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With the introduction of complex lithography schemes like double and multi – patterning and new design principles like
gridded designs with cut masks the requirements for mask to mask overlay have increased dramatically. Still, there are
some good news too for the mask industry since more mask are needed and qualified. Although always confronted with
throughput demands, latest writing tool developments are able to keep pace with ever increasing pattern placement specs
not only for global signatures but for in-die features within the active area. Placement specs less than 3nm (max. 3
Sigma) are expected and needed in all cases in order to keep the mask contribution to the overall overlay budget at an
accepted level. The qualification of these masks relies on high precision metrology tools which have to fulfill stringent
metrology as well as resolution constrains at the same time.
Furthermore, multi-patterning and gridded designs with pinhole type cut masks are drivers for a paradigm shift in
registration metrology from classical registration crosses to in-die registration metrology on production features. These
requirements result in several challenges for registration metrology tools. The resolution of the system must be
sufficiently high to resolve small production features. At the same time tighter repeatability is required. Furthermore,
tool induced shift (TIS) limit the accuracy of in-die measurements.
This paper discusses and demonstrates the importance of low illumination wavelength together with low aberrations for
best contrast imaging for in-die registration metrology. Typical effects like tool induced shift are analyzed and evaluated
using the ZEISS PROVE® registration metrology tool. Additionally, we will address performance gains when going to
higher resolution. The direct impact on repeatability for small features by registration measurements will be discussed as
well.
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