Environmentally Friendly Synthesis and Aerosol Jet Printing of hBN Films for Thermal Packaging Applications

Gallium oxide transistors are a candidate to improve the efficiency of power switching with lower losses. Unfortunately, the material has a very low thermal conductivity, requiring novel packaging solutions to reduce the thermal limitations. Hexagonal boronitride (hBN) is a two-dimensional material with a higher thermal conductivity than would be expected for an electrically insulating material. Using an aerosol jet printer to deposit hBN ink on top of gallium oxide devices has been shown to greatly improve their performance. Commercial hBN flakes were dispersed in a low-cost, environmentally friendly aqueous solvent mixture. Liquid phase exfoliation was employed to break up the particles and allow for the addition of functional groups to the edges of the particles. This was accomplished by using a vibrating probe tip sonicator to add energy to the solution. The energy broke some of the bonds in the hBN particles, and it also broke apart some of the molecules in the solvent, forming functional groups that could then attach to the edges of the hBN particles. Films were created by drop casting the ink and through the use of an Optomec aerosol jet printer. Film thicknesses were determined through the usage of a stylus profilometer and a Keyence VK-X1000 laser confocal microscope. An analysis of the material’s electrical properties was conducted through the creation of parallel plate capacitors. The dielectric constant of the films was obtained. This was to ensure that the added functional groups in the hBN material would not make the material likely to short out transistors if it were employed in packaging. A scanning electron microscope was used to determine particle size. While hBN is a two-dimensional material, the printer does not deposit perfect layers of hBN. Rather, the final film appears like a large pile of plates thrown on top of each other. While the discontinuities in hBN layers make the conductivity of the film lower than perfect hBN films, the haphazard nature of the stack also reduces the anisotropy of the film, allowing heat to radiate in all directions. An analysis of the thermal conductivity of the printed hBN material was also undertaken to determine how different it is from the pure, pristine material. Finally, a hexagonal boronitride film was deposited onto a gallium oxide device and was tested at high temperature to evaluate its performance.