Low Temperature Direct Cu-Cu Immersion Bonding for 3D Integration
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Low Temperature Direct Cu-Cu Immersion Bonding for 3D Integration Rahul Agarwal1 and Wouter Ruythooren1 1 IMEC, 75 Kapeldreef, Leuven, 3001, Belgium ABSTRACT High yielding and high strength Cu-Cu thermo-compression bonds have been obtained at temperatures as low as 175ºC. Plated Cu bumps are used for bonding, without any surface planarization step or plasma treatment, and bonding is performed at atmospheric condition. In this work the 25µm diameter bumps are used at a bump pitch of 100µm and 40µm. Low temperature bonding is achieved by using immersion bonding in citric acid. Citric acid provides in-situ cleaning of the Cu surface during the bonding process. The daisy chain electrical bonding yield ranges from 84%-100% depending on the bonding temperature and pressure. INTRODUCTION Die-to-die stacking is a key enabler in 3-D integration with high density and high speed interconnections. At IMEC direct metal-to-metal bonding for die stacking is investigated as an alternative to solder bonding due to its advantages, such as low processing cost due to fewer processing steps and predictable reliability because of single metal joints. Unlike solder bonding [1,2], in direct metal-to-metal bonding there is no solder reflow which makes this technology very useful for tighter pitch bump formation. In this paper we present the results of the immersion thermo-compression bonding for direct (as plated) Cu-Cu interconnects. High yield and high strength direct Cu-Cu thermo-compression bonds are obtained at temperatures as low as 175°C and results from high density micro-bumps are presented. Cu-Cu thermo-compression bonding requires higher temperature and pressure to make electrical connections as compared to solder bonding (for eg. Cu-Sn). In literature several processes have been discussed earlier where Cu-Cu metallic bonding is performed at lower temperature and/or lower pressure by conditioning the plated Cu bumps. Since there is no reflow of metal the surface roughness plays an important role and hence most of the low temperature Cu-Cu bonding results presented in literature have relied on surface planarization steps like CMP or diamond bit cutting to obtain a surface roughness of a few nanometers [3-5]. In the processes developed by Dr. Suga and his group the smooth metallic surface is first activated in plasma and bonding is performed at room temperature but at ultra high vacuum [6]. The disadvantage in all these processes is that an extra planarization step is required to make the plated surface smooth. In the immersion bonding method presented here citric acid is present between the samples being bonded, providing in-situ cleaning of the Cu surface during the bonding. As plated Cu bumps with average roughness of more than 270nm are successfully bonded at temperatures as low as 175ºC. Bonding is performed on two different test devices. First test device with 40µm pitch peripheral array (480 interconnections distributed over 2 daisy chains) have 100% yielding devices at temperatures as low as 175ºC and 10
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