In-situ spectroscopic reflectometry for polycrystalline silicon thin film etch rate determination during reactive ion etchinc |
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Authors: | Tyrone E Benson Leonard I Kamlet Pete Klimecky Fred L Terry |
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Affiliation: | (1) Department of Electrical Engineering & Computer Science and the Center for Display Technology & Manufacturing, The University of Michigan, 48109-2122 Ann Arbor, MI |
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Abstract: | Accurate film thickness monitors are important for the development of real-time feedback control of dry etch processes and
are very useful for run-to-run process control and process diagnostics. Technologically important films such as polycrystalline
Si, which can have process-dependent refractive indices and/or surface roughness, pose significant challenges for low-cost,
high-speed film thickness measurement systems. We have used spectroscopic reflectometry (SR) to make accuratein-situ, high-speed film thickness measurements during plasma etching of polycrystalline Si. The SR system determines the film thickness
using a least squares regression algorithm that fits the theoretical reflectance to the experimental reflectance vs wavelength
data. We have included physically based models for the variation of the polycrystalline Si bulk refractive indices and surface
roughness in the fitting procedure. The parameters of the refractive index models are adjusted at the beginning of each run
to account for wafer-to-wafer variationswithout the use of additional ex-situ measurements. We have usedex-situ spectroscopic ellipsometry to validate the models used and to check the accuracy of our SR measurements. Currently, our SR
system can acquire data in 40 ms and the software can calculate the polycrystalline Si thickness in less than 55 ms per measurement,
so that a new film thickness and etch rate estimate can be obtained in less than 100 ms. The methods used for analysis of
polycrystalline Si are also directly useful for improving the accuracy of microscope-based spectral reflection measurement
systems commonly used for in-line measurements. Using similar optical modeling concepts, the SR technique can also be used
to accurately measure film thicknesses and etch rates of other thin films with process-dependent optical constants, such as
deposited dielectrics and compound semiconductors. |
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Keywords: | In-situ thin film metrology polysilicon reactive ion etching (RIE) reflectometry |
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