Scanner performance is influenced by the quality of its illumination, mechanical and optical elements and the impact of
these factors on the printed wafer. Isolation of the aggregated scanner errors from other sources of error on the printed wafer
is a challenging task since the total error budget of the lithography process consists of many dynamic sources, such as wafer
planarity and film stack properties.
The mask is conceptually part of the scanner optics and integral to the imaging process. Therefore the mask error
contribution to the overall error becomes relevant for any advanced lithography process.
Discrete mask measurement techniques are currently used to create across mask CDU maps. By subtracting these maps from
their final wafer measurement CDU map counterparts, it is possible to assess within certain limitations the real scanner
induced printed errors. The current discrete measurement methods are time consuming and some overlook errors other than
linewidth variations, such as transmission and phase variations, all of which influence the final printed CD variability.
In this paper we present a methodology, which leverages Applied Materials Aera2<sup>tm</sup>mask inspection tool, based on a socalled
IntenCD aerial imaging produces maps by scanning the mask at high speed, offer full mask coverage and accurate
assessment of all mask induced errors simultaneously, making it ideal for mask CDU characterization and scanner
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.
Die-to-Model (D2M) inspection is an innovative approach to running inspection based on a mask design layout data. The
D2M concept takes inspection from the traditional domain of mask pattern to the preferred domain of the wafer aerial
image. To achieve this, D2M transforms the mask layout database into a resist plane aerial image, which in turn is
compared to the aerial image of the mask, captured by the inspection optics.
D2M detection algorithms work similarly to an Aerial D2D (die-to-die) inspection, but instead of comparing a die to
another die it is compared to the aerial image model. D2M is used whenever D2D inspection is not practical (e.g., single
die) or when a validation of mask conformity to design is needed, i.e., for printed pattern fidelity. D2M is of particular
importance for inspection of logic single die masks, where no simplifying assumption of pattern periodicity may be
done. The application can tailor the sensitivity to meet the needs at different locations, such as device area, scribe lines
In this paper we present first test results of the D2M mask inspection application at a mask shop. We describe the
methodology of using D2M, and review the practical aspects of the D2M mask inspection.
The difficulties encountered during lithography of state-of-the-art 2D patterns are
formidable, and originate from the fact that deep sub-wavelength features are being
printed. This results in a practical limit of <i>k</i><sub>1</sub> ≥0.4 as well as a multitude of complex
restrictive design rules, in order to mitigate or minimize lithographic hot spots. An
alternative approach, that is gradually attracting the lithographic community's
attention, restricts the design of critical layers to straight, dense lines (a 1D grid), that
can be relatively easily printed using current lithographic technology. This is then
followed by subsequent, less critical trimming stages to obtain circuit functionality.
Thus, the 1D gridded approach allows hotspot-free, proximity-effect free lithography
of ultra low- <i>k</i><sub>1</sub> features. These advantages must be supported by a stable CD control
mechanism. One of the overriding parameters impacting CDU performance is photo
mask quality. Previous publications have demonstrated that IntenCD<sup>TM</sup> - a novel,
mask-based CDU mapping technology running on Applied Materials' Aera2<sup>TM</sup> aerial
imaging mask inspection tool - is ideally fit for detecting mask-based CDU issues in
1D (L&S) patterned masks for memory production. Owing to the aerial nature of
image formation, IntenCD directly probes the CD as it is printed on the wafer.
In this paper we suggest that IntenCD is naturally fit for detecting mask-based CDU
issues in 1D GDR masks. We then study a novel method of recovering and
quantifying the physical source of printed CDU, using a novel implementation of the
IntenCD technology. We demonstrate that additional, simple measurements, which
can be readily performed on board the Aera2<sup>TM</sup> platform with minimal throughput
penalty, may complement IntenCD and allow a robust estimation of the specific
nature and strength of mask error source, such as pattern width variation or phase
variation, which leads to CDU issues on the printed wafer. We finally discuss the
roles played by IntenCD in advanced GDR mask production, starting with tight
control over mask production process, continuing to mask qualification at mask shop
and ending at in-line wafer CDU correction in fabs.
A new method for accurate CD measurement and precise pattern size extraction from optical images is proposed. The
approach is based on the underlying field theory of optical image generation. The method demonstrates superior
precision compared with traditional edge detection schemes based on the image and not on the field nature of image
creation. The proposed method presents accuracy parallel to that achieved by SEM imaging. Therefore it provides an
alternative to electronic microscopy measurements in certain cases. This new method may be implemented for
applications of accurate mask-CD measurement, as well as for retrieving the mask model from an optical image for a die
to model application.