Imaging properties of the extreme ultraviolet mask

Abstract

The reflective mask used in extreme ultraviolet lithography is based on a multilayer stack reflector, overcoated with a suitable absorber. We show that in order to predict correctly the reflected field it is necessary to describe in detail the diffraction and multiple reflection processes that the propagating field undergoes in the multilayer stack. The finite thickness of the absorber causes the incoming field to diffract before being incident on the multilayer stack and is reflected many times in the stack before re-emerging. The fact that the effective reflecting surface of the multilayer stack is below its physical surface, adds complexity to the reflection process. An absorber line would thus appear wider and a space narrower, i.e., a diffraction bias is introduced in this process. If the incoming field is not incident normally, an asymmetry is observed in the reflected field that is angle dependent. Thus, off-axis illumination results in both an imaging bias and a feature displacement due to the image asymmetry. These effects are highly predictable and can be taken into account when writing the mask pattern.