Hydrocarbon flame synthesis of in-situ carbon nanotubes-copper metal-matrix compositeTools Wong, Hock Yee (2026) Hydrocarbon flame synthesis of in-situ carbon nanotubes-copper metal-matrix composite. PhD thesis, University of Nottingham.
AbstractThe advance packaging of microprocessor integrated circuits (ICs) faced reliability challenges arising from failure caused by joule heating and electromigration. Impressive physical properties of carbon nanotubes (CNTs) in mechanical strength, electrical and thermal conductivity proposed a feasibilities study on producing copper-CNTs composite as solution to address the joint failure due to electromigration at the interconnect’s region. Growing CNTs directly from copper substrate remains a challenge to overcome due to low carbon diffusivity and passivation layer. This study investigates CNT growth on copper substrates via inverse diffusion flame (IDF) synthesis using acid pre-treatment to remove surface oxides and promote nucleation. Three different acids, hydrochloric (HCl), sulfuric (H₂SO₄), and nitric (HNO₃) were prepared at 1 mol/L concentration for 20 minutes. Nickel substrates pre-treated with HNO₃ served as control samples. The effects of methane flow rate (0.616 – 3.196 SLPM) on CNT yield were examined experimentally and numerically. Numerical simulations were used to predict the temperature of gas species, concentration of carbon precursors and rate of soots nucleation to investigate the correlation of methane flow rate and growth yield of CNTs. The growth control of CNTs was studied under direct current (DC) voltage bias of 0.3V – 30 V at 1.230 SLPM. Data obtained from experimental works from directional growth control serve as reference for the proposal of CNTs numerical growth models. Synthesised CNTs were characterised through various characterisation instrumentations such as Field Emission Scanning Electron Microscope (FESEM), Scanning Transmission Electron Microscope (STEM), High-Resolution Transmission Electron Microscope (HR-TEM), and Energy Dispersive X-ray (EDX). The numerical simulations of CNTs growth models would be separated into two categories, combustion of flame synthesis by ANSYS FLUENT and CNTs numerical growth models by MATLAB R2023a Simulink. Growth of CNTs was observed on nickel substrate pre-treated with HNO3 and copper substrates pre-treated with three different acids of HCl, H2SO4 and HNO3 with IDF synthesis. Both nickel and copper substrates without acid pre-treatment showed no evident of CNTs growth. Copper substrate pre-treated with HCl, H2SO4 and nickel substrate pre-treated with HNO3 showed same trend of CNTs growth yield by varying the methane flow rate with the optimum growth yield of CNTS observed to be at methane flow rate of 1.230 SLPM. Combustion numerical models were developed and validated with data from literature. The pre-treatment of substrate with acid suggested no effects on controlling growth direction of CNTs for both copper and nickel substrates for IDF configuration. The control towards growth direction of CNTs on copper and nickel substrate was achieved at voltage bias of 30V DC. The results from CNTs growth models showed close prediction to experimental data obtained in this research. These results suggested that growing CNTs on copper are promising and warrant further optimisation in growth area and more uniform distribution of CNTs diameters to satisfy minimum specification requirement for applications of ICs. This research demonstrates that with acid pre-treatment, specific IDF conditions, and voltage bias, well aligned CNTs could grow directly on copper substrates, offering a scalable pathway for future IC interconnect applications.
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