Manufacturing Bits: Jan. 17

GOOI FETs
The next-generation power semiconductor market is heating up. Two wide-bandgap technologies—gallium nitride (GaN) on silicon devices and silicon carbide (SiC) MOSFETs—are ramping up in the power semi market. In addition, the industry is also exploring various futuristic technologies, such as bulk vertical GaN, diamond FETs and others.

Purdue University has demonstrated another future candidate based on a compound called beta gallium oxide. With the technology, researchers have devised a gallium oxide on insulator field effect transistor or GOOI FET.

The schematic at the left shows a transistor based on beta gallium oxide. At the right is an AFM image of the semiconductor. (Purdue University image/Peide Ye)

Like GaN and SIC, gallium oxide is a wide-bandgap technology. Basically, wide-bandgap technologies are faster and provide higher breakdown voltages than traditional silicon-based devices. GaN has a bandgap of 3.4 electronvolts (eV). SiC has a bandgap of 3.3 eV. In comparison, silicon has a bandgap of 1.1 eV.

Suddenly, beta gallium oxide is creating a buzz for use in power semi applications. The technology has a bandgap of 4.9 eV, resulting in a critical field strength around 8 MV/cm, according to researchers.

In the works for years, the technology has some challenges, however. “Choices of gate dielectric are limited for MOS applications, p-type doping is not expected, and directional dependence of transport and optical properties is not fully understood,” according to the U.S. Air Force Office of Scientific Research, which recently held a workshop on the technology. “β-Ga2O3 also has relatively poor thermal conductivity, which may prove to be a difficult barrier to overcome for certain applications.”

In response, Purdue has developed a possible breakthrough in the arena. Researchers devised a GOOI FET technology using a new method that costs just pennies.

Today, the challenge is depositing layers of gallium oxide on top of a substrate using an epitaxial tool. The price for a 1- x 1.5-cm piece of beta gallium oxide produced using epitaxy is about $6,000, according to Purdue.

In its method, Purdue used adhesive tape to peel off layers of the semiconductor from a single crystal. The process can be used to cut films of beta gallium oxide material into belts or nano-membranes, according to Purdue. This, in turn, can be transferred to a substrate.

Researchers devised GOOI FETs with drain currents of 600/450 mA/mm. Enhancement-mode GOOI FETs with source-to-drain spacing of 0.9μm demonstrates a breakdown voltage of 185V and an average electric field (E) of 2 MV/cm, according to researchers.

Gallium oxide epi
The recent workshop on beta gallium oxide, sponsored by the Air Force, attempted to explore the state of the technology and identify the gaps and challenges.

The industry has made progress in terms of developing beta gallium oxide substrate technology and devices. But the epitaxial technology that deposits gallium oxide layers on top of a substrate is still in its infancy.

According to Kyma Technologies, there are three competing epi approaches in the arena–molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).

Kyma itself has devised an HVPE technology, enabling the company to produce beta gallium oxide epi wafers. The company has demonstrated growth rates of >3 microns/hour and epi-layers of several microns in thickness. “Researchers (are) working hard to understand the potential of Ga2O3 materials to support next generation volume commercial applications as well as certain niche applications,” said Kyma’s Chief Science Officer, Jacob Leach.

Kyma’s ß-Ga2O3 epiwafers (Source: company)

EU patterning program
The European Union (EU) has established a program that will provide training for European researchers in two fields–focused electron beam induced deposition (FEBID) and extreme ultraviolet (EUV) lithography.

The program is called the Marie Curie Training Network “ELENA”. Empa is one of the project partners, together with 13 universities, three research institutes and five industrial partners.

Over the next four years, the EU will make available about €4 million for ELENA. The program will explore new photoresists for EUV.

Still in R&D, FEBID makes use of an electron beam from a scanning electron microscope. Basically, it decomposes gaseous molecules, which, in turn, deposit materials and structures on a surface at the nanoscale.

One of the big applications is a futuristic manufacturing technology called selective deposition. This involves a process of depositing materials and films in exact places. Selective deposition can be used to deposit metals on metals and dielectrics on dielectrics on a device

For this application, Empa uses FEBID to pattern a granular cobalt compound with special magnetic properties. This is done on a silicon oxide layer between several gold electrodes. In addition, nanophotonics is another application. Gold Me2Au(tfa) was used to write an optical lattice on a vertical cavity surface emitting laser.

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