The influence of spatial resolution on linewidth measurements in the critical dimension scanning electron microscope (CD-SEM) was investigated experimentally. Measurement bias variation and measurement repeatabilities of four edge detection algorithms were evaluated with a series of images at varying focus in order to determine the effect of resolution variation. Three of these algorithms, maximum derivative, regression to baseline, and sigmoidal fit, are commonly used on commercial CD-SEMs, and the other is a model-based library (MBL) approach that detects the line edge by comparing CD-SEM line scans to a library of simulated line scans. MBL is able to take into account beam size and other parameters (including sidewall angle of the line structure). These algorithms were applied to images of polycrystalline silicon lines with various sidewall angles taken under different focus conditions. In general, it was observed that repeatability is degraded by defocus, and bias varied with focus and target shape. These results indicate that if two or more tools had different resolutions, measurement results would have different biases depending on target shape. The amounts of errors depend on the algorithms, with MBL the most stable against focus variation. However, it still has some systematic errors and outliers far from best focus. Investigations of electron distributions and the effect of electron incident angle were performed for a better model.
This study presents a method of extracting three-dimensional metrological information for etched gate structures from top-down SEM images for use in critical dimension analysis. The variations in sidewall angle and bottom corner (footing) roundness are quantified as feature indices by multiple parameter profile characterization (MPPC), and are used as the main indicators of device performance. A stable algorithm developed based on simulation and experimental results partitions the SEM image signal into the sidewall and footing based on the first derivative of the image signal. The width of the sidewall is used as an index of the sidewall angle, and the width of the footing is used as an index of the footing roundness. The validity of the MPPC method is confirmed through experiments using actual poly-Si gate wafers, and is shown to have a 3σ accuracy of ±0.9º for sidewall angles deviating by more than 2º. The sidewall angle index and its distribution map are useful for evaluating the etching process, and are particularly effective for revealing subtle macro variations like asymmetry, while the footing roundness index is useful for screening out bad wafers. As MPPC employs only top-down SEM images, no throughput loss will be incurred in comparison with conventional CD measurements.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.