In-situ spectroscopic reflectometry for polycrystalline silicon thin film etch rate determination during reactive ion etching

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 accurate in-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 che fitting procedure. The parameters of the refractive index models are adjusted at the beginning of each run to account for wafer-to-wafer variations without the use of additional ex-situ measurements. We have used ex-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.