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.
This paper describes a method combining rule-based shot dose assignment with model-based shot size correction. This
combination proves to be very efficient in correcting mask linearity errors while also improving dose edge slope of small
Shot dose assignment is based on tables assigning certain dose levels to a range of feature sizes. The dose to feature size
assignment is derived from mask measurements in such a way that shape corrections are kept to a minimum. For
example, if a 50nm drawn line on mask results in a 45nm chrome line using nominal dose, a dose level is chosen which
is closest to getting the line back on target. Since CD non-linearity is different for lines, line-ends and contacts, different
tables are generated for the different shape categories.
The actual dose assignment is done via DRC rules in a pre-processing step before executing the shape correction in the
MPC engine. Dose assignment to line ends can be restricted to critical line/space dimensions since it might not be
required for all line ends. In addition, adding dose assignment to a wide range of line ends might increase shot count
which is undesirable. The dose assignment algorithm is very flexible and can be adjusted based on the type of layer and
the best balance between accuracy and shot count. These methods can be optimized for the number of dose levels
available for specific mask writers.
The MPC engine now needs to be able to handle different dose levels and requires a model which accurately predicts
mask shapes at all dose levels used. The calibration of such a model is described in a separate paper .
In summary this paper presents an efficient method for combining rule-based VSB shot dose assignment with modelbased
shape corrections in MPC. This method expands the printability of small features sizes without the need for
increasing the base dose of the e-beam writer which reduces backscattering and increases the lifetime of the electron gun
of the writer.