JP2975496B2 - Method of forming element isolation structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an element isolation structure for isolating an element formation region by selectively growing a field oxide film on a surface of a semiconductor substrate.

[0002]

2. Description of the Related Art In order to electrically isolate a plurality of elements formed on a semiconductor substrate, a LOCOS (Loss) has conventionally been used.
cal Oxidation of Silicon) method is widely used.
In the LOCOS method, a pad oxide film is formed on a semiconductor substrate, and a nitride film covering an element formation region is pattern-formed on the pad oxide film. Then, a field oxide film is grown around the element formation region by the thermal oxidation treatment using the nitride film as a mask, and the field oxide film achieves isolation between the element formation regions.

However, the LOCOS method has a drawback that the bird's beak at the edge of the field oxide film is formed to extend long in the element formation region, and therefore, the substantial area of the element formation region is reduced. A typical prior art solution to this problem is called the LOPOS method, and is shown in FIGS. First, a pad oxide film 2 is formed on the surface of a silicon substrate 1 as shown in FIG. Then, as shown in FIG. 3B, a polysilicon film 3 is formed on the pad oxide film 2. Polysilicon film 3 has a thickness of, for example, 1200 °. On the polysilicon film 3, as shown in FIG. 3C, silicon nitride (Si ₃ N
₄ ) The film 4 is formed. On this silicon nitride film 4,
A resist 9 is patterned in a region above the element forming region 8.

[0004] Next, as shown in FIG.
Is used as a mask to perform anisotropic etching. Thereby, the silicon nitride film 4 is patterned so that the silicon nitride film 4 remains on the element formation region 8. At this time, the polysilicon film 3 is simultaneously etched until the film thickness becomes about half the initial film thickness (for example, 600 °).

Next, when an oxidation process is performed using the silicon nitride film 4 as a mask, a field oxide film 5 grows in a region not covered with the silicon nitride film 4, as shown in FIG. At this time, oxygen does not permeate the polysilicon film 3 immediately below the silicon nitride film 4 very quickly.
The bird's beak 7 does not extend too long into the element forming region 8.

[0006] Finally, as shown in FIG. 4 (f), the silicon nitride film 4, the polysilicon film 3 and the pad oxide film 2 are removed, and an element formation region 8 separated by the field oxide film 5 is obtained. In this way, bird's beak 7
Since the field oxide film 5 having a short length can be formed, a wide element formation region 8 can be obtained.

[0007]

However, in the above-mentioned prior art, the process of etching the polysilicon film 3 together with the silicon nitride film 4 in the step of FIG. As a result, the thickness of the remaining film 3a of the polysilicon film 3 varies within the wafer surface. Therefore, the length of Bird's Beak 7
Variations occur in the wafer plane. That is, the bird's beak 7 becomes short in a place where the residual film 3a is thick, and becomes long in a place where the residual film 3a is thin. As a result, there is a problem that the area of the element forming region 8 becomes non-uniform in the wafer plane.

[0008] In order to solve this problem, it is conceivable to improve the uniformity of the etching rate in the wafer plane. However, for that purpose, fundamental measures such as improving the uniformity of plasma in the etching apparatus and improving the in-plane uniformity of the temperature of the wafer are required. Therefore, a significant increase in production cost is unavoidable and is not practical.

Therefore, an object of the present invention is to solve the above-mentioned technical problems and control the length of a bird's beak with high accuracy.
An object of the present invention is to provide a method for forming an element isolation structure capable of separating an element formation region with high dimensional accuracy.

[0010]

According to the present invention, there is provided a method for forming an element isolation structure, comprising the steps of: forming a first polysilicon film on a semiconductor substrate; Forming a second polysilicon film on the etching stopper layer; forming an oxidation-resistant film on the second polysilicon film; The oxidation-resistant film and the second polysilicon film are selectively etched until the etching stopper layer is exposed so as to leave the oxidation-resistant film.
With the second polysilicon film left between the layers,
Stopping the etching, and growing a field oxide film in a region other than the region where the oxidation-resistant film is formed by selective oxidation using the oxidation-resistant film as a mask. I do.

Preferably, the method further includes a step of forming an oxide film on the surface of the semiconductor substrate before the step of forming the first polysilicon film. In this case, the first polysilicon film is formed on the surface of the oxide film. According to the above method, the oxidation-resistant film and the second
When selectively etching the polysilicon film, the etching stops at the etching stopper layer. Therefore, the first polysilicon film below the etching stopper layer can have a uniform thickness at each part of the semiconductor substrate. As a result, when the field oxide film is grown by selective oxidation using the oxidation resistant film as a mask, the length of the bird's beak of the field oxide film is controlled with high precision. Therefore, the element formation region can be separated with high dimensional accuracy.

[0012] The first polysilicon film, the etching stopper layer and the second polysilicon film may be formed as follows.
, May be formed continuously using the same CVD apparatus. In this case, the step of forming the element isolation structure can be simplified. Note that the first polysilicon film has a thickness of 200 ° to 1000 °.
Is preferably within the range. If the thickness of the first polysilicon film is smaller than the above range, the bird's beak may extend long and narrow the element formation region. Also, the first
When the thickness of the polysilicon film is larger than the above range, the edge of the field oxide film rises steeply, and a clear step may be generated between the semiconductor substrate and the surface of the field oxide film. That is, in order to form the bird's beak into an appropriate shape, the thickness of the polysilicon film is preferably set to a value within the above range.

For the same reason, the sum of the thickness of the first polysilicon film and the thickness of the second polysilicon film is 1
It is preferable that the angle is 500 ° or less. Similarly, when the etching stopper layer is composed of a silicon oxide film,
In order to form a bird's beak in an appropriate shape, it is preferable that the thickness of this etching stopper layer be 300 ° or less.

[0014]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 1 and 2 are sectional views showing a method of forming an element isolation structure according to an embodiment of the present invention in the order of steps. First, as shown in FIG. 1A, the surface of the silicon substrate 11 is thermally oxidized in an oxidizing atmosphere at 950 ° C.
A first silicon oxide film 12 as a pad oxide film is formed. The thickness of the first silicon oxide film 12 is, for example,
It is about 500 °.

Next, as shown in FIG. 1B, a first polysilicon film 1 of about 600 ° is formed on the silicon oxide film 12.
3 is formed thereon, a second silicon oxide film 14 of about 200 ° is formed thereon as an etching stopper layer, and a second polysilicon film 15 of about 600 ° is further formed thereon.
Is formed. The formation of the first polysilicon film 13, the second silicon oxide film 14, and the second polysilicon film 15 is continuously performed using a low pressure CVD (chemical vapor deposition) apparatus. That is, the temperature in the film forming chamber is set to about 700 ° C., and the SiH ₄ gas is thermally decomposed to form the first polysilicon film 1.
Form 3 Subsequently, the SiH ₄ gas and the N ₂ O gas are simultaneously supplied into the film forming chamber, and the mixed gas is thermally decomposed to form the second silicon oxide film 14. Then, the supply of the N ₂ O gas is stopped, and the SiH ₄ gas is deposited on the second silicon oxide film 14.
Second polysilicon film 15 obtained by thermal decomposition of gas
Is deposited. In this manner, a film having a three-layer structure of the polysilicon film, the silicon oxide film, and the polysilicon film is continuously formed.

Next, as shown in FIG. 1C, a silicon nitride film 16 as an oxidation resistant film is formed on the second polysilicon film 1.
5 is deposited. The formation of the silicon nitride film 16
For example, it is performed using a low pressure CVD apparatus. At this time, the temperature in the film forming chamber is set to 780 ° C., and SiH ₂ Cl
_{The two} gases and the NH ₃ gas are supplied as source gases into the film formation chamber. Then, a silicon nitride film 16 is formed by thermal decomposition of these source gases. Silicon nitride film 16
Is, for example, about 3000 °. On the silicon nitride film 16, a resist 18 is pattern-formed in a region immediately above a device forming region 17 where a device such as a transistor is to be formed.

Next, as shown in FIG. 2D, the silicon nitride film 16 is patterned by anisotropic etching (eg, reactive ion etching) using the resist 18 as a mask. At this time, resist 1
The second polysilicon film 15 in the region where 8 is not formed is also removed at the same time, and the second silicon oxide film 14 is exposed.
In other words, the anisotropic etching is performed until the second silicon oxide film 14 is exposed.
4 to stop. At this time, the second polysilicon film
15 is a silicon nitride film 16 and a second silicon oxide film 14
It is in the state left between.

Next, the resist 18 is removed, and FIG.
As shown in (e), the field oxide film 19 is grown in a region where the silicon nitride film 16 is not formed by the selective thermal oxidation using the silicon nitride film 16 as a mask. In this thermal oxidation treatment, the thickness of the field oxide film 19 is 800
The process is performed until the angle becomes about 0 °. Note that, before growing the field oxide film 19, the exposed portion of the second silicon oxide film 14 may be removed by etching using hydrofluoric acid or the like.

Thereafter, as shown in FIG. 2 (f), the first silicon oxide film 12, the first polysilicon film 13, the second silicon oxide film 14, the second silicon oxide film
The polysilicon film 15 and the silicon nitride film 16 are removed. In this manner, the element forming region 17 is separated by the field oxide film 19. As described above, in the present embodiment, the first and second polysilicon films 13 and 15 with the silicon oxide film 14 interposed therebetween are interposed between the silicon nitride film 16 and the silicon oxide film 12 for the pad. ing. Therefore, when the silicon nitride film 16 and the second polysilicon film 15 are etched, the etching is reliably stopped by the silicon oxide film 14. As a result, the thickness of the first polysilicon film 13 thereunder can be reliably made uniform within the wafer surface.

Therefore, the length of the bird's beak 20 of the field oxide film 19 becomes uniform in the wafer surface, whereby the area of the element formation region 17 can be made uniform in the wafer surface. In other words, the element formation region can be formed with high dimensional accuracy. In addition, since it is not necessary to make the etching rate uniform, a special etching apparatus is not required. Therefore, there is no possibility that the production cost is excessively increased.

Further, the first polysilicon film 13, the second
Since the silicon oxide film 14 and the second polysilicon film 15 are formed continuously using a low pressure CVD apparatus, the production process is also simple. That is, the element isolation structure can be formed in the same number of steps as in the conventional LOPOS method. Note that the second silicon oxide film 14 may be formed by thermally oxidizing the surface of the first polysilicon film 13.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various design changes can be made without departing from the spirit of the present invention. It is possible. For example, in the above example,
Although a silicon oxide film is used as the etching stopper layer, a film other than the silicon oxide film may be used.

The film thickness of each film shown in the above embodiment is merely an example, and the film thickness of each film can be appropriately set as needed. However, the second silicon oxide film 14 is preferably formed as thin as possible, and preferably has a thickness of 300 ° or less. If the second silicon oxide film 14 is formed thick, the corners 21 (see FIG. 2F) formed on the upper surface of the field oxide film 19 may grow large, and a large step may be generated. .

The thickness of the first polysilicon film 13 is 2
It is preferable that the angle is 00 to 1000 °. When the film thickness is smaller than this range, the bird's beak 20 may extend long and narrow the element formation region 17, and when the film thickness is larger than the above range, the edge of the field oxide film 19 rises steeply, There is a possibility that a clear step may occur between the silicon substrate 11 and the surface of the field oxide film 19. That is, in order to form the bird's beak 20 into an appropriate shape, the thickness of the first polysilicon film 13 is preferably set to a value within the above range.

For the same reason, the total thickness of the first and second polysilicon films 13 and 15 is preferably 1500 ° or less.

[0026]

As described above, according to the method for forming the element isolation structure of the present invention, the first polysilicon film remaining after selectively etching the oxidation-resistant film and the second polysilicon film is formed on the semiconductor substrate. Can have a uniform film thickness in each part. As a result, the length of the bird's beak of the field oxide film can be controlled with high accuracy. This allows
The element formation region can be separated with high dimensional accuracy.

Furthermore, if the first polysilicon film, the etching stopper layer, and the second polysilicon film are successively formed by using the same CVD apparatus, a favorable element isolation structure can be formed by a relatively simple process.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a method of forming an element isolation structure according to an embodiment of the present invention in the order of steps.

FIG. 2 is a cross-sectional view showing a manufacturing process following the process of FIG.

FIG. 3 is a sectional view showing a prior art method in the order of steps.

FIG. 4 is a sectional view showing a step that follows the step of FIG. 3C in the order of steps;

[Explanation of symbols]

 Reference Signs List 11 silicon substrate 12 first silicon oxide film 13 first polysilicon film 14 second silicon oxide film 15 second polysilicon film 16 silicon nitride film 17 element formation region 18 resist 19 field oxide film 20 bird's beak

Claims (6)

(57) [Claims] A step of forming a first polysilicon film on a semiconductor substrate; a step of forming an etching stopper layer on the first polysilicon film; and forming a second polysilicon film on the etching stopper layer. Forming an oxidation-resistant film on the second polysilicon film; and etching the oxidation-resistant film and the second polysilicon film with the etching stopper so as to leave the oxidation-resistant film in a predetermined region. Selectively etch until the layer is exposed .
The second policy between the film and the etching stopper layer.
Stopping the etching while leaving the silicon film, and growing a field oxide film in a region other than the region where the oxidation resistant film is formed by selective oxidation using the oxidation resistant film as a mask. A method for forming an element isolation structure, comprising: 2. The method according to claim 1, further comprising: before forming the first polysilicon film, forming an oxide film on a surface of the semiconductor substrate, wherein the first polysilicon film is formed on a surface of the oxide film. 2. The method for forming an element isolation structure according to claim 1, wherein: 3. The semiconductor device according to claim 1, wherein the first polysilicon film, the etching stopper layer, and the second polysilicon film have the same
3. The method according to claim 1, wherein the device isolation structure is formed continuously using a VD apparatus. 4. The film thickness of the first polysilicon film is 200
4. The method for forming an element isolation structure according to claim 1, wherein the thickness is in the range of {1000}. 5. A method according to claim 1, wherein said first polysilicon film has a thickness equal to said second polysilicon film.
5. The method for forming an element isolation structure according to claim 1, wherein a total of the thickness of the polysilicon film and the thickness of the polysilicon film is 1500 ° or less. 6. The method according to claim 1, wherein said etching stopper layer is made of a silicon oxide film and has a thickness of 300 ° or less.