Ellipsometry: Modeling, Measurement and Process Control
Tuesday, May 1, 2001 10:50 AM in Room Sunset
C6-1-8 New Techniques for Monitoring and Control of Optical Thin Film Deposition by In Situ Spectroscopic Ellipsometry
B. Johs, J.S. Hale, C.M. Herzinger, J.A. Woollam (J.A. Woollam Co., Inc.)
Traditionally, intensity-based reflection and transmission (R/T) measurements have been used to monitor and control the deposition of optical thin films. In addition to the relatively simple instrumentation requirements (as compared to spectroscopic ellipsometers), R/T measurements also have the advantage of more directly measuring the properties of interest for an optical coating structure. In situ spectroscopic ellipsometry can potentially offer better measurement precision and accuracy (compared to R/T), and the ellipsometric Delta parameter provides phase information which enhances the measurement sensitivity for ultra-thin films and also enables the unambiguous calculation of film index and thickness (in some cases). However, quantitatively analyzing the ellipsometric data to extract the thin film properties can be complicated, due to potential film non-idealities such as surface/interfacial roughness and index grading, as well as the sheer complexity of typical multi-layer coating structures. In this paper, new ellipsometric monitoring techniques specifically developed for optical interference coatings are presented. One technique utilizes a spectral signature in the ellipsometric data to infer when the 1/4 wave thickness condition is reached (without the model-based regression analysis approach used in traditional spectroscopic ellipsometric data analysis). Another technique extends the virtual interface approach of Aspnes@footnote 1@ to multi-layer dielectric structures; this enables the extraction of thickness and index from the topmost deposited layer, independent of the previous growth history. The significant advantages of using rotating compensator-based spectroscopic ellipsometer instrumentation when implementing these new techniques will be discussed. Experimental results demonstrating robust control of various optical thin film structures will also be shown. @FootnoteText@ @footnote 1@D.E. Aspnes, J. Vac. Sci. Technol. A 14 (1996) 960.
C6-1-10 Measurement of Very Thin Films Using Spectroscopic Ellipsometry
G.E. Jellison (Oak Ridge National Laboratory)
The measurement of the thickness and other optical properties of very thin films is extremely important for many applications. For example, the semiconductor industry is currently making gate oxides that are 5-10 nm thick, and metrology for quality control is a pressing problem. Ideally, one would like to measure both film thickness and film refractive index, but this becomes increasingly difficult when the film thickness is very small. Spectroscopic Ellipsometry and reflectometry are often the best way to determine the film thicknesses, but these measurements are indirect, and the accuracy depends on both errors and the model of the sample surface. When the ratio of the film thickness to wavelength (d/@lambda@) is very small, ellipsometry and reflectometry are sensitive only to the product (d/@lambda@)*(n@super 2@-1)/n@super 2@, where n is the refractive index of the film, and it is not possible to separately determine both the film thickness and its refractive index. If two or more very thin films are stacked on the sample surface [where (d@sub 1@+d@sub 2@)/@lambda@ is small], then it is impossible to determine, to first order in (d@sub 1@+d@sub 2@)/@lambda@, which film is on top. In this paper, the accuracy limits of spectroscopic ellipsometry measurements of very thin films will be discussed. Some systematic errors, such as those due to errors in the alignment of optical elements, can be minimized if the sample is known to be isotropic and the measurements are made using a generalized ellipsometer, such as the 2-modulator generalized ellipsometer. Other systematic errors, such as the angle of incidence and numerical aperture, can also play an important role in the accuracy of the determined film properties.