SWHL Nanotech team have implemented effective algorithms and developed scalable software allowing to synthesize holographic masks for various lithography applications including MEMS, MOEMS and high-end IC production. Most of the technical problems of state-of-art projection photolithography such as 3D-imaging, quality optimization were stated and solved as a completely numerical problems in the case of holographic lithography approach. Nanotech SWHL team developed effective algorithms of the holographic mask synthesis based on FFT with the complexity of 𝑂(𝑁 𝑙𝑛 𝑁), which allowed to synthesize holographic masks for any IC layer. We developed the continuous phase-shifting optimization method based on WFS, DFS and gradient descent, in which a hologram is synthesized not for the original pattern, but for a pattern with altered amplitude and phase distribution. Like in other RET, the holographic mask synthesized for the properly altered pattern provides a much better-quality image of the original pattern. Thus, today it is possible to use modern computing clusters for the synthesis of holographic masks and to implement them in inexpensive and sustainable devices for holographic photolithography.
We propose a scheme of maskless holography as a base of a novel lithographic technic. Maskless schemes based on reflective SLMs with planar and non-planar layouts are considered. Several effective methods of phase hologram synthesis adapted to the layouts are also introduced. These methods are based on the holographic lithography approach and calculation algorithms that have been developed by Nanotech SWHL. The aim of the work is to find the proper scheme and SLM characteristics, e.g. micromirror size, flatness, dynamic stability, etc. The non-flat optical schemes based on array of reflective SLMs will allow to achieve high NA up to 0.9 without immersion. That investigation allows to design a photolithographic tool with high diffraction efficiency and high-end capabilities.
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.