Manufacturing Bits: Nov. 19

Toothpick Fab Tools
NASA’s Goddard Space Flight Center in Greenbelt, Md. has developed a specialized atomic layer deposition (ALD) system and a “virtual toothpick” to enable ultra-thin films on chips and systems.

NASA has built an ALD reactor chamber, which measures three inches in diameter and two feet in length. The system can deposit films inside pores and cavities, giving ALD the ability to coat in and around 3D objects.

NASA technologist Vivek Dwivedi, who has distinguished himself as the go-to-engineer for atomic layer deposition, has assembled a new reactor at NASA’s Goddard Space Flight Center that he plans to use for thin-film experimentation. He is inserting one of his “virtual toothpick” technologies. Image Credit: NASA Goddard/Bill Hrybyk

Researchers also have devised a suite of monitoring tools. The tools include a series of so-called “virtual toothpicks,” which resemble long probes. The probes monitor the process in real time. It also determines how much gas to deposit on the substrate or component. The probes can provide many other functions, as well.

“What we do is place the substrate or sample inside the reactor and follow the recipe. We also cross our fingers and hope the recipe is successful. It can be a time-consuming process,” said Vivek Dwivedi, a technologist at NASA’s Goddard Space Flight Center, on the agency’s Web site.

“These tools save time,” Dwivedi added.

Space Nanotubes
Carbon nanotubes are taking off—literally. These devices are being used as part of a critical experiment within NASA’s CubeSat Launch initiative (CSLI), a class of small research spacecraft called nanosatellites.

The carbon nanotube arrays are part of ALICE, a CubeSat micro-satellite built by the Air Force Institute of Technology. On a mission scheduled for Dec. 5, ALICE will ride into space on an Atlas V rocket being used to launch a separate and much larger payload. Just 10- x 10- x 30-cm in size, ALICE will be part of an array of eight CubeSats.

In the program, carbon nanotube arrays could provide a more efficient micro-propulsion system for ALICE. Researchers will test the carbon nanotube arrays as electron emitters.

Georgia Tech produced the carbon nanotube arrays. They used a technology that grows bundles of vertically-aligned nanotubes embedded in chips. Researchers used deposition and etching steps to fabricate the arrays. Each one-centimeter square array contains as many as 50,000 nanotube bundles.

Georgia Tech researchers Jud Ready (left) and Graham Sanborn pose with equipment used to grow carbon nanotubes at the Georgia Tech Research Institute (GTRI) in Atlanta. The nanotubes are being tested for potential use in future electrically-powered ion propulsion systems. (Georgia Tech Photo: Rob Felt)

Electrons emitted from the array tips may be used to ionize a gaseous propellant such as xenon. The ionized gas would then be ejected through a nozzle to provide thrust for moving a satellite in space, according to Georgia Tech. In contrast, existing ion thrusters rely on thermionic cathodes. These devices require large amounts of electricity to generate the heat, and they consume a portion of the propellant for their operation.

If the carbon nanotube arrays can be used as electron emitters, they would operate at lower temperatures with less power. “The mission will characterize how well these field emission electron sources operate in the space environment relative to how well they work on the ground in vacuum chamber,” said Jud Ready, a Georgia Tech principal research engineer, on the university’s Web site. “Launch vibrations and exposure to a space environment that includes atomic oxygen and micrometeorites could have some unusual effects on the arrays. This mission will help us evaluate whether these carbon nanotube electron emitters could be used in ion thrusters.”

Nanotube Metrology
Single-walled carbon nanotubes are promising technologies for electronics and other fields. These structures are identified by a pair of chirality indices, which determine the electronic properties of each species.

According to the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California at Berkeley, there are two basic challenges for carbon nanotube research–achieving chirality-controlled growth and understanding chirality-dependent device physics.

Two techniques, direct optical imaging and spectroscopy, are promising methods to help solve these issues. But these techniques fall short at the single nanotube level, “due to the small nanotube signal and unavoidable environment background,” according to Berkeley Labs.

To solve the problem, Berkeley Labs and others have devised a new technique. Using polarization-based microscopy combined with supercontinuum laser illumination, researchers have achieved high-throughput, real-time optical imaging and broadband in situ spectroscopy of individual carbon nanotubes on various substrates and in field-effect transistor devices.

The technique enables the chirality profiling of hundreds of individual carbon nanotubes. This includes both semiconducting and metallic on a growth substrate. In devices, researchers observed that high-order nanotube optical resonances are broadened by electrostatic doping. This unexpected behaviour points to strong interband electron–electron scattering processes, which could dominate the dynamics of excited states in carbon nanotubes, according to researchers.

“To fully understand field-effect devices or optoelectronic devices made from single-walled carbon nanotubes, it is critical to know what species of carbon nanotube is in the device,” said Feng Wang, a condensed matter physicist with Berkeley Lab’s Materials Sciences Division, on the agency’s Web site. “In the past, such information could not be obtained and researchers had to guess as to what was going on.”