Atom interferometers; EUV power recycling.
The Massachusetts Institute of Technology (MIT) has devised one of the world’s most accurate atom interferometers.
Interferometry is a common measurement technique. Basically, the technology looks at electromagnetic waves. The waves are superimposed to extract information.
One interferometry technology type, called an atom interferometer, utilizes the waves of atoms. They measure the difference in phases between atomic matter. These systems are used to measure gravitational and inertial forces, according to MIT.
The most sensitive atom interferometers use matter called Bose-Einstein condensates. Bose-Einstein condensates are clusters of atoms. They are cooled near absolute zero. Then, they inhabit the same quantum state.
Typically, a Bose-Einstein condensate interferometer makes use of a cloud of atoms, or a condensate, in a chamber. Then, a laser beam is fired into the chamber, thereby producing so-called standing waves.
In the process, the standing wave divides the condensate into approximately equal-sized clusters of atoms, according to MIT. This technique enables accurate measurements, but the problem is the division of the condensate into separate clusters is not perfectly even, according to MIT.
In response, MIT has described a way to make atom interferometry with Bose-Einstein condensates even more precise. “We demonstrate a new way to extend the coherence time of separated Bose-Einstein condensates that involves immersion into a superfluid bath,” according to MIT in an abstract within the recent issue of Physical Review Letters.
“When both the system and the bath have similar scattering lengths, immersion in a superfluid bath cancels out inhomogeneous potentials either imposed by external fields or inherent in density fluctuations due to atomic shot noise,” according to MIT in the journal. This effect, which we call superfluid shielding, allows for coherence lifetimes beyond the projection noise limit. We probe the coherence between separated condensates in different sites of an optical lattice by monitoring the contrast and decay of Bloch oscillations. Our technique demonstrates a new way that interactions can improve the performance of quantum devices.”
EUV power recycling
Extreme ultraviolet (EUV) lithography is expected to be inserted into mass production at 7nm and/or 5nm, but the technology must overcome some major challenges, namely the power source.
In EUV, a power source converts plasma into light at 13.5nm wavelengths. Then, the light bounces off several mirrors before hitting the wafer. Today, EUV can print tiny features on a wafer, but the big problem is the power source—it doesn’t generate enough power to enable an EUV scanner go fast enough or make it economically feasible.
Today’s EUV sources are based on a laser-produced-plasma (LPP) technology. In operation, a pre-pulse laser hits the spherical tin droplet in the EUV chamber and turns it into a pancake-like shape. Then, the laser unit fires again, representing the main pulse. The main pulse hits the pancake-like tin droplet and vaporizes it.
“The plasma scatters and emits a large amount of out-of-band radiation—especially the 10.6-um drive laser wavelength and the pre-pulse wavelength—which is harmful to lithography processes and must be separated out of the in-band EUV illumination,” said Kenneth C. Johnson from KJ Innovation, in a recent paper that was published in the Journal of Vacuum Science & Technology.
The paper, entitled “Extreme-ultraviolet plasma source with full, infrared to vacuum ultraviolet spectral filtering, and with power recycling,” proposed a technology to solve the problem.
Typically, an LPP source uses a diffraction grating on the collection mirror, according to Johnson. This is designed to enable spectral purity filtering via diffractive scattering of infrared (IR) radiation, he said.
The grating, according to Johnson, can eliminate at least one IR wavelength via zero-order extinction. “But it is not generally possible to extinguish the zero-order over the full out-of-band spectrum while maintaining high zero-order efficiency over the in-band EUV. The deep ultraviolet (DUV) spectrum, in particular, is inadequately filtered by the IR grating,” he said in the paper.
KJ Innovation proposes a different approach—A new EUV source module that improves spectral filtering and power recycling efficiency. “An EUV-diffracting, phase-Fresnel collection mirror can selectively direct in-band EUV radiation from an LPP source into an intermediate-focus aperture, eliminating all out-of-band radiation in the EUV output without incurring much loss of in-band collection efficiency,” he said.
The technology proposes an alternative approach “in which an EUV-blazed diffraction grating diverts in-band EUV radiation into the intermediate-focus aperture, rather than diverting IR out of the aperture,” Johnson said in the paper. “The full out-of-band spectrum, from long-wave IR to vacuum ultraviolet, can be fully excluded by this method regardless of whether the radiation is undiffracted or diffractively scattered by the grating. Much of the out-of-band radiation—especially the IR—is undiffracted, and this radiation can be efficiently returned to the plasma via retroreflection to enhance IR-to-EUV conversion. Plasma-emitted radiation that does not intercept the collection mirror can also be recycled back to the plasma via retroreflection to further boost EUV output.”
If this power recycling technology works, it could be a key enabler for EUV. “Existing (EUV scanners from) ASML could be field-upgraded to the zero-OOB system by swapping out the collector mirror,” he said. “The only change from current collectors would be a different mirror surface topography.”
For more information, contact KJ Innovation at: [email protected]
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