Manufacturing Bits: Sept. 10

Rock Around The Clock
National Institute of Standards and Technology’s two experimental atomic clocks have set a new record for stability. Resembling a pendulum or metronome, NIST’s atomic clocks can swing back and forth with perfect timing for a period comparable to the age of the universe.

The clocks are based on ytterbium atoms. The clock ticks are stable to within less than two parts in 1 quintillion (1 followed by 18 zeros). This is 10 times better than the previous best published results for other atomic clocks, according to NIST.

NIST’s ultra-stable ytterbium lattice atomic clock. Ytterbium atoms are generated in an oven (large metal cylinder on the left) and sent to a vacuum chamber in the center of the photo to be manipulated and probed by lasers. Laser light is transported to the clock by five fibers (such as the yellow fiber in the lower center of the photo).
Credit: Burrus/NIST

At present, the NIST-F1, a cesium fountain clock at NIST’s site in Boulder, Colorado, serves as the primary time standard in the United States. The NIST-F1 must be averaged for about 400,000 seconds to achieve its best performance. The new ytterbium clocks, according to NIST, achieve that same result in about one second of averaging time.

Each ytterbium-based clock relies on about 10,000 rare-earth atoms cooled to 10 microkelvin or 10 millionths of a degree above absolute zero. Then, the atoms are trapped in an optical lattice, where are a series of pancake-shaped wells made of laser light. “Another laser that ticks 518 trillion times per second provokes a transition between two energy levels in the atoms,” according to NIST.

“The stability of the ytterbium lattice clocks opens the door to a number of practical applications of high-performance timekeeping,” said physicist Andrew Ludlow, on NIST’s Web site.

Scotch Tape Lithography
The University of Minnesota, Argonne National Laboratory and Seoul National University have discovered a new patterning technique—atomic layer lithography.

The technology combines atomic layer deposition (ALD) and simple adhesive-tape-based planarization. In this technology, a layer of metal fills the patterns over the wafer. Scotch Tape is used to remove the excess metal, thereby exposing the gaps.

Using this method, researchers have devised vertical gaps in opaque metal films along the entire contour of a millimeter-sized pattern. The gaps widths were as narrow as 9.9 angstroms, and pack 150,000 such devices on a 4-inch wafer.

In a breakthrough study to improve the manufacturing of optical and electronic devices, University of Minnesota researchers introduced a new patterning technology, atomic layer lithography, based on a layering technique at the atomic level. A layer of metal fills the nano-patterns over an entire wafer and simple Scotch Magic tape was used to remove the excess metal on the surface and expose the atomic scale nano-gaps. Source: University of Minnesota

Electromagnetic waves pass through the nanogaps, enabling background-free transmission measurements. Researchers observed resonant transmission of near-infrared waves through 1.1nm-wide gaps (λ/1,295) and measures an effective refractive index of 17.8.

They also observe resonant transmission of waves through 1.1nm-wide gaps (λ/4,000,000) and inferred an unprecedented field enhancement factor of 25,000. “Our technology, called atomic layer lithography, has the potential to create ultra-small sensors with increased sensitivity and also enable new and exciting experiments at the nanoscale like we’ve never been able to do before,” said Sang-Hyun Oh, professor of electrical and computer engineering at the University of Minnesota, on the entity’s Web site. “This research also provides the basis for future studies to improve electronic and photonic devices.”

Measuring Nanowires
A group of researchers led by Drexel University has developed a breakthrough laser spectroscopy technique. Using laser, or photocurrent spectroscopy, researchers have obtained a direct measurement of individual III-V core-shell nanowires.

Core–shell semiconducting nanocrystals are a class of materials. The nanocrystals consist of a quantum dot core and a shell of a material. The core and the shell are based on III-V materials.

III–V coaxial core-shell semiconducting nanowire heterostructures possess unique characteristics and challenges for future devices. For example, the properties are not well understood without a direct measurement of band alignment in individual nanowires. And current methods for measuring this step height are not practical.

In the lab, researchers devised GaAs/AlGaAs and GaAs/AlAs core–shell nanowire systems. They demonstrated how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets.

“Using the interface within a co-axial core-shell semiconductor nanowire as a model system, we made direct measurements of the band offset for the first time in nanowire electronics,” said Guannan Chen, a graduate student in Drexel’s Materials Science and Engineering department, on the university’s Web site. “This is a significant cornerstone to freely design new nanowire devices such as solar cells, LEDs, and high speed electronics for wireless communications. This work can also extend to broader material systems which can be tailored for specific application.”

—Mark LaPedus