Measuring layer-specific depth-of-focus requirements

Abstract

As the Rayleigh equations already tell us, improvements in imaging resolution often come at the price of a depth-offocus loss. Often we balance the resolution versus DoF dilemma without regard of the imaging layers location in the overall film stack. E.g. often several via or metal layers are processed with the same optical settings despite facing different amount of depth-of-focus requirements. In actuality, however, substrate induced focus variation can vary greatly from layers at the bottom of a film stack to the layers higher up in the film stack. In the age of super-low k1 lithography this variance needs to be taken into account on a layer specific basis when evaluating the resolution versus DoF tradeoff. We have studied substrate induced focus variation for a 45nm technology test-site as function of film stack sequence and spatial frequency, combining various measurement techniques into an overall topography spectrum. These techniques include data extraction from the exposure tools optical leveling sensor, a mechanical air gauge to calibrate the former and interferometric profiling tools. As a result, we can quantify our DoF requirement for a given layer and product and use this information to optimize our process design on a layer-by-layer basis. This work was performed by the Research Alliance Teams at various IBM Research and Development Facilities.