JPH04278587A - Manufacture of semiconductor memory - Google Patents

Abstract

PURPOSE:To provide a manufacturing method for a floating gate type semiconductor memory in which a program voltage can be lowered without deteriorating memory characteristic and deterioration upon rewriting can be reduced. CONSTITUTION:A step of forming an insulating film 10 containing at least a silicon nitride film on one conductivity type semiconductor substrate 1, a step of hydrogen plasma processing, a step of forming a floating gate electrode 11 on the film 10, and a step of forming a control gate electrode 13 on the film 11 through an insulating film 12, are provided. Since hydrogen content of the nitride film for constituting the film 10 is increased by the hydrogen plasma processing, electric conductivity is reduced, and even if the silicon nitride film is used as a tunneling insulating film, memory characteristic is not deteriorated.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor memory device having excellent memory retention characteristics and comprising a field effect transistor having a floating gate structure.

0002

[Prior Art] Conventionally, electrically writable and erasable EE
PROM (Electrically Erasab
(Leand Programmable ROM)
One type of known semiconductor memory device is a floating gate structure that can be written and erased by tunneling injection. This semiconductor memory device with a floating gate structure tunnels charges from the semiconductor substrate side through a thin silicon oxide film, accumulates charges in the floating gate electrode on the insulating film, and changes the threshold voltage of the transistor. The principle is

A conventional method for manufacturing a semiconductor memory device will be described below. FIG. 3 is a cross-sectional view showing a typical example of a conventional floating gate structure semiconductor memory device. A drain region 32 and a source region 33 made of an N-type diffusion layer are formed in a P-type silicon substrate 31. A relatively thick silicon oxide film 34 is formed spanning the drain region 32 and source region 33. This silicon oxide film 34 has a hole formed in a portion above the drain region 32, and a thin silicon oxide film 35 that can serve as a tunneling medium is formed in this hole. Further, on the upper surface of the silicon oxide film 35, a floating gate electrode 36, a silicon oxide film 37, and a control gate electrode 38 are sequentially laminated.

In recent years, in semiconductor memory devices configured as described above, a thin silicon oxide film 35 (tunneling insulation Instead of a thin silicon nitride film, a two-layer film of silicon oxide film-silicon nitride film, a three-layer film of silicon oxide film-silicon nitride film-silicon oxide film, or a silicon nitride film such as an oxynitride film, It has been proposed to use a tunneling insulating film (for example, IDM Technical Digest 1
December 985, page 620: IEDM Tech. Di
g. , Dec. 1985, p. 620).

[0005]

[Problems to be Solved by the Invention] However, in the semiconductor memory device using the conventional silicon nitride film-based tunneling insulating film, the floating gate electrode 3
6 and the control gate electrode 38, a high-temperature heat treatment of approximately 900°C to 1000°C is required after forming the tunneling insulating film, and a silicon nitride film-based tunneling insulating film is required. This has had the problem of significantly deteriorating the film quality and deteriorating memory retention characteristics.

[0006] The present invention solves the above-mentioned conventional problems by realizing a lower programming voltage without deteriorating memory retention characteristics in a semiconductor memory device having a floating gate structure, and thereby reducing deterioration of memory characteristics due to rewriting. An object of the present invention is to provide a method for manufacturing a semiconductor memory device that prevents this.

[0007]

Means for Solving the Problems In order to achieve this object, the method of manufacturing a semiconductor memory device of the present invention includes the steps of: forming an insulating film containing at least a silicon nitride film on a semiconductor substrate of one conductivity type; a step of hydrogen plasma treatment; a step of forming a floating gate electrode on the insulating film;
The method includes a step of forming a control gate electrode on the floating gate electrode with an insulating film interposed therebetween.

[0008]

[Operation] According to the research conducted by the present inventors, the deterioration of memory retention characteristics due to high temperature heat treatment after forming a silicon nitride film strongly depends on the heat treatment conditions. If the temperature is below the formation temperature of the silicon nitride film, the memory retention characteristics will hardly deteriorate, but if the temperature exceeds the formation temperature of the silicon nitride film, the memory retention characteristics will deteriorate.

Furthermore, the degree to which the memory retention characteristics deteriorate is particularly related to the content of hydrogen contained in the silicon nitride film, particularly the content of Si--H bonds. That is, by heat-treating a silicon nitride film containing many Si--H bonds at a high temperature higher than the temperature at which the silicon nitride film was grown, the Si--H bonds are severed, increasing the number of unstable traps inside the film. Therefore,
It has been found that the electrical conductivity of the silicon nitride film increases and the memory retention characteristics of the semiconductor device deteriorate. That is, deterioration in memory retention characteristics due to high-temperature heat treatment after forming a silicon nitride film largely depends on the hydrogen content at the time of forming the silicon nitride film.

The present invention has been made based on the above facts, and it is possible to increase the hydrogen content of a silicon nitride film by performing hydrogen plasma treatment after forming a silicon nitride film-based tunneling insulating film. can. Therefore, the electrical conductivity is reduced, and excellent memory retention characteristics of the semiconductor device can be obtained without deterioration of the memory retention characteristics.

Furthermore, in a method for manufacturing a semiconductor memory device having a floating gate structure, after forming a tunneling insulating film, a floating gate electrode made of a high melting point metal such as a polysilicon film is usually formed thereon, and a source region, A drain region or an interlayer insulating film is formed. In this case, in order to activate and diffuse impurities in the source and drain regions, and to further densify the interlayer insulating film, heat treatment is performed at a high temperature higher than the temperature at which a silicon nitride film, which is a tunneling insulating film, is formed. It will be done. Therefore, it is necessary to perform hydrogen plasma treatment after all heat treatments at high temperatures are completed, and by doing so, the maximum effect of the present invention can be obtained.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a cross-sectional view of the manufacturing process of a semiconductor memory device according to an embodiment of the present invention. First, as shown in FIG. 1(A), a silicon oxide film 2 is formed on the entire surface of a P-type silicon substrate 1.
500 Å thick, and then a silicon nitride film 3 of 1000 Å thick.
A predetermined region for element isolation is etched using a known photoetching technique to expose the surface of the silicon substrate 1. Next, as shown in FIG. 1B, a field oxide film 4 of about 7000 Å is formed by normal thermal oxidation. Next, as shown in FIG. 1(C), the silicon nitride film 3
and the silicon oxide film 2 thereunder are sequentially etched to expose the silicon substrate 1. Thereafter, a silicon oxide film 5 having a thickness of 500 Å is formed by a normal thermal oxidation method. After that, using the photoresist 6 as a mask, phosphorus ions 7 were applied at 100 keV.
The implantation is performed under the condition of 1×10 15 cm −2 to form N-type diffusion layers 8 and 9. Next, as shown in FIG. 1D, the photoresist 6 is removed and a hole is opened in a portion of the silicon oxide film 5 above the N-type diffusion layer 9. A silicon nitride film 10 is formed on this silicon oxide film 5. Silicon nitride film 10
is formed by a reduced pressure vapor phase growth method based on a chemical reaction between dichlorosilane (SiH2Cl2) and ammonia (NH3) under the condition of NH3/SiH2Cl2 = 5,750°C, so that the film thickness is 120 Å. Next, as shown in FIG. 1E, a polysilicon film doped with about 2×10 20 cm −3 of phosphorus is formed on the silicon nitride film 10 to a thickness of about 3000 Å by a known vapor phase growth method.
Thereafter, a floating gate electrode 11 made of a polysilicon film is formed using a photoetching technique. next,
A silicon oxide film 12 is formed on the floating gate electrode 11 to a thickness of about 400 Å by a normal thermal oxidation method. After that, add about 2×1020c of phosphorus
A polysilicon film doped to about m-3 is grown to a thickness of about 4000 Å by a known vapor phase growth method, and a control gate electrode 13 made of the polysilicon film is formed by photo-etching. Using the control gate electrode 13 and field oxide film 4 as a mask, arsenic ions are applied at 50 keV.
Then, N-type diffusion layers 14 and 15 in the source and drain regions are formed by implanting 4×10 15 cm −2 . next,
As shown in FIG. 1F, after the silicon oxide film 16 is grown in a vapor phase over the entire surface, in order to activate the impurities in the source and drain regions and to densify the silicon oxide film 16,
Heat treatment is performed for 20 minutes in a nitrogen atmosphere at 1000°C. Next, hydrogen plasma treatment is performed. In this example, the hydrogen plasma treatment was carried out at a substrate temperature of 400°C and a gas pressure of 0.5 Torr.
, RF power was 100W. Finally, Figure 1
As shown in (G), in order to provide electrodes in the N-type diffusion layers 14 and 15 constituting the source and drain regions, a contact hole is formed in the silicon oxide film 16 by a known photoetching technique, and an aluminum electrode 17 is formed in the silicon oxide film 16. By forming this, a floating gate type semiconductor memory device is completed.

FIG. 2 is a memory retention characteristic diagram of a floating gate type semiconductor memory device manufactured through the steps described above, in which the vertical axis represents the threshold voltage and the horizontal axis represents the memory retention time. In FIG. 2, solid line A is the memory retention characteristic of the semiconductor memory device according to this embodiment, and broken line B is the memory retention characteristic of the conventional semiconductor memory device. Comparing the two, it can be seen that the threshold voltage retention time is clearly longer in the case of this example, and the memory retention characteristics are superior.

The case where a single-layer silicon nitride film is used as the tunneling insulating film has been described above, but this also applies to a two-layer film of silicon nitride film-silicon oxide film, or a two-layer film of silicon oxide film-silicon nitride film-silicon oxide film. It goes without saying that similar effects can be obtained by using a tunneling insulating film based on a silicon nitride film such as a three-layer film or an oxynitride film.

[0016]

As described above, the present invention provides a hydrogen plasma treatment step after forming an insulating film for tunneling, thereby reducing the programming voltage even when a silicon nitride film is used as the insulating film. Accordingly, it is possible to realize an excellent method for manufacturing a semiconductor memory device that can reduce deterioration of memory retention characteristics due to rewriting.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the manufacturing process of a semiconductor memory device according to an embodiment of the present invention.

[Fig. 2] Memory retention characteristic diagram of a semiconductor memory device manufactured by the manufacturing method of the present invention

[Figure 3] Cross-sectional view of a conventional semiconductor memory device

[Explanation of symbols]

1 P-type silicon substrate (semiconductor substrate) 10 Silicon nitride film (insulating film containing silicon nitride film) 11 Floating gate electrode 12 Silicon oxide film (insulating film) 13 Control gate electrode

Claims (2)

[Claims] 1. A step of forming an insulating film including at least a silicon nitride film on a semiconductor substrate of one conductivity type, a step of performing hydrogen plasma treatment, and a step of forming a floating gate electrode on the insulating film, forming a control gate electrode on the floating gate electrode with an insulating film interposed therebetween. 2. The step of forming the insulating film containing the silicon nitride film is a step of forming the insulating film containing the silicon nitride film and then performing heat treatment at a temperature higher than the formation temperature of the silicon nitride film. A method for manufacturing a semiconductor memory device.