JPH06283423A - Method of forming polycrystalline silicon film - Google Patents

Abstract

PURPOSE:To obtain a polycrystalline silicon film with superior film uniformity having a larger grain diameter by a method wherein the surface of a silicon oxide film to be a ground is washed with a liquid having an etching effect beforehand, and, after this pre-treatment, an amorphous silicon film is deposited, and the film is crystalized by heat-treatment. CONSTITUTION:The surface of a silicon wafer 1 is oxidized using steam of 950 deg.C to form a silicon oxide film 2 with a thickness of about 600nm. Then, the surface of this silicon oxide film 2 is washed with diluted hydrofluolic acid as a pre-treatment. After that, an amorphous silicon film 3 with a thickness of 50nm is deposited on the silicon oxide film 2 by a CVD method using SiH6 at a temperature of 480 deg.C. After that, this amorphous silicon film 3 is processed in a form of island and undergone the heat-treatment for about 3h at a temperature of 600 deg.C. Thus, it is possible to obtain a polycrystalline silicon film 4, thereby reducing the density of crystal grains to about 1/100.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device, and more particularly to a method for forming a polycrystalline silicon film used for a channel of a thin film transistor (TFT).

[0002]

2. Description of the Related Art Conventionally, when a polycrystalline silicon film is formed in order to form a thin film transistor on a silicon wafer, the silicon oxide film formed on the silicon wafer as a base is continuously formed (other processing steps are performed). After depositing an amorphous silicon film (without intervening), the amorphous silicon film is crystallized by heat treatment to form a polycrystalline silicon film to be used for the channel of the thin film transistor.

FIG. 4 is a process chart showing the steps of forming a polycrystalline silicon film on a silicon oxide film as described above. First, as shown in FIG.
Approximately 600 nm thick silicon oxide film (SiO ₂ ) on top of
2 as a base, and as shown in FIG. 4B, an amorphous silicon film (a-Si) 3 having a thickness of about 50 nm is immediately deposited on the silicon oxide film 2.

Then, the amorphous silicon film 3 is crystallized by placing it in a heat treatment furnace (not shown) and performing heat treatment (annealing) at a temperature of, for example, 600 ° C. for about 3 hours. ), As shown in FIG.
A polycrystalline silicon film 4 is formed on the above.

As a method of depositing the amorphous silicon film 3 on the silicon oxide film 2, a known method such as a low pressure CVD method or a vacuum evaporation method is generally used. One of the important issues for practical application is stable off current of 0.2.
5 pA / gate or less, that is, standby current consumption is 1 μm
The following must be built on a silicon wafer.

Therefore, as a method of forming a polycrystalline silicon film in order to solve such a problem, for example, "NIKKEI MI" is used.
CRODEVICES "(March 1990 issue pp. 64
-71) introduces the following two methods.

That is, the first method is a technique for increasing the crystal grain size of the polycrystalline silicon film provided for the channel to reduce the crystal grain boundaries through which the off current flows. Low temperature (480 ℃ -5
It is a method of decreasing the generation rate of nuclei by depositing at 50 ° C. The second method is a method of reducing the off-current by making the crystal grain sizes of the polycrystalline silicon film provided for the channel appropriate and thinning them.

Further, in the above-mentioned reference, instead of the silane (SiH ₄ ) gas for forming the thin film transistor in the first method, disilane (Si) having a low activation energy is used.
₂ H ₆ ) gas is used and the deposition temperature of the amorphous silicon film is set to about 480 ° C.
It has been reported that a polycrystalline silicon film having a certain degree can be obtained.

[0009]

As described above, the conventional method for forming a polycrystalline silicon film is as follows. Immediately after forming a silicon oxide film, an amorphous silicon film is deposited on this silicon oxide film and heat-treated. It was crystallized,
Since a state where nuclei are likely to be generated at the interface between the silicon oxide film and the amorphous silicon film during heat treatment, many nuclei are generated during the heat treatment and the crystal grain size after crystallization is about 1 μm at most. There was a problem that it was suppressed by.

Further, as shown in the above-mentioned reference,
Although it is possible to obtain a polycrystalline silicon film having a large grain size by the conventional method of forming a polycrystalline silicon film (the above-mentioned first method), the inventors have conducted a comparative experiment to reduce the amorphous silicon film to a reduced pressure type. Using a CVD apparatus, Si ₂ H ₆ gas at 480 ° C. and a film forming rate of 1 nm / min.
After depositing m, the polycrystalline silicon film was formed by heat treatment at 600 ° C. for 3 hours (normally performed heat treatment time). As a result, the crystal grain size in this polycrystalline silicon film was 1 μm.
It has been confirmed that it is necessary to form a film at a film forming rate of 5 nm / min or more in order to grow the crystal grain size to about 3 μm.

Therefore, in the method for forming a polycrystalline silicon film shown in the above-mentioned reference, the film forming rate is 1 nm / min because the film forming rate is high even if a polycrystalline silicon film having a large grain size can be formed. However, there was a problem that the film thickness uniformity was ± 3%, but became ± 10% or more when the film formation rate was 5 nm / min or more.

The present invention has been made to solve the above problems, and in particular, as a technology for practical use of thin film transistors, the number of nuclei generated at the interface between the underlying silicon oxide film and the amorphous silicon film is reduced. Then, it is an object of the present invention to provide a method for forming a polycrystalline silicon film having a larger crystal grain size and excellent film uniformity after heat treatment.

[0013]

A method of forming a polycrystalline silicon film according to the present invention is a technique aimed at forming a polycrystalline silicon film to be used for a channel of a semiconductor device, particularly a thin film transistor, and a silicon oxide film as a base. The surface is previously washed with a liquid material having an etching action, for example, hydrofluoric acid, ammonia-hydrogen peroxide mixture (hereinafter, this treatment is referred to as a pretreatment step), and after the pretreatment step, an amorphous silicon film is deposited, It is characterized in that a polycrystalline silicon film is formed by heat treatment and crystallization.

In the pretreatment step, the surface of the silicon oxide film is washed with the liquid having the etching action, and the surface of the silicon oxide film is exposed to a plasma atmosphere containing an etching gas, water vapor, or hydrogen. Alternatively, it may be washed with water.

The above amorphous silicon film is composed of at least silane (SiH ₄ ) and disilane (Si).
₂ H ₆ ) or trisilane (Si ₃ H ₈ )
One of them is formed by the CVD method, and impurities are doped in the formation process.

Further, the silicon oxide film as the base is obtained by oxidizing the surface of the silicon wafer or is deposited by CVD to a predetermined thickness.

[0017]

In the method of forming a polycrystalline silicon film according to the present invention, prior to the step of forming an amorphous silicon film, which has been performed immediately after the formation of the underlying silicon oxide film, the surface of the underlying silicon oxide film is Liquid material having an etching action such as hydrofluoric acid and ammonia-hydrogen peroxide, gas having an etching action such as hydrofluoric acid gas, water, water vapor,
Alternatively, it is characterized in that the pretreatment step is performed in advance in a plasma atmosphere containing hydrogen.

As described above, the reason why the above-described pretreatment process is performed prior to the step of depositing the amorphous silicon film on the silicon oxide film is that the surface state of the silicon oxide film and the amorphous silicon film deposited on the surface of the silicon oxide film. As a result of studying the relationship with the crystal grain size of polycrystalline silicon obtained by crystallizing the film, increasing the amount of OH groups bonded to silicon atoms on the surface of the silicon oxide film results in formation of crystal nuclei from the amorphous silicon film. This is because the inventors newly discovered that the formation can be suppressed.

That is, the outermost surface of the silicon oxide film which is usually formed by exposing the surface of a silicon wafer to an oxygen atmosphere is covered with Si--H or distorted Si--O.

When an amorphous silicon film is deposited on this and heat-treated at about 600 ° C., silicon atoms in the amorphous silicon film are immediately distorted on the oxide film surface of Si.
The -O bond is broken, and an array of several silicon atoms (this is a crystal nucleus) matching the silicon oxide film is created.

When the inventors deposit an amorphous silicon film immediately after oxidation (immediately after forming a silicon oxide film on a silicon wafer) and heat-treat it at 600 ° C. or more to form a polycrystalline silicon film, this polycrystalline silicon film is used. It was confirmed that the number of crystal nuclei in the crystalline silicon film reaches 100 / μm ² .

Further, when the crystal grain size of the polycrystalline silicon film is increased as described above, the impurities (such as phosphorus) doped in the polycrystalline silicon film diffuse into the underlying silicon oxide film. It is possible to reduce the crystal grain boundaries in which a high concentration of impurities is present.

Therefore, in the present invention, before the amorphous silicon film is deposited, the above-described pretreatment step is performed in advance so as to terminate the silicon atoms on the surface of the silicon oxide film with OH groups, thereby reducing the number of crystal nuclei generated. As a result, the crystal grain density after crystallization is reduced, and a large-grain polycrystalline silicon film can be formed.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the method for forming a polycrystalline silicon film according to the present invention will be described below with reference to FIGS.

FIG. 1 is a process diagram for explaining one embodiment of the method for forming a polycrystalline silicon film according to the present invention, and particularly shows a process for forming a polycrystalline silicon film used for a channel of a thin film transistor (TFT). But first,
As a first embodiment of the present invention, before depositing an amorphous silicon film, the surface of the underlying silicon oxide film is
The case of cleaning with dilute hydrofluoric acid as a liquid substance having an etching action will be described.

As shown in FIG. 1 (a), the surface of the silicon wafer 1 is oxidized with water vapor at 950 ° C. to form a film thickness 600.
A silicon oxide film 2 having a thickness of about nm is formed. Next, the surface of the silicon oxide film 2 is washed with dilute hydrofluoric acid as a pretreatment step (in this case, the surface of the silicon oxide film 2 is about 0.1).
.About.0.2 nm is etched).

After that, as shown in FIG.
A film thickness of about 5 is formed on the silicon oxide film 2 formed by the VD method.
After depositing a 0 nm amorphous silicon film 3 using Si ₂ H ₆ gas at 480 ° C., the amorphous silicon film 3 is processed into an island shape and subjected to heat treatment at a temperature of 600 ° C. for about 3 hours. As shown in (c), a polycrystalline silicon film 4 is obtained. However, the polycrystalline silicon film 4 may be heat-treated at a temperature of 600 ° C. for about 3 hours and then processed into an island shape.

As shown in FIG. 2C, the crystal grain density in the polycrystalline silicon film 4 obtained through the above steps is the polycrystalline silicon formed by the conventional forming method (shown in FIG. 4). Crystal grain density in film 4 ((a) of FIG. 2)
In comparison with the above, the crystal grain size can be reduced to about 1/100, and the crystal grain size of the polycrystalline silicon film 4 formed by the conventional forming method is 0 as shown in FIG. Although it was 0.8 μm ((a) in FIG. 3), it could be confirmed that it was increased to about 3 μm (FIG. 3).
(B)).

Thereafter, as shown in FIG. 3D, a gate oxide film 5 is formed by a CVD method, and then a gate electrode 6 is formed.
Are formed ((e) in the figure). The gate electrode 6 is formed of a polycrystalline silicon film or a silicide film.

Then, as shown in FIG. 3F, finally, the source region 7 and the drain region 8 are formed in a self-aligned manner using the gate electrode 6 as a mask, and then a TFT is formed through a wiring process.

Next, as a second embodiment of the present invention, a case where a gas having an etching action in the pretreatment step is hydrofluoric acid gas will be described with reference to FIG. 1 similarly to the first embodiment described above. To do.

As shown in FIG. 1A, the surface of the silicon wafer 1 is oxidized with water vapor at 950 ° C. to form a film thickness 600.
After the silicon oxide film 2 having a thickness of about 10 nm is formed, the surface of the silicon oxide film 2 is used as a pretreatment process at room temperature at a partial pressure of 0.
It is exposed to the hydrofluoric acid gas of 01 for 10 s (in this case, the surface of the silicon oxide film 2 is etched by about 0.1 to 0.2 nm).

After that, as shown in FIG.
After depositing an amorphous silicon film 3 with a film thickness of about 50 nm on the silicon oxide film 2 formed by the above CVD method using disilane gas at a temperature of 600 ° C., a heat treatment is performed at a temperature of 600 ° C. for about 3 hours. As shown in (c), a polycrystalline silicon film 4 is obtained (the following steps are the same as in the case of the first embodiment described above).

As a result of obtaining the polycrystalline silicon film 4 through the above steps, the crystal grain size thereof is about 3.1 μm (the crystal grain size of the polycrystalline silicon film formed by the conventional forming method is about 0.8 μm). ) Has been confirmed.

Next, as a third embodiment of the present invention, in the case of cleaning with ammonia hydrogen peroxide solution in the pretreatment step, as in the first and second embodiments described above, FIG.
Will be explained.

As shown in FIG. 1A, the surface of the silicon wafer 1 is oxidized in an oxygen atmosphere at 900 ° C.
A 0 nm silicon oxide film 2 is formed. Then, the surface of the silicon oxide film 2 is used as a pretreatment step in the ammonia hydrogen peroxide solution (NM ₄ OH: H ₂ O ₂ : H ₂ O) at 70 ° C.
= 1: 1: 5 (volume ratio) for 1 minute, further washed with water and dried.

After that, as shown in FIG.
After depositing an amorphous silicon film 3 having a film thickness of 50 nm on the silicon oxide film 2 formed by the above CVD method using Si ₂ H ₆ gas at a temperature of 600 ° C., a heat treatment is performed at 600 ° C. for about 3 hours. A polycrystalline silicon film 4 is obtained as shown in FIG.
The same as in the case of the above embodiment).

As a result of obtaining the polycrystalline silicon film 4 through the above steps, when the crystal grain size is washed with ammonia hydrogen peroxide solution, it is about 3.5 μm (the polycrystalline silicon film formed by the conventional forming method). It was confirmed that the crystal grain size in 1 was as large as about 0.8 μm).

Next, as the fourth and fifth embodiments of the present invention, FIG. 1 is used in the case of washing with water or steam in the pretreatment step as in the first to third embodiments described above. Explain.

As shown in FIG. 1A, the surface of the silicon wafer 1 is oxidized with oxygen at 900 ° C. to have a film thickness of 20 nm.
After forming a silicon oxide film 2 to a degree, the surface of the silicon oxide film 2 is washed with water having a resistance ratio of 16 MΩ or a hydrogen is burned at 200 ° C. to obtain water vapor as a pretreatment step. Exposure for a minute (in this case, the surface of the silicon oxide film 2 is not etched).

After that, as shown in FIG.
After depositing an amorphous silicon film 3 with a film thickness of about 50 nm on the silicon oxide film 2 formed by the above CVD method using Si ₂ H ₆ gas at a temperature of 600 ° C. for 3 hours.
By performing a heat treatment to some extent, a polycrystalline silicon film 4 is obtained as shown in FIG. 7C (the following steps are the same as those in the above-described first embodiment).

As a result of obtaining the polycrystalline silicon film 4 through the above steps, the crystal grain size is about 2.1 μm when washed with water.
It was confirmed that the grain size increased to m (the crystal grain size in the polycrystalline silicon film formed by the conventional forming method was about 0.8 μm) and also increased to about 2.1 μm when exposed to water vapor.

The fourth and fifth embodiments are shown in FIG.
As shown in (b) of the above, the crystal grain density is about 10 times as high as that of the first, second and third embodiments described above, but a sufficient effect is obtained as compared with the conventional forming method. Needless to say (the crystal grain density is reduced to about 1/10).

Next, as a sixth embodiment of the present invention, the case where plasma containing hydrogen is used in the pretreatment step will be described with reference to FIG. 1 similarly to the above-mentioned first to fifth embodiments.

As shown in FIG. 1A, the surface of the silicon wafer 1 is oxidized with water vapor at 950 ° C. to form a film thickness 600.
After the silicon oxide film 2 having a thickness of about nm is formed, the surface of the silicon oxide film 2 is subjected to a pretreatment step to heat the silicon substrate to 300 ° C. and expose it to a hydrogen plasma atmosphere for 3 minutes.

Thereafter, as shown in FIG.
After depositing an amorphous silicon film 3 with a film thickness of about 150 nm on the silicon oxide film 2 formed by the above-mentioned CVD method using disilane gas at a temperature of 600 ° C. for 3 hours.
By performing a heat treatment to some extent, a polycrystalline silicon film 4 is obtained as shown in FIG. 7C (the following steps are the same as those in the above-described first embodiment).

As a result of obtaining the polycrystalline silicon film 4 through the above steps, the crystal grain size is about 2.8 μm (the crystal grain size in the polycrystalline silicon film formed by the conventional forming method is about 0.8 μm). ) Has been confirmed.

Next, as a seventh embodiment of the present invention, in the case where the silicon oxide film 2 to be subjected to the pretreatment step is a CVD oxide film (in the above-mentioned embodiments, the surface of the silicon wafer 1 is oxidized to form a silicon oxide film). The film 2 is formed) will be described with reference to FIG. 1 similarly to the above-described embodiment.

As shown in FIG. 1A, the surface of the silicon wafer 1 is formed at 820 ° C. by using a SiH ₄ gas and a nitrous oxide gas to form a base silicon oxide film 2 having a film thickness of about 200 nm. After the formation, the surface of the silicon oxide film 2 is washed with ammonia hydrogen peroxide solution at 70 ° C. for 1 minute as a pretreatment step.

After that, as shown in FIG.
By depositing an amorphous silicon film 3 having a film thickness of 50 nm on the silicon oxide film 2 formed by the above CVD method using Si ₂ H ₆ gas at a temperature of 600 ° C., and then performing a heat treatment at a temperature of 600 ° C. for about 3 hours. As shown in FIG. 3C, a polycrystalline silicon film 4 is obtained (note that the following steps are the same as those in the first embodiment described above).

As a result of obtaining the polycrystalline silicon film 4 through the above steps, the crystal grain size is about 2.9 μm (the crystal grain size in the polycrystalline silicon film formed by the conventional forming method is about 0.8 μm). ) Has been confirmed.

Although ammonia hydrogen peroxide solution was used as the pretreatment step in the seventh embodiment, hydrofluoric acid aqueous solution, water, hydrofluoric acid gas, or steam was used instead of the ammonia hydrogen peroxide solution. Has already confirmed that there is a similar effect.

The polycrystalline silicon film is doped with impurities when it is used for, for example, a channel or a gate electrode. By increasing the crystal grain size of the polycrystalline silicon film, impurities (for example, phosphorus) are doped. It has already been confirmed by the inventors that the diffusion of (i.e., etc.) into the silicon oxide film can be suppressed. Therefore, the method for forming a polycrystalline silicon film according to the present invention can be applied to the manufacturing process of all semiconductor devices. Technology.

[0054]

As described above, according to the present invention, the liquid material (hydrofluoric acid or ammonia-hydrogen peroxide mixture) having an etching action is previously formed on the surface of the underlying silicon oxide film before depositing the amorphous silicon film. Etc.) or gas (hydrofluoric acid gas, etc.), and further pretreatment with water, water vapor, or hydrogen plasma to suppress nucleation by heat treatment of the amorphous silicon film that is deposited after that, and to reduce grain size after crystallization. There is an effect that the diameter can be increased and off current of the thin film transistor can be reduced.

Furthermore, the on / off ratio of a thin film transistor using such a large-grain polycrystalline silicon film can be obtained in seven digits as compared with four digits in the conventional method. Therefore, by using this thin film transistor, it is possible to form a highly integrated memory cell.

[Brief description of drawings]

FIG. 1 is a process chart for explaining one embodiment of a method for forming a polycrystalline silicon film according to the present invention.

FIG. 2 is a diagram for comparing a crystal grain density in a polycrystalline silicon film formed according to the present invention with a crystal grain density in a polycrystalline silicon film formed by a conventional forming method.

FIG. 3 is a diagram for comparing a crystal grain size in a polycrystalline silicon film formed according to the present invention with a crystal grain size in a polycrystalline silicon film formed by a conventional forming method.

FIG. 4 is a process chart for explaining a conventional method for forming a polycrystalline silicon film.

[Explanation of symbols]

1 ... Silicon wafer, 2 ... Silicon oxide film, 3 ... Amorphous silicon film, 4 ... Polycrystalline silicon film, 5 ... Gate oxide film, 6 ... Gate electrode, 7 ... Source region, 8 ... Drain region.

Claims (8)

[Claims] 1. A method for forming a polycrystalline silicon film, comprising depositing an amorphous silicon film on a base silicon oxide film and then heat-treating the amorphous silicon film to crystallize the amorphous silicon film, before depositing the amorphous silicon film. In addition, a method of forming a polycrystalline silicon film, characterized in that the surface of the silicon oxide film is washed with a liquid having an etching action. 2. A method for forming a polycrystalline silicon film, comprising depositing an amorphous silicon film on a base silicon oxide film and then heat-treating the amorphous silicon film to crystallize the amorphous silicon film, before depositing the amorphous silicon film. A method for forming a polycrystalline silicon film, which comprises exposing the surface of the silicon oxide film to a gas having an etching action. 3. A method for forming a polycrystalline silicon film, comprising depositing an amorphous silicon film on a base silicon oxide film and then heat-treating the amorphous silicon film to crystallize the amorphous silicon film, before depositing the amorphous silicon film. A method for forming a polycrystalline silicon film, which comprises rinsing the surface of the silicon oxide film with water or exposing it to water vapor. 4. A method for forming a polycrystalline silicon film, comprising depositing an amorphous silicon film on a base silicon oxide film and then heat-treating the amorphous silicon film to crystallize the amorphous silicon film, before depositing the amorphous silicon film. A method of forming a polycrystalline silicon film, which comprises exposing the surface of the silicon oxide film to a plasma atmosphere containing hydrogen. 5. A polycrystalline silicon film serving as a channel is formed on the surface of a silicon oxide film formed as a base on a silicon wafer, and then a gate electrode is formed on the polycrystalline silicon film and the polycrystalline silicon film is formed on the polycrystalline silicon film. In a manufacturing process of a thin film transistor in which a source region and a drain region are formed in a self-aligned manner, the polycrystalline silicon film has a pretreatment process performed on the surface of the silicon oxide film by the method according to any one of claims 1 to 4. After that, deposit an amorphous silicon film,
A method for forming a polycrystalline silicon film, which is characterized in that the amorphous silicon film is heat-treated and crystallized. 6. The method for forming a polycrystalline silicon film according to claim 1, wherein the amorphous silicon film is doped with impurities. 7. The amorphous silicon film according to claim 1, wherein the amorphous silicon film is deposited by a CVD method using at least one of silane, disilane, and trisilane. Method for forming crystalline silicon film. 8. The base silicon oxide film is obtained by oxidizing the surface of a silicon wafer, or C
6. The method for forming a polycrystalline silicon film according to claim 1, wherein the polycrystalline silicon film is a VD oxide film.