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MEEF, or Mask Error Enhancement Factor, is simply defined as the ratio of the change in printed wafer
feature width to the change in mask feature width scaled to wafer level. It is important in chip
manufacturing that leads to the amplification of mask errors, creating challenges with both achieving
dimensional control tolerances and ensuring defect free masks, as measured by on-wafer image quality. As
lithographic imaging continues to be stressed, using lower and lower k1 factor resolution enhancement
techniques, the high MEEF areas present on advanced optical masks creates an environment where the
need for increased mask defect sensitivity in high-MEEF areas becomes more and more critical.
There are multiple approaches to mask inspection that may or may not provide enough sensitivity to detect
all wafer-printable defects; the challenge in the application of these techniques is simultaneously
maintaining an acceptable level of mask inspectability. The higher the MEEF, the harder the challenge will
be to achieve and appropriate level of sensitivity while maintaining inspectability…and to do so on the
geometries that matter.
The predominant photomask fabrication inspection approach in use today compares the features on the
reticle directly with the design database using high-NA optics. This approach has the ability to detect small
defects, however, when inspecting aggressive OPC, it can lead to the over-detection of inconsequential, or
nuisance defects. To minimize these nuisance detections, changing the sensitivity of the inspection can
improve the inspectability of a mask inspected in high-NA mode, however, it leads to the inability to detect
subtle, yet wafer-printable defects in High-MEEF geometry, due to the fact that this ‘desense’ must be
applied globally. There are also ‘lithography-emulating’ approaches to inspection that use various means to
provide high defect sensitivity and the ability to tolerate inconsequential, non-printing defects by using
scanner-like conditions to determine which defects are wafer printable. This inspection technique is
commonly referred to as being ‘lithography plane’ or ‘litho plane,’ since it’s assessing the mask quality
based on how the mask appears to the imaging optics during use, as proposed to traditional ‘reticle plane’
inspection which is comparing the mask only with its target design.
Regardless of how the defects are detected, the real question is when should they be detected? For larger
technology nodes, defects are considered ‘statistical risks’…i.e., first they have to occur, and then they
have to fall in high-MEEF areas in order to be of concern, and be below the detection limits of traditional
reticle-plane inspection. In short, the ‘perfect storm’ has to happen in order to miss printable defects using
well-optimized traditional inspection approaches. The introduction of lithographic inspection techniques
has revealed this statistical game is a much higher risk than originally estimated, in that very subtle waferprintable
CD errors typically fall into the desense band for traditional reticle plane inspection. Because printability is largely influenced by MEEF, designs with high-MEEF values are at greater risk of traditional
inspection missing printable CD errors. The question is… how high is high… and at what MEEF is optical
inspection at the reticle plane sufficient? This paper will provide evaluation results for both reticle-plane
and litho-plane inspections as they pertain to varying degrees of MEEF. A newly designed high-MEEF
programmed defect test mask, named VAMPIRE, will be introduced. This test mask is based on 7 nm node
technology and contains intentionally varying degrees of MEEF as well as a variety of programmed defects
in high-MEEF environments…all of which have been verified for defect lithographic significance on a
Zeiss AIMS system.
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