JPH04357877A - Thin film transistor - Google Patents

Abstract

PURPOSE:To form a gate insulating film of a TFT at 150 deg.C or lower by forming at least part of the film of a layer of an organic polymer material. CONSTITUTION:Any type of polymer for constituting a layer of an organic polymer material can be used limited for use if it has a heat resistance of about 150 deg.C. And, as a material in combination with the polymer material, a silicon oxide or silicon nitride is desirable, and a film is formed of it at 150 deg.C or lower by a vacuum vapor-deposition method or a sputtering method. As a method for forming an organic polymer material layer, a vacuum vapor- deposition method, a sputtering method or a polymerizing method may be used. Polyimide and polyester of polymers to be used, are desirably formed as a film by the vacuum vapor-deposition method or the sputtering method.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive element used in an active matrix drive type liquid crystal display, and in particular to a structure of a gate insulating film of an insulated gate thin film transistor (TFT) and a method for manufacturing the same.

0002

2. Description of the Related Art Currently, research and development of MOS type field effect transistors using amorphous thin film semiconductors used in liquid crystal displays as substrates is being actively conducted. For example, by using it in laptop computers, liquid crystal televisions, etc., it is expected that detailed images that operate at high speed can be obtained. However, various problems still exist with this thin film MOS field effect transistor and its manufacturing method. One of these problems is the formation of a gate insulating film. Since the gate insulating film is involved in the electrical characteristics of the transistor and its long-term stability, various methods for forming the gate insulating film are provided. Typical methods include plasma CVD silicon nitride (SiN), plasma CVD silicon oxide film, thermal oxide film, and anodic oxidation. However, silicon oxide or nitride films made by plasma CVD can be formed at low temperatures and can be used to form continuous gate insulating films, but amorphous silicon films have low effective carrier mobility and poor frequency characteristics. However, since the structure is unstable, there is a problem in that it changes significantly over time when the device is driven. On the other hand, in the case of the thermal oxide film method, a dry thermal oxide film that is oxidized in an O2 gas atmosphere is generally used. However, in the case of polycrystalline silicon, the oxidation temperature requires a high temperature of 1000° C. or more. Oxidation at low temperatures or humid oxidation at high temperatures causes the substrate to become polycrystalline, causing protrusions called aspirites to grow from the substrate into the gate oxide film, where electric field concentration occurs and breakdown voltage deteriorates. . Furthermore, even when amorphous silicon is used, the thermal oxidation reaction needs to be carried out at a temperature of 300° C. or higher. As described above, it is necessary to use a high reaction temperature to form a gate insulating film by thermal oxidation, so it has been desired to lower the oxidation temperature from the viewpoint of the substrate material and the jig. For example, at an oxidation reaction temperature of 1,000°C or higher, a quartz substrate must be used as the material, and at a temperature of about 300°C, an inexpensive ordinary glass substrate can be used. Diffusion of substrate impurities occurs, and in order to avoid this, a protective film is required to prevent substrate impurities from entering the TFT, and forming a gate insulating film at such high temperatures increases costs. was. In addition, in recent years, so-called PF-LCD, in which a display is made by sandwiching a liquid crystal between plastic films instead of a glass substrate, has been attracting attention in order to improve the visibility characteristics of LCDs, reduce weight, and reduce costs. The temperature limit is about 150° C., and the conventional method of forming a gate insulating film by thermal oxidation cannot be used. For this reason, various methods are being developed for the purpose of forming gate insulating films at low temperatures. For example, JP-A-59-31068, JP-A-59-31068;
-40580 and 60-241268. However, these oxidation reaction temperatures are 50
The temperature is around 0°C, and even the latter is around 200°C, so it cannot be used for PF-LCDs and the like.

0003

[Object] The present invention aims to provide a method for forming a gate insulating film of a TFT at a low temperature of 150° C. or lower, and a gate insulating film obtained thereby.

0004

The present invention is characterized in that at least a portion of the gate insulating film of a TFT is composed of a layer made of an organic polymer material. The gate insulating film of the present invention and its manufacturing method will be explained based on FIGS. 1 to 3. 1 is a substrate made of glass and plastic (eg, polyethylene terephthalate, polyether sulfone). 2 is a gate electrode, which is made of a metal such as Cr or Ti, and is formed by vacuum evaporation or sputtering, for example, to a thickness of 500 to 2000 Å.
The film is formed to a thickness of 100 mL and patterned using a photolithographic etching method. 3 is a gate insulating film related to the present invention, and although it is shown as a single layer in FIG. 1, this layer may be formed of multiple layers, but at least one of the layers is made of an organic polymer material. It consists of

[0005] The type of polymer constituting the layer of the organic polymer material of the present invention can be used without any particular restriction as long as it has a heat resistance of about 150°C. As the material to be combined, silicon oxide or silicon nitride is preferable, and these are formed into a film at a temperature of 150° C. or lower by a vacuum evaporation method or a sputtering method. As a method for forming the organic polymer material layer, a vacuum evaporation method, a sputtering method, and a polymerization method can be used. Among the polymers used in the present invention, polyimide and polyester are preferably formed into layers by vacuum evaporation or sputtering. By polymerization method, RF
Alternatively, microwave plasma can be used to polymerize, for example, styrene, methyl acrylate, or ethylene to form a polymer layer, or by heating, for example, a dimer of paraxylene can be evaporated and decomposed and polymerized on a substrate. In some cases, the temperature of the evaporation section is 300 to 500°C, and the temperature of the decomposition part is 500 to 700°C (Figures 2 and 3). Regardless of which of these methods is adopted, the substrate is kept at room temperature, the temperature rise is at most several tens of degrees Celsius even when radiant heat is taken into account, and the process temperature is 150 degrees Celsius or less. The total thickness of these organic polymer material layers or a combination of this layer and other layers is 5.
00 to 5000 Å, preferably 1000 to 3000 Å. 4 is a semiconductor layer, which may be polycrystalline or amorphous, but an amorphous silicon thin film is mainly used. As for its manufacturing method, for example, S
iH4 gas under a pressure of 10-3 to 101 Torr, 1
It is decomposed by 3.56 MHZ RF or 1.5 GHZ microwave, and is formed to have a film thickness of 500 to 5000 Å. Furthermore, the semiconductor layer may be of P type or N type, and may also have a composition other than silicon. Reference numeral 6 denotes a source/drain electrode, which is formed in the same manner as 2. However, the film thickness is 3000Å to 1μ
m is preferred. 5 is an ohmic layer that connects 4 and 6 ohmically. For example, P is added in the gas of 4.
It is obtained by mixing H3 gas into 0.1-10% SiH4 gas. Although the TFT shown in FIG. 1 has an inverted staggered structure, the TFT of the present invention is not limited to this type.

[0006]

[Effects] According to the present invention, since at least a part of the gate insulating film of the TFT is composed of a layer of an organic polymer material, the TFT can be manufactured at a low temperature of 150° C. or lower. In addition, this TFT has a mobility of 0.03 cm2/v
・It had sufficient characteristics for LCD switching at s or more. In particular, in the polymerization method, the mobility is 0.05.
cm2/v·s or more can be obtained. There was no significant difference in initial characteristics between the gate insulating film with a stack of organic and inorganic materials and the gate insulating film with only organic materials, but in the former case, silicon oxide or silicon nitride was added to the semiconductor side. The former was superior to the latter in terms of long-term life. That is, the source after 500 hours of continuous voltage application.
The current between the drains (Isd) was 90% of the initial value in the former case, while it was approximately 80% in the latter case.

[Brief explanation of drawings]

FIG. 1 shows the basic structure of a specific example of a thin film transistor of the present invention.

FIG. 2 is a specific example of an apparatus for manufacturing a gate insulating film using a plasma polymerization method.

[Figure 3] Part 1 of a gate insulating film manufacturing device using the evaporation decomposition method
Concrete example.

[Explanation of symbols]

1 Board 2 Gate electrode 3 Gate insulating film 4 Semiconductor layer 5 Ohmic layer 6 Source/drain electrode 7 RF power supply 8 Heater 9 Raw materials

Claims (3)

[Claims] 1. An insulated gate thin film transistor, wherein at least a part of the gate insulating film is made of a layer of an organic polymer material. 2. The thin film transistor according to claim 1, wherein the gate insulating film includes a silicon oxide or nitride layer and an organic polymer material layer. 3. The thin film transistor according to claim 1, wherein the layer of organic polymer material constituting the gate insulating film is formed by vapor deposition, sputtering, or polymerization.