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As the process generation migrate to advanced and smaller dimension or pitch, the mask
and resist 3D effects will impact the lithography focus common window severely because of
both individual depth-of-focus (iDOF) range decrease and center mismatch. Furthermore,
some chemical or thermal factors, such as PEB (Post Exposure Bake) also worsen the usable
depth-of-focus (uDOF) performance. So the mismatch of thru-pitch iDOF center should be
considered as a lithography process integration issue, and more complicated to partition the
3D effects induced by optical or chemical factors.
In order to reduce the impact of 3D effects induced by both optical and chemical issues, and
improve iDOF center mismatch, we would like to propose a mask absorber thickness offset
approach, which is directly to compensate the iDOF center bias by adjusting mask absorber
thickness, for iso, semi-iso or dense characteristics in line, space or via patterns to enlarge
common process window, i.e uDOF, which intends to provide similar application as
Flexwave[1] (ASML trademark).
By the way, since mask absorber thickness offset approach is similar to focus tuning or
change on wafer lithography process, it could be acted as the process tuning method of
photoresist (PR) profile optimization locally, PR scum improvement in specific patterns or to
modulate etching bias to meet process integration request.
For mass production consideration, and available material, current att-PSM blank, quartz,
MoSi with chrome layer as hard-mask in reticle process, will be implemented in this
experiment, i.e. chrome will be kept remaining above partial thru-pitch patterns, and act as the
absorber thickness bias in different patterns. And then, from the best focus offset of thru-pitch
patterns, the iDOF center shifts could be directly corrected and to enlarge uDOF by increasing
the overlap of iDOF. Finally, some negative tone development (NTD) result in line patterns will
be demonstrated as well.
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