JPH05259271A - Method for fabricating semiconductor device and semiconductor device fabricated using the same - Google Patents

Abstract

PURPOSE:To shorten a bird's beak on a protruding extension extending from an active region to correctly define a pattern of the active region with respect to formation of a field oxide film for separating a narrow island-shaped or an island-shaped active region. CONSTITUTION:A silicon oxide film and a silicon nitride film are formed on a surface of a silicon substrate. The silicon nitride film is selectively removed to form a silicon nitride film pattern with an auxiliary pattern added for mutually connecting a protruding extension extending from an island-shaped active region expected region with a protruding extension extending from an adjacent island-shaped active region expected region. A first oxidation process for oxidizing the silicon substrate other than parts coated by the silicon nitride film pattern is performed. The silicon nitride film on the auxiliary pattern part is removed. The silicon substrate is oxidized, and a second oxidation process for forming a field oxide film for mutually separating the active region expected regions is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a semiconductor device and a semiconductor device manufactured by using the same, and more particularly to a method of forming a field oxide film for isolating an elongated island-shaped or peninsular-shaped active region and a method thereof. The present invention relates to a semiconductor device used.

As devices have been miniaturized, the so-called bird's beak, which is generated along with the formation of a field oxide film, has been a problem in that the accuracy of defining active regions is reduced. Therefore, there is a need for a field oxide film forming technique that does not reduce the accuracy of defining the active region.

[0003]

2. Description of the Related Art FIGS. 10A and 10B are views showing a conventional example 1, FIG. 10A is a plan view of a DRAM, FIG. 10B is a sectional view taken along line XX ', and FIG. 10C is a sectional view taken along line YY'. It is a figure.

In the figure, WL is a word line, and B. C. Is a bit line contact, S. C. Is storage contact, FO
X is a field oxide film. However, the bit line, storage electrode, and counter electrode are omitted.

As a method for forming a field oxide film, LO
The COS method is known. In this LOCOS method, as shown in FIG. 10 as a conventional example 1, a field oxide film (F
There is a problem that the active region is reduced by bird's beaks generated when forming OX).

In particular, when the active region has a thin island shape, a long bird's beak is generated at the convex end of the active region. As a result, as shown in FIG.
C. However, there was a problem that it could not be opened sufficiently.

As a method for solving the problem of the conventional example 1,
As in Conventional Example 2 shown in FIG. 11, for example, in a folded bit line structure DRAM, a pattern in which a silicon nitride film pattern which is a selective oxidation mask is extended in a convex shape is proposed.

[0008]

As the width of the active region becomes narrower with the miniaturization of the device, even the silicon nitride film pattern shown in the conventional example 2 has a pattern as shown in FIG.
The bird's beaks that occur when forming a field oxide film tend to become longer, and have come to invade convex extensions. As a result, there has been a problem that the active area is reduced.

In order to suppress the bird's beak from penetrating into the convex extension, it is effective to make the pad oxide film thin. In that case, as shown in FIG. 13, a silicon substrate in the active region is newly added. There is a problem that holes are generated on the surface of.

This hole indicates the generation of a strong stress due to the LOCOS process, and it is presumed that the hole is generated due to the elasticity of the silicon nitride film pushed up by the bird's beak having a long convex portion.

It has been found that this stress causes crystal defects, and that if a hole is formed on the surface of the silicon substrate, the breakdown voltage of the gate oxide film is significantly lowered. The present invention provides a method for forming a field oxide film for element isolation, in which the bird's beak of a convex portion is shortened and the pattern of an active region can be more accurately defined, without producing such a hole, and a method for manufacturing the same. An object of the present invention is to provide a semiconductor device with reduced junction leakage and improved gate oxide film reliability.

[0012]

In order to achieve the above object, the present invention is configured as follows. (1) A method of forming a field oxide film for isolating island-shaped active regions from each other, which comprises a step of forming a silicon oxide film on the surface of a silicon substrate and a step of forming a silicon nitride film on the silicon oxide film. And a convex extension extending from the island-shaped active region formation scheduled region pattern by selectively removing the silicon nitride film, and a convex extension extended from an adjacent island-shaped active region formation scheduled region pattern. A step of forming a silicon nitride film pattern, which includes an auxiliary pattern for interconnecting with each other, a first oxidation step of oxidizing the portion of the silicon substrate not covered by the silicon nitride film pattern, and the silicon nitride film. Of the pattern, a step of removing the silicon nitride film in the auxiliary pattern portion and a step of oxidizing the silicon substrate to separate active region formation planned regions from each other. Configured to include a second oxidation step of forming a field oxide film.

(2) A method of forming a field oxide film for separating island-shaped active regions from each other, which comprises a step of forming a first silicon oxide film on the surface of a silicon substrate and a first step on the silicon oxide film. Step of forming a silicon nitride film, selectively removing the first silicon nitride film to form a convex extension extending from an island-shaped active region formation scheduled region pattern, and an adjacent island-shaped active region formation scheduled An auxiliary pattern is added to connect the convex extension extending from the area pattern to each other.
A step of forming a silicon nitride film pattern, a first oxidation step of oxidizing the portion of the silicon substrate not covered by the silicon nitride film pattern, a step of removing the entire silicon nitride film pattern, and a surface of the silicon substrate A step of sequentially forming a second silicon oxide film and a second silicon nitride film on the silicon substrate, a step of selectively removing the second silicon nitride film in a region including the region where the auxiliary pattern was present, and the silicon substrate And a second oxidation step of forming a field oxide film that oxidizes and separates active region formation planned regions from each other.

(3) A method of forming a field oxide film for separating a peninsular active region, which comprises a step of forming a silicon oxide film on a surface of a silicon substrate and a silicon nitride film on the silicon oxide film. And a step of selectively removing the silicon nitride film to extend to the peninsular active region formation scheduled region pattern by at least half the width of the peninsular active region formation scheduled region pattern. Forming a silicon nitride film pattern, a first oxidation step of oxidizing a portion of the silicon substrate which is not covered by the silicon nitride film pattern, and a step of oxidizing the silicon nitride film pattern. A step of removing the silicon nitride film in the auxiliary pattern part and a field oxide film for oxidizing the silicon substrate to separate the active region formation planned region are formed. Configured to include a second oxidation step of forming.

(4) A method of forming a field oxide film for separating a peninsular active region, which comprises a step of forming a first silicon oxide film on a surface of a silicon substrate, and a first silicon nitride film on the silicon oxide film. A step of forming a film and selectively removing the first silicon nitride film to form a peninsular active region formation scheduled region pattern from the peninsular active region formation scheduled region pattern to at least 1 / A step of forming a silicon nitride film pattern by adding an auxiliary pattern extending by 2 lengths, a first oxidation step of oxidizing a portion of the silicon substrate which is not covered by the silicon nitride film pattern, and the silicon nitriding step. There is a step of removing the entire film pattern, a step of forming a second silicon oxide film and a second silicon nitride film on the surface of the silicon substrate, and the auxiliary pattern. A step of selectively removing the second silicon nitride film in a region containing a region, and a second oxidation process of oxidizing the silicon substrate to form a field oxide film for separating active region formation planned regions from each other. To include.

(5) In one of the above (1) to (4), a polysilicon layer is formed below the silicon nitride film. (6) In one of the above (1) to (5), the auxiliary pattern is formed only in the isolation region between the N-type diffusion layers, and boron is introduced before the second oxidation step. To configure.

(7) A semiconductor memory device, wherein memory cells are separated from each other by a field oxide film formed by one of the methods (1) to (6). ..

(8) A dynamic semiconductor memory device having a folded bit line system in which two word lines pass through an element isolation region between active regions adjacent to each other in the longitudinal direction of active regions including memory cells. The element isolation region is formed of a silicon oxide film thinner than the field oxide film of the other portion and a channel stop impurity having a higher concentration than that of the other portion.

(9) The method for manufacturing a dynamic semiconductor memory device according to (8), wherein the element isolation region is formed by the method according to (1) or (2).

(10) In the method of manufacturing a dynamic semiconductor memory device according to (8), the element isolation region is covered with a silicon nitride film when the field oxide film is formed, and the channel stop impurities are thresholded. It is introduced in a step adjacent to the step of introducing impurities for adjusting the value, and the silicon oxide film in the element isolation region is formed simultaneously with the gate oxide film.

[0021]

The operation of the present invention will be described below in correspondence with the above numbers. (1) In the first oxidation step, between the convex extension extending from the island-shaped active region formation scheduled region pattern and the adjacent protrusion extended from the island-shaped active region formation scheduled region pattern, Long bird's beaks do not occur as they are covered by the auxiliary pattern.

In the second oxidation step, since the oxidation amount can be distributed to the extent that the thickness of the field oxide film already formed is added, the convex extension portion extending from the island-shaped active region formation planned region pattern is formed. The bird's beak can be kept short. As a result, the stress can be suppressed small.

Therefore, it becomes possible to accurately define the pattern of the active region. (2) In the first oxidation step, between the convex extension extending from the island-shaped active region formation scheduled region pattern and the convex extension extending from the adjacent island-shaped active region formation scheduled region pattern, Long bird's beaks do not occur as they are covered by the auxiliary pattern.

In the second oxidation step, the second silicon nitride film is formed in a state where the region including the region where the auxiliary pattern was present is selectively removed. That is, since there is no convex extension pattern extending from the island-shaped active region formation planned area pattern, the island-shaped active area formation pattern is oxidized, and therefore the island is formed by the relatively thick second silicon oxide film and the relatively thin second silicon nitride film. The bird's beak of the convex extended portion extending from the region-shaped active region formation-scheduled region pattern can be suppressed short. As a result, the stress can be suppressed small.

Therefore, it becomes possible to accurately define the pattern of the active region. (3) This makes it possible to apply the present invention even when the active region cannot be connected to the adjacent portion in a pattern.

The process for forming the field oxide film is the same as the above (1). (4) This also makes the present invention applicable even when the active region cannot be connected to the adjacent portion in a pattern.

The process for forming the field oxide film is the same as the above (2). (5) This is the so-called poly buffer LOCO of the present invention.
This is an application to the S method. According to this method, it is possible to suppress the occurrence of an abnormal shape due to the grain of polysilicon and the bird's beak of the convex extension extending from the island-shaped active region formation planned region pattern.

(6) When the channel stop impurity is introduced into the auxiliary pattern portion, the auxiliary pattern is provided only in the region where the N-type diffusion layer is to be formed, and the auxiliary pattern is removed, so that P
Since it is not necessary to mask the channel region, an additional mask process can be omitted.

(7) By having the field oxide film formed by one of the methods (1) to (6), crystal defects and silicon oxide film defects can be reduced, so that the memory retention characteristics can be reduced. An excellent semiconductor memory device can be obtained.

(8) In the folded bit line system, the two word lines passing over the element isolation region between the active regions adjacent to each other in the longitudinal direction of the active region including the memory cell are at the same time H It never reaches the level. Therefore,
It is possible to achieve sufficient isolation with relatively weak element isolation.

(9) This is the method of manufacturing a semiconductor memory device according to (8). The element isolation region through which the two word lines pass is formed by the method of forming a field oxide film described in (1) or (2) above.

(10) This is also the method of manufacturing a semiconductor memory device according to (8). The element isolation region through which the two word lines pass is covered with a silicon nitride film when the field oxide film is formed, and a channel stop impurity is introduced in a step adjacent to the impurity introduction step for threshold adjustment. , A silicon oxide film is formed at the same time as the gate oxide film.

Therefore, although the thickness of the silicon oxide film in the element isolation region is almost the same as the thickness of the gate oxide film, a high-concentration channel stop impurity is introduced under the silicon oxide film and passes over it. Because of the operating principle, the two word lines do not become H level at the same time, it is possible to perform sufficient element isolation.

[0034]

Embodiments of the present invention will be described below with reference to the drawings. [Embodiment 1, FIG. 1 and FIG. 3] Hereinafter, steps will be described in order.

[Step 1, FIG. 1 (a)] A thickness of 100 is formed on the surface of the silicon substrate by thermal oxidation.
Å Silicon oxide film is formed.

A 1200 Å-thick silicon nitride film is formed on the surface of the silicon oxide film by the CVD method. After applying a resist on the surface of the silicon nitride film, a resist pattern is formed by photolithography technology. The typical value of the pattern width is 0.4 μm. At this time, a pattern is formed in which an auxiliary pattern for connecting the island-shaped active region formation-scheduled region pattern and the convex extension extending from the adjacent island-shaped active region formation-scheduled region pattern to each other is added.

The silicon nitride film is etched by reactive ion etching using CF ₄ as a main reaction gas. Strip the resist. As a result, a pattern of the silicon nitride film as shown in FIG. 1A is formed.

The first oxidation step is performed. This is performed by forming a 2500 Å-thick silicon oxide film in a portion not covered with the silicon nitride film by thermal oxidation at 900 ° C. in a hydrogen combustion furnace.

[Step 2, FIG. 1 (b)] After applying a resist on the entire surface, a resist pattern exposing only the auxiliary pattern is formed by photolithography.

The auxiliary pattern portion of the silicon nitride film is etched and removed by reactive ion etching using CF ₄ as a main reaction gas. Strip the resist.

The second oxidation step is performed. This is performed by forming a 2000 Å-thick silicon oxide film on the portion not covered with the silicon nitride film by thermal oxidation at 900 ° C. in a hydrogen combustion furnace.

[Step 3, FIG. 1 (c)] The silicon nitride film is removed by etching with hot phosphoric acid at 150 ° C. The field oxide film according to the present embodiment is completed through the above steps. As a result, as shown in FIG.
A thick field oxide is defined and a convex edge is defined by a thin field oxide, resulting in an accurately defined active region.

The auxiliary pattern may be formed not only in the shape shown in FIG. 1 (b) but also in the shape shown in FIG. [Embodiment 2, FIG. 2 and FIG. 4] Hereinafter, the steps will be described in order.

[Step 1, FIG. 2 (a)] A silicon substrate having a thickness of 100 is formed on the surface by thermal oxidation.
A Å first silicon oxide film is formed.

CVD is performed on the surface of the first silicon oxide film.
A first silicon nitride film having a thickness of 1200Å is formed by the method. After applying a resist on the surface of the first silicon nitride film, a resist pattern is formed by a photolithography technique. Typical value of pattern width is 0.4 μm
Is. At this time, a pattern is formed in which an auxiliary pattern for connecting the island-shaped active region formation-scheduled region pattern and the convex extension extending from the adjacent island-shaped active region formation-scheduled region pattern to each other is added.

The first silicon nitride film is etched by reactive ion etching using CF ₄ as a main reaction gas. Strip the resist. As a result, a pattern of the first silicon nitride film as shown in FIG. 2A is formed.

The first oxidation step is performed. This is performed by forming a 3000 Å-thick silicon oxide film on the portion not covered with the first silicon nitride film by thermal oxidation at 900 ° C. in a hydrogen combustion furnace.

[Step 2, FIG. 2 (b)] The entire silicon nitride film pattern is removed by reactive ion etching using CF ₄ as a main reaction gas.

A second silicon oxide film having a thickness of 200 Å is formed on the entire surface by thermal oxidation. A 1200 Å-thick second silicon nitride film is deposited on the entire surface by the CVD method.

After the resist is applied on the entire surface, a resist pattern exposing a region including the region where the auxiliary pattern existed is formed by a photolithography technique. The exposed silicon nitride film is removed by reactive ion etching using CF ₄ as a main reaction gas.

The resist is peeled off. The second oxidation step is performed. This is performed by forming a 2000 Å-thick silicon oxide film on the portion not covered with the silicon nitride film by thermal oxidation at 900 ° C. in a hydrogen combustion furnace.

[Step 3, FIG. 2 (c)] The second silicon nitride film is etched and removed by hot phosphoric acid at 150 ° C.

Through the above steps, the field oxide film according to the present embodiment is completed. As a result, as shown in FIG. 2C, an active region whose width is defined by a thick field oxide film and whose convex end is defined by a thin field oxide film, and whose shape is accurately defined, is obtained.

The auxiliary pattern may be formed not only in the shape shown in FIG. 2 (b) but also in the shape shown in FIG. [Embodiment 3, FIG. 5] This is a case where the present invention is applied when the active region cannot be connected to the adjacent portion in a pattern.

In the present embodiment, silicon nitriding is added to the peninsular active region formation scheduled region pattern with a pattern extending from this peninsular active region formation scheduled region pattern by at least ½ of its width. Form a film pattern.

After that, the first oxidation step is carried out. The second oxidation step is performed by the method of Example 1 or 2. As a result, as shown in FIG. 5, an active region whose width is defined by a thick field oxide film and whose peninsula-shaped end portions are defined by a thin field oxide film and whose shape is accurately defined is obtained.

[Embodiment 4, FIG. 6] This is one in which the present invention is applied to a so-called poly-buffer LOCOS method, in which a poly-oxide film is formed between a silicon oxide film and a silicon nitride film formed on a silicon substrate. It forms a silicon layer.

The polysilicon layer is formed by the following method. 30 by the CVD method using SiH ₄ as the main raw material gas
A 0Å thick polysilicon layer is formed. Alternatively, by a CVD method using SiH ₆ as a main raw material gas,
Amorphous silicon layer with a thickness of 00Å is formed,
It is annealed at 0 ° C. in N ₂ atmosphere for 3 hours to be crystallized to form a polysilicon layer.

The first oxidation step and the second oxidation step are performed in the same manner as in Example 1 or 2. As a result, the abnormal bird's beak that has conventionally penetrated into the active region as shown in FIG. 6 is eliminated, and an active region with a precisely defined shape is obtained as shown in the lower part of FIG.

Example 5, FIG. 7 This is an example of introducing a channel stop impurity under a thin field oxide film.

As shown in FIG. 7, an auxiliary pattern of a silicon nitride film is provided only in a region where an N type diffusion layer is to be formed, and it is removed after the first oxidation step, and B ⁺ ions are ion-implanted to stop the channel. To form. Ion implantation conditions are
For example, acceleration energy 15 keV, dose amount 1 × 10
^{It is 13} cm ^-2 .

As a result, it is not necessary to mask the P-channel region, and it is possible to omit an additional mask step. [Embodiment 6, FIG. 8] This is a folded bit line system in which two word lines are formed on the element isolation region between the active regions adjacent to each other in the longitudinal direction of the active regions including the memory cells. This is an example applied to a DRAM having a structure of passing through.

In the folded bit line system, the two word lines passing over the element isolation region between the active regions adjacent to each other in the longitudinal direction of the active region including the memory cell are operated according to the principle of operation.
It does not become H level at the same time. Therefore, it is possible to achieve sufficient isolation with relatively weak element isolation.

Therefore, in this embodiment, as shown in FIG. 8, the element isolation region through which the two word lines pass is made of a thin field oxide film. This thin field oxide film is formed by the method of the first or second embodiment.

[Embodiment 7, FIG. 9] The present invention is also based on the folded bit line method, in which the element isolation region between the active regions adjacent to each other in the longitudinal direction of the active regions including the memory cells is formed in two regions.
This is an example applied to a DRAM having a structure through which a word line of a book passes.

A step of adjoining the element isolation region through which the two word lines pass through with a silicon nitride film during the formation of the field oxide film so that the channel stop impurities are adjacent to the impurity introducing step for adjusting the threshold value. And is formed by forming a silicon oxide film at the same time as the gate oxide film.

Therefore, although the thickness of the silicon oxide film in the element isolation region is almost the same as the thickness of the gate oxide film, a high-concentration channel stop impurity is introduced below it and passes over it. Because of the operating principle, the two word lines do not become H level at the same time, it is possible to perform sufficient element isolation.

[0068]

The method of forming a field oxide film according to the present invention has the following effects. Since it is not necessary to make the pad oxide film thinner than necessary, no holes are formed in the silicon substrate.

Bird's beaks entering the convex extension extending from the active region can be shortened. As a result, the pattern of the active area can be accurately defined.

Further, the semiconductor device manufactured by using the method for forming a field oxide film according to the present invention has the following effects. Junction leakage is reduced.

The reliability of the gate oxide film is improved. As a result, it is possible to realize a highly reliable device that can be highly integrated.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment.

FIG. 2 is a diagram showing a second embodiment.

FIG. 3 is a diagram showing a modification of the first embodiment.

FIG. 4 is a diagram showing a modification of the second embodiment.

FIG. 5 is a diagram showing a third embodiment.

FIG. 6 is a diagram showing a fourth embodiment.

FIG. 7 is a diagram showing a fifth embodiment.

FIG. 8 is a diagram showing a sixth embodiment.

FIG. 9 is a diagram showing Example 7.

FIG. 10 is a diagram showing a first conventional example.

FIG. 11 is a diagram showing a second conventional example.

FIG. 12 is a diagram showing Problem 1 of Conventional Example 2.

FIG. 13 is a diagram showing Problem 2 of Conventional Example 2.

Claims (10)

[Claims] 1. A method for forming a field oxide film for isolating island-shaped active regions from each other, the method comprising the steps of forming a silicon oxide film on the surface of a silicon substrate, and forming a silicon nitride film on the silicon oxide film. And a step of selectively removing the silicon nitride film to form a convex extension extending from the island-shaped active region formation-scheduled region pattern and a convex-shaped extension extending from an adjacent island-shaped active region formation-scheduled region pattern. Forming a silicon nitride film pattern by adding an auxiliary pattern for connecting the extension part to each other; first oxidizing step of oxidizing a portion of the silicon substrate not covered by the silicon nitride film pattern; Of the nitride film pattern, the step of removing the silicon nitride film of the auxiliary pattern portion and the oxidation of the silicon substrate to separate active region formation planned regions from each other. And a second oxidation step of forming a field oxide film according to the present invention. 2. A method for forming a field oxide film for isolating island-shaped active regions from each other, comprising a step of forming a first silicon oxide film on a surface of a silicon substrate, and a first silicon film on the silicon oxide film. Step of forming a nitride film, selectively removing the first silicon nitride film to form a convex extension extending from an island-shaped active region formation scheduled region pattern, and an adjacent island-shaped active region formation scheduled region A step of forming a silicon nitride film pattern to which an auxiliary pattern for interconnecting a convex extension extending from the pattern is added, and a step of oxidizing the silicon substrate in a portion not covered by the silicon nitride film pattern An oxidation step, a step of removing the entire silicon nitride film pattern, a step of sequentially forming a second silicon oxide film and a second silicon nitride film on a surface of a silicon substrate, A step of selectively removing the second silicon nitride film in a region including a region where the auxiliary pattern was present; and a step of oxidizing the silicon substrate to form a field oxide film for separating active region formation planned regions from each other. Second
And a step of oxidizing the semiconductor device. 3. A method of forming a field oxide film for isolating a peninsular active region, which comprises a step of forming a silicon oxide film on the surface of a silicon substrate and a step of forming a silicon nitride film on the silicon oxide film. And selectively removing the silicon nitride film to form a peninsular active region formation scheduled region pattern from the peninsular active region formation scheduled region pattern.
Forming a silicon nitride film pattern by adding an auxiliary pattern extending at least ½ of the width; and a first oxidation for oxidizing a portion of the silicon substrate not covered by the silicon nitride film pattern. A step of removing the silicon nitride film of the auxiliary pattern portion of the silicon nitride film pattern, and a second oxidation of oxidizing the silicon substrate to form a field oxide film separating the active region formation planned region. And a step of manufacturing the semiconductor device. 4. A method for forming a field oxide film for separating a peninsular active region, the method comprising:
A step of forming a silicon oxide film, a step of forming a first silicon nitride film on the silicon oxide film, and a step of selectively removing the first silicon nitride film to form a peninsular active region formation scheduled region pattern. A step of forming a silicon nitride film pattern, in which an auxiliary pattern extending at least ½ of the width is added from the peninsular active region formation planned region pattern; A first oxidization step of oxidizing the silicon substrate in an unetched portion, a step of removing the entire silicon nitride film pattern, and a step of forming a second silicon oxide film and a second silicon nitride film on the surface of the silicon substrate. Selectively removing the second silicon nitride film in a region including the region where the auxiliary pattern was present; and oxidizing the silicon substrate to activate the active region. The method of manufacturing a semiconductor device which comprises a second oxidation step of forming a field oxide film for separating the forming region to each other. 5. A method of manufacturing a semiconductor device according to claim 1, wherein a polysilicon layer is formed under the silicon nitride film. 6. The method according to claim 1, wherein the auxiliary pattern is formed only in the isolation region between the N-type diffusion layers, and boron is introduced before the second oxidation step. A method of manufacturing a semiconductor device, which is characterized. 7. A semiconductor memory device, wherein memory cells are isolated from each other by a field oxide film formed by the method according to claim 1. Description: 8. A dynamic semiconductor memory device having a structure in which two word lines pass through an element isolation region between active regions adjacent to each other in the longitudinal direction of an active region including a memory cell in a folded bit line system. A semiconductor device characterized in that the element isolation region is formed of a silicon oxide film thinner than a field oxide film of another portion and a channel stop impurity having a higher concentration than that of the other portion. 9. The method for manufacturing a semiconductor device according to claim 8, wherein the element isolation region is formed by the method according to claim 1 or 2. 10. The method of manufacturing a semiconductor device according to claim 8, wherein the element isolation region is covered with a silicon nitride film when the field oxide film is formed, and the channel stop impurities are used for threshold adjustment. The method of manufacturing a semiconductor device, comprising: introducing the impurity in a step adjacent to the step of introducing the impurity, and forming the silicon oxide film in the element isolation region at the same time as the gate oxide film.