Mask Error Enhancement Factor (MEEF) metrology using automated scripts in CATS

During the development of process generations with critical dimensions (CDs) around or even far below the wavelength used for printing, the wafer exposure step has become more and more complicated due to the non-linearities that occur within this subwavelength regime. These non-linearities give rise to a variety of optical-proximity effects (OPE) that must then be solved using optical-proximity correction (OPC). At the same time, the reticle errors are consuming a larger part of the total error budget on the wafer than we have seen in the past. As a consequence, detailed assessments of the reticle quality are gaining more importance and the average number of measurements for qualification of a high-end reticle is rapidly increasing. Within-die CD measurements are becoming common practice for such reticles. Quite often, these measurement points are selected randomly. However, regions within the design with relatively low aerial-image contrast during imaging will be subject to the largest amplification of the mask errors (highest MEEF).

In this work, a solution is presented in which measurement points with high MEEF values can be selected from any given layout data. The method is embedded into the standard mask data preparation flow using CATSTM. Focus is on the automation necessary to generate set-up files for CD metrology tools, such as the Leica-MueTec M5k. The methodology has been applied in a CMOS volume production site.

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