A computational method for optimal application specific lens control in microlithography

Application specific aberration as a result of localized heating of lens elements during exposure has become more significant in recent years due to increasing low k1 applications. Modern scanners are equipped with sophisticated lens manipulators that are optimized and controlled by scanner software in real time to reduce this aberration. Advanced lens control options can even optimize lens manipulators to achieve better process window and overlay performance for a given application. This is accomplished by including litho metrics as part of the lens optimization process. Litho metrics refer to any lithographic properties of interest (i.e., CD variation, image shift, etc...) that are sensitive to lens aberrations. But, there are challenges that prevent effective use of litho metrics in practice. There are often a large number of critical device features that need monitoring and the associated litho metrics (e.g., CD) generally show strong non-linear response to Zernikes. These issues greatly complicate the lens control algorithm, making real-time lens optimization difficult. We have developed a computational method to address these issues. It transforms the complex physical litho metrics into a compact set of linearized "virtual" litho metrics, ranked by their importance to process window. These new litho metrics can be readily used by the existing scanner software for lens optimization. Both simulations and experiments showed that the litho metrics generated by this method improved aberration control.