BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a liquid crystal display device. In particular, the invention relates to a semiconductor device and a liquid crystal display device each controlling liquid crystal molecules by generating an electric field almost parallel to a substrate.

2. Description of the Related Art

One of technical development strategies of a liquid crystal display device is widening a viewing angle. As a technique for realizing a wide viewing angle, a mode has been used in which gray scales are controlled by generating an electric field almost parallel to a substrate to move liquid crystal molecules within a surface parallel to the substrate. IPS (In-Plane Switching) and FFS (Fringe-Field Switching) are given as such a mode. In an FFS mode, a second electrode having an opening pattern (e.g., a pixel electrode of which a voltage is controlled per pixel) is provided below liquid crystals, and a first electrode (e.g., a common electrode of which a voltage common to all pixels is supplied) is provided below the opening pattern. An electric field is applied between the pixel electrode and the common electrode, so that the liquid crystals are controlled. Since an electric field is applied to the liquid crystals in a direction parallel to a substrate, liquid crystal molecules can be controlled by using the electric field. That is, since liquid crystal molecules which are aligned in parallel to the substrate (so-called homogeneous alignment) can be controlled in a direction parallel to the substrate, a viewing angle is widened.

The first electrode (the common electrode) is formed to be in direct contact with a glass substrate, and a gate electrode in an inversely staggered transistor is also formed to be in direct contact with the glass substrate. An insulating film functioning as a gate insulating film in the inversely staggered transistor is formed to be in direct contact therewith. In addition, the second electrode (the pixel electrode) is formed thereover (see Reference 1: Japanese Published Patent Application No. 2000-89255).

Alternatively, the first electrode (the common electrode) is formed to be in direct contact with the insulating film functioning as the gate insulating film in the inversely staggered transistor. Note that a semiconductor film, a source electrode, and a drain electrode are also formed to be in direct contact with the insulating film functioning as the gate insulating film in the inversely staggered transistor. In addition, an insulating layer is formed to be in direct contact therewith. Further, the second electrode (the pixel electrode) is formed thereover (see Reference 1: Japanese Published Patent Application No. 2000-89255).

SUMMARY OF THE INVENTION

In the aforementioned conventional technique, electrodes which drive liquid crystals are provided with one insulating film interposed therebetween. Therefore, even when a distance between the electrodes is made to be increased, there has been limitation. If the thickness of the insulating film interposed between electrodes is increased, for example, since a gate insulating film in a transistor is thickened there is an adverse effect such that current drive capability of the transistor is decreased.

Further, optimal values for an arrangement interval and width of an opening pattern of a pixel electrode change depending on a distance between the pixel electrode and a common electrode. Thus, when the distance between the pixel electrode and the common electrode cannot be freely set, the arrangement interval and width of the opening pattern of the pixel electrode have to be extremely limited values. Therefore, the size and a direction of an electric field applied to liquid crystal molecules are inadequate.

In view of the foregoing problems, it is an object of the invention to provide a display device which can improve degree of freedom of an interval between two electrodes of a display element and can apply an optimal electric field between the electrodes, and a manufacturing method thereof.

In order to solve the aforementioned problems, a semiconductor device in accordance with the Invention includes a first electrode formed over a substrate, a first insulating film formed over the first electrode, a semiconductor film formed over the first insulating film, a second Insulating film formed over the semiconductor film, a conductive film formed over the second insulating film, a third insulating film formed over the conductive film, and a second electrode formed over the third insulating film and having an opening pattern.

A liquid crystal display device in accordance with the invention includes a first electrode formed over a substrate, a first insulating film formed over the first electrode, a semiconductor film formed over the first insulating film, a second insulating film formed over the semiconductor film, a conductive film formed over the second insulating film, a third insulating film formed over the conductive film, a second electrode formed over the third insulating film and having an opening pattern, and liquid crystals provided over the second electrode.

In accordance with each of the semiconductor device and the liquid crystal display device, the first electrode is formed over the substrate, that is, below the semiconductor film. In addition, since the second electrode is provided over the conductive film (e.g., a gate electrode or a source electrode of a transistor) and the third insulating film, an interval between the first electrode and the second electrode can be increased compared with a conventional device. Further, even when thickness of the first insulating film is changed, it does not affect another element such as a transistor very much. Therefore, the thickness thereof can be optionally changed, so that the interval between the first electrode and the second electrode can be freely set. Accordingly, degree of freedom of the interval between the first electrode and the second electrode is improved. Then, a gradient of an electric field applied between the electrodes can be controlled, so that, for example, an electric field parallel to the substrate can be easily increased. That is, since liquid crystal molecules which are aligned in parallel to the substrate (so-called homogeneous alignment) can be controlled in a direction parallel to the substrate in a display device using liquid crystals, a viewing angle is widened by applying an optimal electric field.

Note that the opening pattern is provided for generating an electric field almost parallel to the substrate between the first electrode and the second electrode. Therefore, the opening pattern can be various shapes as log as it can generate the electric field which is almost parallel to the substrate.

Therefore, the opening pattern includes not only a closed opening pattern such as a slit but also includes a space which is located between conductor patterns and in which the conductor patterns are not formed such as a space between comb-shaped portions in a comb-shaped electrode, for example. That is, the opening pattern may be any pattern as long as a gap or an interval is provided between electrodes. The same can be said hereinafter.

A semiconductor device in accordance with the invention includes a first electrode formed over a substrate, a first insulating film formed over the first electrode, a semiconductor film formed over the first insulating film, a conductive film formed over the semiconductor film, a second insulating film formed over the conductive film, and a second electrode formed over the second insulating film and having an opening pattern.

In accordance with each of the semiconductor device and the liquid crystal display device, the first electrode is formed over the substrate, that is, below the semiconductor film. In addition, since the second electrode is provided over the conductive film (e.g., a source electrode) and the insulating films, an interval between the first electrode and the second electrode can be increased compared with a conventional device. Further, even when thickness of the first insulating film is changed, it does not affect another element such as a transistor very much. Therefore, the thickness thereof can be optionally changed, so that the interval between the first electrode and the second electrode can be freely set. Accordingly, degree of freedom of the interval between the first electrode and the second electrode is improved. Then, a gradient of an electric field applied between the electrodes can be controlled, so that, for example, an electric field parallel to the substrate can be easily increased. That is, since liquid crystal molecules which are aligned in parallel to the substrate (so-called homogeneous alignment) can be controlled in a direction parallel to the substrate in a display device using liquid crystals, a viewing angle is widened by applying an optimal electric field.

A semiconductor device in accordance with the invention includes a first electrode formed over a substrate, a first insulating film formed over the first electrode, a conductive film formed over the first insulating film, a semiconductor film formed over the conductive film, a second insulating film formed over the semiconductor film, and a second electrode formed over the second insulating film and having an opening pattern.

In accordance with each of the semiconductor device and the liquid crystal display device, the first electrode is formed over the substrate, that is, below the semiconductor film and the conductive film (e.g., a gate electrode). In addition, since the second electrode is provided over the second insulating film, an interval between the first electrode and the second electrode can be increased compared with a conventional device. Further, even when thickness of the second insulating film is changed, it does not affect another element such as a transistor very much. Therefore, the thickness thereof can be optionally changed, so that the interval between the first electrode and the second electrode can be freely set. Accordingly, degree of freedom of the interval between the first electrode and the second electrode is improved. Then, a gradient of an electric field applied between the electrodes can be controlled, so that, for example, of an electric field parallel to the substrate can be easily increased. That is, since liquid crystal molecules which are aligned in parallel to the substrate (so-called homogeneous alignment) can be controlled in a direction parallel to the substrate in a display device using liquid crystals, a viewing angle is widened by applying an optimal electric field.

In a semiconductor device in accordance with the invention, the first electrode is a common electrode and the second electrode is a pixel electrode in the aforementioned structure.

In a semiconductor device in accordance with the invention, the first electrode is a pixel electrode and the second electrode is a common electrode in the aforementioned structure.

A liquid crystal display device in accordance with the invention includes a first electrode formed over a substrate, a first insulating film formed over the first electrode, a semiconductor film formed over the first insulating film, a conductive film formed over the semiconductor film, a second insulating film formed over the conductive film, a second electrode formed over the second insulating film and having an opening pattern, and liquid crystals provided over the second electrode.

A liquid crystal display device in accordance with the invention includes a first electrode formed over a substrate, a first insulating film formed over the first electrode, a conductive film formed over the first insulating film, a semiconductor film formed over the conductive film, a second insulating film formed over the semiconductor film, a second electrode formed over the second insulating film and having an opening pattern, and liquid crystals provided over the second electrode.

In a liquid crystal display device in accordance with the invention, the liquid crystals are controlled by an electric field between the first electrode and the second electrode in the aforementioned structure.

In a liquid crystal display device in accordance with the invention, the first electrode is a common electrode and the second electrode is a pixel electrode in the aforementioned structure.

In a liquid crystal display device in accordance with the invention, the first electrode is a pixel electrode and the second electrode is a common electrode in the aforementioned structure.

Note that various types of switches can be used as a switch shown in the invention, and an electrical switch, a mechanical switch, or the like is given as an example. That is, any element can be used as long as it can control a current flow, without limiting to a certain element. For example, it may be a transistor, a diode (e.g., a PN diode, a PIN diode, a Schottky diode, or a diode-connected transistor), or a logic circuit combining such elements. In the case of using a transistor as a switch, the polarity (a conductivity type) of the transistor is not particularly limited to a certain type because it operates just as a switch. However, a transistor of polarity with smaller off-current is preferably used when small off-current is preferable. A transistor provided with an LDD region, a transistor with a multi-gale structure, or the like is given as an example of a transistor with smaller off-current. In addition, it is preferable that an N-channel transistor be used when a potential of a source electrode of the transistor operating as a switch is closer to a low-potential-side power supply (e.g., Vss, GND, or 0 V), while a P-channel transistor be used when the potential of the source electrode is closer to a high-potential-side power supply (e.g., Vdd). This is because the absolute value of a gate-source voltage of the transistor can be Increased, so that the transistor can easily operate as a switch. Note that a CMOS switch may also be employed by using both N-channel and P-channel transistors. By employing the CMOS switch, the switch can be operated appropriately even when a voltage output through the switch (i.e., an input voltage) is changed such that it becomes higher or lower than an output voltage. Note that although a TFT which controls a pixel electrode, a switching element used for a driver circuit portion, or the like can be given as the switch in the invention, the switch can also be used at another portion as long as it is a portion which is necessary to control a current flow.

Note that in the invention, description “being connected” includes the case where elements are electrically connected and the case where elements are directly connected. Accordingly, in structures disclosed in the invention, another element which enables an electrical connection (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, or a diode) may be interposed between elements having a predetermined connection relation. Alternatively, the elements may be provided without interposing another element therebetween. Note that the case where two conductive films are not electrically connected without interposing another element which enables an electrical connection therebetween is described as “being directly connected”. Note also that description “being electrically connected” includes the case where elements are electrically connected and the case where elements are directly connected.

Note that a display element, a display device, and a light-emitting device of the invention can employ various types and include various elements. In the invention, a liquid crystal element can be used. A liquid crystal element is an element which controls transmission or nontransmission of light by an optical modulation action of liquid crystals and includes a pair of electrodes and liquid crystals. Display devices using liquid crystals element include a liquid crystal display, a transmissive liquid crystal display, a semi-transmissive liquid crystal display, a reflective liquid crystal display, and the like. In addition, for example, a display medium, the contrast of which changes by an electromagnetic action, such as an EL element (an EL element means an element including a light-emitting layer which can obtain luminescence generated by applying an electric field. Further, an EL element includes an organic EL element, an inorganic EL element, or an EL element containing both organic and inorganic materials), an electron-emissive element, electronic ink, a grating light valve (GLV), a plasma display panel (PDP), a digital micromirror device (DMD), a piezoelectric ceramic display, or a carbon nanotube can be included. Note that display devices using an EL element include an EL display; display devices using an electron-emissive element include a field emission display (FED), an SED-type flat panel display (SED: Surface-conduction Electron-emitter Display), and the like; and display devices using electronic ink include electronic paper.

Note that in the invention, various types of transistors can be applied to a transistor without limiting to a certain type. Accordingly, a thin film transistor (TFT) using a non-single crystalline semiconductor film typified by amorphous silicon or polycrystalline silicon, a transistor formed by using a semiconductor substrate or an SOI substrate, a MOS transistor, a junction transistor, a bipolar transistor, a transistor using a compound semiconductor such as ZnO or a-InGaZnO, a transistor using an organic semiconductor or a carbon nanotube, or other transistors can be applied. In addition, various types of substrates can be used as a substrate over which a transistor is formed without limiting to a certain type. Therefore, for example, the transistor can be formed over a glass substrate, a plastic substrate, a paper substrate, a cellophane substrate, a stone substrate, or the like. Further, in the case of manufacturing a reflective display, a single crystalline substrate or an SOI substrate can also be used. Moreover, a transistor may be formed over one substrate, and then, the transistor may be transferred to another substrate.

Note that as described above, various types of transistors can be used for the transistor in the invention and the transistor can be formed over various types of substrates. Accordingly, all circuits may be formed over a glass substrate or a plastic substrate. In the case of manufacturing a reflective display, the transistor may be formed over a single crystalline substrate or an SOI substrate. That is, the transistor may be formed over any substrate. By forming all of the circuits over the same substrate, the number of component parts can be reduced to cut cost, or the number of connections to the circuit components can be reduced to improve reliability. Alternatively, a part of the circuits may be formed over one substrate and another part of the circuits may be formed over another substrate. That is, not all of the circuits are required to be formed over the same substrate. For example, a part of the circuits may be formed with transistors over a glass substrate and another part of the circuits may be formed over a single crystalline substrate, so that an IC chip thereof may be connected to the glass substrate by COG (Chip On Glass). Alternatively, the IC chip may be connected to the glass substrate by TAB (Tape Automated Bonding) or a printed wiring board. By forming a part of the circuits over the same substrate in this manner, the number of component parts can be reduced to cut cost, or the number of connections to the circuit components can be reduced to improve reliability. In addition, by forming a portion with a high driving voltage or a portion with high driving frequency, which consumes large power, over another substrate, increase in power consumption can be prevented.

A structure of a transistor can be various modes without limiting to a certain structure. For example, a multi-gate structure having two or more gate electrodes may be used. By using the multi-gate structure, off-current can be reduced; the withstand voltage of the transistor can be increased to improve reliability; or a drain-source current does not fluctuate very much even when a drain-source voltage fluctuates in a saturation region so that flat characteristics can be obtained. In addition, a structure where gate electrodes are formed over and below a channel may be used. By using the structure where gate electrodes are formed over and below the channel, a channel region is enlarged to increase a current flowing therethrough, or a depletion layer can be easily formed to decrease the S value. Further, a structure where a gate electrode is formed over a channel, a structure where a gate electrode is formed below a channel, a staggered structure, or an inversely staggered structure may be used. Further, a channel region may be divided into a plurality of regions and the divided regions may be connected in parallel or in series. A source electrode or a drain electrode may overlap with a channel (or a part of it). By using the structure where the source electrode or the drain electrode may overlap with the channel (or a part of it), a problem in that an electric charge is accumulated in a part of the channel so that an operation becomes unstable can be prevented. Moreover, an LDD region may be provided. By providing the LDD region, off-current can be reduced and the withstand voltage of the transistor can be increased to improve reliability, or a drain-source current does not fluctuate very much even when a drain-source voltage fluctuates in a saturation region so that flat characteristics can be obtained.

Note also that one pixel corresponds to one element which can control brightness in the invention. Therefore, for example, one pixel corresponds to one color element and brightness is expressed with the one color element. Accordingly, in the case of a color display device having color elements of R (Red), G (Green), and B (Blue), a minimum unit of an image is formed of three pixels of an R pixel, a G pixel, and a B pixel. Note that the color elements are not limited to three colors, and color elements with more than three colors may be employed. RGBW (W means white), or RGB plus yellow, cyan, and/or magenta is given as an example. Alternatively, as another example, in the case of controlling brightness of one color element by using a plurality of regions, one region corresponds to one pixel. Therefore, for example, in the case of performing area gray scale display, a plurality of regions which controls brightness are provided in each color element and gray scales are expressed with the whole regions. In this case, one region which controls brightness corresponds to one pixel. Thus, in that case, one color element includes a plurality of pixels. Further, in that case, regions which contribute to display may be different depending on each pixel. Moreover, in a plurality of regions which control brightness in each color element, that is, in a plurality of pixels which form one color element, a viewing angle may be increased by slightly varying signals supplied to the plurality of the pixels. Note that description “one pixel (for three colors)” corresponds to the case where three pixels of R, Q and B are considered as one pixel. Meanwhile, description “one pixel (for one color)” corresponds to the case where a plurality of pixels are provided for each color element and collectively considered as one pixel.

Note also that in the invention, pixels may be provided (arranged) in matrix. Here, description that pixels are provided (arranged) in matrix includes the case where the pixels are provided in stripes in a so-called grid pattern combining vertical stripes and lateral stripes. In addition, in the case of performing full color display with three color elements (e.g., RGB), dots of the three color elements may be provided in a so-called delta pattern. Further, dots of the three color elements may also be provided in Bayer arrangement. Note that the color elements are not limited to three colors, and color elements with more than three colors may be employed. RGBW (W means white), or RGB plus yellow, cyan, and/or magenta is given as an example. Moreover, the sizes of light-emitting regions may be different per color element.

A transistor is an element including at least three terminals of a gate, a drain, and a source, and includes a channel region between a drain region and a source region. Here, since the source and the drain of the transistor change depending on the structure, the operating condition, or the like of the transistor, it is difficult to define which is a source or a drain. Therefore, in the invention, one of regions functioning as a source and a drain is described as a first terminal and the other region is described as a second terminal.

A gate means all of or a part of a gate electrode and a gate wiring (also called a gate line, a gate signal line, or the like). A gate electrode means a conductive film which overlaps with a semiconductor forming a channel region, an LDD (Lightly Doped Drain) region, or the like with a gate insulating film interposed therebetween. A gate wiring means a wiring for connecting gate electrodes of respective pixels or for connecting a gate electrode to another wiring.

However, there is also a portion functioning as both a gate electrode and a gate wiring. Such a region may be called either a gate electrode or a gate wiring. That is, there is a region in which a gate electrode and a gate wiring cannot be clearly distinguished from each other. For example, in the case where a channel region is formed overlapping with an extended gate wiring, the overlapping region functions as both a gate wiring and a gate electrode. Accordingly, such a region may be called either a gate electrode or a gate wiring.

In addition, for example, a region formed of the same material as a gate electrode and electrically connected to the gate electrode may also be called a gate electrode. Similarly, a region formed of the same material as a gate wiring and electrically connected to the gate wiring may also be called a gate wiring. In a strict sense, such a region does not overlap with a channel region, or does not have a function of connecting to another gate electrode in some cases. However, there is a region formed of the same material as a gate electrode or a gate wiring and electrically connected to the gate electrode or the gate wiring, for reduction of manufacturing cost or steps, simplification of layout, or the like. Accordingly, such a region may also be called either a gate electrode or a gate wiring.

In a multi-gate transistor, for example, a gate electrode of one transistor is often connected to a gate electrode of another transistor by using a conductive film which is formed of the same material as the gate electrode. Since such a region is a region for connecting a gate electrode to another gate electrode, it may be called a gate wiring, while it may also be called a gate electrode because a multi-gate transistor can be considered as one transistor. That is, a region which is formed of the same material as a gate electrode or a gate wiring and electrically connected thereto may be called either a gate electrode or a gate wiring. In addition, for example, a conductive film which connects a gate electrode and a gate wiring may also be called either a gate electrode or a gate wiring.

Note that a gate terminal means a part of a region of a gate electrode or a region which is electrically connected to a gate electrode.

Note also that a source means all of or a part of a source region, a source electrode, and a source wiring (also called a source line, a source signal line, or the like). A source region means a semiconductor region containing a large amount of p-type impurities (e.g., boron or gallium) or n-type impurities (e.g., phosphorus or arsenic). Accordingly, a region containing a slight amount of p-type impurities or n-type impurities, namely, an LDD (Lightly Doped Drain) region is not included in a source region. A source electrode is a conductive layer formed of a different material from a source region, and electrically connected to the source region. However, a source electrode and a source region are collectively called a source electrode in some cases. A source wiring is a wiring for connecting source electrodes of pixels or for connecting a source electrode to another wiring.

However, there is also a portion functioning as both a source electrode and a source wiring. Such a region may be called either a source electrode or a source wiring. That is, there is a region in which a source electrode and a source wiring cannot be clearly distinguished from each other. For example, in the case where a source region overlaps with an extended source wiring, the overlapping region functions as both a source wiring and a source electrode. Accordingly, such a region may be called either a source electrode or a source wiring.

In addition, a region formed of the same material as a source electrode and electrically connected to the source electrode, or a portion for connecting a source electrode to another source electrode may also be called a source electrode. A portion which overlaps with a source region may also be called a source electrode. Similarly, a region formed of the same material as a source wiring and electrically connected to the source wiring may also be called a source wiring. In a strict sense, such a region does not have a function of connecting to another source electrode in some cases. However, there is a region formed of the same material as a source electrode or a source wiring and electrically connected to the source electrode or the source wiring, for reduction of manufacturing cost or steps, simplification of layout, or the like. Accordingly, such a region may also be called either a source electrode or a source wiring.

In addition, for example, a conductive film which connects a source electrode and a source wiring may also be called either a source electrode or a source wiring.

Note that a source terminal means a part of a region of a source electrode or a region which is electrically connected to a source electrode.

Note also that a drain means all of a drain region, a drain electrode, and a drain wiring. Words used for a drain in this specification are similar to those used for a source. Words used for a drain terminal are also similar to those used for a source terminal.

Note also that in the invention, a semiconductor device means a device having a circuit including a semiconductor element (e.g., a transistor or a diode). The semiconductor device may also include all devices that can function by utilizing semiconductor characteristics. In addition, a display device means a device having a display element (e.g., a liquid crystal element or a light-emitting element). Note that the display device may include a display panel itself where a plurality of pixels including display elements such as liquid crystal elements or EL elements are formed over the same substrate as a peripheral driver circuit for driving the pixels. The display device may also include a flexible printed circuit (FPC) or a printed wiring board (PWB) attached to the display panel. Further, a light-emitting device means a display device having a self-luminous display element, particularly, such as an EL element or an element used for an FED. A liquid crystal display device means a display device having a liquid crystal element.

In the invention, description that an object is “formed on” or “formed over” another object does not necessarily mean that the object is in direct contact with another object. The description includes the case where the two objects are not in direct contact with each other, that is, the case where another object is sandwiched therebetween. Accordingly, for example, when it is described that a layer B is formed on (or over) a layer A, it includes both of the case where the layer B is formed in direct contact with the layer A, and the case where another layer (e.g., a layer C or a layer D) is formed in direct contact with the layer A and the layer B is formed in direct contact with the layer C or D. Similarly, when it is described that an object is formed above another object, it does not necessarily mean that the object is in direct contact with another object, and another object may be sandwiched therebetween. Accordingly, for example, when it is described that a layer B is formed above a layer A, it includes both of the case where the layer B is formed in direct contact with the layer A, and the case where another layer (e.g., a layer C or a layer D) is formed in direct contact with the layer A and the layer B is formed in direct contact with the layer C or D. Similarly, when it is described that an object is formed below or under another object, it includes both of the case where the objects are in direct contact with each other, and the case where the objects are not in contact with each other. Note that here when it is described that an object is formed above or over another object, a substrate over which an electrode is formed is used as a base and a side over which the electrode is formed corresponds to a direction described as “above” or “over”.

By employing the invention, an interval between the first electrode and the second electrode can be increased and controlled without affecting another element, so that degree of freedom of the interval therebetween is improved. Accordingly, since optimal values for an arrangement interval and width of an opening pattern of a pixel electrode change depending on a distance between the pixel electrode and a common electrode, the size, the width, and the interval of the opening pattern can also be freely set. Then, a gradient of an electric field applied between the electrodes can be controlled, so that, for example, an electric field parallel to a substrate can be easily increased. In particular, since liquid crystal molecules which are aligned in parallel to the substrate (so-called homogeneous alignment) can be controlled in a direction parallel to the substrate in a display device using liquid crystals, a viewing angle is widened by applying an optimal electric field.

