Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
2001-06-12
2003-12-30
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S072000, C257S350000
Reexamination Certificate
active
06670636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to the technical filed of a substrate device, such as a TFT array substrate device, and a semiconductor substrate device on which thin film transistors (hereinafter, referred to as TFTs) are formed. More particularly, the present invention relates to the technical filed of a substrate device that is appropriately provided with an electro-optical device, such as a liquid crystal device.
2. Description of Related Art
Substrate devices of this type include, for example, a polysilicon film having a source region, a drain region and a channel region, or a semiconductor film, such as amorphous silicon formed on a substrate, such as a quartz substrate. A thermally-oxidized film or the like made by dry oxidation or wet oxidation is formed on the surface of the semiconductor film, or a gate insulation film is formed thereon from an HTO film, a TEOS film, or a plasma oxidation film. Further, a TFT is constructed on the substrate by forming a gate electrode film on the gate insulation film. The TFT is used as a pixel switching element in a TFT array substrate device by being built in each pixel in the image display region of an electro-optical device, such as a liquid crystal device. Otherwise, the TFT also is used as a part of the drive circuit of the substrate device by being built in a peripheral region in the circumference of the image display region.
The substrate device, in which the TFT is built as described above, has been widely used in various types of electro-optical devices, including a liquid crystal device employing a TFT active matrix drive system, and the like.
SUMMARY OF THE INVENTION
However, according to a study performed by the inventors of this application, it has been found that when the above-mentioned substrate device is operated for, for example, about 5000 hours, transistor characteristics are deteriorated. More specifically, even if a TFT built in the substrate device exhibits excellent transistor characteristics at the beginning of manufacture of the substrate device, a threshold voltage in the TFT increases in a lifetime of, for example, the 5000 hours which are required in an ordinary product life, and the TFT does not function as a switching element with an ordinary drive voltage, or an off current (that is, a leak current) increases in the TFT and a prescribed duty ratio cannot be sufficiently handled. When the transistor characteristics are deteriorated as described above, a problem arises in that the life of an electro-optical device, which is finally constructed by including, for example, the substrate device, is reduced because the quality of an image, such as a contrast ratio and brightness in the electro-optical device, is lowered. In particular, it also has been found that when various electro-optical devices, including the substrate device, are used in very humid regions, such as South East Asia, and when strong light beams are projected to the substrate device through a light bulb of a projector, the characteristics of a TFT are liable to be more deteriorated because the TFT is affected by high humidity and high temperature. That is, the substrate device has a problem, which is serious to manufacturers in practical applications, in that the life of the device cannot be definitely determined by the affect of the environment in which the device is used, such as humidity and temperature.
An object of the present invention, which was made in view of the above problems, is to provide a substrate device, which includes a TFT, capable of maintaining excellent transistor characteristics for a long period and which is difficult to be affected by an environment in which it is used, such as humidity and temperature and the life of which can be relatively easily increased. It is also an object of the present invention to provide a method of manufacturing the substrate device, as well as to provide an electro-optical device including the substrate device.
To solve the above problems, a substrate device of the present invention includes a substrate, a semiconductor layer formed on the substrate and having a source region, a channel region, and a drain region, a gate insulation film formed on the semiconductor layer in at least the channel region, and a gate electrode film formed on the gate insulation film. The gate insulation film includes a silicon oxide film. Nitrogen atoms exist in at least one of the silicon oxide film and an interface between the silicon oxide film and the semiconductor layer.
According to the substrate device of the present invention, a TFT is constructed by laminating the semiconductor layer, including the channel region and the like, the gate insulation film, and the gate electrode film on the substrate. Here, nitrogen atoms particularly exist in at least one of the silicon oxide film constituting the gate insulation film and the interface between it and the semiconductor layer. As a result, the time-varying deterioration of transistor characteristics, in particular, the deterioration of the transistor characteristics when a transistor is used in an environment of high humidity, high temperature, and the like, can be reduced as compared with a conventional substrate device in which no nitrogen atom exists. With this arrangement, the life of the substrate device can be increased by the provision of the TFT the performance, which can be stably maintained for a long period of time regardless of an environment in which the TFT is used.
Reasons why the deterioration of the transistor characteristics can be lowered in the substrate device of the present invention will be explained below.
First, according to the study of the inventors of this application, when a conventional substrate device, using a dry oxidation film as a gate insulation film, is continuously operated in an ordinary manner, a phenomenon has been confirmed that the threshold value Vth of a gate voltage for turning on a TFT is dislocated toward an enhancement side regardless of whether the TFT is a p-channel TFT or an n-channel TFT. In a substrate device, which includes a power supply circuit capable of supplying a gate voltage up to a predetermined voltage to the TFT, when the threshold value Vth exceeds the predetermined voltage due to the dislocation, it is impossible to operate the substrate device. The phenomenon of dislocation of the threshold value Vth is conspicuous particularly in the p-channel TFT. First, it is contemplated that the transistor characteristics are deteriorated by that the hydrogen atoms in the interface between a gate insulation film and a semiconductor layer are separated therefrom by the heat generated by a channel resistance when a TFT is in operation (for example, the temperature of the transistor increases to about 400° C. in the vicinity of a drain region) and the degree of movement of the nitrogen atoms is lowered in the transistor characteristics. Second, a hot carrier implantation phenomenon (that is, a channel hot electron implantation phenomenon and a drain avalanche hot carrier implantation phenomenon) is contemplated as a cause of the deterioration. Third, it is contemplated that the deterioration is caused by water entering the gate insulation film while the substrate device is being manufactured, or after it has been manufactured, and H
2
O molecules disperse to the vicinity of the interface between the gate insulation film and the semiconductor layer so as to generate a positive charge.
When the gate insulation film is formed of the silicon oxide film, as in the substrate device of the present invention, if oxygen enters between single crystal silicons, they are expanded and Si—Si bonding and deformed Si—O—Si bonding are generated and act as hole traps. In contrast, when water (hydrogen atoms) enters the vicinity of the interface between the gate insulation film and the semiconductor layer, Si—H bonding is generated and acts as a positive charge of Si
+
. Further, when water (oxygen atoms and hydrogen atoms) enters the vicinity of the interface, Si—O
Hayashi Tomohiko
Koide Kiyotaka
Ngo Ngan V.
Oliff & Berridg,e PLC
Seiko Epson Corporation
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