Manufacturing Bits: Oct. 30
AFM Advancements
The NIST Center for Nanoscale Science and Technology has developed on-chip optomechanical sensors for atomic force microscopy (AFM).
This could extend the range of mechanical properties found in commercial AFM cantilevers, potentially enabling the use of this technology to study a wide variety of physical systems.
AFM is used for surface metrology that measures local tip-surface interactions by scanning a flexible cantilever probe over a surface. But the bulky free-space optical system commonly used to sense the motion of the probe imposes limits on the tool’s sensitivity and versatility.
Top: Scanning electron micrograph of the chip-based optomechanical sensor. Bottom: Schematic of the disk-cantilever sensor geometry. Source: NIST.
NIST (the National Institute of Standards and Technology) showed that geometric scaling of both the cantilever and the optical resonator dimensions can enable a variation in the cantilever spring constant by over four orders of magnitude, ranging from devices that are ten times softer than the original design to ones that are one thousand times stiffer.
These cantilevers maintain their high displacement sensitivity and achieve measurement response times that are hundreds of times faster than commercial cantilevers with similar spring constants. Future work will focus on integrating this sensor platform into a commercial AFM system.
Big Blue’s Carbon Nanotubes
IBM has demonstrated the initial steps towards the commercial fabrication of carbon nanotubes.
For the first time, IBM placed and tested more than 10,000 carbon nanotube devices in a single chip. Until now, researchers have been able to place at most a few hundred carbon nanotube devices at a time, not nearly enough to address key issues for commercial applications.
Carbon nanotubes are single atomic sheets of carbon rolled up into a tube. The carbon nanotube forms the core of a transistor device that will work in a fashion similar to the current silicon transistor.
Earlier this year, IBM demonstrated carbon nanotube transistors that can operate as switches at 10nm. There are challenges for carbon nanotubes, due in part to the purity and placement of the devices.
To overcome these barriers, IBM developed a method based on ion-exchange chemistry. This in turn allows precise and controlled placement of aligned carbon nanotubes on a substrate at a high density—two orders of magnitude greater than previous experiments.
The process starts with carbon nanotubes mixed with a surfactant. The substrate is comprised of two oxides with trenches made of chemically modified hafnium oxide (HfO2) and the rest of silicon oxide (SiO2). The substrate gets immersed in the carbon nanotube solution and the nanotubes attach via a chemical bond to the HfO2 regions while the rest of the surface remains clean.
As a result, IBM researchers are able to fabricate more than 10,000 transistors on a single chip. “Carbon nanotubes have largely been laboratory curiosities as far as microelectronic applications are concerned. We are attempting the first steps towards a technology by fabricating carbon nanotube transistors within a conventional wafer fabrication infrastructure,” said Supratik Guha, director of physical sciences at IBM Research, on the company’s Web site. “The motivation to work on carbon nanotube transistors is that at extremely small nanoscale dimensions, they outperform transistors made from any other material. However, there are challenges to address such as ultra high-purity of the carbon nanotubes and deliberate placement at the nanoscale. We have been making significant strides in both.”
3D Insulators
Boston College is exploring the nature of 3D topological insulators. In topological insulators, electrons can behave more like photons. Applications include quantum computing and spintronics.
Researchers report that the placement of tiny ripples on the surface of 3D insulators using bismuth telluride materials can modulate so-called Dirac electrons. As a result, the electrons flow in a pathway that mirrors the topography of the crystal’s surface.
Boston College used scanning tunneling microscopy to determine the effects of 1D buckling on the electronic properties of the materials. By tracking spatial variations of the interference patterns generated by the Dirac electrons, researchers showed that buckling imposes a periodic potential, which locally modulates the surface-state dispersion.
This suggests that forming 1D and 2D ripples is a viable method for creating nanoscale potential landscapes that can be used to control the properties of Dirac electrons in topological insulators.
“We did not expect the electrons to follow the topography,” said Vidya Madhavan, an associate professor of physics, on Boston College’s Web site. “The topography imposes a sinusoidal potential upon the waves. The ripples create that potential by giving the electrons a landscape to follow. This is a way of possibly manipulating these electrons in topological insulators.”
Copper Paste
The Lockheed Martin Space Systems Advanced Technology Center (ATC) has developed a copper-based electrical interconnect material, or solder, that can be processed at about 200 degrees Celsius.
The material, dubbed CuantumFuse, is expected to produce joints with up to 10 times the electrical and thermal conductivity, compared to current tin-based materials. Applications include military and commercial systems.
In the past, nearly all solders contained lead, but there is now a need for lead-free solder because of a worldwide effort to phase out hazardous materials in electronics. The principal lead-free replacement, a combination of tin, silver and copper (Sn/Ag/Cu), has proven acceptable to the consumer electronics industry.
However, multiple issues have arisen with this combination. For example, the high tin content can lead to tin whiskers, which can grow over time and cause short circuits. In addition, fractures are common in challenging environments.
“To address these concerns, we realized a fundamentally new approach was needed to solve the lead-free solder challenge,” said Alfred Zinn, materials scientist at the ATC. “Rather than finding another multi-component alloy, our team devised a solution based on the well-known melting point depression of materials in nanoparticle form. Given this nanoscale phenomenon, we’ve produced a solder paste based on pure copper.”
—Mark LaPedus
Tags: atomic force microscopy, Boston College, carbon nanotubes, Copper, copper solder paste, IBM, Lockheed Martin, National Institute of Standards and Technology, NIST, optomechanical sensors, topological insulators