As the advanced IC device process shrinks to below sub-micron dimensions (90 nm, 65 nm and beyond), the process window (Depth of Focus, DOF) decreases to as low as < 0.2 μm. The impact of defocus on final CD results cannot be ignored any more or be compensated by adjusting exposure energy without side effect because the optical behavior of focus is totally different from that of exposure energy. We have developed an advanced control system to detect and correct for variations in both focus and exposure energy. A library of focus exposure matrix (FEM) models is first set up with pattern profiles of different pitches. The inline photoresist profiles of features of various pitches are then fitted to the database in the FEM models. The exposure and focus with which those features have been processed can be estimated. This approach utilizes information of both resist profile and proximity through focus behaviors and therefore gives more accurate extrapolation of defocus than using profile or proximity alone. The approach can also be used to distinguish process drifts caused by exposure and focus from those caused by other process parameters such as PEB temperature, developing parameters and illumination.
As critical-dimension shrink below 0.13 μm, the SPC (Statistical Process Control) based on CD (Critical Dimension) control in lithography process becomes more difficult. Increasing requirements of a shrinking process window have called on the need for more accurate decision of process window center. However in practical fabrication, we found that systematic error introduced by metrology and/or resist process can significantly impact the process window analysis result. Especially, when the simple polynomial functions are used to fit the lithographic data from focus exposure matrix (FEM), the model will fit these systematic errors rather than filter them out. This will definitely impact the process window analysis and determination of the best process condition. In this paper, we proposed to use a calibrated first principle model to do process window analysis. With this method, the systematic metrology error can be filtered out efficiently and give a more reasonable window analysis result.
In this paper, we evaluate several approaches for proximity matching on a 193nm scanner system such as image contrast tuning, illumination tuning and photoresist tuning. Both experimental and simulation studies are carried out to reveal the differences between approaches. We find that it is very important to determine the root cause of proximity mismatch before attempting proximity matching, and that spectroscopic scatterometry is an excellent tool for OPC tuning