U.S. patent application number 11/567812 was filed with the patent office on 2007-04-26 for process for the fabrication of thin-film device and thin-film device.
Invention is credited to Akihiko Asano, Tomoatsu Kinoshita.
Application Number | 20070090404 11/567812 |
Document ID | / |
Family ID | 33113192 |
Filed Date | 2007-04-26 |
United States Patent
Application |
20070090404 |
Kind Code |
A1 |
Kinoshita; Tomoatsu ; et
al. |
April 26, 2007 |
PROCESS FOR THE FABRICATION OF THIN-FILM DEVICE AND THIN-FILM
DEVICE
Abstract
A thin-film device is fabricated by forming a protective layer
and a thin-film device layer one by one on a first substrate and
bonding a second substrate on the thin-film device layer via a
first adhesive layer or a coating layer and first adhesive layer,
removing the first substrate at least in a part thereof by etching
with a chemical solution, bonding the protective layer, which
covers the thin-film device layer on a side of the first substrate,
to a third substrate via a second adhesive layer, and removing the
second substrate. The protective layer is formed of at least two
layers having resistance to the chemical solution used upon removal
of the first substrate.
Inventors: |
Kinoshita; Tomoatsu;
(Kanagawa, JP) ; Asano; Akihiko; (Kanagawa,
JP) |
Correspondence
Address: |
David R. Metzger;SONNENSCHEIN NATH & ROSENTHAL
Wacker Drive Station, Sears Tower
P.O. Box 061080
Chicago
IL
60606-1080
US
|
Family ID: |
33113192 |
Appl. No.: |
11/567812 |
Filed: |
December 7, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10782658 |
Feb 19, 2004 |
7172708 |
|
|
11567812 |
Dec 7, 2006 |
|
|
|
Current U.S.
Class: |
257/211 ;
257/E21.413; 257/E29.278; 257/E29.295 |
Current CPC
Class: |
H01L 29/66757 20130101;
H01L 29/78603 20130101; H01L 27/3244 20130101; H01L 27/1266
20130101; H01L 29/78621 20130101; H01L 27/1214 20130101; G02F
1/13613 20210101 |
Class at
Publication: |
257/211 |
International
Class: |
H01L 27/10 20060101
H01L027/10 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2003 |
JP |
P2003-046774 |
Claims
1. A thin-film device formed by performing the following steps:
forming a protective layer and a thin-film device layer one by one
on a first substrate, and bonding a second substrate on said
thin-film device layer via at least a first adhesive layer,
removing said first substrate at least in a part thereof by etching
with a chemical solution, bonding said protective layer, which
covers said thin-film device layer on a side of said first
substrate, to a third substrate via a second adhesive layer, and
removing said second substrate; wherein said protective layer is
formed of at least two layers having resistance to said chemical
solution used upon removal of said first substrate.
2. The process for the fabrication of a thin-film device as claimed
in claim 1, wherein said second substrate is bonded on said
thin-film device layer via a coating layer and said first adhesive
layer.
3. A thin-film device formed by performing the following steps:
forming a protective layer and a thin-film device layer one by one
on a first substrate, and bonding a second substrate on said
thin-film device layer via at least a first adhesive layer,
separating said first substrate at least in a part thereof by
etching with a chemical solution, bonding said protective layer,
which covers said thin-film device layer on a side of said first
substrate, to a third substrate via a second adhesive layer, and
separating said second substrate; wherein said protective layer is
formed of at least two layers having resistance to said chemical
solution used upon separation of said first substrate.
4. The process for the fabrication of a thin-film device as claimed
in claim 3, wherein said second substrate is bonded on said
thin-film device layer via a coating layer and said first adhesive
layer.
Description
RELATED APPLICATION DATA
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/782,658, filed Feb. 19, 2004, the entirety of which is
incorporated herein by reference to the extent permitted by law.
The present application claims priority to Japanese patent
application No. 2003-046774 filed in the Japanese Patent Office on
Feb. 25, 2003, the entirety of which also is incorporated by
reference herein to the extent permitted by law.
BACKGROUND OF THE INVENTION
[0002] This invention relates to a fabrication process of a
thin-film device and also to a thin-film device, and specifically
to a process for fabricating a thin-film device, which is useful in
a liquid crystal display, organic electroluminescence display or
the like, by forming the thin-film device on an original substrate
having high heat resistance and then transferring it onto an
application substrate and also to the thin-film device obtained by
the fabrication process.
[0003] In recent years, thin-film devices are receiving a demand
for thickness reduction, weight reduction and solidness under the
influence of a move toward smaller equipment in which they are
used. A limitation is, however, imposed on substrates for use in
fabrication because thin-film devices are fabricated in a
high-temperature vacuum environment. Employed, for example, in a
liquid crystal display which makes use of thin-film transistors are
silica substrates capable of withstanding temperatures up to
1,000.degree. C. or glass substrates capable of withstanding
temperatures up to 500.degree. C. Thickness reductions of these
substrates have been studied but, insofar as silica substrates or
glass substrates are used, the substrates have to be reduced in
size to cope with a reduction in rigidness so that the productivity
is reduced. Further, a reduction in the thickness of a substrate
immediately leads to a significant reduction in solidness, thereby
developing a practical problem. As is appreciated from the
foregoing, there is a difference between the performance required
for an original substrate and the performance required upon
actually using the thin-film device. Attempts have also been made
to fabricate thin-film transistors directly on plastic substrates
which permit thickness reduction, weight reduction and solidness
improvement. These attempts, however, involve significant
difficulties from the standpoint of the maximum withstand
temperatures of the plastic substrates.
[0004] Investigations have, therefore, been made on techniques for
transferring a thin-film device, which has been formed on an
original substrate having a high maximum withstand temperature,
onto an application substrate. For this transfer, it is necessary
to separate only the thin-film device from the original substrate.
Certain methods have been proposed for this purpose, including
provision of a removable layer and subsequent etching of the
removable layer with a chemical solution to separate a thin-film
device layer and an original substrate from each other (see, for
example, PCT International Application No. WO02/084739, page 9 and
FIG. 2) and removal of an original substrate in its entirety by
etching (see, for example, ibid., page 9 and FIG. 1D).
[0005] For the removal of an original substrate with a chemical
solution subsequent to the formation of a thin-film device on the
original substrate, it is necessary to form a protective layer with
a thin film before the formation of the device layer such that the
chemical solution is prevented from penetrating to the device
layer. However, a thin-film layer formed by sputtering, vapor
deposition or CVD is generally accompanied by a problem that it
contains more or less pinholes and a chemical solution may
penetrate to a device layer through the pinholes to damage the
device layer with the chemical solution.
SUMMARY OF THE INVENTION
[0006] An object of the present invention is, therefore, to prevent
penetration of a chemical solution, which is used upon removal or
separation of an original substrate, to a device layer upon
transferring the device layer from the original substrate onto an
application substrate.
[0007] In one aspect of the present invention, there is thus
provided a process for the fabrication of a thin-film device. The
process includes the following steps: forming a protective layer
and a thin-film device layer one by one on a first substrate, and
bonding a second substrate on the thin-film device layer via at
least a first adhesive layer, removing the first substrate at least
in a part thereof by etching with a chemical solution, bonding the
protective layer, which covers the thin-film device layer on a side
of the first substrate, to a third substrate via a second adhesive
layer, and removing the second substrate. The protective layer is
formed of at least two layers having resistance to the chemical
solution used upon removal of the first substrate.
[0008] According to the above-described process, the protective
layer for protecting the device layer from the chemical solution to
be employed upon removal of the first substrate is formed of at
least two layers having resistance to the chemical solution.
Practically speaking, the protective layer is hence formed of two
or more layers. Even if the chemical solution reaches the inner
protective layer (second protective layer) through pinholes formed
in the protective layer (first protective layer) closer to the
chemical solution, any further penetration of the chemical solution
toward the device layer can be prevented by the second protective
layer. The pinholes in each of the two layers having resistance to
the chemical solution to be employed upon removal of the first
substrate are very small and occur at random. Accordingly, the
probability that the pinholes in one of the two layers and those in
the other layer would be formed at the same locations is extremely
low. The above-described anti-penetration measure makes use of this
extremely low probability. Let's assume, for example, that pinholes
are formed in the first protective layer having resistance to the
chemical solution to be used upon removal of the first substrate.
Even if the chemical solution penetrates through the first
protective layer, there is the second protective layer which has
resistance to the chemical solution employed upon removal of the
first substrate. The chemical solution is, therefore, blocked
there. With the above-described at least two layers having
resistance to the chemical solution to be used upon removal of the
first substrate, penetration of the chemical to the device layer
can be surely prevented. Accordingly, transfer-related defects can
be decreased. The process of the present invention, therefore,
makes it possible to fabricate thin-film devices of high quality
while assuring a high production yield.
[0009] In another aspect of the present invention, there is also
provided a thin-film device formed by performing the following
steps: forming a protective layer and a thin-film device layer one
by one on a first substrate, and bonding a second substrate on the
thin-film device layer via at least a first adhesive layer,
removing the first substrate at least in a part thereof by etching
with a chemical solution, bonding the protective layer, which
covers the thin-film device layer on a side of the first substrate,
to a third substrate via a second adhesive layer, and removing the
second substrate. The protective layer is formed of at least two
layers having resistance to the chemical solution used upon removal
of the first substrate.
[0010] In further aspect of the present invention, there is thus
provided a process for the fabrication of a thin-film device. The
process includes the following steps: forming a protective layer
and a thin-film device layer one by one on a first substrate, and
bonding a second substrate on the thin-film device layer via at
least a first adhesive layer, separating the first substrate at
least in a part thereof by etching with a chemical solution,
bonding the protective layer, which covers the thin-film device
layer on a side of the first substrate, to a third substrate via a
second adhesive layer, and separating the second substrate. The
protective layer is formed of at least two layers having resistance
to the chemical solution used upon separation of the first
substrate.
[0011] In yet further aspect of the present invention, there is
also provided a thin-film device formed by performing the following
steps: forming a protective layer and a thin-film device layer one
by one on a first substrate, and bonding a second substrate on the
thin-film device layer via at least a first adhesive layer,
separating the first substrate at least in a part thereof by
etching with a chemical solution, bonding the protective layer,
which covers the thin-film device layer on a side of the first
substrate, to a third substrate via a second adhesive layer, and
separating the second substrate. The protective layer is formed of
at least two layers having resistance to the chemical solution used
upon separation of the first substrate.
[0012] The thin-film device is free from the penetration of the
chemical solution, because it can bring about similar effects and
advantage as the above-described fabrication process. Thin-film
devices according to the present invention can, therefore, be
fabricated with a high production yield. Use of thin-film devices
makes it possible to provide liquid crystal displays or organic EL
displays of excellent quality.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIGS. 1A through 1E show a partial fabrication flowchart of
a process according to a first embodiment of the present invention
for the fabrication of a liquid crystal display;
[0014] FIG. 2 is a fragmentary schematic cross-sectional view of a
transmission active matrix substrate formed in the step illustrated
in FIG. 1A;
[0015] FIGS. 3A through 3D are fragmentary schematic
cross-sectional views depicting penetration of hydrofluoric acid
through pinholes in two protective layers in an etching step shown
in FIG. 1D;
[0016] FIGS. 4A through 4C show a partial fabrication flowchart of
the process according to the first embodiment of the present
invention, and illustrate steps after the step depicted in FIG. 1E,
that is, steps for transferring the transmission active matrix
substrate onto an application substrate made of plastics to obtain
an active substrate;
[0017] FIGS. 5A through 5E show a similar partial fabrication
flowchart as in FIGS. 1A through 1E, but illustrates a process
according to a second embodiment of the present invention for the
fabrication of a liquid crystal display;
[0018] FIG. 6 is a similar fragmentary schematic cross-sectional
view as in FIG. 2, but depicts a reflection active matrix substrate
formed in the step illustrated in FIG. 5A;
[0019] FIG. 7 is a fragmentary schematic cross-sectional view
depicting a positional relationship between pinholes in one of two
protective layers and those in the other protective layer in the
etching step illustrated in FIG. 5D;
[0020] FIGS. 8A through 8E show a similar partial fabrication
flowchart as in FIGS. 1A through 1E, but illustrates a process
according to a third embodiment of the present invention for the
fabrication of a liquid crystal display;
[0021] FIG. 9 is a similar fragmentary schematic cross-sectional
view as in FIG. 2, but depicts a thin-film device which is formed
in the step illustrated in FIG. 8A and includes from a TFT layer to
an organic EL layer;
[0022] FIGS. 10A and 10B are fragmentary schematic cross-sectional
views depicting penetration of a mixed acid through pinholes in two
protective layers in an etching step shown in FIG. 8D; and
[0023] FIGS. 11A through 11E show a partial fabrication flowchart
of a process according to a fourth embodiment of the present
invention for the fabrication of a liquid crystal display.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] With reference to FIGS. 1A through 4C, a description will be
made about a process according to a first embodiment of the present
invention for the fabrication of a thin-film device, specifically
about steps for forming a transmission active substrate on a
plastic substrate by a transfer method for the fabrication of a
liquid crystal display.
[0025] As illustrated in FIG. 1A, a glass substrate or silica
substrate of approximately 0.4 to 1.1 mm thickness is used as a
first substrate 101 which will serve as an original substrate. A
first anti-HF layer 102 having resistance to hydrogen fluoride and
hydrofluoric acid, a first insulating layer 103, a second anti-HF
layer 104 having resistance to hydrogen fluoride and hydrofluoric
acid, and a second insulating layer 105 are formed one by one from
the lowest layer on the first substrate 101 (for example, a glass
substrate of 0.7 mm thickness) to provide a protective layer 106.
The first anti-HF layer 102 is arranged by forming, for example, a
molybdenum (Mo) thin film to an illustrative thickness of 500 nm.
The first insulating layer 103 is provided by forming, for example,
a silicon oxide (SiO.sub.2) layer to an illustrative thickness of
100 nm. The second anti-HF layer 104 is arranged by forming, for
example, a molybdenum (Mo) thin film to an illustrative thickness
of 1,000 nm. The second insulating layer 105 is provided by
forming, for example, a silicon oxide (SiO.sub.2) layer to an
illustrative thickness of 500 nm. The molybdenum thin films can be
formed by sputtering, while the silicon oxide layers can be formed
by plasma CVD (chemical vapor deposition). It is to be noted that
the above-described "HF" means hydrogen fluoride or hydrofluoric
acid in this specification.
[0026] TFT is then formed as a thin-film device layer by a
low-temperature polysilicon bottom-gate thin-film transistor (TFT)
process such as that described, for example, in "'99 Latest Liquid
Crystal Process Technology", 53-59, Press Journal Inc., Tokyo, 1998
or "Flat Panel Displays 1999", 132-139, Nikkei Business
Publications, Inc., Tokyo, 1998.
[0027] As illustrated in FIG. 2, a gate electrode 107 is firstly
formed on the protective layer 106, for example, with a molybdenum
film the thickness of which is 100 nm, for example. This gate
electrode 107 can be formed by conventional photolithography and
etching techniques subsequent to the formation of the molybdenum
film. By plasma CVD, for example, a gate insulation film 108 is
formed with a silicon oxide (SiO.sub.2) layer or with a laminate
composed of a silicon oxide (SiO.sub.2) layer and a silicon nitride
(SiN.sub.x) layer such that the gate electrode 107 is covered by
the gate insulation film 108. In continuation with the gate
insulation film 108, an amorphous silicon layer (thickness: 30 nm
to 100 nm) is formed further. A pulsed XeCl excimer laser beam with
a wavelength of 308 nm is irradiated to the amorphous silicon layer
to melt and recrystallize it such that a polysilicon layer is
formed as a crystalline silicon layer. Using this polysilicon
layer, a polysilicon layer 109 is formed to provide a
channel-forming region. Formed on each side of the polysilicon
layer 109 are a polysilicon layer 110 composed of an n.sup.- dope
region and a polysilicon layer 111 composed of an n.sup.+ dope
region. As appreciated from the foregoing, each active region is
formed as an LDD (lightly doped drain) structure compatible with
both a high on-current and a low off-current. On the polysilicon
layer 109, a stopper layer 112 is formed, for example, with a
SiO.sub.2 layer to protect the channel upon implantation of n.sup.-
phosphorus ions.
[0028] By plasma CVD, a passivation film 113 is formed further with
a silicon oxide (SiO.sub.2) layer or with a laminate composed of a
silicon oxide (SiO.sub.2) layer and a silicon nitride (SiN.sub.x)
layer. On the passivation film 113, a source electrode 114 and
drain electrode 115 are formed, for example, with aluminum such
that these electrodes are connected to the individual polysilicon
layers 111.
[0029] To protect the device and also to effect planarization, a
planarization layer 116 is then formed, for example, with a
methyl-methacrylate-based resin, for example, by the spin coating
method such that the source electrode 114, the drain electrode 115
and the like are covered by the planarization layer 116. In the
planarization layer 116, a contact hole is formed extending to the
source electrode 111. By sputtering, for example, a transparent
conductive film (for example, indium tin oxide,
In.sub.2O.sub.3+SnO.sub.2; hereinafter called "ITO") is then formed
on the planarization layer 116 to provide a pixel electrode 117
such that the pixel electrode 117 is connected to the source
electrode 114 via the contact hole.
[0030] By the above-described steps, a transmission active matrix
substrate can be formed on the first substrate 101. A top-gate
polysilicon TFT or amorphous TFT can be formed likewise although
the bottom-gate polysilicon TFT was formed in the above
description. A description will next be made about steps for
transferring the thin-film device layer from the first substrate
101 onto a plastic substrate.
[0031] As described above with reference to FIG. 1A, an
intermediate construction has been obtained by forming the first
anti-HF layer 102, the first insulating layer 103, the second
anti-HF layer 104, the second insulating layer 105 and a thin-film
device layer 121 on the first substrate 101. Reference is now had
to FIG. 1B. While heating the intermediate construction at
80.degree. C. to 140.degree. C. on a hot plate 122, a first
adhesive layer 123 is formed by coating, for example, a hot melt
adhesive to an illustrative thickness of 1 mm or so.
[0032] As shown in FIG. 1C, a second substrate 124 is next mounted
on the first adhesive layer 123 and, while pressing the second
substrate 124 toward the first substrate 101, the resulting
intermediate construction is allowed to cool down to room
temperature. As the second substrate 124, a molybdenum (Mo)
substrate of 1 mm thickness can be used, for example. As an
alternative, it is possible to coat a hot melt adhesive on the
second substrate 124 and then to mount the first substrate 101, on
which the layers ranging from the first anti-HF layer 102 to the
thin-film device layer 121 have been formed, on the thus-coated hot
melt adhesive with the thin-film device layer 121 being directed
toward the second substrate 124.
[0033] Reference is next had to FIG. 1D. An intermediate
construction with the second substrate 124 bonded thereto via the
first adhesive layer 123 interposed therebetween is then dipped in
hydrofluoric acid (HF) 125 to perform etching of the first
substrate 101. Because the molybdenum layer as the first anti-HF
layer 102 is not etched with the hydrofluoric acid 125, the etching
automatically stops at the first anti-HF layer 102. Illustrative of
the hydrofluoric acid 105 employed here is one having a weight
concentration of 50%. With such an etchant, the etching time can be
set, for example, at 3.5 hours. The concentration of the
hydrofluoric acid 125 and the etching time with the hydrofluoric
acid 125 may be changed without any problem insofar as the glass of
the first substrate 101 can be fully etched off.
[0034] As a result of the above-described etching with the
hydrofluoric acid 125, the first substrate 101 (see, for example,
FIG. 1D already referred to in the above) is fully etched off so
that the first anti-HF layer 102 is exposed (see FIG. 1E).
[0035] When pinholes 132 are contained in the molybdenum layer as
the first anti-HF layer 102 as shown in FIG. 3A, the first
insulation film 103 made of silicon oxide is etched with
hydrofluoric acid at locations where the first insulating layer 103
is facing the pinholes 132. Specifically, the pinholes 132 are
formed extending through the first insulation film 103. There is,
however, an extremely low probability in the formation of the
pinholes 134 in the molybdenum layer as the second anti-HF layer
104 at the same locations as the pinholes 132 formed in the first
anti-HF layer 102. Hydrofluoric acid, therefore, does not penetrate
to the side of the thin-film device layer 121 beyond the second
anti-HF layer 104. If the second anti-HF layer 104 were not formed,
the hydrofluoric acid which has penetrated through the pinholes 132
would penetrate to the thin-film device layer 121 so that the
hydrofluoric acid would damage the thin-film device layer 121.
[0036] As illustrated in FIG. 3B, the molybdenum layer (thickness:
500 nm) as the first anti-HF layer 102 (see FIG. 3A) is then etched
off with a mixed acid [for example, phosphoric acid
(H.sub.3PO.sub.4) 72 wt %+nitric acid (HNO.sub.3) 3 wt %+acetic
acid (CH.sub.3COOH) 10 wt %]. It takes about 1 minute to etch off
the molybdenum layer of 500 nm thickness with the mixed acid. As
this mixed acid does not etch the silicon oxide as the first
insulating layer 103, the etching automatically stops at the first
insulating layer 103. At the locations corresponding to the
pinholes 132 (see FIG. 3A) formed in the first anti-HF layer 102,
however, the first insulating layer 103 has been etched with the
penetrated hydrofluoric acid to form pinholes, through which the
mixed acid is allowed to penetrate to the second anti-HF layer 104.
Accordingly, the pinholes 132 are each formed extending to an
intermediate height in the second anti-HF layer 104. As the second
anti-HF layer 104 is formed thicker than the first anti-HF layer
102, the etching of the second anti-HF layer 104 has, however, not
been completed even after the etching of the first anti-HF layer
102 was completed. When the mixed acid etching is terminated at
this stage, for example, by washing the intermediate construction
with water, the mixed acid cannot penetrate to the thin-film device
layer 121.
[0037] Reference is next had to FIG. 3C. With buffered hydrofluoric
acid [BHF: for example, ammonium fluoride (NH.sub.4F)
6%+hydrofluoric acid (HF) 1%+H.sub.2O 93%], the first insulating
layer 103 (see FIG. 3B) made of silicon oxide is etched off. This
buffered hydrofluoric acid requires about 1 minute to etch off the
silicon oxide (SiO.sub.2) when its thickness is, for example, 100
nm. As the buffered hydrofluoric acid does not etch the second
anti-HF layer 104 made of molybdenum, the etching automatically
stops at the second anti-HF layer 104. If the second anti-HF layer
104 contains pinholes 134, however, the buffered hydrofluoric acid
still penetrates toward the thin-film device layer 121 through the
pinholes 134 even after completion of the etching of the first
insulating layer 103 (see FIG. 3B). As the thickness of the second
insulating layer 105 is 500 nm which is greater than that of the
first insulating layer 103, the buffered hydrofluoric acid,
however, cannot penetrate the second insulating layer 105 beyond
its intermediate height so that the buffered hydrofluoric does not
reach as far as the thin-film device layer 121.
[0038] As shown in FIG. 3D, the second anti-HF layer 104 (see FIG.
3C) formed of the molybdenum layer of 1,000 nm thickness is next
etched off with a similar mixed acid as that described above. This
mixed acid requires about 2 minutes to etch off the molybdenum
layer of 1,000 nm thickness. As this mixed acid does not etch the
second insulating layer 105 made of silicon oxide (SiO.sub.2), the
etching with the mixed acid automatically stops at the second
insulating layer 105. The mixed acid does not reach as far as the
thin-film device layer 121, because the pinholes 134 formed in the
second insulating layer 105 with the buffered hydrofluoric acid
have not reached as far as the thin-film device layer 121.
[0039] As the second insulating layer 105 is made of silicon oxide
which is an insulating material transparent to visible light, it is
unnecessary to specifically remove the second insulating layer 105.
Removal of the second insulating layer 105 is, therefore, not
conducted in the fabrication process according to the first
embodiment of the present invention.
[0040] In the above-described first embodiment, molybdenum is used
as anti-HF layers for both the first and second anti-HF layers 102,
104. However, any material can be used insofar as it is equipped
with resistance to HF, including tungsten, amorphous silicon,
polycrystalline silicon, aluminum oxide (Al.sub.2O.sub.3),
magnesium fluoride (MgF.sub.2), calcium fluoride (CaF.sub.2),
silicon carbide (SiC), amorphous diamond, benzocyclobutene-based
resins, and polyimides. When such a material is used for the first
and second anti-HF layers 102, 104, it is necessary to conduct wet
etching with such chemical solutions as enabling removal of the
respective layers or dry etching with such etching gases as
permitting removal of the respective layers. When polycrystalline
silicon is used, for example, the first and second anti-HF layers
102, 104 can be removed by dry etching such as reactive ion etching
(RIE) which makes use of an etching gas composed in combination of
tetrafluoromethane (CF.sub.4) and oxygen (O.sub.2). When the first
anti-HF layer 102 and second anti-HF layer 104 are etched by using
the same chemical in common, the etching must be conducted such
that the etching of the first anti-HF layer 102 is completed
earlier. This is to avoid etching of the second anti-HF layer 104
in its entirety during etching of the first anti-HF layer 102 when
a chemical solution penetrates to the second anti-HF layer 104
through the pinholes 132 in the first anti-HF layer 102 upon
etching the first anti-HF layer 102 with the chemical solution.
[0041] As illustrated in FIG. 4A, a second adhesive layer 127 is
formed on the side of the back side of the thin-film device layer
121, in other words, on the surface of the second insulating layer
105 after the above-described etching by coating, for example, an
UV curable adhesive, for example, by a spin coating technique.
[0042] Referring next to FIG. 4B, a third substrate 128 is bonded
to the second adhesive layer 127 subsequent to the formation of the
second layer 127. As this third substrate 128, a plastic substrate
is used. By using, for example, a polycarbonate film of 0.2 mm
thickness as the plastic substrate and exposing it to ultraviolet
rays, the second adhesive layer 127 made of the UV curable adhesive
is cured. Although the polycarbonate substrate is used as the
plastic substrate in the above-described first embodiment, the
plastic substrate is not limited to such a polycarbonate substrate
and instead, other plastic substrates may also be used.
[0043] Reference is next had to FIG. 4C. The intermediate
construction prepared as described above is dipped in an alcohol
(not shown) to remove the second substrate 124 by dissolving the
first adhesive layer 123 made of the hot melt adhesive. As a
result, an active substrate is obtained with the thin-film device
layer 121 mounted over the third adhesive layer 128 with the second
adhesive layer 127 and second insulating layer 105 interposed
therebetween.
[0044] Although not shown in any drawing, the active substrate is
subsequently combined with an opposite substrate, and into openings
formed between the substrates, a liquid crystal is injected to
provide a liquid crystal cell. This work can be performed by
conventional assembling steps for liquid crystal cells.
[0045] In the above-described fabrication process of the thin-film
device, the etch rate of the first insulating layer 103 may
desirably be lower than that of the first substrate 101 when the
first insulating layer 103 is removed with the same chemical
solution as the chemical solution (hydrofluoric acid) employed upon
removal of the first substrate. When the first and second anti-HF
layers 102, 104 are made of a material or materials commonly
etchable with the same chemical solution (for example, the
above-described mixed acid) and are etched with the same chemical
solution (for example, the above-described mixed acid), it is
desired that the etching of the first anti-HF layer 102 located
closer to the first substrate 101 is completed earlier than that of
the second anti-HF layer 104. In the above-described first
embodiment, the first anti-HF layer 102 is formed thinner than the
second anti-HF layer 104.
[0046] In the above-described fabrication process of the thin-film
device, the protective layer 106 which serves to protect the
thin-film device layer 121 from the chemical solution is formed of
at least two layers having resistance to hydrofluoric acid used
upon removal of the first substrate 101, specifically the first and
second anti-HF layers 102, 104 made of molybdenum. Practically
speaking, the protective layer is hence arranged as two or more
layers. Even if hydrofluoric acid reaches the inner protective
layer (second anti-HF layer 104) through the pinholes 132 formed in
the outer protective layer (first anti-HF layer 102) located closer
to a side where the intermediate construction is maintained in
contact with hydrofluoric acid, it is still possible to prevent at
the second anti-HF layer 104 any further penetration of
hydrofluoric acid to the side of the thin-film device layer 121.
The pinholes 132, 134 in the first and second anti-HF layers 102,
104, which have resistance to hydrofluoric acid to be employed upon
removal of the first substrate 101, are very small and occur at
random. Accordingly, the probability that the pinholes 132 and 134
would be formed at the same locations is extremely low. The
above-described anti-penetration measure makes use of this
extremely low probability. Let's assume, for example, that pinholes
132,134 are formed in the first and second anti-HF layers 102, 104
having resistance to hydrofluoric acid to be used upon removal of
the first substrate 101. Even if the hydrofluoric acid penetrates
through the first anti-HF layer 102, there is the second anti-HF
layer 104 as the second layer. The hydrofluoric acid is, therefore,
blocked there. With the first and second anti-HF layers 102, 104
having resistance to hydrofluoric acid, penetration of hydrofluoric
acid to the thin-film device layer 121 can be surely prevented.
Accordingly, transfer-related defects can be decreased.
[0047] With reference to FIGS. 5A through 7, a description will
next be made about a process according to a second embodiment of
the present invention for the fabrication of a thin-film device,
specifically about steps for forming a reflection active substrate
on a plastic substrate by a transfer method for the fabrication of
a liquid crystal display.
[0048] As illustrated in FIG. 5A, a glass substrate or silica
substrate of approximately 0.4 to 1.1 mm thickness is used as a
first substrate 201 which will serve as an original substrate. A
first anti-HF layer 202 having resistance to hydrogen fluoride and
hydrofluoric acid, a second anti-HF layer 203 having resistance to
hydrogen fluoride and hydrofluoric acid, and a first insulating
layer 204 are formed one by one from the lowest layer on the first
substrate 201 (for example, a glass substrate of 0.7 mm thickness)
to provide a protective layer 205. The first anti-HF layer 202 is
arranged by forming, for example, a molybdenum (Mo) thin film to an
illustrative thickness of 500 nm. The second anti-HF layer 203 is
arranged by forming, for example, an amorphous silicon (a-Si) layer
to an illustrative thickness of 200 nm. The first insulating layer
204 is provided by forming, for example, a silicon oxide
(SiO.sub.2) layer to an illustrative thickness of 500 nm. The
molybdenum thin film can be formed by sputtering, while the
amorphous silicon layer and silicon oxide layer can be formed by
plasma CVD (chemical vapor deposition).
[0049] As illustrated in FIG. 6, a thin-film transistor (TFT) is
then formed by a low-temperature polysilicon bottom-gate thin-film
transistor (TFT) process in a similar manner as in the first
embodiment. The second embodiment is different from the first
embodiment in that, because the thin-film device is the reflection
active substrate useful for the fabrication of the liquid crystal
display, a pixel electrode 216 uses silver (Ag) instead of ITO and
this silver (Ag) is used as a reflector. The second embodiment is
also different from the first embodiment in that upon formation of
a planarization layer 215 with a methyl-methacrylate-based resin,
the planarization layer is provided with ruggedness to also provide
silver (Ag), which is to be formed over the planarization layer,
with ruggedness so that regular reflection of light can be
prevented.
[0050] Described specifically, a gate electrode 206 is formed on
the protective layer 205, for example, with a molybdenum film the
thickness of which is 100 nm, for example. This gate electrode 206
can be formed by conventional photolithography and etching
techniques. By plasma CVD, for example, a gate insulation film 207
is formed with a silicon oxide (SiO.sub.2) layer or with a laminate
composed of a silicon oxide (SiO.sub.2) layer and a silicon nitride
(SiN.sub.x) layer such that the gate electrode 206 is covered by
the gate insulation film 207. In continuation with the gate
insulation film 207, an amorphous silicon layer (thickness: 30 nm
to 100 nm) is formed further. A pulsed XeCl excimer laser beam with
a wavelength of 308 nm is irradiated to the amorphous silicon layer
to melt and recrystallize it such that a polysilicon layer is
formed as a crystalline silicon layer. Using this polysilicon
layer, a polysilicon layer 208 is formed to provide a
channel-forming region. Formed on each side of the polysilicon
layer 208 are polysilicon layers 209 composed of an n.sup.- dope
region and a polysilicon layer 210 composed of an n.sup.+ dope
region. As appreciated from the foregoing, each active region is
formed as an LDD (lightly doped drain) structure compatible with
both a high on-current and a low off-current. On the polysilicon
layer 208, a stopper layer 211 is formed, for example, with a
silicon oxide (SiO.sub.2) layer to protect the channel upon
implantation of n.sup.- phosphorus ions.
[0051] By plasma CVD, a passivation film 212 is formed further with
a silicon oxide (SiO.sub.2) layer or with a laminate composed of a
silicon oxide (SiO.sub.2) layer and a silicon nitride (SiN.sub.x)
layer. On the passivation film 212, a source electrode 213 and
drain electrode 214 are formed, for example, with aluminum such
that these electrodes are connected to the individual polysilicon
layers 210.
[0052] To protect the device and also to effect planarization, a
planarization layer 215 is then formed, for example, with a
methyl-methacrylate-based resin, for example, by the spin coating
method such that the source electrode 213, the drain electrode 214
and the like are covered by the planarization layer 215. To provide
a pixel electrode, which is to be formed over the planarization
layer 215, with ruggedness, the planarization layer 215 is provided
at a surface thereof with ruggedness. In the planarization layer
215, a contact hole is also formed extending to the source
electrode 213. By forming, for example, silver (Ag) into a film by
sputtering, for example, a pixel electrode 216 is then formed on
the planarization layer 215 such that the pixel electrode 215 is
connected to the source electrode 213 via the contact hole.
[0053] By the above-described steps, a reflection active matrix
substrate can be formed on the first substrate 201. A description
will next be made about steps for transferring the thin-film device
layer from the first substrate 201 onto a plastic substrate.
[0054] As described above with reference to FIG. 5A, an
intermediate construction has been obtained by forming the first
anti-HF layer 202, the second anti-HF layer 203, the first
insulating layer 204 and a thin-film device layer 221 on the first
substrate 201. Reference is now had to FIG. 5B. While heating the
intermediate construction at 80.degree. C. to 140.degree. C. on a
hot plate 222, a first adhesive layer 223 is formed by coating, for
example, a hot melt adhesive to an illustrative thickness of 1 mm
or so.
[0055] As shown in FIG. 5C, a second substrate 224 is next mounted
on the first adhesive layer 223 and, while pressing the second
substrate 224 toward the first substrate 201, the resulting
intermediate construction is allowed to cool down to room
temperature. As the second substrate 224, a molybdenum (Mo)
substrate of 1 mm thickness can be used, for example. As an
alternative, it is possible to coat a hot melt adhesive on the
second substrate 224 and then to mount the first substrate 201, on
which the layers ranging from the first anti-HF layer 202 to the
thin-film device layer 221 have been formed, on the thus-coated hot
melt adhesive with the thin-film device layer 221 being directed
toward the second substrate 224.
[0056] Reference is next had to FIG. 5D. An intermediate
construction with the second substrate 224 bonded thereto via the
first adhesive layer 223 is then dipped in hydrofluoric acid 225 to
perform etching of the first substrate 201. Because the molybdenum
layer as the first anti-HF layer 202 is not etched with the
hydrofluoric acid 225, the etching automatically stops at the first
anti-HF layer 202. Illustrative of the hydrofluoric acid 225
employed here is one having a weight concentration of 50%. With
such an etchant, the etching time can be set, for example, at 3.5
hours. The concentration of the hydrofluoric acid 225 and the
etching time with the hydrofluoric acid 225 may be changed without
any problem insofar as the glass of the first substrate 201 can be
fully etched off.
[0057] As a result of the above-described etching with the
hydrofluoric acid 225, the first substrate 201 (see FIG. 5D) is
fully etched off so that the first anti-HF layer 202 is exposed
(see FIG. 5E).
[0058] When pinholes 232 are contained in the molybdenum layer as
the first anti-HF layer 202 as shown in FIG. 7, hydrofluoric acid
tries to penetrate further to the second anti-HF layer 203 through
the pinholes 232. There is, however, an extremely low probability
in the formation of the pinholes 233 in the second anti-HF layer
203 at the same locations as the pinholes 332 formed in the first
anti-HF layer 202. Hydrofluoric acid, therefore, does not penetrate
to the side of the thin-film device layer 221 beyond the second
anti-HF layer 203. If the second anti-HF layer 203 were not formed,
the hydrofluoric acid which has penetrated through the pinholes 232
would penetrate to the thin-film device layer 221 so that the
hydrofluoric acid would damage the thin-film device layer 221.
[0059] As the thin-film device fabricated by the process according
to the second embodiment is for use in a reflection liquid crystal
display, no problem arises in display performance even if three is
the opaque layer underneath the thin-film device layer 221.
Therefore, the active substrate is subsequently combined with an
opposite substrate, and between the substrates, a liquid crystal is
injected to provide a liquid crystal cell. This work can be
performed by conventional assembling steps for liquid crystal
cells.
[0060] In the above-described process according to the second
embodiment for the fabrication of the thin-film device, the first
anti-HF layer 202 and second anti-HF layer 203 may desirably be
made of materials at least one of which is not etchable with any
common chemical solution (the mixed acid).
[0061] In the above-described process according to the second
embodiment for the fabrication of the thin-film device, the
protective layer 205 which serves to protect the thin-film device
layer 221 from the chemical solution is formed of at least two
layers having resistance to hydrofluoric acid used upon removal of
the first substrate 201, specifically the first anti-HF layer 202
made of molybdenum and the second anti-HF layer 203 made of
amorphous silicon. Practically speaking, the protective layer is
hence arranged as two or more layers. Even if hydrofluoric acid
reaches the inner protective layer (second anti-HF layer 203)
through the pinholes 232 formed in the outer protective layer
(first anti-HF layer 202) located closer to a side where the
intermediate construction is maintained in contact with
hydrofluoric acid, it is still possible to prevent at the second
anti-HF layer 203 any further penetration of hydrofluoric acid to
the side of the thin-film device layer 221. The pinholes 232, 233
in the first and second anti-HF layers 202,203, which have
resistance to HF to be employed upon removal of the first substrate
201, are very small and occur at random. Accordingly, the
probability that the pinholes 232 and 233 would be formed at the
same locations is extremely low. The above-described
anti-penetration measure makes use of this extremely low
probability. Let's assume, for example, that pinholes 232, 233 are
formed in the first and second anti-HF layers 202,203 having
resistance to hydrofluoric acid to be used upon removal of the
first substrate 201. Even if the hydrofluoric acid penetrates
through the first anti-HF layer 202, there is the second anti-HF
layer 203 as the second layer. The hydrofluoric acid is, therefore,
blocked there. With the first and second anti-HF layers 202,203
having resistance to hydrofluoric acid, penetration of hydrofluoric
acid to the thin-film device layer 221 can be surely prevented.
Accordingly, transfer-related defects can be decreased.
[0062] With reference to FIGS. 8A through 10B, a description will
next be made about a process according to a third embodiment of the
present invention for the fabrication of a thin-film device,
specifically about steps for forming an active matrix substrate on
a plastic substrate by a transfer method for the fabrication of an
active-matrix-type organic electroluminescence (EL) display.
[0063] As illustrated in FIG. 8A, a glass substrate or silica
substrate of approximately 0.4 to 1.1 mm thickness is used as a
first substrate 301 which will serve as an original substrate. A
first anti-HF layer 302 having resistance to hydrogen fluoride and
hydrofluoric acid, a second anti-HF layer 303 having resistance to
hydrogen fluoride and hydrofluoric acid, and a first insulating
layer 304 are formed one by one from the lowest layer on the first
substrate 301 (for example, a glass substrate of 0.7 mm thickness)
to provide a protective layer 305. The first anti-HF layer 302 is
arranged by forming, for example, a molybdenum (Mo) film to an
illustrative thickness of 500 nm. The second anti-HF layer 303 is
arranged by forming, for example, aluminum oxide (Al.sub.2O.sub.3)
film to an illustrative thickness of 200 nm. The first insulating
layer 304 is provided by forming, for example, a silicon oxide
(SiO.sub.2) layer to a thickness of 500 nm. The molybdenum film and
aluminum oxide film can be formed by sputtering, while the silicon
oxide layer can be formed by plasma CVD (chemical vapor
deposition).
[0064] A thin-film transistor (TFT) is then formed as a thin-film
device layer by a low-temperature polysilicon bottom-gate thin-film
transistor (TFT) process such as that described, for example, in
"'99 Latest Liquid Crystal Process Technology", 53-59, Press
Journal Inc., Tokyo, 1998 or "Flat Panel Displays 1999", 132-139,
Nikkei Business Publications, Inc., Tokyo, 1998.
[0065] Described specifically, as shown in FIG. 9, a gate electrode
306 is formed on the protective layer 305, for example, with a
molybdenum film. This gate electrode 306 can be formed by
conventional photolithography and etching techniques. A gate
insulation film 307 is formed, for example, with a silicon oxide
(SiO.sub.2) layer or with a laminate composed of a silicon oxide
(SiO.sub.2) layer and a silicon nitride (SiN.sub.x) layer such that
the gate electrode 306 is covered by the gate insulation film 307.
In continuation with the gate insulation film 307, an amorphous
silicon layer (thickness: 30 nm to 100 nm) is formed further. A
pulsed XeCl excimer laser beam with a wavelength of 308 nm is
irradiated to the amorphous silicon layer to melt and recrystallize
it such that a polysilicon layer is formed as a crystalline silicon
layer. Using this polysilicon layer, a polysilicon layer 308 is
formed to provide a channel-forming region. Formed on each side of
the polysilicon layer 308 are polysilicon layers 309 composed of an
n.sup.- dope region and a polysilicon layer 310 composed of an
n.sup.+ dope region. As appreciated from the foregoing, each active
region is formed as an LDD (lightly doped drain) structure
compatible with both a high on-current and a low off-current. On
the polysilicon layer 308, a stopper layer 311 is formed, for
example, with a silicon oxide (SiO.sub.2) layer to protect the
channel upon implantation of n.sup.- phosphorus ions.
[0066] By plasma CVD, a passivation film 312 is formed further with
a silicon oxide (SiO.sub.2) layer or with a laminate composed of a
silicon oxide (SiO.sub.2) layer and a silicon nitride (SiN.sub.x)
layer. On the passivation film 312, a source electrode 313 and
drain electrode 314 are formed, for example, with aluminum such
that these electrodes are connected to the individual polysilicon
layers 310.
[0067] A protective insulation layer 315 is then formed, for
example, with a methyl-methacrylate-based resin, for example, by
the spin coating method such that the source electrode 313, the
drain electrode 314 and the like are covered by the protective
insulation layer 315. To permit connection of the source electrode
313 with an anode of an organic EL element to be formed
subsequently, the protective insulation layer 315 is then removed
by a conventional photolithographic technique and etching technique
at a portion thereof located adjacent the source electrode 313 and
the anode.
[0068] On the protective insulation layer 315, an organic EL
element is then formed. The organic EL element is composed of an
anode 316, an organic layer, and a cathode 319. As the anode 316,
an aluminum (Al) film is formed, for example, by sputtering such
that the aluminum (Al) film is connected to the source electrodes
313 of individual TFTs to permit feeding of a current independently
to the source electrodes 313.
[0069] The organic layer has a structure in the form of a laminate
of an organic hole transport layer 317 and an organic emitter layer
318. The organic hole transport layer 317 can be formed, for
example, to a thickness of 30 nm by vapor deposition of copper
phthalocyanine. As the organic emitter layer 318, Alq3
[tris(8-hydroxyquinolinolato)aluminum(III)] can be applied as a
green color to a thickness of 50 nm, Bathocuproine
(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) as a blue color to
a thickness of 14 nm, and BSB-BCN
[2,5-bis{4-(N-methoxyphenyl)-N-phenylamino]styryl}benzene-1,4-dicarbonitr-
ile] as a red color to a thickness of 30 nm, all by vapor
deposition.
[0070] As the cathode 319, ITO can be used.
[0071] In the above-described fabrication process according to the
third embodiment, the organic EL element adopted the
above-described structure. As the electrodes, however, it is
possible to use conventionally known structures making combined use
of electron transport layers, hole transport layers, electron
injection layers, hole injection layers, electron blocking layers,
hole blocking layers and emitter layers.
[0072] Further, a passivation film 320 is formed to cover the
cathode electrode 319. In this embodiment, a silicon nitride
(Si.sub.3N.sub.4) film can be formed as the passivation film 320,
for example, to a thickness of 300 nm by sputtering. As an
alternative, the passivation film 320 may also be formed by CVD,
vapor deposition or the like.
[0073] The construction ranging from the TFT layer to the organic
EL layer will hereinafter be called "the thin-film device layer". A
description will next be made about steps for transferring the
thin-film device layer from the first substrate 301 onto a plastic
substrate.
[0074] As described above with reference to FIG. 8A, an
intermediate construction has been obtained by forming the first
anti-HF layer 302, the second anti-HF layer 303, the first
insulating layer 304 and the thin-film device layer 321 on the
first substrate 301. Reference is now had to FIG. 8B. While heating
the intermediate construction at 80.degree. C. to 140.degree. C. on
a hot plate 322, a first adhesive layer 323 is formed by coating,
for example, a hot melt adhesive to an illustrative thickness of 1
mm or so.
[0075] As shown in FIG. 8C, a second substrate 324 is next mounted
on the first adhesive layer 323 and, while pressing the second
substrate 324 toward the first substrate 301, the resulting
intermediate construction is allowed to cool down to room
temperature. As the second substrate 324, a molybdenum (Mo)
substrate of 1 mm thickness can be used, for example. As an
alternative, it is possible to coat a hot melt adhesive on the
second substrate 324 and then to mount the first substrate 301, on
which the layers ranging from the first anti-HF layer 302 to the
thin-film device layer 321 have been formed, on the thus-coated hot
melt adhesive with the thin-film device layer 321 being directed
toward the second substrate 324.
[0076] Reference is next had to FIG. 8D. An intermediate
construction with the second substrate 324 bonded thereto via the
first adhesive layer 323 is then dipped in hydrofluoric acid 325 to
perform etching of the first substrate 301. Because the molybdenum
layer as the first anti-HF layer 302 is not etched with the
hydrofluoric acid 325, the etching automatically stops at the first
anti-HF layer 302. Illustrative of the hydrofluoric acid 325
employed here is one having a weight concentration of 50%. With
such an etchant, the etching time can be set, for example, at 3.5
hours. The concentration of the hydrofluoric acid 325 and the
etching time with the hydrofluoric acid 325 may be changed without
any problem insofar as the glass of the first substrate 301 can be
fully etched off.
[0077] As a result of the above-described etching with the
hydrofluoric acid 325, the first substrate 301 (see FIG. 8D) is
fully etched off so that the first anti-HF layer 302 is exposed
(see FIG. 8E).
[0078] When pinholes 332 are contained in the molybdenum layer as
the first anti-HF layer 302 as shown in FIG. 10A, hydrofluoric acid
tries to penetrate further to the second anti-HF layer 303 through
the pinholes 332. There is, however, an extremely low probability
in the formation of the pinholes 333 in the second anti-HF layer
303 at the same locations as the pinholes 232 formed in the first
anti-HF layer 302. Hydrofluoric acid, therefore, does not penetrate
to the side of the thin-film device layer 321 beyond the second
anti-HF layer 303. If the second anti-HF layer 303 were not formed,
the hydrofluoric acid which has penetrated through the pinholes 332
would penetrate to the thin-film device layer 321 so that the
hydrofluoric acid would damage the thin-film device layer 321.
[0079] As illustrated in FIG. 10B, the molybdenum (thickness: 300
nm) as the first anti-HF layer 302 (see FIG. 10A) is then etched
off with a mixed acid [for example, phosphoric acid
(H.sub.3PO.sub.4) 72 wt %+nitric acid (HNO.sub.3) 3 wt %+acetic
acid (CH.sub.3COOH) 10 wt %]. It takes about 1 minute to etch off
the molybdenum layer of 500 nm thickness with the mixed acid. As
this mixed acid does not etch the silicon oxide as the second
anti-HF layer 303, the etching automatically stops at the second
anti-HF layer 303.
[0080] In the above-described third embodiment, the first anti-HF
layer 302 was removed by using a solvent (the mixed acid). It may
be removed by dry etching. Molybdenum as the first anti-HF layer
302 can be removed by dry etching while using a gas composed in
combination of sulfur hexafluoride (SF.sub.6) and oxygen (O.sub.2)
(for example, by reactive ion etching). Under this condition, the
aluminum oxide layer as the second anti-HF layer 303 is not etched,
so that the etching automatically stops at the second anti-HF layer
303.
[0081] Further, the second anti-HF layer 303 and the first
insulating layer 304 are both transparent to visible light and are
both insulating layers. Their removal is not needed, accordingly.
In the above-described third embodiment, the second anti-HF layer
303 and the first insulating layer 304 were not removed.
[0082] Compared with the second embodiment, the third embodiment
has a merit that owing to the formation of the second anti-HF layer
303 with the transparent material, its etching can be obviated to
result in the fewer etching steps.
[0083] The first anti-HF layer 302 was formed with molybdenum, but
it can be formed with any appropriate material other than the
material of the second anti-HF layer 303. Usable examples include
tungsten, amorphous silicon, polycrystalline silicon, aluminum
oxide, magnesium fluoride (MgF.sub.2), calcium fluoride
(CaF.sub.2), silicon carbide (SiC), amorphous diamond,
benzocyclobutene-based resins, polyimides and the like. For the
second anti-HF layer 303, on the other hand, aluminum oxide was
used. It is, however, possible to use a transparent material such
as magnesium fluoride (MgF.sub.2), calcium fluoride (CaF.sub.2) or
a transparent polyimide. There is, however, one condition in this
respect, that is, the first anti-HF layer 302 and the second
anti-HF layer 303 are not etched by the same chemical solution.
[0084] The subsequent steps ranging from the combination with a
plastic substrate to the removal of the hot melt adhesive, can be
conducted in a similar manner as in the above-described, first
embodiment or second embodiment. Subsequent to the formation of the
thin-film device layer on the plastic substrate through the
above-described steps, conventional fabrication steps for organic
EL elements can be conducted to provide a display.
[0085] In the above-described process according to the third
embodiment for the fabrication of the thin-film device, the
protective layer 305 which serves to protect the thin-film device
layer 321 from the chemical solution is formed of at least two
layers having resistance to hydrofluoric acid used upon removal of
the first substrate 301, specifically the first anti-HF layer 302
made of molybdenum and the second anti-HF layer 303 made of
aluminum oxide. Practically speaking, the protective layer is hence
arranged as two or more layers. Even if hydrofluoric acid reaches
the inner protective layer (second anti-HF layer 303) through the
pinholes 332 formed in the outer protective layer (first anti-HF
layer 302) located closer to a side where the intermediate
construction is maintained in contact with hydrofluoric acid, it is
still possible to prevent at the second anti-HF layer 303 any
further penetration of hydrofluoric acid to the side of the
thin-film device layer 321. The pinholes 332,333 in the first and
second anti-HF layers 302,303, which have resistance to
hydrofluoric acid to be employed upon removal of the first
substrate 301, are very small and occur at random. Accordingly, the
probability that the pinholes 332 and 333 would be formed at the
same locations is extremely low. The above-described
anti-penetration measure makes use of this extremely low
probability. Let's assume, for example, that pinholes 332,333 are
formed in the first and second anti-HF layers 302,303 having
resistance to hydrofluoric acid to be used upon removal of the
first substrate 301. Even if the hydrofluoric acid penetrates
through the first anti-HF layer 302, there is the second anti-HF
layer 303 as the second layer. The hydrofluoric acid is, therefore,
blocked there. With the first and second anti-HF layers 302,303
having resistance to hydrofluoric acid, penetration of hydrofluoric
acid to the thin-film device layer 321 can be surely prevented.
Accordingly, transfer-related defects can be decreased.
[0086] With reference to the fabrication flowchart shown in FIGS.
11A through 11E, a description will next be made about a process
according to a fourth embodiment of the present invention for the
fabrication of a thin-film device, specifically about steps for
forming a transmission active substrate on a plastic substrate by a
transfer method for the fabrication of a liquid crystal
display.
[0087] As illustrated in FIG. 11A, a glass substrate or silica
substrate of approximately 0.4 to 1.1 mm thickness is used as a
first substrate 401 which will serve as an original substrate. By
sputtering, for example, an isolation layer 402 is then formed on
the first substrate 401 (for example, a glass substrate of 0.7 mm
thickness). This isolation layer 402 can be formed by depositing,
for example, a molybdenum (Mo) film to a thickness of 1,000 nm. A
first anti-HNO.sub.3 layer 403 and a second anti-HNO.sub.3 layer
404 are then formed one by one from the lower layer on the
isolation layer 402. The first and second anti-HNO.sub.3 layers
403,403 have resistance to nitric, acid will serve as a protective
layer 405 upon removal of the isolation layer 402 with nitric acid.
The first anti-HNO.sub.3 layer 403 is arranged, for example, by
forming an aluminum oxide (Al.sub.2O.sub.3) film to an illustrative
thickness of 200 nm. The second anti-HNO.sub.3 layer 404 is
provided by forming, for example, a silicon oxide (SiO.sub.2) film
to an illustrative thickness of 500 nm. The molybdenum film and the
aluminum oxide film can be formed by sputtering, while the silicon
oxide layer can be formed by plasma CVD (chemical vapor
deposition).
[0088] As illustrated in FIG. 11B, a thin-film device layer 421
with a thin-film transistor (TFT) included therein is then formed
on the protective layer 405 by a low-temperature polysilicon
bottom-gate thin-film transistor (TFT) process in a similar manner
as in the first embodiment. The fourth embodiment is different from
the first embodiment in that the thin-film device layer 421 does
not extend to the end faces of the substrate and the end faces of
the thin-film device layer 421 are covered by an upper protective
layer 407 made of a methyl-methacrylate-based resin.
[0089] Reference is now had to FIG. 11A. In a similar manner as in
the first embodiment, an intermediate construction has been
obtained by forming the isolation layer 402, the protective layer
405, the thin-film device layer 421 and the upper protective layer
407 on the first substrate 401. While heating the intermediate
construction at 80.degree. C. to 140.degree. C. on a hot plate (not
shown), a first adhesive layer 408 is formed by coating, for
example, a hot melt adhesive to an illustrative thickness of 1 mm
or so. As illustrated in FIG. 11C, a second substrate 409 is next
mounted on the first adhesive layer 408 and, while pressing the
second substrate 409 toward the first substrate 401, the resulting
intermediate construction is allowed to cool down to room
temperature. As the second substrate 409, a molybdenum (Mo)
substrate of 1 mm thickness can be used, for example. As an
alternative, it is possible to coat a hot melt adhesive on the
second substrate 409 and then to mount the first substrate 401, on
which the layers ranging from the isolation 402 to the upper
protective layer 407 have been formed, on the thus-coated hot melt
adhesive with the upper protective layer 407 being directed toward
the second substrate 409.
[0090] Reference is next had to FIG. 11D. An intermediate
construction with the second substrate 409 bonded thereto via the
first adhesive layer 408 is then dipped in nitric acid to perform
etching of the molybdenum of the isolation layer 402. In this
etching, neither the protective layer 405 nor the upper protective
layer 407 is etched. The isolation layer 402 is soaked through the
end faces thereof with nitric acid to remove only the isolation
layer 402. As a result, the first substrate 401 and the remaining
layers on the side of the thin-film device layer 421 are separated
from each other.
[0091] When pinholes 433 are contained in the aluminum oxide layer
as the first anti-HNO.sub.3 layer 403 in the protective layer 405
as shown in FIG. 11E, nitric acid penetrates through the pinholes
433 to the second anti-HNO.sub.3 layer 404 made of silicon oxide.
There is, however, an extremely low probability in the formation of
pinholes 434 in silicon oxide film as the second anti-HNO.sub.3
layer 404 at the same locations as the pinholes 433 formed in the
first anti-HNO.sub.3 layer 403. Nitric acid, therefore, does not
penetrate to the side of the thin-film device layer 421 beyond the
second anti-HNO.sub.3 layer 404. If the second HNO.sub.3 layer 404
were not formed, the nitric acid which has penetrated through the
pinholes 433 would penetrate to the thin-film device layer 421 so
that the nitric acid would damage the thin-film device layer
421.
[0092] Further, the first anti-HNO.sub.3 layer 403 and the second
anti-HNO.sub.3 layer 404 are both transparent to visible light and
are both insulating layers. Their removal is not needed,
accordingly. In the above-described fourth embodiment, the
anti-HNO.sub.3 layer 403 and the second anti-HNO.sub.3 layer 404
were not removed. In this manner, an active substrate is
formed.
[0093] The subsequent steps are similar to the corresponding steps
in the above-described first embodiment. Namely, the active
substrate is combined with an opposite substrate, and between the
active substrate and the opposite substrate, a liquid crystal is
injected to provide a liquid crystal cell. This work can be
performed by conventional assembling steps for liquid crystal
cells.
[0094] In the above-described process according to the fourth
embodiment for the fabrication of the thin-film device, the
protective layer 405 which serves to protect the thin-film device
layer 421 from the chemical solution is formed of at least two
layers having resistance to nitric acid used upon separation of the
first substrate 401, specifically the first anti-HNO.sub.3 layer
403 made of aluminum oxide and the second anti-HNO.sub.3 layer 404
made of silicon oxide. Practically speaking, the protective layer
is hence arranged as two or more layers. Even if nitric acid
reaches the inner protective layer (second anti-HNO.sub.3 layer
404) through the pinholes 433 formed in the outer protective layer
(first anti-HNO.sub.3 layer 403) located closer to a side where the
intermediate construction is maintained in contact with nitric
acid, it is still possible to prevent at the anti-HNO.sub.3 layer
404 any further penetration of nitric acid to the side of the
thin-film device layer 421. The pinholes 433,434 in the first and
second anti-HNO.sub.3 layers 403,404, which have resistance to
nitric acid to be employed upon separation of the first substrate
401, are very small and occur at random. Accordingly, the
probability that the pinholes 433 and 434 would be formed at the
same locations is extremely low. The above-described
anti-penetration measure makes use of this extremely low
probability. Let's assume, for example, that the pinholes 433,434
are formed in the first and second anti-HNO.sub.3 layers 403,404
having resistance to nitric acid to be used upon separation of the
first substrate 401. Even if the nitric penetrates through the
first anti-HNO.sub.3 layer 403, there is the second anti-HNO.sub.3
layer 404 as the second layer. The nitric acid is, therefore,
blocked there. With the first and second anti-HNO.sub.3 layers
403,404 having resistance to nitric acid, penetration of nitric
acid to the thin-film device layer 421 can be surely prevented.
Accordingly, transfer-related defects can be decreased.
[0095] Using FIGS. 5A to 5E which illustrate the fabrication
process according to the second embodiment, a description will next
be made about a process according to a fifth embodiment of the
present invention for the fabrication of a thin-film device. In the
fifth embodiment, a reflection active substrate is formed on a
plastic substrate by a transfer method for the fabrication of a
liquid crystal display. Up to the etching of a first substrate, the
fifth embodiment is similar to the second embodiment.
[0096] As the fifth embodiment is directed to the reflection liquid
crystal display, no problem arises even if there is an opaque layer
(a first anti-HF layer 202) underneath a thin-film device layer 221
as illustrated in FIG. 5B. Removal of a molybdenum film (the first
anti-HF layer 202), which is included in a protective layer 205,
makes it possible to avoid the problem that an interconnection in
the thin-film device layer 221 and the molybdenum in the protective
layer 205 may be short-circuited, and hence, has possibility of
achieving an improvement in production yield. In this fifth
embodiment, the protective layer 205 is removed.
[0097] Firstly, the molybdenum film (thickness: 500 nm) as the
first anti-HF layer 202 is etched off with a mixed acid [phosphoric
acid (H.sub.3PO.sub.4) 72 wt %+nitric acid (HNO.sub.3) 3 wt
%+acetic acid (CH.sub.3COOH) 10 wt %]. It takes about 1 minute to
etch off the molybdenum layer of 500 nm thickness with the mixed
acid. As this mixed acid does not etch an amorphous silicon (a-Si)
layer as a second anti-HF layer 203, the etching automatically
stops at the first insulating layer 103. Even if pinholes are
contained in the second anti-HF layer 203, the mixed acid
automatically stops at a first insulating layer 204 because it does
not etch silicon oxide (SiO.sub.2) as the first insulating layer
204.
[0098] The amorphous silicon (a-Si) layer (thickness: 100 nm) as
the second anti-HF layer 203 is etched off with a potassium
hydroxide (KOH) solution (concentration: 30%, for example). It
requires about 1 minute and 30 seconds to etch off the amorphous
silicon layer of 100 nm thickness. Because this potassium hydroxide
(KOH) solution does not etch silicon oxide (SiO.sub.2) as the first
insulating layer 204, the etching automatically stops at the first
insulating layer 204. As the first insulating layer 204 is made of
the insulating material, it is unnecessary to specifically separate
the first insulating layer 204. Separation of the first insulating
layer 204 is, therefore, not conducted in the fabrication process
according to the fifth embodiment of the present invention.
[0099] In the above-described fifth embodiment, the first anti-HF
layer 202 and the second anti-HF layer 203 are not etched by the
same chemical in common so that the insertion of an insulating
layer as an etching stopper between the first anti-HF layer 202 and
the second anti-HF layer 203 can be obviated. Compared with the
first embodiment, the second embodiment has a merit in that it
requires a fewer number of film-forming steps and a fewer number of
etching steps.
[0100] In this fifth embodiment, molybdenum and amorphous silicon
are used for the first anti-HF layer 202 and the second anti-HF
layer 203, respectively. However, any material can be used insofar
as it is equipped with resistance to HF, including tungsten,
polycrystalline silicon, aluminum oxide (Al.sub.2O.sub.3),
magnesium fluoride (MgF.sub.2), calcium fluoride (CaF.sub.2),
silicon carbide (SiC), amorphous diamond, benzocyclobutene-based
resins, and polyimides. There is, however, one condition in this
respect, that is, the second anti-HF layer 203 is not etched by the
chemical which is employed upon removal of the first anti-HF
layer.
[0101] The subsequent steps can each be conducted in a similar
manner as in Example 1.
[0102] The above-described fifth embodiment can bring about similar
effects and advantages as the second embodiment, and in addition,
can avoid the problem that the molybdenum in the protective layer
205 may be short-circuited.
[0103] In each of the thin-film devices obtained by the fabrication
processes according to the first to fifth embodiments of the
present invention, respectively, the protective layer employed in
the fabrication steps is composed of at least two layers having
resistance to a chemical solution used upon removal of the first
substrate, specifically the first anti-HF layer and second anti-HF
layer or the first anti-HNO.sub.3 layer and second anti-HNO.sub.3
layer. Since the thin-film devices can enjoy similar effects and
advantages as their fabrication processes, the thin-film devices
can be fabricated with high production yield while maintaining
their thin-film device layers free from the attack by a chemical
solution used upon removal of their substrates, for example,
hydrofluoric acid or nitric acid. Accordingly, the use of such
thin-film devices makes it possible to provide liquid crystal
displays or organic EL displays of excellent quality.
[0104] While a preferred embodiment of the present invention has
been described using specific terms, such description is for
illustrative purpose only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the following claims.
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