Manufacturing Bits: May 24

Microbunching EUV
Researchers at the SLAC National Accelerator Laboratory have provided a status report on its ongoing efforts to develop a steady-state microbunching (SSMB) technology.

SSMB is a technology used within a storage ring, which is a large-scale, circular particle accelerator. An SSMB mechanism produces a high-power radiation source within the ring. This, in turn, could enable a high-power storage ring source for use in extreme ultraviolet (EUV) lithography. In theory, the technology could enable a kilowatt EUV source at 13.5nm wavelengths.

Others have come up with similar ways to generate high-power EUV sources in a storage ring, such as free-electron lasers (FELs) and traditional synchrotrons.

For example, some have proposed FELs in a storage ring for EUV sources. FELs have a high peak power, but they have a low average power, according a recent presentation from Alex Chao and Daniel Ratner, who are both researchers at SLAC.

In comparison, a traditional synchrotron storage ring has high repetition rates. But the electrons all radiate individually, Chao and Ratner said.

So, the solution to the problem is to find high-power coherent radiation with high peak power and a repetition rate. The answer? SSMB.

“The SSMB microbunching mechanism is the same as a conventional storage ring except that the microwave RF system is replaced by a modulator system consisting of an infrared seed laser of wavelength λm and an undulator,” Chao said in a recent presentation. “The beam must be microbunched. The microbunched beam must be in steady state. It maintains the microbunched state on a turn-by-turn basis. This combination leads to steady-state microbunching.”

Two years ago, researchers from SLAC proposed the idea of using SSMB inside the SPEAR3. The Stanford Positron Electron Accelerating Ring (SPEAR) is a colliding-beam storage ring for use in particle physics research. SPEAR consists of a single ring, which is about 80 meters in diameter. SPEAR3, a next-generation system, is a 3-GeV storage ring.

SLAC researchers are moving away from using SPEAR3. Now, instead of using SPEAR3, researchers from SLAC have come up with another plan for SSMB. “Its design is evolving towards a much smaller ring,” Chao said in an e-mail. “Instead of a 200 meter circumference like SPEAR, it is now 50 meters. Instead of 1-GeV, it is now 400-MeV. The result is that we need only a 1-kW seed laser.

“The EUV power remains at 1 kilowatt per tool. It also reduces the cost to about 1% of the FEL EUV source,” he said. “The same small ring can also be used to produce multi-kilowatt IR or DUV radiations by changing the seed laser and the undulators.”

What’s next? “Proof of principle tests are being negotiated with a few European laboratories with small electron storage rings,” he said.

But as before, the problem is money. “Since we don’t have dedicated funding, the progress is slow,” he said.

Still, the industry should take a hard look at the technology. Needless to say, EUV lithography is delayed. Chipmakers hope to insert EUV at the 7nm node, but that’s not a given. As before, the big problem is the EUV light source. So far, the source can’t generate enough power to enable the required throughput for EUV in mass production.

Polishing X-ray mirrors
For some time, a consortium from Europe has been building a giant free-electron laser facility.

The X-ray laser research facility, called the European XFEL, is under construction in the Hamburg area of Germany. When the facility is completed in 2017, the laser will generate X-ray flashes at 27,000 times per second, or roughly a billion times higher than the best conventional X-ray radiation sources.

Recently, a mirror was delivered to the European XFEL. It is the first of several of its kind needed for the system. Mirrors of this series will be used to deflect the X-rays by up to a few tenths of a degree into the European XFEL’s six scientific instruments in its underground experiment hall.

The mirror is 95cm long and 5.2cm wide. The mirror is flat and does not deviate from its surface quality by more than one nanometer. The reflective mirror is made from a single crystal of silicon.

A scientist inspects a mirror for the XFEL project, which does not deviate from a perfect surface by more than a billionth of a meter. (Source: European XFEL)

To polish the mirror, the group made use of a polishing technology from JTEC. This involves a pressurized fluid bath capable of stripping atom-thick layers off of the crystal. The polishing technique alone took nearly a year.

“When we first started working on these optics, we were looking for something that simply didn’t exist at anywhere near this precision,” said Harald Sinn, who leads the European XFEL X-Ray Optics group. “Now we have the first ever mirror at this extreme specification.”

Soft X-Rays
Japan’s Riken has upgraded its X-ray free electron laser system, adding a soft X-ray laser light capability to the mix.

X-ray lasers are tools for analyzing tiny substances. Soft X-ray lasers are ideal for analyzing catalysts and purifying gases. In theory, the technology can also be used as a source for EUV lithography.

Meanwhile, Riken has upgraded its Riken SPring-8 Center (RSC). RSC offers both an X-ray free electron laser (SACLA facility) and a Synchrotron Radiation (SPring-8 facility) at the same location.

At one time, the SACLA facility had three beamlines for researchers. Two provided hard X-rays, while the other was for soft X-rays. But for various reasons, the soft X-ray laser was often off-line.

Suddenly, there is demand for the soft X-ray laser. To bring this technology back to the mix, Riken made use of an older 60 meter laser. It was actually the prototype of the current 700-meter-long SACLA laser.

The older device is the core upgrade of the existing beamline to produce soft X-ray laser light. It accelerates electrons independently of SACLA’s operation. It passes them through its own undulators. These are magnets that change the path of the electrons, making them emit a sharp beam of light.

The SCSS accelerator in its new home (Source: Riken)

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