Hsiang-Yao Hsiao

Unidirectional growth of microbumps on (111)-oriented and nanotwinned copper.

Tiny Tinny Bumps One challenge in moving to three-dimensional integrated circuit architectures is the need for aligned interconnects to join neighboring layers. Hsiao et al. (p. 1007) applied rapid stirring to the direct current electroplating of copper to produce films with oriented copper grains that have a high density of nanotwin defects. The resulting material was an excellent platform for the growth of copper-tin intermetallic compounds in the form of arrays of microbumps potentially suitable for the soldering of electronic components. Oriented copper grains grown using direct-current electroplating serve as a template for intermetallic microbumps. Highly oriented [111] Cu grains with densely packed nanotwins have been fabricated by direct-current electroplating with a high stirring rate. The [111]-oriented and nanotwinned Cu (nt-Cu) allow for the unidirectional growth of Cu6Sn5 intermetallics in the microbumps of three-dimensional integrated-circuit packaging; a uniform microstructure in a large number of microbumps of controlled orientation can be obtained. The high-density twin boundaries in the nt-Cu serve as vacancy sinks during the solid-state reaction between Pb-free solder and Cu and greatly reduce the formation of Kirkendall (or Frenkel) voids.

Transition from flip chip solder joint to 3D IC microbump: Its effect on microstructure anisotropy

As microelectronic industry develops 3D IC on the basis of through-Si-vias (TSV) technology, the processing and reliability of microbumps, which are used to interconnect the stacking chips, is being actively investigated. Due to the reduction in size of microbumps, the diameter is about one order of magnitude smaller than that of flip chip solder joints, and the volume is 1000 times smaller. Its microstructure and in turn its properties will be anisotropic because the number of grains in a microbump becomes very small. Its statistical failure will have a wide distribution because of anisotropy, including early failure. This review addresses this issue and the remedy.

Thermomigration in flip-chip SnPb solder joints under alternating current stressing

Thermomigration in flip-chip solder joints is investigated using alternating currents and infrared microscopy to decouple it from electromigration effect. It is found that the thermal gradient in solder bump can be as high as 2143°C∕cm when 9.2×104A∕cm2 was applied at 100°C. Markers fabricated by focus ion beam are employed to measure the thermomigration rate. The thermomigration flux is measured to be 3.3×1013at.∕cm2. With the known thermal gradient, the molar heat of 26.8kJ∕mole has been obtained for the transport of Pb.

Thermomigration in Pb-free SnAg solder joint under alternating current stressing

Lead-free solders have been adopted by the microelectronics industry. However, their thermomigration behaviors are unclear. Thermomigration in eutectic SnAg3.5 solder joints was investigated using an alternating current (ac) of 0.57 A at 100 °C. The ac eliminates the electromigration effect and creates a thermal gradient of 2829 °C/cm, facilitating the study of thermomigration. Arrays of tiny markers fabricated by a focused ion beam are employed to measure the thermomigration rate. It is found that Sn atoms migrated toward the hot end. The thermomigration flux and molar heat of transport are measured to be 5.0×1012 atoms/cm2 and 1.36 kJ/mole, respectively.

Direct measurement of hot-spot temperature in flip-chip solder joints under current stressing using infrared microscopy

Several simulation studies reported that a hot spot exists in flip-chip solder bumps under accelerated electromigration. Yet, there are no experimental data to verify it. In this paper, the temperature distribution during electromigration in flip-chip SnAg3.5 solder bumps is directly inspected using infrared microscopy. Two clear hot spots are observed in the bump. One is located at the region with peak current density and the other one is at the bump edge under the current-feeding metallization on the chip side. Under a current stress of 1.06×104A∕cm2, the temperature in the two hot spots are 161.7 and 167.8°C, respectively, which surpass the average bump temperature of 150.5°C. In addition, the effect of under-bump-metallization (UBM) thickness on the hot spots is also examined. It indicates that the hot-spot temperature in the solder bump increases for the solder joints with a thinner UBM. Electromigration test indicates that these hot spots have significant influence on the initial failure location.

To inhibit the consumption of Cu during multiple reflows of Pb-free on Cu

We report an effective approach to inhibit the consumption of Cu during multiple reflows of SnAg2.3 solder on Cu. By depositing a very thin layer of the solder on Cu and followed by a 10-min reflow, the scallop-type morphology of the interfacial Cu6Sn5 intermetallic compounds (IMCs) became flat and the channels between them closed up. When additional solder was deposited on the sample and reflowed again, the consumption of Cu as well as the growth the IMCs was retarded.

Study of interaction between Cu-SnAg and Ni-SnAg interfacial reactions by low solder volume in 3D IC packaging

Metallurgical reactions in Ni/ SnAg2.3 solder/ Cu system are investigated by varying the solder thickness from 40µm to 10µm. We found that the growth rate of the interfacial intermetallic compounds (IMCs) strongly depend on the solder thickness. In the Ni/ 40-µm solder/ Cu samples, the (Cu, Ni)6Sn5 IMCs on the Ni side grew slightly faster than those on the Cu side. However, the trend reverses as the solder thickness decreases below 20µm. The (Cu, Ni)6Sn5 on the Ni side even stop growing after 4-min reflow at 260°C in the Ni/10µm solder/ Cu samples. Yet, the IMCs on the Cu side grew thicker than that in Ni/ 40µm solder/ Cu samples. Compositional analysis reveals that the Cu and Ni concentrations in the solder increases with the decreasing in solder thickness. The changes in the Ni and Cu concentration in the solder plays crucial role on the growth rates at the IMCs in the Ni/ SnAg solder/ Cu system.