Satoshi Tsuji

Quantitative evaluation of “mura” in liquid crystal displays

The visual performance of liquid crystal displays (LCDs) has usually been evaluated by visual inspection during the manufacturing process. One of the visual problems hardest to recognize are regions of low contrast and nonuniform brightness called mura. The accurate and consistent detection of the mura is extremely difficult because there are various shapes and sizes of mura and the inspection results tend to depend on the operators. We conducted a study on the quantitative evaluation of mura based on visual analysis, intending to clarify the de- tection method and create an automated mura inspection process. We developed an algorithm and a hardware system based on a commer- cially available charge-coupled-device camera and a personal computer system with an image processor board. This system can successfully identify and evaluate mura. The algorithm was developed from research on visual analysis and human perception. We converted the front-of- screen images from the LCDs into distributions of luminance information, and the mura regions were distinguished from the background area us- ing our novel algorithm. Our identification method can also distinguish between the muras caused by flaws in the LCD cells and the intentionally designed nonuniform luminance distribution of the backlight.

Application of focused ion beam milling to cross-sectional TEM specimen preparation of industrial materials including heterointerfaces

Specimen preparation is always a crucial question of transmission electron microscope (TEM) investigation. Recently new specimen preparation technique using focused ion beam (FIB) milling was developed. We applied this technique to the fabrication of cross-sectional TEM specimens of industrial materials including heterointerfaces. The following investigations were carried out: multilayered structures in thin-film transistors for the liquid crystal display, interfaces in a hot dip galvanized steel and microstructures of a diamond film on silicon nitride. The largest benefit of FIB lies in its application to heterointerface analysis at a particular position in submicron scale. This technique extends the use of TEM analysis into new areas of characterization of industrial materials.

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.

Extraction and evaluation of mura in liquid crystal displays

The visual performance of liquid crystal displays (LCDs) has been usually inspected and evaluated by sensory analysis at the manufacturing process. One of the most indistinct visual problems is low-contrast non-uniform brightness region called muras. The accurate and consistent detection of the muras is extremely difficult because there are various shapes and sizes of muras and the inspection results tend to depend on the operators. We conducted a study on the quantitative evaluation of muras based on visual analysis and human perception. We converted the front of screen (FOS) images from the LCDs into distributions of luminance information, and the mura regions were distinguished from the background area using our novel algorithm. This approach also led to a weighting function for the categories of muras that appear in the panels. Our identification method can also distinguish between the muras caused by flaws in the LCD cells and the intentionally designed non- uniform luminance distribution of the backlight.

Transmission electron microscopy of thin-film transistors on glass substrates

Abstract The recent attention paid to sample preparation of multilayered structure on glass substrates indicates that cross-sectional transmission electron microscopy (X-TEM) are of considerable interest for their potential effectiveness in thin-film transistor liquid crystal display (TFT-LCD) application. We describe the significant features of our failure analysis problem which involves some difficulties: fragile location, anomalous, and rare defect. An application of focused ion beam (FIB) etching to specimen preparation for X-TEM observation is introduced. We demonstrated that the FIB technique, which allows very localized etching without inducing mechanical stress to thin TEM sample, is very convenient for this delicate location. We also demonstrate that our FIB etching technique is capable of high thinning rates, which allows rapid production of high-quality TEM specimen. Finally, we demonstrate how faulty TFTs including long and fragile protrusions such as whiskers, can be characterized by means of FIB and X-TEM combination.

Application of cross-sectional transmission electron microscopy to thin-film-transistor failure analysis

The locations of process-induced defects in hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs), which are used as elements of active-matrix liquid crystal displays, were investigated by combining focused ion beam (FIB) techniques with cross-sectional transmission electron microscopy (X-TEM). The FIB technique is applied to TFT failure-analysis problems which require very localized etching without inducing mechanical stress. We demonstrate how these techniques are used to characterize TFT defects such as thin layers formed from etching residue, microvoids in the multilayers, fragile aluminum whisker protrusions on the electrodes, or portions of the TFT multilayer damaged by mechanical stress.

Investigation of thin-film transistors using a combination of focused ion beam etching and cross-sectional transmission electron microscopy observation

Abstract The nanometer-scale multilayered structure of hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) used as elements for active matrix liquid crystal displays is investigated using a combination of focused ion beam (FIB) etching and cross-sectional transmission electron microscopy (X-TEM). This paper presents how faulty TFTs fabricated on a glass substrate can be characterized by X-TEM to achieve pinpoint accuracy. X-TEM specimens are prepared by FIB etching from TFTs including relatively large microvoids in their multilayers, without dropping of delicate edges. Cross-sectional bright-field TEM micrographs clearly show the details around the microvoids. This pinpoint X-TEM identifies faults and gives an analysis of their causes. In this paper, conventional low-angle ion milling and FIB etching are compared, Ion irradiation damage during FIB etching is also discussed.

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.

Electron microscopy observation of whiskers and hillocks formed on an Al film deposited on to a glass substrate

Abstract Hillocks and whiskers formed on an Al film deposited on to a glass substrate have been characterized by plan-view as well as cross-sectional transmission electron microscopy. Most of the hillocks and whiskers were nucleated in the middle of the bottom grains and few dislocations were observed inside the bottom grains. Hillocks inherit the orientations from the bottom grains, while whiskers do not. A new formation mechanism of hillocks and whiskers where surface diffusion and impurity-enhanced grain-boundary diffusion play important roles has been proposed.

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.