Dose reduction strategies for low-n PSM absorber using pupil and mask bias optimization at 0.33NA EUV lithography

To maintain manufacturable low levels of defectivity and stochastics, the solutions to sub-0.5k1 patterning at Extreme Ultraviolet Lithography (EUVL) wavelength are dose (throughput) expensive. Subsequently, the lower the k1 imaging, the higher the dose required to ensure sufficient yield for high-volume manufacturing (HVM) using EUV lithography. Over the past decade, many high reflectivity, phase shifting masks (PSM) for EUV lithography have been proposed to pattern images with high aerial image contrast at low exposure doses. In this study, a process has been developed and characterized to produce a test mask with a low-n, low-k absorber material using a leading-edge Multi Beam Mask Writer (MBMW). By using the inherent high contrast of this novel low-n mask absorber, a method to reduce the dose to size (DtS) without compromising the aerial image contrast is proposed at sub-0.5k1 pitches (P32 and P36). Through the application of high contrast PSM in combination with mask bias, ~15% reduction in DtS is experimentally verified. Comparison of simulation and experimentally verified results show that the experimental DtS of low-n PSM is higher than the TaBN mask absorber at constant mask bias which was not predicted. Evaluation of line edge roughness (LER) shows similar LER performance at dose optimized mask bias for low-n versus TaBN mask absorber. The mask writing capability using the MBMW is also discussed using performance parameters such as mask CD-uniformity, linearity and registration.