### OptiChar Module of OptiLayer Thin Film Software

OptiChar enables the optical characterization of single layer films via spectro-photometric and/or ellipsometric data.

In OptiChar, a thin film is represented by a model including spectral dependencies of refractive index and extinction coefficient, film thickness, dependence of optical parameters on the thickness of a thin film (bulk inhomogeneity), thickness of surface overlayer, and porosity.

OptiChar allows you to determine:

The thin-film model can be described by a vector of model parameters $$X$$. The coordinates of the vector $$X$$ can be the parameters describing wavelength dependencies of the refractive indices/extinction coefficients, film thickness, degree of bulk inhomogeneity, porosity, thickness of the surface overlayer etc. The model parameters are determined by the minimization of the discrepancy function $$DF$$ estimating the closeness between experimental and model data:

$DF^2(X)=\sum\limits_{j=1}^N \left[\frac{S(X;\lambda_j)-\hat{S}(\lambda_j)}{\Delta_j}\right]^2,$

where $$S$$ is the spectral characteristic of the model film, $$\hat{S}$$ is the corresponding experimental spectral characteristic, $$\{\lambda_j\}$$ is the wavelength grid in the experimental spectral range, $$\Delta_j$$ are measurement tolerances.

If spectral characteristics (transmittance, reflectance) are measured in %, and then the default $$\Delta_j$$ values for OptiLayer, OptiChar, OptiRE is 1%. In this case, the value of $$DF$$ around one means that RMS deviation of model data from the experiment one is about 1%. This value should be compared with expected accuracy of your measurements. This expected accuracy should include not only random noise, but also systematic errors (drifts, offsets).

A special interface provides an opportunity for a flexible and well-grounded choice of the specific thin film model depending on the available experimental data, its accuracy, and a priori information about the optical properties of the thin film under consideration.

Sophisticated mathematical algorithms enable you to reliably study even the most fine effects in thin films caused by a small absorption, small bulk and surface inhomogeneities.

Using OptiChar you can process:

• Spectral photometric data;
• Spectral ellipsometric data;
• Any combinations of these data

There are multiple factor affecting accuracy of the characterization results even in simplest characterization problems. Among them are:

• material type
• substrate parameters and quality
• thickness of the layer (should not be too thin, or too thick)
• possible inhomogeneity
• possible light scattering
• surfaces quality
• measurement wavelength range
• measurement angles of incidence
• measurements quality: slit size, noise, calibration, presence of drifts, etc.

Important: you should avoid so-called over-fitting problem. It means that you should not try to minimize the discrepancy function $$DF$$ below the accuracy of measurements. For example, if you estimate your accuracy of measurements as 0.5% (including also systematic deviations), you should not try to obtain $$DF$$ below 0.5 (if measurement tolerances in the Measurement Editor Tolerance column is 1%).

If you specify Tolerances as real expected accuracy at each wavelength (Woollam Ellipsometers provide this information, for example), then you should not try to minimize $$DF$$ below 1. When you reach this threshold value, you should stop and try to analyze the results.

### All our characterization models and methodology have been verified in the frame of collaboration with scientists from world leading research groups.

References:

1.  T. V. Amotchkina, M.K. Trubetskov, A.V. Tikhonravov, I.B. Angelov, V. Pervak, Reliable optical characterization of e-beam evaporated TiO2 films deposited at different substrate temperatures, Appl. Opt., Vol. 53, Issue 4, pp. A8-A15 (2014)
2. T. Amotchkina, M. Trubetskov, A. Tikhonravov, V. Janicki, J. Sancho-Parramon, O. Razskazovskaya, and V. Pervak, "Oscillations in spectral behavior of total losses (1 - R - T) in thin dielectric films," Opt. Express 20, 16129-16144 (2012).
3. A. Tikhonravov, T. Amotchkina, M. Trubetskov, R. Francis, V. Janicki, J. Sancho-Parramon, H. Zorc, and V. Pervak. "Optical characterization and reverse engineering based on multiangle spectroscopy." Appl. Opt. 51, 245-254 (2012).
4. T.V. Amotchkina, M.K. Trubetskov, A.V. Tikhonravov, I.B. Angelov, V. Pervak, "Reliable characterization of e-beam evaporated TiO2 films", in Optical Interference Coatings, OSA Technical Digest (Optical Society of America, 2013), paper FA.6.
5. T.V. Amotchkina, V. Janicki, J. Sancho-Parramon, A.V. Tikhonravov, M.K. Trubetskov, and H. Zorc. "General approach to reliable characterization of thin metal films." Appl. Opt. 50, 10, 1453-1464 (2011).
6. A. Tikhonravov, M. Trubetskov, T. Amotchkina, G. DeBell, V. Pervak, A. Sytchkova, M. Grilli, and D. Ristau, "Optical parameters of oxide films typically used in optical coating production," Appl. Opt. 50, C75-C85 (2011).
7. T.V. Amotchkina, M.K. Trubetskov, A. V. Tikhonravov, V. Janicki, J. Sancho-Parramon, and H. Zorc. "Comparison of two techniques for reliable characterization of thin metal-dielectric films." Appl. Opt. 50, 6189-6197 (2011).
8. T. V. Amotchkina, D. Ristau, M. Lappschies, M. Jupe, A. V. Tikhonravov, and M. K. Trubetskov, "Optical Properties of TiO2 -SiO2 Mixture Thin Films Produced by Ion-Beam Sputtering," in Optical Interference Coatings, OSA Technical Digest (CD) (Optical Society of America, 2007), paper TuA8.
9. A. V. Tikhonravov, M. K. Trubetskov, O. F. Prosovskiy, and M. A. Kokarev, "Optical Characterization of Thin Metal Films," in Optical Interference Coatings, OSA Technical Digest (CD) (Optical Society of America, 2007), paper WDPDP2.
10. A. A. Tikhonravov, A. V. Tikhonravov, and M. K. Trubetskov, "Accurate formulas for estimating the effect of surface micro-roughness on ellipsometric angles of dielectric thin films," in Optical Interference Coatings, OSA Technical Digest Series (Optical Society of America, 2004), paper WE4.
11. A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, A. A. Tikhonravov, D. Ristau, S. Günster, Reliable determination of wavelength dependence of thin film refractive index, Proc. SPIE. 5188, Advanced Characterization Techniques for Optics, Semiconductors, and Nanotechnologies 331 (2003)
12. A. V. Tikhonravov, M. K. Trubetskov, A. A. Tikhonravov, and A. Duparre', "Effects of interface roughness on the spectral properties of thin films and multilayers ," Appl. Opt. 42, 5140-5148 (2003)
13. A. V. Tikhonravov, M. K. Trubetskov, M. A. Kokarev, T. V. Amotchkina, A. Duparr, E. Quesnel, D. Ristau, and S. Gunster, "Effect of systematic errors in spectral photometric data on the accuracy of determination of optical parameters of dielectric thin films ," Appl. Opt. 41, 2555-2560 (2002).
14. D. Ristau, S. Güunster, S. Bosch, J. Ferrée-Borrull, F. Peiróo, A. Duparrée, E. Masetti, G. Kiriakidis, E. Quesnel, and A. Tikhonravov, "UV-Optical and microstructural properties of MgF2- and LaF3-coatings deposited by IBS and PVD processes," in Optical Interference Coatings, OSA Technical Digest Series (Optical Society of America, 2001), paper ThA5.
15. A. Tikhonravov, M. K. Trubetskov, A. A. Tikhonravov, and A. Duparre, "Impact of surface roughness on spectral properties of thin films and multilayers," in Optical Interference Coatings, OSA Technical Digest Series (Optical Society of America, 2001), paper ThB5.
16. A. Tikhonravov, M. K. Trubetskov, M. A. Kokarev, T. V. Amotchkina, and A. Duparre, "Influence of systematic errors in spectral photometric measurements on the determination of optical thin film parameters," in Optical Interference Coatings, OSA Technical Digest Series (Optical Society of America, 2001), paper TuD2.
17. A. V. Tikhonravov, M.K. Trubetskov, G. Clarke, B. T. Sullivan, J. A. Dobrowolski, Ellipsometric study of optical properties and small inhomogeneities of Nb2O5 films, Proc. SPIE. 3738, Advances in Optical Interference Coatings 183 (1999).
18. A. V. Tikhonravov, M. K. Trubetskov, Program package for the ellipsometry of inhomogeneous layers, Proc. SPIE. 2046, Inhomogeneous and Quasi-Inhomogeneous Optical Coatings 167 (1993).