Electronic Packaging and Interconnections 2021: 3D Microelectronic Packaging and Emerging Interconnects
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Mehran Maalekian, Materials & Metallurgy Expertise; Christopher Gourlay, Imperial College London; Babak Arfaei, Ford Motor Company; Praveen Kumar, Indian Institute of Science; Sai Vadlamani, Intel Corporation; Kazuhiro Nogita, University of Queensland; David Yan, San Jose State University

Monday 2:00 PM
March 15, 2021
Room: RM 22
Location: TMS2021 Virtual

Session Chair: Praveen Kumar , Indian Institute of Science; Nilesh Badwe, Intel Corp.


2:00 PM  
Electromigration of Cu-Cu Bonds Fabricated by Instant Bonding Using <111>-oriented Nanotwinned Cu Microbumps: Kai-Cheng Shie1; Po-Ning Hsu1; Yu-Jin Li1; King-Ning Tu1; Chih Chen1; 1National Chiao Tung University
    Failure modes of electromigration (EM) tests in Cu-Cu bonds with 30 μm in diameter were investigated. The chips were fabricated by instant bonding which bonding time was shorter than 30 s. The bonding temperature was 300 ℃, and the bonding pressure was from 31 MPa to 90 MPa. The temperature and current density of the EM test were 150 ℃ and 2.12×10^5 A⁄cm^2, respectively. EM Lifetime (20 % resistance change) was longer than 1500 h for the bonding condition: 300 ℃/90 MPa/30 s, while the lifetime of Cu/Sn/Cu solder joints with the same diameter was 448 h. Due to the structure of bumps, current crowding happened at the passivation opening and voids formed there. Computed tomography images of 3D X-ray indicated that voids formed at the high current density (>10^6 A⁄cm^2) region, which implied that current crowding enhanced EM damages.

2:20 PM  
Enhancement on the Bonding Strength of Instantly-bonded Cu-Cu Joints by Post Annealing: Jia Juen Ong1; Chih Chen1; King-Ning Tu1; 1National Chiao Tung University
    This study investigates bonding time and temperature on shear strength in copper direct bonding with highly <111>-oriented nanotwinned copper. Instant bonding process was performed under 10 and 30 seconds and post-bonding annealing was executed at 300 °C. Before bonding process, we use CMP to polish the surface of microbumps to reduce surface roughness. The advantage of the two steps bonding is that we can obtain enough bonding strength in extremely short time due to very large diffusivity of our highly <111> oriented nanotwinned copper in the first bonding process. We further increase the bonding strength of the samples by the second-step annealing process by grain growth. By now, we successfully to fabricate the sample with only first bonding step process in 10 seconds and the shear strength can reach to 55MPa and the bonding strength can increase to above 110MPa after 300 °C/150 N post bonding annealing in one hour.

2:40 PM  
High Electromigration Resistance of Nanotwinned Cu Redistribution Lines for Fan-out Packaging: I-Hsin Tseng1; Chih Chen1; Benson Lin2; Chia-Cheng Chang2; 1National Chiao Tung University; 2MediaTek Inc
     In Fan-out package, the line width of redistribution layers (RDLs) decreases continuously. Therefore, the RDLs are under larger current density than before. In previous research, there are several methods to enhance electromigration lifetime, such as doping a few percent of metal atoms in Cu, adding a thin layer of metal layer at the top of RDLs to inhibit Cu surface diffusion and change the microstructure of Cu to increase the mechanical strength of RDLs. In this study, we adopt highly <111> oriented nanotwinned Cu (nt-Cu) to enhance the electromigration resistance in RDLs. We compared the electromigration lifetime ofnt-Cu and regular Cu with polyimide capping. The microstructure changes of both two samples were observed by focused ion beam. The results show that the electromigration lifetime of nt-Cu lines is 4 times higher than regular Cu lines. Failure mechanisms will be presented in the conference.

3:00 PM  
Investigation of Interdiffusion In Micro Solder Joint with a Fine Pitch Copper Pillar Subjected to Electromigration Stressing: Hossein Madanipour1; Yi Ram Kim1; Allison Osmanson1; Mohsen Tajedini1; Choong-Un Kim1; 1University of Texas at Arlington
    This study concerns the effect of electromigration (EM) on the micro solder joint when testing is conducted under excessively high current densities. The solder joint used in our study was a lead-free Sn-Ag-Cu alloy placed between a fine pitch Cu pillar and a lead frame. Samples tested in a customized oil bath to minimize the potential artifacts of Joule heating. The results showed that EM makes the conversion of the solder joint to Cu6Sn5 and then Cu3Sn IMC phase to proceed faster because of enhanced atomic flux of Cu. Further, it is found that the interdiffusion between Cu and Sn continues even after full conversion of the solder to IMC because of EM forces Cu to migrate to IMC, which in turn results in counter flux of Sn directed toward Cu. This process is found to occur along the grain boundaries, clear evidence of which is obtained in our study.

3:20 PM  
On the 3D Evolution of the Nanoporous Structure of Sintered Ag on a Cu Substrate During Isothermal Aging Observed by In-situ X-ray NanoTomography: Kokouvi N'Tsouaglo1; Azdine Nait-Ali1; Mikael Gueguen1; Pascal Gadaud1; Loic Signor1; Juan Creus2; Marc Legros3; Yijin Liu4; Xavier Milhet1; 1Prime Institute CNRS ENSMA; 2LASIE Université La Rochelle; 3CEMES CNRS; 4SLAC-SSRL
    The most advanced power modules are developped around SiC or GaN chips, allowing better performance at the expense of higher operating temperature. The reliabilty of those modules depends on both the numerous materials and interfaces composing their complex architecture. Indeed, each of these materials have different thermal mismatch coefficients, mechanical and thermal properties, microstructure evolution and all of which influence the adhesion at the interfaces. In particular, the evolution of the microstructure of sintered silver joint (S-Ag), used as bonding material, is one of the key parameter for the sustainability and the performance of the modules. In this context, the pore struture of S-Ag specimens during isothermal aging in the range 200°C to 350°C were monitored in 3D using in-situ transmission X-ray imaging (BL 6.2C at SLAC SSRL, USA). Drastic evolution differences were observed between pure S-Ag and S-Ag specimens containing an interface with a Cu substrate.

3:40 PM  
Low Temperature Cu Direct Bonding with (111)-oriented Nanotwinned Copper Films on Metal Substrates: Hung-Che Liu1; Chih Chen1; 1National Chiao Tung Universality
    Nowadays, the required performance of electronic device continues to increase, making the joule heating and heat dissipation are critical issues to be solved. One of the solutions is to use heat pipes to dissipate heat. Usually, the heat pipes made of two patterned Cu foils, and they are bonded by compressive bonding at 600-800 C for 1 hour. However, the high bonding temperature soften the Cu foils. In this study, we are able to electroplate (111)-oriented nanotwinned copper on the copper substrates with randomly-oriented surface grains, and performed polishing on the Cu surfaces. Then we successful bond the two metal foils at 300℃ in 10 min.

4:00 PM  
Effect of Annealing on Mechanical Properties of nt-Cu Lines in Fan-out Wafer Level Packaging: Wei-You Hsu1; Yu-Jin Li1; I-Hsin Tseng1; Benson Tzu-Hung Lin2; Chia-Cheng Chang2; Chih Chen1; 1National Chiao Tung University; 2MediaTek Inc.
    In this study, we annealed nanotwinned Cu (nt-Cu) and regular Cu lines at different temperatures to examine their toughness. The results show the toughness of as-fabricated nt-Cu lines is at least 33.6MJ/m3, and its tensile strength is 833 MPa. However the toughness of asfabricated regular Cu is only 12 MJ/m3. And after annealing at 250°C for 3h, the toughness of nt-Cu increased into 65 MJ/m3, and the tensile strength is 600 MPa. In contrast, the toughness of regular Cu lines becomes 13 MJ/m3. In addition, we annealed the both Cu lines at 150°C and 200°C for 3h and we observed the toughness of nt-Cu lines is better than that of regular Cu lines regardless of annealing conditions. Finally, the toughness of nt-Cu can increase after different annealing conditions, which make nt-Cu to be a promising material as redistribution lines for next generation fan-out wafer-level packaging.

4:20 PM  
Direct Bonding Process of (111) Nanotwinned Copper Thin Films: Jing-Yi Zhong1; Yung-Ting Tai1; Fan-Yi Ouyang1; 1National Tsing Hua University
    As integration density on very large scale integrated-circuits(VLSI) increases, micro solder bump may lead to mechanical damage. Thus metal - metal direct bonding has been proposed as an alternative technology. In this study, we fabricated highly (111)-preferred orientation nanotwinned Cu thin films by sputtering system. The resistivity of the deposited film is 2.1 µΩ-cm and the percentage of (111) orientation remained almost 100% after annealing at 200°C for 2 hours. Furthermore, the Cu-Cu direct bonding process can be achieved at 250°C for 30 minutes with shear strength of 16.06 MPa without any CMP process. To simulate the package in 3D IC, we also performed bump - film and bump - bump bonding process. Both samples were passed by the reliability test such as Shear test and Thermal Cycle Test (TCT). Microstructure evolution and the bonding mechanism would also be discussed in this talk.