Manufacturing Bits: July 20

Interference EUV lithography
ESOL has developed a standalone interference extreme ultraviolet (EUV) lithography tool for use in R&D applications.

The system, called EMiLE (EUV Micro-interference Lithography Equipment), is primary used to speed up the development of EUV photoresists and related wafer processes.

The system is different than ASML’s EUV lithography scanners, which are used in the production of chips at the 7nm node and beyond. Incorporating a 0.33 numerical aperture lens with a 13.5nm wavelength, ASML’s EUV scanners have 13nm resolutions with a throughput from 135 to 145 wafers per hour.

Used for R&D purposes, ESOL’s EMiLE system is a standalone exposure machine. It is designed as a compact table-top unit with an EUV source. “EUV interference lithography (IL) equipment can expose only simple lines and spaces, and it is different from ASML’s EUV scanners,” explained BG Kim, CEO of ESOL. “EUV IL does not use a mask and cannot expose a random pattern. Only lines and spaces can be exposed. This is still meaningful because the same EUV resolution (16nm half-pitch) is possible.”

Based on a high-harmonic EUV source, ESOL’s EUV IL system creates a pattern through the diffraction and interference phenomenon of EUV light passing through a grating. There is no need for a mask or expensive mirrors.

Imec and Paul Scherrer Institute have also developed interference EUV lithography equipment. ESOL is said to be the world’s first standalone EUV IL unit.

Regardless, interference EUV lithography can solve several problems. “One of the biggest challenges of EUV lithography is EUV photoresists,” Kim said. “EUV photoresists with better resolution at lower dose is strongly required in the industry. The stochastic problem is caused by EUV resists.”

There are other issues. Suppliers of EUV resists develop and synthesize these materials. Then, the resists are tested using an EUV scanner. “Many cycles of testing are required to test the actual performance of the resists,” Kim said. “But access to the EUV scanners is sometimes difficult, so testing is possible only a limited number of times.”

That’s where EMiLE fits in. The system can help develop and test EUV resists. It can speed up the R&D process. “It should be noted that most of the photoresist development period requires only line/space patterning,” Kim said. “(Using IL exposure equipment), development can be accelerated at least several dozen times. EUV IL standalone equipment provides a good cost-of-ownership. This is definitely an innovative method in material development.”

There are other advantages. “In addition, the system can not only pattern line/space, but also contact hole arrays. One-dimensional and two-dimensional resist properties can be tested. This is only possible with grating IL,” Kim said. “Since the table-top EUV light source usually requires exposure for a long time, many people expected that the resist line and space would not be obtained. However, ESOL solved this problem with vibration compensation technology. The first results of ESOL are noteworthy, and it is possible to obtain equipment performance that is ready to be commercialized.”

Better EUV microscopy
The Advanced Research Center for Nanolithography (ARCNL) and Vrije Universiteit Amsterdam have developed a new class of diffractive optical elements that paves the way towards more widespread use of EUV microscopy.

Researchers have demonstrated multi-spectral ptychography using structured EUV beams. Generally, ptychography is a promising lensless, X-ray coherent imaging technique. In a system, this technique generates X-ray images of a sample. This is done by diffracting or scattering a beam on a sample. The beam bounces off the sample and hits a detector. The data captured by the detector has the information required to reconstruct high-resolution images of the sample or inside the sample.

Ptychography can also be applied to today’s EUV microscopes. Today’s EUV tabletop microscopes are promising diagnostic tools for a host of applications, but there are challenges.

“A very practical problem with using EUV light for imaging purposes is that almost every material on earth absorbs most of the radiation. Therefore, we cannot use lenses to focus EUV light,” said ARCNL group leader Stefan Witte.

There is a solution, though. “We can use diffraction. If you send light through an object with slits, it will bend. If the slits are arranged the right way, it is possible to focus the radiation, just like you would focus visible light with a lens,” Witte said.

EUV light can be diffracted using a Fresnel zoneplate (ZP). This zoneplate resembles a disc with a circular pattern of slits. In operation, the plate diffracts the light, but there are still issues.

“With a conventional zoneplate, this results in different focus points for each wavelength in the beam, but we can only use one of them without having to move the sample. Moreover, it is impossible to collect spectral data of a sample when you only send one wavelength of light through it. The material properties of the sample that we could unveil with EUV spectroscopy thus stay hidden,” Witte said.

Researchers found a solution to overcome these problems–they developed “imperfect” zoneplates. “In this work, we propose the concept of spatial entropy minimization as a computational design principle for both mono- and polychromatic focusing optics. We show that spatial entropy minimization yields conventional ZPs for monochromatic radiation. For polychromatic radiation, we observe a previously unexplored class of diffractive optical elements, allowing for balanced spectral efficiency,” said Lars Loetgering, a researcher from the ARCNL, in Optica, a technology journal.

“This new type of diffractive optical elements is not only paving the way towards widespread use of tabletop EUV microscopy, but we can also use it to take a step back and try to make our EUV sources more efficient,” Witte said. “We are looking for the ideal combination of light and diffraction, which can be different depending on the information you are searching for. The technique is currently limited by the efficiency of the sources and the restriction to single wavelength radiation. There is still a lot of work to be done, but with our approach I expect we can optimize the technique further so that it can be used in metrology or materials science. For example, researchers who are now dependent on large synchrotron facilities will be able to do their experiments in their own lab with a tabletop EUV microscope.”