JPH0334672B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides excellent nonvolatile performance, particularly excellent memory retention characteristics, in a nonvolatile memory device having a basic configuration of an MNOS (metal mononitride film-silicon dioxide film-semiconductor) field transistor structure. The present invention relates to a high-performance nonvolatile memory device and a method for manufacturing the same.

Conventionally, it has been usual to use an Al electrode as the gate electrode of an MNOS type nonvolatile memory device.

In recent years, as semiconductors have become smaller in size, more highly integrated, and faster, there has been a trend towards using high melting point materials such as polysilicon as gate electrodes of MOS structural elements. However, in the MNOS type structure, high-temperature treatment of about 1000° C. is usually required when forming the high-melting point metal material on the gate electrode, which is known to deteriorate the memory retention characteristics of the MNOS element.

MNOS type non-volatile memory devices utilize charge transfer from the semiconductor side through the ultra-thin silicon dioxide film to traps distributed at the interface between a silicon nitride film and an ultra-thin silicon dioxide film, or in the bulk of the silicon nitride film. Tunneling injection and its accumulation change the threshold voltage Vth of the transistor and store information.Ensuring the memory retention characteristics is the biggest challenge for MNOS type nonvolatile memory devices, and it is difficult to put it into practical use. This has become an important issue.

An object of the present invention is to provide an MNOS insulating film structure having a high melting point metal gate electrode and a method for manufacturing the same, which can dramatically improve memory retention characteristics in terms of high integration and high precision.
That is, the present invention is characterized in that two or more silicon nitride films having different hydrogen contents are stacked when forming the gate insulating film.

According to the experiments of the present inventors, the deterioration of memory retention characteristics due to high-temperature heat treatment is related to the hydrogen contained in silicon nitride in the MNOS structure, especially the content of Si-H bonds. When a silicon nitride film is subjected to high-temperature treatment at 900° C. or higher, the number of Si--H bonds decreases, unstable traps are added and increased, and memory retention characteristics deteriorate. On the other hand, it has been found that a silicon nitride film with few Si--H bonds hardly produces the unstable traps even when subjected to high-temperature treatment at 900 DEG C. or higher, so that there is little change in memory retention characteristics. In other words, it has been found that the deterioration in characteristics due to high temperature treatment after forming a silicon nitride film largely depends on the hydrogen content during the formation of the silicon nitride film.

Furthermore, according to the study results of the present inventors, the hydrogen content in a silicon nitride film strongly depends on the temperature during vapor phase growth using silane and ammonia; (1) The higher the growth temperature, the lower the total hydrogen content. content, and Si−
H-bonds tend to decrease (e.g. at 700℃
(approximately 8% at 900℃, approximately 6% at 900℃).

(2) When the growth temperature is 900°C or higher, almost no Si-H bonds exist.

It was discovered that This result is consistent with the results of other researchers (PSPeery ital, J.
Elictrorils Mat′, 8.11, (1979)).

The present invention has been made based on the above facts. The MNOS structure according to the present invention will be explained below. In the embodiment of the present invention, Si-
A silicon nitride film with many H bonds is formed, and then
On this silicon nitride film, high temperature (900 to 1000℃)
By stacking silicon nitride films with few Si-H bonds, the first silicon nitride film is treated at high temperature during the growth of the second silicon nitride film, creating an effective trap. A silicon nitride film with low hydrogen content can be formed thereon. By adopting the structure of the present invention, it is possible to appropriately increase the size of the threshold voltage window (△Vth) necessary for the memory characteristics of MNOS, and at the same time, the second layer of silicon nitride Since the film is grown at high temperature (~1000°C) during film formation, even if high temperature treatment is performed after depositing the gate electrode on the silicon nitride film, the memory retention characteristics will remain the same.
It was found that there was almost no deterioration.

Next, a specific example of a method for manufacturing MNOS having the structure of the present invention will be described with reference to the drawings.

As shown in FIG. 1A, a silicon dioxide film 2 with a thickness of 500 Å is formed on the entire surface of a P-type substrate 1, and a silicon nitride film 3 is further formed with a thickness of about 1200 Å, and predetermined portions are etched using a well-known photoetching technique for device isolation. Perform etching with.

Next, as shown in FIG. 1B, a field oxide film 4 of about 1 μm is formed by a normal thermal oxidation method.

Next, as shown in FIG. 1C, after sequentially etching the silicon nitride film 3 and the underlying silicon dioxide film 2, a thin silicon dioxide film 5 of about 20 Å is etched.
Formed by oxidation at 800℃ in an oxygen atmosphere.

Thereafter, as shown in FIG. 1D, NH ₃ /SiH ₄ is deposited on the silicon dioxide film 5 by a vapor phase growth method based on a chemical reaction between silane (SH ₄ ) and ammonia (NH ₃ ). A silicon nitride film 6 is formed to a thickness of 50 Å under conditions of =500 and 800°C. Furthermore, NH ₃ /SiH ₄ =1000,
A silicon nitride film 7 having a thickness of 500 Å is formed by vapor phase growth at 950°C. Next, a polysilicon film with a thickness of about 4000 Å is formed on the entire surface, and then the polysilicon is patterned by photo-etching, leaving only a portion that can become a gate, to form a gate electrode 8. Furthermore, using the gate electrode 8 and field oxide film 4 as a mask, phosphorus is implanted (100 KeV, 4×10 ¹⁵ cm ² ) to form a polysilicon gate electrode and source/drain regions 9 and 10.

Next, as shown in FIG. 1E, a silicon dioxide film 11 is deposited on the entire surface by a well-known vapor phase growth method, and then a silicon dioxide film 11 is deposited on the entire surface by a 1,000-μm film in order to push in the source/drain and to make the silicon dioxide film 11 denser. ℃ for 30 minutes
Heat treatment is performed in an _N2 atmosphere. Finally, in order to provide electrodes in the source/drain regions 9 and 10, a silicon dioxide film 11, a silicon nitride film 6,
7, and the silicon dioxide film 5 is etched to form contact holes, and source/drain electrodes 1 are formed.
2 and 13, an N-channel silicon gate MNOS type nonvolatile memory device can be manufactured.

FIG. 2 shows an example of the memory retention characteristics of the MNOS type nonvolatile memory device obtained as described above. The horizontal axis is the threshold voltage, and the vertical axis is the decay rate of the accumulated charge (∂Vth/∂logt; Vth: threshold voltage, t:
time). The smaller the slope of the line, the
This shows that it has excellent memory retention properties. As shown in FIG. 2, the memory retention characteristics (straight line 14) of the nonvolatile memory device formed according to the present invention are compared with the memory retention characteristics (straight line 15) of the conventional P-channel Al gate MNOS type nonvolatile memory device. However, we were able to fabricate a good nonvolatile memory device with comparable memory capacity and low decay rate.

In addition to this example, as a result of examining silicon nitride films with various hydrogen contents, we found that
It was confirmed that the effects of the present invention can be fully demonstrated by stacking a silicon nitride film grown under a high temperature condition of 900 to 1000 degrees Celsius on a silicon nitride film grown under a relatively low temperature condition of ~900 degrees Celsius. It was done.

In this example, the case where an N-channel nonvolatile memory device is formed using a P-type substrate has been explained, but it goes without saying that a P-channel type MNOS can also be used, and polysilicon can be used as a high-melting point metal gate electrode. I showed an example using
Needless to say, a high melting point metallic material such as Mo may be used.

As described above, the present invention provides a method for manufacturing an MNOS type nonvolatile memory device, in which even if high-temperature treatment is required after forming a silicon nitride film, when forming a silicon nitride film as a gate insulating film,
A silicon nitride film is formed using a vapor phase growth method under relatively low temperature conditions (700 to 900°C), and then nitriding is performed on the silicon nitride film using a vapor growth method under high temperature conditions (900 to 1000°C). By forming a silicon film and stacking silicon nitride films having different hydrogen contents, an excellent nonvolatile memory device without deterioration of memory retention characteristics can be manufactured. In particular, the present invention aims to improve the performance of MNOS type nonvolatile memory devices using polysilicon high-melting point metallic materials as gate electrodes, and to achieve high integration by using high-melting point metallic materials such as polysilicon as gate electrodes. This will greatly contribute to the

[Brief explanation of the drawing]

1A to 1E are process sectional views for explaining an embodiment of the present invention, and FIG. 2 is a characteristic diagram for explaining the effects of the present invention. 1...Silicon substrate, 2...Silicon dioxide film, 3...Silicon nitride film, 4...Silicon dioxide film, 5...Silicon dioxide film, 6...Silicon nitride film, 7...Silicon nitride film, 8... ... Polysilicon film, 9, 10 ... Source and drain, 11
...Silicon dioxide film, 12,13...Source
drain electrode.

Claims (1)

[Scope of Claims] 1. A silicon oxide film formed on a semiconductor substrate, a first silicon nitride film formed on the silicon oxide film, and the first silicon nitride film formed on the first silicon nitride film. a second silicon nitride film having a lower hydrogen content than the first silicon nitride film;
1. A nonvolatile memory device comprising an electrode film made of a high melting point metal material formed on a silicon nitride film. 2 forming a silicon oxide film on the main surface of the semiconductor substrate; growing a first silicon nitride film on the silicon oxide film in a vapor phase at a first temperature; and forming a first silicon nitride film on the first silicon nitride film. The method is characterized by forming a second silicon nitride film at a second temperature higher than the first temperature, and forming an electrode film made of a high melting point metal material on the second silicon nitride film. A method for manufacturing a non-volatile storage device. 3 The first temperature is 700℃-900℃, the second temperature is
3. The method of manufacturing a nonvolatile memory device according to claim 2, wherein the temperature is in the range of 900°C to 1000°C.