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Traditional approach to model based optical proximity correction method is to collect a set of 1-D and
2-D test pattern data, calibrate a scalar or vector model at constant or variable threshold and modify the
physical layout to obtain the desired layout. Optical proximity corrected layout is obtained by minimizing
the error between the target and the printed image iteratively using a calibrated single model to generate a
simulated print image of mask pattern of variety of field polarity. A similar approach can be extended to
incorporate the final silicon image using a lumped model or tandem photo-resist development and etch
process models. Recently, some have begun to incorporate differing models at specific regions of the
layout.
The basic underlying assumption of a model-based OPC requires one to generate a simulated contour
that provides close approximation of wafer image using a calibrated model. During iterative OPC
procedure, not all of the regions of OPC polygons are simulated. That is, sparse sampling of each polygon
is performed to reduce the number of error calculations required and such calculation points are referred to
as an evaluation site. A careful selection of sampling site must be performed to capture optical proximity
effect and obtain the desired OPC. In this paper, utilization of multiples models to generate contour to
accurately define the 2D pattern locally, and implementation of its models throughout the layout is
presented in order to improve accuracy of variety of contact pattern types present in a layout. Hence, the
basic concept is to apply differing models at localized region and achieve greater OPC accuracy than a
single calibrated model.
In particular, a target layout may contain a contact and bar-type structures for the purpose of device
fabrication process step simplifications. Essentially, two different pattern types need to be OPCed, and in
order to perform model based OPC on such a layout, a model for each contact type is generated separately
using a best-fit adaptive search method of optical illumination conditions, aerial image diffusion parameter
and double Gaussian mask loading terms as a main regression parameters. As it terms out, it is difficult to
generate a single model that calibrates to both the contact and bar-type structures and a distinct shift in
empirically calibrated threshold levels exists, and a preferred method is to generate models suited for
contact and bar-type structures separately in order to improve the model and OPC accuracy. However,
each model type needs to be applied at specific locations of a pattern, and a proper OPC recipe for handling
biasing of each pattern type is needed as well as correction scheme suitable for each pattern type is required.
In this paper, we describe an OPC methodology for merged direct contact layout using a proposed pattern
specific modeling and correction technique, and the experimental results indicate that this methodology provides ADI 3s target skew value of 14 nm and ACI 3σ target skew value of 17 nm on a 60 nm half pitch
node.
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