Method for accurately calibrating a constant-angle reflection-interference spectrometer (caris) for measuring photoresist thickness

Abstract

A method is described for determining more accurate Cauchy coefficients for a constant-angle reflection-interference spectrometer (CARIS). This allows photoresist thicknesses for product wafers to be measured more accurately. The method for determining the Cauchy coefficients consists of coating monitor wafers with photoresist layers having various thicknesses formed by varying the spin speed during photoresist coating. The photoresist layers are then patterned using monochromatic radiation through a mask and developing photoresist. The monochromatic radiation has a dose sufficient to just clear the photoresist layers from the surface of the wafers during development. The linewidths of the photoresist are measured and plotted as a function of photoresist thickness to generate a critical dimension (CD) swing curve having an essentially sinusoidal shape that results from interference between the transmitted and reflected monochromatic radiation in the photoresist. The monitor wafer for a predetermined minimum in the CD swing curve is used to calculate more precisely the Cauchy coefficients for the refractive index for the photoresist. The refractive index as a function of frequency (Cauchy equation) is used is used with CARIS to measure photoresist thickness more accurately for product wafers.