Method of selectively transferring LED die to a backplane using height controlled bonding structures
Selective transfer of dies including semiconductor devices to a target substrate can be performed employing local laser irradiation. Coining of at least one set of solder material portions can be employed to provide a planar surface-to-surface contact and to facilitate bonding of adjoining pairs of bond structures. Laser irradiation on the solder material portions can be employed to sequentially bond selected pairs of mated bonding structures, while preventing bonding of devices not to be transferred to the target substrate. Additional laser irradiation can be employed to selectively detach bonded devices, while not detaching devices that are not bonded to the target substrate. The transferred devices can be pressed against the target substrate during a second reflow process so that the top surfaces of the transferred devices can be coplanar. Wetting layers of different sizes can be employed to provide a trapezoidal vertical cross-sectional profile for reflowed solder material portions.
1. A method of transferring devices to a target substrate, comprising:
providing a supply coupon comprising a combination of a source substrate and devices thereupon;
providing a target substrate that includes bonding sites;
forming first bonding material portions on one of surfaces of the devices or surfaces of the bonding sites of the target substrate;
coining the first bonding material portions to form first bonding material pads having a flatter bonding surface than that of the first bonding material portions;
bonding a first set of the first bonding material pads with respective bonding structures to form a first set of bonded material portions, wherein the first set of the first bonding material pads is located on one of a first set of devices or the first set of bonding sites of the target substrate, and the bonding structures are located on another one of the first set of devices or the first set of the bonding sites of the target substrate, wherein the step of bonding the first set of the first bonding material pads with respective the first set of bonding structures comprises selectively reflowing the first set of the first bonding pads of the first set of devices without reflowing a second set of the first bonding material pads of the second set of devices, and wherein the selective reflowing is performed by irradiating a laser beam on the first set of the first bonding material pads; and
detaching the first set of devices from the source substrate, wherein the first set of the devices is bonded to the bonding sites of the target substrate by the first set of bonded material portions, while a remaining second set of devices remains on the source substrate.
2. The method of claim 1 , wherein the first bonding material portions and the first bonding material pads are formed on the devices.
3. The method of claim 1 , wherein the first bonding material portions and the first bonding material pads are formed on the bonding sites of the source substrate.
4. The method of claim 1 , wherein the bonding structures comprise second bonding material pads and wherein the devices comprise LEDs.
5. The method of claim 4 , further comprising:
forming second bonding material portions on the other one of surfaces of the devices or surfaces of the bonding sites of the target substrate; and
coining the second bonding material portions to form the second bonding material pads having a flatter bonding surface than that of the second bonding material portions.
6. The method of claim 5 , wherein coining the first bonding material portions and coining the second bonding material portions comprises pressing the first bonding material portions against the second bonding material portions at an elevated temperature.
7. The method of claim 5 , further comprising:
reflowing a second set of bonding pads on a second set of the bonding sites that are not bonded to the devices to increase a thickness of the second set of the bonding pads without increasing a thickness of the first set of bonded material portions;
providing a second supply coupon comprising a combination of a second source substrate and third and fourth sets of devices thereupon;
bonding the third set of devices to the second set of bonding pads on the second set of the bonding sites on the target substrate such that a clearance space exists between the second source substrate and the first set of devices; and
detaching the third set of devices from the second source substrate, wherein the third set of the devices is bonded to the target substrate, while the fourth set of devices remains on the second source substrate.
8. The method of claim 1 , wherein the first set of bonded material portions have a trapezoidal vertical cross-sectional profile.
9. A method of transferring devices to a target substrate, comprising:
providing a target substrate that includes first and second sets of bonding sites, wherein a first set of devices is bonded to the first set of bonding sites by a first set of bonded material portions, and a second set of bonding sites contains a second set of bonding pads that are not bonded to devices;
reflowing the second set of bonding pads to increase their thickness without increasing a thickness of the first set of bonding material portions, wherein bonding the second set of devices to the second set of bonding sites comprises irradiating a laser beam on bonding material pads located between each pair of a device from the second set of devices and a bonding site from the second set of bonding sites;
providing a supply coupon comprising a combination of a source substrate and second and third sets of devices thereupon;
bonding the second set of devices to the second set of bonding sites on the target substrate such that a clearance space exists between the source substrate and the first set of devices; and
detaching the second set of devices from the source substrate, wherein the second set of devices is bonded to the second set of bonding sites of the target substrate, while the second set of devices remains on the source substrate.
10. The method of claim 9 , further comprising pushing the first set of devices and the target substrate together during the step of reflowing the second set of bonding pads.
11. The method of claim 9 , wherein the devices comprise LEDs and wherein reflowing the second set of bonding pads to increase their thickness comprises reflowing the second set of bonding pads to increase their thickness by at least 50%.
12. The method of claim 9 , wherein gaps between devices are located on the source substrate which correspond to locations of the first set of devices on the target substrate.
13. A method of transferring devices to a target substrate, comprising:
providing a supply coupon comprising a combination of a source substrate and devices thereupon;
providing a target substrate that includes bonding sites;
forming first bonding structures on a set of surfaces of the devices, each of the first bonding structures comprising a first wetting layer having a first lateral dimension;
forming second bonding structures on a set of surfaces of the bonding sites of the target substrate, each of the second bonding structures comprising a second wetting layer having a second lateral dimension that is greater than the first lateral dimension;
bringing the first bonding structures and the second bonding structures into physical contact, wherein each adjoining pair of a first bonding structure and a second bonding structure includes at least one bonding material portion; and
bonding a subset of adjoining pairs of a first bonding structure and a second bonding structure to form a bonded material portion having a trapezoidal vertical cross-sectional profile, wherein the trapezoidal vertical cross-sectional profile is formed by a self-aligning wetting process in which a first surface of the bonded material portion is aligned to a periphery of first wetting layer and an opposite second surface of the bonded material portion is aligned to a periphery of the second wetting laver, and wherein the bonding is performed by reflowing the at least one bonding material portion by irradiating the at least one bonding material portion with laser radiation.
14. The method of claim 13 , wherein:
each adjoining pair of the first bonding structure and the second bonding structure comprises first and second bonding material portions having different melting temperatures; and
during the step of bonding, the first bonding material portion having a lower melting temperature is reflowed while the second bonding material portion having a higher melting temperature is not reflowed, followed by pressing the first and the second bonding material portions together to form the bonded material portion having the trapezoidal vertical cross-sectional profile.
15. The method of claim 13 , wherein:
the first bonding structure contains a first bonding material portion and the second bonding structure contains a second bonding material portion;
the second bonding material portion has a larger width than the first bonding material portion;
the first bonding material portion has a smaller width than the first wetting layer; and
the second bonding material portion has a smaller width than the second wetting layer.