Abstract
Miniaturization of electronic devices has led to the development of 3D IC packages which require ultra-small-scale interconnections. Such small interconnects can be completely converted into Cu-Sn based intermetallic compounds (IMCs) after reflow. In an effort to improve IMC based interconnects, an attempt is made to add Ni to Cu-Sn-based IMCs. Multilayer interconnects consisting of stacks of Cu/Sn/Cu/Sn/Cu or Cu/Ni/Sn/Ni/Sn/Cu/Ni/Sn/Ni/Cu with Ni = 35 nm, 70 nm, and 150 nm were electrodeposited sequentially using copper pyrophosphate, tin methanesulfonic, and nickel Watts baths, respectively. These multilayer interconnects were investigated under room temperature aging conditions and for solid-liquid reactions, where the samples were subjected to 250 °C reflow for 60 s and also 300 °C for 3600 s. The progress of the reaction in the multilayers was monitored by using X-ray Diffraction, Scanning Electron Microscope, and Energy dispersive X-ray Spectroscopy. FIB-milled samples were also prepared for investigation under room temperature aging conditions. Results show that by inserting a 70 nanometres thick Ni layer between copper and tin, premature reaction between Cu and Sn at room temperature can be avoided. During short reflow, the addition of Ni suppresses formation of Cu3Sn IMC. With increasing Ni thickness, Cu consumption is decreased and Ni starts acting as a barrier layer. On the other hand, during long reflow, two types of IMC were found in the Cu/Ni/Sn samples which are the (Cu,Ni)6Sn5 and (Cu,Ni)3Sn, respectively. Details of the reaction sequence and mechanisms are discussed.
Highlights
As the electronic industry is moving towards miniaturization, three-dimensional (3-D) IC packages are being aggressively pursued
The motivation is to prevent premature reaction between Cu and Sn prior to reflow. Another motivation is to form, during reflow, Cu-Sn intermetallic compounds (IMCs) alloyed with Ni which are expected to lead to better mechanical properties and improved reliability of interconnects
Cu/Ni/Sn system, IMC was seen to form at the Ni/Sn interface and along Sn grain boundaries after one day of room temperature aging (Figure 1c,d)
Summary
As the electronic industry is moving towards miniaturization, three-dimensional (3-D) IC packages are being aggressively pursued. Addition of Ni to interconnects has been done in the form of nanoparticles in the recent past It has been shown [3] that Ni nanoparticles of ~20 nm size undergoes reactive dissolution during reflow leading to in-situ alloying. It is, expected that if thin nanometric layers of Ni is introduced in Cu/Sn based interconnects in the form of multilayers, the former can get intermixed with Cu and. The motivation is to prevent premature reaction between Cu and Sn prior to reflow Another motivation is to form, during reflow, Cu-Sn IMCs alloyed with Ni which are expected to lead to better mechanical properties and improved reliability of interconnects. The effect of Ni in the Cu/Sn system is studied after both short and long reflow to investigate solid-liquid state reactions
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