Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-09-18
2003-02-04
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S059000, C257S072000, C257S347000
Reexamination Certificate
active
06515336
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by thin-film transistors (hereinafter abbreviated as TFTs) formed on a substrate having an insulating surface and to a method of fabricating the same. More particularly, the invention relates to an electro-optical device as represented by a liquid crystal display device having a pixel portion and a driver circuit formed in the periphery thereof on the same substrate, and technique that can be favorably utilized to electronic equipment mounting the electro-optical device. In this specification, the semiconductor device stands for devices that work by utilizing the semiconductor characteristics, in general, and encompasses the above-mentioned electro-optical device and electronic equipment mounting the electro-optical device.
2. Description of the Related Art
Technique for constituting switching elements and active circuits by using TFTs has been developed in connection with the electro-optical devices as represented by an active matrix-type liquid crystal display device. TFT is the one in which a semiconductor film is formed on a substrate such as of a glass by a vapor-phase growing method and is used as an active layer. As the semiconductor film, there is preferably used silicon or a material comprising chiefly silicon such as silicon-germanium. The semiconductor films can be classified into amorphous silicon films and crystalline silicon films as represented by polycrystalline silicon depending upon the method of formation.
The TFT using the amorphous semiconductor (typically, amorphous silicon) film is not capable of exhibiting an electric field effect mobility of not smaller than several square centimeters per Vsec due to electro-physical properties caused by the amorphous structure. In the liquid crystal display device of the active matrix type, therefore, the switching elements (pixel TFTs) can be formed for driving the liquid crystals in the pixel portion, but the driver circuit cannot be formed for displaying the picture. Therefore, the driver circuit is employing a technique for mounting a driver IC relying upon a TAB (tape automated bonding) system or a COG (chip on glass) system.
On the other hand, the TFT using, as an active layer, a semiconductor film including a crystal structure (hereinafter referred to as crystalline semiconductor) (typically, crystalline silicone or polycrystalline silicone), exhibits a high electric field effect mobility and makes it possible to form circuits of various functions on the same glass substrate and, hence, makes it possible to realize, in addition to pixel TFTs, a shift register circuit in a driver circuit, a level shifter circuit, a buffer circuit and a sampling circuit. Such circuits have been formed relying upon a CMOS circuit constituted by n-channel TFTs and p-channel TFTs. Owing to the above technique for mounting the driver circuits, it is becoming obvious that the TFTs having a crystalline semiconductor layer as an active layer is suited for forming the driver circuits in addition to the pixel portion on the same substrate, from the standpoint of decreasing the weight and thickness of the liquid crystal display device.
From the standpoint of TFT characteristics, it is better to use the crystalline semiconductor layer as an active layer. From the standpoint of fabricating the TFTs suited for various circuits in addition to the pixel TFTs, however, the steps of fabrication become complex and, besides, the number of the steps increase. It is obvious that an increase in the number of the steps drives up the cost of production and, besides, decreases the yield of production.
Further, in order to stabilize the operation of these circuits fabricated by using n-channel TFTs and p-channel TFTs, the threshold voltages and sub-threshold constants (S-values) of the TFTs must be confined within a predetermined range. For this purpose, the TFTs must be studied from both the structure and the materials constituting them.
SUMMARY OF THE INVENTION
The present invention deals with a technique for solving the above problems, and provides an electro-optical device as represented by a liquid crystal display device of the active matrix type fabricated by using the TFTs and a semiconductor device, wherein the TFTs arranged in the circuits have suitable structures depending upon the circuit functions, in order to improve operation characteristics and reliability of the semiconductor device, to decrease the consumption of electric power, to decrease the number of the steps, to decrease the cost of production and to improve the yield.
In order to decrease the cost of production and to improve the yield, the number of the steps may be decreased as one of the means. Concretely speaking, it is necessary to decrease the number of the photomasks required for the production of the TFTs. In the photolithographic technique, the photomask is used for forming, on the substrate, a resist pattern that serves as a mask in the etching step. Therefore, when the photomask is used, it means that there must be executed the steps of forming a film and etching before and after the step of using the photomask, as well as the steps of peeling the resist, washing and drying, as well as complex steps of applying the resist, prebaking, exposure to light, developing and post-baking even during the photolithography.
While decreasing the number of the photomasks, the TFTs arranged in various circuits have suitable structures depending upon the functions of the circuits. Concretely speaking, it is desired that the TFTs for the switching elements have a structure placing more importance on decreasing the off current than on the operation speed. For this structure, a multi-gate structure is employed. On the other hand, the TFTs used in the driver circuit that requires a high-speed operation should have a structure that places importance on the operation speed and on suppressing the deterioration caused by the injection of hot carriers, which is a serious problem. To accomplish this structure, contrivance is made to the LDD region of the TFT. That is, in the LDD region provided between the channel-forming region and the drain region, gradient is imparted to the concentration such that the concentration of impurity element for controlling the conduction type gradually increases toward the drain region. This constitution is effective in relaxing the concentration of the electric field in the depletion layer near the drain region.
In order to form the LDD region having such a gradient in the concentration of the impurity element according to this invention, the ionized impurity element for controlling the conduction type is accelerated through an electric field and is passed through the gate-insulating film (in this invention, both the gate-insulating film provided between the gate electrode and the semiconductor layer being intimately contacted thereto, and an insulating film extending in the peripheral region thereof from the gate-insulating film, are referred to as gate-insulating film) so as to be added to the semiconductor layer. In this specification, the method of adding the impurity element is called “through doping method” from the standpoint of convenience. In the through doping method of the invention, the gate electrode has a so-called tapered shape in which the thickness of the gate electrode gradually increases toward the inside from an end portion thereof. The gate-insulating film, too, has a tapered shape in which the thickness gradually increases toward a portion that comes in contact with the gate electrode. Upon conducting the through doping method, therefore, the concentration of the impurity element added to the semiconductor layer is controlled by the thickness of the tapered gate insulating film, making it possible to form an LDD region in which the concentration of the impurity element gradually changes in the direction of channel length of the TFT.
The gate electrode is formed by using a heat-resistant electrically condu
Ohnuma Hideto
Ono Koji
Suzawa Hideomi
Yamagata Hirokazu
Ngo Ngan V.
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
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