The importance of being homogeneous: on the influence of illumination inhomogeneity on AIMS images

Defect disposition and qualification with stepper simulating AIMSTM tools on advanced masks of the 90 nm node and below is key to match the customer's expectations for "defect free" masks, i.e. masks containing only nonprinting design variations. For defect dispositioning usually printability studies are carried out using the same illumination settings at the AIMSTM tool as later on at the steppers in the wafer fab. These studies then establish an AIMSTM criterion (e.g., CD variation or transmission deviation) a structure deviation must not exceed. For ever more advanced technologies the accessible process window gets smaller and thus more and more complex apertures have to be used to allow for a still suitable contrast and reliable printing of the patterns. This results in more time-consuming printability studies and tighter AIMSTM specs. Simulations of the printing of mask defects could potentially help to decrease the amount of time for printability studies and also the time for defect disposition in the production. However, usually simulations in their first approximation do not account for effects such as flare, aberrations or illumination inhomogeneities of the AIMSTM tool. This makes it difficult to derive the AIMSTM criterion by simulations. In this paper we show that a homogeneous aperture illumination is crucial for the image contrast and the defect disposition. We present a method to characterize the pupil illumination and investigate the impact of illumination inhomogeneities on various structures and their orientation employing two different aperture types. The experimental results are compared to simulations with both homogeneous illumination and the real illumination distribution. It turns out that for correct simulation predictions on experimental results it is important to provide the correct illumination distribution to the simulations.