Low temperature three dimensional wafer level copper thermo-compression bonding

• Main Author: Lim, Dau Fatt.
• Other Authors: Tan Chuan Seng
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: http://hdl.handle.net/10356/52550
• Institution: Nanyang Technological University

As semiconductor technology scales, integrated circuit performance is shifting from device limited to interconnect limited. Interconnect delay is unavoidably worsen due to longer, thinner interconnect wire and tighter pitch. Three dimensional (3D) integration provides a simple solution to alleviate this problem. 3D integration is a method of stacking chips in a vertical direction and interconnect them through vertical interconnects. In this way, the long horizontal interconnects can be replaced by a shorter vertical interconnects. Cu thermocompression bonding is one of the methods to realize 3D integration. It is a method utilizing pressure and temperature to promote inter-diffusion of atoms to form a strong bond. Cu thermocompression bonding is attractive as it can form mechanical support and provide electrical conductivity path in a single process step. Low temperature bonding (<300oC) is preferred to reduce the thermal stress, improve the alignment accuracy and lower cost. However, bonding of untreated Cu surface often requires temperature up to 350oC to form good bonding. This is mainly due to Cu is easily oxidized and tends to attract contaminants that render low temperature bonding not feasible. A self-assembled monolayer passivation (SAM) is employed to temporarily passivate the Cu surface from excessive oxidation and particle contamination. Alkanethiols of six (C6) and twelve (C12) carbon chain lengths are used to passivate the Cu surface. SAM is found to be able to reduce Cu oxidation and surface particle contamination. SAM is also found to be effectively desorbed away via thermal annealing. Longer carbon chain length shows enhance protection in oxidation and degradation in ambient mainly due to stronger intermolecular attractions. Compared with Benzotriazole (BTA), the most common passivation in Cu, SAM exhibits higher protection properties and ease in removal by thermal desorption. Metal-oxide-semiconductor (MOS) capacitor is fabricated to serve as sensor to determine the side effects arising from SAM passivation. It is found that the flat-band voltage shift and the interface state density after SAM passivation can be effective controlled by conventional forming gas annealing. Cu-Cu bonding using SAM passivation is tested. Cross-sectional analysis at the bonding interface reveals that effective cross-diffusion of atoms is observed only for C6 passivation. Careful examination shows that incomplete desorption has occurred during thermal annealing due to temperture variation on the top wafer. Since C12 has a longer carbon chain length, lower temperature during annealing results in incomplete desorption and hence the sample has a limited inter-diffusion at the interface. Mechanical, electrical and hermeticity properties with C6 SAM passivation shows enhancement in performance compared to Cu bonding without SAM passivation.