JP2632293B2 - Selective removal method of silicon native oxide film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon wafer formed on a surface of a silicon wafer, a surface of a polysilicon film, or a surface of an amorphous silicon film (hereinafter, these surfaces are collectively referred to as "silicon layer surface"). The present invention relates to a method for selectively removing an oxide film.

[0002]

2. Description of the Related Art In a semiconductor device manufacturing process,
Various efforts have been made to clean wafers at each stage of the manufacturing process, as it is possible that various types of contamination may occur that adversely affect the operating characteristics of the device.

[0003] A natural oxide film (SiO ₂ ) formed on the surface of a silicon wafer is also considered to be one of the contaminants on the wafer. However, this natural oxide film only leaves the silicon wafer in the air. But the surface of the wafer
It is easily formed to a thickness of 20 °, and is formed secondarily in various cleaning / etching steps in a semiconductor device manufacturing process.

Here, for example, it is known that the electrical characteristics of a thin gate oxide film are greatly affected by the pretreatment of a silicon wafer. Therefore, when a thin oxide film such as a gate oxide film is formed on a silicon wafer in a semiconductor device manufacturing process, it is necessary to remove the natural oxide film in advance. Also, if a natural oxide film remains on the base on which electrodes such as a source and a drain are to be formed, a normal electrode function cannot be obtained, and a contact resistance is reduced when a metal electrode is formed. In addition, the natural oxide film must be completely removed from the silicon wafer surface. Furthermore, when performing epitaxial growth of silicon, it is necessary to remove the natural oxide film on the substrate surface in advance.

[0005] As described above, the natural oxide film formed on the surface of the silicon wafer is used in a semiconductor device manufacturing process.
In particular, before film formation by CVD, sputtering, or the like, it must be removed from the wafer surface.

As a method of removing a silicon oxide film from the surface of a silicon wafer, various methods have been conventionally proposed and implemented, but recently, hydrogen fluoride (H) has been used.
F) A method of cleaning the surface of a silicon wafer by a gas phase method using gas has been studied. For example, "7th Super LS
As disclosed in I Ultra Clean Technology Symposium Proceedings "Submicron ULSI Process Technology", Realize, Inc., 1988-7, pp. 173-194, anhydrous hydrogen fluoride gas with a very small amount of water was converted to nitrogen. Alternatively, argon gas is sent into the reaction chamber as a carrier gas, and Si on the silicon wafer is
A method of removing a silicon oxide film by reacting O ₂ and hydrogen fluoride,
As disclosed in Japanese Patent No. 502930, a method has been proposed in which an anhydrous hydrogen fluoride gas is supplied together with water vapor, and the silicon oxide film is removed by exposing the surface of a silicon wafer to them.

[0007]

By the way, anhydrous hydrogen fluoride in a gaseous state, particularly at a temperature of 80 ° C. or less,
Forms a 2-6 mers, but is extremely low reactivity with SiO _2, the moisture coexist, hydrogen fluoride fluorine ions F and ionic dissociation ^- generate, the F ^- ions silicon oxide The film acts on the film and the etching reaction of SiO ₂ proceeds. Thus, the moisture is S
Although it plays an important role in the etching reaction of iO ₂ , etching of a silicon oxide film with anhydrous hydrogen fluoride in an atmosphere with a very small amount of water as described above is technically difficult in controlling the reaction and the like. There is a side. Moreover, the reaction between hydrogen fluoride and silicon dioxide generates a large amount of water as an auxiliary component as shown in Chemical formula 1.

[0008]

Embedded image 4HF + SiO ₂ → SiF ₄ + 2H ₂ O

For this reason, in the method of removing a silicon oxide film using an anhydrous hydrogen fluoride gas in an atmosphere having an extremely low moisture content, it is very difficult to control the reaction, and it is difficult to obtain reproducibility in the process. And so on. Furthermore, in the method described in the above-mentioned proceedings “Submicron ULSI process technology”, it is necessary to obtain, for example, an anhydrous hydrogen fluoride diluent gas having a water content of about 0.01 ppm. In order to make the amount of water extremely small, there is a problem that the water must be removed with great effort.

On the other hand, in the method of removing a silicon oxide film by adding water vapor to anhydrous hydrogen fluoride gas,
An undesired side reaction proceeds as shown in FIG.

[0011]

Embedded image SiO ₂ + 2H ₂ O → Si (OH) ₄

Alternatively, as shown in Chemical formula ₃ , a reaction opposite to the etching occurs, and colloidal metasilicate H ₂ SiO ₃ and silicon dioxide SiO ₂ adhere and remain on the wafer surface, causing new contamination. Problem.

[0013]

Embedded image _{2SiF 4 + 2H 2 O → SiO} 2 + SiF 6 2- + 2H
⁺ 2HF

In the semiconductor manufacturing process, various film forming processes are performed on the surface of a silicon wafer. On the silicon wafer, for example, a silicon thermal oxide film or a silicon nitride film is formed in addition to the silicon natural oxide film. Since the insulating film is also formed, there is a problem that even when it is desired to remove only the silicon native oxide film, the insulating film formed on the wafer is also removed at that time. is there.

According to the present invention, there is provided the above-mentioned prior art 2 in which the silicon natural oxide film is removed using anhydrous hydrogen fluoride gas.
It is an object of the present invention to provide a method that can solve the respective problems of the two methods together and that can selectively remove only a silicon native oxide film formed on the surface of a silicon layer. .

[0016]

According to the method for selectively removing a silicon native oxide film according to the present invention, a substrate is housed in a container which is hermetically isolated from the outside air, and anhydrous hydrogen fluoride and methanol are placed in the container. An alcohol such as ethanol or isopropyl alcohol is supplied at a concentration of 4% or less of anhydrous hydrogen fluoride, and the surface of the silicon layer of the substrate is exposed to them to selectively remove a natural oxide film. is there.

[0017]

[Function] When anhydrous hydrogen fluoride and alcohol are supplied to the surface of the silicon layer in a container which is airtightly isolated from the outside air,
By the reaction shown in Chemical formula 4, silicon dioxide (SiO ₂ ) is removed from the surface of the silicon layer.

[0018]

Embedded image SiO ₂ (s) + 4HF (g) → SiF ₄ (g) + 2H
₂ O (g)

In this case, in the method according to the present invention, an alcohol such as methanol, ethanol, or isopropyl alcohol is supplied together with the hydrogen fluoride, and the hydrogen fluoride is ion-dissociated into F ⁻ , and the F ⁻ ion is converted into SiO 2.
₂ effectively acts, and the above reaction proceeds promptly, and the ability to remove SiO ₂ is enhanced.

In addition, since alcohols such as methanol and ethanol, which have particularly low molecular weights, and water are infinitely soluble, H ₂ O produced as a by-product by the above-mentioned reaction is dissolved in the alcohol, and the water content is reduced. It is efficiently taken out of the reaction system and removed. Further, unlike the method described in the above-mentioned proceedings "Submicron ULSI process technology", the moisture in the atmosphere is removed, and therefore, such unfavorable results as shown in the above chemical formulas 2 and 3 due to water are removed. There is no such thing as a side reaction or a reverse reaction of etching which causes contamination. Of course, even under an atmosphere from which water has been removed, the above reaction proceeds without any problem.

On the other hand, it is known that the growth of a native oxide film requires the presence of both oxygen and moisture (for example, see IEICE Technical Report Vol. 89 No. 11).
1: IEICE Technical Report P11-12, “Control of Si Natural Oxide Film Formation”, IEICE 198
However, in the method according to the present invention, the amount of water contained in the hydrogen fluoride and the alcohol supplied into the container is kept low, so that the method for cleaning the silicon wafer is performed. Even if alcohol is used together with anhydrous hydrogen fluoride, the surface of the silicon wafer after the SiO ₂ is removed by etching as in the case where the silicon oxide film is removed by adding water vapor to anhydrous hydrogen fluoride gas. Re-oxidation by the adsorbed moisture and oxygen in the atmosphere hardly occurs.

Further, since alcohol is used for cleaning the silicon wafer, at the stage where the removal of SiO ₂ from the wafer surface is completed, the alcohol remains so as to cover the surface of the silicon wafer in a layered manner. However, since the alcohol itself easily creates a reducing atmosphere and has an extremely low oxidizing ability, even if the silicon surface is exposed after cleaning, the surface is protected by the alcohol and reoxidation is prevented.

The alcohol layer formed so as to cover the surface of the silicon wafer can be easily detached from the surface of the silicon wafer by evacuation, irradiation with ultraviolet light, heating, or the like, which makes post-processing difficult. No.

By setting the concentration of the anhydrous hydrogen fluoride gas in the total gas supplied to the substrate to 4% or less,
At this concentration, for the silicon native oxide film,
Although the etching action as shown in Chemical formula 1 is not hindered so much, the etching reaction of other insulating films such as a silicon thermal oxide film and a silicon nitride film proceeds only slightly. Although the reason for the difference in the reactivity between the natural oxide film and the insulating film is not clearly understood, the natural oxide film can be selectively removed by utilizing the difference in the reactivity.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below.

FIG. 1 is a schematic configuration diagram showing an example of a silicon wafer cleaning apparatus for carrying out the method of the present invention. In the figure, a container 12 in which a silicon wafer 10 to be cleaned is stored is formed using Teflon,
The inside is hermetically isolated from the outside air. This container 12
Has a supply port 14 and an exhaust port 16 connected to a gas / steam supply pipe 20 and an exhaust pipe 22, respectively.
Further, a bypass pipe 18 is provided. Although not shown in FIG. 1, a door for loading and unloading the silicon wafer is provided on the side of the container 12, and a load lock chamber is provided adjacent to the door, and other doors are provided. It is also possible to adopt a configuration in which the process apparatus and the transfer line are connected to be inlined.

A supply line communicating with the supply port 14 of the container 12
At 20, three pipes 24, 26 and 28 join, and each pipe
At the ends of 24, 26, and 28, a supply source 30 of anhydrous hydrogen fluoride, a supply source 32 of alcohol, and a supply source 34 of nitrogen gas as a carrier gas are provided, respectively. In addition, branch lines 36 and 38 of a carrier gas line 28 communicate with the supply lines 24 and 26 of the anhydrous hydrogen fluoride gas and the alcohol vapor, respectively.

A mass flow controller 40, 42, 44 is inserted in each of the pipes 24, 26, 28, respectively, and a temperature controller 46, 48 are provided respectively. The supply amounts of the anhydrous hydrogen fluoride gas and the carrier gas (nitrogen) are adjusted by the mass flow controllers 40 and 44 provided in the supply pipes 24 and 28, respectively. The supply amount of the alcohol vapor is adjusted by controlling the flow rate of nitrogen as a carrier gas and the temperature of the alcohol (liquid) (accordingly, the alcohol vapor pressure). Further, in order to prevent dew condensation from occurring in each supply system of the anhydrous hydrogen fluoride gas and the alcohol vapor, the respective supply pipes 24 and 26 are provided with heat insulating materials 50 and
Insulated by 52.

Next, a description will be given of an experimental example relating to the present invention, which was performed using the above-described cleaning apparatus. In this experiment, a phosphorus-doped n-type silicon wafer having a resistivity of 2 to 8 Ω · cm was used. In the experiments, ultra-high-purity anhydrous hydrogen fluoride manufactured by Showa Denko KK, ultra-high-purity nitrogen manufactured by Nippon Sanso Corporation, and EL grade manufactured by Kanto Chemical Co., Ltd. were used as cleaning gases and reagents. Methanol, ethanol and isopropyl alcohol (IPA) were used. The water content of each gas / reagent is 55 pp for anhydrous hydrogen fluoride.
m (purity of anhydrous hydrogen fluoride is 99.9925%), nitrogen is on the order of ppb (dew point is -70 ° C or less), and methanol is 500 ppm or less. Then, the silicon wafer 10 is loaded and placed in the container 12 of the apparatus shown in FIG. 1, the inside of the container 12 is completely sealed, and then the supply pipes 28, 20 are supplied from the nitrogen gas supply source 34. , High-purity N ₂ gas was fed into the vessel 12 at a flow rate of 15 l / min, and the inside of the vessel 12 was purged for 30 seconds. Then, the temperature of the silicon wafer 10 was adjusted in the range of room temperature to 40 ° C. Under the above conditions, anhydrous hydrogen fluoride gas, methanol (or ethanol, isopropyl alcohol) vapor and nitrogen gas are supplied into the container 12 to perform an etching process, and after the process is completed, the inside of the container 12 is purged with N ₂ gas for 30 seconds. did. Then, when the concentration of the anhydrous hydrogen fluoride gas in the total gas (CH ₃ OH vapor + N ₂ gas for bubbling CH ₃ OH + anhydrous hydrogen fluoride gas + N ₂ carrier gas) was changed, FIGS. The results shown were obtained. The flow rate of all gases is 7 liters per minute, and the temperature is 25 ° C. Also,
An ellipsometer was used to measure the thickness of the oxide film.

FIG. 2 shows the etching rate on the thermal oxide film of silicon, in which the vertical axis shows the etching rate and the horizontal axis shows the concentration of the anhydrous hydrogen fluoride gas. shows the difference in CH ₃ OH N ₂ gas flow rate for bubbling (corresponding to the flow rate of CH ₃ OH since the vapor pressure of CH ₃ OH are held constant). Of these, the flow rate of the N ₂ gas for bubbling was 1,000 cc /
In the case of min, Δ is 1,500 cc / min, □ is 3,000 cc / min, ◎ is 4,500 cc / mi
The case of n is shown.

FIG. 3 shows the flow rate of N ₂ gas for bubbling CH ₃ OH on the vertical axis and the concentration of anhydrous hydrogen fluoride gas on the horizontal axis with respect to etching of a natural silicon oxide film. In the figure, × indicates the possibility of etching, and the case where the silicon native oxide film could be reduced by at least 7Å or more by etching for 15 seconds was indicated by 、, and the film thickness could be reduced only to 7Å or less. The case where there was no is indicated by x.

The following is clear from FIG. 2 and FIG.

For example, when the concentration of anhydrous hydrogen fluoride gas is 2 vol.
l /%, bubbling N ₂ gas flow rate 3,000 cc / mi
When etching is performed under the condition of n, as shown in FIG. 2, the etching rate of the thermal oxide film of silicon is 4 ° /
min or less, it is 4 × (15/6) for 15 seconds.
0) = 1 ° or less. In the etching of the native oxide film under the same conditions, as can be seen from FIG. 3, complete etching is performed in 15 seconds (circle in the figure, and the film thickness is reduced by at least 7 mm as described above). You. That is, a selectivity of at least 1: 7 = 1/7 is observed between the thermal oxide film and the natural oxide film.

As described above, in the low concentration range of the anhydrous hydrogen fluoride gas, the natural oxide film is more easily etched than the thermal oxide film of silicon. For example, when the concentration of the anhydrous hydrogen fluoride gas is 4 vol /% or less, the selectivity of etching with respect to the natural oxide film is high.

Having the etching selectivity in the low-concentration region of the anhydrous hydrogen fluoride gas means that the region where the thermal oxide film is formed on the wafer, such as a process of removing a natural oxide film formed in a contact hole, is used. When a region where a natural oxide film is formed is mixed and the natural oxide film is to be removed, the natural oxide film can be efficiently removed without damaging the thermal oxide film as much as possible.

FIG. 4 is a schematic diagram showing an apparatus suitable for carrying out this method, showing another example different from that shown in FIG. In the device shown in FIG.
A nitrogen gas supply source 56 is connected via a mass flow controller 58 to a pipe 54 that is connected to the alcohol storage tank of the alcohol supply source 32 so that the anhydrous hydrogen fluoride gas and methanol vapor can be timed. It can be supplied to the container 12 by mistake, other configurations are
This is similar to the device shown in FIG.

According to the apparatus shown in FIG. 4, as shown in the time chart of FIG. 5A, the supply of the anhydrous hydrogen fluoride gas and the supply of the methanol vapor are simultaneously started, and the supply is simultaneously stopped. In addition to the cleaning method shown in FIG.
As shown in (2), after the silicon wafer 10 is accommodated in the container 12 and purged with N ₂ gas, first, methanol vapor is introduced into the container 12, and the methanol wafer is introduced for a predetermined time (for example, 10 to 60 seconds).
After the passage of time, the supply of anhydrous hydrogen fluoride gas is started, the anhydrous hydrogen fluoride gas and methanol vapor are simultaneously supplied for a predetermined time to perform a cleaning process, and then the anhydrous hydrogen fluoride gas and methanol vapor are simultaneously supplied. It can be stopped. In this way, the silicon wafer
If the entire surface of the silicon wafer 10 is covered with methanol vapor before etching with anhydrous hydrogen fluoride gas, the affinity between the oxide film on the silicon wafer 10 surface and anhydrous hydrogen fluoride increases, and the etching becomes more uniform. Progressed to. In addition, even when a minute amount of organic substance contamination is present on the surface of the silicon wafer, the methanol vapor removes the organic substance from the wafer surface and discharges the organic substance by the methanol beforehand. The properties could be further improved.

Further, as shown in FIG. 5C, the supply of the anhydrous hydrogen fluoride gas and the supply of the methanol vapor were simultaneously started, the washing was performed, and the supply of the anhydrous hydrogen fluoride gas was stopped. Thereafter, a predetermined time (for example, 10 to 30)
Second) It is also possible to supply only methanol vapor and then stop the supply. By doing so, it was confirmed from the result of ESCA measurement that the remaining fluorine concentration was lower.

Further, as shown in FIG.
When the processing is performed in the order of starting supply of methanol vapor into the inside, starting supply of anhydrous hydrogen fluoride gas, stopping supply of anhydrous hydrogen fluoride gas, and stopping supply of methanol vapor, (2) and (3) in FIG. As a result, a stable cleaning process having the above-described features in the case of the supply method shown in FIG.

The method according to the present invention is as described above. The method for cleaning a silicon wafer before forming a thermal oxide film, an electrode, epitaxial growth, silicidation or the like by a CVD method, sputtering, or the like. When this method is applied, the natural oxide film on the wafer surface can be efficiently removed, which is effective in improving the interface characteristics of each.

Although this method can exert its effect sufficiently by itself, it may be carried out in combination with another cleaning method. It is also effective to remove the natural oxide film formed on the surface of the silicon wafer by using this method after performing cleaning for removing contamination other than the oxide film, that is, organic substances, metals, particles, and the like. .

Further, as an example in which the cleaning process according to this method is used, the process after the cleaning proceeds extremely well, for example, there is selective CVD of tungsten. That is, when the method according to the present invention is performed as a pretreatment of a substrate, good selectivity can be obtained over the entire surface of the wafer when tungsten selective CVD is subsequently performed.

In the above embodiment, the anhydrous hydrogen fluoride and the alcohol are supplied in a vapor state into the container accommodating the silicon wafer, but they are supplied in the form of mist (fine particle droplets). It is also possible to spray on the surface of the silicon wafer or to carry out as a steam bath. Furthermore, it is also possible to carry out by using a mixed solution of hydrogen fluoride and alcohol by a conventional wet cleaning technique that is usually performed.

Further, the above embodiment relates to the removal of the natural silicon oxide film on the surface of the silicon wafer with the vapor of anhydrous hydrogen fluoride and methanol.
The invention is not so limited, as described below.

That is, alcohol such as ethanol may be used instead of methanol vapor. However, the alcohol is anhydrous as shown in the above examples.

Further, the present invention is not limited to the case where the present invention is applied to the removal of a silicon native oxide film on the surface of a silicon wafer, but may be applied to the removal of a silicon native oxide film formed on the surface of a polysilicon film or an amorphous silicon film. Incidentally, such a polysilicon film or an amorphous silicon film is not limited to a film formed on a silicon wafer, and may be, for example, on various semiconductor wafers such as a gallium / arsenic wafer, or on a glass substrate or a ceramic substrate. May be formed on various substrates. As described above, the “silicon layer surface” in the configuration of the present invention includes not only the surface of a silicon wafer but also the surface of a polysilicon film or an amorphous silicon film formed on various substrates.

[0047]

Since the present invention is constructed and operates as described above, when removing the silicon native oxide film on the silicon layer by the method according to the present invention,
The formation of the silicon natural oxide film on the surface of the silicon layer can be effectively prevented without impairing the prompt removal of the silicon natural oxide film. Therefore, the natural oxide film can be completely removed from the surface of the silicon wafer. In addition, water, which is a by-product of the reaction in removing the natural oxide film, can be dissolved in alcohol and removed together with the system, and even if a small amount of water is present in the reaction system, the water can be removed. Similarly, since it can be dissolved in alcohol and removed out of the system, undesirable side reactions and reverse reactions of etching do not occur and cause contamination, controllability of the reaction, and reproducibility in the process Is extremely good. Further, the method according to the present invention can selectively remove the natural oxide film without removing the insulating film or the like formed on the substrate at the same time.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an example of an apparatus for cleaning a silicon wafer for carrying out a method of the present invention.

FIG. 2 is a diagram showing characteristics of an etching rate of a silicon thermal oxide film corresponding to a difference in an anhydrous hydrogen fluoride gas concentration.

FIG. 3 is a diagram showing whether a native oxide film can be etched according to the difference between the flow rate of N ₂ gas for bubbling CH ₃ OH and the concentration of anhydrous hydrogen fluoride gas.

FIG. 4 is a schematic configuration diagram showing another example of an apparatus for performing the method of the present invention.

5 is a time chart for explaining a method of supplying anhydrous hydrogen fluoride gas and methanol vapor when cleaning a silicon wafer by the apparatus shown in FIG. 4;

[Explanation of symbols]

 10 Silicon wafer 12 Container 30 Source of anhydrous hydrogen fluoride 32 Source of alcohol 34, 56 Source of carrier gas (nitrogen gas)

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Keiji Kirei 4-chome Tenjin, Horikawa-dori-Teranouchi, Kamigyo-ku, Kyoto 1 Daikita Screen Manufacturing Co., Ltd. (72) Inventor Shinjun Watanabe Kyoto 136, Uguisudai, Nagaokakyo-shi, Nagano (72) Inventor Zheng Yongbao, 394, 14, Shigen-dori, Senbon-dori, Kamigyo-ku, Kyoto 1-601 Nishijin Grand Heights 601

Claims (1)

(57) [Claims] 1. A substrate is accommodated in a container which is airtightly isolated from the outside air, and anhydrous hydrogen fluoride and alcohol are supplied into the container at a concentration of anhydrous hydrogen fluoride of 4% or less. A method for selectively removing a silicon native oxide film, wherein a silicon native oxide film formed on a surface of a silicon layer is selectively removed.