Effects of Nanometer-Scale Surface Roughness and Applied Load on the Bond Strength and Contact Resistance of Cu-Cu Bonde
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1036-M02-05
Effects of Nanometer-Scale Surface Roughness and Applied Load on the Bond Strength and Contact Resistance of Cu-Cu Bonded 3D ICs Hoi Liong Leong1,2, Chee Lip Gan1,2, Carl V. Thompson1,3, Kin Leong Pey1,4, and Hongyu Li5 1 Advanced Materials for Micro- and Nano-Systems, Singapore-MIT Alliance, 4 Engineering Drive 3, Singapore, 117576, Singapore 2 School of Materials Science and Eingineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139 4 School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore 5 Institute of Microelectronics, 11 Science Park Road, Singapore, 117685, Singapore ABSTRACT The effects of total surface roughness of Cu bond layers and applied load during bonding on the bond strength and the contact resistance of the bonded interface were studied for threedimensional (3D) devices. The 3D structures were fabricated by thermocompression bonding of two wafers, each fabricated with a Cu damascene process. The average bond strength of the samples was found to increase with increasing load, and with decreasing roughness. On the other hand, the average contact resistance increased as the total surface roughness of the bonded interface increased, as well as when the applied load decreased. The increase in bond strength (and also the dicing yield) is related to a smaller true contact area, which can be estimated using a model based on contact mechanics theory. The theory takes into account the roughness of the bonded surface, the applied load during bonding and the nominal bond area. A contact resistance model can also be used to estimate the contact resistance of bonded interfaces based on their true contact area. However, within each set of measurements, a significant spread about the average value of bond strength and contact resistance was observed, suggesting issues of contact and bonding non-uniformity of the wafers. Nonetheless, the contact resistance values given by the model were in good agreement with the minimum values observed in experiments, which may represent the ideal cases of bonding of rough surfaces. Our results show that the impact of surface roughness and applied load on the contact resistance of Cu-Cu thermocompression bonds can be quantified experimentally, and understood in the context of established theory for contact mechanics. INTRODUCTION With increasing device density and continuous shrinking of dimensions for each generation of silicon chip, interconnect delay has been identified as the bottleneck for faster future integrated circuits (ICs). Even the introduction of Cu and low-k dielectrics to replace Al and silicon dioxide as the materials for metallization and intra/inter-level dielectric respectively, the new metal/dielectric interconnect system is still unable to meet the requirements as stated in the technology roadmap. Three-dimensional (3D) ICs fabricated through
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