JPH0318357B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to low-temperature formation of a gate insulating film that influences the switching characteristics of a thin film transistor (TFT).

The general structure of an insulated gate thin film transistor is
They are a substrate - 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, ceramics, etc. as the substrate, but in that case, processing at high temperatures becomes difficult. . Currently, gate insulating films used in semiconductor technology include _SiO2 , etc.;
Thermal oxidation method is mainly used. However, if the temperature is restricted by the substrate, it is difficult to use commonly used thermal oxidation. Other insulating film formation methods include physical vapor deposition (PVD) and chemical vapor deposition (CVD), but compared to thermal oxide film,
Film uniformity, insulation, defects in the film, impurity density,
It is inferior in terms of interface state density, etc. Furthermore, in the case of gate insulating films using thermal oxidation, oxides of semiconductor films have been used.

The present invention utilizes anodic oxidation to form a film having film uniformity, insulation properties, impurity density, and interface state density comparable to those of thermal oxide films at low temperatures. Non-insulating thin films, such as semiconductor films, formed at low temperatures by PVD and CVD on insulating substrates have a high sheet resistance, so conventional methods of anodic oxidation are difficult to use, so source and drain electrodes are prepared in advance. ,
A first layer semiconductor thin film formed on a substrate is patterned, a second layer semiconductor thin film is further formed as a non-insulating thin film on top of the first layer semiconductor thin film, and the above electrode is used as an anode, and the second layer semiconductor thin film is used as an anode. It is oxidized and used as a gate insulating film.

In the case of conventional thermal oxidation, the oxide of the semiconductor film is used as the gate, but in the thin film transistor structure described above, the gate insulating film can be formed without being affected by the composition of the semiconductor film, and until now it has not been oxidized. It is possible to use even bulky materials by forming electrodes and by providing a first layer semiconductor thin film to facilitate the application of voltage between the opposing electrode and the source and drain electrodes. Layered semiconductor thin film
The electrode-gate insulating film structure expands the scope of use of semiconductor films and gate insulating film materials, and provides high-performance thin film transistors. That is, in the present invention, a first layer semiconductor thin film is first formed on a substrate, patterned source and drain electrodes are formed on the semiconductor film, and a second layer semiconductor thin film, which is a non-insulating thin film, is formed on this. In this method, a thin film transistor is manufactured by forming a thin film transistor, and using the electrode as an anode and using anodic oxidation.
In the present invention, a semiconductor film that becomes a channel,
The method is characterized in that a non-insulating thin film that becomes a gate oxide film is separately formed, and the non-insulating thin film, for example, a semiconductor film, is anodized to form the gate oxide film.

The present invention will be explained in detail below using the drawings.

The drawings all show embodiments of the present invention, and FIGS. 1A, B, C, and D show the manufacturing process of a thin film transistor. In the process of FIG. 1A, 1 is a substrate; A first layer semiconductor thin film 2 is formed thereon.

Next, patterned source and drain electrodes 3 are formed on the first layer semiconductor thin film 2. This electrode is used as an anode during anodic oxidation, and also as a source and drain electrode of a thin film transistor.

In the process shown in FIG. 1B, 4 is a film in which a second layer semiconductor thin film as a non-insulating film is formed on the patterned electrode 3 and the first layer semiconductor thin film 2, and is oxidized by anodic oxidation. do.

In the step shown in FIG. 1C, the second layer semiconductor thin film 4 is oxidized to form an oxide film 5. 6 indicates an anticathode for anodic oxidation.

In the step shown in FIG. 1D, a gate electrode 7 is formed.

An example of an actual anodizing device is shown in FIG. 2, where 21 is an electrolyte, 22 is a substrate, and 2
3 is a first layer semiconductor thin film, 24 is a patterned electrode that doubles as a source and drain electrode and is used as an anode, 25 is a non-insulating thin film to be anodized, 26 is a cathode, 27 is a power source used for anodic oxidation.

For example, as the electrolytic solution 21, a mixed solution of N-methylacetamide and 0.04N potassium nitrate solution, a mixed solution of nitrates and halides of tetrahydrofurfuryl alcohol and ethylene glycol, and the substrate 2
2: quartz glass, pyrex glass,
The first layer semiconductor thin film 23 is a silicon film, the source and drain electrodes are high melting point metals such as Mo and Ta, the second layer semiconductor thin film 24 is a silicon film, etc., and the cathode 6 is a platinum electrode. As the power source 27, a constant current-voltage power source is used. The formation of an oxide film by cathodic oxidation under the above conditions will be explained in more detail. In this example, the electrode 3 was formed to have a width of 10 μm. Also, the inter-electrode distance W between the source electrode and the drain electrode
was set to 10 μm, and the length L of the source electrode and drain electrode was set to 100 μm in order to stably maintain the driving ability of the transistor.

Next, as shown in FIG. 1B, amorphous silicon (a-Si) is deposited on the electrode 3 and the first layer semiconductor thin film 2 as a second layer semiconductor thin film 4.
It was formed to a thickness of 150 nm using the CVD method. With this design value, the a-Si resistance (R _SD ) between the source electrode and the drain electrode is expressed as R _SD =ρ·W/(tL). When the resistivity ρ of a-Si 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
It spreads laterally. The resistance of the anodic oxide film formed this time is 1× when a voltage of 1 V is applied at a thickness of 100 nm.
Since it is 10 ¹⁶ Ω or more, it is two orders of magnitude larger than the resistance of a-Si between the source electrode and the drain electrode. 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 second layer semiconductor thin film 4, the anodic oxide film 5 has a thickness of 100 nm on the source electrode and the drain electrode.
To prevent this formation, a constant voltage was applied between the anode (source electrode and drain electrode) and the cathode (counter electrode 6) during anodic oxidation.

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 of Si is smaller than the resistance of the anodic oxide film, and the 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 = 150n
m, and ρ=10 ¹⁰ Ω・cm, R _SD ≒0.7×
10 ¹⁵ ×W/L, and if W/L is approximately smaller than 1, R _SD will be 1/10 or less 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 10μ.
Almost satisfactory results were obtained when the length of the electrode was at least approximately 10 μm.
However, when considering a-Si TFT, the thickness of the a-Si film is determined from the viewpoints of stability of characteristics, resistance, and reduction in resistance due to light, etc., and the thickness of the anodic oxide film (gate insulating film) is determined by the characteristics. It is determined by the stability, dielectric constant, pinhole, etc., and each is adjusted within a range of about 50 to 100 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 W/L
A uniform anodic oxide film can be formed if it satisfies that . Although this embodiment deals with anodic oxidation in the liquid phase, it is of course possible to use the present invention in anodic oxidation in the gas phase.

As described above, according to the present invention, instead of newly forming electrodes for use in anodic oxidation, source and drain electrodes, which are naturally required as electrodes of a transistor, are used, thereby contributing to process simplification. Further, by providing the first layer semiconductor thin film, problems such as the adhesion of the electrodes can be solved, and furthermore, the voltage applied between the electrodes during anodization becomes constant. In order to provide the second layer semiconductor thin film and form the anodic oxide film, the first layer
The oxide film of the layer may have the same composition, or may have a different composition. By providing an electrode and a first layer semiconductor thin film, even for semiconductor films that were conventionally difficult to be anodized, it is possible to form a uniform film at low temperatures, and the high performance and range of thin film transistors can be improved. .

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

[Brief explanation of the drawing]

1A, B, C, and D are manufacturing process diagrams of a thin film transistor showing an embodiment of the present invention, and FIG. 2 is a configuration diagram of an anodizing apparatus. 1, 22... Substrate, 2, 23... First layer semiconductor thin film, 3... Patterned source and drain electrodes, 4, 24... Second layer semiconductor thin film (non-insulating film), 5... Anodized non-insulating film, 6,2
6... Counter electrode, 7... Gate electrode, 21... Electrolyte, 27... Power supply.

Claims (1)

[Claims] 1. In the manufacturing process of a thin film transistor, a step of forming a first layer semiconductor thin film on a substrate;
forming patterned source and drain electrodes on the semiconductor thin film; forming a second layer semiconductor thin film on the patterned metal film; and forming patterned source and drain electrodes on the patterned metal film. A method for manufacturing a thin film transistor, comprising the steps of anodizing the second layer semiconductor thin film as an anode.