Study On Interfacial Bonding Mechanism And Enhanced Heat Transfer Of Graphene/Nano Copper Composites

The third generation semiconductor materials are wide bandgap semiconductor materials represented by gallium nitride(Ga N)and silicon carbide(Si C).They are superior to silicon-based semiconductor in bandgap width,thermal conductivity,working temperature,working voltage and switching loss.With continuous development and increasing application of wide bandgap power devices,thermal management is one of the big challenges to ensure the high performance and reliability.With the thorough analysis of the state-of-art die attach materials,a graphene reinforced copper composite fabrication method was developed to improve heat dissipation of wide bandgap power electronic packaging with sintering process at low temperature and low pressure.In this thesis,a molecular level interface engineering method was proposed to enhance the properties of die attach materials.Oxygen plasma treatment was adopted to introduce oxygen-containing functional groups on the surface of graphene and improve the hydrophilicity and wettability of graphene.Then,copper nanoparticles were decorated on oxygen plasma treatmented graphene(OPTG)with controllable size and density.It was confirmed that C-O-Cu covalent bond is formed at the interface between graphene and copper.Graphene reinforced copper die attach materials were fabricated with C-O-Cu bond already formed at interface of Cu NP@OPTG.With sintering conditions at low temperature and low pressure(255??5MPa),thermal conductivity of 250.8 W/m·K was achieved with0.1wt% Cu NP@OPTG in copper matrix,which was 70.4% higher than that of the sintered pure copper powder(147.2 W/m·K),and 91.9% higher than that of the sintered copper with0.1wt% pristine graphene(130.7 W/m·K).In addition,the shear strength of the sintered copper with 0.1wt% Cu NP@OPTG is 34.2MPa,which is improved by 36.3% compared with that of sintered pure copper(25.1MPa).The coefficient of thermal expansion(CTE)of the novel developed composite material decreases with the increase of the amount of Cu NP@OPTG,which matches better with that of the chip and help improve the reliability of the wide bandgap power electronic devices.It was proved by experimental results and analysis that molecular level interface engineering helps form C-O-Cu bond at interface between graphene and copper before die attach sintering.Therefore,even at low temperature and low pressure sintering condition,significant improved thermal conductivity,shear strength and reliability can be achieved.The developed graphene reinforced composite is promising to be used as die attach material for wide bandgap power electronic packaging.Transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FTIR)were used to analyze and confirm C-O-Cu bond formed at the interface between graphene and copper.The mechanism of heat transfer was discussed and clarify the reason of Cu NP@OPTG enhancing the thermal conductivity of graphene reinforced copper composite,which can be used for further developing composites with high thermal conductivity.