Two-dimensional Layered Hexagonal Boron Nitride For Application In High-Power Electronic Devices As Thermally But Not Electrically Conductive Interface Material

The development of communications, automotive electronics, consumer electronics, military and aerospace electronics required high performance, miniaturization, micromation, multifunction and low cost of microelectronic products. In a short span of 20 years, the electronic devices and their application have become the fastest growing areas. Besides the feature size decreases from micron to nanometer, there are a lot of new practices in terms of encapsulation integration, such as multi-core architecture, three-dimensional chip stack and etc. All of them provide possible solutions to improve computing performance. However, the technology development and emerging application also bring the great challenge to thermal management, which concerns the application limitation, implementation scope and the overall feasibility of electronic devices. Both in a single transistor or a chip or an overall encapsulation, effective heat dissipation capacity is needed to strengthen. The thermal performance of new materials is very important to promote the thermal management characteristic in the next generation of electronic devices. However, many applications require interface materials which are thermal conductive but not electrical conductive, such as CPU microprocessor, integrated electronic chips, high-power LED lamps, battery pack cooling design, etc. Thus, based on the above situation, this paper tried to use the emerging two-dimensional material – hexagonal boron nitride(h BN) to solve the insulation and heat dissipation problems in the high-power electronic devices.Monolayer h BN grown by chemical vapor depositon(CVD) method and few-layer h BN synthesized by liquid phase exfoliation method, separately transferred to the surface of the thermal test chip designed and fabricated by our group in different methods. On the one hand, they can replace the traditional insulation material with poor thermal conductivity such as silicon dioxide(Si O2), to act as the electrical insulated protective layer of the chip. On the other hand, due to their special two-dimensional structure, their superior lateral heat conduction capacity can be employed to rapidly spread the heat generated from the local hot spots of high-power electronic devices in lateral direction. The local maximum temperature can be reduced, so as to improve the lifetime and reliability of the devices. The studies showed that monolayer h BN could realize both the aims of insulation and heat dissipation. However, the cooling effect has a close relation with the growth quality of the material itself and the transfer quality from the growth substrate to the chip surface. Therefore, this technology has poor repeatability and mass production feasibility. In this thesis, a specific percentage of ethanol/water solution was selected as the liquid phase exfoliation solvent. Few-layers h BN were synthesized through ultrasonic and centrifugal steps. The way of droplets evaporation coating was adopted to form a heat spreader on the chip surface, which has good repeatability and low cost. The insulation property of few-layer h BN is very well, and at the heat flux density of 1000 W/cm2 the temperature of the hotspot can be decreased by about 3-5°C.In order to further improve the thermal conductivity of two-dimensional layered h BN-based film, a stack structure of graphene and h BN and a composite structure of them were separately synthesized. It can be found that the first one does not have a good insulated property. While with a certain content of graphene, the composite film showed a better heat dissipation effect as well as a good insulation. At the heat flux density of 1000 W/cm2 the temperature of the hotspot can be decreased by about 8-10°C. The heat dissipation effect of graphene-enhanced two-dimensional layered h BN-based film is benefit from the better continuity and flatness of the composite film. This result enhanced the adhesive between the h BN-based film and the chip surface, so as to reduce the thermal contact resistance in the thermal conduct path. Finite element simulation was used to analyze the experimental test structure, a similar conclusion as the experimental result can be derived from the simulation. That is to say, thermal contact resistance between the film and the chip is an important factor limiting the heat dissipation effect of the high thermal conductive film. Using some functionalized processes to reduce the thermal contact resistance between the heat spreader and the chip surface, would be an effective way to enhance the heat dissipation capacity.