Manufacturing Bits: June 8

Maskless EUV lithography; selective e-beam resists.


Maskless EUV lithography
At this week’s 2020 EUVL Workshop, KJ Innovation will present more details about its efforts to develop a maskless extreme ultraviolet (EUV) lithography technology.

Still in R&D, KJ Innovation’s maskless EUV technology involves a high-numerical aperture (high-NA) system with 2 million individual write beams. The 0.55 NA technology is targeted for direct-write lithography and EUV mask-writing applications. It’s unclear when the system will appear in the market.

Maskless lithography is different than traditional optical and EUV lithography systems used in today’s production fabs. Both optical and EUV use a photomask to pattern a chip. In the process flow, a chipmaker first designs an IC, which is then translated into a file format. Then, in a photomask facility, a mask is produced based on that format. The mask is a master template for an IC design.

In a fab, the mask as well as a wafer are inserted in a lithography scanner. A photoresist, a light-sensitive material, is applied on the wafer. In operation, the scanner generates light, which is transported through a set of projection optics and the mask in the system. Light then hits the resist, creating patterns on the wafer.

Meanwhile, for years, the industry has been selling lithography systems, which do not require an expensive photomask. This is commonly called direct-write or maskless lithography. Direct-write lithography makes use of an e-beam tool that directly patterns tiny features on a wafer without using a mask.

E-beam systems are also used in different applications. For years, e-beam tools have been used to pattern the features in photomasks. The latest systems use multiple beams to pattern a photomask. Today, IMS sells a multi-beam e-beam tool for photomask writing. NuFlare is also working on the technology.

Meanwhile, KJ Innovation is targeting its technology for both direct-write lithography and mask-writing applications. “We are definitely looking at direct-write litho as well as mask writing. It could be an attractive alternative to e-beam for low-volume wafer patterning, especially for prototyping and process development for EUV high-volume manufacturing,” said Ken Johnson, president of KJ Innovation. “One of the main applications for maskless EUVL will probably be EUV mask writing. Maskless EUVL would not necessarily compete with mask-projection EUVL, certainly not for high-volume manufacturing. On the contrary, it could be an enabler for EUV high-volume manufacturing by lowering the cost and/or improving the quality of EUV masks.”

KJ Innovation’s maskless technology makes use of a source power unit, scanner optics and other components. The system makes use of Adlyte’s laser-produced plasma (LPP) source.

“It uses projection optics like mask-projection EUVL,” explained Johnson. “But the mask is replaced with an array of point-focus EUV beams, which are imaged onto the wafer. The wafer is raster-scanned across the point array as the points are modulated to create a digitally synthesized exposure image. There are about 2 million individual write beams in the current design. The exposure wavelength is 13.5nm, but the system could probably also be adapted for use at 6.7nm.”

The point-focus beams are created by a microlens array. This involves EUV zone-plate lenses formed on a microchannel plate with conical holes for beam transmission. MEMS shutters can be located at the beam foci for individual beam modulation.

“One unique characteristic of this system is that the lenses can be designed to nullify projection system aberrations, making it possible to use relatively simple projection optics with only two projection mirrors. However, the zone-plate lenses exhibit chromatic effects (their optical power increases with wavelength). What’s different in my new design is how it handles the chromatic effects,” he said.

In the previous design, the system used a complicated two-stage microlens design. “The lenses had a high-efficiency ‘blazed’ structure, which would require complex multi-level or grayscale lithography processes,” he said. “In my current design, I use a simple one-stage microlens array with binary-optic zone plates similar to EUV lenses that CXRO has been making for about 20 years. The zone plates are formed with a one-layer lithography process, minimum half-pitch 75nm. The lenses are not achromatic, but their chromatic effects can be neutralized by using a diffractive mirror in the projection optics. The diffractive structure is formed by applying an ion-beam figuring (IBF) process to a standard EUV mirror coating. IBF is routinely used for surface shaping in optics manufacturing, so the mirror manufacturing technology is fairly straightforward.”

What’s next? KJ Innovation is exploring the possibility of developing “holographic” EUV mask-projection lithography, which is not maskless.

Selective e-beam resists
Sci-Tron—a startup that was spun out from the University of Manchester–has introduced a new negative tone e-beam resist with high etch selectivity.

The resist, called nEBL3, is flexible and adaptable for a number of market sectors. The resist can be used for direct-write lithography applications. The resist is also ideal for the production of next-generation EUV photomasks. Current e-beam resists have difficulties in terms of meeting the specifications for EUV masks or are difficult to use.

Based on a modular design, Sci-Tron’s heterometallic resist (HMR) technology enables new lithographic device architectures. The modular approach enables nEBL3 to be used with other solvents and developers. This in turn opens the door for manufacturers to incorporate the negative tone resist into their existing fabrication processes.

It also enables high etch selectivity in structures. “The etch selectivity to silicon achieved is so high that we have been studying 15nm thick films of nEBL3, spun from tert-butyl methyl ether (TBME) for high-resolution plasma etching,” said Scott Lewis, director and senior technical advisor at Sci-Tron.

With the resist, the company has demonstrated etch selectivity of >100:1 at a 15nm half-pitch. “However, we have achieved an etch selectivity of >60:1 even down to 8nm resolution,” Lewis said. “We believe that nEBL3 has the potential to become the benchmark negative tone resist to use for lithographers who need to fabricate narrow, deep features.”

Richard Winpenny, director and chief scientific officer at Sci-Tron, added: “nEBL3 is a production-ready negative tone resist as it stands, yet also a ‘launch-pad’ solution for any manufacturers seeking to leverage this technology in their production processes.”

To date, the company has tested its resists on Raith’s 5000+ e-beam lithography systems as well as their latest 5200 tool. The results are cited above. The work was verified at California Institute of Technology (Caltech) in the Kavli Nanoscience Institute. Sci-Tron is also working with a III-V foundry on prototyping the latest GaN CMOS integrated circuits. The foundry is using the resists to pattern these devices.

In 2014, Sci-Tron was spun out from the University of Manchester. The most recent results were achieved in an ongoing collaboration between Sci-Tron, the University of Manchester and the Kavli Nanoscience Institute at CalTech.

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