JP4844077B2 - Manufacturing method of semiconductor device - Google Patents

Description

  The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for manufacturing a semiconductor device using a trench structure as a MOS gate.

  In a semiconductor device using a trench (groove) structure as a MOS gate, a trench is formed by etching a silicon substrate from its surface. Then, the corner of the trench opening formed on the surface of the silicon substrate and the corner of the bottom of the trench formed inside the silicon substrate may have a right angle or an angular shape such as an acute angle.

If the corner of the trench opening or the corner of the bottom of the trench has such an angular shape, the gate oxide film tends to break down easily because the electric field concentrates on the corner and the gate oxide film is likely to deteriorate. There is.
Furthermore, if the corner has such an angular shape, the film thickness of the gate oxide film formed in the corner of the trench opening, the corner of the bottom of the trench, and the vicinity of those corners is formed in other portions. It may be thinner than the thickness of the gate oxide film. As is well known, a portion where the gate oxide film is formed thinly has a higher electric field than a portion where the gate oxide film is formed thick, and therefore, it tends to be easily broken down.
For example, in Japanese Patent Laid-Open No. 7-263692 (Patent Document 1), in order to form a gate oxide film having a uniform film thickness, the corners of the trench opening and the corner of the trench bottom are rounded in advance. An invention for forming a surface is disclosed.

  According to the invention disclosed in Patent Document 1, the step of forming the sacrificial oxide film on the inner wall surface of the trench is performed twice under different environments, thereby rounding the corners of the trench opening and the bottom of the trench. ing.

That is, the step of forming the first sacrificial oxide film is performed in an oxygen atmosphere of 1000 ° C. or higher (first temperature), and thereby, roundness is mainly formed at the corner of the trench opening. In addition, the step of forming the second sacrificial oxide film is performed in a steam atmosphere at 950 ° C. (second temperature), and thereby, roundness is mainly formed at the corners of the trench bottom.
Japanese Patent Laid-Open No. 7-263692

  By the way, according to the knowledge of the inventors, the second sacrificial oxide film 8 is formed when the second sacrificial oxide film 8 is formed at 950 ° C. (second temperature) described in Patent Document 1. However, the film thickness is significantly reduced in the vicinity of the corner 6a of the trench opening. At this time, the biting of the silicon accompanying the formation of the second sacrificial oxide film reduces the roundness (curvature radius) of the corner 6a of the trench opening formed in the step of forming the first sacrificial oxide film. It works (in the direction of the arrow in FIG. 4).

  As described above, when the roundness of the corner 6a of the trench opening is reduced, the electric field is remarkably concentrated in that portion. Further, when the degree is deteriorated to become a right angle or an acute shape such as an acute angle, in addition to the above, As described in Kaihei 7-263692, when a gate oxide film is formed on the inner wall surface of a trench, the film thickness becomes thin, which is not preferable.

  Accordingly, an object of the present invention is to form a second sacrificial oxide film on the inner wall surface of the trench without significantly reducing the corner roundness of the trench opening and rounding the corner at the bottom of the trench. It is an object of the present invention to provide a method for manufacturing a semiconductor device and a semiconductor device capable of forming a gradual and gentle surface.

In order to solve the above problems, a method of manufacturing a semiconductor device according to claim 1 includes:
A first step of forming a trench in a silicon substrate;
A second step of performing thermal oxidation at a first temperature to form a first sacrificial oxide film extending from the inner wall surface of the trench along the surface of the silicon substrate to a thickness of 100 nm ;
A third step of removing the first sacrificial oxide film;
After the third step, thermal oxidation is performed at a second temperature to form a second sacrificial oxide film extending from the inner wall surface of the trench along the surface of the silicon substrate to a thickness of 50 nm . And the process of
A fifth step of removing the second sacrificial oxide film;
Forming a gate oxide film along the surface of the silicon substrate from the inner wall surface of the trench from which the second sacrificial oxide film has been removed ,
The fourth step is performed in an oxygen atmosphere or a water vapor atmosphere,
The first temperature is 1150 ° C.
The second temperature is higher than 1080 ° C. and 1120 ° C. or lower,
The curvature radius ratio of the trench opening before forming the second sacrificial oxide film and the curvature radius ratio of the trench opening after forming the second sacrificial oxide film (the radius of curvature after forming the second sacrificial oxide film) / The radius of curvature before forming the second sacrificial oxide film) is greater than 1.0,
The ratio of the radius of curvature of the trench bottom before forming the second sacrificial oxide film and the radius of curvature of the trench bottom after forming the second sacrificial oxide film (the radius of curvature after the formation of the second sacrificial oxide film / the first The radius of curvature before the formation of the sacrificial oxide film 2 is larger than 1.0 .

Thus, the second sacrificial oxide film can be formed without significantly impairing the corner roundness of the trench opening (fourth step). In addition, a rounded gentle surface is formed at the corner of the trench bottom .

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  FIG. 1A to FIG. 1G are cross-sectional views of a semiconductor device illustrating a method for manufacturing a semiconductor device according to this embodiment in the order of steps. In FIG. 1, reference numeral 1 is a silicon substrate, 2 is a thermal oxide film, 3 is a silicon nitride film, 4 is a photoresist film mask, 5 is a trench formation region, 6 is a trench, 6a is a corner of the opening of the trench 6, 6b is the bottom corner of the trench 6, 6c is the sidewall of the trench 6, 6d is the bottom of the trench 6, 7 is the first sacrificial oxide film, 8 is the second sacrificial oxide film, and 9 is the gate oxide film (silicon oxide film) ). Although not provided with reference numerals, the corner 6a of the opening of the trench 6, the corner 6b of the bottom, the side wall 6c, and the bottom 6d are collectively referred to as the inner wall surface of the trench 6.

  In the step shown in FIG. 1A, a thermal oxide film 2 (eg, SiO 2, SiON, etc.) having a thickness of about 500 mm is formed on the surface of the silicon substrate 1 by thermal oxidation. Thereafter, a silicon nitride film 3 (for example, Si3N4) having a thickness of about 150 to 200 nm serving as a stopper layer is formed on the entire surface of the thermal oxide film 2 by LP-CVD (Low Pressure Chemical Vapor Deposition).

  Next, in the step shown in FIG. 1B, a photoresist film that covers the active region where the semiconductor element is to be formed on the surface of the silicon substrate 1 and opens the region where the trench 6 is to be formed is formed by a photolithography step. A mask 4 is formed. Thereafter, the silicon nitride film 3 and the thermal oxide film 2 are patterned by dry etching using the photoresist film mask 4 to expose the region 5 in the silicon substrate 1 where the trench 6 is to be formed.

  Next, in the step shown in FIG. 1C, the photoresist film mask 4 is removed by a method such as ashing. Thereafter, dry etching is performed on the silicon substrate 1 using the patterned silicon nitride film 3 as a mask, an opening is formed on the surface of the silicon substrate 1 in the exposed region 5 of the silicon substrate 1, and the silicon substrate 1 is exposed. A trench 6 having a bottom 6d is formed inside. FIG. 3A shows an enlarged view of the trench 6 after performing this process. As shown in FIG. 3A, the corner 6a of the trench opening and the corner 6b of the trench bottom are both angular. Further, a damage layer (polycrystalline portion) (not shown) is generated on the inner wall surface of the trench.

  Next, in the step shown in FIG. 1D, the silicon nitride film 3 is removed by etching with phosphoric acid, and then the thermal oxide film 2 is removed by etching with dilute HF (hydrofluoric acid). The surface of the silicon substrate 1 is exposed by removing the thermal oxide film 2 and the silicon nitride film 3.

  Next, in the step shown in FIG. 1E, the inner wall surface of the trench 6 and the surface of the silicon substrate 1 are thermally oxidized by a first temperature (eg, 1150 ° C.) of 970 ° C. or higher in an oxygen atmosphere. A first sacrificial oxide film 7 having a thickness of about 50 to 200 nm, for example, about 100 nm is formed. At this time, the first sacrificial oxide film 7 takes in a damaged layer (polycrystalline portion) generated on the inner wall surface of the trench 6 described above.

  Here, the reason why the first temperature is set to 970 ° C. or more is that, at 970 ° C., the corner 6a of the trench opening and the first sacrificial oxide film 7 formed in the vicinity thereof start to viscously flow at that temperature. This is because the internal stress is relaxed, and the temperature acts to form roundness at the corner 6a of the trench opening. In this step, the first temperature was 1150 degrees. This is because the viscous flow of the first sacrificial oxide film 7 is further activated, so that the internal stress is quickly relaxed and the most suitable temperature for forming a roundness in the corner 6a of the trench opening. .

  Therefore, at the first temperature, the first sacrificial oxide film 7 flows in a viscous manner, so that an increase in internal stress that causes a decrease in the oxidation rate is suppressed. In particular, at the corner 6a of the trench opening, the internal stress due to the viscous flow of the first sacrificial oxide film 7 is relaxed, so that the oxidizing agent is supplied more abundantly than the sidewall 6c and the bottom 6d of the trench 6. A high speed state is maintained. Then, the first sacrificial oxide film 7 acts so that the corner 6a of the trench opening becomes a rounded and gentle surface.

  Thereafter, the first sacrificial oxide film 7 formed on the inner wall surface of the trench 6 and the surface of the silicon substrate 1 is removed by dilute HF (hydrofluoric acid). At this time, the damaged layer (polycrystalline portion) taken in by the first sacrificial oxide film 7 is also removed. Then, the inner wall surface of the trench 6 is smoothed, which contributes to the uniform thickness of the gate oxide film 9 formed in the process shown in FIG.

  FIG. 3B shows an enlarged view of the trench 6 after performing this process. Although a rounded and gentle surface is formed at the corner 6a of the trench opening, an angular portion remains at the corner 6b at the bottom of the trench.

  Next, in the step shown in FIG. 1F, the thickness of the inner wall surface of the trench 6 and the surface of the silicon substrate 1 is about 50 to 200 nm, for example, by thermal oxidation at a second temperature in an oxygen atmosphere. A second sacrificial oxide film 8 is formed so as to have a thickness of about 50 nm. The second temperature is set to a temperature, for example, 1050 ° C., at which the viscous flow is generated in the second sacrificial oxide film 8 but is not as active as the first temperature (eg, 1150 ° C.).

  By setting the second temperature in this way, the high internal stress is excessively relaxed at and near the corner 6b of the trench bottom and its vicinity, which is higher than the side wall 6c and the bottom 6d of the trench 6 because of the concave corner. Therefore, roundness can be formed in the angular portion remaining in the corner 6b at the bottom of the trench, and the roundness (curvature radius) of the corner 6a of the trench opening formed in the step of forming the first sacrificial oxide film. Can be suppressed. Note that the atmosphere for forming the second sacrificial oxide film is not limited to an oxygen atmosphere, and may be, for example, a water vapor atmosphere.

  Thereafter, the inner wall surface of the trench 6 and the second sacrificial oxide film 8 formed on the surface of the silicon substrate 1 are removed by dilute HF (hydrofluoric acid).

  An enlarged view of the trench 6 after this step is performed is shown in FIG. By forming the second sacrificial oxide film 8 at the second temperature on the inner wall surface of the trench 6 and the surface of the silicon substrate 1, the roundness of the corner 6a of the trench opening is not significantly reduced, and the bottom of the trench is reduced. The corner 6b is formed with a rounded gentle surface.

  Next, in the step shown in FIG. 1G, for example, thermal oxidation is performed in an oxygen atmosphere at about 1000 ° C., and a gate having a thickness of about 600 mm is formed on the inner wall surface of the trench 6 and the surface of the silicon substrate 1. An oxide film 9 is formed. Since the manufacturing steps shown in FIGS. 1A to 1F are performed, the thickness of the gate oxide film 9 formed in this step is almost uniform.

  By the way, although explanation and illustration about the manufacturing process after the process shown in FIG. 1 (g) are omitted, for example, a P-doped polysilicon film is buried in the trench 6 by using the LP-CVD method and the surface of the silicon substrate 1 is used. A step of selectively removing a desired position by dry etching the P-doped polysilicon film deposited on the surface of the silicon substrate 1 using a photoresist film mask, and forming a gate electrode; A semiconductor device using the trench structure as a MOS gate is completed through steps such as forming a drain, an interlayer insulating film, wiring, and a passivation film.

  Next, the experimental results obtained by the inventors in earnest experiments will be described with reference to FIG.

  First, FIG. 2A will be described. FIG. 2A shows the radius-of-curvature ratio (second sacrificial oxidation) of the corner 6b at the bottom of the trench before and after performing the thermal oxidation step for forming the second sacrificial oxide film 8 (step shown in FIG. 1F). The temperature at which the thermal oxidation process for forming the second sacrificial oxide film 8 is performed with the vertical axis representing the radius of curvature after the film is formed / the radius of curvature before the second sacrificial oxide film is formed (vertical axis). It is the graph which showed the 2nd temperature on the horizontal axis.

  As shown in FIG. 2A, when the second temperature is set to exceed about 1120 ° C., the radius-of-curvature ratio of the corner 6b at the bottom of the trench is about 1.0. This is because when the thermal oxidation process for forming the second sacrificial oxide film 8 is performed in an environment where the second temperature exceeds about 1120 ° C., the corner at the bottom of the trench is smaller than before the thermal oxidation process is performed. The curvature radius (roundness) of 6b represents almost no change.

  On the other hand, when the second temperature is set to about 1120 ° C. or less, the curvature radius ratio of the corner 6b at the bottom of the trench exceeds 1.0. This is because when the thermal oxidation process for forming the second sacrificial oxide film 8 is performed in an environment where the second temperature is about 1120 ° C. or lower, the corner 6b at the bottom of the trench is compared with that before performing the thermal oxidation process. It shows that the radius of curvature (roundness) of increases.

  That is, FIG. 2A shows that the roundness of the corner 6b at the bottom of the trench does not decrease even if the thermal oxidation process for forming the second sacrificial oxide film 8 is performed regardless of the second temperature. Represents.

  Subsequently, FIG. 2B will be described. 2B shows a curvature radius ratio of the corner 6a of the trench opening before and after performing the thermal oxidation step for forming the second sacrificial oxide film 8 (the radius of curvature after the second sacrificial oxide film is formed). / A graph in which the radius of curvature before forming the second sacrificial oxide film) is plotted on the vertical axis, and the temperature (second temperature) at the time of performing the step of forming the second sacrificial oxide film 8 is plotted on the horizontal axis. It is.

  As shown in FIG. 2B, when the second temperature is set to about 1080 ° C. or less, the radius of curvature ratio of the corner 6a of the trench opening is 1.0 or less. For example, if the second temperature is set to about 1000 ° C., the curvature radius ratio of the corner 6a of the trench opening is about 0.3, and further, if the second temperature is set to about 970 ° C. or less, The curvature radius ratio of the corner 6a of the trench opening is about 0.2. That is, in FIG. 2B, when the second temperature is set to about 1080 ° C. or less and the thermal oxidation process for forming the second sacrificial oxide film 8 is performed, compared to before the thermal oxidation process is performed. This indicates that the radius of curvature (roundness) of the corner 6a of the trench opening decreases.

  On the other hand, when the second temperature is set to exceed about 1080 ° C., the curvature radius ratio of the corner 6a of the trench opening exceeds 1.0. This is because when the second temperature is set to about 1080 ° C. or higher and the thermal oxidation process for forming the second sacrificial oxide film 8 is performed, the trench opening portion is compared with that before the thermal oxidation process is performed. This indicates that the radius of curvature (roundness) of the corner 6a increases.

  That is, in FIG. 2B, the thermal oxidation process for forming the second sacrificial oxide film 8 is performed, so that the roundness of the corner 6a of the trench opening increases or decreases depending on the second temperature. Represents what to do.

  Therefore, the inventors formed a rounded corner 6b at the bottom of the trench when forming the second sacrificial oxide film 8 based on the experimental results shown in FIGS. 2 (a) and 2 (b). However, the second temperature range capable of suppressing a significant decrease in the radius of curvature (roundness) of the corner 6a of the trench opening is 1000 ° C. or higher and 1120 ° C. or lower, preferably 1000 ° C. or higher and 1100 ° C. Hereinafter, more preferably set to 1000 ° C. or higher and 1080 ° C. or lower.

  If the second sacrificial oxide film 8 is formed in this temperature range, the curvature radius ratio of the corner 6a of the trench opening is not significantly lowered. Thereby, it is possible to prevent the electric field from being concentrated on a specific portion of the gate oxide film 9 and causing dielectric breakdown. Furthermore, since the progress of the deterioration of the gate oxide film 9 can be prevented, the characteristics and reliability of the semiconductor device can be improved.

  Although the best mode for carrying out the present invention has been described above, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. is there.

1A is a cross-sectional view of a semiconductor device illustrating a method of manufacturing a semiconductor device according to an embodiment of the present invention in the order of steps; FIG. 3A is a diagram in which a thermal oxide film and a silicon nitride film are stacked on a silicon substrate; FIG. The figure which formed the resist mask, (c) is the figure which formed the trench, (d) is the figure which removed the mask and exposed the surface of the silicon substrate, (e) is the first sacrificial oxide film formed The figure (f) which formed the 2nd sacrificial oxide film, and the figure (g) which formed the gate oxide film. FIG. 5 is a diagram showing a relationship between a second temperature when performing a step of forming a second sacrificial oxide film and a curvature radius ratio of a corner of an opening of a trench before and after the step of forming a second sacrificial oxide film. (A) is a graph showing the relation between the radius of curvature of the corner of the bottom of the trench and the second temperature before and after the step of forming the second sacrificial oxide film, and (b) 2 is a graph showing a relationship between a radius of curvature ratio of a corner of an opening of a trench and a second temperature before and after the step of forming a sacrificial oxide film of FIG. 2A and 2B are enlarged views of a trench of a semiconductor device, in which FIG. 1A is a view before forming a first sacrificial oxide film, FIG. 2B is a view before forming a second sacrificial oxide film, and FIG. 2 is a view after forming and removing a sacrificial oxide film 2. FIG. It is sectional drawing of a semiconductor device which showed a mode that squareness generate | occur | produced in the corner of the trench opening part of a semiconductor device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Silicon substrate, 2 ... Thermal oxide film, 3 ... Silicon nitride film, 4 ... Photoresist film mask, 5 ... Trench formation area, 6 ... Trench, 6a ... Corner of the opening of the trench 6, 6 b... Corner of the bottom of the trench 6, 6 c .. sidewall of the trench 6, 6 d... Bottom of the trench 6, 7. .... Second sacrificial oxide film, 9 ... Gate oxide film (silicon oxide film)

Claims (1)

A first step of forming a trench in a silicon substrate;
A second step of performing thermal oxidation at a first temperature to form a first sacrificial oxide film extending from the inner wall surface of the trench along the surface of the silicon substrate to a thickness of 100 nm ;
A third step of removing the first sacrificial oxide film;
After the third step, thermal oxidation at a second temperature is performed to form a second sacrificial oxide film extending from the inner wall surface of the trench along the surface of the silicon substrate to a film thickness of 50 nm. And a fourth step to
A fifth step of removing the second sacrificial oxide film;
Forming a gate oxide film along the surface of the silicon substrate from the inner wall surface of the trench from which the second sacrificial oxide film has been removed ,
The fourth step is performed in an oxygen atmosphere or a water vapor atmosphere,
The first temperature is 1150 ° C .;
The second temperature is higher than 1080 ° C and not higher than 1120 ° C,
The ratio of the radius of curvature of the trench opening before the formation of the second sacrificial oxide film and the radius of curvature of the trench opening after the formation of the second sacrificial oxide film (formation of the second sacrificial oxide film) (The radius of curvature after the process / the radius of curvature before the formation of the second sacrificial oxide film) is greater than 1.0,
The ratio of the curvature radius of the trench bottom before forming the second sacrificial oxide film to the curvature radius of the trench bottom after forming the second sacrificial oxide film (after forming the second sacrificial oxide film) (Radius of curvature / curvature radius before forming the second sacrificial oxide film) is larger than 1.0 .

Images