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Cu-Sn Solid-State-Diffusion Bonding For 3D Integration

Posted on:2018-09-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:J Q WangFull Text:PDF
GTID:1318330518471780Subject:Microelectronics and Solid State Electronics
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3D integration has been acknowledged as one of the candidates to extend Moore's Law with the advantages of smaller form factor,lower power consumption,higher bandwidth,and heterogeneous integration.Vertical interconnection between wafers or chips is a key technology in 3D integration,and it can be realized by various bonding approaches.With the increasing requirements of reliability and density,low temperature and fine pitch bonding is a trend for future's 3D integration.Because of low cost and high process compatibility,Cu and Sn are the main materials for bonding.Generally,solid-liquid-interdiffusion?SLID?technology is used for traditional Cu-Sn bonding to accomplish vertical interconnection.However,Sn overflow usually occurs in Cu-Sn SLID bonding and leads to risk of electrical short between adjacent bumps as bump pitch reduces to tens of micrometers or below.It is indispensable that the bonding temperature is reduced to below the melting point of Sn?232??and even lower than the eutectic temperature?227??.Thus,low temperature?150?-220??Cu-Sn solid-state-diffusion?SSD?bonding has become an attractive approach to avoid the effect of Sn overflow and scale down the bump pitch.Moreover,driving force for atomic diffusion would be directly affected by low bonding temperature.Owing to easy oxidation of Cu and Sn to form kinetic barriers for Cu-Sn reaction,surface pretreatment prior to bonding would be extremely critical for accomplishing high-quality Cu-Sn SSD interconnection.In this work,fine-pitch bumps were fabricated by high-efficiency and low-cost electroplating process.The pretreatment with Ar mixed 5%H2 plasma?Ar?5%H2??was developed to improve surface properties of electroplated bumps.Low temperature Cu-Sn bonding was perormed using SSD technology.In order to increase storage time,shorten bonding time and improve bonding strength,a novel pretreatment of plasma combined self-assembled monolayer?PcSAM?was also proposed.Finally,the theory about low temperature Cu-Sn SSD bonding was then investigated.The main research contents and results are as follows:1.The design,optimization,and fabrication of fine-pitch bumps have been studied.According to test purpose,micro bumps were distributed around the peripheral.The thickness of Cu and Sn,the current density of electroplating,and the exposure time of thick resist lithography are optimized by correlative experiments.The results of bumping process showed bright and smooth surface.The bump pitch is 20 ?m,average thickness of Cu bump is 4.9 ?m,and that of Sn bump is 2.9 ?m.The bump uniformity of top wafer and bottom wafer were 2.36%and 3.17%.Thus,micro bumps with low roughness and good uniformity were obtained at the end.which met the demand of low temperature Cu-Sn bonding.2.The pretreatment on bump surface with Ar?5%H2?plasma has been studied.The effects of Ar?5%H2?plasma pretreatment were characterized by surface characterization techniques,including auger electron spectroscopy?AES?,atomic force microscope?AFM?,X-ray photoelectron spectroscopy?XPS?,contact angle?CA?analyzer,and so on.According to the theoretical analysis of pretreatment results,Ar?5%H2?plasma would be able to remove the oxide layer of electroplated Cu,smoothen and activate the surface,and suppress the oxygen adsorption.Meanwhile,the effects of Ar?5%H2?plasma on electroplated Sn were similar to that of electroplated Cu.Prior to bonding,optimal Ar?5%H2?plasma pretreatment was used to treat bumps.The subsequent low temperature Cu-Sn bonding with considerably improved performance was obtained,i.e.,void-free bonding interface and high bonding strength.3.The realiazation of low temperature Cu-Sn SSD bonding and the corresponding characteristics have been studied.Generally,bonding is consisted of two stages,i.e.,pre-bonding and annealing.With previous surface pretreatment,Cu-Sn SSD bonding was firstly performed with a pressure of 6.7 MPa at 200? for 60 min under a vacuum of 10-5 mbar?10-3 Pa?.Interfacial microstructure,bonding strength,and channel resistance were investigated by SEM with EDS,die shear tester,and probing station.The as-bonded interface had no Sn overflow and consisted of Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu.Annealing at 200? for 60 min resulted in a three-layer interface of Cu/Cu3Sn/Cu.The corresponding average bonding strength has become 11.4 MPa.The daisy-chain resistance was also on the same level as the theoretical value.This Cu/Sn/Cu interconnection had no significant change after thermal cycling?TC?test and Electromigration?EM?test.Based on these results,low temperature Cu-Sn bonding can be realized by SSD technology and revealed a high-quality interconnection,which would be beneficial for future's 3D integration with higher density.4.The improvement of low temperature Cu-Sn bonding has been studied.Since Cu bumps with high activity would be easily oxidized and contaminated again.a novel pretreatment of PcSAM was then proposed to improve surface properties of electroplated Cu.The same characterizations as plasma pretreatment were also used to investigate the effects of this pretreatment.Measurement results showed that the surface oxygen content was reduced to a lower level,and the clean surface was effectively protected by self-assembled monolayer?SAM?.Subsequently,low temperature Cu-Sn bonding was completed within a short time of 30min.Besides the perfect bonding interface observed by SEM.a much higher bonding strength of about 70 MPa was measured by die shear tester.Thus,low temperature Cu-Sn bonding was finally improved by performing PcSAM pretreatment.5.The basic theory of low temperature Cu-Sn bonding has been studied.The formation,growth,and evolution of intermetallic compound?IMC?were characterized by SEM.The effect relating to bump size was also investigated.The Cu6Sn5 has formed at room temperature,and its nucleation occurred randomly.The growth of Cu6Sn5 was based on the consumation of pure Sn,i.e.,6Cu+5Sn?Cu6Sn5,yet the growth of Cu3Sn was consuming the Cu6Sn5,i.e.,Cu6Sn5+9Cu?5Cu3Sn.Moreover,the morphology of Cu3Sn was flatter than that of Cu6Sn5 with "scallop" shape.The mechanism for Cu6Sn5 and Cu3Sn were both mainly controled by lattice diffusion.Kirkendall void at the interface of Cu/Cu3Sn was closely related to the consumation of Cu6Sn5.Additionally,the size effect of bump was also favorable for the realization of fine pitch low temperature Cu-Sn bonding.
Keywords/Search Tags:3D integration, Low temperature Cu-Sn bonding, Solid-state-diffusion(SSD), Plasma pretreatment, Self-sassembled monolayer(SAM)
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