1. Field of the Invention
The present invention relates to a thin film transistor for use in a liquid crystal display device and a method for fabricating the transistor and, particularly, to a thin film transistor having an LDD (Lightly Doped Drain) structure and a manufacturing method therefor.
2. Description of the Prior Art
As a display for a wall type TV, a projection type TV or an OA (Office Automation) device, the development of a display device using a liquid crystal panel has been performed recently. Among liquid crystal panels, an active matrix liquid crystal display constructed by incorporating thin film transistors, which are active elements, in a liquid crystal display device is specifically expected as useful in a display device for high class OA device or for high definition TV in view of the merit thereof that contrast and response speed are not reduced even if the number of scan lines is increased. According to the active matrix type liquid crystal panel, a large size display can be easily realized in the projection type display such as liquid crystal projection.
It is usual in an active matrix type liquid crystal display device using a light bulb for a liquid crystal projection in which lights passing through pixels are controlled correspondingly to a pixel information by irradiating small elements with intense lights and performing ON/OFF operation of each pixel by switching liquid crystal by means of thin film transistors (TFT""s) and the passed light is enlarged and projected onto a screen, etc., through an optical element such as a lens system. In such case, when an active layer of the TFT is formed of polysilicon, a leakage current thereof in OFF time becomes a problem due to optical excitation in a channel portion of the TFT by influence of not only the incident light but also reflection light from the optical system.
Such active matrix type liquid crystal display device using a light bulb usually includes a first light shield film provided on a TFT substrate and a second light shielding film provided on the TFT on the side of an opposing substrate or the TFT substrate. That is, when light is incident on the TFT from the side of the opposing substrate through a liquid crystal layer, the incident light is shielded by the second light shield film and reflection light from an underlying glass substrate or the optical system is shielded by the first light shield film.
The light shield films are formed on the respective substrates by laminating them on the TFT""s, a wiring layer and interlayer films and the light shield films are preferably about 0.01 to 0.1 xcexcm thick, respectively, since, if the film thickness is too large, breakage or short-circuit of the wiring may occur, and is preferably formed of a material having light shielding characteristics which is enough against intense light of in the order of several millions in lx.
Metal or metal silicide, which is stable in a heating step of a fabrication process of the liquid crystal panel, is usually used as the material of the light shielding film. However, the reflectivity of metal or metal silicide is high and, when the TFT""s are sandwiched between the light shielding films, random reflection occurs within the TFT substrate due to reflection by surfaces of the layers. Therefore, a new problem that a portion of reflected light reaches the TFT""s, causing leakage of light to occur. Intensity of such reflected light is not negligible under condition of intense illumination light from the light bulb and, in order to reduce optical leak current, a countermeasure to reflected light must be taken.
The TFT having the LDD structure is usually formed by using a gate electrode thereof as a mask. FIG. 1 is a plan view of a TFT having a conventional LDD structure, FIG. 2 is a partial cross section taken along a line Axe2x80x94A in FIG. 1 and FIG. 3 is a partial cross section taken along a line Bxe2x80x94B in FIG. 1.
As shown in FIG""s. 1 to 3, showing the conventional TFT for liquid crystal display, the first light shielding film 42 is formed on a transparent insulating substrate 41 of such as glass or quartz. On the first light shielding film 42, a boron-doped polysilicon layer 44, which becomes an active layer of the TFT, is formed through the first interlayer film 43 and a gate oxide film 50 is formed on the boron-doped polysilicon layer 44. The lamination is patterned suitably. A patterned resist is formed in regions of a channel region 45 and an LDD 46 on the gate oxide film 50 and a source region 47 and a drain region 48 are formed by ion-injection through the resist as a mask. After the resist is removed, a gate electrode 51 is formed and the channel region 45 and the LDD regions 46 are formed between the source region and the drain region by injecting ions again through the gate electrode as a mask. On the channel region 45 and the LDD regions 46, a data line 54 is formed through the second interlayer film 53. Furthermore, the second light shielding film 56 is formed through the third interlayer film 55. On the second light shielding film 56, the fourth interlayer film 57 is formed and a liquid crystal panel is completed by forming pixel electrodes 62, a liquid crystal layer 60 and an opposing substrate 61 thereon in the order. In FIG. 1, only one pixel electrode 62 is shown at the central picture cell and other corresponding pixel electrodes for other picture cells are not shown to avoid confusion of the drawing.
In this case, light incident on the side of the liquid crystal layer is blocked by the second light shielding film 56 and reflection light from the substrates and the optical system is blocked by the first light shielding film 42. However, a portion of rear surface reflection light is further reflected by a rear surface of the second light shielding film 56. A resultant secondary reflection light irradiates the LDD regions 46 which is not covered by the gate wiring, causing optical leakage current by which the display quality is degraded.
An object of the present invention is to provide a novel thin film transistor for a liquid crystal display device capable of reducing optical leakage current by covering LDD regions of the thin film transistor by a gate electrode thereof and a manufacturing method of the same thin film transistor.
In order to achieve the above object, the present invention basically employs a technical construction to be described.
According to the first aspect of the present invention, a thin film transistor for a liquid crystal display device formed on a transparent substrate, which comprises a gate electrode, a source region and a drain region formed on both sides of a channel region immediately below the gate electrodes, respectively, and LDD regions formed between the source region and the channel region and between the channel region and the drain region, respectively, is featured by that light shielding portions covering all of the LDD regions are provided on the gate electrodes, respectively.
According to the second aspect of the present invention, the thin film transistor of the liquid crystal display device having the above mentioned construction is featured by a provision of a light shielding portion partially covering the LDD region.
According to the third aspect of the present invention, the width of the light shielding portion is larger than a channel width of the thin film transistor.
According to the fourth aspect of the present invention, a cross sectional configuration of the light shielding portion has a taper such that the thickness of a front portion of the light shielding portion is reduced gradually.
A manufacturing method of the thin film transistors for the liquid crystal display device formed on a transparent substrate, which comprises a gate electrode, a source region and a drain region formed on both sides of a channel region immediately below the gate electrode, respectively, and LDD regions formed between the channel region and the source region and between the channel region and the drain region, respectively, according to the present invention, is featured by comprising at least the first step of forming the first light shielding film on the transparent substrate and forming the first interlayer film on the first light shielding film, the second step of forming the first polysilicon layer on the first interlayer film, patterning it to a predetermined configuration and forming a gate oxide film on the first polysilicon layer, the third step of covering portions of the first polysilicon layer, which becomes the channel region and the LDD regions, respectively, and injecting ions into regions which become the source region and the drain region, the fourth step of covering only the channel region and forming the drain region, the drain region and the LDD regions by further injecting ions into portions which become the source region, the drain region and the LDD regions, the fifth step of forming the second polysilicon layer, patterning it such that it covers the channel region and the LDD regions, forming the gate electrode and forming a gate line on the gate electrode and the sixth step of forming the second interlayer film on the gate line, forming a data line on the second interlayer film and forming the second light shielding film on the data line through the third interlayer film.
According to the second aspect of a manufacturing method of the thin film transistors, according to the present invention, the method is featured by comprising at least the first step of forming the first light shielding film on the transparent substrate and forming the first interlayer film on the first light shielding film, the second step of forming the first polysilicon layer on the first interlayer film, patterning it to a predetermined configuration and forming a gate oxide film on the first polysilicon layer, the third step of depositing the second polysilicon layer on the gate oxide film and forming gate electrode by patterning it to cover a channel region and LDD regions, the fourth step of forming a metal wiring layer, which becomes a gate line, on the gate electrode, the fifth step of simultaneously forming a source region, a drain region and the LDD regions by injecting ions by using the gate electrode and the metal wiring layer as a mask such that low concentration ions are injected to the LDD regions covered by the gate electrode and high concentration ions are injected to the region, which is not covered by the gate electrode, and the sixth step of forming the second interlayer film on the gate line, forming a data line on the second interlayer film and forming the second light shielding film on the data line through the third interlayer film.
According to the third aspect of a manufacturing method of the thin film transistors, according to the present invention, the method is featured by comprising at least the first step of forming the first light shielding film on the transparent substrate and forming the first interlayer film on the first light shielding film, the second step of forming a first polysilicon layer on the first interlayer film, patterning it to a predetermined configuration and forming a gate oxide film on the first polysilicon layer, the third step of depositing the second polysilicon layer on the gate oxide film, forming a gate electrode by patterning it to cover a channel region and LDD regions and, simultaneously therewith, tapering thickness of the gate electrode such that the thickness of a top end portion thereof is reduced gradually, the fourth step of simultaneously forming a source region, a drain region and the LDD regions by injecting ions with using the gate electrode as a mask, the fifth step of forming a metal wiring layer, which becomes a gate line, on the gate electrode and the sixth step of forming a second interlayer film on the gate line, forming a data line on the second interlayer film and forming the second light shielding film on the data line through the third interlayer film.