Reflection control in hyper-NA immersion lithography

The impact of bottom reflection on critical dimension (CD) processing window is intensively investigated with a simulation using a full diffraction model (FDM) in which the effective reflectivity is calculated from standing wave amplitude. Most importantly, the optical phase shift of the reflection is used as a design criterion and was found to be the primary factor that affects the UV distribution, and, hence, has a strong impact on exposure latitude and depth of focus. Foot exposure (FE) is introduced as a new metric to characterize the phase shift. Some single-layer and dual-layer bottom anti-reflective coating (BARC) designs were implemented with an Exitech MS-193i immersion micro-stepper (NA=1.3) for 45-nm dense lines at the Resist Test Center (RTC) at International SEMATECH, Albany, New York. The experimental results show that FE is closely related to the CD processing window. In contrast to conventional BARC usage, a small amount of substrate reflection with a controlled optical phase shift dramatically improves CD processing window.