Christof Bodendorf

Impact of measured pupil illumination fill distribution on lithography simulation and OPC models

Increasing miniaturization and decreasing k1 factors impose continuously growing demands on optical lithography. These requirements are reflected in the need for increasingly accurate lithography simulations, which are prerequisite for successful optical proximity correction (OPC) of the mask layout. Therefore, the physical conditions of the lithography tools and their impact on the resulting printed image have to be carefully considered. The illumination distribution in scanners and steppers is commonly simplified by a top-hat (rectangular cross-section) function. The illuminator is therefore assumed to consist of either completely dark or homogeneously bright areas. In this paper, we investigate the effect of using the measured source, which can deviate significantly from a simple top-hat function, on simulation results and OPC treatment. We compare simulations with measurement and show that there are cases where significant improvements occur by using the real source distribution.

Benchmark of numerical versus analytical proximity curve calculations

For the technology development of microlithography various optical simulation tools are established as a planning and development tool. Depending on the application, various numerical approximation schemes are used to tradeoff accuracy versus speed. Determining the correct numerical setting is often a tricky task as it is a compromise between these two contrary properties. In our study, we compare the numerical accuracy of two optical simulators, Solid-E as a representative for simulators for technology development and Mentor Calibre as design-for-manufacturing and optical proximity correction (OPC) tool. Calibre uses a coherent kernel approximation for performing fast simulations. As a measure for the simulation accuracy, we use the root-mean-square error criterion of a linearity curve compared to an analytical reference simulation.

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.