The main uses of high-brightness LEDs include lights, signal lights, and backlight displays. Although the cost of high-brightness LEDs is high, its superiority makes LED applications acceptable. High brightness LEDs also have the potential for general illumination. In order to reduce the cost of high-brightness LEDs to be used in living rooms, offices, and parking lots, it is necessary to increase the production efficiency of high-brightness LED manufacturing to achieve the goal of reducing single-bit lumen costs. Such high-brightness LEDs can replace incandescent lamps and various fluorescent lamps that are still being produced. A significant advantage of high brightness LEDs is that their lifetime is measured in decades.
Photons of a variety of high-brightness LEDs are emitted in all directions, including below the substrate. If the substrate has a smaller band gap than the LED illumination area, the substrate will absorb about half of the reflected light, which greatly reduces the light output. If a wafer containing a light-emitting diode is bonded to a substrate wafer having a high reflectivity surface, the substrate wafer can also dissipate heat, and the light that is directed toward the substrate will be reflected back and passed through the emitter region, which is greatly Increased total light output.
Many materials, such as silicon, gallium arsenide, gallium nitride, gallium phosphide, and sapphire, can be used to make LEDs. A photonic layer is grown on the composite semiconductor and transferred to a support wafer of silicon or similar material, and the back side of the wafer is exposed. Composite semiconductors are now only as large as 4 inches. This limits the size of the LED wafer to wafer bond to only 2 to 4 inches in diameter. Another problem is that the two crystals that need to be bonded have different heats.
The coefficient of expansion causes the bonding process to be very slow. Since only one pair of wafers can be bonded at a time, this limits the productivity of high-brightness LEDs and the unit cost is also high. These limitations can be overcome by using the newly developed SUSS wafer bonding device, which enables simultaneous bonding of multiple pairs of wafers.
There are two methods for producing high-brightness LEDs based on gallium nitride: gold-gold hot pressing and gold-tin eutectic bonding. In the gold-gold thermocompression bonding process, a layer of 1 to 3 micron thick gold and a barrier bonding layer is applied to each wafer. In order to eliminate the surface contamination affecting the solid state diffusion mechanism, several steps of cleaning (ultraviolet ozone or chemical wet treatment) are required. The temperature at the time of bonding is 250° to 400°, the pressure is 1 to 7 MPa, and the time is from several minutes to several hours. Increase the time and pressure at low temperatures. If time and pressure are not sufficient, there is usually only a partial bond between the wafer and the wafer.
The tin-gold eutectic method achieves bonding by forming an intermetallic alloy by diffusion of solid and liquid. One wafer is coated with a thin layer of gold and the other wafer is coated with a layer of gold tin up to 5 microns thick. A diffusion barrier layer can be applied if necessary. In order to avoid oxidation of tin at high temperatures, wafer bonding needs to be carried out in a gas, such as in a nitrogen-hydrogen mixture (95% N2, 5% H2). This method requires only a low pressure and a temperature slightly higher than the melting point to complete the bond in a few minutes.
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