Chih Chen

Recent progress of molecular organic electroluminescent materials and devices

Electroluminescent devices based on organic materials are of considerable interest owing to their attractive characteristics and potential applications to flat panel displays. After a brief overview of the device construction and operating principles, a review is presented on recent progress in organic electroluminescent materials and devices. Small molecular materials are described with emphasis on their material issues pertaining to charge transport, color, and luminance efficiencies. The chemical nature of electrode/organic interfaces and its impact on device performance are then discussed. Particular attention is paid to recent advances in interface engineering that is of paramount importance to modify the chemical and electronic structure of the interface. The topics in this report also include recent development on the enhancement of electron transport capability in organic materials by doping and the increase in luminance efficiency by utilizing electrophosphorescent materials. Of particular interest for the subject of this review are device reliability and its relationship with material characteristics and interface structures. Important issues relating to display fabrication and the status of display development are briefly addressed as well.

Microstructure-electromigration correlation in a thin stripe of eutectic SnPb solder stressed between Cu electrodes

Room-temperature electromigration occurs in a thin stripe of eutectic SnPb solder stressed by a current density of 105 amp/cm2. Hillocks and voids grow at the anode and the cathode, respectively. While the dominant diffusion species is Sn in this two-phase alloy, the growth of the hillocks, surprisingly, originates from the Pb grains.

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.

Electromigration in Sn–Pb solder strips as a function of alloy composition

Using thin film solder strips, we have investigated the electromigration of six different compositions of Sn–Pb solders at current density of 105 A/cm2 near ambient temperature. The six compositions are pure Sn, Sn80Pb20, Sn70Pb30, Sn62Pb38 (eutectic), Sn40Pb60, and Sn5Pb95. The eutectic alloy, with the lowest melting point and a high density of lamella interfaces, was found to have the fastest hillock growth. As composition moving toward the two terminal phases, the hillock growth rate decreases; but it increases again in pure Sn. The interface between Sn and Pb, being the fastest kinetic path of mass transport, also serves as the place to initiate hillock and void formation.

Effect of current crowding on vacancy diffusion and void formation in electromigration

In multilevel interconnects, current crowding occurs whenever the current changes direction, such as when passing through a via. We propose that in current crowding, the current-density gradient can exert a driving force strong enough to cause excess vacancies (point defects) to migrate from high to low current-density regions. This leads to void formation in the latter. This is a key feature of electromigration-induced damage in very large scale integrated interconnects.

Electromigration failure mechanisms for SnAg3.5 solder bumps on Ti/Cr-Cu/Cu and Ni(P)/Au metallization pads

54.5° C, and the thermal gradient reached 365 °C/c mwhen stressed by 13 10 4 A/c m 2 . This induced thermal gradient may cause atoms to migrate from the chip side to the substrate side, contributing to the failure in the anode/chip side. Moreover, the formation of intermetallic compounds in the anode/chip side may also be responsible for the failure in the anode/chip side.

Infrared microscopy of hot spots induced by Joule heating in flip-chip SnAg solder joints under accelerated electromigration

Joule heating effect in solder joints was investigated using thermal infrared microscopy and modeling in this study. With the increase of applied current, the temperature increased rapidly due to Joule heating. Furthermore, modeling results indicated that a hot spot existed in the solder near the entrance point of the Al trace, and it became more pronounced as the applied current increased. The temperature difference between the hot spot and the solder was as large as 9.4°C when the solder joint was powered by 0.8A. This hot spot may play an important role in the initial void formation during electromigration.

Electromigration in eutectic SnPb solder lines

We have prepared eutectic SnPb solder lines for electromigration study by a process of solder reflow into V-grooves etched on (001) Si wafer surfaces. They are thick lines and are highly reproducible. We report here results of lines of 100 μm in width and 150 to 800 μm in length, stressed by a current density of 2.8×104 A/cm2 at 150 °C in ambient. The accumulation of a large lump of solder, rather than hillocks of Sn and Pb, was observed at the anode, and depletions and voids were observed at the cathode. By measuring the volume of the lump, we have calculated the average effective charge number of electromigration in the eutectic solder to be 33, which is close to the reported value of 47 for self-electromigration in bulk Pb. Using x-ray dispersive analysis, we found that Pb is the dominant diffusing species.

DIRECT CORRELATION BETWEEN MECHANICAL FAILURE AND METALLURGICAL REACTION IN FLIP CHIP SOLDER JOINTS

We tested flip chip solder bonded Si samples under tensile and shear loading as a function of annealing time at 200 °C. The solder bump was eutectic SnPb and the underbump thin film metallization was Cu/Cr deposited on oxidized Si. We found that the failure mode is interfacial fracture and the fracture strength decreases rapidly with annealing time. From scanning electric microscope observations, the fracture occurs at the Cu–Sn/Cr interface. We conclude that it is the metallurgical reaction that has brought the solder into direct contact with the Cr surface. The weak joint is due to the spalling of Cu–Sn compound grains from the Cr surface, especially near the edges and corners of the joint.

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.