Timbre's Optical Digital Profilometry (ODP) system is a scatterometry-based metrology. In lithography applications, the critical dimension (CD) is often patterned photoresist (PR) on an anti-reflective coating (ARC). When a patterned PR is exposed to the broadband light of the optical metrology tool, a change in reflectance may occur. For "sensitive" film stacks, the changing optical signals then produce changing ODP CD, sidewall angle, and film thickness measurements. This report summarizes the results of several resist and ARC stacks subjected to the repeated broadband light exposure of a Therma-Wave CCD-i reflectometry system. The purpose is to determine which resist-ARC stacks are significantly affected by repeated measurement exposure, and to quantify these effects. Our analysis shows that very little metrology exposure-induced change occurs for ArF resists. For KrF resists, the change is closely related to the type of KrF resist used; acetal-types incur large spectral changes upon repeated exposure, whereas ESCAP (Environmentally Stable Chemically Amplified Photoreist) resists measurements are very stable. Significant reduction of metrology induced spectral and CD change as achieved by incorporating a long-pass filter into the system. The changes due to a single measurement are negligible, however, they can be substantial for a sensitive material when characterizing metrology repeatability. Thus, it is recommended to use stable materials, such as oxide gratings, for metrology characterization.
Scatterometry provides a new, vibration-tolerant technique of overlay metrology. Gauge repeatability and reproducibility is improved by an order of magnitude over imaging-based overlay metrology. To measure the overlay of patterned layers A and B by scatterometry, one line grating is placed in layer A and another in layer B. The two gratings overlap when they are viewed in the direction that is normal to the wafer. The line gratings in layers a and b are of equal pitch and their lines are parallel. In one method, overlay is measured by fitting the optical properties of the target with spectra calculated using a model of the target and rigorous coupled wave analysis. A faster and simpler method obtains overlay by applying a linear estimator to a difference of spectra. Optical properties of targets were measured by a normal incidence spectroscopic reflectometer. Test wafers representing three overlay applications were fabricated: contact mask to shallow-trench, first metal mask to contact, and gate-mask to shallow-trench. Overlay measured by scatterometry agree with imaging-based measurements and offsets intentionally written to the reticle.
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