JP2506726B2 - Method of manufacturing nonvolatile memory device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a metal-silicon nitride film-silicon oxide film-
A method for manufacturing a nonvolatile memory device comprising a semiconductor (hereinafter abbreviated as MNOS) type field effect transistor, and a method for manufacturing a MNOS nonvolatile memory device having excellent memory retention characteristics with good controllability and high productivity. It is provided.

2. Description of the Related Art The MNOS nonvolatile memory device has an electrical charge injection from the semiconductor side through the silicon oxide film that can serve as a tunneling medium into the interface between the silicon nitride film and the silicon oxide film or into the silicon nitride film. And, by accumulating the information, the threshold voltage (Vth) of the transistor is changed to store information. In particular, securing the memory retention characteristic is the biggest practical problem of the MNOS nonvolatile memory device. Has become.

The memory retention characteristic of the MNOS type nonvolatile memory device can be generally expressed by the following equation.

However, Q _N ; Amount of charge accumulated in silicon nitride, J ₀ ;
Current that flows in a semiconductor substrate by tunneling, J _N : current that flows in a gate electrode through a transition between traps or a conduction band or a valence band of silicon nitride due to thermal excitation.

As can be seen from the above equation, in order to improve the memory retention characteristics, the charge components J ₀ and J _N flowing out to the substrate and the gate electrode should be made as small as possible, but J ₀ and J _N are
The parameters also depend on the thickness of the silicon oxide film, the distribution of traps in which charges are accumulated, the amount of accumulated charges, the state of the silicon oxide-semiconductor interface, and the state of the silicon oxide film-silicon nitride film interface. There is a significant dependence on the electrical conductivity of the silicon nitride film that is the gate insulating film, and the lower the electrical conductivity, the better the memory retention characteristics of the MNOS nonvolatile memory device.

Problems to be Solved by the Invention However, the amount of charge traps and the electrical conductivity of the silicon nitride film are in a substantially proportional relationship, and for example, a sufficient ΔVth
In order to obtain (amount of change in threshold voltage), a silicon nitride film with a large amount of charge traps, that is, a high electric conductivity must be used, and a significant improvement in memory retention characteristics cannot be expected with the conventional structure. is the current situation.

A first object of the present invention is to provide a manufacturing method for remarkably improving the memory retention characteristic of the MNOS type nonvolatile memory device with respect to this problem.

Means for Solving the Problems To achieve this object, according to the present invention, when forming a silicon nitride film, a silicon nitride film having a relatively high electrical conductivity and a relatively low electrical conductivity having different chemical composition ratios are provided. A method for manufacturing a non-volatile memory device including a step of stacking a silicon nitride film. It is appropriate that the laminated silicon nitride films having different chemical composition ratios are continuously vapor-phase grown under reduced pressure without being exposed to the atmosphere.

Effect According to the present invention, a silicon nitride film having a relatively high electrical conductivity and a silicon nitride film having a relatively low electrical conductivity, which have different chemical composition ratios, are continuously laminated under a reduced pressure so that It is possible to obtain memory retention characteristics superior to those of a single-layer silicon nitride film formed by vapor deposition under reduced pressure.

Examples Hereinafter, specific examples of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view of a semiconductor device manufactured according to an embodiment of the present invention. In the device of FIG. 1, first, for example, a source region 2 and a drain region 3 are formed on an N-type silicon substrate 1 by a well-known selective diffusion technique, and a predetermined portion of a silicon oxide film 4 formed at the time of selective diffusion is formed by a normal method. After the holes are formed by photoetching, a thin silicon oxide film 5 having a thickness of about 20 Å is formed by oxidizing the holes at 800 ° C. in an oxygen atmosphere.

Then, the silicon nitride films 6 and 7 having different film qualities are continuously formed on the silicon oxide film 5 by a vapor phase growth method based on a chemical reaction of dichlorosilane (SiH ₂ Cl ₂ ) and ammonia (NH ₃ ) under reduced pressure. To form. In this example, first, the first silicon nitride film 6 having a relatively high electric conductivity under a reduced pressure of 750 ° C., NH ₃ / SiH ₂ Cl ₂ = 5, and a pressure of 300 to 400 mTorr.
For about 200Å, and then immediately after the growth is completed, evacuation is performed, and after reaching a sufficient degree of vacuum (for example, 1 mTorr or less), the reaction gas is introduced again, and the temperature is 750 ° C., NH ₃ / SiH ₂ Cl ₂ = 5
A second silicon nitride film 7 having a relatively low electric conductivity was formed at a pressure of 500 to 600 mTorr at a pressure of about 300 Å. By evacuation, the mixing ratio of the reaction gas can be once returned to the initial state. Furthermore, since the reaction gas is drawn in when the reaction gas is introduced, a uniform flow rate ratio of the reaction gas can be obtained. As a result, even when a large number of MNOS nonvolatile memory devices are manufactured,
Stable memory retention characteristics can be realized. Finally, as the gate electrode, the aluminum thin film 8 is deposited by about 1.0 μm by the usual vapor deposition method. In the other steps, the MNOS type nonvolatile memory device was formed according to a normal MOS process.

An example of the memory retention characteristics of the MNOS type nonvolatile memory device obtained as described above is shown in FIG. The horizontal axis is the threshold voltage, and the vertical axis is the decay rate of the accumulated charge (2Vth / 2logt, Vth:
Threshold voltage, t: time). The smaller the slope of the straight line in this figure, the better the memory retention characteristics. As shown in FIG. 2, the memory retention characteristic of the nonvolatile memory device formed by the manufacturing method of the present invention shows a characteristic of a straight line 9, and this characteristic is obtained by using only the vapor phase growth method under atmospheric pressure. Compared to any one of the straight line 10 in the case of a single-layer silicon nitride film and the straight line 11 in the case of a single-layer silicon nitride film using only the vapor phase growth method under reduced pressure, the slope is small and excellent. It can be seen that it has a memory retention characteristic.

On the other hand, when comparing the yields as an integrated circuit in the case where the step of laminating the silicon nitride film is continuously performed under reduced pressure and the case where the step of exposing the layer to the atmospheric pressure once after forming the first layer is compared, the yield is reduced. In the case where the layers were successively laminated below, a higher yield was shown, on the order of 5 to 10% on average. The difference is that when exposed to atmospheric pressure, local abnormalities in the interface occur due to the adhesion of fine particles such as fine dust, and the chemical composition changes in the surface state due to exposure to atmospheric pressure. That is the main reason. Therefore, by successively stacking the silicon nitride films under reduced pressure, it is possible to manufacture a semiconductor memory device with good controllability and high productivity.

The temperature and the reaction gas flow rate ratio during the growth of the silicon nitride film are not limited to those in this example, and the temperature is 700 ° C. to 800 ° C. under a reduced pressure and the flow rate ratio (NH
₃ / SiH ₂ Cl ₂ ) is 0.1 to 100 when the first layer is formed, 10 when the second layer is formed.
Various types of silicon nitride films were stacked under the conditions of up to 1000, and the study was conducted. In any case, the effect of the present invention was found. In this embodiment, an example is shown in which two layers of silicon nitride films having different chemical composition ratios are laminated, but three or more layers of silicon nitride films may be laminated under reduced pressure.

In this embodiment, the case of forming the P-channel type MNOS nonvolatile memory device in which the source and the drain are formed by selective diffusion has been described.
It can also be used in the MNOS type non-volatile storage device. Alternatively, a high-melting point metal such as polysilicon or tungsten silicide may be used as the gate electrode to form a gate insulating film by the manufacturing method of the present invention, and the source and drain may be formed by a known self-alignment technique.

As described above, according to the present invention, when forming a silicon nitride film that is a gate insulating film of a MNOS type nonvolatile memory device, a silicon nitride film having a relatively high chemical conductivity and a relatively high electrical conductivity is used. Compared to the MNOS nonvolatile memory device using a single-layer silicon nitride film formed only under atmospheric pressure or only under reduced pressure by successively stacking silicon nitride films having low electric conductivity under reduced pressure, A MNOS nonvolatile memory device can be manufactured which has excellent memory retention characteristics and can be manufactured with good controllability and high productivity when forming a multilayer silicon nitride film. High performance,
This greatly contributes to high productivity.

[Brief description of drawings]

FIG. 1 is a sectional view of a semiconductor device obtained in one embodiment of the present invention, and FIG. 2 is a characteristic diagram for explaining the effect of the present invention. 1 ... Silicon substrate, 2, 3 ... Source and drain, 4
...... Silicon oxide film, 5 ... Ultra-thin silicon oxide film, 6,7
...... Silicon nitride film, 8 …… Aluminum film.

Claims (1)

(57) [Claims] 1. A step of forming a thin silicon oxide film, which can serve as a tunneling medium for holes or electrons, on a surface of a semiconductor substrate of one conductivity type, and a silicon nitride film having a high electric conductivity is formed on the silicon oxide film. (SiH ₂ Cl ₂ ) and ammonia (NH ₃ ) at a flow rate ratio NH ₃ / SiH ₂ Cl ₂ = 0.1 to 100, vapor phase growth, vacuuming, and silicon nitride having high electrical conductivity. Dichlorosilane and ammonia for vapor-depositing highly conductive silicon nitride are included, including the step of vapor-depositing a silicon nitride film with low electrical conductivity at a flow rate ratio of NH ₃ / SiH ₂ Cl ₂ = 10 to 1000. The method for manufacturing a non-volatile memory device, wherein the flow rate ratio is smaller than the flow rate ratio when vapor-depositing silicon nitride having low electric conductivity.