Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SELECTIVE TRANSFER OF MICRO DEVICES
FIELD OF THE INVENTION
[0001] The present disclosure relates to device integration into system
substrates. More
specifically, the present disclosure relates to selective transfer of micro
devices from a donor
substrate to a receiver substrate.
BRIEF SUMMARY
[0002] 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.
[0003] Some embodiments further comprise weakening the donor force bonding the
micro
devices to the donor substrate to assist micro device transfer.
[0004] 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.
[0005] Some embodiments further comprise weakening the donor force using laser
lift off.
[0006] Some embodiments further comprise weakening the donor force by heating
an
area of the donor substrate.
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[0007] Some embodiments further comprise modulating the force by magnetic
field.
[0008] Some embodiments further comprise modulating the receiver force by
heating the
receiver substrate.
[0009] 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.
[0010] Some embodiments further comprise performing an operation on the
receiver
substrate so that the contact pads permanently bond with the selected micro
devices.
[0011] 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.
[0012] 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.
[0013] In some embodiments the push force is created by a sacrificial layer
deposited
between the selected micro device and the donor substrate.
[0014] 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
devices 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 deposite
after the micro device is transferred to the receiver substrate.
[0015] 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.
[0016] 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
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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
[0017] The foregoing and other advantages of the disclosure will become
apparent upon
reading the following detailed description and upon reference to the drawings.
[0018] Figure 1 shows a donor substrate and a receiver substrate before the
transfer
process begins.
[0019] Figures 2A shows 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.
[0020] Figures 2B shows 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.
[0021] Figures 2C shows 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.
[0022] Figure 3A-3E shows different steps for transferring devices based on
1000A. Similar
steps can be used for 1000B and 1000C and combination of 1000A, 1000B, 1000C.
[0023] Figure 3A shows the step of aligning the donor and receiver substrates
[0024] Figure 3B shows the step of moving the substrates together within a
defined
distance margin.
[0025] Figure 3C-1 shows one embodiment of modulating the forces by applying
receiver
forces selectively.
[0026] Figure 3C-2 shows one embodiment of modulating the forces by weakening
the
donor force selectively and applying receiver force globally.
[0027] Figure 3D shows one embodiment of modulating the forces by applying
receiver
and weakening donor forces selectively.
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[0028] Figures 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.
[0029] Figures 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.-4C show
arrangements with
different pitches of micro devices and contact pads.
[0030] Figures 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
devic types is aligned with each micro devices accordingly so multiple
transferring process is
needed to transfer all different types of micro devices.
[0031] Figure 5A shows selective and global heating elements incorporated into
substrates.
[0032] Figure 5B .shows one embodiment for pattering selective and global
heating
elements incorporated into substrates.
[0033] Figure 5C shows use of external sources to selectively heat up at least
one
substrate.
[0034] Figure 6A shows a flowchart of method 1100 for selectively transferring
micro
devices from a donor substrate to a receiver substrate.
[0035] Figures 6B-6G show one method of implementing steps described in method
1100.
[0036] Figure 6B shows the step of preparing the donor and receiver substrates
for
selective transfer.
[0037] Figure 6C shows the step of aligning the substrates.
[0038] Figure 6D shows the step of moving the substrates toward each other
within a
predefined distance margin.
[0039] Figure 6E shows the step of creating receiver forces by curing the
adhesive (e.g.
applying pressure or heat). This can be globally or selectively.
[0040] Figure 6F shows the step of reducing donor forces if needed. This can
be globally or
selectively.
[0041] Figure 6G shows the step of moving the substrates away from each other.
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[0042] Figure 7A shows other possible arrangements of adhesive on receiver
substrate.
[0043] Figure 7B shows a contact pad with a cut out before and after
application of an
adhesive.
[0044] Figure 8 shows a stamping process that can be used to apply adhesive to
contact
pads.
[0045] Figure 9 shows a flowchart of method 1200 for selectively transferring
micro
devices from a donor substrate to a receiver substrate.
[0046] Figure 10 shows a donor substrate and a receiver substrate setup to
perform
method 1200.
[0047] Figures 11A-11E show one embodiment for implementing steps in method
1200.
[0048] Figure 11A shows the step of aligning donor and receiver substrates.
[0049] Figure 11B shows the step of moving donor and receiver substrates to a
defined
distance margin while mechanical force is loose.
[0050] Figure 11C shows the step of increasing mechanical forces.
[0051] Figure 11D shows the step of reducing donor forces if needed (this step
can be
done in advance as well).
[0052] Figure 11E shows moving the donor and receiver substrates away from
each other.
[0053] Figure 12A shows a flowchart of method 1300 for selectively
transferring micro
devices from a donor substrate to a receiver substrate.
[0054] Figure 12B shows a donor substrate and a receiver substrate setup to
perform
method 1300.
[0055] Figures 13A-13E show one embodiment for implementing steps in method
1300.
[0056] Figure 13A shows the step of aligning the donor and receiver
substrates.
[0057] Figure 13B shows the step of moving the substrates within a predefined
distance
margin.
[0058] Figure 13C shows the step of creating receiver force by applying
potential to
electrostatic elements. This can be done selectively or globally.
[0059] Figure 13D shows the step of reducing the donor force if needed. This
can be done
globally or selectively. .
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100601 Figure 13E shows the step of moving the substrates away.
[0061] Figures 14A shows another alternative placements for electrostatic
layer.
[0062] Figures 146 shows another alternative placements for electrostatic
layer.
[0063] Figures 14C shows another alternative placements for electrostatic
layer.
[0064] Figures 14D shows another alternative placements for electrostatic
layer.
[0065] Figure 15A shows another alternative geometries for micro devices and
contact
pads.
[0066] Figure 15B shows another alternative geometries for micro devices and
contact
pads.
[0067] Figure 15C shows another alternative geometries for micro devices and
contact
pads.
[0068] Figure 15D shows another alternative geometries for micro devices and
contact
pads.
[0069] Figure 15E shows another alternative geometries for micro devices and
contact
pads. =
[0070] Figure 16 shows a flowchart of method 1400 for selectively transferring
micro
devices from a donor substrate to a receiver substrate.
[0071] Figure 17A-17E show one embodiment for implementing steps in method
1400.
[0072] Figure 17A shows the step of aligning the donor and receiver
substrates.
[0073] Figure 17B shows the step of moving the substrates to a predefined
distance margin
from each other.
[0074] Figure 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.
[0075] Figure 17D shows applying a push force to the micro devices from the
donor
substrate. The push force from donor substrate should be selective.
[0076] Figure 17E shows the step of moving substrate away.
[0077] Figure 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.
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[0078] Figure 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.
[0079] Figure 19 shows a simplified biasing condition of receiver substrate
for testing the
micro devices for defect and performance analysis.
[0080] Figure 20A shows a selective liftoff process to modulate the force on
the donor
substrate using shadow mask.
[0081] Figure 20B shows a selective liftoff process to modulate the force on
the donor
substrate using a patterned mask.
[0082] Figure 21 shows a donor substrate with a sacrificial layer between
micro devices
and the substrate.
[0083] Figure 22 shows a donor substrate with force modulating elements on the
donor
substrate.
[0084] Figure 23 shows a donor substrate with a force modulating element and a
biasing
pads.
[0085] Figure 24A shows an embodiment for force modulating element by changing
the
capacitance.
[0086] Figure 24B shows an embodiment for force modulating element by using a
shield
electrode.
[0087] Figure 25 shows an embodiment that change the distance between selected
devices and system substrate with the distance between unselected device and
system
substrate.
[0088] Figure 26A shows an embodiment using membrane for moving the micro
device
backward or forward.
[0089] Figure 26B shows an embodiment where the membrane moves the selected
device
closer to the system substrate.
[0090] Figure 27A shows an embodiment using a cantilever (membrane) for moving
the
micro devices forward or backward.
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[0091] Figure 27B shows an embodiment using a cantilever (membrane) for moving
the
selected device closer to the system substrate.
[0092] Figure 28 shows an embodiment for confining the transfer force on the
selected
device by diverging the force from adjacent devices.
[0093] Figure 29 shows an embodiment for confining the transfer force on the
selected
devices by reducing the effect of the force on the adjacent devices.
[0094] 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 modificaiions, equivalents, and alternatives falling within
the spirit and scope of
an invention as defined by the appended claims.
DETAILED DESCRIPTION
[00951 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:
[0096] Figure 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 Figure
1A or can be
the same structure as shown in Figure 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
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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 Figure 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.
[0097] 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
=
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[0098] 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:
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).
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;
c. generating a receiver force that acts to hold the selected micro devices to
their contact pads;
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 system substrate during
steps b and c.
[0099] In some cases, the contact pad can be deposited after the device is
transferred to
the system substrate. In this case the
[00100] 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.
[00101] Figures 2A-2C show exemplary flowcharts of selective transfer
methods
1000A-1000C. Figure 1 shows a donor substrate 100 and a receiver substrate 200
suitable for
=
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performing any of methods 1000. Method 1000A will be described with reference
to Figures
3A-3E. Methods 100013 and 1000C are analogous variations of method 1000A. One
can use the
combination of methods 1000A-1000C to further enhance the transfer process.
[00102] 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 Figure 3A. Micro device 102c is not to be transferred so, although
shown as aligned, it
may or may not align with contact pad 202c.
[00103] 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 Figure 33. 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.
[00104] 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.
[00105] 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.
1. Weaken FD to be less than FR on micro devices selected for transfer
2. Strengthen FR to be greater than FD on micro devices selected for transfer.
=
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3. Weaken FR to be less than FD on micro devices NOT selected for transfer
4. Strengthen FD to be greater than FR on micro devices NOT selected for
transfer
[00106] 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 Figure
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 Figure 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.
[00107] 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 Figure 3D.
This may be done,
for example, using laser lift off techniques, lapping or wet/dry etching. 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.
[00108] 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.
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[00109] 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 Figure 3E. 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 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.
[00110] Figure 2B shows method 1000B; an alternative embodiment of
method
1000A.
[00111] 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.
[00112] 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.
[00113] 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.
[00114] 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.
[00115] 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.
[00116] At 1012B, optional post processing is applied to selected micro
devices
102a, 102b. Once donor substrate 100 is separated from receiver substrate 200,
further
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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.
[00117] Figure 2C shows method 1000C; an alternative embodiment of
method
1000A.
[00118] 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.
[00119] 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.
[00120] At 1006C, donor substrate 100 and receiver substrate 200 are
moved
together until the selected micro devices 102a, 102b touch contact pads 202a,
202b.
[00121] 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.
[00122] 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
[00123] 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
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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.
[00124] When this is not possible, the pitches of each array can be set
proportionally. Figures 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.
[00125] 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 Figure 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
receivers substrate is required between each transfer step.
[00126] 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
[00127] 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.
CA 02936523 2016-07-19
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[00128] Figure 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
plan view in Figure
5B.
[00129] 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.
[00130] 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.
[00131] 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
Figure 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
CA 02936523 2016-07-19
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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
[00132] 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 fqr
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.
[00133] Figure 6A shows a flowchart of method 1100, a modified version
of method
1000 specific to the use of adhesive to generate FR. Figure 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.
[00134] Receiver substrate 200 has an array of contact pads 212
attached. Although
Figure 6B shows the force modulation element 500 connected to the contact pads
212, they
can be physically separated.
[00135] As shown in Figure 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
CA 02936523 2016-07-19
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moved together, micro devices 102a, 102b will make contact with adhesive 500
as well as
contact pads 212a, 212b.
[00136] Method 1100 will be explained with reference to Figures 6B-6F.
At 1102,
adhesive is selectively applied as shown in Figure 6B.
[00137] 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 Figure 6C.
[00138] 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 Figure 6D.
[00139] At 1108, receiver force, FR, is generated, as shown in Figure
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.
[00140] 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 Figure 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.
[00141] 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 Figure 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
CA 02936523 2016-07-19
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display, a transparent electrode layers, fillers, planarization layers and
other optical layers can
be deposited.
[00142] 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.
[00143] 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. Figure 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.
[00144] In another embodiment, one or more cut-outs can be provided for
the
adhesive 500. Figure 78 shows a contact pad 212 with a cut out (I) before and
(II) after
application of an adhesive.
[00145] 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, Figure 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 (1), 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
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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.
[00146] 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
[00147] 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.
[00148] 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.
[00149] Figure 9 shows a flowchart of method 1200, a modified version of
method
1000A suitable for mechanical generation of FR. Figure 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 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. Micro devices 102 and donor substrate 100 are shown as
connected to
ground 244.
[001501 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
CA 02936523 2016-07-19
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and the pitch of the array of micro devices 102 be proportional as this
facilitates the transfer of
multiple devices simultaneously.
[00151] Method 1200 will be described with reference to Figures 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 Figure 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.
[00152] 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 Figure 118.
[00153] 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 Figure 118. 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.
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[00154] 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 Figure 11C.
Selectivity can be
achieved by selectively turning off selective heaters 304.
[00155] 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 Figure 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.
[00156] 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 Figure 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
[00157] 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 case
of magnetic force a
current passes through a conductive layer instead of charging a conductive
layer for
electrostatic force.
[00158] Here, 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
=
CA 02936523 2016-07-19
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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.
[00159] Figure 12A shows a flowchart of method 1300, a modified version
of
method 1000 suitable for electrostatic generation of FR. Figure 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
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. Micro devices 102 and donor substrate 100
are shown as
connected to ground 244.
[00160] The. landing area on the receiver substrate 200 has at least a
contact pad
232 attached and a force modulation element 234.
[00161] 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, Hf0 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 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
CA 02936523 2016-07-19
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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.
[00162] Method 1300 will be explained in conjunction with Figures 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
Figure 13A.
[00163] 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
Figure 13B.
[00164] At 1306, a receiver force, FR, is generated, as shown in Figure
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 300nm 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.
[00165] At 1308, donor force FD is selectively weakened for selected
micro devices
102a, 102b, so that FD' is less than FR, as shown in Figure 13D. This may be
done, for example,
using laser lift off techniques, lapping or wet/dry etching.
[00166] 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 Figure 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
CA 02936523 2016-07-19
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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.
[00167] In other embodiments, electrostatic layer 234 can take on other
configurations. Figure 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.
[00168] In other embodiments, the geometry of contact pads 232,
electrostatic layer
234 and micro devices 102 can be changed to varying effect. Figure 15
illustrates some possible
alternative geometries. Figure 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. Figure 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 Figure 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.
[00169] Figure 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. Figure
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.
CA 02936523 2016-07-19
- 27 -
Transfer of micro devices of different heights
[00170] 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.
[00171] Figure 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 Figure
17A. Note that micro device 102a is of a different height than micro device
102b.
[00172] 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 Figure 17B,
substrates 100,
200 may stop approaching when some micro devices 102 make contact with contact
pads 232.
CA 02936523 2016-07-19
- 28 -
[00173] At 1406, a receiver force, FR, is generated, as shown in Figure
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.
[00174] 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.
[00175] 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 Figure 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;
[00176] 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
CA 02936523 2016-07-19
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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.
[001771 The LEDs conventionally are manufactured in the form of single
color LEDs
on a wafer and patterned to individual micro-devices by different process 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
system 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
system 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.
[00178] In another method, the LED connection-force modulation happens
after the
LED substrate and the backplane substrate are in contact and the system
substrate forces to
LED are selectively modulated by the aforementioned methods presented here.
The LED
CA 02936523 2016-07-19
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=
substrate force modulation can be done prior to the backplane substrate force
modulation as
well.
[00179] 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.
[00180] 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 system 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
[00181] 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.
[00182] 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
CA 02936523 2016-07-19
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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.
[00183] 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.
[00184] It should be understood that various embodiments in accordance
with and
as variations of the above are contemplated.
[00185] 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
CA 02936523 2016-07-19
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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 differ. In another embodiment, the contact pads are
concave. In
CA 02936523 2016-07-19
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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 are 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.
[00186] 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.
[00187] 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.
[00188] 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.
[00189] 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 Figure 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.
CA 02936523 2016-07-19
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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. A opaque film 2012 is
deposited on the
substrate and then it is patterned to create opening 2002-1 for the laser. The
opaque 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.
[00190] In another embodiment shown in Figure 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 system substrate 2108 can apply a transfer force
2110 to attract and
hold the selected devices.
[00191] 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 system substrate 2208
acts more
effectively on selected devices 2206-a. The selected micro devices 2206-a are
moved into
system substrate 2208 while the remaining devices 2206-b on the donor
substrate 2204 can be
used to populate the rest of system substrate 2208 or another system
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 system 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. Figure 23
shows an exemplary
electrostatic holding force 2302 with a biasing pads 2306 on the donor
substrate 2304. Here,
the electrostatic electrodes 2308 are used to pick up the array of devices.
After moving to
system substrate, the electrostatic force for selected micro devices is
reduced by changing the
CA 02936523 2016-07-19
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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
system substrate or a different system substrate. Figure 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 devices. In another case, a shield layer 2410 can shield the
electrostatic electrode
of donor substrate 2404 from the selected devices. All the above methods can
be applied to
electromagnetic force as well.
[00192] In one
embodiment, distance between selected micro devices 2510-a and
system substrate is reduced compared to the distance between unselected micro
devices
2510-b and system substrate. Here, the devices 2510 can move forward or
backward 2506 by
using proper structure 2502 in donor substrate. In one case shown in Figure
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 technique can be
used for moving
the micro devices closer to the system 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 system substrate 2614. Here, the
selected
devices 2610-a are moved closer to the system substrate 2614. Here, the force
modulation
elements 2616 on the system substrate create transfer force for picking the
selected devices
CA 02936523 2016-07-19
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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-a and unselected devices 2610-b to enhance selective transfer.
[00193] In another embodiment demonstrated in Figure 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 system
substrate
2714. Here, the selected devices 2710-a are moved closer to the system
substrate 2714. Here,
the force modulation elements 2716 on the system 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.
[00194] In one embodiment, the transfer force of system substrate is
confined by
using another adjacent force. For the example shown in Figure 28, the system
substrate 2802
uses transfer force as form of electrostatic 2810. In this case, another
electrodes 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.
[00195] In another embodiment, the force of system substrate is
confined by using
different dielectric layer. As shown in Figure 29, to reduce the electrostatic
force from the
system 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.
[00196] 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-devices. This information can
be used to repair
the faulty device if needed. Also, this information can be used to control the
transfer faulty
CA 02936523 2016-07-19
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devices by increasing the donor force or reducing the transfer for 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.
[00197] In all embodiment referred in this applications, more than one
force
modulation element can be used for each micro devices. 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.
[00198] Referring to Figure 30, in one embodiment, the electrostatic
electrode has
two electrically separate parts 3004 and 3005. In one example, as shown in
Figure 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
Figure 31, voltage
sources 3006 and 3007 may form continuous or alternative opposite electric
fields 3008. In this
case, to transfer microdevice 3002, it may be not required to ground the
microdevice during
the transfer process. In these embodiments, opposite electric fields 3008
separates electric
charges inside the conductive electrode 3003. A benefit of this configuration
is the reduction of
the unwanted electrostatic force on adjacent microdevices.
[00199] In another embodiment shown in Figure 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.
=
CA 02936523 2016-07-19
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[00200] In all the embodiment, planarization layer can be deposited
between
structure on system substrate and force modulation elements. This can improve
the surface
profile and so make the transfer easier.
[00201] 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.
=
