Manufacturing Bits: Jan. 29

Thermal lithography
Using a technique called thermal scanning probe lithography, New York University (NYU) and others have reported a breakthrough in fabricating 2D semiconductors.

With the technology, researchers have devised metal electrodes with vanishing Schottky barriers on 2D semiconductors based on molybdenum disulfide (MoS₂). Thermal scanning probe lithography, sometimes called t-SPL, has several advantages over conventional patterning methods, namely e-beam or direct-write lithography.

Besides NYU, the other contributors to this work are Columbia University, SwissLitho, National Institute of Materials Science, National Research Council, and the Swiss École Polytechnique Fédérale de Lausanne (EPFL). The research was supported by the U.S. Army Research Office, the U.S. Department of Energy, the National Science Foundation, and the European Union’s Horizon 2020 Research and Innovation Program.

2D materials could enable a new class of field-effect transistors (FETs) with some intriguing electrical properties. In 2004, graphene was the first 2D material isolated. Other 2D materials include boron nitride and the transition-metal dichalcogenides (TMDs). One TMD, called MoS2, is gaining interest in the market.

Typically, e-beam lithography is used to pattern metal electrodes on these materials. At times, though, e-beam lithography is problematic. It can create non-ohmic contacts and high Schottky barriers, according to NYU and others.

Instead, researchers from NYU and others used thermal scanning probe lithography. This in turn can pattern metal electrodes at sub-10nm resolution with high throughputs. Thermal scanning probe lithography was invented at IBM. Some time ago, the technology was licensed to a Swiss startup called SwissLitho, which in turn brought out a direct-write or maskless tool called the NanoFrazor.

This is a subtractive technique, in which material is selectively removed from a surface using a tiny metal tip. The system makes use of heated silicon tips. The tip is similar to the kind used in atomic force microscopes (AFM). It is attached to a bendable cantilever. The tip can remove substrate material based on predefined patterns. It allows sub-20nm lateral and sub-2nm vertical resolutions.

In operation, t-SPL reduces the power consumption in the patterning process. It eliminates the need to produce high-energy electrons in an ultra-high vacuum.

Vanadium dioxide switches
KAUST has developed an inkjet printing method to develop reconfigurable RF switches based on vanadium dioxide.

A smartphone consists of digital and RF chips. The RF components are integrated into a RF front-end module, which handles the transmit/receive functions.

The front-end module consists of a number of components, including power amplifiers, antenna tuners, low-noise amplifiers (LNAs), filters and RF switches. RF switches route signals from one component to another.

Typically, RF switches are field-effect transistors (FETs), which are fabricated based on an RF SOI process. RF MEMS is a competing technology. Recently, vanadium dioxide is gaining interest for RF switches. This is a phase‐change material, where the electrical properties can be tuned with heat or current, according to researchers from KAUST. It has a phase transition close to room temperature.

In the lab, researchers synthesized vanadium-dioxide nanoparticles, which in turn created vanadium oxide ink. Then, using an inkjet printer, researchers devised an RF switch based on the material. RF switches with up to 40GHz in performance and switching speeds of 0.4µs have been achieved.

“Radio-frequency switches are the key to realizing cost- and space-saving frequency-tunable antennas and filters,” says KAUST Ph.D. student Shuai Yang. “When fully printed electronics become mature for industrialization, our switch will be useful for mass-producing smartphones and other wireless devices.”

Piezoelectric printing
Virginia Tech has developed a new 3D printing technique to print piezoelectric materials in a variety of shapes and sizes.

Piezoelectrics are specialized materials. These materials create electricity in response to a mechanical stress. The materials include crystals, certain ceramics and biological matter.

Researchers have developed a technology, which allows them “to manipulate and design arbitrary piezoelectric constants.” This in turn enables materials that generate “electric charge movement in response to incoming forces and vibrations from any direction via a set of 3D printable topologies.”

“We have developed a design method and printing platform to freely design the sensitivity and operational modes of piezoelectric materials,” said Xiaoyu ‘Rayne’ Zheng, assistant professor at Virginia Tech. “By programming the 3D active topology, you can achieve pretty much any combination of piezoelectric coefficients within a material and use them as transducers and sensors that are not only flexible and strong, but also respond to pressure, vibrations, and impacts via electric signals that tell the location, magnitude, and direction of the impacts within any location of these materials.”