Bonding interface for hybrid TFT-based micro display projector
For small, high-resolution, light-emitting diode (LED) displays, such as for a near-eye display in an artificial-reality headset, LEDs are spaced closely together. A backplane can be used to drive an array of LEDs in an LED display. A plurality of interconnects electrically couple the backplane with the array of LEDs. The backplane can have a different coefficient of thermal expansion (CTE) than the array of LEDs. During bonding of the backplane to the array of LEDs, CTE mismatch can cause misalignment of bonding sites. The higher the bonding temperature, the greater the misalignment of bonding sites. Lower temperature bonding, using materials with lower melting or bonding temperatures, can be used to mitigate misalignment during bonding so that interconnects can be more closely spaced, which can allow LEDs to be more closely spaced, to enable a higher-resolution display.
1. An apparatus comprising:
a semiconductor die including an array of light emitting diodes (LEDs), each LED of the array of LEDs comprising a layered epitaxial structure including a first doped semiconductor layer, a second doped semiconductor layer, and a light-emitting layer between the first doped semiconductor layer and the second doped semiconductor layer;
a thin-film circuit layer deposited on the array of LEDs, wherein the array of LEDs is a support structure for depositing the thin-film circuit layer, and the thin-film circuit layer comprises thin-film transistor circuitry for controlling operation of LEDs in the array of LEDs;
a plurality of metal bonds; and
a backplane coupled with the thin-film circuit layer using the plurality of metal bonds, wherein the backplane includes complementary metal-oxide-semiconductor (CMOS) transistor circuitry for supplying electrical current to the thin-film circuit layer through the plurality of metal bonds, and wherein a number of metal bonds of the plurality of metal bonds coupling the backplane with the thin-film circuit layer is less than a number of LEDs in the array of LEDs.
2. The apparatus of claim 1 , wherein the plurality of metal bonds comprises nanoporous copper.
3. The apparatus of claim 1 , wherein spacing between metal bonds of the plurality of metal bonds is equal to or greater than 5 microns and equal to or less than 18 microns.
4. The apparatus of claim 1 , wherein:
the array of LEDs comprises a count of LEDs,
the plurality of metal bonds includes a count of metal bonds, and
the count of metal bonds is at least two orders of magnitude less than the count of LEDs.
5. The apparatus of claim 1 , wherein the array of LEDs occupies a footprint, and the plurality of metal bonds is dispersed over the footprint.
6. The apparatus of claim 1 , wherein each LED in the array of LEDs is formed of a crystalline semiconductor structure, and the thin-film circuit layer is not lattice matched to the crystalline semiconductor structure of the array of LEDs.
7. The apparatus of claim 6 , wherein the thin-film circuit layer comprises a semiconductor material having an amorphous or polycrystalline structure.
8. The apparatus of claim 1 , wherein the thin-film circuit layer includes material comprising at least one of: c-axis aligned crystal Indium-Gallium-Zinc oxide (CAAC-IGZO), amorphous indium gallium zinc oxide (a-IGZO), low-temperature polycrystalline silicon (LTPS), or amorphous silicon (a-Si).
9. The apparatus of claim 1 , wherein the array of LEDs includes material comprising at least one of: Gallium Nitride (GaN), Indium Gallium Arsenide (InGaAs), Indium Gallium Phosphide (AlInGaP), or Gallium Arsenide (GaAs).
10. The apparatus of claim 1 , wherein the CMOS transistor circuitry in the backplane is formed in single crystal silicon.
11. The apparatus of claim 1 , wherein the plurality of metal bonds comprises a metal material that has a melting point or bonding temperature less than 300 degrees Celsius.
12. A method comprising:
obtaining a semiconductor structure, wherein the semiconductor structure is a layered epitaxial structure including a first doped semiconductor layer, a second doped semiconductor layer, and a light-emitting layer between the first doped semiconductor layer and the second doped semiconductor layer;
forming an array of light emitting diodes (LEDs) from the semiconductor structure;
depositing a thin-film circuit layer on the semiconductor structure, wherein the semiconductor structure is a support structure for depositing the thin-film circuit layer;
forming thin-film transistor circuitry in the thin-film circuit layer for controlling operation of LEDs in the array of LEDs;
obtaining a backplane, the backplane comprising complementary metal-oxide-semiconductor (CMOS) transistor drive circuitry for supplying electrical current to the thin-film circuit layer through a plurality of metal bonds;
forming a plurality of metal bumps on the thin-film circuit layer or on the backplane; and
bonding the backplane to the thin-film circuit layer using the plurality of metal bumps, wherein the plurality of metal bumps becomes the plurality of metal bonds after bonding, and wherein a number of metal bonds of the plurality of metal bonds is less than a number of LEDs in the array of LEDs.
13. The method of claim 12 , wherein the bonding the backplane to the thin-film circuit layer uses a temperature of no more than 300 degrees Celsius.
14. The method of claim 12 , wherein multiple LEDs in the array of LEDs are configured to receive electrical current from the backplane through one metal bond of the plurality of metal bonds after bonding the backplane to the thin-film circuit layer.
15. The method of claim 12 , wherein:
the array of LEDs is divided into a plurality of tiles, each tile of the plurality of tiles comprises a plurality of rows of LEDs; and
rows of the plurality of rows of LEDs are configured to be activated at different times.
16. The method of claim 12 , wherein spacing between metal bonds of the plurality of metal bonds is equal to or greater than 5 microns and equal to or less than 18 microns.
17. The method of claim 12 , wherein forming circuitry in the thin-film circuit layer comprises forming a plurality of transistors in the thin-film circuit layer and one control line electrically coupled with the plurality of transistors.
18. A system comprising:
a frame of a head-mounted device; and
a display integrated with the frame, wherein the display is configured to present content to a user of the head-mounted device, the display comprising:
project optics configured to couple light from a light source into a waveguide; and
the light source, wherein the light source comprises:
a semiconductor die including an array of light emitting diodes (LEDs), each LED of the array of LEDs comprising a layered epitaxial structure including a first doped semiconductor layer, a second doped semiconductor layer, and a light-emitting layer between the first doped semiconductor layer and the second doped semiconductor layer;
a thin-film circuit layer deposited on the array of LEDs, wherein the array of LEDs is a support structure for depositing the thin-film circuit layer, and the thin-film circuit layer comprises thin-film transistor circuitry for controlling operation of LEDs in the array of LEDs;
a plurality of metal bonds; and
a backplane coupled with the thin-film circuit layer using the plurality of metal bonds, wherein the backplane includes complementary metal-oxide-semiconductor (CMOS) transistor circuitry for supplying electrical current to the thin-film circuit layer through the plurality of metal bonds, and wherein a number metal bonds of the plurality of metal bonds coupling the backplane with the thin-film circuit layer is less than a number of LEDs in the array of LEDs.
19. The system of claim 18 , wherein the plurality of metal bonds comprises a metal material that has a melting point or bonding temperature less than 300 degrees Celsius.
20. The system of claim 18 , wherein the backplane comprises silicon, and the layered epitaxial structure comprises III-V or III-nitride material.