U.S. patent application number 15/249615 was filed with the patent office on 2016-12-15 for semiconductor device and method for manufacturing the semiconductor device.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Yoshiharu HIRAKATA, Jun KOYAMA, Shunpei YAMAZAKI.
Application Number | 20160365040 15/249615 |
Document ID | / |
Family ID | 44186992 |
Filed Date | 2016-12-15 |
United States Patent
Application |
20160365040 |
Kind Code |
A1 |
YAMAZAKI; Shunpei ; et
al. |
December 15, 2016 |
SEMICONDUCTOR DEVICE AND METHOD FOR MANUFACTURING THE SEMICONDUCTOR
DEVICE
Abstract
An object is to provide a liquid crystal display device which
can recognize image display even when the liquid crystal display
device is used in a dim environment. In one pixel, a pixel
electrode including both of a region where incident light through a
liquid crystal layer is reflected and a transmissive region is
provided, and image display can be performed in both modes: the
reflective mode where external light is used as an illumination
light source; and the transmissive mode where the backlight is used
as an illumination light source. When there is external light with
insufficient brightness, that is, in a dim environment, the
backlight emits weak light and an image is displayed in the
reflective mode, whereby image display can be performed.
Inventors: |
YAMAZAKI; Shunpei; (Tokyo,
JP) ; KOYAMA; Jun; (Kanagawa, JP) ; HIRAKATA;
Yoshiharu; (Kanagawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
44186992 |
Appl. No.: |
15/249615 |
Filed: |
August 29, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12978770 |
Dec 27, 2010 |
9448433 |
|
|
15249615 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/1337 20130101;
G02F 2001/133302 20130101; G02F 1/134309 20130101; G09G 2310/0237
20130101; G09G 2320/064 20130101; G02F 2201/121 20130101; G02F
1/136286 20130101; G09G 2320/0261 20130101; G09G 2320/103 20130101;
G02F 2201/123 20130101; G09G 3/3413 20130101; G09G 3/2003 20130101;
G02F 1/1368 20130101; G09G 2320/0247 20130101; G09G 3/3406
20130101; G02F 2203/04 20130101; G09G 3/3677 20130101; G02F
1/133345 20130101; G09G 2320/0646 20130101; G02F 1/133514 20130101;
G02F 1/1334 20130101; G02F 1/133555 20130101; G09G 2310/08
20130101; G09G 2330/022 20130101; G09G 2330/021 20130101; G09G
2300/0426 20130101; G02F 2203/01 20130101 |
International
Class: |
G09G 3/34 20060101
G09G003/34; G02F 1/1343 20060101 G02F001/1343; G02F 1/1335 20060101
G02F001/1335; G02F 1/1337 20060101 G02F001/1337; G09G 3/20 20060101
G09G003/20; G09G 3/36 20060101 G09G003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2009 |
JP |
2009-298290 |
Claims
1. A display device comprising: a display panel; a backlight
portion; and an image processing circuit, wherein the display panel
comprises a plurality of pixels each including a pixel electrode
provided with a transmissive region and a reflective region and
configured to control an alignment state of liquid crystal and a
transistor connected to the pixel electrode, and wherein the image
processing circuit comprises a memory circuit storing an image
signal, a comparison circuit configured to compare the image signal
stored in the memory circuit with image signals in a series of
frame periods and to calculate a difference, a first driver circuit
controlling the display panel, and a second driver circuit
controlling light-emission of the backlight portion, the display
device comprising: a moving-image display mode in which the
comparison circuit determines that the series of frame periods in
which a difference is detected is a moving image period, the image
processing circuit outputs a first signal to the display panel, the
display panel is driven by the driver circuit, the image processing
circuit outputs a second signal to the backlight portion, and the
backlight portion is driven by the second driver circuit; and a
still-image display mode in which the comparison circuit determines
that the series of frame periods in which a difference is not
detected is a still image period, the image processing circuit
stops output of a signal to the display panel and the backlight
portion.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/978,770, filed Dec. 27, 2010, now allowed, which claims the
benefit of a foreign priority application filed in Japan as Serial
No. 2009-298290 on Dec. 28, 2009, both of which are incorporated by
reference.
TECHNICAL FIELD
[0002] The present invention relates to a semiconductor device
having a circuit including a thin film transistor (hereinafter
referred to as TFT) and a method for manufacturing the
semiconductor device. For example, the present invention relates to
an electronic device on which an electro-optical device typified by
a liquid crystal display panel is mounted as a component.
[0003] In this specification, a semiconductor device generally
means a device which can function by utilizing semiconductor
characteristics, and an electrooptic device, a semiconductor
circuit, and electronic equipment are all semiconductor
devices.
BACKGROUND ART
[0004] In a liquid crystal display device, an active matrix liquid
crystal display device, in which pixel electrodes are provided in
matrix and transistors are used as switching elements connected to
respective pixel electrodes in order to obtain an image with high
quality, has attracted attention.
[0005] An active matrix liquid crystal display device, in which
transistors formed using a metal oxide for a channel formation
region are used as switching elements connected to respective pixel
electrode, has already been known (Patent Document 1 and Patent
Document 2).
[0006] It is known that an active matrix liquid crystal display
device is classified into two major types: transmissive type and
reflective type.
[0007] In the transmissive liquid crystal display device, a
backlight such as a cold cathode fluorescent lamp or the like is
used and an optical modulation operation is utilized to choose one
between the two states: a state in which light from the backlight
passes through liquid crystal to be output to the outside of the
liquid crystal display device and a state in which light is not
output, whereby bright and dark images are displayed; further,
image display is performed in combination of these.
[0008] Since the backlight is utilized in the transmissive liquid
crystal display device, it is difficult to recognize display in the
environment with strong external light, for example, outdoors.
[0009] In the reflective liquid crystal display device, the optical
modulation action of liquid crystal is utilized to choose one
between the two states: a state in which external light, that is,
incident light is reflected from a pixel electrode to be output to
the outside of the device and a state in which incident light is
not output to the outside of the device, whereby bright and dark
images are displayed; further, image display is performed in
combination of these.
[0010] Compared to the transmissive liquid crystal display device,
the reflective liquid crystal display device has the advantage of
low power consumption since the backlight is not used; therefore, a
demand for the reflective liquid crystal display device as a
portable information terminal has increased.
[0011] Since external light is utilized in the reflective liquid
crystal display device, the reflective liquid crystal display
device is suited for image display in the environment with strong
external light, for example, outdoors. On the other hand, it is
difficult to recognize display when the liquid crystal display
device is used in a dim environment, that is, in the environment
with weak external light.
REFERENCE
[0012] [Patent Document 1] Japanese Published Patent Application
No. 2007-123861
[0013] [Patent Document 2] Japanese Published Patent Application
No. 2007-096055
DISCLOSURE OF INVENTION
[0014] An object of an embodiment of the present invention is to
provide a liquid crystal display device which can recognize image
display even when the liquid crystal display device is used in a
dim environment.
[0015] An object of an embodiment of the present invention is to
provide a liquid crystal display device which can perform image
display in both modes: a reflective mode where external light is
used as an illumination light source; and a transmissive mode where
a backlight is used.
[0016] In one pixel, a pixel electrode including both of a region
where incident light through a liquid crystal layer is reflected
and a transmissive region is provided, and image display can be
performed in both modes: the reflective mode where external light
is used as an illumination light source; and the transmissive mode
where the backlight is used as an illumination light source.
[0017] When there is external light with enough brightness, this
liquid crystal display device is put in the reflective mode and a
still image is displayed, whereby power consumption can be
reduced.
[0018] When there is external light with insufficient brightness,
that is, in a dim environment, the backlight emits weak light and
an image is displayed in the reflective mode, whereby image display
can be performed.
[0019] When external light is weak or there is no external light,
the backlight emits light in the transmissive mode, and image
display can be performed.
[0020] A sensor for detecting brightness of the surroundings of the
liquid crystal display device is preferably provided. The
reflective mode, the transmissive mode, or on/off of the backlight
is preferably performed in accordance with data obtained using the
sensor, and the amount of light is preferably adjusted in
accordance with data obtained using the sensor.
[0021] For a light source of the backlight, it is preferable to use
a plurality of light-emitting diodes (LEDs) in which power
consumption can be further reduced as compared to the cold cathode
fluorescent lamp and which can control the strength and weakness of
light. The use of LEDs for the backlight partly controls the
strength and weakness of light, whereby image display with high
contrast and high color visibility can be performed.
[0022] An embodiment of the present invention disclosed in this
specification is a display device comprising a display panel, a
backlight portion, and an image processing circuit. In the display
device, the display panel comprises a plurality of pixels each
including a pixel electrode provided with a transmissive region and
a reflective region and configured to control an alignment state of
liquid crystal and a transistor connected to the pixel electrode,
and the image processing circuit comprises a memory circuit storing
an image signal, a comparison circuit configured to compare the
image signal stored in the memory circuit with image signals in a
series of frame periods and to calculate a difference, a first
driver circuit controlling the display panel, and a second driver
circuit controlling light-emission of the backlight portion. The
display device comprises a moving-image display mode in which the
comparison circuit determines that the series of frame periods in
which a difference is detected is a moving image period, the image
processing circuit outputs a first signal to the display panel, the
display panel is driven by the driver circuit, the image processing
circuit outputs a second signal to the backlight portion, and the
backlight is driven by the second driver circuit; and a still-image
display mode in which the comparison circuit determines that the
series of frame periods in which a difference is not detected is a
still image period, the image processing circuit stops output of a
signal to the display panel and the backlight portion.
[0023] In the above structure, the pixel electrode includes a
reflective electrode and a transparent electrode (hereinafter
referred to as a transparent electrode) partly in contact and
overlapping with the reflective electrode.
[0024] In an embodiment of the present invention, a plurality of
structures is provided, a reflective electrode is provided above a
side surface of the structure, and the pixel electrode including a
transparent electrode above a top surface of the structure is used.
An embodiment of the present invention is a liquid crystal display
device comprises a plurality of structures over a substrate; a
reflective layer covering side surfaces of the plurality of
structures; an insulating layer provided between adjacent side
surfaces of the plurality of structures and covering the reflective
layer; a pixel electrode including a reflective region overlapping
with the reflective layer with the insulating layer provided
therebetween, and a transmissive region overlapping with a top
surface of the structure; and a transistor electrically connected
to the pixel electrode.
[0025] In the above structure, two inclined planes facing each
other at a cross section of the structures are included, and an
angle .theta.T formed by an inclination of the inclined plane of
the structure and an inclination of the inclined plane facing the
inclined plane is less than 90.degree. , preferably greater than or
equal to 20.degree. and less than or equal to 60.degree..
[0026] In the above structure, the reflective region is a
reflective electrode including a curving surface, and an angle
.theta.R at a point where the reflective electrode is most curved
at the cross section of the reflective electrode, formed by two
inclined planes facing each other is greater than or equal to
90.degree., or preferably greater than or equal to 100.degree. and
less than or equal to 120.degree..
[0027] In the above structure, an upper portion of the structure is
positioned above a surface of the reflective electrode and
projected so that a distance from the upper portion of the
structure to an end portion of the reflective electrode is greater
than or equal to 0.1 .mu.m and less than or equal to 3 .mu.m,
preferably greater than or equal to 0.3 .mu.m and less than or
equal to 2 .mu.m.
[0028] A liquid crystal display device in which image display can
be performed in accordance with an environment of various
brightness levels of external light can be provided. Further, low
power consumption can be realized in displaying of a still
image.
BRIEF DESCRIPTION OF DRAWINGS
[0029] In the accompanying drawings:
[0030] FIG. 1 is a block diagram of an embodiment of the present
invention;
[0031] FIG. 2 illustrates an example of an equivalent circuit of a
pixel which is an embodiment of the present invention;
[0032] FIGS. 3A to 3C are timing charts of an embodiment of the
present invention;
[0033] FIG. 4 is a perspective view of a liquid crystal module
which is an embodiment of the present invention;
[0034] FIGS. 5A and 5B are a top view and a cross section of an
embodiment of the present invention;
[0035] FIG. 6 illustrates a cross-sectional structure of an
embodiment of the present invention;
[0036] FIG. 7 illustrates a cross-sectional structure of an
embodiment of the present invention;
[0037] FIGS. 8A and 8B are a top view and a cross section of an
embodiment of the present invention;
[0038] FIGS. 9A to 9D illustrate an embodiment of a transistor
which can be applied to a liquid crystal display device;
[0039] FIGS. 10A to 10E illustrate an embodiment of a method for
manufacturing a transistor which can be applied to a liquid crystal
display device;
[0040] FIG. 11 illustrates an example of a top view of a pixel of
an embodiment of the present invention;
[0041] FIGS. 12A to 12D each illustrate a cross section of a pixel
which is an embodiment of the present invention;
[0042] FIG. 13 illustrates a cross-sectional structure of a pixel
which is an embodiment of the present invention; and
[0043] FIGS. 14A and 14B illustrate an external view and a block
diagram of a display device which is an embodiment of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0044] Hereinafter, embodiments of the present invention are
described in detail with reference to the accompanying drawings.
However, the present invention is not limited to the description
below, and it is easily understood by those skilled in the art that
modes and details disclosed herein can be modified in various ways
without departing from the spirit and the scope of the present
invention. Therefore, the present invention is not construed as
being limited to description of the embodiments.
Embodiment 1
[0045] In this embodiment, a liquid crystal display device
including a still-image mode and a moving-image mode is described
with reference to FIG. 1. Note that in this specification, a mode
performed in such a way that a display device determines image
signals input to the display device as a still image is described
as a still-image mode, and a mode performed in such a way that the
display device determines the image signals input to the display
device as a moving image is described as a moving-image mode.
[0046] A display device 100 of this embodiment includes an AID
converter circuit 102, an image processing circuit 110, a display
panel 120, and a backlight portion 130 (see FIG. 1).
[0047] The image processing circuit 110 includes a memory circuit
111, a comparison circuit 112, a display control circuit 113, and a
selection circuit 115.
[0048] The display panel 120 includes a driver circuit portion 121
and a pixel portion 122. In the pixel portion 122, a plurality of
pixels 123 each connected to a scan line and a signal line are
arranged in matrix.
[0049] The pixel 123 includes a transistor, a pixel electrode
connected to the transistor, and a capacitor. A liquid crystal
layer is sandwiched between the pixel electrode and a counter
electrode facing the pixel electrode, so that a liquid crystal
element is formed. The pixel electrode includes a region where
incident light through the liquid crystal layer is reflected and a
transmissive region.
[0050] An example of liquid crystal elements is an element which
controls transmission and non-transmission of light by optical
modulation action of liquid crystals. The element can include a
pair of electrodes and a liquid crystal layer. The optical
modulation action of liquid crystals is controlled by an electric
field applied to the liquid crystals (that is, a vertical electric
field). Note that specifically, the following can be used for a
liquid crystal element, for example: a nematic liquid crystal, a
cholesteric liquid crystal, a smectic liquid crystal, a discotic
liquid crystal, a thermotropic liquid crystal, a lyotropic liquid
crystal, a low-molecular liquid crystal, a high-molecular liquid
crystal, polymer dispersed liquid crystal (PDLC), a ferroelectric
liquid crystal, an anti-ferroelectric liquid crystal, a main-chain
liquid crystal, a side-chain high-molecular liquid crystal, a
banana-shaped liquid crystal, and the like. In addition, the
following can be used as a diving method of a liquid crystal: a TN
(twisted nematic) mode, an STN (super twisted nematic) mode, an OCB
(optically compensated birefringence) mode, an ECB (electrically
controlled birefringence) mode, an FLC (ferroelectric liquid
crystal) mode, an AFLC (anti-ferroelectric liquid crystal) mode, a
PDLC (polymer dispersed liquid crystal) mode, a PNLC (polymer
network liquid crystal) mode, a guest-host mode, and the like.
[0051] The backlight portion 130 includes a backlight control
circuit 131 and a backlight 132. Light emitted from the backlight
132 may include the three primary colors of light, and the
backlight 132 includes a white light-emitting element 133, for
example.
[0052] Next, a signal flow in the display device described in this
embodiment is described.
[0053] An analog image signal is input to the display device 100
from an image signal supply source 101. The analog image signal
includes an image signal such as a signal corresponding to red (R),
a signal corresponding to green (G), and a signal corresponding to
blue (B).
[0054] The A/D converter circuit 102 converts the analog image
signal into a digital image signal and outputs the signal to the
image processing circuit 110. When the image signal is converted
into a digital signal in advance, detection of a difference of the
image signals that is to be performed later can be easily
performed, which is preferable.
[0055] The image processing circuit 110 generates an LC image
signal and a backlight signal from the digital image signal which
is input. The LC image signal is an image signal used for
controlling the display panel 120, and the backlight signal is a
signal used for controlling the backlight portion 130.
[0056] The memory circuit 111 provided in the image processing
circuit 110 includes a plurality of frame memories for storing
image signals of a plurality of frames. The number of frame
memories included in the memory circuit 111 is not particularly
limited as long as the image signals of a plurality of frames can
be stored. Note that the frame memory may be formed using a memory
element such as dynamic random access memory (DRAM) or static
random access memory (SRAM).
[0057] The number of frame memories is not particularly limited as
long as the image signal can be stored for each frame period.
Further, the image signals stored in the frame memories are
selectively read out by the comparison circuit 112 and the display
control circuit 113.
[0058] The comparison circuit 112 selectively reads out the image
signals in successive frame periods stored in the memory circuit
111, compares the image signals in the successive frame periods in
each pixel, and detects a difference thereof.
[0059] Depending on whether a difference is detected, operations in
the display control circuit 113 and the selection circuit 115 are
determined. When a difference is detected in any of the pixels by
comparing the image signals in the comparison circuit 112,
successive frame periods during which the difference is detected
are determined as a moving image period. On the other hand, when a
difference is not detected in all the pixels by comparing the image
signals in the comparison circuit 112, successive frame periods
during which no difference is detected are determined as a still
image period. In other words, depending on whether a difference is
detected by the comparison circuit 112, whether the image signals
in the successive frame periods are image signals for displaying a
moving image or image signals for displaying a still image is
determined.
[0060] Note that the difference obtained by the comparison may be
set so as to be determined as a difference to be detected when it
is over a predetermined level. The comparison circuit 112 may be
set so as to determine detection of a difference by the absolute
value of the difference regardless of the value of the
difference.
[0061] Note that in this embodiment, a still image period or a
moving image period is determined by detecting a difference between
the image signals in successive frame periods with the comparison
circuit 112; however, a signal used for switching between the still
image and the moving image may be supplied externally, so that the
still image or the moving image may be displayed in accordance with
the switching signal.
[0062] Note that by switching of a plurality of images which is
time-divided into a plurality of frames at high speed, the images
are recognized as a motion image by human eyes. Specifically, by
switching of images at least 60 times (60 frames) per second, the
images are recognized as a moving image with less flicker by human
eyes. In contrast, unlike a moving image or a partial moving image,
a still image refers to image signals which do not change in
successive frame periods, for example, in an n-th frame and an
(n+1)th frame though a plurality of images which is time-divided
into a plurality of frame periods is switched at high speed.
[0063] The selection circuit 115 includes a plurality of switches,
for example, switches formed using transistors. The selection
circuit 115 selects the image signals from the frame memories in
the memory circuit 111 where the image signals are stored, and the
selection circuit 115 outputs the image signals to the display
control circuit 113 when a difference is detected by calculation
with the comparison circuit 112, that is, when images displayed in
successive frame periods are recognized as a moving image.
[0064] Note that the selection circuit 115 does not output the
image signals to the display control circuit 113 when a difference
between the image signals is not detected by calculation with the
comparison circuit 112, that is, when images displayed in
successive frame periods are recognized as a still image. When a
still image is displayed, the selection circuit 115 does not output
the image signals from the frame memories to the display control
circuit 113, resulting in a reduction in power consumption.
[0065] In the display device of this embodiment, a mode performed
in such a way that the comparison circuit 112 determines the image
signals as a still image is described as the still-image mode, and
a mode performed in such a way that the comparison circuit 112
determines the image signals as a moving image is described as the
moving-image mode.
[0066] The image processing circuit described in this embodiment
may have a mode-switching circuit. The mode-switching circuit has a
function of switching between the moving-image mode and the
still-image mode in such a manner that a user of the display device
selects an operation mode of the display device by hand or using an
external connection device.
[0067] The selection circuit 115 can output the image signals to
the display control circuit 113 in accordance with signals input
from the mode-switching circuit.
[0068] For example, in the case where a user switches an operation
mode and a mode-switching signal is input to the selection circuit
115 from the mode-switching circuit while an operation is performed
in a still-image display mode, even when the comparison circuit 112
does not detect the difference of the image signals in successive
frame periods, the user can carry out a mode in which the image
signals which are input are sequentially output to the display
control circuit 113, that is, a moving-image display mode. In the
case where a user switches an operation mode and a mode-switching
signal is input to the selection circuit 115 from the
mode-switching circuit while an operation is performed in a
moving-image display mode, even when the comparison circuit 112
detects the difference of the image signals in successive frame
periods, the user can carry out a mode in which only the image
signal of one selected frame is output, that is, a still-image
display mode. One frame in a moving image is displayed as a still
image in the display device of this embodiment.
[0069] The display control circuit 113 is a circuit configured to
supply the image signal selected by the selection circuit 115 in
accordance with detection of the difference in the comparison
circuit 112 and supply a signal for controlling the driver circuit
portion 121 of the display panel 120 and the backlight control
circuit 131 of the backlight portion 130.
[0070] Specifically, the display control circuit 113 supplies a
signal controlling switching between supply and stop of a control
signal such as a start pulse SP or a clock signal CK to the display
panel 120. In addition, the display control circuit 113 supplies a
signal for controlling on and off of the backlight to the backlight
control circuit 131.
[0071] When the comparison circuit 112 determines that a moving
image is displayed, an image signal is read out from the memory
circuit 111 through the selection circuit 115 and supplied to the
driver circuit portion 121 from the display control circuit 113,
and a control signal is supplied to the driver circuit portion 121
from the display control circuit 113
[0072] On the other hand, when the comparison circuit 112
determines that a still image is displayed, an image signal is not
supplied to the display control circuit portion 121 from the
display control circuit 113, and supply of each control signal to
the driver circuit portion 121 and the backlight control circuit
131 is stopped.
[0073] In addition, the display device described in this embodiment
as an example includes a backlight lighting circuit and a
photometric circuit. The backlight lighting circuit and the
photometric circuit may be connected to the backlight control
circuit 131. The backlight lighting circuit is configured to input
a signal for lighting and extinction of the backlight performed
manually or using an external connection apparatus by a user of the
display device. The photometric circuit is a circuit configured to
measure brightness of an environment where the display device is
used.
[0074] For example, in the case where the display device described
in this embodiment is used in a dim environment, a signal is input
to the backlight control circuit 131 via the backlight lighting
circuit or the photometric circuit, so that the backlight can be
lit. Note that a threshold value may be set for the photometric
circuit so that the backlight is lit when the brightness of a usage
environment is less than the threshold value.
[0075] In a period in which a still image is displayed using the
structure of this embodiment, frequent writings of the image
signals can be reduced. In addition, power consumption is extremely
low because the still image can be displayed without use of the
backlight.
[0076] The display device described in this embodiment can display
not only a still image with reduced power consumption but also a
full-color image and a moving image.
[0077] When human eyes see an image formed by writing the image
signals plural times, the human eyes see images which are switched
plural times, which might cause eye strain. As described in this
embodiment, the number of writings of the image signals is reduced,
whereby there is an effect of reducing eye strain.
Embodiment 2
[0078] In this embodiment, a driving method of a liquid crystal
display device is described using a pixel connection diagram, a
timing chart, and the like. First, FIG. 2 is a schematic view of a
display panel of a liquid crystal display device. In FIG. 2, the
display panel includes a pixel portion 201, a scan line 202 (also
referred to as a gate line), a signal line 203 (also referred to as
a data line), a pixel 210, a common electrode 218, a capacitor line
219, a scan line side driver circuit 206, and a signal line side
driver circuit 207.
[0079] The pixel 210 includes a pixel transistor 212, a liquid
crystal element 213, and a capacitor 214. A gate of the pixel
transistor 212 is connected to the scan line 202, a first terminal
serving as one of a source and a drain of the pixel transistor 212
is connected to the signal line 203, and a second terminal serving
as the other of the source and the drain of the pixel transistor
212 is connected to one electrode of the liquid crystal element 213
and a first electrode of the capacitor 214. The other electrode of
the liquid crystal element 213 is connected to the common electrode
218. A second electrode of the capacitor 214 is connected to the
capacitor line 219. The pixel transistor 212 is preferably formed
using a thin film transistor (a TFT) having a thin oxide
semiconductor layer.
[0080] Note that a thin film transistor is an element having at
least three terminals of gate, drain, and source. The thin film
transistor includes a channel region between a drain region and a
source region, and current can flow through the drain region, the
channel region, and the source region. Here, since the source and
the drain may change depending on the structure, the operating
condition, and the like of the transistor, it is difficult to
define which is a source or a drain. Therefore, in this document
(the specification, the claims, the drawings, and the like), a
region functioning as a source and a drain is not called the source
or the drain in some cases. In such a case, for example, one of the
source and the drain may be referred to as a first terminal and the
other thereof may be referred to as a second terminal.
Alternatively, one of the source and the drain may be referred to
as a first electrode and the other thereof may be referred to as a
second electrode. Further alternatively, one of the source and the
drain may be referred to as a source region and the other thereof
may be referred to as a drain region.
[0081] Note that the scan line side driver circuit 206 and the
signal line side driver circuit 207 are preferably provided over
the substrate over which the pixel portion 201 is formed; however,
these are not necessarily formed over the substrate over which the
pixel portion 201 is formed. When the scan line side driver circuit
206 and the signal line side driver circuit 207 are provided over
the substrate over which the pixel portion 201 is formed, the
number of the connection terminals for connection to the outside
and the size of the liquid crystal display device can be
reduced.
[0082] Note that the pixels 210 are provided (arranged) in matrix.
Here, description that pixels are provided (arranged) in matrix
includes the case where the pixels are arranged in a straight line
and the case where the pixels are arranged in a jagged line, in a
longitudinal direction or a lateral direction. Therefore, for
example, in the case of performing full color display with three
color elements (e.g., R, G, and B), a case where color filters are
arranged in stripes and a case where dots of the three color
elements are arranged in a delta pattern are included.
[0083] Note that when it is explicitly described that "A and B are
connected," the case where A and B are electrically connected, the
case where A and B are functionally connected, and the case where A
and B are directly connected are included therein.
[0084] Next, the operation of the display panel together with the
operation of the backlight is described with reference to FIG. 3A.
As described in the above embodiment, the operation of the display
panel is roughly divided into a moving-image display period 301 and
a still-image display period 302.
[0085] The cycle of one frame period (or frame frequency) is
preferably less than or equal to 1/60 sec (more than or equal to 60
Hz) in the moving-image display period 301. The frame frequency is
increased, so that flickering is not sensed by a viewer of an
image. In the still-image display period 302, the cycle of one
frame period is extremely long, for example, longer than or equal
to one minute (less than or equal to 0.017 Hz), so that eye strain
can be reduced compared to the case where the same image is
switched plural times.
[0086] When an oxide semiconductor is used for a semiconductor
layer of the pixel transistor 212, the number of carriers in the
oxide semiconductor can be extremely small; therefore, the
off-state current can be reduced. Accordingly, an electrical signal
such as the image signal can be held for a longer period in the
pixel, and a writing interval can be set longer. Therefore, the
cycle of one frame period can be increased, and the frequency of
refresh operations in the still-image display period 302 can be
reduced, whereby an effect of suppressing power consumption can be
further increased.
[0087] In the moving-image display period 301 shown in FIG. 3A, an
image signal is distributed to each pixel, and a driver circuit
control signal for displaying a moving image is supplied to the
driver circuit so that the driver circuit operates. In addition, in
the moving-image display period 301 shown in FIG. 3A, a backlight
with white light is operated by the backlight control signal. One
example of the display panel as an example is configured to
transmit light having a specific wavelength through color filters
of R (red), G (green), and B (blue), whereby color display of a
moving image can be performed.
[0088] As described in the above embodiment, in the still-image
display period 302 illustrated in FIG. 3A, a driver circuit control
signal for writing the image signal of a still image is supplied to
the driver circuit because color display is performed due to
transmission or non-transmission of reflected light, whereby the
driver circuit operates. When the driver circuit control signal is
not supplied in the period other than the period of writing the
image signal to the driver circuit, power consumption can be
reduced. In the still-image display period 302 illustrated in FIG.
3A, display comes to be visible utilizing reflected external light;
therefore, the backlight is not operated by the backlight control
signals. Then, a color still image can be displayed on the display
panel.
[0089] Note that in the still-image display period 302, when a
still image is displayed by transmission or non-transmission of
reflection light, still images may be displayed in a grayscale
depending on placement of the color filter. In this case, an image
signal for displaying images in a grayscale may be supplied as the
image signal input.
[0090] Next, the moving-image display period 301 and the
still-image display period 302 of FIG. 3A is described in details
with reference to timing charts of FIG. 3B and FIG. 3C,
respectively. The timing charts illustrated in FIG. 3B and FIG. 3C
are exaggerated for description, and signals do not operate in
synchronization, except for the case where there is
[0091] First, FIG. 3B is described. FIG. 3B illustrates a clock
signal GCK which is supplied to the scan line side driver circuit
106, a start pulses GSP which is supplied to the scan line side
driver circuit 106, a clock signal SCK which is supplied to the
signal line side driver circuit 107, a start pulse SSP which is
supplied to the signal line side driver circuit 107, image signal
data, and a lighting state of the backlight in the moving-image
display period 301 as an example. Note that low power consumption
and life extension can be attempted by using a white LED as the
backlight.
[0092] In the moving-image display period 301, the clock signal GCK
becomes a clock signal which is always supplied. The start pulse
GSP becomes a pulse corresponding to vertical synchronizing
frequency. The clock signal SCK becomes a clock signal which is
always supplied. The start pulse GSP becomes a pulse corresponding
to one gate selection period. In the moving-image display period
301, the image signal is written to each pixel corresponding to R
(red), G (green), or B (blue), and transmission or non-transmission
of light from the backlight is controlled, so that a viewer can see
color display of a moving image.
[0093] Next, FIG. 3C is described. In FIG. 3C, the still-image
display period 302 is divided into a still-image writing period 303
and a still-image holding period 304.
[0094] In the still-image writing period 303, the clock signal GCK
serves as a clock signal for writing for one screen. The start
pulse GSP serves as a pulse for writing for one screen. The clock
signal SCK serves as a clock signal for writing for one screen. The
start pulse SSP serves as a pulse for writing for one screen. In
the still-image writing period 303, a still image is displayed
using the image signal RGB for performing color display utilizing
reflected light; therefore, the backlight is not turned on.
[0095] In the still-image holding period 304, supply of the clock
signals GCK, the start pulse GSP, the clock signal SCK, and the
start pulse SSP is stopped in order to step the operation of the
signal line side driver circuit and the scan line side driver
circuit. Therefore, in the still-image holding period 304, power
consumption can be reduced. In the still-image holding period 304,
the image signals written to the pixel in the still-image writing
period 303 are held by the pixel transistor with extremely low
off-state current; therefore, a color still image can be held for
longer than or equal to one minute. In the still-image holding
period 304, before the image signal held is decreased as a given
period passes, another still-image writing period 303 is provided,
and an image signal which is the same as the image signal of the
previous period is written (refresh operation), and the still-image
holding period 304 may be provided again.
[0096] Note that in the still-image holding period 304, the
backlight is not turned on in order to reduce power consumption.
When the color still image is difficult to be seen due to reflected
external light, the backlight is turned on and display is
continuously performed, whereby visibility can be improved. As a
result, convenience of the liquid crystal display device can be
increased.
[0097] In the liquid crystal display device described in this
embodiment, power consumption can be reduced when a still image is
displayed.
[0098] This embodiment can be implemented in combination with the
structure described in Embodiment 1, as appropriate.
Embodiment 3
[0099] FIG. 4 illustrates a structure of a liquid crystal display
module 190. The liquid crystal display module 190 includes a
backlight portion 130, a color filter provided in a position
overlapping with the backlight portion 130, a display panel 120 in
which liquid crystal elements are arranged in a matrix, and a
polarizing plate 125a and a polarizing plate 125b which are
provided with the display panel 120 positioned therebetween. The
backlight portion 130 is a surface-emitting backlight portion which
emits uniform white light. For example, the backlight portion 130
may include a white light-emitting element 133 placed in an end
portion of a light guide plate and a diffusing plate 134 provided
between the light guide plate and the display panel 120. In
addition, a flexible printed circuit (FPC) 126 serving as an
external input terminal is electrically connected to a terminal
portion provided in the display panel 120.
[0100] In FIG. 4, light 135 of three colors is schematically
denoted by arrows (R, G, and B). Light emitted from the backlight
portion 130 is modulated by a liquid crystal element overlapping
with the color filter of the display panel 120 and reaches a viewer
through the liquid crystal display module 190, so that the viewer
perceives an image.
[0101] Further, FIG. 4 schematically illustrates a state in which
external light 139 is transmitted through the liquid crystal
element over the display panel 120 and reflected by an electrode
below the liquid crystal element. The intensity of the light
transmitted through the liquid crystal element is modulated by an
image signal; therefore, a viewer can perceive an image also by
reflection light of the external light 139.
[0102] FIG. 5A is a plan view of a display region and illustrates
one pixel thereof. FIG. 5B is a cross-sectional view taken along
lines Y1-Y2 and Z1-Z2 of FIG. 5A.
[0103] In FIG. 5A, a plurality of source wiring layers (including a
source electrode layer 405a or a drain electrode layer 405b) is
arranged in parallel (extends in the vertical direction in the
drawing) to be spaced from each other. A plurality of gate wiring
layers (including a gate electrode layer 401) is provided to extend
in a direction generally perpendicular to the source wiring layers
(the horizontal direction in the drawing) and to be spaced from
each other. Capacitor wiring layers 408 are arranged adjacent to
the plurality of gate wiring layers and extend in a direction
generally parallel to the gate wiring layers, that is, in a
direction generally perpendicular to the source wiring layers (in
the horizontal direction in the drawing).
[0104] The liquid crystal display device in FIGS. 5A and 5B is a
semi-transmissive liquid crystal display device in which a pixel
region includes a reflective region 498 and a transmissive region
499. In the reflective region 498, a reflective electrode layer 446
is stacked as a pixel electrode layer over a transparent electrode
layer 447, and in the transmissive region 499, only the transparent
electrode layer 447 is provided as a pixel electrode layer. Note
that an example in which the transparent electrode layer 447 and
the reflective electrode layer 446 are stacked in this order over
an interlayer film 413 is illustrated in FIGS. 5A and 5B; however,
a structure in which the reflective electrode layer 446 and the
transparent electrode layer 447 are stacked in this order over the
interlayer film 413 may be employed. An insulating layer 407, a
protective insulating layer 409, and the interlayer film 413 are
provided over a transistor 450. The transparent electrode layer 447
and the reflective electrode layer 446 are electrically connected
to the transistor 450 through an opening (a contact hole) provided
in the insulating layer 407, the protective insulating layer 409,
and the interlayer film 413. In the transmissive region 499, a
coloring layer 416 functioning as a color filter layer is provided
between the protective insulating layer 409 and the interlayer film
413.
[0105] As illustrated in FIG. 5B, a common electrode layer 448
(also referred to as a counter electrode layer) is formed on a
second substrate 442 and faces the transparent electrode layer 447
and the reflective electrode layer 446 over a first substrate 441
with a liquid crystal layer 444 provided therebetween. Note that in
the liquid crystal display device in FIGS. 5A and 5B, an alignment
film 460a is provided between the transparent electrode layer 447
and the reflective electrode layer 446, and the liquid crystal
layer 444. An alignment film 460b is provided between the common
electrode layer 448 and the liquid crystal layer 444. The alignment
films 460a and 460b are insulating layers having a function of
controlling the alignment of liquid crystal and therefore, are not
necessarily provided depending on a material of the liquid
crystal.
[0106] The transistor 450 is an example of a bottom-gate
inverted-staggered transistor and includes a gate electrode layer
401, a gate insulating layer 402, an oxide semiconductor layer 403,
the source electrode layer 405a, and the drain electrode layer
405b. In addition, the capacitor wiring layer 408 which is formed
in the same step as the gate electrode layer 401, the gate
insulating layer 402, and a conductive layer 449 which is formed in
the same step as the source electrode layer 405a or the drain
electrode layer 405b are stacked to form a capacitor. Note that the
reflective electrode layer 446 which is formed using a reflective
conductive film of aluminum (Al), silver (Ag), or the like is
preferably provided to cover the capacitor wiring layer 408.
[0107] The semi-transmissive liquid crystal display device in this
embodiment performs color display of moving images in the
transmissive region 499 and monochrome (black and white) display of
still images in the reflective region 498 by control of turning on
and off the transistor 450.
[0108] In the transmissive region 499, image display is performed
by incident light from a backlight provided on the first substrate
441 side. When a coloring layer functioning as a color filter is
provided in the liquid crystal display device, light from the back
light is transmitted through the coloring layer, whereby color
display can be performed in the transmissive region. For example,
in the case of performing full-color display, the color filter may
be formed using a material showing red (R), green (G), or blue (B),
or may be formed using another material showing yellow, cyan,
magenta, or the like.
[0109] In FIGS. 5A and 5B, the coloring layer 416 functioning as a
color filter is provided between the protective insulating layer
409 and the interlayer film 413. Since the coloring layer 416
functions as a color filter, a light-transmitting resin layer which
is formed using a material transmitting only light colored with
chromatic color may be used. An optimal thickness of the coloring
layer 416 may be adjusted as appropriate in consideration of
relation between the concentration of a coloring material included
and the transmittivity of light. In the case where the thickness of
the light-transmitting chromatic color resin layer varies depending
on the chromatic colors or in the case where there is surface
unevenness due to a transistor, an insulating layer which transmits
light in a visible wavelength range (a so-called colorless,
transparent insulating layer) may be stacked for planarization of
the surface of the interlayer film.
[0110] In the case where the coloring layer 416 is directly formed
on the first substrate 441 side, the formation region can be
controlled more precisely and this structure is adjustable to a
pixel with a minute pattern. Alternatively, the coloring layer 416
can be used as an interlayer film.
[0111] The coloring layer 416 may be formed using a photosensitive
or a non-photosensitive organic resin by a coating method.
[0112] On the other hand, in the reflective region 498, image
display is performed by reflecting external light incident from the
second substrate 442 side by the reflective electrode layer
446.
[0113] Examples in which the reflective electrode layer 446 is
formed to have unevenness in the liquid crystal display device are
illustrated in FIG. 6 and FIG. 7. FIG. 6 illustrates an example in
which a surface of the interlayer film 413 in the reflective region
498 is fowled to have an uneven shape so that the reflective
electrode layer 446 has an uneven shape. The uneven shape of the
surface of the interlayer film 413 may be formed by performing
selective etching. For example, the interlayer film 413 having the
uneven shape can be formed, for example, by performing a
photolithography step on a photosensitive organic resin. FIG. 7
illustrates an example in which projected structures are provided
over the interlayer film 413 in the reflective region 498 so that
the reflective electrode layer 446 has an uneven shape. Note that
in FIG. 7, the projected structures are formed by stacking an
insulating layer 480 and an insulating layer 482. For example, an
inorganic insulating layer of silicon oxide, silicon nitride, or
the like can be used as the insulating layer 480, and an organic
resin such as a polyimide resin or an acrylic resin can be used as
the insulating layer 482. First, a silicon oxide film is formed
over the interlayer film 413 by a sputtering method, and a
polyimide resin film is formed over the silicon oxide film by a
coating method. The polyimide resin film is etched with the use of
the silicon oxide film as an etching stopper. The silicon oxide
film is etched with the use of the etched polyimide resin layer as
a mask, so that the projected structures including a stack of the
insulating layer 480 and the insulating layer 482 can be formed as
illustrated in FIG. 7.
[0114] When the reflective electrode layer 446 has an uneven
surface as illustrated in FIG. 6 and FIG. 7, incident light from
the outside is irregularly reflected, so that more favorable image
display can be performed. Accordingly, the visibility of image
display is improved.
[0115] Note that FIGS. 5A and 5B, FIG. 6, and FIG. 7 each
illustrate an example in which monochrome display is performed in
the reflective region 498; however, color display can also be
performed in the reflective region 498. FIGS. 8A and 8B illustrate
an example in which full-color display is performed in both the
transmissive region 499 and the reflective region 498.
[0116] FIGS. 8A and 8B illustrate an example in which a color
filter 470 is provided between the second substrate 442 and the
common electrode layer 448. By providing the color filter 470
between the reflective electrode layer 446 and the second substrate
442 on a viewer side, light reflected by the reflective electrode
layer 446 is transmitted through the color filter 470, so that
color display can be performed.
[0117] The color filter may be provided on an outer side of the
second substrate 442 (on a side opposite to the liquid crystal
layer 444).
[0118] Note that also in FIG. 6 and FIG. 7, if the color filter 470
is provided as illustrated in FIG. 8B instead of the coloring layer
416, full-color display can also be performed in the reflective
region 498.
[0119] This embodiment can be freely combined with Embodiment 1 or
2.
Embodiment 4
[0120] In this embodiment, an example of a transistor which can be
applied to a liquid crystal display device disclosed in this
specification is described. There is no particular limitation on a
structure of a transistor which can be applied to a liquid crystal
display device disclosed in this specification. For example, a
top-gate structure or a bottom-gate structure such as a staggered
type and a planar type can be used. The transistor may have a
single-gate structure in which one channel formation region is
formed, a double-gate structure in which two channel formation
regions are formed, or a triple-gate structure in which three
channel formation regions are formed. Alternatively, the transistor
may have a dual-gate structure including two gate electrode layers
positioned above and below a channel region with a gate insulating
layer provided therebetween. Note that examples of a
cross-sectional structure of a transistor illustrated FIGS. 9A to
9D are described below. Transistors illustrated in FIGS. 9A to 9D
are transistors including an oxide semiconductor as a
semiconductor. An advantage of using an oxide semiconductor is that
high mobility and low off-state current can be obtained in a
relatively easy and low-temperature process: however, it is
needless to say that another semiconductor may be used.
[0121] A transistor 410 illustrated in FIG. 9A is one of
bottom-gate thin film transistors, and is also referred to as an
inverted-staggered thin film transistor.
[0122] The transistor 410 includes, over a substrate 400 having an
insulating surface, a gate electrode layer 401, a gate insulating
layer 402, an oxide semiconductor layer 403, a source electrode
layer 405a, and a drain electrode layer 405b. In addition, an
insulating layer 407 which covers the transistor 410 and is stacked
over the oxide semiconductor layer 403 is provided. A protective
insulating layer 409 is provided over the insulating layer 407.
[0123] A transistor 420 illustrated in FIG. 9B is one of
bottom-gate thin film transistors referred to as a
channel-protective (channel-stop) thin film transistor and is also
referred to as an inverted-staggered thin film transistor.
[0124] The transistor 420 includes, over the substrate 400 having
an insulating surface, the gate electrode layer 401, the gate
insulating layer 402, the oxide semiconductor layer 403, an
insulating layer 427 functioning as a channel protective layer
which covers a channel formation region of the oxide semiconductor
layer 403, the source electrode layer 405a, and the drain electrode
layer 405b. The protective insulating layer 409 is formed so as to
cover the transistor 420.
[0125] A transistor 430 illustrated in FIG. 9C is a bottom-gate
thin film transistor, and includes, over the substrate 400 having
an insulating surface, the gate electrode layer 401, the gate
insulating layer 402, the source electrode layer 405a, the drain
electrode layer 405b, and the oxide semiconductor layer 403. The
insulating layer 407 which covers the transistor 430 and is in
contact with the oxide semiconductor layer 403 is provided. The
protective insulating layer 409 is provided over the insulating
layer 407.
[0126] In the transistor 430, the gate insulating layer 402 is
provided on and in contact with the substrate 400 and the gate
electrode layer 401, and the source electrode layer 405a and the
drain electrode layer 405b are provided on and in contact with the
gate insulating layer 402. Further, the oxide semiconductor layer
403 is provided over the gate insulating layer 402, the source
electrode layer 405a, and the drain electrode layer 405b.
[0127] A thin film transistor 440 illustrated in FIG. 9D is one of
top-gate thin film transistors. The transistor 440 includes, over
the substrate 400 having an insulating surface, an insulating layer
437, the oxide semiconductor layer 403, the source electrode layer
405a, the drain electrode layer 405b, the gate insulating layer
402, and the gate electrode layer 401. A wiring layer 436a and a
wiring layer 436b are provided to be in contact with and
electrically connected to the source electrode layer 405a and the
drain electrode layer 405b, respectively.
[0128] In this embodiment, as described above, the oxide
semiconductor layer 403 is used as a semiconductor layer. As an
oxide semiconductor used for the oxide semiconductor layer 403, an
In--Sn--Ga--Zn--O-based oxide semiconductor layer which is an oxide
of four metal elements; an In--Ga--Zn--O-based oxide semiconductor
layer, an In--Sn--Zn--O-based oxide semiconductor layer, an
In--Al--Zn--O-based oxide semiconductor layer, a
Sn--Ga--Zn--O-based oxide semiconductor layer, an
Al--Ga--Zn--O-based oxide semiconductor layer, or a
Sn--Al--Zn--O-based oxide semiconductor layer which are oxides of
three metal elements; an In--Zn--O-based oxide semiconductor layer,
a Sn--Zn--O-based oxide semiconductor layer, an Al--Zn--O-based
oxide semiconductor layer, a Zn--Mg--O-based oxide semiconductor
layer, a Sn--Mg--O-based oxide semiconductor layer, or an
In--Mg--O-based oxide semiconductor layer which are oxides of two
metal elements; an In--O-based oxide semiconductor layer, a
Sn--O-based oxide semiconductor layer, or a Zn--O-based oxide
semiconductor layer can be used. Further, SiO.sub.2 may be
contained in the above oxide semiconductor. Here, for example, an
In--Ga--Zn--O-based oxide semiconductor is an oxide including at
least In, Ga, and Zn, and there is no particular limitation on the
composition ratio thereof Further, the In--Ga--Zn--O-based oxide
semiconductor may contain an element other than In, Ga, and Zn.
[0129] For the oxide semiconductor layer 403, a thin film,
represented by the chemical formula, InMO.sub.3(ZnO).sub.m (m>0)
can be used. Here, M represents one or more metal elements selected
from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga
and Mn, Ga and Co, or the like.
[0130] In the transistors 410, 420, 430, and 440 each including the
oxide semiconductor layer 403, a current value in an off state
(off-state current value) can be reduced. Therefore, an electrical
signal of image data and the like can be held for a longer period,
so that a writing interval can be set longer. Accordingly,
frequency of refresh operation can be reduced, which leads to an
effect of suppressing power consumption.
[0131] Further, in the transistors 410, 420, 430, and 440 each
including the oxide semiconductor layer 403, relatively high
field-effect mobility can be obtained, whereby high-speed operation
is possible. Therefore, by using any of the transistors in a pixel
portion of a liquid crystal display device, high-quality image can
be provided. Since the transistors can be separately formed over
one substrate in a circuit portion and a pixel portion, the number
of components can be reduced in the liquid crystal display
device.
[0132] Although there is no particular limitation on a substrate
used for the substrate 400 having an insulating surface, a glass
substrate of barium borosilicate glass, aluminoborosilicate glass,
or the like is used.
[0133] In the bottom-gate transistors 410, 420, and 430, an
insulating film serving as a base film may be provided between the
substrate and the gate electrode layer. The base film has a
function of preventing diffusion of an impurity element from the
substrate, and can be formed to have a single-layer or
stacked-layer structure using one or more films selected from a
silicon nitride film, a silicon oxide film, a silicon nitride oxide
film, and a silicon oxynitride film.
[0134] The gate electrode layer 401 can be formed to have a
single-layer or stacked-layer structure using a metal material such
as molybdenum, titanium, chromium, tantalum, tungsten, aluminum,
copper, neodymium, or scandium, or an alloy material which contains
any of these materials as its main component.
[0135] The gate insulating layer 402 can be formed to have a
single-layer or stacked-layer structure using any of a silicon
oxide layer, a silicon nitride layer, a silicon oxynitride layer, a
silicon nitride oxide layer, an aluminum oxide layer, an aluminum
nitride layer, an aluminum oxynitride layer, an aluminum nitride
oxide layer, and a hafnium oxide layer by a plasma CVD method, a
sputtering method, or the like. For example, by a plasma CVD
method, a silicon nitride layer (SiN.sub.y (y>0)) with a
thickness of greater than or equal to 50 nm and less than or equal
to 200 nm is formed as a first gate insulating layer, and a silicon
oxide layer (SiO.sub.x (x>0)) with a thickness of greater than
or equal to 5 nm and less than or equal to 300 nm is formed as a
second gate insulating layer over the first gate insulating layer,
so that a gate insulating layer with a total thickness of 200 nm is
formed.
[0136] A conductive film used for the source electrode layer 405a
and the drain electrode layer 405b can be formed using an element
selected from Al, Cr, Cu, Ta, Ti, Mo, and W, an alloy film
containing any of these elements, an alloy film containing a
combination of any of these elements, or the like. Alternatively, a
structure may be employed in which a high-melting-point metal layer
of Ti, Mo, W, or the like is stacked over and/or below a metal
layer of Al, Cu, or the like. In addition, heat resistance can be
improved by using an Al material to which an element (Si, Nd, Sc,
or the like) which prevents generation of a hillock or a whisker in
an Al film is added.
[0137] A material similar to that of the source electrode layer
405a and the drain electrode layer 405b can be used for a
conductive film such as the wiring layer 436a and the wiring layer
436b which are connected to the source electrode layer 405a and the
drain electrode layer 405b, respectively.
[0138] Alternatively, the conductive film to be the source
electrode layer 405a and drain electrode layer 405b (including a
wiring layer formed in the same layer as the source and drain
electrode layers) may be fowled using conductive metal oxide. As
conductive metal oxide, indium oxide (In.sub.2O.sub.3), tin oxide
(SnO.sub.2), zinc oxide (ZnO), indium oxide-tin oxide alloy
(In.sub.2O.sub.3--SnO.sub.2, which is abbreviated to ITO), indium
oxide-zinc oxide alloy (In.sub.2O.sub.3--ZnO), or any of these
metal oxide materials in which silicon oxide is contained can be
used.
[0139] As the insulating layers 407, 427, and 437, typically, an
inorganic insulating film such as a silicon oxide film, a silicon
oxynitride film, an aluminum oxide film, or an aluminum oxynitride
film can be used.
[0140] As the protective insulating layer 409, an inorganic
insulating film such as a silicon nitride film, an aluminum nitride
film, a silicon nitride oxide film, or an aluminum nitride oxide
film can be used.
[0141] In addition, a planarization insulating film may be formed
over the protective insulating layer 409 in order to reduce surface
unevenness due to the transistor. As the planarization insulating
film, an organic material such as polyimide, acrylic, or
benzocyclobutene can be used. Other than such organic materials, it
is also possible to use a low-dielectric constant material (a low-k
material) or the like. Note that the planarization insulating film
may be formed by stacking a plurality of insulating films formed
using these materials.
[0142] Thus, in this embodiment, a high-performance liquid crystal
display device can be provided by using a transistor including an
oxide semiconductor layer.
Embodiment 5
[0143] In this embodiment, an example of a transistor including an
oxide semiconductor layer and an example of a method for
manufacturing the transistor including an oxide semiconductor layer
are described in detail with reference to FIGS. 10A to 10E. The
same portions as those in the above embodiments and portions having
functions similar to those of the portions in the above embodiments
and steps similar to those in the above embodiments may be handled
as in the above embodiments, and repeated description is omitted.
In addition, detailed description of the same portions is not
repeated.
[0144] FIGS. 10A to 10E illustrate an example of a cross-sectional
structure of a transistor. A transistor 510 illustrated in FIGS.
10A to 10E is a bottom-gate inverted-staggered thin film transistor
which is similar to the transistor 410 illustrated in FIG. 9A.
[0145] An oxide semiconductor used for a semiconductor layer in
this embodiment is an i-type (intrinsic) oxide semiconductor or a
substantially i-type (intrinsic) oxide semiconductor. The i-type
(intrinsic) oxide semiconductor or substantially i-type (intrinsic)
oxide semiconductor is obtained in such a manner that hydrogen,
which is an n-type impurity, is removed from an oxide
semiconductor, and the oxide semiconductor is highly purified so as
to contain as few impurities that are not main components of the
oxide semiconductor as possible. In other words, a highly-purified
i-type (intrinsic) semiconductor or a semiconductor close thereto
is obtained not by adding impurities but by removing impurities
such as hydrogen or water as much as possible. Accordingly, the
oxide semiconductor layer included in the transistor 510 is an
oxide semiconductor layer which is highly purified and made to be
electrically i-type (intrinsic).
[0146] In addition, a highly-purified oxide semiconductor includes
extremely few carriers (close to zero), and the carrier
concentration thereof is less than 1.times.10.sup.14 /cm.sup.3,
preferably less than 1.times.10.sup.12 /cm.sup.3, more preferably
less than 1.times.10.sup.11 /cm.sup.3.
[0147] Since the oxide semiconductor includes extremely few
carriers, off-state current can be reduced in the transistor. The
smaller the amount of off-state current is, the better.
[0148] Specifically, in the transistor including the oxide
semiconductor layer, off-state current density per micrometer in a
channel width at room temperature can be less than or equal to 10
aA/.mu.m (1.times.10.sup.-17 A/.mu.m), further less than or equal
to 1 aA/.mu.m (1.times.10.sup.-18 A/.mu.m), or still further less
than or equal to 10 zA/.mu.m (1.times.10.sup.-20 A/.mu.m).
[0149] When a transistor whose current value in an off state (an
off-state-current value) is extremely small is used as a transistor
in the pixel portion of Embodiment 1, refresh operation in a still
image region can be performed with a small number of times of
writing image data.
[0150] In addition, in the transistor 510 including the oxide
semiconductor layer, the temperature dependence of on-state current
is hardly observed, and off-state current remains extremely
small.
[0151] Steps of manufacturing the transistor 510 over a substrate
505 are described below with reference to FIGS. 10A to 10E.
[0152] First, a conductive film is formed over the substrate 505
having an insulating surface, and then, a gate electrode layer 511
is formed through a first photolithography step. Note that a resist
mask may be formed by an inkjet method. Formation of the resist
mask by an inkjet method needs no photomask; thus, manufacturing
cost can be reduced.
[0153] As the substrate 505 having an insulating surface, a
substrate similar to the substrate 400 described in Embodiment 4
can be used. In this embodiment, a glass substrate is used as the
substrate 505.
[0154] An insulating film serving as a base film may be provided
between the substrate 505 and the gate electrode layer 511. The
base film has a function of preventing diffusion of an impurity
element from the substrate 505, and can be formed with a
single-layer structure or a stacked structure using one or more of
a silicon nitride film, a silicon oxide film, a silicon nitride
oxide film, and a silicon oxynitride film.
[0155] In addition, the gate electrode layer 511 can be formed to
have a single-layer or stacked structure using a metal material
such as molybdenum, titanium, tantalum, tungsten, aluminum, copper,
neodymium, or scandium, or an alloy material which contains any of
these materials as its main component.
[0156] Next, a gate insulating layer 507 is formed over the gate
electrode layer 511. The gate insulating layer 507 can be formed to
have a single-layer structure or a stacked structure using a
silicon oxide layer, a silicon nitride layer, a silicon oxynitride
layer, a silicon nitride oxide layer, an aluminum oxide layer, an
aluminum nitride layer, an aluminum oxynitride layer, an aluminum
nitride oxide layer, or a hafnium oxide layer, by a plasma CVD
method, a sputtering method, or the like.
[0157] As the oxide semiconductor layer in this embodiment, an
oxide semiconductor which is made to be an i-type or substantially
i-type by removing impurities is used. Such a highly-purified oxide
semiconductor is extremely sensitive to an interface level or
interface charge; therefore, an interface between the oxide
semiconductor layer and the gate insulating layer is important. For
that reason, the gate insulating layer that is to be in contact
with a highly-purified oxide semiconductor needs to have high
quality.
[0158] For example, a high-density plasma CVD method using
microwaves (e.g., a frequency of 2.45 GHz) is preferably adopted
because an insulating layer can be dense and can have high
withstand voltage and high quality. When a highly-purified oxide
semiconductor and a high-quality gate insulating layer are in close
contact with each other, the interface level can be reduced and
interface characteristics can be favorable.
[0159] It is needless to say that another deposition method such as
a sputtering method or a plasma CVD method can be employed as long
as a high-quality insulating layer can be formed as a gate
insulating layer. Moreover, it is possible to use as the gate
insulating layer an insulating layer whose quality and
characteristics of an interface with an oxide semiconductor are
improved with heat treatment performed after the formation of the
insulating layer. In any case, an insulating layer that can reduce
interface level density with an oxide semiconductor to form a
favorable interface, as well as having favorable film quality as
the gate insulating layer, is formed.
[0160] Further, in order that hydrogen, a hydroxyl group, and
moisture might be contained in the gate insulating layer 507 and an
oxide semiconductor film 530 as little as possible, it is
preferable that the substrate 505 over which the gate electrode
layer 511 is formed or the substrate 505 over which layers up to
the gate insulating layer 507 are formed be preheated in a
preheating chamber of a sputtering apparatus as pretreatment for
deposition of the oxide semiconductor film 530 so that impurities
such as hydrogen and moisture adsorbed to the substrate 505 are
eliminated and exhaustion is performed. As an exhaustion unit
provided in the preheating chamber, a cryopump is preferable. Note
that this preheating treatment can be omitted. This preheating step
may be similarly performed on the substrate 505 over which
components up to and including a source electrode layer 515a and a
drain electrode layer 515b are formed before formation of an
insulating layer 516.
[0161] Next, the oxide semiconductor film 530 having a thickness of
greater than or equal to 2 nm and less than or equal to 200 nm,
preferably greater than or equal to 5 nm and less than or equal to
30 nm is formed over the gate insulating layer 507 (see FIG.
10A).
[0162] Note that before the oxide semiconductor film 530 is formed
by a sputtering method, powder substances (also referred to as
particles or dust) which are generated at the time of the
deposition and attached on a surface of the gate insulating layer
507 are preferably removed by reverse sputtering in which an argon
gas is introduced and plasma is generated. The reverse sputtering
refers to a method in which, without application of a voltage to a
target side, an RF power source is used for application of a
voltage to a substrate side in an argon atmosphere to generate
plasma in the vicinity of the substrate to modify a surface. Note
that instead of an argon atmosphere, a nitrogen atmosphere, a
helium atmosphere, an oxygen atmosphere, or the like may be
used.
[0163] As an oxide semiconductor used for the oxide semiconductor
film 530, an oxide semiconductor described in Embodiment 4, such as
an oxide of four metal elements, an oxide of three metal elements,
an oxide of two metal elements, an In--O-based oxide semiconductor,
a Sn--O-based oxide semiconductor, or a Zn--O-based oxide
semiconductor can be used. Further, SiO.sub.2 may be contained in
the above oxide semiconductor. In this embodiment, the oxide
semiconductor film 530 is deposited by sputtering with the use of
an In--Ga--Zn--O-based oxide semiconductor target. A
cross-sectional view of this stage is shown in FIG. 10A.
Alternatively, the oxide semiconductor film 530 can be formed by a
sputtering method in a rare gas (typically, argon) atmosphere, an
oxygen atmosphere, or a mixed atmosphere of a rare gas and
oxygen.
[0164] As a target for manufacturing the oxide semiconductor film
530 by a sputtering method, for example, a target having a
composition ratio of In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1
[molar ratio] can be used. Alternatively, a target having a
composition ratio of In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:2
[molar ratio] or In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:4 [molar
ratio] may be used. The fill rate of the oxide target is higher
than or equal to 90% and lower than or equal to 100%, preferably,
higher than or equal to 95% and lower than or equal to 99.9%. With
use of the metal oxide target with high filling rate, the deposited
oxide semiconductor film has high density.
[0165] It is preferable that a high-purity gas in which an impurity
such as hydrogen, water, a hydroxyl group, or hydride is removed be
used as the sputtering gas for the deposition of the oxide
semiconductor film 530.
[0166] The substrate is placed in a deposition chamber under
reduced pressure, and the substrate temperature is set to a
temperature higher than or equal to 100.degree. C. and lower than
or equal to 600.degree. C., preferably higher than or equal to
200.degree. C. and lower than or equal to 400.degree. C. Deposition
is performed while the substrate is heated, whereby the
concentration of an impurity contained in the oxide semiconductor
layer formed can be reduced. In addition, damage by sputtering can
be reduced. Then, residual moisture in the deposition chamber is
removed, a sputtering gas from which hydrogen and moisture are
removed is introduced, and the above-described target is used, so
that the oxide semiconductor film 530 is formed over the substrate
505. In order to remove the residual moisture in the deposition
chamber, an entrapment vacuum pump, for example, a cryopump, an ion
pump, or a titanium sublimation pump is preferably used. The
evacuation unit may be a turbo pump provided with a cold trap. In
the deposition chamber which is evacuated with the cryopump, for
example, a hydrogen atom, a compound containing a hydrogen atom,
such as water (H.sub.2O), (more preferably, also a compound
containing a carbon atom), and the like are removed, whereby the
concentration of an impurity in the oxide semiconductor film formed
in the deposition chamber can be reduced.
[0167] As one example of the deposition condition, the distance
between the substrate and the target is 100 mm, the pressure is 0.6
Pa, the direct-current (DC) power source is 0.5 kW, and the
atmosphere is an oxygen atmosphere (the proportion of the oxygen
flow rate is 100%). Note that a pulse direct current power source
is preferable because powder substances (also referred to as
particles or dust) generated in deposition can be reduced and the
film thickness can be uniform.
[0168] Next, the oxide semiconductor film 530 is processed into an
island-shaped oxide semiconductor layer through a second
photolithography step. A resist mask for forming the island-shaped
oxide semiconductor layer may be formed by an inkjet method.
Formation of the resist mask by an inkjet method needs no
photomask; thus, manufacturing cost can be reduced.
[0169] In the case where a contact hole is formed in the gate
insulating layer 507, a step of forming the contact hole can be
performed at the same time as processing of the oxide semiconductor
film 530.
[0170] For the etching of the oxide semiconductor film 530, either
one or both of wet etching and dry etching may be employed. As an
etchant used for wet etching of the oxide semiconductor film 530,
for example, a mixed solution of phosphoric acid, acetic acid, and
nitric acid, or the like can be used. In addition, ITO07N (produced
by Kanto Chemical Co., Inc.) may also be used.
[0171] Next, first heat treatment is performed on the oxide
semiconductor layer. The oxide semiconductor layer can be
dehydrated or dehydrogenated by this first heat treatment. The
temperature of the first heat treatment is higher than or equal to
400.degree. C. and lower than or equal to 750.degree. C., or higher
than or equal to 400.degree. C. and lower than the strain point of
the substrate. Here, the substrate is put in an electric furnace
which is a kind of heat treatment apparatus and heat treatment is
performed on the oxide semiconductor layer at 450.degree. C. for
one hour in a nitrogen atmosphere, and then, water or hydrogen is
prevented from entering the oxide semiconductor layer without
exposure to the air; thus, an oxide semiconductor layer 531 is
obtained (see FIG. 10B).
[0172] Note that a heat treatment apparatus is not limited to an
electrical furnace, and may include a device for heating an object
to be processed by heat conduction or heat radiation from a heating
element such as a resistance heating element. For example, a rapid
thermal anneal (RTA) apparatus such as a gas rapid thermal anneal
(GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus
can be used. An LRTA apparatus is an apparatus for heating an
object to be processed by radiation of light (an electromagnetic
wave) emitted from a lamp such as a halogen lamp, a metal halide
lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium
lamp, or a high pressure mercury lamp. A GRTA apparatus is an
apparatus for heat treatment using a high-temperature gas. As the
high temperature gas, an inert gas which does not react with an
object to be treated by heat treatment, such as nitrogen or a rare
gas like argon, is used.
[0173] For example, as the first heat treatment, GRTA in which the
substrate is moved into an inert gas heated to a high temperature
as high as 650.degree. C. to 700.degree. C., heated for several
minutes, and moved out of the inert gas heated to the high
temperature may be performed.
[0174] Note that in the first heat treatment, it is preferable that
water, hydrogen, and the like be not contained in the atmosphere of
nitrogen or a rare gas such as helium, neon, or argon. It is
preferable that the purity of nitrogen or the rare gas such as
helium, neon, or argon which is introduced into a heat treatment
apparatus be set to be 6N (99.9999%) or higher, preferably 7N
(99.99999%) or higher (that is, the impurity concentration is 1 ppm
or lower, preferably 0.1 ppm or lower).
[0175] Further, after the oxide semiconductor layer is heated in
the first heat treatment, a high-purity oxygen gas, a high-purity
N.sub.2O gas, or an ultra-dry air (the dew point is lower than or
equal to -40.degree. C., preferably lower than or equal to
-60.degree. C.) may be introduced into the same furnace. It is
preferable that water, hydrogen, and the like be not contained in
an oxygen gas or an N.sub.2O gas. The purity of the oxygen gas or
the N.sub.2O gas which is introduced into the heat treatment
apparatus is preferably 6N (99.9999%) or more, more preferably 7N
(99.99999%) or more (i.e., the concentration of impurities in the
oxygen gas or the N.sub.2O gas is preferably 1 ppm or less, more
preferably 0.1 ppm or less). By the action of the oxygen gas or the
N.sub.2O gas, oxygen which is a main component included in the
oxide semiconductor and which has been reduced at the same time as
the step for removing impurities by dehydration or dehydrogenation
is supplied, so that the oxide semiconductor layer can be a
highly-purified and electrically i-type (intrinsic) oxide
semiconductor.
[0176] In addition, the first heat treatment of the oxide
semiconductor layer can also be performed on the oxide
semiconductor film 530 which has not yet been processed into the
island-shaped oxide semiconductor layer. In that case, the
substrate is taken out from the heat apparatus after the first heat
treatment, and then a photolithography step is performed.
[0177] Note that the first heat treatment may be performed at any
of the following timings in addition to the above timing as long as
after deposition of the oxide semiconductor layer: after a source
electrode layer and a drain electrode layer are formed over the
oxide semiconductor layer and after an insulating layer is formed
over the source electrode layer and the drain electrode layer.
[0178] Further, the step of forming the contact hole in the gate
insulating layer 507 may be performed either before or after the
first heat treatment is performed on the semiconductor film
530.
[0179] In addition, as the oxide semiconductor layer, an oxide
semiconductor layer having a crystal region with a large thickness
(a single crystal region), that is, a crystal region which is
c-axis-aligned perpendicularly to a surface of the film may be
formed by performing deposition twice and heat treatment twice,
even when any of an oxide, a nitride, a metal, or the like is used
for a material of a base component. For example, a first oxide
semiconductor film with a thickness greater than or equal to 3 nm
and less than or equal to 15 nm is deposited, and first heat
treatment is performed in a nitrogen, an oxygen, a rare gas, or a
dry air atmosphere at a temperature higher than or equal to
450.degree. C. and lower than or equal to 850.degree. C. or
preferably higher than or equal to 550.degree. C. and lower than or
equal to 750.degree. C., so that a first oxide semiconductor film
having a crystal region (including a plate-like crystal) in a
region including a surface is formed. Then, a second oxide
semiconductor film which has a larger thickness than the first
oxide semiconductor film is formed, and second heat treatment is
performed at a temperature higher than or equal to 450.degree. C.
and lower than or equal to 850.degree. C. or preferably higher than
or equal to 600.degree. C. and lower than or equal to 700.degree.
C., so that crystal growth proceeds upward with the use of the
first oxide semiconductor film as a seed of the crystal growth and
the whole second oxide semiconductor film is crystallized. In such
a manner, the oxide semiconductor layer having a crystal region
having a large thickness may be formed.
[0180] Next, a conductive film serving as the source and drain
electrode layers (including a wiring formed in the same layer as
the source and drain electrode layers) is formed over the gate
insulating layer 507 and the oxide semiconductor layer 531. As the
conductive film serving as the source and drain electrode layers,
the material used for the source electrode layer 405a and the drain
electrode layer 405b which is described in Embodiment 4 can be
used.
[0181] A resist mask is formed over the conductive film through a
third photolithography step, and the source electrode layer 515a
and the drain electrode layer 515b are formed by selective etching,
and then, the resist mask is removed (see FIG. 10C).
[0182] Light exposure at the time of the formation of the resist
mask in the third photolithography step may be performed using
ultraviolet light, KrF laser light, or ArF laser light. A channel
length L of a transistor that is completed later is determined by a
distance between bottom end portions of the source electrode layer
and the drain electrode layer, which are adjacent to each other
over the oxide semiconductor layer 531. In the case where light
exposure is performed for a channel length L of less than 25 nm,
the light exposure at the time of the formation of the resist mask
in the third photolithography step may be performed using extreme
ultraviolet having an extremely short wavelength of several
nanometers to several tens of nanometers. Light exposure with
extreme ultraviolet leads to a high resolution and a large depth of
focus. Thus, the channel length L of the transistor that is
completed later can be greater than or equal to 10 nm and less than
or equal to 1000 nm and the operation speed of a circuit can be
increased and furthermore the value of off-state current is
extremely small, so that low power consumption can be achieved.
[0183] In order to reduce the number of photomasks used in a
photolithography step and reduce the number of photolithography
steps, an etching step may be performed with the use of a
multi-tone mask which is a light-exposure mask through which light
is transmitted to have a plurality of intensities. A resist mask
formed with the use of a multi-tone mask has a plurality of
thicknesses and further can be changed in shape by etching;
therefore, the resist mask can be used in a plurality of etching
steps for processing into different patterns. Therefore, a resist
mask corresponding to at least two kinds or more of different
patterns can be formed by one multi-tone mask. Thus, the number of
light-exposure masks can be reduced and the number of corresponding
photolithography steps can be also reduced, whereby simplification
of a process can be realized.
[0184] Note that it is preferable that etching conditions be
optimized so as not to etch and divide the oxide semiconductor
layer 531 when the conductive film is etched. However, it is
difficult to obtain etching conditions in which only the conductive
film is etched and the oxide semiconductor layer 531 is not etched
at all. In some cases, only part of the oxide semiconductor layer
531 is etched to be an oxide semiconductor layer having a groove
portion (a recessed portion) when the conductive film is
etched.
[0185] In this embodiment, since the Ti film is used as the
conductive film and the In--Ga--Zn--O-based oxide semiconductor is
used as the oxide semiconductor layer 531, an ammonium
hydroxide/hydrogen peroxide mixture (a 31 wt % hydrogen peroxide
solution:28 wt % ammonia water:water=5:2:2) is used as an etchant
for etching only the conductive film.
[0186] Next, by plasma treatment using a gas such as N.sub.2O,
N.sub.2, or Ar, water or the like adsorbed to a surface of an
exposed portion of the oxide semiconductor layer may be removed. In
the case where the plasma treatment is performed, the insulating
layer 516 is preferably formed without exposure to the air as a
protective insulating film in contact with part of the oxide
semiconductor layer.
[0187] The insulating layer 516 can be formed to a thickness of at
least 1 nm by a method by which an impurity such as water or
hydrogen does not enter the insulating layer 516, such as a
sputtering method as appropriate. When hydrogen is contained in the
insulating layer 516, entry of the hydrogen to the oxide
semiconductor layer, or extraction of oxygen in the oxide
semiconductor layer by hydrogen may occur, thereby causing the
backchannel of the oxide semiconductor layer to have lower
resistance (to be n-type), so that a parasitic channel may be
formed. Therefore, it is important that a deposition method in
which hydrogen is not used is employed in order to form the
insulating layer 516 containing as little hydrogen as possible.
[0188] In this embodiment, a silicon oxide film is formed to a
thickness of 200 nm as the insulating layer 516 with a sputtering
method. The substrate temperature in deposition may be higher than
or equal to room temperature and lower than or equal to 300.degree.
C. and in this embodiment, is 100.degree. C. The silicon oxide film
can be deposited by a sputtering method in a rare gas (typically,
argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere
containing a rare gas and oxygen. As a target, a silicon oxide
target or a silicon target may be used. For example, the silicon
oxide film can be formed using a silicon target by a sputtering
method in an atmosphere containing oxygen. As the insulating layer
516 which is formed in contact with the oxide semiconductor layer,
an inorganic insulating film which does not include impurities such
as moisture, a hydrogen ion, and OH.sup.- and blocks entry of these
from the outside is used. Typically, a silicon oxide film, a
silicon oxynitride film, an aluminum oxide film, an aluminum
oxynitride film, or the like is used.
[0189] In order to remove residual moisture in the deposition
chamber of the insulating layer 516 as in the case of the
deposition of the oxide semiconductor film 530, an entrapment
vacuum pump (such as a cryopump) is preferably used. When the
insulating layer 516 is deposited in the deposition chamber
evacuated using a cryopump, the impurity concentration in the
insulating layer 516 can be reduced. In addition, as an exhaustion
unit for removing the residual moisture in the deposition chamber
of the insulating layer 516, a turbo pump provided with a cold trap
may be used.
[0190] It is preferable that a high-purity gas in which an impurity
such as hydrogen, water, a hydroxyl group, or hydride is removed be
used as the sputtering gas for the deposition of the insulating
layer 516.
[0191] Next, second heat treatment is performed in an inert gas
atmosphere or oxygen gas atmosphere (preferably at a temperature
higher than or equal to 200.degree. C. and lower than or equal to
400.degree. C., for example, higher than or equal to 250.degree. C.
and lower than or equal to 35020 C.). For example, the second heat
treatment is performed in a nitrogen atmosphere at 250.degree. C.
for one hour. In the second heat treatment, part of the oxide
semiconductor layer (a channel formation region) is heated while
being in contact with the insulating layer 516.
[0192] Through the above process, the first heat treatment is
performed on the oxide semiconductor film so that an impurity such
as hydrogen, moisture, a hydroxyl group, or hydride (also referred
to as a hydrogen compound) is intentionally removed from the oxide
semiconductor layer. Additionally, oxygen which is one of main
components of an oxide semiconductor and is simultaneously reduced
in a step of removing an impurity can be supplied. Accordingly, the
oxide semiconductor layer is made to be a highly purified and
electrically i-type (intrinsic) oxide semiconductor.
[0193] Through the above process, the transistor 510 is formed
(FIG. 10D).
[0194] When a silicon oxide layer having a lot of defects is used
as the oxide insulating layer, heat treatment after formation of
the silicon oxide layer has an effect in diffusing an impurity such
as hydrogen, moisture, a hydroxyl group, or hydride contained in
the oxide semiconductor layer to the oxide insulating layer so that
the impurity contained in the oxide semiconductor layer can be
further reduced.
[0195] A protective insulating layer 506 may be formed over the
insulating layer 516. For example, a silicon nitride film is formed
by an RF sputtering method. Since an RF sputtering method has high
productivity, it is preferably used as a deposition method of the
protective insulating layer. As the protective insulating layer, an
inorganic insulating film which does not include an impurity such
as moisture and prevents entry of these from the outside, such as a
silicon nitride film or an aluminum nitride film is used. In this
embodiment, a protective insulating layer is formed using a silicon
nitride film as the protective insulating layer 506 (see FIG.
10E).
[0196] In this embodiment, as the protective insulating layer 506,
a silicon nitride film is formed by heating the substrate 505 over
which layers up to the insulating layer 516 are formed, to a
temperature of 100.degree. C. to 400.degree. C., introducing a
sputtering gas containing high-purity nitrogen from which hydrogen
and moisture are removed, and using a target of silicon
semiconductor. In this case, the protective insulating layer 506 is
preferably deposited removing moisture remaining in a treatment
chamber, similarly to the insulating layer 516.
[0197] After the formation of the protective insulating layer, heat
treatment may be further performed at a temperature higher than or
equal to 100.degree. C. and lower than or equal to 200.degree. C.
in the air for greater than or equal to 1 hour and less than or
equal to 30 hours. This heat treatment may be performed at a fixed
heating temperature. Alternatively, the following change in the
heating temperature may be conducted plural times repeatedly: the
heating temperature is increased from a room temperature to a
temperature of 100.degree. C. to 200.degree. C. and then decreased
to a room temperature.
[0198] In this manner, with the use of the transistor including a
highly-purified oxide semiconductor layer manufactured using this
embodiment, the value of current in an off state (an off-state
current value) can be further reduced. Accordingly, an electric
signal such as image data can be held for a longer period and a
writing interval can be set longer. Therefore, the frequency of
refresh operation can be reduced, which leads to a higher effect of
suppressing power consumption.
[0199] In addition, since the transistor including a
highly-purified oxide semiconductor layer has high field-effect
mobility, high-speed operation is possible. Accordingly, by using
the transistor in a pixel portion of a liquid crystal display
device, a high-quality image can be provided. In addition, since
the transistor can be separately formed in a driver circuit portion
and a pixel portion over one substrate, the number of components of
the liquid crystal display device can be reduced.
[0200] This embodiment can be implemented combining with another
embodiment as appropriate.
Embodiment 6
[0201] In this embodiment, a pixel structure which enables increase
in the amount of reflected light and transmitted light per one
pixel in a semi-transmissive liquid crystal display device is
described with reference to FIG. 11, FIGS. 12A to 12D, and FIG.
13.
[0202] FIG. 11 is a view illustrating a plan structure of a pixel
described in this embodiment. FIGS. 12A and 12B illustrate
cross-sectional structures of a portion along X1-X2 and a portion
along Y1-Y2 respectively, denoted by dashed lines in FIG. 11. In a
pixel described in this embodiment, a transparent electrode 823 and
a reflective electrode 825 are stacked as a pixel electrode over a
substrate 800. The pixel electrode is connected to a drain
electrode 857 of a transistor 851 through a contact hole 855
provided in an insulating film 827, an insulating film 828, and an
organic resin film 822. The drain electrode 857 overlaps with a
capacitor wiring 853 with the insulating film 827 and the
insulating film 828 provided therebetween, so that a storage
capacitor 871 is formed (see FIG. 12A).
[0203] A gate electrode 858 of the transistor 851 is connected to a
wiring 852, and a source electrode 856 of the transistor 851 is
connected to a wiring 854. The transistor described in other
embodiments can be used as the transistor 851.
[0204] External light is reflected by the reflective electrode 825,
so that the pixel electrode can function as a pixel electrode of a
reflective liquid crystal display device. The reflective electrode
825 is provided with a plurality of openings 826. In the opening
826, the reflective electrode 825 does not exist, and a structure
820 and the transparent electrode 823 are projected. Light from the
backlight is transmitted through the opening 826, so that the pixel
electrode can function as a pixel electrode of a transmissive
liquid crystal display device.
[0205] FIG. 13 is a cross-sectional view illustrating an example
different from that in FIG. 12B, which is one embodiment of the
present invention having a structure in which the structure 820 and
the transparent electrode 823 are not projected in the opening 826.
In FIG. 12B, a backlight exit 841 and the opening 826 have almost
the same size, whereas in FIG. 13, the backlight exit 841 and the
opening 826 have different sizes and different distances from a
backlight entrance 842. Accordingly, the area of a transmissive
region can be made larger in FIG. 12B than in FIG. 13, and it can
be said that the cross-sectional shape in FIG. 12B is
preferable.
[0206] The structure 820 is formed in a lower layer of the opening
826 to overlap with the opening 826 with the organic resin film 822
provided therebetween. FIG. 12B is a cross-sectional view of the
portion along Y1-Y2 in FIG. 11, which illustrates the structures of
the pixel electrode and the structure 820. FIG. 12C is an enlarged
view of a portion 880, and FIG. 12D is an enlarged view of a
portion 881.
[0207] Reflected light 832 is external light reflected at the
reflective electrode 825. The top surface of the organic resin film
822 is a curving surface with an uneven shape. By reflecting the
curving surface with an uneven shape on the reflective electrode
825, the area of the reflective region can be increased, and
reflection of an object other than the displayed image is reduced
so that visibility of the displayed image can be improved. In the
cross-sectional shape, the angle .theta.R at a point where the
reflective electrode 825 having a curving surface is most curved,
formed by two inclined planes facing each other may be greater than
or equal to 90.degree., preferably greater than or equal to
100.degree. and less than or equal to 120.degree. (see FIG.
12D).
[0208] The structure 820 includes the backlight exit 841 on the
opening 826 side and the backlight entrance 842 on a backlight (not
illustrated) side. The upper portion of the structure 820 is
positioned above the surface of the reflective electrode 825 and
protrudes from the end portion of the reflective electrode; that
is, the distance H between the top surface of the structure 820 and
the upper end portion of the reflective electrode is greater than
or equal to 0.1 .mu.m and less than or equal to 3 .mu.m, preferably
greater than or equal to 0.3 .mu.m and less than or equal to 2
.mu.m. The backlight entrance 842 is formed to have a larger area
than that of the backlight exit 841. A reflective layer 821 is
formed on the side surfaces of the structure 820 (surfaces on which
the backlight exit 841 and the backlight entrance 842 are not
formed). The structure 820 can be formed using a material having a
light-transmitting property such as silicon oxide (SiOx), silicon
nitride (SiNx), or silicon oxynitride (SiNO). The reflective layer
821 can be formed using a material with high light reflectance such
as aluminum (Al) or silver (Ag).
[0209] Transmitted light 831 emitted from the backlight enters the
structure 820 through the backlight entrance 842. Some of the
incident transmitted light 831 is directly emitted from the
backlight exit 841, some is reflected toward the backlight exit 841
by the reflective layer 821, and some is further reflected to
return to the backlight entrance 842.
[0210] At this time, according to the shape of a cross section of
the structure 820 through the backlight exit 841 to the backlight
entrance 842, side surfaces on right and left facing each other are
inclined surfaces. The angle .theta.T formed by the side surfaces
is made to be less than 90.degree., preferably greater than or
equal to 10.degree. and less than or equal to 60.degree., so that
the transmitted light 831 incident from the backlight entrance 842
can be guided efficiently to the backlight exit 841.
[0211] In a conventional semi-transmissive liquid crystal display
device, when the area of an electrode in a pixel electrode,
functioning as a reflective electrode is SR and the area of an
electrode in a pixel electrode, functioning as a transmissive
electrode (the area of the opening 826) is ST, the proportion of
the total area of both electrodes is 100% (SR+ST=100%). In a
semi-transmissive liquid crystal display device having a pixel
structure described in this embodiment, since the area ST of the
electrode functioning as a transmissive electrode corresponds to
the area of the backlight entrance 842, the amount of transmitted
light can be increased without increasing the area of the opening
826 or the luminance of the backlight. In other words, the
proportion of the total area of both electrodes in appearance can
be 100% or more (the total of SR+ST is 100% or more).
[0212] By using this embodiment, a semi-transmissive liquid crystal
display device with bright and high-quality display can be obtained
without increasing power consumption.
Embodiment 7
[0213] In this embodiment, an example of an electronic device
including the liquid crystal display device described in any of the
above embodiments is described.
[0214] FIG. 14A illustrates an electronic book reader (also
referred to as an e-book reader) which can include housings 9630, a
display portion 9631, operation keys 9632, a solar battery 9633,
and a charge and discharge control circuit 9634. The electronic
book reader is provided with the solar battery 9633 and a display
panel so that the solar battery 9633 and the display panel can be
opened and closed freely. In the electronic book reader, power from
the solar battery is supplied to the display panel, a backlight
portion, or an image processing circuit. The electronic book reader
illustrated in FIG. 14A has a function of displaying various kinds
of information (e.g., a still image, a moving image, and a text
image) on the display portion, a function of displaying a calendar,
a date, the time, or the like on the display portion, a function of
operating or editing the information displayed on the display
portion, a function of controlling processing by various kinds of
software (programs), and the like. Note that in FIG. 14A, a
structure including a battery 9635 and a DCDC converter
(hereinafter abbreviated as a converter 9636) is illustrated as an
example of the charge and discharge control circuit 9634.
[0215] When a semi-transmissive liquid crystal display device is
used as the display portion 9631, in the case where use under a
relatively bright condition is assumed, the structure illustrated
in FIG. 14A is preferable because power generation by the solar
battery 9633 and charge in the battery 9635 are effectively
performed. Note that a structure in which the solar battery 9633 is
provided on each of a surface and a rear surface of the housing
9630 is preferable in order to charge the battery 9635 efficiently.
When a lithium ion battery is used as the battery 9635, there is an
advantage of downsizing or the like.
[0216] The structure and the operation of the charge and discharge
control circuit 9634 illustrated in FIG. 14A are described with
reference to a block diagram in FIG. 14B. The solar battery 9633,
the battery 9635, the converter 9636, the converter 9637, switches
SW1 to SW3, and the display portion 9631 are shown in FIG. 14B, and
the battery 9635, the converter 9636, the converter 9637, and the
switches SW1 to SW3 correspond to the charge and discharge control
circuit 9634.
[0217] First, an example of operation in the case where power is
generated by the solar battery 9633 using external light is
described. The voltage of power generated by the solar battery is
raised or lowered by the converter 9636 so that the power has a
voltage for charging the battery 9635. Then, when the power from
the solar battery 9633 is used for the operation of the display
portion 9631, the switch SW1 is turned on and the voltage of the
power is raised or lowered by the converter 9637 so as to be a
voltage needed for the display portion 9631. In addition, when
display on the display portion 9631 is not performed, the switch
SW1 is turned off and a switch SW2 is turned on so that charge of
the battery 9635 may be performed.
[0218] Next, operation in the case where power is not generated by
the solar battery 9633 using external light is described. The
voltage of power accumulated in the battery 9635 is raised or
lowered by the converter 9637 by turning on the switch SW3. Then,
power from the battery 9635 is used for the operation of the
display portion 9631.
[0219] Note that although the solar battery 9633 is described as an
example of a means for charge, charge of the battery 9635 may be
performed with another means. In addition, a combination of the
solar battery 9633 and another means for charge may be used.
[0220] This embodiment can be implemented in appropriate
combination with any of the structures described in the other
embodiments.
[0221] This application is based on Japanese Patent Application
serial no. 2009-298290 filed with Japan Patent Office on Dec. 28,
2009, the entire contents of which are hereby incorporated by
reference.
* * * * *