Handbook of Modern Coating Technologies

Spectroscopic ellipsometry

• Introduction

Ellipsometry is a century-old optical measurement technique based on analyzing the change in the polarization state when a beam of polarized light was reflected from the surface or interfaces of coatings, which can be used to extract the thickness, optical properties, and composition of the coatings. Before Rothen named this technique as "ellipsometry” in 1945 [1], the basic theory of ellipsometry was established by Drude in 1887 [2]. Drude also con­ducted the first ellipsometric experimental measurement in 1890 to determine the optical properties of some metals [3]. Although ellipsometry has long history, it developed slowly, especially in the early stage. The ellipsometer with a single wavelength dominated the ellip- sometry field for a long time because of the complex of multiwavelength ellipsometer and the difficulty of the data analysis processes. Moreover, the ellipsometers in the initial stage were too slow to be used to investigate the change of dynamic systems. Till approximately 1970s, benefitting from the implementation of photometric instruments (such as light sources and detectors) and the availability of minicomputers, the ellipsometry developed rapidly. The wavelength range, measurement precision, and speed were improved signifi­cantly. Those developments have been reviewed by Hauge [4], Vedam [5], and Aspnes [6,7].

The first ellipsometric spectrum which covered 300—400 nm was described by Jasperson and Schnatterly [8], indicating spectroscopic ellipsometry (SE) was really established. Comparing with the single-wavelength ellipsometry, SE can deconvolute multiwavelength ellipsometric data with complicated optical models, which is helpful to obtain abundant, pre­cise, and reliable information about the samples. At present, with the advanced optical com­ponents being introduced into the ellipsometers, the attainable wavelength range has been extended deeper into the Terahertz and vacuum ultraviolet, thus some extra information about the samples such as their compositions could be extracted reliably [9]. Besides broad­ening the spectral range, there are also many kinds of SE techniques developed. For exam­ple, variable angle spectroscopic ellipsometry (VASE) carries out the ellipsometric measurements at several incidence angles, which helps to improve the reliability of results of data deconvolution. Mueller matrix spectroscopic ellipsometry is an effective tool to extract

Handbook of Modern Coating Technologies. DOI: https://doi.org/10.1016/B978-0-444-63239-5.00002-0

© 2021 Elsevier B.V. All rights reserved.

the information on anisotropic materials and complex systems by interpreting the SE data with the Mueller—Jones formalism [4,10].

While the wavelength range of ellipsometers enlarged, the spectrum acquisition time of ellipsometers shorten continuously. In 1984 it was reported by Muller et al. [11] that the first rapid-scanning spectroscopic ellipsometer which provided 114nm/s scanning rate within the wavelength range of 370—720 nm. Later, with utilizing multichannel detection systems, the acquisition time was reduced significantly. This kind of the multichannel detector spec­troscopic ellipsometer makes the in situ real-time SE (RTSE) very powerful for characterizing the dynamic evolution of the optical properties or the structure of films [12,13], so that it has been used widely to monitor the growth or damage processes of coatings.

Furthermore multiple-technology coupling is another trend of SE development. Infrared (IR) spectroscopic ellipsometry (IRSE) is a method that combines ellipsometer with a Fourier transform spectrometer, by which not only the thicknesses but also the composition of the films can be determined [14—17]. Internal reflection ellipsometry is based on the measure­ment of reflective light beam which penetrates a prism, so it could be applied in the ambient with strong absorption [18,19]. Especially while internal reflective ellipsometry combines with surface plasmon resonance (SPR), the new method is called total internal reflection ellipsometry (TIRE) and has much higher sensitivity than traditional ellipsometry [20,21]. Imaging ellipsometry (IE) is a technique using a charge—coupled device (CCD) camera as the detector in an ellipsometric configuration and it scans the sample surface point by point with a high spatial resolution, which is capable of visualization and quantification of thin- film thickness distributions [22,23]. IE has been gained significant interest and used in lots of fields such as biotechnology and semiconductor metrology.

With the rapid development of ellipsometry, many comprehensive reviews have appeared. Table 2—1 lists some monographs on ellipsometry. The first classic monograph was published by Azzam and Bashara [24], then other monographs were published gradu­ally. For example, a brief user's guide by Tompkins [25], the treatise of IR ellipsometry by Schubert [26], the comprehensive book written by Fujiwara [27], two collections of mono­graphs edited by Tompkins and Irene [9] and Losurdo and Hingerl [28], respectively. At the same time, as an important characterization method, ellipsometry has also been introduced in the chapters about the topics of thin films, surface, and interfaces. Some chapters are listed in Table 2 2.

Coatings play an important role in various fields, and their characterization techniques are indispensable for the preparation of high-quality coatings. Due to the advantages of high precision and nondestructive operation, ellipsometry has been widely used in vacuum, gas, or liquid ambient to extract the information of coatings, such as the thickness, optical con­stants, and other related properties. This chapter will introduce the principle of ellipsometry, discuss the data analysis procedures, address the analytical information, and sum up the common applications in coating characterization.