Spectroellipsometric Characterization Of Inhomogeneous Films

Spectroellipsometric Characterization of Inhomogeneous FilmsB. A. Tirri, A. Turner, P. C. Van BuskirkOptical Operations Directorate, Perkin -Elmer Corporation77 Danbury Road, Wilton, Connecticut 06859 -0420AbstractDetermination of the compositional depth profile of an inhomogeneous film structure isoften approached via surface analysis techniques used in a sputter depth profiling mode.An alternative is to model the compositional profile to fit optical measurement data, suchas multiple incident angle or spectroscopic ellipsometric data. This paper presents theapplication of a statistical approach to this analysis problem. Multiple incident angleand multiple wavelength ellipsometer measurements have been made on hafnia and aluminasingle films on borosilicate glass substrates. Theoretical sensitivity data of the measur-ed quantities 4', A on the index and thickness of the film is presented and discussed. Themeasurement data is then numerically fit to several models that express small inhomogenei-ties in the film refractive index as a system of multiple homogeneous films. Existence oflocal minima in the value of the chosen merit function and statistical techniques that per-mit location of the global minimum are addressed by this approach. Results are discussed interms of the magnitude of the merit function and compared to spectral reflectance measure-ments made on the coated samples.IntroductionInterest in the characterization of inhomogeneous film systems now exists in numeroustechnology areas. One example involves the need for precise control of doping and interfacecomposition profiles in semiconductor structures. Such structures are being applied forboth electronic and optical detection applications. AlxGa1_XAs is an example of a filmmaterial system which has been sensitively and nondestructively probed using ellipsometry.1Another area of increasing interest in inhomogeneous films exists in optical coating.Optical performance of conventional coating designs can often be highly sensitive to inhomo-geneities in the refractive index of one or several films in the coating. The effects ofslight inhomogeneities can be undesirable or, as in some multilayer anti -reflection coat-ings, crucial to the coating's optical performance. Additionally, there exist unconven-tional coating designs which utilize continuously varying film refractive index profiles.Graded index anti -reflection coatings are an example.2'3 It is evident that the capabilityfor modelling of specific inhomogeneous film systems is very desirable.Ellipsometry has become the primary nondestructive technique available for determining thestructural and optical parameters of thin films. The precision and accuracy inherent in thetechnique give it an advantage over photometric and interferometric techniques. One nor-mally makes a series of measurements over several angles of incidence and /or wavelengthsto generate a set of y, A values. These values are then used along with an assumed model toderive the parameters which describe the model. The approach described below has been usedto derive the best fit parameters of discrete multilayer and inhomogeneous film represent-ed by a discrete structure. Figure 1 shows a schematic of the discrete multiple film modelemployed to represent an inhomogeneous film.