JPH03231472A - Manufacture of thin-film transistor - Google Patents

Abstract

PURPOSE:To manufacture a thin-film transistor having a good characteristic by a low-temperature process by a method wherein a gate insulating film is formed by a sputtering method under an atmosphere in which an oxidizing gas is more than an inert gas. CONSTITUTION:A gate insulating film is formed by a sputtering method. It is desirable that the ratio of an inert gas in a gas used for the sputtering is 50% or lower, especially 20vol.% or lower. That is to say, the gate insulating film is manufactured by executing the sputtering method under an atmosphere in which an oxidizing gas is more than the inert gas. As the sputtering method, an RF sputtering method, a DC sputtering method or the like can be used. As the oxidizing gas, oxygen, ozone, nitrous oxide or the like can be enumerated. Especially, when ozone or oxygen is used, it is possible to obtain the gate oxide film which is very good.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a method for manufacturing a thin film transistor applicable to liquid crystal displays, image sensors, and the like.

"Prior Art" Recently, thin film transistors using non-single crystal semiconductor thin films fabricated by chemical vapor deposition or the like have been attracting attention.

Since this thin film transistor is formed on an insulating substrate using a chemical vapor phase method as mentioned above, it can be formed at a low temperature of about 450°C at maximum, and it can be formed using inexpensive soda glass or borosilicate. Glass or the like can be used as the substrate.

This thin film transistor is a field effect type, so-called M
It has the same function as an OSFET, but as mentioned above, it can be formed at low temperature on an inexpensive insulating substrate, and the maximum area that can be manufactured is limited only by the dimensions of the device that forms the thin film semiconductor. It had the advantage that transistors could be easily fabricated on large-area substrates. Therefore, it is extremely promising as a switching element for matrix-structured liquid crystal displays having a large number of pixels, one-dimensional or two-dimensional image sensors, and the like.

In addition, photolithography, which is an already established technology, can be applied to fabricate this thin film transistor.
So-called microfabrication was possible, and it was also possible to integrate it like ICs and the like.

A typical structure of this conventionally known TPT is schematically shown in FIG.

(20) is an insulating substrate made of glass, (21)
is a thin film semiconductor made of a non-single crystal semiconductor, (22),
(23) is the source/drain region, (24), (2
5) is the source/drain electrode, (26) is the gate insulating film, and (27) is the gate electrode.

A thin film transistor configured in this way has a gate electrode (
By applying a voltage to source-drain (27),
2) and (23) to adjust the current flowing between them.

Gate oxide films used in such thin film transistors have been manufactured by direct thermal oxidation of semiconductor materials, thermal CVD under reduced pressure or normal pressure, and the like.

The device characteristics of this thin film transistor are greatly influenced by the film quality of the semiconductor film in the portion where the channel is formed and the characteristics of the gate insulating film.

It has been strongly desired to produce a gate insulating film with particularly good film quality.

In order to obtain good device characteristics by using the gate insulating film produced by the above method in a thin film transistor, it is necessary to set the manufacturing temperature of the gate insulating film to around 600''C, and therefore, crystallized glass is used.

Very expensive substrate materials such as quartz glass had to be used. That is, it eliminates the characteristics of thin film transistors, which can be manufactured by a low-temperature process of about 450° C. and, as a result, can use inexpensive substrate materials (soda glass, etc.).

Additionally, plasma CVD and sputtering methods are known as methods for producing gate insulators at low temperatures, but in both methods, atoms contained in the starting materials and also present during the reaction (for example, Ar , C1, F, N, etc.) are incorporated into the Dito insulating film, causing the generation of fixed charges in the film. Furthermore, the ionic species of atoms present during the reaction collide with the surface of the active layer of the thin film transistor, forming an interface state near the interface between the gate insulating film and the active layer, and in either case, good characteristics of the thin film transistor are maintained. I haven't gotten it yet.

Furthermore, attempts have been made to fabricate a gate insulating film using the photo-CVD method, which has a level of 2.
Although an interface state density of approximately .

``Objective of the present invention'' The present invention is a method for solving the problems of the conventional method, and provides a method for manufacturing thin film transistors with good characteristics using a low-temperature process.

"Structure of the Invention" The structure of the present invention is that in the process of fabricating a thin film transistor, a gate insulating film is fabricated by a sputtering method, and the proportion of an inert gas in the gas used for sputtering is 50% or less, that is, an oxidizing gas. The feature of this method is that the gate insulating film is fabricated by sputtering in an atmosphere containing more than inert gas.

As the sputtering method used in the present invention, any method such as RF sputtering or DC sputtering can be used.
When the sputtering target is an oxide 2 with poor conductivity, such as 5i02, R
It is preferable to use F magnetron sputtering.

In addition, oxygen, ozone, nitrous oxide, etc. can be cited as oxidizing gases, but especially when ozone or oxygen is used, there are no unnecessary atoms incorporated into the gate insulating film, so the gate insulation film is very good. We were able to obtain an insulating film.

Further, ozone is easily decomposed into O radicals (and generates a large amount of 0 radicals per unit volume), which can contribute to improving the film formation rate.

In the production of gate insulating films by the conventional sputtering method, the amount of Ar, which is an inert gas, is greater than that of oxygen gas, and the amount of oxygen is usually about 0 to 10% by volume. That is, in conventional sputtering, it has been thought that Ar strikes a target material to form a film. This is because there is a high probability that an inert gas such as Ar will knock out the target material (sputtering yield). As a result of intensive studies on the characteristics of gate insulating films fabricated by sputtering, the present inventors found that the interface state at the interface between the active layer and the gate insulating film, which indicates the performance of the gate insulating film, and the fixed charge in the gate insulating film It has been found that the deviation from the ideal value of the flat band voltage, which reflects the number of , greatly depends on the proportion of static gas during sputtering.

FIG. 3 shows the relationship between the proportion of Ar gas in the gas and the interface level. Compared to 100% Ar gas, it can be seen that when the amount of Ar gas is less than the amount of oxidizing gas (oxygen in Figure 3) and is 50% or less, the interface state density is reduced to about 1/10. If the proportion of Ar gas is less than 20%,
The value of the interface state is almost constant and low.

FIG. 4 shows the relationship between the proportion of static gas occupied by gas during sputtering and the amount of deviation in flat band voltage.

The deviation of the flat band voltage from the ideal voltage greatly depends on the proportion of Ar gas, and when the proportion of Ar gas is 20% or less, the value is almost close to the ideal voltage.

From these facts, activated Ar atoms present in the reaction atmosphere during film formation by sputtering affect the film quality of the gate insulating film, and it is recommended to reduce the presence of Ar atoms as much as possible during sputtering film formation. It turned out to be desirable.

The reason is that Ar ions or activated A
It is conceivable that the r atoms collide with the interface, form defects at the interface, and are further taken into the gate insulating film, causing the generation of fixed charges.

Also, since oxygen atoms have a lighter mass than Ar atoms,
Even if a collision occurs near the interface, no serious damage will be caused to the area near the interface. Furthermore, since it is a main component in the film, even if it is incorporated into the film, it will not cause the generation of fixed charges.

In addition, all materials used for sputtering are preferably of high purity; for example, the sputtering target is 4N
Most preferably, the above-mentioned synthetic quartz or silicon having a high purity enough to be used for LSI substrates is used.

That is, it is necessary to minimize the amount of impurities present in the gate insulating film. Similarly, the gas used for sputtering was of high purity (5N or higher) to prevent impurities from entering the gate insulating film as much as possible.

The present invention will be explained in detail below using Examples.

“Example 11 FIG. 1 shows the manufacturing process of the thin film transistor of the present invention.

In this example, inexpensive soda glass was used as the substrate (1). A ■-type non-single crystal semiconductor layer (
2) is formed into an island shape to obtain the state shown in FIG. 1(A).

The manufacturing conditions were as follows.

Substrate temperature 350°C reaction pressure
0.06TorrRf power (13,56M
Hz) 100W Gas used S
iH.

Film thickness: 2000 In addition, in this embodiment, a metal mask was used to form the island shape, but a known photolithography technique may also be used.

Next, as shown in FIG. 1(B), excimer laser light (3
) is irradiated near the device region of the non-single crystal semiconductor (2) to crystallize it into a polycrystalline state with a large grain size or a single crystal state with a size approximately equal to the device region. The irradiation conditions of excimer laser light at this time are shown below.

Laser light wavelength: 284rv (KrF) Irradiation energy amount: 200@J/cm" Shot number: 10 Light pulse width: 30nsNext, after forming an N-type non-single crystal semiconductor layer on the entire surface by a known plasma CVD method, By photolithography, patterning was performed so as to leave the source and drain regions (4) and (5), resulting in the state shown in FIG. 1(C).

The conditions for manufacturing this N-type non-single crystal semiconductor layer are shown below.

Substrate temperature 250″C reaction pressure
0.05TorrRf power (13,56M
H,) 150W gas used 5iH
n + PH3+ H2 film thickness 5
This N-type non-single crystal semiconductor was diluted with a large amount of H2 gas, microcrystallized with high Rf power, and had a low electrical resistance.

Next, a gate insulating film (6) is formed by Rf sputtering method.
After that, the source/drain contact holes (7) and (8) were formed by photolithography to obtain the state shown in FIG. 1(D).

The conditions for manufacturing this gate insulating film are shown below.

Target 5iOz 99.99% reaction gas 0□ 100% reaction pressure
0.5PaRf power 50
0W Substrate temperature 100''C Distance between substrate targets 150mm The characteristics of this gate insulating film are shown below.

1/110HF y-ching speed 67nIII/mi
n Dielectric strength voltage 9.1MV/cm Interface state 2.5X10"eV~'cn+-"Next,
A gate electrode (9), a source electrode (10), and a drain (11) electrode were formed using AI to complete a thin film transistor.

The threshold voltage (vt
h) could be made below 1V, and vth of a similar element made of 100% Ar did not fall below 1V.

In addition, the rate of change in vth after applying the gate voltage for a certain period of time is almost the same as that of a gate insulating film formed by thermal oxidation, and is only 0.3 after 1000 hours.
It can be seen that there is only a slight change, and that almost no localized levels are formed at the interface between the gate insulating film (6) and the non-single crystal semiconductor (2) and in the gate insulating film.

Further, the mobility of the thin film transistor of the present invention is 100
cm2/V-S was obtained.

In this example, the ratio of Ar gas when forming the gate insulating film was set to 0, but if the gate insulating film is formed under conditions where Ar gas is present at a ratio of about 20% or less, there is a particular problem in terms of the characteristics of the thin film transistor. did not occur.

However, when the proportion of Ar was set to O, a thin film transistor with better characteristics could be obtained.

Also, when mixing gas at a ratio of 20% or less, the distance between the target and the substrate is made longer than when fabricating with 0% gas, thereby obtaining a gate insulating film with similar film quality. is possible.

Furthermore, the gate insulating film formed by mixing Ar gas at a ratio of 20% or less is irradiated with excimer laser light and flash annealed to remove Ar taken into the film.
It was also possible to eliminate the cause of fixed charges in the film.

At this time, it is possible to increase the amount of energy applied to the film than excimer laser light and simultaneously annealing the gate insulating film and crystallizing the underlying semiconductor layer, which is an extremely effective means of reducing the number of manufacturing steps. there were.

In this example, the exhaust means for the vacuum equipment used in the process of manufacturing thin film transistors was a turbo molecular pump that does not back-diffuse oil etc. from the exhaust system, and the gate insulating film and other semiconductor layers were I tried not to affect the characteristics.

"Effects" According to the method of the present invention, a thin film transistor with very good characteristics could be easily formed using only a low temperature process.

Furthermore, since the cause of fixed charges existing in the gate insulating film can be reduced, it has become possible to provide a highly reliable thin film transistor with little change in characteristics during long-term use.

[Brief explanation of drawings]

FIG. 1 shows the manufacturing process of the present invention. FIG. 2 shows a schematic diagram of a typical thin film transistor. FIG. 3 shows the relationship between the proportion of Ar gas and the interface state density during the fabrication of the gate insulating film. FIG. 4 shows the relationship between the proportion of Ar gas and the amount of deviation in flat band voltage when forming the gate insulating film.

Claims (1)

[Claims] 1. A method for manufacturing a thin film transistor, characterized in that in the step of manufacturing the thin film transistor, a gate insulating film is formed by sputtering in an atmosphere containing more oxidizing gas than inert gas. 2. The method of manufacturing a thin film transistor according to claim 1, wherein the gate insulating film is formed by sputtering in an atmosphere in which the inert gas is present in a proportion of 20% by volume or less.