U.S. patent application number 15/565976 was filed with the patent office on 2018-04-05 for method of producing etching mask, etching mask precursor, and oxide layer, and method of manufacturing thin film transistor.
The applicant listed for this patent is Japan Advanced Institute of Science and Technology, Sumitomo Seika Chemicals Co., Ltd.. Invention is credited to Nobutaka FUJIMOTO, Kazuhiro FUKADA, Satoshi INOUE, Kiyoshi NISHIOKA, Tatsuya SHIMODA, Masahiro SUZUKI.
Application Number | 20180096853 15/565976 |
Document ID | / |
Family ID | 57126520 |
Filed Date | 2018-04-05 |
United States Patent
Application |
20180096853 |
Kind Code |
A1 |
INOUE; Satoshi ; et
al. |
April 5, 2018 |
METHOD OF PRODUCING ETCHING MASK, ETCHING MASK PRECURSOR, AND OXIDE
LAYER, AND METHOD OF MANUFACTURING THIN FILM TRANSISTOR
Abstract
The etching mask 80 for screen printing according to one
embodiment of the present invention includes aliphatic
polycarbonate. Further, the method of producing an oxide layer (the
channel 44) according to one embodiment of the present invention
includes: an etching-mask forming step of forming a pattern of the
etching mask 80 including aliphatic polycarbonate; a contact step
of, after the etching-mask forming step, contacting the oxide layer
with a solution for dissolving a portion of the oxide layer (the
channel 44) which is not protected by the etching mask 80; and a
heating step of, after the contact step, heating the oxide layer
(the channel 44) and the etching mask 80 to or above a temperature
at which the etching mask 80 is decomposed.
Inventors: |
INOUE; Satoshi; (Nagano,
JP) ; SHIMODA; Tatsuya; (Ishikawa, JP) ;
FUKADA; Kazuhiro; (Hiroshima, JP) ; NISHIOKA;
Kiyoshi; (Hyogo, JP) ; FUJIMOTO; Nobutaka;
(Osaka, JP) ; SUZUKI; Masahiro; (Hyogo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Japan Advanced Institute of Science and Technology
Sumitomo Seika Chemicals Co., Ltd. |
Ishikawa
Hyogo |
|
JP
JP |
|
|
Family ID: |
57126520 |
Appl. No.: |
15/565976 |
Filed: |
March 14, 2016 |
PCT Filed: |
March 14, 2016 |
PCT NO: |
PCT/JP2016/057986 |
371 Date: |
October 12, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/786 20130101;
H01L 29/66969 20130101; H01L 21/3086 20130101; H01L 21/3065
20130101; H01L 27/1288 20130101; H01L 29/7869 20130101 |
International
Class: |
H01L 21/3065 20060101
H01L021/3065; H01L 21/308 20060101 H01L021/308; H01L 29/786
20060101 H01L029/786 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 16, 2015 |
JP |
2015-084467 |
Claims
1. An etching mask for screen printing, comprising aliphatic
polycarbonate.
2. The etching mask according to claim 1, wherein the aliphatic
polycarbonate is substantially eliminated or removed by heating at
180.degree. C. or more.
3. The etching mask according to claim 1, formed from an etching
mask precursor, wherein in the etching mask precursor the
percentage of the aliphatic polycarbonate having a molecular weight
of 6000 or more and 400000 or less is 80% by mass or more relative
to the entire aliphatic polycarbonate.
4. The etching mask according to claim 1, formed from an etching
mask precursor, wherein in the etching mask precursor the aliphatic
polycarbonate has a value of L/(D.times.v.times..eta.) of 0.25
mm.sup.-1Pa.sup.-1 or more in which a cylindrical bar made of
polytetrafluoroethylene and having a diameter "D" is dipped in a
collection of the aliphatic polycarbonate having a zero-shear
viscosity .eta. as measured using a rheometer (Model AR-2000EX, TA
Instruments), and the length "L" of a strand withdrawn from an
outermost surface of the collection of the aliphatic polycarbonate
is then measured when the cylindrical bar is pulled upwardly at a
velocity "v".
5. The etching mask according to claim 3, wherein the contact angle
of the etching mask precursor to a base material is 30.degree. or
more and 36.degree. or less at 30 seconds after the etching mask
precursor is placed on the base material, or the contact angle of
the etching mask to the base material is 26.degree. or more and
32.degree. or less at 120 seconds after the etching mask precursor
is placed on the base material.
6. The etching mask according to claim 1, wherein a target of
etching is an oxide layer, and the etching mask is for forming a
pattern on the oxide layer, and provides a portion of the oxide
layer which shows a rectification behavior, the portion of the
oxide layer having had been protected by the pattern, in a case
where the oxide layer is obtained by bringing the etching mask into
contact with a solution for dissolving the oxide layer and then
heating the etching mask to or above a temperature at which the
etching mask is decomposed.
7. The etching mask according to claim 1, wherein the target of
etching is an oxide precursor layer to be oxidized into an oxide
layer, and the etching mask is for forming a pattern on the oxide
precursor layer, and provides a portion of the oxide layer which
shows a rectification behavior, the portion of the oxide layer
having had been protected by the pattern, in a case where the oxide
layer is obtained by bringing the etching mask into contact with a
solution for dissolving the oxide precursor layer and then heating
the etching mask to or above a temperature at which the oxide layer
is formed.
8. The etching mask according to claim 1, wherein the target of
etching is an oxide layer, and the etching mask is for forming a
pattern on the oxide layer, and provides a portion of the oxide
layer which shows a rectification behavior, the portion of the
oxide layer having had been protected by the pattern, in a case
where the oxide layer is obtained by exposing the etching mask to a
plasma for etching the oxide layer and then heating the etching
mask to or above a temperature at which the etching mask is
decomposed.
9. The etching mask according to claim 1, wherein the target of
etching is an oxide precursor layer to be oxidized into an oxide
layer, and the etching mask is for forming a pattern on the oxide
precursor layer, and provides a portion of the oxide layer which
shows a rectification behavior, the portion of the oxide layer
having had been protected by the pattern, in a case where the oxide
layer is obtained by exposing the etching mask to a plasma for
etching the oxide precursor layer and then heating the etching mask
to or above a temperature at which the oxide layer is formed.
10. The etching mask according to claim 1, wherein the aliphatic
polycarbonate is at least one selected from the group consisting of
polyethylene carbonate and polypropylene carbonate.
11. An etching mask precursor for screen printing, comprising
aliphatic polycarbonate.
12. The etching mask precursor for screen printing according to
claim 11, wherein the aliphatic polycarbonate is substantially
eliminated or removed by heating at 180.degree. C. or more.
13. The etching mask precursor for screen printing according to
claim 11, wherein the percentage of the aliphatic polycarbonate
having a molecular weight of 6000 or more and 400000 or less is 80%
by mass or more relative to the entire aliphatic polycarbonate.
14. The etching mask precursor for screen printing according to
claim 11, wherein the aliphatic polycarbonate has a value of
L/(D.times.v.times..eta.) of 0.25 mm.sup.-1Pa.sup.-1 or more in
which a cylindrical bar made of polytetrafluoroethylene and having
a diameter "D" is dipped in a collection of the aliphatic
polycarbonate having a zero-shear viscosity .eta. as measured using
a rheometer (Model AR-2000EX, TA Instruments), and the length "L"
of a strand withdrawn from an outermost surface of the collection
of the aliphatic polycarbonate is then measured when the
cylindrical bar is pulled upwardly at a velocity "v".
15. A method of producing an oxide layer, the method comprising an
etching-mask forming step of forming a pattern of an etching mask
including aliphatic polycarbonate on the oxide layer by the screen
printing method, a contact step of, after the etching-mask forming
step, contacting the oxide layer with a solution for dissolving a
portion of the oxide layer not protected by the etching mask, and a
heating step of, after the contact step, heating the oxide layer
and the etching mask to or above a temperature at which the etching
mask is decomposed.
16. A method of producing an oxide layer, the method comprising an
etching-mask forming step of forming a pattern of an etching mask
including aliphatic polycarbonate on an oxide precursor layer to be
oxidized into the oxide layer by the screen printing method, a
contact step of, after the etching-mask forming step, contacting
the oxide precursor layer with a solution for dissolving a portion
of the oxide precursor layer not protected by the etching mask, and
a heating step of, after the contact step, heating the oxide
precursor layer and the etching mask to or above a temperature at
which the oxide layer is formed.
17. A method of producing an oxide layer, the method comprising an
etching-mask forming step of forming a pattern of an etching mask
including aliphatic polycarbonate on the oxide layer by the screen
printing method, an exposure step of, after the etching-mask
forming step, exposing the oxide layer to a plasma for etching a
portion of the oxide layer not protected by the etching mask, and a
heating step of, after the exposure step, heating the oxide layer
and the etching mask to or above a temperature at which the etching
mask is decomposed.
18. A method of producing an oxide layer, the method comprising an
etching-mask forming step of forming a pattern of an etching mask
including aliphatic polycarbonate on an oxide precursor layer to be
oxidized into the oxide layer by the screen printing method, an
exposure step of, after the etching-mask forming step, exposing the
oxide precursor layer to a plasma for etching a portion of the
oxide precursor layer not protected by the etching mask, and a
heating step of, after the exposure step, heating the oxide
precursor layer and the etching mask to or above a temperature at
which the oxide layer is formed.
19. The method of producing an oxide layer according to claim 15,
wherein the heating step comprises heating to or above a
temperature at which organic substances are decomposed while
applying ultraviolet rays to the oxide precursor layer and/or the
etching mask.
20. A method of manufacturing a thin film transistor, the method
comprising an oxide layer forming step of forming the portion of
the oxide layer which had been protected by the etching mask formed
via the heating step in the method of producing an oxide layer
according to claim 15 so that the portion of the oxide layer makes
contact with a gate electrode through a gate insulator.
21. The etching mask according to claim 4, wherein the contact
angle of the etching mask precursor to a base material is
30.degree. or more and 36.degree. or less at 30 seconds after the
etching mask precursor is placed on the base material, or the
contact angle of the etching mask to the base material is
26.degree. or more and 32.degree. or less at 120 seconds after the
etching mask precursor is placed on the base material.
22. The method of producing an oxide layer according to claim 16,
wherein the heating step comprises heating to or above a
temperature at which organic substances are decomposed while
applying ultraviolet rays to the oxide precursor layer and/or the
etching mask.
23. The method of producing an oxide layer according to claim 17,
wherein the heating step comprises heating to or above a
temperature at which organic substances are decomposed while
applying ultraviolet rays to the oxide precursor layer and/or the
etching mask.
24. The method of producing an oxide layer according to claim 18,
wherein the heating step comprises heating to or above a
temperature at which organic substances are decomposed while
applying ultraviolet rays to the oxide precursor layer and/or the
etching mask.
25. A method of manufacturing a thin film transistor, the method
comprising an oxide layer forming step of forming the portion of
the oxide layer which had been protected by the etching mask formed
via the heating step in the method of producing an oxide layer
according to claim 16 so that the portion of the oxide layer makes
contact with a gate electrode through a gate insulator.
26. A method of manufacturing a thin film transistor, the method
comprising an oxide layer forming step of forming the portion of
the oxide layer which had been protected by the etching mask formed
via the heating step in the method of producing an oxide layer
according to claim 17 so that the portion of the oxide layer makes
contact with a gate electrode through a gate insulator.
27. A method of manufacturing a thin film transistor, the method
comprising an oxide layer forming step of forming the portion of
the oxide layer which had been protected by the etching mask formed
via the heating step in the method of producing an oxide layer
according to claim 18 so that the portion of the oxide layer makes
contact with a gate electrode through a gate insulator.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method of producing an
etching mask, an etching mask precursor, and an oxide layer, and a
method of producing a thin film transistor.
BACKGROUND ART
[0002] Conventionally, a polycrystalline silicon film or an
amorphous silicon film has typically been adopted as a channel
layer of a thin film transistor, which is an example of an
electronic device. However, when a polycrystalline silicon film is
used, electron scattering at the polycrystalline grain boundary
limits electron mobility, so as to cause variation in transistor
property. When an amorphous silicon film is used, electron mobility
is very low and an element tends to deteriorate with time, so as to
cause extremely low element reliability. In this regard, an oxide
semiconductor has been attracting interests, which is higher in
electron mobility than an amorphous silicon film and has less
variation in transistor property than a polycrystalline silicon
film. Moreover, not only an oxide semiconductor, but also an oxide
conductive material or an oxide insulator made of an oxide is an
essential technical element for realization of an electronic device
made of only an oxide, for example, and thus is attracting very
high industrial interests.
[0003] There has been recently an active attempt to produce an
electronic device on a flexible resin substrate in accordance with
a low-energy production process such as a printing method. There is
an advantage that a semiconductor layer can be patterned directly
on a substrate in accordance with a printing method or the like and
no etching step is thus required for patterning.
[0004] For example, Patent Documents 1 to 3 represent attempts for
producing a coated flexible electronic device including a
conductive polymer or an organic semiconductor. Further, the
present applicant discloses a technology for solving some of the
aforementioned problems (Patent Document 4).
PRIOR ART DOCUMENTS
Patent Documents
Patent Document 1: JP Patent Publication (Kokai) No.
2007-134547A
Patent Document 2: JP Patent Publication (Kokai) No.
2007-165900A
Patent Document 3: JP Patent Publication (Kokai) No.
2007-201056A
Patent Document 4: WO2015/019771
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0005] While the industry and consumers demand various information
terminals and information appliances, a semiconductor is required
to operate at higher speed, to be stable for a long period of time,
and to apply a less environmental load. However, the conventional
technique typically includes a vacuum process, a process according
to the photolithography technique, or the like, which requires a
relatively long time period and/or expensive equipment. Therefore,
these processes lead to very low utilization efficiency of raw
materials and production energy. This is not preferred from the
industrial and mass productivity perspectives. It is very difficult
to form layers under current conditions by applying the above
printing method, such as gravure printing, screen printing, offset
printing, or ink jet printing to silicon semiconductors or other
semiconductors that have been mainly used to date. Even in a case
where any one of the conductive polymer and the organic
semiconductor disclosed in Patent Documents 1 to 3 is adopted, the
electrical property and stability thereof remain insufficient. In
addition, the term "layer(s)" in this application conceptually
includes a layer as well as a film. Similarly, the term "film" in
this application conceptually includes a film as well as a
layer.
[0006] Formation of layers by the above various printing methods,
in particular by the screen printing method which is highly
applicable to various devices such as semiconductor elements and
electronic devices, is now attracting much attention in the
industry in view of flexibilization of the devices including such
layers, and in terms of industrialization and mass productivity
mentioned above.
[0007] However, for example, the thickness of a layer formed in
accordance with a printing method (in particular, by the screen
printing method) differs from the thickness (typically in
submicrons) of a layer (for example, an etching mask) required in
manufacture of the aforementioned various devices such as
semiconductor elements. Specifically, a relatively thick layer is
formed upon patterning by a printing method, but the thickness
required for an etching mask may significantly vary depending on
the material or thickness of a film to be etched.
[0008] Meanwhile, a paste or a solution used in a printing method
is supposed to have a viscosity suitable for patterning, and thus
the viscosity thereof is adjusted with a polymer compound. The
inventors of this application recognized that when an etching mask
used for manufacturing various devices is formed with a paste or a
solution containing aliphatic polycarbonate, for example, by the
screen printing method, the stringiness of the aliphatic
polycarbonate may not be appropriately controlled. This, therefore,
may result in a problem in that a pattern of an etching mask which
is good enough for use in the production processes of the
aforementioned various devices, or suitable for manufacturing the
aforementioned various devices may not be formed. As a more
specific example, in a process of forming a layer of a precursor of
an etching mask (hereinafter may also be referred to as an "etching
mask precursor") by a printing method, a portion of the precursor
may be fibrously withdrawn from the precursor layer which is formed
on a layer to be etched and will form a pattern of the etching
mask. This may disadvantageously disrupt the desired pattern.
[0009] Further, when a pattern of the aforementioned etching mask
precursor is formed, for example, by a printing method, the pattern
which has formed needs to be maintained at least until a material
or region to be etched is removed. Therefore, formation of a
pattern which is resistant to time-dependent change is also one of
the challenges for, in particular, the printing method.
[0010] Meanwhile, the development of an etching mask for forming a
pattern of a thin layer used for various devices such as
semiconductor elements without impairing a function thereof is also
another elemental technology for achieving the thin layer.
Solutions to the Problems
[0011] The inventors of this application have conducted extensive
studies and analyses to develop various metal oxides which can be
formed from liquid materials as well as an etching mask which can
reliably provide a pattern of such a metal oxide and can also
protect that metal oxide as much as possible without impairing a
function thereof. More specifically, the inventors of this
application conducted extensive studies and analyses to develop an
elemental technology and various devices manufactured using the
elemental technology, which can provide an etching mask capable of
forming a pattern by a printing method (in particular, by the
screen printing method), and in addition can reliably maintain or
improve the functions of various layers, or can simplify a process
of producing these layers using the etching mask.
[0012] In the studies and analyses conducted by the present
inventors, a process in which a gel-like layer (hereinafter may
also be referred to as a "gel layer") which can be obtained from
the aforementioned paste or solution formed a pattern was
investigated. As a result, the following finding was obtained:
before aliphatic polycarbonate in a gel layer undergoes
decomposition, i.e., before aliphatic polycarbonate is heated to a
temperature at which the aliphatic polycarbonate is decomposed, the
properties of the aliphatic polycarbonate itself or those of the
paste or solution containing the aliphatic polycarbonate have some
impacts on the height, printed conditions, or stringiness of a
pattern of a gel layer when the pattern is formed, in particular,
by the screen printing method. It is noted that in the present
application, a gel layer suitable for screen printing is desired,
and thus the present gel layer may also be referred to as a gel
layer closer to the liquid state.
[0013] As a result of numerous trials and errors as well as
numerous extensive analyses, the inventors of this application
found that inclusion of aliphatic polycarbonate having a molecular
weight in a specific range or having a specific stringiness can
contribute to formation of a pattern having a desired thickness
which can form an etching mask suitable for, in particular, the
screen printing method. In addition, the inventors of this
application found that the aforementioned desired pattern of a gel
layer with a controllable thickness can be easily formed by a
low-energy manufacturing process such as the screen printing
method.
[0014] More interestingly, the inventors of this application found
that a rectification behavior as an electrical property unique to
semiconductors may be lost or impaired when an etching target is an
oxide semiconductor, and a conventional resist mask is used as an
etching mask for forming a pattern. The inventors of this
application also found that in contrast, the rectification behavior
of an oxide semiconductor which had been protected by an etching
mask was able to be observed in a consistent manner when the
aforementioned desired pattern of a gel layer with a controllable
thickness was used as the etching mask. It is noted that the
etching targets in the present invention include not only oxide
semiconductors but also oxide conductive materials or oxide
insulators, or other semiconductor materials, conductor materials,
and insulator materials.
[0015] Therefore, in a more preferred aspect, an oxide (for
example, a metal oxide) is used in combination of the
aforementioned etching mask, the oxide being formed by annealing an
oxide precursor as a starting material with which a pattern is
difficult to be formed, for example, by the screen printing method
to or above a temperature at which the oxide is formed, i.e.,
annealing it to or above a temperature at which an element
dispersed in a solution containing aliphatic polycarbonate binds
with oxygen. This is because a temperature at which the oxide is
formed is higher than a temperature at which the etching mask is
decomposed, and thus the etching mask has been already decomposed
and removed reliably before the oxide is formed. As a result, at
least the following effects of (1) and (2) can be provided. [0016]
(1) An effect of simplifying a production process where a step of
removing the etching mask needs not be performed separately. [0017]
(2) An effect of maintaining or improving, in particular, the
electrical properties of an oxide where effects on the physical
properties of the oxide due to, for example, exposure to oxygen
plasma or contact (typically immersion) with a chemical liquid for
removing a resist mask can be reduced or eliminated.
[0018] That is, each of the above points of view and schemes can
contribute to further improvement of the performance and
manufacture technology of various devices such as semiconductor
elements and electronic devices manufactured using an etching mask
with a controllable thickness formed by a low-energy manufacturing
process (in particular, by the screen printing method).
[0019] The present invention was made based on each of the above
points of view and extensive analysis.
[0020] It is noted that the term "process from the liquid state to
the gel state" as used herein typically refers to, for example, a
state in which a solvent is removed to some extent (typically 80%
or more by the mass ratio relative to the entire solvent) by heat
treatment, but aliphatic polycarbonate is not substantially
decomposed.
[0021] The etching mask for screen printing according to one
embodiment of the present invention includes aliphatic
polycarbonate.
[0022] The etching mask precursor for screen printing according to
one embodiment of the present invention includes aliphatic
polycarbonate.
[0023] With regard to the above etching mask precursor and etching
mask, aliphatic polycarbonate can serve as a material of an etching
mask which can be formed by the screen printing method. This per se
is a very useful and striking effect. Further, the etching mask can
be removed easily by heating at a decomposition temperature or
above of aliphatic polycarbonate. Therefore, this can significantly
contribute to simplifying processes of manufacturing various
devices such as semiconductor elements and electronic devices.
[0024] It is noted that one preferred example of the above etching
mask for screen printing is formed from an etching mask precursor
including aliphatic polycarbonate in which the percentage of
aliphatic polycarbonate having a molecular weight of 6000 or more
and 400000 or less is 80% by mass or more relative to the entire
aliphatic polycarbonate. Further, another preferred example of the
above etching mask for screen printing is formed from an etching
mask precursor including aliphatic polycarbonate having a value of
L/(D.times.v.times..eta.) of 0.25 mm.sup.-1Pa.sup.-1 or more
wherein a cylindrical bar made of polytetrafluoroethylene and
having a diameter "D" is dipped in a collection of aliphatic
polycarbonate having a zero-shear viscosity .eta. as measured using
a rheometer (Model AR-2000EX, TA Instruments), and the length "L"
of a strand withdrawn from the outermost surface of the collection
of the aliphatic polycarbonate is then measured when the
cylindrical bar is pulled upwardly at a velocity "v".
[0025] Further, one preferred example of the above etching mask
precursor for screen printing includes aliphatic polycarbonate in
which the percentage of aliphatic polycarbonate having a molecular
weight of 6000 or more and 400000 or less is 80% by mass or more
relative to the entire aliphatic polycarbonate. Further, another
preferred example of the above etching mask precursor for screen
printing includes aliphatic polycarbonate having a value of
L/(D.times.v.times..eta.) of 0.25 mm.sup.-1Pa.sup.-1 or more
wherein a cylindrical bar made of polytetrafluoroethylene and
having a diameter "D" is dipped in a collection of aliphatic
polycarbonate having a zero-shear viscosity .eta. as measured using
a rheometer (Model AR-2000EX, TA Instruments), and the length "L"
of a strand withdrawn from the outermost surface of the collection
of the aliphatic polycarbonate is then measured when the
cylindrical bar is pulled upwardly at a velocity "v".
[0026] When the aforementioned preferred examples of the etching
mask for screen printing or the aforementioned preferred examples
of the etching mask precursor for screen printing are used, the
stringiness of aliphatic polycarbonate can be controlled more
reliably in an appropriate manner. As a result, a good pattern of
the etching mask precursor or the etching mask can be formed. It is
noted that the studies and analyses conducted by the inventors of
this application indicate that when aliphatic polycarbonate
included in an etching mask precursor and an etching mask satisfies
a numerical range based on the aforementioned computational
expression, the stringiness of the aliphatic polycarbonate can
appropriately be controlled for a layer formed, for example, by the
screen printing method. Therefore, a good pattern of the etching
mask precursor or the etching mask can be formed when the above
numerical range is satisfied.
[0027] Further, a method of producing an oxide layer according to
one embodiment of the present invention includes an etching-mask
forming step of forming a pattern of an etching mask on an oxide
layer by the screen printing method, the etching mask including
aliphatic polycarbonate; a contact step of, after the etching-mask
forming step, contacting the oxide layer with a solution for
dissolving a portion the oxide layer not protected by the etching
mask; and a heating step of, after the contact step, heating the
oxide layer and the etching mask to or above a temperature at which
the etching mask is decomposed.
[0028] Further, a method of producing an oxide layer according to
another embodiment of the present invention includes an
etching-mask forming step of forming a pattern of an etching mask
including aliphatic polycarbonate on an oxide precursor layer to be
oxidized into the oxide layer by the screen printing method; a
contact step of, after the etching-mask forming step, contacting
the oxide precursor layer with a solution for dissolving a portion
of the oxide precursor layer not protected by the etching mask; and
a heating step of, after the contact step, heating the oxide
precursor layer and the etching mask to or above a temperature at
which the oxide layer is formed.
[0029] Further, a method of producing an oxide layer according to
another embodiment of the present invention includes an
etching-mask forming step of forming a pattern of an etching mask
including aliphatic polycarbonate on an oxide layer by the screen
printing method; an exposure step of, after the etching-mask
forming step, exposing the oxide layer to a plasma for etching a
portion of the oxide layer not protected by the etching mask; and a
heating step of, after the exposure step, heating the oxide layer
and the etching mask to or above a temperature at which the etching
mask is decomposed.
[0030] Further, a method of producing an oxide layer according to
another embodiment of the present invention includes an
etching-mask forming step of forming a pattern of an etching mask
including aliphatic polycarbonate on an oxide precursor layer to be
oxidized into the oxide layer by the screen printing method; an
exposure step of, after the etching-mask forming step, exposing the
oxide precursor layer to a plasma for etching a portion of the
oxide precursor layer not protected by the etching mask; and a
heating step of, after the exposure step, heating the oxide
precursor layer and the etching mask to or above a temperature at
which the oxide layer is formed.
[0031] In each of the aforementioned methods of producing an oxide
layer, when the oxide layer in a region to be etched is etched
using an etching mask, the etching mask after formation of a
pattern of the oxide layer can be removed by the heating step as a
relatively easy treatment regardless of whether the so-called
contact step with a solution (typically an immersion step) or the
exposure step with plasma is used. Further, the etching mask used
in each of the aforementioned production methods can be decomposed
and removed reliably by virtue of aliphatic polycarbonate included
therein. In particular, when the target of etching is an oxide
precursor layer to be oxidized into an oxide layer, the etching
mask is also decomposed and removed during the heating step of
heating the oxide precursor layer to or above a temperature at
which the oxide layer is formed. As a result, this can provide a
unique process-simplifying effect in which a step of removing the
etching mask needs not be performed separately.
[0032] Further, a method of manufacturing a thin film transistor
according to another embodiment of the present invention includes
an oxide-layer forming step of forming an oxide layer so that the
oxide layer is brought into contact with a gate electrode through a
gate insulator, the oxide layer being formed via the heating step
of any one of the aforementioned methods of producing an oxide
layer and being protected by the etching mask.
[0033] According to the method of manufacturing a thin film
transistor described above, the rectification behavior of the oxide
layer can be reliably maintained.
[0034] The term "metal oxide" in this application conceptually
includes an oxide semiconductor, an oxide conductive material, or
an oxide insulator. In addition, an oxide semiconductor, an oxide
conductive material, and an oxide insulator are relative concepts
in view of electroconductivity and thus require no strict
distinction. If metal oxides are supposed to be of the same type,
they can be recognized by a person skilled in the art as oxide
semiconductors or as oxide conductive materials or oxide
insulators, depending on various device requirements. The term
"substrate" in this application is not limited to a base in a plate
shape but includes a base and a base material in different modes.
Furthermore, the term "application" in each of the embodiments to
be described later in this application indicates forming a layer on
a substrate in accordance with a low-energy production process such
as typically a printing method. Further, the term "metal" in this
application encompasses not only typical elemental metals but also
transition metals.
Effects of the Invention
[0035] The etching mask for screen printing according to one
embodiment of the present invention and the etching mask precursor
for screen printing according to one embodiment of the present
invention can sufficiently function as an etching mask. In
addition, the etching mask can be easily removed by heating to a
decomposition temperature or above of aliphatic polycarbonate.
Therefore, this can significantly contribute to simplifying
processes of manufacturing various devices such as semiconductor
elements and electronic devices.
[0036] Further, in the methods of producing an oxide layer
according to one embodiment of the present invention, when a region
to be etched of the oxide layer is etched using an etching mask,
the etching mask after formation of a pattern of the oxide layer
can be removed by the heating step as a relatively easy treatment
regardless of whether the so-called contact step with a solution
(typically an immersion step) or the exposure step with plasma is
used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 shows typical examples of optical photomicrographs of
good patterns realized according to a first embodiment of the
present invention.
[0038] FIG. 2A shows typical examples of optical photomicrographs
of unfavorable patterns formed according to the first embodiment of
the present invention.
[0039] FIG. 2B shows typical examples of optical photomicrographs
of good patterns formed according to the first embodiment of the
present invention.
[0040] FIG. 3A is a graph showing the relationship between each
sample and evaluation parameters indicating stringiness according
to the first embodiment of the present invention.
[0041] FIG. 3B is a graph showing the relationship between each
sample and evaluation parameters indicating stringiness according
to the first embodiment of the present invention.
[0042] FIG. 4A is a graph showing the contact angle between a base
material and a solution and the expansion ratio of the solution on
the base material with respect to changes in the concentration of
2-nitropropane at 30 seconds after the placement of the solution
containing aliphatic polycarbonate on the base material according
to the first embodiment of the present invention.
[0043] FIG. 4B is a graph showing the contact angle between a base
material and a solution and the expansion ratio of the solution on
the base material with respect to changes in the concentration of
2-nitropropane at 120 seconds after the placement of the solution
containing aliphatic polycarbonate on the base material according
to the first embodiment of the present invention.
[0044] FIG. 5 is a schematic sectional view showing a process in a
method of producing a thin film transistor according to a second
embodiment of the present invention.
[0045] FIG. 6 is a schematic sectional view showing a process in a
method of producing a thin film transistor according to the second
embodiment of the present invention.
[0046] FIG. 7 is a schematic sectional view showing a process in a
method of producing a thin film transistor according to the second
embodiment of the present invention.
[0047] FIG. 8 is a schematic sectional view showing a process in a
method of producing a thin film transistor according to the second
embodiment of the present invention.
[0048] FIG. 9A is a schematic sectional view showing a process in a
method of producing a thin film transistor according to the second
embodiment of the present invention.
[0049] FIG. 9B is a schematic sectional view showing a process in a
method of producing a thin film transistor according to the second
embodiment of the present invention.
[0050] FIG. 9C shows an image (a plan view photograph) representing
a process described with reference to FIG. 9B according to the
present invention.
[0051] FIG. 9D is a schematic sectional view showing a process in
the method of producing a thin film transistor according to the
second embodiment of the present invention.
[0052] FIG. 9E is an image (a plan view photograph) representing a
process described with reference to in FIG. 9D according to the
present invention.
[0053] FIG. 10A is a schematic sectional view showing a process in
the method of producing a thin film transistor according to the
second embodiment of the present invention.
[0054] FIG. 10B is an image (a plan view photograph) representing a
process described with reference to FIG. 10A according to the
present invention.
[0055] FIG. 11 is a graph showing the TG-DTA properties of an
indium and zinc-containing solution as an example of a material of
an oxide semiconductor precursor for forming a channel of the thin
film transistor according to the second embodiment of the present
invention.
[0056] FIG. 12 is a graph showing the TG-DTA properties of a
solution of polypropylene carbonate as an example of a solution
containing only aliphatic polycarbonate as a solute for forming a
component of the thin film transistor according to the second
embodiment of the present invention (a typical example of an
etching mask precursor).
[0057] FIG. 13 is a graph showing the rectification behavior of a
channel constituting a part of the thin film transistor according
to the second embodiment of the present invention.
[0058] FIG. 14 is a schematic sectional view showing a process in
the method of producing a thin film transistor according to the
second embodiment of the present invention.
[0059] FIG. 15 is a schematic sectional view showing a process in
the method of producing a thin film transistor according to the
second embodiment of the present invention.
[0060] FIG. 16A is a schematic sectional view showing a process in
the method of producing a thin film transistor according to the
modification example (1) of the second embodiment of the present
invention.
[0061] FIG. 16B is a schematic sectional view showing a process in
the method of producing a thin film transistor according to the
modification example (1) of the second embodiment of the present
invention.
[0062] FIG. 17A is a plan view photograph showing the result from
etching treatment of a chromium (Cr) layer using the etching mask
according to the first embodiment in the alternative embodiments
(1) of the present invention.
[0063] FIG. 17B is a plan view photograph showing the result from
etching treatment of a chromium (Cr) layer using an etching mask (a
known resist mask) as Comparative Example of FIG. 17A.
[0064] FIG. 18 is a plan view photograph showing the result from
etching treatment of a silicon dioxide (SiO.sub.2) layer using the
etching mask according to the first embodiment in the alternative
embodiments (2) of the present invention.
DESCRIPTION OF REFERENCE SIGNS
[0065] 10 Substrate [0066] 24 Gate electrode [0067] 32 Gate
insulator precursor layer [0068] 34 Gate insulator [0069] 42
Channel precursor layer [0070] 44 Channel [0071] 50 ITO layer
[0072] 56 Drain electrode [0073] 58 Source electrode [0074] 80
Etching mask [0075] 100 Thin film transistor
EMBODIMENTS OF THE INVENTION
[0076] An etching mask precursor, an etching mask, an oxide
precursor, an oxide layer, a semiconductor element, and an
electronic device according to embodiments of the present
invention, and a method of producing these will be described in
detail with reference to the accompanying drawings. It is noted
that in this disclosure, common parts are denoted by the same
reference numbers throughout the drawings unless otherwise stated.
Further, components in the drawings used in the present embodiments
are not necessarily illustrated in accordance with relative
scaling. Moreover, some of the reference numbers may be omitted for
clarity of the drawings.
First Embodiment
[0077] 1. Etching Mask Precursor, Configurations of Etching Mask,
and Method of Producing these
[0078] The "etching mask precursor" according to this embodiment is
in a state where aliphatic polycarbonate (which may include
inevitable impurities. The same shall apply hereafter) is dissolved
in a certain solvent (typically an organic solvent). Further, the
"etching mask" according to this embodiment is in a state
(typically a "gel state") where the solvent has been removed by
heating the etching mask precursor thereof to an extent where it
can be used for screen printing.
[0079] The etching mask precursor or etching mask according to this
embodiment mainly includes aliphatic polycarbonate, but may include
a compound, composition, or material other than aliphatic
polycarbonate. It is noted that there is no particular limitation
for the lower limit of the content of aliphatic polycarbonate in
the etching mask precursor or etching mask, but the mass percentage
of aliphatic polycarbonate to the total amount of a solute is
typically 80% or more. Further, there is no particular limitation
for the upper limit of the content of aliphatic polycarbonate in
the etching mask precursor or etching mask, but the mass percentage
of aliphatic polycarbonate to the total amount of a solute is
typically 100% or less. In addition, after the target of etching is
subjected to etching treatment using a pattern formed, for example,
by the screen printing method, aliphatic polycarbonate is to be
decomposed and/or removed primarily in a heating step.
[0080] With regard to the etching mask precursor and etching mask
according to this embodiment, aliphatic polycarbonate can serve as
a material of an etching mask which can be formed by the screen
printing method. This per se is a very useful and striking effect.
As described above, the etching mask can be removed very easily by
heating at a decomposition temperature or above of aliphatic
polycarbonate. This can significantly contribute to simplifying
processes for manufacturing various devices such as semiconductor
elements and electronic devices. Further, in order to reliably
remove the etching mask by heating at a decomposition temperature
or above of aliphatic polycarbonate, no other compounds,
compositions, or materials having a decomposition temperature
higher than that of aliphatic polycarbonate are preferably included
in the etching mask precursor solution or in the etching mask.
[0081] In this embodiment, the percentage of aliphatic
polycarbonate having a molecular weight of 6000 or more and 400000
or less in the etching mask precursor or the etching mask is 80% by
mass or more relative to the entire aliphatic polycarbonate.
(Aliphatic Polycarbonate and Solution Containing the Aliphatic
Polycarbonate)
[0082] Next, with regard to aliphatic polycarbonate used in this
embodiment, aliphatic polycarbonate and a solution containing the
aliphatic polycarbonate (that is, the "etching mask precursor")
will be described in detail.
[0083] In this embodiment, endothermically decomposable aliphatic
polycarbonate having good pyrolysability is used. It is noted that
whether the thermal decomposition reaction of aliphatic
polycarbonate is an endothermic reaction or not can be determined
by the differential thermal analysis (DTA). Such aliphatic
polycarbonate has a high oxygen content, and can be decomposed into
low-molecular weight compounds at a relatively low temperature, and
thus actively contributes to reduction of the amount of residual
impurities in a metal oxide such as carbonous impurities.
[0084] Further, in this embodiment, there is no particular
limitation for the organic solvent which can be used for the
"etching mask precursor" as a solution containing aliphatic
polycarbonate, as long as it is an organic solvent in which
aliphatic polycarbonate can be dissolved. Specific examples of the
organic solvent include diethylene-glycol-monoethyl ether acetate
(hereinafter, may also be referred to as "DEGMEA"),
.alpha.-terpineol, .beta.-terpineol, N-methyl-2-pyrrolidone,
2-nitropropane, isopropyl alcohol, diethylene-glycol-monobutyl
ether acetate, diethylene glycol monobutyl ether, toluene,
cyclohexane, methyl ethyl ketone, dimethyl carbonate, diethyl
carbonate, propylene carbonate, and the like. Among these organic
solvents, preferably used are diethylene-glycol-monoethyl ether
acetate, .alpha.-terpineol, N-methyl-2-pyrrolidone, 2-nitropropane,
and propylene carbonate in view of their suitably high boiling
points and low vaporizability at room temperature. It is noted that
in this embodiment, the organic solvent will be eventually removed
as an unwanted substance after etching the target of etching using
an etching mask including aliphatic polycarbonate. Therefore, a
solvent mixture of DEGMEA and 2-nitropropane is preferably used
because a pattern is only required to persist for a relatively
short time from pattern formulation to decomposition or
removal.
[0085] Further, a dispersing agent, a plasticizing agent, and the
like can further be added, if desired, to the etching mask
precursor as a solution containing aliphatic polycarbonate.
[0086] Specific examples of the dispersing agent include:
[0087] polyhydric alcohol esters such as glycerin and sorbitan;
[0088] polyether polyols such as diethylene glycol, triethylene
glycol, dipropylene glycol, tripropylene glycol, polyethylene
glycol, and polypropylene glycol; amines such as
polyethyleneimine;
[0089] (meth)acrylic resins such as polyacrylic acid and
polymethacrylic acid;
[0090] copolymers of isobutylene or styrene with maleic anhydride,
and amine salts thereof; and the like.
[0091] Specific examples of the plasticizing agent include
polyether polyol, phthalate ester, and the like.
[0092] Further, there is no particular limitation for the method of
forming an etching mask precursor layer according to this
embodiment. In one preferred aspect, a layer is formed by a
low-energy manufacturing process. More specifically, an etching
mask precursor layer is preferably formed by application to a base
material in accordance with screen printing as a particularly
simple method.
(Aliphatic Polycarbonate)
[0093] In each experimental example as described later,
polypropylene carbonate is adopted as an example of aliphatic
polycarbonate, but examples of the aliphatic polycarbonate adopted
in this embodiment are not particularly limited. For example,
according to a preferably adoptable aspect of this embodiment,
aliphatic polycarbonate obtained by polymerization reaction between
epoxide and carbon dioxide is used. Adoption of such aliphatic
polycarbonate obtained by polymerization reaction between epoxide
and carbon dioxide exerts an effect such that aliphatic
polycarbonate having a desired molecular weight can be obtained, by
which endothermic decomposition properties can be improved through
the control of the structure of the aliphatic polycarbonate. The
aliphatic polycarbonate is preferably at least one member selected
from the group consisting of a polyethylene carbonate and a
polypropylene carbonate in terms of a high oxygen content and
decomposition into a low molecular weight compound at a relatively
low temperature. With the use of any aliphatic polycarbonate
described above having a molecular weight within the above numeric
range, effects similar to those of this embodiment can be
exerted.
[0094] The epoxide is not particularly limited if the epoxide
undergoes a polymerization reaction with carbon dioxide to form
aliphatic polycarbonate having a structure including aliphatic
groups on the main chain. Adoptable examples of the epoxide in this
embodiment include ethylene oxide, propylene oxide, 1-butene oxide,
2-butene oxide, isobutylene oxide, 1-pentene oxide, 2-pentene
oxide, 1-hexene oxide, 1-octene oxide, 1-decene oxide, cyclopentene
oxide, cyclohexene oxide, styrene oxide, vinylcyclohexene oxide,
3-phenylpropylene oxide, 3,3,3-trifluoropropylene oxide,
3-naphthylpropylene oxide, 3-phenoxypropylene oxide,
3-naphthoxypropylene oxide, butadiene monoxide, 3-vinyloxypropylene
oxide, and 3-trimethylsilyloxypropylene oxide. Among these
epoxides, ethylene oxide and propylene oxide are preferably used in
terms of their high polymerization reactivity with carbon dioxide.
These epoxides may each be used singly or may be used in
combination of two or more thereof.
[0095] The aliphatic polycarbonate mentioned above has a
mass-average molecular weight of preferably from 5000 to 1000000
and more preferably from 10000 to 500000. The aliphatic
polycarbonate having a mass-average molecular weight of less than
5000 may become unsuitable as a material for the screen printing
method because of influences due to a decrease in viscosity, for
example. The aliphatic polycarbonate having a mass-average
molecular weight of more than 1000000 may also become unsuitable as
a material for the screen printing method due to lower
dissolubility of the aliphatic polycarbonate into an organic
solvent. In addition, the numerical values of the mass-average
molecular weight can be calculated by the following method.
[0096] Specifically, a chloroform solution containing the aliphatic
polycarbonate at a concentration of 0.5% by mass is prepared and
measured using high performance liquid chromatography. After the
measurement, the molecular weight is calculated by comparison with
polystyrene having a known mass-average molecular weight measured
under the identical conditions. The measurement conditions are as
follows.
[0097] Type: HLC-8020 (Tosoh Corporation)
[0098] Column: GPC column (Trade name: TSK GEL Multipore HXL-M,
Tosoh Corporation)
[0099] Column temperature: 40.degree. C.
[0100] Eluent: chloroform
[0101] Flow speed: 1 mL/minute
[0102] The aliphatic polycarbonate mentioned above can be
exemplarily produced by polymerization reaction between the epoxide
and carbon dioxide in the presence of a metal catalyst.
[0103] A production example of aliphatic polycarbonate is as
follows.
[0104] The atmosphere of the system of an autoclave having a volume
of 1 L and equipped with a stirrer, a gas introduction tube, and a
thermometer was preliminarily replaced with an atmosphere of
nitrogen, and was then charged with a reaction solution containing
an organozinc catalyst, hexane, and propylene oxide. The atmosphere
of the reaction system was then replaced with an atmosphere of
carbon dioxide by adding carbon dioxide with stirring, and the
autoclave was filled with carbon dioxide until the pressure of the
reaction system reached about 1.5 MPa. The temperature of the
autoclave was subsequently raised to 60.degree. C. and
polymerization reaction was carried out for several hours while
supplying carbon dioxide to be consumed by the reaction. After
completion of the reaction, the autoclave was cooled and
depressurized and its content was filtrated. The filtrated product
was then dried under reduced pressure to obtain polypropylene
carbonate.
[0105] Specific examples of the metal catalyst include an aluminum
catalyst and a zinc catalyst. Among these metal catalysts, the zinc
catalyst is preferably used in terms of high polymerization
activity in polymerization reaction between epoxide and carbon
dioxide. An organozinc catalyst is particularly preferred among the
zinc catalysts.
[0106] Specific examples of the organozinc catalyst include:
[0107] organozinc catalysts such as zinc acetate, diethyl zinc, and
dibutyl zinc; and
[0108] organozinc catalysts obtained by reaction between a zinc
compound and compounds such as primary amine, dihydric phenol,
divalent aromatic carboxylic acid, aromatic hydroxy acid, aliphatic
dicarboxylic acid, and aliphatic monocarboxylic acid.
[0109] According to a preferred aspect, an organozinc catalyst
among these organozinc catalysts is adopted, which is obtained by
reaction of a zinc compound with an aliphatic dicarboxylic acid and
an aliphatic monocarboxylic acid, due to higher polymerization
activity.
[0110] A production example of an organozinc catalyst is as
follows.
[0111] A four-necked flask equipped with a stirrer, a nitrogen gas
introduction tube, a thermometer, and a reflux condenser was
charged with zinc oxide, glutaric acid, acetic acid, and toluene.
After the atmosphere of the reaction system was replaced with an
atmosphere of nitrogen, the temperature of the flask was raised to
55.degree. C. and the materials were stirred at this temperature
for 4 hours so as to perform reaction of the respective materials.
The temperature of the flask was then raised to 110.degree. C. and
the materials were stirred at this temperature for 4 hours so as to
cause azeotropic dehydration for removal of only water. The flask
was then cooled to room temperature to obtain a reaction solution
containing an organozinc catalyst. The organozinc catalyst, which
was obtained by partially fractionating and filtrating the reaction
solution, was subjected to IR measurement (Thermo Nicolet Japan
Inc., trade name: AVATAR360). As a result, no peak based on
carboxylic groups was confirmed.
[0112] The amount of the metal catalyst used for the polymerization
reaction is preferably 0.001 to 20 parts by mass and more
preferably 0.01 to 10 parts by mass with respect to 100 parts by
mass of the epoxide. The polymerization reaction may be unlikely to
progress if the amount of the used metal catalyst is less than
0.001 parts by mass. In contrast, if the amount of the used metal
catalyst exceeds 20 parts by mass, effects in accord with the
amount of use may not be obtained, which may be economically
undesirable.
[0113] A reaction solvent to be used as necessary in the
polymerization reaction is not particularly limited. Any type of an
organic solvent can be adopted as the reaction solvent. Specific
examples of the organic solvent include:
[0114] aliphatic hydrocarbon solvents such as pentane, hexane,
octane, decane, and cyclohexane;
[0115] aromatic hydrocarbon solvents such as benzene, toluene, and
xylene;
[0116] halogenated hydrocarbon solvents such as chloromethane,
methylene dichloride, chloroform, carbon tetrachloride,
1,1-dichloroethane, 1,2-dichloroethane, ethyl chloride,
trichloroethane, 1-chloropropane, 2-chloropropane, 1-chlorobutane,
2-chlorobutane, 1-chloro-2-methylpropane, chlorobenzene, and
bromobenzene; and carbonate solvents such as dimethyl carbonate,
diethyl carbonate, and propylene carbonate.
[0117] The amount of the used reaction solvent is preferably 500
parts by mass or more and 10000 parts by mass or less with respect
to 100 parts by mass of the epoxide in terms of smooth
reaction.
[0118] The method of the reaction between an epoxide and carbon
dioxide in the presence of a metal catalyst is not particularly
limited in the polymerization reaction described above. For
example, an adoptable method includes charging in an autoclave the
epoxide, the metal catalyst, and a reaction solvent as necessary,
mixing these components, and then injecting carbon dioxide under
pressure for reaction.
[0119] The pressure under which the carbon dioxide is used in the
polymerization step is not particularly limited. Typically, the
pressure is preferably from 0.1 MPa to 20 MPa, more preferably from
0.1 MPa to 10 MPa, and even more preferably from 0.1 MPa to 5 MPa.
If the carbon dioxide is used at a pressure exceeding 20 MPa,
effects in accord with the amount of use may not be obtained, which
may be economically undesirable.
[0120] The polymerization reaction temperature in the above
polymerization reaction is not particularly limited. The typical
polymerization reaction temperature is preferably from 30.degree.
C. to 100.degree. C. and more preferably from 40.degree. C. to
80.degree. C. If the polymerization reaction temperature is lower
than 30.degree. C., the polymerization reaction may take a long
period. In contrast, if the polymerization reaction temperature
exceeds 100.degree. C., side reaction may occur with a lower yield.
The period of the polymerization reaction is typically preferred to
be from 2 to 40 hours, although the period differs depending on the
polymerization reaction temperature and cannot be determined
generally.
[0121] After the completion of the polymerization reaction, the
aliphatic polycarbonate may be obtained by filtration or the like,
and washing with a solvent or the like as necessary, followed by
drying.
[Correlation Between Molecular Weight of Aliphatic Polycarbonate
and Stringiness and Shape of Pattern]
[0122] The inventors of this application extensively studied and
analyzed the correlation between the molecular weight of aliphatic
polycarbonate and the stringiness and the shape of a pattern
typically by conducting the following experiments. As a result, the
inventors of this application found that the stringiness of an
etching mask in the "gel state" can be controlled, and a good
pattern can be formed when the percentage of aliphatic
polycarbonate having a molecular weight of 6000 or more and 400000
or less as a representative example of the molecular weight of
aliphatic polycarbonate is 80% by mass or more relative to the
entire aliphatic polycarbonate. It is noted that a type of
aliphatic polycarbonate used in the following Experimental Examples
is polypropylene carbonate (hereinafter may also be referred to as
"PPC").
Experimental Example 1
[0123] As examples of aliphatic polycarbonate, samples of the
etching mask precursors in which at least one of 4 types of PPCs
having different mass-average molecular weights was dissolved were
prepared as shown in the following (1) to (10). It is noted that
each of the following mass-average molecular weights represents
values for aliphatic polycarbonate only.
[0124] (1) Only a PPC having mass-average molecular weight of 30000
(hereinafter may also be referred to as "Sample A")
[0125] (2) Only a PPC having mass-average molecular weight of 90000
(hereinafter may also be referred to as "Sample B")
[0126] (3) A 1:1 mixture of Sample A and Sample B (hereinafter may
also referred to as "Sample AB")
[0127] (4) Only a PPC having mass-average molecular weight of
230000 (hereinafter may also be referred to as "Sample C")
[0128] (5) Only a PPC having mass-average molecular weight of
590000 (hereinafter may also be referred to as "Sample D")
[0129] (6) A 1:1 mixture of Sample C and Sample D (hereinafter may
also referred to as "Sample CD")
[0130] (7) A 1:1 mixture of Sample A and Sample C (hereinafter may
also referred to as "Sample AC")
[0131] (8) A 1:1 mixture of Sample B and Sample C (hereinafter may
also referred to as "Sample BC")
[0132] (9) A 1:1 mixture of Sample A and Sample D (hereinafter may
also referred to as "Sample AD")
[0133] (10) A 1:1 mixture of Sample B and Sample D (hereinafter may
also be referred to as "Sample BD")
[0134] For the above samples, the percentage of aliphatic
polycarbonate having a molecular weight of 6000 or more and 400000
or less was determined as follows. That is, a solution of 0.5% by
mass of aliphatic polycarbonate prepared in chloroform was
subjected to high performance liquid chromatography under the
following conditions, and compared with a polystyrene having a
known molecular weight to determine the molecular weight
distribution. [0135] Model: HLC-8020 (Tosoh Corporation) [0136]
Column: GPC column (Trade name: TSK GEL Multipore HXL-M, Tosoh
Corporation) [0137] Column temperature: 40.degree. C. [0138]
Eluent: chloroform [0139] Flow rate: 1 mL/minute
[0140] The ratio of the area corresponding to the region of the
molecular weight of 6000 or more and 400000 or less to the area
corresponding to the entire chromatogram region was calculated from
a chromatogram obtained by the above method wherein the horizontal
axis represents a molecular weight (log of molecular weight) and
the vertical axis represents an elution rate (dwt/d (log of
molecular weight)). The results obtained are shown in Table 1.
TABLE-US-00001 TABLE 1 Percentage % by mass of aliphatic
polycarbonate having a molecular weight of Sample 6000 to 400000
(1) 81 (2) 93 (3) 87 (4) 79 (5) 58 (6) 69 (7) 80 (8) 86 (9) 70 (10)
76
[0141] Next, the etching mask precursor was subjected to a heating
and stirring step at about 50.degree. C. for about 72 hours under
an atmosphere of air, and then subjected to a defoaming step for
about 30 minutes to form an etching mask precursor in the "gel
state" (or a gel state closer to a liquid form) having a viscosity
suitable for the screen printing method. Subsequently, rectangular
patterns of the above etching mask were formed on a glass substrate
"Eagle XG" (200.times.150.times.0.7 tmm.sup.3) by the screen
printing method.
[0142] Further, preliminary annealing of the patterns was then
performed at 150.degree. C. for 30 minutes in an atmosphere of air,
and then the patterns were evaluated for stringiness under an
optical microscope and an atomic force microscope (AFM).
[0143] The summary of the above experimental results is shown in
Table 2.
TABLE-US-00002 TABLE 2 Height of Shape of pattern Sample pattern
Stringiness (.mu.m) (1) Good Good 1.4 (2) Good Good 1.5 (3) Good
Good 1.8 (4) Good Bad 2.6 (5) Bad Bad 4 (6) Bad Bad Unmeasurable
(7) Good Good 2.1 (8) Good Good 2.1 (9) Bad Bad 5.3 (10) Bad Bad
5.3
[0144] The "shape of pattern" in Table 2 represents the fidelity of
the patterns formed by the printing method. Accordingly, the
expression "bad" with regard to "shape of pattern" means a
situation in which a pattern of the etching mask is not formed to
an extent where it can be used for device production. On the other
hand, the expression "good" with regard to "shape of pattern" means
a situation in which a pattern of the etching mask is reproduced to
an extent where it can be used for device production. Moreover, the
expression "bad" with regard to "stringiness" in Table 2 means a
state in which a portion of the etching mask layer with which a
pattern has been formed by the printing method is drawn as a strand
from the etching mask layer, and the desired pattern is disturbed.
Furthermore, the expression "good" with regard to "stringiness"
means a state in which almost no or completely no stringiness is
observed. In addition, "height of pattern" in Table 2 indicates the
highest height of a pattern as measured under an atomic force
microscope (AFM). Furthermore, the expression "unmeasurable" with
regard to "height of pattern" for Sample (6) means a situation in
which a pattern itself was not essentially formed.
[0145] In addition to Table 2 above, the results of the above (1),
(7), and (8) are shown in FIG. 1, as examples of typical optical
photomicrographs showing the realization of good patterns. The
results of the above (5), (9), and (10) are shown in FIG. 2A as
examples of typical optical photomicrographs showing unfavorable
patterns formed as a result of uncontrolled stringiness.
[0146] As depicted in Table 2, FIG. 1, and FIG. 2A, relatively
low-molecular-weight aliphatic polycarbonate, (1) "Sample A", (2)
"Sample B", and (3) "Sample AB", and moderate-molecular-weight
aliphatic polycarbonate, (7) "Sample AC" were found to have good
"shape of pattern" and "stringiness". In particular, Sample C ((4)
in Table) is indicated to have "bad" stringiness in Table 2,
however, stringiness was observed only partially on the
pattern.
[0147] The thus obtained results indicate that "shape of
patterning" or "stringiness" becomes worse due to an increase in
molecular weight. Meanwhile, for example, a pattern formed by a
screen printing method preferably has a "height" at a level or
higher. It was also found that the adoption of aliphatic
polycarbonate having a significantly low molecular weight is not
preferred in order to achieve "height of pattern" at a level or
higher while maintaining good "shape of pattern" and
"stringiness".
[0148] In addition, the inventors of this application consider the
cause of "bad" "shape of patterning" and "bad" "stringiness" of (9)
"Sample AD" and (10) "Sample BD" as follows.
[0149] As depicted in "height of pattern" in Table 2, each height
of pattern (5.3 .mu.m) of (9) "Sample AD" and (10) "Sample BD" is
almost the same as the sum of the height of pattern (1.4 .mu.m) of
"Sample A" and the height of pattern (4 .mu.m) of "Sample D", or,
the sum of the height of pattern (1.5 .mu.m) of "Sample A" and the
height of pattern (4 .mu.m) of "Sample D". Accordingly, it is
considered that the use of a sample prepared by dissolving only one
type of PPC as a binder results in a situation where
high-molecular-weight aliphatic polycarbonate and
low-molecular-weight aliphatic polycarbonate become immiscible due
to namely, phase separation, when a difference in molecular weight
between the two exceeds a level. More specifically, there can be a
situation where high-molecular-weight aliphatic polycarbonate is
placed on low-molecular-weight aliphatic polycarbonate, or a
situation opposite to that. Meanwhile, (7) "Sample AC" or (8)
"Sample BC" having relatively a small difference in molecular
weight were considered to be in a state in which the
high-molecular-weight aliphatic polycarbonate and the
low-molecular-weight aliphatic polycarbonate are mixed
appropriately without undergoing namely, phase separation. This is
considered to be the reason of their good "shape of patterning" and
"stringiness".
[0150] Therefore, the above experimental results can be said to
indicate that even when materials of the same kind were used,
specifically, when multiple types of aliphatic polycarbonate having
different mass-average molecular weights were used, simple
equalization of the mass-average molecular weights of the multiple
types of aliphatic polycarbonate cannot always lead to a molecular
weight appropriate for obtaining good "shape of patterning" and
"stringiness".
[0151] Each of the above results and the results of examination and
analyses made by the inventors of this application concerning other
molecular weights together indicate that good "shape of pattern"
and "stringiness" can be realized by adopting aliphatic
polycarbonate having a molecular weight of 6000 or more and 400000
or less and constituting 80% by mass or more of the entire
aliphatic polycarbonate.
[0152] Furthermore, the results from (1) to (10) revealed that the
data from the samples of the oxide semiconductor precursors in
which one of the 4 types of PPCs having different mass-average
molecular weights was dissolved as a binder, or the samples of the
oxide semiconductor precursors in which two of the PPCs as binders
were dissolved in combination showed a similar trend. It is noted
that each of the samples of the oxide semiconductor precursors
contains 5% by mass of a solution containing 0.2 mol/kg of indium.
Moreover, the sample numbers used in Table 3 are the same as those
used in Table 1 for clarifying the correspondence relationship with
(1) to (10) in Table 1.
[0153] Production examples of oxide precursors are as follows. It
is noted that in each of the following experimental examples, an
oxide semiconductor precursor, i.e., a precursor to be oxidized
into an oxide semiconductor is used as a representative
example.
[0154] First, indium acetylacetonate and propionic acid were
gradually mixed in a 50 mL flask with stirring to obtain an
indium-containing solution which will eventually form indium
oxide.
[0155] Next, polypropylene carbonate was dissolved in a solvent
mixture of DEGMEA and 2-nitropropane in a 50 mL eggplant-shaped
flask to obtain a solution of 25 wt % polypropylene carbonate.
[0156] Subsequently, the above indium-containing solution was
gradually added to the above solution of polypropylene carbonate to
obtain the above oxide semiconductor precursor.
[0157] Experimental results shown in Table 3 will be described. (1)
"Sample A", (2) "Sample B", and (3) "Sample AB" as aliphatic
polycarbonate having a relatively low molecular weight, and (7)
"Sample AC" as aliphatic polycarbonate having a medium molecular
weight were found to show good "shape of pattern" and
"stringiness". Interestingly, Samples (9) and (10) having a
relatively large molecular weight were also found to show good
pattern shapes and stringiness. FIG. 2B represents an image taken
under an optical microscope showing the results from (9) and (10)
in which good patterns were formed as a result of successful
control of stringiness. The good pattern shapes and stringiness for
the samples (9) and (10) can likely be explained in part by a
somewhat decreased overall concentration of PPC due to addition of
the indium-containing solution to the samples used in Table 2.
However, this does not appear to be the predominant factor.
TABLE-US-00003 TABLE 3 Height of Shape of pattern Sample pattern
Stringiness (.mu.m) (1) Bad Good 1 (2) Bad Good Unmeasurable (3)
Good Good 1.2 (4) Bad Bad 4.4 (5) Bad Bad Unmeasurable (6) Bad Bad
Unmeasurable (7) Good Good 3.2 (8) Good Good 2.6 (9) Good Good 3.7
(10) Good Good 5.3
Experimental Example 2
[0158] Next, in the following experiment, the inventors of this
application measured the average length of drawn out strands (mm)
and zero-shear viscosity (Pas) of the samples (1) to (10) used in
experimental example 1.
[0159] In this experiment, first, regarding the average length of
drawn-out strands (mm), a cylindrical bar made of
polytetrafluoroethylene and having a diameter "D" of 2.9 mm was
dipped in a collection of aliphatic polycarbonate, which had been
formed using each sample, within a container. When the cylindrical
bar was then raised at a velocity "v" of 5 mm/second, and then the
length "L" (mm) of a strand drawn out from the outermost surface of
the collection of the aliphatic polycarbonate was measured. In
addition, the zero-shear viscosity .eta. of each sample was
measured using a rheometer (TA Instruments, AR-2000EX). Each of the
above values was substituted as an evaluation parameter into the
following formula for calculation.
"Evaluation parameter"
(mm.sup.-1Pa.sup.-1)=L/(D.times.v.times..eta.) <Formula>
[0160] Table 4 shows the relationship between each sample, and
average length of drawn-out strands (mm) and zero-shear viscosity
(Pas) in this Experimental example. FIG. 3A is a graph showing the
relationship between each sample and evaluation parameter
representing stringiness in this Experimental example.
TABLE-US-00004 TABLE 4 Average length of Zero-shear Evaluation
drawn-out strands viscosity parameter Sample (mm) (Pa s) (mm.sup.-1
Pa.sup.-1) (1) 2.62 0.14 1.31 (2) 2.74 0.41 0.47 (3) 2.59 0.27 0.67
(4) 113.51 58.7 0.13 (5) 129.58 392.5 0.02 (6) 124.77 279.4 0.03
(7) 51.37 5.7 0.62 (8) 54.80 7.7 0.49 (9) 38.36 11.8 0.22 (10)
65.45 29.4 0.15
[0161] The results in Table 4 and FIG. 3A showed that for a sample
prepared by dissolving only one type of PPC as a binder, good
"shape of patterning" and "stringiness" can be obtained when the
value of the "evaluation parameter" (mm.sup.-1Pa.sup.-1) calculated
by the above formula is 0.4 mm.sup.-1Pa.sup.-1 or more.
[0162] Meanwhile, as indicated in Table 5 below, the results from
(1) to (10) showed that the data from the samples of the oxide
semiconductor precursors in which only one of the 4 types of PPCs
having different mass-average molecular weights was dissolved as a
binder, or the samples of the oxide semiconductor precursors in
which two of the PPCs as binders were dissolved in combination had
a similar trend. It is noted that the sample numbers used in Table
5 are the same as those used in Table 1 for clarifying the
correspondence relationship with (1) to (10) in Table 1.
[0163] As shown in Tables 4 and 5 and FIGS. 3A and 3B, the
"evaluation parameter" generally showed a larger value when the
above samples of the oxide semiconductor precursors were used as
compared with a case where samples having only one type of PPC
dissolved as a binder were used. However, the inventors of this
application assume that an impact of indium acetylacetonate, which
is a solute contained in a very small amount, is very small. That
is, the inventors of this application conclude that the results
shown in Table 4 and FIG. 3A represent authentic and more
generalizable experimental results.
[0164] Therefore, the results in Tables 3 to 5 and FIGS. 3B and 2B
taken together for analysis based on the results from (9) and (10)
in Table 5 revealed that good "shape of patterning" and
"stringiness" were able to be obtained when the value of the
"evaluation parameter" (mm.sup.-1Pa.sup.-1) was 0.25
mm.sup.-1Pa.sup.-1 or more (more strictly, 0.29 mm.sup.-1Pa.sup.-1
or more). It is noted that there is no particular limitation for
the upper limit of the "evaluation parameter" for obtaining good
"shape of patterning" and "stringiness," but it is preferably 1.2
or less (more strictly, 0.9 or less) in order to obtain the height
of a pattern more reliably.
TABLE-US-00005 TABLE 5 Average length of Zero-shear Evaluation
drawn-out strands viscosity parameter Sample (mm) (Pa s) (mm.sup.-1
Pa.sup.-1) (1) 2.33 0.10 1.56 (2) 2.65 0.29 0.64 (3) 2.57 0.21 0.84
(4) 92.71 54.9 0.12 (5) 126.65 340.5 0.03 (6) 115.49 96.9 0.08 (7)
29.69 2.5 0.83 (8) 36.47 3.4 0.73 (9) 30.26 7.2 0.29 (10) 65.96
15.6 0.29
(Other Preferable Molecular Weight Ranges of Aliphatic
Polycarbonate)
[0165] In addition, the molecular weight range of aliphatic
polycarbonate in this embodiment is not limited to the numeric
range disclosed in each of the above experimental examples. As a
result of analyses made by the inventors of this application, for
example, aliphatic polycarbonate having a molecular weight of 6000
or more and 300000 or less constitutes 75% by mass or more of the
entire aliphatic polycarbonate in order to more reliably control
stringiness and form a good pattern in a further preferred
aspect.
Experimental Example 3
(Evaluation of Contact Angle and Expansion Ratio)
[0166] As already described, aliphatic polycarbonate which can
serve as an etching mask is to be decomposed and/or removed
primarily in a heating step after the target of etching is etched
using a pattern formed, for example, by the screen printing method.
Therefore, the etching mask is only required for a limited time
period. Nonetheless, the inventors of this application assumed that
evaluation of the contact angle between an etching mask precursor
and a base material when a solution containing aliphatic
polycarbonate (that is, an "etching mask precursor") is placed on
the base material can provide an indicator suitable for determining
whether or not the solution has a pattern formability (in other
words, well-balanced viscosity or wettability) enough for
persisting during the limited time period.
[0167] Accordingly, the inventors of this application conducted the
following study: a solution of polypropylene carbonate obtained by
dissolving polypropylene carbonate (25 wt %) in a solvent mixture
of DEGMEA and 2-nitropropane was placed on a base material (in this
experimental example, a glass substrate), and then the contact
angle between the base material and the solution and the expansion
ratio of the solution on the base material were determined at 30
seconds and 120 seconds after placement to investigate how they
changed over time. It is noted that the above evaluation was
performed as a function of the concentration (wt %) of
2-nitropropane, which is a solvent suitable for aliphatic
polycarbonate, in order to facilitate the study about how the
contact angle changes. Further, the above "expansion ratio" was
calculated based on the actual size of a pattern with respect to
the designed value.
[0168] FIG. 4A is a graph showing the contact angle between a base
material and a solution, and expansion ratio of the solution on the
base material with respect to changes in the concentration of
2-nitropropane at 30 seconds after placement of the solution
containing aliphatic polycarbonate on the base material.
Furthermore, FIG. 4B is a graph showing the contact angle between a
base material and a solution and expansion ratio of the solution on
the base material with respect to changes in the concentration of
2-nitropropane at 120 seconds after placement of the solution
containing aliphatic polycarbonate on the base material.
[0169] As depicted in FIG. 4A and FIG. 4B, the contact angle
between the base material and the solution, and expansion ratio of
the solution on the base material were found to be inversely
correlated to each other. Specifically, it was revealed that as the
concentration of 2-nitropropane increased, the contact angle
increased and the expansion ratio decreased. However, as depicted
in FIG. 4B, even when the concentration of 2-nitropropane was high
(for example, 75%), there was a case in which the contact angle did
not increase. Moreover, it was revealed that particularly the
contact angle was observed to tend to sharply increase when the
concentration of 2-nitropropane reached a level (typically, 55% or
more and 60% or less), as depicted in both FIG. 4A and FIG. 4B.
[0170] According to further research and analyses made by the
inventors of this application, it was found that a case in which
the contact angle exceeds 36.degree. at the stage of 30 seconds
after placement, or, a case in which the contact angle exceeds
32.degree. at the stage of 120 seconds after placement results in a
situation where a solution containing aliphatic polycarbonate is
repelled from the base material, making it difficult to
sufficiently form a pattern that can be kept for a temporary time.
Accordingly, a solution containing aliphatic polycarbonate is
preferably prepared to achieve the above contact angle or less.
[0171] On the other hand, the results also revealed that when the
contact angle at 30 seconds after the placement was less than
30.degree., or the contact angle at 120 seconds after the placement
was less than 26.degree., the reproducibility of patterns was poor,
and thus a pattern substantially persisting for a temporary time
was difficult to be formed. Therefore, the solution containing
aliphatic polycarbonate is preferably prepared so as to show an
angle larger than each of the above contact angles. Considering all
of the above, the contact angle of an etching mask precursor and a
base material is preferably 30.degree. or more and 36.degree. or
less at 30 seconds after placement of the etching mask precursor on
the base material, or the contact angle of an etching mask
precursor and a base material is preferably 26.degree. or more and
32.degree. or less at 120 seconds after placement of the etching
mask precursor on the base material.
[0172] Furthermore, the results depicted in FIG. 4A and FIG. 4B
above were confirmed to correlate with the results of samples of
the oxide semiconductor precursor explained in "preparation step
for each experiment", which were each prepared by dissolving one of
4 types of PPC differing in mass-average molecular weight as a
binder, or, samples of the oxide semiconductor precursor prepared
by dissolving a combination of any 2 out of the 4 types of the same
as a binder.
[0173] As described above, it can be confirmed that the result of a
solution containing aliphatic polycarbonate, but containing no
oxide precursor in the experimental example was approximately
equivalent to that of a solution containing aliphatic polycarbonate
and an oxide precursor in the Example.
2. Oxide Precursor, Configuration of Oxide Layer, and Method of
Producing these
[0174] In this embodiment, an "oxide precursor" represents a
typical aspect in which aliphatic polycarbonate is mixed with a
compound of a metal to be oxidized into a metal oxide. Therefore, a
typical example of the above oxide precursor is a solution
containing aliphatic polycarbonate in which a compound of a metal
to be oxidized into a metal oxide is dispersed. Further, as already
described, examples of the above metal oxide include oxide
semiconductors, oxide conductive materials, or oxide insulators. It
is noted that once a pattern is formed, for example, by the screen
printing method, aliphatic polycarbonate may be considered as an
impurity with respect to the metal oxide finally obtained, and thus
it will be decomposed and/or removed primarily in a heating
step.
[0175] It is noted that examples of the metal oxide in this
embodiment include oxide semiconductors, oxide conductive
materials, or oxide insulators. Further, in this embodiment, the
percentage of aliphatic polycarbonate having a molecular weight of
6000 or more and 400000 or less is 80% by mass or more relative to
the entire aliphatic polycarbonate as in the case of the etching
mask precursor or etching mask. In addition, materials and
production methods which can be used for the etching mask precursor
or etching mask may also be used except that the aforementioned
metal compound is mixed. Moreover, as a typical example, a method
of producing an oxide semiconductor precursor and an oxide
semiconductor layer are disclosed in detail in Patent Document 4
(WO2015-019771) filed by the applicant of this application.
Therefore, descriptions common with the etching mask precursor or
etching mask will be omitted.
Second Embodiment
1. Overall Structure of Thin Film Transistor According to Present
Embodiment
[0176] FIGS. 5 to 9B, 9D, 10A, 14, and 15 are each a schematic
sectional view of a process in the method of producing a thin film
transistor 100 as an exemplary semiconductor element. It is noted
that FIG. 15 is a schematic sectional view of a process in the
method of producing the thin film transistor 100 according to the
present embodiment and the overall structure of the thin film
transistor. As shown in FIG. 15, the thin film transistor 100
according to the present embodiment includes a substrate 10, on
which a gate electrode 24, a gate insulator 34, a channel 44, a
source electrode 58, and a drain electrode 56 are stacked from the
lower side in the mentioned order. Further, production or
implementation of an electronic device including the above
semiconductor element (e.g. a mobile terminal, an information
appliance, or any other publicly known electric appliance) can be
fully understood without any further explanation by a person
skilled in the art who understands the semiconductor element
according to the present embodiment. Further, various steps for
forming an oxide precursor layer, which are described below, are
included in the "oxide precursor layer forming step" in this
application.
[0177] The thin film transistor 100 has the so-called bottom gate
structure, although this embodiment is not limited to this
structure. A person skilled in the art having ordinary technical
knowledge can thus form the top gate structure by changing the
order of the steps with reference to the description of this
embodiment. Temperatures indicated in this application are surface
temperatures of the heating surface of a heater, which comes into
contact with a substrate. Patterning of an extraction electrode
from each electrode is not depicted in order for simplification of
the drawings.
[0178] There is no particular limitation for the substrate 10
according to the present embodiment, but a substrate commonly used
for semiconductor elements may be used. Various base materials may
be used, including highly heat resistant glass, an SiO.sub.2/Si
substrate (i.e. a substrate in which an oxide silicon film is
formed on a silicon substrate), an alumina (Al.sub.2O.sub.3)
substrate, an STO (SrTiO) substrate, an insulating substrate in
which an STO (SrTiO) layer is formed over a surface of an Si
substrate through a SiO.sub.2 layer and a Ti layer, and the like,
and semiconductor substrates (e.g. a Si substrate, a SiC substrate,
and a Ge substrate). It is noted that examples of the insulating
substrate include films or sheets made of materials such as
polyesters including polyethylene terephthalate and polyethylene
naphthalate; polyolefins; cellulose triacetate; polycarbonate;
polyamide; polyimide; polyamide imide; polysulfone; aramid; and
aromatic polyamide. Further, there is no particular limitation for
the thickness of a substrate, but it is, for example, 3 .mu.m or
more and 300 .mu.m or less. Moreover, the substrate may be hard, or
may be flexible.
2. Method of Manufacturing Thin Film Transistor According to
Present Embodiment
(1) Formation of Gate Electrode
[0179] In this embodiment, a compound of a metal to be oxidized
into an oxide conductive material (hereinafter, may also simply be
referred to as an "oxide conductive material") may be used as a
material for the gate electrode 24. In this case, the gate
electrode 24 in this embodiment is formed by annealing a layer of
an oxide conductive material precursor (hereinafter, may also be
referred to as an "oxide conductive material precursor layer") in
which an oxide conductive material (may include inevitable
impurities. The same shall apply to oxides of other materials in
addition to the oxide of this material) is dispersed in a solution
containing aliphatic polycarbonate. In this embodiment, as shown in
FIG. 5, a gate electrode precursor layer 22 can be formed by
applying a gate electrode precursor solution as a starting material
onto a SiO.sub.2/Si substrate 10 as a base material (hereinafter,
may also simply be referred to as a "substrate") by a low-energy
production process (for example, a printing method or a spin
coating method).
[0180] Subsequently, an annealing step for annealing the gate
electrode precursor layer 22 in air for a predetermined time period
(for example, 10 minutes to 1 hour) at 450.degree. C. to
550.degree. C. is performed, for example. As a result, as depicted
in FIG. 6, the gate electrode 24 is formed on the substrate 10. In
addition, the thickness of the layer of the gate electrode 24 in
this embodiment is about 100 nm, for example.
[0181] Here, one example of the above oxide conductive material is
a material having a structure (typically a complex structure) in
which a ligand is coordinated with a metal to be oxidized into an
oxide conductive material. Examples of the oxide conductive
material according to the present embodiment can include metal
organic acid salts, metal inorganic acid salts, metal halides, and
various metal alkoxides. It is noted that an example of a metal to
be oxidized into an oxide conductive material is ruthenium (Ru). In
this embodiment, an annealing step is performed to form ruthenium
oxide as an oxide conductive material, with which the gate
electrode 24 can be formed, the annealing step including annealing
a gate electrode precursor solution, for example, in air for a
predetermined time period (for example, 10 minutes to 1 hour) at
about 450.degree. C. to about 550.degree. C., the gate electrode
precursor solution being prepared using a solution as a starting
material in which ruthenium (III) nitrosylacetate is dissolved in a
solvent mixture of propionic acid and 2-aminoethanol containing
aliphatic polycarbonate.
[0182] In this embodiment, in particular, when a gate electrode
precursor solution containing the aliphatic polycarbonate described
in the first embodiment is used, a good pattern of the gate
electrode precursor layer 22 can be formed by a printing method.
More specifically, the stringiness of aliphatic polycarbonate in
the gate electrode precursor solution can be controlled in an
appropriate manner, leading to formation of a good pattern of the
gate electrode precursor layer 22.
[0183] In this embodiment, instead of the above gate electrode 24,
high melting metal such as platinum, gold, silver, copper,
aluminum, molybdenum, palladium, ruthenium, iridium, or tungsten, a
metal material such as an alloy thereof, a pt silicon layer, or an
n.sup.+-silicon layer can be applied, for example. In this case,
the gate electrode 24 can be formed on the substrate 10 in
accordance with a known sputtering technique or a known CVD
technique.
(2) Formation of Gate Insulator
[0184] Furthermore, in this embodiment, a gate insulator 34 can be
formed by annealing a layer of an oxide insulator precursor
(hereinafter, may also be referred to as an "oxide insulator
precursor layer") in which a compound of a metal to be oxidized
into an oxide insulator (hereinafter may also simply be referred to
as an "oxide insulator") is dispersed in a solution containing
aliphatic polycarbonate.
[0185] Specifically, as depicted in FIG. 7, a gate insulator
precursor layer 32 is formed by applying the above oxide insulator
precursor onto the gate electrode 24 in accordance with a
low-energy production process (e.g. a printing method or a spin
coating method).
[0186] The gate insulator precursor layer 32 in a gel state is then
subjected to the annealing (main annealing) step for annealing at
about 450.degree. C. to about 550.degree. C. for a predetermined
time period (e.g. 10 minutes to 1 hour) in air, for example,
thereby forming an oxide of lanthanum (La) and zirconium (Zr),
which is an oxide insulator. As a result, as depicted in FIG. 8,
the gate insulator 34 can be formed. In addition, the thickness of
the layer of the gate insulator 34 of this embodiment ranges from
about 100 nm to about 250 nm, for example.
[0187] An example of the above oxide insulator is a material having
a structure (typically a complex structure) wherein a ligand is
coordinated with a metal to be oxidized into an oxide insulator.
Examples of the oxide insulator of this embodiment can include
metal organic acid salts, metal inorganic acid salts, metal
halides, and various metal alkoxides, or, other organic acid salts,
inorganic acid salts, halides, or various alkoxides.
[0188] It is noted that a typical example of the oxide insulator is
an oxide including lanthanum (La) and zirconium (Zr). This oxide
can be used as the gate insulator 34. In this embodiment, a first
solution is prepared in which lanthanum (III) acetate is dissolved
in propionic acid (a solvent) containing aliphatic polycarbonate,
and a second solution is prepared in which zirconium butoxide is
dissolved in propionic acid (a solvent) containing aliphatic
polycarbonate. A gate insulator precursor solution as a starting
material in which the first solution and the second solution are
mixed can be subjected to an annealing step for annealing, for
example, in air for a predetermined time period (for example, 10
minutes to 1 hour) at about 450.degree. C. to about 550.degree. C.
to form an oxide insulator.
[0189] In this embodiment, in particular, when an oxide insulator
precursor containing the aliphatic polycarbonate described in the
first embodiment is used, a good pattern of the gate insulator
precursor layer 32 can be formed by a printing method. More
specifically, the stringiness of aliphatic polycarbonate in the
oxide insulator precursor can be controlled in an appropriate
manner, leading to formation of a good pattern of the gate
insulator precursor layer 32.
[0190] Moreover, in this embodiment, oxide silicon or silicon
oxynitride can be applied, for example, instead of the above gate
insulator 34. In this case, the gate insulator 34 can be formed on
the gate electrode 24 by a known CVD technique, for example.
(3) Formation of Channel
[0191] Further, in this embodiment, a channel 44 can be formed by
annealing a layer of an oxide semiconductor precursor (hereinafter,
may also be referred to as an "oxide semiconductor precursor
layer") in which a compound of a metal to be oxidized into an oxide
semiconductor (hereinafter, may also simply be referred to as
"oxide semiconductor") is dispersed in a solution containing
aliphatic polycarbonate. In this embodiment, a channel precursor
layer 42 can be formed by applying a channel precursor solution as
a starting material onto the gate insulator 34 by a low-energy
production process (for example, a printing method or a spin
coating method) as shown in FIG. 9A.
[0192] Subsequently, the channel precursor layer 42 is subjected to
an annealing step described below to form the channel 44 as shown
in FIG. 10A.
[0193] Here, an example of the oxide semiconductor is a material
having a structure (typically a complex structure) in which a
ligand is coordinated with a metal to be oxidized into an oxide
semiconductor. Examples of a material to be used for forming the
oxide semiconductor according to the present embodiment can include
metal organic acid salts, metal inorganic acid salts, metal
halides, and various metal alkoxides. It is noted that a typical
example of the oxide semiconductor is indium-zinc oxide. For
example, indium-zinc oxide (hereinafter, may also be referred to as
"InZnO") as an oxide semiconductor can be formed by performing an
annealing step of annealing a solution in which indium
acetylacetonate and zinc chloride are dissolved in propionic acid
including aliphatic polycarbonate (may also be referred to as an
"InZn solution") at 450.degree. C. to 550.degree. C. for a
predetermined time (for example, 10 minutes to 1 hour) in air. As a
result, the channel 44 can be formed.
[0194] Examples of the metal to be oxidized into an oxide
semiconductor include one, two, or more members selected from the
group consisting of indium, tin, zinc, cadmium, titanium, silver,
copper, tungsten, nickel, indium-zinc, indium-tin,
indium-gallium-zinc, antimony-tin, and gallium-zinc. In terms of
element performance, stability, and the like, indium-zinc is
preferably adopted as the metal to be oxidized into an oxide
semiconductor.
[0195] In this embodiment, in particular, when a channel precursor
solution containing the aliphatic polycarbonate described in the
first embodiment is used, a good pattern of the channel precursor
layer 42 can be formed by a printing method. More specifically, the
stringiness of aliphatic polycarbonate in the channel precursor
solution can be controlled in an appropriate manner, leading to
formation of a good pattern of the channel precursor layer 42.
[0196] Moreover, in this embodiment, in particular, when the
channel 44 as an oxide semiconductor layer is formed, a method of
producing a metal oxide according to one embodiment of the
invention which has been invented by the inventors of this
application, for example, a method disclosed in Patent Document 4,
can be used as a preferred example.
[0197] A typical method of forming the channel 44 involves: a
precursor layer forming step of forming an oxide semiconductor
precursor on or above a substrate to form a layer, the oxide
semiconductor precursor having a compound of a metal to be oxidized
into the oxide semiconductor dispersed in a solution containing
aliphatic polycarbonate; and an annealing step of heating the
precursor layer to a first temperature at which 90 wt % or more of
aliphatic polycarbonate is decomposed, and then annealing the
precursor layer at a temperature higher than the first temperature
and equal to or higher than a second temperature at which the metal
binds to oxygen, and where an exothermic peak is observed as
determined by differential thermal analysis (DTA) of the precursor
or the metal compound.
<TG-DTA (Thermogravimetry and Differential Thermal Analysis)
Properties>
[0198] More specifically, FIG. 11 is a graph showing the TG-DTA
properties of an indium-zinc-containing solution as an example of a
material for forming the oxide precursor according to the first
embodiment, which will serve as an oxide semiconductor precursor
for forming a channel of a thin film transistor. Further, FIG. 12
is a graph showing an example of the TG-DTA properties of a
solution of polypropylene carbonate as an example of a solution (a
typical example of an etching mask precursor) containing aliphatic
polycarbonate alone as a solute for forming a component (for
example, a channel) of a thin film transistor. It is noted that as
shown in FIGS. 11 and 12, solid lines in the figures represent
results from thermogravimetric (TG) measurements, and dotted lines
in the figures represent results from differential thermal analysis
(DTA) measurements.
[0199] Results from the thermogravimetric measurements in FIG. 11
showed a significant weight loss at around 120.degree. C. probably
due to evaporation of the solvent. The results also showed an
exothermic peak at around 330.degree. C. in the graph of the
differential thermal analysis of the InZn solution as indicated by
(X) in FIG. 11. This thus indicates that indium and zinc are bound
with oxygen at around 330.degree. C. Therefore, this temperature of
330.degree. C. corresponds to the aforementioned second
temperature.
[0200] In contrast, the results in FIG. 12 from the
thermogravimetric measurements showed a significant weight loss at
around 140.degree. C. to 190.degree. C. due to partial
decomposition or elimination of polypropylene carbonate itself as
the solvent in the solution of polypropylene carbonate disappeared.
It is noted that polypropylene carbonate appears to be converted
into carbon dioxide and water by the above decomposition. The
results in FIG. 12 also showed that 90 wt % or more of aliphatic
polycarbonate was decomposed and removed at around 190.degree. C.
Therefore, this temperature of 190.degree. C. corresponds to the
aforementioned first temperature. More detailed analysis revealed
that 95 wt % or more of aliphatic polycarbonate was decomposed at
around 250.degree. C., and almost all (99 wt % or more) of
aliphatic polycarbonate was decomposed at around 260.degree. C.
Therefore, use of an etching mask precursor including a type of
aliphatic polycarbonate which can be substantially or predominantly
eliminated or removed by heat treatment at 250.degree. C. or more
(more preferably 260.degree. C. or more) can reliably reduce or
prevent effects on a layer to be etched (typically, adverse effects
on electrical properties). It is noted that the above results were
obtained when aliphatic polycarbonate was decomposed by relatively
brief heat treatment. However, aliphatic polycarbonate was found to
be substantially decomposed even at a lower temperature (for
example, 180.degree. C.) when a longer heat treatment is performed.
Therefore, use of an etching mask precursor including a type of
aliphatic polycarbonate which can be substantially or predominantly
eliminated or removed by heat treatment at 180.degree. C. or more
can reduce or prevent effects on a layer to be etched (typically,
adverse effects on electrical properties).
[0201] Further, studies conducted by the inventors of this
application showed that when heated (for example, using a known
heater) while applying ultraviolet rays (for example, a
low-pressure mercury lamp (Model UV-300 H-E, SAMCO) is used, and
the ultraviolet rays have wavelengths of 185 nm and 254 nm), the
first temperature was able to be lowered by tens of degrees (for
example, about 30.degree. C.) as compared with a temperature when
heated without use of ultraviolet irradiation. Therefore, in a
heating step for decomposing aliphatic polycarbonate, heating is
preferably performed while applying ultraviolet rays to each oxide
layer precursor and/or etching mask at or above a temperature where
organic substances, for example, the aforementioned organic
solvent, the organic portion of the aforementioned complex
structure, or the aforementioned aliphatic polycarbonate are
decomposed. This is because the final oxide layer preferably
includes the organic portions of the aforementioned solute, the
aforementioned solvent, or the aforementioned aliphatic
polycarbonate as less as possible. It is noted that the ultraviolet
rays do not necessarily need to be applied after decomposition of
the organic substances in the above heating step, but in a more
preferred aspect, heating is performed while applying ultraviolet
rays to each oxide precursor layer and/or etching mask at or above
a temperature where the oxide layer is formed. It is noted that
there is no particular limitation for the wavelengths of the
ultraviolet rays. Ultraviolet rays having wavelengths other than
185 nm or 254 nm can show a similar effect.
[0202] The temperature (the second temperature) where an exothermic
peak is observed in differential thermal analysis (DTA) at which a
metal in an oxide semiconductor binds to oxygen is preferably
sufficiently higher than the temperature at which aliphatic
polycarbonate is decomposed, or the temperature at which aliphatic
polycarbonate is decomposed is sufficiently lower than the
temperature (the second temperature) at which the exothermic peak
is observed. If that is the case, 90 wt % or more (more preferably
95 wt % or more, even more preferably 99 wt % or more, and most
preferably 99.9 wt % or more) of aliphatic polycarbonate can be
decomposed more reliably.
[0203] Further, an etching mask can be reliably eliminated or
removed when heat treatment is performed at a temperature where
aliphatic polycarbonate is decomposed (typically, at the first
temperature or above, preferably 180.degree. C. or above, more
preferably 250.degree. C. or above, and even more preferably
260.degree. C. or above). Strikingly, this can reliably reduce or
eliminate effects on the electrical properties of an oxide
(including not only a semiconductor but also a conductive material
or an insulator) formed by heating at the second temperature or
above.
[0204] According to the research and analyses conducted by the
inventors of this application, a difference between the first
temperature and the second temperature is 10.degree. C. or higher,
more preferably 50.degree. C. or higher, and further preferably
100.degree. C. or higher, leading to the reduction of impurities
represented by carbonous impurities remaining in the oxide
layer.
[0205] The oxide semiconductor is not particularly limited in terms
of its phase state. For example, the oxide semiconductor may be in
any one of the crystal form, the polycrystalline form, and the
amorphous form. The present embodiment can also cover a phase state
where grown crystal has a branch form or a scale form. Furthermore,
the present embodiment is obviously not particularly limited by a
patterned shape (e.g. a spherical shape, an elliptical shape, or a
rectangular shape).
(Step of Forming and Annealing Pattern of Channel Precursor
Layer)
[0206] Next, a specific method of forming a pattern of the channel
44 will be described. It is noted that this method of forming a
pattern of the channel 44 can also be applied partially or almost
completely to manufacture of the aforementioned oxide conductive
materials or oxide insulators (that is, various oxide layers).
[0207] As already described, in this embodiment, the channel
precursor layer 42 is formed by applying a channel precursor
solution onto the gate insulator 34 in accordance with a low-energy
production process (e.g., a spin coating method) as shown in FIG.
9A. It is noted that there is no particular limitation for the
thickness (wet) of the channel precursor layer 42 as an oxide
semiconductor precursor layer.
[0208] Subsequently, a channel precursor layer 42 having a
thickness of about 600 nm is formed by heating, for example, at
150.degree. C. for a predetermined period (e.g. 3 minutes) in the
preliminary annealing (may also be referred to as the "first
preliminary annealing") step. The first preliminary annealing step
is performed mainly for the purpose of promoting gelation and
fixation of the channel precursor layer 42 on the gate insulator
34.
[0209] In this embodiment, a pattern of an etching mask 80 formed
from the etching mask precursor according to the first embodiment
is then formed on the channel precursor layer 42 by the screen
printing method (an etching-mask forming step) as shown in FIGS. 9B
and 9C. It is noted that in FIG. 9C as well as FIGS. 9E and 10B
described below, only the "step of forming and annealing a pattern
of a channel precursor layer" is selected and imaged (plan view
photographs) for clarity.
[0210] Subsequently, a pattern of the channel precursor layer 42 is
formed using the etching mask 80. Specifically, the etching mask 80
and the channel precursor layer 42 are brought into contact with a
commercially available etching solution (Product name: "ITO-02,"
Kanto Chemical Co., Inc.) in which the oxide precursor layer can be
dissolved (typically, immersed into the etching solution) (a
contact step). As a result, a portion of the channel precursor
layer 42 not protected by the pattern of the etching mask 80 is
etched and removed as shown in FIGS. 9D and 9E. On the other hand,
a portion of the channel precursor layer 42 protected by the
pattern of the etching mask 80 is not etched, and remains on the
gate insulator 34.
[0211] Subsequently, a second preliminary annealing step (a drying
step) is performed at a predetermined temperature (the first
temperature) in order to decompose the etching mask 80 (in
particular, aliphatic polycarbonate) and aliphatic polycarbonate in
the channel precursor layer 42 with which the pattern has been
formed.
[0212] In the second preliminary annealing step according to the
present embodiment, heating is performed at a temperature where 90
wt % or more of aliphatic polycarbonate is decomposed. Through the
second preliminary annealing step and the main annealing (the
annealing step) described below, the etching mask 80 itself and
impurities contained in the final channel precursor layer 42 such
as, in particular, carbonous impurities originated from aliphatic
polycarbonate can be mostly eliminated. In order to reliably reduce
residual impurities in the channel 44 such as in particular
carbonous impurities originated from aliphatic polycarbonate, the
first temperature is preferably such that 95 wt % or more of
aliphatic polycarbonate is preferably decomposed at that
temperature, and more preferably such that 99 wt % or more of
aliphatic polycarbonate is decomposed at that temperature.
[0213] The second preliminary annealing step is not limited to
include drying at normal temperature under normal pressure. For
example, the second preliminary annealing step can include
treatment such as heating or decompressing such as drying by
heating, decompression drying, or decompression drying by heating,
as long as the substrate, the gate insulator, and the like are not
adversely affected. The second preliminary annealing step can
possibly influence the increase or decrease in surface roughness of
the oxide semiconductor layer. Because behavior during drying
differs depending on the solvent, conditions such as the
temperature (first temperature) in the second preliminary annealing
step are set appropriately in accordance with the type of the
solvent.
[0214] The second preliminary annealing according to the present
embodiment can be exemplified by heating the channel precursor
layer 42 at a temperature in the range of from 180.degree. C. or
higher to 300.degree. C. or lower for a predetermined period (e.g.
30 minutes). The above preliminary annealing is performed in an
oxygen atmosphere or in the atmosphere (hereinafter, also
collectively called an "oxygen-containing atmosphere"), for
example. According to an adoptable aspect, the second preliminary
annealing step is performed in a nitrogen atmosphere.
[0215] Subsequently, in the main annealing, i.e., "the annealing
step," the etching mask 80 and the channel precursor layer 42 are
heated, for example, under an oxygen-containing atmosphere for a
predetermined time period in a range of 180.degree. C. or above,
more preferably 250.degree. C. or above, even more preferably
300.degree. C. or above, and still more preferably 500.degree. C.
or above in terms of electrical properties. As a result, the
channel 44 as an oxide semiconductor layer is formed on the gate
insulator 34 as shown in FIGS. 10A and 10B. Strikingly, the etching
mask 80 is also decomposed and/or removed in the main annealing
step (the heating step in this embodiment). This means that in this
embodiment, removal of an etching mask and formation of an oxide
layer (the oxide-layer forming step) can be achieved
simultaneously. Therefore, use of the etching mask 80 can
significantly simplify the production process. It is noted that the
final thickness of an oxide semiconductor layer after the main
annealing is typically 0.01 .mu.m or more and 10 .mu.m or less. In
particular, it is worth noting that the resulting layer is
resistant to cracks even when it has a thickness as thin as about
0.01 .mu.m (or 10 nm).
[0216] Here, the preset temperature in the above annealing step is
selected so that it corresponds to a temperature at which a ligand
of an oxide semiconductor is decomposed in the oxide semiconductor
forming process, and a metal thereof is allowed to bind to oxygen,
and further so that it is equal to or above a temperature where an
exothermic peak is observed in differential thermal analysis (DTA)
as described above (the second temperature). In the above annealing
step, aliphatic polycarbonate, a dispersant, and an organic solvent
in the channel precursor layer 42 are to be reliably decomposed
and/or removed. It is noted that in a preferred aspect, the second
temperature is higher than the first temperature by 10.degree. C.
or more in order to reliably reduce residual impurities such as
carbonous impurities in the oxide semiconductor layer after the
main annealing. In addition, when the second temperature is higher
than the first temperature by 50.degree. C. or more, such residual
impurities can be reduced even more reliably. In the most preferred
aspect, the second temperature is higher than the first temperature
by 100.degree. C. or more in view of the controllability of the
thickness and/or the thinning of the final oxide semiconductor
layer as well as reduction of residual impurities. Nonetheless,
there is no particular limitation for the maximum difference
between the second temperature and the first temperature.
[0217] Our analysis suggests that aliphatic polycarbonate is mostly
decomposed when heated to the first temperature, and thus in the
subsequent annealing step (the main annealing) at the second
temperature, almost no decomposition reaction of aliphatic
polycarbonate takes place, and a reaction of bonding a metal with
oxygen takes place almost exclusively. That is, ideally, two
different functions of the first temperature and the second
temperature can allow cracks to be less generated even in the case
of a very thin layer as described above.
[0218] Meanwhile, the channel 44 protected by the etching mask 80
was found to show a rectification behavior as a semiconductor when
etching treatment was performed using the etching mask 80 according
to the present embodiment. Interestingly, a channel protected by
using a commercially available resist as an etching mask was,
however, found to show no rectification behavior under some
circumstances even though the conditions were the same except for
the material of the etching mask.
[0219] FIG. 13 is a graph showing the rectification behavior of the
channel 44 which is a part of the thin film transistor 100
according to the present embodiment. As shown in this graph, the
rectification behavior as a semiconductor can be obtained when
etching treatment was performed using the etching mask 80 according
to the present embodiment. This can be attributed to reliable
removal of the etching mask 80 in the heating step when forming a
pattern of the oxide layer, or in other words reliable removal of
residual impurities which may affect the electrical properties of a
semiconductor. Further, another striking advantage of the etching
mask 80 according to the present embodiment is that use of the
etching mask 80 according to the present embodiment can eliminate
any effect on the so-called back channel region, which otherwise
may occur upon detaching a resist. It is noted that the mixing
ratio (by the mass ratio) of indium and zinc used in the graph is
adjusted to 1:1 by adjusting the molar ratio per kg of indium
acetylacetonate and zinc chloride. However, the mixing ratio of
indium and zinc is not limited to the above ratio. For example, the
inventors of this application observed the rectification behavior
of the channel 44 even when the ratio of indium to zinc was 2:1.
Therefore, in a preferred aspect, the method of manufacturing a
thin film transistor includes an oxide-layer forming step of
forming an oxide layer (the channel 44) protected by the etching
mask 80 in a way such that it makes contact with the gate electrode
24 through the gate insulator 34, the oxide layer being formed via
the heating step in the method of producing an oxide layer
according to the present embodiment.
[0220] None of the first preliminary annealing step, the second
preliminary annealing step, and main annealing (the annealing step)
described above is particularly limited in terms of its
heating/annealing method. The heating method can be exemplified by
a conventional heating method with use of a thermostat, an electric
furnace, or the like. Particularly in a case where the substrate is
less heat-resistant, it is preferred to adopt a method of
heating/annealing only the oxide semiconductor layer by means of
ultraviolet rays, electromagnetic waves, or a lamp for prevention
of heat transfer to the substrate.
[0221] In the processes of removing the etching mask 80 and forming
the channel 44, aliphatic polycarbonate can contribute to reducing
or eliminating decomposition products remaining in the oxide
semiconductor layer after the annealing decomposition, and further
contribute to forming minute patterns of the oxide semiconductor
layer. Therefore, in a preferred aspect of the present embodiment,
aliphatic polycarbonate is used.
[0222] The studies conducted by the inventors of this application
also demonstrated that in this embodiment, the final thickness of
the channel 44 was able to be controlled by varying the weight
ratio of aliphatic polycarbonate and a compound of a metal to be
oxidized into an oxide semiconductor, or varying the concentration
of aliphatic polycarbonate or a compound of a metal. For example,
our studies showed that the channel 44 having a thickness ranging
from 10 nm to 50 nm, which can be considered as a very thin layer,
was able to be formed without generating cracks. It is noted that
in addition to a thin layer as described above, a layer having a
thickness of 50 nm or more can be formed relatively easily by
appropriately adjusting the thickness of the channel precursor
layer 42, the aforementioned weight ratio, and the like. In
general, a layer used for a channel has a thickness of 0.01 .mu.m
(i.e. 10 nm) or more and 1 .mu.m or less. This means that the oxide
semiconductor precursor and the oxide semiconductor layer according
to the present embodiment, in which the final thickness of the
channel 44 can be controlled, are suitable as a material for
manufacturing a thin film transistor.
[0223] In addition, when the oxide semiconductor precursor
according to the present embodiment is used, the resulting oxide
semiconductor precursor layer, which may be considerably thick
(e.g. 10 .mu.m or more) at the initial stage, can be made extremely
thin (e.g. 10 nm to 100 nm) after annealing because aliphatic
polycarbonate and the like are reliably decomposed in the
subsequent annealing step. Strikingly, no crack is developed, or
development of cracks is reliably reduced even when the resulting
layer is very thin as described above. This indicates that the
oxide semiconductor precursor and the oxide semiconductor layer
according to the present embodiment, which can be formed to have
both a sufficiently large initial thickness and a very thin final
thickness, are very suitable for a low-energy production process or
a known imprinting process. Further, use of the oxide semiconductor
layer, in which no crack is developed, or development of cracks is
reliably reduced even when it is very thin, will significantly
improve the stability of the thin film transistor 100 according to
the present embodiment.
[0224] Further, in this embodiment, the electrical properties and
stability of the oxide semiconductor layer which will form a
channel can be improved by appropriately selecting the type and
combination of the oxide semiconductors, and adjusting the mixing
ratio of the oxide semiconductor(s) and aliphatic
polycarbonate.
(4) Formation of Source Electrode and Drain Electrode
[0225] Subsequently, a layer of the etching mask 80 according to
the present embodiment patterned on the channel 44 was further
formed in a way similar as in the method of forming the channel 44.
Then, as shown in FIG. 14, an ITO layer 50 is formed on the channel
44 and the etching mask 80 by the publicly known sputtering method.
The target material used in this embodiment includes an ITO
containing, for example, 5 wt % of tin oxide (SnO.sub.2), and is
formed under conditions of room temperature to 100.degree. C. Once
the etching mask 80 is then removed by heating to about 250.degree.
C., a drain electrode 56 and a source electrode 58 made of the ITO
layer 50 is formed on the channel 44. As a result, the thin film
transistor 100 is manufactured as shown in FIG. 15. It is noted
that in another conceivable aspect, a pattern of a publicly known
resist layer formed by the conventional photolithographic method
may also be used in place of the etching mask 80.
[0226] Furthermore, in this embodiment, a method of forming a drain
electrode pattern and a source electrode pattern using paste silver
(Ag) or paste ITO (indium tin oxide) according to a printing
method, for example, instead of the above drain electrode 56 and
source electrode 58, is an adoptable aspect. Moreover, a gold (Au)
or aluminum (Al) drain electrode pattern and a gold (Au) or
aluminum (Al) source electrode pattern formed by a known
evaporation method may also be employed instead of the drain
electrode 56 and the source electrode 58.
Modification Example (1) of Second Embodiment
[0227] The thin film transistor according to this embodiment is
similar to the thin film transistor 100 according to the second
embodiment in terms of the production process and configuration
thereof except that etching treatment was performed using the
etching mask 80 after the step (the main annealing) of annealing a
channel according to the second embodiment. Therefore, descriptions
common with the first or second embodiment will be omitted.
[0228] In this embodiment, a step of forming a pattern of the
channel 44 is performed after forming the channel 44 as an oxide
semiconductor layer. Therefore, in this modification example (1),
etching of the oxide precursor layer to be oxidized into an oxide
layer as performed in the second embodiment is not performed.
[0229] FIGS. 16A and 16B are each a schematic sectional view of a
process in the method of producing a thin film transistor as an
example of a semiconductor element. It is noted that the structure
shown in FIG. 9A according to the second embodiment has been formed
before the step shown in FIG. 16A. Further, the structure shown in
FIG. 10A according to the second embodiment will be formed after
the step shown in FIG. 16B.
[0230] In a conceivable aspect, the channel 44 after the main
annealing is formed before performing etching treatment as shown in
this modification example (1). As shown in FIG. 16A, a pattern of
the etching mask 80 as described in the second embodiment is formed
on the channel 44. Then, the etching mask 80 and the channel 44 are
exposed to a solution for dissolving the channel 44 as an oxide
layer (typically immersed into this etching solution), and then
heated to or above a temperature at which the etching mask 80 is
decomposed. As a result, the channel 44 on which a pattern is
formed can be obtained as shown in FIG. 16B. It is noted that in
this modification example (1), a portion of the oxide layer (the
channel 44) protected by the etching mask 80 (more specifically, a
pattern of the etching mask 80) can also show a rectification
behavior as a semiconductor.
Modification Example (2) of Second Embodiment
[0231] Further, in the second embodiment or in the modification
example (1) of the second embodiment, a pattern of the channel 44
is formed via the contact step of allowing contact with a
predetermined etching solution as a solution for dissolving an
oxide precursor layer or a solution for dissolving an oxide layer,
but in the second embodiment, it is not limited to a treatment in
which the etching solution is used. For example, in a conceivable
aspect, an exposure step of allowing exposure to a publicly known
gaseous plasma for etching the channel precursor layer 42 as an
oxide semiconductor precursor layer according to the second
embodiment can be used to etch a region of the channel precursor
layer 42 which is not protected by the etching mask 80. As a
result, a region of the channel precursor layer 42 which has not
been exposed to the plasma will serve as the channel 44 after the
subsequent heating step. In addition, the etching mask 80 can be
decomposed and/or eliminated in the heating step. As a result,
decomposition and/or removal of the etching mask 80 can be achieved
in one treatment step (the heating step) along with formulation of
the channel 44. Further, the modification example (2) can also be
applied to the examples described in the modification example (1)
of the second embodiment. Specifically, in another conceivable
aspect, the step of forming a pattern of the channel 44 via the
exposure step of exposure to plasma may be performed after forming
the channel 44 as an oxide semiconductor layer. In all of the
aforementioned cases, the channel 44 on which a pattern is formed
can show a rectification behavior as a semiconductor.
Modification Example (3) of Second Embodiment
[0232] In the second embodiment, the etching mask 80 formed by the
screen printing method is used to form a pattern of the channel 44.
In another conceivable aspect, the etching mask 80 formed by the
screen printing method may be used to form a pattern of other
layers (for example, an oxide conductive material layer or an oxide
insulator layer). Even when the etching mask 80 is used for the
aforementioned other layers, the resulting oxide conductive
material layer on which a pattern is formed can show electric
conductivity, and the resulting oxide insulator layer on which a
pattern is formed can show insulation performance.
Alternative Embodiments (1)
[0233] In each of the above embodiments, an oxide precursor layer
or an oxide layer is a target of the etching treatment, but each of
the above embodiments is not limited to these examples. For
example, the etching mask used in each of the above embodiments can
be used to form a pattern of a metal by etching a portion of that
metal.
[0234] FIG. 17A is a plan view photograph showing the result of
etching treatment of a chromium (Cr) layer using the etching mask
according to the first embodiment. Further, FIG. 17B is a plan view
photograph showing the result of etching treatment of a chromium
(Cr) layer using an etching mask (a known resist mask) as
Comparative Example of FIG. 17A.
[0235] As shown in FIG. 17A, residues of the etching mask were not
visually observed when the etching treatment of a chromium (Cr)
layer was performed using the etching mask according to the first
embodiment. However, when the etching treatment of a chromium (Cr)
layer was performed using the resist mask according to Comparative
Example, many residues of the resist mask were visually observed as
shown in FIG. 17B. It is noted that this etching treatment was
found not to substantially alter the electrical properties
(electric conductivity) as a metal. Further, even when other metals
are etched, similar effects as the above are observed. Therefore,
use of the etching mask according to the first embodiment enables
the etching mask after etching treatment to be reliably removed
even when the target of etching treatment is a metal.
Alternative Embodiments (2)
[0236] Further, the etching mask used in each of the above
embodiments can be used, for example, to form a pattern of an
insulator by etching a portion of that insulator. FIG. 18 shows a
plan view photograph showing the result of etching treatment of a
silicon dioxide layer (SiO.sub.2) using the etching mask according
to the first embodiment.
[0237] As shown in FIG. 18, when the etching treatment of the
silicon dioxide (SiO.sub.2) layer was performed using the etching
mask according to the first embodiment, no residue of the etching
mask was visually observed. It is noted that this etching treatment
was found not to substantially alter the electrical properties
(insulation performance) of the silicon dioxide (SiO.sub.2) layer.
Further, even when other insulators are etched, similar effects as
the above are observed. Therefore, use of the etching mask
according to the first embodiment enables the etching mask after
etching treatment to be reliably removed even when the target of
etching treatment is an insulator.
Alternative Embodiments (3)
[0238] Meanwhile, in the second embodiment and the modification
examples thereof, a thin film transistor having a so-called
inverted-staggered structure is described, but each of the above
embodiments is not limited to this structure. For example, in
addition to a thin film transistor having the staggered structure,
a thin film transistor having the so-called planar structure in
which a source electrode, a drain electrode, and a channel are
disposed on the same plane can also show similar effects as does
each of the above embodiments. Further, in another conceivable
embodiment, the channel (that is, the oxide semiconductor layer) in
each of the above embodiments may be formed on a base material or a
substrate.
[0239] As described above, the above embodiments and experimental
examples have been disclosed not for limiting the present invention
but for describing these embodiments and experimental examples.
Furthermore, modification examples made within the scope of the
present invention, inclusive of other combinations of the
embodiments, will also be included in the scope of the patent
claims.
INDUSTRIAL APPLICABILITY
[0240] The present invention can be applied broadly to the fields
of electronic devices including a mobile terminal, an information
appliance, a sensor, or any other publicly known electric appliance
including various semiconductor elements, MEMS (Micro Electro
Mechanical Systems) or NEMS (Nano Electro Mechanical Systems), and
medical devices, for example.
* * * * *