U.S. patent application number 17/184071 was filed with the patent office on 2021-07-08 for using underfill or flux to promote placing and parallel bonding of light emitting diodes.
The applicant listed for this patent is Facebook Technologies, LLC. Invention is credited to Daniel Brodoceanu, Patrick Joseph Hughes, Thiago Martins Amaral, Alexander Udo May, Karsten Moh, Pooya Saketi, Oscar Torrents Abad.
Application Number | 20210210667 17/184071 |
Document ID | / |
Family ID | 1000005462126 |
Filed Date | 2021-07-08 |
United States Patent
Application |
20210210667 |
Kind Code |
A1 |
Brodoceanu; Daniel ; et
al. |
July 8, 2021 |
USING UNDERFILL OR FLUX TO PROMOTE PLACING AND PARALLEL BONDING OF
LIGHT EMITTING DIODES
Abstract
Embodiments relate to using flux or underfill as a trapping
layer for temporarily attaching light emitting diodes (LEDs) to a
substrate and heating to simultaneously bond multiple LEDs onto the
substrate. The flux or underfill may be selectively coated at the
ends of electrodes of the LEDs prior to placing the LEDs on the
substrate. Due to adhesive properties of the flux or underfill,
multiple LEDs can be placed on and attached to the substrate prior
to performing the bonding process. Once LEDs are placed on the
substrate, the flux or underfill facilitates formation of metallic
contacts between electrodes of the LED and contacts of the
substrate during the bonding process. By using the flux or
underfill, the formation of metallic contacts can be performed even
without applying pressure.
Inventors: |
Brodoceanu; Daniel; (Cork,
IE) ; Martins Amaral; Thiago; (Saarbrucken, DE)
; Saketi; Pooya; (Cork, IE) ; Hughes; Patrick
Joseph; (Cork, IE) ; May; Alexander Udo; (St.
Ingbert, DE) ; Moh; Karsten; (Saarbrucken, DE)
; Torrents Abad; Oscar; (Cork, IE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Facebook Technologies, LLC |
Menlo Park |
CA |
US |
|
|
Family ID: |
1000005462126 |
Appl. No.: |
17/184071 |
Filed: |
February 24, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
16425866 |
May 29, 2019 |
10964867 |
|
|
17184071 |
|
|
|
|
62742838 |
Oct 8, 2018 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/67144 20130101;
B23K 1/0016 20130101; H01L 33/56 20130101; B23K 2101/36 20180801;
H01L 2933/005 20130101; H01L 25/0753 20130101; B23K 1/0056
20130101; H01L 33/30 20130101; H01L 21/67259 20130101; H01L
2933/0066 20130101; H01L 33/62 20130101; B23K 1/203 20130101 |
International
Class: |
H01L 33/62 20060101
H01L033/62; H01L 21/67 20060101 H01L021/67; H01L 33/56 20060101
H01L033/56; H01L 25/075 20060101 H01L025/075; B23K 1/20 20060101
B23K001/20; B23K 1/005 20060101 B23K001/005; B23K 1/00 20060101
B23K001/00 |
Claims
1. A light emitting assembly comprising: a substrate with contacts
on a side of the substrate; and a plurality of light emitting
diodes (LEDs) having electrodes connected to the contacts of the
substrate by metallic contacts, a metallic contact for a first
light emitting diode (LED) die formed by: placing at least the
first LED die on the substrate with a flux or underfill as a
trapping layer between an electrode of the first LED die and a
contact of the substrate; and heating the contact, the electrode,
and the flux or underfill to form the metallic contact between the
first LED die and the substrate.
2. The light emitting assembly of claim 1, wherein forming the
metallic contact further comprises detaching a pick-up head for
placing the first LED die on the substrate from the first LED die
after placing the first LED die on the substrate, adhesive forces
of the flux or underfill securing the first LED die on the
substrate during detaching of the pick-up head from the first LED
die.
3. The light emitting assembly of claim 1, wherein forming the
metallic contact further comprises placing a second LED die on the
substrate prior to the heating.
4. The light emitting assembly of claim 1, wherein the metallic
contact is formed without applying external pressure on the first
LED die towards the substrate during the heating.
5. The light emitting assembly of claim 1, wherein forming the
metallic contact further comprises: placing a platform with an
elastomer pad on the first LED die; and applying pressure on the
first LED die towards the substrate by applying pressure on the
platform with the elastomer pad.
6. The light emitting assembly of claim 1, wherein the electrode,
the contact, and the flux or underfill are selectively heated by
focusing laser light.
7. The light emitting assembly of claim 1, wherein the flux or
underfill is rosin.
8. (canceled)
9. The light emitting assembly of claim 3, wherein the second LED
die is placed on the substrate simultaneously with the placing of
the first LED die on the substrate.
10. The light emitting assembly of claim 1, wherein the flux or
underfill is provided on the electrode, and at least a tip of the
electrode is coated with the flux or underfill by dipping the tip
into a flux or underfill layer.
11. The light emitting assembly of claim 1, wherein forming the
metallic contact further comprises subsequent to placing the first
LED die on the substrate, repositioning the first LED die to align
the electrode with the contact, the flux or underfill remaining
between the electrode and the contact.
12. A non-transitory computer readable storage medium with
instructions that, when executed by at least one processor, cause
the processor to: align electrodes of a first light emitting diode
(LED) die with contacts of a substrate, flux or underfill provided
on at least the electrodes or the contacts; place the first LED die
on the substrate with the flux or underfill as a trapping layer
between the electrodes and the contacts; and heat the electrodes,
the contacts, and the flux or underfill to form metallic contacts
between the first LED die and the contacts.
13. The non-transitory computer readable storage medium of claim
12, further comprising an instruction to detach a pick-up head for
placing the first LED die on the substrate from the first LED die
after placing the first LED die on the substrate, adhesive forces
of the flux or underfill securing the first LED die on the
substrate during detaching of the pick-up head from the first LED
die.
14. The non-transitory computer readable storage medium of claim
12, further comprising an instruction to place a second LED die on
the substrate prior to the heating.
15. The non-transitory computer readable storage medium of claim
14, wherein the second LED die is placed on the substrate
simultaneously with the placing of the first LED die on the
substrate.
16. The non-transitory computer readable storage medium of claim
12, wherein the flux or underfill is provided on the electrodes,
and at least tips of the electrodes are coated with the flux or
underfill by dipping the tips into a flux or underfill layer.
17. The non-transitory computer readable storage medium of claim
12, wherein the metallic contacts are formed without applying
external pressure on the first LED die towards the substrate during
the heating.
18. The non-transitory computer readable storage medium of claim
12, further comprising instructions to: place a platform with an
elastomer pad on the first LED die; and apply pressure on the first
LED die towards the substrate by applying pressure on the platform
with the elastomer pad.
19. The non-transitory computer readable storage medium of claim
12, wherein the electrodes, the contacts, and the flux or underfill
are selectively heated by focusing laser light.
20. The non-transitory computer readable storage medium of claim
12, wherein the flux or underfill is rosin.
21. The non-transitory computer readable storage medium of claim
12, further comprising an instruction to, subsequent to placing the
first LED die on the substrate, repositioning the first LED die to
align the electrodes with the contacts, the flux or underfill
remaining between the electrodes and the contacts.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. application Ser. No.
16/425,866, filed May 29, 2019, which claims the benefit of U.S.
Provisional Application No. 62/742,838, filed Oct. 8, 2018. The
subject matter of all of the foregoing is incorporated herein by
reference in their entirety.
BACKGROUND
[0002] The present disclosure relates to placing and bonding light
emitting diodes (LEDs) on a substrate, and specifically, to
applying a flux or underfill between the LEDs and substrate to
promote the placing and bonding processes.
[0003] In display fabrication, LEDs may be moved from one substrate
to another. For example, micro-LEDs (.mu.LEDs) for emitting
different colors of light may be transferred from native substrates
(on which the micro-LEDs) are fabricated or carrier substrates to a
display substrate including control circuits for the .mu.LEDs to
manufacture an electronic display. As the form factor of LEDs
decreases, the placing of LEDs into desired arrangements without
damaging the LED dies becomes increasingly difficult.
[0004] Furthermore, LEDs are bonded to a substrate by
thermocompression (TC) bonding to form metallic contacts between
the LED and substrate. TC bonding forms metallic contacts between
two metals by simultaneously applying force and heat. To ensure
each LED is bonded correctly, the placing and bonding process is
applied to one LED at a time. Specifically, once an LED is placed
on a substrate, it is bonded to the substrate before another LED is
placed on the substrate. As a result, the substrate and previously
bonded LEDs undergo multiple heating cycles. Repeated high
temperature heating cycles is time consuming, increases the risk of
damaging LEDs, and can lead to the formation of oxide layers at the
metallic contacts.
SUMMARY
[0005] Embodiments relate to using flux or underfill as a trapping
layer for temporarily attaching light emitting diodes (LEDs) to a
substrate and heating to simultaneously bond multiple LEDs onto the
substrate. Electrodes of a light emitting diode (LED) die are
aligned with contacts of a substrate. Flux or underfill is provided
on at least the electrodes or the contacts. The LED die is placed
on the substrate with the flux or underfill as a trapping layer
between the electrodes and the contacts. The electrodes, the
contacts, and the flux or underfill are heated to form a metallic
contact between the LED die and the substrate.
[0006] In some embodiments, a pick-up head for placing the LED die
on the substrate is detached from the first LED die after placing
the LED die on the substrate. Adhesive forces of the flux or
underfill secure the LED die on the substrate during the detachment
of the pick-up head from the LED die.
[0007] In some embodiments, another LED die is placed on the
substrate prior to the heating process. The other LED die may be
placed on the substrate simultaneously with the placing of the LED
die on the substrate.
[0008] In some embodiments, the metallic contact is formed without
applying external pressure on the first LED die towards the
substrate during the heating.
[0009] In some embodiments, pressure is applied on the first LED
die towards the substrate by placing the first LED die and the
substrate in a high-pressure chamber during the heating.
[0010] In some embodiments, a platform with an elastomer pad is
placed on the first LED die. Pressure is applied on the first LED
die towards the substrate by applying pressure on the platform with
the elastomer pad.
[0011] In some embodiments, the flux or underfill is provided on
the electrodes, and at least tips of the electrodes are coated with
the flux or underfill by dipping the tips into a flux or underfill
layer.
[0012] In some embodiments, the flux or underfill is rosin or
Benzocyclobutene (BCB).
[0013] In some embodiments, subsequent to placing the first LED die
on the substrate, the first LED die is repositioned to align the
electrodes with the contacts. The flux of underfill remains between
the electrodes and the contact.
[0014] In some embodiments, the first LED die is repositioned based
on image signals received from a camera. The camera captures images
of the first LED die through a microscope lens.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1A is a cross sectional view of LED dies bonded to a
substrate by metallic contacts, according to one embodiment.
[0016] FIG. 1B is a cross sectional view of LED dies placed on the
substrate prior to bonding, according to one embodiment.
[0017] FIGS. 2A-2C illustrate a sequence of schematic diagrams for
bonding electrodes of LED dies and contacts of the substrate,
according to one embodiment.
[0018] FIGS. 3A-3G illustrate a sequence of schematic diagrams for
applying flux or underfill on electrodes of the LED dies and
removing a native substrate from the LED dies, according to one
embodiment.
[0019] FIG. 4A is a cross sectional view of a platform with
elastomer pads attached to LEDs on a substrate, according to one
embodiment.
[0020] FIG. 4B is a bottom view of the platform with elastomer
pads, according to one embodiment.
[0021] FIG. 5 is a cross sectional view of a high-pressure chamber
containing LEDs on a substrate, according to one embodiment.
[0022] FIG. 6 is a flow chart illustrating a method for using flux
or underfill to temporarily attach and bond an LED to a substrate,
according to one embodiment.
[0023] FIG. 7 is a schematic diagram illustrating an operation of a
pick-up head placing an LED onto a substrate, according to one
embodiment.
[0024] FIG. 8 is a block diagram of the controller, according to
one embodiment.
[0025] FIG. 9 is a block diagram of software modules in the memory
of the controller, according to one embodiment.
[0026] FIG. 10 is a circuit diagram of a testing circuit on a
substrate, according to one embodiment.
[0027] FIGS. 11A through 11C are schematic cross sections of a
micro (.mu.LED), according to some embodiments.
[0028] The figures depict various embodiments of the present
disclosure for purposes of illustration only.
DETAILED DESCRIPTION
[0029] In the following description of embodiments, numerous
specific details are set forth in order to provide more thorough
understanding. However, note that the embodiments may be practiced
without one or more of these specific details. In other instances,
well-known features have not been described in detail to avoid
unnecessarily complicating the description.
[0030] Embodiments are described herein with reference to the
figures where like reference numbers indicate identical or
functionally similar elements. Also in the figures, the left most
digits of each reference number correspond to the figure in which
the reference number is first used.
[0031] Embodiments relate to using flux or underfill as a trapping
layer for temporarily attaching light emitting diodes (LEDs) to a
substrate and heating to simultaneously bond multiple LEDs onto the
substrate. The flux or underfill may be selectively coated at the
ends of electrodes of the LEDs prior to placing the LEDs on the
substrate. Due to adhesive properties of the flux or underfill,
multiple LEDs can be placed on and attached to the substrate prior
to performing the bonding process. Once LEDs are placed on the
substrate, the flux or underfill facilitates formation of metallic
contacts between electrodes of the LED and contacts of the
substrate during the bonding process. By using the flux or
underfill, the formation of metallic contacts can be performed
without applying pressure.
[0032] FIG. 1A is a cross sectional view of LED dies 110 bonded to
a display substrate 120 by metallic contacts 130, according to one
embodiment. FIG. 1B is a cross sectional view of LED dies 110
placed on the display substrate 120 prior to bonding, according to
one embodiment. The electrodes 140 of the LED dies 110 are aligned
with and temporarily attached to conducting contacts 150 of the
display substrate 120 by the flux or underfill 160. During bonding,
part of the electrodes 140, part of the contacts 150, and flux or
underfill 160 melt to form the metallic contacts 130. This can be
done in parallel for multiple LEDs 110 placed on the display
substrate 120.
[0033] In FIG. 1B, the display substrate 120 is on top of a hot
plate 170 to heat the electrodes 140, the contacts 150 and the flux
or underfill 160. However, different mechanisms may be used to heat
the assembly such as exposing the electrodes 140, the contact 150,
and the flux or underfill 160 to a laser beam.
[0034] An LED 110 is a surface-mounted device (SMD) that emits
light if a voltage difference is applied between the electrodes
140. The electrodes 140 can be made of a single metal (e.g., gold
(Au)) or alloys (e.g., copper (Cu) and tin (Sn) or gold (Au) and
tin (Sn)). The electrodes 140 may be nanoporous. An LED 110 can
have an epitaxial structure formed from, among other examples,
Gallium nitride (GaN), gallium arsenide (GaAs), or gallium
phosphide (GaP). In some embodiments, the LED 110 is a micro-LED
(.mu.LED) die. The LED 110 may also be embodied as a
vertical-cavity surface-emitting laser (VCSEL) that emits infrared
wavelengths.
[0035] The electrodes 140 of the LED 110 can be aligned with and
paced on contacts of 150 the display substrate 120 by a pick-up
head. Alternatively, an array of LEDs 110 and their electrodes 140
can be aligned with and placed on an array of contacts 150 in a
single step (this may be referred to as a monolithic approach). In
some embodiments, the circuits in the display substrate 120 are
powered so that the LED 110 emits light 128 as soon as electrical
contact is established during the placement process, as described
with reference to FIGS. 7-10.
[0036] The display substrate 120 mechanically supports electronic
components (such as the LEDs 110) and electrically connects the
electronic components using traces (not shown) and contacts 150.
For example, the display substrate is a semiconductor substrate
with traces, contacts and other electronic components fabricated
using complementary metal-oxide-semiconductor (CMOS) technology. In
some embodiments, the contacts 150 are alloys that include copper
(Cu). The display substrate 120 can support any number of LEDs 110.
The display substrate 120 may include circuits that are completed
once one or more LEDs 110 are placed onto the display substrate
120. In some embodiments, the display substrate 120 can include a
control circuit that drives current in the display substrate 120.
For example, the display substrate 120 is a display substrate of an
electronic display. In this example, the LEDs may be placed (e.g.,
by a pick-up head) at pixel or sub-pixel locations to connect the
LED dies to control circuits in the display substrate. In this way,
the control circuit can drive the electronic display by applying
current to the LED dies 110.
[0037] The metallic contacts 130 are electrical connections between
the electrodes 140 of the LEDs 110 and the contacts 150 of the
display substrate 120. The metallic contacts 130 are formed by
bonding the electrodes 140 to the contacts 150. Depending on the
material of the electrodes and the contacts 150, the metallic
contacts 130 may be a metallic bond (e.g., a pure gold or copper
bond) or an intermetallic bond (e.g., a gold-tin or copper-tin
alloy bond). The metallic contacts 130 are formed during the
bonding process by heating the electrodes 140, contacts 150, and
flux or underfill 160. Pressure may also be applied to form the
metallic contacts 130. When both heat and pressure are applied, the
process is referred to as thermocompression (TC) bonding. However,
applying pressure on LEDs 110 during bonding may cause
misalignment. Hence, in some embodiments, the metallic contacts 130
are formed without applying external pressure or applying only
reduced pressure to the LEDs 110 towards the display substrate 120
during the bonding process.
[0038] The hot plate 170 is a plate that can control the
temperature of the electrodes 140, contacts 150, and flux or
underfill 160 by heating or cooling the display substrate 120. The
hot plate 170 may be advantageous for bonding the contacts 150 to
the electrodes 140. For example, due to heat from the hot plate
170, the electrodes 140, contacts 150, and flux or underfill 160
melt to form the metallic contact 130. The hot plate 170 may be a
Peltier cell when bonding temperatures are low (e.g., -20.degree.
C. to 90.degree. C.). The bonding temperatures may be low when the
flux or underfill includes rosin. In some embodiments, the
temperature of the electrodes 140, contacts 150, and flux or
underfill 160 is controlled by another (or additional) method or
apparatus, such as a laser beam that locally heats the electrodes
140, contacts 150, and flux or underfill 160. For example, after
multiple LEDs 110 are placed, a laser setup may be employed to
selectively heat individual LEDs 110. A laser setup may be employed
when bonding temperatures are high (e.g., 250.degree. C. to
300.degree. C.). Bonding temperatures may be high to bond metals
(e.g., to form a copper-tin bond).
[0039] The flux or underfill 160 promotes bonding of the electrodes
140 to the contacts 150 to form the metallic contacts 130. The flux
or underfill 160 placed between the electrodes 140 and contacts 150
may be referred to herein as a `trapping layer` because the flux or
underfill 150 temporarily holds the electrodes 140 in place so that
the electrodes 140 can bond with the contacts 150. The flux or
underfill 160 is any combination of underfill material, flux
material, and underfill material with flux properties. Examples of
flux or underfill 160 include, but are not limited to, rosin or
other similar flux types, epoxy based materials, and
Benzocyclobutene (BCB) based materials. Examples of other materials
that can be used as a trapping layer include, but are not limited
to, conductive paste formulations (e.g., silver nanoparticle ink),
low melting point metals (e.g., indium), nanoporous gold, and
eutectic alloys. Flux material remove oxides (e.g., Cu or Sn
oxides) during the bonding process because the oxides may prevent
formation the metallic contacts 130. Thus, in embodiments where
oxidation does not occur or is reduced during the bonding process
(e.g., the electrodes 140 and contacts 150 are gold or the bonding
process occurs in a reduced atmosphere environment), the flux or
underfill 160 may not include flux material. In some embodiments,
the flux or underfill 160 mechanically strengthens the bond
structure. During the bonding process, the flux or underfill 160
may turn liquid around 50.degree. C., may become active by removing
oxides around 80-110.degree. C., and may assist bonding by
decreasing surface tension around 230.degree. C.
[0040] In some embodiments, if flux or underfill 160 is placed
between the electrodes 140 and contacts 150, the bonding process
can form metallic contacts 130 without applying external pressure
on the LEDs 110 towards the display substrate 120. During heating,
the flux or underfill 160 can decrease the surface tension of the
melted electrodes 140 and contacts 150. The flux or underfill 160
can pull the opposing surfaces of the electrodes 140 and contacts
150 together due to capillary forces and mass loss during thermal
decomposition. This may particularly occur when the dimensions of
the LED dies 110 are on the order of micrometers. The flux or
underfill 160 can also allow effective wetting of both surfaces of
the electrodes 140 and contacts 150. Adhesive properties of the
flux or underfill 160 can maintain the position of the placed LED
110 on the substrate (e.g., the flux or underfill 160 maintains
contact between the electrodes 140 and contacts 150) during the
bonding process. Due to any combination of these features, the
metallic contacts 130 can be formed without applying external
pressure to the LEDs 110.
[0041] In some embodiments, the flux or underfill functions as the
trapping layer having adhesive properties to assist in the
temporary placement of the LEDs 110 on the display substrate 120.
For example, flux or underfills 160 can be become solid below
certain temperatures (e.g., rosin is solid below 50.degree. C.),
allowing multiple LEDs 110 to be placed on the display substrate
120 prior to the bonding process. For example, after placement of
an LED 110 by a pick-up head, the pick-up head can detach from the
LED 110 due the adhesive forces of the flux or underfill 160
keeping the LED 110 attached to the display substrate 120, so that
any number of LEDs 110 can form metallic contacts 130 with the
display substrate 120 during a single bonding process. Among other
advantages, a single bonding process for forming metallic contacts
130 can reduce a need to remove oxide layers formed on metallic
contacts 130 after multiple thermal cycles. A single bonding
process can also reduce a risk of damaging the LEDs 110 from
multiple thermal cycles. A single bonding process can also be less
time consuming than multiple bonding processes. A single bonding
process can also reduce a risk of dendrites formation.
[0042] Furthermore, the temporary attachment of LEDs 110 to the
contacts 150 by the flux or underfill 160 can allow LEDs 110 to be
repositioned after alignment. For example, after multiple LEDs 110
are aligned and placed, misplaced LEDs 110 can be repositioned
prior to bonding.
[0043] FIGS. 2A-2C illustrate a sequence of schematic diagrams for
bonding electrodes 140 of LED dies 110 and contacts 150 of the
display substrate 120, according to one embodiment. FIG. 2A is a
cross sectional view of flux or underfill 160 applied to the
electrodes 140 prior to placement of the LEDs 110 on the display
substrate 120, according to one embodiment. FIG. 2B is a cross
sectional view of flux or underfill 160 applied to the contacts 150
prior to placement of the LEDs 110 on the display substrate 120,
according to one embodiment. Among other advantages, application of
the flux or underfill 160 on the electrodes 140 or the contacts 150
can reduce cleaning of the flux or underfill 160 from the display
substrate 120 after the bonding process. FIG. 2C is a cross
sectional view of flux or underfill 160 coated on the display
substrate 120 prior to placement of the LEDs 110 on the display
substrate 120, according to one embodiment. The flux or underfill
can be applied to the display substrate 120 by spin coating. After
the LEDs 110 are bonded to the display substrate 120, remaining
flux or underfill may be removed, such as by etching or application
of a solvent.
[0044] FIGS. 3A-3G illustrate a sequence of schematic diagrams for
applying flux or underfill 160 on electrodes 140 of the LED dies
110 (e.g., as seen in FIG. 2A) and removing a native substrate 310
from the LED dies 110, according to one embodiment. FIG. 3A is a
cross sectional view of a flux or underfill layer 330 on a slide
320 and LEDs 110 attached to a native substrate 310, according to
one embodiment. The native substrate 310 can be the substrate that
the LEDs 110 were formed on. For example, the native substrate is a
sapphire substrate (e.g., GaAs). The flux or underfill layer 330
can be deposited on the slide 320 by spin coating. For example, the
slide 320 is made of glass slide coated with rosin as the flux or
underfill layer 330.
[0045] FIG. 3B is a cross sectional view of electrodes 140 of the
LEDs 110 dipped into the flux or underfill layer 330, according to
one embodiment. FIG. 3C is a cross sectional view of LEDs 110
removed from the flux or underfill layer 330, according to one
embodiment. As result of dipping the electrodes 140 of the LEDs 110
into the flux or underfill layer 330, the flux or underfill 160 is
applied to at least tips of the electrodes 140.
[0046] FIG. 3D is a cross sectional view of LEDs 110 attached to
the native substrate 310 embedded in a polymer 340 on a carrier
substrate 350, according to one embodiment. FIG. 3E is a cross
sectional view of LEDs 110 embedded in a polymer 340 on the carrier
substrate 350 and detached from the native substrate 310, according
to one embodiment. The native substrate 310 can be removed by wet
etching or laser lift off (LLO), for example. After the native
substrate 310 is removed, the carrier substrate 350 with the LEDs
110 can be moved to a different facility or location for further
processing of the LEDs 110. During such moving process, the polymer
340 firmly holds the LEDs 110 in place.
[0047] FIG. 3F is a cross sectional view of LEDs 110 attached to an
etched polymer 360, according to one embodiment. After the native
substrate 310 is removed and the carrier substrate 350 is moved to
a desired processing facility or location, portions of the polymer
340 are removed. Portions can be removed by etching, such as radio
frequency (RF) dry etching, to create the etched polymer 360.
Alternatively, portions of the polymer 340 can be removed by
application of a solvent that dissolves the polymer 340.
[0048] After removing portions of the polymer 340, the LEDs 110 can
be detached from the etched polymer 360, as illustrated in FIG. 3G.
By removing portions of the polymer 340, the polymer 340 no longer
holds the LEDs 110 firmly in place, and enables the LEDs 110 to be
detached from the carrier substrate 340 (e.g., by a pick-up head
370). After detaching from the etched polymer 360, the LEDs 110
with the flux or underfill 160 can be aligned and placed on a
display substrate 120. A pick-up head 370 and aligning and placing
LEDs 110 on a display substrate 120 are further described with
reference to FIGS. 7-10.
[0049] Due to adhesive properties of the flux or underfill 160, the
flux or underfill 160 allows multiple LEDs (e.g., all desired LEDs)
to be placed prior to bonding. After an LED 110 is placed, adhesive
properties of the flux or underfill 160 can temporarily hold the
LED 110 in place, attached to the display substrate 120. The
adhesive properties of the flux or underfill 150 allow a pick-up
head 370 to detach from an LED 110 placed on the display substrate
120 and perform subsequent pick and place operations multiple
times. After the LEDs 110 are aligned and placed on the display
substrate 120, the LEDs 110 can be simultaneously bonded in
parallel to the display substrate 120 by forming metallic contacts
130. In some embodiments, pressure is applied to the LEDs 110 and
display substrate 120 during the bonding process to assist in
forming metallic contacts 130, as described below in detail with
reference to FIGS. 4A-5.
[0050] FIG. 4A is a cross sectional view of a platform 410 with
elastomer pads 420 attached to LEDs 110 on a display substrate 120,
according to one embodiment. FIG. 4B is a bottom view of the
platform 410 with elastomer pads 420, according to one embodiment.
The platform 410 with elastomer pads 420 can be used to apply
pressure (e.g., uniformly) on the LEDs 110 towards the display
substrate 120 by applying pressure on the platform 410. Alternative
pick and place methods can be used other than a platform with
elastomer pads, such as mechanical gripers or vacuum chucks.
[0051] In some embodiments, each elastomer pad 420 is in contact
with a single LED 110. Due to the discrete elastomer pads 420,
lateral movement of the LEDs 110 on the display substrate 120 can
be proportional to the coefficient of thermal expansion (CTE) of
the platform 410. Thus, if the platform has a negligible CTE, such
as fused silica platform, the lateral movement of the LEDs 110
during the bonding process can be reduced.
[0052] FIG. 5 is a cross sectional view of a high-pressure chamber
510 containing LEDs 110 on a display substrate 120, according to
one embodiment. By increasing the internal pressure of the
high-pressure chamber 510, the ambient pressure on the LEDs 110 and
display substrate 120 increases. Due to Pascal's law, the increase
in ambient pressure results in a net downward force on the LEDs
110, due to a difference in surface area between the bottom and top
surfaces of the LEDs 110.
[0053] During the bonding process, a hot plate 170 or heating
system (neither shown in FIG. 5) can heat the gas within the
high-pressure chamber 150. For example, a heating system increases
the gas in the chamber to 300.degree. C. Among other advantages,
since no solid object applies pressure on the LEDs 110, the
high-pressure chamber can reduce lateral movement of the LEDs 110
during the bonding process. For example, lateral movement can be
caused by a CTE mismatch when a solid object applies pressure on
the LEDs 110. Thus, lateral alignment of the placed LEDs 110 on the
display substrate 120 can be preserved when forming the metallic
contacts 130.
[0054] FIG. 6 is a flow chart illustrating a method for using flux
or underfill to temporarily attach and bond an LED to a substrate,
according to one embodiment. The steps of method may be performed
in different orders, and the method may include different,
additional, or fewer steps.
[0055] Electrodes of a first light emitting diode (LED) die are
aligned 610 with contacts of a substrate. Flux or underfill is
provided on at least the electrodes or the contacts. In some
embodiments, the flux or underfill is rosin.
[0056] The first LED die is placed 620 on the substrate. The flux
or underfill forms a trapping layer between the electrodes and the
contacts. In some embodiments, the LED die is aligned with and
placed on the substrate by a pick-up head. After placing the LED
die on the substrate, the pick-up head is detached from the LED die
due to adhesive forces of the flux or underfill securing the first
LED die to the substrate. For example, the pick-up head is
performing a pick and place operation for multiple LED dies.
[0057] The electrodes, contacts, and flux or underfill are heated
630 to form a metallic contact between the first LED die and the
substrate. In some embodiments, the electrodes, the contacts, and
the flux or underfill are selectively heated by focusing laser
light.
[0058] In some embodiments, the flux or underfill is provided on
the electrodes, and at least tips of the electrodes are coated with
the flux or underfill by dipping the tips into a flux or underfill
layer. In some embodiments, the electrodes having at least the tips
coated with the flux or underfill are embedded in a polymer. After
embedding the electrodes in a polymer, a native substrate is
removed from the first LED die. The native substrate is a substrate
on which the first LED die was fabricated. The native substrate is
removed prior to aligning the electrodes with the contacts. In some
embodiments, portions of the polymer surrounding the electrodes are
etched. After etching the portions of the polymer, the first LED
die is picked up by a pick-up head for aligning and placing the
first LED die on the substrate.
[0059] FIG. 7 is a cross sectional view illustrating an operation
of a pick-up head 370 placing an LED 110 onto a display substrate
120, according to one embodiment. The pick-up head 370 is attached
to the LED 110 and places the LED 110 onto the display substrate
120 by aligning contacts 150 of the display substrate 120 with
electrodes 140 of the LED 110. If a voltage difference is applied
between the contacts 150 and if the LED 110 is properly placed, the
LED 110 can emit light 728 from an emission surface 730. A hot
plate 170 is connected to the display substrate 120. A camera 712
is placed above a microscope lens 710 to capture images of the LED
110 being placed onto the display substrate 120 from the top. The
camera 712 generates image signals 714 that function as real time
feedback to correct improper LED 110 placement during the placement
process. The camera 712 sends image signals 714 to the controller
716. Using the image signals 714, the controller 716 sends control
signals 718 to the actuator 720. The actuator is attached to the
pick-up head 370 via a mount 708. In some embodiments, FIG. 7
includes different and/or other components than those shown in FIG.
7. For example, the LED 110 can include an elastomeric material
layer that allows the LED 110 to be adhesively attached to a
pick-up surface of the pick-up head 370. In another example, the
LED 110 can be temporarily attached to the pick-up head by
mechanical gripers or vacuum chucks.
[0060] The pick-up head 370 places LEDs 110 onto the display
substrate 120. The pick-up head 370 may also be referred to as a
pick and place head. The pick-up head 370 can support any number of
LEDs 110 and can place multiple LEDs 110 onto the display substrate
120 at once. For example, an array of LEDs 110 and their electrodes
140 can be aligned with and placed on an array of contacts 150 in a
single step (this may be referred to as a monolithic approach).
Before placing the LED 110, the pick-up head 370 may pick up the
LED 110 from a native substrate or a carrier substrate 350. Picking
up an LED 110 from a native or carrier substrate 350 and aligning
and placing the LED 110 on the display substrate 120 can be
referred to as a pick and place operation. Due to the flux or
underfill 160, the pick-up head 370 can perform multiple pick and
place operations without bonding each LED 110 to the display
substrate 120. In some embodiments, a portion of the pick-up head
370 is transparent to allow the camera 712 to capture images of the
LED 110 through the pick-up head 370. In some embodiments, one or
more LEDs 110 are repositioned once they are positioned on the
display substrate 120, for example, because the electrodes 140 are
misaligned with the contacts 150. In these embodiments, the flux or
underfill can be flexible enough to stay between the electrodes 140
and the contacts 150.
[0061] The mount 708 is an actuated slide that supports the pick-up
head 370. The mount 708 can support multiple pick-up heads 706. For
example, the mount 708 supports two pick-up heads 706 such that two
LEDs 110 can be placed at once. In some embodiments, the mount 708
is made of a transparent material, such as glass.
[0062] The actuator 720 is connected to the mount 708 and controls
movement of the mount 708. By moving the mount 708, the actuator
720 aligns the pick-up head 370 with the display substrate 120.
This allows the pick-up head 370 to place one or more LEDs 110 on
the display substrate 120 by aligning the electrodes 140 with the
contacts 150. In some embodiments, the actuator 720 is a multiple
degree of freedom actuator, such as an actuator that is configured
to move the mount 708 up and down, left and right, forward and
back. The actuator can also adjust yaw, tilt, or rotate the mount
708. In some embodiments, multiple actuators 720 connected to
multiple mounts 708 perform LED 110 placement tasks in parallel to
increase throughput.
[0063] The camera 712 is an image capturing device that captures
the images of the LED 110. In some embodiments, the camera 712
captures images to determine whether the LED 110 is emitting light
728. The camera 712 can also capture images to determine the
placement location and angle of a placed LED 110. The camera 712
can also enable detection of luminance of the light 728 emitted by
the LED 110.
[0064] The microscope lens 710 magnifies the LED 110 for the camera
712. The microscope lens 710 can allow the camera 712 to view and
distinguish light 728 from LEDs 110.
[0065] The controller 716 is a computing device that controls the
placement of LEDs 110 by providing control signals 718 to the
actuator 720. The control signals 718 are determined by the
controller 716 and can be based on the image signals 714 received
from the camera 712. The controller 716 can analyze the emitting
state or the placement of the LED 110 to determine if the placement
of the LED 110 should be adjusted. The controller 716 is further
described with reference to FIGS. 8 and 9.
[0066] FIG. 8 is a block diagram of the controller 716, according
to one embodiment. The controller 716 may include, among other
components, a processor 802, a memory 804, a user interface 806, a
video interface 870, and a control interface 808. The modules 802
through 808 communicate via a bus 874. Some embodiments of the
controller 716 have different and/or other components than those
shown in FIG. 8.
[0067] The controller 716 is a computer device that may be a
personal computer (PC), a video game console, a tablet PC, a
smartphone, or any machine capable of executing instructions
(sequential or otherwise) that specify actions to be taken by that
device. The controller 716 can operate as a standalone device or a
connected (e.g., networked) device that connects to other machines.
Furthermore, while only a single device is illustrated, the term
"device" shall also be taken to include any collection of devices
that individually or jointly execute instructions to perform any
one or more of the methodologies discussed herein.
[0068] The processor 802 is a processing circuitry configured to
carry out instructions stored in the memory 804. For example, the
processor 802 can be a central processing unit (CPU) and/or a
graphics processing unit (GPU). The processor 802 may be a
general-purpose or embedded processor using any of a variety of
instruction set architectures (ISAs). Although a single processor
802 is illustrated in FIG. 8, the controller 716 may include
multiple processors 802. In multiprocessor systems, each of the
processors 802 may commonly, but not necessarily, implement the
same ISA. The processor 802, or a part of it, may be specifically
designed for efficient processing of graphical images, such as
those received in the image signals 714. For example, the processor
802 may perform one or more image processing steps to determine an
emitting state of an LED 110.
[0069] The memory 804 is a non-transitory machine-readable medium
on which is stored data and instructions (e.g., software) embodying
any one or more of the methodologies or functions described herein.
For example, the memory 804 may store instructions that when
executed by the processor 802 configures the processor 802 to
perform the method described below in detail with reference to FIG.
6. Instructions may also reside, completely or at least partially,
within the processor 802 (e.g., within the processor's cache
memory) during execution thereof by the controller 716.
[0070] The term "machine-readable medium" should be taken to
include a single medium or multiple media (e.g., a centralized or
distributed database, or associated caches and servers) able to
store instructions. The term "machine-readable medium" shall also
be taken to include any medium that is capable of storing
instructions for execution by the device and that cause the device
to perform any one or more of the methodologies disclosed herein.
The term "machine-readable medium" includes, but is not limited to,
data repositories in the form of solid-state memories, optical
media, and magnetic media.
[0071] The user interface 806 is hardware, software, firmware, or a
combination thereof that enables a user to interact with the
controller 716. The user interface 806 can include an alphanumeric
input device (e.g., a keyboard) and a cursor control device (e.g.,
a mouse, a trackball, a joystick, a motion sensor, or other
pointing instrument). For example, a user uses a keyboard and mouse
to select placement parameters for placing a set of LEDs 110 on the
display substrate 120.
[0072] The control interface 808 transmits control signals 718 to
the actuator 720. For example, the control interface 808 is a
circuit or a combination of circuits and software that interfaces
with the actuator 720 to transmit the control signals 718.
[0073] The video interface 870 is a circuit or a combination of
circuit and software that receives image data via the image signals
714 from the camera 712 and transfers the image data to the memory
804 and/or processor 802 to be stored and processed.
[0074] The controller 716 executes computer program modules for
providing functionality described herein. As used herein, the term
"module" refers to computer program instructions and/or other logic
used to provide the specified functionality. Thus, a module can be
implemented in hardware, firmware, and/or software. In some
embodiments, program modules formed of executable computer program
instructions are loaded into the memory 804, and executed by the
processor 802. For example, program instructions for the method 700
describe herein can be loaded into the memory 804, and executed by
the processor 802.
[0075] FIG. 9 is a block diagram of software modules in the memory
804 of the controller 716, according to one embodiment. The memory
804 may store, among other modules, an actuator control module 902,
a temperature control module 904, a vision recognition module 906,
and a parameter adjuster module 908. The memory 804 may include
other modules not illustrated in FIG. 9.
[0076] The actuator control module 902 provides instructions for
generating control signals 718 to control the actuator 720 to
perform pick and place operations and adjust one or more placement
parameters. The placement parameters are parameters that relate to
placing one or more LEDs 110 on the display substrate 120. The
placement parameters include a placing location, a placing angle
(e.g., including a rotation angle and three tilt angles), a placing
pressure, a placing temperature, and a placing time. The placing
location is the location of the LED 110 on the display substrate
120. The placing angle is the angle of the LED 110 relative to the
display substrate 120. The placing pressure is the pressure applied
to the LED 110 by the pick-up head 370 once it is placed on the
display substrate 120. The placing time is the amount of time that
the placing pressure and the placing temperature are applied to the
LED 110. The placing temperature is the temperature of the display
substrate 120 or a temperature change of the display substrate 120
during the placing of the LED 110.
[0077] The temperature control module 904 sets the temperature of
the hot plate 170. As such, the temperature control module 904 sets
the placing temperature. The temperature control module 904 can
also set the temperature of the hot plate 170 during the bonding
process.
[0078] Parameters that relate to placement and bonding include
heating ramp profile, flux or underfill behavior, underfill
behavior, the influence of lateral and vertical movements (e.g.
caused by thermal expansion), the influence of metal oxides,
allowable pressure range, and allowable temperature range.
[0079] The heating ramp profile represents the temperature
evolution during bonding. For example, the temperature can increase
at a rate of 3.degree. Celsius per second (C/s) up to 750.degree.
C., then increase at a rate of 70.degree. C./s up to 270.degree.
C., then remain constant for five minutes (so called dwell time),
then decrease at a controlled rate of 2.degree. C./s. The heating
ramp profile can be optimized experimentally and/or based on
theoretical simulations.
[0080] Since underfills and fluxes can be liquid and freely move
(e.g., when heated), their presence and evolution during bonding
can be optically monitored during the bonding process (e.g., using
the same optical feedback system for LED alignment and
placement).
[0081] Lateral and vertical movement can occur as a result of heat
expansion during the bonding process. For example, the display
substrate 120 and hot plate 170 can expand as their respective
temperatures increase. The amount of expansion can depend on the
coefficient of thermal expansion (CTE) of each material (e.g. it is
proportional to the temperature and occurs in all directions).
[0082] Lateral and vertical movements may be monitored and
controlled during the bonding process. For example, when applying
pressure by a platform 410 with elastomer pads 420, vertical
movements may be monitored such that the pressure between the
display substrate 120 and the LED 110 remains constant during
bonding.
[0083] The vision recognition module 906 performs analysis on the
image data in the image signals 714 to determine the emitting
states of the LED 110 and the placement of the LED 110. The vision
recognition module 906 can determine whether an emitting state
fails one or more criteria. The criteria can form a standard for
determining proper placement of one or more LEDs 110. For example,
one of the criteria relates to whether the LED 110 emits light 728
or the LED 110 emits an amount of lumens above a threshold. An
emitting state can fail the criteria for any number of reasons,
such as, for example, an LED 110 is placed outside a target placing
location, placing angle, placing time, placing pressure, placing
temperature, etc.
[0084] The parameter adjuster module 908 provides instructions for
monitoring the placement and bonding parameters and adjusting them
in real time as needed. For example, the parameter adjuster module
908 adjusts the placement parameters in response to one or more
emitting states failing criteria. The parameter adjuster module 908
may determine which parameters to adjust based on the failed
criteria. For example, if an LED 110 is incorrectly placed on the
display substrate 120 (e.g., between contacts 150), the placing
location can be adjusted. In another example, if the LED 110 moves
after placement, the placing time and pressure may be adjusted. The
adjusted parameters can be temporarily adjusted for the LED 110
currently being placed or permanently adjusted for the current and
future placement of LEDs 110. The parameter adjuster module 908 can
continually adjust the placement parameters until one or more
emitting states satisfy the criteria. This can allow for optimizing
the placement parameters of the placement process and allow insight
into root causes of failed LED 110 placement.
[0085] FIG. 10 is a circuit diagram of a testing circuit 1000 on a
display substrate 120, according to one embodiment. The testing
circuit 1000 can be used for experimental purposes, for example, to
test a new placing scheme or new placement parameters. The
arrangement of the testing circuit 1000 allows parallel testing of
the LEDs 110 on the circuit using only two connecting wires. The
testing circuit 1000 includes a top wire 1004, a bottom wire 1006,
LEDs 110a through 704e (represented as diodes), and resistors 1002
electrically connected between the top wire 1004 and LEDs 110a
through 704e. The dashed lines around LEDs 110b and 704d represent
shorts between the contacts 150 on the display substrate 120.
[0086] By applying a high supply voltage V.sub.a to the top wire
1004 and low supply voltage V.sub.b to the bottom wire 1006
(V.sub.a>V.sub.b), a voltage bias can be applied across the LEDs
110. As a result, LEDs 110a, 704c, and 704e will emit light 728.
However, LEDs 110b and 704d will not emit light 728, due to the
shorts. Despite the local circuit shorts near LED 110b and LED
110d, the resistors 1002 prevent the entire testing circuit 1000
from shorting. For example, the resistors 1002 each have a
resistance of 750 K.OMEGA..
[0087] Furthermore, the relative voltage levels of applied voltages
V.sub.a, V.sub.b can be reversed to apply a negative voltage bias
across the top and bottom wires (V.sub.a<V.sub.b). By doing so,
reverse current I.sub.Rev flows from the bottom wire 1006 to the
upper wire 1004 via the shorted LEDs 110b and 704d. The number of
shorted LEDs in the testing circuit 1000 can be estimated by
measuring I.sub.Rev. For example, if each shorted LED allows 70
.mu.A of current to pass through, 20 .mu.A of reverse current
I.sub.Rev may indicate that two LEDs are shorted.
[0088] Assuming that the LEDs 110 are functioning properly (e.g.,
they were tested before being picked up by the pick-up head 370),
the results of the positive and negative voltage bias can be used
in combination with the images captured by the camera 712 to
determine the number of improperly placed LEDs 110. For example, if
no I.sub.Rev is measured, yet one or more LEDs 110 do not emit
light 728 when current is in the forward direction
(V.sub.a>V.sub.b), then it may be determined that one or more
LEDs 110 were improperly placed on the display substrate 120.
[0089] FIGS. 11A through 11C show schematic cross sections of a
.mu.LED 1100, according to some embodiments. The .mu.LED 1100 is an
example of a visible or non-visible LED that may be positioned on a
surface of a display substrate (e.g., display substrate 120) to
emit collimated visible or invisible light. The feature size of the
.mu.LED 1100 (e.g., the diameter) may range from sub-micrometers to
tens of micrometers (e.g., from 0.1 .mu.m to 10 .mu.m). The pitch
(e.g., spacing between .mu.LEDs) may similarly range from
sub-micrometers to tens of micrometers.
[0090] The .mu.LED 1100 may be formed on a substrate layer 1102,
and may include, among other components, a gallium semiconductor
layer 1104 disposed on the substrate layer 1102, a dielectric layer
1114 disposed on the gallium semiconductor layer 1104, a p-contact
1116 disposed on a first portion of the dielectric layer 1114, and
an n-contact 1118 disposed on a second portion of the gallium
semiconductor layer 1104. In some embodiments, the gallium
semiconductor layer 1104 is grown on the substrate layer 1102 as an
epitaxial layer.
[0091] As illustrated in FIG. 11B, the substrate layer 1102 may be
removed from the surface of the gallium semiconductor layer 1104 of
the .mu.LED 1100 to reveal a light emitting face 1110 of the
.mu.LED 1100. In some embodiments, the substrate layer 1102 is
separated from the gallium semiconductor layer 1104 using a laser
lift-off (LLO) process.
[0092] In some embodiments, the gallium semiconductor layer 1104 is
shaped into a mesa 1106. An active (or light emitting) layer 1108
(or "active light emitting area") is included in the structure of
the mesa 1106. The mesa 1106 has a truncated top, on a side opposed
to the light transmitting or emitting face 1110 of the .mu.LED
1100. The mesa 1106 also has a near-parabolic shape to form a
reflective enclosure for light generated within the .mu.LED
1100.
[0093] FIG. 11C illustrates the .mu.LED 1100 after removal of the
substrate layer 1102. Upon removal of the substrate layer 1102, the
.mu.LED 1100 may be placed on a display substrate (not shown), and
operated to emit light. The arrows 1112 show how light emitted from
the active layer 1108 is reflected off the p-contact 1116 and
internal walls of the mesa 1106 toward the light emitting face 1110
at an angle sufficient for the light to escape the .mu.LED device
1100 (i.e., within an angle of total internal reflection). During
operation, the p-contact 1116 and the n-contact 1118 connect the
.mu.LED 1100 to a display substrate (not shown).
[0094] In some embodiments, the parabolic shaped structure of the
.mu.LED 1100 results in an increase in the extraction efficiency of
the .mu.LED 1100 into low illumination angles when compared to
unshaped or standard LEDs. For example, standard LED dies generally
provide an emission full width half maximum (FWHM) angle of
120.degree., which is dictated by the Lambertian reflectance from a
diffuse surface. In comparison, the .mu.LED 1100 can be designed to
provide controlled emission angle FWHM of less than standard LED
dies, such as around 60.degree.. This increased efficiency and
collimated output of the .mu.LED 1100 can produce light visible to
the human eye with only nano-amps of drive current.
[0095] The .mu.LED 1100 may include an active light emitting area
that is less than standard LEDs, such as less than 2,000
.mu.m.sup.2. The .mu.LED 1100 directionalizes the light output from
the active light emitting area and increases the brightness level
of the light output. The .mu.LED 1100 may be less than 20 .mu.m in
diameter with a parabolic structure (or a similar structure) etched
directly onto the LED die during the wafer processing steps to form
a quasi-collimated light beam emerging from the light emitting face
1110 of the .mu.LED 1100.
[0096] As used herein, "directionalized light" includes collimated
and quasi-collimated light. For example, directionalized light may
be light that is emitted from a light generating region of a LED
and at least a portion of the emitted light is directed into a beam
having a half angle. This may increase the brightness of the LED in
the direction of the beam of light.
[0097] A .mu.LED 1100 may include a circular cross section when cut
along a horizontal plane as shown in FIGS. 11A-11C. A .mu.LED 1100
may have a parabolic structure etched directly onto the LED die
during the wafer processing steps. The parabolic structure may
comprise a light emitting region of the .mu.LED 1100 and reflects a
portion of the generated light to form the quasi-collimated light
beam emitted from the light emitting face 1110.
[0098] As discussed above, the substrate layer 1102 may correspond
to a glass or sapphire substrate. The gallium semiconductor layer
1104 may include a p-doped GaN layer, an n-doped GaN layer, and the
active layer 1108 between the p-doped and n-doped GaN layers. The
active layer may include a multi-quantum well structure. The
substrate layer 1102 is transparent to a laser projected by the
laser projector 126, which may be applied through the substrate
layer 1102 to the gallium semiconductor layer 1104. In other
embodiments, the substrate layer 1102 may comprise a gallium
compound, as such GaAs. The gallium semiconductor layer 1104 may
include a p-doped GaAn layer, an n-doped GaAs layer, and the active
layer 1108 between the p-doped and n-doped GaAs layers. In some
embodiments, the .mu.LED 1100 includes a Gallium phosphide (GaP)
substrate 1102 for increased transparency relative to GaAs, such as
for red visible LEDs. In some embodiments, the substrate layer 1102
is a semiconductor substrate, such as a silicon substrate. When a
non-transparent substrate layer 1102 is used, a laser may be
applied at the interface of the substrate layer 1102 and the
gallium semiconductor layer to separate the layers and form the
gallium material to facilitate pick and place.
[0099] While particular embodiments and applications have been
illustrated and described, it is to be understood that the
invention is not limited to the precise construction and components
disclosed herein and that various modifications, changes and
variations which will be apparent to those skilled in the art may
be made in the arrangement, operation and details of the method and
apparatus disclosed herein without departing from the spirit and
scope of the present disclosure.
* * * * *