JPH0123938B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for treating the surface of a substrate exposed by dry etching, which is one of photoetching techniques.

Conventionally, in the process of manufacturing semiconductor devices such as integrated circuits, an aqueous solution containing hydrofluoric acid as the main component was used to etch a silicon oxide film, and heat of about 150°C was used to etch a silicon nitride film. So-called wet etching techniques, such as using phosphoric acid, have been used. In recent years, instead of these methods, so-called etching methods have been developed in which silicon oxide films and silicon nitride films are etched in plasma formed using a gas such as a mixed gas of _{CF 4 and} H ₂ or a Freon gas such as C 3 F 8 or CHF ₃ . Dry etching techniques began to be used. This new dry etching technology exhibits so-called anisotropic etching properties, in which etching progresses only in the direction perpendicular to the substrate surface using the resist as a mask, and etching does not progress in the lateral direction. This method has the advantage that patterns of silicon oxide films and silicon nitride films can be formed with high accuracy according to resist dimensions. However, the properties of the silicon oxide film formed by oxidizing the surface of the silicon substrate etched and exposed by this method and the interface between the substrate and the silicon are metal ( M etal) - oxide film (Silicon - Oxide ) -
So-called MOS in which silicon is laminated
When used as an element, there are problems such as a low dielectric breakdown voltage and a large number of surface states, and there are many problems in putting it into practical use. For this reason, when etching a silicon oxide film, dry etching is not performed until the silicon substrate is exposed, the etching is finished leaving a very thin silicon oxide film, and the remaining thin silicon oxide film is etched with an aqueous solution containing hydrogen fluoride. The expedient method of removing is used.

According to research conducted by the present inventors, large amounts of carbon and fluorine were observed by Auger electron spectroscopy on the silicon surface after dry etching of these fluorocarbons. Furthermore, in the case of a silicon surface that has been subjected to reactive sputter etching using carbon fluoride gas by incorporating the sputtering effect, carbon and fluorine are distributed not only on the so-called silicon surface but also within the silicon substrate to a depth of about 30 Å. This was confirmed.

For this reason, the inventor of the present invention has developed an etching method under which the damaged layer on the silicon surface after etching can be removed by etching approximately 100 Å on a normal silicon substrate.
It is removed with an etching solution consisting of HNO ₃ −HF−CH ₃ −COOH, and the silicon surface is oxidized to form a MOS
The electrical characteristics of the were evaluated. The dielectric strength voltage of the gate insulating film is 3 to 6 MV/cm, which is actually worse than the dielectric strength voltage of 6 to 8 MV/cm when the surface is oxidized without being etched with the above etching solution.
Of course, the result was much worse than the dielectric breakdown voltage of 9.5 to 10.2 MV/cm for a silicon oxide film formed by oxidizing a clean silicon surface, and the surface layer of the silicon substrate after etching was simply removed by any method. We confirmed that it is not just a matter of just doing it.

It is an object of the present invention to remove the damaged layer on the surface of the silicon substrate after dry etching by applying an appropriate surface treatment after etching, thereby making it compatible with the subsequent semiconductor integrated circuit manufacturing process, and making it easier to manufacture. The objective is to improve the electrical properties of the device.

According to the present invention, a silicon oxide film or a silicon nitride film on a silicon substrate is coated with CF ₄ +H ₂ , CHF ₃ ,
Parallel plate plasma etching, reactive sputter etching, reactive ion shower using carbon fluorochloride gas such as C ₂ F ₆ , C ₃ F ₈ , C ₄ F ₈ , CClF ₃ +H ₂ CCl ₄ A method for manufacturing a semiconductor device is obtained, which comprises etching a silicon substrate until it is exposed by a dry etching technique such as etching, and then performing surface treatment in an oxygen plasma gas containing at least 10 mol% of CF ₄ at most. .

Oxygen acts to oxidize the silicon surface, causing CF ₄
etches the formed oxide film. In the present invention, the CF ₄ content is only 10 mol % at most, so the etching rate is low and the oxidation rate is high. If the etching rate is too high, the substrate will be deeply etched and the surface will lose its flatness, making it impossible to use it in the manufacturing process of semiconductor integrated circuits. However, as can be seen from the following examples, the etching rate is low, which is advantageous. Furthermore, as mentioned above, since oxidation is faster than etching, the surface is always covered with an oxide film, which serves as surface protection.

Next, as an example, a case will be described in which CF ₄ is added to oxygen in silicon surface treatment after dry etching.

The data illustrated in Figure 1 shows that a silicon substrate is placed inside a cylindrical plasma device with a diameter of 20 cm and a length of 30 cm, which is separated from the external discharge area by an aluminum cylinder with many minute holes. to this device
This figure shows the dependence of the thickness of the silicon oxide film formed on the silicon substrate surface on the concentration of CF ₄ when a mixed gas of CF ₄ and O ₂ is introduced and a discharge is generated under the conditions of 0.3 Torr and 200 W.

As described above, in the method of the present invention, the surface is always covered with an oxide film, and the etching gas etches the oxide film. What is shown in FIG. 1 is the thickness of the oxide film covering the surface.

The silicon oxide film thickness is in the range of 30 to 56 Å, and no similar dependence on CF ₄ concentration is observed. CF ₄
Addition of O ₂ has a greater effect on the etching rate of the silicon substrate, oxide film, and nitride film than on the silicon oxide film.

When surface treatment is performed in the manufacturing process of semiconductor integrated circuits, the method must not create large irregularities on the surface. This is because the unevenness increases the risk that the electrodes and wiring formed thereon will be disconnected. Furthermore, since surface treatment is usually aimed at extremely thin layers of several tens of angstroms, controllability will be lost if the processing speed is too high. Although the present invention requires a low etching rate, as shown below, the etching rate is sufficiently low so that it can be used for manufacturing semiconductor integrated circuits.

The data illustrated in Figure 2 shows the dependence of the etching rate of the silicon substrate (1 in the figure), the silicon oxide film (2 in the figure), and the silicon nitride film (3 in the figure) on the CF ₄ concentration. . The etching rate was measured as follows. A silicon substrate whose surface is partially covered with a glass plate to prevent etching.
A silicon substrate with a silicon oxide film formed by the CVD method and a silicon substrate with a silicon nitride film formed on the surface by the CVD method are prepared. Each substrate is used to measure the etching rate of silicon, silicon oxide film, and silicon nitride film, respectively. Next, the treatment of this example is performed for 30 minutes to 1 hour. Since the etching speed is low, processing is performed for a certain amount of time to improve measurement accuracy. After processing, the silicon substrate has a difference in level between the part covered by the glass plate and the part not covered by the glass plate, so the etching rate (Å/min) can be determined by measuring it with a surface roughness meter and dividing it by the processing time. Note that although the etched surface is covered with an oxide film, the thickness is extremely thin as shown in FIG. 1, so if the processing time is lengthened, there will not be a large error. The thickness of the silicon oxide film and silicon nitride film before and after treatment may be measured using an ellipsometer. The etching rate is approximately proportional to _{the CF 4 concentration} up to 10%.
If CF ₄ is added in an amount of 10% or more, the silicon substrate surface becomes rough, and mirror etching of the silicon substrate surface becomes impossible.

If the surface becomes rough and an oxide film is formed there, the dielectric strength will be low and it cannot be used as an integrated circuit. Whether the surface is mirror-like or not is determined visually by shining a light onto the substrate surface. Even if you shine light on it, if you can't see the light flux on the substrate, it's a mirror surface, and if you can see it, it's not a mirror surface. If it's not a mirror surface, the parts that are hit by the light will look white. This method is a well-known method used by silicon substrate manufacturers and the like.

The above experiments were conducted at a substrate temperature in the range of 60 to 70°C.

As a representative example of the case where 3 mol % of CF ₄ was added, the etching rates of the silicon substrate and the silicon oxide film were 6.5 Å/min and 10 Å/min, respectively.

This difference can be explained by the fact that the oxide film is etched while the silicon substrate is oxidized, taking into consideration the respective densities.

In the case of this example, the thickness of the oxide film grown on the surface of the silicon substrate is approximately 40 Å. This indicates that the silicon substrate surface up to a depth of 20 Å is converted into a silicon oxide film during the very early stage of surface treatment. Therefore, considering that the etching rate of silicon is 6.5 Å/min, the damage layer of 30 Å deep on the silicon surface after dry etching observed by the above-mentioned Auger electron spectroscopy disappears in about 2 minutes. I will do it.

CF ₄ ; 100 SCCM in a parallel plate type reactive sputter etching device with a diameter of 40 cm and an electrode spacing of 10 cm.
The dielectric strength voltage of the gate insulating film of the MOS diode created on the silicon substrate surface was etched for 2 minutes under the conditions of introducing H ₂ ; 20 SCCM gas and generating a high frequency discharge of 13.56 MHz under a pressure of 10 Pa. The average value is 6.9MV/cm, and the standard deviation is
It was 0.85MV/cm.

In addition, after the above-mentioned reactive sputter etching, the surface treatment using the above-mentioned gas containing 3 mol% CF ₄ was performed for 2 minutes on the silicon substrate surface.
In a MOS diode, the breakdown voltage of the gate insulating film is 10.5MV/cm, and its standard deviation is 0.42MV/cm.
It was hot. Furthermore, on a silicon substrate without any special treatment, the values were 9.5 MV/cm and 1.5 MV/cm, respectively. This means that surface treatment after etching improves the dielectric breakdown voltage and reduces deviation.
This shows that it is possible to produce products with uniform properties and to obtain better results than untreated products.

In this experimental process, we simultaneously exposed two specimens, one that had just been dry etched, one that had not been subjected to any treatment, and one that had been subjected to the surface treatment of the present invention after dry etching, in oxygen at 950°C.
When the film was oxidized to a thickness of 400 Å at a temperature of °C, a film thickness of 400 ± 5 Å was obtained as desired in the untreated film and in the surface treated film according to the above embodiment of the present invention after dry etching. The film thickness was 325±5 Å after dry etching alone, and the result supported the Auger electron spectroscopy finding that impurities such as carbon were present on the silicon surface after dry etching.

The first feature of the present invention is that surface treatment can be carried out at a low temperature of 60 to 70°C, as shown in the Examples. The substrate temperature during reactive sputter etching of silicon oxide and silicon nitride films was 80-90°C under the same conditions as in the above embodiments of the present invention.
The temperature rises to ℃. In the surface treatment method of the present invention, the temperature is 60 to 70°C as described above, so the surface treatment is performed at a lower temperature than that during etching. Therefore, unlike high-temperature annealing in an inert gas, for example, it has the advantage that there are no defects that are thermally expanded and generated from the damaged layer caused by dry etching, which was limited only to the surface layer.

The second feature of the present invention is, as shown in the examples,
This means that the process can be completed in a dry state and in a short time of about 2 minutes.

The third feature of the present invention is that F, Cl, and
This is a problem in the manufacture of semiconductor devices such as
It contains a halogen element that forms a salt that is stable with alkali metals such as Na, Ka, Li, etc. and easily dissolves in the subsequent water washing process. Therefore,
The present invention can be carried out easily anywhere without worrying about alkali metal contamination. However, excessive addition of fluorochloride-based gas roughens the silicon surface, which not only lowers the dielectric strength voltage of the insulating film formed on the surface but also induces the generation of secondary defects. Therefore, it is desirable to limit the amount to about 10 mol% at most.

The fourth feature of the present invention is that the etching rate of silicon oxide films and silicon nitride films used in semiconductor integrated circuits can be easily selected to be less than about 10 to 50 Å, which is faster than that obtained by conventional photoetching techniques. The purpose is to not substantially change the pattern shape to the extent that it can be described as blinking.

Although a cylindrical plasma device was used in the above embodiment, since the surface treatment of the present invention is based on a chemical reaction of reactive active species generated in plasma, a plasma device other than a cylindrical shape may be used as long as it generates the same type of reactive active species. A similar effect can be obtained even with a plasma device.

[Brief explanation of drawings]

FIG. 1 shows an example of the CF ₄ concentration dependence of a silicon oxide film formed on a silicon substrate in one embodiment of the present invention. FIG. 2 also shows a silicon substrate (1 in the figure) in an embodiment of the present invention;
This figure shows an example of the CF ₄ concentration dependence of the etching rate of a silicon oxide film (2 in the figure) and a silicon nitride film (3 in the figure).

Claims (1)

[Claims] 1. Following the step of dry etching a silicon nitride film or silicon oxide film on a silicon substrate with an etching gas containing at least carbon and either fluorine or chlorine, the exposed silicon substrate surface is etched with CF of up to 10 mol%. 4. A method for manufacturing a semiconductor device, comprising a step of surface treatment at low temperature in oxygen gas plasma containing ₄ .