JPH06168936A - Oxidizing method of semiconductor substrate - Google Patents

Abstract

PURPOSE:To improve the uniformity and the reproducibility of an oxide film, by treating an initial oxide thin film of a semiconductor substrate, with mixed solution of acidic solution and alkaline solution, in the oxidizing method of a semiconductor substrate wherein the initial oxide film is formed before a practical oxide film is formed. CONSTITUTION:After a silicon wafer 1 is dipped in solution of BF:H2O=1:4 for one minute, and unnecessary oxide film which has been formed on the surface is completely eliminated, the wafer is washed in flowing pure water for about 15 minutes. The thickness of an oxide film formed on the silicon wafer 1 by the above washing is about 2-3Angstrom . Dry oxygen is made to flow at a rate of 3 liter/min in a quartz diffusion tube 3 kept at 800 deg.C by an electric furnace 4 for heating, from a processing gas feeding inlet 3A of one end of the tube 3. After the silicon wafer 1 is inserted into the tube 3 and held for 10 minutes, the wafer is taken out from the tube 3. The thickness of an initial oxide film formed on the silicon wafer 1 at this time is 20Angstrom .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for oxidizing a semiconductor substrate, and more particularly to a method for oxidizing a semiconductor substrate which can improve the uniformity and reproducibility of an oxide film formed on the surface of the semiconductor substrate. .

[0002]

2. Description of the Related Art It is well known that in a MOS transistor, the thickness of a gate oxide film is an important parameter that influences characteristics such as its threshold voltage (V _T ).

In recent years, with the high integration of MOSLSI, the gate oxide film has been made thinner. As a result, making the film thickness uniform has become an important issue in the manufacturing process.

As a method for forming a gate oxide film, (1) the film thickness reproducibility is superior to other methods. (2) The device is simple. For these reasons, the thermal oxidation method in which the semiconductor substrate is heat-treated in an oxidizing atmosphere gas is generally used.

Normally, a gate oxide film such as MOSLSI is formed after completely removing an unnecessary oxide film in the gate formation region on the surface of the semiconductor substrate formed in the process before the gate oxidation with a hydrofluoric acid-based solution. The heat treatment is performed on the semiconductor substrate in an oxidizing atmosphere gas.

[0006]

However, this method has a problem that there is a large variation in the film thickness within the surface of the semiconductor substrate, within the lot, and between lots, and the uniformity and reproducibility are not sufficient.

That is, in the conventional method, (1) the thickness of the oxide film to be formed differs depending on the position of the substrate in the thermal oxidation furnace. (2) Even within a single substrate, the thickness of the oxide film varies. (3) Even if each lot is processed under the same conditions, the thickness will vary.

There are drawbacks such as An object of the present invention is to eliminate the above-mentioned inconveniences and drawbacks of the prior art, and to provide a method for oxidizing a semiconductor substrate with good uniformity and reproducibility of an oxide film.

[0009]

In order to achieve the above-mentioned object, in the present invention, before the semiconductor substrate is heat-treated to form a substantial oxide film, it is uniformly grown at a rate slower than the growth rate of the substantial oxide film. In the method of oxidizing a semiconductor substrate to form an initial oxide film having a uniform thickness, the initial oxide thin film is formed by chemical treatment of the semiconductor substrate with a mixed solution of an oxidizing solution and an alkaline solution. is there.

[0010]

According to the above structure, since the initial oxide film is formed to have a uniform film thickness, variations in the final oxide film thickness are reduced.

[0011]

The present invention will be described in more detail below.

As a result of detailed investigation of the form of film thickness variation,
It was found that the variation in film thickness mainly occurred in the early stage of oxidation. It was also found that the variation in film thickness was greater as the growth rate of oxidation was higher.

It is considered that this is because, as described above, the oxide film on the surface of the semiconductor substrate is completely removed by the hydrofluoric acid-based solution at the initial stage of the oxidation step, and the extremely active surface is exposed. To be

This is because, as is well known, when the surface thus activated is exposed to a high temperature oxidizing atmosphere, the oxidation proceeds at an extremely high rate. Moreover, the initial oxide film formed at this time has extremely large variations.
Then, this variation becomes more remarkable as the oxidation rate is higher.

However, once an oxide film having a thickness of about 20 to 30 angstroms is formed, thereafter, oxidation with relatively little variation proceeds.

Therefore, a uniform oxide film should be formed if there is no variation in film thickness at the initial stage of oxidation. That is, if an oxide thin film having a uniform thickness is formed in advance before the normal oxidation process for forming a substantial oxide film, the final variation in oxide film thickness should be greatly reduced. The present invention has been devised based on the above consideration.

As a method of forming an oxide thin film having a uniform thickness before the oxidation treatment for forming a substantial oxide film, it is considered to reduce the oxidation rate in the range where the film thickness in the initial stage of oxidation varies as described above. That is, before heat-treating a semiconductor substrate in an oxidizing atmosphere gas, (1) a method of heat-treating at a lower temperature than this, or (2) a partial pressure of oxygen or water is lowered as compared with the oxidizing atmosphere gas A method in which heat treatment is performed in an atmosphere gas may be considered. According to these methods, since the growth rate of the initial oxide film growing on the surface of the semiconductor substrate is lower than the growth rate in the step of forming the substantial oxide film, it is necessary to form an initial oxide thin film having a relatively uniform thickness. You can

Another method of forming an initial oxide thin film having a uniform thickness before the oxidation step of forming a substantial oxide film is as follows.
A method is conceivable in which the semiconductor substrate is treated with a hydrofluoric acid-based solution to completely remove the unnecessary oxide film in a desired portion and then immersed in an oxidizing solution.

In this case, a suitable oxidizing solution is
Examples thereof include nitric acid, hydrochloric acid, sulfuric acid, and aqua regia. Further, it may be a mixed solution of these solutions and an alkaline solution (for example, ammonia water). The alkaline solution has an etching effect on silicon, but the mixed solution may be oxidizable as a whole.

As an example, if a silicon substrate is immersed in nitric acid kept at 100 ° C. for 20 minutes, a uniform oxide film of about 20 angstroms grows. If this silicon wafer is heat-treated in an oxidizing atmosphere gas, an oxide film with extremely good uniformity can be formed.

FIG. 1 is a sectional view showing the structure of a thermal oxidation apparatus suitable for carrying out the method of the present invention. A wafer holder 2 is loaded inside the quartz diffusion tube 3, and the wafer holder 2
On top of this, a large number of silicon substrates 1 are placed in parallel at intervals. An electric furnace 4 for heating is arranged on the outer periphery of the quartz diffusion tube 3. Further, the quartz diffusion tube 3 has a processing gas introduction port 3A.

Next, the examples carried out by the present inventors will be specifically described. Experimental Example 1 The semiconductor substrate used in this experiment has a plane orientation (10
0), n-type conductivity, resistivity 10 Ωcm, diameter 76 mm, thickness 500 μm silicon wafer 1 (20 pieces). First, this silicon wafer is immersed in a solution of HF: H ₂ O = 1: 4 for 1 minute to completely remove an unnecessary oxide film formed on the surface, and then washed with running pure water for about 15 minutes. did. The oxide film formed on the silicon wafer by this cleaning had a thickness of 2 to 3 angstroms.

Next, dry oxygen was flowed at a rate of 3 liters / minute into the quartz diffusion tube 3 kept at 800 ° C. by the heating electric furnace 4 from the processing gas inlet 3A at one end of the quartz diffusion tube 3. After the silicon wafer 1 was inserted and held for 10 minutes, it was taken out from the quartz diffusion tube 3. At this time, the initial oxide film formed on the silicon wafer was 20 Å.

Next, as a step of forming a substantial oxide film, 1
Dry oxygen was flowed from the end 3A at a rate of 3 l / min into the quartz diffusion tube kept at 000 ° C., and the silicon wafer on which the initial oxide thin film was formed was inserted and held for 60 minutes. After that, the silicon wafer 1 is taken out from the quartz diffusion tube 3,
The oxidizable film thickness was measured with an ellipsometer.

The thickness of each wafer was measured at one point at the center and four points at the periphery (positions having an angle of 90 ° with each other at a point of 10 mm from the edge of the silicon wafer) for a total of five points.
In this way, all wafers (20 sheets) were measured. Furthermore,
The same experiment was repeated 10 lots, and the results of evaluation of reproducibility are shown by graph A in FIG.

For comparison, heat treatment at 800 ° C. (formation treatment of the initial oxide thin film) is not performed (other conditions are the same as those of the present experimental example), and 10 lots of the conventional method are used. It is shown by graph B in FIG.

Table 1 shows a comparison of film thickness variations between the present experimental example and the conventional method, divided into the wafer surface and the lot.

[0028]

[Table 1] As is clear from the comparison between the graphs A and B in FIG. 2 and Table 1, in the experimental example according to the present invention, the variation in the wafer surface, within the lot, and between lots is larger than that in the conventional method.
It was reduced to 1/2 or less, and the effect of the present invention was confirmed. Experimental Example 2 Next, a second experimental example of the present invention will be described. The semiconductor substrate used is the same as in the first experimental example, and the pretreatment cleaning of the silicon wafer is also the same.

First, the quartz diffusion tube 3 kept at 950 ° C.
Oxygen 0.3 liter / min and nitrogen 2.
After flowing a mixed gas of 7 liters / minute, the silicon wafer 1 was inserted therein and kept for 10 minutes, and then the inflowing gas was switched to only 3 liters / minute of oxygen and kept for 60 minutes.

Then, the silicon wafer 1 was taken out from the quartz diffusion tube 3 and the film thickness was evaluated by the same method as in the first experimental example. Note that the initial oxide film thickness formed on the silicon wafer by the heat treatment for 10 minutes with the mixed gas in the previous period was 40%.
Was ~ 42 Å.

Furthermore, the same experiment was repeated 10 lots, and the reproducibility was evaluated. The result is shown in graph C in FIG. For comparison, heat treatment in a mixed gas of 0.3 liter / min of oxygen and 2.7 liter / min of nitrogen was not performed (other conditions are the same as those of the present experimental example), and 10 lots were also tested by the conventional method. The results are shown in Graph D in FIG.

Table 2 shows a comparison of the film thickness variation between the present experimental example and the conventional example, divided into the wafer surface, the lot, and the lot.

[0033]

[Table 2] As is clear from the comparison between the graphs C and D in FIG. 3 and Table 2, in the experimental example of the present invention, the variation in the wafer surface and between lots was reduced to less than 1/2 in comparison with the conventional method. Therefore, the effect of the present invention was confirmed. Experimental Example 3 Next, a third experimental example of the present invention will be described. The semiconductor substrate used is plane orientation (100), n-type conductivity, and resistivity 2Ω.
It is a silicon wafer (20 sheets) having a cm, a diameter of 100 mm and a thickness of 500 μm.

First, this wafer was subjected to HF: H ₂ O = 1: 4.
It was immersed in the above solution for 1 minute to completely remove the unnecessary oxide film formed on the surface, and then washed with running water for about 15 minutes in pure water. After that, nitric acid (HN
O ₃₎ was immersed 20 minutes in, and washed with running water for about 15 minutes in pure water.

The initial oxide film thickness formed on the silicon wafer by the above process was 19 to 20 angstroms.
The thus cleaned silicon wafer was inserted into a quartz diffusion tube 3 kept at 950 ° C. while flowing a mixed gas of 3 liters / minute of oxygen and 3 liters / minute of nitrogen from one end.
Hold for 00 minutes.

Thereafter, the silicon wafer 1 is attached to the quartz diffusion tube 3
Then, the film thickness of the substantial oxide film was measured in the same manner as in the first experimental example. Furthermore, the same experiment was repeated for 10 lots, and the evaluation results are shown in Graph E in FIG.

For comparison, 10 lots were also tested by the conventional method without leaching in nitric acid (other conditions are the same as those of this experimental example), and the results are shown in graph F in FIG.

Table 3 shows a comparison of film thickness variations between the present experimental example and the conventional method, divided into the wafer surface, the lot, and the lot.

[0039]

[Table 3] As is clear from the comparison between the graphs A and B in FIG. 4 and Table 3, according to this experimental example, the variation is 1/2 in the wafer surface, in the lot, and between lots as compared with the conventional method.
It was reduced to the following, and the effect of the present invention was confirmed. Experimental Example 4 Next, a fourth example of the present invention will be described. The semiconductor substrate used has a plane orientation (100), n-type conductivity, and a resistivity of 8
It is a silicon wafer having an Ωcm, a diameter of 100 mm and a thickness of 500 μm. The pretreatment cleaning of the silicon wafer was performed in the same manner as in the first experimental example.

First, the quartz diffusion tube 3 kept at 950 ° C.
Oxygen 3.0 liters / minute and hydrogen 0.06 liters / minute are made to flow from one end 3A thereof for combustion. The resulting 2.97 l / min oxygen and 0.06 l / min
A silicon wafer was inserted into a mixed gas flow of water vapor (H ₂ O) for 2 minutes and held for 2 minutes.

Thereafter, the flow rate of hydrogen was switched to 1.8 liters / minute while keeping the oxygen at a constant flow rate of 3 liters / minute. (It becomes a mixed gas flow of oxygen of 1 liter / minute) for 10 minutes.

Then, the silicon wafer 1 was taken out from the quartz diffusion tube 3 and the film thickness was evaluated by the same method as in the first experimental example. Further, 10 lots of the same example were repeated and the reproducibility was evaluated. The result is shown as a graph G in FIG.

Further, for comparison, the heat treatment in a mixed air flow of 2.97 liters / min of oxygen and 0.06 liters / min of steam (other conditions are the same as those of this example) was conducted by the conventional method. Also performed 10 lots, and the results are shown in Graph H in Figure 5.
Indicated by.

Table 4 shows a comparison of the film thickness variations between the present experimental example and the conventional method for the wafer surface, the lot, and the lot.

[0045]

[Table 4] As is clear from the comparison between the graphs G and H in FIG. 5 and Table 4, the experimental example of the present invention shows that the variation is 1/2 in the wafer surface, within the lot, and between lots as compared with the conventional method. It was reduced to the following, and the effect of the present invention was confirmed.

Furthermore, according to the invention, a thickness of 20-30
By slowing only the film formation rate until the initial angstrom oxide film is formed, the film thickness of the actual oxide film can be made uniform, so that the film formation time can be shortened and the efficiency of the film formation process can be improved compared to the conventional method. be able to. In addition, it was confirmed that the effect of the present invention was exhibited even when the experiment was conducted under the other conditions described in the claims.

[0047]

As described above, according to the present invention, since there is no variation in the film thickness in the initial stage of growth of the oxide film, an oxide film having an extremely uniform structure and excellent reproducibility can be formed, which in turn results in the characteristics of MOSLSI. The manufacturing yield, and the manufacturing efficiency can be significantly improved.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of a thermal oxidation device suitable for carrying out the method of the present invention.

FIG. 2 is a diagram showing a variation situation of an oxide film thickness manufactured by the method of the present invention in comparison with a variation situation of a conventional method.

FIG. 3 is a diagram showing a variation situation of an oxide film thickness manufactured by the method of the present invention in comparison with a variation situation of a conventional method.

FIG. 4 is a diagram showing a variation situation of an oxide film thickness manufactured by the method of the present invention in comparison with a variation situation of a conventional method.

FIG. 5 is a diagram showing a variation situation of an oxide film thickness produced by the method of the present invention in comparison with a variation situation of a conventional method.

[Explanation of symbols]

1 ... Silicon substrate, 2 ... Wafer holder, 3 ... Quartz diffusion tube, 4 ... Electric furnace for heating, 3A ... Processing gas inlet

Claims (2)

[Claims] 1. A semiconductor substrate at a rate slower than a growth rate of an oxide film in the step of substantially oxidizing the semiconductor substrate before the step of heat-treating the semiconductor substrate in an oxidizing atmosphere gas to form a substantial oxide film. In the method of oxidizing a semiconductor substrate having a step of forming an initial oxide film having a substantially uniform thickness in the step of forming an initial oxide thin film on the semiconductor substrate, the step of forming an initial oxide thin film on the semiconductor substrate is performed with an oxidizing solution and an alkaline solution. A method for oxidizing a semiconductor substrate, which comprises a step of chemical treatment with a mixed solution. 2. The method for oxidizing a semiconductor substrate according to claim 1, wherein the alkaline solution is aqueous ammonia.