Scientists from Meisei University and Waseda University in Japan have developed a room-temperature bonding technique for integrating wide bandgap materials such as gallium nitride (GaN) with thermally-conducting materials such as diamond. This could boost the cooling effect on GaN devices and facilitate better performance through higher power levels, longer device lifetime, improved reliability, and reduced manufacturing costs. The technique could have applications for wireless transmitters, radars, satellite equipment, and other high-power and high-frequency electronic devices. The work was supported by a multidisciplinary university research initiative (MURI) project from the U.S. Office of Naval Research (ONR).
The technique, called surface-activated bonding, uses an ion source in a high-vacuum environment to first clean the surfaces of the GaN and diamond, which activates the surfaces by creating dangling bonds. Introducing small amounts of silicon into the ion beams facilitates forming strong atomic bonds at room temperature, allowing the direct bonding of the GaN and single-crystal diamond to fabricate high-electron-mobility transistors (HEMTs).
The resulting interface layer from GaN to single-crystal diamond is just four nanometers thick, allowing heat dissipation up to two times more efficient than in the state-of-the-art GaN-on-diamond HEMTs by eliminating the low-quality diamond leftover from nanocrystalline diamond growth. Diamond is currently integrated with GaN using crystalline growth techniques that produce a thicker interface layer and low-quality nanocrystalline diamond near the interface. Additionally, the new process can be done at room temperature using surface-activated bonding techniques, reducing the thermal stress applied to the devices.
For high-power electronic applications using materials such as GaN in miniaturized devices, heat dissipation can be a limiting factor in power densities imposed on the devices. By adding a layer of diamond, which conducts heat five times better than copper, engineers have tried to spread and dissipate the thermal energy.
However, when diamond films are grown on GaN, they must be seeded with nanocrystalline particles around 30 nanometers in diameter, and this layer of nanocrystalline diamond has low thermal conductivity – which adds resistance to the flow of heat into the bulk diamond film. In addition, the growth takes place at high temperatures, which can create stress-producing cracks in the resulting transistors.
Samuel Graham, one of the author, commented that in the currently used growth technique, you don’t reach the high thermal conductivity properties of the microcrystalline diamond layer until you are a few microns away from the interface. The materials near the interface just don’t have good thermal properties. This bonding technique allows them to start with ultra-high thermal conductivity diamond right at the interface.
By creating a thinner interface, the surface-activated bonding technique moves the thermal dissipation closer to the GaN heat source.
Zhe Cheng, the first author of the paper, stated that their bonding technique brings high thermal conductivity single-crystal diamond closer to the hotspots in the GaN devices, which has the potential to reshape the way these devices are cooled. And because the bonding takes place near room temperature, they can avoid thermal stresses that can damage the devices.
That reduction in thermal stress can be significant, going from as much as 900 megapascals (MPa) to less than 100 MPa with the room temperature technique. This low-stress bonding allows for thick layers of the diamond to be integrated with the GaN and provides a method for diamond integration with other semiconductor materials.
Beyond the GaN and diamond, the technique can be used with other semiconductors, such as gallium oxide, and other thermal conductors, such as silicon carbide. Graham said the technique has broad applications to bond electronic materials where thin interfacial layers are advantageous.
He added that this new pathway has given them the ability to mix and match materials. This can provide them with great electrical properties, but the clear advantage is a vastly superior thermal interface. They believe this will prove to be the best technology available so far for integrating wide bandgap materials with thermally conducting substrates.
In future work, the researchers plan to study other ion sources and evaluate other materials that could be integrated using the technique.
In addition to the researchers already mentioned, the paper included co-corresponding author Fengwen Mu from Meisei University and Waseda University in Japan, Luke Yates from Georgia Tech, and Tadatomo Suga from Meisei University.
This research was supported by the U.S. Office of Naval Research (ONR) through MURI Grant No. N00014-18-1-2429. Any findings, conclusions, and recommendations are those of the authors and not necessarily of the Office of Naval Research.
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