U.S. patent application number 15/442293 was filed with the patent office on 2017-07-27 for selective transfer of micro devices.
The applicant listed for this patent is VueReal Inc.. Invention is credited to Gholamreza Chaji.
Application Number | 20170215280 15/442293 |
Document ID | / |
Family ID | 59359285 |
Filed Date | 2017-07-27 |
United States Patent
Application |
20170215280 |
Kind Code |
A1 |
Chaji; Gholamreza |
July 27, 2017 |
SELECTIVE TRANSFER OF MICRO DEVICES
Abstract
What is disclosed is a method of selectively transferring micro
devices from a donor substrate to contact pads on a receiver
substrate. Micro devices being attached to a donor substrate with a
donor force. The donor substrate and receiver substrate are aligned
and brought together so that selected micro devices meet
corresponding contact pads. A receiver force is generated to hold
selected micro devices to the contact pads on the receiver
substrate. The donor force is weakened and the substrates are moved
apart leaving selected micro devices on the receiver substrate.
Several methods of generating the receiver force are disclosed,
including adhesive, mechanical and electrostatic techniques.
Inventors: |
Chaji; Gholamreza;
(Waterloo, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VueReal Inc. |
Waterloo |
|
CA |
|
|
Family ID: |
59359285 |
Appl. No.: |
15/442293 |
Filed: |
February 24, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15002662 |
Jan 21, 2016 |
|
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15442293 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 2224/29026
20130101; H01L 2224/27334 20130101; H01L 24/95 20130101; H01L
2224/29011 20130101; H01L 2224/75252 20130101; H01L 2224/97
20130101; H01L 24/83 20130101; H01L 2224/83141 20130101; H01L 22/14
20130101; H01L 2221/68368 20130101; H01L 2224/2732 20130101; H01L
2224/83902 20130101; H01L 2224/83234 20130101; H01L 2221/68322
20130101; H01L 2224/83862 20130101; H05K 13/04 20130101; H01L 24/29
20130101; H01L 24/32 20130101; H01L 2224/8316 20130101; H01L
2224/83192 20130101; H05K 13/0015 20130101; H01L 2224/2919
20130101; H01L 2224/2919 20130101; H01L 2224/75253 20130101; H01L
2224/83005 20130101; H01L 2224/83238 20130101; H01L 2224/95
20130101; H01L 2224/2732 20130101; H01L 2224/29026 20130101; H01L
2224/95 20130101; H01L 2224/97 20130101; H01L 2924/0781 20130101;
H01L 24/75 20130101; H01L 2924/00014 20130101; H01L 2224/83
20130101; H01L 2924/00012 20130101; H01L 2924/00014 20130101; H01L
2924/07802 20130101; H01L 2224/27 20130101; H01L 2924/00012
20130101; H01L 2224/27002 20130101; H01L 2224/29078 20130101; H01L
2224/8318 20130101; H01L 2224/32237 20130101; G01R 31/2635
20130101; H01L 2224/2919 20130101; H01L 22/20 20130101; H01L
2224/83121 20130101; H05K 13/0069 20130101; H01L 2224/95001
20130101; H05K 1/111 20130101; H01L 24/97 20130101; H01L 2224/29019
20130101; H01L 2224/83143 20130101; H01L 21/6835 20130101; H01L
2224/7598 20130101; H01L 2224/83862 20130101; H01L 24/27 20130101;
H01L 25/50 20130101; H01L 2221/68381 20130101; H01L 2224/29006
20130101; H01L 2224/83121 20130101; H01L 2224/8314 20130101; H01L
2224/83191 20130101 |
International
Class: |
H05K 1/11 20060101
H05K001/11; H05K 13/04 20060101 H05K013/04; H05K 13/00 20060101
H05K013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2016 |
CA |
2921737 |
Jul 19, 2016 |
CA |
2936523 |
Claims
1. A method of transferring a micro device, the method comprising:
positioning a donor substrate comprising the micro device proximal
to a receiver substrate, wherein the micro device is affixed to the
donor substrate by a donor force; and transferring the micro device
from the donor substrate to the receiver substrate responsive to
selectively reducing the donor force affixing the micro device to
the donor substrate.
2. The method of claim 1, wherein selectively reducing the donor
force comprises physically shielding the micro device from the
donor force.
3. The method of claim 1, wherein selectively reducing the donor
force comprises changing the bias condition of donor force.
4. The method of claim 1, wherein selectively reducing the donor
force comprises selectively applying a form of light to the micro
device using a shadow mask.
5. The method of claim 1, wherein selectively reducing the donor
force comprises changing a distance between the micro device and a
source of the donor force.
6. A method of transferring a micro device, the method comprising:
positioning a donor substrate comprising the micro device proximal
to a receiver substrate, wherein the receiver substrate comprises a
force modulator element; and transferring the micro device from the
donor substrate to the receiver substrate responsive to selectively
reducing the distance between the micro device and the force
modulator element.
7. The method of claim 6, wherein reducing the distance between the
micro device and the force modulator element comprises moving the
micro device toward the force modulator element using a
membrane.
8. A method of transferring a micro device, the method comprising:
positioning a donor substrate comprising the micro device proximal
to a receiver substrate, wherein the receiver substrate comprises a
force modulator element creating transfer force for transferring
the selected micro devices; and reducing the effect of said force
generated by the force modulator element on unwanted micro
devices.
9. The method of claim 8, wherein selectively reducing the effect
of a force generated by the force modulator element comprises
generating a reverse polarity of force surrounding the force
modulating element.
10. A method of transferring micro devices, the method comprising:
positioning a donor substrate comprising micro devices proximal to
a receiver substrate, wherein the receiver substrate comprises a
current curable bonding layer; and transferring the micro devices
by applying current to the bonding layer of selected micro
devices.
11. The method of claim 10, wherein the current is applied using a
circuit in the receiver substrate.
12. The method of claim 10, wherein the circuit in the receiver
substrate is shared with a circuit associated the driving or
controlling the selected micro devices.
13. The method of claim 10, wherein the bonding layer comprises one
or more contact pads and selectively curing a portion of the
bonding layer comprises curing a portion of the bonding layer
between one or more contact pads.
14. A receiver substrate used to receive a micro device from a
donor substrate, the receiver substrate comprising: an array of one
or more pad structures, wherein each pad structure comprises a
conductive layer and a dielectric layer.
15. The receiver substrate of claim 14, wherein the dielectric
layer can be modulated to be a conductive layer.
16. The receiver substrate of claim 15, wherein the dielectric
layer is modulated to the conductive layer during operation to
couple a circuit in the receiver substrate to the micro device.
17. The receiver substrate of claim 15, wherein the dielectric
layer is modulated to the conductive layer using a doped layer.
18. The receiver substrate of claim 15, wherein the dielectric
layer is modulated to the conductive layer using a laser to induce
dielectric breakdown.
19. The receiver substrate of claim 14, wherein the dielectric
layer creates electrostatic force that attracts the micro device
from the donor substrate.
20. A method of transferring a micro device, the method comprising:
positioning a donor substrate comprising a micro device proximal to
a receiver substrate, wherein the receiver substrate comprises a
pad structure having a dielectric layer and a conductive layer;
transferring the micro device from the donor substrate to the pad
structure; and coupling the first micro device and the conductive
layer by removing the dielectric layer.
21. The method of claim 20, wherein the dielectric layer is removed
by the means of mechanical force.
22. The method of claim 20, wherein the dielectric layer is removed
by the means of thermal force.
23. The method of using a deformable layer on top of the receiver
pad or landing or the micro device to adjust for different height
in the micro devices.
24. The method of claim 23, where the deformable layer consists of
either conductive layer, high resistive layer, or a dielectric
layer.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 15/002,662 filed Jan. 21, 2016, which is
hereby incorporated by reference in its entirety. This application
claims priority to Canadian Application No. 2,921,737 filed Feb.
25, 2016, and Canadian Application No. 2,936,523, filed Jul. 19,
2016, each of which is hereby incorporated by reference herein in
its entirety.
FIELD OF THE INVENTION
[0002] The present disclosure relates to device integration into
system or receiver substrates. More specifically, the present
disclosure relates to selective transfer of micro devices from a
donor substrate to a receiver substrate.
BRIEF SUMMARY
[0003] According to one aspect there is provided, a method of
transferring selected micro devices in an array of micro devices
each of which is bonded to a donor substrate with a donor force to
contact pads in an array on a receiver substrate, the method
comprising: aligning the donor substrate and the receiver substrate
so that each of the selected micro devices are in line with a
contact pad on the receiver substrate (in case contact pad does not
pre-exist, other markers in the receiver substrate can be used for
alignment); moving the donor substrate and the receiver substrate
together until each of the selected micro devices are in contact or
proximity with a respective contact pad on the receiver substrate;
generating a receiver force that acts to hold the selected micro
devices to their contact pads while not affecting other micro
devices in contact with or proximity contact with the receiver
substrate; and moving the donor substrate and the receiver
substrate apart leaving the selected micro devices on the receiver
substrate.
[0004] Some embodiments further comprise weakening the donor force
bonding the micro devices to the donor substrate to assist micro
device transfer.
[0005] In some embodiments, the donor force for the selected micro
devices is weakened to improve selectivity in micro device
transfer. In some embodiments, the receiver force is generated
selectively to improve selectivity in micro device transfer. Some
embodiments further comprise weakening the donor force using laser
lift off. Some embodiments further comprise weakening the donor
force by heating an area of the donor substrate. Some embodiments
further comprise modulating the force by magnetic field. Some
embodiments further comprise modulating the receiver force by
heating the receiver substrate.
[0006] In some embodiments the heating is performed by passing a
current through the contact pads. In some embodiments the receiver
force is generated by mechanical grip. Some embodiments further
comprise performing an operation on the receiver substrate so that
the contact pads permanently bond with the selected micro
devices.
[0007] In some embodiments the receiver force is generated by
electrostatic attraction between the selected micro devices and the
receiver substrate. In some embodiments the receiver force is
generated by an adhesive layer positioned between the selected
micro devices and the receiver substrate. Some embodiments further
comprise removing the donor force; and applying a push force to
selected micro devices to move the devices toward the receiver
substrate.
[0008] In some embodiments the push force is created by a
sacrificial layer deposited between the selected micro device and
the donor substrate.
[0009] According to another aspect there is provided a receiver
substrate structure comprising: an array of landing areas for
holding micro devices from a donor substrate selectively, each
landing area comprising: at least one contact pad for coupling or
connecting a micro device to at least one circuit or a potential in
the receiver substrate; and at least one force modulation element
for creating a receiver force for holding a micro device on the
receiver substrate. For clarity, the area where the micro device
sits on the receiver substrate is called the landing area. The
contact pad can pre-exist on the receiver substrate or be deposited
after the micro device is transferred to the receiver
substrate.
[0010] In some embodiments the force modulation element is an
electrostatic structure. In some embodiments the force modulation
element is a mechanical grip. In some embodiments, for each landing
area, a same element acts as the force modulation element and the
contact pad.
[0011] The foregoing and additional aspects and embodiments of the
present disclosure will be apparent to those of ordinary skill in
the art in view of the detailed description of various embodiments
and/or aspects, which is made with reference to the drawings, a
brief description of which is provided next.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The foregoing and other advantages of the disclosure will
become apparent upon reading the following detailed description and
upon reference to the drawings.
[0013] FIG. 1A shows a donor substrate and a receiver substrate
before the transfer process begins.
[0014] FIG. 1B shows a donor substrate and a receiver substrate
before the transfer process begins.
[0015] FIG. 2A shows a flowchart of modulating at least one of the
donor or receiver forces after donor and receiver substrates are in
contact or proximity with each other.
[0016] FIG. 2B shows a flowchart of modulating the donor forces in
advance and modulating receiver forces if needed after donor and
receiver substrates are in contact or proximity with each
other.
[0017] FIG. 2C shows a flowchart of modulating the receiver forces
in advance and modulating donor forces if needed after donor and
receiver substrates are in contact or proximity with each
other.
[0018] FIGS. 3A-3E show different steps for transferring devices
based on 1000A. Similar steps can be used for 1000B and 1000C and
combination of 1000A, 1000B, 1000C.
[0019] FIG. 3A shows the step of aligning the donor and receiver
substrates.
[0020] FIG. 3B shows the step of moving the substrates together
within a defined distance margin.
[0021] FIG. 3C-1 shows one embodiment of modulating the forces by
applying receiver forces selectively.
[0022] FIG. 3C-2 shows one embodiment of modulating the forces by
weakening the donor force selectively and applying receiver force
globally.
[0023] FIG. 3D shows one embodiment of modulating the forces by
applying receiver and weakening donor forces selectively.
[0024] FIG. 3E shows the step of moving the substrate apart.
[0025] FIG. 4A shows a donor substrate with different micro devices
interleaved and the corresponding contact pads in the receiver
substrate are aligned with each micro devices accordingly enabling
transferring different micro devices at once.
[0026] FIG. 4B shows a donor substrate with different micro devices
in groups and the corresponding contact pads in the receiver
substrate are aligned with each micro devices accordingly enabling
transferring different micro devices at once.
[0027] FIG. 4C shows a donor substrate with different micro devices
interleaved and only one set of the corresponding contact pads in
the receiver substrate with one of the micro device types is
aligned with each micro devices accordingly so multiple
transferring process is needed to transfer all different types of
micro devices.
[0028] FIG. 5A shows selective and global heating elements
incorporated into substrates.
[0029] FIG. 5B shows one embodiment for pattering selective and
global heating elements incorporated into substrates.
[0030] FIG. 5C shows use of external sources to selectively heat up
at least one substrate.
[0031] FIG. 6A shows a flowchart of method 1100 for selectively
transferring micro devices from a donor substrate to a receiver
substrate.
[0032] FIGS. 6B-6G show one method of implementing steps described
in method 1100.
[0033] FIG. 6B shows the step of preparing the donor and receiver
substrates for selective transfer.
[0034] FIG. 6C shows the step of aligning the substrates.
[0035] FIG. 6D shows the step of moving the substrates toward each
other within a predefined distance margin.
[0036] FIG. 6E shows the step of creating receiver forces by curing
the adhesive (e.g. applying pressure or heat). This can be globally
or selectively.
[0037] FIG. 6F shows the step of reducing donor forces if needed.
This can be globally or selectively.
[0038] FIG. 6G shows the step of moving the substrates away from
each other.
[0039] FIG. 7A shows other possible arrangements of adhesive on
receiver substrate.
[0040] FIG. 7B shows a contact pad with a cut out before and after
application of an adhesive.
[0041] FIG. 8 shows a stamping process that can be used to apply
adhesive to contact pads.
[0042] FIG. 9 shows a flowchart of method 1200 for selectively
transferring micro devices from a donor substrate to a receiver
substrate.
[0043] FIG. 10 shows a donor substrate and a receiver substrate
setup to perform method 1200.
[0044] FIGS. 11A-11E show one embodiment for implementing steps in
method 1200.
[0045] FIG. 11A shows the step of aligning donor and receiver
substrates.
[0046] FIG. 11B shows the step of moving donor and receiver
substrates to a defined distance margin while mechanical force is
loose.
[0047] FIG. 11C shows the step of increasing mechanical forces.
[0048] FIG. 11D shows the step of reducing donor forces if needed
(this step can be done in advance as well).
[0049] FIG. 11E shows moving the donor and receiver substrates away
from each other.
[0050] FIG. 12A shows a flowchart of method 1300 for selectively
transferring micro devices from a donor substrate to a receiver
substrate.
[0051] FIG. 12B shows a donor substrate and a receiver substrate
setup to perform method 1300.
[0052] FIGS. 13A-13E show a specific embodiment for implementing
steps in method 1300.
[0053] FIG. 13A shows the step of aligning the donor and receiver
substrates.
[0054] FIG. 13B shows the step of moving the substrates within a
predefined distance margin.
[0055] FIG. 13C shows the step of creating receiver force by
applying potential to electrostatic elements. This can be done
selectively or globally.
[0056] FIG. 13D shows the step of reducing the donor force if
needed. This can be done globally or selectively.
[0057] FIG. 13E shows the step of moving the substrates away.
[0058] FIG. 14A shows another alternative placement for
electrostatic layer.
[0059] FIG. 14B shows another alternative placement for
electrostatic layer.
[0060] FIG. 14C shows another alternative placement for
electrostatic layer.
[0061] FIG. 14D shows another alternative placement for
electrostatic layer.
[0062] FIG. 15A shows another alternative geometry for micro
devices and contact pads.
[0063] FIG. 15B shows another alternative geometry for micro
devices and contact pads.
[0064] FIG. 15C shows another alternative geometry for micro
devices and contact pads.
[0065] FIG. 15D shows another alternative geometry for micro
devices and contact pads.
[0066] FIG. 15E shows another alternative geometry for micro
devices and contact pads.
[0067] FIG. 16 shows a flowchart of method 1400 for selectively
transferring micro devices from a donor substrate to a receiver
substrate.
[0068] FIG. 17A-17E show one embodiment for implementing steps in
method 1400.
[0069] FIG. 17A shows the step of aligning the donor and receiver
substrates.
[0070] FIG. 17B shows the step of moving the substrates to a
predefined distance margin from each other.
[0071] FIG. 17C shows one embodiment for the step of creating a
receiver force if needed. This can be globally or selectively. The
force can be created with different method.
[0072] FIG. 17D shows applying a push force to the micro devices
from the donor substrate. The push force from donor substrate
should be selective.
[0073] FIG. 17E shows the step of moving substrate away.
[0074] FIG. 18A shows a platform for testing by biasing at least
one of the donor substrate or the receiver substrate to enable
testing the micro devices for defects and performance. Here, the
output of the micro device is through the receiver substrate.
[0075] FIG. 18B shows a platform for testing by biasing at least
one of the donor substrate or the receiver substrate to enable
testing the micro devices for defects and performance. Here, the
output of the micro device is through the donor substrate.
[0076] FIG. 19 shows a simplified biasing condition of receiver
substrate for testing the micro devices for defect and performance
analysis.
[0077] FIG. 20A shows a selective liftoff process to modulate the
force on the donor substrate using shadow mask.
[0078] FIG. 20B shows a selective liftoff process to modulate the
force on the donor substrate using a patterned mask.
[0079] FIG. 21 shows a donor substrate with a sacrificial layer
between micro devices and the substrate.
[0080] FIG. 22 shows a donor substrate with force modulating
elements on the donor substrate.
[0081] FIG. 23 shows a donor substrate with a force modulating
element and a biasing pads.
[0082] FIG. 24A shows an embodiment for force modulating element by
changing the capacitance.
[0083] FIG. 24B shows an embodiment for force modulating element by
using a shield electrode.
[0084] FIG. 25 shows an embodiment that change the distance between
selected devices and receiver substrate with the distance between
unselected device and receiver substrate.
[0085] FIG. 26A shows an embodiment using membrane for moving the
micro device backward or forward.
[0086] FIG. 26B shows an embodiment where the membrane moves the
selected device closer to the receiver substrate.
[0087] FIG. 27A shows an embodiment using a cantilever (membrane)
for moving the micro devices forward or backward.
[0088] FIG. 27B shows an embodiment using a cantilever (membrane)
for moving the selected device closer to the receiver
substrate.
[0089] FIG. 28 shows an embodiment for confining the transfer force
on the selected device by diverging the force from adjacent
devices.
[0090] FIG. 29 shows an embodiment for confining the transfer force
on the selected devices by reducing the effect of the force on the
adjacent devices.
[0091] FIG. 30 shows an embodiment which uses an electrostatic
electrode to modulate transfer force.
[0092] FIG. 31 shows an embodiment which uses an electrostatic
electrode to modulate transfer force.
[0093] FIG. 32 shows an embodiment which uses an electrostatic
electrode to modulate transfer force.
[0094] FIG. 33 shows a donor substrate and a receiver substrate
with bonding element formed on each contact pads of the receiver
substrate.
[0095] FIG. 34 shows an aligned donor substrate and receiver
substrate with a current source connected to one of the contact
pads on the receiver substrate and the donor substrate.
[0096] FIG. 35 shows a donor substrate and a receiver substrate
with a transferred and bonded micro device on the receiver
substrate.
[0097] FIG. 36 shows a receiver substrate with contact pads having
two electrically isolated bonding elements and a current source
connecting to these elements.
[0098] FIG. 37 shows a donor substrate and a receiver substrate
with bonding element formed on each micro devices on the donor
substrate.
[0099] FIG. 38 shows a receiver substrate with contact pads having
two electrically isolated bonding elements separate circuitries for
the bonding elements and driving contact pads.
[0100] FIG. 39 shows a receiver substrate with bonding elements
which transform to the driving contact pads after curing. Two
switches control the curing and driving functions.
[0101] FIG. 40 shows a receiver substrate with bonding elements
which transform to the driving contact pads after curing. The same
circuitry controls the curing and driving functions.
[0102] FIG. 41 shows a receiver substrate connection scheme with
bonding elements which is connected to the individual pixel circuit
for the vertical current curing configuration.
[0103] FIG. 42 shows a receiver substrate connection scheme with
bonding elements which is connected to the individual pixel circuit
for the lateral current curing configuration.
[0104] FIG. 43 is a block diagram showing process flow of
transferring micro devices.
[0105] FIG. 44A is a cross sectional view of an array of micro
devices on the donor substrate and contact pads on the acceptor
one.
[0106] FIG. 44B is a cross sectional view of donor and acceptor
substrates in which electrostatic force is applied.
[0107] FIG. 44C is a cross sectional view of transferred micro
device to the receiver substrate.
[0108] FIG. 45A is a process flow of modifying dielectric layer to
form a conductive layer by doping top surface of the dielectric
layer with dopants.
[0109] FIG. 45B is a cross sectional view of the receiver substrate
with dielectric layer in which process of doping top surface of
dielectric layer to form a conductive layer is demonstrated.
[0110] FIG. 45C shows the process for diffusion of dopant through
the dielectric layer with thermal annealing.
[0111] FIG. 45D is a block diagram showing process flow of
modifying dielectric layer to form a conductive layer by doping
bottom surface of the dielectric layer with dopants.
[0112] FIG. 46A shows process flow of creating conductive
interlayer by doping surface of the micro device.
[0113] FIG. 46B shows surface doping of the micro devices.
[0114] FIG. 46C shows transferred surface doped micro devices and
annealing step for dopant penetration from the micro device into
the dielectric layer.
[0115] FIG. 47 shows process flow of modifying electrical
properties of the dielectric layer with laser exposure.
[0116] FIG. 48A is a cross sectional view of transferred micro
device to the receiver substrate.
[0117] FIG. 48B shows laser exposure of micro device from
sidewalls.
[0118] FIG. 49 is a block diagram showing process flow of micro
device transfer using soft material as a dielectric layer.
[0119] FIG. 50 is a cross sectional view of transferred micro
device on top of the soft dielectric layer in which mechanical
stress is applied.
[0120] FIG. 51 is a cross sectional view of formed soft dielectric
layer between the insulator layers on the receiver substrate.
[0121] FIG. 52A is a process flow diagram of transferring patterned
soft dielectric layer to the contact pads by stamping
technique.
[0122] FIG. 52B is schematic illustration of forming patterned soft
dielectric layer on the contact pads by stamping method.
[0123] FIG. 52C is schematic illustration of forming patterned soft
dielectric layer on the contact pads by using patterned
stamper.
[0124] FIG. 53A shows a cross sectional view of the receiver
substrate with the formed dielectric with metal nanoparticles
layer.
[0125] FIG. 53B is a cross sectional view of transferred micro
device on top of the dielectric with metal nanoparticles layer.
[0126] FIG. 53C shows aggregated metal nanoparticles forming
conductive layer between the contact pad and micro device electrode
by mechanical or thermal stress.
[0127] FIG. 54A is a cross sectional view of receiver substrate and
micro devices with different heights on the donor substrate.
[0128] FIG. 54B shows process of transferring micro devices with
different heights to the receiver substrate.
[0129] FIG. 55 shows deformable pads for creating dual function
pads.
[0130] While the present disclosure is susceptible to various
modifications and alternative forms, specific embodiments or
implementations have been shown by way of example in the drawings
and will be described in detail herein. It should be understood,
however, that the disclosure is not intended to be limited to the
particular forms disclosed. Rather, the disclosure is to cover all
modifications, equivalents, and alternatives falling within the
spirit and scope of an invention as defined by the appended
claims.
DETAILED DESCRIPTION
[0131] In one aspect of the invention is transferring a micro
device where the method comprising: positioning a donor substrate
comprising the micro device proximal to a receiver substrate,
wherein the micro device is affixed to the donor substrate by a
donor force; and transferring the micro device from the donor
substrate to the receiver substrate responsive to selectively
reducing the donor force affixing the micro device to the donor
substrate.
[0132] In one case, the donor force is reduced by physically
shielding the micro device from the donor force.
[0133] Alternatively, the selectively reducing the donor force
comprises changing the bias condition of donor force.
[0134] In another method, selectively reducing the donor force
comprises selectively applying a form of light to the micro device
using a shadow mask.
[0135] In another method, reducing the donor force comprises
changing a distance between the micro device and a source of the
donor force.
[0136] Another aspect of the invention is a method of transferring
a micro device, the method comprising: positioning a donor
substrate comprising the micro device proximal to a receiver
substrate, wherein the receiver substrate comprises a force
modulator element; and transferring the micro device from the donor
substrate to the receiver substrate responsive to selectively
reducing the distance between the micro device and the force
modulator element.
[0137] In one case, reducing the distance between the micro device
and the force modulator element comprises moving the micro device
toward the force modulator element using a membrane.
[0138] In another case, reducing the distance is done mechanically
moving the device closer to the receiver substrate.
[0139] Alternatively, a sacrificial layer can be used that changes
volume under some triggers such as temperature, light, or voltage
potential and so moving the micro device closer to the system
substrate.
[0140] Another aspect of the invention is a method of transferring
a micro device, the method comprising: positioning a donor
substrate comprising the micro device proximal to a receiver
substrate, wherein the receiver substrate comprises a force
modulator element creating transfer force for transferring the
selected micro devices; and reducing the effect of said force
generated by the force modulator element on unwanted micro
devices.
[0141] In one aspect of the invention, selectively reducing the
effect of a force generated by the force modulator element
comprises generating a reverse polarity of force surrounding the
force modulating element.
[0142] Another aspect of the invention is a method of transferring
micro devices, the method comprising: positioning a donor substrate
comprising micro devices proximal to a receiver substrate, wherein
the receiver substrate comprises a current curable bonding layer;
and transferring the micro devices by applying current to the
bonding layer of selected micro devices.
[0143] In one case, the current is applied using a circuit in the
receiver substrate.
[0144] In one case, the circuit in the receiver substrate is shared
with a circuit associated the driving or controlling the selected
micro devices.
[0145] In another case, the bonding layer comprises one or more
contact pads and selectively curing a portion of the bonding layer
comprises curing a portion of the bonding layer between one or more
contact pads.
[0146] In another aspect of the invention, a receiver substrate
used to receive a micro device from a donor substrate, the receiver
substrate comprising, an array of one or more pad structures,
wherein each pad structure comprises a conductive layer and a
dielectric layer
[0147] In one case, the dielectric layer can be modulated to be a
conductive layer.
[0148] In another case, the dielectric layer is modulated to the
conductive layer during operation to couple a circuit in the
receiver substrate to the micro device.
[0149] In another case, the dielectric layer is modulated to the
conductive layer using a doped layer.
[0150] In another alternative case, the dielectric layer is
modulated to the conductive layer using a laser to induce
dielectric breakdown.
[0151] In another alternative case, the dielectric layer creates
electrostatic force that attracts the micro device from the donor
substrate.
[0152] Another aspect of the invention is a method of transferring
a micro device, the method comprising: positioning a donor
substrate comprising a micro device proximal to a receiver
substrate, wherein the receiver substrate comprises a pad structure
having a dielectric layer and a conductive layer; transferring the
micro device from the donor substrate to the pad structure; and
coupling transferred micro devices and the conductive layer by
removing the dielectric layer.
[0153] In one case, the dielectric layer is removed by the means of
mechanical force.
[0154] In another case, the dielectric layer is removed by the
means of thermal force.
[0155] Another aspect of the invention is a method of using a
deformable layer on top of the receiver pads or landing area to
adjust for different height in the micro devices.
[0156] In one aspect, the deformable layer is an either conductive
layer, high resistive layer, or a dielectric layer.
[0157] The conformal layer can be used in combination with any of
the transfer methods.
[0158] Many micro devices, including light emitting diodes (LEDs),
Organic LEDs, sensors, solid state devices, integrated circuits,
MEMS (micro-electro-mechanical systems) and other electronic
components, are typically fabricated in batches, often on planar
substrates. To form an operational system, micro devices from at
least one donor substrate need to be selectively transferred to a
receiver substrate.
Substrate and transfer structure:
[0159] FIG. 1 shows a donor substrate 100 and receiver substrate
200, before the transfer process begins. Micro devices 102a, 102b,
102c begin in an array attached to donor substrate 100. The
receiver substrate consists of an array of landing areas 202a,
202b, 202c where the micro devices will sit. The landing areas
202a, 202b, 202c each include at least one force modulation element
204a, 204b, 204c and at least a contact pad 206a, 206b, 206c. The
force modulation element and contact pads can be different as shown
in FIG. 1A or can be the same structure as shown in FIG. 1B. The
micro devices 102 may be coupled or connected to a circuit or a
potential on the receiver substrate 200 through contact pads 206a,
206b, 206c. The force modulation elements 204a, 204b, 204c create a
transfer force to hold the micro device 102a, 102b, 102c
selectively on the receiver substrate 200 and separate them from
the donor substrate 100. The donor substrate 100 is the substrate
upon which micro devices 102 are manufactured or grown or another
temporary substrate onto which they have been transferred. Micro
devices 102 can be any micro device that is typically manufactured
in planar batches including LEDs, OLEDs, sensors, solid state
devices, integrated circuit, MEMS, and other electronic components.
Donor substrate 100 is chosen according to the manufacturing
process for a particular type of micro device 102. For example, in
the case of conventional GaN LEDs, donor substrate 100 is typically
sapphire. Generally, when growing GaN LEDs, the atomic distance of
donor substrate 100 should match that of the material being grown
in order to avoid defects in the film. Each micro device 102 is
attached to donor substrate 100 by a force, FD, determined by the
manufacturing process and the nature of the micro devices 102. FD
will be substantially the same for each micro device 102. Receiver
substrate 200 can be any more desirable location for micro devices
102. It can be, for example, a printed circuit board (PCB), a thin
film transistor backplane, an integrated circuit substrate, or, in
the case of optical micro devices 102 such as LEDs, a component of
a display, for example a driving circuitry backplane. The landing
area on the receiver substrate as shown in FIG. 1B refers to the
location where micro device sits on the receiver substrate and may
consist of at least one contact pad 101a and at least one force
modulation element 101b. Although in some of the figures the
landing area may be the same size as the contact pads 202, the
contact pads 202 can be smaller than the landing area. Contact pads
202 are the locations where micro devices may be coupled or
directly connected to the receiver substrate 200. In this
description, landing area and contact pads are used
interchangeably.
[0160] The goal in selective transfer is to transfer some, selected
micro devices 102, from donor substrate 100 to receiver substrate
200. For example, the transfer of micro devices 102a and 102b onto
contact pads 206a and 206b without transferring micro device 102c
will be described.
Transfer Process
[0161] The following steps describe a method of transferring
selected micro devices in an array of micro devices each of which
is bonded to a donor substrate with a donor force to contact pads
in an array on a receiver substrate:
[0162] a. aligning the donor substrate and the receiver substrate
so that each of the selected micro devices are in line with a
contact pad on the receiver substrate; (in case the contact pad
does not pre-exist on the receiver substrate, the alignment can be
also done using other marks; or device can be aligned to transfer
force modulation element).
[0163] b. moving the donor substrate and the receiver substrate
together until each of the selected micro devices are in contact
with or proximity with at least one contact pad on the receiver
substrate;
[0164] c. generating a receiver force that acts to hold the
selected micro devices to their contact pads;
[0165] d. moving the donor substrate and the receiver substrate
apart leaving the selected micro devices on the receiver substrate
while other non-selected micro devices from donor substrate stays
on donor substrate despite possible contact with or proximity
contact with the receiver substrate during steps b and c.
[0166] In some cases, the contact pad can be deposited after the
device is transferred to the receiver substrate. If the donor force
is too strong for receiver force to overcome for transferring the
micro device to the receiver substrate, the donor force for micro
devices is weakened to assist micro device transfer. In addition,
if the receiver force is applied globally or selective receiver
force is not enough to transfer the micro devices selectively, the
donor force for the selected micro devices is weakened selectively
to improve selectivity in micro device transfer.
[0167] FIGS. 2A-2C show exemplary flowcharts of selective transfer
methods 1000A-1000C. FIG. 1 shows a donor substrate 100 and a
receiver substrate 200 suitable for performing any of methods 1000.
Method 1000A will be described with reference to FIGS. 3A-3E.
Methods 1000B and 1000C are analogous variations of method 1000A.
One can use the combination of methods 1000A-1000C to further
enhance the transfer process.
[0168] At 1002A donor substrate 100 and receiver substrate 200 are
aligned so that selected micro devices 102a, 102b are in line with
corresponding contact pads 202a, 202b, as shown in FIG. 3A. Micro
device 102c is not to be transferred so, although shown as aligned,
it may or may not align with contact pad 202c.
[0169] At 1004A, donor substrate 100 and receiver substrate 200 are
moved together until the selected micro devices 102a, 102b are
positioned within a defined distance of contact pads 202a, 202b, as
shown in FIG. 3B. The defined distance may correspond to full or
partial contact but is not limited thereto. In other words, it may
not be strictly necessary that selected micro devices 102a, 102b
actually touch corresponding contact pads 202a, 202b, but must be
near enough so that the forces described below can be
manipulated.
[0170] At 1006A, forces between selected micro devices 102, donor
substrate 100 and receiver substrate 200 (and contact pads 202) are
modulated so as to create a net force towards receiver substrate
200 for selected micro devices and a net force towards donor
substrate 100 (or zero net force) for other micro devices 102c.
[0171] Consider the forces acting one of the selected micro devices
102. There is a pre-existing force holding it to donor substrate
100, FD. There is also a force generated between micro device 102
and receiver substrate 200, FR, acting to pull or hold micro device
102 towards receiver substrate 200 and cause a transfer. For any
given micro device 102, when the substrates are moved apart, if FR
exceeds FD the micro device 102 will go with receiver substrate
200, while if FD exceeds FR the micro device 102 will stay with
donor substrate 100. There are several ways to generate FR that
will be described in later sections. However, once FR has been
generated, there are at least four (4) possible ways to modulate FR
and FD to achieve transfer of selected micro devices.
[0172] 1. Weaken FD to be less than FR on micro devices selected
for transfer.
[0173] 2. Strengthen FR to be greater than FD on micro devices
selected for transfer.
[0174] 3. Weaken FR to be less than FD on micro devices NOT
selected for transfer.
[0175] 4. Strengthen FD to be greater than FR on micro devices NOT
selected for transfer.
[0176] Different combinations and arrangements of the above are
also possible. Using combinations may, in some cases, be desirable.
For example, if the required change in FD or FR is very high, one
can use a combination of modulation of FD and FR to achieve the
desired net forces for the selected and the non-selected micro
devices. Preferably, FR can be generated selectively and therefore
act only on selected micro devices 102a, 102b, as shown in FIG.
3C-1. FR can also be generated globally and apply across all of
receiver substrate 200 and therefore act on micro devices 102a,
102b, 102c, as shown in FIG. 3C-2 here donor forces may selectively
get weakened. The landing area on the receiver substrate may
include a force modulation element to cause FR force modulation,
fully or partially. Methods for selective and global generation of
FR will be described below, including adhesive, mechanical and
electrostatic and magnetic techniques. Additionally, examples of
force modulation elements in landing area are described below.
However, one of skill in the art knows that different variations of
the force modulation elements that are not listed here are
possible. Moreover, it should be understood that the shapes and
structures of the contact pads and the force modulation elements
are used for explanation and are not limited to the ones used in
this description.
[0177] In one embodiment, donor force FD is selectively weakened
for selected micro devices 102a, 102b, so that FD' is less than FR,
as shown in FIG. 3D. This may be done, for example, using laser
lift off techniques, lapping or wet/dry etching, or temperature. In
case of temperature, localized temperature can change the material
characteristics bonding the devices 102 to donor substrate 100 and
so reducing the donor force FD. In another case, the donor force FD
is weakened globally. This process can be done part of transferring
the devices 102 to a temporarily substrate or it can be done at the
original substrate. In both cases, the donor force FD can be weak
prior to transfer to the receiver substrate or some extra trigger
force or effects may be needed during the transfer to make the
donor force FD weak. The trigger effect can be higher temperature,
pressure, or chemical reaction. In some cases, it may be desirable
to use selective and global generation of FR simultaneously. For
example, it may be infeasible to generate a selective FR of
sufficient magnitude to overcome FD' alone. In that case, the
global component of FR should preferably remain small, ideally less
than FD', while the sum of the global and the selective components
of FR is greater than FD', but less than FD.
[0178] It should also be noted that activities performed during
steps 1002A-1006A can sometimes be interspersed with one another.
For example, selective or global weakening of FD could take place
before the substrates are brought together.
[0179] At 1008A, donor substrate 100 and receiver substrate 200 are
moved apart, leaving selected micro devices 102a, 102b attached to
corresponding contact pads 202a, 202b, as shown in FIG. 3E. Once
donor substrate 100 is separated from receiver substrate 200,
further processing steps can be taken place. For example, donor
substrate 100 and receiver substrate 200 can be re-aligned and
steps 1002A to 1008A can be repeated in order to transfer a
different set of micro devices 102 to a different set of contact
pads 202. Additional layers can also be deposited on top of or in
between micro devices 102, for example, during the manufacture of a
LED display, transparent electrode layers, fillers, planarization
layers and other optical layers can be deposited.
[0180] FIG. 2B shows method 1000B; an alternative embodiment of
method 1000A.
[0181] At 1002B, the force between micro devices 102a, 102b and
donor substrate 100 are modulated globally (for all devices in an
area of donor substrate) or selectively (for selected micro devices
102a, 102b only) so as to weaken donor force, FD.
[0182] At 1004B donor substrate 100 and receiver substrate 200 are
aligned so that selected micro devices 102a, 102b are in line with
corresponding contact pads 202a, 202b.
[0183] At 1006B, donor substrate 100 and receiver substrate 200 are
moved together until the selected micro devices 102a, 102b touch
contact pads 202a, 202b. It may not be strictly necessary that
selected micro devices 102a, 102b actually touch corresponding
contact pads 202a, 202b, but must be near enough so that the forces
described below can be manipulated.
[0184] At 1008B, if needed the forces between selected micro
devices 102 and receiver substrate 200 (and contact pads 202) are
modulated so as to create a net force towards receiver substrate
200 for selected micro devices and a net force towards donor
substrate 100 (or zero net force) for other micro devices 102c.
[0185] At 1010B, donor substrate 100 and receiver substrate 200 are
moved apart, leaving selected micro devices 102a, 102b attached to
corresponding contact pads 202a, 202b.
[0186] At 1012B, optional post processing is applied to selected
micro devices 102a, 102b. Once donor substrate 100 is separated
from receiver substrate 200, further processing steps can be taken.
Additional layers can be deposited on top of or in between micro
devices 102, for example, during the manufacture of a LED display,
transparent electrode layers, fillers, planarization layers and
other optical layers can be deposited. Step 1012B is optional and
may be applied at the conclusion of method 1000A or 1000C as
well.
[0187] FIG. 2C shows method 1000C; an alternative embodiment of
method 1000A.
[0188] At 1002C, contact pads 202a, 202b corresponding to selected
micro devices 102a, 102b are treated to create extra force upon
contact. For example, an adhesive layer may be applied, as
described in greater detail below.
[0189] At 1004C donor substrate 100 and receiver substrate 200 are
aligned so that selected micro devices 102a, 102b are in line with
corresponding contact pads 202a, 202b.
[0190] At 1006C, donor substrate 100 and receiver substrate 200 are
moved together until the selected micro devices 102a, 102b touch
contact pads 202a, 202b.
[0191] At 1008C, if needed the forces between selected micro
devices 102 and donor substrate 100 are modulated so as to create a
net force towards receiver substrate 200 for selected micro devices
and a net force towards donor substrate 100 (or zero net force) for
other micro devices 102c.
[0192] At 1010B, donor substrate 100 and receiver substrate 200 are
moved apart, leaving selected micro devices 102a, 102b attached to
corresponding contact pads 202a, 202b.
Multiple Applications
[0193] Any of the methods 1000A, 1000B, 1000C can be applied
multiple times to the same receiver substrate 200, using different
or the same donor substrates 100 or the same donor substrate 100
using different receiver substrates 200. For example, consider the
case of assembling a display from LEDs. Each pixel may comprise
red, green and blue LEDs in a cluster. However, manufacturing LEDs
is more easily done in batches of a single colour and on substrates
that are not always suitable for incorporation into a display.
Accordingly, the LEDs must be removed from the donor 100 substrate,
possibly where they are grown, and placed on a receiver substrate,
which may be the backplane of a display, in RGB clusters. In case,
the color This is simplest when the pitch of the array of pixels
can be set to match the pitch of the array of LEDs on the donor
substrate.
[0194] When this is not possible, the pitches of each array can be
set proportionally. FIGS. 4A and 4B show arrangements where the
pitch of the LEDs on the donor substrate is one seventh the pitch
of the contact pads on the receiver substrate.
[0195] In general, however, matching the pitch of an array of
pixels to the donor substrate is likely to be infeasible. For
example, one generally tries to manufacture LEDs with the smallest
possible pitch on the donor substrate to maximize yield, but the
pitch of the pixels and the array of contact pads on the receiver
substrate is designed based on desired product specifications such
as size and resolution of a display. In this case, one may not be
able to transfer all the LEDs in one step and repetition of any of
the methods 1000A, 1000B, 1000C will be necessary. Accordingly, it
may be possible to design the donor substrate and the receiver
substrate contact pad array so that a portion of each pixel can be
populated during each repetition of any of methods 1000A, 1000B,
1000C as shown in FIG. 4C. At I, receiver substrate and donor
substrate are not aligned. At II, all red LEDs are transferred. At
III, all green LEDs are transferred. At IV, all blue LEDs are
transferred. Repositioning of donor substrate and receiver
substrate is required between each transfer step.
[0196] Those of skill in the art will now understand that that
additional variations and combinations of methods 1000A, 1000B and
1000C are also possible. Specific techniques and considerations are
described below that will apply to any of methods 1000, alone or in
combination.
Use of Heat for Force Modulation
[0197] Selective and global heating can be used in multiple ways to
assist in method 1000A. For example, heat can be used in step 1008A
to weaken FD or after step 1008A to create a permanent bond between
micro devices 102 and contact pads 202. In one embodiment, heat can
be generated using resistive elements incorporated into donor
substrate 100 and/or receiver substrate 200.
[0198] FIG. 5A shows selective and global heating elements
incorporated into substrates. Selective heating elements 300 and
global heating element 302 may be incorporated into donor substrate
100 while selective heating elements 304 and global heating element
306 may be incorporated into receiver substrate 200. In another
embodiment, selective heating can be achieved using a patterned
global heater, shown in FIG. 5B.
[0199] FD can be weakened by applying heat to the interface between
a micro device 102 and donor substrate 100. Preferably, selective
heating elements 300 are sufficient to heat the interface past a
threshold temperature where micro devices 102 will detach. However,
when this is not feasible, global heater 302 can be used to raise
the temperature to a point below the threshold while selective
heaters 300 raise the temperature further, only for selected micro
devices 102a, 102b above the threshold. An environmental heat
source, e.g. a hot room, can substitute for the global heater.
[0200] Heat can also be used to create a permanent bond between
micro devices 102 and contact pads 202. In this case, contact pads
202 should be constructed of a material that will cure when heated,
creating a permanent bond. Preferably, selective heating elements
304 are sufficient to heat contact pads 202 past a threshold
temperature to cause curing. However, when this is not feasible,
global heater 306 can be used to raise the temperature to a point
below the threshold for curing while selective heaters 304 raise
the temperature for selected contact pads 202a, 202b above the
threshold. An environmental heat source, e.g. a hot room, can
substitute for the global heater. Pressure may also be applied to
aid in permanent bonding.
[0201] Other variations are possible. In some cases, it may be
feasible for micro devices 102 or contact pads 202 to themselves
act as the resistive elements in selective heaters 300, 304. Heat
can also be applied in a selective manner using lasers. In the case
of lasers, it is likely that at least one of the donor substrate
100 and the receiver substrate 200 will have to be constructed of
material that is at least semi-transparent to the laser being used.
As shown in FIG. 5C, in one case, shadow mask can be used to
selectively block the laser from the non-selected devices. Here,
the shadow mask 501 is aligned with the receiver substrate or donor
substrate depending on direction of laser. Then laser can cover the
either substrate partially or fully. In case of partial coverage,
raster scan or step-and-repeat may be used to cover the entire
intended area on the substrate. To further improve the heat
transfer from the laser, a layer with higher laser absorption rate
can be added to the force modulation element. It is possible to use
the contact pad as the force modulation element in the receiver
substrate.
Adhesive Force Modulation
[0202] In another embodiment of selective transfer, FR is generated
by adhesive. Here, the FR is modulated either by selective
application of adhesive to the landing area on the receiver
substrate (or selected micro devices) or by selective curing of an
adhesive layer. This method can be used in combination with
weakening the donor force selectively or globally and is compatible
with any of the methods 1000A, 1000B, and 1000C or any combination
of them. Although, the following description is based on 1000A
similar approaches can be used for 1000B, 1000C and the combination
of the methods. In addition, the order of donor force weakening
step 1110 can be changed in reference to other steps without
affecting the results.
[0203] FIG. 6A shows a flowchart of method 1100, a modified version
of method 1000 specific to the use of adhesive to generate FR. FIG.
6B shows donor substrate 100 and receiver substrate 200 setup to
perform method 1100. Donor substrate 100 is shown in cross section
and receiver substrate 200 is shown in cross section and plan view.
Donor substrate 100 has an array of micro devices 102 attached.
Donor force FD acts to hold micro devices 102 to donor substrate
100.
[0204] Receiver substrate 200 has an array of contact pads 212
attached. Although FIG. 6B shows the force modulation element 500
connected to the contact pads 212, they can be physically
separated.
[0205] As shown in FIG. 6B, contact pads 212a, 212b are surrounded
by a ring of adhesive 500. Adhesive 500 has been applied
selectively to contact pads 212 where transfer of a micro device is
desired so that when donor substrate 100 and receiver substrate 200
are moved together, micro devices 102a, 102b will make contact with
adhesive 500 as well as contact pads 212a, 212b.
[0206] Method 1100 will be explained with reference to FIGS. 6B-6F.
At 1102, adhesive is selectively applied as shown in FIG. 6B.
[0207] At 1104 donor substrate 100 and receiver substrate 200 are
aligned so that selected micro devices 102a, 102b are in line with
corresponding selected contact pads 212a, 212b, as shown in FIG.
6C.
[0208] At 1106, donor substrate 100 and receiver substrate 200 are
moved together until selected micro devices 102a, 102b are in
contact with corresponding selected contact pads 212a, 212b and
adhesive 500, as shown in FIG. 6D.
[0209] At 1108, receiver force, FR, is generated, as shown in FIG.
6E. FR is generated by adhesion between micro devices 102a, 102b,
adhesive 500 and at least one of contact pads 212a, 212b and
receiver substrate 200. FR acts to hold selected micro devices 102
to corresponding selected contact pads 212. Preferably, FR can be
generated selectively by applying adhesive 500 selectively, as
shown.
[0210] At 1110, donor force FD is selectively (or globally)
weakened for selected micro devices 102a, 102b, so that FD' is less
than FR, as shown in FIG. 6F. The may be done, for example, using
laser lift off techniques, lapping or wet/dry etching. In another
case, donor force FD can be weakened for all the micro devices. In
this case, force modulation is done by selective adhesive
application to the selected force element on the receiver
substrate. The order of FD and FR modulation can be changed. This
step may be eliminated if the adhesive force modulation is
selective and FR is larger than FD.
[0211] At 1112, donor substrate 100 and receiver substrate 200 are
moved apart, leaving selected micro devices 102a, 102b attached to
corresponding selected contact pads 212a, 212b, as shown in FIG.
6G. Once donor substrate 100 is separated from receiver substrate
200, further processing steps can be taken. For example, donor
substrate 100 and receiver substrate 200 can be re-aligned and
steps can be repeated in order to transfer a different set of micro
devices 102 to contact pads 212. Additional layers can also be
deposited on top of or in between micro devices 102, for example,
during the manufacture of a LED display, a transparent electrode
layers, fillers, planarization layers and other optical layers can
be deposited.
[0212] One possible additional step, at 1114, is curing adhesive
500. Curing may create a permanent bond between micro devices 102
and contact pads 212. In another embodiment, curing takes place as
part of step 1108 and is part of generating FR. If several sets of
selected micro devices 102 are to be transferred to a common
receiver substrate 200 curing may be done after all the transfers
are complete or after each set is transferred.
[0213] Adhesive 500 can be applied in many ways. For example,
adhesive 500 can be applied to any or all of micro devices 102,
contact pads 212 or receiver substrate 200. It will often be
desirable that an electrical coupling exist between a micro device
102 and its corresponding contact pad 202. In this case, the
adhesive may be selected for its conductivity. However, suitable
conductive adhesives are not always available. In any case, but
especially when a conductive adhesive is not available, adhesives
can be applied near contact pads or may cover only a portion of the
contact pad. FIG. 7A shows some other possible arrangements of
adhesive on receiver substrate 200, (I) including four corners,
(II) opposite sides, (III) center and (IV) one side geometries.
[0214] In another embodiment, one or more cut-outs can be provided
for the adhesive 500. FIG. 7B shows a contact pad 212 with a cut
out (I) before and (II) after application of an adhesive.
[0215] The adhesive 500 can be stamped, printed or patterned onto
the contact pads 212, micro devices 102 or receiver substrate 200
by any normal lithography techniques. For example, FIG. 8 shows a
stamping process that can be used to apply adhesive 500 to, for
example, contact pads 212. Selectivity in generating FR can be
achieved by selecting which contact pads 212 will receive adhesive
500. An analogous procedure can be used to apply adhesive to micro
devices 102 or receiver substrate 200. At (I), a stamp with a
profile matching the desired distribution of adhesive 500 is wet.
At (II), the stamp is brought into contact with the receiver
substrate 200 and selected micro devices 102. At (III), receiver
substrate is now wet with adhesive and ready to receive transfer of
selected micro devices 102. Depending on the needs of the process,
stamps with reverse profiles can also be used. In another
embodiment, both the micro devices 102 and contact pads 212 may be
wet with adhesive.
[0216] Adhesive 500 may be selected so that it will cure when heat
is applied. Any of the techniques described with regard to heating
can be suitably applied by one of skill in the art, according to
the needs of a specific application.
Mechanical Force Modulation
[0217] In another embodiment of selective transfer, FR is generated
by mechanical force. Here, the FR is modulated by application of
mechanical forces between the landing area on the receiver
substrate and the micro device. This method can be used in
combination with weakening the donor force selectively or globally
and is compatible with any of the methods 1000A, 1000B, and 1000C
or any combination of them. Although, the following description is
based on 1000A similar approaches can be used for 1000B, 1000C and
the combination of the methods. In addition, the order of donor
force weakening step 1210 can be changed in reference to other
steps without affecting the results.
[0218] In one example, differential thermal expansion or pressure
force can be used to achieve a friction fit that will hold micro
devices 102 to contact pads 202. In another example, thermal and
pressure can be applied to create a bonding and this bonding can
act as FR as well.
[0219] FIG. 9 shows a flowchart of method 1200, a modified version
of method 1000A suitable for mechanical generation of FR. FIG. 10
shows a donor substrate 100 and a receiver substrate 200 setup to
perform method 1200. Donor substrate 100 is shown in cross section
and receiver substrate 200 is shown in cross section and plain
view. Donor substrate 100 has an array of micro devices 102
attached. Donor force FD acts to hold micro devices 102 to donor
substrate 100. Micro devices 102 and donor substrate 100 are shown
as connected to ground 244.
[0220] Receiver substrate 200 has an array of contact pads 232
attached. In the embodiment shown, the array of contact pads 232 is
of the same pitch as the array of micro devices 102; i.e. there is
one micro device 102 for each contact pad 232. As discussed above,
this need not be true, although it is preferable that the pitch of
the array of contact pads 232 and the pitch of the array of micro
devices 102 be proportional as this facilitates the transfer of
multiple devices simultaneously.
[0221] Method 1200 will be described with reference to FIGS.
11A-11E. At 1202 the substrates are prepared for mechanical force
modulation. In case of a mechanical grip, the grip is opened by
different means. In one example heat is applied to force modulation
element 222 which can be the same a contact pad on the landing
area. Here, mechanical grip and contact pads are used
interchangeably. However, it is obvious to one of skill in the art
that the mechanical grip and contact pad can be different. It is
possible to integrate the mechanical grip in the micro devices as
well. The heat can be applied globally or selectively using heaters
304 causing the grip to open, as shown by the double arrows in FIG.
11A. Note that contact pads 222 are constructed with a central
depression 224 and peripheral walls 226. It should also be noted
that a combination of selective heaters 304 and global heater 306
or a combination of selective heaters 304 and an environmental heat
source or external heat source in combination or alone could also
be used.
[0222] At 1204, donor substrate 100 and receiver substrate are
aligned so that selected micro devices 102a, 102b are in line with
corresponding contact pads 222a, 222b, as shown in FIG. 11B.
[0223] At 1206, donor substrate 100 and receiver substrate 200 are
moved together until the selected micro devices 102a, 102b fit into
the space defined by the peripheral walls of corresponding
mechanical grip as shown in FIG. 11B. As noted above, each contact
pad 222 is constructed with a central depression 224 and peripheral
walls 226. These features of contact pads 222 are sized so as to
fit snugly around a micro device 102. The material of the
mechanical grips is chosen, in part, due to thermal properties;
specifically, so that the mechanical grips have a higher
coefficient of thermal expansion than micro devices 102.
Accordingly, when heat is applied to the mechanical grips they
expand more than a micro device 102 would expand at the same
temperature so that the central depression and peripheral walls
will be able to accommodate a micro device 102 with a gap 228. The
expanded size of mechanical grip allows micro devices 102 to fit
easily.
[0224] At 1208, a receiver force, FR, is generated. FR is generated
by selectively cooling contact pads 222 corresponding to selected
micro devices 102, causing peripheral walls 226 to contract around
selected micro devices 102, closing gap 228 and exerting a
compressive force on micro device 102, holding it in place, as
shown in FIG. 11C. Selectivity can be achieved by selectively
turning off selective heaters 304.
[0225] At 1210, donor force FD is selectively (or globally)
weakened for selected micro devices 102a, 102b, so that FD' is less
than FR, as shown in FIG. 11D. This may be done, for example, using
laser lift off techniques, lapping or wet/dry etching. In some
embodiments FD is weaker than FR, in which case selective weakening
of FD is not required. This step may be eliminated if the
mechanical force modulation is selective and the FR is larger than
FD.
[0226] At 1212, donor substrate 100 and receiver substrate 200 are
moved apart, leaving selected micro devices 102a, 102b attached to
corresponding contact pads 222a, 222b, as shown in FIG. 11E. Once
donor substrate 100 is separated from receiver substrate 200,
further processing steps can be taken. For example, donor substrate
100 and receiver substrate 200 can be re-aligned and steps can be
repeated in order to transfer a different set of micro devices 102
and to contact pads 222. Additional layers can also be deposited on
top of or in between micro devices 102, for example, during the
manufacture of a LED display, transparent electrode layers,
fillers, planarization layers and other optical layers can be
deposited.
Electrostatic Force Modulation
[0227] In another embodiment of selective transfer, FR is generated
by an electrostatic force or magnetic force. Although the
structures here are used to describe the electrostatic force
similar structures can be used for magnetic force. In embodiments
where magnetic force is used instead of electrostatic force, a
current pass through a conductive layer instead of charging a
conductive layer for electrostatic force.
[0228] Using electrostatic force for selective transfer, the FR is
modulated by application of selective electrostatic forces between
the landing area on the receiver substrate and the micro device.
This method can be used in combination with weakening the donor
force selectively or globally and is compatible with any of the
methods 1000A, 1000B, and 1000C or any combination of them.
Although, the following description is based on 1000A similar
approaches can be used for 1000B, 1000C and the combination of the
methods. In addition, the order of donor force weakening step 1410
can be changed in reference to other steps without affecting the
results.
[0229] FIG. 12A shows a flowchart of method 1300, a modified
version of method 1000 suitable for electrostatic generation of FR.
FIG. 12B shows a donor substrate 100 and a receiver substrate 200
setup to perform method 1300. Donor substrate 100 is shown in cross
section and receiver substrate 200 is shown in cross section and in
plain view. Donor substrate 100 has an array of micro devices 102
attached. Donor force FD acts to hold micro devices 102 to donor
substrate 100. Micro devices 102 and donor substrate 100 are shown
as connected to ground 244.
[0230] The landing area on the receiver substrate 200 has at least
a contact pad 232 attached and a force modulation element 234.
[0231] Contact pads 232 are surrounded by a ring of
conductor/dielectric bi-layer composite, hereinafter called an
electrostatic layer 234. The shape and location of force modulation
element 234 can be changed in the landing area and in relation to
the contact pad. Electrostatic layer 234 has a dielectric portion
236 and a conductive portion 238. Dielectric portion 236 comprises
a material selected, in part, for its dielectric properties,
including dielectric constant, dielectric leakage and breakdown
voltage. The dielectric portion can also be part of the micro
device or a combination of the receiver substrate and the micro
device. Suitable materials may include SiN, SiON, SiO, HfO and
various polymers. Conductive portion 238 is selected, in part, for
its conductive properties. There are many suitable single metals,
bi-layers and tri-layers that can be suitable for use as a
conductive portion 238 including Ag, Au and Ti/Au. Each conductive
portion 238 is coupled to a voltage source 240, via a switch 242.
Note that although conductive portions 238 are shown as connected
in parallel to a single voltage source 240 via simple switches 242,
this is to be understood as an illustrative example. Conductive
portions 238 might be connected to one voltage source 240 in
parallel. Different subsets of conductive portions 238 may be
connected to different voltage sources. Simple switches 242 can be
replaced with more complex arrangements. The desired functionality
is the ability to selectively connect a voltage source 240, having
a potential different than that of the micro devices 102, to
selected conductive portions 238 when needed to cause an
electrostatic attraction between the selected conductive portions
238 and corresponding selected micro devices 102.
[0232] Method 1300 will be explained in conjunction with FIGS.
13A-13E. At 1302, donor substrate 100 and receiver substrate are
aligned so that selected micro devices 102a, 102b are in line with
corresponding contact pads 232a, 232b, as shown in FIG. 13A.
[0233] At 1304, donor substrate 100 and receiver substrate 200 are
moved together until the micro devices 102 come into contact with
contact pads 232, as shown is FIG. 13B.
[0234] At 1306, a receiver force, FR, is generated, as shown in
FIG. 13C. FR is generated by closing switches 242a, 242b that
connect conductive portions 238 of electrostatic layers 234 to
voltage source 240 creating charged conductive portions 238 at the
potential of voltage source 240. Selected micro devices 102a, 102b,
being at a different potential, e.g. ground potential (or other
relative potential), will be electrostatically attracted to
conductive portions 238. The electrostatic charge can be generated
by different potential levels. For example, for a 300 nm
dielectric, to get a proper grip on a micro device, a voltage
difference between 20V to 50V may need to be applied to the
electrostatic force element. However, this voltage can be modified
depending on the device, gap size, and the dielectric constant.
[0235] At 1308, donor force FD is selectively weakened for selected
micro devices 102a, 102b, so that FD' is less than FR, as shown in
FIG. 13D. This may be done, for example, using laser lift off
techniques, lapping or wet/dry etching.
[0236] At 1310, donor substrate 100 and receiver substrate 200 are
moved apart, leaving selected micro devices 102a, 102b attached to
corresponding contact pads 232a, 232b, as shown in FIG. 13E. Once
donor substrate 100 is separated from receiver substrate 200,
further processing steps can be taken and the ground 244 may be
removed. For example, donor substrate 100 and receiver substrate
200 can be re-aligned and steps can be repeated in order to
transfer a different set of micro devices 102 and to contact pads
232. Additional layers can also be deposited on top of or in
between micro devices 102, for example, during the manufacture of a
LED display, transparent electrode layers, fillers, planarization
layers and other optical layers can be deposited. It should be
noted that FR will cease to operate if the connection to voltage
source 240 is removed. Accordingly, further processing steps to
create a permanent bond between micro devices 102 and contact pads
232 are desirable. Curing contact pads 232, as described above, is
a suitable further processing step that will create such a bond and
enable further working or transporting receiver substrate 200.
[0237] In other embodiments, electrostatic layer 234 can take on
other configurations. FIG. 14 shows some alternative placements for
electrostatic layer 234. Possible alternative placements of
electrostatic layer 234 relative to each contact pad 232 include:
(A) four corners, (B) opposite sides, (C) center and (D) one side.
Those of skill in the art will now be able to design a
configuration suitable to particular applications.
[0238] In other embodiments, the geometry of contact pads 232,
electrostatic layer 234 and micro devices 102 can be changed to
varying effect. FIG. 15 illustrates some possible alternative
geometries. FIG. 15A shows an embodiment where electrostatic layer
234 extends above the top of contact pad 232 to form a hollow 240
and micro device 102 has a mesa 242 that will fit within hollow
240. FIG. 15B shows an embodiment where electrostatic layer 234
extends above the top of contact pad 232 to form a hollow 240 and
micro device 102 has an extension 244 attached to it that will fit
within hollow 240. Extension 240 may be made of the same material
as contact pad 232 so that later curing will fuse extension 244 and
contact pad 232. Sloping geometries, as shown in FIG. 15E, are also
possible. Geometries with mesa 242 or extension 244 can help guide
micro devices 102 into contact pads 232 and insure a proper fit and
prevent tilting of micro devices 102 when detaching from donor
substrate 100. Preferably, the geometry of micro devices 102 and
contact pads 232 are chosen to match so as to maximize the
electrostatic force.
[0239] FIG. 15C shows an embodiment where electrostatic layer 234
forms a hollow 240, but conductive portion 238 remains in the same
plane as contact pad 232. FIG. 15D shows an embodiment where
electrostatic layer 234 forms a hollow 240, but also overlaps with
contact pad 232 and conductive portion 238 is in a different plane
than contact pad 232, allowing the fine tuning of the electrostatic
force.
Transfer of Micro Devices of Different Heights
[0240] In another embodiment of selective transfer, the force on
the donor substrate is modulated to push the device toward the
receiver substrate. In one example, after removing the donor force
other forces such as electrostatic forces can be used to push the
device toward the receiver substrate. In another case, a
sacrificial layer can be used to create a push force in presence of
heat or light sources. To selectively create the push force, a
shadow mask can be used for applying a light source (e.g. laser) to
the selected micro devices. In addition, the FR can be generated by
one of aforementioned methods (e.g. mechanical, heating, adhesive,
electrostatic). For example, the FR can be modulated by application
of selective electrostatic forces between landing area on the
receiver substrate and the micro device. This method is compatible
with any of the methods 1000A, 1000B, and 1000C or any combination
of them. Although, the following description is based on 1000A,
similar approaches can be used for 1000B, 1000C and the combination
of the methods. In addition, the order of donor force modulation
step 1410 can be changed in reference to other steps without
affecting the results. However, the most reliable results can be
achieved by applying the FR first and then applying the push force
to the micro device.
[0241] FIG. 16 shows a flowchart of method 1400 based on
electrostatic FR. However, other FR forces can be applied as well.
Method 1400 is a modified version of method 1300 and is
particularly suited to simultaneous transfer of micro devices 102
of different heights. At 1402, donor substrate 100 and receiver
substrate are aligned so that selected micro devices 102a, 102b are
in line with corresponding contact pads 232a, 232b, as shown in
FIG. 17A. Note that micro device 102a is of a different height than
micro device 102b.
[0242] At 1404, donor substrate 100 and receiver substrate 200 are
moved together until the micro devices 102 are close enough for
electrostatic FR to act on micro devices 102. Donor substrate 100
and receiver substrate 200 may be held so that no micro devices 102
make contact with contact pads 232 or, as shown in FIG. 17B,
substrates 100, 200 may stop approaching when some micro devices
102 make contact with contact pads 232.
[0243] At 1406, a receiver force, FR, is generated, as shown in
FIG. 17C. FR is generated by closing switches 242a, 242b that
connect conductive portions 238 of electrostatic layers 234 to
voltage source 240 creating charged conductive portions 238 at the
potential of voltage source 240. Selected micro devices 102a, 102b,
being at a different potential, e.g. ground potential, will be
electrostatically attracted to conductive portions 238.
[0244] At 1408, donor force FD is selectively weakened for selected
micro devices 102a, 102b, so that FD' is less than FR. This may be
done, for example, using laser lift off techniques, lapping or
wet/dry etching. At this point, micro devices 102a, 102b will
detach from donor substrate 100. Micro device 102b will jump the
gap to their corresponding contact pads 232a, 232b on receiver
substrate 200.
[0245] At 1410, donor substrate 100 and receiver substrate 200 are
moved apart, leaving selected micro devices 102a, 102b attached to
corresponding contact pads 232a, 232b, as shown in FIG. 17E. Once
donor substrate 100 is separated from receiver substrate 200,
further processing steps can be taken. For example, donor substrate
100 and receiver substrate 200 can be re-aligned and steps can be
repeated in order to transfer a different set of micro devices 102
and to contact pads 232. Additional layers can also be deposited on
top of or in between micro devices 102, for example, during the
manufacture of a LED display, transparent electrode layers,
fillers, planarization layers and other optical layers can be
deposited. It should be noted that FR will cease to operate if the
connection to voltage source 240 is removed. Accordingly, further
processing steps to create a permanent bond between micro devices
102 and contact pads 232 are desirable. Curing contact pads 232, as
described above, is a suitable further processing step that will
create such a bond and enable further working or transporting
receiver substrate 200.
[0246] One application of this method is development of displays
based on micro-LED devices. An LED display consists of RGB (or
other pixel patterning) pixels made of individual color LEDs (such
as red, green or blue or any other color). The LEDs are
manufactured separately and then transferred to a backplane. The
backplane circuit actively or passively drives these LEDs. In the
Active form each sub-pixel is driven by a transistor circuit by
either controlling the current, the ON time, or both. In the
Passive form, each sub-pixel can be addressed by selecting the
respective row and column and is driven by an external driving
force.
[0247] The LEDs conventionally are manufactured in the form of
single color LEDs on a wafer and patterned to individual micro
devices by different processes such as etching. As the pitch of the
LEDs on their substrate is different from their pitch on a display,
a method is required to selectively transfer them from their
substrate to the backplane. The LEDs' pitch on their substrate is
the minimum possible to increase the LED manufacturing yield on a
wafer, while the LED pitch on the backplane is dictated by the
display size and resolution. According to methods implemented here,
one can modulate the force between the LED substrate and the
micro-LEDs and uses any of the technique presented here to increase
the force between selected LED and backplane substrate. In one
case, the force for LED wafer is modulated first. In this case, the
force between LED devices and substrate is reduced either by laser,
backplane etching, or other methods. The process can selectively
weaken the connection force between selected LEDs for transfer and
the LED substrate or it can be applied to all the devices to reduce
the connection force of all the LED devices to the LED substrate.
In one embodiment, this is accomplished by transferring all LEDs
from their native substrate to a temporary substrate. Here, the
temporary substrate is attached to the LEDs from the top side, and
then the first substrate is removed either by polishing and/or
etching or laser lift off. The force between the temporary
substrate and the LED devices is weaker than the force that the
receiver substrate can selectively apply to the LEDs. To achieve
that a buffer layer may be deposited on the temporary substrate
first. This buffer layer can be a polyamide layer. If the buffer
layer is not conductive, to enable testing the devices after
transfer to the temporary and receiver substrate, an electrode
before or after the buffer layer will be deposited and patterned.
If the electrode is deposited before the buffer layer, the buffer
layer maybe patterned to create an opening for contact.
[0248] In another method, the LED connection-force modulation
happens after the LED substrate and the backplane substrate are in
contact and the receiver substrate forces to LED are selectively
modulated by the aforementioned methods presented here. The LED
substrate force modulation can be done prior to the backplane
substrate force modulation as well.
[0249] As the force holding the LEDs to the backplane substrate
after transfer is temporary in most of the aforementioned methods,
a post processing step may be needed to increase the connection
reliability to the backplane substrate. In one embodiment, high
temperature (and/or pressure can be used). Here, a flat surface is
used to apply pressure to the LEDs while the temperature is
increased. The pressure increases gradually to avoid cracking or
dislocation of the LED devices. In addition, the selective force of
the backplane substrate can stay active during this process to
assist the bonding.
[0250] In one case, the two connections required for the LED are on
the transfer side and the LED is in full contact with the backplane
after the transfer process. In another case, a top electrode will
be deposited and patterned if needed. In one case, a polarization
layer can be used before depositing the electrode. For example, a
layer of polyamide can be coated on the backplane substrate. After
the deposition, the layer can be patterned to create an opening for
connecting the top electrode layer to receiver substrate contacts.
The contacts can be separated for each LED or shared. In addition,
optical enhancement layers can be deposited as well before or after
top electrode deposition.
Testing Process
[0251] Identifying defective micro devices and also characterizing
the micro devices after being transferred is an essential part of
developing a high yield system since it can enable the use of
repair and compensation techniques.
[0252] In one embodiment shown in FIG. 18, the receiver substrate
is put in test mode during a transfer process. If needed, the donor
substrate may be biased for test mode. If the micro device is an
optoelectronic device, a sensor 1810 (or sensor array) is used to
extract the optical characteristics of the transferred devices.
Here, the receiver substrate is biased so that only the selected
device 1802 is activated through selected contact pads 1804. Also,
unselected devices 1806 stay deactivated and unselected pads 1808
stay inactive to prevent any interference. For connectivity
testing, the micro device is biased to be active (for the LED case,
it emits light). If a micro device is not active, the device can be
flagged as defective. In another test, the micro device is biased
to be inactive (for the LED case, it does not emit light). If a
micro device is active, the device can be flagged as defective.
FIG. 19 shows an example of a pixel biasing condition for
activating or deactivating a micro device. Here, the micro device
1906 is coupled 1908 to a bias voltage 1910 (supply voltage) to
become activated. For deactivating the micro device 1906, it is
disconnected from the voltages. Here, the donor substrate 1900 can
be biased for enabling the test. In another case, the micro devices
are tested during post processing. While a surface is used to apply
pressure to the devices to create permanent bonding, the circuit is
biased to activate the micro devices. The surface can be conductive
so that it can act as another electrode of the micro devices (if
needed). The pressure can be adjusted if a device is not active to
improve any malfunction in the connection to the receiver
substrate. Similar testing can be performed to test for open
defective devices. For performance testing, the micro device is
biased with different levels and its performance (for the LED case,
its output light and color point) is measured.
[0253] In one case, the defective devices are replaced or fixed
before applying any post processing to permanently bond the device
into receiver substrate. Here, the defective devices can be removed
before replacing it with a working device. In another embodiment,
the landing area on the receiver substrate corresponding to the
micro devices comprises at least a contact pad and at least a force
modulation element.
[0254] It should be understood that various embodiments in
accordance with and as variations of the above are
contemplated.
[0255] In another embodiment, the net transfer forces are modulated
by weakening the donor force using laser lift off In another
embodiment, the net transfer forces are modulated by weakening the
donor force using selectively heating the area of the donor
substrate near each of the selected micro devices. In another
embodiment, the net transfer forces are modulated by selectively
applying adhesive layer to the micro devices. In another
embodiment, a molding device is used to apply the adhesive layer
selectively. In another embodiment, printing is used to apply the
adhesive layer selectively. In another embodiment, a post process
is performed on the receiver substrate so that the contact pads
permanently bond with the selected micro devices. In another
embodiment, the post process comprises heating the receiver
substrate. In another embodiment, the heating is done by passing a
current through the contact pads. In another embodiment, the method
is repeated using at least one additional set of selected micro
devices and corresponding contact pads. In another embodiment, the
contact pads are located inside an indentation in the receiver
substrate and each selected micro device fits into one such
indentation. In another embodiment, the pitch of the array of micro
devices is the same as the pitch of the array of contact pads. In
another embodiment, the pitch of the array of micro devices is
proportional to the pitch of the array of contact pads. In another
embodiment, each of the selected micro devices comprises a
protrusion and the contact pads comprise a depression sized to
match the protrusion on each micro device. In another embodiment,
the net transfer forces are modulated by generating electrostatic
attraction between the selected micro devices and the receiver
substrate. In another embodiment, the electrostatic forces are
applied to the entire array of micro devices on the donor substrate
by a force element on the receiver substrate or behind the receiver
substrate. In another embodiment, the electrostatic forces are
generated selectively by the force modulation element of the
landing area. In another embodiment, the force modulation element
of the landing area on the receiver substrate comprises a
conductive element near each contact pad, each conductive element
capable of being linked to a voltage source in order to sustain an
electrostatic charge. In another embodiment, each conductive
element comprises one or more sub-elements. In another embodiment,
the sub-elements are distributed around the contact pad. In another
embodiment, each conductive element surrounds a contact pad. In
another embodiment, the force modulation element of the landing
area on the receiver substrate comprises a conductive layer and a
dielectric layer throughout a substantial portion of the landing
area, the conductive layer capable of being linked to a voltage
source in order to sustain an electrostatic charge. In another
embodiment, the donor substrate and the receiver substrate are
brought close together, but the selected micro devices and the
contact pads do not touch until after the net transfer forces are
modulated whereupon the selected micro devices move across the
small gap to the contact pads. In another embodiment, the height of
the selected micro devices differs. In another embodiment, the
contact pads are concave. In another embodiment, the force
modulation element of the receiver substrate generates a mechanical
clamping force. In another embodiment, the mechanical force
modulation element forms part of at least one contact pad. In
another embodiment, the mechanical force modulation element is
separate from the contact pad. In another embodiment, the
mechanical force modulation is created by thermal expansion or
compression of at least one of the force modulation element or
micro device. In another embodiment, each contact pad has a concave
portion and each selected micro device is inserted into a concave
portion of a contact pad.
[0256] In another embodiment, the receiver substrate is heated
before the donor substrate and the receiver substrate are moved
together so that the concave portion of the contact pads expands to
be larger than a selected micro device and the receiver substrate
is cooled before the donor substrate and the receiver substrate are
moved apart so that the concave portion of the contact pads
contracts around the selected micro devices and provides the
receiver force via mechanical clamping of the selected micro
devices.
[0257] In another embodiment, the force modulation element in the
landing area of the receiver substrate is an adhesive layer
positioned between the selected micro devices and the receiver
substrate. In another embodiment, the adhesive layer is conductive.
In another embodiment, a portion of each of the contact pads on the
receiver substrate is coated with an adhesive layer. In another
embodiment, a portion of each of the selected micro devices is
coated with an adhesive layer. In another embodiment, a portion of
the area near the contact pads is coated with an adhesive
layer.
[0258] In another embodiment, the net transfer force is modulated
both on the donor substrate with at least one of the aforementioned
methods and on the receiver substrate with at least one of the
described methods.
[0259] In one embodiment, the force on the donor substrate is
modulated by selectively lifting off the micro devices. In one
case, a shadow mask is used to block the laser from the unwanted
devices. In one case as shown in FIG. 20(a), the shadow mask 2002
is made of an opaque substrate with opening 2002-1 in the substrate
for allowing the laser to pass through. The laser separates the
selected devices 2006 from the donor substrate 2004. The system
(receiver) substrate 2008 can apply a transfer force 2010 to
attract and hold the separated devices 2006. In another case,
shadow mask 2002 can be made with patterning. A transparent
substrate 2014 for the laser is used. An opaque film 2012 is
deposited on the substrate and then it is patterned to create
opening 2002-1 for the laser. The opaque film 2012 can be
combination of few different films. The opaque film can be either
on top or bottom of the substrate 2014. In another case, the laser
is programmed to only target specific area.
[0260] In another embodiment shown in FIG. 21, the donor substrate
2104 has a layer 2102 holding the micro devices 2106. The adhesion
of the holding layer 2102 can change due to temperature,
illumination, or electrical current. Using the adhesion modulation,
the adhesion of the layer 2102 is decreased for selected devices or
for the adhesion of unselected devices is increased. The receiver
substrate 2108 can apply a transfer force 2110 to attract and hold
the selected devices.
[0261] In another embodiment, the donor substrate 2204 is using
either electrostatic or electromagnetic force 2202 to hold an array
of devices 2206. After picking the array of micro devices 2206 from
the original substrate, the force for holding the selected micro
devices 2206-a on the donor substrate is reduced (or the force for
the unselected device 2206-b is increased). As a result, the
transfer force 2210 from receiver substrate 2208 acts more
effectively on selected devices 2206-a. The selected micro devices
2206-a are moved into receiver substrate 2208 while the remaining
devices 2206-b on the donor substrate 2204 can be used to populate
the rest of receiver substrate 2208 or another receiver substrate.
In case of using electrostatic force for holding the array of micro
devices 2206, the force can be changed by either manipulating the
voltage or by changing dielectric characteristic. In case of
manipulating the voltage, the device 2206 may need to be biased. As
a result, either the micro devices 2206 are biased after being in
contact with the receiver substrate 2208 contact pads (it can be
similar to the contact pads described in the landing area or a
different pad) or there is a contact pads on the donor substrate
that bias the micro devices. FIG. 23 shows an exemplary
electrostatic holding force 2302 with a biasing pad 2306 on the
donor substrate 2304. Here, the electrostatic electrodes 2308 are
used to pick up the array of devices. After moving to receiver
substrate, the electrostatic force for selected micro devices is
reduced by changing the voltage across two electrodes 2306 and 2308
(the same method can be used to increase the force for unselected
micro devices). In addition, one can create a repelling force for
selected micro devices by applying similar charges to both
electrodes 2306 and 2308. The remaining micro devices on the donor
substrate can be transferred similarly to either the other area of
receiver substrate or a different receiver substrate. FIG. 24 shows
two exemplary embodiment for changing the changing the
characteristics of the donor substrate 2404 forces. In case of
changing the capacitance 2402 characteristic, one can change the
thickness of the dielectric. In this case, the thickness of
dielectric can be changed by either moving 2406 the electrode 2408
or the dielectric structure. The movement can be done through MEMS
structure or piezo materials. In all the cases, electrostatic
electrode can be continuous or patterned for group or a single
micro device. In another case, a shield layer 2410 can physically
shield the electrostatic electrode of donor substrate 2404 from the
selected devices. All the above methods can be applied to
electromagnetic force as well.
[0262] In one embodiment, distance between selected micro devices
2510-a and receiver substrate is reduced compared to the distance
between unselected micro devices 2510-b and receiver substrate.
Here, the devices 2510 can move forward or backward 2506 by using
proper structure 2502 in donor substrate. In one case shown in FIG.
26, MEMS membrane 2612 is used to create the movement 2606-a for
the selected device 2610-a forward or movement 2606-b for the
unselected devices backward 2610-b. Here, the holding force element
2608 can be on the moving part 2612 or on stationary part. In one
embodiment, the movement can be controlled by electromagnetic force
created by current passing through membrane 2612 or a current
through a wire on the donor substrate 2604. In another embodiment,
the movement is controlled by electrostatic force. In another
embodiment, the movement is controlled by piezo materials. Also,
different techniques can be used for moving the micro devices
closer to the receiver substrate. In another case, micro fluid and
membrane is used to move the devices forward or backward. After
picking the devices 2610 from original substrate, the donor
substrate 2604 moves to receiver substrate 2614. Here, the selected
devices 2610-a are moved closer to the receiver substrate 2614.
Here, the force modulation elements 2616 on the receiver substrate
create transfer force for picking the selected devices 2610-a. The
transfer force can be either the same for all the devices (in this
case the force modulation elements 2616 can be uniform or
patterned) or different for selected devices 2610-and unselected
devices 2610-b to enhance selective transfer.
[0263] In another embodiment demonstrated in FIG. 27, the force
modulation element 2702 and its movement 2706 is controlled by a
free standing cantilever 2712 (the cantilever can be secured in one
or more point or totally free standing). After picking the devices
2710 from original substrate, the donor substrate 2704 moves to
receiver substrate 2714. Here, the selected devices 2710-a are
moved closer to the receiver substrate 2714. Here, the force
modulation elements 2716 on the receiver substrate 2714 create
transfer force for picking the selected devices 2710-a. The
transfer force can be either the same for all the devices (in this
case the force modulation elements 2716 can be uniform or
patterned) or different for selected devices 2710-a and unselected
devices 2710-b to enhance selective transfer.
[0264] In one embodiment, the transfer force of receiver substrate
is confined by using another adjacent force. For the example shown
in FIG. 28, the receiver substrate 2802 uses transfer force as form
of electrostatic 2810. In this case, another electrode 2806
adjacent to the electrostatic electrode 2804 in at least on side of
the electrostatic pad is created. While the electrostatic electrode
2804 create transfer force to attract the selected device 2812-a,
these other pads 2806 redirect the electrostatic force away from
the unselected micro devices 2812-b.
[0265] In another embodiment, the force of receiver substrate is
confined by using different dielectric layer. As shown in FIG. 29,
to reduce the electrostatic force from the receiver substrate 2902
on adjacent devices 2912-a, 2912-b on the donor substrate 2908, the
dielectric layer 2906-a, 2906-b on the side of the electrostatic
pad 2904 has different dielectric constant or different
thickness.
[0266] In one embodiment, the donor substrate or receiver substrate
has sensing devices. As the micro devices are being integrated into
the receiver substrate, the sensing device can test the
functionality of each micro device. This information can be used to
repair the faulty device if needed. Also, this information can be
used to control the transfer faulty devices by increasing the donor
force or reducing the transfer for such faulty devices. In case of
emissive device, the sensing element is a photo-sensor that can
detect the output of emissive micro devices. In this case, the
micro device is biased during the transfer to emit (or be off). The
output is measured by sensing element and so it is used to identify
if the device is normal, always ON, or always OFF, or other stages
of operation. If sensing device is located on the donor substrate,
the testing can be done during the transferring the device to donor
substrate as well. The sensing device can be part of donor or
receiver substrate or extra element added to the substrate.
[0267] In the embodiments associated with FIGS. 24, 25, 26, 27, and
other possible embodiments, distance can be modulated by using
materials that deforms under a trigger conditions. In one case,
piezoelectric material or electroactive polymers can be used. Here
by applying the electric filed the distance modulation layer
deforms and moves the device away from the donor substrate or bring
it closer to the receiver substrate. In another case liquid crystal
materials, and any other type of materials with volume changing
characteristic is used to modulate the distance. Here, applying
electricity can increase or decrease the device volumes and so
modulate the distance of micro device by pushing or polling the
device. The other trigger source can be heat or a form of light.
Selective application of heat can change the shape, volume or
viscosity of certain materials. These characteristics can be used
to change the distance between micro device and receiver or donor
substrates.
[0268] In all embodiment referred in these applications, more than
one force modulation element can be used for each micro device. In
case of electrostatic force for example, one can use two electrodes
for different polarity. In one case, the bias voltage of electrodes
can be DC in another case the bias voltage for the electrodes can
be AC.
[0269] Referring to FIG. 30, in one embodiment, the electrostatic
electrode has two electrically separate parts 3004 and 3005. In one
example, as shown in FIG. 30, electrode 3004 may be a ring
surrounding electrode 3005. During the transfer process electrodes
3004 and 3005 are connected to voltage sources 3006 and 3007. As it
is shown in FIG. 31, voltage sources 3006 and 3007 may form
continuous or alternative opposite electric fields 3008. In this
case, to transfer micro device 3002, it may be not required to
ground the micro device during the transfer process. In these
embodiments, opposite electric fields 3008 separates electric
charges inside the conductive electrode 3003. The voltage polarity
shown in FIG. 31 is an example and one can use different
polarities. Also, the pads can have different configuration. A
benefit of this configuration is the reduction of the unwanted
electrostatic force on adjacent micro devices. Since the devices
are not biased, the spray electrostatic force from pads 3005 will
not affect the unwanted devices.
[0270] In another embodiment shown in FIG. 32, the electrostatic
pad may have two separate parts 3201 and 3202 forming a ring around
the contact pad 3203. During the transfer process electrodes 3004
and 3005 are connected to voltage sources 3006 and 3007 which may
output continuous voltage or alternative voltage at different or
same phase. In one example when voltage sources 3004 and 3005 are
alternative voltage the phase difference may be 180 degrees.
[0271] In all the embodiment, planarization layer can be deposited
between structure on receiver substrate and force modulation
elements. This can improve the surface profile and so make the
transfer easier.
Dual Function Pads for Selective Transfer.
[0272] In another aspect of the present invention, a deformable
bonding layer is positioned between the pads in receiver substrate
and micro devices. The bonding layer can be deformed to accommodate
different height in micro devices. In addition, the bonding layer
can cover most of the landing area and, therefore, relax alignment
accuracy between donor substrate and receiver substrate. In one
embodiment a deformable bonding layer between receiver substrate
and micro devices is used to accommodate the height difference in
micro devices. In one aspect of the invention, the bonding layer is
current curable.
[0273] In one embodiment of the invention, a transfer mechanism is
used for holding devices from donor substrate to receiver substrate
where the bonding layer is cured selectively by applying current to
the bonding layer. Here, either the current is applied selectively
or the bonding layer is applied selectively or both.
[0274] In one aspect of the invention, the current is applied by
circuit in the receiver substrate. In another aspect of the
invention, the circuit applying current is partially or fully
shared with the circuit driving the micro devices in operation
mode. where operation mode can be different for different type of
micro devices. For example, for micro LED devices, operation mode
is where the device generates lights based on driving circuit
output. In another example, for micro sensor device, the operation
mode is where device generate a signal (e.g. charge current,
voltage, impedance, etc) based on a stimulating input signals.
[0275] In another aspect of the invention, the current is applied
selectively by the donor substrate. Here the donor substrate can
control the current of each individual micro devices or groups of
micro devices.
[0276] In one aspect of the invention, the curing current flows
through micro device and receiver substrate. In another aspect of
the invention, the curing current flows between to contact in the
receiver substrate.
[0277] In one embodiment shown in FIG. 33, a receiver substrate
3300 with contact pads 3301 is aligned with a donor substrate 3302
with an array of micro devices 3303 having electrodes 3304. Each
micro device 3003 has at least one corresponding contact pad 3301.
Micro devices 3303 can be smaller, larger or the same size as
bonding layer 3005.
[0278] Still referring to FIG. 33, bonding layer 3305 may be formed
on contact pads 3301. Bonding material 3005 may be an adhesive
layer to promote bonding of the micro device 3303 to the contact
pad 3301. In one case, the curing improves the conductivity of the
bonding layer 3305 and at the same time hold the micro device 3303
into receiver substrate 3300. In another case, upon curing the
bonding layer 3305, this layer may transform from a non-conductive
to a conductive material. In one aspect of this invention, the
bonding layer is curable with electrical current. Here, the bonding
layer 3305 can have conductive nanoparticles. The current passing
through the bonding layer 3305 fuse the nanoparticles together.
[0279] Referring to FIG. 34, in one embodiment, selective current
3401 through receiver substrate 3300 is applied to the selected
pads 3301 after the receiver substrate 3300 and donor substrate
3302 are aligned with the bonding layers 3305. The current 3401
selectively cures the selected bonding layers 3305 and the selected
micro devices 3003 are transferred to the receiver substrate 3000.
In another the bonding layer 3305 is applied selectively to the
selected pads 3001. As a result, curing the bonding layer 3305 can
be global although selective curing as described will work as
well.
[0280] In another embodiment, the micro devices 3303 are
transferred to the receiver substrate 3300 using other selective
transfer techniques (e.g. substrate assisted transfers or
pick-and-place techniques). In this case, the bonding layer 3305
can be also part of the transfer mechanism. Here, during transfer,
the bonding layer 3305 can be dielectric and so that a voltage on
pad 3301 can act as receiver electrostatic force. After transfer,
the current flowing through the bonding layer can cure the bonding
layer 3305 and turn it to a conductive layer. As shown in FIG. 35,
after transferring micro device 3303 to the receiver substrate
3300, a current source (not shown) is used to cure the bonding
layer 3305 by passing the electrical current through the micro
device and the bonding layer 3305. At the beginning of the process,
bonding layer 3305 resist to the flow of the current which cause
the self-heating of this layer. As the curing process continues,
bonding layer 3305 resistance decreases. Upon the completion of the
curing process, bonding layer 3305 transforms to a conductive
interlayer between contact pad 3301 and the micro device electrode
3304. In one example, the bonding layer 3305 material may be an
epoxy resin with metallic conductive nanoparticles (e.g. Au, Ag,
etc) or nonmetallic conductive particles (e.g. carbon nanotubes).
The nanoparticle elements are separated and suspended in the epoxy
resin which results in a high resistance for this material. Upon
current flow, local welding causes the nanoparticles to joint and
at the end, to transform the material to a conductive layer. In
another case, the temperature of the combined donor and receiver
substrates is increased to a level below the temperature required
for temperature. The temperature of pads for selected micro devices
increased selectively to bond the micro devices to the receiver
substrate. Different means can do this process such as applying a
current through the selected pads. In another case, only selected
micro devices contact the pads on the receiver substrate.
Therefore, as the combined temperature increases beyond the bonding
temperature only the selected devices bond to the receiver
substrates. As the donor force can be modulated in advance to be
less than the bonding force globally, the selected devices will
stay on the receiver substrate.
[0281] Referring to FIG. 36, in another embodiment, the contact pad
3602 may be designed to have two separate conductive elements 3601a
and 3601b. In this case, the bonding layer 3305 may be formed
between conductive elements 3301a and 3301b. Here, curing is done
by flowing a current between these two conductive elements and
independent from the micro device.
[0282] Alternatively, as shown in FIG. 37, bonding layer 3305 may
be formed on the top of the micro devices 3304 (or receiver
substrate pad) using methods such as but not limited to stamping,
printing or deposition and patterning. The layers can be applied
selectively or globally. In all the embodiments in this section,
bonding layer 3305 may be formed only for the selected micro
devices 3303 that are transferred. In all the embodiments in this
section, the current source may be DC or an AC current. In all the
embodiments in this section, a voltage source may be used to cure
the bonding layer.
[0283] In one embodiment shown in FIG. 38, the contact pad 3301 and
bonding element 3305 are separated. Driving circuit 3802 is
connected to the contact pad and drives the micro device after
bonding to the receiver substrate 3300. Sub-circuit 3803 is
connected to the bonding element and the switch 3804 is closed for
selected micro devices during the bonding process.
[0284] In another embodiment, referring to FIG. 39, bonding element
and the contact pad 3601 are the same. During the bonding process
both switches 3902 and 3903 may be connected while after bonding,
switches 3902 or 3903 may connect the micro device to the
electronic circuit 3904 of the receiver substrate 3300. In an
alternate embodiment, shown in FIG. 40, the same receiver substrate
pixel circuitry 3604 may be used both for bonding and pixel
driving.
[0285] Referring to FIG. 41, each contact pad 4102 on the receiver
substrate may be connected to the individual pixel circuit 4101.
The bonding element on the contact pads is cured after transferring
the micro device and flowing current through the contact pad 3802
and the micro device. One can cure any array of micro devices or
any individual micro device using signal for columns and rows 4103
and 4104. This may be important for cases where defective micro
devices should not be bonded to the receiver substrate. After the
integration process is done, the pixel is used to program the pixel
for operation mode.
[0286] Alternatively, referring to FIG. 42, the bonding element is
cured by passing current through contact pads elements 4201A and
4201B after closing the switch 4202 connected to the bias 4203.
This configuration allows for a lateral current curing of the
bonding elements. After the integration and other steps is done,
the switches are configured so that the circuit programs the micro
device for operation mode. In one case, the two contact pads are
shorted together and coupled to the circuit. In another case, one
of the contact pads is floating while the other one is coupled to
the circuit.
[0287] FIG. 43 shows process flow of transfer and integration of
micro devices from the donor substrate to the acceptor substrate.
Referring to FIG. 43, block 4300, the first step is the fabrication
of micro devices.
[0288] The next step, block 4302, is preparation of micro devices
for transfer process. This process may be a combination of several
steps. In this step, bonding between the micro devices and native
substrate may be weaken by any appropriate method. In addition,
micro devices may be transferred to the temporary substrate.
[0289] The next step, block 4304, is the formation of dual function
bonding or dielectric layer on top of the contact pad on the
receiver substrate by various methods. Etching step may also be
employed to form dielectric layer on top of the contact pad.
[0290] The next step, block 4306, is the transfer process in which
micro devices are transferred from the donor substrate to the
acceptor/receiver one with the electrostatic or other type of
forces. Here, the dielectric (resistive) layer act as normal
dielectric (or high resistive layer) and create electrostatic force
(or thermal force) in combination with the contact pad.
[0291] After the micro device transfer step, referring to block
4308, several post processing steps such as cleaning,
planarization, formation of additional layer, etc. may be employed
on the receiver substrate.
[0292] After that the dielectric property is changed to couple the
contact pads to the micro device at step 4310.
[0293] Further post processing steps 4312 such as deposition of
electrode, light confinement, and other process can be
performed.
[0294] Referring to FIG. 44A, micro devices 4404 are attached to
donor substrate 4402. Receiver substrate 4400 contains array of
contact pads 4406. Dual function dielectric layer 4408 is formed on
top of contact pad 4406 by various methods. Additional steps
including patterning and etching may be employed to remove the
dielectric layer from the unwanted area. Here, dual function pad
structure is a combination of conductive contact pad and dielectric
layer on top of it.
[0295] In an embodiment, attraction force between the landing pad
and micro device is an electrostatic force. Referring to FIG. 44B,
an array of micro devices is attached to donor substrate 4402.
Contact pads 4406 on the acceptor substrate 4400 are connected to a
voltage source. Here, voltage bias can be applied to all pads at
the same time or to the desired individual pad for selective
transfer purpose as it is shown in FIG. 44B. Donor substrate 4402
with attached micro devices and receiver substrate are brought
together so that surface of the micro devices contacts to the
landing pad. Differential potential in the dielectric layer creates
an attractive electrostatic force, which pulls the micro device
toward the receiver substrate. Referring to FIG. 44B, separating
the donor substrate from the receiver substrate detaches the micro
device from the donor substrate.
[0296] Dielectric layer 4408 between the contact pad and micro
device is insulator and prevents electrical biasing of the micro
device during normal operation. Referring to process 4310, one can
modulate the dielectric properties of this layer to make it
conductive in order to electrically connect the micro device to the
contact pad for operation biasing.
[0297] In one embodiment, one can partially dope the dielectric
layer during process 4304 and modulate its dielectric properties to
conductive in process 4310.
[0298] FIG. 45A shows process flow for modifying the dielectric
layer to form a conductive layer by doping top surface of the
dielectric layer with dopants.
[0299] Referring to FIG. 45A, process 4504 is a combination of two
steps 4500 and 4502. The first step is a formation of the
dielectric (resistive) layer on the receiver substrate. This step
may be done with variety of methods such as deposition, stamping
and spin coating, etc. Additional steps such as patterning and
etching steps may be required to remove the dielectric layer from
the unwanted area. At the end of this process, dielectric
(resistive) layer is formed on top of the contact pad. The next
step shown in block 4502 is doping top surface of the dielectric
layer to form a highly doped surface. The third and fourth steps
shown in block 4506 and 4508 are the transfer of the micro device
from the donor substrate to the receiver substrate and post
processing step. The next step, process 4510, is modulation of
dielectric (resistive) layer to conductive layer, which is a
thermal annealing of the dielectric layer. This will allow
diffusion of dopants into the dielectric layer making this layer
conductive.
[0300] In any of the above embodiment, a blocking layer can be used
on micro devices or receiver substrate to prevent migration of
unwanted materials to the micro devices or receiver substrate.
[0301] Referring to FIG. 45B, prior to the micro device transfer,
top side of the dielectric film 4518 is doped 4513 with dopants by
various techniques for example ion bombardment method to form
highly doped layer 4514 on the received substrate 4510. In case of
ion bombardment method, dopant concentration, energy of doping, and
uniformity of doping profile can be controlled precisely. In an
embodiment, one can form dielectric layer 4518 on top of the
receiver substrate followed by doping 4513 of the entire surface
and subsequent etching of this layer to selectively form dielectric
layer 4518 on top of the contact pads 4516. In another embodiment,
one can form dielectric layer 4518 on the receiver substrate 4510
followed by an etching step and then a doping step to dope surface
of the dielectric layer 4518. Prior to the doping process in the
latter case additional steps such as applying photoresist and
patterning may be employed to create a mask for prevention of
doping of unwanted area on the receiver substrate. A wide variety
of dopants can be used in order to dope the top layer of the
dielectric layer. Dopant includes but not limited to boron,
phosphorus, indium, arsenic, antimony, gold, aluminum, Si, Ge,
titanium, chromium.
[0302] Referring to FIG. 45C, following the transfer of micro
device, thermal annealing step 4510 at elevated temperature is used
to bond the top surface of doped dielectric layer 4518 to the micro
device electrode 4514 and in the meantime allow diffusion 4533 of
dopant into the dielectric layer 4518 forming a conductive layer.
Conductive layer connects the contact pad 4516 on the receiving
substrate 4520 to the micro device electrode. In this particular
design, dielectric layer 4518 can be made of an organic/inorganic
materials, semiconductors and polymers. Examples are silicon oxide,
silicon nitride, polyamide and organic polymers and small molecules
organic materials. Materials can be deposited by various methods
such as sputtering, CVD, PECVD, sol.quadrature.gel, spin coating,
inkjet printing and thermal evaporation techniques. Dielectric
thickness is within a range of tens of nanometers to micrometers.
For sintering the dielectric layer 4518, depending on the material
that is used the micro device can be heated from 50.degree. C. to
600.degree. C. enabling annealing of this layer and dopant
diffusion through the film.
[0303] In another embodiment, referring to the process flow steps
shown in FIG. 45D, bottom side of the dielectric layer is doped
with the dopant. Here, formation of dual function dielectric layer
on the contact pads, process 4545, is a combination of three steps.
The first step 4540 is a formation of thin dielectric layer on the
receiver substrate. The next process, block 4542, is doping the
thin dielectric layer. The third step, block 4544 is formation of
highly resistive low impurity second dielectric layer on top of the
doped layer. Here, patterning and etching steps for removal of the
dielectric layer from the unwanted area can be employed at any
stage depending on the dielectric layer formation method, before
the micro device transfer process. Following the micro device
transfer step 4546, subsequent sintering 4548 of the dielectric
layer, process 4550, allows diffusion of dopant from the bottom
side of the layer to the top side and modulation of dielectric
layer to conductive layer.
[0304] In another embodiment, prior to the micro device transfer,
side of the micro device (e.g. either top or bottom side), which
requires to be connected to the contact pad is doped first and then
the micro device is transferred to the receiver substrate. FIG. 46A
shows the process flow of transferring the surface doped micro
devices to the receiver substrate. Similar to the previous process
flows, the dielectric layer is formed 4602 on the receiver
substrate on top of the contact pad. During process 4604, surface
of the micro device that requires to be attached to the landing pad
is doped by an appropriate method. Doping is following by micro
device transfer 4306 and thermal annealing 4608.
[0305] Referring to FIG. 43B, doping 4606 creates highly doped
surface 4617 on micro devices. During process 4306, micro devices
4614 are transferred to the receiver substrate 4610. After the
micro device transfer step, during process 4304, thermal annealing
4608 allows penetration of dopants from the surface of micro device
4617 into the dielectric layer 4618.
[0306] Referring to FIG. 46C, annealing step 4608 also allows for
diffusion of the penetrated dopants from the surface of the micro
device 4617 through the dielectric layer 4618. One can also form a
very thin dielectric layer on the micro device acting as a
protective layer and dope the layer instead of doping the surface
of the micro device directly to prevent any damage that might be
imposed on the micro device surface during doping process.
[0307] In an embodiment, another method to change dielectric layer
to conductive layer is to expose this layer to the laser beam. FIG.
47 shows process flow of modifying dielectric layer to form a
conductive layer by a laser exposure. The first step 4704 similar
to the previous process is the formation of dielectric layer on the
receiver substrate. The second step 4706 is the transfer of micro
devices on top of the dielectric layer. The third step 4708 is
post-processing. The last step 4710 is laser exposure of the
dielectric layer to induce dielectric breakdown.
[0308] Laser beam exposure induces breakdown of the dielectric
layer (i.e. insulator) and shorting between the contact pad and
device electrode. One can also use laser as a heating source to
melt the dielectric layer. In another embodiment, laser beam may be
used to heat the metal contact and melt the contact allowing
diffusion of metal ions into the dielectric layer or promoting
reaction of metal with the dielectric layer. At least one contact,
either micro device electrode or receiving substrate contact pad
requires to be transparent allowing a penetration of laser into the
dielectric layer.
[0309] FIG. 48A shows cross sectional view of the structure in
which laser 4805 is exposed to the dielectric layer 4818 from the
top or bottom side perpendicular to the micro device 4804.
Transparent electrode 4802 may be made from a thin metal film or
transparent conductive oxide. Highly resistive dielectric film
without impurity is used for the dielectric interlayer 4818. Laser
beam wavelength can be chosen in a way so that it can be absorbed
by the dielectric material 4818. In addition, power, beam diameter
and exposure pulse are defined to precisely control the process and
preventing damage of the other layers. Dielectric layer 4818 can be
formed from wide variety of organic and inorganic materials such as
SiO.sub.2, SiN, etc. Metal contact may be made from aluminum,
tungsten, molybdenum, etc. Reflective metal on the contact pad 4806
or micro device electrode can be also used as a protective layer to
prevent damage of the substrate 4810 and/or underlying layers.
[0310] Referring to FIG. 48B, in an embodiment, in the case of two
metal reflective electrodes on each side of the dielectric layer
4818 (i.e. contact biasing pad 4806 and micro device electrode
4802) in which the laser beam 4850 is blocked when is exposed
perpendicular to the surface of electrodes, light confinement
structure 4824 may be used to reflect the laser beam 4850 and
direct it to the dielectric layer.4818. Light confinement structure
4824 may be a combination of several different layers and
materials. The top surface of light confinement structure 4824 is
made from highly reflective metal 4826. Incident laser beam on the
light confinement structure 4824, where reflective metal presents,
is reflected toward the sidewalls of the micro device 4804.
Reflected laser beam 4850 is absorbed by dielectric layer 4818
resulting a modification of its electrical properties. Here, light
confinement structure 4824 may be made before transferring the
micro devices 4804 or after the transfer process.
[0311] In an embodiment, mechanical or thermal force may be used to
remove partially or fully the dielectric layer 4818 to provide
electrical connection between the contact pad 4816 and micro device
electrode 4802.
[0312] In an embodiment, soft material may be used to form a
dielectric layer for landing pad. The soft dielectric layer can be
made from organic or inorganic materials such as polymers and
polyimide, etc. Here the term "soft materials" refers to any
materials that can be easily modified or deformed by a mechanical
or thermal force such as gels and polymers. Depending on the
material specific characteristics, variety of methods such as
thermal evaporation, spin coating, inkjet printing, stamping, spray
coating, etc. can be used to form the dielectric layer.
[0313] FIG. 49 shows process flow of the formation of soft
dielectric layer and subsequent post processing steps for removing
this layer. Referring to FIG. 49, process 4900, first step is
preparation of substrate such as cleaning steps for removal of
residuals and impurities on top of the surface of the receiver
substrate allowing a better and stronger attachment of the
dielectric layer to the surface. During process 4902, dielectric
layer from soft material is formed by employing a proper method on
top of the system or receiver substrate. Afterwards, referring to
step 4904, the formed dielectric layer is patterned to remove the
material from the unwanted area resulting formation of this layer
on top of the contact pad. Following formation of the dielectric
layer and subsequent patterning step, the micro device is
transferred 4906 from the donor or carrier substrate to the system
or receiver substrate. Here, one can employ previous methods to
modify the dielectric properties of the formed layer to conductive
layer. In addition, one can employ a mechanical or thermal force,
referring to process 4908, to remove the dielectric layer between
the contact and micro device electrode. In the case of mechanical
force, it can be applied to the micro device while receiver
substrate is fixed. In another case mechanical force can be applied
to the back of the receiver substrate or to both the micro device
and receiver substrate at the same time. The last step, block 4910,
includes sequence of cleaning steps for removing the material
residual on the receiver substrate.
[0314] FIG. 50 shows the process step 4908 in which mechanical
stress is applied to the micro device. Here applied mechanical
stress 4908 allows removal of soft material 4922 between the
contact pad 4916 on the receiver substrate 4920 and micro device
electrode 4914 from the sidewalls where the opening exist. In an
embodiment, mechanical force or stress can be applied to the
individual micro device or to the several micro devices at the same
time. This can be done either at the end of each micro device
transfer step or at the end of the transfer step of several micro
devices.
[0315] In another embodiment, after the micro device transfer step,
one can put thermal force on the dielectric layer, referring again
to block 4908 in FIG. 49. Thermal force results evaporation of the
dielectric layer between the contact pad and the micro device
electrode, which allows a physical contact between them. Depending
on the material evaporation temperature, variety of methods such as
direct heating, conventional heating or laser beam can be used to
heat the dielectric layer. In addition, thermal force can be used
to soften or melt the material that can be easily removed from the
opening area on the sidewalls between the contacts. Additional
cleaning step may be used to remove the remaining residual on the
receiver substrate. One can also use mechanical force during
heating step to accelerate the dielectric removal process. Thermal
force may also be used to reduce the required magnitude of the
mechanical force in order to prevent catastrophic damages on the
receiver substrate and micro devices.
[0316] Referring to process 4904, for formation of dual functional
dielectric layer from soft material, different methods can be used
to pattern the dielectric layer. In one case, dielectric layer is
formed on the entire surface of the receiver substrate and then
patterning steps are employed. In another case, referring to FIG.
51, separator layer 5106 that can also be a light confining layer
is first formed on the receiver substrate 5100 and then dielectric
layer 5108 is formed on the receiver substrate. In case of
formation of dielectric layer 5108 on top of the separator layer
5106, additional steps may be used to remove the dielectric layer
on top of the separator layer 5106. Separator layer 5106 isolates
adjacent landing pads 5116. In FIG. 51, for example a well shape
structure is formed around the contact biasing pad 5116.
[0317] In another embodiment, stamping technique can be used to
pattern the dielectric layer. FIG. 52A shows process flow of
forming patterned soft dielectric layer on the contact pad by
stamping technique. As a first step 5200, a dummy substrate is
received and prepared (e.g. cleaned). As a second step 5202, a soft
dielectric layer is formed on the dummy substrate. As a third step
5204, the dielectric layer is patterned. As a fourth step 5206, a
stamper is attached to the dummy substrate. As a fifth step 5208,
the stamper and associated dielectric layer is removed from the
dummy substrate. As a sixth step 5210, the dielectric layer is
stamped on a contact pad on the receiver substrate. As a final step
5212, the stamper is removed from the contact pad and receiver
substrate.
[0318] Referring to FIG. 52B, one method is to form 5202 dielectric
layer 5212 on dummy substrate 5214 and pattern 5204 it in
accordance to the contact bias pads pattern on the receiver
substrate. Then stamper 5216 or carrier substrate is attached 5206
to patterned dielectric layer 5212 on the dummy substrate. The
stamper can be made from wide variety of materials such as PDMS.
Subsequently, stamper 5216 and associated the dielectric layer 5212
is detached or removed 5208 from the dummy substrate 5214. The
stamper is 5216 is then used to stamp 5210 the dielectric layer
5212 on the contact pads 5226 on the receiver substrate 5230.
[0319] In another embodiment, referring to FIG. 52C, the stamper
5217 or carrier substrate is patterned first in accordance to the
formation of contact pads on the receiver substrate. For example, a
PDMS mold can be formed on the stamper 5217 in any specific desired
shape. The dielectric layer 5212 is prepared 5200 and patterned on
the dummy substrate 5214. The patterned stamper 2917 is then
attached 5206 to the dielectric layer 5212 on the dummy substrate
5212. The patterned stamper 2917 and associated dielectric layer
5212 is then removed 5208 from the dummy substrate and used to
stamp 5210 the dielectric layer 5212 on contact pads 5226 on the
receiver substrate 5230.
[0320] In an embodiment, soft dielectric layer can be formed
directly on the micro devices rather than receiver substrate. Here,
micro devices may be on the growth substrate or on the carrier one.
Dielectric layer is formed on the surface of the micro device,
which requires to be attached to the contact pad. Methods such is
immersion, spray coating or spin coating may be used to form this
layer. The next step is the transfer of the device to the receiver
substrate.
[0321] Following the transfer, disclosed methods are used to remove
or modify the dielectric layer. In an embodiment, mechanical or
thermal force can be used to partially remove the dielectric layer.
Here, soft material, which includes metal nanoparticles, can be
used as the dual functional dielectric layer. Metal nanoparticles
such as gold or silver are first dispersed in the dielectric
material or solvent. Referring to FIG. 53A, insulating layer 5380
or in one case light confining layer is formed on the receiver
substrate prior to the formation of dielectric/conductive layer.
Dielectric layer 5318 including metal nanoparticles 5313 forms
layer 5317 on the receiver substrate 5300. Insulating layer 5380
prevents formation of this layer on the entire surface and
subsequent lateral shorting between the contact pads 5306. Since
nanoparticles 5313 are dispersed in the dielectric layer 5318, the
formed layer 5317 is not conductive and acts as a dielectric layer
5318 in the micro device transfer step. FIG. 53B shows transferred
micro device 5304 on layer 5317.
[0322] After the micro device transfer process, referring to FIG.
21C, mechanical stress 5350 or thermal stress 5355 or both is
applied to remove dielectric material 5317 and aggregate the metal
nanoparticles between the contact pad 5306 and micro device
electrode 5304. Here, isolation layer 5380 prevents lateral
electrical shorting between the contact pads 5306 on the receiver
substrate 5300 due to the aggregated metal nanoparticles. One can
also employ thermal stress 5355 to melt the nanoparticles allowing
stronger bonding between the electrode 5304 and contact pad
5306.
[0323] In an embodiment, referring to FIG. 54A, soft dielectric
layer 5418 can be used to transfer micro devices 5404, 5405 with
different heights from the donor substrate 5403 to the acceptor
substrate 5400. Here the donor or carrier substrate may hold micro
devices with different heights. During micro device transfer
process, when hard dielectric layer is used and donor substrate is
brought in close proximity of the receiver substrate, taller micro
devices 5405 may prevent transfer of shorter micro devices 5405 to
the receiver substrate 5400. For example, device 3100 increases the
critical distance between the electrode of shorter height micro
device 5404 and landing pad 5402 and therefore transfer force may
not be strong enough to transfer the micro device to the contact
pad 5406. Referring to FIG. 22B, formed soft dielectric layer on
the receiver substrate on the other hand, during micro device
transfer process allows penetration of taller devices into this
layer which in turn allows contact between the electrode of shorter
height device and landing pad 3102.
[0324] In an embodiment, referring to FIG. 55, a dielectric layer
5418 is used on top of the contact pads 5226. This dielectric
deforms under pressure and/or temperature. In one case, the
deformation is assisted by the softening of the pad material 5226
underneath the dielectric layer 5418. In another case, the
dielectric layer has at least one hole 5418-2 (or it does not cover
the entire pad 5226 area). Thus, the soften pad material 5226 can
be squeezed through the holes 5418-2 and create a conductive layer
with the micro device 5404. In another case, the soften pad
material 5226 can be used for bonding the micro device 5404 to the
substrate 5400.
[0325] While particular implementations and applications of the
present disclosure have been illustrated and described, it is to be
understood that the present disclosure is not limited to the
precise construction and compositions disclosed herein and that
various modifications, changes, and variations can be apparent from
the foregoing descriptions without departing from the spirit and
scope of an invention as defined in the appended claims.
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