Strategies for predictive control of chrome stress-induced registration errors

Abstract

The focus of this paper is on the development and implementation of a correction strategy that enables mask manufacturers to maintain the yields at current levels while simultaneously reducing registration errors by several nanometers. An alternate consequence is that yields at current registration specifications are improved. Previous work has shown that one source of image placement error is the chrome stress relief caused by etching. This can cause over 25 nm of distortion from the resist pattern to the final etched chrome pattern. Theoretical and experimental data have shown that the distortion has a radial signature, which can be significantly reduced by traditional magnification correction. If the magnitude of this correction term can be predicted before patterning, the magnification can be implemented as a correction term in the writing process, minimizing registration errors. Studies have shown that the percent clear area of the mask, x-field size, y-field size, and chrome stress are the key parameters that will affect the correction term. Data based on finite element simulations were first fit to these parameters to obtain a predictive curve based upon theory. Experimental reticles were then written to test the theoretical prediction. The predictions were found to coincide well with the experimental data; registration improvements of over 20 nm were observed. The correlation was then applied to a set of production reticles. There was an observable improvement in registration after the correlation was implemented, although less than that seen in the experimental reticles.