Holographic EUV lithography achieves 40 nm features without projection optics
This paper shows that a lensless holographic approach can print arbitrary nanoscale patterns at the extreme ultraviolet (EUV) wavelength of 13.5 nm with critical dimensions down to 40 nm. Instead of the complex and expensive multi-mirror projection optics used in commercial EUV scanners, the researchers use a single computer-designed holographic mask that diffracts coherent EUV light to form the desired image on a wafer.
The team developed an inverse-design framework for computer-generated holograms (CGHs). Inverse design means computing the mask pattern directly from the desired target layout. Their optical model captures the main three-dimensional diffraction effects of the mask by treating the mask response as a shift-invariant convolution and then propagating the field to the wafer using angular-spectrum methods. The computed masks were fabricated by direct-write electron-beam lithography in hydrogen silsesquioxane (HSQ) on an 80 nm silicon-nitride membrane, with a 200 nm HSQ absorber layer.
Exposures were carried out with coherent EUV light from a synchrotron beamline (bandwidth ~4% full width at half maximum). The masks were transmissive, with the HSQ layer transmitting about 13.5% of the 13.5 nm light. Using HSQ also as the imaging resist on the wafer, the authors report printed, non-periodic curvilinear patterns with features as small as 40 nm. In their setup, exposure times were about 10–20 seconds, and simulations indicate aerial-image contrast of roughly 40% for 80 nm pitch / 40 nm features and up to 50% for larger pitches.
Why this matters: interference-based EUV methods are simple but can only make periodic patterns. Projection EUV scanners can print arbitrary layouts but need expensive optics. Holographic EUV lithography (EUV-HL) combines the lensless simplicity of interference lithography with the freedom to make arbitrary, non-periodic designs at sub-50 nm scales. The authors also argue the method provides a route toward patterning at beyond-EUV (BEUV) wavelengths, where interference methods do not apply.