Multiple Films

Double films formed on InP were studied by applying ex situ ellipsometry after successive etching steps of the thermally formed oxide by an HF solution. From calculations based on a four-phase model (see Appendix A1 for treatment of two and more layers), the existence of an easily etched outer layer with smaller refractive index and a more durable inner layer with larger index was shown, which is consistent with XPS analysis. In the same work, in situ ellipsometry data was shown to be explained by the presence of a liquid layer when a microscopic model based on the Lorentz-Lorenz relation was used. Other ellipsometric studies of double films, such as double films of Si02 and Si3N4 on Si, have been reported. 

Ellipsometry as a useful technique that can be applied to studies of electrochemical systems is reviewed in this chapter. 

The optical principles and equations used in the technique are concisely summarized. The combined reflectance-ellipsometry (three-parameter ellipsometry) method and spectroscopic ellip-sometry are expected to be applied to an increasing number of studies in interfacial electrochemistry. The importance of proper experimental conditions, especially the proper choice of incidence angle is emphasized. Instrumentation, experimental methods, and error and sensitivity problems are dealt with. Some typical and recent applications in electrochemistry are reviewed. 

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The relative positions of the lines on the photographs were determined with use of a steel millimeter scale. The intensities of the lines on the NaZn13 photographs were determined visually, by the multiple-film technique, with the use of no external standards. 

With an MLR system, linewidth variations caused by resist thickness differences can be eliminated because of the planarization step. The reflections from either topographic structures or multiple film layers can be eliminated by using an absorbant material for the bottom layer. Either an inherently absorbant polymer material can be chosen or a dye can be added... 

When diffracted X-ray beams fall on a photographic film at different angles, as the different layer lines in a cylindrical-film rotation photograph do, it is necessary to correct for the absorption of X-rays in different thicknesses of film. (Since double-coated films are normally used, the effect on the back layer depends on the absorption in the film.) This was first considered by Cox and Shaw (1930) Whittaker (1953) gives a formula which is more accurate and deals with greater obliquity and a thicker film Grenville-Wells (1955) gives the corrections when the multiple film method is used. 

Powder photographs were taken with a 57.3-mm radius Debye-Scherrer camera and nickel-filtered Cu Ka radiation (Xmean = 1.5418 A). For intensity work, the multiple-film technique was used. The lattice constant, derived from 23 reflections from 42.56° to 73.47° 20 and corrected for film shrinkage with a parallel film of a reference substance, was Oo = 15.02 A with an estimated standard deviation of 0.01 A. 

In the last few years rapid advances have been made in the field of computational crystallography, so that it is now possible to produce highly refined computer models of a wide variety of polymeric materials using X-ray diffraction data. Unfortunately, these achievements have been negated to some extent because the techniques used to collect the data for such refinement programs have not advanced at a comparable rate. In this contribution we describe a computer program which facilitates the reduction of intensity and d-spacing data obtained by the multiple film-pack method, and attempts to quantify the errors associated with such measurements. 

The refinement (2) proceeded in the same way as for the x-ray work, except that o and S q were refined as additional variables. We assumed that the scattering was kinematic. The cross sectional dimensions of the microfibrils are 200xl0C)X and our previous work on synthetic polymer single crystals showed that the kinematic approximation was adequate for such small crystallites. Intensity measurement presented considerable difficulty in that multiple film exposures could not be obtained. Sequential exposures of the same area of the specimen led to problems of beam damage, and patterns from different areas were not comparable due to differences in the preferred orientation. As a result, only the 28 strongest non-meridional intensities could be measured. These were all for reflections which could be indexed by the Meyer and Misch unit cell, and thus the two chain unit cell was used for the refinement. 

Figure 7 illustrates the discretized liquid film for C02 transfer across the vapor and liquid films. Here Y is gas phase composition, X is liquid phase composition, T is temperature, I is interface, V is vapor, and L is liquid. Note that the liquid film is discretized into multiple film segments to accurately model the nonhomogeneous film layer. 

Newer technology allows for opaque, translucent, transparent samples as well as multiple film thicknesses for both reflectance and transmittance measurements. The newer technology provides a spectral match (matches spectral curve of the sample at each wavelength across the visible spectrum 360-740 nm). The advantage of spectral matching is its ability to match both color and opacity from a single database. 

Directly from the in situ observations, the multiple film cracking is analyzed through the evolution of the crack density with the longitudinal strain applied to the system. Before any deformation, a representative zone of the sample is chosen. Its total length is 0.55 mm, extended parallel to the longitudinal axis of the sample. This initial length is denoted For any further deformation, it is denoted X(e) and corresponds to ... 

One quality control application of near-IR spectroscopy is the nondestructive determination of tablet hardness. Near-IR prediction of tablet hardness has been used and investigated by Drennen for a number of years. In 1991, the first publication of the near-IR technique for this application appeared [26]. Ciurczak and Drennen published similar results in 1992 [60] and Drennen and Lodder in 1993 [61]. Results of a study presented at the 1994 annual meeting of the American Association of Pharmaceutical Scientists were published in a 1995 paper [62]. In that paper, Kirsch and Drennen identified the utility of the technique in the determination of multiple film-coated tablet properties, including tablet hardness. In a review paper regarding the use of near-IR in the analysis of solid dosage forms, Kirsch and Drennen discussed the historical aspects of near-IR prediction of tablet hardness [27]. 

In addition to the modified electrodes described in the previous sections, which usually involve a conductive substrate and a single film of modifying material, more complicated structures have been described. Typical examples (Figure 14.2.4) include multiple films of different polymers (e.g., bilayer structures), metal films formed on the polymer layer (sandwich structures), multiple conductive substrates under the polymer film (electrode arrays), intermixed films of ionic and electronic conductor (biconductive layers), and polymer layers with porous metal or minigrid supports (solid polymer electrolyte or ion-gate structures) (6,7). These often show different electrochemical properties than the simpler modified electrodes and may be useful in applications such as switches, amplifiers, and sensors. 

Radoslovich tried to answer these problems using a crystal structure refinement technique involving X-ray diffraction intensity data collected by the multiple-film Weissenberg method. His refinement involved a modest use of an electronic computer with two-dimensional Fourier syntheses using intensity data transferred to perforated tape... 

The development work on films has been done primarily with the small cell of Figure 2. The effective area of this cell is 0.012 sq. foot. It is envisioned that a commercial desalinization unit utilizing these membranes would somewhat resemble a plate and frame filter press with a close stacking arrangement of membranes. To test the feasibility of this concept a larger cell (effective film area 0.2 sq. foot), capable of multiple film arrangement, was designed and fabricated. 

Commercially available are various types of aluminium front surface mirrors to suit different requirements. For the visible spectral range, there are standard mirrors such as Alflex A . If improved reflection is required, a multiple film mirror Alflex B can be used. Both types of mirrors are provided with a hard and resistant dielectric protection coating. Such mirrors were first made by Hass et al. [73, 74]. The aluminium film on the surface mirror Alflex is even protected by an interference film system, which also enhances the reflectance for the visible range. In the visible and infrared, the spectral curve of the reflectance is approximately the same for Alflex A as that of an unprotected aluminium surface. With a mirror type Alflex B. the increase in reflection in the visible, with a maximum at 550 nm, can be clearly seen in Fig. 12. If required, this maximum can also be shifted to other wavelengths in the visible spectrum. 

The determination of anisotropy in a film is shown by Haitjema and Woerlee (1989). In general multiple films give high parameter correlations when all parameters of every film are to be determined. Often, optical properties are quite well known, e.g., a Si02 coating on Si, so in that case ellipsometry is mainly used for film thickness determinations. 

Kunstler W, Xia Z, Weinhold T, Pucher A, Gerhard-Mrrlthanpt R (2000) Piezoelectricity of porous polytetrafluoroethylene single- and multiple-film electrets containing high charge densities of both polarities. Appl Phys A 70 5-8... 

The selected clothing depends on the severity of exposure (duration and concentration). Dupont s SafeSPEC interactive software-based tool provides the data required for selection of protective clothing. If selected parameters are for low concentration and short duration of exposure, then the program suggests use of a general protection fabric with serged seam in coverall style. If a person is exposed to severe conditions (high concentration for several hours), then a multiple film barrier material with a broad level of chemical protection and a taped or double taped seam is selected. The chemical resistance of various materials is discussed in the next section of this chapter. Selection should be made by a professionally trained person. 

The overall reflection coefficients can be obtained by summing all the intensities of the light beams emerging in the direction of reflection in the ambient medium by the successive multiple reflections and transmissions. Alternatively, they can be obtained by matrix operations relating the waves in the successive media. The latter alternative is more elegant and convenient in dealing with multiple films. This matrix method of deriving the overall reflection... 

The three-phase model just described is a reasonably good approximation for thin surface films formed on electrodes in most cases. The optical relations for a duplex film or a multiple-film system will be dealt with briefly in Appendix Al. Other cases where the refractive index of the film varies along the depth of the film or the surface has roughness in the microscale will be discussed later. 

Derivation of the reflection coefficient expressions and the ellipsometry equations for film-covered surfaces employing matrix operations will be shown here. The treatment is similar to Heavens and Hayfield and White. This matrix method is less cumbersome, especially when multiple films are involved, compared to another frequently used method of deriving the reflection coefficients, in which amplitudes of individual beams resulting from multiple reflections and transmissions at the interfaces are summed. 

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