Jörg Thiele

Improved manufacturability by OPC based on defocus data

The paper describes the advantages of optical proximity correction (OPC) based on defocus data instead of best focus data. By additionally acepting asymmetric variations of the dimension of different patterns e.g. for an isolated line that can become wider than its nominal width this method can deliver structures much more robust against opens and shorts than in the standard OPC approach which is based on data taken at best process conditions. The differences of both OPC methods are compared based on simulations and checked against experimental data of characteristic IC patterns.

High-accuracy simulation-based optical proximity correction

In times of continuing aggressive shrinking of chip layouts a thorough understanding of the pattern transfer process from layout to silicon is indispensable. We analyzed the most prominent effects limiting the control of this process for a contact layer like process, printing 140nm features of variable length and different proximity using 248nm lithography. Deviations of the photo mask from the ideal layout, in particular mask off-target and corner rounding have been identified as clearly contributing to the printing behavior. In the next step, these deviations from ideal behavior have been incorporated into the optical proximity correction (OPC) modeling process. The degree of accuracy for describing experimental data by simulation, using an OPC model modified in that manner could be increased significantly. Further improvement in modeling the optical imaging process could be accomplished by taking into account lens aberrations of the exposure tool. This suggests a high potential to improve OPC by considering the effects mentioned, delivering a significant contribution to extending the application of OPC techniques beyond current limits.

Verifying high NA polarization OPC treatment on wafer

High NA scanners with adjustable polarization are becoming commercially available. Linear polarization has been shown to significantly improve imaging performance of preferentially oriented lines. Azimuthal and tangential polarization are now becoming commercially available. The latter has less asymmetry in its imaging and can resolve critical features oriented in multiple directions at the same time. Linear y-oriented or vertical polarization was used, since at the time of this work, azimuthal and tangential polarization were not available. Such x- and y-oriented linear polarization could be used in double exposure imaging, for example. Just as for unpolarized imaging, OPC models are required for polarized imaging that are accurate in (a) fitting and predicting experimental CD values, (b) fragmenting layout, and (c) correcting the fragmented layout to target. This paper describes the results of such a first OPC verification loop. Experimental proximity data in X- and Y-orientation were measured. Source polarization and wafer stack thin film effects were included in the empirically fit OPC simulation model. A parallel investigation was undertaken using an unpolarized source. It served as the reference case. Simple test patterns as well product-like 2D layout was treated with the vertically polarized and unpolarized OPC models. A test mask was written and wafer printing results obtained. They demonstrated the validity of the approach and pointed to further OPC model improvements.

OPC with customized asymmetric pupil illumination fill

Sophisticated designs of the pupil illumination fill of scanners and steppers permit considerable improvements of the resolution and the quality of the optical projection for certain critical patterns. However, the mask layout can have quite different requirements for the resolution as well as the shape of the critical patterns in the two spatial directions. For instance, typical DRAM designs have one orientation with much higher requirements than the other orientation. This asymmetry can be accounted for with a corresponding pupil fill that has a reduced symmetry as well. It is for example possible to combine high resolution and high contrast of the most critical pattern in one spatial orientation at the cost of the other orientation. Unfortunately, this leads to an asymmetric source distribution with x-y dependent optical proximity effects. Therefore the transfer of one and the same pattern from the mask to the wafer will differ if this pattern is rotated by 90 degrees. But fortunately, this anisotropic mapping can be compensated by applying an appropriate optical proximity correction (OPC) which is anisotropic as well. In the current work, we measure on silicon the orientation dependent proximity effect for a customized and strongly asymmetric pupil illumination fill design. With this input data, we build a lithography simulation model which is able to reproduce this anisotropy well. We further perform full chip anisotropic OPC and present the actual success of this resolution enhancement technique with various measurement results and printed wafer images. We also discuss the challenges and problems of this method.

Simulation based OPC for contact pattern using 193 nm lithography

For state of the art technologies, rule based optical proximity correction (OPC) together with conventional illumination is commonly used for contact layers, because it is simple to handle and processing times are short. However, as geometries are getting smaller it becomes more difficult to accurately control critical dimension (CD) variations influenced by nearby pattern. This applies in particular for irregularly arranged contact holes. Here simulation based OPC is more effective. We present a procedure for application of simulation based OPC for a 193 nm lithography contact hole layer with rectangular contact holes of different sizes in different proximities, using attenuated phase shift masks. In order to further improve the accuracy of the simulation based OPC process, characteristics of the mask, like mask corner rounding are incorporated in the OPC process. We build an OPC model, use it for OPC processing of DRAM design data and investigate the process window of the printing contacts. The results show an overlapping process window for length and width of isolated and dense small contact holes of different length and width, which is sufficient for volume production.