Thin film characterization techniques

The development of AW thin-film characterization techniques has occurred largely because of the interest by various research groups in developing chemical sensors based on coated AW devices (see Chapter 5). Thus, many of the film characterization techniques described here were developed in an effort to characterize sensor coatings or to interpret the observed responses from AW chemical sensors in operation. 

Macrocyclic Compounds in Analytical Chemistry. Edited by Yury A. Zolotov Surface-Launched Acoustic Wave Sensors Chemical Sensing and Thin-Film Characterization. By Michael Thompson and David Stone Modern Isotope Ratio Mass Spectrometry. Edited by T. J. Platzner High Performance Capillary Electrophoresis Theory, Techniques, and Applications. Edited by Morteza G. Khaledi... 

The contributions of this volume were presented at the meeting and selected for publication in Progress in Colloid and Polymer Science covering a representative spectrum of surface sensitive techniques and their application to polymer surface and thin film characterization as well as recent examples of technologically relevant materials and process development. 

AW device sensitivity to viscoelastic parameters and electrical pnqieities can be used to advantage in some film characterization techniques. In these situations, a comparison of the AW device response to a model of the AW/thin film interaction is often crucial to the effective evaluation of thin film parameters. These additional interaction mechanisms typically involve changes in both the wave velocity and the wave attenuation for SAW, APM and FPW devices, and changes in both resonant frequency and admittance magnitude in TSM devices. In contrast, mass loading does not contribute to wave attenuation or decreases in admittance since moving mass involves no power dissipation (see Chapter 3). 

For some film-characterization techniques, the sensitivity of AW devices to film mass density allows these devices to be used as sensitive microbalances (nanogram mass changes can be effectively quantified). This thin-film mass balance can be used to monitor absorption into polymers and adsorption onto sur-... 

Depth profiling techniques applied to thermodynamically equilibrated thin films characterize the compositions of coexisting phases and the spatial extent of the separating interface. This procedure repeated at different temperatures yields the coexistence curve and the corresponding temperature variation of the interfacial width. Determined coexistence curves are well described by the mean field theory with composition-dependent bulk interaction parameter [74]. The same interaction parameter also seems to generate the interfacial widths in accordance with results presented here [107] (Sect. 2.2.2) and elsewhere [88, 96, 129]. These predictions may however need to be aided by capillary wave contributions to fit another observations [95, 97, 98], especially those tracing the change of the interfacial width with film thickness [121,130] (see Sect. 3.2.2). 

Thin films form an important category of materials that find applications in a wide variety of industrial applications [31], Optimization of thin film properties reqnires techniques, which can directly characterize the same. SAW devices are ideally suited to thin-film characterization due to their extreme sensitivity to thin-film properties (Equation 4.1). The sensitivity of SAW devices to a variety of film properties such as mass density, viscoelasticity, and condnctivity makes them versatile characterization tools. The ability of SAW devices to rapidly respond to changes in thin-film properties allows for monitoring dynamic processes such as film deposition, chemical modification, and diffusion of species in and out of the film. The thin-film focus should not be viewed as a limitation of SAW devices. Bulk material properties can be derived from thin-film data, although such extrapolations should be performed with care [1], In this chapter, approximate expressions showing applicability of SAW devices to characterize physical and chemical properties of thin-film materials are derived. 

J. Stumpe, Polymer Interfaces and Ultra-thin Films Characterized by Optical Evanescent Wave Techniques, Makromoleku-lare Chemie-Macromolecular Symposia 48/49, 363 (1991)... 

The semiconductor industry has largely driven the development of mass spectrometry (MS) instrumentation for thin film characterization, and secondary ion mass spectrometry (SIMS) is the reference technique for sensitive, quantitative depth profiling of implanted species in semiconductors [1], Applications of MS techniques to thin and thick film analysis are now found in many fields as the spectral information obtainable, both elemental and molecular, helps to address the most complex problems. 

Although optical vibrational techniques are less sensitive than electron-based spectro-metric methods, these techniques are employed extensively for thin-film characterization because of the specific and characteristic vibrational spectrum shown by various functional groups and molecules present in the film. The most commonly used vibrational spectroscopic techniques are infrared (IR) and Raman spectroscopy. Because of the interference caused by absorption of IR by the underlying substrate, IR reflection-adsorption spectroscopy (IRRAS) and its polarization modulation (PM) analog, PM-IRRAS, which uses the polarization selectivity of surface adsorption, are typically employed to characterize thin films (Gregoriou and Rodman, 2006). 

Thin films on substrates provide a characterization challenge the whole system must be measured at once and substrate properties can easily mask those of the thin film. Other techniques have been employed to study these complex systems. Fryer et al. used local thermal analysis to probe polystyrene (PS) and PMMA on two different substrates. These results were comparable to ellipsometry values establishing local thermal analysis as an effective technique. Again, PS did not have a favorable interaction with the polar or nonpolar silicon surface and both PS systems showed a decrease in Tg with decreasing film thickness. PMMA showed similar behavior on the nonpolar surface. However, on the polar substrate the Tg increased as the PMMA films became thinner. Porter saw similar effects measuring PMMA on silica with a differential scanning... 

Gas barrier properties of polymeric membranes are strongly dependent on thickness, making it critical to verify thickness measurement with multiple methods. Several techniques for obtaining film thickness were reviewed here to demonstrate the benefits and challenges associated with each. Although ellipsometry is a common technique for non-contact and non-destructive thin film characterization, the other methods are necessary to confirm the measured thickness. 

In the case of Langmuir monolayers, film thickness and index of refraction have not been given much attention. While several groups have measured A versus a, [143-145], calculations by Knoll and co-workers [146] call into question the ability of ellipsometry to unambiguously determine thickness and refractive index of a Langmuir monolayer. A small error in the chosen index of refraction produces a large error in thickness. A new microscopic imaging technique described in section IV-3E uses ellipsometric contrast but does not require absolute determination of thickness and refractive index. Ellipsometry is routinely used to successfully characterize thin films on solid supports as described in Sections X-7, XI-2, and XV-7. 

Transmission electron microscopy (TEM) can resolve features down to about 1 nm and allows the use of electron diffraction to characterize the structure. Since electrons must pass through the sample however, the technique is limited to thin films. One cryoelectron microscopic study of fatty-acid Langmuir films on vitrified water [13] showed faceted crystals. The application of TEM to Langmuir-Blodgett films is discussed in Chapter XV. 

For applied work, an optical characterization technique should be as simple, rapid, and informative as possible. Other valuable aspects are the ability to perform measurements in a contactless manner at (or even above) room temperature. Modulation Spectroscopy is one of the most usehil techniques for studying the optical proponents of the bulk (semiconductors or metals) and surface (semiconductors) of technologically important materials. It is relatively simple, inexpensive, compact, and easy to use. Although photoluminescence is the most widely used technique for characterizing bulk and thin-film semiconductors. Modulation Spectroscopy is gainii in popularity as new applications are found and the database is increased. There are about 100 laboratories (university, industry, and government) around the world that use Modulation Spectroscopy for semiconductor characterization. 

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