Proceedings Article | 7 October 2009
KEYWORDS: Inspection, Photomasks, Calibration, Image processing, Data modeling, Scanners, Process modeling, Airborne remote sensing, Optical lithography, Databases
Advanced immersion lithography is enabled by a combination of optimized off-axis illumination,
highly complex design patterns, and photo-mask technologies with several transmission and phase
levels. The pattern on the mask, for 45nm half pitch and below, shows little resemblance to the target
printed pattern, which is revealed only when illuminated with the correct aerial exposure conditions.
The main pattern is modified or surrounded by OPC and SRAF features which are comparatively
much smaller. The small size and irregularity of these features present a challenge to mask inspection
process, both due to their size and the mask manufacturing process sensitivity. While most masks are
inspected using a die-to-die scheme, single-die masks use an alternative detection scheme based on
comparing the mask image to mask design data. In high-resolution inspection tools, the resolution
must be sufficient to resolve the sub-resolution features, and compare them to the mask design. In
aerial inspection tools, which have optics that mimic the illumination and collection exposure
conditions over the mask as in a scanner, the inspection image depicts the mask at the scanner
resolution. As a consequence, in the aerial image, as in the scanner, sub-resolution features are not
resolved and do not develop. Therefore, a conventional comparison to a database is not possible.
Here, we present a single die detection scheme that takes a new approach - an optical model is
calculated from the mask design information, based on an optical modeling of the inspection optics
response. The result is an aerial model image, which predicts the aerial image created by the
inspection tool, and may be directly compared to the real image captured by the inspection machine.
We describe herein the theoretical foundation of the Die-to-Model scheme, and the practical
computational implementation. As a consequence of the high quality modeling, the detection scheme
employed for single die inspection performance is identical to the die-to-die scheme,. This new die to
model scheme, implemented on the Aera2 aerial mask inspection tool is successfully implemented in
4x memory and 32nm generation logic mask production at leading mask shops.
