Model based OPC has been generally used to correct proximity effects down to ~50 nm critical dimensions at
k1 values around 0.3. As design rules shrink and k1 drops below 0.3, however; it is very hard to obtain enough process
window and acceptable MEEF (Mask Error Enhancement Factor) with conventional model based OPC. Recently, ILT
(Inverse Lithography Technology) has been introduced and has demonstrated wider process windows than conventional
OPC. The ILT developed by Luminescent uses level-set methods to find the optimal photo mask layout, which
maximizes the process window subject to mask manufacturing constraints.
We have evaluated performance of ILT for critical dimensions of 55nm, printed under conditions
corresponding to k1 ~ 0.28. Results indicated a larger process window and better pattern fidelity than obtained with
other methods. In this paper, we present the optimization procedures, model calibration and evaluation results for 55 nm
metal and contact layers and discuss the possibilities and the limitations of this new technology.
In the past when design rule is not tight, CD-based OPC modeling was acceptable. But shrinkage of design rule
eventually led to small process window, which in part increased MEEF(Mask Error Enhancement Factor). Hence, data
for OPC modeling have also become more complex and diverse in order to characterize the critical OPC models. The
number of measurement points for OPC model evaluation has increased to several hundred points per layer, and
metrology requests for realizing pattern shapes on the wafer are no longer simple one-dimensional measurements.
Traditional CD-based OPC modeling is based on 1 dimensional parameter fitting and has limited information. Due to
this reason, the accuracy of the model has intrinsic limitations. Recently, development of modeling methodology
resulted in SEM image calibration. SEM image calibration use SEM image to calibrate large volume 2 dimensional
information. SEM image calibration is based on real SEM image which has several thousands of CD information. It
needs only SEM images instead of several hundred CD data, so data feedback is more easy. But this approach makes it
difficult to achieve confidential level for predictability because SEM image is restricted to local region. And modeling
accuracy is highly dependent on SEM image quality and local position.
In this paper, we propose SEM image calibration method that feeds back SEM image calibrated model to
model-based verification. By using this method, modeling accuracy is increased and better post OPC verification can be
made. We will discuss the application result on sub-60nm device and the feasibility of this approach.
New generation DRAM devices such as high speed Graphic DRAMs demand smaller size transistors and very precise CD control. However, the application of very high NA and aggressive Resolution Enhancement Techniques (RETs) increases Isolated-dense bias and leaves very small process window for isolated transistor patterns. It implies that a very aggressive and also very delicate OPC work is required for these new generation devices.
A novel measurement system which can compare CD SEM image with CAD data has been developed and we were able to systematically calibrate OPC modeling and verify modeling accuracy by connecting this measurement system with OPC tools. In this paper, the functions of the novel measurement system are presented and the application to the OPC calibration and OPC accuracy verification are presented. This novel measurement system was very useful for 2D model calibration. We were able to enhance OPC accuracy through this systematic OPC calibration and verification methodology.