Manufacturing Bits: Jan. 23

Looking inside memristors
E-beam inspection is gaining steam. Using this type of technology, the National Institute of Standards and Technology (NIST) has been able to see the inner workings of the memristor.

The memristor is a type of ReRAM, which works by changing the resistance of materials. In a memristor, an electric current is applied to a material, changing the resistance of that material. The resistance state can then be measured. This principle is called “memristor” and relies on the principle of hysteresis.

The use of specialized hardware like ReRAM or memristors can be used for neuromorphic computing. This involves machine learning, which makes use of neural networks. In neural networking, the system crunches data and identifies patterns. Then, it learns which of those attributes are important.

Many of these machine learning systems use FPGAs or GPUs with SRAM-based memory. In R&D, the industry is working on ReRAM for this segment. ReRAM is much denser than the GPU/SRAM architecture.

Hewlett-Packard attempted to commercialize the memristor, but the technology has run into roadblocks. One problem is that the industry lacks a detailed understanding of how these devices work, according to NIST.

For memristors, HP used a bi-level titanium dioxide thin-film to store the resistance. In one possible breakthrough, NIST has demonstrated electron beam-induced current measurements, which, in turn, monitor the resistive switching in devices based on titanium dioxide.

In the inspection process, the beam from the e-beam knocks some of the free electrons off the structure. Then, the e-beam also induced four different currents to flow within the structure.

With the technology, NIST found several dark spots, which are regions of conductivity. The dark spots are also places where the memristor are prone to leakage. Leakage usually resides outside the core of the device. In memristors, the leakage takes place where it switches between the low and high resistance levels.

Illustration shows an electron beam impinging on a section of a memristor, a device whose resistance depends on the memory of past current flow. (Source: NIST)

With the findings, NIST suggests that reducing the size of a memristor could minimize or even eliminate some of the unwanted current pathways. Electron beam-induced current measurements “is a way of generating much stronger intuition about what might be a good way to engineer memristors,” said Brian Hoskins of NIST, on the agency’s Web site. “You’re probably not going to start seeing some really big improvements until you reduce dimensions of the memristor on that scale.”

Imaging quantum dots
The University of Bochum, the University of Basel and the Swiss Nanoscience Institute have developed an optical microscopy technique that allows the imaging of quantum dots in semiconductor chips.

In the 1980s, researchers stumbled upon a tiny particle or nanocrystal with electrical properties. These nanocrystals, based on semiconductor materials, were later named quantum dots. Quantum dots were curiosity items until 2013, when Sony launched the world’s first LCD TV using these inorganic semiconductor nanocrystals. When inserted into an LCD TV, quantum dots can boost the color gamut in the display, enabling vivid picture quality with relatively little capital.

Based on 10nm to 2nm feature sizes, quantum dots are produced using a chemical process. The dots are bundled and sold in the form of glass tubes or films. They are tunable and can convert short-wavelength light into colors spanning the visible spectrum.

Besides LCD TVs, quantum dots are also being explored for use in quantum computing and other applications.

The problem? The industry wants to look inside quantum dots. But traditional microscopy methods are limited in terms of resolution.

Researchers have overcome these limitations in recent times. One way is to use lasers of various wavelengths in a system. This, in turn, triggers a fluorescence effect in molecules, enabling researchers to image structures at the nano-scale.

The development of this method is called stimulated emission depletion (STED). Researchers from the University of Basel and others have developed a similar technique using far-field nanoscopy. Nanoscopy allows the imaging of nanoscale objects, such as a quantum mechanical two-level system. In the lab, researchers used the technology to excite the atoms with a pulsed laser. The atoms change color during each pulse, causing the atom’s fluorescence to switch on and off.

STED works by implementing at least four different energy levels in response to the laser excitation. In comparison, researchers from Basel and others developed a technique that works in two energy states.

Image of quantum dots in a semiconductor: whereas the image taken with a normal microscope is blurry (left), the new method (right) clearly shows four quantum dots (bright yellow spots). (Image: University of Basel, Department of Physics)

Two-state systems are key for modeling systems in quantum mechanics. Unlike STED microscopy, the new nanoscopy method also releases no heat. “This is a huge advantage, as any heat released can destroy the molecules you’re examining,” said Richard Warburton, a researcher. “Our nanoscope is suitable for all objects with two energy levels, such as real atoms, cold molecules, quantum dots, or color centers.”

Cryomodules
The SLAC National Accelerator Laboratory has received one of the first building blocks to enable a new superconducting X-ray laser that stretches three miles in length.

Built by a collaboration of national laboratories, the first cryomodule has arrived at SLAC, based in Menlo Park, Calif. These 40-foot-long sections, called cryomodules, are building blocks for a major upgrade called the LCLS-II. The modules will amplify the performance of the lab’s X-ray free-electron laser, the Linac Coherent Light Source (LCLS).

Linked together and chilled to nearly absolute zero, 37 of these segments will accelerate electrons to almost the speed of light. Inside the cryomodules, strings of super-cold niobium cavities will be filled with electric fields.

This superconducting technology will allow LCLS-II to fire X-rays that are, on average, 10,000 times brighter than the LCLS in pulses that arrive up to a million times per second. With the technology, researchers hope to examine the details of complex materials and other technologies.

LCLS-II is expected to be operational by the early 2020s.