Research On Wafer-level Cu-Sn Solid-state Diffusion Bonding And Electro-migration Simulation

As an extension of Moore’s Law,3D integration technology has been widely used in integrated circuits.By extending interconnection to vertical direction,3D interconnection has become one of the key technologies to realize 3D integration.Cu–Sn bonding is one of the most potential eutectic bonding methods and has become an important way to realize 3D interconnection.In the process of Cu–Sn bonding,the pitch of bumps decreases continuously,so that the problem of electric short circuit caused by Sn overflow becomes more and more serious.After bonding,the electrical,thermal and mechanical distribution of the bonding joint greatly affects the stability of the circuit during the real circuit service.In this paper,a low-temperature and fine-pitch wafer-level Cu–Sn solid-state diffusion bonding is proposed,and the electromigration failure of the bonding joint is simulated.The main research contents are as follows:First of all,the size,pitch and layout of the micro bumps are designed,and the thickness of the metal layer of the bumps was dimensioned;the process flow of the micro bumps was designed,and the corresponding photolithographic plates were drawn;physical vapor deposition was used to fabricate the micro bumps;by using plasma pretreatment,the surface oxides of the bumps were removed and the surface roughness of the bumps was reduced.The measurement results show that Cu bumps with a thickness of 1.56μm and Cu/Sn bumps with a thickness of 1.56/1.63μm were obtained;the average height difference,root mean square roughness and thickness of the Sn layer were 404.28 nm,98.96 nm and 1.62μm respectively,and the consistency deviation was 3.70%;the Contact Angle of Cu surface was reduced from29°to 7°after Ar（5%H₂）plasma treatment.Secondly,low-temperature wafer-level Cu–Sn solid-state diffusion bonding was realized,then the interface composition,quality,mechanical and electrical properties of the bonded joint were characterized and tested,and the diffusion growth kinetics of Cu–Sn was analyzed.The results show that after pre-bonding,Sn was completely consumed,forming a Cu/Cu₃Sn/Cu₆Sn₅/Cu₃Sn/Cu five-layer interface.After annealing,Cu₆Sn₅ was exhausted,forming a Cu/Cu₃Sn/Cu three-layer interface.The average shear strength of annealed samples was up to 27 MPa,which has some advantages compared with other studies at the same temperature;the average resistance of daisy chain channel was 20.0Ω,which was consistent with the theoretical value;the parabolic growth constant of Cu₃Sn was as high as 1.86×10^-15m²/s.Finally,a daisy-chain packaging model was established to simulate the multi-physical field coupling distribution of the bonding structure under the input current condition,and the influence of bump size,pitch and shape on the electrical,thermal and mechanical distribution was discussed.The results show that the current accumulation effect was easy to occur at the position where the aluminum wire is connected to the bump,which led to the increase of temperature and stress;when 2 m A current was applied,the maximal temperature,temperature gradient,stress and current density of 20-μm square bump reach 110.85℃,61.54 K/m,231.83MPa,and 4.03×10³ A/cm²,respectively;the electromigration stability of large-pitch bumps was weak;the input current suitable for square bump was 6 m A,that for spherical bump was 8 mA.