Manufacturing Bits: Sept. 4

Flat diamond chips
Kanazawa University and the National Institute of Advanced Industrial Science and Technology (AIST) have developed a process that solves a big issue for diamond semiconductors in power applications.

Researchers have developed a water vapor annealing technique that creates atomically flat diamond surfaces. This brings diamond semiconductors one step closer to becoming more commercially viable.

In the works for years, diamond field-effect transistors (FETs) have many intriguing properties in power applications. These are wide-bandgap devices with a high breakdown field and thermal conductivity. Diamond has a wide bandgap (5.45 eV), a high breakdown field (10MV/cm), and a high thermal conductivity (22W/cmK). In comparison, silicon has a bandgap of 1.1 eV. Wide bandgap refers to higher voltage electronic band gaps in devices, which are larger than 1 electronvolt (eV).

The problem? Diamond semiconductors suffer from cost, surface property problems and other issues.

Generally, the diamond structure must be stabilized in the production process. This is done using a surface termination technique via a hydrogen process. This, however, creates unwanted two-dimensional hole gas layers (2DHG). But removing these layers causes roughness on the diamond surface. It also leads to degradation in device performance, according to researchers.

Schematic illustration of the formation of OH-terminated diamond (111) surface by water vapor annealing of H-terminated one (Source: Kanazawa University, AIST)

To solve the problem, researchers first developed synthetic single-crystalline diamond substrates. Then, using microwave plasma chemical vapor deposition (MPCVD), diamond films were grown on the substrates. The diamond samples were exposed to hydrogen plasma in the MPCVD chamber.

The next step is to form hydroxyl-terminated surfaces. For this, hydrogen-terminated diamond samples were exposed to a new water vapor annealing process from Kanazawa University and AIST. The annealing system consists of a quartz tube in an electric furnace. The annealing process took place in an atmosphere of nitrogen, which bubbled through the ultrapure water in the furnace.

During this process, the carbon–hydrogen (C–H) bonds remained on the diamond surface. The annealing processing was done at a temperature below 400°C. Two-dimensional hole gas layers (2DHG) were still present.

But it was a different story when the annealing temperatures were above 500°C. The process removed the 2DHG, but it also maintained the surface morphology of the diamond surfaces. “Water vapor annealing above 500°C removed C–H bonds from the diamond surface, indicating the disappearance of the 2DHG,” said Ryo Yoshida of AIST.

“Compared with conventional techniques to remove 2DHG, such as wet chemical oxidation, water vapor annealing offers the advantage of maintaining an atomically flat surface,” added Norio Tokuda from Kanazawa University.

Diamond finFETs
HRL Laboratories has recently developed the industry’s first diamond finFET.

Diamond finFETs are tolerant to high power usage, heat and radiation, making them ideal for extreme-environment electronics in future spacecraft, according to HRL, a joint R&D venture between Boeing and General Motors.

Making diamond FETs is challenging. It requires a hydrogen-terminated channel, which has a hydrogen-treated surface through the transistor. This, in turn, enables conductivity for the device. But there are some reliability issues for these devices when they run in high-temperature conditions.

To solve the problem, HRL developed a fully-depleted diamond finFET device with a 100nm fin. The device showed a greater than 3000 on/off ratio. Devices were characterized at room temperature and at 150 °C, according to researchers.

A diamond FinFET with source, drain, and fin channel (Source: HRL, Nature Scientific Reports)

To make diamond finFETs, researchers started with a 3mm × 3mm diamond substrate. An epitaxial layer was grown on the top, according to HRL in the journal Nature Scientific Reports. Then, the device was patterned and dry etched, which, in turn, defined the contacts and channels. A Ti/Pt/Au mix was used to form a contact, according to HRL in Nature Scientific Reports.

“Our goal was to vastly increase the power performance of transistors. The target application for these transistors is radio frequency or RF electronics,” said Biqin Huang, a principal investigator at HRL. “Typically for digital electronics in the semiconductor industry, to increase computing performance the transistor is scaled down, which has been done according to Moore’s Law for decades. This increases the speed but reduces the power capability of the device. To increase power performance in RF electronics for certain frequencies, the only option is to increase the breakdown voltage of the semiconductor material you are using. Because this is an intrinsic property in the material, to change it requires using a different material. We chose diamond because it has a breakdown voltage greater than silicon or gallium nitride semiconductors roughly by a factor of 3. So the potential increased power density is very desirable.”

Strong alloys
Sandia National Laboratories has devised a platinum-gold alloy said to be the most wear-resistant metal in the world.

The alloy is 100 times more durable than high-strength steel, putting it in the same class as diamond and sapphire for wear-resistant materials. “We showed there’s a fundamental change you can make to some alloys that will impart this tremendous increase in performance over a broad range of real, practical metals,” said Nic Argibay, a materials scientist at Sandia.