Whole Wafer Macro CD Metrology
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Whole Wafer Macro CD Metrology Scott Balak (Rudolph Technologies, Inc.) & Byeong-Ok Cho, Ph.D (Samsung SEC) For over ten years automated macro defect inspection systems have been utilized by lithography cells to monitor random litho track defectivity. These systems focus on throughput over sensitivity to ensure a large population of samples can be cost-effectively inspected quick enough to react to highlight track defect excursions. In addition to litho track-induced defects, macro defect inspection systems can also detect scanner-induced defocus detects. Defocus defects are characterized by a large variation in the width of exposed pattern lines. Although useful for monitoring defect excursions, the application provides little use for predicting a trend towards a future excursion or predicting the range of the pattern variation across the field. These applications require an expensive, high resolution CD-SEM or OCD system which typically measure five points and extrapolate results for whole wafer variation. For highly repetitive structures under the proper illumination conditions, there can be a direct correlation between Grayscale Value (GSV) and variation in linewidth critical dimension. Figure 1 displays an array of simple test structures with CD varied from +5nm to -5nm of nominal. This direct correlation can then be utilized by a high speed macro defect inspection system to provide a single solution for both defect and metrology results. A whole wafer image can be mapped to the corresponding GSV range for each distinct CD range. Figure 3 shows a zoomed in area of CD variation along the transition of a FEM. Figure 3-CD variation detail at the transition The technique was generated through cooperation between Rudolph Technologies and Samsung SEC utilizing the latest generation AXi 940 macro inspection module. Figure 1 For this example, there is a clear linear correlation between CD variation and GSV response from the camera as displayed in Figure 2.