Author: Sumant Sood, SUSS MicroTec Inc., Waterbury Center, VT, Ray Thomas, Sonoscan Inc., Elk Grove Village
Bonding of Wafers in High-Brightness LED Production <br>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 coefficients of thermal expansion and the bonding process is 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 bonding temperature is 250° to 400°, the pressure is 1 to 7 MPa, and the time is from a few 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 is carried out in a gas such as 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.
Devices that have been developed for simultaneous bonding of multiple pairs of wafers are based on an 8-inch wafer platform. To increase throughput, the 8-inch diameter platform accepts eight 2", four 3-inch, or three 4-inch wafers at a time (Figure 1) and can be bonded simultaneously.
Figure 1: Three pairs of 4-inch wafer synchronous bonding devices developed by SUSS Microtec.
This device has a cavity in which the cavity can be heated and pressurized and can be inserted into the fixture. The fixture is composed of upper and lower plates made of silicon carbide. The upper plate is detachable and the lower plate has a thermocouple to control the temperature. The flatness of each board is required to be within 2 microns. The plane meshing between the two plates is required to be no more than 20 microns in the range of 200 mm.
The two wafers to be bonded are placed between the upper and lower plates of the fixture, and then the two wafers are aligned by hand or pneumatically. A layer of easily meshable material, such as graphite, may be placed between the wafer and the upper plate to achieve optimum pressure distribution (Figure 2). The upper plate of the device is close to the wafer and the two wafers are fixed. The positioning device is inserted into the cavity by the loader, and the lower plate is brought up to the positioning device so that the loader can be removed, and the device cavity is sealed and bonded. The process begins.
Figure 2: Eight pressure sheets between 2 inches and the upper plate reveal a very uniform pressure distribution during the bonding process.
The optimum time for LED wafer bonding, temperature and pressure parameters are determined by the process to meet metal and device requirements. The key to successfully developing processes and converting technology to production is to quickly and accurately combine wafers. The surface appears as an image, and the image is used to find the location of the pores and cracks.
Detection of Bonding of High-Brightness LED Wafers with Acoustic Microscopy <br> Bonded wafers can be tested with laboratory manual ultrasound microscopes or highly efficient automated systems for on-line use. Sonoscan supplies these two ultrasound microscopy systems with similar resolutions. The automated system removes the bonded wafer by the robot, analyzes it, and then puts it back into the wafer cassette after drying. Image data can be stored in a database, and image parameters can be used to control ultrasound parameters to adjust the image of the sample.
The ultrasonic sensor scans the wafer surface back and forth. When scanning, the sensor emits pulse waves to the two bonded wafers at a frequency of several thousand times per second, and receives the reflected echo sound waves. Each pulse echo becomes the sound image. One pixel.
Pulsed ultrasound is not reflected when it travels in a homogeneous material, but is partially or fully reflected back when it touches the interface. The amplitude of the echo reflected by the interface depends on the acoustic properties (sound velocity, density) of the two materials on the interface. When two high-brightness LED wafers are bonded by gold-gold thermocompression, the first interface encountered by the ultrasound is a silicon-gold interface. A portion of the ultrasound is reflected and the extent of the reflection depends on the acoustic properties of the two solids.
The image points reflected back from the silicon gold interface have a signal strength from zero (no signal) to total reflection (near 100%). In fact, the air or other gases in the stratification or pores are filled with gas to cause the ultrasound to totally reflect. Assume that there is no bond between the two gold layers and is separated by air or other gases. The acoustic properties of gas and solid are very different, so that the sound waves are almost completely reflected, resulting in high amplitude image points. The sensitivity of ultrasound to differences in acoustic performance explains why acoustic microscopy enables precise imaging of tiny anomalies. Usually the resolution of the image increases as the frequency of the sensor increases. High-brightness LED bonded wafers typically use high-frequency sensors (up to 400 MHz) to obtain high-resolution images.
Figure 3: Sonoscan's D-9000 sensor focuses on the ultrasound image of the adhesive surface. A void (yellow arrow) appears on the bond interface.
Figure 3 is a sonogram of a bonded interface between a 2-inch silicon wafer for producing high-brightness LEDs and another 2-inch GaAs wafer. If no ultrasound image provides a depth analysis of the bond interface and defects within the material, the defects will continue to the next process without being discovered, which can lead to horrible and time consuming failures. This image is obtained by focusing the ultrasound on the back side of the gallium arsenide wafer below. The straight line feature (red arrow) in the figure is a crack in a fragile gallium arsenide wafer. These cracks are caused by internal stresses generated by cooling too quickly after bonding. The two wafers are bonded at 300 ° C.
The elliptical or circular feature (yellow arrow) in Figure 3 is the pore at the gold-gold bond interface. Figure 4 shows the sound image focused on the same bonded wafer interface and shows the details of the pores. Since the fine cracks in gallium arsenide are in the material below the bonding interface, they are not shown in the interface diagram.
Figure 4: Ultrasound focusing on the bonding surface reveals details of the pores.
Figure 5: Ultrasound image of a 4-inch tantalum wafer bonded to a gallium arsenide wafer by gold-tin. The quality of the entire wafer bond varies greatly due to the organic contamination of the interface, and only the dark blue areas are well bonded.
Figure 5 is an interface ultrasound image of a 4-inch twin crystal wafer bonded to another 4-inch gallium arsenide wafer by gold-tin bonding. Like other ultrasound images, this ultrasound high-brightness LED sound image was obtained using the C mode of the D-9000 scanning ultrasound microscope. Because the benefits of the product depend on the quality of the wafer bond, the ultrasound microscopy image of the image reveals subtle changes in the bond layer, which is important for the production of high-brightness LEDs.
Dark blue is a well-bonded area. The periphery of the bonded wafer and a large portion of the center area are not bonded (red areas). There are also several small circular pores (red) that are approximately circular, and a few small circular areas in the center are bonded. There is also a significant change in the blue region, and the change in amplitude of these signals represents a change in the quality of the bond. The quality of bonding has a great impact on the production efficiency of high-brightness LEDs. For comparison, Figure 6 shows a 4-inch wafer bonded to a gallium arsenide wafer by gold-tin. The entire interface is evenly bonded together.
Figure 7 is an ultrasound image of an interface between a 2-inch silicon wafer and a 2-inch sapphire wafer bonded by gold-tin. The wafer is divided into bonded devices by the dividing lane. SST-AP/Taiwan
Figure 6: Similar wafer pairs, but the edge-free bond is uniform.
Figure 7: A sonogram of a 2-inch grid sapphire wafer bonded to a 2-inch silicon wafer by gold-tin. The light gray area is the bonded device, the divider of the black stripe cutting device.
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