Formulation and characterization of micro/nano Cu paste for electronics bonding

• Main Author: Ang, Kok Yong
• Other Authors: Gan Chee Lip
• Format: Final Year Project
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Materials
• Online Access: http://hdl.handle.net/10356/74020
• Institution: Nanyang Technological University

Three-dimensional integrated circuits (3D-IC) technology was introduced to solve the interconnect delay and huge power dissipation problems found in two-dimensional integrated circuits (2D-IC). Copper is a popular material that is used for 3D-IC interconnection applications. Metallic pastes are used as interconnects as they can be sintered at much lower temperature than the bulk metal. Copper nano-particles show a lower sintering temperature than copper micro-particles. However, cracks have been observed on the surface of nano-copper paste. Micro-copper paste also shows high porosity due to the gaps between the micro-particles. A method that mixes both copper nano- and micro-particles has been developed in an attempt to solve the problems found on both types of copper particles. Mixed pastes of 1:1, 2:1, 3:1 and 5:1 (copper nano- to micro-particles, weight ratio) were formulated and nano-copper paste was made for comparison. SEM micrographs were taken and cracks were observed on the nano-copper paste. 1:1 mixed paste shows the highest porosity among other mixed pastes. Sintering at different temperatures and shear testing was done for each mixed paste. For sintering at 280°C, 5:1 mixed paste provides the highest average bond strength of 3.88 MPa and the bond strength decreases with decreasing nano- to micro-particles weight ratio. SEM micrographs were taken for the same paste after sintering at different temperatures. It shows that the porosity decreases at higher sintering temperature. Shear testing of the same mixed paste at different temperatures shows that the mixed paste sintered at higher temperature could achieve higher bond strength. 3:1 mixed paste sintered at 280°C could achieve the average bond strength of 3.64 MPa as compared to 3.53 MPa and 2.37 MPa at 250°C and 220°C, respectively. However, the number of data obtained during the shear testing is insufficient to draw a reliable conclusion. Surface failure analysis was conducted to determine the failure mechanism for different mixed pastes. The dominant failure mode for 5:1 mixed paste is cohesion failure. The failure occurs between the adhesives for 5:1 mixed paste that suggests great bonding quality between the die and the subtrate that leads to high bond strength. The results show the formulation of mixed paste at specific ratios and sintering temperature and time could be utilized for 3D interconnects application via low temperature metal-metal bonding method.