Frontiers of Cu Electrodeposition and Electroless Plating for On-chip Interconnects
In the electronics industry, interconnect is defined as a conductive connection between two or more circuit elements. It interconnects elements (transistor, resistors, etc.) on an integrated circuit or components on a printed circuit board. The main funct
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Frontiers of Cu Electrodeposition and Electroless Plating for On-chip Interconnects James F. Rohan and Damien Thompson
5.1 Introduction In the electronics industry, interconnect is defined as a conductive connection between two or more circuit elements. It interconnects elements (transistor, resistors, etc.) on an integrated circuit or components on a printed circuit board. The main function of the interconnect is to contact the junctions and gates between device cells and input/output (I/O) signal pads. These functions require specific material properties. For performance or speed, the metallization structure should have low resistance and capacitance. For reliability, it is important to have the capability of carrying high current density, stability against thermal annealing, resistance against corrosion and good mechanical properties. Over the past 40 years, the continuous improvements in microcircuit density and performance predicted by Moore’s Law has led to reduced interconnect dimensions. Until the mid-1990s, Al interconnect was sufficient for VLSI circuit processing [1]. Further developments in miniaturisation of IC interconnect required a more conductive material than Al to minimise the RC (resistance– capacitance) delay which is in effect a time-delay between the input and output for a signal or potential applied to a circuit. When coupled with the poor resistance to electromigration (transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms) and poor mechanical properties for application in ultra large-scale integrated (ULSI) circuits, it was clear that an alternative to Al was required [2]. Only three metals, Ag, Au and Cu have lower resistivity than Al. For practical applications, it is clear that using Cu rather than Ag or Au is more realistic. J. F. Rohan (&) D. Thompson Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland e-mail: [email protected]
K. Kondo et al. (eds.), Copper Electrodeposition for Nanofabrication of Electronics Devices, Nanostructure Science and Technology 171, DOI: 10.1007/978-1-4614-9176-7_5, Springer Science+Business Media New York 2014
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J. F. Rohan and D. Thompson
The resistivity of Cu is 1.67 lX cm, about 40 % lower than Al, which when coupled with the new low k dielectrics introduced for the processing led to significant improvement in the RC delay. Cu also has advantages of higher melting point 1083 C by comparison with 660 C for Al and higher barrier to migration of an atom from its lattice position in a crystal, Al (1.4 eV) and Cu (2.2 eV). Despite these advantages Cu had not been used for on-chip interconnect to that point because of device reliability concerns and processing difficulties. It can diffuse rapidly through SiO2 in the presence of an electric field [3], decreasing transistor reliability. It also oxidises significantly at low temperatures but unlike Al it does not self-passivate [4]. However, one of the
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