Manufacturing Bits: Aug. 27

Holographic lithography; EUV storage ring; immersion printing.


Holographic lithography
Switzerland’s Nanotech SWHL GmbH has come out of stealth mode and disclosed its initial technology—a holographic lithography system.

Founded in 2015, Nanotech SWHL has developed a sub-wavelength holographic lithography system that generates and prints 3D images on surfaces with one mask at one exposure. Still in R&D, the system is initially targeted for advanced packaging, MEMS and sensors. In the future, it is also said to be capable of patterning devices at 7nm and below.

If it works, the technology promises to reduce the cost-of-ownership for lithography. It is said to have a 4x lower cost at the same resolution and throughput than conventional systems.

The technology is different than conventional lithography tools, which use light sources and optics to print tiny features on a wafer. It also makes use of a photomask, which is the template of an IC design.

Holographic lithography is a different concept. A hologram is an image, which looks like a three-dimensional shape. It uses laser light for illuminating the 3D image. In simple terms, holographic images appear following two phases–recording and reconstruction. An object and a reference wave are recorded on photosensitive plate. Then, the plate is illuminated with a reconstruction wave.

Based on roughly the same idea, Nanotech SWHL has developed a holographic stepper and holographic masks. The company is quick to point out that it’s not developing interferential lithography, which creates periodic images on a surface.

The holographic stepper produces any image you need. The system doesn’t use a projection lens. Instead, the system consists of an illuminator with three simple lenses and a holographic mask.

The technology is based on the principles of wave optics. It also uses diffraction to create images. Sub-wavelength holographic lithography “uses coherent light that diffracts going through a holographic mask, and then interferes on a wafer with photoresists, thus creating the desired image,” according to a white paper from Nanotech SWHL.

The holographic mask combines the functions of a projection lens and the mask itself. “The holographic mask is a diffractive optical element (DOE), i.e., a transparent plate on which a diffraction microstructure is fabricated, changing the amplitude and/or phase of the incident electromagnetic wave in the specified way. The pattern of the diffraction microstructure results from the corresponding synthesis, so the holographic mask is a computer-generated hologram (CGH),” according to the paper.

In the lab, the R&D holographic stepper has reconstructed an image with CDs at 250nm. The image was generated by a reconstruction wave with NA of 0.53 at wavelength 441.6nm.

In operation, the holographic stepper generates a light beam via a laser. This is guided by a system of mirrors. Then, the light moves into the beam expander, which consists of a micro objective, a pinhole and a parabolic mirror.

“The focal lengths of the micro objective to the focal length of the parabolic mirror refers to 1:50. The Gaussian beam is expanded to illuminate the pupil of the main lens with the intensity at the edge of the aperture of the holographic mask at 20% level of the intensity in the center. A holographic mask can be adapted to work with the uneven illumination,” according to the paper. “The extended beam reflects into the main lens by using the flat mirror. The main lens forms a converging beam with near to spherical wavefront. The converging beam illuminates the holographic mask, and thus reconstructs an image. The image can be either exposed on a silicon wafer coated with a photoresist or captured by a CMOS camera in real time.”

The company has devised a way to synthesize holographic masks into 3D images, such as an array of cylinders and microlenses with varying diameters. This in turn could be used in patterning MEMS structures, microfluidic systems and others.

Nanotech SWHL’s development partners include EMPA, CSCS, Fraunhofer, ENAS and Venture Kick. Nanotech SWHL is seeking Series A funding to complete its holographic tool, and to fund a high-end development program.

EUV storage ring
A research group has provided an update on its ongoing efforts to develop a storage ring technology that may one day be used as a power source for extreme ultraviolet (EUV) lithography.

At the Metrology Light Source (MLS) in Berlin, the group has demonstrated a “proof-of-principle” test using a technology called steady-state microbunching (SSMB). The experiment was conducted in collaboration with Tsinghua University and MLS.

The results are promising for SSMB in the field of EUV. Today’s EUV lithography scanners from ASML make use of an integrated power source. The company’s latest system has a 246-watt power source with 13nm resolutions. The tool has a throughput of 170 wafers an hour (wph).

Long term, the industry wants higher EUV power sources. ASML is developing these systems in R&D, while others are exploring the use of giant storage rings to power the EUV scanner.

In 2014, one group devised a EUV storage concept based on SSMB. In theory, an SSMB storage ring for EUV would be 50 meters in circumference and operate at 400-MeV. A ring could consist of two to six EUV scanners as needed. Each EUV tool could produce 1-kW of power.

“The basic idea of SSMB is to manipulate the beam’s dynamics in a storage ring so that its distribution is not the conventional Gaussian, but microbunched,” said Alex Chao, who is spearheading the research. “The whole concept of SSMB lies in the invention of a way to make the beam microbunched and stay microbunched in the turn-by-turn environment of a storage ring.”

Chao, who was a professor of physics at SLAC at Stanford University, retired this year.

In 2017, Chao and others began to collaborate with Tsinghua and MLS. MLS is located near the BESSY II storage ring facility in Berlin-Adlershof, Germany. It is run by the Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute.

Now, the group has demonstrated the first proof-of-principle test of the SSMB. It was carried out in an experiment at a small storage ring at MLS. The experiment is a collaboration between Tsinghua and MLS.

“It uses MLS as a testbed for the first SSMB principle. A laser-induced microbunched electron beam is made to circulate around the ring,” Chao said. “The main result is that we successfully detected the laser radiation in the next revolution, establishing the first step toward the SSMB mechanism.”

There is one caveat. “The proof-of-principle test did not use EUV. The radiation was at 1-micron radiation. So we are not claiming EUV was available. But stay tuned,” said Chao. “We consider this a critical step for SSMB. The signals are preliminary. It is expected that frequency filtering will enhance the signal-to-background by a large factor. The experiment is continuing at the MLS.”

SSMB signals Source: Alex Chao

More signals! Source: Alex Chao

Immersion printing
The Singapore University of Technology and Design has developed a 3D printing method to fabricate 3D porous models in one step.

The technology, called immersion precipitation 3D printing (ip3DP), involves specialized inks containing polymers. Using a 3D printer, the inks are printed in a bath of a non-solvent. Then, the printed ink solidifies via immersion precipitation. This in turn generates porous structures at micro-to-nano scales.

“This work is the first demonstration of three-dimensionally controlled immersion precipitation based on digitally controlled depositions of materials,” said Rahul Karyappa, a researcher.

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