FIELD OF THE INVENTION

This invention relates to the field of light emitting devices, and in particular to a system that reduces damage to the light emitting devices during laser lift-off of the sapphire substrate upon which the light emitting element is grown.

BACKGROUND OF THE INVENTION

Semiconductor devices, including semiconductor light emitting elements, are formed/grown on a substrate, sapphire wafer substrates being common. In the example of a light emitting element, a GaN nucleation layer may be formed on a sapphire substrate, followed by one or more n-type layers, one or more active layers, and one or more p-type layers. Metallic conductors may be formed through and upon the layers to provide coupling of the n-type and p-type layers to an external source of power to activate the active layer(s) of the light emitting element, via contact pads above the uppermost (p-type) layer.

Because the metallic contact pads are generally opaque or reflective, the light emitting element is designed to emit lights from the surface opposite the contact pads and through the substrate. To improve light extraction efficiency, the substrate may be removed, exposing the semiconductor surface. The semiconductor surface may be processed to further enhance the light extraction efficiency. In some cases one or more contact pads may be placed on the light emitting side of the device.

Laser lift-off is a process that is commonly used to remove the sapphire substrate from the light emitting element. A laser pulse is projected through the sapphire substrate and is absorbed by the semiconductor layer at the Sapphire-semiconductor interface, producing a localized explosive shockwave, due to the instant thermal decomposition of the semiconductor layer at the interface.

If the laser lift-off (LLO) is performed at the wafer level, the wafer-size sapphire substrate is removed after the whole wafer has been processed. If, on the other hand, the laser lift-off is performed for each individual die, the dies are flip-chip mounted on a sub-mount tile, with the sapphire facing upward. The laser is applied to each die, and the die-size sapphire chips pop up into a “Sapphire Collector”, or “Confetti Catcher” immediately after the laser is incident on each die, leaving the semiconductor structure on the sub-mount tile. The sub-mount tile is subsequently processed to create, for example, lens elements over each die, then sliced/diced to provide the individual light emitting devices.

Between the time that the sapphire is removed and the dies are covered, the relatively fragile semiconductor surface is exposed, and susceptible to mechanical damage. During an example set of production runs, the yield lost to such mechanical damage has been measured to be about 0.236%.

SUMMARY OF THE INVENTION

It would be advantageous to reduce the likelihood of mechanical damage to a light emitting element after laser lift-off.

To better address this concern, in an embodiment of this invention, one or more features, both structural and parametric, are included in a Sapphire Collector (SC) for capturing the die-size sapphire chips that are removed from a semiconductor structure during die-level laser lift-off (LLO). These features are designed to increase the likelihood that each sapphire chip is securely captured by the Sapphire Collector immediately after it is released from the semiconductor structure. The Sapphire Collector includes a vacuum-enhance collector with a pickup element that lifts each released chip into the collector, and air pushers that direct the chips further into the collection tunnel leading to a discard bin.

In embodiments of this invention, the features that enhance the likelihood of a released sapphire chip being securely captured by the Sapphire Collector include one or more of the following.

To reduce the likelihood of the chip striking a top surface of the collector and bouncing back exiting the pickup element, an angled air pusher may be situated near the top of the collector to direct the chips away from the top surface and farther into the collection tunnel. At the same time, a complementary angled air pusher may be situated near the bottom of the collector to also direct the picked-up chips farther into the collection tunnel, and further direct any ricocheted chips away from the pickup element. To further enhance the efficiency of these air pushers, the air pushers may be shaped as air knives with high velocity, low volume output.

The entry to the collection tunnel may be flared to maximize the collection cross-section area, and to reduce the likelihood of a chip ricocheting back toward the pickup area. A trench may be created around the interior of the pickup element, to prevent any chips that manage to slide out of the flared tunnel opening, or otherwise coming near to the pickup element from exiting the pickup element.

The exterior surface of the collector facing the dies on the sub-mount may be chamfered, to reduce the likelihood of a ‘wild chip’ ricocheting repeatedly between the sub-mount and the lower surface of the collector. The pickup element, and the provided vacuum may also be designed to optimize the likelihood that a released chip will be forced to enter the collector.

In an example embodiment of this invention, the yield loss due to mechanical damage after laser lift-off was reduced by an order of magnitude, from 0.236% to 0.023%.

