C. R. Kao

Micromechanical behavior of single-crystalline Cu6Sn5 by picoindentation

The micromechanical behavior of single-crystalline Cu6Sn5 is studied by micropillar compression in a picoindenter. The compound Cu6Sn5 is important because it is used as a structural material in microbumps of advanced electronic packages. Micropillars of Cu6Sn5 with known crystallographic orientations were fabricated by focused ion beam machining. Pillars with the c-axis perpendicular to the load direction tended to possess higher strain to failure and lower Young’s modulus. Measured Young’s modulus from micropillar compression was compared with results from nanoindentation measurements. The modulus values from micropillar compression were consistently smaller than those from nanoindentation measurements.

In situ observations of micromechanical behaviors of intermetallic compounds for structural applications in 3D IC micro joints

For 3D IC application, solder volume in micro joint is much smaller than in the conventional joint. IMCs can easily occupied micro joints and dominate the properties of micro joint. Thus, it is anticipated that IMCs are to be used as structural materials in commercial scale in a few years. However, mechanical property data for reliability modelling are lacking. To characterize mechanical properties of IMCs becomes an urgent issue. In this study, we investigated the micromechanical behaviors of single crystalline IMCs generally formed in micro joints. Testing structures were produced by focused ion beam machining and subsequently tested by picoindenter in SEM which can simultaneously observe the sequence of failure of test structure. Our results are helpful on understanding the fracture modelling of IMCs.

Chip-to-Chip Direct Interconnections by Using Controlled Flow Electroless Ni Plating

A low-temperature and pressureless process using controlled flow electroless Ni plating is developed for bonding Cu pillar bumps for chip-stacking applications. The bonding temperature can be as low as 70°C, which is the lowest among competing processes. A low bonding temperature reduces the thermomechanical stress and enhances reliability, and pressureless bonding eliminates the potential of damaging the delicate devices on chips. In this process, polydimethylsiloxane technology, which is commonly used for microelectromechanical systems fabrication, is introduced to fabricate the airtight fixture for bonding, and Ni ions are supplied by peristaltic pumping the electroless plating solution into the microchannel of a test vehicle with controlled flow. The bonded pillars were analyzed by scanning electron microscope. Besides, to further confirm and to avoid the damage created by polishing steps, a focused ion beam was also used for the observation. The results also show that dome-shaped copper pillars present void-free and seamless interconnections after plating with a 0.16xa0mL/min flow rate at 80°C for 2.5xa0h.

Analyses and design for electrochemical migration suppression by alloying indium into silver

Silver electrochemical migration (ECM) is a serious reliability issue for fine pitch as well as power electronic devices that employ silver as interconnection materials. In this study, a method to suppress the ECM behavior of silver was proposed and implemented. Solid solutions of Ag–In and intermetallic compounds were fabricated and their ECM properties were studied through the water drop test (WDT). The WDT was performed on alumina substrate in deionized water at 3xa0V, where the leakage current was measured. The results show that Ag–In alloy samples have considerably longer lifetime before reaching short circuit than pure silver samples. As indium concentration in Ag–In solid solution increases, the resistance to ECM also increases. The migration process is completely inhibited when the indium concentration reaches 19xa0at.%. The morphologies of the dendrites and surfaces of the anode and cathode were also investigated. A model was established to explain the anti-ECM mechanism of Ag–In alloys. By forming indium oxide on the surface of the anode under electric field in a humid environment, the anode gets passivated and the silver dissolution path is completely blocked, and thus inhibiting the ECM process. It was discovered that the quality of electrode surfaces is essential in realizing the full potential of the inherent anti-ECM ability of Ag–In alloys. In conclusion, the results of this investigation have shown that Ag–In alloys can be promising and valuable candidates for conductors, metallization, and interconnect materials in fine pitch and high power electronic devices.

Effects of Aspect Ratio on Microstructural Evolution of Ni/Sn/Ni Microjoints

Under the simultaneous influence of volume shrinkage and surface solder diffusion, the microstructural evolution in Ni/Sn/Ni microjoints exhibits a conformable tendency among different joint sizes. This conformable tendency is the necking of the Sn layer and the initiation of voids which occur only near the periphery of the Sn layer. The reasons for this joint-size dependency have been theoretically evaluated and proposed in this study. This dependency remains applicable until the Ni3Sn4 layers growing in opposite directions impinge on each other. Afterward, microvoids are able to form everywhere along the centerline of the joints. It is hypothesized that the electrical and mechanical performance of microjoints is significantly related to the microstructures in these joints. With the help of the joint-size dependency proposed in this study, it is expected that the dimensional design of microjoints is capable of being optimized to assure high reliability.

Reaction Within Ni/Sn/Cu Microjoints for Chip-Stacking Applications

The reaction within Ni/Sn/Cu microjoints has been investigated with a 10xa0μm initial thickness of Sn so that Sn was quickly consumed in the early stage of the reaction, transforming the joints into full intermetallic joints. Accordingly, the emphasis of the present study is on the materials interactions in such full intermetallic joints. Ni/Sn/Cu sandwiches were prepared by thermal compression bonding. High-temperature storage tests were conducted at 150°C, 180°C, and 200°C for different time periods. The hardness and Young’s modulus of the intermetallic joints were measured using nanoindentation. The key finding of this study is that, once Sn was exhausted, the originally planar (Cu,Ni)3Sn grew preferably along the grain boundary of (Cu,Ni)6Sn5, creating a highly nonuniform (Cu,Ni)3Sn growth front. Both the hardness and elastic modulus increased with the Ni concentration in the intermetallics.

Grain Boundary Penetration of Various Types of Ni Layer by Molten Metals

The grain boundary penetration of three types of Ni layer, Ni foil, electroplated Ni, and electroless Ni, by molten Pb and 95Pb5Sn (wt.%) is investigated. The average grain sizes of Ni foil and electroplated Ni are 10xa0μm and 1xa0μm, respectively, while the electroless Ni is amorphous. The purpose of using two molten metals is to study the effect of intermetallic formation on grain boundary penetration. Molten Pb was able to penetrate or disintegrate all three types of Ni, including amorphous Ni, which does not contain any grain boundaries. On the other hand, the addition of merely 5xa0wt.% Sn into molten Pb was able to slow the penetration down substantially for all three types of Ni layer, with the greatest suppression found in electroless Ni where a grain boundary penetration event did not take place. The mechanism for the Sn effect is due to the formation of a protective Ni3Sn4 intermetallic compound at the interface acting as a barrier against grain boundary penetration.

Effect of Ag concentration on Ni/Sn-xAg/Ni micro joints under space confinement

Previous studies have shown that voids were observed in 3D IC-scaled micro joints of Ni/Sn/Ni solid state reaction under severe space confinement and Ag addition can effectively eliminate the void formation. Therefore, the present study is aimed at investigation of the lower limit of Ag concentration which prevents void formation and also the effect of Ag concentration when primary Ag3Sn appears. The solid state diffusion couple, Ni/Sn-xAg/Ni, was prepared by thermal compression with six different Ag concentrations from 0 to 8.0 wt.%. After thermal compression bonding, samples were placed at 200°C for isothermal aging process. Subsequent observation of cross-section showed that as-bonded Ag3Sn initially formed as fine particles dispersed in Sn matrix. Then, Ag3Sn particles coarsened during aging and were likely heterogeneously located at the Ni3Sn4 layer. After Sn was completely consumed, Ag3Sn particles were located at the middle of sandwich structure and microstructure of various Ag concentrations were distinct. A nearly void-free sandwich structure could be established by adding more than 3.5 wt. % Ag in solder when Sn was fully consumed. In addition, the relation between the area fraction of continuous Ag3Sn layer and Ag concentration was found linear.