Effects of residual aberrations on line-end shortening in 193-nm lithography

Abstract

This paper extends and further validates the methodology for calibrating 193nm chemically amplified resist models and applying the models to line-end shortening simulation in the presence of image imperfections. SPLAT, an imaging simulator, is used to simulate the light intensity at the bottom of resist film and predict the resulted wafer patterns in the presence of lens aberrations. The mask critical dimensions (CD) were measured to exclude the mask CD error effects. The experiments were conducted at Texas Instruments on a 193nm scanner. The mask CD errors proved a major contribution to isolated-dense line CD bias on the wafer. The lens aberrations were shown to be critical to the choice of optimal imaging location and the through-focus CD variation. By finding the optimal image location and threshold photoacid concentration, this model can predict line CD through focus, pitch and feature size, with a RMS error of 5nm. However, this model is not adequate in predicting the narrow space between line ends due to the poor resist response in very low contrast areas. A variable threshold model based on trajectory dissolution rate assumption is proposed to predict the wafer CD in low contrast areas, which resulted in a RMS error of 24nm. Considering the large SEM measurement noise on 193nm resists, this error is reasonable and sufficient for OPC applications.