U.S. patent application number 12/824568 was filed with the patent office on 2010-10-21 for field effect transistor manufacturing method.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to HIDEO HOSONO, TATSUYA IWASAKI, Toshio Kamiya, KENJI NOMURA, MASAFUMI SANO, HISATO YABUTA.
Application Number | 20100267198 12/824568 |
Document ID | / |
Family ID | 36461431 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100267198 |
Kind Code |
A1 |
YABUTA; HISATO ; et
al. |
October 21, 2010 |
FIELD EFFECT TRANSISTOR MANUFACTURING METHOD
Abstract
Provided is a novel method for manufacturing a field effect
transistor. Prior to forming an amorphous oxide layer on a
substrate, ultraviolet rays are irradiated onto the substrate
surface in an ozone atmosphere, plasma is irradiated onto the
substrate surface, or the substrate surface is cleaned by a
chemical solution containing hydrogen peroxide.
Inventors: |
YABUTA; HISATO; (TOKYO,
JP) ; SANO; MASAFUMI; (FUJISAWA-SHI, JP) ;
IWASAKI; TATSUYA; (TOKYO, JP) ; HOSONO; HIDEO;
(YAMATO-SHI, JP) ; Kamiya; Toshio; (Kawasaki-shi,
JP) ; NOMURA; KENJI; (TOKYO, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
1290 Avenue of the Americas
NEW YORK
NY
10104-3800
US
|
Assignee: |
CANON KABUSHIKI KAISHA
TOKYO
JP
TOKYO INSTITUTE OF TECHNOLOGY
TOKYO
JP
|
Family ID: |
36461431 |
Appl. No.: |
12/824568 |
Filed: |
June 28, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11269641 |
Nov 9, 2005 |
|
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12824568 |
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Current U.S.
Class: |
438/104 ;
257/E21.461 |
Current CPC
Class: |
H01L 29/78693 20130101;
H01L 29/78618 20130101; H01L 29/66969 20130101 |
Class at
Publication: |
438/104 ;
257/E21.461 |
International
Class: |
H01L 21/36 20060101
H01L021/36 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 10, 2004 |
JP |
2004326686 |
Claims
1-6. (canceled)
7. A method of manufacturing a field-effect transistor comprising:
a first step of preparing a substrate; a second step of depositing
on the substrate an insulating layer or an active layer comprising
an amorphous oxide semiconductor comprising one element selected
from at least In, Zn and Sn; and third step of thermal processing,
at a higher temperature than a deposition temperature of the active
layer or the insulating layer in the second step, in an atmosphere
of at least one of ozone and nitrogen oxide to reduce defect levels
in the amorphous oxide semiconductor, wherein the temperature of
the thermal processing is higher than the deposition temperature of
the active layer or the insulating layer, and is not more than
200.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for manufacturing
a field effect transistor.
[0003] 2. Related Background Art
[0004] In recent years, flat image display devices (Flat Panel
Display: FPD) have been developed for practical use as a result of
the progress made in technologies such as liquid crystals and
electroluminescence (EL). These FPDs are driven by the active
matrix circuitry of field effect thin-film transistors (Thin Film
Transistor: TFT) which use an amorphous silicon thin-film or a
polycrystalline thin-film on a glass substrate in the active
layer.
[0005] Meanwhile, testing is being carried out into the use of
resin substrates which are light-weight and flexible in place of a
glass substrate in order to make such FPDs even thinner, lighter
and have better their shatter resistance.
[0006] However, the manufacture of such an above-described
transistor which uses a silicon thin-film requires a relatively
high temperature thermal process, whereby direct formation onto a
resin substrate, which usually has a low thermal resistance, is
difficult.
[0007] Therefore, development is being actively conducted into TFTs
which are capable of deposition at low temperatures and which use,
for example, ZnO as a material for the oxide semiconductor
thin-film (Japanese Patent Application Laid-Open No.
2003-298062).
[0008] From the knowledge of the present inventors that ZnO cannot
generally form a stable amorphous phase, existing mainly in a
polycrystalline phase, carriers would therefore be scattered at the
interface between polycrystalline particles. As a result of this
fact, it was learned that electron mobility cannot be
increased.
[0009] That is, a method has been sought for producing an amorphous
oxide which can be preferably used in the active layer of a field
effect transistor.
SUMMARY OF THE INVENTION
[0010] In consideration of the above-described background, it is an
object of the present invention to provide a method for
manufacturing a novel field effect transistor.
[0011] The present invention will now be explained in further
detail.
(First Aspect of the Present Invention: Deposition Pre-Treatment to
Deposition Post-Treatment)
[0012] The method for manufacturing a field effect transistor
according to the present invention comprises:
[0013] a first step of preparing a substrate; and
[0014] a second step of forming on the substrate an active layer
comprising an amorphous oxide; wherein
[0015] prior to the second step, at least one of:
[0016] a step of irradiating ultraviolet rays onto the substrate
surface in an ozone atmosphere; or
[0017] a step of irradiating plasma onto the substrate surface;
or
[0018] a step of cleaning the substrate surface with a chemical
solution containing hydrogen peroxide is carried out.
[0019] Further, the method for manufacturing a field effect
transistor according to the present invention comprises:
[0020] a first step of preparing a substrate; and
[0021] a second step of forming on the substrate an active layer
comprising an amorphous oxide; wherein
[0022] the second step is carried out in an atmosphere comprising
at least one selected from the group consisting of ozone gas,
nitrogen oxide gas, an oxygen-containing radical, elemental oxygen,
oxygen ion and an oxygen radical.
[0023] Further, the method for manufacturing a field effect
transistor according to the present invention comprises:
[0024] a first step of preparing a substrate; and
[0025] a second step of forming on the substrate an active layer
comprising an amorphous oxide; wherein
[0026] subsequent to the second step, the method comprises at least
one step of:
[0027] a step of thermal processing at a higher temperature than
the deposition temperature of the active layer in the second step;
and
[0028] a step of irradiating an oxygen-containing plasma onto the
substrate comprising the active layer.
[0029] The present invention also comprises, subsequent to the
second step, at least one of the steps of: thermal processing;
irradiating oxygen-containing plasma onto the oxide film; mask
deposition for patterning of the film; and etching for patterning
of the film.
[0030] The present invention also comprises, subsequent to the
second step, subjecting the substrate comprising an amorphous oxide
to:
[0031] thermal processing in an atmosphere containing ozone; or
[0032] thermal processing in an atmosphere containing nitrogen
oxide; or
[0033] thermal processing in an atmosphere containing water
vapor.
[0034] The present invention also comprises, subsequent to the
second step, subjecting the substrate comprising an amorphous oxide
to:
[0035] thermal processing in an atmosphere containing an oxygen
radical;
[0036] irradiating an oxygen-containing plasma onto the amorphous
oxide; or
[0037] irradiating oxygen-containing plasma onto the amorphous
oxide in a state wherein the substrate has been heated.
[0038] The present invention also comprises, subsequent to the
second step, subjecting the amorphous oxide to:
[0039] irradiation with an oxygen-containing radical beam; or
[0040] mask deposition for patterning of the amorphous oxide;
or
[0041] an etching step for patterning of the amorphous oxide.
(Second Aspect of the Present Invention: Deposition (or Film
Formation) Method)
[0042] The method for manufacturing a field effect transistor
according to the present invention comprises:
[0043] a first step of preparing a substrate; and
[0044] a second step of forming on the substrate an active layer
comprising an amorphous oxide; wherein
[0045] the second step is carried out by:
[0046] resistance heating deposition; or
[0047] electron beam deposition; or
[0048] chemical vapor deposition; or
[0049] line-beam laser deposition; or
[0050] electrodeposition.
[0051] Resistance heating deposition can include, for example,
resistance heating deposition using a Knudsen cell. Chemical vapor
deposition includes methods having means for promoting source
material decomposition by plasma, as well as means for promoting
source material decomposition by a catalyst.
(Third Aspect of the present Invention: Substrate Temperature)
[0052] The method for manufacturing a field effect transistor
according to the present invention comprises:
[0053] a first step of preparing a substrate; and
[0054] a second step of forming on the substrate an active layer
comprising an amorphous oxide; wherein
[0055] the second step is carried out at a deposition temperature
of 70.degree. C. or more.
[0056] A lower limit for the deposition temperature may be set as
appropriate, although preferably it is lower than the thermal
deformation temperature of the substrate.
[0057] Here, the thermal deformation temperature is, for example,
from 100.degree. C. to 200.degree. C., inclusive thereof.
Therefore, the above-described deposition temperature is preferably
70.degree. C. or more to 200.degree. C. or less.
[0058] The amorphous oxide produced in the above three aspects of
the present invention is characterized, for example, in having an
electron carrier density of less than 1.times.10.sup.18/cm.sup.3,
or, in being an amorphous oxide in which electron mobility tends to
increase as electron carrier density increases.
[0059] Such an amorphous oxide is an oxide comprising at least one
of In, Zn and Sn, or, is an oxide comprising In, Zn and Ga.
[0060] The above-described first to third aspects of the present
invention may include a separate step in between the first and
second steps. In the present invention, while "depositing an
amorphous oxide on a substrate" obviously includes direct
deposition onto the substrate, this phrase also includes deposition
of the amorphous oxide onto the substrate via another layer(s).
[0061] According to the present invention, a method for
manufacturing a novel field effect transistor comprising an
amorphous oxide is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0062] FIG. 1 is a graph illustrating the relationship between the
electron carrier density of an In--Ga--Zn--O system amorphous film
deposited by pulsed laser deposition and the oxygen partial
pressure during deposition;
[0063] FIG. 2 is a graph illustrating the relationship between the
electrical conductivity of an In--Ga--Zn--O system amorphous film
deposited by sputtering using argon gas and the oxygen partial
pressure during deposition;
[0064] FIG. 3 is a graph illustrating the relationship between the
number of electron carriers of an In--Ga--Zn--O system amorphous
film deposited by pulsed laser deposition and electron
mobility;
[0065] FIG. 4 is a graph illustrating the change in electrical
conductivity, carrier density and electron mobility with respect to
the x value in InGaO.sub.3(Zn.sub.1-xMg.sub.xO) deposited by pulsed
laser deposition in an atmosphere having an oxygen partial pressure
of 0.8 Pa;
[0066] FIG. 5 is a schematic diagram illustrating a top-gate type
MISFET device structure;
[0067] FIG. 6 is a graph illustrating the current-voltage
characteristics of a top-gate type MISFET device;
[0068] FIG. 7A is a schematic diagram for explaining the third
aspect of the present invention;
[0069] FIG. 7B is a schematic diagram for explaining the third
aspect of the present invention;
[0070] FIG. 8 is a schematic diagram of an apparatus for carrying
out deposition by PLD; and
[0071] FIG. 9 is a schematic diagram of an apparatus for carrying
out deposition by sputtering.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0072] First, the above-described first to third aspects of the
present invention will be explained with reference to a first,
second and third embodiment.
[0073] After this explanation, the amorphous oxide according to the
present invention and matters common to each of the embodiments
will be described.
First Embodiment: Pre-Deposition to Post-Deposition
[0074] 1-A The method for manufacturing a field effect transistor
according to the present embodiment is characterized by, subsequent
to preparing a substrate yet prior to forming on the substrate an
active layer comprising an amorphous oxide, carrying out any of the
following steps:
[0075] irradiating ultraviolet rays onto the substrate surface in
an ozone atmosphere; or
[0076] irradiating plasma onto the substrate surface; or
[0077] cleaning the substrate surface with a chemical solution
containing hydrogen peroxide; or
[0078] coating with a film comprising silicon and oxygen.
[0079] As a result of the above surface treatment process of the
substrate, contaminants adhered to the substrate surface are
removed, whereby the substrate surface is cleaned.
[0080] As a result of the above process, performance deterioration
due to contaminants diffusing into the film constituting a TFT
(thin-film transistor), or other such field effect transistor, can
be reduced.
[0081] Further, as a result of removing adhered matter from the
substrate surface, adhesion between the substrate and the film
constituting the transistor can be improved. [0082] 1-B The method
for manufacturing a field effect transistor according to the
present invention is characterized by, subsequent to preparing a
substrate to be used for deposition, depositing an amorphous oxide
in a prescribed atmosphere.
[0083] Such a prescribed atmosphere comprises at least one selected
from the group consisting of ozone gas, nitrogen oxide gas, an
oxygen-containing radical, elemental oxygen, oxygen ion and an
oxygen radical.
[0084] The ozone gas, nitrogen oxide gas, an oxygen-containing
radical, and oxygen radical can be fed into the deposition chamber
from outside of the deposition chamber.
[0085] By irradiating an oxygen-containing plasma onto the
substrate, elemental oxygen, oxygen ion and oxygen radicals can be
generated in the deposition chamber.
[0086] Since the above-described ozone gas etc. is more strongly
oxidative than oxygen in a molecular state, such substances are
suitable when trying to obtain an amorphous oxide which has little
oxygen deficiency.
[0087] When the above-described amorphous oxide is used as the
active layer of a field effect transistor, according to
above-described present invention, unnecessary oxygen deficiency
can be reduced, whereby deterioration in transistor characteristics
due to defect level formation can be suppressed.
[0088] The present invention also encompasses the case where the
amorphous oxide is used as an insulating layer. If the insulating
layer is formed in accordance with the above-described method, the
advantageous effect that the insulating properties increase can be
achieved.
[0089] The present invention further encompasses the case where
during amorphous oxide deposition oxygen molecules are also
incorporated into the above-described atmosphere. [0090] 1-C The
present invention is also characterized in carrying out, subsequent
to preparing a substrate (first step) and a second step of forming
on the substrate an active layer comprising, an amorphous oxide, at
least one step (post-treatment step) of the below-described
post-treatment steps.
[0091] That is, a step of thermal processing at a higher
temperature than the deposition temperature of the active layer in
the second step; or
[0092] a step of irradiating an oxygen-containing plasma onto the
substrate comprising the active layer.
[0093] The deposition temperature is, for example, room
temperature. Specifically, the deposition temperature is in the
range of 0.degree. C. to 40.degree. C.
[0094] In some cases, such as when performing deposition at room
temperature, deposition is intentionally carried out without
heating the substrate during deposition of the active layer.
[0095] The above-described thermal processing step can be carried
out as appropriate as long as such step is after amorphous oxide
formation.
[0096] Obviously, the thermal processing step can be carried out
after the gate insulating film has been formed on the substrate, or
after the electrode films (the drain electrode, source electrode,
gate electrode etc.) have been formed.
[0097] In particular, when an oxide is used as the above-described
electrode film, it is preferable to carry out the thermal
processing step after electrode film formation.
[0098] The thermal processing step can be carried out in an
ozone-containing atmosphere, a nitrogen oxide-containing
atmosphere, a water vapor-containing atmosphere, an oxygen
radical-containing atmosphere and the like.
[0099] The temperature in the thermal processing step is, for
example, greater than room temperature and 600.degree. C. or less.
Preferably, the temperature is 200.degree. C. or less. When a
flexible substrate such as PET (polyethylene terephthalate) is
used, the temperature is 200.degree. C. or less, preferably
100.degree. C. or less, and more preferably 50.degree. C. or
less.
[0100] According to the above, unnecessary oxygen deficiency can be
reduced, whereby deterioration of transistor characteristics due to
defect level formation can be reduced.
[0101] When an insulating layer comprising an oxide is formed on
the substrate, the insulating properties thereof can be
increased.
[0102] In addition, the step of irradiating an oxygen-containing
plasma may be carried out as appropriate, as long as such step is
after amorphous oxide formation.
[0103] Specifically, this refers to after the amorphous oxide
active layer deposition, after gate insulating film deposition
(when using an oxide for the gate insulating film), or after
electrode film deposition (when using an oxide for the drain
electrode, source electrode, or gate electrode).
[0104] Plasma irradiation can also be carried out while heating the
substrate.
[0105] As a result of such plasma irradiation, unnecessary oxygen
deficiency can be reduced, whereby deterioration of transistor
characteristics due to defect level formation can be suppressed.
Further, when an insulating layer is comprised on the substrate,
insulating properties can be increased.
[0106] Furthermore, after the second step, the formed film can be
subjected to patterning in order to construct a field effect
transistor such as a TFT.
[0107] Specifically, a mask layer to be used for patterning is
deposited. Alternatively, after the film has been deposited,
etching can be carried out after undergoing resist coating and
lithography steps.
[0108] By carrying out the above, the number of steps during TFT
device formation can be reduced, whereby circuitry and devices can
be obtained having little variation in characteristics between
devices.
(Second Aspect of the Present Invention: Deposition Method)
[0109] The method for manufacturing a field effect transistor
according to the present embodiment comprises:
[0110] preparing a substrate (first step), followed by a second
step of forming on the substrate an active layer comprising an
amorphous oxide, wherein the second step is carried out by:
[0111] resistance heating deposition; or
[0112] electron beam deposition; or
[0113] chemical vapor deposition; or
[0114] line-beam laser deposition; or
[0115] electrodeposition.
[0116] The present invention encompasses preparing a substrate,
followed by, when forming on the substrate at least one selected
from the group consisting of an amorphous oxide active layer of a
field effect transistor, a source electrode, a drain electrode, a
gate insulating film and a gate electrode, forming by the
above-mentioned resistance heating deposition, electron beam
deposition, chemical vapor deposition, line-beam laser deposition
or electrodeposition.
[0117] From this, an active layer, an electrode film or an
insulating film can be obtained having equal to or better quality
than that of conventional pulsed laser deposition. Further,
according to the invention in accordance with the present
embodiment, the amorphous oxide can be deposited onto a substrate
having the same or greater surface are than that of a sputtering
technique.
[0118] Although it depends on the apparatus used to carry out
deposition, the conditions relating to oxygen (e.g. oxygen partial
pressure) when depositing an amorphous oxide by the above-described
techniques can be set, for example, in the below range.
[0119] For resistance heating deposition and electron beam
deposition, the oxygen partial pressure or the total pressure is
set in the range of from 10.sup.-3 to 10 Pa.
[0120] For chemical vapor deposition, half of the chamber internal
total pressure, for example, can be set as the oxygen partial
pressure. For line-beam laser deposition, the oxygen partial
pressure range can be set, for example, from 4.5 Pa to less than
6.5 Pa.
[0121] Line-beam laser deposition is a deposition technique which
uses a laser employed in pulsed laser deposition (described below),
to which a line optical system is attached for generating a laser
line beam having a prescribed width.
(Third Aspect of the Present Invention: Substrate Temperature)
[0122] The method for manufacturing a field effect transistor
according to the present embodiment comprises preparing a substrate
(first step), and carrying out deposition at a deposition
temperature of 70.degree. C. or higher during a second step of
forming on the substrate an active layer comprising an amorphous
oxide.
[0123] Here, the term deposition temperature refers to, for
example, the temperature of the substrate, the temperature of the
uppermost surface of the substrate (the surface on which the film
is being grown), the temperature near the substrate, or the
temperature indicated by a chamber internal thermometer installed
in the respective film deposition apparatus.
[0124] Therefore, even when deposition is carried out with the
atmosphere temperature set to room temperature (e.g. using a
heater, or especially when conducting deposition without heating),
cases where the temperature of the substrate itself or the
temperature of the uppermost surface of the substrate is 70.degree.
C. or higher are within the range of the invention according to the
present embodiment.
[0125] The lower limit for the deposition temperature (e.g.
substrate temperature) may be set as appropriate, although
preferably it is lower than the thermal deformation temperature of
the substrate, for example.
[0126] Although the thermal deformation temperature is dependent on
the substrate, it is from 100.degree. C. or more to 200.degree. C.
or less (inclusive thereof), for example.
[0127] By setting the deposition temperature (e.g. substrate
temperature) during deposition to 70.degree. C. or higher,
variation in film characteristics, which occurs in processes
subsequent to formation of the amorphous oxide, is less likely to
happen, which ultimately results in reduced variation in the device
characteristics. Here, examples of device characteristics include
electron mobility, on/off ratio, voltage between drain and source,
gate threshold voltage and the like, which exist in a transistor
fabricated using the above-described amorphous oxide.
[0128] Further, the reason for setting the temperature to
70.degree. C. or higher is because, when using an amorphous oxide
to form the transistors for a display apparatus or similar device,
there are cases where in subsequent processes heating to about
60.degree. C. is carried out, or the device heats up during use to
about 60.degree. C.
[0129] In addition, the stability of the device improves during
high-temperature operation or after high-temperature environment
storage. This concept is illustrated in FIGS. 7A and 7B.
[0130] FIG. 7A illustrates the relationship between a typical
(variation in device characteristics after storage for 10 hours at
60.degree. C.)/(variation in device characteristics before storage)
on the vertical axis, and substrate temperature during deposition
of the amorphous oxide on the horizontal axis. It can be seen that
if the temperature is set to 70.degree. C. or higher, the variation
in characteristics decreases.
[0131] A preferable range for the substrate temperature will depend
on the deposition method and deposition techniques. However,
although since in a sputtering method high-energy particles are
irradiated onto the substrate surface, a sputtering method is a
preferable deposition method as deposition is good at even
comparatively low temperatures.
[0132] Here, although strictly speaking "substrate temperature"
refers to the temperature of the substrate surface during
deposition, in cases where it is difficult to directly measure the
temperature during deposition, the temperature can be taken to be
the average value of the substrate temperature prior to deposition
and the substrate temperature immediately after deposition.
Substrate temperature may be measured using an arbitrary
thermometer, such as a radiation thermometer or a thermocouple.
[0133] The deposition temperature (e.g. substrate temperature) is
preferably lower than that of the substrate thermal deformation
temperature. Especially in cases where a resin substrate is used,
and when deposition is carried out at a higher temperature than the
thermal deformation temperature, film separation and film damage
can occur.
[0134] That is, fabrication yield decreases. FIG. 7B is a schematic
diagram illustrating the relationship between yield on the vertical
axis and substrate temperature during deposition on the horizontal
axis. It can be seen that yield decreases if the substrate
temperature is set higher than the thermal deformation
temperature.
[0135] Using a substrate which has a substrate thermal deformation
temperature of 100.degree. C. or more to 200.degree. C. or less is
preferable from the viewpoint of device stability and substrate
flexibility.
[0136] The deposition temperature (e.g. substrate temperature) in
the invention according to the present embodiment is preferably
70.degree. C. or more to 200.degree. C. or less, and more
preferably from 70.degree. C. or more to 100.degree. C. or less,
although this does depend on the kind of substrate that is
used.
[0137] Further, in terms of fabricating a TFT on a flexible
substrate, preferable conditions include using a material with a
substrate deformation temperature of between about 120 to
150.degree. C., and depositing at a substrate temperature of about
80 to 100.degree. C.
[0138] The thermal deformation temperature of typical resin
substrates is about 75.degree. C. for acrylic resin (PMMA),
70.degree. C. for PET and 150.degree. C. for PC (polycarbonate),
although the temperature will vary depending on factors such as the
production method and the mixture. For example, by strengthening
with glass fiber or similar, materials do exist which have their
deformation temperature raised to about 200.degree. C., even for
PET based materials.
[0139] Here, thermal deformation temperature can be evaluated in
accordance with JIS K7206 testing standards.
[0140] A glass substrate, plastic substrate or a plastic film
substrate can be used as a substrate to be formed with a
transparent film. The kinds of plastic which can be used include an
arbitrary resin such as polyethylene terephthalate (PET),
polyimide, acryl (PMMA), epoxy and the like.
[0141] It is noted that in addition to the deposition method
explained for the second embodiment, the deposition method
according to the present embodiment can also be appropriately
selected from among, for example, pulsed laser deposition (PLD) and
sputtering (SP).
[0142] The amorphous oxide which can be applied in the
above-describe first to third embodiments will now be
explained.
(Amorphous Oxide)
[0143] The electron carrier density of the amorphous oxide
according to the present invention is the value when measured at
room temperature. Room temperature is, for example, 25.degree. C.,
and more specifically can be selected as appropriate from the range
of about 0.degree. C. to 40.degree. C. The electron carrier density
of the amorphous oxide according to the present invention does not
have to be less than 10.sup.18/cm.sup.3 over the whole range of
0.degree. C. to 40.degree. C. For instance, it is acceptable if
electron carrier density is less than 10.sup.18/cm.sup.3 at
25.degree. C. If electron carrier density is further decreased to
1.times.10.sup.17/cm.sup.3 or less, and more preferably
1.times.10.sup.16/cm.sup.3 or less, a normally-off TFT can be
obtained at a good yield.
[0144] Additionally, the "less than 10.sup.18/cm.sup.3" means
preferably less than 1.times.10.sup.18/cm.sup.3, and more
preferably less than 1.0.times.10.sup.18/cm.sup.3.
[0145] Measurement of electron carrier density can be obtained from
the Holl effect measurement.
[0146] In the present invention the term "amorphous oxide" refers
to an oxide in which a halo pattern can be observed and does not
show a specific diffraction line in its X-ray diffraction
spectrum.
[0147] The lower limit of the electron carrier density for the
amorphous oxide according to the present invention is not
particularly restricted, as long as the amorphous oxide can be
employed as the channel layer of a TFT. The lower limit is, for
example 1.times.10.sup.12/cm.sup.3.
[0148] Accordingly, in the present invention electron carrier
density is set at, for example, 1.times.10.sup.12/cm.sup.3 or more
to less than 1.times.10.sup.18/cm.sup.3 by controlling the
materials, composition ratio, production conditions and similar
factors of the amorphous oxide, as shown in the below Examples.
More preferable is the range of 1.times.10.sup.13/cm.sup.3 or more
to 1.times.10.sup.17/cm.sup.3 or less, and still more preferable is
the range of from 1.times.10.sup.15/cm.sup.3 or more to
1.times.10.sup.16/cm.sup.3 or less.
[0149] In addition to InZnGa oxide, the amorphous oxide can also be
appropriately selected from among In oxides, In.sub.xZn.sub.1-x
oxides (0.2.ltoreq.x.ltoreq.1), In.sub.xSn.sub.1-x oxides
(0.8.ltoreq.x.ltoreq.1), or In.sub.x(Zn,Sn).sub.1-x oxides
(0.15.ltoreq.x.ltoreq.1).
[0150] In.sub.x(Zn,Sn).sub.1-x oxide can also be written as
In.sub.x(Zn.sub.ySn.sub.1-y).sub.1-x oxide, wherein the range of y
is from 1 to 0.
[0151] For the case of an indium oxide which does not contain zinc
or tin, a part of the indium may be substituted with gallium (i.e.
the case of In.sub.xGa.sub.1-x oxide (0.ltoreq.x.ltoreq.1)).
[0152] Amorphous oxides having an electron carrier density of less
than 1.times.10.sup.18/cm.sup.3 which the present inventors were
successful in fabricating will now be explained.
[0153] One of the above oxides comprised In--Ga--Zn--O, wherein the
composition of its crystalline state can be expressed as
InGaO.sub.3(ZnO).sub.m (m is a natural number of less than 6),
characterized in that the electron carrier density was less than
1.times.10.sup.18/cm.sup.3.
[0154] Another of the above oxides comprised In--Ga--Zn--Mg--O,
wherein the composition of its crystalline state can be expressed
as InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.m (m is a natural number of
less than 6; 0.ltoreq.x.ltoreq.1), characterized in that the
electron carrier density was less than
1.times.10.sup.18/cm.sup.3.
[0155] It is preferable that the electron mobility in the film
constituted from these oxides is designed to exceed 1
cm.sup.2/(Vsec).
[0156] If the above-described film is used for the channel layer,
transistor characteristics can be realized wherein the gate current
when the transistor is off is a normally-off of less than 0.1
microamperes, and the on/off ratio is more than 10.sup.3. Further,
such layer is transparent or translucent with respect to visible
light, whereby a flexible TFT can be realized.
[0157] The above-described film is characterized by the fact that
electron mobility increases in conjunction with an increase in the
number of electrons being conducted. A glass substrate, plastic
substrate or a plastic film substrate can be used as a substrate to
be formed with a transparent film.
[0158] When the above-described amorphous oxide is employed for the
channel layer, at least one of Al.sub.2O.sub.3, Y.sub.2O.sub.3, or
HfO.sub.2, or a mixed compound consisting of at least two thereof,
can be applied as the gate insulating film.
[0159] Further, intentionally not dosing into the amorphous oxide
impurity ions for increasing electric resistance, and carrying out
deposition in an atmosphere containing oxygen gas is also a
preferable embodiment.
[0160] The present inventors discovered the unique characteristic
that, in this semi-insulating oxide amorphous thin-film, electron
mobility increases in conjunction with an increase in the number of
electrons being conducted. In addition, the present inventors
discovered that if a TFT is fabricated using this film, transistor
characteristics, such as on/off ratio, saturation current in a
pinch-off state and switching speed, improve even further. That is,
the present inventors discovered that, using an amorphous oxide, a
normally-off type TFT can be realized.
[0161] If an amorphous oxide thin film is used as the film
transistor channel layer, electron mobility can be made to exceed 1
cm.sup.2/(Vsec), and preferably exceed 5 cm.sup.2/(Vsec).
[0162] When the electron carrier density is less than
1.times.10.sup.18/cm.sup.3, and preferably less than
1.times.10.sup.16/cm.sup.3, the current between the drain and
source when in an off state (no applied gate voltage) can be made
to be less than 10 microamperes, and preferably less than 0.1
microamperes.
[0163] In addition, if the above film is used, and when electron
mobility exceeds 1 cm.sup.2/(Vsec), and preferably exceeds 5
cm.sup.2/(Vsec), the saturation current after pinching-off can be
made to exceed 10 microamperes, and the on/off ratio can be made to
exceed 10.sup.3.
[0164] In a TFT, in the pinched-off state, a high voltage is
applied to the gate terminal, whereby electrons are present in a
high density in the channel.
[0165] Accordingly, according to the present invention, the
saturation current value can be further increased by just the
amount that the electron mobility increases. As a result, an
improvement in transistor characteristics, such as greater on/off
ratio, higher saturation current and faster switching speed can be
expected.
[0166] In contrast, in a conventional compound, if the number of
electrons increases, electron mobility decreases due to the
electrons colliding into each other.
[0167] Structures which can be used for the above-described TFT
include a staggered (top-gate) structure which forms a gate
insulating layer and a gate terminal in that order on a
semiconductor channel layer, and a inversely staggered
(bottom-gate) structure which forms a gate insulating layer and a
semiconductor channel layer in that order on a the gate
terminal.
(First Deposition Method: PLD)
[0168] An amorphous oxide having a composition in its crystalline
state which can be expressed as InGaO.sub.3(ZnO).sub.m (m is a
natural number of less than 6) can be stably maintained in an
amorphous state until a high temperature of 800.degree. C. or
higher when the value of m is less than 6, but as the value of m
increases, i.e. as the ratio of ZnO to InGaO.sub.3 increases,
becoming more like a ZnO composition, the oxide crystallizes more
easily.
[0169] Therefore, as an amorphous TFT channel layer, the value of m
is preferably less than 6.
[0170] The deposition method preferably uses a vapor deposition
method when the target is a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.m composition. Among vapor deposition methods,
sputtering and pulsed laser deposition are suitable. From a
mass-production viewpoint, sputtering is the most suitable.
[0171] However, if an amorphous film is fabricated under ordinary
conditions, oxygen deficiency occurs to a large extent, whereby
until now it was impossible to make the electron carrier density to
less than 1.times.10.sup.18/cm.sup.3 and electrical conductivity to
10 S/cm or less. Further, when such a film was used, it is
impossible to construct a normally-off transistor.
[0172] The present inventors fabricated In--Ga--Zn--O produced by
pulsed laser deposition using the apparatus illustrated in FIG.
8.
[0173] Deposition was carried out using a pulsed laser deposition
apparatus such as that illustrated in FIG. 8.
[0174] In FIG. 8, reference numeral 701 denotes a RP (rotary pump),
702 denotes a TMP (turbo molecular pump), 703 denotes a preparation
chamber, 704 denotes a RHEED electron gun, 705 denotes substrate
support means for rotating and vertical movement of the substrate,
706 denotes a laser entrance window, 707 denotes a substrate, 708
denotes a target, 709 denotes a radical source, 710 denotes a gas
entrance window, 711 denotes target support means for rotating and
vertical movement of the target, 712 denotes a bias line, 713
denotes a main line, 714 denotes a TMP (turbo molecular pump), 715
denotes a RP (rotary pump), 716 denotes a titanium getter pump and
717 denotes a shutter. Further, in FIG. 8 reference numeral 718
denotes an IG (ion gauge), 719 denotes a PG (Pirani gauge), 720
denotes a BG (Baratron gauge) and 721 denotes a growth chamber
(chamber).
[0175] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film was deposited on a SiO.sub.2 glass substrate (1737,
manufactured by Corning Incorporated) by pulsed laser deposition
employing a KrF excimer laser. As a pre-deposition treatment,
degreasing cleaning of the substrate by ultrasound was conducted
using acetone, ethanol and pure water (each for 5 minutes), and
then drying in air at 100.degree. C.
[0176] For the above polycrystalline target, an
InGaO.sub.3(ZnO).sub.4 sintered body target (size 20 mm.phi. 5 mmt)
was used. This was obtained by subjecting
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO (each a 4N reagent) as the
source material to wet blending (solvent: ethanol), and then
calcining (1,000.degree. C. for 2 hours), dry grinding and
sintering (1,550.degree. C. for 2 hours). The electrical
conductivity of the thus-obtained target was 90 (S/cm).
[0177] The degree of vacuum of the growth chamber was set to
2.times.10.sup.-6 (Pa), and deposition was carried out by
controlling the oxygen partial pressure during growth to 6.5
(Pa).
[0178] The oxygen partial pressure in the chamber 721 was 6.5 Pa
and the substrate temperature was 25.degree. C.
[0179] The distance between the target 708 and the substrate 707
which was to undergo deposition was 30 (mm), and the power of the
KrF excimer laser injected from the entrance window 716 was within
the range of 1.5 to 3 (mJ/cm.sup.2/pulse). Pulse width was 20
(nsec), and repetition frequency was 10 (Hz). Irradiation spot
diameter was set at 1.times.1 (mm angle).
[0180] Deposition was conducted in such a manner at a deposition
rate of 7 (nm/min).
[0181] Small angle X-ray scattering method (SAXS) (thin-film
method, incidence angle 0.5 degrees) of the obtained thin-film
showed that the fabricated In--Ga--Zn--O system thin film could be
called amorphous, in view of the fact that a clear diffraction peak
could not be observed.
[0182] It was learned from analysis of the pattern obtained from
X-ray reflectivity measurement that the root-mean square roughness
(Rrms) of the film was approximately 0.5 nm and that film thickness
was about 120 nm. Fluorescent X-ray (XRF) analysis showed that the
metal composition ratio of the thin-film was
In:Ga:Zn=0.98:1.02:4.
[0183] Electrical conductivity was less than about 10.sup.-2 S/cm.
Electron carrier density could be estimated to be approximately
10.sup.16/cm.sup.3 or less, and electron mobility to be 5
cm.sup.2/(Vsec).
[0184] Analysis of the optical absorption spectrum showed that the
optical bandgap energy of the fabricated amorphous thin-film was
about 3 eV. From the above results, it was learned that the
fabricated In--Ga--Zn--O system thin film existed in an amorphous
phase close to a composition of crystalline InGaO.sub.3(ZnO).sub.4,
that there was little oxygen deficiency, and that the thin-film was
a transparent and flat thin-film which had low electrical
conductivity.
[0185] This will now be specifically explained with reference to
FIG. 1. FIG. 1 illustrates the change in electron carrier density
of the deposited oxide if the oxygen partial pressure is varied
when a transparent amorphous oxide thin-film constituted from
In--Ga--Zn--O, in which the composition is expressed as
InGaO.sub.3(ZnO).sub.m (m is a natural number of less than 6), and
which is assumed to have a crystalline state, is fabricated under
the same conditions as the present embodiment.
[0186] Under the same conditions as those of the present
embodiment, electron carrier density was able to be reduced to less
than 1.times.10.sup.18/cm.sup.3 as illustrated in FIG. 1, by
carrying out deposition in an atmosphere having a high oxygen
partial pressure which exceeded 4.5 Pa. In this case, the
temperature of the substrate was intentionally not raised, being
maintained at approximately room temperature. When using a flexible
plastic film as the substrate, it is preferable to maintain the
substrate temperature to below 100.degree. C.
[0187] If the oxygen partial pressure is still further increased,
it is possible to reduce the electron carrier density still
further. For example, as illustrated in FIG. 1, for an
InGaO.sub.3(ZnO).sub.4 thin-film deposited at a substrate
temperature of 25.degree. C. and an oxygen partial pressure of 5
Pa, it was possible to further reduce electron carrier density to
1.times.10.sup.16/cm.sup.3.
[0188] As illustrated in FIG. 2, the obtained thin-film had an
electron mobility of more than 1 cm.sup.2/(Vsec). However, with the
pulsed laser deposition method of the present embodiment, if the
oxygen partial pressure is set to 6.5 Pa or more the surface of the
deposited film is uneven, making it difficult to use as the channel
layer of a TFT.
[0189] Therefore, if a transparent amorphous oxide thin-film in
which the composition in a crystalline state is expressed as
InGaO.sub.3(ZnO).sub.m (m is a natural number of less than 6) is
employed in an atmosphere wherein the oxygen partial pressure is
more than 4.5 Pa, and preferably more than 5 Pa, but less than 6.5
Pa, it is possible to construct a normally-off transistor.
[0190] In addition, the electron mobility of this thin-film was
more than 1 cm.sup.2/Vsec, and the on/off ratio could be increased
to more than 10.sup.3.
[0191] Thus, as explained above, when carrying out deposition of an
InGaZn oxide by PLD under the conditions shown in the present
embodiment, it is preferable to control the oxygen partial pressure
to be from 4.5 Pa or more, but less than 6.5 Pa.
[0192] Further, the realization of an electron carrier density of
less than 1.times.10.sup.18/cm.sup.3 is dependent on factors such
as the oxygen partial pressure conditions, the structure of the
deposition apparatus and the materials and composition which are
deposited.
[0193] Next, an amorphous oxide was produced in the above-described
apparatus under conditions of an oxygen partial pressure of 6.5 Pa,
and the top-gate type MISFET device illustrated in FIG. 5 was
fabricated. Specifically, first, a 120 nm thick semi-insulating
amorphous InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer
2 was formed on a glass substrate 1 by the above-described
amorphous In--Ga--Zn--O thin-film fabricating method.
[0194] Next, the oxygen partial pressure in the chamber was set to
less than 1 Pa, and high-electrical conductivity
InGaO.sub.3(ZnO).sub.4 and gold film were each laminated on top of
this layer to a 30 nm thickness by pulsed laser deposition. A drain
terminal 5 and source terminal 6 were formed by photolithography
and a lift-off technique. Finally, a Y.sub.2O.sub.3 film was
deposited as a gate insulating film 3 by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
Gold was deposited on top of this film, and a gate terminal 4 was
formed by photolithography and a lift-off technique.
MISFET Device Characteristics Evaluation
[0195] FIG. 6 illustrates the current-voltage characteristics of
the MISFET device measured at room temperature. It can be seen that
the channel is an n-type semiconductor from the fact that the drain
current I.sub.DS increases in conjunction with an increase in the
drain voltage V.sub.DS. This does not contradict the fact that an
amorphous In--Ga--Zn--O system semiconductor is an n-type. This
shows the behavior of a typical transistor wherein the I.sub.DS is
V.sub.DS=about 6 V and is saturated (pinched-off). A check of the
gain characteristics showed that the threshold of the gate voltage
V.sub.GS when V.sub.DS=4 V applied was approximately -0.5 V.
Further, when V.sub.G=10 V, current of I.sub.DS=1.0.times.10.sup.-5
A flowed. This matches with the fact that carriers were able to be
induced in the In--Ga--Zn--O system amorphous semiconductor
thin-film of the insulating body from the gate bias.
[0196] The transistor on/off ratio was more than 10.sup.3.
Calculation of the field effect mobility from the output
characteristics showed that a field effect mobility of about 7
cm.sup.2 (Vs).sup.-1 was obtained in the saturated region. Although
the same measurements were performed by irradiating visible light
on the fabricated device, no change in the transistor
characteristics could be confirmed.
[0197] According to the present embodiment, electron carrier
density is small, so that a thin-film transistor can be realized
having a high electric resistance and a channel layer in which
electron mobility is large.
[0198] The above-described amorphous oxide comprises the excellent
characteristics of electron mobility increasing in conjunction with
an increase in electron carrier density, and expression of
degenerating conduction.
[0199] Although in the present embodiment a thin-film transistor
was formed on a glass substrate, since the deposition itself can be
conducted at room temperature, a plastic sheet, film or similar
substrate can also be used.
[0200] The amorphous oxide obtained in the present embodiment
showed hardly any optical absorption of visible light, whereby a
transparent flexible TFT can be realized.
(Second Deposition Method: Sputtering (SP method))
[0201] Deposition by a high-frequency SP method employing argon gas
as the atmospheric gas will be now explained.
[0202] The SP method was carried out using the apparatus
illustrated in FIG. 9. In FIG. 9, reference numeral 807 denotes a
substrate to undergo deposition, 808 denotes a target, 805 denotes
substrate support means equipped with a cooling mechanism, 814
denotes a turbo molecular pump, 815 denotes a rotary pump, 817
denotes a shutter, 818 denotes an ion gauge, 819 denotes a Pirani
gauge, 821 denotes a growth chamber (chamber) and 830 denotes a
gate valve.
[0203] As the substrate 807 to undergo deposition, a SiO.sub.2
glass substrate (1737, manufactured by Corning Incorporated) was
prepared. As a pre-deposition treatment, degreasing cleaning of the
substrate by ultrasound was conducted using acetone, ethanol and
pure water (each for 5 minutes), and then drying in air at
100.degree. C.
[0204] For the target material, polycrystalline sintered body
target (size 20 mm.phi. 5 mmt) comprising an InGaO.sub.3(ZnO).sub.4
was used.
[0205] This sintered body was produced by subjecting
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO (each a 4N reagent) as the
source material to wet blending (solvent: ethanol), and then
calcining (1,000.degree. C. for 2 hours), dry grinding and
sintering (1,550.degree. C. for 2 hours). The electrical
conductivity of this target 808 was 90 (S/cm), which was a
semiconducting state.
[0206] The degree of vacuum of the growth chamber 821 was set to
1.times.10.sup.-4 (Pa), and the total pressure of the oxygen gas
and argon gas during deposition was set to a fixed value in the
range of from 4 to 1.times.10.sup.-1 (Pa). The oxygen partial
pressure was varied in the range of 10.sup.-3 to 2.times.10.sup.-1
(Pa) by varying the partial pressures of the argon gas and the
oxygen.
[0207] The substrate temperature was set to room temperature, and
the distance between the target 808 and the substrate 807 which was
to undergo deposition was 30 (mm).
[0208] The injected power was RF 180 W, and the deposition rate was
10 (nm/min).
[0209] Small angle X-ray scattering method (SAXS) (thin-film
method, incidence angle 0.5 degrees) of the obtained thin-film
showed that the fabricated In--Ga--Zn--O system thin film was an
amorphous film, in view of the fact that a clear diffraction peak
could not be observed.
[0210] It was further learned from analysis of the pattern obtained
from X-ray reflectivity measurement that the root-mean square
roughness (Rrms) of the film was approximately 0.5 nm and that film
thickness was about 120 nm. Fluorescent X-ray (XRF) analysis showed
that the metal composition ratio of the thin-film was
In:Ga:Zn=0.98:1.02:4.
[0211] While varying the oxygen partial pressure of the atmosphere
during deposition, the electrical conductivity of the obtained
amorphous oxide film was measured. The results are shown in FIG.
3.
[0212] As shown in FIG. 3, by carrying out deposition in an
atmosphere having a high oxygen partial pressure of more than
3.times.10.sup.-2 Pa, the electrical conductivity was able to be
reduced to less than 10 S/cm.
[0213] By increasing the oxygen partial pressure still further, it
is possible to reduce the electron carrier density even more.
[0214] For example, as illustrated in FIG. 3, for an
InGaO.sub.3(ZnO).sub.4 thin-film deposited at a substrate
temperature of 25.degree. C. and an oxygen partial pressure of
10.sup.-1 Pa, it was possible to further reduce electric
conductivity to about 10.sup.-10 S/cm. On the other hand, the
electrical conductivity of an InGaO.sub.3(ZnO).sub.4 thin-film
deposited at an oxygen partial pressure of more than 10.sup.-1 Pa
could not be measured as the electric resistance was too high. In a
case such as this, although it was impossible to measure electron
mobility, the electron mobility was estimated to be about 1
cm.sup.2/Vsec by extrapolating from the value from a film having a
large electron carrier density.
[0215] That is, using a transparent amorphous oxide thin-film
constituted from In--Ga--Zn--O produced using a sputtering
deposition method, in which the composition in a crystalline state
is expressed as InGaO.sub.3(ZnO).sub.m (m is a natural number of
less than 6), a normally-off transistor having an on/off ratio of
more than 10.sup.3 could be fabricated in an argon gas atmosphere
having an oxygen partial pressure exceeding 3.times.10.sup.-2 Pa,
and preferably exceeding 5.times.10.sup.-1 Pa.
[0216] When the apparatus and materials illustrated in the present
embodiment are used, the oxygen partial pressure during deposition
by sputtering is, for example, in the range of 3.times.10.sup.-2 Pa
or more and 5.times.10.sup.-1 Pa or less. As illustrated in FIG. 2,
a thin-film fabricated by pulsed laser deposition or sputtering has
an electron mobility which increases in conjunction with an
increase in the number of electrons being conducted.
[0217] As explained above, controlling the oxygen partial pressure
allows oxygen deficiency to be reduced, thereby enabling electron
carrier density to be reduced. Unlike in a polycrystalline state,
since particle interfaces inherently do not exist in an amorphous
state, an amorphous thin-film having a high electron mobility can
be obtained.
[0218] It is noted that even when a 200 .mu.m thick polyethylene
terephthalate (PET) film was used in place of a glass substrate,
the obtained InGaO.sub.3(ZnO).sub.4 amorphous oxide film showed the
same characteristics.
[0219] If polycrystalline InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.m (m
is a natural number of less than 6; 0<x.ltoreq.1) is used, a
high-resistance InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.m amorphous
film can be obtained even if the oxygen partial pressure is less
than 1 Pa.
[0220] For example, when a target is used with 80 atom % of its Zn
substituted with Mg, the electron carrier density of a film
obtained by pulsed laser deposition can be below made to be
1.times.10.sup.16/cm.sup.3 (electric resistance value is about
10.sup.-2 S/cm).
[0221] Although the electron mobility of such a film is lower than
that of a Mg-free film, the difference is not large, wherein the
room temperature electron mobility is about 5 cm.sup.2/(Vsec).
Compared with amorphous silicon, this is a value larger by about
one order. When deposition is conducted under the same conditions,
electrical conductivity and electron mobility both decrease in
relation to the increase in Mg content. Thus, Mg content is
preferably more than 20%, and is less than 85% (taking the content
as x, 0.2<x<0.85).
[0222] As described above, by controlling oxygen partial pressure,
oxygen deficiency can be reduced, whereby electron carrier density
can be reduced without adding a specific impurity ion. Further,
unlike a polycrystalline state, particle interfaces do not
inherently exist in an amorphous state, which allows for an
amorphous thin-film having a high electron mobility to be obtained.
In addition, since the number of electrons being conducted can be
reduced without the addition of a specific impurity ion, there is
no scattering due to impurities, whereby electron mobility can be
maintained at a high level.
[0223] For a thin-film transistor which employs the above-described
amorphous oxide, Al.sub.2O.sub.3, Y.sub.2O.sub.3, or HfO.sub.2, or
a mixed compound consisting of at least two thereof, is preferably
used as the gate insulating film.
[0224] If a defect is present in the interface between the gate
insulating thin-film and the channel layer thin-film, a reduction
in electron mobility and hysteresis in the transistor
characteristics occurs. Further, leak current significantly differs
depending on the type of gate insulating film. For these reasons,
it is necessary to select a gate insulating thin-film which is
suitable for the channel layer. If an Al.sub.2O.sub.3 film is used,
leak current can be lowered. If a Y.sub.2O.sub.3 film is used,
hysteresis can be reduced. If a high dielectric constant HfO.sub.2
film is used, electron mobility can be increased. If a mixed
crystal of these films is used, a TFT can be formed in which leak
current and hysteresis is small, while electron mobility is large.
Further, since the gate insulating thin-film forming process and
the channel layer forming process can be carried out at room
temperature, a TFT structure can be formed in either a staggered
structure or an inverse staggered structure.
[0225] A TFT formed in this manner is a three-terminal device
comprising a gate terminal, a source terminal and a drain terminal,
which uses a semiconductor thin-film deposited on an insulating
substrate, such as ceramic, glass, plastic or the like, as a
channel layer through which electrons or holes move, wherein by
applying a voltage to the gate terminal, the current flowing in the
channel layer is controlled. A TFT formed in this manner is thus an
active device which has the function of switching the current
between the source terminal and drain terminal.
[0226] It is important in the present invention to control the
oxygen deficiency amount in order to achieve a prescribed electron
carrier density.
[0227] Although in the above-described structure the control of the
oxygen amount (oxygen deficiency amount) of the amorphous oxide is
achieved by carrying out deposition in an atmosphere comprising a
prescribed density of oxygen, other preferable methods include
controlling (decreasing or increasing) the oxygen deficiency amount
by subjecting the oxide film to a post-treatment in an atmosphere
which contains oxygen.
[0228] To effectively control the oxygen deficiency amount, the
temperature of the oxygen-containing atmosphere is set to 0.degree.
C. or more to 300.degree. C. or less, preferably from 25.degree. C.
or more to 250.degree. C. or less, and more preferably from
100.degree. C. or more to 200.degree. C. or less.
[0229] Obviously, it is acceptable to carry out deposition in an
oxygen-containing atmosphere, and then also carry out the
post-deposition post-treatment in an atmosphere which contains
oxygen. If the prescribed electron carrier density
(1.times.10.sup.18/cm.sup.3) can be attained, it is also acceptable
to not control the oxygen partial pressure during deposition, and
carry out a post-deposition post-treatment in an atmosphere which
contains oxygen.
[0230] The lower limit of the electron carrier density in the
present invention is, for example, 1.times.10.sup.14/cm.sup.3,
although this depends on what kind of device, circuit or apparatus
the obtained oxide film is to be used for.
(Expansion of the Material System)
[0231] As a result of progress in research by broadening the
material system, it was discovered that an amorphous oxide
comprising an oxide of at least one element selected from the group
consisting of Zn, In and Sn can be used to fabricate an amorphous
oxide film having low electron carrier density and high electron
mobility.
[0232] It was further discovered that such an amorphous oxide film
has the unique characteristic that electron mobility increases in
conjunction with an increase in the number of electrons being
conducted.
[0233] A normally-off type TFT having excellent transistor
characteristics, such as on/off ratio, saturation current in a
pinched-off state and switching speed, can be fabricated by
fabricating a TFT which employs such film.
[0234] A complex oxide can be constituted comprising the below
elements in the amorphous oxide which comprises at least one
element selected from among the above-described Zn, In and Sn.
[0235] Such elements include at least one element selected from the
group consisting of: group 2 elements M2 (M2 denoting Mg and Ca)
which have an atomic number below that of zinc; group 3 elements M3
(M3 denoting B, Al, Ga and Y) which have an atomic number below
that of indium; group 4 elements M4 (M4 denoting. Si, Ge and Zr)
which have an atomic number below that of tin; group 5 elements M5
(M5 denoting V, Nb and Ta); and Lu and W.
[0236] In the present invention, an oxide can be employed which has
the below characteristics (a) through (h). [0237] (a) An amorphous
oxide having an electron carrier density at room temperature of
less than 1.times.10.sup.18/cm.sup.3. [0238] (b) An amorphous oxide
wherein electron mobility increases in conjunction with an increase
in the number of electrons being conducted.
[0239] Here, room temperature refers to a temperature of from about
0.degree. C. to 40.degree. C. The term "amorphous oxide" refers to
a compound in which only a halo pattern (no specific diffraction
lines shown) can be observed in its X-ray diffraction spectrum.
Furthermore, "electron mobility" as used here refers to the
electron mobility obtained from Holl effect measurement. [0240] (c)
The amorphous oxide described in the above (a) or (b), wherein
electron mobility at room temperature exceeds 0.1 cm.sup.2/(Vsec).
[0241] (d) The amorphous oxide described in the above (b) to (c)
which expresses degenerating conduction. Here, the term
"degenerating conduction" refers to the condition where thermal
activation energy in the temperature dependency of electric
resistance is 30 meV or less. [0242] (e) The amorphous oxide
described in the above (a) to (d), which comprises at least one
element selected from the group consisting of Zn, In and Sn as a
structural component. [0243] (f) An amorphous oxide film, wherein
the amorphous oxide described in the above (e) comprises at least
one element selected from the group consisting of: group 2 elements
M2 (M2 denoting Mg and Ca) which have an atomic number below that
of zinc; group 3 (group 13) elements M3 (M3 denoting B, Al, Ga and
Y) which have an atomic number below that of indium; group 4
elements M4 (M4 denoting Si, Ge and Zr) which have an atomic number
below that of tin; group 5 elements M5 (M5 denoting V, Nb and Ta);
and Lu and W. (g) The amorphous oxide film described in any of the
above (a) to (f), wherein the composition of its crystalline state
is the simple compound In.sub.10-xM3.sub.xO.sub.3
(Zn.sub.1-yM2.sub.yO).sub.m (0.ltoreq.x, y.ltoreq.1; m is zero or a
natural number of less than 6), or a mixture of compounds in which
m is different. M3 is, for example, Ga, and M2 is, for example, Mg.
[0244] (h) The amorphous oxide film described in any of the above
(a) to (g) provided on a glass substrate, a metal substrate, a
plastic substrate or a plastic film substrate.
[0245] Further, the present invention is a field effect transistor
which employs the amorphous oxide or amorphous oxide film described
in the above (10) for the channel layer.
[0246] Constituted is a field effect transistor which employs an
amorphous oxide film having an electron carrier density of less
than 1.times.10.sup.18/cm.sup.3 but more than
1.times.10.sup.15/cm.sup.3 for the channel layer, and which is
provided with a gate terminal via a source terminal, a drain
terminal and a gate insulating film. When about 5 V is applied
between the source and drain terminals, the current between the
source and drain terminals when no gate voltage is applied can be
made to be 10.sup.-7 amperes.
[0247] The electron mobility of an oxide crystal increases as the
overlap of the metal ion s orbitals increases, so that an oxide
crystal of Zn, In or Sn, which have a high atomic number, has a
large electron mobility of from 0.1 to 200 cm.sup.2/(Vsec).
[0248] Further, in oxides the oxygen and the metal ion are
ionically bonded.
[0249] For that reason, the chemical bond has no orientation,
whereby the structure is random. Thus, even for an amorphous state,
in which bond orientation is nonuniform, it is possible for
electron mobility to be about the same magnitude as the electron
mobility of a crystalline state.
[0250] On the other hand, by substituting the Zn, In or Sn atom
with an element having a lower atomic number, electron mobility is
reduced, whereby the electron mobility of the amorphous oxide
according to the present invention is about 0.01 cm.sup.2/(Vsec) to
20 cm.sup.2/(Vsec).
[0251] When fabricating the channel layer of a transistor using the
above-described oxide, it is preferable that Al.sub.2O.sub.3,
Y.sub.2O.sub.3, or HfO.sub.2, or a mixed compound consisting of at
least two thereof, serves as the gate insulating film.
[0252] If a defect is present in the interface between the gate
insulating thin-film and the channel layer thin-film, a reduction
in electron mobility and hysteresis in the transistor
characteristics occurs. Further, leak current significantly differs
depending on the type of gate insulating film. For these reasons,
it is necessary to select a gate insulating thin-film which is
suitable for the channel layer. If an Al.sub.2O.sub.3 film is used,
leak current can be lowered. If a Y.sub.2O.sub.3 film is used,
hysteresis can be reduced. If a high dielectric constant HfO.sub.2
film is used, field effect mobility can be increased. If a mixed
crystal of these films is used, a TFT can be formed in which leak
current and hysteresis is small, while field effect mobility is
large. Further, since the gate insulating thin-film forming process
and the channel layer forming process can be carried out at room
temperature, a TFT structure can be formed in either a staggered
structure or an inverse staggered structure.
[0253] The In.sub.2O.sub.3 oxide film can be deposited by a
vapor-phase method, wherein an amorphous film can be obtained by
charging the atmosphere during deposition with about 0.1 Pa of
moisture.
[0254] While for ZnO and SnO.sub.2 it is difficult to obtain an
amorphous film, an amorphous film can be obtained by adding about
20% by atomic weight of In.sub.2O.sub.3 in the case of ZnO, and
about 90% by atomic weight of In.sub.2O.sub.3 in the case of
SnO.sub.2. In particular, to obtain a Sn--In--O system amorphous
film, it is preferable to charge about 0.1 Pa of nitrogen gas into
the atmosphere.
[0255] The above-described amorphous film can be dosed with an
element constituting at least one complex oxide selected from the
group consisting of: group 2 elements M2 (M2 denoting Mg and Ca)
which have an atomic number below that of zinc; group 3 elements M3
(M3 denoting B, Al, Ga and Y) which have an atomic number below
that of indium; group 4 elements M4 (M4 denoting Si, Ge and Zr)
which have an atomic number below that of tin; group 5 elements M5
(M5 denoting V, Nb and Ta); and Lu and W.
[0256] Dosing with such elements allows for better stabilization of
the amorphous film at room temperature, and enables a broader range
of compositions which the amorphous film can be obtained.
[0257] In particular, dosing with the strongly covalent B, Si or Ge
is effective in stabilizing the amorphous phase. Complex oxides
which are constituted from ions having a large difference in ionic
radius have a stabilized amorphous phase.
[0258] For example, in an In--Zn--O system, although it is hard to
obtain an amorphous film if indium is not present in excess of
about 20 atom % of the composition, a stable amorphous film can be
obtained by dosing with magnesium in an amount equivalent to that
of the indium, with the indium making up about 15 atom %.
[0259] In deposition by a vapor-phase method, an amorphous oxide
film can be obtained in which the electron carrier density is less
than 1.times.10.sup.18/cm.sup.3 and more than
1.times.10.sup.15/cm.sup.3 by controlling the atmosphere.
[0260] It is preferable to use a vapor-phase method, such as pulsed
laser deposition (PLD), sputtering (SP) and electron beam
deposition, as the deposition method for the amorphous oxide. Among
vapor-phase methods, PLD is suitable from the viewpoint of easy
control of the materials system composition, while sputtering is
suitable from a mass-production viewpoint. However, the deposition
method is not limited to these methods.
(Deposition of an In--Zn--Ga--O Amorphous Oxide Film by PLD)
[0261] An In--Zn--Ga--O system amorphous oxide semiconductor film
was deposited by PLD employing a KrF excimer laser onto glass
substrates (1737, manufactured by Corning Incorporated) with
polycrystalline sintered bodies having an InGaO.sub.3(ZnO) and an
InGaO.sub.3(ZnO).sub.4 composition serving as the respective
targets.
[0262] The apparatus illustrated in FIG. 9 was used as the
deposition apparatus. The deposition conditions were the same as
when the apparatus was used.
[0263] The substrate temperature was 25.degree. C. Small angle
X-ray scattering method (SAXS) (thin-film method, incidence angle
0.5 degrees) of the obtained films showed that the In--Zn--Ga--O
system films fabricated from the two kinds of target were amorphous
films, since clear diffraction peaks could not be observed.
[0264] It was learned from analysis of the patterns obtained from
X-ray reflectivity measurement that the root-mean square roughness
(Rrms) of the In--Ga--Zn--O system amorphous oxide films on the
substrate was approximately 0.5 nm and that film thickness was
about 120 nm.
[0265] Fluorescent X-ray (XRF) analysis showed that the metal
composition ratio of the film obtained using a polycrystalline
sintered body having an InGaO.sub.3(ZnO) composition as the target
was In:Ga:Zn=1.1:1.1:0.9, while the metal composition ratio of the
film obtained using a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition as the target was
In:Ga:Zn=0.98:1.02:4.
[0266] Varying the partial pressure of oxygen, the electron carrier
density of the amorphous oxide film obtained using a
polycrystalline sintered body having an InGaO.sub.3(ZnO).sub.4
composition as the target was measured. The results are shown in
FIG. 1. Depositing in an atmosphere having an oxygen partial
pressure in excess of 4.2 Pa allowed the electron carrier density
to be reduced to less than 1.times.10.sup.18/cm.sup.3. In this
case, the substrate temperature was maintained at roughly room
temperature by intentionally not heating. When the oxygen partial
pressure was less than 6.5 Pa, the surface of the obtained
amorphous oxide film was flat.
[0267] When the oxygen partial pressure was 5 Pa, the electron
carrier density of the amorphous oxide film obtained using a
polycrystalline sintered body having an InGaO.sub.3(ZnO).sub.4
composition as a target was to 10.sup.16/cm.sup.3, and electrical
conductivity was to 10.sup.-2 S/cm. Electron mobility was estimated
to be about 5 cm.sup.2/Vsec. Analysis of the optical absorption
spectrum showed that the optical bandgap energy of the fabricated
amorphous oxide film was about 3 eV.
[0268] Further increasing the oxygen partial pressure enabled the
electron carrier density to be further reduced. As shown in FIG. 1,
in an In--Zn--Ga--O system amorphous oxide film deposited at
substrate temperature of 25.degree. C. and a oxygen partial
pressure of 6 Pa, the electron carrier density could be reduced to
8.times.10.sup.15/cm.sup.3 (electrical conductivity of about
8.times.10.sup.-3 S/cm). Electron mobility was estimated to be
about 1 cm.sup.2/Vsecond. However, using PLD, if the oxygen partial
pressure is set at 6.5 Pa or more, the surface of the deposited
film becomes uneven, whereby it is difficult to use as the TFT
channel layer.
[0269] The electron carrier density and electron mobility were
examined for In--Zn--Ga--O system amorphous oxide films deposited
at different oxygen partial pressures using a polycrystalline
sintered body having an InGaO.sub.3(ZnO).sub.4 composition as the
target. The results are shown in FIG. 2. If the electron carrier
density is increased from 1.times.10.sup.16/cm.sup.3 to
1.times.10.sup.20/cm.sup.3, electron mobility showed an increase
from about 3 cm.sup.2/Vsec to about 11 cm.sup.2/Vsec. In addition,
amorphous oxide films obtained using a polycrystalline sintered
body having an InGaO.sub.3(ZnO) composition as the target also
showed the same trend.
[0270] Even when a 200 .mu.m polyethylene terephthalate (PET) film
was used in place of the glass substrate, the obtained
In--Zn--Ga--O system amorphous oxide film showed the same
characteristics.
[0271] (Deposition of an In--Zn--Ga--Mg--O Amorphous Oxide Film by
PLD)
[0272] Using polycrystalline InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.4
(0<x.ltoreq.1) as a target, an
InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.4 (0<x.ltoreq.1) film was
deposited by PLD onto a glass substrate.
[0273] The apparatus illustrated in FIG. 8 was used as the
deposition apparatus.
[0274] A SiO.sub.2 glass substrate (1737, manufactured by Corning
Incorporated) was prepared as the substrate to undergo deposition.
As a pre-deposition treatment on the substrate, degreasing cleaning
by ultrasound was conducted using acetone, ethanol and pure water
(each for 5 minutes), and then the substrate was dried in air at
100.degree. C. As a target, InGa(Zn.sub.1-xMg.sub.xO).sub.4 (x=1-0)
sintered body (size: 20 mm.phi. 5 mmt) was used.
[0275] The target was fabricated by subjecting
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO:MgO (each a 4N reagent) as the
source material to wet blending (solvent: ethanol), and then
calcining (1,000.degree. C. for 2 hours), dry grinding and
sintering (1,550.degree. C. for 2 hours).
[0276] The growth chamber degree of vacuum was set to
2.times.10.sup.-6 (Pa), and the oxygen partial pressure during
growth was set to 0.8 (Pa). Deposition was carried out at a
substrate temperature of 25.degree. C. The distance between the
target and the substrate which was to undergo deposition was 30
(mm).
[0277] The power of the KrF excimer was 1.5 (mJ/cm.sup.2/pulse).
Pulse width was 20 (nsec), and repetition frequency was 10 (Hz).
Irradiation spot diameter was set at 1.times.1 (mm angle).
[0278] The deposition rate was 7 (nm/min).
[0279] The atmosphere had an oxygen partial pressure of 0.8 Pa, and
substrate temperature was 25.degree. C. Small angle X-ray
scattering method (SAXS) (thin-film method, incidence angle 0.5
degrees) of the obtained film showed that the fabricated
In--Zn--Ga--Mg--O system film was an amorphous film, since a clear
diffraction peak could not be observed. The surface of the obtained
film was flat.
[0280] The x value dependency of electrical conductivity, electron
carrier density and electron mobility were examined for
In--Zn--Ga--Mg--O system amorphous oxide films deposited at an
oxygen partial pressure of 0.8 Pa using different x value
targets.
[0281] The results are shown in FIG. 4. It is shown that, when the
x value exceeded 0.4, an electron carrier density of less than
1.times.10.sup.18/cm.sup.3 was possible at an atmosphere having an
oxygen partial pressure of 0.8 Pa. Further, for an amorphous oxide
film having an x value of more than 0.4, the electron mobility was
more than 1 cm.sup.2/Vsec.
[0282] As illustrated in FIG. 4, when a target is used in which the
Zn is substituted with 80 atom % of Mg, and at an atmosphere having
an oxygen partial pressure of 0.8 Pa, the electron carrier density
of a film deposited by pulsed laser deposition can be made to be
less than 1.times.10.sup.16/cm.sup.3 (electrical resistance of
about 10.sup.-2 S/cm). Although the electron mobility of such a
film is lower than that of a Mg-free film, the difference is not
large, wherein the room temperature electron mobility is about 5
cm.sup.2/(Vsec). Compared with amorphous silicon, this is a value
larger by about one order. When deposition is conducted under the
same conditions, electrical conductivity and electron mobility both
decrease in relation to the increase in Mg content. Thus, Mg
content is preferably more than 20 atom %, and is less than 85 atom
% (taking the content as x, 0.2<x<0.85). Even more preferable
is 0.5<x<0.85.
[0283] Even when a 200 .mu.m polyethylene terephthalate (PET) film
was used in place of the glass substrate, the obtained
InGaO.sub.3(Zn.sub.1-xMg.sub.xO).sub.4 (0<x.ltoreq.1) amorphous
oxide film showed the same characteristics.
(Deposition of an In.sub.2O.sub.3 Amorphous Oxide Film by PLD)
[0284] An In.sub.2O.sub.3 film was deposited by PLD using a KrF
excimer laser onto a 200 .mu.m PET film with an In.sub.2O.sub.3
polycrystalline sintered body serving as a target.
[0285] The apparatus illustrated in FIG. 8 was used as the
apparatus. A SiO.sub.2 glass substrate (1737, manufactured by
Corning Incorporated) was prepared as the substrate to undergo
deposition.
[0286] As a substrate pre-deposition treatment, degreasing cleaning
by ultrasound was conducted using acetone, ethanol and pure water
(each for 5 minutes), and then drying in air at 100.degree. C.
[0287] As a target, an In.sub.2O.sub.3 sintered body (size: 20
mm.phi. 5 mmt) was used. The target was prepared by subjecting an
In.sub.2O.sub.3 (4N reagent) source material to calcining
(1,000.degree. C. for 2 hours), dry grinding and sintering
(1,550.degree. C. for 2 hours).
[0288] The growth chamber degree of vacuum was set to
2.times.10.sup.-6 (Pa), the oxygen partial pressure during growth
was set to 5 (Pa), and the substrate temperature was set to room
temperature.
[0289] The oxygen partial pressure was set to 5 Pa and the water
vapor partial pressure to 0.1 Pa. Oxygen radicals were generated by
applying 200 W to an oxygen radical generator.
[0290] The distance between the target and the substrate which was
to undergo deposition was 40 (mm). The power of the KrF excimer was
0.5 (mJ/cm.sup.2/pulse). Pulse width was 20 (nsec), and repetition
frequency was 10 (Hz). Irradiation spot diameter was set at
1.times.1 (mm angle).
[0291] The deposition rate was 3 (nm/min).
[0292] Small angle X-ray scattering method (SAXS) (thin-film
method, incidence angle 0.5 degrees) of the obtained film showed
that the fabricated In--O system film was an amorphous film, since
a clear diffraction peak could not be observed. Film thickness was
80 nm.
[0293] The obtained In--O system amorphous oxide film had an
electron carrier density of 5.times.10.sup.17/cm.sup.3 and an
electron mobility of about 7 cm.sup.2/(Vsec).
(Deposition of an In--Sn--O System Amorphous Oxide Film by PLD)
[0294] An In--Sn--O system oxide film was deposited by PLD using a
KrF excimer laser onto a 200 .mu.m PET film with an
(In.sub.0.9Sn.sub.0.1)O.sub.3.1 polycrystalline sintered body
serving as a target.
[0295] Specifically, a SiO.sub.2 glass substrate (1737,
manufactured by Corning Incorporated) was prepared as the substrate
to undergo deposition.
[0296] As a substrate pre-deposition treatment, degreasing cleaning
by ultrasound was conducted using acetone, ethanol and pure water
(each for 5 minutes), and then drying in air at 100.degree. C.
[0297] As a target, an In.sub.2O.sub.3--SnO.sub.2 sintered body
(size: 20 mm.phi. 5 mmt) was prepared. The target was prepared by
subjecting an In.sub.2O.sub.3--SnO.sub.2 (4N reagent) source
material to material to wet blending (solvent: ethanol), calcining
(1,000.degree. C. for 2 hours), dry grinding and sintering
(1,550.degree. C. for 2 hours).
[0298] The substrate temperature was at room temperature. The
oxygen partial pressure was set to 5 (Pa) and the nitrogen partial
pressure was set to 0.1 (Pa). Oxygen radicals were generated by
applying 200 W to an oxygen radical generator.
[0299] The distance between the target and the substrate which was
to undergo deposition was 30 (mm). The power of the KrF excimer was
1.5 (mJ/cm.sup.2/pulse). Pulse width was 20 (nsec), and repetition
frequency was 10 (Hz). Irradiation spot diameter was set at
1.times.1 (mm angle).
[0300] The deposition rate was 6 (nm/min).
[0301] Small angle X-ray scattering method (SAXS) (thin-film
method, incidence angle 0.5 degrees) of the obtained film showed
that the fabricated In--Sn--O system film was an amorphous film,
since a clear diffraction peak could not be observed.
[0302] The obtained In--Sn--O system amorphous oxide film had an
electron carrier density of 8.times.10.sup.17/cm.sup.3 and an
electron mobility of about 5 cm.sup.2/(Vsec). Film thickness was
100 nm.
(Deposition of an In--Ga--O Amorphous Oxide Film by PLD)
[0303] A SiO.sub.2 glass substrate (1737, manufactured by Corning
Incorporated) was prepared as the substrate to undergo
deposition.
[0304] As a pre-deposition treatment of the substrate, degreasing
cleaning by ultrasound was conducted using acetone, ethanol and
pure water (each for 5 minutes), and then drying in air at
100.degree. C.
[0305] As a target, an
(In.sub.2O.sub.3).sub.1-x--(Ga.sub.2O.sub.3).sub.x (X=0-1) sintered
body (size: 20 mm.phi. 5 mmt) was used When, x=0.1, for example,
the target would be an (In.sub.0.9Ga.sub.0.1).sub.2O.sub.3
polycrystalline sintered body.
[0306] The target was prepared by subjecting an
In.sub.2O.sub.3--Ga.sub.2O.sub.2 (4N reagent) source material to
wet blending (solvent: ethanol), calcining (1,000.degree. C. for 2
hours), dry grinding and sintering (1,550.degree. C. for 2 hours).
The growth chamber degree of vacuum was set to 2.times.10.sup.-6
(Pa), and the oxygen partial pressure during growth to 1 (Pa).
Deposition was carried out with a substrate temperature at room
temperature. The distance between the target and the substrate
which was to undergo deposition was 30 (mm). The power of the KrF
excimer was 1.5 (mJ/cm.sup.2/pulse). Pulse width was 20 (nsec), and
repetition frequency was 10 (Hz). Irradiation spot diameter was set
at 1.times.1 (mm angle). The deposition rate was 6 (nm/min).
[0307] Substrate temperature was 25.degree. C. and the oxygen
partial pressure was 1 Pa. Small angle X-ray scattering method
(SAXS) (thin-film method, incidence angle 0.5 degrees) of the
obtained film showed that the fabricated In--Ga--O system film was
an amorphous film, since a clear diffraction peak could not be
observed. Film thickness was 120 nm.
[0308] The obtained In--Ga--O system amorphous oxide film had an
electron carrier density of 8.times.10.sup.16/cm.sup.3 and an
electron mobility of about 1 cm.sup.2/(Vsec).
(Fabrication of a TFT Device Using an In--Zn--Ga--O System
Amorphous Oxide Film (Glass Substrate))
TFT Device Fabrication
[0309] The top-gate type TFT device illustrated in FIG. 5 was
fabricated.
[0310] First, an In--Ga--Zn--O system amorphous oxide film was
fabricated onto a glass substrate 1 using the above-described PLD
apparatus under an oxygen partial pressure of 5 Pa with a
polycrystalline sintered body having an InGaO.sub.3(ZnO).sub.4
composition serving as the target. A 120 nm thick In--Ga--Zn--O
system amorphous oxide film to be used as a channel layer 2 was
formed.
[0311] Next, the oxygen partial pressure in the chamber was set to
be less than 1 Pa, and high-electrical-conductivity In--Ga--Zn--O
system amorphous oxide film and gold film were each laminated on
top of this layer to a 30 nm thickness by PLD. A drain terminal 5
and source terminal 6 were formed by photolithography and a
lift-off technique.
[0312] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 was deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
Gold was deposited on top of this film, and a gate terminal 4 was
formed by photolithography and a lift-off technique. Channel length
was 50 .mu.m and channel width was 200 .mu.m.
TFT Device Characteristics Evaluation
[0313] FIG. 6 illustrates the current-voltage characteristics of a
TFT device measured at room temperature. It can be seen that the
channel is an n-type conductor from the fact that the drain current
I.sub.DS increases in conjunction with an increase in the drain
voltage V.sub.DS.
[0314] This does not contradict the fact that an amorphous
In--Ga--Zn--O system amorphous oxide film is an n-type
semiconductor. This shows the behavior of a typical transistor
wherein the I.sub.DS is V.sub.DS=about 6 V and is saturated
(pinched-off). A check of the gain characteristics showed that the
threshold of the gate voltage V.sub.GS when V.sub.DS=4 V applied
was approximately -0.5 V.
[0315] Further, when V.sub.G=10 V, a current of
I.sub.DS=1.0.times.10.sup.-5 A flowed. This matches with the fact
that carriers were able to be induced in the In--Ga--Zn--O system
amorphous oxide film of the insulating body from the gate bias.
[0316] The transistor on/off ratio was more than 10.sup.3.
Calculation of the field effect mobility from the output
characteristics showed that a field effect mobility of about 7
cm.sup.2 (Vs).sup.-1 was obtained in the saturated region. Although
the same measurements were performed by irradiating visible light
on the fabricated device, no change in the transistor
characteristics could be confirmed.
[0317] Further, setting the electron carrier density of the
amorphous oxide to be less than 1.times.10.sup.18/cm.sup.3 allows
application as a channel layer. This electron carrier density was
preferably 1.times.10.sup.17/cm.sup.3 or less, and was more
preferably 1.times.10.sup.16/cm.sup.3 or less.
(Fabrication of a TFT Device Using an In--Zn--Ga--O System
Amorphous Oxide Film (Amorphous Substrate))
[0318] The top-gate type TFT device illustrated in FIG. 5 was
fabricated. First, a 120 nm thick In--Zn--Ga--O system amorphous
oxide film to be used as a channel layer 2 was formed onto a
polyethylene terephthalate (PET) substrate 1 by PLD under an oxygen
partial pressure of 5 Pa with a polycrystalline sintered body
having an InGaO.sub.3(ZnO) composition serving as the target.
[0319] Next, the oxygen partial pressure in the chamber was set to
be less than 1 Pa, and high-electrical-conductivity In--Zn--Ga--O
system amorphous oxide film and gold film were each laminated on
top of this layer to a-30 nm thickness by PLD. A drain terminal 5
and source terminal 6 were formed by photolithography and a
lift-off technique. Finally, a Y.sub.2O.sub.3 film to be used as a
gate insulating film 3 was deposited by electron beam deposition.
Gold was deposited on top of this film, and a gate terminal 4 was
formed by photolithography and a lift-off technique. Channel length
was 50 .mu.m and channel width was 200 .mu.m. Three kinds of TFT
were fabricated having the above-described structure employing
Y.sub.2O.sub.3 (thickness: 140 nm), Al.sub.2O.sub.3 (thickness: 130
.mu.m) and HfO.sub.2 (thickness: 140 .mu.m) as the gate insulating
film.
TFT Device Characteristics Evaluation
[0320] The current-voltage characteristics of the TFT devices
formed on a PET film measured at room temperature were the same as
that in FIG. 6. That is, it can be seen that the channels were an
n-type conductor from the fact that the drain current I.sub.DS
increases in conjunction with an increase in the drain voltage
V.sub.DS. This does not contradict the fact that an amorphous
In--Ga--Zn--O system amorphous oxide film is an n-type
semiconductor. This shows the behavior of atypical transistor
wherein the I.sub.DS is V.sub.DS=about 6 V and is saturated
(pinched-off). Further, when V.sub.G=0, a current of
I.sub.ds=10.sup.-8 A flowed, and when V.sub.G=10 V, a current of
I.sub.DS=2.0.times.10.sup.-5 A flowed. This matches with the fact
that carriers were able to be induced in the In--Ga--Zn--O system
amorphous oxide film of the insulating body from the gate bias.
[0321] The transistor on/off ratio was more than 10.sup.3.
Calculation of the field effect mobility from the output
characteristics showed that a field effect mobility of about 7
cm.sup.2(Vs).sup.-1 was obtained in the saturated region.
[0322] Although the same measurements for transistor
characteristics were performed by bending the devices fabricated on
a PET film at a 30 mm radius of curvature, no change in the
transistor characteristics could be confirmed. In addition, the
same measurements were performed by irradiating visible light on
the fabricated devices, although no change in the transistor
characteristics could be confirmed.
[0323] Even the TFT using an Al.sub.2O.sub.3 film as the gate
insulating film showed transistor characteristics similar to those
illustrated in FIG. 6, although when V.sub.G=0, a current of
I.sub.ds=10.sup.-6 A flowed, and when V.sub.G=10 V, a current of
I.sub.DS=5.0.times.10.sup.-6 A flowed. The transistor on/off ratio
was more than 10.sup.2. Calculation of the field effect mobility
from the output characteristics showed that a field effect mobility
of about 2 cm.sup.2(Vs).sup.-1 was obtained in the saturated
region.
[0324] Even the TFT using an HfO.sub.2 film as the gate insulating
film showed transistor characteristics similar to those illustrated
in FIG. 6, although when V.sub.G=0, a current of I.sub.ds=10.sup.-6
A flowed, and when V.sub.G=10 V, a current of
I.sub.DS=1.0.times.10.sup.-6 A flowed. The transistor on/off ratio
was more than 10.sup.2. Calculation of the field effect mobility
from the output characteristics showed that a field effect mobility
of about 10 cm.sup.2(Vs).sup.-1 was obtained in the saturated
region.
(Fabrication of a TFT Device Using an In.sub.2O.sub.3 Amorphous
Oxide Film by PLD)
[0325] The top-gate type TFT device illustrated in FIG. 5 was
fabricated. First, an 80 nm thick In.sub.2O.sub.3 amorphous oxide
film to be used as a channel layer 2 was formed onto a polyethylene
terephthalate (PET) substrate 1 by PLD.
[0326] Next, the oxygen partial pressure in the chamber was set to
be less than 1 Pa, and the applied voltage to the oxygen radical
generator was set to zero. High-electrical-conductivity
In.sub.2O.sub.3 amorphous oxide film and gold film were each
laminated on top of the above layer to a 30 nm thickness by PLD. A
drain terminal 5 and source terminal 6 were formed by
photolithography and a lift-off technique. Finally, a
Y.sub.2O.sub.3 film to be used as a gate insulating film 3 was
deposited by electron beam deposition. Gold was deposited on top of
this film, and a gate terminal 4 was formed by photolithography and
a lift-off technique.
TFT Device Characteristics Evaluation
[0327] The current-voltage characteristics of a TFT device formed
on a PET film were measured at room temperature. It can be seen
that the channel is an n-type semiconductor from the fact that the
drain current I.sub.DS increases in conjunction with an increase in
the drain voltage V.sub.DS. This does not contradict the fact that
an amorphous In--O system amorphous oxide film is an n-type
conductor. This shows the behavior of a typical transistor wherein
the I.sub.DS is V.sub.DS=about 5 V and is saturated (pinched-off).
Further, when V.sub.G=0, a current of I.sub.ds=2.times.10.sup.-8 A
flowed, and when V.sub.G=10 V, a current of
I.sub.DS=2.0.times.10.sup.-6 A flowed. This matches with the fact
that carriers were able to be induced in the In--O system amorphous
oxide film of the insulating body from the gate bias.
[0328] The transistor on/off ratio was about 10.sup.2. Calculation
of the field effect mobility from the output characteristics showed
that a field effect mobility of about 10 cm.sup.2(Vs).sup.-1 was
obtained in the saturated region. A TFT device fabricated on a
glass substrate also showed the same characteristics.
[0329] Although the same measurements of transistor characteristics
were performed by bending the device fabricated on a PET film at a
30 mm radius of curvature, no change in the transistor
characteristics could be confirmed.
(Fabrication of a TFT Device Using an In--Sn--O System Amorphous
Oxide Film by PLD)
[0330] The top-gate type TFT device illustrated in FIG. 5 was
fabricated. First, a 100 nm thick In--Sn--O system amorphous oxide
film to be used as a channel layer 2 was formed onto a polyethylene
terephthalate (PET) substrate 1 by PLD. Next, the oxygen partial
pressure in the chamber was set to be less than 1 Pa, and the
applied voltage to the oxygen radical generator was set to zero.
High-electrical-conductivity In--Sn--O system amorphous oxide film
and gold film were each laminated on top of the above layer to a 30
nm thickness by PLD. A drain terminal 5 and source terminal 6 were
formed by photolithography and a lift-off technique. Finally, a
Y.sub.2O.sub.3 film to be used as a gate insulating film 3 was
deposited by electron beam deposition. Gold was deposited on top of
this film, and a gate terminal 4 was formed by photolithography and
a lift-off technique.
TFT Device Characteristics Evaluation
[0331] The current-voltage characteristics of a TFT device formed
on a PET film were measured at room temperature. It can be seen
that the channel is an n-type semiconductor from the fact that the
drain current I.sub.DS increases in conjunction with an increase in
the drain voltage V.sub.DS. This does not contradict the fact that
In--Sn--O system amorphous oxide film is an n-type conductor. This
shows the behavior of a typical transistor wherein the I.sub.DS is
I.sub.DS=about 6 V and is saturated (pinched-off). Further, when
V.sub.G=0, a current of I.sub.ds=5.times.10.sup.-8 A flowed, and
when V.sub.G=10 V, a current of I.sub.DS=5.0.times.10.sup.-5 A
flowed. This matches with the fact that carriers were able to be
induced in the In--Sn--O system amorphous oxide film of the
insulating body from the gate bias.
[0332] The transistor on/off ratio was about 10.sup.3. Calculation
of the field effect mobility from the output characteristics showed
that a field effect mobility of about 5 cm.sup.2(Vs).sup.-1 was
obtained in the saturated region. A TFT device fabricated on a
glass substrate also showed the same characteristics.
[0333] Although the same measurements of transistor characteristics
were performed by bending the device fabricated on a PET film at a
30 mm radius of curvature, no change in the transistor
characteristics could be confirmed.
(Fabrication of a TFT Device Using an In--Ga--O System Amorphous
Oxide Film by PLD)
[0334] The top-gate type TFT device illustrated in FIG. 5 was
fabricated. First, a 120 nm thick In--Ga--O system amorphous oxide
film to be used as a channel layer 2 was formed onto a polyethylene
terephthalate (PET) substrate 1 using the deposition method
illustrated in Example 6. Next, the oxygen partial pressure in the
chamber was set to be less than 1 Pa, and the applied voltage to
the oxygen radical generator was set to zero.
High-electrical-conductivity In--Ga--O system amorphous oxide film
and gold film were each laminated on top of the above layer to a 30
nm thickness by PLD. A drain terminal 5 and source terminal 6 were
formed by photolithography and a lift-off technique. Finally, a
Y.sub.2O.sub.3 film to be used as a gate insulating film 3 was
deposited by electron beam deposition. Gold was deposited on top of
this film, and a gate terminal 4 was formed by photolithography and
a lift-off technique.
TFT Device Characteristics Evaluation
[0335] The current-voltage characteristics of a TFT device formed
on a PET film were measured at room temperature. It can be seen
that the channel is an n-type semiconductor from the fact that the
drain current I.sub.DS increases in conjunction with an increase in
the drain voltage V.sub.DS. This does not contradict the fact that
In--Ga--O system amorphous oxide film is an n-type conductor. This
shows the behavior of a typical transistor wherein the I.sub.DS is
V.sub.DS=about 6 V and is saturated (pinched-off). Further, when
V.sub.G=0, a current of I.sub.ds=1.times.10.sup.-6 A flowed, and
when V.sub.G=10 V, a current of I.sub.DS=1.0.times.10.sup.-6A
flowed. This matches with the fact that carriers were able to be
induced in the In--Ga--O system amorphous oxide film of the
insulating body from the gate bias.
[0336] The transistor on/off ratio was about 10.sup.2. Calculation
of the field effect mobility from the output characteristics showed
that a field effect mobility of about 0.8 cm.sup.2(Vs).sup.-1 was
obtained in the saturated region. A TFT device fabricated on a
glass substrate also showed the same characteristics.
[0337] Although the same measurements of transistor characteristics
were performed by bending the device fabricated on a PET film at a
30 mm radius of curvature, no change in the transistor
characteristics could be confirmed.
[0338] Further, setting the electron carrier density of the
amorphous oxide to be less than 1.times.10.sup.18/cm.sup.3 allows
application as a channel layer. This electron carrier density is
preferably 1.times.10.sup.17/am.sup.3 or less and more preferably
10.sup.16/cm.sup.3 or less.
[0339] While explanation will now proceed mainly relating to an
In--Ga--Zn--O system oxide compound, the present invention
according to the first to third aspects is not limited to the
examples illustrated below.
[0340] First, examples relating to the first aspect of the present
invention (from deposition pre-treatment to post-treatment) will be
explained.
Example 1-1
[0341] First, a PET substrate is placed in the chamber of a
UV/O.sub.3 surface treatment apparatus, and the substrate surface
is irradiated with ultraviolet rays.
[0342] The chamber that this apparatus has conducts deposition in
an oxygen-containing atmosphere under atmospheric pressure. Ozone
forms in the chamber from the ultraviolet ray irradiation.
Contaminants on the substrate surface are removed by the ozone and
the ultraviolet rays, whereby a clean surface can be obtained.
[0343] On a substrate which had undergone surface treatment using
this method, an In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited by pulsed laser deposition employing a KrF
excimer laser with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0344] The deposition conditions are appropriately set within the
above-mentioned range.
[0345] Next, the top-gate type MISFET device illustrated in FIG. 5
will be fabricated. Specifically, the device is fabricated in the
following manner.
[0346] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0347] Next, high-electrical-conductivity InGaO.sub.3(ZnO).sub.4
and gold film are each laminated on top of this layer to a 30 nm
thickness by pulsed laser deposition, and a drain terminal 5 and
source terminal 6 are formed by photolithography and a lift-off
technique. Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15). Gold is
deposited on top of this film, and a gate terminal 4 is formed by
photolithography and a lift-off technique. From the above steps, a
field effect transistor is obtained.
Example 1-2
[0348] First, a glass substrate (1737, manufactured by Corning
Incorporated) is placed in the chamber of a parallel-plate
atmospheric-pressure plasma apparatus, and low-energy plasma is
irradiated onto the substrate surface.
[0349] This apparatus removes contaminants on the substrate surface
by irradiating low-energy plasma onto the substrate surface,
whereby the state of the substrate top surface can be made to
change.
[0350] On a substrate which had undergone surface treatment using
this method, an In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited by pulsed laser deposition employing a KrF
excimer laser with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0351] It can be confirmed from a peeling test carried out on the
obtained amorphous oxide film that the adhesion between the
substrate and the amorphous oxide film is extremely good.
[0352] An amorphous oxide obtained in this manner can be used to
fabricate a transistor such as that illustrated in Example 1, for
example.
Example 1-3
[0353] First, a glass substrate (1737, manufactured by Corning
Incorporated) is immersed in an aqueous solution consisting of 5%
hydrogen peroxide and 5% ammonia (APM), and subjected to ultrasonic
cleaning for 5 minutes.
[0354] The substrate is removed from the APM, then immersed in pure
water and subjected to ultrasonic cleaning for 5 minutes. After
this, the substrate is immersed in an aqueous solution consisting
of 5% hydrogen peroxide and 5% hydrogen chloride (HPM), and
subjected to ultrasonic cleaning for 5 minutes.
[0355] Aqueous hydrogen fluoride or a mixed aqueous solution of
hydrogen fluoride and hydrogen peroxide can also be used in place
of the HPM. The substrate is removed from the HPM, then immersed in
pure water and subjected to ultrasonic cleaning for 5 minutes. The
substrate is then dried using dry nitrogen.
[0356] Contaminants on the substrate surface are removed by the
above cleaning process, whereby a clean surface can be
obtained.
[0357] Using this method, the above-described In--Ga--Zn--O system
amorphous oxide semiconductor thin-film is deposited on a substrate
which has undergone surface treatment.
[0358] It can be confirmed from a peeling test carried out on the
obtained amorphous oxide film that the adhesion between the
substrate and the amorphous oxide film is extremely good.
Example 1-4
[0359] First, a siloxane-based condensate liquid is thinly coated
onto a PET substrate by spin coating.
[0360] A substrate obtained in this manner is well dried at room
temperature and under low humidity conditions.
[0361] Alternatively, a PET substrate or PET film product which
have undergone a hard-coating treatment may also be used.
[0362] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film, for example, is deposited on a substrate which has
undergone surface treatment using the above-described method. A
transistor can be formed by employing the thin-film obtained in
this manner.
Example 1-5
[0363] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited on a glass substrate (1737, manufactured by
Corning Incorporated) by pulsed laser deposition employing a KrF
excimer laser with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0364] An ozone generating device is installed in the chamber,
wherein deposition is conducted while introducing an
ozone-containing oxygen gas in place of the conventional O.sub.2
gas.
[0365] The oxygen partial pressure in the chamber containing ozone
can be set, for example, to 6 Pa and the substrate temperature, for
example, to 25.degree. C. The thin-film obtained in this manner is
used to fabricate a FET.
[0366] Specifically, the top-gate type MISFET device illustrated in
FIG. 5 will be fabricated.
[0367] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0368] Next, the oxygen partial pressure in the chamber is set to
be less than 1 Pa, and high-electrical-conductivity
InGaO.sub.3(ZnO).sub.4 and gold film are each laminated on top of
this layer to a 30 nm thickness by pulsed laser deposition. A drain
terminal 5 and source terminal 6 are formed by photolithography and
a lift-off technique.
[0369] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
[0370] An ozone generating device is also installed in the electron
beam deposition apparatus, wherein deposition is conducted while
feeding ozone and O.sub.2 gas.
[0371] Gold is deposited on top of this film, and a gate terminal 4
is formed by photolithography and a lift-off technique.
[0372] Thus, insulating properties can be improved by employing
ozone also during gate insulating film formation.
Example 1-6
[0373] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited on a glass substrate (1737, manufactured by
Corning Incorporated) by pulsed laser deposition employing a KrF
excimer laser with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0374] In addition to a conventional O.sub.2 gas line, a N.sub.2O
gas line is installed in the chamber, wherein deposition is
conducted while feeding O.sub.2 gas and N.sub.2O gas into the
chamber in equal flow amounts.
[0375] In place of the N.sub.2O, NO.sub.2 or NO may also be
used.
[0376] The O.sub.2+N.sub.2O pressure in the chamber is set to about
6 Pa and the substrate temperature to 25.degree. C.
[0377] Using this apparatus, the top-gate type MISFET device
illustrated in FIG. 5 will be fabricated by depositing an amorphous
oxide onto a substrate.
[0378] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method. Next, the oxygen
partial pressure in the chamber is set to be less than 1 Pa, and
high-electrical-conductivity InGaO.sub.3(ZnO).sub.4 and gold film
are each laminated on top of the formed layer to a 30 nm thickness
by pulsed laser deposition. A drain terminal 5 and source terminal
6 are formed by photolithography and a lift-off technique.
[0379] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
[0380] A N.sub.2O line is also installed in the electron beam
deposition apparatus, wherein deposition is conducted while feeding
N.sub.2O and O.sub.2 gas. Gold is deposited on top of this film,
and a gate terminal 4 is formed by photolithography and a lift-off
technique.
[0381] Thus, insulating properties can be improved by employing
ozone also during gate insulating film formation.
Example 1-7
[0382] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited by pulsed laser deposition employing a KrF
excimer laser onto a glass substrate (1737, manufactured by Corning
Incorporated) with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0383] A radical generator is installed in the chamber, whereby
oxygen radicals are fed onto the substrate by passing O.sub.2 gas
through the radical generator.
[0384] The oxygen partial pressure in the chamber is set to 6 Pa
and the substrate temperature to 25.degree. C.
[0385] A FET will be fabricated using the above-described
thin-film. The top-gate type MISFET device illustrated in FIG. 5
will be fabricated.
[0386] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0387] Next, the oxygen partial pressure in the chamber is set to
be less than 1 Pa, and high-electrical-conductivity
InGaO.sub.3(ZnO).sub.4 and gold film are each laminated on top of
this layer to a 30 nm thickness by pulsed laser deposition. A drain
terminal 5 and source terminal 6 are formed by photolithography and
a lift-off technique.
[0388] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm). A
radical generator is similarly installed in the electron beam
deposition apparatus as well, whereby deposition is carried out
*while feeding oxygen radicals. Gold is deposited on top of the
resulting film, and a gate terminal 4 is formed by photolithography
and a lift-off technique.
[0389] An FET having extremely good insulating properties for the
gate insulating film is thereby realized.
Example 1-8
[0390] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited by pulsed laser deposition employing a KrF
excimer laser onto a glass substrate (1737, manufactured by Corning
Incorporated) with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0391] An ECR plasma generator is installed in the chamber, whereby
oxygen plasma is fed onto the substrate by passing O.sub.2 gas
through the ECR plasma generator.
[0392] As the plasma generator, an RF plasma generator or a DC
plasma generator is acceptable. The oxygen partial pressure in the
chamber is set to 6 Pa and the substrate temperature to 25.degree.
C.
[0393] Using the thin-film obtained in this manner, the top-gate
type MISFET device illustrated in FIG. 5 will be fabricated.
[0394] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0395] Next, the oxygen partial pressure in the chamber is set to
be less than 1 Pa, and high-electrical-conductivity
InGaO.sub.3(ZnO).sub.4 and gold film are each laminated on top of
this layer to a 30 nm thickness by pulsed laser deposition. A drain
terminal 5 and source terminal 6 are formed by photolithography and
a lift-off technique.
[0396] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm). An
ECR plasma generator is similarly installed in the electron beam
deposition apparatus as well, whereby deposition is carried out
while feeding oxygen plasma. Gold is deposited on top of the
resulting film, and a gate terminal 4 is formed by photolithography
and a lift-off technique.
[0397] An FET having extremely good insulating properties for the
gate insulating film is thereby realized.
Example 1-9
[0398] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited by pulsed laser deposition employing a KrF
excimer laser onto a glass substrate (1737, manufactured by Corning
Incorporated) with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0399] The oxygen partial pressure in the chamber is set to 6 Pa
and the substrate temperature to 25.degree. C.
[0400] A substrate which has undergone thin-film deposition is
subjected to thermal processing for 2 hours in air at 150.degree.
C. using an electric furnace.
[0401] Using the thin-film obtained in this manner, the top-gate
type MISFET device illustrated in FIG. 5 will be fabricated.
[0402] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0403] Next, the oxygen partial pressure in the chamber is set to
be less than 1 Pa, and high-electrical-conductivity
InGaO.sub.3(ZnO).sub.4 and gold film are each laminated on top of
this layer to a 30 nm thickness by pulsed laser deposition. A drain
terminal 5 and source terminal 6 are formed by photolithography and
a lift-off technique.
[0404] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
After Y.sub.2O.sub.3 film deposition as well, thermal processing is
performed for 2 hours in air at 150.degree. C. using an electric
furnace. Gold is deposited on top of this film, and a gate terminal
4 is formed by photolithography and a lift-off technique.
[0405] An FET having extremely good insulating properties for the
gate insulating film is thereby realized.
[0406] In the present Example 1-9, the thermal processing after
amorphous oxide film deposition and after Y.sub.2O.sub.3 film
deposition can also be carried out in an ozone atmosphere by
installing an ozone generator in the electric furnace.
[0407] Further, in the present Example 1-9, the thermal processing
after amorphous oxide film deposition and after Y.sub.2O.sub.3 film
deposition can also be carried out in a N.sub.2O+O.sub.2 atmosphere
by providing a N.sub.2O gas line and an oxygen gas line in the
electric furnace.
[0408] Further, in the present. Example 1-9, the thermal processing
after amorphous oxide film deposition and after Y.sub.2O.sub.3 film
deposition can also be carried out in air having an almost
saturated water vapor pressure in a water-vapor oxidation electric
furnace.
[0409] Further, in the present Example 1-9, the thermal processing
after amorphous oxide film deposition and after Y.sub.2O.sub.3 film
deposition can also be carried out by generating oxygen radicals
from a radical generating device provided in the deposition
chamber, and heating the substrate to 200.degree. C. with a
substrate heater while feeding generated oxygen radicals.
[0410] Further, in the present Example 1-9, the thermal processing
after amorphous oxide film deposition and after Y.sub.2O.sub.3 film
deposition can also be carried out using oxygen plasma generated by
an ECR plasma generator provided in the deposition chamber. As the
oxygen plasma generator, either a RF plasma generator or a DC
plasma generator is acceptable. The plasma generator is held, for
example, for 2 hours while irradiating the oxygen plasma onto the
substrate.
[0411] During plasma irradiation onto the oxygen substrate,
deposition may be carried out while heating the substrate to
200.degree. C. with a substrate heater.
Example 1-10
[0412] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited by pulsed laser deposition employing a KrF
excimer laser onto a glass substrate (1737, manufactured by
Corning. Incorporated) with a polycrystalline sintered body having
an InGaO.sub.3(ZnO).sub.4 composition serving as the target. The
oxygen partial pressure in the chamber is set to 6 Pa and the
substrate temperature to 25.degree. C. Using the thin-film obtained
in this manner, the top-gate type MISFET device illustrated in FIG.
5 will be fabricated.
[0413] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0414] Next, a metal mask provided with apertures in the shape of
the drain terminal 5 and the source terminal 6 is mounted so as to
closely adhere to the InGaO.sub.3(ZnO).sub.4 film surface which has
been deposited. The resulting structure is placed in a chamber, and
the oxygen partial pressure in the chamber is set to be less than 1
Pa. High-electrical-conductivity InGaO.sub.3(ZnO).sub.4 and gold
film are each laminated to a 30 nm thickness by pulsed laser
deposition. A drain terminal 5 and source terminal 6 are then
formed by removing the metal mask. Finally, a Y.sub.2O.sub.3 film
to be used as a gate insulating film 3 is deposited by electron
beam deposition (thickness: 90 nm; relative dielectric constant:
about 15; leak current density: 10.sup.-3 A/cm.sup.2 when applying
0.5 MV/cm) in such a way as to be deposited between the drain
terminal 5 and source terminal 6, i.e. on the channel. Gold is
deposited on top of this film. The metal mask is then removed, to
thereby form a gate terminal 4. By using a metal mask, a TFT device
can be formed without going through a lithography process.
Example 1-11
[0415] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited by pulsed laser deposition employing a KrF
excimer laser onto a glass substrate (1737, manufactured by Corning
Incorporated) with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0416] The oxygen partial pressure in the chamber is set to 6 Pa
and the substrate temperature to 25.degree. C.
[0417] Using the thin-film obtained in this manner, the top-gate
type MISFET device illustrated in FIG. 5 will be fabricated.
[0418] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0419] The oxygen partial pressure in the chamber is set to be less
than 1 Pa, and a gold film is laminated to a 30 nm thickness by
pulsed laser deposition. A drain terminal 5 and source terminal 6
are formed by photolithography and wet etching using aqueous
KI+I.sub.2.
[0420] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
Gold is deposited on top of this film, and a gate terminal 4 is
formed by photolithography and plasma dry etching using CF.sub.4+Ar
gas.
[0421] In this manner, a TFT having little variation between TFT
devices formed on the substrate can be fabricated.
[0422] Next, examples relating to the second aspect of the present
invention (deposition method) will be explained.
Example 2-1
[0423] An In--Ga--Zn mixture or alloy is placed in a deposition
apparatus which uses a tungsten boat as a resistance heating
evaporation source.
[0424] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited on a glass substrate (1737, manufactured by
Corning Incorporated) which is arranged facing a heating
evaporation source, by heating an In--Ga--Zn mixture or alloy in an
oxygen atmosphere to cause evaporation. The oxygen partial pressure
in the chamber is set to about 0.1 Pa and the substrate temperature
to 25.degree. C.
[0425] The composition of the In--Ga--Zn mixture or alloy placed in
the tungsten boat is adjusted so that the composition of the film
deposited on the glass substrate by deposition is formed into a
desired composition.
[0426] As well as a boat, a filament or basket may also be used as
the resistance heating evaporation source, and the material for
such object may be molybdenum, tantalum or similar substance.
[0427] In this manner, a thin-film of an amorphous oxide is formed
on the substrate.
[0428] A transistor such as that illustrated in FIG. 5 will be
fabricated using this film.
Example 2-2
[0429] Deposition is carried out using a molecular beam epitaxy
(MBE) system having three Knudsen cells and a gas inlet port.
[0430] Each of the Knudsen cells is provided with indium, gallium
and zinc simple metal, and the Knudsen cell heaters are heated.
[0431] The indium, gallium and zinc are thereby made to evaporate.
Oxygen gas is simultaneously fed from the gas inlet port, whereby
an In--Ga--Zn--O system amorphous oxide semiconductor thin-film is
deposited on a glass substrate (1737, manufactured by Corning
Incorporated) arranged in the direction in which the Knudsen cells
and the gas inlet port are facing.
[0432] The chamber internal pressure is set to 0.005 Pa and the
substrate temperature to 25.degree. C.
[0433] The heating temperature for the Knudsen cells is adjusted so
that the composition of the film deposited on the glass substrate
is formed into a desired composition.
[0434] The oxygen gas fed from the gas inlet port may be ordinary
O.sub.2 molecular gas, although ozone gas can also be used.
[0435] In addition, oxygen radicals may also be fed.
[0436] Using the thin-film obtained by the above-described method,
the top-gate type MISFET device illustrated in FIG. 5 will be
fabricated.
[0437] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0438] Next, while maintaining the chamber internal pressure to
0.005 Pa, the amount of oxygen gas being fed is reduced to one-half
of that during the thin-film fabrication described above.
High-electrical-conductivity InGaO.sub.3(ZnO).sub.4 is formed on
top of this layer to a 30 nm thickness by the above-described
thin-film fabrication method. Gold film is formed on top of this to
a 30 nm thickness by resistance heating evaporation using a
tungsten boat, and a drain terminal 5 and source terminal 6 are
formed by photolithography and a lift-off technique.
[0439] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
Gold is deposited on top of this film, and a gate terminal 4 is
formed by photolithography and a lift-off technique.
[0440] In this manner, the FET illustrated in FIG. 5 can be
obtained.
Example 2-3
[0441] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited on a glass substrate (1737, manufactured by
Corning Incorporated) by electron beam deposition while feeding
oxygen gas so as to radiate towards the substrate, wherein an
In.sub.2O.sub.3--Ga.sub.2O.sub.3--ZnO oxide sintered body serves as
a target.
[0442] The chamber internal pressure is set to 0.01 Pa and the
substrate temperature to 25.degree. C.
[0443] The composition of the In.sub.2O.sub.3--Ga.sub.2O.sub.3--ZnO
oxide sintered body is adjusted so that the composition of the film
deposited on the glass substrate is formed into a desired
composition.
[0444] Using a thin-film of the amorphous oxide obtained in this
manner, an FET can be fabricated by the method illustrated in
Example 2-2.
Example 2-4
[0445] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited on a substrate (1737, manufactured by
Corning Incorporated) by chemical vapor deposition (CVD), wherein
trimethylgallium (TMG), trimethylindium (TMI), dimethylzinc (DMZ)
and oxygen serve as a source gas.
[0446] The chamber internal pressure is set to 1 Pa and the
substrate temperature to 200.degree. C.
[0447] The source gas flow rate is adjusted so that the composition
of the film deposited on the glass substrate is formed into a
desired composition.
[0448] Triethylgallium (TEG), triisobutylgallium (TIBG) or gallium
alkoxide can be used in place of TMG.
[0449] Further, triethylindium (TEI) or indium alkoxide can be used
in place of TMI, and triethylzinc (TEZ) or zinc alkoxide can be
used in place of DMZ.
[0450] The oxygen gas may be ordinary O.sub.2 molecular gas,
although ozone gas can also be used. In addition, the oxygen may
also be fed into the chamber as oxygen radicals.
[0451] In addition, an oxidizing gas such as NO.sub.2 or N.sub.2O
may also be used.
[0452] Using a thin-film of the amorphous oxide obtained in this
manner, a FET can be fabricated by the method illustrated in
Example 2-2.
Example 2-5
[0453] In Example 2-4, generating a plasma in the chamber during
formation of the amorphous In--Ga--Zn--O thin-film by CVD enables a
film having little residual organic matter to be formed at a lower
substrate heating temperature.
[0454] Specifically, an In--Ga--Zn--O system amorphous oxide
semiconductor thin-film is deposited at a chamber internal pressure
of 0.1 Pa and a substrate temperature of 100.degree. C. using the
same source gas as that in Example 2-4, by using an ECR plasma
generator to feed plasma into the chamber.
Example 2-6
[0455] In Example 2-4, during formation of the amorphous
In--Ga--Zn--O thin-film by CVD, immediately after the source gas is
fed into the chamber, but before the source gas reaches the
substrate, source gas is passed through a tungsten mesh heated to
1,000.degree. C. or more, and then made to arrive at the substrate.
According to this method, it is possible to form a film having
little residual organic matter at a lower substrate heating
temperature, since the source gas is decomposed to a greater extent
by the tungsten catalyst.
[0456] Platinum, molybdenum, tantalum and the like can also be used
in place of the tungsten mesh.
[0457] As an example, a tungsten mesh heated to 1,500.degree. C. is
introduced into the chamber, whereby an In--Ga--Zn--O system
amorphous oxide semiconductor thin-film is deposited at a chamber
internal pressure of 1 Pa and a substrate temperature of
100.degree. C. using the same source gas as that in Example
2-4.
Example 2-7
[0458] Deposition is carried out by line-beam pulsed laser
deposition using a 100 mm width laser line beam generated by adding
a line optical system to a KrF excimer laser.
[0459] A polycrystalline sintered body having a size 100 mm wide
and which has an InGaO.sub.3(ZnO).sub.4 composition is used as the
target.
[0460] An amorphous oxide semiconductor thin-film is deposited onto
a 100 mm.times.100 mm glass substrate while moving the substrate in
a vertical direction with respect to the beam line so that the film
to be grown is uniformly deposited within the substrate
surface.
[0461] The oxygen partial pressure in the chamber is set to 6 Pa
and the substrate temperature to 25.degree. C.
[0462] Using the amorphous oxide thin-film obtained in this
manner,.the top-gate MISFET device illustrated in FIG. 5 will be
fabricated.
[0463] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0464] Next, the oxygen partial pressure in the chamber is set to
less than 1 Pa, and high-electrical-conductivity
InGaO.sub.3(ZnO).sub.4 and gold film are each laminated on top of
this layer to a 30 nm thickness by line-beam pulsed laser
deposition. A drain terminal 5 and source terminal 6 are formed by
photolithography and a lift-off technique.
[0465] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by line-beam pulsed laser deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
Gold is deposited on top of this film, and a gate terminal 4 is
formed by photolithography and a lift-off technique. In this
manner, the FET illustrated in FIG. 5 is formed.
Example 2-8
[0466] Deposition of an amorphous oxide carried out by an
electrodeposition method will now be explained.
[0467] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited on a substrate (1737, manufactured by
Corning Incorporated) with an aqueous solution containing indium
nitrate, gallium nitrate, zinc nitrate and dimethylamineborane
(DMAB) serving as the raw material.
[0468] First, after undergoing non-electric field deposition,
electrodeposition is carried out by using an external power source
to apply an electric field on an aqueous solution that does not
contain dimethylamineborane (DMAB).
[0469] The temperature of the aqueous solution is set from
60.degree. C. (during no electric field) to 85.degree. C. (during
electrodeposition).
[0470] The aqueous solution serving as the raw material is adjusted
so that the composition of the film deposited on the glass
substrate is formed into a desired composition.
[0471] Using the method illustrated in Example 2-4, a FET is
realized which uses the thin-film of an amorphous oxide produced by
electrodeposition.
[0472] Next, examples relating to the third aspect of the present
invention (deposition temperature) will be explained.
Example 3-1
[0473] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is deposited on a glass substrate (1737, manufactured by
Corning Incorporated) by pulsed laser deposition employing a KrF
excimer laser with a polycrystalline sintered body having an
InGaO.sub.3(ZnO).sub.4 composition serving as the target.
[0474] The partial oxygen pressure in the chamber is set to 6 Pa
and the substrate temperature to 70.degree. C.
[0475] Using a thin film of the amorphous oxide obtained in this
manner, the top-gate MISFET device illustrated in FIG. 5 is
fabricated.
[0476] First, a 120 nm thick semi-insulating amorphous
InGaO.sub.3(ZnO).sub.4 film to be used as a channel layer 2 is
formed on a glass substrate 1 by the above-described amorphous
In--Ga--Zn--O thin-film fabricating method.
[0477] Next, the oxygen partial pressure in the chamber is set to
less than 1 Pa, and high-electrical-conductivity
InGaO.sub.3(ZnO).sub.4 and gold film are each laminated on top of
this layer to a 30 nm thickness by pulsed laser deposition. A drain
terminal 5 and source terminal 6 are formed by photolithography and
a lift-off technique.
[0478] Finally, a Y.sub.2O.sub.3 film to be used as a gate
insulating film 3 is deposited by electron beam deposition
(thickness: 90 nm; relative dielectric constant: about 15; leak
current density: 10.sup.-3 A/cm.sup.2 when applying 0.5 MV/cm).
Gold is deposited on top of this film, and a gate terminal 4 is
formed by photolithography and a lift-off technique.
[0479] The FET illustrated in FIG. 5 is thus obtained.
[0480] In addition, the substrate temperature during deposition of
the In--Ga--Zn--O system amorphous oxide semiconductor thin-film
can be set to, for example, 120.degree. C.
Example 3-2
[0481] Transparent polycarbonate (PC) is used as the substrate.
[0482] Although a 0.3 mm thick substrate is used here, a resin film
of about 10 .mu.m to 100 .mu.m can be used. Further, a resin
substrate or resin film coated on its surface with a silicon oxide
film, silicon nitride film or the like may also be used.
[0483] An In--Ga--Zn--O system amorphous oxide semiconductor
thin-film is fabricated by sputtering deposition in an argon gas
atmosphere having an oxygen partial pressure exceeding
3.times.10.sup.-1 Pa, and preferably exceeding 5.times.10.sup.-1
Pa. The substrate temperature during deposition is set to
120.degree. C. By depositing in a state heated in this manner, the
stability of a device when made to operate, for example, in a
60.degree. C. constant temperature can be increased.
[0484] If substrate temperature during deposition is higher than
the PC substrate distortion temperature (150.degree. C.), variation
in TFT device properties (gate voltage V.sub.GS threshold value or
I.sub.DS etc.) increases.
[0485] By using the amorphous oxide according to the present
invention for a channel layer, a transistor, and in particular, a
normally-off type FET, can be realized.
[0486] Such a transistor can be employed as a switching device for
a liquid crystal display (LCD) or an organic EL display.
[0487] In addition, since the amorphous oxide can be formed on a
flexible substrate including plastic films, the present invention
can be broadly applied in such products as flexible displays as
well as IC cards, ID tags and other devices.
[0488] This application claims priority from Japanese Patent
Application No. 2004-326686 filed on Nov. 10, 2004, which is hereby
incorporated by reference herein.
* * * * *