Proceedings PaperSwing curve prediction from reflectance spectra: a new method to predict optimal resist thicknesses and compare processes
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A new general method to predict resist maxima and minima of linewidth swing curves using UV reflectance spectra near the actinic wavelength is proposed. In this technique, the reflectance spectra in the vicinity of the exposure wavelength are used to determine the effective phase shift due to the underlying substrate. With this parameter, resist thicknesses yielding minima and maxima linewidths can be predicted. In principal, resist minima and maxima can be determined from the UV spectra of a single wafer. This technique was used to predict minima and maxima on both simple (bare silicon) and multifilm (polybuffer LOCOS and gate) stacks for 2 different resists, with and without topography. Three of the film stacks included a top antireflecting coating. Results were compared with swing curves determined from measured linewidths versus resist thicknesses. In general, predicted resist maxima and minima were within 50 Angstroms of the corresponding values measured from linewidths which was well within the experimental error. Additionally, the peak height of the reflectance spectra in the vicinity of the actinic wavelength was generally correlated to linewidth swing determined from multiple wafers. This implies that process stability with respect to cd swing can be compared directly from reflectance spectra. Theoretically, results determined from a nearly normal incident UV reflectometer require a correction to account for the finite NA of the lens. Simulation was used to determine this correction. However, two different simulation programs (Prolith 6.0 and Solid C 6.3) overcorrected the case of an i-line stepper with 0.63NA and 0.65 partial coherence. In this case, agreement with experiment was better with a "half" correction determined from simulation.