U.S. patent application number 11/583022 was filed with the patent office on 2007-04-26 for field-effect transistor including transparent oxide and light-shielding member, and display utilizing the transistor.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Katsumi Abe, Ryo Hayashi, Hideya Kumomi, Kojiro Nishi, Masafumi Sano.
Application Number | 20070090365 11/583022 |
Document ID | / |
Family ID | 37984496 |
Filed Date | 2007-04-26 |
United States Patent
Application |
20070090365 |
Kind Code |
A1 |
Hayashi; Ryo ; et
al. |
April 26, 2007 |
Field-effect transistor including transparent oxide and
light-shielding member, and display utilizing the transistor
Abstract
A field-effect transistor includes a substrate, a source
electrode, a drain electrode, a gate electrode, a gate-insulating
film, and an active layer. The active layer contains an oxide
having a transmittance of 70% or more in the wavelength range of
400 to 800 nm. A light-shielding member is provided as a
light-shielding structure for the active layer, for example, on the
bottom face of the substrate.
Inventors: |
Hayashi; Ryo; (Yokohama-shi,
JP) ; Sano; Masafumi; (Yokohama-shi, JP) ;
Abe; Katsumi; (Kawasaki-shi, JP) ; Kumomi;
Hideya; (Tokyo, JP) ; Nishi; Kojiro;
(Yokohama-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
37984496 |
Appl. No.: |
11/583022 |
Filed: |
October 19, 2006 |
Current U.S.
Class: |
257/72 |
Current CPC
Class: |
G02F 1/1368 20130101;
H01L 29/7869 20130101; H01L 27/1225 20130101; H01L 29/78633
20130101; H01L 29/78693 20130101; H01L 2251/5338 20130101; H01L
27/3248 20130101; H01L 27/3272 20130101 |
Class at
Publication: |
257/072 |
International
Class: |
H01L 29/04 20060101
H01L029/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 20, 2005 |
JP |
2005-305950 |
Claims
1. A field-effect transistor comprising a substrate, a source
electrode, a drain electrode, a gate electrode, a gate-insulating
film, and an active layer, wherein the active layer contains an
oxide having a transmittance of 70% or more in the wavelength range
of 400 to 800 nm; and a light-shielding structure is provided
between the substrate and the active layer or is provided on the
surface of the substrate on the side opposite the active layer.
2. The field-effect transistor according to claim 1, wherein the
light-shielding structure has a light-shielding property that
prevents entry of light from the substrate side toward the active
layer.
3. The field-effect transistor according to claim 1, wherein the
light-shielding structure has a light-shielding property that
prevents entry of light having a wavelength range of 400 to 800
nm.
4. The field-effect transistor according to claim 1, wherein the
light-shielding structure has a light-shielding property that
prevents entry of at least light or an electromagnetic wave having
a wavelength range of 300 to 500 nm.
5. The field-effect transistor according to claim 1, wherein the
oxide is an amorphous oxide.
6. The field-effect transistor according to claim 1, wherein the
oxide contains at least one of In, Zn, and Sn.
7. The field-effect transistor according to claim 1, wherein the
oxide is an amorphous oxide containing at least one of In, Zn, and
Ga.
8. A display comprising a plurality of pixel parts each provided
with a field-effect transistor according to claim 1 and a
liquid-crystal layer or a light-emitting layer.
9. A field-effect transistor comprising a substrate, a source
electrode, a drain electrode, a gate electrode, a gate-insulating
film, and an active layer, wherein the active layer contains an
oxide having a transmittance of 70% or more in the wavelength range
of 400 to 800 nm; and the substrate has a light-shielding
property.
10. The field-effect transistor according to claim 9, wherein the
substrate has a light-shielding property that prevents entry of
light from the substrate side toward the active layer.
11. The field-effect transistor according to claim 9, wherein the
substrate has a light-shielding property that prevents entry of
light having a wavelength range of 400 to 800 nm.
12. The field-effect transistor according to claim 9, wherein the
substrate has a light-shielding property that prevents entry of at
least light or an electromagnetic wave having a wavelength range of
300 to 500 nm.
13. The field-effect transistor according to claim 9, wherein the
oxide is an amorphous oxide.
14. The field-effect transistor according to claim 9, wherein the
oxide contains at least one of In, Zn, and Ga.
15. The field-effect transistor according to claim 9, wherein the
oxide is an amorphous oxide containing at least one of In, Zn, and
Ga.
16. A display comprising a plurality of pixel parts each provided
with a field-effect transistor according to claim 9 and a
liquid-crystal layer or a light-emitting layer.
17. A field-effect transistor comprising a substrate, a source
electrode, a drain electrode, a gate electrode, a gate-insulating
film, an active layer, and a light-shielding structure, wherein the
active layer contains an oxide having a transmittance of 70% or
more in the wavelength range of 400 to 800 nm; and the
light-shielding structure is provided over the active layer.
18. The field-effect transistor according to claim 17, wherein the
light-shielding structure has a light-shielding property that
prevents entry of light from the active layer side toward the
substrate.
19. The field-effect transistor according to claim 17, wherein the
light-shielding structure has a light-shielding property that
prevents entry of light at angles of less than 90 degrees to the
surface of the substrate toward the active layer.
20. The field-effect transistor according to claim 17, wherein the
light-shielding structure overlaps the source electrode and the
drain electrode.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to transistors using amorphous
oxides and displays utilizing the transistors.
[0003] 2. Description of the Related Art
[0004] Recently, technologies in which transparent amorphous oxide
semiconductor films composed of indium, gallium, zinc, and oxygen
are applied to channel layers of thin-film transistors (TFTs) have
been developed. For example, International Publication No. WO
2005/088726 (Patent Document) discloses a technology for using an
InGaZn system transparent amorphous oxide film as the channel layer
of a TFT.
[0005] This transparent amorphous oxide semiconductor film can be
formed at a low temperature and is transparent to visible light.
Therefore, a flexible and transparent TFT can be formed on a
substrate such as a plastic sheet or film.
[0006] In Nature (2004), 432, 488-492 (Non-Patent Document), it is
disclosed that a transparent amorphous oxide semiconductor film has
a visible light transmittance of about 80% or more when the
composition ratio by fluorescent X-ray analysis is
In:Ga:Zn=1.1:1.1:0.9, and that it is possible to produce a
transparent TFT.
[0007] The present inventors have conducted studies in order to
produce transparent field-effect transistors by using transparent
amorphous oxide semiconductor films and have found adventitiously a
phenomenon that electrical conductivity changes under visible light
having a certain wavelength.
[0008] In order to investigate the phenomenon in detail, the
present inventors have conducted experiments for measuring
electrical conductivity under exposure to spectral light as
described below (spectral sensitivity measurement experiments). As
a result, a change (increase) in electrical conductivity caused by
light absorption was observed in a region at the shorter-wavelength
side of visible light (FIG. 8).
[0009] The results shown in FIG. 8 suggest that when a thin-film
transistor (TFT) is irradiated with visible light, the OFF current
of the TFT changes significantly depending on, in particular, the
irradiation intensity of visible light at the shorter-wavelength
side. Such a change may affect the stable performance of the
TFT.
[0010] That is, it has been found for the first time that, in a
transparent amorphous oxide which should be transparent to visible
light, a change in electrical conductivity occurs, namely,
photocarriers are practically generated by irradiation with light
in a certain visible light region.
[0011] The present inventors have arrived at the understanding on
the basis of the finding of the above-mentioned phenomenon that
when a material which is generally recognized as a transparent
oxide is used for the active layer of a TFT, it is preferable that
the TFT be provided with a light-shielding structure for shielding
the oxide from light in order to operate the TFT with higher
stability. On the basis of the above, the present invention
relating to transistors including a light-shielding structure has
been accomplished.
[0012] However, the light-shielding structure may be unnecessary
depending on the use of a TFT, i.e., when visible light at the
shorter-wavelength side does not enter the TFT or when the incident
light does not highly affect the total stability of a device even
if the light enters the device.
SUMMARY OF THE INVENTION
[0013] The present invention provides a field-effect transistor
including a light-shielding structure and provides a display
provided with the transistor.
[0014] A field-effect transistor according to a first aspect of the
present invention includes a substrate, a source electrode, a drain
electrode, a gate electrode, a gate-insulating film, and an active
layer. The active layer contains an oxide having a transmittance of
70% or more in the wavelength range of 400 to 800 nm. As a
light-shielding structure, a light-shielding layer is provided
between the substrate and the active layer or is provided on the
surface of the substrate on the side opposite the active layer, or
the substrate has a light-shielding property.
[0015] A field-effect transistor according to a second aspect of
the present invention includes a substrate, a source electrode, a
drain electrode, a gate electrode, a gate-insulating film, and an
active layer. The active layer contains an oxide having a
transmittance of 70% or more in the wavelength range of 400 to 800
nm. As a light-shielding structure, a light-shielding layer is
provided over the active layer. The light-shielding structure
shields light entering from all directions which form angles of
less than 90 degrees with the direction along the surface of the
substrate toward the active layer.
[0016] A field-effect transistor according to a third aspect of the
present invention includes a substrate, a source electrode, a drain
electrode, a gate electrode, a gate-insulating film, an active
layer, and a light-shielding film. The active layer contains an
oxide having a transmittance of 70% or more in the wavelength range
of 400 to 800 nm. As a light-shielding structure, a light-shielding
film is provided over the active layer.
[0017] The light-shielding structure according to the first to the
third aspects of the present invention is a film having a
light-shielding property to light having a wavelength range of 400
to 800 nm or a film having a light-shielding property to light or
an electromagnetic wave having a wavelength range of around 400 nm
(wavelengths ranging at least from 300 nm to 500 nm).
[0018] A display according to a fourth aspect of the present
invention includes a plurality of pixel parts each provided with
the above-described field-effect transistor and a liquid-crystal
layer or a light-emitting layer.
[0019] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic cross-sectional view of a field-effect
transistor according to the present invention.
[0021] FIG. 2 is a schematic cross-sectional view of a field-effect
transistor according to the present invention.
[0022] FIG. 3 is a schematic cross-sectional view of a field-effect
transistor according to the present invention.
[0023] FIGS. 4A to 4C are schematic cross-sectional views of
field-effect transistors according to the present invention.
[0024] FIG. 5 is a schematic cross-sectional view of a display
utilizing a field-effect transistor according to the present
invention.
[0025] FIG. 6 is a schematic cross-sectional view of a display
utilizing a field-effect transistor according to the present
invention.
[0026] FIGS. 7A to 7D are schematic cross-sectional views of
examples of field-effect transistors to which the present invention
can be applied.
[0027] FIG. 8 is a diagram showing results of spectral sensitivity
measurement experiments for describing the present invention.
DESCRIPTION OF THE EMBODIMENTS
[0028] In general, the term "visible light" indicates light having
a wavelength range of about 400 nm to about 800 nm. In addition,
generally, a material having a light transmittance of 70% or more
is recognized as a transparent material. FIG. 2 in the
above-mentioned Non-Patent Document shows that the amorphous oxide
relating to the present invention has a transmittance of 70% or
more.
[0029] Therefore, in the present invention, an oxide having a light
transmittance of 70% or more in the wavelength range of 400 to 800
nm (visible light) is defined as a transparent oxide. The
transparent oxides in the present invention include not only oxides
having a light transmittance of 70% or more throughout the
above-mentioned light wavelength range but also oxides having a
light transmittance of 70% or more in a part of the above-mentioned
light wavelength range.
[0030] In addition, the transmittance in the wavelength of the
above-mentioned range is preferably 80% or more, more preferably
90% or more.
First Embodiment: Light Shield to Incident Light from Substrate
Side
[0031] Light entering from the substrate side toward the active
layer is shielded when light-shielding films are provided at the
positions shown in FIGS. 1 to 3.
[0032] Preferably, the light-shielding film has a light-shielding
property to visible light having a wavelength range of 400 to 800
nm. More preferably, the light-shielding film further has a
light-shielding property to light or an electromagnetic wave having
a wavelength of 400 nm or less (for example, the wavelength range
of 100 to 400 nm).
[0033] The transparent oxide (for example, transparent amorphous
oxide) in the present invention causes a phenomenon of photocarrier
generation in the shorter-wavelength region of visible light.
Therefore, in particular, it is preferable that the light-shielding
film have a light-shielding property to at least light or an
electromagnetic wave having a wavelength range of 300 to 500
nm.
[0034] In addition, the light-shielding film is not required to
have a transmittance of 0% as long as the light-shielding film has
a light-shielding property to light having the above-mentioned
wavelength. The transmittance is preferably 30% or less, more
preferably 10% or less, more preferably 5% or less, and further
preferably 0.01% or less.
[0035] The material for the light-shielding film in the present
invention is not specifically limited. The light-shielding property
of the material may be low, provided that the light-shielding
property which is equivalent to the above-mentioned transmittance
can be achieved by increasing the thickness of the film.
[0036] The present invention will now be specifically
described.
[0037] FIG. 1 shows an example of an inverted-staggered TFT
provided on a substrate. In FIG. 1, reference numeral 1000
represents a substrate, reference numeral 1020 represents an active
layer, reference numeral 1030 represents a source electrode,
reference numeral 1040 represents a drain electrode, reference
numeral 1050 represents a gate-insulating film, and reference
numeral 1060 represents a gate electrode. In addition, a
light-shielding film 1090 is provided on the bottom face of the
substrate 1000 (on the surface of the substrate 1000 on the side
opposite the active layer 1020) as a light-shielding structure.
[0038] In order to shield light from the direction perpendicular to
the surface of the substrate 1000 (i.e., from directly below the
active layer 1020), the width L of light-shielding film 1090 should
be equal to or longer than the width 1 of the active layer 1020.
Particularly, in order to sufficiently shield obliquely incident
light, the width L of the light-shielding film 1090 should be 2
times or more the width 1 of the active layer 1020, preferably 4
times more the width 1. The light-shielding film 1090 may be
provided on the entire surface of the substrate 1000.
[0039] FIG. 1 is a schematic cross-section view of a TFT. The
length of the light-shielding film 1090 in the direction vertical
to the surface of the paper on which FIG. 1 is drawn, in the depth
direction, is equal to or larger than (preferably 2 times, more
preferably 4 times) that of the active layer 1020.
[0040] In FIG. 1, an inverted-staggered TFT is exemplarily
described, but the structure of a TFT is not limited to the
inverted-staggered type as described below.
[0041] FIG. 2 shows an example that a light-shielding film is
provided on the top face of a substrate 1000 (on the surface of the
substrate 1000 on the active layer 1020 side). This case also has
the same relationship between the width 1 of the active layer 1020
and the width L of the light-shielding film 1090 as that in the
above case. In addition, the TFT shown in FIG. 2 may have an
additional light-shielding film at the same position shown in FIG.
1 so that light-shielding films are provided on both surfaces of
the substrate 1000.
[0042] FIG. 3 shows a case that a substrate 1091 itself has a
light-shielding property.
Second Embodiment: Light Shield to Incident Light from the Side
Opposite the Substrate
[0043] In order to shield the incident light from the side opposite
the substrate, a light-shielding film 4090 is provided as shown in
FIG. 4A.
[0044] Preferably, the light-shielding film 4090 has a
light-shielding property to visible light having a wavelength range
of 400 to 800 nm. More preferably, the light-shielding film further
has a light-shielding property to an electromagnetic wave having a
wavelength of 400 nm or less (for example, the wavelength range of
100 to 400 nm).
[0045] The transparent oxide (for example, amorphous oxide) in the
present invention causes a phenomenon of photocarrier generation in
the shorter-wavelength region of visible light. Therefore, in
particular, it is preferable that the light-shielding film have a
light-shielding property to at least light or an electromagnetic
wave having a wavelength range of 300 to 500 nm.
[0046] In addition, the light-shielding film is not required to
have a transmittance of 0% as long as the light-shielding film has
a light-shielding property to light having the above-mentioned
wavelength. The transmittance is preferably 30% or less, more
preferably 10% or less, more preferably 5% or less, and further
preferably 0.01% or less.
[0047] Specifically, as shown in FIG. 4A, a light-shielding film is
further provided independent of a source electrode 1030, a drain
electrode 1040, and a gate electrode 1060. With such a structure,
the active layer is shielded from light which cannot be shielded by
only the electrodes such as the source electrode. In FIG. 4A, an
example of an inverted-staggered TFT is described as an example.
The present invention can be applied to TFTs having other
configurations as described below. In addition, when the
light-shielding film is made of a material having a high electrical
conductivity, it is necessary to interpose an insulating layer (not
shown) between the light-shielding film 4090 and the electrodes
such as the source electrode.
[0048] Furthermore, as shown in FIG. 4A, light irradiated toward
the active layer 1020 from all directions which form angles
(indicated by .theta. in the figure) of less than 90 degrees with
the direction along the surface of the substrate 1000 can be
shielded by providing the light-shielding film 4090 at the upper
side of the active layer 1020.
[0049] In an inverted-staggered TFT shown in FIG. 4B, a
light-shielding film 4090 serves as the light-shielding structure,
and an insulating layer 4095 is interposed between the
light-shielding film 4090 and a source electrode 1030, an active
layer 1020, and a drain electrode 1040. In the structure shown in
FIG. 4B, the light-shielding film 4090 is further provided
independent of the electrodes such as the source electrode 1030. It
is preferable that the light-shielding film 4090 and the source
electrode 1030 (and the drain electrode 1040) partially overlap one
another when viewed from the direction (indicated by reference
numeral 1205 in the figure) perpendicular to the direction
(indicated by reference numeral 1204 in the figure) along the
surface of the substrate 1000. The width (m) of the overlapped
portion is preferably equal to or larger than the thickness of the
insulating film 4095 provided directly on the source electrode
1030. It is preferable that the source electrode 1030 and the drain
electrode 1040 are completely covered with the light-shielding film
4090 when viewed from the vertical direction (indicated by
reference numeral 1205 in the figure).
[0050] FIG. 4C shows an example of a staggered TFT provided with a
light-shielding film 4090 as a light-shielding structure. As in
this example, the light-shielding structure may be provided so as
to cover the gate-insulating layer 1050 at the portion not being
covered with the gate electrode 1060 and to cover the side faces of
the active layer (amorphous oxide) 1020.
[0051] In addition, the structure in the second embodiment may
further include the same constitution as in the first embodiment.
Such a structure is included in the scope of the present invention
and provides a TFT having a light-shielding property to incident
light from the substrate side and from the active layer side.
[0052] The first and second embodiments describe examples using a
light-shielding member (or a film or a substrate having a
light-shielding property) as the light-shielding structure. The
light-shielding structure may be a film, layer, or member which
achieves the light shielding by absorbing or reflecting light
having a predetermined wavelength range. In addition, the light
shielding may be achieved by a combination of the absorption and
reflection of light. The light-shielding structure may be a
multilayer of light-shielding films, light-shielding layers, or
light-shielding members. In addition, the light-shielding structure
may be a photonic crystal having an optical two- or
three-dimensional refractive index difference.
Third Embodiment: Display
[0053] A display provided with a field-effect transistor
(specifically TFT) described in the first or second embodiment will
now be described.
[0054] The structure used in a display is as follows:
[0055] The drain electrode functioning as an output terminal of a
TFT is connected to an input electrode of a light-emitting device
such as an electroluminescence device using an organic or inorganic
material, a light-transmittance-controlling device of a
liquid-crystal cell or an electrophoretic particle cell, or a
light-reflectance-controlling device.
[0056] For example, as shown in FIG. 5, an amorphous oxide
semiconductor film 5002, a source electrode 5003, a drain electrode
5004, a gate-insulating film 5005, and a gate electrode 5006 are
deposited and patterned on a substrate 5001.
[0057] The drain electrode 5004 is connected to an electrode 5008
via an interlayer-insulating film 5007. The electrode 5008 is in
contact with a light-emitting layer 5010 which is in contact with
an electrode 5011. A current flowing into the light-emitting layer
5010 can be controlled by the current value flowing from the source
electrode 5003 to the drain electrode 5004 through a channel formed
in the amorphous oxide semiconductor film 5002. This control is
conducted by the voltage of the gate electrode 5006 of a TFT. Here,
the light-emitting layer 5010 is an inorganic or organic
electroluminescence device.
[0058] In such a device structure, the interlayer-insulating film
5007 or the electrode 5008 serves as a light-insulating film so
that the amorphous oxide semiconductor film 5002 is not irradiated
with visible light and light or an electromagnetic wave having a
wavelength shorter than that of visible light.
[0059] The light irradiation from the substrate side is shielded by
providing a light-shielding member 5009 on the top or bottom face
of the substrate 5001. FIG. 5 shows a case that the light-shielding
member 5009 is provided on the bottom.
[0060] In addition, the gate electrode 5006 may have a function as
a light-shielding film so that it is unnecessary to separately
provide the light-shielding member 5009.
[0061] When an inorganic or organic electroluminescence device has
a top emission structure, it is preferably that the electrode 5008
function as a light-shielding layer.
[0062] A liquid-crystal display will now be described with
reference to FIG. 6.
[0063] Reference numeral 6001 represents a substrate, reference
numeral 6002 represents an active layer made of an amorphous oxide,
reference numeral 6003 represents a source electrode, reference
numeral 6004 represents a drain electrode, reference numeral 6005
represents a gate-insulating electrode, and reference numeral 6006
represents a gate electrode.
[0064] As shown in FIG. 6, the drain electrode 6004 is extended and
thereby also serves as an electrode 6008. A
light-transmittance-controlling device or
light-reflectance-controlling device composed of a liquid-crystal
cell or an electrophoretic particle cell 6013 is interposed between
high-resistive films 6012 (for example, oriented films of a
polyimide).
[0065] A voltage is applied to the liquid-crystal cell or the
electrophoretic particle cell 6013 by the electrodes 6008 and 6011.
With such a structure, the voltage applied between the electrode
6008 and the electrode 6011 can be controlled by controlling the
voltage of the gate electrode 6006 of a TFT.
[0066] An interlayer-insulating film 6007 serving as a
light-shielding layer or a gate electrode 6006 made of an opaque
metal such as Al having a light-shielding function may be used so
that the active layer 6002 made of amorphous oxide is not
irradiated with visible light and not irradiated with light or an
electromagnetic wave having a wavelength shorter than that of
visible light.
[0067] The light irradiation from the substrate 6001 side is
preferably shielded by providing a light-shielding film 6009 on the
top or bottom face of the substrate 6001. FIG. 6 shows a case that
the light-shielding film 6009 is provided on the bottom.
[0068] When a device has a structure such that the substrate is not
required to have transparency to visible light, the substrate may
be formed of a light-shielding member.
[0069] The display provided with the transistor described in the
first or second embodiment may be a transparent type, a reflective
type, or a combination thereof.
(1) Material for Light-Shielding Structure Applied to the First to
the Third Embodiments
[0070] The present invention is characterized by, as described
above, that a light-shielding member is provided so that the active
layer is not irradiated with visible light and light or an
electromagnetic wave having a wavelength shorter than that of
visible light from the outside of a TFT.
[0071] The light-shielding structure has the following
constitution: (a) the substrate itself of a TFT is a
light-shielding member, or a layer of a light-shielding member is
provided on the top or bottom face of the substrate; (b) a
light-shielding layer is provided on the upper portion (the side
opposite the substrate) of a TFT (the lower electrode of a
light-emitting layer or the high-resistive layer of a
liquid-crystal device may also serve as a light-shielding layer);
(c) the interlayer-insulating film is formed of a light-shielding
member; or (d) some or all electrodes, i.e., the source electrode,
drain electrode, and gate electrode, of a TFT are formed so as to
have a function as a light-shielding member. The present invention
is achieved by a combination optionally selected from the
above-mentioned constitution.
[0072] Any structure can be optionally used as long as the
structure has a light-shielding property. In particular, it is
preferable that the transmittance for visible light and light or an
electromagnetic wave having a wavelength shorter than that of
visible light (wavelength range of 300 to 800 nm) be less than
0.01%.
[0073] A deviation in the OFF-current of a TFT caused by light
irradiation can be decreased to 1/100 or less by satisfying a
condition that the transmittance is less than 0.01%.
[0074] Examples of the material of the light-shielding member
include metals such as Al, Cr, and Ni; alloys thereof; and
silicides. The structure of the light-shielding film may be a
multilayer film composed of different materials. In an example
composed of three layers, the center layer is made of a material
having a high electrical conductivity and a high light-shielding
property, and the layers at both sides are made of a material whose
light-shielding property is inferior to that of the center layer,
but the electrical conductivity is sufficiently lower than that of
the center layer.
[0075] The light-shielding film may be formed of a high-melting
point metal such as Ti, Cr, W, Ta, Mo, or Pb, an alloy containing
such a metal, or a silicide. In addition, the light-shielding film
may be formed of WSi, WSiN, TiN, WN, amorphous silicon, or
polycrystal silicon. Furthermore, the light-shielding film may be
formed of an organic material (for example, a resin such as a
rubber shielding visible light).
[0076] The thickness of the light-shielding film is, for example,
in the range of several tens of nanometers to several tens of
micrometers.
(2) Types of Field-Effect Transistors to which the First to the
Third Embodiments can be Applied
[0077] The TFTs described in the above-mentioned two examples are
bottom-gate inverted-staggered and top-gate staggered types, but
the present invention is not limited to these. In the present
invention, the TFT may be a coplanar type, an inverted-coplanar
type, or other structures as long as a light-shielding member is
provided.
[0078] Examples of the structures of TFTs to which the present
invention can be applied include, as shown in FIGS. 7A to 7D, a
staggered TFT (FIG. 7A), an inverted-staggered TFT (FIG. 7B), a
coplanar TFT (FIG. 7C), and an inverted-coplanar TFT (FIG. 7D). In
the figures, reference numeral 1 represents a substrate, reference
numeral 2 represents an active layer, reference numeral 3
represents a source electrode, reference numeral 4 represents a
drain electrode, reference numeral 5 represents a gate-insulating
film, and reference numeral 6 represents a gate electrode.
(3) Transparent Oxide Material Applied to the First to the Third
Embodiments
[0079] Examples of transparent oxides in the present invention
include single-crystal oxides, polycrystal oxides, amorphous
oxides, and mixtures thereof. The polycrystal oxides may be, for
example, ZnO or ITO.
[0080] Amorphous oxides which can be applied to the present
invention are described in the above-mentioned Patent Document in
detail. Hereinafter, cases that an amorphous oxide is used as a
material of the active layer will be described.
[0081] The active layer for a normally-off TFT may be an oxide film
having an electronic carrier concentration of lower than
10.sup.18/cm.sup.3.
[0082] Specifically, such an oxide film may have a structure in an
In--Ga--Zn--O system, and the composition in the crystalline state
is represented by InGaO.sub.3(ZnO).sub.m (wherein m is an integer
less than 6).
[0083] In addition, the oxide film may have a structure in an
In--Ga--Zn--Mg--O system, and the composition in the crystalline
state is InGaO.sub.3(Zn.sub.1-XMg.sub.XO).sub.m (wherein m is an
integer less than 6, and X is denoted by 0<X.ltoreq.1).
[0084] The electron mobility of the material for the oxide
characteristically increases with the number of conduction
electrons. Examples of the substrate for a TFT include glass
substrates, plastic resin substrates, and plastic films.
[0085] An amorphous oxide film having a low electron carrier
concentration and a large electron mobility may be formed of an
amorphous oxide composed of an oxide of at least one element
selected from Zn, In, and Sn.
[0086] The electron mobility of this amorphous oxide film
characteristically increases with the number of conduction
electrons. A normally-off TFT can be produced by using this film.
The normally-off TFT has excellent transistor characteristics such
as an ON/OFF ratio, a saturation current in the pinch-off state,
and a switching speed.
[0087] The semiconductor layer may be an amorphous oxide containing
at least one element selected from Sn, In, and Zn.
[0088] In addition, when Sn is selected as the at least one element
of the amorphous oxide, the Sn may be substituted with
Sn.sub.1-XM4.sub.X (wherein X is denoted by 0<X<1, and M4 is
a Group 4 element having an atomic number less than that of Sn
selected from the group consisting of Si, Ge, and Zr).
[0089] When In is selected as the at least one element of the
amorphous oxide, the In may be substituted with In.sub.1-YM3.sub.Y
(wherein Y is denoted by 0<Y<1, and M3 is Lu or a Group 3
element having an atomic number less than that of In selected from
the group consisting of B, Al, Ga, and Y).
[0090] When Zn is selected as the at least one element of the
amorphous oxide, the Zn may be substituted with Zn.sub.1-ZM2.sub.Z
(wherein the Z is denoted by 0<Z<1, and M2 is a Group 2
element having an atomic number less than that of Zn selected from
the group consisting of Mg and Ca).
[0091] Specifically, examples of the amorphous material which can
be applied to the present invention include Sn--In--Zn oxides,
In--Zn--Ga--Mg oxides, In oxides, In--Sn oxides, In--Ga oxides,
In--Zn oxides, Zn--Ga oxides, and Sn--In--Zn oxides. The
composition ratio of the constituting materials is not limited to
1:1. The amorphous phase of Zn or Sn by itself alone may not be
readily produced, but are readily produced by adding In. For
example, in an In--Zn system, it is preferable that the composition
contains about 20 at % or more of In as an atomic ratio excluding
oxygen. In a Sn--In system, it is preferable that the composition
contain about 80 at % or more of In as an atomic ratio excluding
oxygen. In a Sn--In--Zn system, it is preferable that the
composition contains about 15 at % or more of In as an atomic ratio
excluding oxygen.
[0092] Amorphousness of a film is determined by confirming that no
clear diffraction peak is observed (namely, a halo pattern is
observed) by X-ray diffraction analysis with a low incident angle
of about 0.5 degrees. In addition, in the present invention, when
the above-mentioned materials are used in channel layers of
field-effect transistors, the channel layers may contain a
microcrystalline material. The existence of a microcrystal in an
amorphous oxide can be confirmed by observation with a transmission
electron microscope, for example.
(4) Substrate and Electrode Materials Applied to the First to the
Third Embodiments.
[0093] The electrodes of a transistor according to the present
invention are formed of materials, such as Al and Au, which have a
light-shielding property as previously described. The substrate may
be a light-shielding substrate such as an Al-metal substrate, a
silicon substrate, or a flexible substrate such as a plastic or PET
substrate.
EXAMPLE
(Spectral Sensitivity Evaluation Experiment)
[0094] First, spectral sensitivity measurement experiments for
amorphous oxides according to the present invention will be
described in detail.
[0095] An oxide of an amorphous In--Ga--Zn system was formed on a
substrate by sputtering.
[0096] Specifically, the amorphous oxide was deposited on a glass
substrate (1737: manufactured by Corning Inc.) so as to have a
thickness of 50 nm by high-frequency sputtering in an atmosphere of
a gas mixture of oxygen and argon. The target material was a
sintered body composed of In:Ga:Zn=1:1:1. The ultimate vacuum in a
growth chamber was 8.times.10.sup.-4 Pa, the total pressure of
oxygen and argon was 5.3.times.10.sup.-1 Pa, and the oxygen partial
pressure was 1.8.times.10.sup.-2 Pa.
[0097] The substrate during the forming of the film was not
specifically heated. The chamber temperature during the forming of
the film was about 30.degree. C. The resulting films were examined
by X-ray diffraction analysis (thin film method) with X-ray having
an incident angle of 0.5 degrees with respect to the film surface.
No clear diffraction peak was detected, and the results confirmed
that all the resulting In--Zn--Ga--O films were amorphous.
[0098] Furthermore, the pattern analysis of the film was conducted
by X-ray reflectometry to confirm that the mean square roughness
(Rrms) and the thickness of the thin film were about 0.5 nm and
about 50 nm, respectively.
[0099] The metal composition ratio of the thin film by fluorescent
X-ray analysis (XRF) was In:Ga:Zn=1.00:0.94:0.65. Light-absorption
analysis confirmed that the width of the forbidden energy band of
the resulting amorphous thin film was about 3.1 eV.
[0100] An electrode having a diameter of 1 mm was formed on the
thus obtained amorphous oxide film. Specifically, electrodes made
of laminated metals of Au (40 nm) and Ti (5 nm) were deposited by a
masked evaporation method at intervals of 2 mm. Thus, samples of
measurements were prepared. The laminated electrode had the
surfacemost layer of Au.
[0101] The electrical conductivity of the samples were measured
(spectral sensitivity characteristics evaluation) by using a
spectrum (at intervals of 10 nm) of a constant light intensity (2.5
mW/cm.sup.2) and a bias voltage of 10 V. The measurement was
conducted by using a spectral-sensitivity analyzing system,
CEP-2000.
[0102] FIG. 8 shows the results.
[0103] As shown in FIG. 8, the photoinduced carrier generation and
an increase in the electrical conductivity were observed in the
amorphous film when the wavelength was shorter than about 450 nm
(about 2.8 eV), which is almost the same as an energy of about 3.1
eV corresponding to the width of the forbidden energy band. In
addition, the amount of the photoinduced carrier generation in the
spectral sensitivity characteristics evaluation depended on the
irradiated light intensity.
[0104] The above-described experiments have revealed for the first
time that it is necessary that a TFT is provided with a
light-shielding film in order to be more stably operated even when
an amorphous oxide which is thought to be transparent to visible
light is used, for example, for the active layer of the TFT.
Example 1
[0105] In this EXAMPLE, a staggered (top-gate) MISFET device shown
in FIG. 7A was produced.
[0106] Firstly, a gold film was laminated on a glass substrate 1 so
as to have a thickness of 30 nm and then formed into a drain
terminal 4 and a source terminal 3 by photolithography and
lift-off. Then, an amorphous film to be used as a channel layer 2
having a metal composition ratio of In:Ga:Zn=1.00:0.94:0.65 was
formed by sputtering so as to have a thickness of 30 nm. The
conditions for forming the amorphous oxide film were the same as
those in the above-described evaluation experiments.
[0107] Lastly, a Y.sub.2O.sub.3 film to be used as a
gate-insulating film was formed by electron beam evaporation, and
thereon a gold film was formed. The gold film was formed into a
gate terminal by photolithography and lift-off.
[0108] Then, a light-shielding member made of aluminum foil was
provided on the surface of the glass substrate on the side opposite
the TFT so that the TFT part was not irradiated with light from the
outside. The light-shielding member of the aluminum foil had a
transmittance of less than 0.01% to visible light and light or an
electromagnetic wave having a wavelength of less than that of
visible light.
[0109] The resulting MISFET device was evaluated for I-V
characteristics under irradiation with light of a fluorescent lamp
from the surface of the glass substrate. The light emitted from the
fluorescent lamp had a wavelength range of 350 to 750 nm. The
results were that the electron field-effect mobility was 7
cm.sup.2/Vs and the ON/OFF ratio was higher than 10.sup.5. In
addition, characteristics of the device were measured in the dark
instead of the irradiation with the light of a fluorescent lamp,
and no changes were observed in the electron field-effect mobility
and the ON/OFF ratio.
[0110] For a comparative experiment, a MISFET device sample was
prepared. The sample was the same as the above-described staggered
(top-gate) MISFET device except that a light-shielding member was
not provided.
[0111] This MISFET device was evaluated for I-V characteristics
under irradiation with light of a fluorescent lamp from the surface
side of the glass substrate. It was confirmed that the ON/OFF ratio
was decreased by an order of magnitude.
[0112] The above-described results have revealed that a
light-shielding structure is necessary for stable operation even
when an amorphous oxide which is recognized to be transparent is
used for the active layer of a TFT.
[0113] The field-effect transistors in accordance with the present
invention can be utilized as switching devices of liquid crystal
displays and inorganic or organic EL displays. In addition, the
transistors can be formed on flexible substrates such as plastic
films by a low-temperature process, and therefore can be widely
applied to not only flexible displays but also IC cards and ID
tags. According to the present invention, a novel transistor
provided with a light-shielding structure can be provided as a
field-effect transistor using an oxide which is recognized to be
transparent for the active layer.
[0114] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all modifications, equivalent
structures and functions.
[0115] This application claims the benefit of Japanese Application
No. 2005-305950 filed Oct. 20, 2005, which is hereby incorporated
by reference herein in its entirety.
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