JPH0642550B2 - MIS type nonvolatile memory and method of manufacturing the same - Google Patents

Description

The present invention relates to a MIS type non-volatile memory and a method for manufacturing the same.

[Conventional technology]

Conventionally, in a MNOS integrated circuit which constitutes a MIS type nonvolatile memory, a gate insulating film is composed of a silicon oxide film and a silicon nitride film, and this silicon nitride film is generally NH ₃ and SiH ₂ Cl ₂ or NH It was formed by the CVD method with a fixed flow ratio of ₃ and SiH ₄ .

[Problems to be solved by the invention]

However, in the above-described conventional method for forming the silicon nitride film of the MNOS integrated circuit, since the flow rate ratio of the raw material gas is fixed, the composition ratio of the silicon nitride film becomes constant, and the write characteristic of injecting charges into the gate insulating film, Optimum of each memory characteristic such as erase characteristic for releasing injected charge, retention characteristic for injected charge (hereinafter referred to as memory characteristic), and deterioration of characteristic due to repeated write / erase (hereinafter referred to as fatigue characteristic) Could not get the characteristics.

Therefore, when the composition ratio r (r = N / Si) of the silicon nitride film is reduced in order to improve writing and erasing characteristics, Si- is formed at the interface between the silicon oxide film and the silicon nitride film.
There were drawbacks such as increased N-bonding and reduced fatigue properties.

An object of the present invention is to provide a MIS type non-volatile memory having improved memory characteristics and a manufacturing method thereof.

[Means for solving problems]

A MIS type nonvolatile memory according to a first aspect of the present invention is a MIS having a gate insulating film formed on a semiconductor substrate and having a two-layer structure including a silicon oxide film and a silicon nitride film, and a gate electrode provided on the gate insulating film. In the non-volatile memory, the composition of the silicon nitride film continuously changes in the depth direction, and the nitrogen concentration increases as it gets closer to the gate electrode.

In the method for manufacturing a non-volatile memory of the second invention, a silicon oxide film and a silicon nitride film are sequentially formed on a semiconductor substrate and then patterned to form a gate insulating film composed of two layers of a silicon oxide film and a silicon nitride film. A method of manufacturing a MIS non-volatile memory, wherein the composition of the silicon nitride film is continuously changed by a CVD method in which a flow rate ratio of a reaction gas is changed.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

1A to 1D are MISs according to the first embodiment of the present invention.
6A to 6C are cross-sectional views of the semiconductor chip in process order, for explaining the method for manufacturing the nonvolatile memory.

First, as shown in FIG. 1A, a field oxide film 2 for element isolation having a thickness of 1000 Å or more is formed on a silicon substrate 1 and then a lower gate insulating film is formed in an active region where a MNOS transistor is formed. Silicon oxide film 3 to 10
Form a few Å thickness.

Next, as shown in FIG. 1 (b), a silicon nitride film 4 serving as an upper gate insulating film is formed by a low pressure CVD method to a thickness of several hundred Å by continuously changing the composition ratio r in the thickness direction. To do. The silicon nitride film having the changed composition ratio r (N / Si) can be formed, for example, by changing the flow rate ratio R (NH ₃ / SiH ₂ Cl ₂ ) of NH ₃ and SiH ₂ Cl ₂ used in the low pressure CVD method. .

FIG. 2 is a diagram showing the relationship between the flow rate ratio R of the reaction gas and the thickness of the silicon nitride film when the silicon nitride film is formed in the first embodiment.

In FIG. 2, the thickness t from the interface of the silicon oxide film 3 is
_{At the} initial stage of formation of the silicon nitride film 4, which is the region of ₁ , the flow rate ratio R is set to several hundreds, and the flow rate ratio R is rapidly decreased to 10 or less. By this operation, the silicon oxide film 3
Then, a silicon nitride film with few Si—N bonds is formed at the interface of the silicon nitride film 4, and the fatigue characteristics are improved.

Then, in the region of t ₂ , the flow rate ratio R is made to react at an appropriate value of 10 or less. At this time, the silicon nitride film 4 becomes a film having many Si unbonded hands, traps for trapping charges are increased, and writing characteristics are improved.

Next, in the region of t ₃ , the flow rate ratio R is gradually increased,
(Finally, R ≧ 100) The silicon nitride film 4 is made to have a film quality with few Si end bonds. As a result, the charge transfer to the gate electrode side is reduced and the storage characteristics are improved.

Next, after forming the silicon nitride film 4 with different compositions in this way, a polycrystalline silicon film 5 is formed on the entire surface by a low pressure CVD method as shown in FIG. 1 (c).

Then, as shown in FIG. 1D, a gate electrode 5A made of polycrystalline silicon is formed by a dry etching technique by a photolithography process, and then a source / drain diffusion layer 6 of the MNOS transistor is formed by an ion implantation technique.
To form. Then, the MNOS transistor is completed according to a conventional method.

FIG. 3 is a flow rate ratio R of NH ₃ and SiH ₂ Cl ₂ at the time of forming a silicon nitride film for explaining the second embodiment of the present invention.
FIG. 6 is a relational diagram between the thickness and the thickness of the silicon nitride film.

That is, when the same operation as in the first embodiment shown in FIGS. 1A to 1D is performed, the flow rate ratio R is set to 10 or less in the region of the film thickness t ₂ ′ of the silicon nitride film. In the region of t ₃ ′, the flow rate ratio R is gradually increased (R ≧ 10
0) and react to form a silicon nitride film.

In the second embodiment, since the gate insulating film having silicon nitride, which is very easy to write and the memory characteristic is taken into consideration, is formed, it is possible to write at a low voltage, and it is possible to write the program only once. The so-called one-time PROM has an advantage that optimum memory characteristics can be obtained.

〔The invention's effect〕

As described above, according to the present invention, in a MIS type nonvolatile memory having a gate insulating film composed of a silicon oxide film and a silicon nitride film, the composition of the silicon nitride film is continuously changed to form a memory characteristic. There is an effect that can be improved.

[Brief description of drawings]

1 (a) to 1 (d) are sectional views of a semiconductor chip shown in the order of steps for explaining the first embodiment of the present invention, and FIG.
FIG. 3 is a diagram showing the relationship between the flow rate ratio of the reaction gas and the film thickness when the silicon nitride film is formed in the first embodiment, and FIG. 3 is the second diagram of the present invention.
FIG. 6 is a diagram showing the relationship between the flow rate ratio of the reaction gas and the film thickness when the silicon nitride film is formed in the example of FIG. 1 ... Silicon substrate, 2 ... Field oxide film, 3 ... Silicon oxide film, 4 ... Silicon nitride film, 5 ... Polycrystalline silicon film, 5A ... Gate electrode, 6 ... Source / drain region.

Claims (2)

[Claims] 1. A MIS having a gate insulating film formed of two layers of a silicon oxide film and a silicon nitride film formed on a semiconductor substrate, and a gate electrode provided on the gate insulating film.
Type non-volatile memory, wherein the composition of the silicon nitride film continuously changes in the depth direction, and the nitrogen concentration increases as it approaches at least the gate electrode. 2. A method of manufacturing a MIS non-volatile memory in which a silicon oxide film and a silicon nitride film are sequentially formed on a semiconductor substrate and then patterned to form a gate insulating film composed of two layers of a silicon oxide film and a silicon nitride film. At
A method of manufacturing a MIS type non-volatile memory, characterized in that the silicon nitride film is formed by continuously changing the composition by a CVD method in which a flow rate ratio of a reaction gas is changed.