Eiji Iwamura

A study of hillock formation on AlTa alloy films for interconnections of TFT-LCDs

Abstract The relationship between hillock formation and microstructure was studied in AlTa alloy films for interconnections of thin-film transistor-liquid-crystal displays. In-situ scanning electron microscopy observation of hillock formation, transmission electron microscopy studies of microstructures of both the hillock and film, and in-situ stress measurements during isothermal annealing were carried out on Al-2at.%Ta alloy films deposited on glass substrates by d.c. magnetron sputtering. Hillock size increased with annealing temperature and time during variable temperature and isothermal annealing, respectively. Macroscopic hillock number density saturated soon after the appearance of hillocks. New hillocks were observed on the outer perimeter of old hillocks. The distance between hillocks ranged from 20 μm to 100 μm, an extremely large distance in comparison with the grain size. The relationship between hillock formation and microstructures on AlTa alloy films can be explained by a model in which hillock formation due to lateral diffusion, i.e. diffusion in the film plane, results in compressive stress relaxation in a large area around the hillock. The fine-grained film structure caused by the addition of Ta plays an important role in reducing hillock density.

Influence of adding transition metal elements to an aluminum target on electrical resistivity and hillock resistance in sputter‐deposited aluminum alloy thin films

Effects of adding group VIII transition metals (Fe, Co, and Ni) to an aluminum target on electrical resistivity and hillock suppression of sputter‐deposited Al alloy films were studied. It was found that the group VIII transition metals and Al formed a complete series of metastable solid solutions in the as‐deposited state. The solid solutions were decomposed into intermetallic compounds, and the microstructures of the Al alloy films changed during annealing up to 300 °C. The electrical resistivities of Al–Fe, Al–Co, and Al–Ni alloy films changed corresponding to the change in their microstructures, and markedly decreased to 5.0 μΩ cm at the precipitation points. The film stresses also changed corresponding to the change in the microstructure of the Al alloy films, and were abruptly relieved by precipitation of intermetallic compounds and grain growth. Since Al–Fe, Al–Co, and Al–Ni alloy films did not yield, they are highly resistant to hillock formation. The low electrical resistivity and the excellent r...

Effects of Nd content in Al thin films on hillock formation

The effect of Nd content in Al films on hillock formation was investigated for applications of interconnections for thin film transistor liquid crystal displays. It was found that the hillock density of Al–Nd alloy films was strongly dependent on the Nd content, and the hillocks were completely suppressed in Al–2.0–6.0 at. % Nd alloy films. X-ray diffractometry, x-ray absorption fine-structure spectroscopy, and transmission electron microscopy indicated that the microstructures of Al–Nd alloy films strongly depend on the Nd content, and Al–2.0–6.0 at. % Nd alloy films form a stable solid solution with a polycrystalline α-Al-like structure. The results obtained from microstructural analysis were largely in agreement with hillock formation behavior. In this study, it was found that the excellent hillock resistance of Al–2.0–6.0 at. % Nd alloy films originates from solid-solution hardening by the strongly distorted α-Al-like structure.

Effect of aluminium oxide caps on hillock formation in aluminium alloy films

Abstract The effect of surface oxide layers on thermally induced hillock formation was examined in AlTa and AlCu alloy films. An anodic oxide or a sputter-deposited oxide layer was intentionally formed on the top of the Al alloy films, and subsequently annealed in a vacuum of less than 1×10 −4 Pa. Hillock formation on the encapsulated films, the dependence of hillock density on types and thickness of the oxides, and film stresses were investigated. It was observed that hillocks preferentially formed under the oxides and extruded out of the films, breaking through them. SEM and cross-sectional TEM micrographs revealed hillock growth along with the oxide/metal interface and deformation of the surface oxides following the change of surface topography by hillock formation. More than 100 nm in thickness of anodic oxide caps or a 230-nm thick sputter oxide were necessary to suppress hillock formation. An identical hillock density was obtained in each Al alloy film with encapsulation up to 62 nm in thickness, independent of the thickness and type of the oxide cap. The results indicate that surface conditions are unlikely to determine hillock density, and hillock suppression in the encapsulated films was presumably achieved by lower film stresses at elevated temperature resulting from higher initial tensile stresses induced by anodization and a smaller gradient of the stress-temperature curve of the metal/oxide multilayered films.

Morphology of sputter deposited Al alloy films

Abstract Morphology of Al–2.0at%Ta and Al–2.0 at.% Nd alloy films before and after annealing was investigated for applications of interconnections for liquid crystal displays. It was found that the morphology and the microstructure of Al–2.0 at.% Nd alloy films changed markedly by annealing at the temperature region from 200°C to 300°C, while the morphology of Al–2.0 at.% Ta alloy films did not change by annealing up to 400°C. For the case of Al–2.0 at.% Nd alloy films, the incline of the fiber texture to the substrate normal was observed during annealing. Structural characteristics of the Al films were investigated by TEM, SAD and XRD to determine the influence of alloying elements on the morphology and the fiber texture. From these results, it was concluded that the microstructures strongly influence the morphology and the grain orientation of Al alloy films.

Hillock morphology and density in AlTa and AlCu alloy films encapsulated with aluminum oxide layers

The effect of surface oxide layers on annealed hillock formation was examined in 1 μm thick films of AlTa and AlCu with artificial aluminum oxide caps. An anodic oxide layer or a sputter-deposited oxide layer was formed on the top of the Al alloy films, and the test samples were annealed at 573 K for 3.6 ks in a vacuum of less than 1×10−4 Pa. How hillocks formed on the films and the dependence of hillocking density on oxide thickness were investigated. It was observed that hillocks formed under the oxides or broke through them. Scanning electron micrographs and cross-sectional transmission electron micrographs provided a clear view of the flexibility of the surface oxides. AlTa films exhibited a much lower hillock density than AlCu films. There was no significant dependence of hillock density on oxide thickness or type of surface oxide. It was determined that the hillock density was independent of defects or of the properties of the thin surface oxide layers.The effect of surface oxide layers on annealed hillock formation was examined in 1 μm thick films of AlTa and AlCu with artificial aluminum oxide caps. An anodic oxide layer or a sputter-deposited oxide layer was formed on the top of the Al alloy films, and the test samples were annealed at 573 K for 3.6 ks in a vacuum of less than 1×10−4 Pa. How hillocks formed on the films and the dependence of hillocking density on oxide thickness were investigated. It was observed that hillocks formed under the oxides or broke through them. Scanning electron micrographs and cross-sectional transmission electron micrographs provided a clear view of the flexibility of the surface oxides. AlTa films exhibited a much lower hillock density than AlCu films. There was no significant dependence of hillock density on oxide thickness or type of surface oxide. It was determined that the hillock density was independent of defects or of the properties of the thin surface oxide layers.

A study of hillock suppression in Al alloy interconnections for liquid crystal displays

Hillock formation mechanism and hillock suppression by alloying Al films is summarized. The hillock growth occurs at elevated temperature and under compressive film stress by migration of Al atoms along grain boundaries to film surface. Microstructure and its change upon heating of Al films directly affect stress relaxation behavior, and strongly affect hillock formation behavior. Therefore, microstructural altering and stress relaxation controlling of Al films by alloying are effective in suppressing hillocks. It is confirmed that Al-Ta, Al-Ta-Si and Al-Nd alloy films have strong effect in suppressing hillocks, and are most appropriate materials for interconnections of liquid crystal displays.