Optimization of the envelope method for characterization of optical thin film on substrate specimens


As a result of widespread applications of thin films in optics, optoelectronics and magnetooptics, several methods are used for characterization of thin films, which compute the average thickness da, the thickness non-uniformity Δd, and the complex refractive index of the film, in different spectral regions. The choice of the characterization method of a thin film on a glass substrate specimen, from its normal incidence transmittance spectrum T(λ), depends mainly on whether this spectrum contains interference pattern with several maxima and minima. In the spectral region without interference pattern of T(λ), the film is either strongly absorbing or very thin, and it is most often characterized by the spectroscopic ellipsometry method, which assumes validity of dispersion model for the complex refractive index. In the spectral region with interference pattern of T(λ), scanned by UV-VIS-NIR spectrophotometer, the film can be either dielectric, semiconducting or insulating, with average thickness da ~ [500,5000] nm, and its more accurate characterization is achieved by the envelopes method (EM). The higher accuracy of EM, in comparison with the spectroscopic ellipsometry method, is mainly due to including the interference pattern equation, whereas EM has the additional advantage of not using any dispersion model. However, EM assumes existence of a wide spectral region of quasi-transparency of the film, it contains three subjectively chosen parameters, and employs different algorithms for films with uniform and with non-uniform thickness.

Researchers at Technical University of Sofia, Bulgaria: Assoc. Prof. Dorian Minkov (College of Energetics and Electronics) and Assoc. Prof. George Angelov (Department of Microelectronics), together with Prof. Emilio Marquez from the University of Cadiz in Spain and his Spanish co-workers, proposed optimization of EM (OEM). Their research work is currently published in the journal, Thin Solid Films.

The advantages of OEM compared to EM are: it always results in more accurate characterization of the film, regardless of whether the film has or does not have a wide spectral region of quasi-transparency; optimizes the three subjectively chosen parameters of EM; and employs the same algorithm for uniform and for non-uniform films.

In OEM, the optimization is performed by using original error metrics. Due to inability of EM to separate accurately the absorption from the thickness non-uniformity of the film, this team of researchers used models of four specimens, containing thin films with significantly different absorption and thickness non-uniformity, to investigate the effectiveness of OEM. The OEM characterizations of these model films showed, that the errors in the computations are 0.1% for the average film thickness da, and 0.15% for the refractive index of the film.

Furthermore, transmittance spectra T(λ) have been scanned and used of two specimens containing a-Si films without wide spectral region of quasi-transparency, and with average film thickness da close to 780 nm and 3920 nm measured by electron microscope. A comparison of these data with film characterizations results showed that the average thickness errors are 38% and 25% for EM characterization, and less than 0.4% for OEM characterization. Achieving such a high accuracy of film characterization by OEM is especially significant for the thicker film, since it seems to be the thickest film ever characterized from its spectrum T(λ).

envelope method for characterisation of optical thin film on substrate specimens-Advances-in-Engineering

envelope method for characterization of optical thin film-Advances in Engineering

About the author

Dr. Dorian Minkov is currently an Assoc. Prof. at the Technical University of Sofia. He started his career as a researcher of light emitting structures and devices at the Institute of Microelectronics and Optoelectronics in Botevgrad. In 1991, he joined the group of the founder of EM – Prof. R.Swanepoel, at the Rand Afrikaans University in Johannesburg, where he worked on characterization of thin films by EM. Between 1996 and 2011, he was Assoc. Prof. and Senior Researcher in Japan, at Tohoku University, National Institute of Physiological Sciences, and Ritsumeikan University, where he performed research on electromagnetic characterization of surface cracks in metals, light induced changes of DNA, and light generation from targets in electron storage rings.

His main current research interest is in optical characterization of thin films, whereas he is also interested in the broad fields of applied optics and materials research.

About the author

George Angelov started working in 1999 at the Technology Center-Institute of Microelectronics, Sofia, Bulgaria where he was engaged in IC and PCB design and various aspects of semiconductor device modeling. From 2007 to 2012 he was Asst. Prof. and since 2013 he has been Assoc. Prof. with the Department of Microelectronics, Technical University of Sofia, whereas in 2015 he was elected as Chair of the Department of Microelectronics.

Since 2016 he has been selected as Head of MINOLab (MicroNanoLab) at the Sofia Tech Park and CEO of the Bulgarian Cluster of companies in Microelectronics and Industrial Electronics Systems. His research interests include semiconductor device & circuit modeling, novel semiconductor devices, signal processing, bioelectronics, optical thin film characterization and renewable energy sources.


Minkov, D., Gavrilov, G., Angelov, G., Moreno, J., Vazquez, C., Ruano, S., & Marquez, E. (2018). Optimization of the envelope method for characterization of optical thin film on substrate specimens from their normal incidence transmittance spectrumThin Solid Films645, 370-378.


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