Liquid crystal display device and method of manufacturing the same

A liquid crystal display device includes a pair of transparent substrates facing each other through a liquid crystal layer disposed therebetween; a gate insulating film formed so as to cover a gate electrode formed in the pixel regions, disposed closer to the liquid crystal layer, of one of the transparent substrates; a semiconductor film provided on the gate insulating film, for forming a thin-film transistor; a first electrode provided on the semiconductor film through the first insulating film and the second insulating film; a second electrode provided on the first electrode through a third insulating film; and a contact hole formed collectively in the first insulating film, the second insulating film, and the third insulating film on the first electrode, where a second electrode is formed on the contact hole. A floating electrode is formed in the peripheral region of the contact hole.

TECHNICAL FIELD

The present invention relates to a liquid crystal display device, particularly to one based on a technology called IPS (in-plane switching), and to a method of manufacturing the devices.

BACKGROUND ART

A liquid crystal display device based on a technology called IPS has a pair of transparent substrates disposed facing each other through a liquid crystal. Each pixel region of one of the transparent substrates closer to the liquid crystal has a pixel electrode; and a common electrode for generating an electric field (lateral electric field) parallel to the transparent substrates, between the pixel electrode and the common electrode. The amount of light transmitting through a region between the pixel electrode and the common electrode is regulated by controlling driving the liquid crystal according to an electric field. Such a liquid crystal display device is known as being capable of providing unchanged display images even if viewed from a diagonal direction with respect to the screen surface (excellent in so-called wide viewing angle characteristics).

Conventionally, in such a liquid crystal display device, a pixel electrode and a common electrode have been formed of a conductive layer that does not transmit light. In recent years, however, the following type has been known. That is, common electrodes made of transparent electrodes are formed on the entire area of the region excluding around the pixel regions, and strip-shaped pixel electrodes are formed on the common electrodes through an insulating film.

With a liquid crystal display device thus structured, a lateral electric field is generated between a pixel electrode and a common electrode, which provides excellent wide viewing angle characteristics and a higher aperture ratio (refer to patent literature 1 for example).

Meanwhile, a liquid crystal display device with the diagonal electric field method has been developed. In the device, pixel electrodes and common electrodes for applying an electric field to the liquid crystal layer are disposed on different layers through an insulating film. The device provides a wider viewing angle and a higher contrast than the IPS method, and further the device can be driven at low voltage and has a high transmittance, thereby featuring bright display.

However, the device involves the following problems. That is, the potential difference between a drain signal line and a pixel electrode causes orientation misalignment, which produces a region that does not contribute to display near a signal line to decrease the aperture ratio. Further, coupling capacitance produced between a signal line and a pixel electrode likely degrades display quality (e.g. crosstalk).

Hence, a liquid crystal display device is devised in which pixel electrodes and common electrodes are disposed on an interlayer resin film in order to reduce such influence by potential of a signal line (refer to patent literatures 2 and 3 for example).

However, a request has been made for providing a liquid crystal display device with a higher aperture ratio (transmittance) and a method of manufacturing the device at low cost.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

A liquid crystal display device of the present invention includes a pair of transparent substrates; a gate insulating film; a switching element; a first electrode; a second electrode; and a contact hole. The pair of transparent substrates facing each other through a liquid crystal layer is disposed therebetween. The gate insulating film is formed so as to cover the gate electrode formed in the pixel regions, disposed closer to the liquid crystal layer, of one of the transparent substrates. The switching element is formed of a thin-film transistor provided on the gate insulating film. The first electrode is provided on the switching element through an insulating film. The second electrode is provided on the first electrode through an insulating film and is connected to the electrode of the switching element. The contact hole is collectively formed in the insulating film on the switching element and in the insulating film on the first electrode. The second electrode is formed at contact hole. The liquid crystal display device has a floating electrode (simultaneously with the first electrode) formed in the peripheral region of the contact hole in the insulating film on the switching element.

A method of manufacturing a liquid crystal display device, of the present invention is one manufacturing a device that includes a pair of transparent substrates; a gate insulating film; a switching element; a first electrode; a second electrode; and a contact hole.

The pair of transparent substrates of the liquid crystal display device facing each other through a liquid crystal layer is disposed therebetween. The gate insulating film is formed so as to cover a gate electrode formed in the pixel regions, disposed closer to the liquid crystal layer, of one of the transparent substrates. The switching element is formed of a thin-film transistor provided on the gate insulating film. The first electrode is provided above the switching element through an insulating film. The second electrode is provided over the first electrode through an insulating film and is connected to the electrode of the switching element. The contact hole is collectively formed in the insulating film on the switching element and the insulating film on the first electrode for accommodating the second electrode to be formed.

The method of manufacturing liquid crystal display devices is as follows. After an insulating film is formed on a switching element, a first electrode is patterned on the insulating film while a floating electrode is formed in a peripheral region where the contact hole is formed. Then after an insulating film is formed on the first electrode, the contact hole is collectively formed in the multiple insulating films to expose part of the electrode of the switching element outside, and then the electrode of the switching element is connected to the second electrode.

In this way, the present invention allows providing a liquid crystal display device with a high aperture ratio (transmittance) and low cost and a method of manufacturing the device.

DESCRIPTION OF EMBODIMENT

Exemplary Embodiment

Hereinafter, a description is made of a liquid crystal display device and a method of manufacturing the device according to an embodiment of the present invention usingFIGS. 1 through 4E.

FIG. 1is a plan view showing the structure of the substantial part for one pixel, of a liquid crystal display device according to an embodiment of the present.FIG. 2is an outline sectional view of the switching element inFIG. 1, taken along line2-2.FIG. 3is an outline sectional view of the liquid crystal layer inFIG. 1, taken along line3-3. The liquid crystal display device shown in the figures is of an active matrix type, where multiple pixels are arranged in a matrix.

As shown inFIGS. 1,2, and3, a pair of transparent substrates1and12facing each other through liquid crystal layer13are disposed therebetween. Multiple gate electrodes2are formed in the predetermined pattern in the pixel regions of insulating, transparent substrate1(e.g. a glass substrate) closer to liquid crystal layer13, directly or through a base layer. Then, gate insulating film3is formed on transparent substrate1so as to cover gate electrode2. Gate insulating film3has semiconductor film4formed thereon. Source/drain electrode5is formed on semiconductor film4to form a thin-film transistor as a switching element.

Here, semiconductor film4is desirably formed of an amorphous oxide semiconductor of InGaZnOxincluding In—Ga—Zn—O. To form a film of the semiconductor, vapor phase deposition such as sputtering and laser deposition can be used with a polycrystalline sintered body having a composition of InGaO3(ZnO)4for example as a target.

Gate electrode2is connected to signal line2a. Source/drain electrode5is connected to signal line5a. Signal lines2aand5aare formed so as to cross each other isolated by gate insulating film3. Gate electrode2is formed integrally with signal line2athat becomes a scanning signal line. Part of signal line5aof source/drain electrode5combines as a video signal line, where both lines are connected to each other. Here, gate electrode2, source/drain electrode5, and signal lines2aand5aare formed of a single metal of Al, Mo, Cr, W, Ti, Pb, Cu, or Si; of a composite lamination (e.g. Ti/Al) of some of these metals; or of a metal compound layer (e.g. MoW, AlCu). In this embodiment, gate electrode2and source/drain electrode5are formed of Cr; alternatively, they may be formed of different materials.

On source/drain electrode5(i.e. a switching element), first insulating film6, second insulating film7, first electrode8as a common electrode, third insulating film9, and second electrode10as a pixel electrode are successively laminated. Second electrode10is connected to source/drain electrode5(i.e. a thin-film transistor) through contact hole11collectively formed in the three-layered films: first insulating film6, second insulating film7, and third insulating film9. In other words, first electrode8is provided on a switching element through first insulating film6and second insulating film7as insulating films. Second electrode10is provided on first electrode8through third insulating film9as an insulating film and is connected to the electrode of the switching element. The wall surface of contact hole11is covered with second electrode10. Floating electrode19is formed in the peripheral region of contact hole11.

First electrode8, second electrode10, and floating electrode19are formed of a transparent conductive film such as ITO (indium tin oxide). In the process of forming first electrode8, floating electrode19is simultaneously formed so as to be present around the region where contact hole11is formed. First electrode8is supplied with a common potential that is different from a potential applied to second electrode10. Hence, first electrode8, second electrode10, and third insulating film9form a retentive capacity that is in addition transparent, thereby increasing the aperture ratio during transmission display.

Here, third insulating film9is ideally a silicon nitride film formed by plasma CVD (chemical vapor deposition). A silicon nitride film has a dielectric constant higher than a coated insulating film made of an organic or inorganic material, and than a silicon oxide film, thereby increasing the retentive capacitance. Third insulating film9is desirably made closely packed by being formed at high temperature.

Second insulating film7is a coated insulating film made of an organic or inorganic material that is a SOG (spin on glass) material having Si—O bonds. As described later, using an SOG material for second insulating film7allows using collective dry etching of first insulating film6and third insulating film9, thereby simplifying the manufacturing process. Further, film formation can be made by a common coater, which reduces the film forming cost itself compared to an inorganic insulating film such as first insulating film6and third insulating film9formed by a vacuum device. Further, a film thicker than an inorganic insulating film can be easily formed, thereby increasing flatness and reducing parasitic capacitance. Second insulating film7is formed of an SOG material having Si—O bonds, which has a heat resistance high enough to form third insulating film9at 240° C. or higher, thereby forming more reliable third insulating film9.

As shown inFIG. 3, at the side of displaying images, insulating transparent substrate12as the common substrate, made of such as a glass substrate is disposed so as to face transparent substrate1, and liquid crystal layer13is disposed between transparent substrates1and12. Second electrode10, which becomes a surface contacting liquid crystal layer13of transparent substrate1, has oriented film14formed thereon. At the side contacting liquid crystal layer13of transparent substrate12, oriented film14is disposed as well. The inner surface where oriented film14of transparent substrate12is formed has color filter15and black matrix16formed thereon. Then, overcoat17is formed so as to cover color filter15and black matrix16, and oriented film14is formed on overcoat17.

The outer surfaces of transparent substrates1and12have polarizing plate18disposed thereon. InFIG. 1, polarizing plate18is not shown. Further, such as a phase difference plate may be disposed on at least one of transparent substrates1and12as required.

Here, in a liquid crystal display device according to the embodiment, second electrode10has a linear part and is formed in a comb-teeth shape. First electrode8is formed in a sheet shape. Then, the liquid crystal display device generates an electric field parallel with transparent substrates1and12between second electrode10and first electrode8to drive liquid crystal layer13for displaying.

Next, a description is made of an example method of manufacturing liquid crystal display devices, according to an embodiment of the present invention usingFIGS. 4A through 4E.

First, as shown inFIG. 4A, transparent substrate1is prepared to form a metal film made of such as Cr over the entire surface of substrate1by sputtering for example. Then, selectively the metal film is etched by photolithography technique to form gate electrode2together with signal lines.

Next, as shown inFIG. 4B, gate insulating film3is formed made of an SiN film over the entire surface of transparent substrate1including gate electrode2by plasma CVD or sputtering for example. At this moment, as film forming conditions, the film forming temperature (substrate temperature) is 380° C. and the film thickness is 300 nm. Further, successively an a-Si layer (or an a-Si layer doped with n-type impurities) is formed over the entire surface of gate insulating film3by CVD for example. Furthermore, a metal film made of such as Cr is formed over the entire surface of the a-Si layer by sputtering for example. Then, selectively the a-Si layer and the metal film are etched simultaneously by photolithography technique to form semiconductor film4for a thin-film transistor (hereinafter, abbreviated as TFT) and source/drain electrode (including signal lines)5.

Next, as shown inFIG. 4C, first insulating film6made of SiN is formed over the entire surface of transparent substrate1including source/drain electrode5(channel region) by such as plasma CVD and sputtering. Further, the entire surface of first insulating film6is applied with an SOG material having Si—O bonds, and then is baked them at 250° C. for 60 minutes in an oven for heat curing process to form second insulating film7. The thickness of second insulating film7formed here is preferably 1.5 to 4.0 μm. A thickness of less than 1.5 μm unpreferably causes uneven parts at positions where such as TFTs are present, and furthermore at first electrode8and second electrode10formed in the following step. A thickness of more than 4.0 μm unpreferably increases the light absorption rate due to second insulating film7to decrease the brightness of the display area.

Further, an ITO film is formed over the entire surface of second insulating film7by sputtering for example. Then, selectively the ITO film is etched by photolithography technique to form first electrode8and floating electrode19with a thickness of 55 nm. Here, first electrode8is electrically connected to the common wiring wired on the frame region of the liquid crystal display device. Floating electrode19is formed so as to be present in the peripheral region of contact hole11processed in the subsequent step.

Next, as shown inFIG. 4D, third insulating film9made of SiN is formed, which has a favorable insulation performance, for example, over the entire surface of second insulating film7including first electrode8by such as plasma CVD and sputtering. At this moment, as film forming conditions, the film forming temperature (substrate temperature) can be 230° C. to 300° C. since second insulating film7at the layer lower than third insulating film9is an SOG material with a higher heat-resisting temperature. Hence, third insulating film9can be formed that is more closely packed and more reliable than the case where second insulating film2is made of a conventional resin film.

At this moment, the gas flow ratio of mono-silane (SiH4) to ammonia (NH3) (both are material gases for forming a film by plasma CVD) is set to 1:6 when forming a regular bulk layer of an insulating film. Then, halfway through the process, the gas flow amount of ammonia (NH3) is increased to make the ratio 1:16 for example. In this way, the etching rate near the surface of the insulating film is desirably higher than that at the other part (bulk layer). The film thickness of the part with the higher etching rate is desirably between 5% and 30% of that of the insulating film, and more desirably approximately between 8% and 12%. By thus forming a film (recess layer) with a high etching rate near the surface, contact hole11can be formed in a normal tapered shape. In other words, as shown inFIG. 4D, contact hole11can be formed so that hole11closer to its opening has a larger diameter compared to its bottom.

To obtain desired moisture resistance and insulation performance of the channel region of TFTs and source/drain electrode5, the thickness of third insulating film9is appropriately 100 nm or more. A thickness exceeding 1,000 nm produces a lower capacitance between first electrode8and second electrode10, which unpreferably prevents sufficient write voltage to be applied to the liquid crystal and requires a higher voltage for driving liquid crystal molecules.

After that, photosensitive resist mask20is formed on third insulating film9. Next, contact hole11is formed for each pixel by dry etching so as to collectively penetrate the three-layered insulating films (i.e. first insulating film6covering source/drain electrode5, second insulating film7, and third insulating film9), and part of source/drain electrode5is exposed once again. A mixed gas of O2and one of such as SF6, CHF3, and CF4is used as an etching gas for dry etching. As a result that the three-layered insulating films are thus collectively etched, some manufacturing steps such as a photolithography step are eliminated and the load of an exposing step (exposure, photo-reaction process) is reduced to lower costs, compared to conventional liquid crystal display devices that are produced by patterning (forming a contact hole) by photolithography technique using a photosensitive resin material as second insulating film7.

Further, second insulating film7interposed between first insulating film6and third insulating film9, both inorganic insulating films made of such as SiN, is an SOG material having Si—O bonds. Hence, uneven parts are not generated in each layer after dry etching. In addition, as a result that the selection ratio of second insulating film7to photoresist is 2.5 or more and the etching rate of second insulating film7is 500 nm/min or higher, plasma does not damage second insulating film7, thereby allowing stable patterning.

In third insulating film9, floating electrode19is formed simultaneously with first electrode8in the peripheral region of contact hole11, where this floating electrode19prevents hole11to broaden when hole11is formed. Accordingly, even when contact hole11is formed by collectively etching the three-layered insulating films, highly accurate hole11can be formed.

As shown inFIG. 4E, after forming contact hole11, resist mask20is removed. After that, the entire of third insulating film9and contact hole11are coated with a transparent conductive material made of ITO so as to cover them. Then, second electrode (pixel electrode)10is formed by photolithography and etching, where the film thickness is 75 nm. In this case, part of the transparent conductive material is film-formed inside contact hole11, which causes second electrode (pixel electrode)10to be electrically connected to source/drain electrode5.

In this embodiment, a SiN film is used as third insulating film9; alternatively, an insulating film containing oxygen (e.g. SiO2, SiON) as third insulating film9at least contacting the ITO may be used in order to reliably avoid whitish turbidness on the ITO.

The description is made of the case where first insulating film6is formed on source/drain electrode5; however, first insulating film6is not necessarily required depending on such as the degree of reliability demanded. The present invention exhibits an advantage of increasing the retentive capacity even with second insulating film7formed directly on source/drain electrode5. Even with such a structure, an SOG material as second insulating film7provides a higher reliability than a resin material. Further, the description is made of the case where a SiN film is formed as an insulating film, but not limited to the case. A laminated film containing SiO2, SiO, or SiN may be formed in such as a two-layer structure made from SiO2and SiN.

In third insulating film9, floating electrode19is formed simultaneously with first electrode8in the peripheral region of contact hole11. Hence, even when contact hole11is formed by collectively etching the three-layered insulating films, highly accurate hole11can be formed, and so can be second electrode10.

Industrial Applicability

The present invention is useful in that it provides a liquid crystal display device with a high aperture ratio (transmittance) at low cost.

REFERENCE MARKS IN THE DRAWINGS

13Liquid crystal layer