The model calibration process, in a resolution enhancement technique (RET) flow, is one of the most
critical steps towards building an accurate OPC recipe. RET simulation platforms use models for predicting
latent images in the wafer due to exposure of different design layouts. Accurate models can precisely
capture the proximity effects for the lithographic process and help RET engineers build the proper recipes
to obtain high yield. To calibrate OPC models, test geometries are created and exposed through the
lithography environment that we want to model, and metrology data are collected for these geometries.
This data is then used to tune or calibrate the model parameters. Metrology tools usually provide critical
dimension (CD) data and not edge placement error (EPE - the displacement between the polygon and resist
edge) data however model calibration requires EPE data for simulation. To work around this problem, only
symmetrical geometries are used since, having this constraint, EPE can be easily extracted from CD measurements.
In real designs, it is more likely to encounter asymmetrical structures as well as complex 2D structures that
cannot easily be made symmetrical, especially when we talk about technology nodes for 65nm and beyond.
The absence of 2D and asymmetric test structures in the calibration process would require models to
interpolate or extrapolate the EPE's for these structures in a real design.
In this paper we present an approach to extract the EPE information from both SEM images and contours
extracted by the metrology tools for structures on test wafers, and directly use them in the calibration of a
55nm poly process. These new EPE structures would now mimic the complexity of real 2D designs. Each
of these structures can be individually weighed according to the data variance. Model accuracy is then
compared to the conventional method of calibration using symmetrical data only. The paper also illustrates
the ability of the new flow to extract more accurate measurement out of wafer data that are more immune to
errors compared to the conventional method.