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As the semiconductor industry moves toward immersion lithography using numerical apertures above 1.0 the quality of
the photomask becomes even more crucial. Photomask specifications are driven by the critical dimension (CD)
metrology within the wafer fab. Knowledge of the CD values at resist level provides a reliable mechanism for the
prediction of device performance. Ultimately, tolerances of device electrical properties drive the wafer linewidth
specifications of the lithography group. Staying within this budget is influenced mainly by the scanner settings, resist
process, and photomask quality. Tightening of photomask specifications is one mechanism for meeting the wafer CD
targets. The challenge lies in determining how photomask level metrology results influence wafer level imaging
performance. Can it be inferred that photomask level CD performance is the direct contributor to wafer level CD
performance? With respect to phase shift masks, criteria such as phase and transmission control are generally tightened
with each technology node. Are there other photomask relevant influences that effect wafer CD performance?
A comprehensive study is presented supporting the use of scanner emulation based photomask CD metrology to predict
wafer level within chip CD uniformity (CDU). Using scanner emulation with the photomask can provide more accurate
wafer level prediction because it inherently includes all contributors to image formation related to the 3D topography
such as the physical CD, phase, transmission, sidewall angle, surface roughness, and other material properties.
Emulated images from different photomask types were captured to provide CD values across chip. Emulated scanner
image measurements were completed using an AIMSTM45-193i with its hyper-NA, through-pellicle data acquisition
capability including the Global CDU MapTM software option for AIMSTM tools. The through-pellicle data acquisition
capability is an essential prerequisite for capturing final CDU data (after final clean and pellicle mounting) before the
photomask ships or for re-qualification at the wafer fab. Data was also collected on these photomasks using a
conventional CD-SEM metrology system with the pellicles removed. A comparison was then made to wafer prints
demonstrating the benefit of using scanner emulation based photomask CD metrology.
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