JPH0955380A - Method for manufacturing MOS semiconductor device - Google Patents

Abstract

(57) Abstract: Heterogeneous atoms are shallowly introduced into the surface of a silicon substrate to form a high quality ultrathin oxide film. SOLUTION: A silicon substrate 401 is oxidized to form a thermal oxide film 402 with a thickness of about 350 Å, and SiF ions 403 are injected through the thermal oxide film 402 with an implantation energy of 20K.
Implantation is performed at eV and a dose amount of 1 × 10 ¹⁵ / cm ² . As a result, an amorphous layer 404 containing fluorine atoms is formed under the thermal oxide film 402. After removing the thermal oxide film 402 by immersing it in a buffered hydrofluoric acid solution, the silicon substrate 401 was thermally oxidized in oxygen to form an oxide film 405 with a film thickness of about 70Å on the surface of the silicon substrate 401.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a MOS type semiconductor device, and more particularly to a manufacturing method characterized by the step of forming a gate oxide film thereof.

[0002]

2. Description of the Related Art A silicon oxide film (SiO ₂ ; hereinafter simply referred to as an oxide film) formed by thermally oxidizing a silicon substrate is usually used as a gate oxide film forming a MOS semiconductor device. With the miniaturization and high integration of MOS devices, the gate oxide film is becoming thinner. The film thickness of the gate oxide film is, for example, 100Å, 0.25 μm in 64M DRAM of 0.35 μm process technology.
In the process technology 256M DRAM, the one of 80Å is used. In addition, CMOS logic semiconductor devices aiming at higher performance are required to be thinned one generation earlier than DRAMs, and CMOS of 0.25 μm process technology requires 6 layers.
A 0Å thick gate oxide film is used. In such an ultra-thin oxide film, the influence of certain defects (structural defects) existing in the film cannot be ignored. For this reason, a problem that has not been observed in the conventional thick gate oxide film occurs, and it becomes difficult to secure the reliability of the device.

Explaining structural defects in the oxide film,
Although oxide film is an amorphous structure having a short range order of Si-O ₄ tetrahedral structure (tetrahedra), structural defects points to the portion where the short-range order is shifted from the tetrahedral structure is destroyed. In the structural defect portion, there exist silicon dangling bonds in which the chemical bonds of oxygen atoms bridging between silicon atoms are broken, and incomplete bonds such as Si—Si and Si—OH bonds. Although the cause of generation of such structural defects is not clear, it has been pointed out that the surface state of the silicon substrate before oxidation has an influence. Bulk silicon is a very high quality single crystal, but there are many uncontrollable factors (surface defects) on its surface. For example, very small irregularities, natural oxide film, impurities adsorbed on the surface (water, heavy metals, carbides, particles, chemical residues,
Mobile ions). It is impossible to perfect the surface condition of the substrate, and surface defects remain even after pretreatment with various chemicals. It is considered that such surface defects are incorporated into the film by oxidation to form structural defects. This type of unstable bond is broken by the application of high electric field stress. As a result, the generated dangling bonds serve as electron trap sites, causing a change in characteristics over time and reducing reliability.

As described above, as the thickness of the oxide film is reduced, the structure of the interface between the silicon substrate and the oxide film has a great influence on the characteristics of the oxide film, and the interface structure controls the characteristics of the oxide film. Therefore, in order to obtain a highly reliable ultrathin oxide film, it is necessary to control the interface structure at the atomic level. Several methods have been proposed to achieve such an objective.

For example, JP-A-6-45320, JP-A-5-315318 and JP-A-3-25782.
Japanese Patent Publication No. 8 discloses a method of introducing nitrogen atoms into the interface between an oxide film and silicon. Japanese Unexamined Patent Publication No. 6-45320 discloses a method in which excited nitrogen is added to an oxidizing atmosphere and introduced into an oxide film. JP-A-6-315
Japanese Patent No. 318 discloses a method of implanting nitrogen atoms into the surface of a silicon substrate before oxidation by an ion implantation method and oxidizing the surface to introduce nitrogen atoms into an oxide film. Japanese Unexamined Patent Publication No. 6-257828 discloses a method in which a silicon substrate is thermally oxidized and then heat-treated in a nitrogen atmosphere to introduce nitrogen atoms into an oxide film. JP-A-6-85278, JP-A-6-13372, and JP-A-5-747.
Japanese Patent Laid-Open No. 62-62 discloses a method of introducing a fluorine atom or a chlorine atom, which has a strong bonding force to silicon, into an oxide film from a vapor phase.

The above method introduces atoms other than silicon and oxygen (hereinafter referred to as "heteroatoms") into the oxide film to form terminal ends of dangling bonds or a more stable bond state, whereby structural defects at the interface are formed. Is intended to obtain a highly reliable oxide film. In these methods, the heteroatoms are introduced into the oxide film by a method such as heat treatment after oxidation or ion implantation.

[0007]

In the method of introducing a heteroatom from the gas phase into the oxide film through the surface of the oxide film by the heat treatment after oxidation, Si / Si, which requires the termination of the bond, is most necessary.
Up to near the O ₂ interface, it depends on thermal diffusion. this is,
It cannot be said that it is a method of efficiently supplying different kinds of atoms near the Si / SiO ₂ interface. Further, the heat treatment conditions may change depending on the thickness of the oxide film. Furthermore, it is not preferable to increase the number of heat treatment steps in the future because device miniaturization and process temperature reduction will progress.

It can be said that the method in which the heteroatoms are introduced into the silicon substrate by ion implantation and then oxidized is excellent in that the desired amount of the heteroatoms can be introduced at a substantially desired depth. Considering the use for forming an ultra-thin oxide film, it is desirable that foreign atoms are introduced shallowly into the surface of the silicon substrate, and Si / Si is formed when the oxide film is formed.
It is desirable to concentrate in the vicinity of the O ₂ interface. However, in the ion implantation method, the range and the spread of the implanted ions in the silicon substrate during the ion implantation are almost determined by the mass of the ion species when the implantation energy is determined. Ion energy by ion implanter is usually 20
Since it is up to 200 KeV, when a heteroatom such as nitrogen, chlorine or fluorine is ion-implanted, it is deeply implanted, and it is difficult to form the above state. The heteroatoms that have been deeply introduced into the silicon substrate generate defects in the crystal when they are injected, and do not contribute to modification of the oxide film.

As described above, conventionally, a method of introducing a heteroatom to modify an oxide film has been proposed, but the amount of the heteroatom to be introduced and its introduction position must be highly controllable. I won't. In this respect, it can be said that the conventional method is insufficient. Although the present invention is a method of introducing a heteroatom to modify an oxide film, the heteroatom is introduced shallowly on the surface of a silicon substrate, and when the oxide film is formed, it is near the Si / SiO ₂ interface. The present invention provides a method for forming a high quality ultrathin oxide film by concentrating on

[0010]

According to the present invention, in order to form a gate oxide film, a step of ion-implanting a silicon halide on the surface of a silicon substrate, and the silicon substrate after the ion implantation in an oxidizing gas atmosphere. And a step of oxidizing the surface of the silicon substrate by heat treatment. An example of a silicon halide is one or both of SiF and SiCl.
In the conventionally proposed method, nitrogen, chlorine, fluorine atoms, etc. are ion-implanted into the surface of the silicon substrate before oxidation.However, in the present invention, when the heteroatom is introduced into the silicon substrate, it is halogenated instead of the heteroatom alone. The difference is that ion implantation is performed as silicon ions.

In FIG. 1, SiF ions are implanted into a silicon substrate at an implantation energy of 20 KeV and a dose of 1 × 10 ¹⁵ / cm ^2.
The results of comparing the concentration distributions in the depth direction are shown for the case of implanting in (2) (solid line) and the case of implanting fluorine ions under the same conditions (broken line). It can be seen that SiF ions are distributed at a higher concentration in a shallower region from the surface than F ions. This is because the range of implanted ions is determined by the mass number of implanted ions. Therefore, it can be said that the silicon halide ion having a large mass number is more suitable than the single halogen ion in order to form a high quality ultrathin oxide film. In order for the implanted silicon halide to have the effect of terminating dangling bonds in the oxide film in the thermal oxide film, the impurity concentration of halogenated silicon in the silicon is 10 ¹⁹ to 10 ²⁰ / cm ³ . It is preferable.

For the heat treatment in the oxidizing gas atmosphere after the injection step, a method such as a dry oxidation method using an ordinary electric furnace or a wet oxidation method can be used. In a preferred aspect of the present invention, an oxide film is formed on the surface of the silicon substrate before the silicon halide is ion-implanted, and the silicon halide is ion-implanted into the silicon substrate through the oxide film. As a result, the implantation depth of the implanted ions can be adjusted by the oxide film, and a thinner oxide film can be formed. The oxide film for adjusting the implantation depth of the ion implantation may be formed by oxidizing the silicon substrate or may be deposited by the CVD method.

In a further preferred aspect, the oxide film for adjusting the implantation depth of the ion implantation contains nitrogen atoms. This has the effect of terminating dangling bonds in the oxide film by a so-called knockout phenomenon, in which the silicon halide ions repel nitrogen atoms in the oxide film toward the silicon substrate side during implantation of the silicon halide ion. Nitrogen atoms can be supplied from the silicon surface. This method is also a method corresponding to the formation of a thinner oxide film.
As the oxide film containing nitrogen atoms, a SiON film formed by a CVD method may be used, and a thermal oxide film formed by a thermal oxidation method may be subjected to thermal annealing or plasma treatment in an NH ₃ or N ₂ O atmosphere. You may use.

[0014]

【Example】

(Embodiment 1) An embodiment based on the methods of claims 1 and 2 is shown in FIG. (A) shows a silicon substrate 201. (B) SiF ions 202 are implanted into a silicon substrate 201 with an implantation energy of 20 KeV and a dose of 1 × 10 ¹⁵ / cm ² . SiF ions can be obtained by using SiF ₄ as a source gas for ion implantation. By this implantation, fluorine atoms F are formed on the surface of the silicon substrate 201.
Amorphous layer 203 containing is formed. (C) Thereafter, the oxide film 204 is formed on the surface of the silicon substrate 201 by thermally oxidizing the silicon substrate 201 in oxygen.
To form At this time, the thickness of the oxide film 204 is about 300.
Was Å. FIG. 3 shows the results of mass spectrometric analysis of ions that can be extracted when SiF ₄ is used as a source gas for implanting SiF ions into the silicon substrate 201. From the results of FIG. 3, it can be seen that SiF ions can be extracted with sufficient strength.

(Embodiment 2) An embodiment based on the present invention of claim 3 is shown in FIG. (A) shows a silicon substrate 401. (B) The silicon substrate 401 is oxidized in a mixed gas atmosphere of oxygen and hydrogen, and the thermal oxide film 4 is formed on the surface of the silicon substrate 401.
02 is formed to a thickness of about 350Å. (C) Next, SiF ions 403 are implanted through the thermal oxide film 402 with an implantation energy of 20 KeV and a dose of 1 × 10 ¹⁵ /
Inject at cm ² . As a result, an amorphous layer 404 containing fluorine atoms is formed under the thermal oxide film 402. (D) Next, the silicon substrate 401 is immersed in a buffered hydrofluoric acid solution to remove the thermal oxide film 402. (E) Then, the silicon substrate 401 is thermally oxidized in oxygen to form an oxide film 40 on the surface of the silicon substrate 401.
5 was formed. At this time, the thickness of the oxide film 405 is about 70.
Was Å.

(Embodiment 3) An embodiment corresponding to claim 4 will be described with reference to FIG. (A) shows a silicon substrate 501. (B) S is formed on the silicon substrate 501 by the plasma CVD method.
The iON film 502 is formed to a thickness of about 350Å. (C) Next, SiF ions 503 are implanted through the SiON film 502 with an implantation energy of 20 KeV and a dose of 1 × 10 ^15.
/ Cm ² injection. As a result, the amorphous layer 504 containing F is formed under the SiON film 502. Then, near the interface of the amorphous layer 504 on the SiON film 502 side, nitrogen atoms N knocked out from the SiON film 502 are included. (D) Next, the silicon substrate 501 is immersed in a buffered hydrofluoric acid solution to remove the SiON film 502. (E) Next, the silicon substrate 501 is thermally oxidized in oxygen to form an oxide film 505 on the surface of the silicon substrate 501.
Was formed. At this time, the thickness of the oxide film 505 is about 70Å
Met.

[0017]

According to the present invention, an oxide film is formed by implanting silicon halide ions into the surface of a silicon substrate and then performing thermal oxidation. By using halogenated silicon ions for implantation instead of halogen ions alone as in the conventional method, it is possible to dope at a high concentration into a shallow region from the surface of the silicon substrate, and achieve a higher film thickness and thinner than conventional methods. It becomes possible to form a high quality oxide film. Furthermore, by implanting into the substrate through the oxide film at the time of ion implantation, it becomes possible to form a high-quality oxide film with a thinner film thickness. At that time, if the oxide film contains nitrogen atoms, the nitrogen atoms are knocked out into the silicon substrate, and the dangling bonds in the oxide film formed in the subsequent thermal oxidation step are terminated by the nitrogen atoms, and the oxidation is performed. Effective for improving the quality of the film.

The effect of the oxide film forming method of the present invention on the characteristics of the oxide film will be specifically described. FIG. 6 shows the Si-2p spectrum of the oxide film by X-ray photoelectron spectroscopy (XPS). The horizontal axis represents the binding energy and the vertical axis represents the photoelectron intensity. Of the two large peaks, Si ⁺⁴ represents silicon from the oxide film, and Si ⁰ represents the contribution from the silicon carrier of the substrate. The intensity of the region between Si ⁰ and Si ⁺⁴ is a contribution of Si ⁺³ , Si ^+2, etc., which means that dangling bonds of silicon exist in the oxide film. In FIG. 6, what is indicated by a broken line in the region between the two peaks is the result of the present invention according to Example 1 of FIG. 2, and the solid line is the result when fluorine ions are implanted under the same conditions as in Example 1. Is. The shaded portion of the region is the effect of the present invention, and represents that dangling bonds are reduced.

It is considered that such dangling bonds of silicon in the oxide film cause deterioration of the oxide film. Therefore,
To evaluate the amount of dangling bonds of silicon in the oxide film,
Ratio of the area integral of the Si ⁺⁴ peak and the remaining intensity (suboxide; Sub) obtained by subtracting the area integral of Si ⁰ and Si ⁺⁴ from the total intensity of the photoelectron intensity (total area integral) Sub / Si ⁺⁴
FIG. 7 shows the result of the comparison of the above. Example 1 is a conventional example
Fluorine ions are ion-implanted under the same conditions as. From the results of FIG. 7, it is understood that the present invention can reduce dangling bonds of silicon in the oxide film. FIG. 8 is a comparison of the interface state density between the conventional method and the method of the present invention. From the results of FIG. 8, it can be seen that the interface state density is also reduced by the present invention. The reduction of the interface state density is effective in preventing the deterioration of device characteristics due to the shift of the threshold value.

[Brief description of drawings]

FIG. 1 is Si when ion implantation is performed on a silicon substrate with an implantation energy of 20 KeV and a dose amount of 1 × 10 ¹⁵ / cm ^2.
It is a figure which compares the concentration distribution in the depth direction of F ion and F ion.

2A to 2C are process cross-sectional views illustrating a first embodiment.

FIG. 3 is a mass spectrum showing an ion beam that can be taken out when SiF ₄ gas is used as an ion source.

FIG. 4 is a process cross-sectional view showing a second embodiment.

FIG. 5 is a process sectional view showing a third embodiment.

FIG. 6 shows the XPS of an oxide film in the conventional method and the method of the present invention.
It is a figure which compares the Si-2p spectrum obtained by.

FIG. 7 is a diagram comparing the ratio Sub / Si ⁺⁴ between the sub oxide amount Sub and the Si ⁺⁴ amount in the Si-2p spectrum of XPS between the method of the present invention and the conventional method.

FIG. 8 is a diagram comparing the interface state density of an oxide film between the method of the present invention and the conventional method.

[Explanation of symbols]

201, 401, 501 Silicon substrate 202, 403, 503 SiF ion beam 203, 404, 504 Silicon layer amorphized by ion implantation 204, 405, 505 Oxide film 402 Oxide film for adjusting ion implantation depth

Claims (4)

[Claims] 1. A method of manufacturing a MOS type semiconductor device comprising forming a gate oxide film on a surface of a silicon substrate and using the gate oxide film as a constituent element, wherein a halogenated film is formed on the surface of the silicon substrate to form the gate oxide film. A method of manufacturing a MOS type semiconductor device, comprising: a step of ion-implanting silicon; and a step of oxidizing the surface of the silicon substrate by heat-treating the ion-implanted silicon substrate in an oxidizing gas atmosphere. . 2. Silicon halides are SiF and SiCl.
2. The method for manufacturing a MOS semiconductor device according to claim 1, wherein one or both of them. 3. A method of manufacturing a MOS type semiconductor device, comprising forming a gate oxide film on a surface of a silicon substrate and using the gate oxide film as a constituent element, wherein a silicon oxide is formed on the surface of the silicon substrate to form the gate oxide film. A step of forming a film, a step of ion-implanting silicon halide into the silicon substrate through the silicon oxide film, a step of removing the silicon oxide film, and then a heat treatment of the silicon substrate in an oxidizing gas atmosphere. And a step of oxidizing the surface of the silicon substrate. 4. The method for manufacturing a MOS semiconductor device according to claim 3, wherein the silicon oxide film contains nitrogen atoms.