LED assembly having maximum metal support for laser lift-off of growth substrate
View Patent ↗Described is a process for forming an LED structure using a laser lift-off process to remove the growth substrate (e.g., sapphire) after the LED die is bonded to a submount. The underside of the LED die has formed on it anode and cathode electrodes that are substantially in the same plane, where the electrodes cover at least 85% of the back surface of the LED structure. The submount has a corresponding layout of anode and cathode electrodes substantially in the same plane. The LED die electrodes and submount electrodes are ultrasonically welded together such that virtually the entire surface of the LED die is supported by the electrodes and submount. Other bonding techniques may also be used. No underfill is used. The growth substrate, forming the top of the LED structure, is then removed from the LED layers using a laser lift-off process. The extremely high pressures created during the laser lift-off process do not damage the LED layers due to the large area support of the LED layers by the electrodes and submount.
1. A method of fabricating a light emitting diode (LED) structure comprising:
growing an n-type layer, an active layer, and a p-type layer over a growth substrate, forming LED layers;
forming openings in the p-type layer and active layer to expose areas of the n-type layer;
forming a first metal portion extending through the openings to electrically contact the n-type layer;
forming a second metal portion to electrically contact the p-type layer;
wherein the first metal portion and the second metal portion form a first bonding metal on a back surface of an LED die, the first bonding metal covering at least 85% of the back surface of the LED die;
forming a second bonding metal on a submount generally corresponding to the first bonding metal on the LED die;
interconnecting the first bonding metal to the second bonding metal, with substantially no gaps between the first bonding metal and the second bonding metal, so that the back surface of the LED die is supported by the first bonding metal, the second bonding metal, and the submount, wherein at least 85% of the back surface of the LED die is directly supported by the first bonding metal, the second bonding metal, and the submount in a direction perpendicular to the back surface of the LED, whereby the second bonding metal underlies at least 85% of the back surface of the LED; and
removing the substrate from the LED die, during a laser lift-off process, by exposing the LED die to a laser beam to allow the substrate to be lifted off the LED die without using an underfill between the submount and the LED layers to support the LED layers during the laser lift-off process, wherein the LED layers form a substantially rectangular surface, wherein the first bonding metal extends completely to an edge of the rectangular surface such that a perimeter of the rectangular surface is directly supported by at least portion of the first bonding metal, the second bonding metal, and the submount.
2. The method of claim 1 wherein forming the first metal portion and the second metal portion comprises plating metal areas electrically contacting the n-type layer and the p-type layer to form the first bonding metal on the back surface of the LED die.
3. The method of claim 1 wherein the first metal portion and the second metal portion comprise metal having a thickness exceeding 20 microns to provide support of the LED layers during the laser lift-off process.
4. The method of claim 1 wherein the first metal portion and the second metal portion are separated for electrical insulation by a gap less than 50 microns.
5. The method of claim 1 wherein the LED layers form a substantially rectangular surface, wherein the first bonding metal substantially extends to an edge of the rectangular surface.
6. The method of claim 1 wherein the first bonding metal and the second bonding metal are substantially thermally matched so as to have uniform expansion behavior for limiting thermally induced stresses on the LED layers.
7. The method of claim 1 wherein the step of forming a second bonding metal on a submount generally corresponding to the first bonding metal on the LED die comprises forming the second bonding metal to have gaps between anode and cathode portions that precisely match gaps in a layout of the first bonding metal.
8. The method of claim 1 wherein the step of forming a second bonding metal on a submount generally corresponding to the first bonding metal on the LED die comprises forming the second bonding metal to have gaps between anode and cathode portions larger than gaps in a layout of the first bonding metal.
9. The method of claim 1 wherein the second metal portion comprises a reflective metal for reflecting light towards the n-type layer.
10. The method of claim 1 wherein anode and cathode portions of the first bonding metal portion form interdigitated metal fingers.
11. A method of fabricating a light emitting diode (LED) structure comprising: growing an n-type layer, an active layer, and a p-type layer over a growth substrate, form LED layers:
forming opening in the p-type layer and active layer, and a p-type layer over a grow substrate, forming LED layers;
forming a first metal portion extending through the openings to electrically contact the n-type layer;
forming a second metal portion to electrically contact the p-type layer;
wherein the first metal portion and the second metal portion form a first bonding metal on a back surface of an LED die, the first bonding metal covering at least 85% of the back surface of the LED die;
forming a second bonding metal on a submount generally corresponding to the first bonding metal on the LED die;
interconnecting the first bonding metal to the second bonding metal with substantially no gaps between the first bonding metal to the second bonding metal, so that the back surface of the LED die is supported by the first bonding metal, and the submount, wherein at least 85% of the back surface of the LED die is directly supported by the first bonding metal, the second bonding metal, and the submount in a direction perpendicular to the back surface of the LED, whereby the second bonding metal underlies at least 85% if the back surface of the LED; and
removing the substrate from the LED die, during a laser lift-off process, by exposing the LED die to a laser beam to allow the substrate to be lifted off the LED die without using an underfill between the submount and the LED layers to support the LED layers during the laser lift-off process, wherein the LED layers form a substantially rectangular surface, wherein a perimeter of the rectangular surface is supported by a photoresist layer that contacts the submount during the laser lift-off process.