Low temperature, Low pressure and Fine - pitch Cu - Cu Thermocompression bonding for Three Dimensional Integration applications

Panigrahi, Asisa Kumar (2017) Low temperature, Low pressure and Fine - pitch Cu - Cu Thermocompression bonding for Three Dimensional Integration applications. PhD thesis, Indian Insitute of Technology Hyderabad.

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Abstract

Over the past four decades, improvement in Integrated Circuit (IC) performance was primarily achieved by continuously scaling down the device dimensions as per Moore’s law. ICs have essentially remained a planar platform throughout the period of rigorou s scaling. Two challenges have emerged in the recent years. The first one is that the device dimension scaling has almost hit a road block. The second and the significant one is that the interconnect performance is limiting the performance of the entire sy stem. The interconnect delay is comparable to that of device and has become the real bottleneck. In order to enhance system performance demands, new interconnect materials as well as innovative architectures need to be developed. Even though Cu and low - k m aterials offered enhancements in system performance by improving global interconnect RC delay, their contributions were limited. The focus therefore shifted towards developing new integration approaches that reduce the interconnect delays. Flip chip techno logy, Low Temperature Co - fired Ceramic (LTCC) technology are couple of examples that achieved success albeit partially. Three Dimension al (3D) Integration technology is one of the best approaches among others which suits CMOS applications wherein multiple layers of devices are stacked with high density inte rconnects between the layers . Apart from this, another major advantage of 3D IC is its ability to achieve heterogeneous integration. 3D integration is typically achieved using various stacking options pro minent among them being Wafer - on - Wafer (WoW), Chip - on - Chip (CoC) and Chip - on - Wafer (CoW) . Among all the methods, Cu - Cu WoW thermocompression bonding is considered as a frontrunner in 3D IC technology, due to excellent electrical conductivity, thermal condu ctivity, and impressive electromigration resistance. The principle of bond formation in thermocompression is diffusion of atom and grain growth of two contacted Cu surfaces. The key bottleneck in achieving the low temperature and low pressure bonding is th e presence of native oxide and other contaminants which precludes diffusion of Cu across the boundary. vii This thesis proposes a low temperature, low pressure WoW fine pitch (25μm) Cu - Cu bonding using dual damascene compatible oxidation resistive metal - alloy based passivation technique. To start with, our first efforts were focused on utilizing a porous metal Ti as Cu surface passivation layer. We have systematically studied the effects of Ti thickness on bonding quality via its effects on surface roughness, o xidation prevention and interdiffusion of Cu atoms. Through this study, we have found that a Ti thickness of 3 nm not only results in excellent bonding but also reduces the need of thermal stress for reliable Cu - Cu bonding. However, specific contact resis tance of Ti passivated bonded structure is comparably better than reported literatures. A careful observation reveals few challenges of Ti as passivation material. Ti is susceptible to oxidation once it's exposed to atmosphere over an extended amount of ti me. Even though Cu - Cu bonding with optimized Ti passivation provides high quality bonding at low temperature and pressure with better specific contact resistance compared to available literature, but further improvement is essential for high density Cu - Cu interconnect bonding. Furthermore, Ti is not dual damascene process compatible which limits the applications of multilayer heterogeneous integration. Constantan, a Cu rich Cu - Ni metal - alloy is explored as an alternative as it does not get oxidized even at higher temperature up to 300°C and also miniscule oxidation happens at room temperature. Furthermore, Unlike Ti, Constantan is damascene process compatible. Systematically optimized ultrathin, constantan (2 nm) on Cu surface not only protected Cu from oxid ation but also reduced the surface roughness to about 0.7 nm, which led to Cu - Cu blanket bonding at a temperature as low as 150 ̊C at 4 bar pressure. The quest for lowering the bonding temperature motivated us to explore alternative Cu based Manganin metal - alloy which has dual role of protecting Cu surface from oxidation even at higher temperature (< 300 ̊C) and Manganin alloy passivated Cu surface has diffusion enhancement due to higher <111> oriented Cu plane formation as compared to Constantan passivated Cu surface. This led to even high quality Cu - Cu thermocompression blanket bonding at 140 ̊C tempe rature and at a nominal contact force of 4 bar. In addition the high density fine pitch (25 μm) Cu - Cu bonding having daisy chain, mechanical stability structure, grid structure, and SEM line structures were successfully demonstrated using optimized ultrath in constantan and Manganin viii passivation layer at 200° C with a nominal contact pressure of 4 bar. Furthermore, electrical performance of the Cu - Cu bonded structures using various passivation materials are investigated under various tests including, tempera ture cycling test, multiple current stressing test, and under harsher environment conditions as per the ITRS (International Technology Roadmap for Semiconductors) road map. This method of alloy based passivation is confirmed to be effective and enable low temperature, low pressure Cu - Cu wafer on wafer bonding for 3D integration applications.

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