Hiroshi Takatsuji

Mo-capped Al-Nd alloy for both gate and data bus lines of liquid crystal displays

Abstract Aluminum–neodymium (Al–Nd) alloy and molybdenum (Mo)-capped structures were applied to both gate and data bus lines of liquid crystal displays (LCD). We investigated the hillock resistance and electrical properties of Al–Nd alloys. Their structures were studied by cross-sectional transmission electron microscopy (TEM) and SEM. Adding 2 at.% Nd to Al effectively prevented the Al film from forming hillock and whisker. The electrical resistivity varied with the annealing temperature after deposition: the resistivity was 4.3 μΩ cm with annealing at 350°C and 10.0 μΩ cm with annealing at 250°C. Mo is used not only to suppress hillocks, but also for taper etching of lines and as a contact layer with other materials. We investigated the effects of adding Nd to Al, and found that the Nd addition improved the step coverage, thermal resistance, patternability, and mechanical strength of the lines. By applying a common structure and metals for both gate and data bus lines, we could increase the productivity of TFT-LCDs.

Evaluation of Al(Nd)-alloy films for application to thin-film-transistor liquid crystal displays

Higher-resolution, larger-diagonal active-matrix liquid crystal displays (AMLCDs) will require the use of low-resistivity gate metal such as aluminum transition-metal alloys. Al(Nd ≥ 3 wt.%) alloy films are adequate for AMLCD fabrication because of their low resistivity and their tendency not to form hillocks during thermal processing. The use of both optical light scattering and nanoindentation for the rapid evaluation of hillock formation has been demonstrated, along with the use of ramped resistance measurements for observing the process of discontinuous precipitation (the combination of Al grain growth and Al-Nd compound precipitation). Al(Nd) films were further characterized by TEM and AFM to confirm the effect of their finely dispersed Al-Nd compound precipitates on decreased grain size and decreased hillock formation.

Pinholes in Al thin films: their effects on TFT characteristics and a taguchi method analysis of their origins

Abstract The effects of pinholes in the aluminum gate electrode on the characteristics of thin-film transistors (TFTs) were investigated. It was found that the TFT characteristics are degraded by pinholes in the aluminum thin film in the case of an etching-stopper-type inverted staggered TFT fabricated by using a back-side exposure process. By means of Taguchi methods, it was found that the generation of pinholes in the aluminum thin film is strongly related to the sputtering parameters, and that the pinhole density is related to the surface roughness of the Al thin film.

Pure Al thin film protective layer to prevent stress migration in Al wiring for thin-film transistors

Abstract The anti-stress migration property of layered structure aluminum (Al) thin films overcoated with pure Al was investigated for application of such films as interconnect materials in large arrays of high-resolution thin-film transistor liquid crystal displays (TFT-LCDs). It was found that no hillock or whisker generation occurred in a pure Al thin film with a sputter-deposited fine-grained polycrystalline pure Al layer after exposure to mechanical and 300°C thermal stresses. Atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM) analyses revealed the morphology of the layered structure thin film and the mechanism for the prevention of stress migration in the film.

Al-based sputter-deposited films for large liquid-crystal-display

Abstract The relationship between the properties of Al thin film sputter-deposited on large glass substrate (550×650 mm) and the sputtering process parameters were clearly identified by means of Taguchi methods. The number of magnet passes, the substrate temperature, and the Ar pressure have significant effects on the microstructure of Al thin film. Taguchi methods contribute to the optimization of the sputtering process by reducing the number of test trials from 324 to 18. The results indicate good agreement between the properties of the simulated and actual sputter-deposited Al film.

Effects of surface damaging and overcoating on the formation of hillocks and whiskers on pure Al films deposited on to a glass substrate

Abstract Conditions of the outermost surfaces of Al films deposited on to a glass substrate were modified systematically in two different ways. In one, the surfaces of the Al films were indented by a Vickers indenter with different loads. In the other, overcoating layers with different thicknesses were deposited on to the original Al films. The effects of the Vickers indentation and the overcoating layer on the formation of hillocks and whiskers were studied systematically. Modification of the outermost surfaces leads to both suppression and enhancement of formation of hillocks and whiskers. It is concluded that the strength of the natural oxide layer governs whether or not whiskers and hillocks are formed.

Advanced time-of-flight secondary ion mass spectrometry analyses for application to TFT-LCD

Abstract Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis was applied to TFT-LCD failure analysis. We have demonstrated that the TOF-SIMS analysis using large-area sample mapping function is a powerful method to distinguish good regions from defect regions, and that selective field data analysis, which are obtained by reconstructing the raw-data of initial mapping measurement helps to identify the source of chemical and particulate contamination. We have successfully detected the reason of the failure.

In-Situ Tem Characterization of Whiskers on Al Electrodes for Thin-Film Transistors

The recent attention paid to stress migration of aluminum (Al) electrodes in thin-film transistor liquid crystal display (TFT-LCD) applications indicates that wiring materials with low electrical resistivities are of considerable interest for their potential use in higher-resolution displays. In this paper, we firstly describe how as-grown Al whiskers on Al electrodes fabricated on a LCD-grade glass substrate can be characterized by means of a high-voltage transmission electron microscope (HV-TEM) operated at 1 MV. The whiskers ranging from 300 to 400 nm in diameter are sufficient to be transparent to high-voltage electrons. This allows detailed observation of whisker characteristics such as its morphology and crystallography. In most cases, the as-grown Al whiskers in our study had straight rod shapes, and could be regarded as single crystals. Secondly, we report on the in-situ fabrication and observation of Al whiskers at elevated temperature with the HV-TEM. Since relatively thick TEM samples (up to about 1 mm) can be set on a sample holder in the HV-TEM, various growth stages of Al whiskers can be investigated under various heating conditions. Finally, we demonstrate a TEM sample preparation method for the cross-section of an individual Al whisker, using focused ion beam (FIB) etching. This technique makes it possible to reduce the thickness of an Al whisker close to the root. Both bright- and dark-field TEM images provide nanostructural information on the whisker/Al thin-film interface.

The influence of cooling water flowing in the sputtering target on aluminum based thin film nanostructure deposited on glass substrates

Abstract The influence of the cooling efficiency of the sputtering cathode on the nanostructure of an aluminum based (Al-based) thin film sputterdeposited on a large glass substrate (550 × 650 mm) was investigated, With high cooling efficiency, a highly (111)-textured pure-Al film was formed. On the other hand, low cooling efficiency contribute to the formation of a three-layer structure Al-Cu thin film that showed strong resistance to hillock and whisker generation.

Nitrogen-added Al rare-earth alloys for thin film transistors

Abstract The effects of nitrogen addition to aluminum-rare-earth alloys were investigated. Yttrium and gadolinium were employed as respective alloy components. The electrical properties and hillock densities of alloy films were investigated, and their nanostructures were studied by cross-sectional transmission electron microscopy. Nitrogen effectively decreases the grain size, and causes the columnar structure, that is generally present in aluminum-based alloys, to disappear. Nitric aluminum-rare-earth alloys have a strong resistance to hillock formation, and formed no micro-voids even after annealing at 450°C. An N 2 flow rate of 1.3–10% in the sputtering gas gives a low hillock density of 2.0 E +1–7.7 E −1 pcs/mm 2 after annealing at 350°C in both nitric Al-rare-earth alloys. In the case of patterned nitric aluminum-yttrium alloys, an N 2 flow rate of less than 1.3% causes large side-hillocks after annealing at over 350°C. As an optimum value, an N 2 flow rate of 2.5% results in a low hillock density of 1.9 E +1 pcs/mm 2 and a low resistivity of 8.6 μΩ cm after annealing at 350°C.