Semiconductor device manufacturing method

Abstract

To provide a method for manufacturing a semiconductor device capable of suppressing occurrence of a big deformation and a defect in a field oxide film and the bird's beak thereof even if a base oxide film is thinned to reduce the bird's beak and forming a semiconductor element easier to obtain desired electric characteristics.SOLUTION: A method for manufacturing a semiconductor device 100 includes steps of: forming a base oxide film 13 on a surface of a silicon semiconductor substrate 11 (P-type well region 12); forming a thick film part 13a having at least a prescribed width W on an element separation region B side from a boundary C and a thin film part 13b thinner than a film thickness ta of the thick film part 13a in an activation region A and the element separation region B other than the thick film part 13a, the thick film part and the thin film part provided along the boundary C between the activation region A and the element separation region B with respect to the base oxide film 13; forming a silicon nitride film 14 on surfaces of the thick film part 13a and the thin film part 13b; and selectively removing the silicon nitride film 14 of the element separation region B by over-etching processing.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a semiconductor device. By law Regarding. [Background technology]

[0002] In a semiconductor device having a plurality of semiconductor elements, an element isolation structure is provided to prevent electrical interference between the elements. One method for providing this element isolation structure is the Local Oxidation of Silicon (LOCOS) method, which selectively forms a thick silicon oxide film (sometimes called a "field oxide film") in an element isolation region (sometimes called a "field") to isolate a plurality of elements formed in an active region.

[0003] As an example of the LOCOS method, first, a silicon nitride film that is difficult to pass oxidizing species and is sufficiently less susceptible to thermal oxidation than silicon is formed on a silicon semiconductor substrate. In addition, before forming the silicon nitride film, a base oxide film is formed between the silicon semiconductor substrate and the silicon nitride film as a stress buffer film. Next, the silicon nitride film is selectively removed by etching to form a mask pattern, and then a field oxidation process is performed. As a result, an element isolation structure is provided with a field oxide film that is thickened by oxidizing the surface of the silicon semiconductor substrate that is not covered with the silicon nitride film, and a plurality of elements are electrically isolated.

[0004] However, during the field oxidation process, the oxidizing species diffuse from the openings of the mask pattern through the underlying oxide film and under the silicon nitride film, resulting in a structure in which the edge of the field oxide film extends under the silicon nitride film. The cross-sectional shape of the edge of the field oxide film resembles a bird's beak, so it is sometimes called a "bird's beak." This bird's beak extends into the active region, narrowing the effective area of ​​the active region and potentially adversely affecting electrical characteristics. Therefore, in order to prevent the diffusion of oxidizing species into the silicon semiconductor substrate below the silicon nitride film, the base oxide film is made as thin as possible, thereby making the bird's beak smaller and ensuring a large effective area for the active region, thereby enabling the placement of more semiconductor elements.

[0005] However, when the oxide film is thinned, dislocations are more likely to occur in the silicon semiconductor substrate due to shear stress between the oxide film and the silicon nitride film. The occurrence of dislocations can lead to deterioration of characteristics due to hot carriers, and can significantly reduce the number of years of service (device life). Therefore, in order to alleviate this shear stress, a method has been proposed in which the thickness of the base oxide film is set to 10 nm to 100 nm, and the heating temperature of the heat treatment performed on the base oxide film is set to 1,050°C or higher to form a field oxide film with a thickness of 400 nm or more (see Patent Document 1). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-21424 Summary of the Invention [Problem to be solved by the invention]

[0007] In the semiconductor device manufacturing method described in Patent Document 1, when the base oxide film is thinned, the problem of reduced durability due to dislocations caused by stress is solved. However, in addition, when the silicon nitride film is selectively removed by dry etching, minute holes called microtrenches may be formed.

[0008] Specifically, in this dry etching process, when high-frequency power is applied to an etching gas such as SF6 to generate ions and radicals to selectively remove the silicon nitride film, highly reactive radicals tend to accumulate near the periphery of the etched surface. This can result in the formation of micro-trenches with defects in the surrounding silicon, or even if no micro-trenches are formed, the etching damage can cause defects in the silicon. If a silicon semiconductor substrate is field-oxidized under such conditions, defects in the silicon remain along with deformation of the field oxide film and its bird's beak, and the semiconductor elements formed in the active region may not have the desired electrical characteristics due to leakage currents through these defects.

[0009] Therefore, one aspect of the present invention aims to provide a method for manufacturing a semiconductor device that can form a semiconductor element that is likely to have desired electrical characteristics by suppressing the occurrence of large deformations or defects in the field oxide film and its bird's beak even if the underlying oxide film is made thin in order to reduce the bird's beak. [Means for solving the problem]

[0010] A method for manufacturing a semiconductor device according to an embodiment of the present invention includes the steps of: 1. A method for manufacturing a semiconductor device having an activation region for forming a semiconductor element and an element isolation region for electrically isolating the semiconductor element, comprising the steps of: A base oxide film is formed on the surface of a silicon semiconductor substrate; forming a thick film portion provided along a boundary between the activation region and the element isolation region and having a predetermined width from at least the boundary toward the element isolation region, and a thin film portion having a thickness thinner than that of the thick film portion in the activation region and the element isolation region other than the thick film portion, in the base oxide film; forming a silicon nitride film on a surface of the thick film portion and the thin film portion; the silicon nitride film in the element isolation region is selectively removed by overetching; selectively forming a field oxide film on the surface of the silicon semiconductor substrate in the element isolation region by a field oxidation process using the silicon nitride film in the activation region as a mask; This includes: Effect of the Invention

[0011] According to one aspect of the present invention, it is possible to provide a method for manufacturing a semiconductor device that can form a semiconductor element that is likely to have desired electrical characteristics by suppressing the occurrence of large deformation or defects in the field oxide film and its bird's beak even if the underlying oxide film is made thin in order to reduce the bird's beak. [Brief description of the drawings]

[0012] [Figure 1] 1A to 1C are schematic cross-sectional views illustrating a method for manufacturing a semiconductor device according to the first embodiment. [Diagram 2] 2A to 2C are schematic cross-sectional views illustrating a method for manufacturing the semiconductor device according to the first embodiment. [Diagram 3] 3A to 3C are schematic cross-sectional views illustrating a method for manufacturing the semiconductor device according to the first embodiment. [Figure 4] 4A to 4C are schematic cross-sectional views illustrating a method for manufacturing the semiconductor device according to the first embodiment. [Diagram 5] 5A to 5C are schematic cross-sectional views illustrating a method for manufacturing the semiconductor device according to the first embodiment. [Figure 6] 6A to 6C are schematic cross-sectional views illustrating a method for manufacturing a semiconductor device according to the second embodiment. [Figure 7] 7A to 7C are schematic cross-sectional views illustrating a method for manufacturing a semiconductor device according to the third embodiment. [Figure 8] FIG. 8 is a schematic cross-sectional view showing a conventional method for manufacturing a semiconductor device. [Figure 9] FIG. 9 is a schematic cross-sectional view showing a conventional method for manufacturing a semiconductor device. [Figure 10] FIG. 10 is a schematic cross-sectional view showing a conventional method for manufacturing a semiconductor device. [Figure 11]FIG. 11 is a schematic cross-sectional view showing a conventional method for manufacturing a semiconductor device. [Figure 12] FIG. 12 is a schematic cross-sectional view showing another conventional method for manufacturing a semiconductor device. [Figure 13] FIG. 13 is a schematic cross-sectional view showing another conventional semiconductor device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] A method for manufacturing a semiconductor device according to an embodiment of the present invention is a method for manufacturing a semiconductor device having an activation region for forming a semiconductor element and an isolation region for electrically isolating the semiconductor element. In this method for manufacturing a semiconductor device, first, an underlying oxide film is formed on a surface of a silicon semiconductor substrate. Next, a thick film portion is formed on the underlying oxide film along the boundary between the activation region and the isolation region, and has a predetermined width from the boundary toward the isolation region, and a thin film portion is formed in the activation region and the isolation region other than the thick film portion and has a thickness thinner than that of the thick film portion. Next, a silicon nitride film is formed on the surfaces of the thick film portion and the thin film portion, and the silicon nitride film in the isolation region is selectively removed by an overetching process. Then, a field oxide film is selectively formed on the surface of the silicon semiconductor substrate in the isolation region by a field oxidation process using the silicon nitride film in the activation region as a mask.

[0014] In this method of manufacturing a semiconductor device, when the silicon nitride film in the element isolation region is selectively removed by an overetching process using dry etching, a thick film portion of the base oxide film is formed on the periphery of the etching surface where the etching is likely to be deep, i.e., from the boundary between the activation region and the element isolation region toward the element isolation region. As a result, even if the thin film portion of the base oxide film is made thin to reduce the bird's beak of the silicon semiconductor substrate, the thick film portion prevents the etching from reaching the silicon semiconductor substrate. This makes it possible to obtain a semiconductor device in which large deformations and defects of the field oxide film and its bird's beak can be suppressed and semiconductor elements having desired electrical characteristics can be formed.

[0015] However, it is generally impractical to directly identify such a reduction in defects in the silicon locally near the bird's beak.

[0016] Here, "over-etching" refers to etching that can sufficiently remove the silicon nitride film that remains due to "in-plane variations" even after just etching. The above-mentioned "just etching" refers to etching that removes most of the silicon nitride film and is performed until the etching product in the plasma changes, causing a large change in the spectrum of plasma emission.

[0017] Moreover, the term "base oxide film" refers to a silicon oxide film formed on the surface of a silicon semiconductor substrate (or, for example, a P-type well layer formed on a silicon semiconductor substrate as in the following embodiment), and refers to an oxide film that serves as a base for a silicon nitride film used as a mask during field oxidation. This base oxide film is generally formed by a thermal oxidation method, but may also be an oxide film deposited by CVD. Furthermore, this base oxide film may have any shape as long as it is formed on a silicon semiconductor substrate as a base for a silicon nitride film.

[0018] Hereinafter, a conventional embodiment and an embodiment of the present invention will be described in detail with reference to the drawings. In the drawings, the same components are denoted by the same reference numerals, and duplicated explanations may be omitted. In the drawings, the X, Y, and Z directions are perpendicular to each other. The direction including the X direction and the opposite direction of the X direction (-X direction) is called the "X-axis direction", the direction including the Y direction and the opposite direction of the Y direction (-Y direction) is called the "Y-axis direction", and the direction including the Z direction and the opposite direction of the Z direction (-Z direction) is called the "Z-axis direction" (height direction, thickness direction). In this regard, in the following embodiments, the surface of each film on the Z direction side may be called the "surface". The drawings are schematic, and the ratios of width, length, depth, etc. are not as shown in the drawings.

[0019] First, before describing the embodiments of the present invention, a conventional embodiment will be described. All of these embodiments are methods for manufacturing a semiconductor device that provide an activation region in which a semiconductor element is formed and an element isolation region that electrically isolates the semiconductor element.

[0020] 8 to 10 are schematic cross-sectional views showing a conventional method for manufacturing a semiconductor device. In the conventional manufacturing method shown in Fig. 8 to 10, the base oxide film is made as thin as possible to prevent the passage of oxidizing species during field oxidation, thereby preventing the bird's beak from becoming large.

[0021] In a conventional method for manufacturing a semiconductor device, first, as shown in FIG. 8, a P-type well region 12 is formed on a surface of a P-type silicon semiconductor substrate 11 by implanting impurities and thermal diffusion, and then a base oxide film 13 having a thickness t1 is formed on the P-type well region 12 by thermal oxidation treatment.

[0022] Next, as shown in FIG. 9, a silicon nitride film 14 which serves as a mask during field oxidation is formed over the entire surface of the base oxide film 13, and then a photoresist film 15 which serves as a mask during silicon nitride film etching is formed over the entire surface of the silicon nitride film 14.

[0023] Then, as shown in FIG. 10, in the element isolation region B, the photoresist film 15 is selectively removed by exposure processing, and then the silicon nitride film 14 is selectively removed by etching processing. In the etching process of the silicon nitride film 14, when dry etching is used, the selectivity ratio of the silicon nitride film 14 to the underlying oxide film 13 is generally relatively small, about 2 to 3. Moreover, the silicon nitride film 14 is selectively removed by over-etching in consideration of the film thickness variation of the silicon nitride film 14 and the in-plane variation in etching. At this time, the etching surface reaches the underlying oxide film 13 by the over-etching process, and the film thickness of the underlying oxide film 13 becomes t2, which is thinner than t1. Furthermore, radicals tend to accumulate on the peripheral edge of this etching surface (i.e., the boundary C between the activation region A and the element isolation region B) toward the element isolation region B, and the etching rate tends to be locally high, so that the etching reaches the P-type well region 12, and a micro-trench MT is formed.

[0024] Specifically, in the overetching process of the silicon nitride film 14, the etching rate in the vicinity of the periphery of the etched surface may be about 1.2 times the etching rate in the other areas of the etched surface. In practice, the conditions of the overetching process were set to a constant film thickness, and the overetching amount of the base oxide film 13 was set to 10 nm, and the silicon nitride film 14 with a film thickness of 150 nm was selectively removed by dry etching. As a result, the base oxide film 13 was overetched by about 22 nm near the periphery of the etched surface, and by about 10 nm other than the periphery of the etched surface. From this result, if the selection ratio of the silicon nitride film 14 to the base oxide film 13 is set to 2 and converted into silicon nitride film 14, it is considered that the silicon nitride film 14 was etched by about 194 nm to 216 nm near the periphery of the etched surface, and by about 170 nm to 180 nm other than the periphery. Therefore, from these values, it can be seen that the etching rate near the periphery of the etched surface is about 1.2 times that of the etching rate other than the periphery of the etched surface.

[0025] Moreover, under these overetching conditions, the width of the region deeply etched from the periphery of the etched surface toward the element isolation region B was about 0.3 μm.

[0026] Furthermore, as shown in FIG. 10, when field oxidation is performed in a state where the micro-trench MT is formed in the P-type well region 12, the surface area on which the oxidizing species reacts becomes large, and the formation region W of the bird's beak 16a is enlarged. b1 As the width of the active region A increases, the field oxide film 16 and its bird's beak 16a are deformed. As a result, the semiconductor element formed in the active region A may not have the desired electrical characteristics.

[0027] Therefore, it may be possible to prevent the formation of the micro-trench MT in the P-type well region 12 by making the thickness of the base oxide film 13 sufficiently large. Specifically, as shown in FIG. 12, by setting the thickness of the base oxide film 13 to t3 which is sufficiently thicker than t1, it is possible to prevent the micro-trench MT from being formed in the P-type well region 12. In this case, as shown in FIG. 13, when field oxidation is performed in this state, field oxide film 16 and its bird's beak 16a are not deformed, but since film thickness t3 of base oxide film 13 is thicker than film thickness t1 of base oxide film 13 shown in FIG. 10, the oxidizing species easily passes through base oxide film 13. Then, the oxidizing species easily diffuses into P-type well region 12 below silicon nitride film 14, and the formation region W of bird's beak 16a is expanded. b2 becomes wider.

[0028] Thus, in the conventional semiconductor device manufacturing method, it was necessary to adjust the film thickness of base oxide film 13 so that the formation area of ​​bird's beak 16a would not become too wide and so that field oxide film 16 and its bird's beak 16a would not be deformed, but this adjustment was extremely difficult. Therefore, in the embodiment of the present invention, a semiconductor device can be provided in which a semiconductor element 100 having desired electrical characteristics can be formed without significant deformation of the field oxide film 16 or its bird's beak 16a, even if the base oxide film 13 is thinned to make the bird's beak 16a smaller.

[0029] (First embodiment) 1 to 5 are schematic cross-sectional views showing a method for manufacturing a semiconductor device according to a first embodiment. The semiconductor device 100 according to the first embodiment is manufactured by the method for manufacturing a semiconductor device according to the first embodiment. In the method for manufacturing a semiconductor device according to the first embodiment, an activation region A for forming a semiconductor element and an element isolation region B for electrically isolating the semiconductor element are provided. The method for manufacturing this semiconductor device will be described in detail below.

[0030] In the method for manufacturing a semiconductor device according to the present embodiment, first, as shown in FIG. 1, a P-type well region 12 is formed on the surface of a P-type silicon semiconductor substrate 11 by implanting impurities and thermal diffusion, and then the P-type well region 12 is thermally oxidized to a thickness of ta An underlying oxide film 13 is formed.

[0031] Next, as shown in FIG. 2, the thick film portion 13a and the thin film portion 13b are formed on the base oxide film 13. Specifically, the thick film portion 13a and the thin film portion 13b have a thickness of t a The photoresist film is then applied to the area where the thin film portion 13b is to be formed so as to maintain the state of the photoresist film. a Thinner than b The undercoat oxide film 13 is subjected to half-etching so as to form the above-mentioned.

[0032] The thick film portion 13a is provided along the boundary C between the activation region A and the element isolation region B, and has a predetermined width W at least from the boundary C toward the element isolation region B. This thick film portion 13a functions as a stopper to prevent over-etching up to the P-type well region 12 when the silicon nitride film 14 in the element isolation region B is selectively removed by dry etching. This is because the etching speed becomes locally faster near the periphery of the etching surface where radicals are likely to accumulate, so the thick film portion 13a is provided along the boundary C between the activation region A and the element isolation region B so as to include the vicinity of the periphery of the etching surface.

[0033] The thickness t of the thick film portion 13a a There is no particular limitation on the thickness t as long as the etching does not reach the P-type well region 12 when the silicon nitride film 14 is selectively removed, and the thickness t can be appropriately selected according to the purpose. a However, since if the thickness is too thick, the bird's beak formation region tends to spread, it is preferable to make the thickness thin enough that the etching does not reach the P-type well region 12.

[0034] The predetermined width W of the thick film portion 13a is not particularly limited as long as it is equal to or greater than the width that is deeply etched near the periphery of the etching surface when the silicon nitride film 14 is selectively removed, and can be appropriately selected depending on the purpose. However, a width that is deeply etched near the periphery of the etching surface is preferable. In this embodiment, from the viewpoint of stably forming the thick film portion 13a along the boundary C between the activation region A and the element isolation region B, the predetermined width W of the thick film portion 13a is set to 1 μm.

[0035] The position of thick film portion 13a is not particularly limited and can be appropriately selected according to the purpose as long as it can be provided along boundary C between activation region A and element isolation region B so as to include the region to be deeply etched near the periphery of the etching surface, but a position including the width to be deeply etched is preferable in element isolation region B. Also, from the viewpoint of reducing the bird's beak, it is preferable that thick film portion 13a is not included in activation region A. In this embodiment, the thick film portion 13a is positioned such that, of the 1 μm of the predetermined width W of the thick film portion 13a, 0.3 μm is included in the activation region A and 0.7 μm is included in the element isolation region B.

[0036] The thin film portion 13b is formed in the active region A and the element isolation region B other than the thick film portion 13a. a Thinner film thickness t b has. The thickness t of the thin film portion 13b b There is no particular restriction on the thickness of the thin film portion 13b, and it can be appropriately selected according to the purpose as long as the etching does not reach the P-type well region 12 when the silicon nitride film 14 is selectively removed. However, the thinner the thickness, the more difficult it is for oxidizing species to pass through during field oxidation, and the more difficult it is to prevent diffusion under the silicon nitride film 14, so that 35 nm or less is preferable. b If the thickness is too thin, dislocations occur in the silicon semiconductor substrate 11 due to stress, and the durability of the silicon semiconductor substrate 11 is easily reduced, so that the thickness is preferably 20 nm or more.

[0037] Considering the film thickness of the thick film portion 13a and the thin film portion 13b from the film thickness of the silicon nitride film 14, consider the case where the silicon nitride film 14 is etched to a depth 1.2 times deeper than the film thickness t of the silicon nitride film 14 in order to completely selectively remove the silicon nitride film 14. In this case, if the selectivity ratio of the silicon nitride film 14 to the underlying oxide film 13 is estimated conservatively as 2, the thin film portion 13b of the underlying oxide film 13 is etched to a thickness of 0.1t. From this, the film thickness t of the thin film portion 13b is b must be 0.1t or more. In addition, since the etching depth is 1.2 times deeper than the thin film portion 13b near the periphery of the etching surface, the thick film portion 13a of the base oxide film 13 is etched to a depth of 0.22t. a The weight must be at least 0.22t.

[0038] Specifically, if the selection ratio of the silicon nitride film 14 to the underlying oxide film 13 is 2, then the following formula, (t+2t a ) / (t+2t b ) ≧1.2. For example, if the thickness t of the silicon nitride film 14 is 150 nm, the thickness t of the thin film portion 13b b 0.1t or more, and the thickness t a is set to 0.22t or more, and the thickness t b The preferred range of t b Considering that the thickness t a is set to 39 nm or more, and the film thickness t b is 20 nm or more.

[0039] Next, as shown in FIG. 3, a silicon nitride film 14 is formed by CVD on the entire surface of the base oxide film 13 including the thick portion 13a and the thin portion 13b, and then a photoresist film 15 is formed on the entire surface of the silicon nitride film 14.

[0040] The thickness of the silicon nitride film 14 is not particularly limited as long as it has a thickness that serves as a mask during field oxidation, and can be appropriately selected depending on the purpose, but a thickness that allows easy selective removal is preferable. In this embodiment, the thickness t of the silicon nitride film 14 is set to 150 nm.

[0041] 4, the photoresist film 15 in the element isolation region B is selectively removed by exposure processing, and then the silicon nitride film 14 in the element isolation region B is selectively removed by over-etching processing using dry etching. At this time, the thickness t c is the thickness t of the thick film portion 13a a and the thickness t of the thin film portion 13b b It will be thinner than.

[0042] Next, a field oxide film 16 is selectively formed on the surface of the P-type well region 12 in the element isolation region B as shown in FIG. 5 by a field oxidation process using the silicon nitride film 14 in the activation region A shown in FIG. 4 as a mask.

[0043] After the field oxide film 16 is formed, the silicon nitride film 14 and the base oxide film 13 in the active region A are removed, and a semiconductor element such as a MOSFET is formed in the active region A.

[0044] In this manner, in the method for manufacturing a semiconductor device according to the first embodiment, etching does not reach the P-type well region 12, and thus no micro-trench MT is generated. Therefore, in this method for manufacturing a semiconductor device, even if the base oxide film 13 is thinned to reduce the size of the bird's beak 16a, the occurrence of large deformations or defects in the field oxide film 16 and its bird's beak 16a is suppressed, and a semiconductor element that is likely to have the desired electrical characteristics can be formed.

[0045] Second embodiment Fig. 6 is a schematic cross-sectional view showing a method for manufacturing a semiconductor device according to the second embodiment. Fig. 6 also shows a state similar to the state after the photoresist film 15 shown in Fig. 3 in the first embodiment is formed.

[0046] As shown in FIG. 6, the second embodiment is similar to the first embodiment, except that the thick film portion 13a in the first embodiment is not formed to extend into the active region A. As a result, in the second embodiment, since the thick film portion 13a is not formed to extend into the activation region A, it becomes difficult for oxidizing species to pass under the silicon nitride film 14 in the activation region A during field oxidation, and the bird's beak can be made smaller.

[0047] (Third embodiment) Fig. 7 is a schematic cross-sectional view showing a method for manufacturing a semiconductor device according to the third embodiment. Fig. 7 also shows a state similar to the state after the photoresist film 15 shown in Fig. 3 in the first embodiment is formed.

[0048] As shown in FIG. 7, the third embodiment is similar to the first embodiment, except that the thick film portion 13a in the first embodiment is formed to extend over the entire element isolation region B. As a result, in the third embodiment, the thick film portion 13a is formed extending over the entire area of ​​the element isolation region B, so that alignment can be easily performed when performing the exposure process of the photoresist film 15 and the overetching process of the silicon nitride film 14.

[0049] As described above, the method for manufacturing a semiconductor device according to one embodiment of the present invention is a method for manufacturing a semiconductor device having an activation region for forming a semiconductor element and an isolation region for electrically isolating the semiconductor element. In this method for manufacturing a semiconductor device, first, a base oxide film is formed on the surface of a silicon semiconductor substrate. Next, a thick film portion is formed on the base oxide film along the boundary between the activation region and the isolation region, and has a predetermined width from the boundary toward the isolation region, and a thin film portion is formed in the activation region and the isolation region other than the thick film portion and has a thickness thinner than that of the thick film portion. Next, a silicon nitride film is formed on the surfaces of the thick film portion and the thin film portion, and the silicon nitride film in the isolation region is selectively removed by an overetching process. Then, a field oxide film is selectively formed on the surface of the silicon semiconductor substrate in the isolation region by a field oxidation process using the silicon nitride film in the activation region as a mask.

[0050] As a result, even if the base oxide film is made thin to reduce the bird's beak, it is possible to provide a semiconductor device in which large deformation or defects in the field oxide film and its bird's beak can be suppressed and semiconductor elements having desired electrical characteristics can be formed.

[0051] The present invention has been specifically described above based on each embodiment, but the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the present invention.

[0052] For example, in each of the above embodiments, a P-type well region is formed on a P-type silicon semiconductor substrate, but this is not limiting, and both may be N-type, or either one may be N-type. Furthermore, in each of the above embodiments, a P-type well region is formed, but the present invention is not limited to this, and it is not necessary to form a P-type well region. Furthermore, in each of the above embodiments, the thick film portion and the thin film portion are formed by half etching processing, but this is not limiting, and the thick film portion may be formed by field oxidation processing. [Explanation of symbols]

[0053] 11 Silicon semiconductor substrate 12 P-type well region 13 Undercoat oxide film 13a Thick film part 13b Thin film part 14 Silicon nitride film 15 Photoresist film 16 Field oxide 100 Semiconductor device A activation area B Element isolation area C boundary t Silicon nitride film thickness t a Thick film thickness t b Thin film thickness W (at the thick part) given width Wb (bird's beak) formation region

Claims

1. 1. A method for manufacturing a semiconductor device having an activation region for forming a semiconductor element and an element isolation region for electrically isolating the semiconductor element, comprising the steps of: A base oxide film is formed on the surface of a silicon semiconductor substrate; forming a thick film portion provided along a boundary between the activation region and the element isolation region and having a predetermined width from at least the boundary toward the element isolation region, and a thin film portion having a thickness thinner than that of the thick film portion in the activation region and the element isolation region other than the thick film portion, in the base oxide film; forming a silicon nitride film on surfaces of the thick film portion and the thin film portion; the silicon nitride film in the element isolation region is selectively removed by overetching; selectively forming a field oxide film on the surface of the silicon semiconductor substrate in the element isolation region by a field oxidation process using the silicon nitride film in the activation region as a mask; A method for manufacturing a semiconductor device comprising the steps of:

2. 2. The method for manufacturing a semiconductor device according to claim 1, wherein the predetermined width is set by measuring in advance a width of a region that is deeply etched from the boundary toward the element isolation region in the base oxide film having a constant thickness under the condition of the over-etching process that selectively removes the silicon nitride film.

3. 3. The method for manufacturing a semiconductor device according to claim 1, wherein in the overetching treatment of the silicon nitride film, the thick film portion is formed extending toward the activation region.

4. 4. The method for manufacturing a semiconductor device according to claim 1, wherein the thick film portion is formed over the entire element isolation region in the overetching process of the silicon nitride film.

5. 5. The method for manufacturing a semiconductor device according to claim 1, wherein the thick portion and the thin portion are formed by a half etching process.

6. 6. The method for manufacturing a semiconductor device according to claim 1, wherein the thick film portion and the thin film portion are formed by a field oxidation process.

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