Yumi Fukuda

Luminescence Properties of Eu2+-Doped Green-Emitting Sr-Sialon Phosphor and Its Application to White Light-Emitting Diodes

We developed a green-emitting phosphor Sr3Si13Al3O2N21:Eu2+ that is highly luminescent under excitation by blue light. It shows a highly efficient green luminescence whose external quantum efficiency reaches 67% for 460 nm excitation, and has small thermal quenching. Using this phosphor, we obtained white light-emitting diodes (LEDs) whose luminous efficacy and color rendering index Ra at 5330 K are 62 lm/W and 87, respectively. These features show that this green-emitting phosphor has high potential for application to white LEDs.

Luminescence Properties of Eu2+-Doped Red-Emitting Sr-Containing Sialon Phosphor

We developed a Eu2+-doped red-emitting Sr-containing sialon phosphor Sr2Si7Al3ON13:Eu2+ that could play a very important role in high color rendering of white light-emitting diodes (LEDs) for solid-state lighting. It realizes both high efficiency and small thermal quenching under excitation by blue light, which are essential for operation in a high-temperature atmosphere. It shows a highly efficient red luminescence whose external quantum efficiency reaches 73% for 450 nm excitation. These features show that this red-emitting phosphor has high potential for application to white LEDs.

White Light-Emitting Diodes for Wide-Color-Gamut Backlight Using Green-Emitting Sr-Sialon Phosphor

We have successfully developed a white light-emitting diode (LED) for a wide-color-gamut backlight composed of a green-emitting phosphor Sr3Si13Al3O2N21:Eu2+ combined with a blue LED and a red-emitting phosphor CaAlSiN3:Eu2+. This white LED showed a discrete spectrum with distinct separation of red, green, and blue primary colors due to a narrow emission band of around 525 nm for the green phosphor. 94.2% of the wide color gamut of the National Television System Committee standard was attained by applying typical color filters of LCDs. The power LED module composed of 16 of these white LEDs revealed their excellent power dependence. The LED is expected to replace cold cathode fluorescent lamp (CCFL), and find a suitable application as a backlight in large-scale LCDs for in-vehicle use or for flat-panel television sets.

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.

Cation and anion ordering in Sr2Si7Al3ON13 phosphors

A series of photoluminescent Ce3+ doped samples with compositions close to Sr2Si7Al3ON13:Ce have been studied by neutron powder diffraction to determine the Si4+/Al3+ and N3−/O2− site ordering. Contrary to a commonly held assumption that the edge sharing tetrahedral sites in this structure are occupied exclusively by Al3+, we find a partial occupancy of Al3+ on these site but also an unexpected preference for Al3+ to occupy 2 other tetrahedral sites which are only corner sharing. From the crystal structures and local structures, as determined by pair distribution function (PDF) analysis, we also find evidence for alternating Si–Al site ordering within the edge sharing chains as well as dimerization of the Si4+ and Al3+ cations within these chains. The O2− are found to be partially ordered onto 2 of the anion sites, although small amounts of O2− are found on other sites as well. The cation and anion ordering found by neutron diffraction is supported by theoretical calculations. Understanding cation and anion ordering is essential for optimizing the photoluminescence properties of this promising class of phosphor materials.

White Light-Emitting Diodes Using Sr-Containing Sialon Phosphors

We have successfully developed white light-emitting diodes (LEDs) that are highly luminescent with excellent color rendering by utilizing a green-emitting phosphor (Sr3Si13Al3O2N21:Eu2+) and a red-emitting phosphor (Sr2Si7Al3ON13:Eu2+). The LEDs have luminous efficacy of 53–60 lm/W and color rendering index Ra of 95–98 in a wide range of tunable correlated color temperatures (3200–6100 K). We found that white LEDs using these phosphors have excellent properties at high power output. Hence, these phosphors have potential applications in white LEDs for lighting.

Synthesis and Characterization of β-SiAlON Phosphor Powder Prepared by Reduction Nitridation of a Zeolite

Rare-earth activated oxynitride or nitride luminescent materials have attracted considerable attention due to their potential applications as phosphors and pigments. Eu2+-doped -sialon has been reported to represent a new class of green phosphors with high efficiency. In this study, -sialon phosphor was synthesized by reduction nitridation of a zeolite. Eu ion-exchanged zeolite was fired at 1400 °C for 1 hour under NH3 gas containing 0.5 vol%C3H8. As a result, formation of -sialon with green emission under UV irradiation was confirmed.