JPH07106246A - Formation of polycrystalline silicon thin film - Google Patents

Abstract

PURPOSE:To obtain a polycrystalline silicon very thin film by a method wherein a first insulating layer, a first amorphous silicon layer, a second insulating layer and a second amorphous silicon layer are formed in order on an insulating substrate and an energy beam is emitted from the side of the second amorphous silicon layer. CONSTITUTION:An SiD2 film 2 which is a first insulating layer, a first amorphous silicon layer 3 which is used as a channel region, a second insulating layer 4, a second amorphous silicon layer 5 and a third insulating layer 6 are formed in order on an insulating substrate 1. Then, annealing is performed in a nitrogen atmosphere for eliminating hydrogen in the layers 3 and 5. Then, an energy beam, such as an Ar ion laser beam, is emitted from the side of the layer 5 and the layers 3 and 5 are molten, solidified and crystallized. In such a way, the second silicon layer acts as a heat storage layer and there is no need to thicken the first silicon layer for forming a channel layer and the like of a thin film transistor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a polycrystalline silicon thin film, and more particularly to a method for forming a polycrystalline silicon thin film which can be suitably used for a channel region of a thin film transistor.

[0002]

2. Description of the Related Art A conventional method for forming a polycrystalline silicon thin film used as a channel region of a thin film transistor is shown in FIG.

First, as shown in FIG. 1A, a first insulating layer 22, an amorphous silicon layer 23 to be a channel region of a thin film transistor, and a second insulating layer 24 are sequentially formed on an insulating substrate 21 made of glass or the like. Form. At this time, the film thickness of the amorphous silicon layer 23 is 200 nm or more, usually about 500 nm.

Next, as shown in FIG.
The amorphous silicon layer 23 is crystallized by melting and solidifying the amorphous silicon layer 23 by irradiating it with energy rays from the 4 side. An argon ion laser, an excimer laser, or the like is used as the energy ray.

In the method described above, about 50 of the silicon layer 23 is used.
Among the film thickness of 0 nm, a huge crystal having a thickness of about 300 nm and a grain size of several tens of μm is formed from the side in contact with the first insulating layer 22. The remaining approximately 200 nm has a particle size of 100 n
It becomes a polycrystal of m. The reason why crystals having such a huge grain size are formed at the bottom of the amorphous silicon layer 23 is that the film thickness of the amorphous silicon layer 23 is sufficiently large. That is, since the absolute amount of heat accumulated in the silicon layer 23 is large and the effect of heat dissipation to the atmosphere is relatively small,
This is because the crystallization speed of silicon on the side in contact with the first insulating layer 22 becomes slow.

Amorphous silicon layer 23 is 200 nm
When the thickness is made thinner, the entire silicon layer 23 becomes polycrystalline with a grain size of several 100 nm. This is because the amount of heat released to the atmosphere is large relative to the amount of heat stored, and the temperature of the silicon layer 23 drops rapidly. Only when the thickness of the amorphous silicon layer 23 is 200 nm or more, crystals having a diameter of several μm can be formed on the side in contact with the first insulating layer 22.

[0007]

As is apparent from the above description, the amorphous silicon layer 23 needs a certain thickness in order to stably form crystals having a huge grain size. In the conventional technology, the film thickness is set to about 500 nm, and about 3
A polycrystalline silicon film having a thickness of 00 nm was obtained. However, if the silicon layer 23 is thicker than necessary, it is not preferable in forming a transistor, and the stepped portion near the channel region of the transistor may adversely affect the insulating property of the insulating layer or cause disconnection of the metal wiring.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a method for forming a polycrystalline silicon thin film having a crystal grain size of several tens of μm and a film thickness of 200 nm or less. And

[0009]

In order to achieve the above object, in the present invention, a first insulating layer, a first amorphous silicon layer, a second insulating layer and a second amorphous silicon are provided on an insulating substrate. Layers are sequentially formed, and an energy ray is irradiated from the second amorphous silicon layer side to form the first amorphous silicon layer.
The amorphous silicon layer and the second amorphous silicon layer are melted and crystallized.

[0010]

With the above structure, the total thickness of the silicon layer can be increased by increasing the thickness of the second silicon layer. Therefore, the first silicon layer can be made thin enough not to hinder device formation. Further, at this time, if the thickness of the second silicon layer is 200 nm or more, the amount of heat storage per unit area is sufficiently larger than the amount of heat radiation, and the crystallization speed can be slowed. Therefore, even if the first silicon layer has a film thickness of 200 nm or less, it is possible to form a polycrystalline silicon thin film having a huge grain size of several tens of μm.

[0011]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is a diagram for explaining a method for forming a polycrystalline silicon thin film according to the present invention, in which 1 is an insulating substrate made of glass or the like.

First, as shown in FIG.
A silicon dioxide (SiO ₂ ) film 2, which is a first insulating layer,
A first amorphous silicon layer 3 to be a channel region,
The second insulating layer 4, the second amorphous silicon layer 5, and the third insulating layer 6 are sequentially formed. The first insulating layer 2 is formed to a thickness of about 500 nm by a plasma CVD method using, for example, silane gas (SiH ₄ ) and laughing gas (N ₂ O), and the first amorphous silicon layer 3 is formed by, for example, silane gas. By a plasma CVD method using
It is formed to have a thickness of about 0 nm. The second insulating layer 3 includes, for example, silicon dioxide, tantalum oxide (TaO _x ), silicon nitride (Si
_x N _y ), etc. are used. When formed with tantalum oxide, the sputtering method is used. When formed with silicon dioxide, the plasma CVD method using silane gas and laughing gas is used. When further formed with silicon nitride, silane gas is used. And is formed to a thickness of about 50 nm by the plasma CVD method using ammonia gas (NH ₃ ). The second amorphous silicon layer 4 is formed to have a thickness of 200 nm or more by, for example, a plasma CVD method using silane gas, and the third insulating layer 6 is formed.
Is made of silicon dioxide or the like, and is formed to have a thickness of about 5 nm by, for example, a plasma CVD method using silane gas and laughing gas. The third insulating layer 6 prevents impurities from mixing into the amorphous silicon layers 3 and 5 from the outside air when the amorphous silicon layers 3 and 5 are crystallized, or the amorphous silicon layer 5 scatters. The third insulating layer 6 is not always necessary when only the first amorphous silicon layer 3 is used for the device.

Before irradiating with energy rays, in order to remove hydrogen in the first amorphous silicon layer 3 and the second amorphous silicon layer 5, annealing is performed at 600 ° C. in a nitrogen atmosphere for about 1 hour.

Next, as shown in FIG. 1B, the first amorphous silicon layer 3 and the second amorphous silicon layer 5 are melted and solidified by irradiating an energy beam such as an Ar ion laser. Crystallize. At this time, the power of the Ar ion laser is 0.1 to 1.0 W
Do it in a degree. The laser beam is divided into two beams, and the interval is adjusted to be several tens of μm. This is for crystallizing from the central part. As shown in FIG. 2, the laser beam scans perpendicularly to the dividing direction at a speed of 1 to 10 cm / sec.

By increasing the film thickness of the second amorphous silicon layer 5 to 200 nm or more, the amount of heat storage per unit area can be increased, and it is possible to increase the amount of heat stored in the atmosphere or the insulating substrate 1.
The influence of heat radiation on the heat can be relatively reduced. This can reduce the solidification rate of silicon,
It is possible to form a polycrystalline silicon thin film having a grain size of several tens of μm. When a thin film transistor is formed using this, the influence of grain boundaries is reduced and mobility is increased. Further, since the current flowing through the grain boundary can be reduced, the OFF current of the transistor can be reduced.

As described above, after irradiating the third insulating layer 6 with energy rays to polycrystallize the first amorphous silicon layer 3 and the third amorphous silicon layer 5, the third insulating layer 6 is formed. , The second silicon layer 5 and the second insulating layer 4 are removed by etching to polycrystallize the second silicon layer 3
Is used as a channel layer of a transistor, for example. When etching the third insulating layer 6 and the second insulating layer 4, an etching solution such as hydrofluoric acid is used. When etching the polycrystallized second silicon layer 5, hydrofluoric acid and nitric acid are used. A mixed solution or the like is used.

[0017]

As described above, according to the method for forming a polycrystalline silicon thin film of the present invention, the first insulating layer, the first amorphous silicon layer, the second insulating layer, and
Since the second amorphous silicon layer is sequentially formed and the second amorphous silicon layer is irradiated with energy rays, the first amorphous silicon layer and the second amorphous silicon layer are melted and crystallized. , Second
This silicon layer acts as a heat storage layer, and it is not necessary to thicken the first silicon layer for forming the channel layer of the thin film transistor. As a result, a thin polycrystalline silicon thin film can be formed. When this is used as the channel layer of a thin film transistor, the adverse effect on the insulating property of the insulating layer at the step portion is reduced, and the disconnection of the metal wiring can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a method for forming a polycrystalline silicon thin film according to the present invention.

FIG. 2 is a diagram for explaining crystallization of silicon by an argon ion laser.

FIG. 3 is a diagram for explaining a conventional method for forming a polycrystalline silicon thin film.

[Explanation of symbols]

1 ... Insulating substrate, 2 ... First insulating layer, 3 ... First amorphous silicon layer, 4 ... Second insulating layer,
5 ... Second morphous silicon layer

Claims (2)

[Claims] 1. A first insulating layer, a first amorphous silicon layer, a second insulating layer, and a second amorphous silicon layer are sequentially formed on an insulating substrate, and from the second amorphous silicon layer side. A method of forming a polycrystalline silicon thin film, which comprises irradiating an energy ray to melt and crystallize the first amorphous silicon layer and the second amorphous silicon layer. 2. The method for forming a polycrystalline silicon thin film according to claim 1, wherein the thickness of the second amorphous silicon layer is 200 nm or more.