  This paper proposes a new model for Cluster-tools with two load locks.
Cluster-tools are widely used to automate single wafer processing in
semiconductor industry. The load locks are the entry points into the vacuum of
the Cluster-tool's mainframe. Usually there are two of them available. Each lot
being processed, is dedicated to a single load-lock. Therefore at most two
different lots (with possibly different processing times and qualification) can
be processed simultaneously. This restriction is one of the major potential
bottlenecks.
  Capacity planning is one of the possible applications for the proposed model
and the paper demonstrates the integration into a more general framework that
considers different tool types and different operational modes.
  The paper also generalizes an earlier model that is limited to three
processing chambers. The proposed modeling approach is based on makespan
reductions by parallel processing. It turns out that the performance of the new
approach is similar, when compared to the generalized model for three chambers,
but the new approach outperforms the generalized model for four and more
chambers.
