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As the semiconductor industry moves to the 45nm node and beyond, the tolerable
lithography process window significantly shrinks due to the combined use of high NA
and low k1 factor. This is exacerbated by the fact that the usable depth of focus at 45nm
node for critical layer is 200nm or less. Traditional Optical Proximity Correction (OPC)
only computes the optimal pattern layout to optimize its patterning at nominal process
condition (nominal defocus and nominal exposure dose) according to an OPC model
calibrated at this nominal condition, and this may put the post-OPC layout at nonnegligible
patterning risk due to the inevitable process variation (defocus and dose
variations). With a little sacrifice at the nominal condition, process variation aware OPC
can greatly enhance the robustness of post-OPC layout patterning in the presence of
defocus and dose variation. There is also an increasing demand for through process
window lithography verification for post-OPC circuit layout. The corner stone for
successful process variation aware OPC and lithography verification is an accurately
calibrated continuous process window model which is a continuous function of defocus
and dose. This calibrated model needs to be able to interpolate and extrapolate in the
usable process window. Based on Synopsys' OPC modeling software package ProGen,
we developed and implemented a novel methodology for continuous process window
(PW) model, which has two continuous adjustable process parameters: defocus and dose.
The calibration of this continuous PW model was performed in a single calibration
process using silicon measurement at nominal condition and off-focus-off-dose
conditions which are sparsely sampled within the measured entire focus exposure matrix
(FEM). The silicon data at the off-focus-off-dose conditions not used for model
calibration was utilized to validate the accuracy and stability of PW model during model
interpolation and extrapolation. We demonstrated this novel continuous PW modeling
approach can achieve very good performance both at nominal condition and at
interpolated or extrapolated off-focus-off-dose conditions.
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