JPH0817210B2 - Method for manufacturing semiconductor memory device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor memory device including a floating gate type field effect transistor.

Conventional technology Conventionally, EEPROM (Electric
As one of ally Erasable and Programmable ROM,
A semiconductor memory device having a floating structure in which writing and erasing are performed by tunnel injection is well known. In this floating gate type semiconductor memory device, charges are tunnel-injected through a thin insulating film on a diffusion layer, charges are accumulated in a floating gate electrode on the insulating film, and a threshold voltage of a transistor is changed. The principle is to store information.

FIG. 2 shows a cross-sectional structure diagram of a typical floating gate type semiconductor memory device. As shown in FIG. 2, a source region 2 and a drain region 3 made of an N type diffusion layer are formed in a P type silicon substrate 1, and the source region 2,
A relatively thick silicon oxide film 4 is formed straddling the drain region 3, and only a part of the silicon oxide 4 is opened, and a thin silicon oxide film 5 that can serve as a tunnel medium is formed in this opening, and the silicon oxide film 5 is oxidized. The floating gate electrode 6, the silicon oxide film 7 and the control gate electrode 8 are sequentially laminated on the silicon films 4 and 5.

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 part of the silicon oxide film 4 is formed by a known photoetching technique. To open the drain region 3 by
It is necessary to form a very thin silicon oxide film 5 of about 100Å so that writing and erasing can be performed with a program voltage of 15-20V. In order to form this very thin silicon oxide film 5 with good controllability and stability. In addition, it is usually formed by oxidation at a relatively low temperature of about 900 ° C. on the other hand,
After forming the tunnel insulating film, it is necessary to form a floating electrode on the tunnel insulating film and further form a control gate electrode on the floating gate electrode via the insulating film. However, the floating electrode and the control gate electrode are formed of polysilicon. Since high-melting point metals such as etc. are usually used, the heat treatment after forming the tunnel insulating film is 1000 ° C-
It was usual to use a relatively high temperature heat treatment process of about 1100 ° C.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention A floating gate type semiconductor memory device rewrites data by tunneling charges through a very thin tunnel insulating film, and the number of times of rewriting is ensured in the floating gate type semiconductor memory device. Although it is the biggest problem of the device and is an important problem in practical use, in the floating gate type semiconductor memory device manufactured by the conventional manufacturing method as described above, it is very easy to break if rewriting is repeated, There is a problem that it is very difficult to secure a stable number of rewrites with high accuracy.

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

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 the substrate from the surface to the inside of a conductivity type semiconductor silicon substrate, and a surface of the substrate. A step of forming a first insulating film on the first insulating film, a step of forming an opening at a predetermined portion of the first insulating film so as to reach the diffusion layer, and a tunnel on the surface of the diffusion layer of the opening. Forming a second insulating film that can serve 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, the second insulating film is formed at a temperature of 1050 ° C. or lower, and all the heat treatment steps after the formation of the second insulating film are performed by the second insulating film. film Forming temperature + 50 ℃ is given in the following.

Action According to the study by the present inventor, the number of repeated rewritings leading to breakdown is largely dependent on the heat treatment step after forming the second insulating film, and particularly, the heat treatment after forming the second insulating film. It has been found that when the temperature is higher than the formation temperature of the second insulating film by more than 50 ° C., the rewriting characteristics are extremely deteriorated.

The present invention has been made based on the above facts. By setting the heat treatment temperature after forming the tunnel insulating film to the formation temperature of the tunnel insulating film + 50 ° C. or less as in the present invention, it is less likely to be destroyed even if repeated rewriting It is very easy to secure reliability. Although details of the mechanism by which such repetitive rewriting characteristics are dramatically improved are unknown, the temperature of the heat treatment after the formation of the second insulating film to be the tunnel medium is set to the second insulating film formation temperature + 50 ° C.
It is presumed that the following will alleviate the thermal stress on the thin second insulating film, reduce the occurrence of strain and traps in the second insulating film, and make it less likely to be destroyed even after repeated rewriting. It It was also found that when the formation temperature of the second oxide film is 1050 ° C. or higher, the effect of the present invention is almost lost, and the formation temperature of the tunnel oxide film needs to be 1050 ° C. or lower.

Example A specific example of the present invention will be described with reference to the drawings.

1A to 1D are sectional views in order of the processes, showing an embodiment of the present invention.

First, as shown in FIG. 1A, a source region 2 and a drain region 3 made of an N-type diffusion layer are formed on a P-type silicon substrate 1 by a known selective diffusion technique, and then a silicon oxide film 4 is usually formed. It is formed by the thermal diffusion method. The thickness of the silicon oxide film 4 is required to be thick so that a tunnel from the substrate does not occur, and is about 500Å in this embodiment. Next, a predetermined portion of the silicon oxide film 4 on the drain region 3 is etched by a known photoetching technique to form an opening which will be 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 forming temperature must be 1050 ° C. or lower. In this example, the process was performed at 1000 ° C. in a dry oxygen atmosphere diluted with argon. Also, to effectively use the tunnel effect,
It is necessary to set the thickness of the silicon oxide film 5 to about 50-150Å, and in this embodiment, 100Å was formed. Further, all the heat treatment temperatures after the formation of the tunnel silicon oxide film 5 must be set to the silicon oxide film formation temperature of the tunnel medium + 50 ° C. or less, and in this embodiment, as described below, 950 ° C. or less. It was carried out in.

First, as shown in FIG. 1C, a floating gate electrode 6 made of a polysilicon film is formed on the silicon oxide film 5.
However, in the present embodiment, in order to form at a temperature as low as possible, first, by a reduced pressure vapor phase growth method using a mixed gas of silane (SiH ₄ ) and phosphine (PH ₃ ),
A polysilicon film doped with phosphorus (about 6 × 10 ²⁰ cm ^-3 )
About 5000 Å was formed at 600 ° C, and then phosphorus was activated at 900 ° C. After that, the floating gate electrode 6 made of a polysilicon film is formed by a well-known photo etching 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 in a dry oxygen atmosphere diluted with argon at 950 ° C. It was controlled to be about 500Å on the 6th. After that, phosphorus was doped (about 6 ×) by a reduced pressure vapor deposition method using a mixed gas of silane (SiH ₄ ) and phosphine (PH ₃ ).
A polysilicon film of 10 ²⁰ cm ^-3 ) was formed at 600 ° C. to about 5000 Å, and then phosphorus was activated at 900 ° C. Then,
The control gate electrode 8 made of a polysilicon film is formed by a well-known photo etching technique.

Next, as shown in FIG. 1D, a silicon oxide film 9 is deposited on the entire surface at 450 ° C. by the atmospheric pressure vapor deposition method by a chemical reaction between silane (SiH ₄ ) and oxygen, and then the source and drain are pushed in. And 950 ° C., 30 for densification of the silicon oxide film 9.
Min, heat treatment was performed in a nitrogen atmosphere. After that, a contact hole is formed in the silicon oxide film 9 by a known photo-etching technique to form an aluminum electrode 10, and FIG.
A floating gate type semiconductor memory device as described above can be manufactured.

FIG. 3 shows an example of 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. As shown in FIG. 3, the rewriting characteristic (solid line 11) of the semiconductor memory device manufactured by the manufacturing method of the above-described embodiment is much higher than the rewriting characteristic (solid line 12) of the conventional semiconductor memory device. It turns out to be excellent.

EFFECTS OF THE INVENTION As is apparent from the above description, according to the manufacturing method of the present invention, the occurrence of strain and traps in the silicon oxide film of a very thin tunnel medium of about 100 Å is reduced, and stable and accurate. The number of times of rewriting can be ensured, which greatly contributes to the high reliability of the floating gate type semiconductor memory device.

[Brief description of drawings]

FIG. 1 is a sectional view in order of steps for explaining a manufacturing method of an embodiment of the present invention, FIG. 2 is a sectional view for explaining a structure of a floating gate type semiconductor memory device, and FIG. 3 is a sectional view of the present invention. It is a characteristic view for explaining an effect. 1 ... P-type silicon substrate, 2 ... source region, 3 ... drain region, 4 ... silicon oxide film, 5 ... thin silicon oxide film that can be a tunnel medium, 6 ... floating gate electrode, 7 ... oxidation Silicon film, 8 ... Control gate electrode, 9 ... Silicon oxide film, 10 ... Aluminum electrode.

Claims (1)

[Claims] 1. A step of forming a diffusion layer having a conductivity type opposite to that of the same conductivity type semiconductor substrate on a surface portion thereof, a step of forming a first insulating film on a surface of the semiconductor substrate, and the first step. Forming an opening at a predetermined portion of the insulating film reaching the diffusion layer; forming a second insulating film that can serve as a tunnel medium on the surface of the diffusion layer at the opening; A method of manufacturing a semiconductor memory device, comprising at least a step of forming a floating gate electrode on a second insulating film and a step of forming a control gate electrode on the floating gate electrode via a third insulating film. A semiconductor characterized in that the second insulating film is formed at a temperature of 1050 ° C. or lower, and all the heat treatment steps after the second insulating film is formed are performed at a temperature of the second insulating film + 50 ° C. or lower. Storage device manufacturing method