Manufacturing Bits: Jan. 5

Gallium oxide chips

The National Renewable Energy Laboratory (NREL), the Colorado School of Mines, and Saint-Gobain Crystals have teamed up to develop manufacturing technologies and devices based on an emerging material called gallium oxide.


This work is part of a three-year program, dubbed the Oxide Electronic Devices for Extreme Operating Environments project, which is funded by the U.S. Department of Energy.


The goal is to develop low-cost, all-oxide circuit elements. These devices could withstand high temperatures, corrosive atmospheres and mechanical stresses. Applications include automotive, energy systems, power devices, among others.


Still in R&D, crystalline beta gallium oxide is a promising wide bandgap semiconductor material, which is used for power semiconductor applications. Gallium oxide has a large bandgap of 4.8–4.9 eV with a high breakdown field of 8 MV/cm. The technology has a high voltage figure of merit, which is more than 3,000 times greater than silicon, more than 8 times greater than silicon carbide (SiC) and more than 4 times greater than that of gallium nitride (GaN).


Gallium oxide is still in its infancy. The DOE project is carrying out a host of activities, including crystal growth of gallium oxide, improving the manufacturing of gallium oxide single-crystal wafers, and device fabrication.


Leading-edge GaN foundry

HRL Laboratories has released its multi-project wafer (MPW) schedule for its new leading-edge GaN foundry technology in 2021 and 2022.


HRL’s T3 GaN monolithic microwave integrated circuit (MMIC) technology features a 40nm gate length with a high cutoff frequency (fT=200GHz), a high breakdown voltage (>50V), and low on-resistance (Ron<0.9Ohm.mm).


In RF GaN, the most advanced gate length is 90nm or so, that is, until now. Vendors are mainly shipping RF GaN chips with gate lengths at 0.15µm to 0.5µm. GaN, a III-V material, is also a wide bandgap technology, which refers to the amount of energy required for an electron to break free from its orbit. GaN has a bandgap of 3.4 eV, while silicon is 1.1 eV.


GaN devices handle more power with better characteristics than traditional silicon-based devices. GaN also enables higher instantaneous bandwidths.


Designed to develop power amplifiers and low-noise amplifiers with low-noise figures, HRL’s T3 technology is primarily used at ka-band frequencies and higher (30GHZ to 150GHz). Applications include wireless communications, high-resolution radar imaging, and many others.


HRL’s early access MPW program enables customers to develop high-performance, low-cost GaN devices. HRL processes GaN wafers in a 10,000-square-foot ISO Class 4 cleanroom, and is a U.S. Department of Defense Trusted Foundry.


“Having collaborated with DARPA to successfully complete three MPW runs in the past 18 months, we are opening this foundry model with more frequent fabrication runs,” said Florian Herrault, GaN strategy lead at HRL, an R&D venture between The Boeing Company and General Motors. “The foundry is available to universities and businesses of any size.”


Growing GaN

The National Institute for Materials Science (NIMS) and the Tokyo Institute of Technology have developed a technique for growing GaN crystals with fewer defects than the traditional techniques.


In this case, GaN is targeted for power semiconductor devices. As stated, GaN power semis are based on wideband-gap technologies, which are more efficient with higher breakdown electric field strengths than silicon.


In GaN, the first step is to develop GaN substrates, which are then processed into devices.


To develop GaN substrates, vendors make use of a GaN single crystal growth process. During the flow, gaseous raw material is sprayed onto a substrate, but this process sometimes causes the formation of atomic-scale defects and dislocations in the crystal. The resulting devices are prone to leakage or even damage.


“To address this issue, intensive efforts have been made to develop two alternative crystal synthesis techniques: the ammonothermal method and the sodium flux method,” according to NIMS and the Tokyo Institute of Technology. “In both methods, a crystal is grown in a solution containing raw materials for crystal growth. While the Na flux method have proven to be effective in minimizing the formation of dislocations, a new problem has been identified: a growing crystal incorporates inclusions (clumps of the constituents of the solution.)”


In response, researchers grew a GaN crystal while coating the GaN-seed substrate with a liquid alloy composed of raw materials for crystal growth. This in turn prevented inclusions from being trapped within the growing crystal. “In addition, this technique was found to be effective in significantly reducing the formation of dislocations, resulting in the synthesis of high-quality crystals,” according to NIMS and the Tokyo Institute of Technology. “This technique allows the fabrication of a high-quality GaN substrate through a very simple process within approximately one hour.”


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