Solid-state light-emitting devices such as light emitting diodes (“LEDs”) are attractive candidates for replacing conventional light sources such as incandescent and fluorescent lamps. LEDs typically have substantially higher light conversion efficiencies than incandescent lamps, and have longer lifetimes than conventional light sources. Some types of LEDs have higher conversion efficiencies than fluorescent light sources and even higher conversion efficiencies have been demonstrated in the laboratory. For LEDs to be accepted in various lighting applications, it is important to optimize every step of the processing and achieve the highest efficiencies possible. The light conversion efficiency for an LED die is typically determined by the internal quantum efficiency and the light extraction efficiency. While the internal quantum efficiency is related to how efficiently the electrical current is converted to photons, the light extraction efficiency is related to how much of the produced photons can successfully escape or exit from the die. To fabricate an LED die, a wafer is processed by a semiconductor process through photolithographic steps and depositing and etching various metal, oxide, and nitride layers. The dicing or singulation of LED die is performed at the end of the wafer processing and is an important step that impacts the LED device performance in a number of ways.
A challenge associated with manufacturing or producing LED devices is to dice or separate LED devices from a typical semiconductor wafer. A conventional approach of separating a solid-state device or LED die from a semiconductor wafer is to employ a dicing mechanism such as diamond scribing and breaking. There are a number of drawbacks associated with diamond scribing. One drawback is low throughput partially due to the inherently slow scribing speed. Another drawback associated with the diamond scribing is the limitation of wafer thickness which is typically limited to about 100 μm. Furthermore, the diamond tool wears quickly and requires frequent replacement increasing the cost of manufacturing. When scribing the front side, the street, which is space used to separate LED devices from each other, is required to be free of GaN-based layers and needs to be wide enough to allow the tip of the diamond tool to scribe between the devices. When scribing the backside, the break through the bulk of substrate is not always well-controlled resulting in non-negligible chipping and cracks into the device structures causing significant yield loss.
Technological advancement in diode-pumped solid-state (“DPSS”) laser enabled laser scribing has overcome some of the above-mentioned drawbacks and has become increasingly popular. However, the high energy exerted by the laser beam can negatively impact the device performance if laser scribing is not performed correctly. To avoid or minimize damage to the active regions of an LED device and maintain a high process yield, scribing has to be placed at the center of a sufficiently wide dicing street. Laser scribing can be done either on the epi/front side or on the substrate/back side.
Some LED chips have opaque metallization on the substrate back surface to facilitate die-attach with solder and/or to provide a mirror surface. The mirror surface is used to reflect internal light back from the mirror surface to the front side of LED device thereby increase overall efficiency of light extraction. However, the opaque metal coating on the back of the substrate creates a scribing problem since the front side of the devices can not be seen precisely from the backside; therefore, scribing lines on the backside can be risky. A conventional approach is to use a backside camera to view the front side while scribing is performed at the backside. Alternatively, scribing on dicing streets from the front side of device can be easily performed as the wafer is scribed. A drawback with the front alignment for laser scribing, however, is that it reduces light output due to the contamination over the lighting areas of the devices. One approach to reduce the front side contamination is to use a thin protective coating prior to laser scribing at the expense of added processing steps and complexity. When the cleaving/breaking along the street is initiated from the front side scribing, the backside metal does not separate precisely along the street. Due to the flexible nature of thin metal layers, some or all of the metal layers on the backside may simply bend while the substrate and the front-side epitaxial layers are completely separated after the breaking step. As a result, some dies may miss some metals while other dies may have extra metals hanging over from the neighboring die(s). These problems can degrade the LED device performance and cause yield loss. Furthermore, incomplete separation of the backside metal layer causes inconsistent and random spacing of dies when the tape is expanded for separation. Another drawback is that laser scribe can block light passage due to absorbing scribe walls created during the process of laser scribing in the vicinity of light emitting regions.
To reduce the drawbacks or problems relating to the contamination as well as absorbing scribe wall(s), a conventional process of laser scribing is performed at the back side of a wafer. To implement back side laser scribing, a second high resolution camera with precise alignment with the top side camera is installed under the wafer. In order to securely hold a wafer and allow the viewing of the front side, which is hidden from the top side due to the opaque backside metal on the substrate, the wafer holding chuck needs to be transparent and have holes to provide vacuum suction. Transparent wafer holding chuck and vacuum suction add complexity to the laser scribing system since the wafer chuck is moved while securely holding the wafer allowing the viewing of the wafer through the wafer holding chuck.
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