Characterization of Thin Film Materials

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. 

As described in Chapter 3, mass detection limits for AW devices are typically at or below one ng/cm. These low detection limits translate into hundredths of a monolayer of atoms and film thicknesses of hundredths of nanometers. This 

1) and surface area and pore size distribution (Section 4.3.1.2). In addition, it is useful for real-time monitoring of processes such as film deposition (Section 4.4.1), materials modification (Sections 4.4.2 and 4.4.5), corrosion (Section 4.4.3), and diffusion (Section 4.2.2). It can also be used to monitor adsorption at surfaces from both gases and liquids (Section 4.3). 

Using AW devices to monitor dynamic processes such as diffusion and corrosion can dramatically reduce the time required to quantify these processes. For example, as discussed in Section 4.2.2, diffusion equilibration times typically increase with the square of the diffusional length. For a thin film, this length scale, the film thickness (h), is very small. This enables the quantification of diffusion coefficients as low as 10 cm /sec in less than one day, whereas months would be required using many conventional techniques that use thick films or bulk samples. For corrosion monitoring, the dramatic decrease in mass detection limits obtainable using coated AW devices, as compared with conventional balances and sample coupons, allows detectable mass changes to be achieved in minutes or hours rather than days or months (Section 4.4.3). 

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). 

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Figure 1 Characterization of thin-film material. (From Itoh, 1986.)...

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