JP2738369B2 - Method for manufacturing semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for manufacturing a semiconductor device using a silicon nitride film, and more particularly to a method for manufacturing a semiconductor device in which a silicon nitride film is prevented from being left unetched at a step portion.

[0002]

2. Description of the Related Art Since a silicon nitride film has good insulation and oxidation resistance, various techniques for manufacturing a semiconductor device utilizing this characteristic have been proposed. For example, JP-A-6-2
In the method of manufacturing a nonvolatile semiconductor memory device described in Japanese Patent No. 59094, a silicon nitride film is used as an oxidation mask for embedding a diffusion layer. That is, FIG. 6 is a diagram showing a part of the process. First, FIG.
After the element isolation oxide film 2 is formed on the surface of the silicon substrate 1 and the silicon oxide film 3 is formed, a silicon nitride film 4 is formed on these silicon oxide films 2 and 3 as shown in FIG. Next, as shown in FIG.
The channel region is covered with the silicon nitride film 4 by patterning the silicon nitride film 4 by anisotropic etching using the mask as a mask. Next, as shown in FIG. 6C, the diffusion layer 9 is formed by ion implantation using the silicon nitride film 4 as a mask, and the diffusion layer 9 is formed by thermal oxidation using the silicon nitride film 4 as a mask. Embedded in

[0003]

In the method of manufacturing a semiconductor memory device using such a silicon nitride film, the following problems occur. That is, as shown in FIG. 6B, when the silicon nitride film 4 is etched,
If a step is formed between the element isolation oxide film 2 and the silicon oxide film 3 in the transistor region, the silicon nitride film 4 may be left on the step due to anisotropy of etching. When the silicon nitride film 4 is left at the step, the remaining silicon nitride film 4 is present as foreign matter in a later process, and since the silicon nitride film has oxidation resistance, FIG. As shown in (c), the thickness of the silicon oxide film 10 at the step portion by the thermal oxidation method does not become sufficiently large, and the filling of the diffusion layer 9 at this portion becomes incomplete, and the It will affect the shape.

[0004] Isotropic etching can be considered to remove the etching residue at the step. However, when isotropically etching the silicon nitride film, phosphoric acid is used. Therefore, it cannot be used as a mask, and it is difficult to adopt this method. In addition, if the etching time is increased in order to prevent the silicon nitride film from being left unetched, FIG.
The silicon nitride film 4 patterned as shown in FIG.
7 tends to have an inversely tapered shape, or the silicon nitride film 4 becomes thinner as shown in FIG. 7B, which tends to cause dimensional errors, making it difficult to perform fine processing in a later step.

An object of the present invention is to provide a method of manufacturing a semiconductor device capable of improving a manufacturing yield without generating an etching residue of a silicon nitride film at a step portion.

[0006]

According to a method of manufacturing a semiconductor device of the present invention, a step of forming a first silicon oxide film on a surface of a semiconductor substrate having a step, and a step of forming a silicon film on the first silicon oxide film. Forming a nitride film, forming a second silicon oxide film on the silicon nitride film,
A step of etching and removing the second silicon oxide film in a region including the step portion where the silicon nitride film is not to be left, and a step of isotropically etching the silicon nitride film using the remaining second silicon oxide film as a mask And a step of removing the second silicon oxide film, and a step of forming the silicon nitride film into a desired pattern by anisotropic etching. Here, the isotropic etching of the silicon nitride film preferably has an etching selectivity to the silicon oxide film of 10 or more. Further, the present invention is effective when a silicon nitride film is not left on a step formed by an edge of an oxide film for element isolation formed of a thick silicon oxide film formed on a surface of a semiconductor substrate.

[0007]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a basic technique of the present invention, and is a cross-sectional view showing a method of selectively forming a silicon nitride film in an element region in the order of steps. First, FIG.
As shown in FIG. 1A, after a silicon oxide film 2 for element isolation is formed on the surface of a silicon substrate 1, 50 nm is formed in an element formation region.
A gate oxide film 3 made of a silicon oxide film having the following thickness is formed by a thermal oxidation method or a CVD method. Further, a silicon nitride film 4 having a thickness of 400 to 500 nm is
A silicon oxide film 5 having a thickness of 10 to 20 nm is formed thereon by a CVD method.

Next, as shown in FIG. 1B, a photoresist 6 is applied to the silicon oxide film 5 in a region including a step formed at the boundary between the device isolation oxide film 2 and the gate oxide film 3. It is removed by isotropic etching using as a mask. Then, as shown in FIG. 1C, the silicon nitride film 4 is isotropically etched using the etched silicon oxide film 5 as a mask. At this time, phosphoric acid is used as an etching material. By this isotropic etching, the silicon nitride film 4 at the step is completely removed by etching.

Next, as shown in FIG. 1D, after removing the silicon oxide film 5, a photoresist 7 is selectively formed in a region where the silicon nitride film is to be left, and the photoresist 7 is used as a mask to form a silicon film. The nitride film 4 is etched by anisotropic dry etching to form a pattern of the silicon nitride film 4. At this time, in the anisotropic dry etching of the silicon nitride film, if the etching conditions are such that the selectivity with respect to the silicon oxide film is at least 1 and the selectivity with respect to silicon is at least 4 or more, the silicon substrate 1 The silicon nitride film 4 and the gate oxide film 3 can be etched into a desired pattern without substantially etching the surface of the substrate.

Therefore, according to this method, the silicon nitride film 4 at the step portion on the silicon substrate 1 is removed by isotropic etching in the previous step, so that a part of the silicon nitride film 4 remains at the step portion. This is not considered a foreign substance. In this isotropic etching, since a silicon oxide film having etching selectivity to phosphoric acid as an etching material is used as a mask, the silicon oxide film can function effectively as a mask. As a result, the silicon nitride film does not need to be over-etched, and the pattern does not become thin or the tapered cross section does not occur.

2 to 5 are sectional views showing an embodiment in which the present invention is applied to a method of manufacturing a nonvolatile semiconductor memory device in the order of steps. First, as shown in FIG. 2A, a silicon oxide film 12 for element isolation is formed thick on a silicon substrate 11, and a silicon oxide film 12 is formed on an element region including a memory cell region and a peripheral region.
A gate oxide film 13 made of a silicon oxide film having a thickness of not more than nm is formed by a thermal oxidation method or a CVD method. Next, polysilicon 14 is formed on the entire surface by a CVD method,
An n-type impurity is introduced.

Next, as shown in FIG. 2B, the polysilicon 1 in the memory cell region is formed by photolithography.
4 is patterned to form a floating gate FG. At this time, the polysilicon is left in the peripheral region for protecting the substrate. FIG. 2 is a sectional view taken along line AA of FIG. Next, as shown in FIG. 2C, a silicon oxide film 15 of 5 to 50 nm is formed on the entire surface by a thermal oxidation method or a CVD method, and 10 to 30 nm is formed thereon.
Is formed by the CVD method, and an n-type impurity is introduced. Then, a silicon oxide film 17 of 50 to 300 nm is formed on the surface by a thermal oxidation method or a CVD method.

Then, as shown in FIG. 3A, the silicon oxide film 17 and the polysilicon 16 in the memory cell region are etched using a photoresist 30 to form a control gate CG. Next, as shown in FIG. 3B, a silicon oxide film 18 having a thickness of 5 to 50 nm is formed on the entire surface by a thermal oxidation method or a CVD method. Is formed by a CVD method, and a silicon oxide film 20 of 10 nm or more is formed on the surface by a CVD method. Next, FIG.
As described above, the memory cell region is covered with the photoresist 31, and the silicon oxide film 20 and the silicon nitride film 19 in the peripheral circuit region are respectively removed by isotropic etching. At this time, the isotropic etching of the silicon nitride film 19 is performed under such etching conditions that the selectivity with respect to the silicon oxide film 20 is 10 or more.

Next, as shown in FIG. 4A, the silicon oxide film 20 is removed by isotropic etching.
As shown in (b), the silicon nitride film 19 in the memory cell region is etched back by anisotropic etching, and the side wall 19 of the silicon nitride film 19 is formed in the memory cell region.
Form A. At this time, the selectivity with respect to polysilicon is 5
The above anisotropic etching is used. Alternatively, after a silicon oxide film having a thickness of 20 nm or less is formed on the surface by a thermal oxidation method before etching back, the silicon nitride film is etched back using anisotropic etching having a selectivity of 5 or more to the silicon oxide film. The sidewall of the silicon nitride film described above may be formed.

Next, as shown in FIG. 5A, the polysilicon 1 is formed using the silicon oxide film 17 on the control gate CG and the side wall 19A of the silicon nitride film as a mask.
4 is removed by etching to remove the floating gate FG.
Is separated. At this time, the polysilicon 14 in the peripheral region is simultaneously etched away. Next, as shown in FIG. 5B, an erase gate (erase gate) oxide film 21 and a gate oxide film 22 in the peripheral region are simultaneously formed by a thermal oxidation method or a CVD method, and polysilicon is deposited to form an n-type impurity. After the introduction, the pattern is formed by photolithography to form the erase gate EG and the peripheral gate 23 at the same time. As a result, a nonvolatile memory cell is formed.

Therefore, in the nonvolatile memory cell formed in such a process, the breakdown voltage is maintained by using the silicon nitride film as the insulating film between the control gate and the erase gate without leaving the silicon nitride film on the peripheral step. A thin film can be formed as it is, which is advantageous for miniaturization.

Although this embodiment is an example in which the present invention is applied to the manufacture of a nonvolatile memory cell, any semiconductor device having a step on the surface of a semiconductor substrate and using a silicon nitride film may be used. The invention is equally applicable.

[0018]

As described above, according to the present invention, the first silicon oxide film, the silicon nitride film, and the second silicon oxide film are formed sequentially, and the second silicon oxide film is formed in a region where the silicon nitride film is not desired to remain. The silicon oxide film is removed and the remaining second
The silicon nitride film is isotropically etched using the silicon oxide film as a mask, and then the second silicon oxide film is removed and the silicon nitride film is formed into a desired pattern by anisotropic etching. The silicon nitride film can be isotropically etched without deterioration of the mask, and the etching time does not need to be made longer than necessary. As a result, the silicon nitride film can be left in a desired region, and the silicon nitride film in an unnecessary region can be reliably removed, and the silicon nitride film residue does not cause foreign matter to lower the yield of the semiconductor device. By forming a silicon nitride film having excellent oxidation resistance only in a desired region, the characteristics of the semiconductor device can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a first embodiment of the present invention in the order of manufacturing steps.

FIG. 2 is a first sectional view showing a second embodiment of the present invention in the order of manufacturing steps.

FIG. 3 is a second sectional view showing the second embodiment of the present invention in the order of manufacturing steps.

FIG. 4 is a third sectional view showing the second embodiment of the present invention in the order of manufacturing steps.

FIG. 5 is a first cross-sectional view showing a second embodiment of the present invention in the order of manufacturing steps.

FIG. 6 is a sectional view showing a conventional manufacturing method in the order of steps.

FIG. 7 is a cross-sectional view for explaining a conventional problem.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Element isolation oxide film 3 Gate oxide film 4 Silicon nitride film 5 Silicon oxide film 6,7 Photoresist 11 Silicon substrate 12 Element isolation oxide film 13 Gate oxide film 14 Polysilicon 15 Silicon oxide film 16 Polysilicon 17 Silicon oxide Film 18 Silicon oxide film 19 Silicon nitride film 20 Silicon oxide film 21 Erase gate oxide film 22 Gate oxide film 23 Gate electrode FG Floating gate CG Control gate EG Erase gate

──────────────────────────────────────────────────続 き Continued on front page (51) Int.Cl. ⁶ Identification code FI H01L 29/792

Claims (5)

(57) [Claims] A first portion provided on a surface of a semiconductor substrate having a step portion;
Forming a silicon oxide film on the first silicon oxide film, forming a silicon nitride film on the first silicon oxide film, forming a second silicon oxide film on the silicon nitride film, A step of etching and removing the second silicon oxide film in a region where the silicon nitride film containing the silicon nitride film is not to be left; and a step of isotropically etching the silicon nitride film using the remaining second silicon oxide film as a mask; A method of manufacturing a semiconductor device, comprising: a step of removing the second silicon oxide film; and a step of forming the silicon nitride film into a desired pattern by anisotropic etching. 2. The method according to claim 1, wherein the isotropic etching of the silicon nitride film has an etching selectivity with respect to the silicon oxide film of 10 or more. 3. The method of manufacturing a semiconductor device according to claim 1, wherein the step is an edge of an oxide film for element isolation formed of a thick silicon oxide film formed on a surface of the semiconductor substrate. 4. A step of forming a gate oxide film on a surface of a semiconductor substrate, a step of forming a first polysilicon on the gate oxide film and forming the first polysilicon in a required pattern, Forming a silicon oxide film, a second polysilicon, and a silicon oxide film on the first polysilicon;
Forming a control gate with a second polysilicon by patterning them, forming a first silicon oxide film, a silicon nitride film, and a second silicon oxide film sequentially on the entire surface; Removing the second silicon oxide film in a region including a surface step portion of the semiconductor substrate except for the step of removing the silicon nitride film by isotropic etching using the remaining second silicon oxide film as a mask Performing the step of removing the second silicon oxide film; the step of anisotropically etching the silicon nitride film to leave the sidewall only on the side surface of the control gate; and the step of removing the sidewall of the silicon nitride film. Forming a floating gate by etching the first polysilicon again using the mask as a mask;
Forming an erase gate oxide film on the side surface of the floating gate, forming a third polysilicon on the entire surface, patterning the third polysilicon, and forming the third polysilicon through the side wall of the silicon nitride film and the erase gate oxide film; A method of manufacturing a semiconductor device for forming a non-volatile memory cell including a step of forming an erase gate opposed to a gate. 5. A step of forming a gate of a transistor in a peripheral region by using the first polysilicon while leaving a part of the first polysilicon in a peripheral region at the time of initial etching. The method for manufacturing a semiconductor device according to claim 4, wherein