JPH0389556A - Manufacture of semiconductor memory - Google Patents

Abstract

PURPOSE:To remarkably improve repetitively writing characteristic by conducting all heat treating steps after the formation of a second insulating film at +50 deg.C or lower for forming temperature of the film. CONSTITUTION:A source region 2 made of an N-type diffused layer and a drain region 3 are formed on a P-type silicon substrate 1, and a silicon oxide film 4 is formed. The thickness of the film 4 is required to be so increased as not to cause a tunnel. Then, an opening to become a tunnel region is formed by etching. A thin silicon oxide film 5 is formed in the opening, and its forming temperature is necessary to by 1050 deg.C or lower. All heat treating temperatures after the formation of the film 5 are set to (oxide silicon film forming temperature) +50 deg.C or lower. Strain or traps are hardly generated in the silicon oxide film of a very thin tunnel medium of about 100Angstrom , and an accurately and stably writing number can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method of manufacturing a semiconductor memory device comprising a floating gate field effect transistor.

BACKGROUND OF THE INVENTION Conventionally, EEPROM (El
electrically Erasable and P
2. Description of the Related Art A semiconductor memory device with a floating structure in which writing and erasing is performed by tunnel injection is well known as one type of programmable ROM. This floating gate type semiconductor memory device tunnel-injects charges through a thin insulating film on the diffusion layer, accumulates charges in the floating gate electrode on the insulating film, and changes the threshold voltage of the transistor. The principle is to memorize information.

FIG. 2 shows a cross-sectional structural diagram of a typical floating gate type semiconductor memory device. As shown in FIG. 2, a source region 2 consisting of an N-type diffusion layer is formed in a P-type silicon substrate l.
and a drain region 3 are formed, and a relatively thick silicon oxide film 4 is formed over the source region 2 and the drain region 3.
is formed, and only a portion of this silicon oxide 4 is opened, a thin silicon oxide film 5 that can serve as a tunnel medium is formed in this opening, and a floating gate electrode 6. is formed on the silicon oxide film 4.5. It has a structure in which a silicon oxide film 7 and a control gate electrode 8 are sequentially laminated.

Conventionally, when manufacturing a floating gate type semiconductor memory device as shown in FIG. 2, a relatively thick silicon oxide film 4 is usually formed on the drain region 3, and a portion of this silicon oxide film 4 is etched using a known photoetching technique. A hole is opened to reach the drain region 3, and usually 1
In order to be able to write and erase with a program voltage of 5-20V, it is necessary to form a very thin silicon oxide film 5 of about 100 A, but in order to form this very thin silicon oxide film 5 stably with good controllability. Usually 900
It was formed by oxidation at a relatively low temperature of around °C.

On the other hand, after forming the tunnel insulating film, it is necessary to form a floating gate electrode on the tunnel insulating film and further to form a control gate electrode on the floating gate electrode via an insulating film. Since the electrode usually uses a high melting point metal such as polysilicon, a relatively high temperature heat treatment process of about 1000° C. to 1100° C. is usually used as the heat treatment after forming the tunnel insulating film.

Problems to be Solved by the Invention Floating gate type semiconductor memory devices perform rewriting by performing charge tunneling through a very thin tunnel insulating film. The biggest problem with storage devices, and one of the most important in practical terms, is that floating gate semiconductor storage devices manufactured using the conventional manufacturing method described above are extremely susceptible to destruction if repeatedly rewritten. However, the problem was that it was very difficult to ensure an accurate and stable number of rewrites.

The present invention solves the above-mentioned conventional problems, and an object of the present invention is to provide a manufacturing method for a semiconductor memory device having a floating gate structure, which can dramatically improve the repeatable rewriting characteristics. It is something.

Means for Solving the Problems In order to achieve the above object, the present invention provides a step of forming a diffusion layer of a conductivity type opposite to that of the substrate from the surface to the inside of a semiconductor silicon substrate of one conductivity type, and a step of forming a diffusion layer on the surface of the substrate. forming a first insulating film in the first insulating film, forming an opening in a predetermined portion of the first insulating film to reach the diffusion layer, and forming a tunnel on the surface of the diffusion layer in the opening. forming a second insulating film that serves as a medium; forming a floating gate electrode on the second insulating film; and forming a control gate electrode on the floating gate electrode via a third insulating film. In the method for manufacturing a semiconductor memory device including at least the step of forming the second insulating film, all heat treatment steps after forming the second insulating film are performed at a temperature of +50° C. or less for forming the second insulating film.

According to the study conducted by the present inventor, it has been found that the number of repeated rewrites leading to destruction is largely dependent on the heat treatment process after forming the second insulating film, and in particular, the heat treatment process after forming the second insulating film. It has been found that when the temperature is set to a temperature higher than the formation temperature of the second insulating film by more than 50° C., the repeated rewriting characteristics deteriorate significantly.

The present invention has been made based on the above fact, and by setting the heat treatment temperature after forming the tunnel insulating film as in the present invention to 50°C or less above the formation temperature of the tunnel insulating film, it becomes difficult to break down even when repeatedly rewritten. , it becomes very easy to ensure reliability. Although the details of the mechanism by which such repeated rewriting characteristics are dramatically improved are unknown, the temperature of the heat treatment after forming the second insulating film, which will serve as a tunnel medium, is increased by 50° to the second insulating film formation temperature.
It is estimated that by keeping the temperature below ℃, the thermal stress on the thin second insulating film will be alleviated, causing less distortion and traps in the second insulating film, and making it less likely to be destroyed even after repeated rewriting. be done. Further, when the formation temperature of the second oxide film becomes 1050°C or higher, the effect of the present invention is almost lost, and the formation temperature of the tunnel oxide film becomes 1050°C or higher.
It was also found that it is necessary to keep the temperature below ℃.

Example Specific embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a step-by-step sectional view showing an embodiment of the present invention. First, as shown in FIG. 1A, on a P-type silicon substrate 1,
A source region 2 and a drain region 3 made of N-type diffusion layers are formed by a known selective diffusion technique, and then a silicon oxide film 4 is formed by a normal thermal diffusion method. The thickness of the silicon oxide film 4 needs to be thick so that tunneling from the substrate does not occur, and in this example, it was set to about 500A. Next, a predetermined portion of the silicon oxide film 4 on the drain region 3 is etched using a known photoetching technique to form an opening that will become a tunnel region.

Next, as shown in FIG. 1B, a thin silicon oxide film 5 that can serve as a tunnel medium is formed in the opening, but the formation temperature must be 1050° C. or lower. This example was carried out at 1000° C. in a dry oxygen atmosphere diluted with argon. Further, in order to effectively utilize the tunnel effect, the thickness of the silicon oxide QI 5 needs to be approximately 50-150A, and in this example, it was formed to be 100A.

Furthermore, the temperature of all heat treatments after the formation of the tunnel silicon oxide film 5 must be lower than the silicon oxide film formation temperature of the tunnel medium + 50°C, and in this example, the temperature is 950°C or lower as described below. did.

First, as shown in FIG. 1C, a floating gate electrode 6 made of a polysilicon film is placed on a silicon oxide film 5.
However, in this example, in order to form it at as low a temperature as possible, phosphorus was doped (approximately 6 x 10 "cm-3)" polysilicon film was formed at 600°C and approximately 5000A,
Thereafter, phosphorus was activated at 900°C. Thereafter, a floating gate electrode 6 made of a polysilicon film is formed using a well-known photoetching technique. Next, the surface of the floating gate electrode 6 is oxidized to form the silicon oxide film 7. In this embodiment, the silicon oxide film 7 is formed at 950° C. in a dry oxygen atmosphere diluted with argon.
The thickness was controlled to be about 50 OA on the floating gate electrode 6. Then, silane (SiH
4) and phosphine (PHs) by a low pressure vapor phase growth method using a mixed gas of
m-') polysilicon film at 600℃ and approximately 5000A.
After formation, phosphorus activation was performed at 900°C. Next, a control gate electrode 8 made of a polysilicon film is formed using a well-known photoetching technique.

Next, as shown in the first diagram, a silicon oxide film 9 is deposited on the entire surface at 450°C by atmospheric pressure vapor phase growth using a chemical reaction between silane (SiH4) and oxygen, and then the source and drain are pressed in. 95 to make the silicon oxide film 9 denser.
Heat treatment was performed at 0° C. for 30 minutes in a nitrogen atmosphere. Thereafter, a contact hole is formed in the silicon oxide film 9 by a known photoetching technique, and an aluminum electrode 10 is formed to form a floating gate type semiconductor memory device as shown in the first diagram.

FIG. 3 shows an example of the repeated rewriting characteristics of the floating gate type semiconductor memory device obtained as described above. The vertical axis is the cumulative defective rate, and the horizontal axis is the number of rewrites. Third
As shown in the figure, the rewrite characteristics (solid line 11) of the semiconductor memory device manufactured by the manufacturing method of the above-described example are much better than the rewrite characteristics (solid line 12) of the conventional semiconductor memory device. I know that there is.

As is clear from the detailed explanation of the invention, according to the manufacturing method of the present invention, distortion and traps are less likely to occur in the silicon oxide film of the tunnel medium, which is very thin at about 100A, and is stable with high precision. The number of rewrites can be ensured, and this greatly contributes to increasing the reliability of floating gate type semiconductor memory devices.

[Brief explanation of drawings]

1 is a step-by-step sectional view for explaining the manufacturing method of an embodiment of the present invention, FIG. 2 is a sectional view for explaining the structure of a floating gate type semiconductor memory device, and FIG. 3 is a sectional view for explaining the structure of a floating gate type semiconductor memory device. It is a characteristic diagram for detailed explanation. 1... P-type silicon substrate, 2... Source region, 3... Drain region, 4...
Silicon oxide film, 5... Thin silicon oxide film that can serve as a tunnel medium, 6... Floating gate electrode, 7... Silicon oxide film, 8...
... Control gate electrode, 9 ... Silicon oxide film, 10 ... Aluminum electrode.

Claims (2)

[Claims] (1) A step of forming a diffusion layer of a conductivity type opposite to that of the same substrate on the surface of a semiconductor substrate of one conductivity type, a step of forming a first insulating film on the surface of the semiconductor substrate, and a step of forming the first insulating film on the surface of the semiconductor substrate. forming an opening that reaches the diffusion layer in a predetermined portion of the film; forming a second insulating film that can serve as a tunnel medium on the surface of the diffusion layer in the opening; forming a floating gate electrode on the insulating film;
In the method for manufacturing a semiconductor memory device including at least the step of forming a control gate electrode on the floating gate electrode via a third insulating film, all the heat treatment steps after forming the second insulating film are A method for manufacturing a semiconductor memory device, characterized in that the process is carried out at a temperature lower than the temperature for forming an insulating film in step 2 +50°C. (2) The method for manufacturing a semiconductor memory device according to claim 1, wherein the temperature at which the second insulating film is formed is 1050° C. or lower.