Miloslav Ohlídal

IV: Scattering of Light from Multilayer Systems With Rough Boundaries

Publisher Summary This chapter discusses the scattering of light from multilayer systems with rough boundaries. Multilayer systems are employed in various branches of applied optics and the optics industry. The chapter reviews the important theoretical and experimental results concerning the optics of rough multilayer systems. The physical models of rough boundaries (surfaces) play an important role in the optics of rough multilayer systems. The theoretical approaches enable us to calculate optical quantities corresponding to the rough boundaries and rough thin film systems. The interpretation of experimental data using the theoretical results introduced has been described. Relevant methods for the optical characterization of rough surfaces, rough single layers and rough multilayer systems have been outlined. The chapter also focuses on the scattering of X-rays from rough surfaces and thin film systems with rough boundaries.

Measurement of the thickness distribution and optical constants of non-uniform thin films

In this paper, an original method for the complete optical characterization of thin films exhibiting area thickness non-uniformity is presented. This method is based on interpreting experimental data obtained using an original imaging spectroscopic photometer operating in the reflection mode at normal incidence of light. A CCD camera is employed as a detector of the photometer. The spectral dependences of the reflectance measured simultaneously by individual pixels of the CCD camera correspond to the local reflectance of small areas of the non-uniform thin films characterized. These areas form a matrix along a relatively large part of the substrate covered with the non-uniform film. The spectral dependences of the local reflectance measured by the individual pixels are treated separately by means of the formulae for the reflectance valid for uniform thin films. The reason is that the local areas corresponding to the pixels are sufficiently small so that the film characterized can be considered to be uniform within these local areas. Using this approach, it is possible to determine the values of the local thickness and local optical constants for every small area of the matrix. Thus, in principle it is possible to determine the distributions (maps) of the local thickness and the local optical constants of the non-uniform films simultaneously. This method is used to characterize carbon-nitride thin films exhibiting only the thickness area non-uniformity.

Optical characterization of non-uniform thin films using imaging spectrophotometry

In this paper the method of imaging spectrophotometry enabling us to characterize non-absorbing thin films non-uniform in the optical parameters is described. This method is based on interpreting the spectral dependences of the local absolute reflectances measured at the normal incidence of light. It is shown how to determine the area distribution of thickness and refractive index of the non-absorbing non-uniform thin films by treating these reflectances. Moreover, the generalization of the method for the optical characterization of slightly absorbing non-uniform thin films is also indicated. Furthermore, the two-channel imaging spectrophotometer enabling us to apply the method of imaging reflectometry is described. The procedure for determining the spectral dependences of the local absolute reflectance in the points aligned in a matrix situated on the illuminated area of the non-uniform thin film by means of the spectrophotometer is also presented. The practical advantages of the method are specified. The method is illustrated by means of the optical characterization of a selected epitaxial ZnSe thin film prepared using molecular beam epitaxy onto gallium arsenide single-crystal substrate.

Optical analysis by means of spectroscopic reflectometry of single and double layers with correlated randomly rough boundaries

Optical analysis of rough single and double layers is performed by interpreting the spectral dependences of the measured coherent reflectance. Formulas for the coherent reflectance of these systems derived within the scalar theory of diffraction of light are used for this interpretation. Possibilities and limitations of the method utilized are illustrated using several concrete samples of both the rough single and double layers represented by models corresponding to fully correlated (identical), partially correlated, and fully uncorrelated boundaries. It is shown that the values of the optical parameters, the root-mean-square (rms) values of the heights of the boundary irregularities and the values of the cross-correlation coefficients of the boundaries characterizing the layered systems mentioned can be determined within the method described.

Mapping of properties of thin plasma jet films using imaging spectroscopic reflectometry

The construction of a normal-incidence imaging spectrophotometer for mapping of thin film properties is described. It is based on an on-axis reflective imaging system, utilising a telescope-like arrangement. A charge-coupled device camera is used as the detector, permitting measurements in the spectral range of 275-1100 nm with resolution of 37 mu m. The performance of the instrument is demonstrated by optical characterisation of highly non-uniform thin films deposited from hexamethyldisiloxane on silicon substrates by a single capillary plasma jet at atmospheric pressure. The imaging spectrophotometry is used as a self-sufficient technique for the determination of both the film optical constants and maps of local thickness. The thickness maps are compared with the results of conventional thickness profile characterisation methods, profilometry and atomic force microscopy and the differences and errors are discussed.

Measurement of thickness distribution, optical constants, and roughness parameters of rough nonuniform ZnSe thin films

Epitaxial ZnSe thin films exhibiting two important defects, i.e., boundary roughness and thickness nonuniformity, prepared on GaAs substrates, are optically characterized using a combination of variable-angle spectroscopic ellipsometry, spectroscopic near-normal reflectometry, and imaging spectroscopic reflectometry (ISR). The influence of boundary roughness is incorporated into optical quantity formulas by the Rayleigh-Rice theory. Thickness nonuniformity is included using averaging of the unnormalized Mueller matrices. The dispersion model of the optical constants of the ZnSe films is based on parametrization of the joint density of electronic states. Very thin overlayers represented by thin films with identically rough boundaries are taken into account on the upper boundaries of the ZnSe films. Standard optical techniques are used to determine the spectral dependencies of the optical constants of the ZnSe films, together with the parameters of roughness and thickness nonuniformity. ISR is then used to find the maps of the local thickness and local rms value of height irregularities. The values of roughness parameters, determined using the standard techniques and ISR, are verified by a comparison with results obtained by atomic force microscopy.

New Method for the Complete Optical Analysis of Thin Films Nonuniform in Optical Parameters

In this paper, a new optical method for characterizing thin films exhibiting area nonuniformity in optical parameters is described. This method is based on interpreting the spectral dependences of the reflectance measured using the special experimental arrangement described in detail. Using this method, the distribution of both the optical parameters, i.e. the local thickness and local refractive index, describing the thin film studied can be determined along a large area of the substrate. It is shown that the method presented can be employed for determining strong nonuniformities in the optical parameters of the films studied. The method is illustrated through the optical analysis of strongly nonuniform thin films formed by a mixture of CNx and SiOy deposited onto silicon single-crystal substrates.

Comparison of the two-dimensional Fraunhofer and the two-dimensional Fresnel approximations in the analysis of surface roughness by angle speckle correlation. I: Theory

Abstract In this paper we discuss some new theoretical results which we reached in the analysis of a random surface roughness in the framework of the two-dimensional (2D) Fresnel approximation. A comparison with 2D Fraunhofer approximation results is made. It is shown for experimentally reasonable values of the geometrical arrangement parameters of the method that the surface roughness measurement can be interpreted in the 2D Fraunhofer approximation only for surfaces with high slopes of surface irregularities. Surfaces with small slopes of irregularities require interpretation of the surface roughness measurement in the 2D Fresnel approximation. In this case, not only the standard deviation of surface height fluctuations but also the correlation length of the surface can be determined.

Optical characterization of nonabsorbing and weakly absorbing thin films with the wavelengths related to extrema in spectral reflectances.

In this contribution a new efficient modification of a method that enables us to perform the optical characterization of nonabsorbing and weakly absorbing thin films without using the absolute values of the reflectances measured is presented. Namely, this modification is based on determining the values of the wavelengths corresponding to touching the spectral dependences of the reflectances of the studied films measured for several angles of incidence with the envelopes of maxima and minima of these spectral dependences. By means of combining the explicit formulas containing the wavelengths mentioned and the suitable iteration procedure one can evaluate the values of the thicknesses and spectral dependences of the refractive indices of the films analyzed in reliable and precise ways.

Study of Surfaces Generated by Abrasive Waterjet Technology

The paper deals with results obtained by means of contactless optical shadow method and by commercial methods, namely by using an optical commercial profilometer MicroProf (FRT) and a contact profilometer HOMMEL TESTER T8000. The main emphasis is put on the analysis of results for defining the process of creation of a new surface generated by the stream of abrasive waterjet, including its geometric parameters and mechanisms of cutting tool-material interaction. New possibilities of the surface quality evaluation and optimizing the technological parameters selection of the cutting process appear.