7 March 2008 Improvement of the common DoF across field for hole-structure process layers
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It is well-known that the available depth of focus (DoF) tend to decrease for each advancing technology node. Moreover the leveling control on wafer topography has become a challenge to affect the focus control on exposure tool capability, especially for the critical hole-structure layer of the back end of line (BEoL). In this study, we used the via layer from the real products as an example of optimizing the exposure tool's leveling system to reduce process-related influences to improve the intra-field focus control range. First, the focus-exposure matrices (FEMs) were applied to a wafer in different leveling modes. Then, patterns' critical dimension (CD) in different locations within the same field were measured to produce the Bossung curves required to determine the best focus. The same steps were repeated on a bare wafer to illustrate how the process reduced the common depth of focus range. We also introduced the non-optical leveling sensor, which measured the wafer by the use of physical methods. Since it does not interact with the film stack or the pattern density, the measurement accuracy will be insensitive to process variation. Therefore, it can be used to compensate the optically induced errors from the optical leveling system and to expand the useful depth of focus for improving CD uniformity. Finally, we briefly summarize the improvement ratio achieved of the common DoF using these optical and non-optical leveling systems with different leveling modes.
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Shu Huei Hou, Shu Huei Hou, Edgar Huang, Edgar Huang, Aroma Tseng, Aroma Tseng, Met Yeh, Met Yeh, Bill Lin, Bill Lin, Chun Chi Yu, Chun Chi Yu, Eason Lin, Eason Lin, "Improvement of the common DoF across field for hole-structure process layers", Proc. SPIE 6924, Optical Microlithography XXI, 692434 (7 March 2008); doi: 10.1117/12.772171; https://doi.org/10.1117/12.772171

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