U.S. patent application number 14/256267 was filed with the patent office on 2014-11-27 for thin-film transitor and method for manufacturing the same.
This patent application is currently assigned to Japan Display Inc.. The applicant listed for this patent is Japan Display Inc.. Invention is credited to Masayoshi Fuchi, Masato HIRAMATSU, Arichika Ishida.
Application Number | 20140346497 14/256267 |
Document ID | / |
Family ID | 51934791 |
Filed Date | 2014-11-27 |
United States Patent
Application |
20140346497 |
Kind Code |
A1 |
HIRAMATSU; Masato ; et
al. |
November 27, 2014 |
THIN-FILM TRANSITOR AND METHOD FOR MANUFACTURING THE SAME
Abstract
According to one embodiment, a thin-film transistor comprises an
oxide semiconductor layer formed on a part of a substrate, a first
gate insulator film of a silicon dioxide film formed on the oxide
semiconductor layer and by the CVD method with a silane-based
source gas, a second gate insulator film of a silicon dioxide film
formed on the first gate insulator film by the CVD method with a
TEOS source gas, and a gate electrode formed on the second gate
insulator film.
Inventors: |
HIRAMATSU; Masato; (Tokyo,
JP) ; Fuchi; Masayoshi; (Tokyo, JP) ; Ishida;
Arichika; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Japan Display Inc. |
Minato-ku |
|
JP |
|
|
Assignee: |
Japan Display Inc.
Minato-ku
JP
|
Family ID: |
51934791 |
Appl. No.: |
14/256267 |
Filed: |
April 18, 2014 |
Current U.S.
Class: |
257/43 ;
438/104 |
Current CPC
Class: |
H01L 21/02271 20130101;
H01L 21/0262 20130101; H01L 21/02214 20130101; H01L 29/66969
20130101; H01L 21/02164 20130101; H01L 21/465 20130101; H01L
29/78696 20130101; H01L 29/4908 20130101; H01L 21/02211 20130101;
H01L 29/24 20130101; H01L 21/443 20130101; H01L 29/7869 20130101;
H01L 21/02565 20130101 |
Class at
Publication: |
257/43 ;
438/104 |
International
Class: |
H01L 29/786 20060101
H01L029/786; H01L 29/66 20060101 H01L029/66 |
Foreign Application Data
Date |
Code |
Application Number |
May 21, 2013 |
JP |
2013-107376 |
Claims
1. A thin-film transistor comprising: an oxide semiconductor layer
formed on a part of a substrate; a first gate insulator film of a
silicon dioxide film formed on the oxide semiconductor layer and
the substrate by the CVD method with a silane-based source gas; a
second gate insulator film of a silicon dioxide film formed on the
first gate insulator film by the CVD method with a TEOS source gas;
and a gate electrode formed on the second gate insulator film.
2. The thin-film transistor of claim 1, wherein the oxide
semiconductor layer is an oxide of an alloy of indium, gallium and
zinc (IGZO).
3. A method for manufacturing a thin-film transistor comprising:
forming an oxide semiconductor layer on a part of a substrate;
forming a first gate insulator film of a silicon dioxide film on
the oxide semiconductor layer and the substrate by the CVD method
with a silane-based source gas; forming a second gate insulator
film of a silicon dioxide film on the first gate insulator film by
the CVD method with a TEOS source gas; and forming a gate electrode
on the second gate insulator film.
4. The method of claim 3, wherein gas consisting mainly of
SiH.sub.4 and N.sub.2O is used as the silane-based source gas.
5. The method of claim 3, wherein an oxide of an alloy of indium,
gallium and zinc (IGZO) is used as the oxide semiconductor
layer.
6. The method of claim 3, wherein the forming of the oxide
semiconductor layer includes forming the oxide semiconductor layer
on the substrate and subsequently processing the oxide
semiconductor layer by wet etching into a shape of an island.
7. A method for manufacturing a thin-film transistor comprising:
forming an oxide semiconductor layer on a part of a substrate;
forming a gate insulator film of a silicon dioxide film on the
oxide semiconductor layer and the substrate by performing a surface
treatment with oxygen plasma and by forming a film by the CVD
method with a TEOS source gas; and forming a gate electrode on the
gate insulator film.
8. The method of claim 7, wherein an oxide of an alloy of indium,
gallium and zinc (IGZO) is used as the oxide semiconductor
layer.
9. The method of claim 7, wherein the forming of the oxide
semiconductor layer includes forming the oxide semiconductor layer
on the substrate and subsequently processing the oxide
semiconductor layer by wet etching into a shape of an island.
10. The method of claim 7, wherein the forming of the gate
insulator film includes performing the surface treatment with the
oxygen plasma on the oxide semiconductor layer and subsequently
forming the gate insulator film by the CVD method.
11. The method of claim 10, wherein an oxide of an alloy of indium,
gallium and zinc (IGZO) is used as the oxide semiconductor
layer.
12. The method of claim 10, wherein the forming of the oxide
semiconductor layer includes forming the oxide semiconductor layer
entirely on the substrate and processing the oxide semiconductor
layer by wet etching into a shape of an island.
13. The method of claim 7, wherein the forming of the gate
insulator film includes performing the surface treatment with the
oxygen plasma on the oxide semiconductor layer and forming the gate
insulator film by the CVD method simultaneously.
14. The method of claim 13, wherein an oxide of an alloy of indium,
gallium and zinc (IGZO) is used as the oxide semiconductor
layer.
15. The method of claim 13, wherein the forming of the oxide
semiconductor layer includes forming the oxide semiconductor layer
entirely on the substrate and subsequently processing the oxide
semiconductor layer by wet etching into a shape of an island.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2013-107376,
filed May 21, 2013, the entire contents of which are incorporated
herein by reference.
FIELD
[0002] Embodiments described herein relate generally to a thin-film
transistor using a channel of an oxide semiconductor and a method
for manufacturing the same.
[0003] BACKGROUND Recently, for higher definition and lower power
consumption of a liquid crystal display, a thin-film transistor
(TFT) comprising a channel layer of an oxide semiconductor such as
an oxide of an alloy of indium, gallium, and zinc (IGZO) is in
actual use. In a coplanar TFT of this kind, it is necessary to
laminate a gate insulator film and a gate electrode material on an
island-shaped IGZO film. However, because of the influence of a
step in an end portion of the island-shaped IGZO, it is difficult
to form the gate insulator film and gate electrode material
conformally in coating. Generally, a step even larger than the
already existing step of the IGZO is created after the gate
insulator film is formed, and therefore the gate electrode material
deposited thereon can not be connected well in the end portion of
the IGZO (step-disconnection).
[0004] In the case of a polysilicon thin film transistor, when the
polysilicon film is processed by a dry etching method with an
etching gas, the etching conditions are adjusted such that its end
portion becomes oblique to some extent with respect to the
substrate. In this way, the coating of the gate insulator film can
be conformal. In the case of an IGZO film, however, since wet
etching with oxalic acid or the like is mainly employed, only a
substantially vertical end portion can be made. As a result, the
step-disconnection of the gate electrode material occurs in the end
portion. When the width of the channel is narrow, the
step-disconnection results in disconnection of the gate portion of
the transistor from the lead-out wiring portion for forming a
circuit. This leads to a problem of incapability of transistor
operation.
[0005] As a film formation which enables conformal coating, silicon
dioxide film formation by the chemical vapor deposition (CVD)
method using a source gas of tetraethyl orthosilicate (TEOS) is
well known. This material is used as a gate insulator film disposed
on a polycrystalline silicon film having large convexoconcave, and
can moderate the convexoconcave of the polycrystalline silicone
film thereunder.
[0006] However, the following was found. That is, when a silicon
dioxide film was formed with gaseous TEOS on the IGZO film, oxygen
was lost from the IGZO surface by a chemical reaction between the
gaseous TEOS and the IGZO surface. As a result, the IGZO film,
which was originally a semiconductor, behaved as if it was a metal
at the interface with the gate insulator film and became unable to
perform the transistor operation. This was similarly observed in
the cases where metal oxides other than IGZO were used as the
semiconductor layer.
[0007] As described above, conventionally, when a silicon dioxide
gate insulator film is formed with gaseous TEOS on an oxide
semiconductor, the oxide semiconductor behaves as if it is a metal
at the interface with the gate insulator film, causing a problem of
incapability of transistor operation.
SUMMARY
[0008] In general, according to one embodiment, a thin-film
transistor comprises an oxide semiconductor layer formed on a part
of a substrate, a first gate insulator film of a silicon dioxide
film formed on the oxide semiconductor layer and by the CVD method
with a silane-based source gas, a second gate insulator film of a
silicon dioxide film formed on the first gate insulator film by the
CVD method with a TEOS source gas, and a gate electrode formed on
the second gate insulator film.
[0009] Further, a method for manufacturing a thin-film transistor
comprising forming an oxide semiconductor layer on a part of a
substrate, forming a first gate insulator film of a silicon dioxide
film on the semiconductor layer and the substrate by the CVD method
with a silane-based source gas, forming a second gate insulator
film of a silicon dioxide film on the first gate insulator film by
the CVD method with a TEOS source gas, and forming a gate electrode
on the second gate insulator film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a plan view showing a schematic structure of a
thin-film transistor according to a first embodiment;
[0011] FIG. 2 is a sectional view (sectional view along the length
of a channel) taken along arrow I-I' of FIG. 1;
[0012] FIG. 3 is a sectional view (sectional view along the width
of a channel) taken along arrow II-II' of FIG. 1;
[0013] FIGS. 4A to 4D are sectional views each showing a process of
manufacturing a thin-film transistor according to the first
embodiment;
[0014] FIG. 5 is a characteristic diagram of a thin-film transistor
according to the first embodiment in comparison with conventional
examples;
[0015] FIGS. 6A and 6B are sectional views each showing a process
of manufacturing a thin-film transistor according to a second
embodiment; and
[0016] FIGS. 7A and 7B are sectional views each showing a process
of manufacturing a thin-film transistor according to a third
embodiment.
DETAILED DESCRIPTION
[0017] The basic concept of the present embodiments will be
described prior to the detailed descriptions of the
embodiments.
[0018] As mentioned above, for the prevention of the
step-disconnection of the gate insulator film, it is effective to
form a silicon dioxide film with gaseous TEOS. However, the use of
gaseous TEOS causes the reduction of the IGZO film thereunder.
Here, the inventors of the present invention have used the
excellent coating of the silicon dioxide film with the gaseous TEOS
and taken measures to prevent the metallization of the IGZO film.
More specifically, a treatment for preventing the reduction of the
IGZO surface was carried out and the silicon dioxide film with the
gaseous TEOS later was formed in an experiment.
[0019] As a result, it was found to be effective to form a silicon
dioxide film with a gaseous mixture of TEOS and oxygen after oxide
plasma processing. It was also found to be advantageous to form a
silicon dioxide film with the gaseous mixture of TEOS and oxygen
after forming a silicon dioxide film just thin enough to cover the
surface without using TEOS.
[0020] The above effectiveness can be similarly obtained also in
the cases of oxide semiconductors in addition to IGZO. Further, the
present invention is especially effective to a coplanar TFT in
which a gate electrode is formed via a gate insulator film on an
island-shaped oxide semiconductor layer.
[0021] A thin-film transistor according to an embodiment will now
be described with reference to the accompanying drawings.
First Embodiment
[0022] FIGS. 1 to 3 are diagrams each showing a schematic structure
of a thin-film transistor, especially, a coplanar TFT using IGZO as
a channel, according to a first embodiment. FIG. 1 is a plane view,
FIG. 2 is a sectional view taken along arrow I-I' of FIG. 1, and
FIG. 3 is a sectional view taken along arrow II-II' of FIG. 1. This
TFT is used, for example, for pixel switching in a liquid crystal
display.
[0023] Reference number 10 in the diagrams denotes, for example, a
transparent glass substrate which constitutes a part of a liquid
crystal panel. On a part of the substrate 10, an oxide
semiconductor layer 20 of IGZO is formed. The oxide semiconductor
layer 20 is provided in a shape of an island to fit with a coplanar
TFT formation region. The side surface of the end portion of the
oxide semiconductor layer 20 is substantially vertical.
[0024] A gate insulator film 30 of a silicon dioxide film is formed
on the substrate 10 in such a manner as to cover the oxide
semiconductor layer 20. Here, the gate insulator film 30 has a
laminated structure of a first silicon dioxide film (first gate
insulator film) 31 and a second silicon dioxide film (second gate
insulator film) 32, which will be described later. In the first
silicon dioxide film 31, the step of the oxide semiconductor layer
20 thereunder is reflected, thus creating a steep step. Further,
the second silicon dioxide film 32 is formed conformally in the
step portion of the first silicon dioxide film 31, and therefore, a
step in a portion corresponding to the end portion of the oxide
semiconductor layer 20 is gentle.
[0025] On the gate insulator 30, a gate electrode 40 of MoW or the
like is formed. Then, a source/drain region (not shown) in the
oxide semiconductor layer 20 is formed to sandwich the gate
electrode 40, whereby the coplanar TFT is constructed.
[0026] Further, an interlayer insulator 50 of a silicon dioxide
film or the like is formed to cover the gate electrode 40. In the
interlayer insulator 50, a contact hole is formed. By filling an
electric conductor in this hole, the source/drain electrode 60 is
formed. On the interlayer insulator 50, a wiring layer 70 is formed
to be connected to the source/drain electrode 60.
[0027] FIGS. 4A to 4D are sectional views each showing a process of
manufacturing the TFT according to the present embodiment, and
corresponding to the section of FIG. 3.
[0028] First, as shown in FIG. 4A, the oxide semiconductor layer 20
is formed on the glass substrate 10 to have a thickness of 50 nm,
and is patterned into a shape of the device.
[0029] The oxide semiconductor layer 20 is an oxide of an alloy of
indium, gallium and zinc (IGZO). The IGZO film may be formed, for
example, by a solid reaction method wherein the mixed material
including In, Ga and
[0030] Zn is calcinated and gradually cooled, or by the CVD method
or the metalorganic chemical vapor deposition (MOCVD) method, or
the like. After the oxide semiconductor layer 20 is formed, a mask
of a silicon nitrogen film (not shown) or the like is formed on the
oxide semiconductor layer 20. Then, the oxide semiconductor layer
20 is processed into a desired pattern by wet etching with oxalic
acid. Here, the sidewall of the end portion of the oxide
semiconductor layer 20 is substantially vertical.
[0031] Next, as shown in FIG. 4B, by the CVD method with an
N.sub.2O--SiH.sub.4-based source gas, the first silicon dioxide
film 31 is formed on the substrate 10 to have a thickness of 10 nm
to cover the oxide semiconductor layer 20. Here, in the first
silicon dioxide film 31, the step of the oxide semiconductor layer
20 is reflected, thus creating a steep step.
[0032] Then, as shown in FIG. 4C, the second silicon dioxide film
32 is formed on the first silicon dioxide film 31 to have a
thickness of 70 nm by the CVD method with a TEOS source gas. Here,
by using the gaseous TEOS, the second silicon dioxide film 32 is
formed conformally even in the step portion of the first silicon
dioxide film 31, and therefore a slope on the side surface of the
silicon dioxide film 32 becomes gentle. That is, the gate insulator
film 30 comprising the silicon dioxide films 31 and 32 has a gentle
step even in the end portion of the oxide semiconductor layer 20.
Further, when the silicon dioxide film 32 is formed, the silicon
dioxide film 31 thereunder prevents an escape of oxygen from the
oxide semiconductor layer 20.
[0033] After that, as shown in FIG. 4D, by depositing MoW up to a
thickness of 200 nm on the gate insulator film 30 by a sputtering
method, the gate electrode 40 is formed. Here, since the slope of
the side surface of the gate insulator film 30 is gentle, the
step-disconnection of the gate electrode 40 will not occur.
[0034] From here, the patterning of the gate electrode 40, the
formation of the interlayer insulator 50, the formation of the
source/drain electrode 60, and the like follow to complete the
structure shown in FIGS. 1 to 3.
[0035] Vg-Id characteristics of the TFT manufactured by the above
method and those of TFTs manufactured by conventional methods are
shown in FIG. 5. Reference symbol A in FIG. 5 indicates a case
(before improvement) where a single-layer gate insulator film is
formed with gaseous SiH.sub.4+N.sub.2O. Reference symbol B
indicates a case (after improvement) where a two-layer gate
insulator film is formed with the gaseous SiH.sub.4+N.sub.2O and
the gaseous TEOS according to the present embodiment. Reference
symbol C indicates a case (before improvement) where a single-layer
gate insulator film is formed with the gaseous TEOS.
[0036] In reference symbol A, fine device characteristics can be
obtained, but there is a risk of step-disconnection of a gate
electrode. In reference symbol B, device characteristics
substantially the same as those of reference symbol A can be
obtained, and the step-disconnection of the gate electrode can be
prevented as well. In reference symbol C, though the
step-disconnection of the gate electrode can be prevented, the
threshold voltage shifts to negative, causing degradation of the
device characteristics. From the above, it is understood that a
two-layer gate insulator film of the present embodiment can prevent
step-disconnection of a gate electrode without causing degradation
of device characteristics.
[0037] In addition, in the present embodiment, by thinning the
silicon dioxide film 31, which is the first layer formed with an
N.sub.2O--SiH.sub.4-based gas, capacitance-voltage (C-V)
characteristics substantially the same as those of polysilicon TFT
are obtained.
[0038] Further, as to the shift of the threshold voltage, in
particular, the shift to the negative side becomes small.
Therefore, the current when Vg is zero will not become large,
providing significant advantages also in designing a circuit.
[0039] As mentioned above, in the present embodiment, after the
first silicon dioxide film 31 is formed on the oxide semiconductor
layer 20 with the N.sub.2O--SiH.sub.4-based gas, the second silicon
dioxide film 32 is formed with the gaseous TEOS. Consequently, it
is possible to prevent the reduction of the oxide semiconductor
layer 20 which occurs when the gate insulator film 30 comprising
the silicon dioxide films 31 and 32 is formed. Further, since the
step on the gate insular film 30 is gentle, step-disconnection of
the gate electrode 40 can be prevented. Therefore, the device
characteristics of the TFT using IGZO as a channel can be
improved.
[0040] In addition, since this is a coplanar TFT, the size of the
transistor can be reduced in comparison with that of an inverted
staggered TFT. Further, since the capacitive components of the gate
and the source/drain are reduced, this transistor can operate at
high speed. Therefore, it is especially suited for a
high-definition panel.
Second Embodiment
[0041] FIGS. 6A and 6B are sectional views each showing a process
of manufacturing a thin-film transistor according to a second
embodiment. Note that the portions identical to those of FIGS. 4A
to 4D are denoted with the same reference numbers and will not
described in detail.
[0042] The present embodiment is different from the first
embodiment described earlier in respect of treating the oxide
semiconductor layer with oxygen plasma processing instead of
forming a gate insulator film into two layers.
[0043] First, in a manner similar to that in FIG. 4A of the first
embodiment, the oxide semiconductor 20 of IGZO is formed on the
glass substrate 10 and patterned into the shape of the device.
[0044] Next, as shown in FIG. 6A, a surface treatment with oxygen
plasma is carried out on the oxide semiconductor layer 20. More
specifically, an oxygen radical is produced by generating a plasma
discharge in a chamber into which gaseous N.sub.2O or O.sub.2 is
introduced, and is directed onto the surface of the oxygen
semiconductor layer 20. This oxygen plasma processing prevents the
reduction of the oxygen semiconductor layer 20 which occurs when
the gate insulator film is formed in the subsequent process. That
is, by introducing an excessive amount of oxygen in advance in
consideration of oxygen escaping when TEOS is used, the reduction
of the oxygen semiconductor layer 20 can be inhibited.
[0045] Then, as shown in FIG. 6B, the gate insulator film 30 is
formed on the substrate 10 to cover the oxide semiconductor layer
20 by forming a silicon dioxide film by the CVD method with the
TEOS source gas. Here, by using TEOS, the step portion of the
semiconductor layer 20 is formed conformally and thus the step of
the gate insulator film 30 is gentle. Further, by performing the
surface treatment with oxygen plasma, the reduction of the oxide
semiconductor layer 20 can be inhibited.
[0046] After that, in a similar manner as the first embodiment, the
gate electrode 40 is formed on the gate insulator film 30. Here,
since the slope of the side surface of the gate insulator film 30
is gentle, the step-disconnection of the gate electrode 40 will not
occur.
[0047] From here, in a manner similar to the first embodiment, the
formation of the interlayer insulator 50 and the source/drain
electrode 60, and the like follow to complete the structure of the
transistor.
[0048] As described above, in the present embodiment, the surface
treatment with oxygen plasma is carried out before the gate
insulator film 30 is formed on the oxide semiconductor layer 20 by
the CVD method with the gaseous TEOS. Therefore, the reduction of
the oxide semiconductor layer 20 occurring when the gate insulator
film 30 is formed with the gaseous TEOS can be prevented.
Consequently, effects similar to those of the first embodiment can
be obtained.
Third Embodiment
[0049] FIGS. 7A and 7B are sectional views each showing a process
of manufacturing a thin-film transistor according to a third
embodiment. Note that the portions identical to those of FIGS. 4A
to 4D are denoted with the same reference number and will not be
described in detail.
[0050] The present embodiment is different from the second
embodiment described earlier in respect of performing the oxygen
plasma irradiation and the film formation with the gaseous TEOS
simultaneously instead of processing with oxygen plasma in advance
before forming a film with TEOS.
[0051] First, in a manner similar to that in FIG. 4A of the first
embodiment, the oxide semiconductor layer 20 of IGZO is formed on
the glass substrate 10 and is patterned into a shape of the
device.
[0052] Next, as shown in FIG. 7A, the surface treatment with oxygen
plasma is carried out on the oxygen semiconductor layer 20 while a
silicon dioxide film is formed by the CVD method with the TEOS
source gas. In this way, the gate insulator film 30 is formed on
the substrate 10 to cover the oxide semiconductor layer 20. Here,
by using TEOS, the gate insulator film 30 is formed conformally
even in the step portion of the oxide semiconductor layer 20.
Further, by performing the surface treatment with oxygen plasma,
the escape of oxygen from the oxide semiconductor can be
inhibited.
[0053] Then, as shown in FIG. 7B, the gate insulator film 30 is
formed up to a thickness of 80 nm and the slope of the side surface
of the gate insulator film 30 become gentle.
[0054] As described above, in the present embodiment, by performing
the surface treatment with oxygen plasma and the silicon dioxide
film formation with the gaseous TEOS, it is possible to prevent the
escape of oxygen from the oxide semiconductor layer 20 and to make
the slope of the side surface of the gate insulator film 30 gentle.
Therefore, the benefits similar to those of the first embodiment
can be obtained.
Modified Example
[0055] Note that the present invention will not be limited to each
of the embodiments described above.
[0056] Although IGZO is used as the oxide semiconductor layer in
the embodiments, any of metal oxide semiconductors such as ZnO,
In2O3, and InGaO, may be used instead.
[0057] In the first embodiment, the source gas used in forming the
first silicon dioxide film is not necessarily limited to
N.sub.2O--SiH.sub.4 but may be a silane-based gas which can prevent
reduction, such as N.sub.2O--SiH.sub.4. Moreover, the carrier gas
such as Ar or He in addition to the source gas can be
comprised.
[0058] In addition, the present embodiments can be used not only in
liquid crystal displays but also in other displays for pixel
switching. Besides the liquid crystal displays, the present
embodiments are also applicable to various devices comprising a
transistor on an oxide semiconductor layer.
[0059] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms;
[0060] furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
* * * * *