JPH0318356B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a gate insulating film at a low temperature, which influences the switching characteristics of a thin film transistor (TFT).

The structure of an insulated gate thin film transistor is
A semiconductor thin film - an insulating layer - a conductive layer. In order to take advantage of the large area and low cost characteristics of thin film transistors, it is possible to use glass or ceramics as the substrate, but in that case, processing at high temperatures becomes difficult.

Gate insulating films currently used in semiconductor technology include SiO ₂ and Al ₂ O ₃ , but currently thermal oxidation is mainly used. However, the commonly used thermal oxidation is difficult when temperature is restricted by the substrate. Other insulating film formation methods include PVD and CVD, but these methods are inferior to thermal oxide films in terms of film uniformity, insulation, defects in the film, impurity density, interface state density, etc. There is.

The present invention provides a method of forming an insulating film comparable to a thermal oxide film at low temperatures using anodic oxidation.

Conventionally, anodic oxidation was mainly used for metals, and in semiconductor technology, it was used for good conductor films, but it was used for non-insulating thin films, that is,
In the case of high-resistance thin films other than good conductors and insulators, such as amorphous silicon semiconductor films, it is difficult to uniformly apply voltage over a large area using conventional anodic oxidation methods. could not be formed. Therefore, in the present invention, by using a patterned electrode, a uniform film can be formed even in a high resistance film, and since this electrode is used as a source and drain electrode, it is possible to reduce the process and area. It is also suitable for manufacturing thin film transistors.

Methods for forming semiconductor films on insulating substrates include CVD using plasma and low pressure at low temperatures;
Alternatively, a PVD method or the like may be considered, but the formed film has a large sheet resistance, making it difficult to use conventional methods of anodic oxidation. In the present invention, patterned electrodes, such as thin wire electrodes, are formed in advance on a substrate as source and drain electrodes, and a non-insulating thin film, such as a semiconductor film, is formed thereon. This is a method of manufacturing a thin film transistor in which a semiconductor film is used as an anode, a semiconductor film is anodized in layers, the layered anodic oxide film is used as an insulating film, for example, a gate insulating film, and the thin wire electrodes are used as source and drain electrodes. Furthermore, when using glass or ceramics as a substrate, an impurity trap film is formed on the substrate in advance using silicon nitride in order to prevent the diffusion of impurities from the substrate from adversely affecting the anticonductor film. The above-described patterned electrode is formed on top, a non-insulating thin film, for example, a semiconductor film is attached, and the source and drain electrodes are made into a sandwich-like structure and anodized to form a thin film transistor to be used as a gate film and an insulating film. A manufacturing method is also provided. In this case, the impurity trap film is important for preventing adverse effects on the semiconductor film during anodic oxidation, improving the pattern accuracy of the electrode to be patterned, and improving the switching characteristics of the transistor. In addition, by forming the above-mentioned trap film, substrates that are difficult to anodize conventionally and
This makes it possible to use electrodes that are difficult to use, and is effective in improving the adaptability of anodic oxidation. In addition, in manufacturing thin film transistors, which includes the process of anodizing a non-insulating thin film, patterned source and drain electrodes are formed on the substrate in advance, a non-insulating thin film is formed, and a new anode is formed. Instead, by anodic oxidizing a non-insulating thin film using the source and drain electrodes, it is possible to form a uniform and highly insulating film. Next, details of the present invention will be explained using the drawings.

FIG. 1 is a process diagram illustrating the manufacture of a thin film transistor according to an embodiment of the present invention.

FIG. 1A shows the process of forming thin wire electrodes on a substrate. The thin wire electrode can be used for lift-off and
This is done by using etching or the like, and 1 is a substrate, and 2 is a patterned electrode, for example, a thin wire electrode. This thin wire electrode 2 is used as an anodic oxidation electrode, and also as a source and drain electrode.

In this example, the thin wire electrode 2 has a width of 10 μm.
was formed. Further, the inter-electrode distance W between the source electrode and the drain electrode was set to 10 μm, and in order to stably maintain the length L of the source electrode and the drain electrode and the driving ability of the transistor, it was set to 100 μm.

Next, as shown in FIG. 1B, amorphous silicon (a-Si) is deposited on the thin wire electrode 2 and the insulating substrate 1 as a semiconductor film 3 by plasma CVD.
A thickness of 150 nm was formed. With this design value, the a-Si resistance (R _SD ) between the source and drain electrodes is
R _SD =ρ·W/(t·L). a-Si
When the resistivity ρ of is 10 ¹⁰ Ω・cm (commonly usable ones are 10 ⁷ to 10 ¹⁰ Ω・cm), R _SD = 0.7×
10 ¹⁴ Ω.

Then, by performing anodic oxidation as shown in FIG. 1C, a uniform oxide film 5 is formed. Here, the a-Si film is anodized on the source and drain electrodes with the lowest resistance at the initial stage of anodic oxidation, but as the resistance of the anodic oxide film increases, the a-Si film is anodized. The part that expands will expand laterally. The resistance of the anodic oxide film formed this time is 1×10 ¹⁶ Ω or more when a voltage of 1 V is applied at a thickness of 100 nm, which is two orders of magnitude higher than the resistance of a-Si between the source and drain electrodes. As a result, a uniform and good oxide film 5 can be formed between the source electrode and the drain electrode. In addition, in order to leave at least 50 nm of the semiconductor film 3, it is necessary to prevent the anodic oxide film 5 from being formed over 100 nm on the source and drain electrodes between the anode (source electrode and drain electrode) and the cathode (counter electrode 4) during anodic oxidation. A constant voltage was applied to.

In the above explanation, the distance between the source and drain electrodes was 10 μm, and the length of the source and drain electrodes was 100 μm. It is sufficient that the resistance is smaller than the resistance of the anodic oxide film, and a desired anodic oxide film can be formed with a resistance of approximately 1/10 or less. To reduce the resistance ratio to 1/10 or less, t = about 150 nm,
If ρ=10 ¹⁰ Ω・cm, R _SD ≒0.7×10 ¹⁵ ×W/L, and W/L is approximately 1.
If it is smaller, R _SD will be less than 1/10 of the resistance of the anodic oxide film. However, if W and L have too large values, that is, if the area is too large, pinholes will occur and a good oxide film will not be obtained. Therefore, when we determined the conditions, the distance between the electrodes was approximately 2 to 10 μm, and the length of the electrodes was at least approximately 10 μm.
In the range of m, almost satisfactory results were obtained. However, considering a-SiTFT, a-
The thickness of the Si film is determined based on stability of properties, resistance, and reduction in resistance due to light, etc. The thickness of the anodic oxide film (gate insulating film) is determined based on stability of properties, dielectric constant, pinholes, etc. determined, 50 to 100 nm, respectively.
It is adjusted in a range of about 100 to 200 nm. Even in this case, under the conditions that the resistivity of the a-Si film is 10 ¹⁰ Ω・cm or less, the film thickness is 50 nm or more, and the anodic oxide film is 100 nm or more, the above-mentioned W/L must be approximately smaller than 1. For example, it is possible to form a uniform anodic oxide film.

Anodic oxidation can be performed in either liquid phase or gas phase, and in particular, the gas phase method using oxygen plasma has a low conductance in the gas phase, so it is possible to obtain a uniform and high-quality oxide film even on high-resistance materials. . 4 is a counter electrode during anodic oxidation, and 5 is an oxide film formed by anodic oxidation. In FIG. 1D, the semiconductor thin film 3 formed in FIG. 1B and the oxide film 5 formed in FIG. 1C are patterned, and the thin linear electrodes 2 in FIG. 1A are used as source and drain electrodes. , a gate electrode 6 is attached on an oxide film 5 to form an insulated gate thin film transistor.

In FIG. 2, the substrate 1 is
This is a case where a step including forming an impurity trap film 11 of silicon nitride or the like is added to prevent impurity diffusion from zero. FIG. 2A-1 shows the step of forming the impurity trap film 11 in advance. The other steps are the same as in the case of FIG. 1, and in this case, as shown in FIG.
It has a sand German beach structure. Although the process shown here does not involve doping, it is also possible to include a doping process, and the impurity trap of the sandwich structure shown in FIG. It also includes membranes. In addition, 41 is a counter electrode during anodic oxidation, 5
1 is an oxide film, and 61 is a gate electrode.

FIG. 3 is an explanatory diagram showing an outline of an actual anodizing apparatus. 30 is an insulating material used to reduce voltage fluctuations due to the rise and fall of the electrolytic solution 35 and form a uniform film, 34 is a non-insulating film to be anodized, and 33 is a source and A patterned electrode that also serves as a drain and is used as an anode. 32 is a substrate, and 36 is a cathode. 37 is a power source.

As is clear from the above process example, the present invention does not newly form an electrode to be used for anodization, but rather uses a part of a transistor component that is naturally necessary as an electrode for a transistor (MOS). This simplifies the process, allows the formation of a uniform insulating film, and allows the formation of source and drain electrodes at the same time.

This method makes it possible to anodize even non-insulating films that have been difficult to anodize in the past, making it possible to form uniform, high-quality films at low temperatures, and is effective in taking advantage of the advantages of TFT.

The present invention is a technique particularly effective for TFTs on display panel substrates using liquid crystals, etc., and is particularly suitable as a display device for small portable devices such as wristwatches.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing the thin film transistor manufacturing process of the present invention. FIG. 2 is a process diagram showing another manufacturing process of the thin film transistor of the present invention. FIG. 3 is an explanatory diagram showing an outline of the anodizing device. 1, 10, 32... Substrate, 2, 21... Patterned source and drain electrodes, 3, 31... Semiconductor film, 4, 36, 41... Counter electrode during anodic oxidation, 5, 51... ...Oxide film (gate insulating film),
6, 61... Gate electrode, 11... Impurity trap film, 35... Electrolyte, 36... Cathode.

Claims (1)

[Claims] 1. In the manufacturing process of a thin film transistor, a step of forming patterned source and drain electrodes on an insulating substrate, a step of forming a non-insulating thin film on the electrodes, and a step of forming patterned source and drain electrodes on the insulating substrate; A method for manufacturing a thin film transistor, comprising the step of anodizing the non-insulating thin film using source and drain electrodes as anodes. 2. In the manufacturing process of a thin film transistor, a step of forming an impurity trap film on an insulating substrate, a step of forming patterned source and drain electrodes on the impurity trap film, and a step of forming a non-insulating thin film on the electrodes. A method for manufacturing a thin film transistor, comprising the steps of: anodic oxidizing the non-insulating thin film using the patterned source and drain electrodes as anodes.