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For the next years optical lithography stays at 193nm with a numerical aperture of 1.35. Mask design becomes more
complex, mask and lithography specifications tighten. The k1 factor comes close to 0.25 which leads to a tremendously
increased Mask Error Enhancement Factor (MEEF). This means that CD errors on mask are getting highly amplified on
wafer. Process control becomes more important than ever. Accurate process control is a key factor to success to maintain
a high yield in chip production.
One key parameter to ensure a high and reliable functionality for any integrated circuit is the critical dimension
uniformity (CDU). There are different contributors which impact the intra-field CD performance at wafer such as mask
CD uniformity, scanner fingerprint, resist process etc. In the present work we focus on improvement of mask CD
signature which is one of the main contributors to intra-field CD uniformity. The mask CD uniformity has been
measured by WLCD32 which measures the CD based on proven aerial image technology. Based on this CD input the
CD uniformity was corrected by CDC200TM and afterwards verified by WLCD32 measurement. The CDC200TM tool
utilizes an ultrafast femto-second laser to write intra-volume shading elements (Shade-In ElementsTM) inside the bulk
material of the mask. By adjusting the density of the shading elements, the light transmission through the mask is locally
changed in a manner that improves wafer CDU when the corrected mask is printed.
Additionally, the impact of the improved CD uniformity on the lithography process window was investigated. Goal of
the work is to establish a process flow for mask CD uniformity improvement based on mask CD metrology by WLCD32
and mask CD uniformity control by CDC200TM and to verify its impact on the lithography process window. The
proposed process flow will be validated by wafer prints.
It was shown that the WLCD32 has an excellent correlation to wafer data and an outstanding CD repeatability. It
provides a reliable input for CD uniformity correction and is the tool of choice to verify the CD uniformity improvement
after CDC200TM treatment.
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