Comparison of techniques to measure the point spread function due to scatter and flare in EUV lithography systems

The source of flare in EUVL systems is mostly from the mid-spatial frequency roughness (1 /μm - 1 /mm spatial periods) of mirrors. Due to the challenges in polishing mirrors to a small fraction of the wavelength, flare in EUV lithography tools is expected to be greater than flare in current DUV tools. Even though EUV flare is constant across the field, there can be within-die flare variations due to variations in layout density. Hence, it is expected that to meet the CD control requirements for the 32 nm node, Flare Variation Compensation (FVC), akin to Optical Proximity Correction (OPC) would be required. FVC needs the within-die flare level estimated by convolving the Point Spread Function due to scatter (PSFsc) with the mask layout. Thus, accurate knowledge of the system PSFsc is essential for FVC. Experimental results of the Modulation Transfer Function (MTF) technique to estimate flare and the PSFsc of the Engineering Test Stand (ETS) are presented. It was also determined that due to the nature of the PSFsc in EUVL tools a more accurate measure for flare would be to use the 0.5 μm line as opposed to the current 2 μm line standard for measuring flare on DUVL tools.