JP2009158765A - Gate oxide film forming method, and method of manufacturing semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an easy and simple method of forming a plurality of gate oxide films having different film thicknesses while suppressing a size error of an active region almost to an error when the gate oxide films are uniform in film thickness and also suppressing an influence on a substrate. <P>SOLUTION: The gate oxide film forming method includes the steps of: forming an oxide shield film 3 not in a field oxide film formation expected region on the semiconductor substrate 1; then performing oxidation processing using the oxide shield film 3 as a mask to form a field oxide film 5 in the field oxide film formation expected region; then performing etching processing to expose the substrate surface within a first gate oxide film formation expected region 6 and then performing oxidation processing by a first film thickness; etching away the remaining oxide shield film 3 and then performing oxidation processing by a second film thickness less than the first film thickness on at least a surface of a second gate oxide film formation expected region 7. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for forming a gate oxide film and a method for manufacturing a semiconductor device, and more particularly to a method for forming a plurality of gate oxide films having different thicknesses.

  Generally, a high voltage MOS type semiconductor device equipped with an insulation effect transistor (MOS transistor) is composed of a high withstand voltage element having a high withstand voltage and a low withstand voltage element for performing high-speed signal processing in the same semiconductor device. . In a high breakdown voltage element, it is necessary to form a thick gate oxide film, and in a low breakdown voltage element, it is necessary to form a thin gate oxide film.

  Conventionally, as a method of forming a gate oxide film having a plurality of film thicknesses on the same substrate, generally, after forming an element isolation oxide film, the thickest gate oxide film is formed on the entire surface, and thereafter In addition, a method has been used in which a plurality of gate oxide films are formed by etching the thick gate oxide film formed in a predetermined region constituting a thin gate oxide film and then forming a thin gate oxide film. .

  FIG. 7 is a schematic process cross-sectional view showing a manufacturing process when forming a gate oxide film by a conventional method, and is divided into FIGS. 7A to 7E for each process. In the following description, the gate oxide films having different thicknesses are referred to as “first gate oxide film 9” and “second gate oxide film 10”, and the second gate oxide film 10 is formed more than the first gate oxide film 9. It shall be thin.

  First, as shown in FIG. 7A, the entire surface of the semiconductor substrate 1 is thermally oxidized to form a pad oxide film 93, and then a nitride film 4 is formed by a CVD (Chemical Vapor Deposition) method. Thereafter, the nitride film 4 in the region where the field oxide film 5 is to be formed is removed. As a result, the nitride film 4 remains outside the region where the field oxide film 5 is to be formed.

  Note that the pad oxide film 93 shown in FIG. 7A is formed to have a film thickness equivalent to the desired thickness of the thick first gate oxide film 9.

  Next, as shown in FIG. 7B, the entire surface is oxidized. At this time, since the nitride film 4 has strong oxidation resistance, the active region under the nitride film 4 is hardly oxidized, and the oxidation of the active region under the region where the nitride film 4 is not formed proceeds, and the field is in this region. An oxide film 5 is formed.

  Next, as shown in FIG. 7C, the remaining nitride film 4 is removed by etching.

  Next, as shown in FIG. 7D, etching is performed using a resist 81 patterned so as to expose the surface of the region 7 where the second gate oxide film 10 is to be formed. The pad oxide film 93 that has been removed is removed by etching.

  Next, as shown in FIG. 7E, after the resist 81 is removed, an oxidation process is performed on the entire surface. As a result, a thick first gate oxide film 9 is formed in the region 6, and a thin second gate oxide film 10 is formed in the region 7.

  According to the above method, gate oxide films having different film thicknesses can be formed only by adding one photo process. Further, an oxidation process at the time of forming the pad oxide film 93 according to FIG. 7A for forming the first gate oxide film 9 and a process according to FIG. 7E for forming the second gate oxide film 9 are performed. Any of the oxidation processes is an oxidation process in a state in which the nitride film 4 is not formed, so that no extra stress is applied to the semiconductor substrate 1 during the oxidation process. For this reason, it is possible to prevent crystal defects from occurring in the semiconductor substrate 1 during the oxidation step, and to form a highly reliable gate oxide film.

  However, in the conventional method shown in FIG. 7, it is necessary to previously form a thick field oxide film on the assumption that the film thickness is reduced by etching, which causes a problem that the bird's beak becomes large.

  That is, as shown in FIG. 7D, when the substrate surface of the semiconductor substrate 1 is exposed in the second gate oxide film formation region 7, the portion corresponding to the film thickness of the pad oxide film 93 formed in advance is etched. Will be removed. The film thickness of the first gate oxide film 9 is the film thickness of the pad oxide film 93 remaining in FIG. 7D and the film thickness that increases by being oxidized in a later step (that is, the second gate oxide film). 10 film thickness). That is, when it is desired to form the first gate oxide film 9 with a large thickness, it is necessary to form a thick oxide film as the pad oxide film 93 in advance. At this time, in the step of FIG. 7D, it is necessary to remove the thick pad oxide film 93 formed in the region 7.

  For example, when the pad oxide film 93 is formed with a film thickness of about 100 nm, in order to finally form the field oxide film 5 with a film thickness of about 300 nm, a reduction in film thickness of about 100 nm is considered in advance in FIG. At the time of b), it is necessary to form a field oxide film 5 of about 400 nm. When the field oxide film 5 is formed thick in this way, the bird's beak becomes large and the flat portion of the active region becomes small. For this reason, the gap between the design dimension (mask dimension) and the finished dimension increases, and there arises a problem that the variation of the active region at the minimum dimension increases.

  Further, since the active region varies, the resist 81 formed in FIG. 7D also varies, and the exposed second gate oxide film formation region 7 also varies. . As a result, the formation position of the second gate oxide film 10 varies, which may affect the electrical characteristics. In particular, high-speed signal processing is required for a low breakdown voltage element including the thin second gate oxide film 10, and therefore, it is necessary to perform fine processing. For the above reason, the active region and the second gate oxide are required. Variations in the position where the film 10 is formed also hinders speeding up of signal processing.

  As an improvement measure for the above problem, a method described in Patent Document 1 below is disclosed. Hereinafter, description will be given with reference to the drawings.

  8 and 9 are schematic process cross-sectional views showing a manufacturing process when forming a gate oxide film by the method described in Patent Document 1, and FIGS. 8A to 8D are shown for each process. 9 (a) to 9 (c) (they are divided into two drawings for the sake of space). In addition, the same code | symbol is attached | subjected about the component same as FIG.

  First, as shown in FIG. 8A, the entire surface of the semiconductor substrate 1 is thermally oxidized to form a thin pad oxide film 3, and then a nitride film 4 is formed by a CVD method. Thereafter, nitride film 4 in the region where field oxide film 5 is to be formed is removed, and nitride film 4 is left in a region other than the region where field oxide film 5 is to be formed.

  Next, as shown in FIG. 8B, the entire surface is oxidized. As in the case of FIG. 7B, oxidation of the active region below the region where the nitride film 4 is not formed proceeds, and a field oxide film 5 is formed in this region.

  Next, as shown in FIG. 8C, the remaining nitride film 4 is removed by etching.

  Next, as shown in FIG. 8D, a nitride film 51 is formed on the entire surface.

  Next, as shown in FIG. 9A, etching is performed in a state where a resist 82 is formed so as to mask a region other than the region 6 where the thick first gate oxide film 9 is formed. By this etching, the nitride film 51 and the underlying pad oxide film 3 formed in the region 6 are removed, and the substrate surface in the region 6 is exposed.

  Next, after removing the resist 82, as shown in FIG. 9B, an oxidation process corresponding to the thickness of the first gate oxide film 9 (thick film) is performed. At this time, since the nitride film 51 is formed in the second gate oxide film formation scheduled region 7, it functions as an oxidation mask, and the oxidation of the semiconductor substrate 1 in the region does not proceed. Therefore, a thick oxide film 9a is formed only on the semiconductor substrate 1 in the first gate oxide film formation planned region 6. Strictly speaking, an oxidation process corresponding to a value obtained by subtracting the thickness of the second gate oxide film 10 from the thickness of the first gate oxide film 9 is performed.

  Next, as shown in FIG. 9C, after the remaining nitride film 51 is removed, the pad oxide film 3 formed in the region 7 is removed to expose the substrate surface of the semiconductor substrate 1, and the entire surface is exposed. On the other hand, an oxidation process is performed for the desired thickness of the second gate oxide film 10. Thus, a thin second gate oxide film 10 is formed in the region 7, and a first gate oxide film 9 thicker than the second gate oxide film 10 is formed in the region 6.

  According to this method, since the field oxide film 5 is etched away by the thickness of the nitride film 51, the bird's beak in the field oxide film 5 is formed by forming the nitride film 51 with a small thickness. Is formed as originally set. For this reason, the variation of the active region as described above is suppressed. In other words, the field oxide film 5 can be formed in a thin film as compared with the case of FIG.

Japanese Patent Laid-Open No. 3-116968

  However, in the case of the method described in Patent Document 1, an oxidation process for forming a thick gate oxide film in a state where the nitride film 51 is formed in a region other than the first gate oxide film formation scheduled region 6. Is performed (see FIG. 9B). In other words, during the oxidation process, the nitride film 51 is formed from the active region related to the second gate oxide film formation scheduled region 7 to almost the entire surface of the field oxide film 5.

  The nitride film has a lower coefficient of thermal expansion than the oxide film. For this reason, when a nitride film is formed at the time of the thermal oxidation process, the stress generated due to the difference in thermal expansion coefficient is given to the semiconductor substrate, which causes distortion and crystal defects in the substrate. . This stress is concentrated on a stepped portion such as a bird's beak.

  As shown in FIG. 9B, in the case of the method described in Patent Document 1, the nitride film 51 is formed on substantially the entire surface from the upper surface of the field oxide film 5 to the upper surface of the active region in the second gate oxide film formation scheduled region 7. Thermal oxidation treatment is performed under the film-formed state. The oxide film (5, 3) formed under the nitride film 51 is thermally expanded by the heat supplied during the thermal oxidation process. However, as described above, since the nitride film 51 formed in the upper layer has a lower expansion coefficient than the oxide film, the oxide film formed in the lower layer of the nitride film 51 has the nitride film 51 formed thereon. The thermal expansion in the direction that does not. Stress is generated by this thermal expansion, and the stress is concentrated on a stepped portion such as a bird's beak as described above. Since the stress increases as the formation area of the nitride film 51 increases, the nitride film 51 is formed in all regions other than the first gate oxide film formation region 6 as shown in FIG. 9B. When the thermal oxidation treatment is performed under the condition, there is a concern that a large stress is concentrated on the bird's beak and the like, resulting in distortion and crystal defects.

  Furthermore, in the case of the method disclosed in Patent Document 1, the nitride film 4 (FIG. 8A) and the nitride film 51 (FIG. 8D) and the nitride film forming step are required twice, and the film formation cost is reduced. There is a problem of rising.

  In view of the above problems, the present invention suppresses the dimensional error of the active region to the extent of the error when the thickness of the gate oxide film is uniform, and has a different film thickness while suppressing the influence on the substrate. It is an object to provide a method for easily forming a plurality of gate oxide films. Another object of the present invention is to provide a method for manufacturing a semiconductor device including a plurality of gate oxide films having different film thicknesses.

  In order to achieve the above object, a gate oxide film forming method according to the present invention includes a first step of forming an oxide shielding film in addition to a field oxide film formation scheduled region on a semiconductor substrate, and after the first step, A second step of forming a field oxide film in the field oxide film formation scheduled region by performing an oxidation process using the oxidation shielding film as a mask, and after the second step, an etching process is performed in the first gate oxide film formation scheduled region. After exposing the substrate surface, a third step of performing an oxidation process for the first film thickness, and after the completion of the third step, removing the remaining oxide shielding film by an etching process, then forming at least a second gate oxide film It has the 4th process of performing the oxidation process for the 2nd film thickness thinner than the 1st film thickness on the surface of a predetermined field.

  According to the first feature of the gate oxide film forming method according to the present invention, the gate oxide films having different thicknesses are formed in the first gate oxide film formation region and the second gate oxide film formation region. can do. Specifically, the first gate oxide film in the first gate oxide film formation planned region is formed to be thicker than the second gate oxide film in the second gate oxide film formation planned region.

  When the oxidation process according to the third step is performed, the oxide shielding film remaining outside the first gate oxide film formation region is in the state after the field oxide film formation, that is, other than the first gate oxide film formation region. It is only formed from the upper surface of the active region to the outer wall of the field oxide film. Therefore, compared with the method of Patent Document 1 in which the nitride film is formed in all regions other than the region during the oxidation process for the first gate oxide film formation region, the oxidation of the first gate oxide film formation region is performed. The nitride film formation area during processing is greatly reduced. As a result, the stress on the semiconductor substrate (bird's beak, etc.) due to the oxidation treatment (third step) for the region where the first gate oxide film is to be formed can be greatly reduced, and the generation of distortion and crystal defects on the semiconductor substrate is suppressed. .

  Further, since the oxidation process is performed for the first film thickness to be a thick film in the third step, it is not necessary to form a thick oxide film in advance in the active region when the field oxide film is formed. For this reason, it is not necessary to etch the thick film when exposing the substrate surface. Therefore, it is not necessary to form a thick field oxide film in advance in anticipation of significant etching, and variations in the active region can be suppressed.

  Further, according to the first feature, the oxidation shielding film formed for forming the field oxide film in the first step is also used as an oxidation mask film during the oxidation process according to the third step. That is, as compared with the method of Patent Document 1, the number of steps of forming the nitride film can be reduced once, and the manufacturing cost is suppressed.

  In addition to the first feature, the gate oxide film forming method according to the present invention also applies to a part of the field oxide film in the third gate oxide film formation scheduled region in the third step. A second feature is that after the etching, the remaining field oxide film is also oxidized.

  According to the second feature of the method for forming a gate oxide film according to the present invention, it is possible to form a gate oxide film having three or more different film thicknesses without increasing the number of steps as compared with the first feature. it can.

  In addition to the first or second feature, the gate oxide film forming method according to the present invention performs an oxidation process for the second film thickness on the entire surface of the semiconductor substrate in the fourth step. This is the third feature.

  In addition to the first to third features, the gate oxide film forming method according to the present invention includes forming a pad oxide film on the entire surface of the semiconductor substrate before starting the first step. 5 steps, and in the second step, after removing the pad oxide film in the non-formation region of the oxidation shielding film, an oxidation treatment is performed using the oxidation shielding film as a mask. In the fourth step, After removing the oxide shielding film, at least the pad oxide film in the region where the second gate oxide film is to be formed is removed to expose the substrate surface in the region, and then the oxidation treatment for the second film thickness is performed. This is a fourth feature.

  According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, wherein a first gate oxide film is formed in the first gate oxide film formation region by the gate oxide film forming method having any one of the first to fourth characteristics. Forming a second gate oxide film thinner than the first gate oxide film in the region where the second gate oxide film is to be formed, and a first gate electrode and the second gate over the first gate oxide film Forming a second gate electrode on the upper layer of the oxide film, and forming a source / drain diffusion region in the peripheral region under the first gate oxide film and the peripheral region under the second gate oxide film on the semiconductor substrate; The first feature is to have a step of performing.

  According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, wherein a first gate oxide film is formed in the first gate oxide film formation scheduled region by the gate oxide film forming method having the second feature. A second gate oxide film that is thinner than the first gate oxide film is formed in the formation planned region, and a third gate oxide film that is thicker than the first gate oxide film is formed in the third gate oxide film formation planned region. Forming a first gate electrode on the first gate oxide film, forming a second gate electrode on the second gate oxide film, and forming a third gate electrode on the third gate oxide film. Forming a source / drain diffusion region in a peripheral region below the first gate oxide film, a peripheral region below the second gate oxide film, and a peripheral region below the third gate oxide film on the semiconductor substrate; And have The second said Rukoto.

  According to the first or second feature of the method of manufacturing a semiconductor device according to the present invention, a plurality of different withstand voltages can be obtained while minimizing distortion and crystal defects in the semiconductor substrate and variations in the active region. These MOS transistors can be manufactured on the same semiconductor substrate.

  According to the configuration of the present invention, a plurality of gate oxide films having different film thicknesses can be formed by a simple method while suppressing the influence of electrical errors and distortion on the semiconductor substrate.

  Hereinafter, embodiments of a gate oxide film forming method and a semiconductor device manufacturing method according to the present invention (hereinafter, collectively referred to as “method of the present invention” as appropriate) will be described with reference to the drawings.

[First Embodiment]
A first embodiment of the method of the present invention (hereinafter referred to as “this embodiment” as appropriate) will be described with reference to FIGS. In the present embodiment, the case where the first gate oxide film 9 having a large thickness and the second gate oxide film 10 having a smaller thickness than the first gate oxide film 9 are formed on the same semiconductor substrate will be described. .

  1 and 2 are schematic process cross-sectional views showing a manufacturing process when manufacturing a semiconductor device using the method of the present invention according to this embodiment, and FIGS. 1A to 1D are shown for each process. 2 (a) to 2 (c) (they are divided into two drawings for the sake of space). FIG. 3 is a flowchart of the manufacturing process of the method of the present invention according to this embodiment, and each step in the following sentence represents each step of the flowchart shown in FIG.

  Further, the following schematic cross-sectional structural diagrams are schematically shown, and the dimensional ratio on the drawing does not necessarily match the actual dimensional ratio. The same applies to the following embodiments. Further, in the drawings, the same components as those in FIGS. 7 to 9 are denoted by the same reference numerals.

First, as shown in FIG. 1A, the main surface of the semiconductor substrate 1 on which the N ⁻ type well region 2 is formed is oxidized to form a thin pad oxide film 3 having a film thickness of 20 nm, for example (step # 1). ). Thereafter, a nitride film 4 (eg, a silicon nitride film) is formed on the entire surface, for example, with a thickness of 100 nm, and then the nitride film 4 in the region where the field oxide film 5 is to be formed is removed (step # 2). That is, at the end of step # 2, as shown in FIG. 1A, the nitride film 4 is formed outside the region where the field oxide film 5 is to be formed.

  Next, as shown in FIG. 1B, an oxidation process is performed on the entire surface to form a field oxide film 5 having a thickness of, for example, about 400 nm (step # 3). The nitride film 4 has a function of shielding the oxidation of the semiconductor substrate 1 formed therebelow, so that the oxidation of the semiconductor substrate 1 does not proceed below the formation region of the nitride film 4. Thereby, only the oxidation of the semiconductor substrate 1 in the region where the nitride film 4 is not formed proceeds, and the field oxide film 5 is formed in the region.

  Next, etching is performed while masking with a resist 31, thereby exposing the substrate surface of the region 6 where the thick first gate oxide film 9 is to be formed as shown in FIG. 1C (step # 4). ). More specifically, the nitride film 4 and the pad oxide film 3 formed in the region 6 are removed by etching.

  Next, after removing the resist 31, as shown in FIG. 1D, the entire surface is oxidized (step # 5). At this time, since the nitride film 4 is formed in the region 7 where the second gate oxide film 10 is to be formed, the oxidation of the semiconductor substrate 1 in the region does not proceed. By this step # 5, an oxide film 9a is formed in the region 6. Note that the film thickness to be oxidized in step # 5 will be described later.

  Next, after removing the remaining nitride film 4 (step # 6), low concentration N-type impurity ions are implanted into the region 7 where the second gate oxide film 10 is to be formed, thereby forming the N well region 11. (Step # 7). Thereafter, a P body region 12 is formed by implanting low-concentration P-type impurity ions into the region 6 where the first gate oxide film 9 is to be formed (step # 8). Steps # 7 and # 8 may be interchanged.

  Next, after removing the pad oxide film 3 formed in the region 7 to expose the substrate surface of the semiconductor substrate 1 (step # 9), a desired second gate is formed as shown in FIG. An oxidation process corresponding to the thickness of the oxide film 10 is performed (step # 10).

  Specifically, in step # 9, the entire surface is etched back until the surface of the semiconductor substrate in the region 7 is exposed, and then the entire surface is oxidized by an amount corresponding to the thickness of the second gate oxide film 10. At this time, the thickness of the oxide film 9a already formed in the region 6 is similarly decreased in step # 9 and further increased in step # 10. Therefore, the film thickness formed by the oxidation process in step # 5 is d1, the decreased film thickness in step # 9 (corresponding to the film thickness of the pad oxide film 3) is d2, and the increased film thickness in step # 10 (ie, the second film thickness). If d3 is equivalent to the desired film thickness of the gate oxide film 10, the film thickness d4 formed in the region 6 at the end of step # 10 is d4 = d1−d2 + d3. Here, the film thickness d4 formed in the region 6 at the end of step # 10 corresponds to the desired film thickness of the first gate oxide film 9 formed in the region 6. Therefore, by setting the film thickness d1 to be formed in step # 5 to (d4 + d2−d3), the first gate oxide film 9 having the desired film thickness d4 is formed in the region 6 at the end of step # 10. be able to.

  Thus, when step # 10 is completed, the first gate oxide film 9 is in the region 6 and the second gate oxide film 10 having a thickness smaller than that of the first gate oxide film 9 is in the region 7. A film is formed.

  Thereafter, the same process as a normal transistor manufacturing process is performed. That is, as shown in FIG. 2B, after forming gate electrodes 13 and 15 in both regions 6 and 7, respectively, N-type high-concentration impurity ions are implanted to form source / drain regions 16 in the region 6. The source / drain regions 14 are formed in the region 7 respectively.

  2C, an interlayer insulating film 17 is formed on the entire surface, and contact plugs 18 for making electrical contact with the source / drain regions 14 and 16 are formed.

  According to the method of the present invention according to this embodiment, since the pad oxide film 3 can be made thin as in FIG. 8, the thickness of the field oxide film 5 etched in step # 4 can be reduced. . Therefore, it is not necessary to form the thick field oxide film 5 in advance in anticipation of significant etching. Therefore, it is possible to suppress variations in the active region.

  Further, as shown in FIG. 1D, when the oxidation process according to step # 5 is performed, the nitride film 4 extends from the upper surface of the active region other than the first gate oxide film formation region 6 to the outside of the field oxide film 5. Stays formed over the wall. In the case of the method of Patent Document 1, the nitride film 51 is formed in all regions other than the region 6 during the oxidation process on the first gate oxide film formation scheduled region 6 (see FIG. 9B). That is, in the case of the method of the present invention, compared with the method of Patent Document 1, the nitride film formation area during the oxidation process for the first gate oxide film formation scheduled region 6 is greatly reduced. As a result, the stress on the semiconductor substrate 1 (bird's beak, etc.) due to the oxidation process (step # 5) on the first gate oxide film formation planned region 6 can be greatly reduced, and distortion and crystal defects are generated on the semiconductor substrate 1. It is suppressed.

  Furthermore, according to the method of the present invention, the nitride film 4 formed for forming the field oxide film 5 is also used as an oxidation mask film during the oxidation process according to Step # 5. That is, as compared with the method of Patent Document 1, the number of steps of forming the nitride film can be reduced once, and the manufacturing cost is suppressed.

  In the above embodiment, the nitride film 4 is formed in step # 2. However, the film material formed in this step is intended to cover the region where the field oxide film 5 is not formed in order to prevent oxidation of the semiconductor substrate 1 when the field oxide film 5 is formed in the subsequent step # 3. It is what. For this reason, the film material to be formed in step # 2 is not limited to the nitride film as long as the film material has oxidation resistance. The same applies to the following second embodiment.

  In the above embodiment, the nitride film 4 and the pad oxide film 3 in the region 6 are removed in Step # 4. However, only the nitride film 4 is removed and a part or all of the pad oxide film 3 remains. You may let them. In this case, in the oxidation process according to step # 5, the film thickness of the oxide film to be formed may be determined in consideration of the film thickness of the oxide film 3 remaining at step # 4.

  In the above embodiment, the oxide film on the entire surface of the substrate is removed in step # 9. However, only the oxide film 3 formed in the region 7 may be removed. In this case, it is preferable to reverse the order of steps # 7 and # 8. That is, after the body region 12 is formed by performing P-type impurity ion implantation with the region 6 masked with the exposed resist (step # 8), the region 7 is masked with the exposed resist. Impurity ion implantation is performed to form the well region 11 (step # 7). Thereafter, etching is performed with the resist remaining as it is to remove the pad oxide film 3 in the region 7 (step # 9). By performing in this way, it is possible to perform etching of the oxide film on the region 7 by utilizing a resist as a mask at the time of ion implantation.

  In the above embodiment, the case where a P-channel MOS transistor is manufactured has been described as an example. However, the present invention can also be used in the case where an N-channel MOS transistor is manufactured by inverting each polarity. . The same applies to the following second embodiment.

[Second Embodiment]
A second embodiment of the method of the present invention (hereinafter referred to as “this embodiment” as appropriate) will be described with reference to FIGS. In the present embodiment, the thick first gate oxide film 9, the second gate oxide film 10 thinner than the first gate oxide film 9, and the thicker film thickness than the first gate oxide film 9. A case where the third gate oxide film 21 is formed on the same semiconductor substrate will be described.

  4 and 5 are schematic process cross-sectional views showing a manufacturing process when a semiconductor device is manufactured using the method of the present invention according to this embodiment, and FIGS. 4A to 4D are shown for each process. FIG. 5A to FIG. 5C are illustrated separately (divided into two drawings for the sake of space). FIG. 6 is a flowchart showing the manufacturing process of the method of the present invention according to this embodiment, and each step in the following sentence represents each step of the flowchart shown in FIG. In addition, the same step number is attached | subjected about the process same as step # 1- # 10 which concerns on 1st Embodiment, The description is simplified or abbreviate | omitted.

  4 and 5, the same components as those of the first embodiment are denoted by the same reference numerals, and the description thereof is simplified or omitted.

First, the pad oxide film 3, the nitride film 4, and the field oxide film 5 are formed on the semiconductor substrate 1 on which the N ⁻ type well region 2 is formed by the same process as steps # 1 to # 3 (FIG. 4 ( a) and (b)). In the present embodiment, the field oxide film 5 is formed to a thickness of about 400 nm, for example.

  Next, etching is performed in a state where masking is performed with a resist 32 having a pattern shape that exposes the regions 6 and 6 where the first gate oxide film 9 is to be formed and the third gate oxide film 21 is to be formed. As shown in (c), the substrate surface in the region 6 is exposed by etching, and the film thickness of the field oxide film 5 in the region 8 is decreased (step # 4a). The film thickness of the field oxide film 5 in the region 8 to be decreased in step # 4a will be described later.

  Next, after removing the resist 32, as shown in FIG. 4D, the entire surface is oxidized (step # 5a). At this time, since the nitride film 4 is formed in the region 7 where the second gate oxide film 10 is to be formed, the oxidation of the semiconductor substrate 1 in the region does not proceed. By this step # 5a, an oxide film 9a is formed in the region 6, and an oxide film 21a is formed in the region 8.

  Next, after removing the remaining nitride film 4 (step # 6), low concentration N-type impurity ions are implanted into the region 7 where the second gate oxide film 10 is to be formed, thereby forming the N well region 11. (Step # 7). Thereafter, P body regions 12 and 23 are formed by implanting low-concentration P-type impurity ions into the regions 6 and 8 (step # 8a). Steps # 7 and # 8a may be interchanged.

  Next, after removing the pad oxide film 3 formed in the region 7 to expose the substrate surface of the semiconductor substrate 1 (step # 9), a desired second gate is formed as shown in FIG. An oxidation process corresponding to the thickness of the oxide film 10 is performed (step # 10a).

  Specifically, in step # 9, the entire surface is etched back until the surface of the semiconductor substrate in the region 7 is exposed, and then the entire surface is oxidized by an amount corresponding to the thickness of the second gate oxide film 10. At this time, the oxide film 9a already formed in the region 6 and the oxide film 21 already formed in the region 8 are similarly reduced in film thickness in step # 9, and further in step # 10a. In this case, the film thickness increases. Accordingly, when the film thickness of the pad oxide film 3 is d2, the desired film thickness of the second gate oxide film 10 is d3, and the desired film thickness of the first gate oxide film 9 is d4, step # of the first embodiment 5, the film thickness d1 to be oxidized in step # 5a of this embodiment is set to (d4 + d2−d3), so that the first gate oxidation with the desired film thickness d4 is included in the region 6 at the end of step # 10a. A film 9 can be formed.

  On the other hand, the film thickness of the field oxide film 5 formed in step # 3 is d11, the film thickness of the field oxide film 5 in the region 8 to be decreased by the etching related to step # 4a is d12, and the film thickness of the oxide film in step # 5a. Is d1, the film thickness d13 formed in the region 8 at the end of step # 10a is d13 = d11−d12 + d1. Therefore, by setting the film thickness d12 to be etched in step # 4a to (d11 + d1-d13), the third gate oxide film 21 having the desired film thickness d13 is formed in the region 8 at the end of step # 10a. Can do.

  Thereafter, the same process as a normal transistor manufacturing process is performed. That is, as shown in FIG. 5B, after forming gate electrodes 13, 15, and 22 in the regions 6, 7, and 8, respectively, N-type high-concentration impurity ions are implanted and the source in the region 6 is formed. A drain region 16 is formed, a source / drain region 14 is formed in the region 7, and a source / drain region 19 is formed in the region 8.

  Then, as shown in FIG. 5C, after an interlayer insulating film 17 is formed on the entire surface, contact plugs 18 for making electrical contact with the source / drain regions 14, 16, 19 are formed.

  Also in the case of the method of the present invention according to this embodiment, since the pad oxide film 3 can be made a thin film as in the case of the first embodiment, the field oxide film 5 formed outside the region 8 is step # 4a. It is not necessary to form a thick field oxide film 5 in advance in anticipation of being greatly etched by this etching process. Therefore, it is possible to suppress variations in the active region.

  Further, as in the first embodiment, the area of the nitride film 4 formed during the oxidation process according to Step # 5a can be greatly reduced as compared with the method described in Patent Document 1, and therefore, according to Step # 5a. The stress on the semiconductor substrate 1 (bird's beak, etc.) due to the oxidation treatment can be greatly reduced, and the occurrence of distortion and crystal defects on the semiconductor substrate 1 is suppressed. Furthermore, the number of steps of forming the nitride film can be reduced, thereby reducing the manufacturing cost.

  Furthermore, in the case of this embodiment, it is possible to form three different types of gate oxide films without increasing the number of steps compared to the first embodiment. As a result, three types of transistors having different breakdown voltages can be mounted on the semiconductor substrate 1.

[Another embodiment]
In the first embodiment described above, the case where the gate oxide films having two different film thicknesses are formed has been described. However, the substrate surface exposure process according to step # 4 and the oxidation process according to # 5 are repeated a plurality of times. Thus, it is possible to form a gate oxide film having three or more kinds of film thicknesses.

  For example, in the first embodiment, the first gate oxide film 9, the second gate oxide film 10 thinner than the first gate oxide film 9, and the third gate oxide film 21 thicker than the first gate oxide film 9 are formed. The case where it does is demonstrated. In this case, first, in step # 4, after exposing the substrate surface of the semiconductor substrate 1 in the region where the third gate oxide film 21 having the thickest film is formed, oxidation processing is performed in step # 5. Next, in step # 4 again, after exposing the substrate surface of the semiconductor substrate 1 in the region where the first gate oxide film 9 having the next largest film thickness is to be formed, oxidation processing is performed in step # 5. Thereafter, steps after step # 6 are performed. Thereby, three or more types of film thicknesses can be formed also by the method according to the first embodiment. Note that this method can also be used in the method according to the second embodiment. .

Schematic process sectional drawing which shows a part of manufacturing process at the time of forming a gate oxide film using the method of the present invention concerning a 1st embodiment Schematic process sectional drawing which shows another part of manufacturing process at the time of forming a gate oxide film using the method of the present invention concerning a 1st embodiment The flowchart which shows the manufacturing process at the time of forming a gate oxide film using the method of this invention concerning 1st Embodiment in process order. Schematic process sectional drawing which shows a part of manufacturing process at the time of forming a gate oxide film using the method of the present invention concerning a 2nd embodiment Other process sectional drawing which shows a part of manufacturing process at the time of forming a gate oxide film using the method of the present invention concerning a 2nd embodiment. The flowchart which shows the manufacturing process at the time of forming a gate oxide film using the method of the present invention concerning a 2nd embodiment in order of a process. Schematic process cross-sectional view showing the manufacturing process when forming a gate oxide film by a conventional method Schematic process sectional view showing a part of the manufacturing process when forming a gate oxide film by another conventional method Schematic process sectional view showing another part of the manufacturing process when forming a gate oxide film by another conventional method

Explanation of symbols

1: Semiconductor substrate 2: Well region 3: Pad oxide film 4: Nitride film 5: Field oxide film 6: First gate oxide film formation (planned) region 7: Second gate oxide film formation (planned) region 9: First Gate oxide film 9a: Oxide film 10: Second gate oxide film 11: Well region 12: Body region 13: Gate electrode 14: Source / drain region 15: Gate electrode 16: Source / drain region 17: Interlayer insulating film 18: Contact Plug 19: Source / drain region 21: Third gate oxide film 21a: Oxide film 22: Gate electrode 23: Body region 31, 32: Resist 51: Nitride film 81, 82: Resist 93: Pad oxide film

Claims (6)

A first step of forming an oxide shielding film in addition to the field oxide film formation scheduled region on the semiconductor substrate;
After the first step, a second step of forming a field oxide film in the field oxide film formation scheduled region by performing an oxidation process using the oxidation shielding film as a mask;
A third step of performing an oxidation treatment for the first film thickness after exposing the substrate surface in the first gate oxide film formation scheduled region after the second step by etching;
After the third step, the remaining oxide shielding film is removed by etching, and then at least the surface of the region where the second gate oxide film is to be formed is oxidized for a second film thickness that is thinner than the first film thickness. A gate oxide film forming method comprising: a fourth step.   In the third step, after the part of the field oxide film in the third gate oxide film formation scheduled region is etched, the remaining field oxide film is also oxidized. The method for forming a gate oxide film according to claim 1.   3. The gate oxide film forming method according to claim 1, wherein in the fourth step, an oxidation treatment for the second film thickness is performed on the entire surface of the semiconductor substrate. A fifth step of forming a pad oxide film on the entire surface of the semiconductor substrate before starting the first step;
In the second step, after removing the pad oxide film in the non-formation region of the oxidation shielding film, an oxidation treatment is performed using the oxidation shielding film as a mask,
In the fourth step, after removing the oxidation shielding film, further removing at least the pad oxide film in the region where the second gate oxide film is to be formed to expose the substrate surface in the region, 4. The method for forming a gate oxide film according to claim 1, wherein an oxidation treatment for a film thickness is performed. 5. The gate oxide film formation method according to claim 1, wherein a first gate oxide film is formed in the first gate oxide film formation scheduled region, and the second gate oxide film formation scheduled region is formed in the second gate oxide film formation scheduled region. Forming a second gate oxide film that is thinner than the first gate oxide film;
Forming a first gate electrode on the first gate oxide film and forming a second gate electrode on the second gate oxide film;
Forming a source / drain diffusion region in a peripheral region below the first gate oxide film and a peripheral region below the second gate oxide film on the semiconductor substrate. Method. 3. The gate oxide film forming method according to claim 2, wherein a first gate oxide film is formed in the first gate oxide film formation scheduled region, and a thickness thinner than the first gate oxide film in the second gate oxide film formation scheduled region. Forming a third gate oxide film thicker than the first gate oxide film in the region where the third gate oxide film is to be formed,
Forming a first gate electrode on the first gate oxide film, a second gate electrode on the second gate oxide film, and a third gate electrode on the third gate oxide film;
Forming a source / drain diffusion region in a peripheral region under the first gate oxide film, a peripheral region under the second gate oxide film, and a peripheral region under the third gate oxide film on the semiconductor substrate; A method for manufacturing a semiconductor device, comprising:

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