A new approach for improving measurement accuracy and reliability as the VASE approaches its fundamental limits.
Authors: Zhimin Zhu; Xianggui Ye; Sean Simmons; Catherine Frank; Tim Limmer; James Lamb
Brewer Science, Inc. (United States)
A variable-angle spectroscopic ellipsometer (VASE) is an essential tool for measuring the thickness of a thin film, as well as its n and k optical parameters. However, for films thinner than 10 nm, precise measurement is very challenging. In this paper, the root causes of these measurement complexities are discussed, and a new approach is proposed to improve measurement accuracy and reliability as the VASE approaches its fundamental limits, that is, the limitations of the system’s mechanical accuracy and optical coherence. Specifically, twelve different polymer thin films were tested, each with 16 test points. The measured thicknesses (ranging from 3 nm to 19 nm) and n- and k-values strongly depended on test settings and on the data fitting algorithm, incident angle, and wavelength selection. Fitting using equal thickness and n-value of multiple test points was applied as additional criteria to evaluate the consistency of the test results. With this technique, reasonable n-values and thickness values were obtained from films that were as thin as 3 nm. To improve the efficiency of the model fitting, film thickness and n-value are associated to allow these two-dimensional parameters to be fitted into a quasi-one-dimensional model, which reduces central processing unit (CPU) utilization.
Paper available for download for SPIE members or for a fee.
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