U.S. patent application number 12/446477 was filed with the patent office on 2010-08-05 for electronic device having a plastic substrate.
This patent application is currently assigned to KONNKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Eliav Itzhak Haskal.
Application Number | 20100196683 12/446477 |
Document ID | / |
Family ID | 39135351 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100196683 |
Kind Code |
A1 |
Haskal; Eliav Itzhak |
August 5, 2010 |
ELECTRONIC DEVICE HAVING A PLASTIC SUBSTRATE
Abstract
A method of manufacturing a thin-film electronic device
comprises applying a plastic coating to a rigid carrier substrate
(12) using a wet casting process, the plastic coating forming a
plastic substrate (22). The plastic material has a coefficient of
thermal expansion greater in a first direction perpendicular to the
substrate plane than in a second direction parallel to the
substrate plane. Thin film electronic elements are formed over the
plastic substrate and the rigid carrier substrate is released from
the plastic substrate by a heating process which expands the
plastic substrate preferentially in a direction perpendicular to
the substrate plane. The anisotropy of the thermal expansion in the
plastic substrate of the invention enables the expansion of the
substrate during the thermal lift-off process to be in the
perpendicular direction. This has been found to aid the lift-off
process and also protect the components mounted on the upper
surface of the plastic substrate.
Inventors: |
Haskal; Eliav Itzhak;
(Eindhoven, NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONNKLIJKE PHILIPS ELECTRONICS
N.V.
Eindhoven
NL
|
Family ID: |
39135351 |
Appl. No.: |
12/446477 |
Filed: |
October 22, 2007 |
PCT Filed: |
October 22, 2007 |
PCT NO: |
PCT/IB07/54284 |
371 Date: |
April 21, 2009 |
Current U.S.
Class: |
428/212 ;
264/311; 264/334; 264/482; 445/24 |
Current CPC
Class: |
H01L 29/78603 20130101;
Y10T 428/24942 20150115; Y10T 428/24802 20150115; H01L 51/0097
20130101; G02F 1/167 20130101; H01L 27/1214 20130101; G02F 1/133305
20130101; H01L 27/1266 20130101; G02F 1/1362 20130101 |
Class at
Publication: |
428/212 ;
264/334; 264/482; 264/311; 445/24 |
International
Class: |
H01J 9/00 20060101
H01J009/00; B29C 41/22 20060101 B29C041/22; B29C 41/04 20060101
B29C041/04; B32B 7/02 20060101 B32B007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 27, 2006 |
EP |
06123059.5 |
Claims
1. A method of manufacturing a thin-film electronic device, the
method comprising: applying a plastic coating (22) to a rigid
carrier substrate (12) using a wet casting process, the plastic
coating (22) forming a plastic substrate, and comprising a
transparent plastic material having a coefficient of thermal
expansion at least three times greater in a first direction
perpendicular to the substrate plane than in a second direction
parallel to the substrate plane; forming thin film electronic
elements (30, 32, 34, 36, 40) over the plastic substrate (22); and
releasing the rigid carrier substrate (12) from the plastic
substrate, by a heating process which expands the plastic substrate
preferentially in a direction perpendicular to the substrate
plane.
2. A method as claimed in claim 1, wherein the plastic material has
a greatest coefficient of thermal expansion in a direction
substantially perpendicular to the rigid carrier substrate
(12).
3. A method as claimed in claim 1 or 2, wherein the plastic
material has a least coefficient of thermal expansion in a
direction substantially parallel to the rigid carrier substrate
(12).
4. A method as claimed in any preceding claim, wherein the
coefficient of thermal expansion of the plastic material
perpendicular to the rigid carrier substrate (12) is at least five
times that parallel to the rigid carrier substrate (12).
5. A method as claimed in claim 4, wherein the coefficient of
thermal expansion of the plastic material perpendicular to the
rigid carrier substrate (12) is at least ten times that parallel to
the rigid carrier substrate.
6. A method as claimed in any preceding claim, wherein the
coefficient of thermal expansion of the plastic material parallel
to the rigid carrier substrate (12) is less than
3010.sup.-6/.degree. C.
7. A method as claimed in any preceding claim, wherein the plastic
material comprises a polyimide.
8. A method as claimed in claim 7, wherein the plastic material
comprises poly(p-phenylene biphenyltetracarboximide).
9. A method as claimed in any preceding claim, wherein the release
is by thermally delaminating the plastic substrate from the rigid
carrier substrate.
10. A method as claimed in claim 9, wherein the thermal
delamination is performed by exposure to ultraviolet laser
light.
11. A method as claimed in claim 10, wherein the ultraviolet laser
light has a wavelength greater than 200 nm.
12. A method as claimed in any preceding claim, wherein the rigid
carrier substrate (12) comprises a glass substrate.
13. A method as claimed in any preceding claim, wherein the wet
casting process comprises a spin-on process.
14. A method as claimed in any preceding claim, for manufacturing
an active matrix display device, wherein: forming thin film
electronic elements (30, 32, 34, 36, 40) over the plastic substrate
(22) comprises forming an array of pixel circuits over the plastic
substrate, and wherein the method further comprises forming a
display layer (100) over the array of pixel circuits before
releasing the rigid carrier substrate from the plastic
substrate.
15. A method as claimed in claim 14, further comprising
manufacturing a second substrate arrangement (50), and wherein
forming a display layer over the array of pixel circuits comprises
mounting the first and second substrate arrangements with
electro-optic material sandwiched therebetween, the active matrix
display device thereby comprising first and second substrates with
the electro-optic material sandwiched therebetween.
16. A thin-film electronic device, comprising: a plastic substrate
(22) comprising a transparent plastic material, the plastic
substrate having a coefficient of thermal expansion which is at
least three times greater in a direction perpendicular to the
substrate plane than in a direction parallel to the substrate
plane; and thin film electronic elements over the plastic
substrate;
17. A thin-film electronic device as claimed in claim 16, wherein
the plastic substrate (22) has a greatest coefficient of thermal
expansion in a direction substantially perpendicular to the
substrate plane.
18. A thin-film electronic device as claimed in claim 16 or 17,
wherein the coefficient of thermal expansion of the plastic
substrate perpendicular to the substrate plane is at least five
times that parallel to the substrate plane.
19. A thin-film electronic device as claimed in claim 18, wherein
the coefficient of thermal expansion of the plastic substrate
perpendicular to the substrate plane is at least ten times that
parallel to the substrate plane.
20. A thin-film electronic device as claimed in any one of claims
16 to 19, wherein the plastic material comprises a polyimide.
21. A thin-film electronic device as claimed in claim 20, wherein
the plastic material comprises poly(p-phenylene
biphenyltetracarboximide).
Description
FIELD OF THE INVENTION
[0001] The invention relates to the manufacture of an electronic
device, such as an active matrix display device, on a plastic
substrate.
BACKGROUND OF THE INVENTION
[0002] The most common form of active matrix display is an active
matrix liquid crystal display (AMLCD). AMLCD devices are usually
made on large glass substrates that are 0.7 mm thick. Two plates
are needed for a cell, so that completed displays are just over 1.4
mm thick. Mobile phone manufacturers, and some laptop computer
manufacturers, require thinner and lighter displays, and completed
cells can be thinned in an HF (hydrofluoric acid) solution,
typically to about 0.8 mm thick. Mobile phone manufacturers ideally
want the displays to be even thinner, but it has been found that
cells below 0.8 mm thick made by this method are too fragile.
[0003] The HF thinning is not attractive because it is a wasteful
process that uses hazardous chemicals that are difficult to dispose
of safely and economically. There is also some yield loss during
the etching process due to pitting of the glass.
[0004] The attractiveness of light, rugged and thin plastic AMLCDs
as an alternative has long been recognized. Recently, interest in
plastic displays has increased even further, partly due to the
increased use of color AMLCDs in mobile phones and PDAs. There has
been much research recently into AMLCDs and organic light emitting
diode (OLED) displays on plastic substrates. Despite this interest,
there is still a need for a plausible manufacturing route for mass
production of plastic displays.
[0005] A number of different ways have been reported for the
manufacture of thin-film transistors (TFTs) or displays on plastic
substrates.
[0006] One technique is described in WO 05/050754, in which a
substrate arrangement is manufactured comprising a rigid carrier
substrate and a plastic substrate over the rigid carrier substrate.
The rigid carrier substrate is released from the plastic substrate
after forming pixel circuits and display cells over the plastic
substrate. This enables substantially conventional substrate
handling, processing and cell making to be employed.
[0007] To release the plastic substrate from a glass carrier a
heating method is often used. By heating the glass and the plastic
substrate, the plastic substrate and the electronic components
formed on the substrate are released from the glass carrier.
[0008] There are various methods by which the plastic substrate can
be separated from the glass carrier. A release process proposed in
WO 05/050754 is a laser lift-off process. Laser light at
ultraviolet wavelengths is used to cause the lift-off of the
plastic substrate from the underlying carrier. It has been
suggested that the release process is a photoablation process due
to multiple-photon processes, including localized heating. A
suggested material for this process is polyimide, which is chosen
for its high-temperature stability and high absorption of UV
energy.
[0009] There are potential problems in using a heating effect to
lift-off the plastic substrate from the glass. Sufficient energy is
needed to enable lift off to occur, but without damaging either the
plastic substrate or the components formed on it, which may result
from thermal expansion effects.
[0010] When using a laser lift-off process, higher wavelengths
within the UV spectrum are preferable because lower wavelengths are
absorbed more by the glass substrate, making the laser release less
effective. For example commercially available lasers which operate
at 308 nm or 351 nm are preferred.
[0011] At these higher wavelengths, the energy absorbed in the
plastic layer is statistically distributed without complete
thermalisation in the plastic polymer molecules. This gives rise to
localized heating effects, which can in turn result in damage to
the plastic substrate or the components mounted on it. This can
also result in partial or poor lift-off from the carrier.
[0012] There is therefore a need for a substrate material which is
able to withstand heating without cracking or buckling either of
the substrate or the components mounted on it, and which can have
good lift-off from a glass substrate.
SUMMARY OF THE INVENTION
[0013] According to the invention there is provided a method of
manufacturing a thin-film electronic device, the method
comprising:
[0014] applying a plastic coating to a rigid carrier substrate
using a wet casting process, the plastic coating forming a plastic
substrate, and comprising a transparent plastic material having a
coefficient of thermal expansion at least three times greater in a
first direction perpendicular to the substrate plane than in a
second direction parallel to the substrate plane;
[0015] forming thin film electronic elements over the plastic
substrate; and
[0016] releasing the rigid carrier substrate from the plastic
substrate, by a heating process which expands the plastic substrate
preferentially in a direction perpendicular to the substrate
plane.
[0017] The anisotropy of the thermal expansion in the plastic
substrate of the invention enables the expansion of the substrate
during the thermal lift-off process to be in the perpendicular
direction. This has been found to aid the lift-off process and also
protect the components mounted on the upper surface of the plastic
substrate.
[0018] The invention thus provides improved delamination of the
plastic substrate, and with reduced buckling and cracking of
electronic layers (silicon layers and metal layers) on top of the
plastic substrate, through the minimal introduction of lateral
stress in these layers.
[0019] Preferably, the coefficient of thermal expansion
perpendicular to the carrier substrate plane is at least five times
that parallel to the rigid carrier substrate plane. More
preferably, the coefficient of thermal expansion in the
perpendicular direction is at least ten times, and more preferably
at least 15 times greater than in the parallel direction.
[0020] The plastic layer material preferably comprises a polyimide
for example poly(p-phenylene biphenyltetracarboximide). This is a
material which can be arranged to have a coefficient of thermal
expansion of 10510.sup.-6/.degree. C. (105 ppm/.degree. C.) in one
direction and 510.sup.-6/.degree. C. (5 ppm/.degree. C.) in a
second direction. The larger coefficient of thermal expansion is
then arranged to be perpendicular to the glass substrate, and this
results from the wet casting, in particular spin coating,
application process.
[0021] The release process may comprise thermal delamination which
is performed by exposure to ultraviolet laser light. Preferably,
the ultraviolet laser light has a wavelength greater than 200 nm.
The rigid carrier substrate preferably comprises a glass
substrate.
[0022] The plastic is capable of wet casting. The plastic layer can
for example be applied to the rigid substrate by a spin-on process,
and this plastic substrate then becomes the final device substrate.
Alternatively, the plastic can be applied by spreading with a blade
or printing techniques such as offset litho or silk screen
printing. This enables a very thin layer of plastic material to be
applied to a carrier device.
[0023] The method can be used for manufacturing an active matrix
display device, wherein:
[0024] forming thin film electronic elements over the plastic
substrate comprises forming an array of pixel circuits over the
plastic substrate,
[0025] and wherein the method further comprises forming a display
layer over the array of pixel circuits before releasing the rigid
carrier substrate from the plastic substrate.
[0026] The invention thus provides a method for the successful
separation of a carrier substrate from a plastic display substrate
which is formed upon it. This enables substantially conventional
substrate handling, processing and cell making to be employed in
the manufacture of a display. This then enables the manufacturing
process for making active matrix displays on plastic substrates to
be carried out in standard factories, with only minimal extra
equipment needed.
[0027] The plastic displays can be made on standard glass
substrates, and these can be re-used many times. This invention can
be applied for example for LCDs, PLED or OLED displays and
electrophoretic displays, and with amorphous silicon (a-Si) or
low-temperature polycrystalline silicon (LTPS) TFTs.
[0028] The method may further comprise manufacturing a second
substrate arrangement, and wherein forming a display layer over the
array of pixel circuits comprises mounting the first and second
substrate arrangements with electro-optic material sandwiched
therebetween, the active matrix display device thereby comprising
first and second substrates with the electro-optic material
sandwiched therebetween.
[0029] The process essentially allows the TFTs to be fabricated on
plastic layers, interconnects to be made, and some packaging to be
carried out while the plastic layer is still stuck to the glass.
The release is carried out after the cell formation. This is
attractive for all plastic substrate applications, and is a
particularly attractive process for making displays on flexible
substrates.
[0030] The invention also provides a thin-film electronic device,
comprising:
[0031] a plastic substrate comprising a transparent plastic
material, the plastic substrate having a coefficient of thermal
expansion which is at least three times greater in a direction
perpendicular to the substrate plane than in a direction parallel
to the substrate plane; and
[0032] thin film electronic elements over the plastic
substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Examples of the invention will now be described in detail
with reference to the accompanying drawings, in which:
[0034] FIG. 1 shows manufactured displays made in accordance with
the invention being released from a common glass substrate for use
in a mobile phone;
[0035] FIG. 2 shows a known laser release process;
[0036] FIG. 3 shows buckling which can occur in the known release
process, and which can cause cracking of the surface layers;
[0037] FIG. 4 shows a substrate according to the invention;
[0038] FIGS. 5A to 5M show the processing steps for one example of
method of the invention starting with the substrate of FIG. 4 for
producing a first example of display of the invention;
[0039] FIG. 6 shows a second example of manufactured display of the
invention; and
[0040] FIG. 7 shows a third example of manufactured display of the
invention.
DETAILED EMBODIMENTS
[0041] The invention relates generally to the manufacture of
thin-film electronic devices on plastic substrates, and concerns a
process in which a thermal release process of a plastic substrate
from a rigid carrier substrate is used. The invention concerns in
particular the plastic material and arranges a coefficient of
thermal expansion to be greater in a first direction perpendicular
to the substrate plane than in a second direction parallel to the
substrate plane. This improves the thermal lift-off process and
prevents damage to the circuit components carried by the substrate
during the thermal lift-off process.
[0042] The invention has particular application to the manufacture
of an active matrix display device, and an example of the invention
will be described in this context.
[0043] FIG. 1 shows schematically the fabrication of plastic
displays in accordance with the invention, and shows the final
release stage, which is improved by this invention. Completed
displays 10 are released from a glass substrate 12 and are then
used in devices such as a mobile phone 14.
[0044] FIG. 2 shows an example of a known arrangement where a
polyimide layer 22 is spincoated onto a glass carrier 12. The
polyimide layer 22 forms the substrate after it has been separated
from the glass carrier 12 by a laser lift-off process.
[0045] FIG. 3 shows one possible problem which can arise during the
thermal laser lift-off. Particularly when higher wavelength laser
illumination is used, for example with 351-nm excitation, the
absorbed energy from the laser is distributed without complete
thermalisation of the polyimide molecules, leading to dissociation
of the weakest bonds. The localized thermal heating of the material
causes the polyimide layer 22 to buckle as shown in FIG. 3, and
this damages the layers on top of the substrate.
[0046] One example of the method of the invention will now be
described in detail, to show how a display can be made using laser
release from a glass substrate, and with inkjet printing for color
filters of the display, and VALC (Vacuum Alignment with Liquid
Crystal) for cell making. The example shown in detail is for the
manufacture of liquid crystal (LC) display cells, having LC
material sandwiched between two opposite (active and passive)
substrates. The invention can be applied to many other display
technologies and to non-display technologies, and the specific
example is for illustration only.
[0047] FIG. 4 shows a glass carrier plate 12 and a plastic layer 22
which functions as a plastic substrate. A substrate in this form
can form the basis of the active plate and the passive plate.
[0048] The plastic layer 22 must be strong enough to be one of the
walls of the completed cell. In addition, the plastic layer 22
should be transparent and ideally non-birefringent. Furthermore,
the plastic layer 22 should be able to absorb laser energy to
effect lift-off of the plastic layer 22 from the carrier plate
12.
[0049] The invention concerns the material for the plastic layer
22, and proposes the use of a plastic material with characteristics
to make it suitable for a thermally-induced lift-off process.
[0050] The substrate material for the plastic layer 22 should have
a tensile strength of >200 MPa, and be non-birefringent. It
should also be deposited using a wet casting process, for example
spin-coating.
[0051] The spin-coating gives an extremely high quality surface and
can give an extremely thin layer if required. Most importantly, the
plastic layer 22 is capable of wet casting. The plastic layer 22
can thus instead be applied by spreading with a blade or printing
techniques such as offset litho or silk screen printing.
[0052] The substrate formed from the plastic layer 22 can comprise
a number of polymers.
[0053] It has already been recognized that the polymer should be
transparent and which may be wet cast (for example spin coated)
from solution to produce a transparent and preferably
non-birefringent film.
[0054] All materials have a coefficient of thermal expansion.
During the transfer of heat to a material, there is a change to the
energy stored in the intermolecular bonds at an atomic level. As
the energy stored by the bonds between the atom increases, so will
the length of each bond. This gives rise to an expansion of a solid
on heating, and a contraction of the solid on cooling. Materials
with a volumetric coefficient of thermal expansion which is
distributed unequally in the x, y and z planes are called
anisotropic materials.
[0055] The invention is based on the recognition that materials
with large anisotropy in the coefficient of thermal expansion (CTE)
in one direction in comparison to a second direction are
particularly suited to a laser lift-off process.
[0056] In accordance with the invention, materials with a large
anisotropy of CTE are aligned on the glass carrier substrate so
that the largest CTE is perpendicular to the glass substrate. The
CTE is at least three times larger perpendicular to the glass
substrate than parallel to the glass substrate.
[0057] These CTE characteristics allow the plastic substrate to be
thermally delaminated from the glass substrate 12, with the plastic
layer 22 expanding in this perpendicular direction rather than
buckling as shown in FIG. 2. This also reduces potential damage to
the device layers mounted on the plastic substrate.
[0058] FIG. 4 shows a polyimide plastic layer 22 with a
perpendicular CTE higher than the parallel CTE.
[0059] An example of a polyimide to be used as a plastic substrate
with the characteristics as outlined above is poly(p-phenylene
biphenyltetracarboximide). This particular polymer has rigid
rod-like chains. It has significantly higher anisotropy in the
thermal expansion coefficient than shown in other materials.
[0060] In use, the poly(p-phenylene biphenyltetracarboximide) does
not distort, crack or buckle when exposed to excitation
wavelengths, even when these are high wavelengths of the UV
spectrum, for example greater than 200 nm.
[0061] By way of example, poly(p-phenylene
biphenyltetracarboximide) has a CTE value in one direction of 105
ppm/.degree. C. and a CTE in a second direction of approximately 5
ppm/.degree. C. The largest CTE value is aligned to be
perpendicular to the glass carrier plate 12, and parallel to the
direction of the application of the heat or laser.
[0062] The large anisotropy in the CTE is a feature both of the
material (resulting from its crystalline content) and the
deposition process. A spin coating process provides in-plane
orientation of the polymer molecular chains, and this contributes
to the high anisotropy.
[0063] A spin coating process is a costly process, as much of the
material is wasted. A slit or blade coating process may therefore
be preferred, and will again provide the in-plane molecular
alignment giving rise to the required anisotropy. The viscosity of
the polyimide may need to be adjusted to make slit coating
suitable.
[0064] Although the specific example of material given displays a
large anisotropy in the coefficient of thermal expansion (a factor
of 20), the difference in the two values does not need to be so
prominent in order for the invention to function. Preferably, the
CTE of the material perpendicular to the glass carrier is at least
three times that of the CTE parallel to the substrate, i.e. a
factor of at least 3. More preferred arrangements have a factor of
more than 4, for example in the range 4-7. As in the specific
example above, the factor may be more than 10.
[0065] The actual CTE value is preferably less than 30 ppm/.degree.
C. in the parallel direction, for example in the range 20
ppm/.degree. C. to 30 ppm/.degree. C. However, the CTE value in the
parallel direction may be even lower, for example less than 10
ppm/.degree. C. In particular, the CTE can be matched to the
underlying glass substrate, and/or to the layers to be deposited
onto the plastic substrate.
[0066] FIGS. 5A to 5M show schematic diagrams for sequential stages
of one fabrication scheme. For clarity, these figures show only one
display being made, but in practice there would be many displays on
large glass substrates, as shown in FIG. 1.
[0067] FIG. 5A shows the active plate, in which a-Si TFT arrays
have been made on the plastic surface using (almost) standard
processing. The plastic substrate 22 is in accordance with the
invention as outlined above. FIG. 5A also shows an optional release
layer 20, for example an amorphous silicon layer, to assist the
release of the plastic substrate from the glass carrier.
[0068] The maximum processing temperature will depend upon the
plastic layer chosen, but it could be higher than for freestanding
plastic films because the plastic is securely anchored to the rigid
glass substrate 12 and there are no problems with shrinkage.
[0069] The TFT array comprises a gate metal layer 30, a silicon
nitride gate dielectric layer 32 and ITO pixel electrodes 34. The
TFTs are shown schematically as 36.
[0070] FIG. 5B shows the addition of column spacers 40 for the LC
cell. These could either be made by inkjet printing or spinning on
a suitable polymer layer and then patterning by photolithography.
Dispersed glass or plastic beads or rods could also be used
instead, but column spacers that stick to both substrates can give
plastic cells increased mechanical strength and help protect the
cell from separating.
[0071] FIG. 5C shows the fabrication of the passive plate
substrate. The passive plate also comprises a glass substrate 50,
and optional release layer 52 and a plastic substrate 54 (which may
be the same plastic as used for the active plate or may be a
different plastic). FIG. 5C also shows black mask layers 56. This
demonstrates another advantage of fabricating plastic displays in
this way, which is that structures can be built into the substrate.
The black mask 56 could also be made at a later stage using
standard methods.
[0072] As shown in FIG. 5D, a second polymer layer 60 is added to
the passive plate layers. This step is only needed if a buried
black mask layer is used.
[0073] As shown in FIG. 5E, recessed wells 70 are etched into the
passive carrier plate. This step is only needed if color filters
are to be inkjet printed. These wells serve to define accurately
the shape of the color filter pixels. The wells can be etched into
the plastic layer either by photolithography and oxygen plasma,
laser ablation or by stamping with a hard mask.
[0074] FIG. 5F shows the passive carrier plate after inkjet
printing of the color filter layers 80.
[0075] FIG. 5G shows an ITO layer 90 sputtered onto the glass
substrate. Furthermore, discrete display devices are formed over
the common active plate glass substrate by etching away the ITO,
plastic and a-Si release layer.
[0076] At this stage, different process routes may be taken,
depending on whether traditional cell making is used, or the newer
Vacuum Alignment with Liquid Crystal (VALC) method, sometimes
called drop filling. In this drop filling method, LC droplets are
put onto one of the plates before alignment and plate coupling is
carried out under vacuum. By way of example, the following diagrams
are for the VALC process.
[0077] FIG. 5H shows an LC drop 100 put on the active plate area of
the plastic display.
[0078] The assembled panel of FIG. 5I is then formed by using the
VALC process.
[0079] One of the glass plates is then removed from the plastic
layer.
[0080] As detailed above, a laser release process is preferably
used, but other heating methods may be employed, for example lamp
heating through the glass of the release layer or bottom of the
plastic layer, or by heating the glass plate on a hot plate.
[0081] The release process is improved by virtue of the use of a
plastic material of the invention.
[0082] FIG. 5J shows the glass substrate of the passive plate
removed. With a laser release process, the laser is applied at a
wavelength greater than 200 nm to the passive glass carrier plate.
Examples of wavelength for this purpose are 308 nm or 351 nm.
[0083] Once the plastic layer has separated from the carrier plate,
the passive glass carrier plate can then be cleaned to remove all
traces of residues from the passive plate process before being
re-used.
[0084] As shown in FIG. 5K, a polarizer 110 is then added. It is
easier to do it at this stage before final release of the display
because the display still has rigidity due to its coupling to the
glass. The polarizer also gives added strength to the top plastic
layer. A chip-on-glass process can also be done at this stage, or
interconnect foils added. The advantage of doing this at this stage
is that the plastic sheet is still firmly stuck to the glass,
simplifying alignment and fixing.
[0085] As shown in FIG. 5L, the plastic substrate of the active
plate is also released by a similar method described above from the
active plate glass substrate 12, which can also be cleaned and used
again.
[0086] A polarizer film is also applied to the plastic substrate 22
of each active plate. The second polarizer 112 is shown in FIG. 6M,
which shows the completed display.
[0087] The polarizer must be applied display-by-display in this
case. If VALC is not used, then interconnects are made after
formation of the completed cells shown in FIG. 5M.
[0088] The plastic substrates are released from the glass substrate
by laser release from the plastic that is directly in contact with
the glass substrate.
[0089] Laser irradiation (XeCl) can be used through the glass
substrate. A thin layer, <1 .mu.m, is photo-ablated leaving
freestanding polymer films with good mechanical integrity.
[0090] The glass substrate may be cleaned before the wet casting
deposition process so that surface contaminants such as oils and
ions are removed prior to the coating process. Conventional
solvents can be used to perform the cleaning process.
[0091] The examples above relate to the manufacture of an active
matrix display device. In further aspects, the invention relates
more generally to the production of electronic devices comprising
thin-film circuitry on a plastic substrate. Thus, the invention
also applies more generally to forming thin-film electronic
elements over a plastic substrate supported by a rigid carrier
substrate, and then releasing the rigid carrier substrate from the
plastic substrate. These devices may, for example, comprise solar
cells, large area lighting panels, and flexible thin-film
electronic devices for applications in wearable or medical systems.
Again, substantially conventional substrate handling can thus be
used in the processing of a thin-film electronic device (for
example having TFTs) on a plastic substrate. The use of a wet cast
(e.g. spin-on) process gives a flat, high quality surface.
[0092] The wet cast (e.g. spin-on) process enables very thin
substrates to be formed. For example a substrate may be formed with
thickness as low as 3 .mu.m.
[0093] The examples of manufacturing method described in detail
above are in connection with an LCD display. However, it will be
apparent to those skilled in the art that there are many variations
to each of the steps described.
[0094] More generally, the invention provides firstly the
application of plastic to a substrate. The thickness of the plastic
layers will normally be in the thickness range 2 .mu.m to 50 .mu.m.
This plastic will ultimately become the plastic substrate of the
display, for display applications. Suitable wet casting processes
are spin coating, printing and spreading.
[0095] The substrate can be either a standard glass substrate or a
glass substrate coated with a blue-light absorbing layer. The
choice depends on the plastic used and laser release
properties.
[0096] A passivation layer will normally be desired, applied above
the plastic layer. Suitable layer types are silicon nitride or
silicon oxide deposited by plasma-enhanced chemical vapour
deposition (PECVD) or sputtering.
[0097] TFT arrays are then be made on the plastic/passivation
layer. The TFT array fabrication can be carried out under fairly
standard array processing conditions for a-Si or low-temperature
poly-Si (LTPS) TFTs. There may have to be some small process
changes to ensure that the deposited layers do not have high
mechanical stress. The use of standard glass substrates coated with
very thin layers of plastic and standard TFT array processing means
that this process can be used in existing TFT fabrication
plants.
[0098] TFTs can be used as the active device element for
multiplexing several different display types, not only the LCD
example above. Whatever the display type is, the display is
fabricated while the TFT array is still stuck onto the glass. This
means that standard display fabrication tools and techniques can be
used and the presence of a thin layer of plastic will not cause any
significant differences. The display drivers can also be bonded to
the display at this time.
[0099] The laser used to remove the plastic substrate from the
carrier is applied through the glass substrate to hit the bottom of
the plastic. The laser for this purpose will normally have to be
scanned to cover the complete area of the display. Pulsed excimer
lasers with wavelengths of 308 nm and 351 nm can be used.
[0100] The invention enables direct laser release of a clear
plastic substrate, and such a substrate can be used with all
display types, including transmissive and transflective LCDs and
downward emitting organic LEDs (OLEDs), such as a polymer LED.
[0101] As mentioned above, a liquid crystal display is only one
example of display technology which can benefit from the
invention.
[0102] Poly(p-phenylene biphenyltetracarboximide) is mentioned as a
preferred example of a material in one embodiment of the invention.
It will be recognized by the person skilled in the art that a
material displaying similar characteristics with large anisotropy
of coefficient of thermal expansion may also be used to achieve the
same result.
[0103] As an example of alternative display technology, FIG. 6
shows a reflective display device 200 using electrophoretic display
material. An example of this type of display is known as an E-Ink
display. An array of thin film transistors 202 is formed on a
plastics layer 204 on glass. The TFT array is provided over a
silicon nitride passivation layer 206 and a silicon nitride gate
insulator layer 208, and the ITO pixels 210 are formed over a
polymer passivation layer 212. The glass substrate is not shown in
FIG. 6, which shows the final removed display device.
[0104] A layer of electrophoretic material comprises capsules 214
and is laminated onto the TFT array. The capsules are responsive to
a local electric field across the ink foil layer. This layer is
inherently tacky, and is placed on the TFT array and heated, to
approximately 100 degrees Celsius, and rolled. The display modules
are finished with a ITO layer 216 and a plastic protective layer
218.
[0105] Driver chips mounted onto foils are then connected to
lead-in areas, and the laser release step is then carried out.
[0106] FIG. 7 shows an example polymer LED downwardly-emitting
display device 300. An array of encapsulated thin film transistor
circuits 302 are formed on a clear plastic (such as silicone, BCB
or parylene) substrate 304, and with transparent ITO pixel
electrodes 306 formed over a silicon nitride passivation layer
308.
[0107] A hydrophilic polymer wall 310 surrounds the pixel (although
this is not required for organic LEDs) which is defined by the
polymer (or organic) LED material 312. A metal cathode 314, such as
Ca, covers the structure, and is covered by a polymer passivation
layer 316.
[0108] The examples of FIGS. 6 and 7 are manufactured in accordance
with the invention, and it will be appreciated that numerous other
specific display designs, as well as other electronic component
designs, can be made using the approach explained above. For
example, suitable display types include OLED (organic LED), PLED
(polymer LED), EL (electroluminescent) and PDLC (polymer-dispersed
liquid crystal) displays, as well as LCDs.
[0109] Some examples of polyimide are given above. Other examples
are PMDA-PDA and BPDA-PDA. It is also possible to fluorinate high
anisotropy polyimides to increase their transparency. Other
examples of materials which are spincoatable are from the BCB
family (benzocyclobutane) or polybenzoxozole. The invention is thus
not limited to polyimides.
[0110] Various other modifications will be apparent to those
skilled in the art.
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