Yukio Kizaki

A fine pitch COG technique for a TFT-LCD panel using an indium alloy

A fine pitch chip-on-glass (COG) bonding technique for liquid crystal display (LCD) panels has been developed. An IC chip with gold bumps was dipped in a stirred indium alloy bath in a nitrogen atmosphere without flux. Shallow-bowl-shaped In alloy bumps were selectively formed on the Au bumps on the IC electrodes. The minimum bump pitch was 50 /spl mu/m, and the bump size was 31 by 31 /spl mu/m. The In alloy bumps whose minimum pitch was 100 /spl mu/m were connected to molybdenum conductors without flux at low pressure (30 gf/bump or less) and low temperature (110/spl deg/C or less). The temperature was lower than the alloy melting point. The mean contact resistance was 0.78 /spl Omega/. It was found that the calculation of the minimum bump pitch for the bump sizes and the In alloy bump height is useful for designing new ICs with fine pitch bumps. It has been demonstrated through a thermal shock test (TST), a high temperature and high-humidity storage test; and a high-temperature storage test that the contact resistance changes satisfied the specification. Prototype TFT-LCD panels with 80-/spl mu/m pitch driver ICs were successfully developed. >

A Gradient Index Liquid Crystal Microlens Array for Light-Field Camera Applications

We have developed a microlens array (MLA) that utilizes liquid crystal (LC) for switching between light field and normal picturing modes in the camera application. The gradient index (GRIN) profile in an LC layer was obtained by applying the electric field distribution between a pair of molded-and-buried concave and planar electrodes. The concave array was formed by the imprinting method with ultraviolet curing resin. An indium tin oxide layer was deposited on the concave array to make the transparent electrode, which was then buried and flattened with the same resin. The transparency and flatness of the electrodes and resin keep the image quality high without applied voltage in the normal mode, and the electrode in the concave shape causes the LC layer to have a GRIN profile that acts as a MLA when voltage is applied in the light-field mode. The fabricated MLA showed suitable mode-switching operations by applying (for light-field mode) or not applying (for normal mode) a voltage of ±4 V.

The development of repairable Au-Al solid phase diffusion flip-chip bonding

The authors have developed a new repairable chip-on-glass (COG) bonding technique for liquid crystal display (LCD) panels. Gold (Au) bumps on an LSI chip were bonded directly to aluminum (Al) electrodes on a glass substrate by formation of Al-Au intermetallic compounds in the diffusion layer. The developed repairable bonding technique consists a of two-level bonding process. First, the chip was bonded at 250/spl deg/C. Partial interconnection could be obtained at the local contact portions between the Au bump and the Al electrode. If the electrical connection failed, the bonded chip was removed. There was a distribution of the area formed Al-Au intermetallic compounds at local contact portions for 250/spl deg/C bonding. Some areas formed Al-Au intermetallic compounds of the Al electrode were sometimes removed with the chip removal, and an underlying metal layer was locally exposed at the remained surface. Then, a new chip was bonded on the same Al electrodes under the same conditions at 250/spl deg/C. After obtaining the electrical connection, the second bonding was done at 350/spl deg/C. An AlAu4 intermetallic formation was obtained by this bonding in the diffusion layer. Reliability tests of second bonded samples were carried out and the contact resistance between the Au bumps and the Al electrodes was measured by the four-probe resistance measurement. In the case that the exposed area ratio of the underlying metal layer was less than 30% of bonding area for each Al electrode, the stable electrical connection has been kept for a high temperature storage test and a thermal shock test. It was confirmed that a stable electrical connection had been obtained by the proposed repairable bonding process.

An investigation of stable bonding for Au–Al solid phase diffusion bonding techniques

By using a chip that has Au bumps and a substrate that has only a varying Al film thickness, initial bonding strength is made constant in Au–Al solid-phase diffusion bonding. On the other hand, by changing the Au–Al intermetallic compound formed during bonding, a relationship is obtained between the formed intermetallic compound and bonding reliability. Samples obtained when installing ICs under the same conditions on substrates with Al film thicknesses of 350 nm and 1000 nm were left for 1000 hours at 125 °C and the bonding strength and connection resistance were measured. Immediately after bonding, there was no meaningful difference between them. However, after 1000 hours, reduced bonding strength and increased connection resistance were observed in the sample whose Al film thickness was 1000 nm whereas a stable connection was obtained in the sample whose Al film thickness was 350 nm. The difference in reliability in a high-temperature environment results from differences in the Au–Al intermetallic compounds formed at the time of bonding and by the existence of unreacted Al. In the sample whose Al film thickness was 350 nm, there was no Al at the junction at the time of bonding, so that the final product was predominantly Au4Al. On the other hand, in the sample whose Al film thickness was 1000 nm, Al existed at the junction, so that several Au–Al intermetallic compounds were formed which subsequently degrade by diffusion reactions at high temperature.

67.3: Electrochemical Reaction Display with Dual Reflective and Emissive Modes

We have developed an electrochemical reaction display (ECRD) that can be operated in dual reflective and emissive modes in an entire pixel area of a single device. The ECRD utilizes electrochromic (EC) and electrogenerated chemiluminescence (ECL) reactions. These two electrochemical reactions can be controlled independently by adding a luminescent molecule to a liquid electrolyte of an EC cell with three electrodes. In the reflective mode, the ECRD cell exhibits high reflectance (47%) and high contrast ratio (6:1). In the emissive mode, the cell can show luminescent moving images because of the fast response time (10 ms).

Large Area Microencapsulated Reflective Guest-Host Liquid Crystal Displays and Their Applications

We have developed reflective liquid crystal displays using microencapsulated guest-host liquid crystals, whose size was sufficiently large for viewing documents. A high-brightness image can be realized because there is no need for polarizers. Easy fabrication processes, consisting of screen-printing of microencapsulated liquid crystal and film adhesion, have enabled the realization of thinner and lighter cell structures. It has been confirmed that the display is tolerant of the pressures to which it would be subject in actual use. The optimization of fabrication processes has enabled the realization of reflectance uniformity in the display area and reduction of the driving voltage. Our developed display is suitable for portable information systems, such as electronic book applications.

5.1: Analysis of the Anisotropy of Bend Transition for OCB LCD with Low Initialization Voltage

To improve the efficiency of bend transition in OCB, its anisotropy was analyzed. The transition antiparallel (opposite) to the rubbing direction was significantly fast and this anisotropy remained at low temperatures. We propose two effects of LC shear flow on the anisotropy. Based on a pixel design that takes the anisotropy into consideration, 40% reduction of the initialization voltage in a 3-inch TFT-LCD has been achieved.

AU-AL SOLID PHASE DIFFUSION FLIP CHIP BONDING

A new flip chip bonding technique using Au-Al solid phase bonding was developed. In this technique, a diffusion layer of Au and Al is formed by connecting a driver IC with Au bumps to an Al wiring board. In the conventional flip chip mounting method, the solder bumps formed on the LSI electrode are connected to the board by the reflow method. This method is difficult to apply to metal with weak solder wettability such as Al wiring. Furthermore, the reflow method is difficult to apply to electronic equipment constructed with material that has low heat resistance such as liquid crystal display (LCD) modules. Therefore, assuming the application to LCD panels with Al wiring, an investigation of connection conditions, analysis of heat transmission, and identification of the Au-Al intermetallic compounds were performed. The results show that connection is mechanically and electronically stable, the LCD panel is not affected by the connection conditions such as temperature, the intermetallic compounds are Au5Al2 and Au4Al, and the final product is Au4Al. Furthermore, an LCD module was fabricated by bonding a driver IC with 80 μm connection pitch to an LCD panel using Au-Al solid phase diffusion bonding.