JP2009224363A - Manufacturing method of semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce an area of a recess LOCOS structure of a semiconductor device, and to suppress a leakage current. <P>SOLUTION: A recess is formed by digging down the surface of a silicon substrate 50 in a region for forming a LOCOS film by anisotropic etching. Isotropic etching is then performed at the recess formed by anisotropic etching to round a corner 60b defined by the side surface and the footprint of the recess 58b. Thereafter, the silicon substrate 50 in the recess 58b is oxidized selectively, thus forming an LOCOS film filling the recess 58b. Since the corner 60b of the recess 58b is rounded, a concentration of stress by the LOCOS film in the silicon substrate 50 is alleviated at the outside of the corner 60b, and the occurrence of a defect in the silicon substrate 50 is prevented. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a semiconductor device, and more particularly to formation of a local oxidation (LOCOS) film.

In an integrated circuit on the surface of a silicon substrate, a LOCOS structure is used for the purpose of separating elements formed on the surface of the silicon substrate. In basic LOCOS, a silicon substrate surface is covered with an oxidation-resistant mask such as a silicon nitride film (Si ₃ N ₄ film), and the silicon substrate surface in the opening of the mask is selectively thermally oxidized. The LOCOS film, which is an oxide film formed thereby, grows thick due to volume expansion due to oxidation, and about half of the thickness grows below the substrate surface, contributing to element isolation, while about half of the thickness is above the substrate surface. To form a step on the substrate surface. This step has a problem that it causes uneven exposure and step breakage in subsequent steps. The LOCOS film also grows in the lateral direction and extends under the mask to form a bird's beak. There was a problem that stress due to bird's beaks caused defects in the silicon substrate. The spread of bird's beaks in the lateral direction is an obstacle to improving the integration degree of elements.

  A recess LOCOS method is known as a technique for improving these problems. In the recess LOCOS method, a silicon substrate in a region where a LOCOS film is to be formed is dug by etching to form a recess, and a LOCOS film is formed in this recess. Thereby, the level | step difference on the silicon substrate surface can be made low, and the thickness embed | buried in the silicon substrate correspondingly increases. As a result, the same element isolation effect as that of the basic LOCOS method described above can be realized with a smaller amount of oxidation, and the bird's beak can be reduced accordingly.

Note that in a semiconductor device that requires higher integration, a trench element isolation (STI) method in which an insulator is embedded in a trench formed on the surface of a silicon substrate is used instead of the LOCOS method. High integration requires not only miniaturization of the element isolation structure but also miniaturization of various structures, and new technologies have been developed to meet the demand. For example, a tungsten plug structure or a structure in which silicide is formed between the contact and the diffusion layer to reduce the contact resistance in response to the miniaturization of the contact hole is used. Here, the silicide serving as the base film of the contact can be formed in a self-aligned manner on the surface of the silicon substrate surrounded by STI by using a salicide (self-aligned-silicide) technique.
JP 58-112342 A JP 2000-156376 A

  It is possible to reduce the size of the LOCOS in the horizontal direction by forming the recess of the silicon substrate for forming the recess LOCOS by using anisotropic etching. FIG. 10 is a vertical sectional view of the recess 2 of the recess LOCOS formed by anisotropic etching. The recess 2 is formed by performing anisotropic etching on the silicon substrate 4 using the silicon nitride film 6 and the photoresist film 8 laminated on the silicon substrate 4 as an etching mask. For example, reactive ion etching (RIE) is used as the anisotropic etching. By using anisotropic etching, the opening of the recess 2 has a shape and size substantially corresponding to the opening 10 of the etching mask, and expansion in the lateral direction is suppressed. The side surface 12 and the bottom surface 14 of the recess 2 form a sharp corner (corner 16). For example, due to the effect that ions irradiated by etching are attracted to the photoresist film 8, the irradiation density of ions at the edge is higher than the central portion of the recess 2, and the corner 16 may be formed at an acute angle. .

  As described above, the recess 2 formed by anisotropic etching can suppress the lateral spread more than the case of forming by isotropic etching, and accordingly, the area of the LOCOS formed in the recess 2 can also be suppressed. However, when a LOCOS film is grown in the recess 2 by anisotropic etching, there is a problem that the leakage current between the diffusion layer in the vicinity thereof and the substrate is likely to increase. FIG. 11 is a vertical sectional view showing a state in which the LOCOS film 20 is formed in the recess 2. It is observed that when the LOCOS film 20 is grown in the recess 2 by anisotropic etching, defects of the silicon substrate 4 are likely to occur in a region 22 that extends obliquely downward from the portion corresponding to the corner 16 to the outside of the LOCOS film 20 due to the stress. It was done. It is understood that the leakage current described above is due to this defect.

  The present invention has been made to solve the above problems, and it is an object of the present invention to provide a method of manufacturing a semiconductor device capable of forming a recessed LOCOS structure with a small area while suppressing leakage current.

  A method for manufacturing a semiconductor device according to the present invention is a method for manufacturing a semiconductor device having a local oxide film on a surface of a silicon substrate, wherein the silicon substrate surface in a region where the local oxide film is formed is dug down by anisotropic etching. An anisotropic etching step for forming a concave portion, and an isotropic etching step for subjecting the concave portion formed by the anisotropic etching step to isotropic etching and rounding a corner sandwiched between a side surface and a bottom surface of the concave portion. And a local oxidation step of selectively oxidizing the silicon substrate in the recess to form the local oxide film filling the recess after the isotropic etching step.

  According to the present invention, since the recess is dug down basically by anisotropic etching, the size of the opening can be made smaller than when dug down by isotropic etching. Thereby, the area of the LOCOS film grown in the opening is also suppressed. Further, after the recess is dug down, isotropic etching is performed to round the corner sandwiched between the side surface and the bottom surface of the recess, thereby reducing the stress concentration due to the growth of the LOCOS film at the corner. Thereby, generation | occurrence | production of the defect in a silicon substrate is suppressed, and reduction of a leakage current is aimed at.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings. 1 to 8 are schematic vertical cross-sectional views showing the manufacturing process of the semiconductor device according to the embodiment. In this semiconductor device, an element such as a transistor is formed on the surface of a silicon substrate 50, and a recess LOCOS structure is used for the element isolation. FIGS. 1 to 8 show a portion related to the recess LOCOS structure of the semiconductor device, and the structures of various elements and the like are basically omitted.

  Step 1 (see FIG. 1): A silicon nitride film 52 is formed on the surface of the silicon substrate 50 by using a CVD (Chemical Vapor Deposition) method (low pressure CVD method, plasma CVD method, high density plasma CVD method or atmospheric pressure CVD method). Form. The silicon nitride film 52 is formed to a thickness that can function as a mask for a thermal oxidation process for the growth of a LOCOS film described later.

  Before depositing the silicon nitride film 52, a pad oxide film for relaxing the stress between the silicon nitride film 52 and the surface of the silicon substrate 50 may be formed by thinly thermally oxidizing the surface of the silicon substrate 50. Good.

  Step 2 (see FIG. 2): A photoresist is applied on the silicon nitride film 52 by a spin coating method to form a photoresist film 54. The photoresist film 54 is patterned by exposure / development processing to form an opening 56 in a region where LOCOS is to be formed.

  Step 3 (see FIG. 3): Using the photoresist film 54 as a mask, anisotropic etching such as RIE is performed to form the silicon nitride film 52 (the silicon nitride film 52 and the pad oxide film when the pad oxide film is formed). Then, the silicon substrate 50 is further dug down to a depth of about half the thickness of the LOCOS film to be formed, and a recess 58 a is formed in the silicon substrate 50. The depth of the recess 58a can be about 300 mm, for example. Here, the recess 58a formed by anisotropic etching forms a corner 60a whose side and bottom are sharp.

  Step 4 (see FIG. 4): Next, the inner surface of the recess 58a of the silicon substrate 50 is etched by an isotropic etching method such as wet etching using the photoresist film 54 as a mask. By this isotropic etching, a recess 58b having a rounded corner 60b sandwiched between the side surface and the bottom surface is formed. This isotropic etching mainly focuses on rounding the corner 60b of the recess 58b, and the difference in depth between the recess 58b and the recess 58a is basically set smaller than the depth of the recess 58a.

  Step 5 (see FIG. 5): After the photoresist film 54 is peeled off, thermal oxidation is performed at a temperature of about 1100 ° C. in a water vapor atmosphere to form a LOCOS film 62 having a thickness enough to fill the recess 58b.

  Step 6 (see FIG. 6): After the silicon nitride film 52 is removed by wet etching (if the pad oxide film is formed, the pad oxide film remaining under the silicon nitride film 52 is also removed), the LOCOS film 62 A titanium (Ti) film 64 is deposited on the surface of the silicon substrate 50 including the surface by sputtering.

Step 7 (see FIG. 7): A Ti silicide film 66 is formed in a self-aligned manner on the surface of the silicon substrate 50 excluding the surface of the LOCOS film 62 by a salicide process using the Ti film 64. The formation of the Ti silicide film 66 is realized by annealing in a nitrogen atmosphere. The portion of the Ti film 64 that has not been silicided in this annealing process is removed by, for example, a cleaning process using ammonia water. Incidentally, ammonia perwater is a mixed solution of ammonia (NH ₄ OH), hydrogen peroxide (H ₂ O ₂ ), and water (H ₂ O).

  Step 8 (see FIG. 8): An interlayer insulating film 68 is deposited on the silicon substrate 50 and patterned by photolithography to form a contact hole 70 in a region where the LOCOS film 62 is not formed, that is, in the active region. Form. The Ti silicide film 66 is exposed on the bottom surface of the contact hole 70. A Ti film 72 is formed as a barrier metal on the inner surface of the contact hole 70, and a tungsten plug 74 is embedded in the contact hole 70. An aluminum (Al) film is deposited on the interlayer insulating film 68 and patterned to form wirings 76. The wiring 76 is electrically connected to a diffusion layer (not shown) formed in the active region of the silicon substrate 50 through the tungsten plug 74 and the Ti silicide film 66.

  In this semiconductor device, after the recess 58a is dug down by anisotropic etching, the bottom corner of the recess 58a is changed from a shape having a sharp corner (corner 60a) to a shape having a rounded corner (corner 60b) by isotropic etching. To do. As a result, the size of the opening of the recess 58b becomes smaller than when the same depth is all dug by isotropic etching, and the area of the LOCOS film 62 can be reduced.

  Further, since the corner 60b is rounded, stress concentration during growth of the LOCOS film 62 on the silicon substrate 50 outside the corner 60b is alleviated. Thereby, generation | occurrence | production of the defect in the silicon substrate 50 is suppressed, and reduction of leakage current is achieved.

  In particular, when a silicide film (Ti silicide film 66) is formed on the surface of the silicon substrate 50 by the salicide process as in the semiconductor device of this embodiment, the silicide film is disposed just outside the LOCOS film 62. Therefore, when the structure using the Ti silicide film 66 shown in FIG. 8 is applied to the silicon substrate in which the defect-prone region 22 shown in FIG. 11 exists, there is a portion where the distance between the region 22 and the Ti silicide film 66 is short. There is a problem that leakage current between the wiring 76 and the silicon substrate is more likely to occur. In this regard, according to the present invention, by suppressing the occurrence of defects in the silicon substrate 50 in the region 22, the leakage current can be suitably suppressed even in a semiconductor device that forms silicide on the surface of the silicon substrate 50 by the salicide process. That is, in a semiconductor device using LOCOS for element isolation, the contact resistance using silicide can be easily reduced, so that the contact can be miniaturized, which contributes to an increase in the degree of integration of the semiconductor device.

  Note that as the corner 60b is rounded, stress on the outer silicon substrate 50 is reduced and defects are less likely to occur, but on the other hand, the opening of the recess 58b is enlarged due to isotropic etching, and the area of the LOCOS film 62 is reduced. The effect of can be reduced. Therefore, the degree of isotropic etching in step 4 is set in consideration of both the degree of rounding and the enlargement of the opening.

  FIG. 9 is a vertical sectional view schematically showing the shape 80 of the recess 58a formed by anisotropic etching and the shapes 82 and 84 of the recess 58b after isotropic etching. For example, in the anisotropic etching of step 3, the ions are attracted toward the edge of the recess 58 by the electrical interaction between the irradiated ions and the photoresist film 54 and the like, and the etching is performed from the center of the recess 58 at the edge. Can go fast. In this case, the concave portion 58 has a convex bottom surface (convex upward) like the shape 80, and the sharpness of the corner 60 is increased. When isotropic etching is performed on the concave portion 58 of the shape 80, the concave portion 58 changes in the order of the shape 82 and the shape 84 according to the etching time (etching amount). In the shape 82, the corner 60 is rounded, but the bottom of the recess 58 is still convex. On the other hand, in the shape 84, the roundness of the corner 60 is further increased, and the bottom of the concave portion 58 is concave (convex downward). The shape in which the bottom surface of the concave portion 58 is concave as in this shape 84 is such that the orientation of the surface gradually changes over a wide range between the side surface and the bottom surface of the concave portion 58, and stress in the silicon substrate 50 is preferable. To be suppressed. Therefore, the degree of isotropic etching in step 4 can be suitably set by using the bottom surface as a guide.

It is a typical vertical sectional view showing the manufacturing process of the semiconductor device concerning the embodiment of the present invention. It is a typical vertical sectional view showing the manufacturing process of the semiconductor device concerning the embodiment of the present invention. It is a typical vertical sectional view showing the manufacturing process of the semiconductor device concerning the embodiment of the present invention. It is a typical vertical sectional view showing the manufacturing process of the semiconductor device concerning the embodiment of the present invention. It is a typical vertical sectional view showing the manufacturing process of the semiconductor device concerning the embodiment of the present invention. It is a typical vertical sectional view showing the manufacturing process of the semiconductor device concerning the embodiment of the present invention. It is a typical vertical sectional view showing the manufacturing process of the semiconductor device concerning the embodiment of the present invention. It is a typical vertical sectional view showing the manufacturing process of the semiconductor device concerning the embodiment of the present invention. It is a vertical sectional view schematically showing the shape of a recess formed by anisotropic etching and the shape of the recess after isotropic etching. It is a vertical sectional view of a conventional recess formed by anisotropic etching to form a recess LOCOS. FIG. 11 is a vertical sectional view of a conventional recess LOCOS structure in which a LOCOS film is formed in the recess of FIG. 10.

Explanation of symbols

  50 silicon substrate, 52 silicon nitride film, 54 photoresist film, 56 opening, 58a, 58b recess, 60a, 60b corner, 62 LOCOS film, 64, 72 Ti film, 66 Ti silicide film, 68 interlayer insulating film, 70 contact hole 74 Tungsten plug, 76 wiring.

Claims (3)

In a method for manufacturing a semiconductor device having a local oxide film on a silicon substrate surface,
An anisotropic etching step of forming a recess by digging the surface of the silicon substrate in a region for forming the local oxide film by anisotropic etching;
Isotropic etching step for applying isotropic etching to the recess formed by the anisotropic etching step and rounding a corner sandwiched between a side surface and a bottom surface of the recess;
After the isotropic etching step, a local oxidation step of selectively oxidizing the silicon substrate in the recess to form the local oxide film filling the recess;
A method for manufacturing a semiconductor device, comprising: In the manufacturing method of the semiconductor device according to claim 1,
And a salicide step of forming silicide on the surface of the silicon substrate on which the local oxide film is formed by a salicide technique. In the manufacturing method of the semiconductor device according to claim 1 or 2,
The method of manufacturing a semiconductor device, wherein the isotropic etching step forms a bottom surface of the concave portion into a concave surface.

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