JPH06275605A - Formation of element isolation structure - Google Patents

Abstract

PURPOSE:To isolate an element forming region with higher accuracy of dimensions by selectively growing a field oxide film on the surface of a semiconductor substrate. CONSTITUTION:A first silicon oxide film 12, a poly-silicon film 13, a second silicon oxide film 14, and a silicon nitride film 16 are formed on a silicon substrate 11 in a form of lamination. Further, a resist 18 is pattern-formed on a region on the element forming region 17. A silicon nitride film 16 is patterned by anisotropic etching using the resist 18 as a mask. In this instance, the etching is securely stopped at the second silicon oxide film 14. Then, the exposed second silicon oxide film 14 is removed, and, after that, a field oxide film 19 is grown by selective oxidation using the silicon nitride film 16 as a mask. The dispersion of the etching of the silicon nitride film 16 will not be transferred to the poly- silicon film 13 so that it is possible to highly accurately control the length of the bird's beak 20.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In order to electrically separate a plurality of elements formed on a semiconductor substrate, a LOCOS (Lo
The 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 patterned on the pad oxide film. Then, a field oxide film grows around the element forming region by the thermal oxidation process using the nitride film as a mask, and the field oxide film achieves isolation between the element forming regions.

However, the LOCOS method has a drawback in that bird's beaks at the edges of the field oxide film are formed to extend in the element forming region for a long time, and thus the area of the element forming region is substantially reduced. A typical prior art that solves this problem is called the LOPOS method and is shown in FIGS. First, as shown in FIG. 3A, a pad oxide film 2 is formed on the surface of the silicon substrate 1. Then, as shown in FIG. 3B, a polysilicon film 3 is formed on the pad oxide film 2. The film thickness of the polysilicon film 3 is, for example, 1200Å. On the polysilicon film 3, as shown in FIG. 3 (c), 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.

Next, as shown in FIG.
Is used as a mask for anisotropic etching. As a result, the silicon nitride film 4 is patterned so that the silicon nitride film 4 remains on the element forming region 8. At this time, at the same time, the polysilicon film 3 is etched until the film thickness becomes about half the initial film thickness (for example, 600 Å).

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

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 forming 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 forming region 8 can be obtained.

[0007]

However, in the above-mentioned prior art, the silicon nitride film 4 is etched as shown in FIG.
In the step (d), the etching rate of the silicon nitride film 4 varies within the wafer surface due to nonuniformity of plasma in the etching apparatus. Therefore, the nonuniformity of the etching of the silicon nitride film 4 is transferred to the polysilicon film 3 which is subsequently etched after the silicon nitride film 4. As a result, the film thickness of the residual film 3a of the polysilicon film 3 varies within the wafer surface. Therefore, the length of the bird's beak 7 varies within the wafer surface. That is, the bird's beak 7 is formed at a portion where the residual film 3a is thick.
Becomes shorter and the bird's beak 7 becomes longer where the residual film 3a is thin. As a result, there has been a problem that the area of the element formation region 8 becomes non-uniform within the wafer surface, which adversely affects the element characteristics.

In order to solve this problem, it is considered to improve the uniformity of the etching rate within the wafer surface. However, for that purpose, it is necessary to take 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. Therefore, a large increase in production cost is inevitable and impractical.

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

[0010]

The method for forming an element isolation structure according to the present invention for achieving the above object comprises a step of forming a polysilicon film on a semiconductor substrate and an etching on the polysilicon film. Forming a stopper layer, forming an oxidation resistant film on the etching stopper layer, and exposing the oxidation resistant film until the etching stopper layer is exposed so that the oxidation resistant film is left in a predetermined region. And a step of 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. And

It is preferable to further include a step of forming an oxide film on the surface of the semiconductor substrate before the step of forming the polysilicon film. In this case, the polysilicon film is formed on the surface of the oxide film. According to the above method, when the oxidation resistant film is selectively etched, this etching stops at the etching stopper layer. Therefore, the polysilicon film below the etching stopper layer is not affected by variations in etching of the oxidation resistant film. Therefore, the polysilicon film can have a uniform film thickness in 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 accuracy.
Therefore, the element formation region can be separated with high dimensional accuracy.

The polysilicon film and the etching stopper layer may be continuously formed by using the same CVD apparatus as described in claim 4. By doing so, the step of forming the element isolation structure can be simplified.
When forming an oxide film on the surface of the semiconductor substrate,
The thickness of the oxide film is preferably 300 Å or less. This is to prevent the bird's beak from extending too long.

The thickness of the polysilicon film is 30.
It is preferably in the range of 0Å to 1200Å. If the thickness of the polysilicon film is smaller than the above range, the bird's beak may extend for a long time to narrow the element formation region. Further, when the thickness of the polysilicon film is thicker than the above range, the edge of the field oxide film may be sharply raised, and a clear step may occur between the semiconductor substrate and the surface of the field oxide film. . That is, in order to make the bird's beak into an appropriate shape, it is preferable to set the film thickness of the polysilicon film to a value within the above range.

Similarly, when the etching stopper layer is composed of a silicon oxide film, the film thickness of the etching stopper layer is set to 3 in order to make the bird's beak a proper shape.
It is preferably set to 00Å or less.

[0015]

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 1 and 2 are cross-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 is formed as a pad oxide film. The film thickness of the first silicon oxide film 12 is, for example, 3
It is set to about 00Å.

Next, as shown in FIG. 1 (b), a polysilicon film 13 of about 600 Å is formed on the silicon oxide film 12, and a second film of about 300 Å as an etching stopper layer is further formed thereon. The silicon oxide film 14 is formed. The polysilicon film 13 is formed by using a low pressure CVD (chemical vapor deposition) device. 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 polysilicon film 13. Then, the surface of the polysilicon film 13 is thermally oxidized in an oxidizing atmosphere at 900 ° C. to form the second silicon oxide film 14.

The formation of the second silicon oxide film 14 is
It can also be performed using a low pressure CVD apparatus. In this case, after the polysilicon film 13 is formed, the SiH ₄ gas and the N ₂ O gas are continuously supplied into the film forming chamber at the same time, and the mixed gas is thermally decomposed to form a second film on the polysilicon film 13. The silicon oxide film 14 may be deposited. By doing so, the polysilicon film 13 and the second silicon oxide film 14 can be continuously formed.

Next, as shown in FIG. 1C, a silicon nitride film 16 as an oxidation resistant film is deposited on the polysilicon film 13. The silicon nitride film 16 is formed using, for example, a low pressure CVD apparatus. At this time, the temperature inside the film forming chamber is set to, for example, 780 ° C., and SiH ₂ C
The l ₂ gas and the NH ₃ gas are supplied into the film forming chamber as raw material gases. Then, the silicon nitride film 16 is formed by thermal decomposition of these source gases. Silicon nitride film 1
The film thickness of 6 is, for example, about 3000 Å. A resist 18 is patterned on the silicon nitride film 16 in a region directly above an element formation region 17 where an element such as a transistor is to be formed.

Next, as shown in FIG. 1D, the silicon nitride film 16 is patterned by anisotropic etching (eg, reactive ion etching) using the resist 18 as a mask. This anisotropic etching is performed under the condition that the selection ratio of silicon nitride to silicon oxide is high. Then, the etching is performed until the second silicon oxide film 14 is exposed, and is stopped by the second silicon oxide film 14. As a result, even if the etching rate varies within the wafer surface, the silicon nitride film 14 in the portion not covered with the resist 18 is reliably removed. Further, the influence of variations in the etching of the silicon nitride film 16 is completely blocked by the second silicon oxide film 14, and the polysilicon film 1
No more than three.

Next, as shown in FIG. 2 (e), the exposed portion of the second silicon oxide film 14 is removed by etching with hydrofluoric acid (HF) or the like, and the polysilicon is not formed in the region where the silicon nitride film 16 is not formed. The silicon film 13 is exposed. The step of etching away the exposed portion of the second silicon oxide film 14 may be omitted. As described above, the influence of variations in etching of the silicon nitride film 16 is completely blocked by the second silicon oxide film 14, so that the polysilicon film 13 can have a uniform film thickness within the wafer surface.

Next, the resist 18 is removed, and FIG.
As shown in (f), the field oxide film 19 is grown in the region where the silicon nitride film 16 is not formed by the selective thermal oxidation process using the silicon nitride film 16 as a mask. This thermal oxidation treatment is performed, for example, at 980
It is carried out by steam oxidation in a H ₂ O ₂ atmosphere at 0 ° C. until the film thickness of the field oxide film 19 reaches about 8000 Å.

After that, as shown in FIG. 2G, the first silicon oxide film 12, the polysilicon film 13, the second silicon oxide film 14 and the silicon nitride film 16 on the element forming region 17 are removed. To be done. In this way, the element formation region 17 is separated by the field oxide film 19. As described above, in this embodiment, the second silicon oxide film 14 is interposed between the silicon nitride film 16 and the polysilicon film 13. Therefore, when the silicon nitride film 16 is etched, this etching is performed on the silicon oxide film 1
Make sure to stop by 4. As a result, variations in etching of the silicon nitride film 16 are not transferred to the polysilicon film 13. Therefore, the film thickness of the polysilicon film 13 can be 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, and thus the area of the element forming region 17 can be made uniform in the wafer surface. In other words, the element formation region can be formed with high dimensional accuracy. Moreover, since it is not necessary to make the etching rate uniform, there is no need for a special etching device. Therefore, there is no fear that the production cost will increase excessively.

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

Further, 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 film thickness of 300 Å or less. If the second silicon oxide film 14 is formed thick, the corners 21 (see FIG. 2 (g)) formed on the upper surface of the field oxide film 19 may grow significantly, which may cause a large step. .

The thickness of the polysilicon film 13 is 300.
It is preferably Å to 1200Å. When the film thickness is smaller than this range, the bird's beak 20 may extend longer and narrow the element forming region 17, and when the film thickness is larger than the above range, the rising edge of the field oxide film 19 becomes sharp, A clear step may occur between the silicon substrate 11 and the surface of the field oxide film 19. That is, in order to make the bird's beak 20 have an appropriate shape, it is preferable to set the film thickness of the polysilicon film 13 to a value within the above range.

Similarly, in order to prevent the bird's beak 20 from extending too long, the thickness of the first silicon oxide film 12 is preferably set to 300 Å or less.

[0028]

As described above, according to the method for forming the element isolation structure of the present invention, the non-uniformity of the etching of the oxidation resistant film is not transferred to the polysilicon film, so that the polysilicon film is a semiconductor. It is possible to have a uniform film thickness in each part of the substrate. 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.

Further, if the polysilicon film and the etching stopper layer are continuously formed by using the same CVD apparatus, a good element isolation structure can be formed by a relatively simple process.

[Brief description of drawings]

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

2A to 2D are cross-sectional views showing a manufacturing process following the process of FIG.

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

FIG. 4 is a cross-sectional view showing a step that follows the step of FIG. 3D in the order of steps.

[Explanation of symbols]

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

Claims (6)

[Claims] 1. A step of forming a polysilicon film on a semiconductor substrate, a step of forming an etching stopper layer on the polysilicon film, a step of forming an oxidation resistant film on the etching stopper layer, and a predetermined process. The oxidation resistant film is selectively etched until the etching stopper layer is exposed so that the oxidation resistant film is left in the region, and the oxidation resistant film is selectively oxidized by using the oxidation resistant film as a mask. And a step of growing a field oxide film in a region other than the region where the conductive film is formed. 2. The method further comprises the step of forming an oxide film on the surface of the semiconductor substrate before the step of forming the polysilicon film, wherein the polysilicon film is formed on the surface of the oxide film. The method for forming an element isolation structure according to claim 1. 3. The method for forming an element isolation structure according to claim 2, wherein the oxide film has a thickness of 300 Å or less. 4. The element isolation structure according to claim 1, wherein the polysilicon film and the etching stopper layer are continuously formed by using the same CVD apparatus. Forming method. 5. The method for forming an element isolation structure according to claim 1, wherein the thickness of the polysilicon film is in the range of 300Å to 1200Å. 6. The method for forming an element isolation structure according to claim 1, wherein the etching stopper layer is made of a silicon oxide film and has a film thickness of 300 Å or less.