Emerging Interconnect and Pb-free Materials for Advanced Packaging Technology: Cu- and Ag- related Bonding Materials
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Fan-Yi Ouyang, National Tsing Hua University; C. Robert Kao, National Taiwan University; Albert T Wu, National Central University; Fay Hua, Intel Corporation; Yan Li, Intel Corporation; Babak Arfaei, Binghamton University; Kazuhiro Nogita, The University of Queensland

Monday 8:30 AM
February 27, 2017
Room: 30E
Location: San Diego Convention Ctr

Session Chair: C. Robert Kao, National Taiwan University; Kwang-Lung Lin, National Cheng Kung University

8:30 AM  Invited
Low-Temperature Cu-to-Cu Direct Bonding Enabled by Highly (111)-oriented and Nanotwinned Cu: Chih Chen1; Chien-Min Liu1; Tien-Lin Lu1; Han-wen Lin1; Yi Cheng Chu1; Chia-Ling Lu1; Jing-Ye Juang1; Kuan-Neng Chen1; King-Ning Tu2; 1National Chiao Tung University; 2University of California at Los Angeles
    Low-temperature Cu-to-Cu direct bonding appears to be one of the solutions for fine-pitch microbumps for 3D IC packaging. However, the high bonding temperature and pressure are the main problems of this approach. We achieve low-temperature Cu-to-Cu direct bonding at low pressure and ordinary vacuum by highly (111)-oriented and nanotwinned Cu (nt-Cu). The bonding temperature can be lowered to 150C at a compressive stress of 114 psi held for 60 min at 10-3 torr, or at 200C for 30 min. Excellent bonding interface can be obtained by bonding two highly (111)-oriented nt-Cu films. Our breakthrough is based on the finding that the diffusivity of Cu atoms on (111) surfaces is approximately 3-4 orders higher than other major planes. Bonding results for patterned Cu microbumps will be also presented in the conference.

8:50 AM  
The Materials Science of Solder Joints in Cu Pillar/Interposer Geometries: Francis Mutuku1; Mohammed Genanu2; Babak Arfaei3; Eric Cotts2; Eric Perfecto4; 1Universal Instruments; 2Binghamton University; 3Ford Motor Co; 4Global Foundries
    The effect on Pb free solder joint microstructure and performance of the variation of processing parameters such as thermal history, or composition, depends strongly upon geometry. The advent of 2.5/3D technologies in microelectronics has decreased length scales, and changed aspect ratios. This decrease of solder volumes dramatically affects the solidification microstructure of near eutectic SnAgCu solder, and increases the focus on the formation of intermetallic compounds at solder/metallization interfaces. In the current study, relations between processing, microstructure and reliability of assemblies enabled through Cu pillar/interposer technology were examined. The effects of solder cap composition, thickness and volume on the microstructure of Cu pillar assemblies were examined. Precipitate and Sn grain morphologies, intermetallic compound crystal structures and morphologies, were quantified, and correlated with both shear strength and with performance in accelerated thermal cycling tests. Comparisons with results from studies at much larger length scales (ball grid array) are reported.

9:10 AM  
Mechanisms of Copper Pumping and Its Impact on the Reliability of 3D Electronic Devices: Hanry Yang1; Tae-Kyu Lee2; Indranath Dutta1; 1Washington State University; 2Portland State University
    Copper-pumping, which occurs due to plastic deformation of the ends of through-silicon-vias (TSVs) and/or diffusionally accommodated sliding at Cu-barrier layer-Si interfaces, can result in intrusion or protrusion of TSVs during fabrication or service. It may be induced by thermal-cycling and/or passage of an electric current (i.e., electromigration), and can adversely impact the reliability of back-end-of-line (BEOL) and redistribution-layer (RDL) structures. In this paper, we report on the construct of a deformation mechanism map for copper-pumping in a heating rate vs. temperature field, and investigate its impact on damage mechanisms in the RDL/BEOL layers, including distortion and delamination. Finally, we report on the kinetics of copper-pumping due to electromigration, and assess its effect on RDL/BEOL stability. Experimental and simulation results on copper pumping under thermal cycling, with and without applied electric current, will be presented to rationalize the experimental results.

9:30 AM  
Influence of Annealing Conditions on the Microstructure of Cu-filled Through-silicon Vias: Zhao Xuewei1; Limin Ma1; Fu Guo1; 1Beijing University of Technology
    Reliability investigation is a key link in the industrialization of through-silicon via(TSV) technology. One issue of great concern for TSV reliability is the coefficient of thermal expansion(CTE) mismatch between different materials in TSV structures. The CTE mismatch may induce thermal stresses at interfacial region when TSVs experience temperature changes during the processes of manufacture and application. These stresses may affect negatively the performance of production, and possibly lead to failure. By annealing, microstructure changes of TSVs can be investigated and analyzed to explain and optimize the thermo-mechanical behaviors of TSV structures. In this paper, microstructure changes of Cu-TSV samples under different test conditions are observed by FIB-SEM and EBSD method, influences of the temperature and heating rate on microstructure evolution and defect formation are analyzed and compared systematically. Voids in copper, cracks in the interface and intrusion of copper have been observed, and the correlated mechanism will be discussed.

9:50 AM Break

10:10 AM  
The Effect of Interlayer on Abnormal Grain Growth of Nanotwinned Copper Thin Film during Annealing Process: Leh-Ping Chang1; Hsin-Yuan Chen1; Fan-Yi Ouyang1; 1National Tsing Hua University
    Cu nanotwins have been demonstrated to exhibit excellent electrical and mechanical properties as interconnects in the electronic industry. In this study, we investigate the effect of interlayers, CrN and TiN, on microstructure and thermal stability of highly pure nano-twinned Cu thin film fabricated by unbalanced magnetron sputtering system. After deposition, the samples with different interlayers were subject to anneal at 250℃ in vacuum for 30 to 120 minutes. For as-deposited samples, the Cu thin film with high density nanotwin (111) structure was observed inboth interlayer samples. After isothermal annealing, some abnormal (200) grain growth were found in the samples with CrN interlayer; however, there was no significant microstructural change in the samples with TiN interlayer. The results suggest that the thermal stability of nanotwinned Cu thin film is strongly affected to the interlayer and the samples with TiN interlayer shows better thermal stability than CrN interlayers.

10:30 AM  
Fabrication and Characterisation of Electroplated Nanotwinned-copper Films on Polymer Substrates: Liang-Hsien Chang1; Chih Chen1; Dyi-Chung Hu2; Ray Tain3; Yu-Hua Chen3; 1National Chiao Tung University; 2SiPlus Company; 3New Business Development Division Unimicron Technology Corp.
    Nanotwinned copper (nt-Cu) is drawing wide attention for it simultaneously demonstrates high strength and high ductility, characteristics that are usually thought to be mutually exclusive. Also, according to previous studies, nanotwinned Cu may be the best structure for resisting electromigration damage. In this study, we deposit (111)-oriented nt-Cu on a polymer substrate provided by Unimicron Corporation. CuSO4-based electroplating solution with suitable additives was used in the electroplating process. Direct current was applied to electroplate copper on PCB substrates. Randomly-oriented electroless copper has been deposited on the polymer substrates as the seed layer. Current density and plating bath were as variable. After copper electroplating, XRD, FIB, EBSD were used to understand the detail of the microstructure of the films. We found that the as-deposited Cu has >30% (111)-oriented nt-Cu on the polymer substrate. The microstructures of the highly (111)-oriented nt-Cu will be presented in the conference.

10:50 AM  
A Study of Microstructure, Electronic Flame-off Characteristics and Electrical Properties of 15um Ag-Pd-Au-Pt (APAP) Alloy Wires: Che-Wei Hsu1; Fei-Yi Hung1; Truan-Sheng Lui1; 1National Cheng Kung University
    In this study, a 0.6 mil Ag-2.5Pd-1.5Au-0.15Pt alloy wire (APAP) was selected as the bonding wire material. Platinum element is added to have better strength in pull-test and ability to endure high temperature during the electrical test. The elements distribution of the free air ball (FAB) and electrical properties of APAP wire were investigated. Element distribution in the FAB plays a key role in the first bonding process. The results show Pd-rich dendrite can be observed in the FAB, while the other three elements distribute uniformly. In the measurement of electrical properties, the fusion current of APAP wire has little decrease after high current and long-term use. Accordingly, APAP wire can be a promising candidate for wire bonding.

11:10 AM  
In-situ Evolution of the Nanoporous Microstructure of Sintered Ag at High Temperature: Azdine Nait-Ali1; Diouwel Tandiang1; Marc Legros2; Yijin Liu3; Douglas Van Campen3; Xavier Milhet1; 1Institut Pprime CNRS; 2CEMES CNRS; 3SLAC National Accelerator Laboratory
    Silver pastes sintering is a potential candidate for die bonding in power electronic modules. The joints, obtained by sintering, exhibit a significant pore fraction thus reducing the density of the material compared to bulk silver. This was shown to alter drastically the mechanical properties (Young's modulus, yield strength and ultimate tensile stress) at room temperature. However, while careful analysis of the nanoporous structure has been reported in 2D, little is known about its quantitative spatial evolution during thermal ageing and more specifically during temperature jumps. In this context, high temperature evolutions of the 3D nanoporous structure were observed in-situ using a heater fitted into the beamline 6-2C of SSRL. Segmentation of the porosity and subsequent statistical analysis of the tomographic dataset reveal pore shape, size and spatial distributions evolution during continuous heating. Such an analysis provides insight into the microstructural evolution of sintered nanoporous Ag joints in-service.