Display device

A display device having high display quality is provided. In the display device including a plurality of pixels, adjacent pixel electrodes are formed over different insulating layers. Accordingly, when seen in a plan view, the adjacent pixel electrodes can be close to each other without constraints of design rules. Openings (light-emitting regions) of the adjacent pixels can be close to each other, leading to an improvement in graininess of an image. With the use of a step provided between the adjacent pixel electrodes, the resistance of an EL layer across the adjacent pixels can be increased to reduce crosstalk.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. One embodiment of the invention disclosed in this specification and the like also relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the invention disclosed in this specification and the like relates to a display device and a manufacturing method thereof.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device (e.g., a liquid crystal display device and a light-emitting display device), a projection device, a lighting device, an electro-optical device, a power storage device, a memory device, a semiconductor circuit, an imaging device, an electronic device, and the like may be referred to as a semiconductor device. Alternatively, they may include a semiconductor device.

2. Description of the Related Art

An active-matrix display device where a transistor for driving a display element is provided in each pixel is known. For example, an active-matrix liquid crystal display device that includes a liquid crystal element as a display element, an active-matrix light-emitting display device that includes a light-emitting element, such as an organic electroluminescent (EL) element, as a display element, and the like are known. These active-matrix display devices are easier to increase in display size or definition than simple-matrix display devices, and have an advantage in reduced power consumption and the like.

Patent Document 1 discloses a light-emitting display device including an organic EL element as a display element.

REFERENCE

Patent Document

SUMMARY OF THE INVENTION

In recent years, demands for display devices with higher definition and higher aperture ratio are increasing. In addition, display devices are required to have high reliability and be produced at low cost.

An object of one embodiment of the present invention is to provide a display device, electronic device, or the like having favorable visibility. Another object of one embodiment of the present invention is to provide a display device, electronic device, or the like having high display quality. Another object of one embodiment of the present invention is to provide a display device, electronic device, or the like with low power consumption. Another object of one embodiment of the present invention is to provide a display device, electronic device, or the like with high productivity. Another object of one embodiment of the present invention is to provide a display device, electronic device, or the like having high reliability. Another object of one embodiment of the present invention is to provide a novel display device, electronic device, or the like.

In a display device including a plurality of pixels, adjacent pixel electrodes are formed over different insulating layers. Accordingly, when seen in a plan view, the adjacent pixel electrodes can be close to each other without constraints of design rules. Openings (light-emitting regions) of the adjacent pixels can be close to each other, leading to an improvement in graininess of an image. With the use of a step provided between the adjacent pixel electrodes, the resistance of an EL layer across the adjacent pixels can be increased to reduce crosstalk.

One embodiment of the present invention is a display device including a first electrode, a second electrode, a third electrode, a first insulating layer, a second insulating layer, a third insulating layer, and an organic layer. The first insulating layer includes a region covered with the second insulating layer. The first electrode is provided over the first insulating layer. The second electrode is provided over the second insulating layer. The second insulating layer includes a region covering an end portion of the first electrode. The third insulating layer includes a region in contact with part of the first electrode and a region covering an end portion of the second electrode. The organic layer includes a first region overlapping with the first electrode and a second region overlapping with the second electrode. The third electrode includes a region overlapping with the first electrode with the first region positioned therebetween and a region overlapping with the second electrode with the second region positioned therebetween. The organic layer has a function of emitting visible light.

One embodiment of the present invention is a display device including a first transistor, a second transistor, a first electrode, a second electrode, a third electrode, a first insulating layer, a second insulating layer, a third insulating layer, and an organic layer. The first transistor and the second transistor are covered with the first insulating layer. The first insulating layer includes a region covered with the second insulating layer. The first electrode is provided over the first insulating layer. The second electrode is provided over the second insulating layer. The second insulating layer includes a region covering an end portion of the first electrode. The third insulating layer includes a region in contact with part of the first electrode and a region covering an end portion of the second electrode. The organic layer includes a first region overlapping with the first electrode and a second region overlapping with the second electrode. The third electrode includes a region overlapping with the first electrode with the first region positioned therebetween, a region overlapping with the second electrode with the second region positioned therebetween, and a region overlapping with the third insulating layer. The first electrode is electrically connected to the first transistor. The second electrode is electrically connected to the second transistor. The organic layer has a function of emitting visible light.

The first transistor and the second transistor each preferably include an oxide semiconductor in a semiconductor layer in which a channel is formed. The organic layer preferably includes a light-emitting substance.

The display device of one embodiment of the present invention preferably includes coloring layers having a function of changing a hue of the visible light. For example, a first coloring layer is preferably provided in a region overlapping with the first region. A second coloring layer is preferably provided in a region overlapping with the second region.

One embodiment of the present invention can provide a display device, electronic device, or the like having favorable visibility. One embodiment of the present invention can provide a display device, electronic device, or the like having high display quality. One embodiment of the present invention can provide a display device, electronic device, or the like with low power consumption. One embodiment of the present invention can provide a display device, electronic device, or the like with high productivity. One embodiment of the present invention can provide a display device, electronic device, or the like having high reliability. One embodiment of the present invention can provide a novel display device, electronic device, or the like.

DETAILED DESCRIPTION OF THE INVENTION

The position, size, range, and the like of each component illustrated in the drawings and the like are not accurately represented in some cases to facilitate understanding of the invention. Therefore, the disclosed invention is not necessarily limited to the position, size, range, and the like disclosed in the drawings and the like. For example, in the actual manufacturing process, a layer, a resist mask, or the like might be unintentionally reduced in size by treatment such as etching, which is not illustrated in some cases for easy understanding of the present invention.

Especially in a top view (also referred to as a “plan view”), a perspective view, or the like, some components might not be illustrated for easy understanding of the invention. In addition, some hidden lines and the like might not be shown.

Ordinal numbers such as “first” and “second” in this specification and the like are used in order to avoid confusion among components and do not denote the priority or the order such as the order of steps or the stacking order. A term without an ordinal number in this specification and the like might be provided with an ordinal number in a claim in order to avoid confusion among components. A different ordinal number from an ordinal number used to denote a component in this specification and the like might be used to denote the component in a claim. Moreover, a term with an ordinal number in this specification and the like might not be provided with any ordinal number in a claim and the like.

In addition, in this specification and the like, the term such as an “electrode” or a “wiring” does not limit a function of the component. For example, an “electrode” is used as part of a “wiring” in some cases, and vice versa. Furthermore, the term “electrode” or “wiring” can also mean a combination of a plurality of “electrodes” and “wirings” provided in an integrated manner.

Note that the channel length refers to, for example, the distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a plan view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not fixed to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value, in a region where a channel is formed.

The channel width refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor (or a portion where current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other, or a region where a channel is formed. In one transistor, channel widths in all regions are not necessarily the same. In other words, the channel width of one transistor is not fixed to one value in some cases. Therefore, in this specification, the channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

Note that depending on transistor structures, a channel width in a region where a channel is actually formed (hereinafter, also referred to as an “effective channel width”) is different from a channel width shown in a top view of a transistor (hereinafter, also referred to as an “apparent channel width”) in some cases. For example, in a transistor having a gate electrode covering side surfaces of a semiconductor layer, an effective channel width is greater than an apparent channel width, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a gate electrode covering side surfaces of a semiconductor, the proportion of a channel formation region formed in the side surfaces of the semiconductor is increased in some cases. In that case, an effective channel width is greater than an apparent channel width.

In such a case, an effective channel width is difficult to measure in some cases. For example, estimation of an effective channel width from a design value requires an assumption that the shape of a semiconductor is known. Therefore, in the case where the shape of a semiconductor is not known accurately, it is difficult to measure an effective channel width accurately.

Therefore, in this specification, an apparent channel width is referred to as a surrounded channel width (SCW) in some cases. Furthermore, in this specification, in the case where the term “channel width” is simply used, it may represent a surrounded channel width or an apparent channel width. Alternatively, in this specification, in the case where the term “channel width” is simply used, it may represent an effective channel width in some cases. Note that the values of a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by analyzing a cross-sectional TEM image and the like.

Note that in the case where field-effect mobility, a current value per channel width, and the like of a transistor are obtained by calculation, a surrounded channel width may be used for the calculation. In that case, a value different from the one obtained in the case where an effective channel width is used for the calculation is obtained in some cases.

In this specification and the like, in the case where an etching step (removal step) is performed after a resist mask is formed by a photolithography method, the resist mask is removed after the etching step, unless otherwise specified.

In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. The transistor includes a channel formation region between a drain (a drain terminal, a drain region, or a drain electrode) and a source (a source terminal, a source region, or a source electrode), and current can flow between the source and the drain through the channel formation region. Note that in this specification and the like, a channel formation region refers to a region through which current mainly flows.

A transistor described in this specification and the like refers to an enhancement-mode (normally-off) field-effect transistor, unless otherwise specified. A transistor described in this specification and the like also refers to an n-channel transistor, unless otherwise specified. Therefore, the threshold voltage (also referred to as “Vth”) thereof is higher than 0 V, unless otherwise specified.

Note that the Vthof a transistor including a back gate in this specification and the like refers to a Vthobtained when the potential of the back gate is set equal to that of a source or a gate, unless otherwise specified.

Unless otherwise specified, off-state current in this specification and the like refers to drain current of a transistor in an off state (also referred to as a non-conducting state and a cutoff state). Unless otherwise specified, the off state of an n-channel transistor means that a potential difference (hereinafter also referred to as Vg) between its gate and source when the potential of the source is regarded as a reference potential is lower than the threshold voltage Vth, and the off state of a p-channel transistor means that the voltage Vgbetween its gate and source is higher than the threshold voltage Vth. For example, the off-state current of an n-channel transistor sometimes refers to drain current that flows when Vgis lower than the threshold voltage Vth.

The off-state current of a transistor depends on Vgin some cases. Thus, “the off-state current of a transistor is lower than or equal to I” may mean there is Vgat which the off-state current of the transistor is lower than or equal to I. Furthermore, the off-state current of a transistor may refer to the off-state current in an off state at predetermined Vg, the off-state current in an off state at Vgin a predetermined range, the off-state current in an off state at Vgwith which sufficiently reduced off-state current is obtained, or the like.

As an example, the assumption is made of an n-channel transistor where Vthis 0.5 V and the drain current is 1×10−9A at Vgof 0.5 V, 1×10−13A at Vgof 0.1 V, 1×10−19A at Vgof −0.5 V, and 1×10−22A at Vgof −0.8 V. The drain current of the transistor is lower than or equal to 1×10−19A at Vgof −0.5 V or at Vgin the range of −0.8 V to −0.5 V; therefore, it can be said that the off-state current of the transistor is lower than or equal to 1×10−19A. Since there is Vgat which the drain current of the transistor is lower than or equal to 1×10−22A, it may be said that the off-state current of the transistor is lower than or equal to 1×10−22A.

The off-state current of a transistor depends on temperature in some cases. Unless otherwise specified, the off-state current in this specification may be an off-state current at room temperature (RT), 60° C., 85° C., 95° C., or 125° C. Alternatively, the off-state current may be an off-state current at a temperature at which the reliability of a semiconductor device or the like including the transistor is ensured or a temperature at which the semiconductor device or the like is used (e.g., a temperature higher than or equal to 5° C. and lower than or equal to 35° C.). The state in which the off-state current of a transistor is lower than or equal to I may indicate that the off-state current of the transistor at RT, 60° C., 85° C., 95° C., 125° C., a temperature at which the reliability of a semiconductor device or the like including the transistor is ensured, or a temperature at which the semiconductor device or the like is used (e.g., a temperature higher than or equal to 5° C. and lower than or equal to 35° C.) is lower than or equal to I at a certain Vg.

In some cases, the off-state current of a transistor depends on a voltage between its drain and source when the potential of the source is regarded as a reference potential (hereinafter such a voltage is also referred to as Vd). Unless otherwise specified, the off-state current in this specification may be an off-state current at Vdof 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V. Alternatively, the off-state current may be an off-state current at Vdat which the reliability of a semiconductor device or the like including the transistor is ensured or at Vdused in the semiconductor device or the like including the transistor. The state in which the off-state current of a transistor is lower than or equal to I may indicate that the off-state current of the transistor at Vdof 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V, at Vdat which the reliability of a semiconductor device or the like including the transistor is ensured, or at Vdused in the semiconductor device or the like is lower than or equal to I at a certain Vg.

In the above description of off-state current, a drain may be replaced with a source. That is, the off-state current sometimes refers to current that flows through a source of a transistor in the off state.

In this specification and the like, the term “leakage current” sometimes expresses the same meaning as “off-state current”. In this specification and the like, the off-state current sometimes refers to current that flows between a source and a drain of a transistor in the off state, for example.

Note that the term “over” or “below” in this specification and the like does not necessarily mean that a component is placed “directly over” or “directly below” and “directly in contact with” another component. For example, the expression “electrode B over insulating layer A” does not necessarily mean that the electrode B is on and in direct contact with the insulating layer A and can mean the case where another component is provided between the insulating layer A and the electrode B.

In this specification and the like, unless otherwise specified, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, unless otherwise specified, the term “substantially parallel” indicates that the angle formed between two straight lines is greater than or equal to −30° and less than or equal to 30°. The term “perpendicular” or “orthogonal” indicates, unless otherwise specified, that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. In addition, unless otherwise specified, the term “substantially perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 60° and less than or equal to 120°.

In this specification and the like, the terms “identical”, “the same”, “equal”, “uniform”, and the like (including synonyms thereof) used in describing calculation values and actual measurement values allow for a margin of error of ±20% unless otherwise specified.

In this specification and the like, a metal oxide means an oxide of metal in a broad sense. Metal oxides are classified into an oxide insulator, an oxide conductor (including a transparent oxide conductor), an oxide semiconductor (also simply referred to as an OS), and the like. For example, a metal oxide used in an active layer of a transistor is called an oxide semiconductor in some cases. That is, a metal oxide that has at least one of an amplifying function, a rectifying function, and a switching function can be called a metal oxide semiconductor, or OS for short. In addition, an OS FET is a transistor including a metal oxide or an oxide semiconductor.

In this specification and the like, a metal oxide including nitrogen is also called a metal oxide in some cases. Moreover, a metal oxide including nitrogen may be called a metal oxynitride.

In this specification and the like, “c-axis aligned crystal (CAAC)” or “cloud-aligned composite (CAC)” may be stated in some cases. Note that CAAC refers to an example of a crystal structure, and CAC refers to an example of a function or a material composition.

In this specification and the like, a CAC-OS or a CAC metal oxide has a conducting function in a part of the material and has an insulating function in another part of the material; as a whole, the CAC-OS or the CAC metal oxide has a function of a semiconductor. In the case where the CAC-OS or the CAC metal oxide is used in an active layer of a transistor, the conducting function is to allow electrons (or holes) serving as carriers to flow, and the insulating function is to not allow electrons serving as carriers to flow. By the complementary action of the conducting function and the insulating function, the CAC-OS or the CAC metal oxide can have a switching function (on/off function). In the CAC-OS or the CAC metal oxide, separation of the functions can maximize each function.

Furthermore, in the CAC-OS or the CAC metal oxide, the conductive regions and the insulating regions each have a size of more than or equal to 0.5 nm and less than or equal to 10 nm, preferably more than or equal to 0.5 nm and less than or equal to 3 nm and are dispersed in the material, in some cases.

The CAC-OS or the CAC metal oxide includes components having different bandgaps. For example, the CAC-OS or the CAC metal oxide includes a component having a wide gap due to the insulating region and a component having a narrow gap due to the conductive region. In the case of such a composition, carriers mainly flow in the component having a narrow gap. The component having a narrow gap complements the component having a wide gap, and carriers also flow in the component having a wide gap in conjunction with the component having a narrow gap. Therefore, in the case where the above-described CAC-OS or the CAC metal oxide is used in a channel formation region of a transistor, high current drive capability in the on state of the transistor, that is, a high on-state current and high field-effect mobility, can be obtained.

In other words, the CAC-OS or the CAC metal oxide can be called a matrix composite or a metal matrix composite.

A display device100of one embodiment of the present invention will be described with reference to the drawings.

Structure Example

FIG. 1Ais a schematic perspective view of the display device100. In the display device100, a substrate111and a substrate121are bonded to each other. The display device100includes a display region235, a peripheral circuit region232, a peripheral circuit region233, and the like.FIG. 1Aillustrates an example in which the display device100is provided with an FPC124. Thus, the structure illustrated inFIG. 1Acan be regarded as a display module including the display device100and the FPC124.

The peripheral circuit region232and the peripheral circuit region233each include a circuit for supplying a signal to the display region235. The circuits included in the peripheral circuit region232and the peripheral circuit region233are collectively referred to as a peripheral driver circuit in some cases. Examples of the peripheral driver circuit include a scan line driver circuit and a signal line driver circuit.

Part or the whole of the peripheral driver circuit may be mounted as an integrated circuit (IC). The IC including the part or the whole of the peripheral driver circuit may be mounted over the substrate111by a chip on glass (COG) method, a chip on film (COF) method, or the like. The IC may be mounted over the FPC124by a COF method or the like.

A signal and power are input from the outside to the display region235, the peripheral circuit region232, and the peripheral circuit region233through the FPC124.

FIG. 1Aalso illustrates an enlarged view of part of the display region235. In the display region235, a plurality of pixels230are arranged in a matrix. The pixels230are classified into two types, a pixel230aand a pixel230b, in accordance with cross-sectional structures. The pixel230ais provided adjacent to the pixel230b. Note that in this specification and the like, the pixel230refers to both the pixel230aand the pixel230b.

FIG. 1Bis a cross-sectional view taken along the dashed-dotted line A1-A2inFIG. 1A.FIG. 1Billustrates cross sections of part of the display region235, part of the peripheral circuit region233, and part of a region including the FPC124.

Each of the plurality of pixels230includes a light-emitting element170as a display element. In this specification and the like, the light-emitting element170included in the pixel230ais referred to as a light-emitting element170a, and the light-emitting element170included in the pixel230bis referred to as a light-emitting element170b. Note that in this specification and the like, the light-emitting element170refers to both the light-emitting element170aand the light-emitting element170b.

Each of the plurality of pixels230includes a transistor251for driving the display element. The peripheral circuit region232and the peripheral circuit region233each include a plurality of transistors.FIG. 1Billustrates a transistor252as one example of the transistors included in the peripheral circuit region233.

The display device100includes the transistor251, the transistor252, the light-emitting element170, a coloring layer131, a light-blocking layer132, and the like between the substrate111and the substrate121. The substrate111and the substrate121are bonded to each other with an adhesive layer142.

As the adhesive layer142, any of a variety of curable adhesives such as a reactive curable adhesive, a thermosetting curable adhesive, an anaerobic adhesive, and a photocurable adhesive such as an ultraviolet curable adhesive can be used. Examples of these adhesives include an epoxy resin, an acrylic resin, a silicone resin, a phenol resin, a polyimide resin, an imide resin, a polyvinyl chloride (PVC) resin, a polyvinyl butyral (PVB) resin, and an ethylene vinyl acetate (EVA) resin. In particular, a material with low moisture permeability, such as an epoxy resin, is preferred. Alternatively, a two-component resin may be used. Still alternatively, an adhesive sheet or the like may be used.

The substrate121is provided with an adhesive layer122, an insulating layer123, the coloring layer131, the light-blocking layer132, an insulating layer133, and the like. The insulating layer133may function as a planarization layer.

The substrate111is provided with the transistor251, the transistor252, and the light-emitting element170with an adhesive layer112and an insulating layer113provided therebetween.

FIG. 1Cis an enlarged view of the transistor252. Note that the transistor252can have a structure similar to that of the transistor251.

The transistor252includes an electrode221, a semiconductor layer231, an electrode224a, an electrode224b, and an electrode226. The electrode221is provided over the insulating layer113, and an insulating layer211is provided to cover the electrode221. The semiconductor layer231is provided over the insulating layer211. The electrode224aand the electrode224bare provided over the insulating layer211. The electrode224aincludes a region in contact with part of the semiconductor layer231. The electrode224bincludes a region in contact with another part of the semiconductor layer231. One of the electrode224aand the electrode224bcan function as a source electrode, and the other can function as a drain electrode.

An insulating layer210is provided to cover the electrode224a, the electrode224b, and the semiconductor layer231. The electrode226is provided over the insulating layer210. The electrode226includes a region overlapping with the semiconductor layer231. An insulating layer213is provided to cover the electrode226.

InFIG. 1B, the transistor251and the transistor252are bottom-gate transistors. The transistor251is used for controlling current flowing to the light-emitting element170(such a transistor is also referred to as a driving transistor).

An insulating layer115is provided over the insulating layer213. The insulating layer115functions as a planarization layer. The transistor251and the transistor252are covered with the insulating layer213and the insulating layer115. The number of insulating layers covering the transistors is not limited and may be one or two or more.

A material through which impurities such as water and hydrogen do not easily diffuse is preferably used for at least one of the insulating layers that cover the transistors. This is because such an insulating layer can serve as a barrier film. Such a structure can effectively suppress diffusion of the impurities into the transistors from the outside, and a highly reliable display device can be achieved.

In the pixel230aillustrated inFIG. 1B, an electrode171ais provided over the insulating layer115. In the pixel230a, the electrode171ais electrically connected to one of a source electrode and a drain electrode of the transistor251through an opening provided in the insulating layer115.

In the pixel230billustrated inFIG. 1B, an insulating layer116is provided over the insulating layer115. The insulating layer116includes a region overlapping with an end portion of the electrode171a. An electrode171bis provided over the insulating layer116. In the pixel230b, the electrode171bprovided over the insulating layer116is electrically connected to one of the source electrode and the drain electrode of the transistor251through an opening provided in the insulating layer115and the insulating layer116.

An insulating layer114is provided to cover end portions of the electrode171aand the electrode171b. The insulating layer114includes a region overlapping with part of the electrode171aand a region overlapping with part of the electrode171b. InFIG. 1B, the insulating layer114includes a region in contact with part of the electrode171aand a region in contact with part of the electrode171b.

An EL layer172is provided over the electrode171a, the electrode171b, and the insulating layer114, and an electrode173is provided over the EL layer172. The electrode173includes a region overlapping with the electrode171awith part of the EL layer172provided therebetween, a region overlapping with the electrode171bwith another part of the EL layer172provided therebetween, and a region overlapping with the insulating layer114.

The insulating layer114can serve as a partition wall. The insulating layer114is preferably formed so that its sidewall has a tapered shape or a tilted surface with a continuous curvature. The sidewall of the insulating layer114having the above-described shape enables favorable coverage with the electrode173formed later.

In the display device100illustrated inFIG. 1B, a wiring125, an electrode228, and an electrode229are provided. The wiring125and the electrode228are provided over the insulating layer211. The electrode229is electrically connected to the electrode228through an opening that is provided in the insulating layer210and overlaps with the electrode228. The wiring125and the electrode228can be formed concurrently with the electrode224aand the electrode224bin the same step. The electrode229can be formed concurrently with the electrode226in the same step.

The FPC124is electrically connected to the electrode229through a connection layer138. The electrode229is electrically connected to the peripheral driver circuit.

As the connection layer138, an anisotropic conductive film (ACF), an anisotropic conductive paste (ACP), or the like can be used.

The light-emitting element170illustrated inFIG. 1Bis a top-emission light-emitting element. The light-emitting element170ahas a structure in which the electrode171a, the EL layer172, and the electrode173are stacked in this order from the insulating layer115side. The light-emitting element170bhas a structure in which the electrode171b, the EL layer172, and the electrode173are stacked in this order from the insulating layer115side.

The electrode171aand the electrode171bhave a function of reflecting visible light, and the electrode173has a function of transmitting visible light. Part of the electrode173functions as a common electrode of the light-emitting element170aand another part of the electrode173functions as a common electrode of the light-emitting element170b.

The EL layer172includes at least a light-emitting layer. In addition to the light-emitting layer, the EL layer172may further include one or more layers containing any of a substance with a high hole-injection property, a substance with a high hole-transport property, a hole-blocking material, a substance with a high electron-transport property, a substance with a high electron-injection property, a substance with a bipolar property (a substance with a high electron- and hole-transport property), and the like.

The emission color of the light-emitting element170can be changed to white, red, green, blue, cyan, magenta, yellow, or the like depending on the material of the EL layer172.

As a color display method, there are a method in which the light-emitting element170whose emission color is white is combined with a coloring layer and a method in which the light-emitting element170with a different emission color is provided in each pixel. The former method is more productive than the latter method. In other words, the latter method, which requires separate formation of the EL layer172pixel by pixel, is less productive than the former method. However, the latter method can produce the emission color with higher color purity than the emission color produced by the former method. When the light-emitting element170has a microcavity structure in the latter method, the color purity can be further increased.

For the EL layer172, either a low molecular compound or a high molecular compound can be used, and an inorganic compound may also be used. The layers included in the EL layer172can be formed by any of the following methods: an evaporation method (including a vacuum evaporation method), a transfer method, a printing method, an inkjet method, a coating method, and the like.

The EL layer172may contain an inorganic compound such as quantum dots. For example, quantum dots used for the light-emitting layer can serve as light-emitting materials.

The display device100includes the light-emitting element170whose emission color is white. That is, the color of light175aemitted from the light-emitting element170aand the color of light175bemitted from the light-emitting element170bare both white. Note that in this specification and the like, light175refers to both the light175aand the light175b.

The light175is emitted from the light-emitting element170to the substrate121side through the coloring layer131and the like. The wavelength range of the light175that passes through the coloring layer131depends on the material of the coloring layer131. That is, the hue of the light175can be changed to red, green, blue, cyan, magenta, yellow, or the like by making the light175pass through the coloring layer131.

The color display can be achieved when the hue of light varies between pixels. To achieve color display, the colors of the coloring layers combined with the emission colors of the light-emitting element170may be yellow, cyan, and magenta, as well as red, green, and blue. The colors of the combined coloring layers may be determined as appropriate in accordance with the purpose, the uses, or the like.

There is no particular limitation on a material used for the substrate111and the substrate121. The material is determined according to the purpose in consideration of whether it has a light-transmitting property, heat resistance high enough to withstand heat treatment, or the like. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. Alternatively, a semiconductor substrate, a flexible substrate, an attachment film, a base film, or the like may be used.

As the semiconductor substrate, a semiconductor substrate of silicon, germanium, or the like or a compound semiconductor substrate of silicon carbide, silicon germanium, gallium arsenide, indium phosphide, zinc oxide, or gallium oxide, or the like is used, for example. For the semiconductor substrate, a single-crystal semiconductor or a polycrystalline semiconductor may be used.

To increase the flexibility of the display device100, a flexible substrate, an attachment film, a base film, or the like may be used as each of the substrate111and the substrate121.

Examples of materials of the flexible substrate, the attachment film, the base film, and the like include polyester resins such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), a polyacrylonitrile resin, an acrylic resin, a polyimide resin, a polymethyl methacrylate resin, a polycarbonate (PC) resin, a polyethersulfone (PES) resin, polyamide resins (e.g., nylon and aramid), a polysiloxane resin, a cycloolefin resin, a polystyrene resin, a polyamide-imide resin, a polyurethane resin, a polyvinyl chloride resin, a polyvinylidene chloride resin, a polypropylene resin, a polytetrafluoroethylene (PTFE) resin, an ABS resin, and cellulose nanofiber.

When any of the above-described materials is used for the substrates, a lightweight display device can be provided. Furthermore, when any of the above-described materials is used for the substrates, a shock-resistant display device can be provided. Moreover, when any of the above-described materials is used for the substrates, a non-breakable display device can be provided.

The flexible substrate used as the substrate111or121preferably has a lower coefficient of linear expansion because a lower coefficient of linear expansion suppresses deformation due to an environment. The flexible substrate used as the substrate111or121is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10−3/K, lower than or equal to 5×10−5/K, or lower than or equal to 1×10−5/K. In particular, aramid is preferably used for the flexible substrate because of its low coefficient of linear expansion.

As conductive materials for a gate, a source, and a drain of a transistor, and a conductive layer such as a wiring or an electrode included in a display device, a metal element selected from aluminum, chromium, copper, silver, gold, platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium (Hf), vanadium (V), niobium (Nb), manganese, magnesium, zirconium, beryllium, and the like; an alloy containing any of the above metal elements as a component; an alloy containing a combination of the above metal elements; or the like can be used. Alternatively, a semiconductor typified by polycrystalline silicon including an impurity element such as phosphorus, or silicide such as nickel silicide may be used. There is no particular limitation on the formation method of the conductive material, and a variety of formation methods such as an evaporation method, a CVD method, a sputtering method, and a spin coating method can be employed.

The conductive layer can be formed using a conductive material containing oxygen, such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. Moreover, a conductive material containing nitrogen, such as titanium nitride, tantalum nitride, or tungsten nitride, can be used. In addition, a stacked-layer structure formed using a conductive material containing oxygen, a conductive material containing nitrogen, and a material containing any of the above metal elements can be used.

The conductive material that can be used for the conductive layer may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum layer containing silicon, a two-layer structure in which a titanium layer is stacked over an aluminum layer, a two-layer structure in which a titanium layer is stacked over a titanium nitride layer, a two-layer structure in which a tungsten layer is stacked over a titanium nitride layer, a two-layer structure in which a tungsten layer is stacked over a tantalum nitride layer, and a three-layer structure in which a titanium layer, an aluminum layer, and a titanium layer are stacked in this order can be used. Alternatively, an aluminum alloy containing one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used as the conductive material.

The electrode171aand the electrode171bare preferably formed using a conductive material that efficiently reflects light emitted from the EL layer172. Note that the electrode171aand the electrode171bmay have a stacked-layer structure of a plurality of layers without limitation to a single-layer structure. For example, in the case where each of the electrode171aand the electrode171bis used as an anode, a layer in contact with the EL layer172may be a light-transmitting layer, such as an indium tin oxide layer, and a layer having high reflectance (e.g., a layer formed using aluminum, an alloy containing aluminum, or silver) may be provided in contact with the layer.

For the conductive material that reflects visible light, for example, a metal material such as aluminum, gold, platinum, silver, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy containing any of these metal materials can be used. Lanthanum, neodymium, germanium, or the like may be added to the metal material or the alloy. Furthermore, an alloy containing aluminum (an aluminum alloy) such as an alloy of aluminum and titanium, an alloy of aluminum and nickel, or an alloy of aluminum and neodymium; or an alloy containing silver such as an alloy of silver and copper, an alloy of silver, copper, and palladium, or an alloy of silver and magnesium can be used. An alloy containing silver and copper is preferable because of its high heat resistance. Furthermore, a metal film or an alloy film may be stacked with a metal oxide film. For example, when a metal film or a metal oxide film is stacked in contact with an aluminum alloy film, oxidation of the aluminum alloy film can be suppressed. Other examples of the metal film and the metal oxide film are a titanium film and a titanium oxide film, respectively. Alternatively, as described above, a light-transmitting conductive film and a film containing metal materials may be stacked. For example, a stacked film of silver and indium tin oxide (ITO) or a stacked film of indium tin oxide and an alloy of silver and magnesium can be used.

The display device having a top-emission structure is described as an example inFIG. 1B. In the case of a display device having a bottom-emission structure, the electrode171is formed using a conductive material transmitting visible light and the electrode173is formed using a conductive material reflecting visible light. In the case of a display device having a dual-emission structure, the electrode171and the electrode173are formed using a conductive material transmitting visible light.

As a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added, or graphene can be used. Alternatively, as a light-transmitting conductive material, an oxide conductor can be used. Alternatively, a metal material such as gold, silver, platinum, magnesium, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, or titanium, or an alloy material containing any of these metal materials can be used. Alternatively, a nitride of the metal material (e.g., titanium nitride) or the like may be used. In the case of using the metal material or the alloy material (or the nitride thereof), the thickness is set small enough to transmit light. Alternatively, a stacked film of any of the above materials can be used for the conductive layers. For example, a stacked film of indium tin oxide and an alloy of silver and magnesium is preferably used because the conductivity can be increased. They can also be used for conductive layers such as a variety of wirings and electrodes included in a display device, and conductive layers (e.g., conductive layers serving as a pixel electrode or a common electrode) included in a display element.

Here, an oxide conductor, which is one type of metal oxide, is described. In this specification and the like, an oxide conductor may be referred to as OC. For example, the oxide conductor is obtained in the following manner. Oxygen vacancies are formed in a metal oxide, and then hydrogen is added to the oxygen vacancies, so that a donor level is formed in the vicinity of the conduction band. This increases the conductivity of the metal oxide; accordingly, the metal oxide becomes a conductor. The metal oxide having become a conductor can be referred to as an oxide conductor. An oxide semiconductor generally transmits visible light because of its large energy gap. Since an oxide conductor is a metal oxide having a donor level in the vicinity of the conduction band, the influence of absorption due to the donor level is small in the oxide conductor, and the oxide conductor has a visible light transmitting property comparable to that of an oxide semiconductor.

The insulating layers can be formed with a single layer or a stack of layers of one or more materials selected from aluminum nitride, aluminum oxide, aluminum nitride oxide, aluminum oxynitride, magnesium oxide, silicon nitride, silicon oxide, silicon nitride oxide, silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, aluminum silicate, and the like. Alternatively, a material in which two or more materials selected from an oxide material, a nitride material, an oxynitride material, and a nitride oxide material are mixed may be used.

Note that in this specification, a nitride oxide refers to a compound that includes more nitrogen than oxygen. An oxynitride refers to a compound that includes more oxygen than nitrogen. The content of each element can be measured by Rutherford backscattering spectrometry (RBS), for example.

In particular, the insulating layers113and213are preferably formed using an insulating material which is relatively impermeable to impurities. The insulating layers113and213may each be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulating material containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. Examples of such an insulating material that is relatively impermeable to impurities include aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, and silicon nitride.

When the insulating material that is relatively impermeable to impurities is used for the insulating layer113, impurity diffusion from the substrate111side can be suppressed, and the reliability of the transistor can be improved. When the insulating material that is relatively impermeable to impurities is used for the insulating layer213, impurity diffusion from the insulating layer115side can be suppressed, and the reliability of the transistor can be improved.

Furthermore, the insulating layer that can function as the planarization layer can be formed using an organic material having heat resistance, such as polyimide, an acrylic resin, a benzocyclobutene resin, polyamide, or an epoxy resin. Other than such organic materials, it is possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. Note that a plurality of insulating layers formed of these materials may be stacked.

Note that the siloxane-based resin corresponds to a resin including a Si—O—Si bond formed using a siloxane-based material as a starting material. The siloxane-based resin may include an organic group (e.g., an alkyl group or an aryl group) or a fluoro group as a substituent. In addition, the organic group may include a fluoro group.

A surface of the insulating layer or the like may be subjected to CMP treatment. By the CMP treatment, unevenness of the sample surface can be reduced, and coverage with an insulating layer or a conductive layer formed later can be increased.

Examples of a material that can be used for the coloring layers include a metal material, a resin material, and a resin material containing a pigment or dye.

Examples of a material that can be used for the light-blocking layer include carbon black, titanium black, a metal, a metal oxide, and a composite oxide containing a solid solution of a plurality of metal oxides. The light-blocking layer may be a film containing a resin material or a thin film of an inorganic material such as a metal. Stacked films containing the material of the coloring layer can also be used for the light-blocking layer. For example, a stacked-layer structure of a film containing a material of a coloring layer which transmits light of a certain color and a film containing a material of a coloring layer which transmits light of another color can be employed. It is preferable that the coloring layer and the light-blocking layer be formed using the same material because the same manufacturing apparatus can be used and the process can be simplified.

There is no particular limitation on the structure of the transistor included in the display device of one embodiment of the present invention. For example, a planar transistor or a staggered transistor may be used. A top-gate transistor or a bottom-gate transistor may be used. Gate electrodes may be provided above and below a channel.

A transistor included in a peripheral driver circuit and a transistor included in a pixel circuit may have the same structure or different structures. Transistors included in the peripheral driver circuit may have the same structure or a combination of two or more kinds of structures. Similarly, transistors included in the pixel circuit may have the same structure or a combination of two or more kinds of structures.

Note that when one of the gate electrodes provided above and below a channel is referred to as a “gate electrode”, the other is referred to as a “back gate electrode”. In addition, when one of the gate electrodes provided above and below a channel is referred to as a “gate”, the other is referred to as a “back gate”. Note that the gate electrode may be referred to as a “front gate electrode”. Similarly, the gate may be referred to as a “front gate”.

For example, the electrode221included in the transistor252can function as a gate electrode. The electrode226included in the transistor252can function as a back gate electrode. Thus, the insulating layer210and the insulating layer211can each function as a gate insulating layer.

When the gate electrode and the back gate electrode are provided, a semiconductor layer of the transistor can be electrically surrounded by an electric field generated from the gate electrode and an electric field generated from the back gate electrode. Such a transistor structure in which electric fields generated from the gate electrode and the back gate electrode electrically surround the semiconductor layer where the channel is formed can be referred to as a surrounded-channel (S-channel) structure.

The back gate electrode can function in a manner similar to that of the gate electrode. The potential of the back gate electrode may be the same as that of the gate electrode or may be a ground potential or a predetermined potential. By changing the potential of the back gate electrode independently of the potential of the gate electrode, the threshold voltage of the transistor can be changed.

By providing the gate electrode and the back gate electrode and by setting the potentials of the gate electrode and the back gate electrode to be the same, a region of the semiconductor layer through which carriers flow is enlarged in the film thickness direction; thus, the amount of carrier transfer is increased. As a result, the on-state current and the field-effect mobility of the transistor are increased.

Accordingly, the transistor can have a high on-state current for its area. That is, the area occupied by the transistor can be small for required on-state current. Therefore, a semiconductor device having a high degree of integration can be provided.

The use of the transistor having a high on-state current for a display device can reduce signal delay in wirings and can suppress degradation of display quality even in the display device in which the number of wirings is increased because of an increase in size or definition.

Furthermore, the gate electrode and the back gate electrode are formed using conductive layers and thus each have a function of preventing an electric field generated outside the transistor from influencing the semiconductor layer in which the channel is formed (in particular, an electric field blocking function against static electricity and the like). When the back gate electrode is formed larger than the semiconductor layer to cover the semiconductor layer in the plan view, the electric field blocking function can be enhanced.

Since the gate electrode and the back gate electrode each have a function of blocking an electric field from the outside, charges of charged particles and the like generated above and below the transistor do not influence the channel formation region of the semiconductor layer. Thus, degradation due to a stress test (e.g., a negative gate bias temperature (NGBT) stress test where negative voltage is applied to a gate (this stress test is also referred to as NBT or NBTS)) can be reduced. In addition, the gate electrode and the back gate electrode can block an electric field generated from the drain electrode so as not to affect the semiconductor layer. Thus, changes in the rising voltage of on-state current due to changes in drain voltage can be suppressed. Note that this effect is significant when a potential is applied to the gate electrode and the back gate electrode.

Before and after a positive gate bias temperature (PGBT) stress test where positive voltage is applied to a gate (this stress test is also referred to as PBT or PBTS)), a transistor including a back gate electrode has a smaller change in threshold voltage than a transistor including no back gate electrode.

The BT stress test such as NGBT or PGBT is a kind of accelerated test and can evaluate, in a short time, a change by long-term use (i.e., a change over time) in characteristics of transistors. In particular, the amount of change in threshold voltage of the transistor between before and after the BT stress test is an important indicator when examining the reliability of the transistor. When the amount of change in the threshold voltage between before and after the BT stress test is small, the transistor has higher reliability.

By providing the gate electrode and the back gate electrode and by setting the potentials of the gate electrode and the back gate electrode to be the same, the change in threshold voltage is reduced. Accordingly, variation in electrical characteristics among a plurality of transistors is also reduced.

In the case where light is incident on the back gate electrode side, when the back gate electrode is formed using a light-blocking conductive film, light can be prevented from entering the semiconductor layer from the back gate electrode side. Therefore, photodegradation of the semiconductor layer can be prevented and deterioration in electrical characteristics of the transistor, such as a shift of the threshold voltage, can be prevented.

As a semiconductor material used for the semiconductor layer of the transistor, silicon or germanium can be used, for example. Alternatively, a compound semiconductor such as silicon carbide, gallium arsenide, a metal oxide, or a nitride semiconductor, an organic semiconductor, or the like can be used.

As a semiconductor material used for the transistor, polycrystalline silicon (polysilicon) or amorphous silicon can be used, for example. As a semiconductor material used for the transistor, an oxide semiconductor, which is one type of metal oxide, can be used. Typically, an oxide semiconductor containing indium or the like can be used.

In particular, a semiconductor material having a wider band gap and a lower carrier density than silicon is preferably used because off-state current between the source and the drain of the transistor can be reduced.

The semiconductor layer preferably includes, for example, a material represented by an In-M-Zn-based oxide that contains at least indium, zinc, and M (a metal such as aluminum, titanium, gallium, germanium, yttrium, zirconium, lanthanum, cerium, tin, neodymium, or hafnium). In order to reduce variations in electrical characteristics of the transistor including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer in addition to In, Zn, and M.

Examples of the stabilizer, including metals that can be used as M, are gallium, tin, hafnium, aluminum, and zirconium. Other examples of the stabilizer are lanthanoid such as lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

Note that here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main components and there is no limitation on the ratio of In:Ga:Zn. Furthermore, a metal element in addition to In, Ga, and Zn may be contained.

The semiconductor layer of any of the transistors disclosed in one embodiment of the present invention may include a cloud-aligned composite oxide semiconductor (CAC-OS).

The aforementioned non-single-crystal oxide semiconductor or CAC-OS is preferably used for the semiconductor layer of any of the transistors disclosed in one embodiment of the present invention. As the non-single-crystal oxide semiconductor, an nc-OS or a CAAC-OS is preferably used.

FIG. 2Ais an enlarged plan view of part of the display region235.FIG. 2Aillustrates an arrangement example of the electrode171a, the electrode171b, and the insulating layer114.FIG. 2Bis a cross-sectional view taken along the dashed-dotted line B1-B2inFIG. 2A.FIG. 2Billustrates a structure where the components up to the electrode173are formed over the substrate111.

The pixel230aand the pixel230bare alternately arranged in a direction R. The pixel230aand the pixel230bare alternately arranged also in a direction C. That is, the electrode171aand the electrode171bare alternately arranged in both the direction R and the direction C. Note that in the plan view, the direction R is orthogonal to the direction C.

In the display device100of one embodiment of the present invention, the electrode171ais formed over the insulating layer115, and the electrode171bis formed over the insulating layer116. The formation of the electrode171aand the electrode171bover different layers can prevent a short circuit between the electrodes. As illustrated inFIG. 2C, the adjacent electrodes171aand171bcan overlap with each other with the insulating layer116provided therebetween. Thus, a distance G between the adjacent electrodes171aand171bcan be substantially zero.

FIG. 3Ais an enlarged plan view of part of the display region235without the insulating layer116.FIG. 3Aillustrates an arrangement example of the electrode171aand the insulating layer114.FIG. 3Bis a cross-sectional view taken along the dashed-dotted line C1-C2inFIG. 3A.FIG. 3Billustrates a structure where the components up to the electrode173except the insulating layer116are formed over the substrate111. Without the insulating layer116, the electrode171bcannot be provided. Accordingly, the pixel230bcannot be provided.

To prevent a short circuit between the adjacent electrodes171aor an increase in parasitic capacitance in the structure inFIG. 3B, a distance Gr needs to keep a certain length or longer. An increase in the distance Gr leads to an increase in a width Fr of the insulating layer114. However, the structure without the insulating layer116enables a reduction in manufacturing costs and an improvement in the productivity.

Since the distance G in the display device100of one embodiment of the present invention can be substantially zero, a width F of the insulating layer114can be smaller than the width Fr (seeFIG. 2AandFIG. 3A). A decrease in the width F leads to an increase in the proportion of the light-emitting area of the light-emitting element170in the area occupied by the pixel230(also referred to as aperture ratio).

For example, in the case where the constant emission luminance (amount of light) is obtained, emission luminance per unit area can be lowered by increasing the light-emitting area of the light-emitting element170. That is, the deterioration of the light-emitting element170is reduced by increasing the aperture ratio, which leads to an increase in the reliability of the display device. Moreover, the visibility of the display device can be increased. Furthermore, the display quality of the display device can be improved.

The insulating layer114functions as a partition for preventing an electrical short circuit between the electrode171aand the electrode171badjacent to each other (or between the adjacent electrodes171a). In the case of using a metal mask for formation of the EL layer172, the insulating layer114has a function of preventing the contact of the metal mask with a region where the light-emitting element170is formed.

However, in some cases, electric charge is supplied to the adjacent light-emitting elements170through the EL layer172formed along the surface of the insulating layer114to cause undesired light emission (also referred to as crosstalk). As a distance Kr of the EL layer172formed along the surface of the insulating layer114becomes smaller, the crosstalk easily occurs. That is, as the width F and the width Fr become smaller, the crosstalk easily occurs.

Since the display device100of one embodiment of the present invention includes the insulating layer116, a distance K of the EL layer172formed along the surface of the insulating layer114can be kept at a certain length or longer even when the width F becomes small (seeFIG. 2B).

Owing to the insulating layer116, a distance between the vertex of the insulating layer114and the surface of the electrode171a(i.e., a step) can be increased. In addition, with the insulating layer116, the EL layer172is likely to have low step coverage. Thus, the resistance of the EL layer172on the insulating layer114can be increased. With the insulating layer116, the crosstalk can be reduced and the display quality of the display device can be improved.

A head-mounted display device (also referred to as head-mounted display or HMD) used while being mounted on the head of a user is known. The HMD is often used as a display device for virtual reality (VR) because the user can feel a strong sense of immersion.

The user of the HMD sees an image enlarged by a lens. Thus, even when a display device has high resolution, the wide width F makes the user strongly feel graininess of the image because each pixel is independently emphasized. In particular, a display device including a self-luminous element as a display element has a high contrast ratio; thus, the user easily feels strong graininess of the image.

According to one embodiment of the present invention, the width F of a display device can be reduced. Accordingly, graininess of a displayed image can be reduced. With the use of the display device of one embodiment of the present invention, an HMD that can provide a strong sense of immersion can be obtained.

Modification Example 1

FIG. 4illustrates a cross section of a display device100A, which is a modification example of the display device100. The display device100A includes a touch sensor370between the substrate121and the coloring layer131. In this embodiment, the touch sensor370includes a conductive layer374, an insulating layer375, a conductive layer376a, a conductive layer376b, a conductive layer377, and an insulating layer378.

The conductive layers376a,376b, and377are preferably formed using a light-transmitting conductive material. However, a light-transmitting conductive material generally has resistivity higher than that of a conductive material that does not have a light-transmitting property (a light-blocking conductive material). Thus, the conductive layers376a,376b, and377are formed using a metal material having low resistivity in some cases to increase the size and definition of the touch sensor.

When the conductive layers376a,376b, and377are formed using a metal material, the reflection of external light is preferably reduced. Although a metal material generally has high reflectivity, the reflectivity of the metal material can be reduced by oxidation treatment or the like to make the metal material darkened.

The conductive layers376a,376b, and377may be formed using a stack including a metal layer and a layer having low reflectivity (also referred to as a dark layer). The dark layer has high resistivity; thus, the stack including the metal layer and the dark layer is preferable. Examples of the dark layer include a layer containing copper oxide and a layer containing copper chloride or tellurium chloride. Alternatively, the dark layer may be formed using a metal particle such as an Ag particle, an Ag fiber, or a Cu particle, a carbon nanoparticle such as a carbon nanotube (CNT) or graphene, or a conductive high molecule such as PEDOT, polyaniline, or polypyrrole, for example.

As the touch sensor370, an optical touch sensor including a photoelectric conversion element as well as a resistive touch sensor or a capacitive touch sensor may be used. Examples of the capacitive touch sensor include a surface capacitive touch sensor and a projected capacitive touch sensor. Examples of the projected capacitive touch sensor include a self-capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method. The use of a mutual capacitive touch sensor is preferable because multiple points can be sensed simultaneously.

Other components are similar to those of the display device100and thus are not described in detail.

Alternatively, a touch sensor may be provided on the outer side of the substrate121without the formation of the touch sensor370between the substrate121and the coloring layer131. For example, a sheet-like touch sensor176may be provided to overlap with the display region235(seeFIG. 5).

Modification Example 2

FIG. 6illustrates a cross section of a display device100B, which is another modification example of the display device100. In the display device100B, a functional member135is provided over the substrate121.

Examples of the functional member135include optical members such as a polarizing plate, a retardation plate, a light diffusion layer (e.g., a diffusion film), an anti-reflection layer (also referred to as an AR layer), an anti-glare layer (also referred to as an AG layer), and a light-condensing film. Examples of the functional member except the optical members include an antistatic film preventing the attachment of dust, a water repellent film suppressing the attachment of stain, and a hard coat film suppressing generation of a scratch in use. As the functional member135, some of the above members may be used in combination. For example, a circularly polarizing plate in which a linear polarizing plate and a retardation plate are combined may be used.

The AR layer has a function of reducing regular reflection (specular reflection) of external light by utilizing an optical interference effect. When the AR layer is used as the functional member135, the AR layer is formed using a material with a refractive index different from that of the substrate121. The AR layer can be formed using, for example, a material such as zirconium oxide, magnesium fluoride, aluminum oxide, or silicon oxide.

The anti-glare layer (AG layer) may be used instead of the AR layer. The AG layer has a function of reducing regular reflection (specular reflection) by diffusing incident external light.

As a method for forming the AG layer, a method in which fine unevenness is formed on a surface, a method in which materials with different refractive indices are mixed, a combination of both methods, and the like are known. For example, the AG layer can be formed by mixing a light-transmitting resin with nanofibers such as cellulose fibers, inorganic beads formed of silicon oxide or the like, resin beads, or the like.

The AG layer may be provided to overlap with the AR layer. When a stack including the AG layer and the AR layer is used, a function of preventing reflection and glare of external light can be enhanced. With the use of the AR layer and/or the AG layer, for example, the external-light reflectivity of a surface of the display device is preferably less than 1%, further preferably less than 0.3%.

Other components are similar to those of the display device100and thus are not described in detail.

Modification Example 3

FIG. 7illustrates a cross section of a display device100C, which is another modification example of the display device100. The display device100C includes the light-emitting elements170emitting light of different colors in the respective pixels.FIG. 7illustrates a light-emitting element170R emitting red light175R, a light-emitting element170G emitting green light175G, and a light-emitting element170B emitting blue light175B, for example.

The light-emitting element170R includes the electrode171a, an EL layer172R, and the electrode173. The light-emitting element170G includes the electrode171b, an EL layer172G, and the electrode173. The light-emitting element170B includes the electrode171a(which is different from the electrode171aof the light-emitting element170R), an EL layer172B, and the electrode173.

The EL layers172(the EL layer172R, the EL layer172G, and the EL layer172B inFIG. 7) that emit light of different colors are provided in the respective pixels, whereby the emission colors with high color purity can be obtained. Thus, the coloring layer131is not necessarily formed. In addition, as described above, when the light-emitting element170R, the light-emitting element170G, and the light-emitting element170B each have a microcavity structure, the color purity can be further increased.

Modification Example 4

FIG. 8illustrates a cross section of a display device100D, which is another modification example of the display device100. The display device100D includes the light-emitting element170having a microcavity structure.

With the use of the light-emitting elements170each having a microcavity structure, a different emission color for each pixel can be obtained even when the EL layers172emitting light of the same color (e.g., white) are provided in the respective pixels.FIG. 8illustrates the light-emitting element170R emitting the red light175R, the light-emitting element170G emitting the green light175G, and the light-emitting element170B emitting the blue light175B, for example.

In the case where the light-emitting element170has a microcavity structure, the electrode173is formed using a conductive material (a semi-transmissive material) which transmits and reflects a certain proportion of light, and the electrode171is formed by stacking a conductive material having high reflectance (the reflectance of visible light is preferably higher than or equal to 80% and lower than or equal to 100%, further preferably higher than or equal to 90% and lower than or equal to 100%) and a conductive material having high transmittance (the transmittance of visible light is preferably higher than or equal to 70% and lower than or equal to 100%, further preferably higher than or equal to 90% and lower than or equal to 100%). Here, the electrode171has a stacked structure of an electrode171_1formed using a conductive material having a function of reflecting visible light and an electrode171_2formed using a conductive material having a function of transmitting visible light. The electrode171_2is provided between the EL layer172and the electrode171_1that can function as a reflective electrode.

For example, the electrode173may be formed using a conductive material containing silver (Ag) or a conductive material containing aluminum (Al) having a thickness of 1 nm to 30 nm, or preferably 1 nm to 15 nm. For example, the electrode173is formed using a 10-nm-thick conductive material containing silver and magnesium.

The electrode171_1may be formed using a conductive material containing silver (Ag) or a conductive material containing aluminum (Al) having a thickness of 50 nm to 500 nm, or preferably 50 nm to 200 nm, for example. For example, the electrode171_1is formed using a 100-nm-thick conductive material containing silver.

The electrode171_2may be formed using a conductive oxide containing indium (In) or a conductive oxide containing zinc (Zn) having a thickness of 1 nm to 200 nm, preferably 5 nm to 100 nm, for example. For example, the electrode171_2is formed using indium tin oxide. Furthermore, a conductive oxide may be provided under the electrode171_1.

By changing the thickness t of the electrode171_2, a distance d from the interface A between the electrode173and the EL layer172to the interface B between the electrode171_1and the electrode171_2can be set to a given value. In other words, an optical path length from the interface A to the interface B can be set to a given value.

When the wavelength of light generated in the EL layer172is set to λ, the light-emitting element170having a microcavity structure emits light whose optical path length is equal to an integer multiple of λ/2. Thus, light emission with a narrow spectral width can be obtained. When the thickness t of the electrode171_2varies between pixels, light with a different wavelength for each pixel can be obtained even with the use of the EL layers172emitting light of the same color.

The use of a microcavity structure makes it possible to have high color purity of each emission color, and a display device achieves favorable color reproducibility. It is not necessary to independently form the EL layer172in each pixel depending on the emission color; thus, the productivity of a display device can be improved. Furthermore, a high-definition display device can be achieved easily.

Note that a method for adjusting the distance d is not limited to the above method. For example, the distance d may be adjusted by changing the thickness of the EL layer172.

The light-emitting element170having a microcavity structure and the coloring layer131may be used in combination. In that case, glare of light that enters from the outside (also referred to as external light) can be reduced.

For example, the case where the light-emitting element170that emits the red light175R with a microcavity structure and the coloring layer131that transmits red light are used in combination is described. The wavelength range other than a red wavelength range of external light is absorbed by the coloring layer131that transmits the red light. The red external light that passes through the coloring layer131and enters the light-emitting element170almost disappears by the effect of the microcavity structure.

When the light-emitting element170having a microcavity structure and the coloring layer131are used in combination, glare of external light can be reduced, leading to improved visibility of a display device. Furthermore, a display device having a high display quality can be achieved. Moreover, since a functional member for preventing glare is not necessarily provided, the productivity of a display device can be increased.

An example of a method for manufacturing the display device100is described with reference to drawings. In this embodiment, the display region235is focused on in the description of the manufacturing method.

Note that insulating layers, semiconductor layers, conductive layers for forming electrodes and wirings, and the like included in the display device can be formed by any of a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, a pulsed laser deposition (PLD) method, an atomic layer deposition (ALD) method, and the like. As the CVD method, a plasma-enhanced chemical vapor deposition (PECVD) method or a thermal CVD method may be used. As the thermal CVD method, for example, a metal organic chemical vapor deposition (MOCVD) method may be used.

The insulating layers, the semiconductor layers, the conductive layers for forming the electrodes and the wirings, or the like included in the display device may be formed by a method such as spin coating, dipping, spray coating, ink-jetting, dispensing, screen printing, or offset printing, or with a slit coater, a roll coater, a curtain coater, or a knife coater.

By using the PECVD method, a high-quality film can be formed at relatively low temperatures. By using a deposition method that does not use plasma for deposition, such as the MOCVD method, the ALD method, or the thermal CVD method, damage is not easily caused on a surface on which the film is deposited. For example, a wiring, an electrode, an element (e.g., transistor or capacitor), or the like included in a semiconductor device might be charged up by receiving charges from plasma. In that case, accumulated charges might break the wiring, electrode, element, or the like included in the semiconductor device. Such plasma damage is not caused in the case of using a deposition method without using plasma, and thus the yield of a semiconductor device can be increased. In addition, since plasma damage does not occur in the deposition, a film with few defects can be obtained.

Unlike in a deposition method in which particles ejected from a target or the like are deposited, in the CVD method and the ALD method, a film is formed by reaction at a surface of an object. Thus, the CVD method and the ALD method enable favorable step coverage almost regardless of the shape of an object. In particular, the ALD method enables excellent step coverage and excellent thickness uniformity and can be suitably used for covering a surface of an opening with a high aspect ratio, for example. On the other hand, the ALD method has a relatively low deposition rate; thus, it is sometimes preferable to combine the ALD method with another deposition method with a high deposition rate, such as the CVD method.

When the CVD method or the ALD method is used, composition of a film to be formed can be controlled with a flow rate ratio of source gases. For example, by the CVD method or the ALD method, a film with a desired composition can be formed by adjusting the flow rate ratio of the source gases. Moreover, by changing the flow rate ratio of the source gases during deposition by the CVD method or the ALD method, a film whose composition is continuously changed can be formed. In the case where a film is formed while the flow rate ratio of the source gases is changed, the time for deposition can be shorter than in the case where a film is formed using a plurality of deposition chambers because time for transfer and pressure adjustment can be saved. Thus, semiconductor devices can be manufactured with improved productivity in some cases.

In the case where a film is formed by the ALD method, a gas that does not contain chlorine is preferably used as a material gas.

When an oxide semiconductor is formed by a sputtering method, each chamber of a sputtering apparatus is preferably evacuated to a high vacuum (to the degree of approximately 5×10−7Pa to 1×10−4Pa) by an adsorption vacuum pump such as a cryopump so that water and the like acting as impurities for the oxide semiconductor can be removed as much as possible. In particular, the partial pressure of gas molecules corresponding to H2O (gas molecules corresponding to m/z=18) in the chamber in the standby mode of the sputtering apparatus is preferably lower than or equal to 1×10−4Pa, further preferably 5×10−5Pa. The deposition temperature is preferably higher than or equal to RT and lower than or equal to 500° C., further preferably higher than or equal to RT and lower than or equal to 300° C., still further preferably higher than or equal to RT and lower than or equal to 200° C.

In addition, increasing the purity of a sputtering gas is necessary. For example, as an oxygen gas or an argon gas used as a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower is used, whereby entry of moisture or the like into the oxide semiconductor film can be minimized.

In the case where the insulating layers, the conductive layers, the semiconductor layers, or the like are formed by a sputtering method using a sputtering gas containing oxygen, oxygen can be supplied to a layer over which these layers are formed. As the amount of oxygen contained in the sputtering gas increases, the amount of oxygen supplied to the layer over which these layers are formed tends to increase.

The layers (thin films) included in the display device can be processed by a photolithography method or the like. Alternatively, island-shaped layers may be formed by a film formation method using a blocking mask. Alternatively, the layers may be processed by a nanoimprinting method, a sandblasting method, a lift-off method, or the like. Examples of the photolithography method include a method in which a resist mask is formed over a layer (thin film) to be processed, part of the layer (thin film) is selectively removed by using the resist mask as a mask, and the resist mask is removed, and a method in which a photosensitive layer is formed and exposed to light and developed to be processed into a desired shape.

In the case of using light in the photolithography method, any of an i-line (light with a wavelength of 365 nm), a g-line (light with a wavelength of 436 nm), and an h-line (light with a wavelength of 405 nm), or combined light of any of them can be used for exposure. Alternatively, ultraviolet light, KrF laser light, ArF laser light, or the like can be used. Exposure may be performed by liquid immersion exposure technique. As the light for the exposure, extreme ultra-violet (EUV) light or X-rays may be used. Instead of the light for the exposure, an electron beam can be used. It is preferable to use extreme ultra-violet (EUV) light, X-rays, or an electron beam because extremely minute processing can be performed. Note that in the case of performing exposure by scanning of a beam such as an electron beam, a photomask is not needed.

For the removal (etching) of the layers (thin films), a dry etching method, a wet etching method, a sandblast method, or the like can be used. Alternatively, these etching methods may be employed in combination.

Manufacturing Method Example

The display device100described in this embodiment is manufactured by combining a first element substrate181(seeFIG. 9D) and a second element substrate182(seeFIG. 14).

First, a method for forming the first element substrate181is described.

A separation layer332is formed over a support substrate331, and a layer333is formed over the separation layer332(seeFIG. 9A). The support substrate331can be formed using a material similar to that of the substrate111or121. The support substrate331preferably has a function of transmitting ultraviolet light.

In this embodiment, a glass substrate is used as the support substrate331. The support substrate331preferably has mechanical strength high enough to withstand a manufacturing process of the display device100. In other words, the support substrate331preferably has rigidity high enough to facilitate the transfer and has resistance to heat applied in the manufacturing process.

The separation layer332preferably has a function of absorbing light for separation performed later in Step C6. As the separation layer332, a metal layer, a metal oxide layer, or the like can be used. For example, titanium oxide (TAX), molybdenum oxide, aluminum oxide, tungsten oxide, indium tin oxide containing silicon (ITSO), indium zinc oxide, and an In—Ga—Zn oxide can be used for the separation layer332.

There is no particular limitation on a method for forming the separation layer332. For example, the separation layer332can be formed by a sputtering method, a plasma-enhanced CVD method, an evaporation method, a sol-gel method, an electrophoretic method, a spray method, or the like.

In the case where the separation layer332includes a metal oxide, the separation layer332can be formed in such a manner that a metal layer is formed and then oxygen is introduced into the metal layer. At this time, only a surface of the metal layer or the entire metal layer is oxidized. In the former case, the introduction of oxygen into the metal layer forms a structure in which the metal layer and the metal oxide layer are stacked.

The metal layer may be oxidized by heating in an oxygen-containing atmosphere. In this case, the metal layer is preferably heated while an oxygen-containing gas is supplied. The temperature at which the metal layer is heated is preferably higher than or equal to 100° C. and lower than or equal to 500° C., further preferably higher than or equal to 100° C. and lower than or equal to 450° C., still further preferably higher than or equal to 100° C. and lower than or equal to 400° C., yet still further preferably higher than or equal to 100° C. and lower than or equal to 350° C.

The temperature at which the metal layer is heated is preferably lower than or equal to the maximum temperature in manufacturing the transistor. In that case, the maximum temperature in manufacturing the display device can be prevented from increasing. When the temperature at which the metal layer is heated is set to lower than or equal to the maximum temperature in manufacturing the transistor, a manufacturing apparatus for the manufacturing process of the transistor, for example, can also be utilized, which reduces additional capital investment and the like. As a result, the display device can be manufactured at reduced costs. When the manufacturing temperature of the transistor is lower than or equal to 350° C., for example, the temperature of the heat treatment is preferably lower than or equal to 350° C.

Alternatively, the separation layer332may be formed in such a manner that a metal layer is formed and then radical treatment is performed on a surface of the metal layer. In the radical treatment, the surface of the metal layer is preferably exposed to an atmosphere containing an oxygen radical and/or a hydroxyl radical. For example, plasma treatment is preferably performed in an atmosphere containing oxygen and/or water vapor (H2O).

The radical treatment can be performed with a plasma generation apparatus or an ozone generation apparatus. For example, oxygen plasma treatment, hydrogen plasma treatment, water plasma treatment, ozone treatment, or the like can be performed. Oxygen plasma treatment can be performed by generating plasma in an oxygen-containing atmosphere. Hydrogen plasma treatment can be performed by generating plasma in a hydrogen-containing atmosphere. Water plasma treatment can be performed by generating plasma in an atmosphere containing water vapor (H2O). Water plasma treatment is particularly preferable because it makes a large amount of moisture present on the surface of the separation layer332or in the separation layer332.

Plasma treatment may be performed in an atmosphere containing two or more of oxygen, hydrogen, water (water vapor), and an inert gas (typically, argon). Examples of the plasma treatment include plasma treatment in an atmosphere containing oxygen and hydrogen, plasma treatment in an atmosphere containing oxygen and water, plasma treatment in an atmosphere containing water and argon, plasma treatment in an atmosphere containing oxygen and argon, and plasma treatment in an atmosphere containing oxygen, water, and argon. An argon gas is preferably used for the plasma treatment, in which case the separation layer332can be damaged during the plasma treatment.

Two or more kinds of plasma treatment may be performed sequentially without exposure to the air. For example, argon plasma treatment may be followed by water plasma treatment.

In the above manner, hydrogen, oxygen, a hydrogen radical (H*), an oxygen radical (O*), a hydroxyl radical (OH*), and the like can be present on the surface of the separation layer332or in the separation layer332. These elements are changed into H2O by being heated by heat treatment or light irradiation.

The separation layer332preferably has a thickness of, for example, greater than or equal to 1 nm and less than or equal to 200 nm, further preferably greater than or equal to 5 nm and less than or equal to 100 nm, and still further preferably greater than or equal to 5 nm and less than or equal to 50 nm. In the case where the separation layer332is formed by oxidizing the metal layer, the completed separation layer332sometimes becomes thicker than the initially formed metal layer.

Before or during a separation between the separation layer332and the layer333, performed in a later step, the force required for the separation can be reduced by feeding a water-containing liquid to the interface between the separation layer332and the layer333. The smaller the contact angle between the separation layer332and the liquid is, the smaller the force required for the separation can be. Specifically, the contact angle between the separation layer332and the water-containing liquid is preferably greater than 0° and less than or equal to 60°, further preferably greater than 0° and less than or equal to 50°. When the wettability with respect to the water-containing liquid is extremely high (e.g., when the contact angle is approximately 20° or less), it is sometimes difficult to obtain an accurate value of the contact angle. The wettability of the separation layer332with respect to the water-containing liquid is preferably as high as possible. Therefore, the wettability with respect to the water-containing liquid may be high enough to prevent an accurate value of the contact angle from being obtained.

Titanium oxide, tungsten oxide, or the like is suitable for the separation layer332. Titanium oxide is preferably used because the costs can be lower than that when tungsten oxide is used.

The separation layer332may have a photocatalyst function. When the separation layer332having a photocatalyst function is irradiated with light, photocatalytic reaction can be caused. As a result, the bonding strength between the separation layer332and the layer333is weakened to facilitate the separation in some cases. The separation layer332can be appropriately irradiated with light with a wavelength for activating the separation layer332. For example, the separation layer332is irradiated with ultraviolet light. For example, after the separation layer332is formed, the separation layer332may be directly irradiated with ultraviolet light not through another layer. An ultraviolet light lamp can be suitably used for the ultraviolet light irradiation. Examples of the ultraviolet light lamp include a mercury lamp, a mercury xenon lamp, and a metal halide lamp. Alternatively, the separation layer332may be activated through the laser irradiation step which is performed before the separation.

The separation layer332may be formed using titanium oxide to which a metal element or nitrogen is added. When the separation layer332is formed using titanium oxide to which such an element is added, visible light can be used for the separation between the separation layer332and the layer333instead of ultraviolet light.

The layer333can be formed using any of a variety of resin materials (including resin precursors). The layer333is preferably formed using a thermosetting material. The layer333may be formed using a material with photosensitivity or a material without photosensitivity (also called a non-photosensitive material).

When a photosensitive material is used, part of the layer333is removed by a photolithography method, whereby the layer333having a desired shape can be formed.

The layer333is preferably formed using a material containing a polyimide resin or a polyimide resin precursor. The layer333can be formed using, for example, a material containing a polyimide resin and a solvent or a material containing polyamic acid and a solvent. A polyimide is a material suitable for a planarization film or the like of a display device, and therefore, the film formation apparatus and the material can be shared. Thus, there is no need to prepare another apparatus and another material to obtain the structure of one embodiment of the present invention.

Examples of resin materials that can be used to form the layer333include an acrylic resin, an epoxy resin, a polyamide resin, a polyimide-amide resin, a siloxane resin, a benzocyclobutene-based resin, a phenol resin, and precursors of these resins.

The layer333is preferably formed with a slit coater or a spin coater. By a spin coating method, a thin film can be uniformly formed over a large-sized substrate.

The layer333is preferably formed using a solution having a viscosity of greater than or equal to 5 cP and less than 500 cP, further preferably greater than or equal to 5 cP and less than 100 cP, still further preferably greater than or equal to 10 cP and less than or equal to 50 cP. The lower the viscosity of the solution is, the easier the coating is. Furthermore, the lower the viscosity of the solution is, the more the entry of air bubbles can be inhibited and thus the higher the quality of a formed layer can be.

The layer333may be formed using an inorganic material that releases hydrogen by heating. For example, the layer333may be formed using amorphous silicon containing hydrogen.

Next, the layer333is cured by heat treatment. The heat treatment can be performed while a gas containing one or more of oxygen, nitrogen, and a rare gas (e.g., argon) is supplied into a chamber of a heating apparatus, for example. Alternatively, the heat treatment can be performed in an air atmosphere with the use of a chamber of a heating apparatus, a hot plate, or the like.

When the heat treatment is performed in an air atmosphere or performed while a gas containing oxygen is supplied, the layer333is sometimes colored by oxidation to have decreased visible-light transmitting property. For that reason, heating is preferably performed while a nitrogen gas is supplied. Thus, the visible-light transmitting property of the layer333can be increased.

The temperature of the heat treatment is preferably set to lower than or equal to the maximum temperature in manufacturing the transistor. When the manufacturing temperature of the transistor is lower than or equal to 350° C., for example, the temperature of the heat treatment is preferably lower than or equal to 350° C.

The duration of the heat treatment is preferably longer than or equal to 5 minutes and shorter than or equal to 24 hours, further preferably longer than or equal to 30 minutes and shorter than or equal to 12 hours, still further preferably longer than or equal to 1 hour and shorter than or equal to 6 hours, for example. Note that the duration of the heat treatment is not particularly limited to these examples. For example, the duration of the heat treatment that is performed by a rapid thermal annealing (RTA) method may be shorter than five minutes.

As the heating apparatus, it is possible to use, for example, an electric furnace or any apparatus for heating an object by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, an RTA apparatus such as a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTA apparatus is an apparatus for heating an object 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. With such an RTA apparatus, the treatment time can be shortened and thus the RTA apparatus is preferred for mass production. Alternatively, an in-line heating apparatus may be used in the heat treatment.

Before the heat treatment, heat treatment (also referred to as prebaking treatment) for removing the solvent contained in the layer333may be performed. The temperature of the prebaking treatment can be set as appropriate according to the material to be used. For example, the temperature of the prebaking treatment can be higher than or equal to 50° C. and lower than or equal to 180° C., higher than or equal to 80° C. and lower than or equal to 150° C., or higher than or equal to 90° C. and lower than or equal to 120° C. The heat treatment may double as the prebaking treatment, in which case the solvent contained in the layer333can be removed by the heat treatment.

The layer333preferably has a thickness of greater than or equal to 0.01 μm and less than 10 μm, further preferably greater than or equal to 0.1 μm and less than or equal to 5 μm, still further preferably greater than or equal to 0.5 μm and less than or equal to 3 μm. By forming the layer333thin, the display device can be manufactured at low costs.

The layer333preferably has a thermal expansion coefficient of greater than or equal to 0.1 ppm/° C. and less than or equal to 50 ppm/° C., further preferably greater than or equal to 0.1 ppm/° C. and less than or equal to 20 ppm/° C., still further preferably greater than or equal to 0.1 ppm/° C. and less than or equal to 10 ppm/° C. The lower the thermal expansion coefficient of the layer333is, the more the generation of a crack in a layer included in a transistor or the like and breakage of a transistor or the like which are caused owing to the heating can be prevented.

The separation layer332can be physically separated from the layer333instead of being irradiated with light.

In the case where the separation layer332is physically separated from the layer333, a stack including a layer containing a high-melting-point metal material such as tungsten and a layer containing oxide of the metal material is used as the separation layer332, for example. A metal layer that contains a high-melting-point metal material and has a surface oxidized by oxygen plasma treatment or the like may also be used.

In addition, in the case where the separation layer332is physically separated from the layer333, an inorganic insulating material containing oxygen, such as silicon oxide, silicon oxynitride, or silicon nitride oxide, is used for the layer333.

The separation layer332can be separated from the layer333by applying a perpendicular tensile force to the support substrate331over which the separation layer332is formed.

As in the separation by the light irradiation, the physical separation can be easily performed in such a manner that a water-containing liquid such as water or an aqueous solution is fed to the separation interface so that the liquid penetrates into the separation interface. Furthermore, an adverse effect of static electricity caused at the separation on a functional element such as a transistor (e.g., damage to a semiconductor element from static electricity) can be suppressed.

The liquid to be fed can be water (preferably pure water), a neutral, alkaline, or acidic aqueous solution, an aqueous solution in which a salt is dissolved, or the like. Other examples of the liquid include ethanol and acetone. Any of a variety of organic solvents may also be used.

When the separation is performed by the light irradiation, the separation layer332is not necessarily formed. Note that the formation of the separation layer332leads to an increase in the absorption amount of irradiation light. In addition, the formation of the separation layer332leads to an increase in the yield of a separation step. Accordingly, the productivity of the display device can be improved.

The layer333may be formed using a water-soluble resin material. When the layer333is formed using a water-soluble resin material, a cleaning step can also serve as a step of separating the support substrate331. Accordingly, a light irradiation step, a physical separation step, and the like can be omitted. A step of removing the layer333, which is performed later, can also be omitted.

Next, an insulating layer129is formed over the layer333.

Next, the coloring layer131is formed over the insulating layer129and the light-blocking layer132(seeFIG. 9C).

The coloring layer131is formed using a photosensitive material, in which case the processing into an island shape can be performed by a photolithography method or the like. The coloring layer131and the light-blocking layer132are formed as needed. Thus, at least one of the coloring layer131and the light-blocking layer132is not formed in some cases. Note that in the display device100, the light-blocking layer132is formed to overlap with the peripheral circuit region232, the peripheral circuit region233, and the like.

Then, the insulating layer133is formed over the coloring layer131and the light-blocking layer132(seeFIG. 9D).

The insulating layer133preferably functions as a planarization layer. A resin material such as an acrylic resin or an epoxy resin is suitably used for the insulating layer133. An inorganic insulating layer may be used as the insulating layer133.

Through the above steps, the first element substrate181can be formed.

Next, a method for forming the second element substrate182is described.

A separation layer335is formed over a support substrate334, a layer336is formed over the separation layer335, and the insulating layer113is formed over the layer336(seeFIG. 10A). For the support substrate334, a material similar to that of the support substrate331can be used. The separation layer335is formed using a material and a method similar to those of the separation layer332. The layer336is formed using a material and a method similar to those of the layer333. The insulating layer113is formed using a material and a method similar to those of the insulating layer129.

Then, the electrode221is formed over the insulating layer113. The electrode221can be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed (seeFIG. 10B).

Next, the insulating layer211is formed over the insulating layer113and the electrode221(seeFIG. 10C). For the insulating layer211, for example, an inorganic insulating film such as a silicon nitride film, a silicon oxynitride film, a silicon oxide film, a silicon nitride oxide film, an aluminum oxide film, or an aluminum nitride film can be used. Alternatively, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, a neodymium oxide film, or the like may be used. A stack including two or more of the above insulating films may also be used.

An inorganic insulating film is preferably formed at high temperatures because the film can have higher density and a higher barrier property as the deposition temperature becomes higher. The substrate temperature during the deposition of the inorganic insulating film is preferably higher than or equal to room temperature (25° C.) and lower than or equal to 350° C., further preferably higher than or equal to 100° C. and lower than or equal to 300° C.

When an oxide semiconductor is used for the semiconductor layer231, an insulating layer including a region in contact with the semiconductor layer231is preferably an insulating layer from which oxygen is released by heating (hereinafter, such an insulating layer is also referred to as an insulating layer containing excess oxygen). Therefore, when an oxide semiconductor is used for the semiconductor layer231, the insulating layer211is preferably the insulating layer containing excess oxygen.

Note that in this specification and the like, oxygen released from a layer by heating is referred to as excess oxygen. The insulating layer containing excess oxygen may be an insulating layer in which the amount of released oxygen converted into oxygen atoms is 1.0×1018atoms/cm3or more, 1.0×1019atoms/cm3or more, or 1.0×1020atoms/cm3or more, in thermal desorption spectroscopy (TDS) analysis performed under such a condition that a surface of the insulating layer is heated at a temperature higher than or equal to 100° C. and lower than or equal to 700° C., preferably higher than or equal to 100° C. and lower than or equal to 500° C.

Then, the semiconductor layer231is formed (seeFIG. 10C). In this embodiment, an oxide semiconductor layer is formed as the semiconductor layer231. The oxide semiconductor layer can be formed in the following manner: an oxide semiconductor film is formed, a resist mask is formed, the oxide semiconductor film is etched, and the resist mask is removed.

The substrate temperature during the formation of the oxide semiconductor film is preferably lower than or equal to 350° C., further preferably higher than or equal to room temperature and lower than or equal to 200° C., still further preferably higher than or equal to room temperature and lower than or equal to 130° C.

The oxide semiconductor film can be formed using one of an inert gas and an oxygen gas. Note that there is no particular limitation on the flow rate ratio of oxygen (the partial pressure of oxygen) at the time of forming the oxide semiconductor film. To fabricate a transistor having high field-effect mobility, the flow rate ratio of oxygen (the partial pressure of oxygen) at the time of forming the oxide semiconductor film is preferably higher than or equal to 0% and lower than or equal to 30%, further preferably higher than or equal to 5% and lower than or equal to 30%, still further preferably higher than or equal to 7% and lower than or equal to 15%.

An oxide semiconductor film preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained.

The energy gap of the oxide semiconductor is preferably 2 eV or more, further preferably 2.5 eV or more, and still further preferably 3 eV or more. The use of such an oxide semiconductor having a wide energy gap leads to a reduction in off-state current of a transistor.

A semiconductor material having an energy gap of 2.5 eV or more is particularly preferable because of its high visible-light transmittance.

The oxide semiconductor film can be formed by a sputtering method. Alternatively, a PLD method, a PECVD method, a thermal CVD method, an ALD method, a vacuum evaporation method, or the like may be used.

Next, the electrode224a, the electrode224b, and the wiring125are formed (seeFIG. 10D). The electrode224a, the electrode224b, and the wiring125can be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed. Each of the electrode224aand the electrode224bis connected to the semiconductor layer231.

Note that during the formation of the electrode224aand the electrode224b, the semiconductor layer231might be partly etched to be thin in a region not covered by the resist mask.

Next, the insulating layer210is formed (seeFIG. 11A). It is preferable to use an oxide insulating layer formed in an oxygen-containing atmosphere, such as a silicon oxide layer or a silicon oxynitride layer, as the insulating layer210. The oxide insulating layer is formed in an oxygen-containing atmosphere, whereby the insulating layer containing excess oxygen can be formed.

Next, the electrode226is formed over the insulating layer210(seeFIG. 11B). The electrode226includes a region overlapping with the semiconductor layer231. Through the above steps, the transistor251can be formed. Although not illustrated, the transistor252is formed in a similar manner.

Next, the insulating layer213is formed (seeFIG. 11C). The insulating layer213is preferably formed using an insulating material with low oxygen diffusibility and low oxygen permeability, such as silicon nitride.

When the insulating layer210is the insulating layer containing excess oxygen, a stack including the insulating layer210and an insulating film with low oxygen diffusibility and low oxygen permeability is heated, whereby oxygen can be efficiently supplied to the oxide semiconductor layer. As a result, oxygen vacancies in the oxide semiconductor layer can be filled and defects at the interface between the oxide semiconductor layer and the insulating layer210can be repaired, leading to a reduction in defect levels. Accordingly, an extremely highly reliable transistor can be fabricated. The use of such a transistor in a display device can increase the reliability of the display device.

Then, the insulating layer115is formed (seeFIG. 11D). The display element is formed on the insulating layer115in a later step; thus, the insulating layer115preferably functions as a planarization layer.

Next, an opening161that reaches the electrode224ais formed in the insulating layer115, the insulating layer213, and the insulating layer210(seeFIG. 11D).

Then, the electrode171ais formed over a region, which is to be the pixel230a, of the insulating layer115(seeFIG. 12A). The electrode171acan be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed. Note that the electrode171ais electrically connected to the electrode224aincluded in the transistor251in the region to be the pixel230a. The electrode171ais formed using a conductive material that reflects visible light.

Next, the insulating layer116is formed (seeFIG. 12B). The display element is formed on the insulating layer116in a later step; thus, the insulating layer116preferably functions as a planarization layer. The insulating layer116is formed using a material and a method similar to those of the insulating layer115.

Then, an opening162is formed in the insulating layer116(seeFIG. 12B). The opening162is formed to overlap with the opening161in a region to be the pixel230b.

Then, the electrode171bis formed over the region, which is to be the pixel230b, of the insulating layer116(seeFIG. 12C). The electrode171bcan be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed. Note that the electrode171bis electrically connected to the electrode224aincluded in the transistor251in the region to be the pixel230b. The electrode171bis formed using a material and a method similar to those of the electrode171a.

Next, part of the insulating layer116that overlaps with the electrode171ais removed to expose the surface of the electrode171a(seeFIG. 13A).

Then, the insulating layer114that covers end portions of the electrode171aand the electrode171bis formed (seeFIG. 13B). The insulating layer114includes an opening overlapping with the electrode171aand an opening overlapping with the electrode171b.

Next, the EL layer172is formed (seeFIG. 13C). In this embodiment, the EL layer172is formed using an organic EL material. Thus, the EL layer172in this embodiment is an organic layer. The EL layer172can be formed by an evaporation method, a coating method, a printing method, a discharge method, or the like. In the case where the EL layer172is formed for each individual pixel, an evaporation method using a shadow mask such as a metal mask, an ink-jet method, or the like can be used. In the case of sharing the EL layer172by some pixels, an evaporation method not using a metal mask can be used.

Steps after the formation of the EL layer172are performed such that temperatures higher than the upper temperature limit of the EL layer172are not applied to the EL layer172.

Then, the electrode173is formed (seeFIG. 14). Part of the electrode173functions as the common electrode of the light-emitting element170aand another part of the electrode173functions as the common electrode of the light-emitting element170b. The electrode173is formed using a conductive material that transmits visible light.

Through the above steps, the second element substrate182can be formed.

Note that the use of a multi-tone mask in a photolithography process can increase the productivity of a display device. Here, a multi-tone mask is described with reference toFIGS. 15A to 15D. A multi-tone mask is a mask capable of light exposure of three levels to provide an exposed region, a half-exposed region, and an unexposed region. A multi-tone mask is a light-exposure mask through which light is transmitted to have a plurality of intensities. One-time light exposure and development process allows a resist mask with a plurality of thicknesses (typically, two kinds of thicknesses) to be formed. Accordingly, the use of a multi-tone mask can reduce the number of light-exposure masks (photomasks).

As illustrated inFIG. 15A, the gray-tone mask801aincludes a light-transmitting substrate802, and a light-blocking portion803and a diffraction grating804which are formed over the light-transmitting substrate802. The light transmittance of the light-blocking portion803is 0%. On the other hand, in the diffraction grating804, a gap in a light-transmitting portion in a slit form, a dot form, a mesh form, or the like is adjusted to be less than or equal to the resolution limit of light for exposure; thus, the light transmittance can be adjusted. The slits, dots, or mesh of the diffraction grating804can be either periodic or nonperiodic.

As the light-transmitting substrate802, a light-transmitting substrate such as a quartz substrate can be used. The light-blocking portion803and the diffraction grating804can be formed using a light-blocking material which absorbs light, such as chromium or chromium oxide.

When the gray-tone mask801ais irradiated with light for exposure, light transmittance805of the light-blocking portion803is 0% and the light transmittance805of a region where neither the light-blocking portion803nor the diffraction grating804is provided is 100%, as shown inFIG. 15B. The light transmittance of the diffraction grating804can be adjusted in the range from 10% to 70%. The light transmittance of the diffraction grating804can be adjusted by adjusting the gap and pitch of slits, dots, or mesh of the diffraction grating.

As illustrated inFIG. 15C, the half-tone mask801bincludes the light-transmitting substrate802, and a semi-transmissive portion807and a light-blocking portion806which are formed over the light-transmitting substrate802. The semi-transmissive portion807can be formed using MoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-blocking portion806can be formed using a light-blocking material which absorbs light, such as chromium or chromium oxide.

When the half-tone mask801bis irradiated with light for exposure, light transmittance808of the light-blocking portion806is 0% and the light transmittance808of a region where neither the light-blocking portion806nor the semi-transmissive portion807is provided is 100%, as shown inFIG. 15D. The light transmittance of the semi-transmissive portion807can be adjusted in the range from 10% to 70%. The light transmittance of the semi-transmissive portion807can be adjusted with the material of the semi-transmissive portion807.

The resist mask formed using a multi-tone mask is a resist mask including a plurality of regions (here, two regions) with different thicknesses; a region having a large thickness and a region having a small thickness. In the resist mask formed using a multi-tone mask, a region having a large thickness is referred to as a projected portion of the resist mask, and a region having a small thickness is referred to as a depressed portion of the resist mask.

Next, a method for manufacturing the display device100including the first element substrate181and the second element substrate182is described.

The first element substrate181and the second element substrate182are attached to each other with the adhesive layer142so that the coloring layer131faces the light-emitting element170(seeFIG. 16A). At this time, the attachment is performed so that the light-emitting region of the light-emitting element170overlaps with the coloring layer131.

As the adhesive layer142, any of a variety of curable adhesives such as a reactive curable adhesive, a thermosetting adhesive, an anaerobic adhesive, and a photocurable adhesive such as an ultraviolet curable adhesive can be used. Alternatively, an adhesive sheet or the like may be used.

Then, the separation layer335is irradiated with ultraviolet light from the support substrate334side to separate the support substrate334and the separation layer335from the layer336(seeFIG. 16B).

Note that the separation of the support substrate334can be performed by light irradiation or can be physically performed. In both cases, before the separation, a separation trigger may be formed by separating part of the layer336from the support substrate334or the separation layer335. For example, the separation trigger may be formed by inserting a sharp instrument such as a knife between the support substrate334and the layer336. The separation trigger may be formed by a method that uses a laser, such as a laser ablation method.

In this embodiment, the separation by light irradiation is described. The ultraviolet light irradiation is preferably performed with a linear laser apparatus. The linear laser apparatuses are used in the manufacturing lines for low-temperature polysilicon (LTPS) and the like, which enables effective use of the manufacturing lines for LTPS and the like. A linear laser is laser light condensed into a long rectangular shape (shaped into a linear laser beam).

In this embodiment, the linear laser apparatus is used. Specifically, the support substrate334and the linear laser light are moved relative to the direction perpendicular to the long-axis direction of the linear laser light and parallel to the surface of the support substrate334. In a region irradiated with the laser light, the bonding strength between the separation layer335and the layer336is weakened.

The light for the irradiation preferably has a wavelength of greater than or equal to 180 nm and less than or equal to 450 nm. The light for the irradiation particularly preferably has a wavelength of 308 nm or around 308 nm. The energy density of the light is preferably greater than or equal to 250 mJ/cm2and less than or equal to 400 mJ/cm2, further preferably greater than or equal to 250 mJ/cm2and less than or equal to 360 mJ/cm2.

In the case where the light irradiation is performed with a laser apparatus, the number of shots of laser light with which one portion is irradiated can be greater than or equal to 1 and less than or equal to 50, preferably greater than 1 and less than or equal to 10, further preferably greater than 1 and less than or equal to 5.

There are portions with low light intensity on both ends of the short axis of the laser beam. Accordingly, it is preferable that one shot and another shot overlap with each other by greater than or equal to the width of the portion with low light intensity. Therefore, the number of laser shots is preferably greater than or equal to 1.1, further preferably greater than or equal to 1.25.

In this specification, the number of laser shots refers to the number of times a point (region) is irradiated with laser light, and is determined by a beam width, scanning speed, a frequency, an overlap percentage, or the like. There is a portion between a pulse and another pulse when a linear beam is moved in a scanning direction, that is, a portion where one shot and another shot partly overlap with each other, and their overlapping ratio is referred to as an overlap percentage. Note that as the overlap percentage becomes closer to 100%, the number of shots is increased, and as the overlap percentage becomes further from 100%, the number of shots is decreased. The higher the scanning speed, the smaller the number of shots.

The expression “the number of shots of the laser light is 1.1” means that there is an overlap equivalent to approximately one-tenth of the beam width between two successive shots, and can mean that the overlap percentage is 10%. Similarly, the expression “the number of shots of the laser light is 1.25” means that there is an overlap equivalent to approximately one-fourth of the beam width between two successive shots, and can mean that the overlap percentage is 25%.

Note that the energy density of light used for irradiation in the laser crystallization step of LTPS is high, e.g., greater than or equal to 350 mJ/cm2and less than or equal to 400 mJ/cm2. Furthermore, the number of laser shots needs to be large, e.g., greater than or equal to 10 and less than or equal to 100.

Meanwhile, in this embodiment, light irradiation for separating the separation layer335and the layer336from each other can be performed at a lower energy density or with a smaller number of shots than that performed in the laser crystallization step. Accordingly, the number of substrates which can be processed by a laser apparatus can be increased. The running costs of a laser apparatus can be reduced by reducing the frequency of maintenance of the laser apparatus, for example. Consequently, the manufacturing costs of display devices and the like can be reduced.

Since the light irradiation is performed at a lower energy density or with a smaller number of shots than that performed in the laser crystallization step, damage to the substrate caused by the laser light irradiation can be reduced. Accordingly, after being used once, the substrate is less likely to decrease in strength and thus can be reused, which results in cost reduction.

In this embodiment, the separation layer335is provided between the support substrate334and the layer336. Owing to the separation layer335, in some cases, the light irradiation can be performed at a lower energy density or with a smaller number of shots than that in the case where the separation layer335is not used.

If a foreign matter such as dust is adhered to the surface of the support substrate that is subjected to light irradiation at the time of the light irradiation through the support substrate, in some cases, nonuniformity occurs in the light irradiation and part with low separability is generated, leading to a reduction in yield of the step of separating the separation layer335and the layer336from each other. For that reason, it is preferable that the light irradiation surface be cleaned before or during the light irradiation. For example, the light irradiation surface of the support substrate can be cleaned with an organic solvent such as acetone, water, or the like. The light irradiation may be performed while a gas is sprayed with an air knife. Thus, nonuniformity in the light irradiation can be reduced and the yield of the separation can be increased.

Next, the support substrate334and the separation layer335are separated from the layer336(seeFIG. 17A).

Before or during the separation, a water-containing liquid is preferably fed to the separation interface. Water present at the separation interface further reduces adhesion or adhesiveness between the separation layer335and the layer336and reduces the force required for the separation. Furthermore, feeding a water-containing liquid to the separation interface sometimes weakens or breaks a bond between the separation layer335and the layer336. A chemical bond with the liquid is utilized to break a bond between the separation layer335and the layer336, which allows the separation to proceed. For example, in the case where there is a hydrogen bond between the separation layer335and the layer336, it can be assumed that feeding the water-containing liquid forms a hydrogen bond between the water and the separation layer335or the layer336to break the hydrogen bond between the separation layer335and the layer336.

The separation layer335preferably has low surface tension and high wettability with respect to a water-containing liquid. In that case, the water-containing liquid can be distributed over the entire surface of the separation layer335and can be easily fed to the separation interface. Distribution of the water over the entire separation layer335leads to uniform separation.

The water-containing liquid present at the separation interface can inhibit an adverse effect of static electricity that is caused at the time of separation on a functional element included in the second element substrate182(e.g., damage to a semiconductor element from static electricity). Static electricity on a surface of the second element substrate182which is exposed by the separation may be removed with an ionizer or the like.

In the case where a liquid is fed to the separation interface, the surface of the second element substrate182which is exposed by the separation may be dried.

Next, the layer336is removed. The layer336can be removed by a dry etching method, for example (seeFIG. 17B). Accordingly, the insulating layer113is exposed. InFIG. 17B, the removed layer336is shown by a dashed line.

Then, the substrate111is attached to the insulating layer113with the adhesive layer112(seeFIG. 18A). The adhesive layer112is formed using a material similar to that of the adhesive layer142. In this embodiment, a PET film is used as the substrate111. In that case, the display device can be reduced in weight and thickness. The display device using a film substrate such as a PET film is more robust than a display device using glass, metal, or the like. The display device can have higher flexibility.

Then, the separation layer332is irradiated with ultraviolet light from the support substrate331side to separate the support substrate331and the separation layer332from the layer333(seeFIG. 18B). This step is similar to Step C2.

Next, the support substrate331and the separation layer332are separated from the layer333(seeFIG. 19A). This step is similar to Step C3.

Next, the layer333is removed (seeFIG. 19B). Accordingly, the insulating layer123is exposed. InFIG. 19B, the removed layer333is shown by a dashed line. This step is similar to Step C4.

Next, the substrate121is attached to the insulating layer123with the adhesive layer122(seeFIG. 20). The adhesive layer122is formed using a material similar to that of the adhesive layer142. The substrate121is formed using a material similar to that of the substrate111.

Through the above steps, the display device100can be manufactured.

In this embodiment, a display device150having a structure different from the structure of the display device100is described. Note that description is made mainly on portions different from those of the display device100to avoid repeated description.

Structure Example

The display device150includes a transistor having a structure different from the structure of the transistor included in the display device100.FIG. 21Aillustrates an example of a cross section of the display device150.FIG. 21Ais a cross-sectional view taken along the dashed-dotted line A1-A2inFIG. 1A. The display device150includes a transistor251S, a transistor252S, the light-emitting element170, the coloring layer131, the light-blocking layer132, and the like between the substrate111and the substrate121. Note that the transistor251S corresponds to the transistor251, and the transistor252S corresponds to the transistor252.

The transistor251S and the transistor252S included in the display device150are top-gate transistors.FIG. 21Bis an enlarged view of the transistor252S. Note that the transistor252S can have a structure similar to that of the transistor251S.

Manufacturing Method Example

Next, an example of a method for manufacturing the display device150is described. The display device150is manufactured by combining the first element substrate181and a second element substrate182A.

The first element substrate181used for manufacturing the display device150is formed in a manner similar to that of the first element substrate181used for manufacturing the display device100.

A method for forming the second element substrate182A used for manufacturing the display device150is described. Note that description is made mainly on portions different from those of the method for forming the second element substrate182used for manufacturing the display device100to avoid repeated description.

First, the steps up to Step B4in Embodiment 2 are performed (seeFIG. 22A).

Then, an insulating layer225is formed (seeFIG. 22B). The insulating layer225can be formed using a material and a method similar to those of the insulating layer211. The insulating layer225can function as a gate insulating layer of the transistor.

Next, the electrode226including a region overlapping with the semiconductor layer231is formed over the insulating layer225(seeFIG. 22C). The electrode226can be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed.

Then, part of the insulating layer225is selectively removed by using the electrode226as a mask (seeFIG. 22D). Through Step D3, the island-shaped insulating layer225is formed, and a region of the semiconductor layer231that does not overlap with the electrode226is exposed.

Next, an impurity227for increasing conductivity is added to the region of the semiconductor layer231that has been exposed in Step D3(seeFIG. 23A). Since the oxide semiconductor is used for the semiconductor layer231in this embodiment, the impurity for forming an oxygen vacancy to increase the conductivity is added to the semiconductor layer231(the oxide semiconductor layer). As the impurity for forming the oxygen vacancy in the oxide semiconductor layer, one or more of the following can be used, for example: phosphorus, arsenic, antimony, boron, aluminum, silicon, nitrogen, helium, neon, argon, krypton, xenon, indium, fluorine, chlorine, titanium, zinc, and carbon. As a method for adding the impurity, plasma treatment, ion implantation, ion doping, plasma immersion ion implantation, or the like can be used.

When the above element is added as an impurity element to the oxide semiconductor layer, a bond between a metal element and oxygen in the oxide semiconductor layer is cut, so that an oxygen vacancy is formed. Interaction between an oxygen vacancy in the oxide semiconductor layer and hydrogen that remains in the oxide semiconductor layer or is added to the oxide semiconductor layer later can increase the conductivity of the oxide semiconductor layer.

When hydrogen enters an oxide semiconductor in which an oxygen vacancy is formed by addition of an impurity element, hydrogen enters an oxygen vacant site and forms a donor level in the vicinity of the conduction band. As a result, the conductivity of the oxide semiconductor can be increased. In other words, the resistivity of the oxide semiconductor can be reduced.

The region of the semiconductor layer231that has increased conductivity due to the addition of the impurity227can function as a source region or a drain region of the transistor. In this embodiment, the impurity227is added by using the electrode226as a mask. In other words, the source region or the drain region of the transistor is formed in a self-aligned manner.

Next, the insulating layer210is formed, and the insulating layer213is formed over the insulating layer210(seeFIG. 23B). The insulating layer210can be formed using a material and a method similar to those of the insulating layer211.

Note that it is preferable to use an insulating film formed in an oxygen-containing atmosphere, such as a silicon oxide film or a silicon oxynitride film, for the insulating layer210. The insulating layer210is preferably an insulating layer containing excess oxygen.

The insulating layer213is preferably an insulating layer with low oxygen diffusibility and low oxygen permeability, such as a silicon nitride film. An insulating layer formed in an oxygen-containing atmosphere and an insulating layer containing excess oxygen easily release a large amount of oxygen by heating. When a stack including such an insulating layer that releases oxygen and an insulating layer with low oxygen diffusibility and low oxygen permeability is heated, oxygen can be supplied to a channel formation region of the semiconductor layer231. As a result, oxygen vacancies in the channel formation region can be filled and defects at the interface between the channel formation region and the insulating layer225can be repaired, leading to a reduction in defect levels. Accordingly, an extremely highly reliable transistor can be fabricated. Thus, an extremely highly reliable display device can be manufactured.

Next, part of the insulating layer210and part of the insulating layer213are selectively removed to form an opening that reaches the source region of the semiconductor layer231and an opening that reaches the drain region of the semiconductor layer231(seeFIG. 23C).

Then, the electrode224a, the electrode224b, and the wiring125are formed over the insulating layer213(seeFIG. 23D). The electrode224a, the electrode224b, and the wiring125are formed in a manner similar to that of Step B5in Embodiment 2.

Through the above steps, the top-gate transistor251S can be formed. Although not illustrated, the top-gate transistor252S can be formed at the same time. The subsequent steps are performed in a manner similar to that of the steps including and after Step B8in Embodiment 2, whereby the second element substrate182A can be formed.

The first element substrate181and the second element substrate182A are attached to each other with the adhesive layer142so that the coloring layer131faces the light-emitting element170, whereby the display device150can be manufactured.

The display device150including the first element substrate181and the second element substrate182A is manufactured in a manner similar to that of the steps including and after Step C1in Embodiment 2.

In this embodiment, a display device200having a structure different from the structure of the display device100is described. Note that description is made mainly on portions different from those of the display device100to avoid repeated description.

Structure Example

The display device200includes a transistor251T in the display region235that has a structure different from the structure of the transistor251included in the display device100.FIG. 24Aillustrates an example of a cross section of the display device200.FIG. 24Ais a cross-sectional view taken along the dashed-dotted line A1-A2inFIG. 1A. The display device200includes the transistor251T, the transistor252, the light-emitting element170, and the like between the substrate111and the substrate121.FIG. 24Bis an enlarged view of the transistor252T.FIG. 24Cis an enlarged view of the transistor252.

The transistor251T includes an electrode221T, an electrode226T, an electrode224aT, and an electrode224bT. At least one of (preferably, all of) the electrode221T, the electrode226T, the electrode224aT, and the electrode224bT is formed using a conductive material having a function of transmitting visible light. Thus, the transistor251T has a function of transmitting visible light.

The display device200includes the bottom-emission light-emitting element170. Thus, the light175emitted from the light-emitting element170is extracted through the substrate111. As in the above-described display device100C, the EL layers172that emit light of different colors are provided in the respective pixels, so that the coloring layer131is not necessarily formed.

The transistor251T overlapping with the light-emitting element170has a function of transmitting visible light. Thus, the light175emitted from the light-emitting element170can be extracted through the substrate111without being blocked by the transistor251T.FIG. 24Dis a schematic perspective view of the pixel230that illustrates a state where the light175emitted from the light-emitting element170is extracted through the transistor251T.

Although not illustrated, an electrode of a capacitor included in each pixel is preferably formed using a conductive material having a function of transmitting visible light. In addition, conductive layers electrically connected to the transistor, the capacitor, and the like in each pixel are preferably formed using a conductive material having a function of transmitting visible light.

In general, a display device having a bottom-emission structure is difficult to have a high aperture ratio because light emitted from a light-emitting element is blocked by a transistor, a capacitor, and the like included in each pixel. According to one embodiment of the present invention, even a display device having a bottom-emission structure can have a high aperture ratio.

Note that as a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added is used, for example. A conductive material having an energy band gap of 2.5 eV or more is particularly preferable because of its high visible-light transmittance.

However, the light-transmitting conductive material has resistivity higher than that of a light-blocking conductive material such as copper or aluminum. To prevent signal delay, bus lines such as a scan line, a signal line, and a power supply line are preferably formed using a light-blocking conductive material (metal material) having low resistivity. Note that the bus lines may be formed using the light-transmitting conductive material in some cases depending on the size of the display region235, the widths of the bus lines, and the thicknesses of the bus lines, for example.

The transistor252included in the peripheral circuit region233should operate at high speed and thus is preferably formed using a light-blocking conductive material (metal material) having low resistivity.

As described above, part of the electrode173functions as the common electrode of the light-emitting element170aand another part of the electrode173functions as the common electrode of the light-emitting element170b. Thus, the resistivity of the electrode173is preferably as low as possible. In particular, in order to increase the size and definition of a display device, the resistivity of the electrode173is preferably as low as possible. The electrode173included in a display device having a bottom-emission structure can be formed using a light-blocking conductive material having low resistivity; thus, the electrode173can have reduced resistivity.

Modification Example 1

FIG. 25illustrates a cross section of a display device200A, which is a modification example of the display device200. The display device200A includes the coloring layer131between the transistor251T and the insulating layer115. The display device200A also includes the light-emitting element170whose emission color is white.

The light175is emitted from the light-emitting element170to the substrate111side through the coloring layer131and the like. The hue of the light175emitted from the light-emitting element170can be changed to red, green, blue, cyan, magenta, yellow, or the like depending on the material of the coloring layer131.

Modification Example 2

In the case where the electrode171is provided to overlap with the wiring125, the potential of the electrode171is changed unintentionally in some cases in response to a change in the potential of the wiring125. For example, in the case where the wiring125is a wiring for supplying a video signal, after the writing of a video signal to the specific pixel230, the potential written to the pixel230is changed unintentionally in some cases in response to a change in the potential of the wiring125overlapping with the pixel230. Thus, the display quality of the display device is decreased in some cases.

FIG. 26illustrates a cross section of a display device200B, which is a modification example of the display device200A. In the display device200B, an electrode126is formed over a region of the coloring layer131that overlaps with the wiring125. The electrode126is covered with the insulating layer115. The electrode126is supplied with a fixed potential such as a common potential or a power supply potential.

In a region where the electrode171overlaps with the wiring125, the electrode126is provided between the electrode171and the wiring125, whereby an unintentional change in the potential of the electrode171can be suppressed because the potential of the electrode171is not affected by a change in the potential of the wiring125.

Note that an example in which the electrode126is provided over the coloring layer131is described in this embodiment; however, one embodiment of the present invention is not limited to this example. For example, the electrode126may be provided over the insulating layer213or the insulating layer210.

The electrode126may be formed using a light-transmitting conductive material or a light-blocking conductive material.

In this embodiment, a specific structure example of the display device100is described.FIG. 27Ais a block diagram illustrating the display device100. As described in Embodiment 1, the display device100includes the display region235, the peripheral circuit region232, and the peripheral circuit region233.

The circuit included in the peripheral circuit region232functions as, for example, a scan line driver circuit. The circuit included in the peripheral circuit region232functions as, for example, a signal line driver circuit. Some sort of circuit may be provided to face the peripheral circuit region232with the display region235positioned therebetween. Some sort of circuit may be provided to face the peripheral circuit region233with the display region235positioned therebetween. As described above, the circuits included in the peripheral circuit region232and the peripheral circuit region233are collectively referred to as a peripheral driver circuit in some cases.

Any of a variety of circuits such as a shift register, a level shifter, an inverter, a latch, an analog switch, and a logic circuit can be used as the peripheral driver circuit. A transistor, a capacitor, and the like can be used in the peripheral driver circuit. A transistor included in the peripheral driver circuit can be formed in the same steps as the transistors included in the pixel230.

The display device200includes m wirings236which are arranged substantially parallel to each other and whose potentials are controlled by the circuits included in the peripheral circuit region232, and n wirings237which are arranged substantially parallel to each other and whose potentials are controlled by the circuits included in the peripheral circuit region233.

The display region235includes the plurality of pixels230arranged in a matrix. Full-color display can be achieved by making the pixel230that adjusts red light, the pixel230that adjusts green light, and the pixel230that adjusts blue light collectively function as one pixel240and by adjusting the amount of light (emission luminance) emitted from each pixel230. Thus, the three pixels230each function as a sub-pixel. That is, three sub-pixels adjust, for example, the amount of light of red light, green light, and blue light (see FIG.27B1). The light colors adjusted by the three sub-pixels are not limited to red (R), green (G), and blue (B) and may be cyan (C), magenta (M), and yellow (Y) (see FIG.27B2).

Four sub-pixels may collectively function as one pixel. For example, a sub-pixel that adjusts white light may be added to the three sub-pixels that adjust red light, green light, and blue light (see FIG.27B3). The addition of the sub-pixel that adjusts white light can increase the luminance of the display region. Alternatively, a sub-pixel that adjusts yellow light may be added to the three sub-pixels that adjust red light, green light, and blue light (see FIG.27B4). Alternatively, a sub-pixel that adjusts white light may be added to the three sub-pixels that adjust cyan light, magenta light, and yellow light (see FIG.27B5).

When sub-pixels that adjust light of colors such as red, green, blue, cyan, magenta, and yellow are combined as appropriate with more sub-pixels functioning as one pixel, the reproducibility of halftones can be increased. Thus, the display quality can be improved.

The display device of one embodiment of the present invention can reproduce the color gamut of various standards. For example, the display device of one embodiment of the present invention can reproduce the color gamut of the following standards: the Phase Alternating Line (PAL) or National Television System Committee (NTSC) standard used for TV broadcasting; the standard RGB (sRGB) or Adobe RGB standard used widely for display devices in electronic devices such as personal computers, digital cameras, and printers; the International Telecommunication Union Radiocommunication Sector Broadcasting Service (Television) 709 (ITU-R BT.709) standard used for high-definition televisions (HDTV, also referred to Hi-Vision); the Digital Cinema Initiatives P3 (DCI-P3) standard used for digital cinema projection; and the ITU-R BT.2020 (Recommendation 2020 (REC.2020)) standard used for ultra-high-definition televisions (UHDTV, also referred to as Super Hi-Vision); and the like.

Using the pixels240arranged in a matrix of 1920×1080, the display device100can achieve full color display with full high definition (also referred to as 2K resolution, 2K1K, 2K, and the like). Using the pixels240arranged in a matrix of 3840×2160, the display device100can achieve full color display with ultra high definition (also referred to as 4K resolution, 4K2K, 4K, and the like). Using the pixels240arranged in a matrix of 7680×4320, the display device100can achieve full color display with super high definition (also referred to as 8K resolution, 8K4K, 8K, and the like). Using a larger number of pixels240, the display device100can achieve full color display with 16K or 32K resolution.

<Circuit Configuration Example of Pixel230>

FIGS. 28A and 28Billustrate circuit configuration examples of the pixel230. The pixel230includes a pixel circuit431and a display element432.

Each of the wirings236is electrically connected to the n pixel circuits431in a given row among the pixel circuits431arranged in m rows and n columns in the display region235. Each of the wirings237is electrically connected to the m pixel circuits431in a given column among the pixel circuits431arranged in m rows and n columns. Note that m and n are each an integer of 1 or more.

[Example of Pixel Circuit for Light-Emitting Display Device]

FIG. 28Aillustrates an example of a circuit configuration that can be used for the pixel230in the display device100. The pixel circuit431illustrated inFIG. 28Aincludes a transistor436, a capacitor433, the transistor251, and a transistor434. The pixel circuit431is electrically connected to the light-emitting element170that can function as the display element432.

One of a source electrode and a drain electrode of the transistor436is electrically connected to a wiring to which a data signal (also referred to as a video signal) is supplied (hereinafter, referred to as a signal line DL_n). A gate electrode of the transistor436is electrically connected to a wiring to which a gate signal is supplied (hereinafter, referred to as a scan line GL_m). The signal line DL_n and the scan line GL_m correspond to the wiring237and the wiring236, respectively.

The transistor436has a function of controlling whether to write a data signal to a node435.

One of a pair of electrodes of the capacitor433is electrically connected to the node435, and the other of the pair of electrodes of the capacitor433is electrically connected to a node437. The other of the source electrode and the drain electrode of the transistor436is electrically connected to the node435.

The capacitor433functions as a storage capacitor for storing data written to the node435.

One of a source electrode and a drain electrode of the transistor251is electrically connected to a potential supply line VL_a, and the other of the source electrode and the drain electrode of the transistor251is electrically connected to the node437. A gate electrode of the transistor251is electrically connected to the node435.

One of a source electrode and a drain electrode of the transistor434is electrically connected to a potential supply line V0, and the other of the source electrode and the drain electrode of the transistor434is electrically connected to the node437. A gate electrode of the transistor434is electrically connected to the scan line GL_m.

One of an anode and a cathode of the light-emitting element170is electrically connected to a potential supply line VL_b, and the other is electrically connected to the node437.

As the light-emitting element170, an organic electroluminescent element (also referred to as an organic EL element) or the like can be used, for example. Note that the light-emitting element170is not limited thereto and may be an inorganic EL element containing an inorganic material, for example.

As a power supply potential, a potential on the relatively high potential side or a potential on the relatively low potential side can be used, for example. A power supply potential on the high potential side is referred to as a high power supply potential (also referred to as VDD), and a power supply potential on the low potential side is referred to as a low power supply potential (also referred to as VSS). A ground potential can be used as the high power supply potential or the low power supply potential. For example, in the case where a ground potential is used as the high power supply potential, the low power supply potential is a potential lower than the ground potential, and in the case where a ground potential is used as the low power supply potential, the high power supply potential is a potential higher than the ground potential.

A high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other, for example.

In the display device including the pixel circuit431inFIG. 28A, the pixel circuits431are sequentially selected row by row by the circuit included in the peripheral circuit region232, whereby the transistors436and the transistors434are turned on and a data signal is written to the nodes435.

When the transistors436and the transistors434are turned off, the pixel circuits431in which the data has been written to the nodes435are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor251is controlled in accordance with the potential of the data written to the node435. The light-emitting element170emits light with a luminance corresponding to the amount of current flow. This operation is sequentially performed row by row; thus, an image can be displayed.

[Example of Pixel Circuit for Liquid Crystal Display Device]

The pixel circuit431inFIG. 28Bincludes the transistor436and the capacitor433. The pixel circuit431is electrically connected to a liquid crystal element180functioning as the display element432.

The potential of one of a pair of electrodes of the liquid crystal element180is set in accordance with the specifications of the pixel circuit431as appropriate. The alignment state of the liquid crystal element180depends on data written to the node435. A common potential may be applied to one of the pair of electrodes of the liquid crystal element180included in each of the plurality of pixel circuits431. The potential applied to one of the pair of electrodes of the liquid crystal element180in the pixel circuit431may differ between rows.

The liquid crystal element180can employ, for example, a vertical alignment (VA) mode. Examples of the vertical alignment mode include a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, and an advanced super view (ASV) mode.

The liquid crystal element180can employ a variety of modes. For example, a liquid crystal element using, instead of a VA mode, a twisted nematic (TN) mode, an in-plane switching (IPS) mode, a VA-IPS mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, a guest-host mode, or the like can be used.

The liquid crystal element controls transmission or non-transmission of light utilizing an optical modulation action of a liquid crystal. The optical modulation action of the liquid crystal is controlled by an electric field applied to the liquid crystal (including a horizontal electric field, a vertical electric field, and an oblique electric field). As the liquid crystal used for the liquid crystal element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal (PDLC), a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

As the liquid crystal material, a positive liquid crystal or a negative liquid crystal may be used, and an appropriate liquid crystal material can be used depending on the mode or design to be used.

An alignment film can be provided to adjust the alignment of a liquid crystal. In the case where a horizontal electric field mode is employed, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which a chiral material is mixed to account for several weight percent or more is used for the liquid crystal in order to improve the temperature range. The liquid crystal composition which includes a liquid crystal exhibiting a blue phase and a chiral material has a short response time and optical isotropy, which makes the alignment process unneeded. In addition, the liquid crystal composition which includes a liquid crystal exhibiting a blue phase and a chiral material has a small viewing angle dependence. An alignment film does not need to be provided and rubbing treatment is thus not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of a liquid crystal display device in the manufacturing process can be reduced.

Note that when a guest-host mode liquid crystal material is used for the liquid crystal element180, a functional member such as a light diffusion layer or a polarizing plate is not necessarily provided. Accordingly, the productivity of the display device can be improved. In addition, since a functional member such as a polarizing plate is unnecessary, the luminance of reflected light of the liquid crystal element180can be increased. Accordingly, the visibility of the display device can be increased.

The on and off states (bright and dark states) of a reflective liquid crystal display device including a circularly polarizing plate are switched depending on the alignment direction of the major axes of liquid crystal molecules: a direction substantially perpendicular to a substrate or a direction substantially parallel to the substrate. In general, it is difficult to use a liquid crystal element that operates in a horizontal electric field mode such as an IPS mode in a reflective liquid crystal display device because the major axes of liquid crystal molecules in the liquid crystal element are aligned in a direction substantially parallel to a substrate in both of the on and off states.

The on and off states of a liquid crystal element that operates in a horizontal electric field mode such as a VA-IPS mode are switched depending on the alignment direction of the major axes of liquid crystal molecules: a direction substantially perpendicular to a substrate or a direction substantially parallel to the substrate. Thus, when a liquid crystal element that operates in a horizontal electric field mode is used in a reflective liquid crystal display device, the liquid crystal element preferably operates in a VA-IPS mode.

Note that, as described later, a display element other than the light-emitting element170and the liquid crystal element180can be used.

In the pixel circuit431in the m-th row and the n-th column, one of the source electrode and the drain electrode of the transistor436is electrically connected to the signal line DL_n, and the other of the source electrode and the drain electrode of the transistor436is electrically connected to the node435. The gate electrode of the transistor436is electrically connected to the scan line GL_m. The transistor436has a function of controlling whether to write a data signal to the node435.

One of the pair of electrodes of the capacitor433is electrically connected to a wiring to which a specific potential is supplied (hereinafter, referred to as a capacitor line CL), and the other of the pair of electrodes of the capacitor433is electrically connected to the node435. The other of the pair of electrodes of the liquid crystal element180is electrically connected to the node435. The potential of the capacitor line CL is set in accordance with the specifications of the pixel circuit431as appropriate. The capacitor433functions as a storage capacitor for storing data written to the node435.

For example, in the display device including the pixel circuit431inFIG. 28B, the pixel circuits431are sequentially selected row by row by the circuit included in the peripheral circuit region232, whereby the transistors436are turned on and a data signal is written to the nodes435.

When the transistors436are turned off, the pixel circuits431in which the data signal has been written to the nodes435are brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed on the display region235.

The display device of one embodiment of the present invention can employ various modes and include various display elements. The display element includes at least one of an electroluminescent (EL) element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element) including an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), a plasma display panel (PDP), an electron emitter, a liquid crystal element, an electrophoretic element, a display element using micro electro mechanical systems (MEMS) such as a grating light valve (GLV), a digital micromirror device (DMD), a digital micro shutter (DMS) element, a MIRASOL (registered trademark) display, an interferometric modulator display (IMOD) element, and a piezoelectric ceramic display, an electrowetting element, and the like. Other than the above, display media whose contrast, luminance, reflectivity, transmittance, or the like is changed by an electrical or magnetic effect may be included. Alternatively, quantum dots may be used as the display element.

Note that examples of display devices having EL elements include an EL display. Examples of a display device including an electron emitter include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of a display device including a liquid crystal element include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). Examples of a display device including an electrophoretic element include electronic paper. Examples of a display device including quantum dots include a quantum dot display.

In the case of a transflective liquid crystal display or a reflective liquid crystal display, some or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to contain aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes. Thus, the power consumption can be further reduced.

In this embodiment, structure examples of a light-emitting element that can be used as the light-emitting element170are described. Note that an EL layer320described in this embodiment corresponds to the EL layer172described in the other embodiments.

In the light-emitting element311illustrated inFIG. 29A, the EL layer320is provided between a pair of electrodes (an electrode318and an electrode322). The electrode318, the electrode322, and the EL layer320respectively correspond to the electrode171, the electrode173, and the EL layer172of the aforementioned embodiments. Note that the electrode318is used as an anode and the electrode322is used as a cathode as an example in the following description of this embodiment.

The EL layer320includes at least a light-emitting layer and may have a stacked-layer structure including a functional layer other than the light-emitting layer.

As the functional layer other than the light-emitting layer, a layer containing a substance having a high hole-injection property, a substance having a high hole-transport property, a substance having a high electron-transport property, a substance having a high electron-injection property, a bipolar substance (a substance having high electron- and hole-transport properties), or the like can be used. Specifically, functional layers such as a hole-injection layer, a hole-transport layer, an electron-transport layer, and an electron-injection layer can be used in combination as appropriate.

The light-emitting element311illustrated inFIG. 29Aemits light when current flows because of a potential difference generated between the electrode318and the electrode322and holes and electrons are recombined in the EL layer320. That is, the light-emitting region is formed in the EL layer320.

In the present invention, light emitted from the light-emitting element311is extracted to the outside from the electrode318side or the electrode322side. Therefore, one of the electrode318and the electrode322is formed of a light-transmitting substance.

Note that a plurality of EL layers320may be stacked between the electrode318and the electrode322as in a light-emitting element312illustrated inFIG. 29B. In the case where n (n is a natural number of 2 or more) layers are stacked, a charge generation layer320ais preferably provided between an m-th EL layer320and an (m+1)-th EL layer320. Note that m is a natural number greater than or equal to 1 and less than n. The components except the electrode318and the electrode322correspond to the EL layer172of the aforementioned embodiments.

The charge generation layer320acan be formed using a composite material of an organic compound and a metal oxide. Examples of the metal oxide include vanadium oxide, molybdenum oxide, and tungsten oxide. As the organic compound, a variety of compounds can be used; for example, an aromatic amine compound, a carbazole derivative, an aromatic hydrocarbon, and an oligomer, a dendrimer, and a polymer which have a basic skeleton of any of these compounds can be used. As the organic compound, it is preferable to use the organic compound which has a hole-transport property and has a hole mobility of 10−6cm2/Vs or higher. Note that other substances may be used as long as their hole-transport properties are higher than their electron-transport properties. These materials used for the charge generation layer320ahave excellent carrier-injection properties and carrier-transport properties; thus, the light-emitting element312can be driven with low current and with low voltage. Other than the composite material, a material obtained by adding an alkali metal, an alkaline earth metal, a compound of the alkali metal, a compound of an alkaline earth metal, or the like to the above-described composite material can be used for the charge generation layer320a.

Note that the charge generation layer320amay be formed with a combination of a composite material of an organic compound and a metal oxide with another material. For example, a layer containing the composite material of an organic compound and a metal oxide may be combined with a layer containing a compound of a substance selected from electron-donating substances and a compound having a high electron-transport property. Moreover, a layer containing the composite material of an organic compound and a metal oxide may be combined with a transparent conductive film.

The light-emitting element312having such a structure is unlikely to result in energy transfer between the adjacent EL layers320and can easily have high emission efficiency and a long lifetime. Moreover, it is easy to obtain phosphorescence from one light-emitting layer and fluorescence from the other light-emitting layer.

The charge generation layer320ahas a function of injecting holes to one of the EL layers320that is in contact with the charge generation layer320aand a function of injecting electrons to the other EL layer320that is in contact with the charge generation layer320a, when voltage is applied between the electrode318and the electrode322.

The light-emitting element312illustrated inFIG. 29Bcan provide a variety of emission colors by changing the type of the light-emitting substance used for the EL layer320. In addition, a plurality of light-emitting substances emitting light of different colors may be used as the light-emitting substances, whereby light emission having a broad spectrum or white light can be obtained.

In the case of obtaining white light using the light-emitting element312inFIG. 29B, as for a combination of a plurality of EL layers, a structure for emitting white light including red light, blue light, and green light may be used. For example, the structure may include an EL layer containing a blue fluorescent substance as a light-emitting substance and an EL layer containing green and red phosphorescent substances as light-emitting substances. Alternatively, a structure including an EL layer emitting red light, an EL layer emitting green light, and an EL layer emitting blue light may be employed. Further alternatively, with a structure including EL layers emitting light of complementary colors, white light can be obtained. In a stacked-layer element including two EL layers which emit light of complementary colors, the combinations of colors are as follows: blue and yellow, blue-green and red, and the like.

Note that in the structure of the above-described stacked-layer element, by providing the charge generation layer between the stacked light-emitting layers, the element can emit light with high luminance at a low current density, and have a long lifetime.

In this embodiment, an operation mode that can be employed in the display device100is described with reference to FIGS.30A1,30A2,30B, and30C.

In this embodiment, a normal driving mode (normal mode) with a normal frame frequency (typically, higher than or equal to 60 Hz and lower than or equal to 240 Hz) and an idling stop (IDS) driving mode with a low frame frequency will be described below, for example.

Note that the IDS driving mode refers to a driving method in which after image data is written, rewriting of image data is stopped. This increases the interval between writing of image data and subsequent writing of image data, thereby reducing the power that would be consumed by writing of image data in that interval. The IDS driving mode can be performed at a frame frequency which is 1/100 to 1/10 of the normal driving mode, for example. A still image is displayed by the same video signals in consecutive frames. Thus, the IDS driving mode is particularly effective when displaying a still image. When an image is displayed using IDS driving, power consumption is reduced, image flickering (flicker) is suppressed, and eyestrain can be reduced.

FIGS.30A1and30A2are pixel circuit diagrams, andFIGS. 30B and 30Care timing charts showing the normal driving mode and the IDS driving mode. Note that in FIG.30A1, a display element501and a pixel circuit431electrically connected to the display element501are illustrated. The liquid crystal element180is used as the display element501, for example. In the pixel circuit431in FIG.30A1, a signal line SL, a gate line GL, a transistor M1connected to the signal line SL and the gate line GL, and a capacitor CSLCconnected to the transistor M1are illustrated. Note that a node to which one electrode of the display element501, one of a source and a drain of the transistor M1, and the capacitor CSLCare connected is referred to as a node ND1.

FIG.30A2illustrates a display element502and the pixel circuit431electrically connected to the display element502. For example, the light-emitting element170(EL element) is used as the display element502. In the pixel circuit431in FIG.30A2, the signal line SL, the gate line GL, the transistor M1connected to the signal line SL and the gate line GL, a transistor M2connected to the transistor M1and the display element502, and a capacitor CSELconnected to the transistor M1, the transistor M2, and the display element502are illustrated.

Data D1is supplied from the signal line SL. Then, the data D1is supplied to the node ND1through the transistor M1. After that, the transistor M1is turned off, so that the data D1is held at the node ND1. Thus, the transistor M1may become a leakage path of the data D1supplied from the signal line SL to the node ND1. The off-state current of the transistor M1is preferably as low as possible. A transistor including a metal oxide in a semiconductor layer in which a channel is formed is preferably used as the transistor M1. A metal oxide having at least one of an amplification function, a rectification function, and a switching function can be referred to as a metal oxide semiconductor or an oxide semiconductor (abbreviated to an OS). As a typical example of a transistor, a transistor including an oxide semiconductor in a semiconductor layer in which a channel is formed (OS transistor) is described. The OS transistor has a feature of extremely low leakage current (off-state current) in an off state compared with a transistor including polycrystalline silicon or the like. When the OS transistor is used as the transistor M1, electric charges supplied to the node ND1can be held for a long period.

In particular, the response speed of an EL element used as the display element502is higher than that of a liquid crystal element; accordingly, the EL element is sensitive to a change in the voltage of the node ND1. Thus, it is preferable to use the OS transistor as the transistor M1of the pixel circuit431, in which case flickers due to a change in electric charges of the node ND1can be reduced.

In addition, the off-state current of a transistor used as the transistor M2is preferably as low as possible. The use of a transistor having a low off-state current as the transistor M2can suppress a phenomenon of slight light emission in a black display (also referred to as black floating). Accordingly, the OS transistor is preferably used as the transistor M2of the pixel circuit431.

In the circuit diagram illustrated in FIG.30A2, the liquid crystal element used as the display element502becomes a leakage path of the data D1. Therefore, to perform IDS driving appropriately, the resistivity of the liquid crystal element is preferably higher than or equal to 1.0×1014Ω·cm.

Note that for example, an In—Ga—Zn oxide, an In—Zn oxide, or the like can be suitably used for a channel formation region of the above OS transistor. The In—Ga—Zn oxide can typically have an atomic ratio of In:Ga:Zn=4:2:4.1 or a neighborhood thereof.

FIG. 30Bis a timing chart showing waveforms of signals supplied to the signal line SL and the gate line GL in the normal driving mode. In the normal driving mode, a normal frame frequency (e.g., 60 Hz) is used for operation.FIG. 30Bshows periods T1to T3. A scan signal is supplied to the gate line GL in each frame period and the data D1is written from the signal line SL to the node ND1. This operation is performed both to write the same data D1in the periods T1to T3and to write different data in the periods T1to T3.

FIG. 30Cis a timing chart showing waveforms of signals supplied to the signal line SL and the gate line GL in the IDS driving mode. In the IDS driving, a low frame frequency (e.g., 1 Hz) is used for operation. One frame period is denoted by a period T1and includes a data writing period TWand a data retention period TRET. In the IDS driving mode, a scan signal is supplied to the gate line GL and the data D1of the signal line SL is written in the period TW, the gate line GL is fixed to a low-level voltage in the period TRET, and the transistor M1is turned off so that the written data D1is retained. Note that the low frame frequency may be higher than or equal to 0.1 Hz and lower than 60 Hz, for example.

In this embodiment, an example of a method for driving a touch sensor will be described with reference to drawings.

[Example of Sensing Method of Sensor]

FIG. 31Ais a block diagram illustrating the structure of a mutual capacitive touch sensor.FIG. 31Aillustrates a pulse voltage output circuit551and a current sensing circuit552. Note that inFIG. 31A, six wirings X1to X6represent electrodes521to which a pulse voltage is applied, and six wirings Y1to Y6represent electrodes522that sense changes in current.FIG. 31Aalso illustrates a capacitor553that is formed where the electrodes521and522overlap with each other. Note that functional replacement between the electrodes521and522is possible.

The pulse voltage output circuit551is a circuit for sequentially applying a pulse voltage to the wirings X1to X6. By application of a pulse voltage to the wirings X1to X6, an electric field is generated between the electrodes521and522of the capacitors553. When the electric field between the electrodes is shielded, for example, a change occurs in the capacitor553(mutual capacitance). The approach or contact of a sensing target can be sensed by utilizing this change.

The current sensing circuit552is a circuit for sensing changes in current flowing through the wirings Y1to Y6that are caused by the change in mutual capacitance in the capacitor553. No change in current value is sensed in the wirings Y1to Y6when there is no approach or contact of a sensing target, whereas a decrease in current value is sensed when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current.

Note that one or both of the pulse voltage output circuit551and the current sensing circuit552may be formed over the first element substrate181or the second element substrate182. For example, it is preferable to form the pixel circuit, the peripheral driver circuit, and the like at the same time because the process can be simplified and the number of components used for driving the touch sensor can be reduced. One or both of the pulse voltage output circuit551and the current sensing circuit552may be mounted on an IC.

In particular, in the case of using an oxide semiconductor, crystalline silicon such as polycrystalline silicon or single crystal silicon, or the like for the semiconductor layer where a channel is formed in the transistor over the second element substrate182, the driving characteristics of the pulse voltage output circuit551, the current sensing circuit552, or the like are increased and the sensitivity of the touch sensor can be thus increased.

FIG. 31Bis a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated inFIG. 31A. InFIG. 31B, sensing of a sensing target is performed in all the rows and columns in one frame period.FIG. 31Bshows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). Sensed current values of the wirings Y1to Y6are shown as the waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1to X6, and the waveforms of the wirings Y1to Y6change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y1to Y6change uniformly in accordance with changes in the voltages of the wirings X1to X6. The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes.

By sensing a change in mutual capacitance in this manner, the approach or contact of a sensing target can be sensed.

AlthoughFIG. 31Ais a passive matrix touch sensor in which only the capacitor553is provided at the intersection of wirings as a touch sensor, the touch sensor may be an active matrix touch sensor including a transistor and a capacitor.FIG. 32is a sensor circuit included in an active matrix touch sensor.

The sensor circuit includes the capacitor553and transistors561,562, and563. A signal S2is input to a gate of the transistor563. A voltage VRES is applied to one of a source and a drain of the transistor563, and one electrode of the capacitor553and a gate of the transistor561are electrically connected to the other of the source and the drain of the transistor563. One of a source and a drain of the transistor561is electrically connected to one of a source and a drain of the transistor562, and a voltage VSS is applied to the other of the source and the drain of the transistor561. A signal S1is input to a gate of the transistor562, and a wiring ML is electrically connected to the other of the source and the drain of the transistor562. The voltage VSS is applied to the other electrode of the capacitor553.

Next, the operation of the sensor circuit will be described. First, a potential for turning on the transistor563is supplied as the signal S2, and a potential with respect to the voltage VRES is thus applied to a node n connected to the gate of the transistor561. Then, a potential for turning off the transistor563is applied as the signal S2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor553changes owing to the approach or contact of a sensing target such as a finger; accordingly, the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor562is supplied as the signal S1. Current flowing through the transistor561, that is, current flowing through the wiring ML is changed in response to the potential of the node n. By sensing this current, the approach or contact of a sensing target can be sensed.

It is preferable that the transistors561,562, and563each include an oxide semiconductor in a semiconductor layer where a channel is formed. In particular, by using an oxide semiconductor in a semiconductor layer where a channel of the transistor563is formed, the potential of the node n can be held for a long time and the frequency of operation (refresh operation) of resupplying VRES to the node n can be reduced.

[Example of Driving Method of Display Device]

FIG. 33Ais a block diagram illustrating a configuration example of a display device.FIG. 33Aillustrates a gate driver circuit GD (a scan line driver circuit), a source driver circuit SD (a signal line driver circuit), and a display portion including a plurality of pixels pix. InFIG. 33A, gate lines x_1to x_m (m is a natural number) electrically connected to the gate driver circuit GD and source lines y_1to y_n (n is a natural number) electrically connected to the source driver circuit SD are illustrated. Corresponding to these lines, the pixels pix are denoted by (1, 1) to (n, m).

FIG. 33Bis a timing chart of signals supplied to the gate lines and the source lines in the display device illustrated inFIG. 33A. The periods inFIG. 33Bshow the case where data signals are rewritten every frame period and the case where data signals are not rewritten. Note that periods such as a retrace period are not taken into consideration inFIG. 33B.

In the case where data signals are rewritten every frame period, scan signals are sequentially supplied to the gate lines x_1to x_m. In a horizontal scanning period1H, during which the scan signal is at an H level, data signals D are supplied to the source lines y_1to y_n in the columns.

In the case where data signals are not rewritten every frame period, supply of scan signals to the gate lines x_1to x_m is stopped. In the horizontal scanning period1H, supply of data signals to the source lines y_1to y_n in the columns is stopped.

A driving method in which data signals are not rewritten every frame period is effective particularly when an oxide semiconductor is used for the semiconductor layer where a channel is formed in the transistor included in the pixel pix. A transistor including an oxide semiconductor can have much lower off-state current than a transistor including a semiconductor such as silicon. Thus, a data signal written in the previous period can be held without rewriting data signals every frame period, and grayscale of pixels can be held for 1 second or longer, preferably 5 seconds or longer, for example.

In the case where polycrystalline silicon or the like is used for a semiconductor layer where a channel of a transistor included in the pixel pix is formed, the storage capacitance of the pixel is preferably increased in advance. The larger the storage capacitance is, the longer the grayscale of the pixel can be held. The storage capacitance may be determined depending on leakage current of a transistor or a display element which is electrically connected to the storage capacitor. For example, the storage capacitance per pixel is set to 5 fF to 5 pF inclusive, preferably 10 fF to 5 pF inclusive, and further preferably 20 fF to 1 pF inclusive, so that a data signal written in the previous period can be held without rewriting data signals every frame period. For example, grayscale of a pixel can be held for several frame periods or several tens of frame periods.

[Example of Driving Method of Display Portion and Touch Sensor]

FIGS. 34A to 34Dshow examples of the operations in successive frame periods of the touch sensor described with reference toFIGS. 31A and 31Band the display portion described with reference toFIGS. 33A and 33Bthat are driven for 1 sec (one second). InFIG. 34A, one frame period for the display portion is 16.7 ms (frame frequency: 60 Hz), and one frame period for the touch sensor is 16.7 ms (frame frequency: 60 Hz).

In the display device of one embodiment of the present invention, the display portion and the touch sensor operate independently of each other, and the display device can have a touch sensing period concurrent with a display period. That is why one frame period for the display portion and one frame period for the touch sensor can both be 16.7 ms (frame frequency: 60 Hz) as shown inFIG. 34A. The frame frequency for the touch sensor may differ from that for the display portion. For example, as shown inFIG. 34B, one frame period for the display portion may be 8.3 ms (frame frequency: 120 Hz) and one frame period for the touch sensor may be 16.7 ms (frame frequency: 60 Hz). The frame frequency for the display portion may be 33.3 ms (frame frequency: 30 Hz) (not shown).

The frame frequency for the display portion may be changeable, i.e., the frame frequency in displaying moving images may be increased (e.g., 60 Hz or more, or 120 Hz or more), whereas the frame frequency in displaying still images may be decreased (e.g., 60 Hz or less, 30 Hz or less, or 1 Hz or less). With this structure, the power consumption of the display device can be reduced. The frame frequency for the touch sensor may be changeable, and the frame frequency in waiting may differ from the frame frequency in sensing a touch.

Moreover, in the display device of one embodiment of the present invention, the following operation is possible: data signals are not rewritten in the display portion and a data signal written in the previous period is held. In that case, one frame period for the display portion can be longer than 16.7 ms. Thus, as shown inFIG. 34C, the operation can be switched so that one frame period for the display portion is 1 sec (frame frequency: 1 Hz) and one frame period for the touch sensor is 16.7 ms (frame frequency: 60 Hz).

Note that for the operation in which data signals are not rewritten in the display portion and a data signal written in the previous period is held, the above-described IDS driving mode can be referred to. As the IDS driving mode, a partial IDS driving mode may be employed in which data signals are rewritten only in a specific region of the display portion. The partial IDS driving mode is a mode in which data signals are rewritten only in a specific region of the display portion and a data signal written in the previous period is held in the other region.

Furthermore, by the driving method of a touch sensor that is disclosed in this embodiment, the touch sensor can be continuously driven in the case ofFIG. 34C. Thus, data signals in the display portion can also be rewritten when the approach or contact of a sensing target is sensed by the touch sensor, as shown inFIG. 34D.

If rewriting of data signals in a display portion is performed during a sensing period of a touch sensor, noise caused by rewriting of the data signals travels through the touch sensor and the sensitivity of the touch sensor might decrease. For this reason, rewriting of data signals in a display portion and sensing by a touch sensor are preferably performed in different periods.

FIG. 35Ashows an example in which rewriting of data signals in a display portion and sensing by a touch sensor are performed alternately.FIG. 35Bshows an example in which sensing by a touch sensor is performed one time every two rewritings of data signals in a display portion. Note that sensing by a touch sensor may be performed once every three or more rewritings.

In the case where an oxide semiconductor is used for a semiconductor layer where a channel is formed in a transistor included in the pixel pix, the off-state current can be significantly reduced and the frequency of rewriting data signals can be sufficiently reduced. Specifically, a sufficiently long break period can be set between rewritings of data signals. The break period can be 0.5 seconds or longer, 1 second or longer, or 5 seconds or longer, for example. The upper limit of the break period depends on the leakage current of a capacitor or a display element connected to the transistor; for example, 1 minute or shorter, 10 minutes or shorter, 1 hour or shorter, or 1 day or shorter.

FIG. 35Cshows an example in which rewriting of data signals in a display portion is performed once every 5 seconds. A break period for stopping the rewriting operation of a display portion is set inFIG. 35Cbetween rewriting of data signals and next rewriting. In the break period, a touch sensor can be operated at a frame frequency of i Hz (i is more than or equal to the frame frequency of a display device; here, 0.2 Hz or more). Preferably, sensing by a touch sensor is performed in a break period and is not performed in a rewriting period of data signals in a display portion as shown inFIG. 35C, so that the sensitivity of a touch sensor can be increased. When rewriting of data signals in a display portion and sensing by a touch sensor are performed at the same time as shown inFIG. 35D, operation signals can be simplified.

In a break period during which rewriting of data signals in a display portion is not performed, not only the supply of data signals to the display portion, but also the operation of one or both of the gate driver circuit GD and the source driver circuit SD may be stopped. The supply of power to one or both of the gate driver circuit GD and the source driver circuit SD may also be stopped. Thus, noise is further reduced, and the sensitivity of the touch sensor can be further increased. Moreover, the power consumption of the display device can be further reduced.

With the use of the above-described driving method, an excellent display device having reduced power consumption and increased detection sensitivity of a touch sensor can be obtained.

The display device of one embodiment of the present invention includes a display portion and a touch sensor between two substrates. With this structure, the distance between the display portion and the touch sensor can be reduced. At this time, noise is easily transmitted to the touch sensor in driving the display portion, which might reduce the sensitivity of the touch sensor. When the driving method in this embodiment is employed, a display device including a touch sensor, which has both reduced thickness and high sensitivity, can be obtained.

In this embodiment, structure examples of transistors that can be used for the display device of one embodiment of the present invention are described with reference toFIGS. 36A to 36C,FIGS. 37A to 37C, andFIGS. 38A to 38C.

To show a structure example of a transistor, a transistor3200ais described with reference toFIGS. 36A to 36C.FIG. 36Ais a top view of the transistor3200a.FIG. 36Bis a cross-sectional view taken along the dashed-dotted line X1-X2inFIG. 36A, andFIG. 36Cis a cross-sectional view taken along the dashed-dotted line Y1-Y2inFIG. 36A. Note that inFIG. 36A, some components of the transistor3200a(e.g., an insulating layer serving as a gate insulating layer) are not illustrated to avoid complexity. Note that hereinafter, the direction of the dashed-dotted line X1-X2may be called the channel length direction, and the direction of the dashed-dotted line Y1-Y2may be called the channel width direction. As inFIG. 36A, some components are not illustrated in some cases in top views of transistors described below.

The transistor3200aincludes a conductive layer3221over an insulating layer3224; an insulating layer3211over the insulating layer3224and the conductive layer3221; a metal oxide layer3231over the insulating layer3211; a conductive layer3222aover the metal oxide layer3231; a conductive layer3222bover the metal oxide layer3231; an insulating layer3212over the metal oxide layer3231, the conductive layer3222a, and the conductive layer3222b; a conductive layer3223over the insulating layer3212; and an insulating layer3213over the insulating layer3212and the conductive layer3223.

The insulating layers3211and3212have an opening3235. The conductive layer3223is electrically connected to the conductive layer3221in the opening3235.

The insulating layer3211serves as a first gate insulating layer of the transistor3200a. The insulating layer3212serves as a second gate insulating layer of the transistor3200a. The insulating layer3213serves as a protective insulating layer of the transistor3200a. The conductive layer3221serves as a first gate of the transistor3200a. The conductive layer3222aserves as one of a source and a drain of the transistor3200aand the conductive layer3222bserves as the other of the source and the drain. The conductive layer3223serves as a second gate of the transistor3200a.

Note that the transistor3200ais a channel-etched transistor, and has a dual-gate structure.

The transistor3200awithout the conductive layer3223is also available. In that case, the transistor3200ais a channel-etched transistor, and has a bottom-gate structure.

As illustrated inFIGS. 36B and 36C, the metal oxide layer3231faces the conductive layer3221and the conductive layer3223, and is between the conductive layers serving as the two gates. The length of the conductive layer3223in the channel length direction is longer than the length of the metal oxide layer3231in the channel length direction. The length of the conductive layer3223in the channel width direction is longer than the length of the metal oxide layer3231in the channel width direction. The whole metal oxide layer3231is covered with the conductive layer3223with the insulating layer3212positioned therebetween.

In other words, the conductive layers3221and3223are connected to each other in the opening3235provided in the insulating layers3211and3212, and have a region located outside a side end portion of the metal oxide layer3231.

With this structure, the metal oxide layer3231included in the transistor3200acan be electrically surrounded by electric fields of the conductive layers3221and3223. A device structure of a transistor in which electric fields of a first gate and a second gate electrically surround a metal oxide layer where a channel formation region is formed, like in the transistor3200a, can be referred to as a surrounded channel (S-channel) structure.

Since the transistor3200ahas the S-channel structure, an electric field for inducing a channel can be effectively applied to the metal oxide layer3231by the conductive layer3221serving as the first gate; therefore, the current drive capability of the transistor3200acan be improved and high on-state current characteristics can be obtained. Since the on-state current can be increased, it is possible to reduce the size of the transistor3200a. In addition, since the transistor3200ahas a structure in which the metal oxide layer3231is surrounded by the conductive layer3221serving as the first gate and the conductive layer3223serving as the second gate, the mechanical strength of the transistor3200acan be increased.

The metal oxide layer3231preferably includes a region in which the atomic proportion of In is larger than the atomic proportion of M For example, the atomic ratio of In to M and Zn in the metal oxide layer3231is preferably In:M:Zn=4:2:3 or in the neighborhood thereof. As for the range expressed by the term “neighborhood” here, when In is 4, M ranges from 1.5 to 2.5 and Zn ranges from 2 to 4. Alternatively, the atomic ratio of In to M and Zn in the metal oxide layer3231is preferably In:M:Zn=5:1:6 or in the neighborhood thereof.

The metal oxide layer3231is preferably a CAC-OS or a CAC metal oxide. When the metal oxide layer3231is a CAC-OS or a CAC metal oxide and has a region in which the atomic proportion of In is higher than the atomic proportion of M, the transistor3200acan have high field-effect mobility.

Since the transistor3200ahaving the S-channel structure has high field-effect mobility and high driving capability, the use of the transistor3200ain the driver circuit, a typical example of which is a gate driver that generates a gate signal, allows the display device to have a narrow bezel. The use of the transistor3200ain a source driver (particularly in a demultiplexer connected to an output terminal of a shift register included in the source driver) that supplies a signal to a signal line included in the display device can reduce the number of wirings connected to the display device.

Furthermore, the transistor3200ais a channel-etched transistor and thus can be fabricated through a smaller number of steps than a transistor formed using low-temperature polysilicon. In addition, unlike the transistor formed using low-temperature polysilicon, the transistor3200aincluding the metal oxide layer in the channel does not need a laser crystallization step. Accordingly, the manufacturing cost can be reduced even in the case of a display device formed using a large substrate. Transistors having high field-effect mobility like the transistor3200aare preferably used in a driver circuit and a display portion of a large display device having high resolution such as ultra high definition (4K resolution, 4K2K, or 4K) or super high definition (8K resolution, 8K4K, or 8K), in which case writing can be performed in a short time and display defects can be reduced.

The insulating layers3211and3212in contact with the metal oxide layer3231are preferably oxide insulating films, and further preferably includes a region containing oxygen in excess of the stoichiometric composition (oxygen-excess region). In other words, the insulating layers3211and3212are insulating films from which oxygen can be released. In order to provide the oxygen-excess region in the insulating layers3211and3212, the insulating layers3211and3212are formed in an oxygen atmosphere, or the deposited insulating layers3211and3212are subjected to heat treatment in an oxygen atmosphere, for example.

An oxide semiconductor, which is a kind of metal oxide, can be used for the metal oxide layer3231.

In the case where the metal oxide layer3231includes an In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide satisfy In>M The atomic ratio of metal elements in such a sputtering target is, for example, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1, In:M:Zn=5:1:6, In:M:Zn=5:1:7, In:M:Zn=5:1:8, In:M:Zn=6:1:6, or In:M:Zn=5:2:5.

In the case where the metal oxide layer3231is formed using an In-M-Zn oxide, it is preferable to use a target including a polycrystalline In-M-Zn oxide as the sputtering target. The use of the target including a polycrystalline In-M-Zn oxide facilitates formation of the metal oxide layer3231having crystallinity. Note that the atomic ratio of metal elements in the formed metal oxide layer3231varies from the above atomic ratios of metal elements of the sputtering targets in a range of ±40%. For example, when a sputtering target with an atomic ratio of In:Ga:Zn=4:2:4.1 is used for forming the metal oxide layer3231, the atomic ratio of In to Ga and Zn in the formed metal oxide layer3231may be 4:2:3 or in the neighborhood of 4:2:3.

The energy gap of the metal oxide layer3231is 2 eV or more, preferably 2.5 eV or more. The use of such an oxide semiconductor having a wide energy gap leads to a reduction in off-state current of a transistor.

Furthermore, the metal oxide layer3231preferably has a non-single-crystal structure. The non-single-crystal structure includes a c-axis aligned crystalline (CAAC) structure, a polycrystalline structure, a microcrystalline structure, or an amorphous structure, for example. Among the non-single-crystal structures, the amorphous structure has the highest density of defect states, whereas the CAAC structure has the lowest density of defect states.

The metal oxide layer3231formed with a metal oxide film with low impurity concentration and low density of defect states can give the transistor excellent electrical characteristics. Thus, the use of such a metal oxide film is preferable. Here, the state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. Note that impurities in the metal oxide film are typically water, hydrogen, and the like. In this specification and the like, reducing or removing water and hydrogen from the metal oxide film is referred to as dehydration or dehydrogenation in some cases. Moreover, adding oxygen to a metal oxide film or an oxide insulating film is referred to as oxygen addition in some cases, and a state in which oxygen in excess of the stoichiometric composition is contained due to the oxygen addition is referred to as an oxygen-excess state in some cases.

A highly purified intrinsic or substantially highly purified intrinsic metal oxide film has few carrier generation sources, and thus has a low carrier density. Thus, a transistor in which a channel formation region is formed in the metal oxide film rarely has a negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic metal oxide film has a low density of defect states and accordingly has a low density of trap states in some cases. Furthermore, the highly purified intrinsic or substantially highly purified intrinsic metal oxide film has an extremely low off-state current; even when an element has a channel width of 1×106lam and a channel length of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, that is, less than or equal to 1×10−13A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V.

The insulating layer3213includes one or both of hydrogen and nitrogen. Alternatively, the insulating layer3213includes nitrogen and silicon. The insulating layer3213has a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, or the like. The insulating layer3213can prevent outward diffusion of oxygen from the metal oxide layer3231, outward diffusion of oxygen from the insulating layer3212, and entry of hydrogen, water, or the like into the metal oxide layer3231from the outside.

The insulating layer3213can be a nitride insulating film, for example. The nitride insulating film is formed using silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like.

To show a structure example of a transistor, a transistor3200bis described with reference toFIGS. 37A to 37C.FIG. 37Ais a top view of the transistor3200b.FIG. 37Bis a cross-sectional view taken along the dashed-dotted line X1-X2inFIG. 37A, andFIG. 37Cis a cross-sectional view taken along the dashed-dotted line Y1-Y2inFIG. 37A.

The transistor3200bis different from the transistor3200ain that the metal oxide layer3231, the conductive layer3222a, the conductive layer3222b, and the insulating layer3212each have a multi-layer structure.

The insulating layer3212includes an insulating layer3212aover the metal oxide layer3231and the conductive layers3222aand3222b, and an insulating layer3212bover the insulating layer3212a. The insulating layer3212has a function of supplying oxygen to the metal oxide layer3231. That is, the insulating layer3212contains oxygen. The insulating layer3212ais an insulating layer that allows oxygen to pass therethrough. Note that the insulating layer3212aserves also as a film that relieves damage to the metal oxide layer3231at the time of forming the insulating layer3212b.

A silicon oxide, a silicon oxynitride, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm can be used for the insulating layer3212a.

Furthermore, it is preferable that the number of defects in the insulating layer3212abe small and typically, the spin density of a signal that appears at g=2.001 due to a dangling bond of silicon be lower than or equal to 3×1017spins/cm3by electron spin resonance (ESR) measurement. This is because if the density of defects in the insulating layer3212ais high, oxygen is bonded to the defects and the property of transmitting oxygen of the insulating layer3212ais lowered.

Note that not all oxygen that has entered the insulating layer3212afrom the outside moves to the outside of the insulating layer3212abut some oxygen remains in the insulating layer3212a. In some cases, movement of oxygen occurs in the insulating layer3212ain such a manner that oxygen enters the insulating layer3212aand oxygen contained in the insulating layer3212amoves to the outside of the insulating layer3212a. When an oxide insulating layer that can transmit oxygen is formed as the insulating layer3212a, oxygen released from the insulating layer3212bprovided over the insulating layer3212acan be moved to the metal oxide layer3231through the insulating layer3212a.

Note that the insulating layer3212acan be formed using an oxide insulating layer having a low density of states due to nitrogen oxide. Note that the density of states due to nitrogen oxide can be formed between the valence band maximum (Ev_os) and the conduction band minimum (Ec_os) of the metal oxide film. A silicon oxynitride film that releases a small amount of nitrogen oxide, an aluminum oxynitride film that releases a small amount of nitrogen oxide, or the like can be used as the above oxide insulating layer.

Note that a silicon oxynitride film that releases a small amount of nitrogen oxide is a film which releases ammonia more than nitrogen oxide in thermal desorption spectroscopy (TDS) analysis; the amount of released ammonia is typically greater than or equal to 1×1018/cm3and less than or equal to 5×1019/cm3. Note that the amount of released ammonia is the amount of ammonia released by heat treatment with which the surface temperature of the film becomes higher than or equal to 50° C. and lower than or equal to 650° C., preferably higher than or equal to 50° C. and lower than or equal to 550° C.

Nitrogen oxide (NOx; x is greater than 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2), typically NO2or NO, forms levels in the insulating layer3212a, for example. The level is positioned in the energy gap of the metal oxide layer3231. Therefore, when nitrogen oxide is diffused to the interface between the insulating layer3212aand the metal oxide layer3231, an electron is in some cases trapped by the level on the insulating layer3212aside. As a result, the trapped electron remains in the vicinity of the interface between the insulating layer3212aand the metal oxide layer3231; thus, the threshold voltage of the transistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide contained in the insulating layer3212areacts with ammonia contained in the insulating layer3212bin heat treatment, nitrogen oxide contained in the insulating layer3212ais reduced. Therefore, an electron is hardly trapped at the interface between the insulating layer3212aand the metal oxide layer3231.

By using the above oxide insulating layer for the insulating layer3212a, a shift in the threshold voltage of the transistor can be reduced, which leads to reduced fluctuations in the electrical characteristics of the transistor.

The concentration of nitrogen of the above oxide insulating layer measured by SIMS is lower than or equal to 6×1020atoms/cm3.

The above oxide insulating layer is formed by a PECVD method at a substrate temperature higher than or equal to 220° C. and lower than or equal to 350° C. using silane and dinitrogen monoxide, whereby a dense and hard film can be formed.

The insulating layer3212bis an oxide insulating layer that contains oxygen at a higher proportion than the stoichiometric composition. Part of oxygen is released from the above oxide insulating layer by heating. The amount of oxygen released from the oxide insulating layer in TDS is more than or equal to 1.0×1019atoms/cm3, preferably more than or equal to 3.0×1020atoms/cm3. Note that the amount of released oxygen is the total amount of oxygen released by heat treatment in a temperature range of 50° C. to 650° C. or 50° C. to 550° C. in TDS. In addition, the amount of released oxygen is the total amount of released oxygen converted into oxygen atoms in TDS.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 400 nm can be used for the insulating layer3212b.

It is preferable that the number of defects in the insulating layer3212bbe small and typically, the spin density corresponding to a signal that appears at g=2.001 due to a dangling bond of silicon be lower than 1.5×1018spins/cm3, preferably lower than or equal to 1×1018spins/cm3by ESR measurement. Note that the insulating layer3212bis provided more apart from the metal oxide layer3231than the insulating layer3212ais; thus, the insulating layer3212bmay have higher density of defects than the insulating layer3212a.

Furthermore, the insulating layer3212can include insulating layers including the same kind of material; thus, a boundary between the insulating layer3212aand the insulating layer3212bcannot be clearly observed in some cases. Thus, in this embodiment, the boundary between the insulating layer3212aand the insulating layer3212bis shown by a dashed line. Although a two-layer structure including the insulating layers3212aand3212bis described in this embodiment, the present invention is not limited to this. For example, a single-layer structure including only the insulating layer3212aor a multi-layer structure including three or more layers may be employed.

The metal oxide layer3231in the transistor3200bincludes a metal oxide layer3231_1over the insulating layer3211and a metal oxide layer3231_2over the metal oxide layer3231_1. The metal oxide layers3231_1and3231_2contain the same kind of element. For example, it is preferable that the metal oxide layers3231_1and3231_2each independently contain the same element as the element in the metal oxide layer3231that is described above.

Each of the metal oxide layers3231_1and3231_2preferably contains a region where the atomic proportion of In is higher than the atomic proportion of M For example, the atomic ratio of In to M and Zn in each of the metal oxide layers3231_1and3231_2is preferably In:M:Zn=4:2:3 or in the neighborhood thereof. As for the range expressed by the term “neighborhood” here, when In is 4, M ranges from 1.5 to 2.5 and Zn ranges from 2 to 4. Alternatively, the atomic ratio of In to M and Zn in each of the metal oxide layers3231_1and3231_2is preferably In:M:Zn=5:1:6 or in the neighborhood thereof. The metal oxide layers3231_1and3231_2having substantially the same composition as described above can be formed using the same sputtering target; thus, the manufacturing cost can be reduced. When the same sputtering target is used, the metal oxide layers3231_1and3231_2can be formed successively in the same vacuum chamber. This can suppress entry of impurities into the interface between the metal oxide layers3231_1and3231_2.

Here, the metal oxide layer3231_1may include a region whose crystallinity is lower than that of the metal oxide layer3231_2. Note that the crystallinity of the metal oxide layers3231_1and3231_2can be determined by analysis by X-ray diffraction (XRD) or with a transmission electron microscope (TEM), for example.

The region with low crystallinity in the metal oxide layer3231_1serves as a diffusion path of excess oxygen, through which excess oxygen can be diffused into the metal oxide layer3231_2having higher crystallinity than the metal oxide layer3231_1. When a multi-layer structure including the metal oxide layers having different crystal structures is employed and the region with low crystallinity is used as a diffusion path of excess oxygen as described above, the transistor can be highly reliable.

The metal oxide layer3231_2having a region with higher crystallinity than the metal oxide layer3231_1can prevent impurities from entering the metal oxide layer3231. In particular, the increased crystallinity of the metal oxide layer3231_2can reduce damage at the time of processing into the conductive layers3222aand3222b. The surface of the metal oxide layer3231, i.e., the surface of the metal oxide layer3231_2is exposed to an etchant or an etching gas at the time of processing into the conductive layers3222aand3222b. However, when the metal oxide layer3231_2has a region with high crystallinity, the metal oxide layer3231_2has higher etching resistance than the metal oxide layer3231_1. Thus, the metal oxide layer3231_2serves as an etching stopper.

By including a region having lower crystallinity than the metal oxide layer3231_2, the metal oxide layer3231_1sometimes has a high carrier density.

When the metal oxide layer3231_1has a high carrier density, the Fermi level is sometimes high relative to the conduction band of the metal oxide layer3231_1. This lowers the conduction band minimum of the metal oxide layer3231_1, so that the energy difference between the conduction band minimum of the metal oxide layer3231_1and the trap level, which might be formed in a gate insulating film (here, the insulating layer3211), is increased in some cases. The increase of the energy difference can reduce trap of charges in the gate insulating film and reduce variation in the threshold voltage of the transistor, in some cases. In addition, when the metal oxide layer3231_1has a high carrier density, the metal oxide layer3231can have high field-effect mobility.

Although the metal oxide layer3231in the transistor3200bhas a multi-layer structure including two layers in this example, the structure is not limited thereto, and the metal oxide layer3231may have a multi-layer structure including three or more layers.

The conductive layer3222ain the transistor3200bincludes a conductive layer3222a_1, a conductive layer3222a_2over the conductive layer3222a_1, and a conductive layer3222a_3over the conductive layer3222a_2. The conductive layer3222bin the transistor3200bincludes a conductive layer3222b_1, a conductive layer3222b_2over the conductive layer3222b_1, and a conductive layer3222b_3over the conductive layer3222b_2.

For example, it is preferable that the conductive layers3222a_1,3222b_1,3222a_3, and3222b_3contain one or more elements selected from titanium, tungsten, tantalum, molybdenum, indium, gallium, tin, and zinc. Furthermore, it is preferable that the conductive layers3222a_2and3222b_2contain one or more elements selected from copper, aluminum, and silver.

Specifically, the conductive layers3222a_1,3222b_1,3222a_3, and3222b_3can contain an In—Sn oxide or an In—Zn oxide and the conductive layers3222a_2and3222b_2can contain copper.

An end portion of the conductive layer3222a_1has a region located outside an end portion of the conductive layer3222a_2. The conductive layer3222a_3covers a top surface and a side surface of the conductive layer3222a_2and has a region that is in contact with the conductive layer3222a_1. An end portion of the conductive layer3222b_1has a region located outside an end portion of the conductive layer3222b_2. The conductive layer3222b_3covers a top surface and a side surface of the conductive layer3222b_2and has a region that is in contact with the conductive layer3222b_1.

The above structure is preferred because the structure can reduce the wiring resistance of the conductive layers3222aand3222band inhibit diffusion of copper to the metal oxide layer3231.

To show a structure example of a transistor, a transistor3200cis described with reference toFIGS. 38A to 38C.FIG. 38Ais a top view of the transistor3200c.FIG. 38Bis a cross-sectional view taken along the dashed-dotted line X1-X2inFIG. 38A, andFIG. 38Cis a cross-sectional view taken along the dashed-dotted line Y1-Y2inFIG. 38A.

The transistor3200cillustrated inFIGS. 38A to 38Cincludes the conductive layer3221over the insulating layer3224; the insulating layer3211over the conductive layer3221; the metal oxide layer3231over the insulating layer3211; the insulating layer3212over the metal oxide layer3231; the conductive layer3223over the insulating layer3212; and the insulating layer3213over the insulating layer3211, the metal oxide layer3231, and the conductive layer3223. The metal oxide layer3231includes a channel formation region3231ioverlapping with the conductive layer3223, a source region3231sin contact with the insulating layer3213, and a drain region3231din contact with the insulating layer3213.

The insulating layer3213contains nitrogen or hydrogen. The insulating layer3213is in contact with the source region3231sand the drain region3231d, so that nitrogen or hydrogen that is contained in the insulating layer3213is added to the source region3231sand the drain region3231d. The source region3231sand the drain region3231deach have a high carrier density when nitrogen or hydrogen is added thereto.

The transistor3200cmay further include an insulating layer3215over the insulating layer3213, the conductive layer3222aelectrically connected to the source region3231sthrough an opening3236aprovided in the insulating layers3213and3215, and the conductive layer3222belectrically connected to the drain region3231dthrough an opening3236bprovided in the insulating layers3213and3215.

The insulating layer3215can be an oxide insulating film. Alternatively, a multi-layer film including an oxide insulating film and a nitride insulating film can be used as the insulating layer3215. The insulating layer3215can include, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, gallium oxide, or Ga—Zn oxide. Furthermore, the insulating layer3215preferably functions as a barrier film against hydrogen, water, and the like from the outside.

The insulating layer3211serves as a first gate insulating film, and the insulating layer3212serves as a second gate insulating film. The insulating layers3213and3215serve as a protective insulating film.

The insulating layer3212includes an excess oxygen region. Since the insulating layer3212includes the excess oxygen region, excess oxygen can be supplied to the channel formation region3231iincluded in the metal oxide layer3231. As a result, oxygen vacancies that might be formed in the channel formation region3231ican be filled with excess oxygen, which can provide a highly reliable semiconductor device.

To supply excess oxygen to the metal oxide layer3231, excess oxygen may be supplied to the insulating layer3211that is formed below the metal oxide layer3231. However, in that case, excess oxygen contained in the insulating layer3211might also be supplied to the source region3231sand the drain region3231dincluded in the metal oxide layer3231. When excess oxygen is supplied to the source region3231sand the drain region3231d, the resistance of the source region3231sand the drain region3231dmight be increased.

In contrast, in the structure in which the insulating layer3212formed over the metal oxide layer3231contains excess oxygen, excess oxygen can be selectively supplied only to the channel formation region3231i. Alternatively, the carrier density of the source and drain regions3231sand3231dcan be selectively increased after excess oxygen is supplied to the channel formation region3231iand the source and drain regions3231sand3231d, in which case an increase in the resistance of the source and drain regions3231sand3231dcan be prevented.

Furthermore, each of the source region3231sand the drain region3231dincluded in the metal oxide layer3231preferably contains an element that forms an oxygen vacancy or an element that is bonded to an oxygen vacancy. Typical examples of the element that forms an oxygen vacancy or the element that is bonded to an oxygen vacancy include hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, titanium, and a rare gas. Typical examples of the rare gas element include helium, neon, argon, krypton, and xenon. In the case where one or more of the elements that form oxygen vacancies are contained in the insulating layer3213, the one or more of the elements are diffused from the insulating layer3213to the source region3231sand the drain region3231d. One or more of the elements that form oxygen vacancies may be added to the source region3231sand the drain region3231dby impurity addition treatment.

An impurity element added to the oxide semiconductor film cuts a bond between a metal element and oxygen in the oxide semiconductor film, so that an oxygen vacancy is formed. Alternatively, when the impurity element is added to the oxide semiconductor film, oxygen bonded to a metal element in the oxide semiconductor film is bonded to the impurity element, and the oxygen is released from the metal element, whereby an oxygen vacancy is formed. As a result, the oxide semiconductor film has a higher carrier density and thus the conductivity thereof becomes higher.

The conductive layer3221functions as a first gate electrode and the conductive layer3223functions as a second gate electrode. The conductive layer3222afunctions as a source electrode and the conductive layer3222bfunctions as a drain electrode.

As illustrated inFIG. 38C, an opening3237is formed in the insulating layers3211and3212. The conductive layer3221is electrically connected to the conductive layer3223in the opening3237. Thus, the conductive layers3221and3223are supplied with the same potential. Note that different potentials may be applied to the conductive layers3221and3223without providing the opening3237. Alternatively, the conductive layer3221may be used as a light-blocking film without providing the opening3237. For example, light irradiating the channel formation region3231ifrom the bottom can be reduced by the conductive layer3221formed with a light-blocking material.

As illustrated inFIGS. 38B and 38C, the metal oxide layer3231faces the conductive layer3221functioning as the first gate electrode and the conductive layer3223functioning as the second gate electrode and is between the two conductive films functioning as the gate electrodes.

As with the transistors3200aand3200b, the transistor3200chas the S-channel structure. Such a structure enables the metal oxide layer3231included in the transistor3200cto be electrically surrounded by electric fields of the conductive layer3221functioning as the first gate electrode and the conductive layer3223functioning as the second gate electrode.

Since the transistor3200chas the S-channel structure, an electric field for inducing a channel can be effectively applied to the metal oxide layer3231by the conductive layer3221or3223; thus, the current drive capability of the transistor3200ccan be improved and high on-state current characteristics can be obtained. As a result of the high on-state current, it is possible to reduce the size of the transistor3200c. Furthermore, since the transistor3200chas a structure in which the metal oxide layer3231is surrounded by the conductive layers3221and3223, the mechanical strength of the transistor3200ccan be increased.

The transistor3200cmay be called a top-gate self-aligned (TGSA) FET from the position of the conductive layer3223relative to the metal oxide layer3231or the formation method of the conductive layer3223.

The metal oxide layer3231in the transistor3200cmay have a multi-layer structure including two or more layers, as in the transistor3200b.

Although the insulating layer3212is present only in a portion overlapping with the conductive layer3223in the transistor3200c, the structure is not limited thereto, and the insulating layer3212may cover the metal oxide layer3231. Alternatively, the conductive layer3221may be omitted.

In this embodiment, electronic devices that can include the display device of one embodiment of the present invention will be described.

The display device of one embodiment of the present invention can be used for a display portion of an electronic device. As a result, the electronic device can have high display quality, extremely high resolution, or high reliability.

Examples of electronic devices include a television set, a desktop or laptop personal computer, a monitor of a computer or the like, a digital camera, a digital video camera, a digital photo frame, a mobile phone, a portable game machine, a portable information terminal, an audio reproducing device, and a large game machine such as a pachinko machine.

The electronic device of one embodiment of the present invention can be incorporated along a curved inside/outside wall surface of a house or a building or a curved interior/exterior surface of a car.

The electronic device of one embodiment of the present invention may include a secondary battery. It is preferable that the secondary battery be capable of being charged by contactless power transmission.

Examples of the secondary battery include a lithium ion secondary battery such as a lithium polymer battery using a gel electrolyte (lithium ion polymer battery), a nickel-metal hydride battery, a nickel-cadmium battery, an organic radical battery, a lead-acid battery, an air secondary battery, a nickel-zinc battery, and a silver-zinc battery.

The electronic device of one embodiment of the present invention may include an antenna. When a signal is received by the antenna, the electronic device can display an image, data, or the like on a display portion. When the electronic device includes the antenna and a secondary battery, the antenna may be used for contactless power transmission.

The electronic device of one embodiment of the present invention can have a variety of functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of executing a variety of software (programs), a wireless communication function, and a function of reading out a program or data stored in a recording medium.

Furthermore, the electronic device including a plurality of display portions can have a function of displaying image information mainly on one display portion while displaying text information mainly on another display portion, a function of displaying a three-dimensional image by displaying images where parallax is considered on a plurality of display portions, or the like. Furthermore, the electronic device including an image receiving portion can have a function of photographing a still image or a moving image, a function of automatically or manually correcting a photographed image, a function of storing a photographed image in a recording medium (an external recording medium or a recording medium incorporated in the electronic device), a function of displaying a photographed image on a display portion, or the like. Note that the functions of the electronic devices of embodiments of the present invention are not limited thereto, and the electronic devices can have a variety of functions.

The display device of one embodiment of the present invention can display images with extremely high resolution. For this reason, the display device can be used particularly for portable electronic devices, wearable electronic devices (wearable devices), e-book readers, and the like. In addition, the display device can be suitably used for virtual reality (VR) devices, augmented reality (AR) devices, and the like.

FIG. 39Ais an external view of a camera840to which a finder850is attached.

The camera840includes a housing841, a display portion842, an operation button843, a shutter button844, and the like. Furthermore, an attachable lens846is attached to the camera840.

Although the lens846of the camera840here is detachable from the housing841for replacement, the lens846may be built in a housing.

When the shutter button844is pressed, the camera840can take images. In addition, images can be taken by a touch on the display portion842that serves as a touch panel.

The housing841of the camera840has a mount including an electrode, and the finder850, a stroboscope, and the like can be connected to the housing841.

The finder850includes a housing851, a display portion852, a button853, and the like.

The housing851includes a mount for engagement with the mount of the camera840so that the finder850can be connected to the camera840. The mount includes an electrode, and a moving image or the like received from the camera840through the electrode can be displayed on the display portion852.

The button853serves as a power button. The display portion852can be turned on and off with the button853.

The display device of one embodiment of the present invention can be used in the display portion842of the camera840and the display portion852of the finder850.

Although the camera840and the finder850are separate and detachable electronic devices inFIG. 39A, a finder including the display device of one embodiment of the present invention may be built in the housing841of the camera840.

FIG. 39Bis an external view of a head-mounted display860.

The head-mounted display860includes a mounting portion861, a lens862, a main body863, a display portion864, a cable865, and the like. In addition, a battery866is built in the mounting portion861.

Power is supplied from the battery866to the main body863through the cable865. The main body863includes a wireless receiver or the like to receive video data such as image data and display it on the display portion864. The movement of the user's eyeball or eyelid is captured by a camera in the main body863and then the coordinates of the eyepoint are calculated using the captured data to utilize the user's eye as an input means.

The mounting portion861may include a plurality of electrodes so as to be in contact with the user. The main body863may have a function of sensing current flowing through the electrodes with the movement of the user's eyeball to recognize the direction of his or her eyes. The main body863may have a function of sensing current flowing through the electrodes to monitor the user's pulse. The mounting portion861may include sensors such as a temperature sensor, a pressure sensor, or an acceleration sensor so that the user's biological information can be displayed on the display portion864. The main body863may sense the movement of the user's head or the like to move an image displayed on the display portion864in synchronization with the movement of the user's head, or the like.

FIGS. 39C and 39Dare external views of a head-mounted display870.

The head-mounted display870includes a housing871, two display portions872, an operation button873, and a fixing band874.

The head-mounted display870has the functions of the above-described head-mounted display860and includes two display portions.

Since the head-mounted display870includes the two display portions872, the user's eyes can see their respective display portions. Thus, a high-definition image can be displayed even when a three-dimensional display using parallax, or the like, is performed. In addition, the display portion872is curved around an arc with the user's eye as an approximate center. Owing to this, distances between the user's eye and display surfaces of the display portion become equal; thus, the user can see a more natural image. Even when the luminance or chromaticity of light emitted from the display portion varies depending on the viewing angle, the influence of the variation is substantially ignorable and thus a more realistic image can be displayed because the user's eye is positioned in the normal direction of the display surface of the display portion.

The operation button873serves as a power button or the like. A button other than the operation button873may be included.

As illustrated inFIG. 39E, lenses875may be provided between the display portion872and the user's eyes. The user can see magnified images on the display portion872through the lenses875, leading to higher sense of presence. In that case, as illustrated inFIG. 39E, a dial876for changing the position of the lenses and adjusting visibility may be included.

The display device of one embodiment of the present invention can be used for the display portion872. Since the display device of one embodiment of the present invention has extremely high definition, even when an image is magnified using the lenses875as illustrated inFIG. 39E, the pixels are not perceived by the user, and thus a more realistic image can be displayed.

FIGS. 40A and 40Billustrate an example in which the head-mounted display includes one display portion872. This structure can reduce the number of components.

The display portion872can display an image for the right eye and an image for the left eye side by side on a right region and a left region, respectively. Thus, a three-dimensional moving image using binocular disparity can be displayed.

One image which can be seen by both eyes may be displayed on the entire display portion872. A panorama moving image can thus be displayed from end to end of the field of view, which can provide a higher sense of reality.

The lenses875may be provided. Two images may be displayed side by side on the display portion872. Alternatively, one image may be displayed on the display portion872and seen by both eyes through the lenses875.

The display portion872is not necessarily curved and may have a flat display surface as illustrated in an example ofFIGS. 40C and 40Din which the display portion872does not have a curved surface, for example.

In this embodiment, examples of electronic devices including any of the display devices disclosed in this specification and the like are described.

Examples of the electronic devices including any of the display devices of one embodiment of the present invention are as follows: display devices such as televisions and monitors, lighting devices, desktop personal computers, laptop personal computers, word processors, image reproduction devices which reproduce still images and moving images stored in recording media such as digital versatile discs (DVDs), portable CD players, radios, tape recorders, headphone stereos, stereos, table clocks, wall clocks, cordless phone handsets, transceivers, car phones, mobile phones, portable information terminals, tablet terminals, portable game machines, stationary game machines such as pachinko machines, calculators, electronic notebooks, e-book readers, electronic translators, audio input devices, video cameras, digital still cameras, electric shavers, high-frequency heating appliances such as microwave ovens, electric rice cookers, electric washing machines, electric vacuum cleaners, water heaters, electric fans, hair dryers, air-conditioning systems such as air conditioners, humidifiers, and dehumidifiers, dishwashers, dish dryers, clothes dryers, futon dryers, electric refrigerators, electric freezers, electric refrigerator-freezers, freezers for preserving DNA, flashlights, electrical tools such as chain saws, smoke detectors, and medical equipment such as dialyzers. Further examples include the following industrial equipment: guide lights, traffic lights, belt conveyors, elevators, escalators, industrial robots, power storage systems, and power storage devices for leveling the amount of power supply and smart grid. In addition, moving objects and the like driven by fuel engines and electric motors using power from power storage units, and the like may also be included in the range of electronic devices. Examples of the moving objects include electric vehicles (EV), hybrid electric vehicles (HEV) which include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEV), tracked vehicles in which caterpillar tracks are substituted for wheels of these vehicles, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, golf carts, boats, ships, submarines, helicopters, aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft.

An information terminal2910illustrated inFIG. 41Aincludes a housing2911, a display portion2912, a microphone2917, a speaker portion2914, a camera2913, an external connection portion2916, an operation switch2915, and the like. A display panel and a touch screen that use a flexible substrate are provided in the display portion2912. In addition, the information terminal2910includes an antenna, a battery, and the like inside the housing2911. The information terminal2910can be used as, for example, a smartphone, a mobile phone, a tablet information terminal, a tablet personal computer, or an e-book reader.

A notebook personal computer2920illustrated inFIG. 41Bincludes a housing2921, a display portion2922, a keyboard2923, a pointing device2924, and the like. In addition, the notebook personal computer2920includes an antenna, a battery, and the like inside the housing2921.

A video camera2940inFIG. 41Cincludes a housing2941, a housing2942, a display portion2943, operation switches2944, a lens2945, a joint2946, and the like. The operation switches2944and the lens2945are provided for the housing2941, and the display portion2943is provided for the housing2942. In addition, the video camera2940includes an antenna, a battery, and the like inside the housing2941. The housing2941and the housing2942are connected to each other with the joint2946, and the angle between the housing2941and the housing2942can be changed with the joint2946. The orientation of an image on the display portion2943can be changed and display and non-display of an image can be switched depending on the angle between the housings2941and2942.

FIG. 41Dillustrates an example of a bangle-type information terminal. An information terminal2950includes a housing2951, a display portion2952, and the like. In addition, the information terminal2950includes an antenna, a battery, and the like inside the housing2951. The display portion2952is supported by the housing2951having a curved surface. A display panel formed with a flexible substrate is provided in the display portion2952, whereby the information terminal2950can be a user-friendly information terminal that is flexible and lightweight.

FIG. 41Eillustrates an example of a watch-type information terminal. An information terminal2960includes a housing2961, a display portion2962, a band2963, a buckle2964, an operation switch2965, an input/output terminal2966, and the like. In addition, the information terminal2960includes an antenna, a battery, and the like inside the housing2961. The information terminal2960is capable of executing a variety of applications such as mobile phone calls, e-mailing, text viewing and editing, music reproduction, Internet communication, and computer games.

The display surface of the display portion2962is curved, and images can be displayed on the curved display surface. In addition, the display portion2962includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, by touching an icon2967displayed on the display portion2962, application can be started. With the operation switch2965, a variety of functions such as time setting, power on/off, on/off of wireless communication, setting and cancellation of a silent mode, and setting and cancellation of a power saving mode can be performed. For example, the functions of the operation switch2965can be set by setting the operating system incorporated in the information terminal2960.

The information terminal2960can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the information terminal2960and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. Moreover, the information terminal2960includes the input/output terminal2966, and data can be directly transmitted to and received from another information terminal via the input/output terminal2966. In addition, charging via the input/output terminal2966is possible. Note that the charging operation may be performed by wireless power feeding without using the input/output terminal2966.

FIG. 41Fillustrates a tablet personal computer5300including a housing5301, a housing5302, a display portion5303, an optical sensor5304, an optical sensor5305, a switch5306, and the like. The display portion5303is supported by the housing5301and the housing5302. The display portion5303is formed using a flexible substrate and thus has a function of being bendable flexibly. By changing the angle between the housing5301and the housing5302with a hinge5307and a hinge5308, the display portion5303can be folded so that the housing5301and the housing5302overlap with each other. Although not illustrated, an open/close sensor may be incorporated so that the above-described angle change can be used as information about conditions of use of the display portion5303.

FIG. 41Gis a perspective view of a television device9100. The television device9100includes a housing9000, a display portion9001, a speaker9003, an operation key9005(including a power switch or an operation switch), a connection terminal9006, a sensor9007(a sensor having a function of measuring distance, light, temperature, or the like), and the like. In the television device9100, the display portion9001can include a display device having a screen size of, for example, 50 inches or more or 100 inches or more.

The display device of one embodiment of the present invention is included in the display portions of the electronic devices in this embodiment. With the use of the display device and the driving method of one embodiment of the present invention for the display portions of the electronic devices, the electronic devices can have excellent visibility.

This application is based on Japanese Patent Application Serial No. 2016-250228 filed with Japan Patent Office on Dec. 23, 2016, the entire contents of which are hereby incorporated by reference.