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Sensing & Measurement

Choosing techniques for the optical characterization of thin films

Spectroscopic, ellipsometric, interferometric, photothermal, and combined methods that use dispersion and structural models, all allow optical properties to be determined.
4 September 2006, SPIE Newsroom. DOI: 10.1117/2.1200608.0341

The optical properties of thin films are very important for many applications, including interference devices (such as antireflection coatings, laser mirrors, and monochromatic filters), as well as optoelectronics, integrated optics, solar power engineering, microelectronics, and optical sensor technology. The end application of a film determines the reflectance and transmittance properties required during fabrication. Numerous methods are used for characterization and these can be classified in the following categories: spectrophotometric, ellipsometric, interferometric, photothermal, and combined methods. We describe these methods, and how they are best used, briefly here.

Spectrophotometric methods determine the spectral dependencies of reflectance and transmittance for thin films or thin film systems within the spectral regions of interest.1,2 Reflectance and transmittance are measured at near-normal incidence and normal incidence, respectively, using various types of spectrophotometers.

Ellipsometric methods analyze changes in the state of polarized light that has been transmitted through and reflected from films and their systems.3,4 Ellipsometers use oblique incident light within the spectral ranges of interest.

Interferometry uses interference microscopes and interferometers to characterize thin films.5–7 Interferograms, which may result from reflected or transmitted light, are analyzed to determine the geometric quantities of thin films: these include thickness and boundary roughness.

Photothermal methods determine the absorption (including very weak absorption) of materials that form thin films.8–12 Measured changes in temperature, optical, or thermophysical properties of thin films are used to calculate the absorbance. The four main photothermal techniques used in practice include: mirage, photoacoustic gas cell-microphone, photothermal-displacement, and laser calorimetry. For example, laser calorimetry9 is based on an increase in the temperature of the sample that is caused by the absorption of light from a pump laser. The absorption of a material is determined from the the initial-rate increase of sample temperature with time.

Combined methods simply involve the simultaneous use of individual methods belonging to the categories above. One popular such method uses variable angle spectroscopic ellipsometry (VASE) and spectroscopic reflectometry (SR).13–15 The data gained by VASE (including spectral dependences of the ellipsometric quantities measured for several angles of incidence) is combined with that from SR (the spectral dependencies of the reflectance measured at near-normal incidence) and a suitable numerical technique, such as the least-squares method (LSM), is used to extract the results.

Unless the experimental data are to be treated separately at selected wavelengths, suitable dispersion and structural models of the films are needed. These enable extraction of parameters from the data when the spectral dependencies of the optical quantities are treated by the LSM or a similar numerical method.

Dispersion models parameterize the spectral dependencies of the dielectric functions of films or corresponding functions, such as the density of electronic states and joint density of electronic states for amorphous and crystalline film materials, respectively.16,17 Dispersion parameters of films, such as the band gap, can be determined by data treatment.

Structural models are used to evaluate quantities like the volumes and boundaries of films or their systems and include film defects such as boundary roughness, non-uniformity in thickness, volume inhomogeneities (columnar structure), optical inhomogeneities corresponding to refractive index profiles, and transition interfacial layers. Structural models can use information determined from experimental data such as roughness parameters and transition layer thickness.

Complete optical characterization of thin films and their systems requires determining all parameters, including defects. As a result, one must select the best suited methods from those specified above.18

Ivan Ohlidal
Deptartment of Physical Electronics, Masaryk University
Czech Republic
Ivan Ohlidal is a professor in the field of quantum electronics and optics and has focused on the optical properties and optical characterization of thin films for more than 35 years. His key areas of research include the influence of defects, such as boundary roughness, inhomogeneity, non-uniformity and transition layers, on the optical properties and characterization of thin films. He is a member of the committee of SPIE CZ.
Daniel Franta
Laboratory of Plasma Physics and Plasma Sources, Masaryk University
Czech Republic
Daniel Franta has been studying the optical properties and characterization of thin films as well as solid state physics for more than 10 years. His fundamental research focuses on the influence of various defects on the optical characterization of thin films and dispersion models of amorphous and crystalline materials.

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