyte, a simple salt, and water. The adsorbed layer on the solid surface is a three-component phase as well. The adsorbance of polyelectrolytes thus cannot easily be determined from measurements of the refractive index nf of the adsorbed phase. Hence, it was assumed that the adsorbed layer is a homogeneous layer of thickness t and further that nf is represented by the Lorenz-Lorentz equation as follows ... 

The solution was first filtered to remove particulate impurities and the films were heated to 80-85°C to remove the solvent. The thicknesses and refractive indices of the polymers were obtained from ellipsometric measurements on calibration layers, which were spun on BK7 glass substrates. Measurements on each sample were performed at four different wavelengths (63A.8 nm, 753.0 nm, 802.0 nm and 852 nm) in order to minimize the errors in the extrapolated... 

The results of this model calculation can be used to extract unknown film properties, such as the film refractive index and thickness. Normally, however, an ellipsometer will yield only two independent observables (the two ellipsometric angles defined in equation (3.6), for example). For that reason, only two film properties can be extracted from a single measurement. If the film is nonabsorbing with a real refractive index, an ellipsometric measurement can provide the real part of the refractive index, n, and the thickness. For... 

In extreme cases, such as boiling HC1 in contact with stainless steel, inhibitors can still be found that work very well indeed. Ellipsometric measurements show that they can form polymer layers five and six molecular layers thick—a kind of in situ formation of a paint film, built up by successive adsorption, layer upon layer. 

In this section we focus on spectroscopic methods used to determine the real and imaginary parts of h(w). It is not our aim to discuss all the methods used in semiconductor physics, but only those commonly applied to CP, based on reflectance/transmittance and ellipsometric measurements. At the end, we report briefly on other methods that are seldom used or work only in reduced spectral ranges. 

Ellipsometric measurements with processed photoresists showed a long-time swelling over several days. Attempts to investigate the adsorption after addition of surfactant delivered only an continued swelling. It could not be distinguished between the effects of swelling and of the surfactant adsorption. 

It can be summarized that ellipsometric measurements proved the formation of a surfactant adsorption layer on the photoresist surface. At ceg- it is assumed to form a monolayer. To get more information about the adsorption layer and its influence on the surface properties of the photoresist, an electrokinetic characterization of unexposed and processed photoresist in solutions of the cationic surfactant was carried out. The zeta potential of the photoresist layers is given in Fig. 8 as a function of the surfactant concentration. The measurement was performed at pH = 6 in a background electrolyte (KC1) concentration of 10-5 M to ensure the minimum conductivity of the solution necessary for the measurement. 

For SPR-measurements, two layers with a thickness of less than 100 nm were prepared on BK7-prisms (refractive index d = 1.51680, Krombach, Germany) and all other layers were prepared on NPH2-prisms ( d = 1.92286, Ohara, Japan) to fulfil the resonance conditions of the surface plasmons. To prepare layers for the RlfS-measurements, the solutions were spin-coated onto glass-substrates with a Ta205-layer for reflection enhancement. Si-wafers were used for the ellipsometric measurements. 

M contains all the information about the anisotropic optical properties of the medium that supports the electromagnetic fields some of its 36 tensor components are the target of the ellipsometric measurements of anisotropic crystals. This optical matrix M is thus defined by... 

The geometry of the ellipsometric measurement is shown in Fig. 2.13. The anisotropic material medium under study has coordinate z > 0, while the isotropic ambient medium (air) has z < 0 the electromagnetic field components of interest are the tangential components Ex, Ey, Hx, and Hy the plane of incidence is the xz plane. 

Miscellaneous Physical Chemistry. The electrochemical oxidation of /3-carotene by a single-step reversible two-electron transfer has been reported.106 The fluorescence lifetimes of a- and j8-carotene have been measured by the streak camera technique to be 5.5 10 ps (6 x 10 2 mol 1 1 in chloroform).107 Ellipsometric measurements of light-absorbing monomolecular layers of several compounds, including 8 -apo-/3-caroten-8 -oic acid (227) have been described.108 The kinetics of autoxidation of amorphous retinyl acetate have been determined.109... 

Fig. 11.7. Ellipsometric measurement during the interaction of CH3 with an a-C H surface at a temperature of 340 K. The CH3 flux density is j 2 x 1015 cm 2s 1...

Figure 6.4-14 Spectra of the refractive index n, the absorption index k, and the degree of (phase) polarization Ppi, of quartz glass as derived from ellipsometric measurements.

FIGURE 12.13 Ellipsometric measurement of post-CMP film thickness on two wafers, polished with different process parameters, showing center-to-edge polish rate nonuniformity. 

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