JPH03201434A - Formation of silicon oxide film - Google Patents

Abstract

PURPOSE:To form a silicon oxide film, which can be formed at a low temperature, has a few interfacial levels and has little leakage current due to a defect, by a method wherein oxygen ions or atomic oxygen is applied to the cleaned surface of a silicon substrate. CONSTITUTION:Oxygen ions or atomic oxygen is applied to the cleaned surface of a silicon substrate 11 to form an SiO2 film 12 of thickness of 20Angstrom or thereabouts. Then, a molecular beam of silicon is fed on this film 12 to form a polysilicon layer 13 of a thickness of 10Angstrom or thereabouts. When oxygen ions or atomic oxygen is again applied and this layer 13 is oxidized, an oxide film 12 of a thickness of 40Angstrom in total is formed. If this process is repeated, a thick film of a film thickness or thicker, which is decided up by diffusion of oxygen in the SiO2 film, can be formed at a low temperature. According to such a method, even if the SiO2 film is grown at a room temperature, a film, whose breakdown strength, leakage current and interfacial level density are all identical with those of the oxide film formed by a thermal oxidation, can be formed.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a method for forming a silicon oxide film.

(Prior Art and Problems to be Solved by the Invention) Conventionally, the method of forming a silicon oxide film has been thermal oxidation of a silicon substrate or vapor phase growth. By thermally oxidizing a silicon substrate, a high quality silicon oxide film can be obtained, and since the interface is formed within the silicon substrate, the interface state density is also low. However, thermal oxidation requires a high temperature of 800° C. or higher, and there is a drawback that the impurity profile formed in the substrate collapses due to impurity diffusion during the thermal oxidation process. On the other hand, vapor phase growth makes it possible to form an oxide film at low temperatures. However, in this vapor phase growth, a reaction between silicon and oxygen occurs in the vapor phase, resulting in 5i02 particles that accumulate on the substrate, so that many voids exist in the oxide film. Therefore, an oxide film formed by vapor phase growth has a lower breakdown voltage and more leakage current than a thermal oxide film. moreover,
Since the silicon surface is not cleaned, there is a problem that the density of interface states is high and that it cannot be used in areas where a high quality oxide film is required, such as the gate oxide film of a MOS device. Furthermore, gate oxide films of MOS devices tend to become thinner as LSIs become more densely packed, and it is expected that a film thickness of 100A or less will be required in the near future. Particularly in DRAMs, it is difficult to reduce the capacitance of a capacitor in order to prevent soft errors caused by alpha rays, and therefore it is necessary to compensate for the decrease in capacitance due to miniaturization by making the oxide film thinner. Furthermore, as the chip size increases, the area occupied by the gate region becomes larger, and an oxide film with excellent electrical insulation properties without breakdown voltage defects is required over a large area. On the other hand, even if the oxide film becomes thinner, it is practically difficult to lower the operating voltage, and oxide films tend to be used under higher electric field strength than before. However, as the thermal oxide film becomes thinner, many defects such as pinholes and weak spots occur that cause insulation failure. The reason for this is that the influence of the SiOx layer existing at the interface between Si and 5i02 becomes negligible as the oxide film becomes thinner, and also because of surface contamination due to adhesion of fine particles, organic matter, oil, or bacteria. It is thought that there is.

Therefore, the present inventors developed molecular Si I! 1: When oxygen plasma generated by ECR was simultaneously supplied to the substrate, it was found that an oxide film could be formed at a low temperature. Also,
Unlike the deposition of oxide films by vapor phase growth, this method uses molecular beams to oxidize Si on the surface rather than a vapor phase reaction, making it possible to form a denser film compared to vapor phase growth. Ta. Furthermore, SiMBE was used before 5i02 formation.
By growing a Si buffer epitaxial layer, the 5i02/Si interface could be made flat on the atomic order, and the decrease in breakdown voltage caused by electric field concentration due to the unevenness of the interface could be reduced. The breakdown voltage and leakage current of the oxide film thus formed were equivalent to those of a thermal oxide film of the same thickness.

However, in this method, even if the Si substrate surface is cleaned, the upper layer 5i02 and the Si substrate surface are not completely connected, dangling bonds remain at the interface, and interface states are generated due to these dangling bonds. There was a problem. The interface state density is about 10 times higher than that of thermal oxide film.
It could no longer be used as a gate oxide film for an OS. Furthermore, due to the low temperature growth, there are many vacancies in 5i02, and the leakage current due to these defects is larger than that of a thermally oxidized film at a film thickness of 100 Å or less, which is a problem.

It is an object of the present invention to provide a method for forming silicon oxide that eliminates these conventional drawbacks, allows formation at low temperatures, has few interface states, and reduces leakage current due to defects.

(Means for Solving the Problems) The present invention provides oxygen ions (O-), atomic oxygen (O), or oxygen ions or atomic oxygen contained on a semiconductor whose clean surface is exposed in a vacuum chamber. Oxygen molecular beam (O2
), a first silicon oxide film is formed on the surface, and then a thin polysilicon film is formed on the first silicon oxide film in the same vacuum chamber, and oxygen ions (O-) or atomic Silicon oxidation characterized by repeating oxidation of a polysilicon film by irradiating oxygen (O), oxygen ions, or oxygen molecular beams (O2) containing atomic oxygen until a predetermined oxide film thickness is reached. This is a method of forming a film.

The present invention also includes forming a thin polysilicon film before forming the first silicon oxide film and oxidizing this polysilicon film in the same manner, and repeating the formation and oxidation of a thin polysilicon film in the same manner. It will be done.

(Operation) The principle of the present invention will be explained. In conventional thermal oxidation, oxidation occurs at the interface between the 5i02 and the substrate Si crystals, so a lot of energy is required to diffuse oxygen in the 5i02 and break the back bond of the substrate crystalline Si, and this increases the oxidation temperature and time. has been decided.

As shown in Figure 2(a), when molecular Si and atomic oxygen or oxygen ions are simultaneously supplied from the surface side, oxidation always occurs at the surface, and moreover, the back bond of the substrate Si forming the crystal is cut. Since this is not necessary, an oxide film can be formed at low temperatures. However, in the above oxide film formation method, 5i02 is deposited at a low temperature, so as shown in Fig. 2(b).
As shown in , even if the substrate Si surface is cleaned, the upper layer 5i0
2 and the surface of the Si substrate are not completely connected, dangling bonds remain at the interface, and interface states are generated due to these dangling bonds. Further, as shown in FIG. 2(b), there are vacancies 24 in 5i02, and the leakage current due to these defects is larger than that in a thermally oxidized film at a thin film thickness of 100 A or less.

Therefore, as shown in FIG. 3(a), the present inventor irradiated the surface of a silicon substrate that had been cleaned in advance with oxygen ions or atomic oxygen.
As shown in Figure (b), the silicon substrate is oxidized and the surface has 5
It was found that i02 was formed. This 5i02 is formed by oxidizing the substrate, and is equivalent to a thermal oxide film in both film quality and interface state. However, the formation rate of 5i02 decreases as soon as 5i02 becomes thick and oxygen diffusion in 5i02 becomes rate-limiting, and 2OA
It was not possible to form an oxide film with a thickness greater than that. Currently, M.O.
S) The film thickness of the oxide film used as the gate oxide film of the transistor is 60 to 100A, and it is necessary to form an even thicker film at a low temperature.

Therefore, as shown in FIG. 1(a), the present inventor formed 5i02 of about 2OA by irradiating the surface of a cleaned silicon substrate with oxygen ions or atomic oxygen, and then, as shown in FIG. As shown in b), a silicon molecular beam is supplied on top of this to form a polysilicon bath containing about 10 people, and as shown in Fig. 1(C), oxygen ions or atomic oxygen is irradiated again. It has been found that when the polysilicon tank used as a lever is oxidized, an oxide film of a total of 40A is formed. By repeating this process, a film thicker than that determined by the diffusion of oxygen in 5i02 can be formed at low temperatures. If the film thickness of polysilicon is too thick, the oxide film formed by oxidizing it will become thick and the thickness will become diffusion-limiting, so the thickness of polysilicon is preferably several tens of amps or less. Furthermore, since the thickness of the oxide film is determined by the dose of silicon molecules supplied, the controllability of the film thickness is also extremely good.

With this method, even if grown at room temperature, it was possible to create an oxide film with breakdown voltage, leakage current, and interface state density comparable to those formed by thermal oxidation.

In this method, 5i02 can be formed without oxidizing the substrate by sending a Si molecular beam to form a thin polysilicon and then oxidizing it by ECR without performing the step of first oxidizing the substrate by ECR irradiation. Therefore, the substrate is Si
It is not necessary that this is the case, and a similar oxide film was obtained on a compound semiconductor.

(Example) Next, an example will be specifically described. The experiment was 40cc
The experiment was carried out using an MBE apparatus equipped with an electron gun type Si evaporator and a 100 W ECR type plasma source. The sample wafer has a 4-inch n-type 5i (100) 0.01-0.02Ωc
m substrate was used. 98°C NH40H-based cleaning solution (NH4
0H:H2O2:H2O=1:6:20)
Washed for 10 minutes and then water for 10 minutes. After drying, the film was transported into a formation chamber, where 10 A-8I layers were deposited, and then heated at 800°C for 1 minute to clean it, and an epitaxial layer serving as a buffer layer was grown to a thickness of 300 OA at a growth temperature of 500°C. After lowering the substrate temperature to room temperature, the clean surface was irradiated with oxygen plasma from an ECR plasma source to oxidize the surface by about 2 OA. At this time, RH
It was confirmed that the EED pattern changed from 2xl indicating a clean surface to a halo pattern indicating the formation of an amorphous 5i02 layer. The oxygen partial pressure in the passivation film forming chamber was 5×10 Torr. 5xlO
The mean free path of gas at -5 Torr is several tens of cm.
Therefore, there are few reactions in the atmosphere, and the reactions involved in 5i02 formation occur on the surface. After this, increase the substrate temperature to 50
The temperature was raised to 0°C, and a Si molecular beam was supplied from an electron gun type Si evaporator, and 10A was applied to the oxide film formed by oxygen plasma.
A polysilicon layer was formed. At this time, it was confirmed that the RHEED pattern changed from a halo pattern to a nongloss pattern indicating that polycrystalline silicon was formed. After lowering the substrate temperature to room temperature again, the polysilicon surface was irradiated with oxygen plasma from an ECR plasma source to completely oxidize the 20A polysilicon. At this time, the RHEED pattern changed from a ring pattern to a halo pattern.

First, in order to investigate the interface states of the formed oxide film, a MOS capacitor was fabricated as a prototype, and the interface state density was determined using the c-meter measurement (Terman method). Table 1 shows a comparison of the interface states between a conventional example in which molecular Si and oxygen plasma were simultaneously supplied, a case in which oxidation by oxygen plasma and deposition of polysilicon were repeated, and a case in thermal oxidation. The thickness of the oxide film was 60.

Table 1 As shown in Table 1, when molecular Si and oxygen plasma were supplied simultaneously, the interface level, which was 1012 to 1013 cm-2, was repeatedly oxidized by oxygen plasma and deposited polysilicon. In this case, it drops by about one digit to 1011
cm'', and the interface state density could be lowered to the same level as that of a thermal oxide film. Figure 4 shows the I-V of the same sample.
These are the results of the measurement. The leakage current is higher in the film formed using oxygen plasma and Si molecular beam (C) due to the effect of vacancies existing in the film, but in the film formed by repeating oxidation by oxygen plasma and polysilicon deposition (B) ) was found to have fewer defects and less leakage current, similar to the thermal oxide film (a).

Finally, in order to evaluate the flatness of the 5i02/Si interface formed by this method, three
A 00A epitaxial buffer layer was formed into a 50A 5i02 by repeating oxidation using a growth solution, oxygen plasma, and polysilicon deposition, and a cross-sectional lattice image of the interface was observed.

Because the buffer layer was grown by MBE, the interface was extremely flat, and when a normal 5i (100) wafer was used, the only disturbance at the interface was a one-atomic layer step observed every several hundred amps. This is present on the original MBE Kaicho growth fur layer. The density of this one-atomic layer step depends on the slope of the wafer surface and is exactly
When using a surface, a flat terrace of several thousand amperes could be obtained.

Note that although silicon wafers were targeted in this example,
The method of the present invention uses 5O8 (5O8) where silicon exists only on the surface.
Silicon on 5apphire) substrates and more generally 5OI (Silicon on In5ulat) substrates.
or) Of course, it can also be applied to substrates, etc. In addition, this method
Since i is also supplied from the surface side, the substrate does not essentially need to be Si, and it was confirmed that a high-quality oxide film could be obtained on a compound semiconductor as well.

(Effects of the Invention) As described above in detail, according to the present invention, it is possible to form an oxide film with very few interface states that is electrically equivalent to a thermal oxide film at room temperature.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram of the principle of the method of the present invention, Figure 2 is a conceptual diagram of the principle of the prior art, Figure 3 is a conceptual diagram of the principle of the prior art, and Figure 4 is a conceptual diagram of the principle of the prior art. FIG. 5 is a diagram showing dependence on the 5i02 formation method.

Claims (2)

[Claims] (1) Oxygen ions (O^-) or atomic oxygen (O), or oxygen molecular beams containing oxygen ions or atomic oxygen (O_2
), a first silicon oxide film is formed on the surface, and then a thin polysilicon film is formed on the first silicon oxide film in the same vacuum chamber, and oxygen ions (O^-) or atomic Silicon oxide characterized by repeating oxidation of a polysilicon film by irradiating it with oxygen (O), oxygen ions, or oxygen molecular beams (O_2) containing atomic oxygen until a desired oxide film thickness is achieved. How to form a film. (2) A thin polysilicon film is formed before the first silicon oxide film is formed, and oxygen ions (O^-), atomic oxygen (O), or oxygen molecular beams containing oxygen ions or atomic oxygen (O_2) are formed. 2. The method of forming a silicon oxide film according to claim 1, wherein the first silicon oxide film is formed by irradiating the polysilicon film to oxidize the polysilicon film.