Correlation of process with topography evolution during reactive ion etching.

Abstract

We present a low-cost, high-speed, high-accuracy <italic>in situ</italic> thin film measurement system for real-time process monitoring and industrial process control. This sensor, the two-channel spectroscopic reflectometer (2CSR), is a hybrid of spectroscopic ellipsometry (SE) and spectroscopic reflectometry (SR). In 2CSR a polarized beam of white light is directed at the sample. The reflected light is resolved into its two orthogonal components, <italic> s</italic> and <italic>p</italic>, using a Wollaston prism. These data are recorded simultaneously as a function of wavelength using a two-channel spectrometer with linear array detectors. The fact that 2CSR has no moving parts, coupled with the use of the two-channel linear array detectors, enables high accuracy-data acquisition across the sensor's spectral range in 6ms. This makes the 2CSR ideal for real-time high-speed process monitoring and control in an industrial setting. We show accurate <italic>in situ</italic>, high-speed film thickness measurements during the plasma etches of both silicon dioxide and polycrystalline silicon samples. Additionally, we illustrate some of the advantages of <italic> in situ</italic> metrology for statistical process characterization and process control applications. We demonstrate an improved polysilicon optical model which accounts for both the scattering losses due to non-specular reflections from the surface and for the change in the polarizability of the surface roughness layer due to the inclusions of voids. We find that at near normal incidence the scattering effects dominate while at larger angles of incidence the combined model is less sensitive to scattering effects and the effective media effects begin to dominate. Finally, we show the first ever analysis of real-time <italic>in situ </italic> measurements of the reflectance from a grating structure while it is being etched, from which the topography of the grating is extracted. We use the 2CSR to measure the <italic>s</italic> and <italic>p</italic> reflectances in real-time and analyze the data off-line using the Rigorous Coupled Wave Analysis. We performed scanning electron microscopy (SEM) on representative samples which were etched to different depths and successfully compared the micrographs to the RCWA extracted topography down to the limit of our SEM.