JPS59227128A - Oxidation method for semiconductor substrate - Google Patents

Abstract

PURPOSE:To enable the attainment of an oxide film being uniform and having excellent repeatability by a method wherein a process of forming a thin initial oxide film of uniform thickness on a semiconductor substrate is provided before a process of forming a substantial oxide film on the semiconductor substrate. CONSTITUTION:After an unnecessary oxide film formed on the surface of an Si wafer 1 is removed completely, said surface is washed by flowing water. Next, dry oxygen is let to flow from a gas inlet port 3A into a quartz diffusion tube 3 heated by a heating furnace 4, the wafer 1 is inserted into the tube and held for a prescribed time, and thereby a thin initial oxide film of uniform thickness is formed on the wafer 1. Then, as a process of forming a substantial oxide film, dry oxygen is let to flow from one end 3A into the tube 3 maintained at a higher temperature than in the process in which the initial oxide film is formed, and then the wafer 1 with the initial oxide film formed thereon is inserted into the tube and held therein for a prescribed time. According to this oxidation method, the non-uniformity of the thickness of the oxide film in an initial period of growth thereof is eliminated, and thus an oxide film having very excellent uniformity and repeatability can be formed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Application) The present invention relates to a method for oxidizing a semiconductor substrate, and in particular, to a method for oxidizing a semiconductor substrate, which can improve the uniformity and reproducibility of the oxide film thickness formed on the surface of the semiconductor substrate. Concerning a method of oxidizing a substrate.

(Background) In a MOS transistor, the thickness of the wax and gate oxide film influences its threshold voltage (vl) and other characteristics II! It is well known that this is a very important parameter.

In recent years, as MOS LSIs have become more highly integrated, the gate oxide film has become thinner. As a result, uniformity of film thickness has become an important filli in the manufacturing process.

The method for forming the gate oxide film has two advantages: (1) superior reproducibility of film thickness compared to other methods; (2) simple equipment; A thermal oxidation method in which a semiconductor substrate is heat-treated in a gas is generally used.

Normally, when forming a gate shading film for MOS LSI, etc., the unnecessary oxide film in the gate formation region on the surface of the semiconductor substrate, which was formed in the process before gate oxidation, is completely removed using a hydrofluoric acid solution. This is carried out by heat-treating the semiconductor substrate in an oxidizing atmospheric gas.

However, this method has the problem that the film thickness varies greatly within the plane of the semiconductor substrate, within a lot, and between lots, resulting in insufficient uniformity and reproducibility. That is, in the above method: (1) The thickness of the oxide film formed differs depending on where in the thermal oxidation furnace the substrate is removed.

(2) Even within a single substrate, variations occur in the thickness of the oxide film.

(3) Even if each loft is processed under the same conditions, the thickness will vary.

There were drawbacks such as.

(Objective) Therefore, it is an object of the present invention to provide a method for oxidizing a semiconductor substrate, which eliminates the above-mentioned disadvantages and drawbacks and provides a good uniformity and reproducibility of an oxide film.

(Generally speaking, the present invention is characterized by the fact that, before the step of heat-treating the semiconductor substrate in an oxidizing atmosphere gas to form a substantial oxide film, a semiconductor substrate having a uniform thickness is formed.
The reason is that a process is provided to form an extremely thin oxide film.

(Example) The present invention will be explained in more detail below.

As a result of a detailed investigation of the form of film thickness fluctuation, it was found that film thickness fluctuation mainly occurs at the initial stage of oxidation. It was also found that the variation in film thickness increases as the oxidation growth rate increases.

The reason for this is that, as mentioned above, at the beginning of the oxidation process, the oxide film on the surface of the semiconductor substrate is completely removed with a heptazate-based solution.
This is thought to be because the extremely active surface remains exposed.

Because d, the table WIh thus activated.

As is well known, when exposed to a high temperature oxidizing atmosphere,
Oxidation proceeds at an extremely rapid rate. Moreover, the initial oxide film formed at this time has extremely large fluctuations. This variation becomes more noticeable when the oxidation rate is high.

However, once an oxide film of about 20 to 30 λ is formed, oxidation progresses with relatively little variation.

Therefore, a uniform oxide film should be able to be formed unless variations in film thickness occur at the early stage of fermentation. That is, if a thin oxide film with a uniform thickness is formed in advance before a normal oxidation process for forming a substantial oxide film, it should be possible to greatly reduce variations in the final oxide film thickness.

This invention was devised by the present inventor based on the above considerations.

Before the oxidation treatment to form a substantial oxide film, one possible method for forming a dispersed thin film with a uniform thickness is to reduce the oxidation rate in the range where the film thickness varies at the initial stage of oxidation, as described above. That is, before heat-treating a semiconductor substrate in an oxidizing atmospheric gas, (1) a method of heat-treating at a much lower temperature, or (2) a method of lowering the partial pressure of oxygen or moisture than the aforementioned oxidizing atmospheric gas. Possible methods include heat treatment in a heated atmosphere gas.

With Nire et al.'s method, the growth rate of the initial oxide film that grows on the surface of the semiconductor substrate is smaller than the growth rate in the process of forming a substantial oxide film, so it is possible to form an initial oxide thin film with a relatively uniform thickness. can be formed.

Another method for forming an initial thin oxide film with a uniform thickness before the oxidation process to form a substantial oxide film is to treat the semiconductor substrate with a heptazate-based solution to completely remove unnecessary oxide films from desired areas. One possible method is to immerse it in an oxidizing solution after removing it.

Suitable oxidizing solutions in this case include nitric acid, hydrochloric acid,
Examples include sulfuric acid or aqua regia. Alternatively, a mixed solution of these solutions and an alkaline solution (for example, 1 ammonia water, etc.) may be used. Although the alkaline solution has an etching effect on silicon, it is sufficient if the mixed solution as a whole is oxidizing.

For example, if a silicon substrate is immersed in nitric acid maintained at 100° C. for 20 minutes, a uniform oxide film of about 20'A will grow. By heating the silicon urchin in the oxidizing atmospheric gas E1, an oxide film with extremely good uniformity can be formed.

1st Ii! , l are cross-sectional views 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,
A large number of silicon substrates 1 are placed on the wafer holder 2 in parallel and spaced apart from each other. A heating electric furnace 4 is arranged around the outer periphery of the quartz diffusion tube 3 .

Furthermore, the quartz diffusion tube 3 has a treatment scum inlet 3A.

Next, an actual example conducted by the present inventors will be specifically explained.

Experimental Example 1 The conductor substrates used in this experiment were silicon wafers (20 wafers) having a surface orientation of 10 mm, n-type conductivity, resistivity of 10 flcrn, diameter of 76 mm, and thickness of 500 μm.

First, convert this silicon cube into HF:H,O=1:
After being immersed in the solution of No. 4 for 1 minute to completely remove the unnecessary oxide film formed on the surface, it was washed in running pure water for about 15 minutes. Note that the oxide film formed on the silicon wafer by this cleaning had a thickness of 2 to 3^.

Next, the dry WR element is poured 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 at a rate of 31/min, and the silicone WR element is poured into the quartz diffusion tube 3 kept at 800° C. After inserting and holding for 10 minutes, insert the quartz diffusion tube 3
I took it out from inside. At this time, the initial oxide film formed on the silicon cube was 20A.

Next, as a step to form a substantial oxide film, 3t15 of dry oxygen is introduced into the quartz diffusion tube maintained at 100°C from 3A at one end.
A silicon wafer having an initial oxidized film formed thereon was inserted and held for 60 minutes. Thereafter, the silicon wafer 1 was taken out from the quartz diffusion tube 3, and the oxide film thickness was measured using an ellipsometer. 5 positions at an angle of 90 to each other)
It is a point. All wafers (20 wafers) were measured in this manner. Furthermore, the same experiment was repeated for 10 lots and the reproducibility was evaluated. The results are shown in FIG. 2AK.

For comparison, we also conducted 10 lots of experiments using the conventional method without heat treatment at 800°C (initial oxide thin film formation treatment) (other conditions were the same as in this experiment), and the results were reported in the second @
Shown in B.

Table 1 shows a comparison of the film thickness variations between this experimental example and the conventional method.
As is clear from the comparison of B and Table 1, in the experimental example according to the present invention, the variation within the wafer surface, within the Plot, and between lots was reduced to less than i compared to the conventional method. The effectiveness of the invention was confirmed.

Example Next 1 (The 20th example of the present invention will be explained.

The semiconductor substrate used was the same as in the first experimental example,
The pretreatment cleaning of silicon wafers is also the same.

First, one end of the 3
A mixed gas of 0.611 min of oxygen and 2.71 min of nitrogen was flowed through A, and after inserting the silicon wafer 1 into this and holding it for 10 minutes, the gas was switched to only oxygen 6115;+ for 60 minutes. held.

Thereafter, the silicon wafer 1 was taken out from inside the quartz diffusion tube 6, and the film thickness was evaluated in the same manner as in the first experimental example.

In addition, the initial oxide film thickness generated on the silicon wafer by thermoelectric treatment for 10 minutes using the mixed gas # is 40 to 42λ.
Met.

Furthermore, the same experiment was repeated in 10 lots and the reproducibility was evaluated. The results are shown in FIG. 3C. For comparison, we also conducted 10 lots of experiments using the conventional method without heat treatment in a gas mixture of 0.317 minutes of oxygen and 2.7179 hours of nitrogen (the issue of ponds is the same as this experiment example). is shown in the third diagram.

Note that Table 2 shows a comparison of film thickness variations between this experimental example and the conventional method, divided into within the wafer plane, within a lot, and between lots.

Table 2 As is clear from the comparison between Figures C and D and Table 2, in the experimental example of the present invention, the variation within the wafer plane, within the funnel, and between lots was significantly reduced compared to the conventional method. The effect of the present invention was confirmed.

Example Next, a third experimental example of the present invention will be explained.

The semiconductor substrate used had a plane orientation (100), nm conductivity,
These are silicon wafers (20 pieces) with a resistivity of 2Ω, a diameter of 100 wm, and a thickness of 500 μm.

First, this wafer was immersed in a solution of HF:HtO=1:4 for 1 minute to completely remove an unnecessary oxide film formed on the surface, and then washed in running pure water for about 15 minutes.

Furthermore, after that, it was immersed in nitric acid (HNO,) maintained at 100° C. for 20 minutes, and then washed in running pure water for about 15 minutes.

The initial oxide film thickness produced on the silicon wafer by the above process was 19-20 degrees. The thus cleaned silicon wafer was inserted into a quartz diffusion chamber kept at 950° C. in which a mixed gas of 317 minutes of oxygen and 311 minutes of S1 was flowed from one end, and held for 100 minutes.

Thereafter, the silicon wafer 1 was taken out from the quartz diffusion tube 3, and the substantial thickness of the 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 FIG. 4E.

Also for comparison, the process of immersion in nitric acid is not carried out (
Other conditions were the same as in this experiment) Experiments using the conventional method were also conducted in 10 lots, and the results are shown in FIG. 4F.

Furthermore, Table 3 shows a comparison of film thickness variations between this experimental example and the conventional method, divided into within the wafer plane, within a lot, and between lots. Table 3 As is clear from the comparison between Figures A and B in Figure 4 and Table 3, this experimental example reduced the deviation within the wafer surface, within a lot, and between lots to below A compared to the conventional method. Finally, the effects of the present invention were confirmed.

Example Next, a fourth bag test example of the present invention will be explained.

The semiconductor substrate used had a plane orientation (100), n-type conductivity,
Chilled with a silicon wafer with a resistivity of 8Ω, a diameter of 100m, and a thickness of 500μm. The pretreatment cleaning of the silicon wafer was performed in the same manner as in the first experimental example.

First, oxygen 3. ot/l) and 0.061 ml of hydrogen to be combusted. A silicon wafer was inserted into the resulting mixed gas flow of 2.97 ty% oxygen and 0.061/fj- water vapor (HtO) and held for 2 minutes.

After that, while maintaining the oxygen at a constant flow rate of 31791,
The hydrogen flow rate is changed to 1.81z%) (in this case, due to the combustion of hydrogen, 1.8j~) water vapor and 2.11. 10 volumes were held (resulting in a mixed air flow of different oxygen).

After that, the silicon wafer l was taken out from the quartz diffusion tube 3, and the film thickness was evaluated using the same method as in the first experimental example.
The same experiment was repeated 1013 times, and the results of the evaluation of palpitations are shown in Figure 5G.

tta ratioK (') tameK flJ! In 2.97
175> Heat treatment is not performed in a mixed air flow of water RsfAO, oet/min (other conditions are the present Example M and 1141
-) Experiments using the conventional method were also conducted for 10 times (1 unit), and the results were shown on ffi51MH.

In addition, Figure 4 shows a comparison of the M thickness variation between this experimental example and the conventional method. For example, compared to the conventional method, the variation within a wafer, within a lot, and between lots has been reduced to below A.
The effects of the present invention have been confirmed 1. Furthermore, it has been confirmed that the effects of the present invention can be achieved even when experiments are conducted under other conditions described in the claims.

(Effects) As explained above, according to the present invention, film thickness variations in the early stage of oxide film growth are eliminated, so an oxide film with extremely good uniformity and reproducibility can be formed, and as a result, M
The characteristics and manufacturing yield of OS LSI etc. can be significantly improved.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the construction of a thermal oxidation apparatus suitable for carrying out the method of the present invention, and FIGS. 2, 3, 4, and 5 show oxidation FIG. 3 is a diagram showing the state of film thickness variation in comparison with the state of flaking caused by a conventional method. . 1... Silicon ML 2... Ueno 1 holder, 3
...Stone collar diffusion tube, 4...Electric furnace for heating, 3A...
Processing gas inlet representative Patent attorney Michito Hiraki Figure 1 Figure 2 Figure 3 Figure oyL4 Lot number 1S 4 Agony 1 2 3 4 5 6 7 8 9 10 Lot number Figure 5 1 2 3 4 5 6 7 8 9 10 units/site number

Claims (1)

[Scope of Claims] (1) In a method of oxidizing a semiconductor substrate, which is performed by heat-treating a semiconductor substrate in an oxidizing atmospheric gas, the semiconductor substrate is heated in the oxidizing atmospheric gas to form a substantial oxide film. A method for oxidizing a semiconductor substrate, comprising, before the step of forming, a step of forming an initial oxide thin film with a uniform thickness on the semiconductor substrate at a rate slower than the oxidation rate in the step of substantially oxidizing the semiconductor substrate. . (2. In Claim 1, the oxidation of a semiconductor substrate is characterized in that the step of forming an initial oxide thin film on the semiconductor substrate is performed at a temperature lower than the temperature of the step of substantially oxidizing the semiconductor substrate. (3) In claim 1, it is provided that the step of forming an initial oxide thin film on the semiconductor substrate is carried out under conditions with a lower oxygen partial pressure than the atmosphere of the step of substantially oxidizing the semiconductor substrate. A method for oxidizing a semiconductor substrate characterized by: (4) According to claim 1, the step of forming an initial oxide thin film on the semiconductor substrate is performed by an oxidizing chemical treatment. (5) A method for oxidizing a semiconductor substrate. (5) In claim 1, the step of forming an initial oxidized thin film on the semiconductor substrate is performed under conditions where the water vapor partial pressure is lower than the atmosphere of the step of substantially oxidizing the semiconductor substrate. A method of oxidizing a semiconductor substrate, characterized in that it is carried out.