News Column

Patent Issued for Display Device

June 27, 2014



By a News Reporter-Staff News Editor at Energy Weekly News -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventors Yamazaki, Shunpei (Tokyo, JP); Murakami, Satoshi (Tochigi, JP); Osame, Mitsuaki (Kanagawa, JP), filed on June 20, 2013, was published online on June 10, 2014.

The patent's assignee for patent number 8748895 is Semiconductor Energy Laboratory Co., Ltd. (Kanagawa-ken, JP).

News editors obtained the following quote from the background information supplied by the inventors: "The present invention belongs to a technical field related to a display device in which a semiconductor device (typically, a transistor) is used as a device, in particular, a light emitting device represented by an electroluminescence display device, and to a technical field related to an electronic apparatus equipped with the display device in an image display portion.

"In recent years, development of liquid crystal display devices and electroluminescence display devices in which transistors (particularly, thin film transistors) are integrated on a substrate have progressed. Such display devices are respectively characterized in that the transistors are formed on a glass substrate by using a thin film formation technique, and the transistors thus formed are disposed in pixels each arranged in matrix and made to function as a display device for image display.

"A variety of specifications are conceivable, which are required for areas (hereinafter, referred to as pixel portions), in which the image display is performed in the display device. However, the following are given as examples thereof: a large number of dots and high definition are ensured; an area of an effective display region in each pixel is large and bright image display is possible; and the pixel portion involves no defects that may induce point defects or line defects in its entirety. In order to achieve those specifications, not only the performance of the transistors arranged in each pixel should be satisfactory but also a technique of forming the transistors while increasing yield through a stable process is necessary.

"Further, in an organic electroluminescence display device among the electroluminescence display devices, an organic compound is used for a light emitting element serving as a light emitting source. Accordingly, a measure for suppressing deterioration of the organic compound is most highly required in ensuring its reliability. In other words, in order to achieve a highly reliable display device, attention must be paid not only to an influence of an accumulated damage in the process during manufacturing the device but also to the subsequent deterioration with time, which results from the accumulated damage.

"In the above-mentioned circumstances of development, the applicants of the present invention are most concerned, in the present conditions, with problems such as variation and shift of a threshold voltage in the transistors, which arise due to the accumulation of plasma damages on an insulating film etc. in an etching process."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "The present invention has been made in view of the above-mentioned problems and an object of the present invention is to provide a device structure effective in reducing an influence of plasma damage in a manufacturing process of a display device. Further, another object of the present invention is to provide a display device having uniform display characteristics (referring to display characteristics small in luminance variation between adjacent pixels and in degradation degree) obtained by reducing the influence of the plasma damage to suppress variations in threshold voltage of transistors.

"The present invention relates to a display device characterized by including the following structures as a device structure effective in solving the above problems. Note that a light emitting element defined here refers to an element in which a light emitting member (referring to a laminate obtained by laminating a light emitting layer, a carrier injecting layer, a carrier transporting layer, a carrier blocking layer, and other components required for light emission, such as an organic or inorganic compound) is provided between a pair of electrodes (anode and cathode). For example, an electroluminescence element is included in the category of the light emitting element. (1) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a barrier layer formed above the active layer; a planarization layer formed on the barrier layer; and a drain electrode formed on the planarization layer, the light emitting element including: a pixel electrode connected in contact with an upper surface of the drain electrode on the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, and the drain electrode being connected with the active layer through a first opening formed in the gate insulating film and the barrier layer and a second opening formed in the planarization layer.

"Note that the planarization layer of the present invention may be formed of either an inorganic insulating film or an organic resin film formed by sputtering, plasma CVD, low-pressure thermal CVD, or spin-coating. Also, when the planarization layer is formed by sputtering, plasma CVD, or low-pressure thermal CVD, its surface may be polished before the use (inclusive of polishing with a mechanical or chemical action, or a combined action thereof). By using the planarization layer, the surface of a first electrode formed on the planarization layer can be leveled and the light emitting element (EL element) can be prevented from being short-circuited. Also, by providing the barrier layer thereon, impurity diffusion from the EL element can be blocked to protect TFTs and degassing from an organic insulating film can be avoided as well. Further, by forming the barrier layer in the portion close to the active layer of the TFT, the impurity diffusion from the EL element is blocked to effectively protect the TFT. (2) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a planarization layer formed above the active layer; a barrier layer formed on the planarization layer; and a drain electrode formed on the barrier layer, the light emitting element including: a pixel electrode connected in contact with an upper surface of the drain electrode on the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, the drain electrode being connected with the active layer through a first opening formed in the gate insulating film and the barrier layer and a second opening formed in the planarization layer, and the barrier layer covering an upper surface of the planarization layer and a side surface of the second opening formed in the planarization layer.

"Also, according to the structure stated in (2), the barrier layer is formed to thereby prevent the planarization layer from being etched in etching the first electrode and the drain electrode. Also, by covering the planarization layer with the barrier layer, the impurity diffusion from the planarization layer to the light emitting element is avoided. (3) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a planarization layer formed above the active layer; a barrier layer formed on the planarization layer; and a drain electrode formed on the barrier layer, the light emitting element including: a pixel electrode connected in contact with an upper surface of the drain electrode on the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, the drain electrode being connected with the active layer through an opening formed in the gate insulating film, the planarization layer, and the barrier layer, and the barrier layer covering an upper surface of the planarization layer.

"Also, according to the structure stated in (3), the openings formed in the gate insulating film, the planarization layer, and the barrier layer are formed through etching by use of the same masks and can thus be formed with the smaller number of masks than that of the structure in (1) or (2). (4) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a barrier layer formed above the active layer; a planarization layer formed on the barrier layer; and a drain electrode formed on the planarization layer, the light emitting element including: a pixel electrode connected in contact with a lower surface of the drain electrode on the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, and the drain electrode being connected with the active layer through a first opening formed in the gate insulating film and the barrier layer and a second opening formed in the planarization layer. (5) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a planarization layer formed above the active layer; a barrier layer formed on the planarization layer; and a drain electrode formed on the barrier layer, the light emitting element including: a pixel electrode connected in contact with a lower surface of the drain electrode on the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, the drain electrode being connected with the active layer through a first opening formed in the gate insulating film and the barrier layer and a second opening formed in the planarization layer, and the barrier layer covering an upper surface of the planarization layer and a side surface of the second opening formed in the planarization layer. (6) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a planarization layer formed above the active layer; a barrier layer formed on the planarization layer; and a drain electrode formed on the barrier layer, the light emitting element including: a pixel electrode connected in contact with a lower surface of the drain electrode on the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, the drain electrode being connected with the active layer through an opening formed in the gate insulating film, the planarization layer, and the barrier layer, and the barrier layer covering an upper surface of the planarization layer.

"Also, according to the structures stated in (4), (5), and (6), after forming the pixel electrode, the drain electrode is formed, so that the structures are useful in the case where the drain electrode has a large film thickness. When formed after the formation of the drain electrode, the pixel electrode is needed to partially overlap the drain electrode. When the drain electrode has a large film thickness, there is a fear that any coverage failure takes place, such as step-like breakage in the pixel electrode. (7) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a barrier layer formed above the active layer; an insulating layer formed on the barrier layer; a drain electrode formed on the insulating layer; and a planarization layer formed on a source electrode or the drain electrode, the light emitting element including: a pixel electrode formed on the planarization layer and connected with the drain electrode through an opening formed in the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, and the drain electrode being connected with the active layer through a first opening formed in the gate insulating film and the barrier layer and a second opening formed in the insulating layer. (8) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; an insulating layer formed above the active layer; a barrier layer formed on the insulating layer; a drain electrode formed on the barrier layer; and a planarization layer formed on the drain electrode, the light emitting element including: a pixel electrode formed on the planarization layer and connected with the drain electrode through an opening formed in the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, the drain electrode being connected with the active layer through a first opening formed in the gate insulating film and the barrier layer and a second opening formed in the insulating layer, and the barrier layer covering an upper surface of the insulating layer and a side surface of the second opening formed in the insulating layer. (9) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; an insulating layer formed above the active layer; a barrier layer formed on the insulating layer; a drain electrode formed on the barrier layer; and a planarization layer formed on the drain electrode, the light emitting element including: a pixel electrode formed on the planarization layer and connected with the drain electrode through an opening formed in the planarization layer; a light emitting member formed in contact with the pixel electrode; and an opposing electrode facing the pixel electrode through the light emitting member, the drain electrode being connected with the active layer through an opening formed in the gate insulating film, the insulating layer, and the barrier layer, and the barrier layer covering an upper surface of the insulating layer.

"Further, according to the structures stated in (7), (8), and (9), the pixel electrode is connected with the drain electrode through a contact hole and insulated from the other wirings through the planarization layer, so that the area of the pixel electrode can be increased.

"Note that, according to the structures stated in (7) to (9), the upper surface of the planarization layer may be covered with the silicon nitride film. This is because when the organic resin film is used as the planarization layer, in particular, the upper surface thereof (including an inner wall surface thereof when any opening is formed) is covered with the silicon nitride film, so that gases (including a component gas or moisture) generated from the organic resin film can be effectively prevented from diffusing to the light emitting element side.

"Also, it is preferable that end portions (at least, corner portions) of the pixel electrode be covered with the resin film. This is because the electric fields are likely to concentrate in the end portion of the pixel electrode and further, coverage of the film formed thereon is poor, so that it is preferable not to use the end portions in forming the light emitting element. Note that the resin film may be either a nonphotosensitive resin film or a photosensitive resin film. If using the photosensitive resin film, either positive or negative type films can be used. (10) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a barrier layer formed above the active layer; a planarization layer formed on the barrier layer; and a drain electrode formed on the planarization layer, the drain electrode constituting a laminate electrode having a laminate structure of a first metallic film and a second metallic film and including a portion where a part of the second metallic film is removed to expose the first metallic film, the light emitting element including: the portion where the first metallic film is exposed; a light emitting member formed in contact with the portion where the first metallic film is exposed; and an opposing electrode facing the portion where the first metallic film is exposed through the light emitting member, and the drain electrode being connected with the active layer through a first opening formed in the gate insulating film and the barrier layer and a second opening formed in the planarization layer. (11) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a planarization layer formed above the active layer; a barrier layer formed on the planarization layer; and a drain electrode formed on the barrier layer, the drain electrode constituting a laminate electrode having a laminate structure of a first metallic film and a second metallic film and including a portion where a part of the second metallic film is removed to expose the first metallic film, the light emitting element including: the portion where the first metallic film is exposed; a light emitting member formed in contact with the portion where the first metallic film is exposed; and an opposing electrode facing the portion where the first metallic film is exposed through the light emitting member, and the barrier layer covering an upper surface of the planarization layer and a side surface of an opening formed in the insulating layer. (12) A display device according to the present invention includes: a transistor formed on a substrate surface; and a light emitting element connected with the transistor, the transistor including: an active layer formed of a semiconductor; a gate insulating film formed in contact with the active layer; a gate electrode facing the active layer through the gate insulating film; a planarization layer formed above the active layer; a barrier layer formed on the planarization layer; and a drain electrode formed on the barrier layer, the drain electrode constituting a laminate electrode having a laminate structure of a first metallic film and a second metallic film and including a portion where a part of the second metallic film is removed to expose the first metallic film, the light emitting element including: the portion where the first metallic film is exposed; a light emitting member formed in contact with the portion where the first metallic film is exposed; and an opposing electrode facing the portion where the first metallic film is exposed through the light emitting member, a source electrode and the drain electrode being connected with the active layer through an opening formed in the gate insulating film, the insulating layer, and the barrier layer, and the barrier layer covering an upper surface of the planarization layer.

"Note that, according to the structures stated in (10) to (12), an angle between a section of the second metallic film in the portion where a part of the second metallic film is removed to expose the first metallic portion and an upper surface of the second metallic film is preferably an obtuse angle. In other words, the above means that the portion concerned is formed by removing the second metallic film through etching and etching is preferably performed such that the etched section takes a tapered shape. This is because even when the light generated inside the light emitting member propagates laterally in the light emitting member, the light can be reflected by the section and taken out efficiently, provided that the section of the second metallic film has the tapered shape. According to the structures stated in (10) to (12), the emitted light in the lateral direction is reflected by the slope of the second metallic film formed in the step portion of the laminate electrode or condensed there, thereby increasing an amount of the emitted light that is to be taken out in a certain direction (direction in which the light passes through the opposing electrode). Also, for that purpose, it is preferable that the film thickness of the light emitting member be smaller than that of the second metallic film.

"Further, the pixel portion is covered with the resin film except the portion where a part of the second metallic film is removed to expose the first metallic film. This is because the second metallic film is etched by using the resin film, so that the portions other than a part of the second metallic film (removed portion) are all covered with the resin film. However, the resin film is only needed to cover the pixel portion and it is not always required for the resin film to remain in the portions other than the pixel portion (e.g., driver circuit portion). Further, needless to say, a terminal portion for transmitting/receiving signals to/from an external circuit should be uncovered with the resins film.

"Note that the first metallic film is preferably made of a metallic film capable of functioning as an anode of the light emitting element, such as a titanium film or a titanium nitride film. The second metallic film is preferably made of a metallic film such as an aluminum film (including an aluminum alloy film or an aluminum film added with other elements, the same being applied to the following description), which shows high reflectivity. In this example, only a two-layer structure composed of the first metallic film and the second metallic film is shown, but the multi-layer structure including the two or more layers can be adopted.

"In the above structures of the present invention, the silicon nitride film used for the barrier layer is preferably made as fine as possible. The higher the finess, the higher the barrier property. The diffusion blocking effect against the degassing component is thus enhanced. For example, when the organic resin film is used as the planarization layer, the diffusion of the component gas or moisture therefrom to the transistor side or the light emitting element side can be effectively suppressed.

"Further, when the inorganic insulating film (typically, a spin on glass film) formed by spin-coating is used as the planarization layer as well, the above is rather extremely effective in controlling the diffusion of the component gas or moisture. Further, the SOG (spin on glass) film includes an organic SOG film and an inorganic SOG film. When considering the application to the present invention, the inorganic SOG film is preferable because of less degassing. Preferable examples of the inorganic SOG film include an SiOx film, a PSG (phosphorous silicate glass) film, a BSG (boron silicate glass) film, and a BPSG (boron phosphorous silicate glass) film, which are formed by spin-coating. Specifically, the SOG film is represented by OCD series manufactured by Tokyo Ohka Kogyo Co., Ltd.) and it is needless to say that the other known SOG films can be used."

For additional information on this patent, see: Yamazaki, Shunpei; Murakami, Satoshi; Osame, Mitsuaki. Display Device. U.S. Patent Number 8748895, filed June 20, 2013, and published online on June 10, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8748895.PN.&OS=PN/8748895RS=PN/8748895

Keywords for this news article include: Electronics, Semiconductor, Semiconductor Energy Laboratory Co., Semiconductor Energy Laboratory Co. Ltd.

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