# US 10,768,515 B2 · Method for manufacturing ultra-dense LED projector using thinned gallium nitride

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- Number: US 10,768,515 B2
- Document ID: US10768515B2
- Status: Granted
- Kind: B2
- Application number: 16/154,603
- Filed: 2018-10-08
- Published: 2019-06-13
- Granted: 2020-09-08
- Earliest claimed: 2017-12-12
- Assignees: Tectus Corporation
- Applicants: Spy Eye, LLC
- Inventors: Kwong-Hin Henry Choy; Paul Scott Martin
- Web page: https://patentgrep.com/patent/US10768515B2

## CPC

- G03B 21/2033: {LED or laser light sources}
- G02B 27/0172: {characterised by optical features}
- G03B 21/2013: {Plural light sources}
- G03B 21/28: Reflectors in projection beam {(in illumination beam G03B21/2066)}
- H05B 45/00: Circuit arrangements for operating light-emitting diodes [LED]
- H05B 45/30: Driver circuits
- H10H 20/825: containing nitrogen, e.g. GaN
- H10H 20/84: Coatings, e.g. passivation layers or antireflective coatings
- H10W 90/00: Package configurations
- G02C 7/04: Contact lenses for the eyes (disinfection or sterilisation of contact lenses A61L12/00)
- H10H 20/821: of the light-emitting regions, e.g. non-planar junctions
- G03B: APPARATUS OR ARRANGEMENTS FOR TAKING PHOTOGRAPHS OR FOR PROJECTING OR VIEWING THEM; APPARATUS OR ARRANGEMENTS EMPLOYING ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ACCESSORIES THEREFOR (optical parts of such apparatus G02B; photosensitive materials or processes for photographic purposes G03C; apparatus for processing exposed photographic materials G03D)
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- H05B: ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H10H: INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS
- H10W: GENERIC PACKAGES, INTERCONNECTIONS, CONNECTORS OR OTHER CONSTRUCTIONAL DETAILS OF DEVICES COVERED BY CLASS H10
- G02C: SPECTACLES; SUNGLASSES OR GOGGLES INSOFAR AS THEY HAVE THE SAME FEATURES AS SPECTACLES; CONTACT LENSES (trial frames for testing the eyes A61B3/04; goggles or eyeshields not having the same features as spectacles A61F9/00)

## Abstract

A small projector uses an ultra-dense array of gallium nitride (GaN) LEDs. However, epitaxial growth of GaN typically produces a GaN region that is Sum or thicker. To achieve high pixel density, the LEDs have small area, so the resulting LED structures are tall and skinny. This is undesirable because it makes further processing more difficult and has higher optical losses. As a result, it is beneficial to reduce the thickness of the GaN region. In one approach, a wafer with the GaN region on substrate is bonded to a backplane wafer containing LED driver circuits. The substrate is then separated from the GaN region, exposing a buffer layer of the GaN region. The GaN region is thinned and then patterned into individual LEDs. Typically, the buffer layer is removed entirely.

## Claims

### Claim 1

A method for manufacturing a femtoprojector display, the method comprising:
attaching a frontplane die to a backplane die, the backplane die comprising an array of LED driver circuits on a backplane substrate, and the frontplane die comprising a gallium nitride region grown on a frontplane substrate, where attaching the frontplane die to the backplane die comprises bonding a metal layer on the frontplane die to a metal layer on the backplane die, and where the bonding uses a bonding agent selected from aluminum; indium tin oxide; aluminum-doped zinc oxide; aluminum with a surface coating of silicon, germanium or titanium; Sn solder and In solder;
removing the frontplane substrate;
thinning the gallium nitride region; and
patterning the gallium nitride region into individual LED pillars having an active region of width not more than 2 um, where thinning the gallium nitride region reduces a height of the individual LED pillars as measured from the active region to a top of the gallium nitride region to less than 3 um.

### Claim 2 (depends on 1)

The method of claim 1 where the backplane substrate is a silicon substrate, the LED driver circuits are CMOS circuits, and the metal layer on the backplane substrate is a copper layer.

### Claim 3 (depends on 1)

The method of claim 1 where thinning the gallium nitride region reduces a ratio of the height to the width to less than 5:1.

### Claim 4 (depends on 3)

The method of claim 3 where the trenches separating adjacent individual LED pillars have a width of not more than 0.5 um.

### Claim 5 (depends on 3)

The method of claim 3 where the trenches separating adjacent individual LED pillars have a height-to-width aspect ratio of at least 4:1.

### Claim 6 (depends on 1)

The method of claim 1 where thinning the gallium nitride region reduces a thickness of the gallium nitride region by at least 3 um.

### Claim 7 (depends on 1)

The method of claim 1 where patterning the gallium nitride region into individual LED pillars comprises dry etching trenches into the gallium nitride region to pattern the individual LED pillars.

### Claim 8

A method for manufacturing a femtoprojector display, the method comprising:
attaching a frontplane die to a backplane die, the backplane die comprising an array of LED driver circuits on a backplane substrate, and the frontplane die comprising a gallium nitride region grown on a frontplane substrate;
removing the frontplane substrate;
thinning the gallium nitride region;
after removing the frontplane substrate, planarizing the frontplane die before thinning the gallium nitride region; and
patterning the gallium nitride region into individual LED pillars having an active region of width not more than 2 um, where thinning the gallium nitride region reduces a height of the individual LED pillars as measured from the active region to a top of the gallium nitride region to less than 3 um.

### Claim 9 (depends on 8)

The method of claim 8 where growth of the gallium nitride region on the frontplane substrate results in a buffer region in the gallium nitride region adjacent to the frontplane substrate, and thinning the gallium nitride region removes the buffer region.

### Claim 10 (depends on 8)

The method of claim 8 where thinning the gallium nitride region reduces a ratio of the height to the width to less than 5:1.

### Claim 11 (depends on 8)

The method of claim 8 where thinning the gallium nitride region reduces a thickness of the gallium nitride region by at least 3 um.

### Claim 12 (depends on 8)

The method of claim 8 where removing the frontplane substrate comprises using a liftoff process to remove the frontplane substrate.

### Claim 13 (depends on 8)

The method of claim 8 where patterning the gallium nitride region into individual LED pillars comprises dry etching trenches into the gallium nitride region to pattern the individual LED pillars.

### Claim 14 (depends on 13)

The method of claim 13 where the trenches separating adjacent individual LED pillars have a width of not more than 0.5 um.

### Claim 15 (depends on 13)

The method of claim 13 where the trenches separating adjacent individual LED pillars have a height-to-width aspect ratio of at least 4:1.

### Claim 16

A method for manufacturing a femtoprojector display, the method comprising:
attaching a frontplane die to a backplane die, the backplane die comprising an array of LED driver circuits on a backplane substrate, and the frontplane die comprising a gallium nitride region grown on a frontplane substrate;
removing the frontplane substrate;
thinning the gallium nitride region; and
patterning the gallium nitride region into individual LED pillars having an active region of width not more than 2 um, where thinning the gallium nitride region reduces a height of the individual LED pillars as measured from the active region to a top of the gallium nitride region to less than 3 um; where patterning the gallium nitride region into individual LED pillars comprises:
lithographically masking areas of the gallium nitride region to define the individual LED pillars; and
etching trenches into areas of the gallium nitride region that are not masked; and
without any lithography steps:
passivating sidewalls of the individual LED pillars; and
forming electrical contacts to top contacts of the individual LED pillars.

### Claim 17 (depends on 16)

The method of claim 16 where passivating sidewalls of the individual LED pillars comprises:
using atomic layer deposition to deposit a passivation material on the sidewalls and tops of the individual LED pillars; and
without a use of lithography, etching the deposited passivation material to remove the passivation material from the tops of the individual LED pillars but leaving passivation material on the sidewalls of the individual LED pillars.

### Claim 18 (depends on 16)

The method of claim 16 further comprising:
depositing a multi-layer dielectric stack on a top surface of the individual LED pillars, the multi-layer dielectric stack providing wavelength selectivity.

### Claim 19 (depends on 18)

The method of claim 18 further comprising:
depositing a sacrificial layer on the multi-layer dielectric stack;
patterning the multi-layer dielectric stack and sacrificial layer into individual pillars aligned with the individual LED pillars;
depositing fill material in spaces between the individual pillars of the sacrificial layer;
removing the sacrificial layer to create voids defined by the fill material; and
filling at least some of the voids with wavelength-conversion material that converts light produced by the individual LED pillars to a different wavelength, where the multi-layer dielectric stack passes light from the individual LED pillars and reflects light converted to the different wavelength.

### Claim 20 (depends on 19)

The method of claim 19 where the wavelength-conversion material includes quantum dots.

### Claim 21 (depends on 16)

The method of claim 16 where thinning the gallium nitride region reduces a ratio of the height to the width to less than 5:1.

### Claim 22 (depends on 16)

The method of claim 16 where thinning the gallium nitride region reduces a thickness of the gallium nitride region by at least 3 um.

### Claim 23 (depends on 16)

The method of claim 16 where the trenches separating adjacent individual LED pillars have a width of not more than 0.5 um.

### Claim 24 (depends on 16)

The method of claim 16 where the trenches separating adjacent individual LED pillars have a height-to-width aspect ratio of at least 4:1.