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Shrinking feature sizes and the need for tighter CD (Critical Dimension) control require the introduction of new
technologies in mask making processes. One of those methods is the dose assignment of individual shots on VSB
(Variable Shaped Beam) mask writers to compensate CD non-linearity effects and improve dose edge slope. Using
increased dose levels only for most critical features, generally only for the smallest CDs on a mask, the change in mask
write time is minimal while the increase in image quality can be significant. However, this technology requires accurate
modeling of the mask effects, especially the CD/dose dependencies. This paper describes a mask model calibration flow
for Mask Process Correction (MPC) applications with shot dose assignment.
The first step in the calibration flow is the selection of appropriate test structures. For this work, a combination of linespace
patterns as well as a series of contact patterns are used for calibration. Features sizes vary from 34 nm up to
several micrometers in order to capture a wide range of CDs and pattern densities. After mask measurements are
completed the results are carefully analyzed and measurements very close to the process window limitation and outliers
are removed from the data set.
One key finding in this study is that by including patterns exposed at various dose levels the simulated contours of the
calibrated model very well match the SEM contours even if the calibration was based entirely on gauge based CD
values. In the calibration example shown in this paper, only 1D line and space measurements as well as 1D contact
measurements are used for calibration. However, those measurements include patterns exposed at dose levels between
75% and 150% of the nominal dose. The best model achieved in this study uses 2 e-beam kernels and 4 kernels for the
simulation of development and etch effects. The model error RMS on a large range of CD down to 34 nm line CD is
0.71 nm.
The calibrated model is then used to generate 2D contours for line ends, space ends and contacts and those contours are
compared to SEM images. For all patterns, including those very close to the resolution limit, very good contour overlay
is achieved. It appears that by including the various dose levels in the calibration a very good separation of the e-beam
model components from the etch components is possible and that this also results in very accurate 2D model quality.
In conclusion, very accurate mask model calibration is achieved for mask processes using shot dose assignment.
Standard test patterns can be used for calibration if they include the dose variations intended for correction.
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