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.
Since the 1970s, the cost of photolithography in IC manufacturing has reached nearly 70% of the total production cost. The cost of production lithography equipment has grown almost 160 times reaching ~$80M (193i scanner) and exceeding $200M (EUV scanner). Such high costs have caused a significant market concentration in equipment and chip manufacturing.
Historically, the projection exposure technology was developed on the foundation of geometrical optics. A projection mask was a magnified stencil of the projected pattern. To achieve resolution, extensive development efforts have been made to increase the projection optics numerical aperture (NA), reduce exposure wavelength, minimize distortions of the projection lens and destructive diffraction effects. Complicated and highly costly RETs (OPC, PSM, SMO etc.) are used to suppress distortions in the pattern image induced by diffraction and aberrations of the projection optics.
Sub-Wavelength Holographic Lithography (SWHL) is an unconventional imaging approach in photolithography that utilizes diffraction and interference effects to create high-quality images in photoresist, whilst decreasing exposure steps’ cost, including higher resolution nodes. The ability to construct arbitrary wavefronts opens multiple opportunities like printing on various 3-D surfaces. The SWHL is capable of dramatic reduction of the lithography cost and opens new opportunities in 3D imaging. Two proof-of-concept exposure systems were designed and built to verify the SWHL capabilities: the first for imaging patterns with sub-wavelength resolution, the second for printing on 3D surfaces. The fidelity and computation efficiency of mask synthesis and RET algorithms were verified.
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.
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.
Authors of the report have been developing Sub-Wavelength Holographic Lithography (SWHL) methods of aerial image creation for IC layer topologies for the last several years. Sub-wavelength holographic masks (SWHM) have a number of substantial advantages in comparison with the traditional masks, which are used in projection photo-microlithography. The main advantages: there is no one-to-one correspondence between mask and image elements thus the effect of local mask defects almost completely eliminated [1]; holographic mask may consist of single-tipe elements with typical size many times bigger than projection mask elements [2]; technological methods of image quality optimization can be replaced by virtual routines in the process of the holographic mask calculating, that simplifies mask manufacturing and dramatically reduces the mask cost [3]; imaging via holographic mask does not need the projection lens, that significantly simplifies photolithographic tool and reduces ones cost. Our group developed effective methods of holographic mask synthesis and of aerial images modelling and created software package. This methods and calculation results were verified and reported many times [1-3].
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