JPH0429222B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for forming a nitride film by directly nitriding a sample such as a silicon substrate.

[Technical background of the invention and its problems]

Semiconductor integrated circuits are becoming more and more highly integrated year by year. For example, dynamic random access memory (DRAM), a typical example of MOS memory, is rapidly evolving from 16K bits to 64K bits to 256K bits, and 1M bit DRAM is about to be announced soon. In this case, the area of the integrated circuit chip will continue to increase in density without changing much, so the area of the DRAM memory cell will inevitably become smaller and smaller. However, as the memory cell area decreases, the capacity of the MOS capacitor that makes up the memory cell decreases.
This must be avoided in order to ensure a sufficient S/N ratio. To achieve this, it is necessary to make the silicon oxide film (SiO ₂ film) that makes up the MOS capacitor thinner. For example, in 64K bit DRAM, MOS
The SiO ₂ film of the capacitor is about 400 Å thick, which is about 200 Å for 256K bits and about 100 Å for 1M bits.
It is required that the thickness be approximately Å.

However, as the SiO film becomes thinner in this way, the controllability of the film thickness deteriorates, and the withstand voltage characteristics of the SiO ₂ film deteriorate due to an increase in defect density.

In order to solve these problems, it has been conventionally considered to use a silicon nitride film (Si ₃ N ₄ film) instead of the SiO ₂ film as the dielectric film of the MOS capacitor. The dielectric constant of the Si ₃ N ₄ film is approximately twice that of the SiO ₂ film. Therefore, if a MOS capacitor is constructed using a SiN ₄ film, the area and thickness will be approximately the same as when using an SiO ₂ film. This is because twice the capacity can be achieved. Moreover, the Si ₃ N ₄ film has a much greater effect of blocking impurities such as Na than the SiO ₂ film, which is also advantageous for improving the reliability of the device.

Conventionally, the method for forming Si ₃ N ₄ film was to mix monosilane (SiH ₄ ) and ammonia (NH ₃ ) at about 850°C.
Vapor phase deposition (CVD), which involves thermal decomposition at a moderate level, was common. However, with this method, (1) it is difficult to control the film thickness, and (2) there is usually a thickness of 10 to 20 Å on a silicon substrate.
Since the SiO ₂ film exists as a natural oxide film, when a Si ₃ N ₄ film is deposited on it, an MNOS structure is formed, and the surface state density at the Si-Si ₃ N ₄ interface increases to 10 ¹² /cm ² or more. This causes a decrease in the S/N ratio.
There was an essential flaw.

Therefore, as a method for forming a Si ₃ N ₄ film without such drawbacks, a direct nitriding method in which N ₂ or NH ₃ gas and silicon are directly reacted at a high temperature of 1200° C. or higher has been attempted. However, if the Si ₃ N ₄ film is simply nitrided directly at high temperature, the thickness will not exceed about 50 Å.
This is because the quality of the Si ₃ N ₄ film is much denser than that of the SiO ₂ film, and if a very thin Si ₃ N ₄ film is formed on the surface, the supply of N atoms to the silicon substrate will be blocked. caused by. Also by this method SiN ₄
The film quality is island-like and heterogeneous (for example, the paper “Very Thin Silicon Nitride Films
Grown by Direct Thermal Reactive with
Nitrogen” (J.Electrochem.:Solid-State
Science and Technology, March 1978).

On the other hand, a method using glow discharge has been proposed in order to increase the nitriding rate by direct nitriding and obtain a thicker Si ₃ N ₄ film (Takashi Ito: 1973).
28th Spring 2018 Joint Conference of Applied Physics Society 29P-C-
(see 5). The outline of this method will be explained using FIG. 1. 1 is a quartz tube, inside which silicon wafers 4 supported by a susceptor 3 coated with silicon carbide (SiC) are arranged side by side. One end of the quartz tube 1 is covered with a lid 2, and the other end is connected to an exhaust system. A coil 5 is installed on the outer periphery of the quartz tube 1.
is wound around the coil 5, and an RF power source 6 is connected to both ends of the coil 5. 7 is a gas introduction part, from which gas such as NH ₃ , N ₂ +H ₂ or N ₂ is supplied.

Now, set the pressure inside the quartz tube 1 so that the NH ₃ gas is 0.1 to 10 Torr, and set the pressure in the quartz tube 1 to 400KHz from the RF power supply 6.
When RF power is supplied, gas plasma 8 is generated in the quartz tube 1 by glow discharge, and at the same time, the silicon wafer 4 is heated by induction heating. According to reports, nitrogen-containing gas plasma8
A reaction occurs between and the silicon wafer 4, and NH ₃
A Si ₃ N ₄ film of about 100 Å is formed in about 100 minutes under the conditions of a flow rate of 1/min, RF power of 10 KW, and silicon wafer temperature of 1050°C.

However, according to the results of the present inventor's repeated trials of this method, although a Si ₃ N ₄ film of approximately 100 Å was indeed obtained, the Si-Si ₃ N ₄ interface characteristics were unstable. As a result of investigating the cause, it became clear from film thickness measurements that much of the Si ₃ N ₄ film obtained using this method was not deposited by direct nitriding of the silicon wafer, but by CVD. . This is due to Si generated when the inner wall of the quartz tube and silicon wafer are spattered by gas plasma.
This is due to reaction with _NH3 . Furthermore, damage to the silicon wafer surface due to gas plasma is also considered to be an unstable factor for the Si ₃ N ₄ film produced by this method.

Other methods to form _{thicker Si3N4} films include
A method has also been proposed in which N is ion-implanted into a silicon wafer and heat-treated. However, this method also damages the silicon wafer, so stable Si−Si ₃ N ₄
There is a drawback that interfacial properties cannot be obtained.

[Purpose of the invention]

An object of the present invention is to provide a method for forming a nitride film that makes it possible to obtain a nitride film with excellent properties only by a direct nitriding reaction without damaging the sample surface and without involving any deposition phenomenon caused by CVD. shall be.

[Summary of the invention]

The present invention is characterized in that a gas containing nitrogen is introduced into a sample substrate on which a silicon oxide film has been previously formed on the surface of a silicon layer, and light is irradiated at the same time to directly nitride the sample surface.

In the present invention, the light irradiated onto the sample surface is
It is desirable that the ultraviolet light contains a wavelength component of 500 nm or less as a main component.

〔Effect of the invention〕

In the present invention, a high-quality nitride film having a thickness of about 70 Å or more can be formed on the sample surface at a relatively low temperature and in a short time by the effect of light irradiation and the effect of adding a gas containing fluorine. . Addition of a fluorine-containing gas serves to constantly activate the surface of the nitride film being formed, thereby increasing the growth rate of the nitride film. Furthermore, since no glow discharge or ion implantation is used, there is no damage to the sample surface, and there is no deposited film due to CVD, so the interfacial properties of the nitride film are stable. According to the present invention, it is possible to easily form a composite film of a silicon oxide film and a silicon nitride film that has excellent insulating properties and has a higher dielectric constant than a silicon oxide film alone.

Therefore, if the present invention is applied to, for example, the formation of MOS capacitors in DRAM, higher integration will become possible.
It is possible to stabilize the characteristics of DRAM and improve reliability.

In the present invention, the reason why the growth rate of a nitride film by direct nitriding is improved by light irradiation is not yet clear. Recently, CVD and dry etching using optical excitation have been attracting attention, and it has been revealed that optical irradiation is also effective for forming oxide films by thermal oxidation. Light irradiation in these methods is performed to excite and activate the supplied reactive gas. In the case of the present invention, for example, for _N2 gas supplied for nitriding, ultraviolet light of about 300 to 500 nm has insufficient photon energy to excite N atoms, and even if N atoms Even if N atoms are excited, as mentioned above, if a very thin nitride film is formed on the sample surface, the supply of N atoms to the sample surface will be blocked. It is thought that the nitride film growth rate is improved by a special mechanism different from the above.

[Embodiments of the invention]

FIG. 2 is a schematic diagram of a nitride film forming apparatus used in an embodiment of the present invention. Reference numeral 11 denotes a stainless steel chamber, in which a sapphire sample holder 13 supported by a quartz support 12 is placed, and a sample 14 is placed on this sample holder 13. The sample holder 13 has a built-in tungsten heater 15.
The lead wire 16 of No. 5 is led out of the chamber 11 and connected to a DC power source 17. Sample holder 13
The temperature can be raised to approximately 1200℃ using DC resistance heating. A thermocouple 18 is directly fixed to the sample 14 with, for example, alumina cement, and connected to an external temperature sensor 19.

The inside of the chamber 11 is first opened by opening the valve 20 and roughly pumped out from the exhaust port 21 by an oil diffusion pump (not shown), and then closed by closing the valve 20 and draining the gate valve 2.
2 is opened, and the main exhaust port 23 is evacuated using a cryopump (not shown). And the degree of vacuum
After reaching the 10 ^-6 Torr level, the gate valve 22 is closed and a gas containing nitrogen is supplied from the gas inlet 24. 25 is an exhaust port for the supplied gas.

26 is a light source that illuminates the surface of the sample 14;
It is a Hg-Xe lamp. The light from the lamp 26 is focused by a parabolic mirror 27 of revolution, passes through a fused quartz plate 28 provided on the upper lid 30 of the chamber 11, and is irradiated onto the sample 14. 29 ₁ ,
29 ₂ is a clamp for preventing the top cover 30 and the quartz plate 28 from coming off when the pressure inside the chamber 11 is 1 atmosphere or more.

In addition, when performing nitriding under reduced pressure, valve 2
0 to set the pressure in the chamber 11 to a predetermined pressure, and when nitriding is performed at normal pressure, the introduced gas is discharged from the gas outlet 25 with the valve 20 closed. The gas outlet 25 is provided with a check valve (not shown), and this check valve closes under reduced pressure, and when the pressure is normal, the introduced gas flows out from the gas outlet 25. It will be kept at normal pressure.

Using this apparatus, prior to working examples of the present invention, specific experimental data will be described in which silicon wafers were directly nitrided by light irradiation using only N ₂ gas as basic data. The introduced gas is _N2 ,
The flow rate was fixed at 1000c.c./min to maintain normal pressure inside the chamber 11, and while heating the silicon wafer,
At the same time, the surface of the silicon wafer was irradiated with the Hg-Xe lamp 26. The light intensity on the silicon wafer surface is
The power was fixed at 100 mw/cm ² and the nitriding time was 60 minutes.

FIG. 3 shows the spectrum of the Hg-Xe lamp 26 used. As is clear from the figure,
This light consists of a line spectrum of 500 nm or less,
It can be seen that most of the UV rays are 350 nm or less.

FIG. 4 shows the thickness of the Si ₃ N ₄ film obtained in this experimental example in relation to the heating temperature of the silicon wafer. The solid line is an experimental example, and the broken line is a reference example in which no light irradiation was performed.
Film thickness was measured using an ellipsometer. As is clear from the figure, without light irradiation, the silicon wafer is hardly nitrided, even at 1050℃.
The Si ₃ N ₄ film is 20 Å or less. On the other hand, in this experimental example, a film thickness of nearly 100 Å was obtained at 1050° C., and the growth rate was about five times that of the case without light irradiation. In addition, Si ₃ N ₄ formed in this experimental example
Observation with an electric microscope revealed that the surface of the film was smooth and did not form islands.

The Si ₃ N ₄ film obtained in this experimental example has excellent film quality and stability of interfacial properties. Figure 5 shows
These are the results of analyzing the elemental composition in the depth direction of the Si ₃ N ₄ film obtained at 1050°C in this experimental example using an Auger spectrometer. As is clear from the figure, the obtained
The Si ₃ N ₄ film contains almost no O atoms.
The high O concentration on the surface is thought to be the result of the sample surface being exposed to air when the sample is introduced into the Augier analyzer.

As mentioned above, the reason why the growth rate of the Si ₃ N ₄ film increases due to light irradiation is not yet clear. It was also confirmed that the temperature increase on the sample surface due to light irradiation was small, and that the growth rate was not increased by increasing the sample temperature. Figure 6 shows the change in sample temperature when the sample temperature is set at 1000°C and the Hg-Xe lamp 26 is turned on at point A. Even after 60 minutes of light irradiation, the sample temperature remains at 1020°C. It is clear that there is almost no effect of temperature rise.

In the above experimental example, the 90 Å obtained at 1050℃
A MOS capacitor with an area of 1 mm ² was formed using a Si ₃ N ₄ film, and the breakdown voltage distribution was measured. The results are shown in FIG. This breakdown voltage characteristic is extremely superior to that of the conventional method using a Si ₃ N ₄ film. Although basic data using only N ₂ gas has been shown above, in the present invention, it is actually mixed with fluorine-containing gas, for example, one or more gases selected from CF ₄ , NF ₃ , XeF ₂ , etc. can do. This results in a faster nitride film growth rate. This is due to the following reasons. Once a stable nitride film of a certain thickness is formed by direct nitriding, further nitriding reactions are suppressed. However, when fluorine is included in the raw material gas, the bond of the formed nitride film is broken and a constantly active surface state is obtained, which accelerates the nitriding reaction.

Note that the present invention is not limited to the above embodiments. For example, a sample on which a Si ₃ N ₄ film is to be formed by nitridation is not limited to a single-crystal silicon wafer, but may be any substrate that includes a single-crystal, polycrystalline, or amorphous silicon layer at least in the surface portion.
It is also effective when directly nitriding substrates made of other materials.

Next, embodiments according to the present invention will be described. Figure 8a
As shown in FIG. 3, a silicon wafer 31 with a thin SiO ₂ film 32 formed on its surface by thermal oxidation or CVD was used as a sample.
By introducing a mixed gas of N-containing gas and F-containing gas and performing direct nitriding while irradiating with light, a Si ₃ N ₄ film 33 is formed under the SiO ₂ film 32 as shown in FIG.
can be formed. This is confirmed by Augier analysis. According to this embodiment, it is possible to obtain a composite insulating film that has excellent insulating properties and a larger dielectric constant than a silicon oxide film alone. This composite film exhibits excellent properties as, for example, a capacitor insulating film for DRAM and an interlayer insulating film for various ICs.

In the example, N ₂ was used as the introduced gas, but N ₂
NH ₃ can be used instead.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a nitride film forming apparatus using glow discharge, Figure 2 is a schematic diagram of a nitride film forming apparatus used in an embodiment of the present invention, and Figure 3 is used as a light source for the same apparatus. Figure 4 is a diagram showing the emission spectrum of the Hg-Xe lamp obtained in the above example.
A diagram showing the relationship between the film thickness of the Si ₃ N ₄ film and the sample temperature in comparison with a reference example, and Figure 5 is also based on the above example.
FIG. 6 is a diagram showing the results of Auger analysis of the Si ₃ N ₄ film, FIG. 6 is a diagram showing the temperature change of the sample in the above example, and FIG. 7 is a diagram showing the MOS capacitor using the Si ₃ N ₄ film according to the above example. Figures 8a and 8b, which show the breakdown voltage distribution, are diagrams showing the state of Si ₃ N ₄ film formation according to another example. 11... Chamber, 13... Sample holder, 14
... Sample, 15 ... Heater, 24 ... Gas inlet, 25 ... Gas discharge port, 26 ... Hg-Xe lamp, 27 ... Paraboloid of revolution mirror, 28 ... Fused quartz plate.

Claims (1)

[Claims] 1. A sample substrate having at least a silicon layer on the surface and a silicon oxide film is placed in a chamber, and a gas containing nitrogen is introduced into the chamber.
A method for forming a nitride film, comprising directly nitriding the surface of the sample substrate while irradiating the surface of the sample substrate with light. 2 The nitrogen-containing gas is N ₂ or NH ₃ ,
The method for forming a nitride film according to claim 1, wherein the gas is a mixed gas containing at least one gas selected from CF ₄ , NF ₃ , and XeF ₂ .