JPH0455526B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Technical Field> The present invention relates to a method of forming a SiO ₂ /SiN/SiO ₂ film used as a memory capacitor insulating film of a MOS memory.

<Technical background of the invention and its problems> As LSI devices become more highly integrated, DRAM (random access memory), which is a typical example of MOS memory, has its memory capacitor insulating film reduced in size. A thinner film is required. but,
As SiO ₂ films conventionally used become thinner, they not only become difficult to control the film thickness, but also suffer from deterioration of dielectric breakdown voltage characteristics, radiation damage resistance, reaction with the gate metal on the film, and impurity content. Problems such as deterioration of diffusion masking properties have arisen. Therefore, in recent years, a three-layer film with a SiO ₂ /SiN/SiO ₂ structure has been attracting attention instead of the SiO ₂ film. The SiO ₂ /SiN/SiO ₂ three-layer film has excellent breakdown voltage characteristics and TDDB characteristics (Time Dependent
It has superior dielectric breakdown (dielectric breakdown) compared to a single-layer SiO ₂ film, and is used not only as an insulating film for memory capacitors but also for EEPROM, etc.

However, when the SiN film is made thinner,
When attempting to form a SiO ₂ film by thermal oxidation, the thickness of the SiN film is reported to be less than a certain value (approximately 45 Å):
E. SUZUKI, H. HIRAISHI et al.;
IEEETRANSACTIONS ON
ELECTRON DEVICES, VOL.ED−30,
NO.2P.122 (1983)), oxygen permeates through the SiN film, oxidation progresses at the Si interface, and a very thick oxide film grows. Therefore, when forming a thin SiO ₂ /SiN/SiO ₂ film, there is a limit to the thickness of the SiN film.

<Objective of the invention> The object of the invention is to form a SiN film on a thin SiO ₂ film formed by thermal oxidation on a Si substrate, and then remove the SiN film.
By annealing in _N2 atmosphere containing _NH3 ,
Even if the SiN film is extremely thin (~45 Å or less), we provide a method to make the SiN film through which oxygen does not pass during the subsequent thermal oxidation to form the SiO ₂ film, thereby realizing thinning of the entire three-layer film. be.

<Example> A SiN film is formed on a SiO ₂ film formed by heat treating a silicon substrate in an oxidizing atmosphere, and the SiN film is annealed in an N ₂ atmosphere containing NH ₃ .
We have developed a method to modify the film and form a SiN film that does not allow oxygen to pass through during thermal oxidation to form the upper layer SiO ₂ film.
The following examples will be described.

FIG. 1 is a process diagram.

As shown in Figure 1a, a silicon substrate (e.g., P(100), 15-20 Ωcm) 1 is thermally oxidized at 800°C in dry oxygen or in dry oxygen containing HC1, and thermally oxidized to a thickness of 35 Å. After forming the film (SiO ₂ film) 2,
A SiN film 3 of about 30 Å is formed thereon by the LPCVD method. After this, annealing is performed at 1000° C. or higher in an atmosphere containing 5% or more of NH ₃ relative to N ₂ . It is desirable that the annealing temperature be as high as the integrated circuit manufacturing process allows, but if the annealing temperature is 1050℃ or higher,
It was experimentally found that the effect of modifying the SiN film can be achieved by annealing for about 2 hours. Also, _NH3
It was found that a concentration of _N2 of about 2% had no effect on modifying the SiN film, and a concentration of 5% to 8% showed good results. However, care must be taken when handling high concentrations of NH ₃ as there is a possibility that NH ₃ may explode, especially at high temperatures. NH ₃ requires a purifier to remove moisture, and in the experiment shown in this example, a purifier to obtain NH ₃ of 99.999% or more was used.

Using the above method, in an _N2 atmosphere containing _NH3 .
After modifying the SiN film (Fig. 1 b3'), 950
When the SiO ₂ film 4 was formed on the SiN film 3' by thermal oxidation using steam oxidation at .degree. C. for 20 minutes, the SiO ₂ film had grown to a thickness of about 20 Å. On the other hand, containing the above-mentioned _NH3
When the modification with N ₂ is not carried out, or when similar experiments are conducted in an atmosphere containing only N ₂ , the film after thermal oxidation on the SiN film has an SiO ₂ equivalent film with a
The film was grown with a certain precision of about 1.5 Å (value measured by ellipsometry for a film on a Si substrate, assuming a refractive index of 1.46 for a SiO ₂ film). This value was the same as the thickness of the SiO ₂ film formed on the Si substrate by performing steam oxidation at 950° C. for 20 minutes.

These results are shown in FIG.

Figure 2 shows SiO ₂ /
After performing thermal oxidation treatment to form the third layer of SiO ₂ film on the SiN film, SiO ₂ /SiN/
FIG. ₂ is a diagram showing the SiO 2 equivalent film thickness measured by ellipsometry using a SiO ₂ film as a SiO ₂ equivalent film. According to this figure, annealing was performed in an _N2 atmosphere containing _NH3 .
Even if the SiN film is about 30 Å, the film thickness will not increase with subsequent thermal oxidation treatment. On the other hand, if the SiN film is annealed in an N ₂ atmosphere that does not contain NH ₃ or is not annealed at all, the SiN film will be thinner than about 60 to 70 Å. It can be seen that the thickness of the film increases significantly during the subsequent thermal oxidation treatment, and becomes approximately the same thickness as the SiO ₂ film obtained when the Si substrate is directly thermally oxidized.

From the above results, it is considered that the SiN film annealed in an N ₂ atmosphere containing NH ₃ changes its properties to prevent oxygen from passing through during the subsequent heat treatment in an oxidizing atmosphere. It was also found that this effect was produced using an N ₂ atmosphere containing NH ₃ .

<Effects of the Invention> As explained in detail above, by the method of the present invention, even if the SiN film formed on the SiO ₂ film is very thin, oxygen will not pass through during the formation of the thermal oxide film on the SiN film. It can be made into SiN film, SiO ₂ /SiN/
It becomes possible to make the SiO ₂ film thinner. Conventional SiN film thickness 45
For example, SiO ₂ /SiN/
The SiO ₂ film thickness can be set to 20 Å/30 Å/20 Å,
A three-layer film equivalent to 60 Å of SiO ₂ is obtained. This film has extremely low defect density and is suitable for high-density memory applications.
It has become possible to use it as an insulating film for capacitors.

[Brief explanation of the drawing]

Fig. 1 is a process diagram of an embodiment of the present invention, and Fig. ₂ is a process diagram of an example of the present invention.
FIG. 2 is a diagram showing the SiO ₂ equivalent film thickness measured by ellipsometry using the SiO ₂ /SiN/SiO ₂ film on the Si substrate as a SiO ₂ equivalent film after thermal oxidation treatment to form a SiO ₂ film of the layer. . Explanation of symbols, 1...Silicon substrate, 2...
SiO ₂ film, 3... SiN film, 3'... SiN film annealed in an N ₂ atmosphere containing NH ₃ , 4... SiO ₂ film.

Claims (1)

[Claims] 1. After forming an SiO ₂ film on a silicon substrate by thermal oxidation, forming an SiN film with a thickness of about 70 Å or less on it, and then forming an SiO ₂ film on it by thermal oxidation, SiO ₂ /SiN /In the method of forming a SiO ₃ film, after forming the SiN film with a thickness of about 70 Å or less,
Annealing in an _N2 atmosphere containing at least _NH3 ,
A method for forming a SiO ₂ /SiN/SiO ₂ film, characterized in that the SiN film is modified to reduce the oxygen permeability of the SiN film during formation of the SiO ₂ film on the SiN film.