Imaging enhancement (low k1 imaging) in EUV lithography: current status and future resolution enhancement techniques

With the introduction of the NXE:3400 scanner, ASML has brought EUV lithography to high-volume manufacturing (HVM) at the 7-nm logic node and beyond. In this presentation, we will briefly summarize the imaging performance results obtained from the latest NXE:3400 system that is characterized by a numerical aperture (NA) of 0.33 and a pupil-fill ratio (PFR) of 0.2. Then we will discuss enhancement techniques which can be considered to push EUV lithography to finer resolution and lower k1 values. We will do this by first looking back over the last two decades at the enhancement techniques considered and used for low k1 imaging in KrF, ArF and ArFi lithographies and discussing their applicability for EUV lithography. We also will look into the fundamental differences between KrF/ArF and EUV, most notably concerning resist and mask. Both have an impact on the effective NILS. Two main pillars for both DUV and EUV NILS or imaging enhancement are the mask absorber embodiment and the illumination pupil characteristics. We will summarize our findings on mask optimization, which has been carried out by studying EUV lithography at diffraction level. We will show by simulations and experiments how M3D effects (contrast loss, best focus per feature) can be mitigated by either advanced mask embodiments or advanced illumination schemes.