Experimental verification of sub-wavelength holographic lithigraphy (SWHL) concept

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Sub-Wavelength Holographic Lithography (SWHL) was introduced some years ago by Nanotech SWHL GmbH as a disruptive and promising method to replace projection photolithography. SWHL is based on principles of wave optics and uses a computer-generated hologram (CGH) as a photomask for both 2D and 3D imaging. To proof the concept of SWHL first for sub-wavelength critical dimensions (CD) and then for non-flat imaging we designed two experimental optical set-ups. Both set-ups use commercially available 442nm He-Cd gas laser. The holographic masks were designed as a set of windows in an opaque chromium layer on a fused silica blank. The imaging in SWHL does not require any projection optics. Thanks to this the optical system includes only the illuminator of the mask. The illuminator design is very simple, with just a few optical elements. To demonstrate an image with sub-wavelength resolution, we use illumination with NA 0.53. For this NA we generated image with CD 250 nm that is 0.56 of the wavelength 442 nm. To demonstrate 3D imaging capability the demonstration lab tool was developed. The tool provides the illumination of holographic mask with NA 0.24. The mask generated a multi-plane image with a depth of 100 μm and the image resolution of 2 μm. We demonstrated both subwavelength and 3D holographic imaging in experiments and prove the concept of SWHL. All the experiments were made as computer simulations first. The comparison of the simulation and experimental results proved the reliability of our software.