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Pattern Quality Confirmation: Increased CD SEM Sensitivity to Yield Limiting Process Excursions by Richard Dare, Lucent Technologies Amir Azordegan and John Miller, KLA-Tencor Corporation
Feature size has a direct correlation to device performance, process yield, and reliability. Quantifying feature size with a CD SEM is, therefore, an important part of process control. As devices get smaller, feature shape is becoming equally important due to its effect on the transfer functions from photo to etch and the electrical properties of the device after etch. Measurements of the shape of sampled structures are necessary to develop and improve the correlation between process changes and subsequent yield and device reliability. Ideally, measurements of feature size and shape would occur on a single metrology platform. However, the process of measuring feature size with a CD SEM is typically one of reduction, with large amounts of information contained in images and linescans reduced to a single number. Critical information about feature shape is lost, and process excursions are not identified in a timely manner.
Users of top-down CD SEMs know that features which are noticeably different in cross section can produce similar CDs. An example of this would be two lines with similar bottom dimensions but with different sidewall slopes as shown in Figure 1. Traditional CD SEM measurements would detect the increase in the electron signal as the feature is scanned at the onset of the sidewall as the feature “edge”. Since the bottom dimension of the line cross section is similar for these features, this “edge” in the electron signal will give similar CDs for the two lines. Information about the sidewall slope that is contained in the images and linescans produced by the SEM is not utilized. KLA-Tencor has developed Pattern Quality Confirmation (pQC™) to utilize the feature shape information lost in the reduction of a complex feature scan to a single CD measurement. pQC provides value in the form of additional metrics that can be used to track pattern fidelity. These metrics are 46
Spring 2000
Yield Management Solutions
Optimal F-E
CD=543 nm
Strong Defocus
CD=535 nm
F i g u re 1. Optimal and defocussed i-line 0.5 µm ADI metal 1 line CD SEM images and linescans. ρ indicates the correlation between the non-optimal an d optimal images (and linescans). While the defocussed line CD i s within spec, it is clear tha t the defocussed lin e suffers fr om a serious degradati on i n s idewa ll slope. This shape anomaly is reflected in the pQC corr elati on score (per fect correlati on = 100).
derived from the correlation of measured features to user-defined standards for the feature in the form of linescans or images.