Semiconductor device and method for manufacturing the same

Abstract

To provide a semiconductor device that achieves both an improvement in manufacturing efficiency and an improvement in withstand voltage.SOLUTION: A semiconductor device includes a semiconductor chip 2 having a first main surface on one side and a second main surface on the other side, a pn junction extending along the first main surface and formed inside the semiconductor chip 2, a trench 13 passing through the pn junction from the first main surface and defining an element region in the semiconductor chip 2, an insulating film 19 covering sidewalls 16 and 17 and a bottom wall 18 of the trench 13, and an embedded electrode 15 embedded in the trench 13 via the insulating film 19, and the bottom wall 18 of the trench 13 includes a protruding portion 20 protruding from the lower end of the insulating film 19 upward inside the insulating film 19 in the depth direction of the trench 13.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a semiconductor device and a method for manufacturing the semiconductor device. [Background technology]

[0002] Patent Document 1 discloses a semiconductor device including a p-type region, a first p-epitaxial region, an n-type buried region, a second p-epitaxial region, and a deep trench isolation (DTI) structure. The first p-type epitaxial layer is formed on the p-type region. The n-type buried region is formed on the first p-epitaxial region. The second p-epitaxial region is formed on the n-type buried region. The DTI structure surrounds a region where a high-voltage lateral MOS transistor is formed in a plan view. The DTI structure penetrates the second p-epitaxial region, the n-type buried region, and the first p-epitaxial region to reach the p-type region. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-122543 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of a semiconductor device according to an embodiment of the present disclosure is to achieve both improved manufacturing efficiency and improved breakdown voltage. [Means for solving the problem]

[0005] A semiconductor device according to one embodiment of the present disclosure includes a semiconductor chip having a first main surface on one side and a second main surface on the other side, a pn junction extending along the first main surface and formed inside the semiconductor chip, a trench extending from the first main surface through the pn junction and defining an element region in the semiconductor chip, an insulating film covering the side walls and bottom wall of the trench, and an embedded electrode embedded in the trench via the insulating film, wherein the bottom wall of the trench includes a protrusion protruding from a lower end of the insulating film toward an upper interior portion of the insulating film in the depth direction of the trench. [Effects of the Invention]

[0006] According to the semiconductor device according to an embodiment of the present disclosure, it is possible to achieve both improvement in manufacturing efficiency and improvement in breakdown voltage. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic plan view of a semiconductor device according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is an enlarged view of the area surrounded by the two-dot chain line II in FIG. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 4 is an enlarged view of the region surrounded by the two-dot chain line IV in FIG. 3, and shows the first form of the element isolation structure. [Figure 5] FIG. 5 is an enlarged view of the region surrounded by the two-dot chain line IV in FIG. 3, showing a second embodiment of the element isolation structure. [Figure 6A] FIG. 6A is an enlarged view of the area surrounded by the two-dot chain line VI in FIG. 4, showing a first form of the contact portion. [Figure 6B] FIG. 6B is an enlarged view of the area surrounded by the two-dot chain line VI in FIG. 4, showing a second form of the contact portion. [Figure 6C] FIG. 6C is an enlarged view of the area surrounded by the two-dot chain line VI in FIG. 4, showing a third form of the contact portion. [Figure 7]FIG. 7 is a diagram showing a flow of the manufacturing process of the semiconductor device. [Figure 8A] FIG. 8A is a schematic plan view showing a part of the manufacturing process of the semiconductor device. [Figure 8B] FIG. 8B is a schematic cross-sectional view showing a part of the manufacturing process of the semiconductor device. [Figure 9A] FIG. 9A is a diagram showing the next step of FIG. 8A. [Figure 9B] FIG. 9B is a diagram showing the next step of FIG. 8B. [Figure 10A] FIG. 10A is a diagram showing the next step of FIG. 9A. [Figure 10B] FIG. 10B is a diagram showing the next step of FIG. 9B. [Figure 11A] FIG. 11A shows the next step in FIG. 10A. [Figure 11B] FIG. 11B is a diagram showing the next step of FIG. 10B. [Figure 12A] FIG. 12A is a diagram showing the next step of FIG. 11A. [Figure 12B] FIG. 12B is a diagram showing the next step of FIG. 11B. [Figure 13A] FIG. 13A is a diagram showing a modified example of the manufacturing process of the semiconductor device. [Figure 13B] FIG. 13B shows the next step in FIG. 13A. [Figure 14] FIG. 14 is a diagram showing the relationship between the thickness of the sidewall insulating film of the element isolation structure and the magnitude of the breakdown voltage. DETAILED DESCRIPTION OF THE INVENTION

[0008] Next, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The components in the accompanying drawings are not necessarily precisely illustrated, but are shown schematically, and the scales of the drawings do not necessarily match. Fig. 1 is a schematic plan view of a semiconductor device 1 according to an embodiment of the present disclosure. Fig. 2 is an enlarged view of a region surrounded by a two-dot chain line II in Fig. 1. Fig. 3 is a cross-sectional view taken along line III-III in Fig. 2.

[0009] The semiconductor device 1 includes a rectangular parallelepiped semiconductor chip 2. In this embodiment, the semiconductor chip 2 is made of a Si (silicon) chip. The semiconductor chip 2 has a first main surface 3 on one side, a second main surface 4 on the other side, and first to fourth side surfaces 5A to 5D connecting the first main surface 3 and the second main surface 4. The first main surface 3 and the second main surface 4 are formed in a quadrangular shape in a plan view seen from their normal direction Z (hereinafter simply referred to as "plan view"). The normal direction Z is also the thickness direction of the semiconductor chip 2. The first side surface 5A and the second side surface 5B extend in a first direction X along the first main surface 3 and face a second direction Y that intersects (specifically, is perpendicular to) the first direction X. The third side surface 5C and the fourth side surface 5D extend in the second direction Y and face the first direction X.

[0010] The semiconductor device 1 includes a p-type first layer 6, a p-type or n-type second layer 7, and an n-type third layer 8 formed within a semiconductor chip 2. The first layer 6 may be referred to as a "base layer." The second layer 7 may be referred to as a "device-forming layer." The third layer 8 may be referred to as a "buried layer." The first layer 6, the second layer 7, and the third layer 8 may be considered components of the semiconductor chip 2.

[0011] The first layer 6 is formed in a region on the second main surface 4 side within the semiconductor chip 2, and forms part of the second main surface 4 and the first to fourth side surfaces 5A to 5D. The first layer 6 may have a concentration gradient in which the p-type impurity concentration on the first main surface 3 side is lower than the p-type impurity concentration on the second main surface 4 side. Specifically, the first layer 6 may have a stacked structure including a high-concentration layer 6a and a low-concentration layer 6b stacked in this order from the second main surface 4 side.

[0012] The high-concentration layer 6a has a relatively high p-type impurity concentration. The p-type impurity concentration of the high-concentration layer 6a is 1×10 16 cm -3 More than 1×10 20 cm -3The high-concentration layer 6a may have a thickness of 100 μm or more and 100 μm or less. In this embodiment, the high-concentration layer 6a is made of a p-type semiconductor substrate (Si substrate). The low-concentration layer 6b has a p-type impurity concentration lower than that of the high-concentration layer 6a and is stacked on the high-concentration layer 6a. The p-type impurity concentration of the low-concentration layer 6b is 1×10 14 cm -3 More than 1×10 17 cm -3 The low-concentration layer 6b may have a thickness less than that of the high-concentration layer 6a. The low-concentration layer 6b may have a thickness of 0.5 μm or more and 20 μm or less. In this embodiment, the low-concentration layer 6b is made of a p-type epitaxial layer (Si epitaxial layer).

[0013] The second layer 7 is formed in a region on the first main surface 3 side within the semiconductor chip 2, and forms part of the first main surface 3 and the first to fourth side surfaces 5A to 5D. The conductivity type (n-type or p-type) of the second layer 7 is arbitrary and is selected according to the specifications of the semiconductor device 1. In this embodiment, an example in which the second layer 7 has n-type conductivity will be described, but the conductivity type of the second layer 7 is not intended to be limited to n-type.

[0014] The second layer 7 may have a uniform n-type impurity concentration in the thickness direction, or may have an n-type impurity concentration gradient that increases toward the first main surface 3. The n-type impurity concentration of the second layer 7 is 1×10 14 cm -3 More than 1×10 17 cm -3 The second layer 7 may have a thickness of 0.5 μm or more and 20 μm or less. In this embodiment, the second layer 7 is made of an n-type epitaxial layer (Si epitaxial layer).

[0015] The third layer 8 is interposed in the region between the first layer 6 and the second layer 7 within the semiconductor chip 2, and forms part of the first to fourth side surfaces 5A to 5D of the semiconductor chip 2. The third layer 8 forms a pn junction J at the boundary with the first layer 6. That is, within the semiconductor chip 2, a pn junction J (a pn-junction portion) is formed in the middle of the thickness direction between the first main surface 3 and the second main surface 4, extending horizontally along the first main surface 3 (orthogonal to the thickness direction). The pn junction J may also be referred to as a "pn-connection portion" or a "pn-boundary portion."

[0016] The third layer 8 has a higher n-type impurity concentration than the second layer 7. Specifically, the third layer 8 may have a concentration gradient in which the n-type impurity concentration on the first main surface 3 side is higher than the n-type impurity concentration on the second main surface 4 side. More specifically, the third layer 8 may have a stacked structure including a lightly buried layer 8a and a heavily buried layer 8b stacked in this order from the first layer 6 side.

[0017] The lightly buried layer 8a has a relatively low n-type impurity concentration, and is stacked on the lightly buried layer 6b of the first layer 6. The lightly buried layer 8a forms a pn junction J with the lightly buried layer 6b. The lightly buried layer 8a may have a lower n-type impurity concentration than the second layer 7, or may have a higher n-type impurity concentration than the second layer 7. The n-type impurity concentration of the lightly buried layer 8a is 1×10 14 cm -3 More than 1×10 18 cm -3 The low-concentration buried layer 8a may have a thickness of 0.1 μm or more and 5 μm or less. In this embodiment, the low-concentration buried layer 8a is made of an n-type epitaxial layer (Si epitaxial layer).

[0018] The heavily doped buried layer 8b has a higher n-type impurity concentration than the lightly doped buried layer 8a and is stacked on the lightly doped buried layer 8a. The heavily doped buried layer 8b preferably has a higher n-type impurity concentration than the second layer 7. The heavily doped buried layer 8b has an n-type impurity concentration of 1×10 16 cm -3 More than 1×10 21 cm -3 The heavily doped buried layer 8b may have a thickness of 0.1 μm or more and 5 μm or less. In this embodiment, the heavily doped buried layer 8b is made of an n-type epitaxial layer (Si epitaxial layer).

[0019] The semiconductor device 1 includes a plurality of element regions 9 provided on the first main surface 3 (second layer 7). The plurality of element regions 9 are regions in which various functional elements are respectively formed. The plurality of element regions 9 are respectively defined inwardly of the first main surface 3 at intervals from the first to fourth side surfaces 5A to 5D in plan view. The number, arrangement and shape of the element regions 9 are arbitrary and are not limited to a specific number, arrangement and shape.

[0020] The plurality of functional elements may each include at least one of a semiconductor switching element, a semiconductor rectifying element, and a passive element. The semiconductor switching element may include at least one of a JFET (Junction Field Effect Transistor), a MISFET (Metal Insulator Semiconductor Field Effect Transistor), a BJT (Bipolar Junction Transistor), and an IGBT (Insulated Gate Bipolar Junction Transistor).

[0021] The semiconductor rectifying element may include at least one of a pn junction diode, a pin junction diode, a Zener diode, a Schottky barrier diode, and a fast recovery diode. The passive element may include at least one of a resistor, a capacitor, an inductor, and a fuse. In this embodiment, the multiple element regions 9 include at least one transistor region 9A. The structure of the transistor region 9A side will be specifically described below.

[0022] The semiconductor device 1 includes an element isolation structure 10 that defines a transistor region 9A on the first main surface 3. The element isolation structure 10 defines the transistor region 9A having a predetermined shape in a plan view. The element isolation structure 10 may also be referred to as a "trench electrode structure." Referring to FIG. 2, the element isolation structure 10 is formed in a strip shape extending along the transistor region 9A in a plan view. In this embodiment, the element isolation structure 10 is formed in a ring shape (a square ring shape in this embodiment) in a plan view, and defines a transistor region 9A of a predetermined shape (a square shape in this embodiment). In this embodiment, the four corners of the element isolation structure 10 have rounded shapes that curve in a direction away from the transistor region 9A in a plan view. The planar shape of the element isolation structure 10 (the planar shape of the transistor region 9A) is arbitrary. The element isolation structure 10 may be formed in a polygonal ring shape, a circular ring shape, or an elliptical ring shape in a plan view, and define a transistor region 9A that is polygonal, circular, or elliptical in a plan view.

[0023] The element isolation structure 10 has a trench width W1. The trench width W1 is the width in a direction perpendicular to the direction in which the element isolation structure 10 extends in a plan view. The trench width W1 may be 0.5 μm or more and 10 μm or less. The trench width W1 is preferably 2 μm or more and 4 μm or less. 3 , the element isolation structure 10 is formed on the first main surface 3 so as to penetrate the pn junction J, and defines a transistor region 9A on the first main surface 3. Specifically, the element isolation structure 10 penetrates the second layer 7 and the third layer 8 to reach the first layer 6, and defines the transistor region 9A in the second layer 7. In this embodiment, the element isolation structure 10 extends from the first main surface 3 toward the second main surface 4 so as to reach the high-concentration layer 6a of the first layer 6, and penetrates the second layer 7, the third layer 8, and the low-concentration layer 6b of the first layer 6.

[0024] The element isolation structure 10 includes an inner peripheral wall on the transistor region 9A side, an outer peripheral wall on the opposite side of the inner peripheral wall (the peripheral edge side of the semiconductor chip 2), and a bottom wall connecting the inner peripheral wall and the outer peripheral wall. The element isolation structure 10 is electrically connected to the semiconductor chip 2 at the bottom wall and electrically insulated from the semiconductor chip 2 at the side walls (inner peripheral wall and outer peripheral wall). That is, the element isolation structure 10 has a lower end portion electrically connected to the semiconductor chip 2. Specifically, the element isolation structure 10 is electrically connected to the first layer 6 and electrically insulated from the second layer 7 and the third layer 8. That is, the element isolation structure 10 is fixed at the same potential as the first layer 6.

[0025] The element isolation structure 10 includes a trench 13 , a trench insulating film 14 , and a trench electrode 15 . 2, the trench 13 is formed in a ring shape in a plan view. The width of the trench 13 may be the trench width W1 described above. Referring to FIG. 3, the trench 13 is formed on the first main surface 3 side of the semiconductor chip 2 so as to penetrate the pn junction J. Specifically, the trench 13 penetrates the second layer 7 and the third layer 8 to reach the first layer 6. In this embodiment, the trench 13 extends from the first main surface 3 toward the second main surface 4 so as to reach the high-concentration layer 6a of the first layer 6, and penetrates the second layer 7, the third layer 8, and the low-concentration layer 6b of the first layer 6.

[0026] The trench 13 includes an inner peripheral wall 16 on the transistor region 9A side, an outer peripheral wall 17 on the opposite side of the inner peripheral wall 16 (the peripheral edge side of the semiconductor chip 2), and a bottom wall 18 connecting the inner peripheral wall 16 and the outer peripheral wall 17. The inner peripheral wall 16 and the outer peripheral wall 17 may be referred to as the "inner side wall" and the "outer side wall", or as the "first side wall" and the "second side wall", respectively. The trench insulating film 14 covers the inner wall 16 and the outer wall 17 of the trench 13 so as to expose the semiconductor chip 2 from the bottom wall 18 of the trench 13. Specifically, the trench insulating film 14 exposes the first layer 6 from the bottom wall 18 of the trench 13. In this embodiment, the trench insulating film 14 exposes the high-concentration layer 6a of the first layer 6 from the bottom wall 18 of the trench 13. It is preferable that the trench insulating film 14 covers the entire inner wall 16 and the entire outer wall 17 of the trench 13. The trench insulating film 14 may include a silicon oxide film. It is preferable that the trench insulating film 14 includes a silicon oxide film made of an oxide of the semiconductor chip 2.

[0027] The trench electrode 15 is embedded in the trench 13 with the trench insulating film 14 sandwiched therebetween, and is electrically connected to the semiconductor chip 2 at the bottom wall 18 of the trench 13. Specifically, the trench electrode 15 is electrically connected to the first layer 6 and electrically insulated from the second layer 7 and the third layer 8. The trench electrode 15 preferably contains conductive polysilicon. The trench electrode 15 preferably contains conductive polysilicon of the same conductivity type (p-type in this embodiment) as the first layer 6. The p-type impurity of the trench electrode 15 is preferably boron.

[0028] The semiconductor device 1 includes a p-type impurity region 22 formed in a region along the bottom wall 18 of the trench 13 in the semiconductor chip 2. The impurity region 22 is formed in the first layer 6 so as to cover the bottom wall 18 of the trench 13. The impurity region 22 has a higher p-type impurity concentration than the first layer 6. Specifically, the impurity region 22 is formed in the high-concentration layer 6a in the first layer 6, and has a higher p-type impurity concentration than the high-concentration layer 6a.

[0029] In this embodiment, the trench electrode 15 is formed as a supply source of p-type impurities for the first layer 6, and the impurity region 22 contains the p-type impurities of the first layer 6 and the p-type impurities of the trench electrode 15. The impurity region 22 also covers the inner and outer peripheral walls 16 and 17 of the trench 13. The impurity region 22 is preferably formed in the high concentration layer 6a of the first layer 6 at a distance from the low concentration layer 6b of the first layer 6.

[0030] 3, the semiconductor device 1 includes a planar-gate MISFET 30 as an example of a functional element formed in the transistor region 9A. The MISFET 30 is not shown in FIG. 2. The MISFET 30 may take any one of the following forms depending on the magnitude of the drain-source voltage applied between the drain and source: a high-voltage (HV)-MISFET (e.g., 100 V to 1000 V), a middle-voltage (MV)-MISFET (e.g., 30 V to 100 V), and a low-voltage (LV)-MISFET (e.g., 1 V to 30 V). While this embodiment describes an example in which the MISFET 30 is an HV-MISFET, it is not intended to limit the form of the MISFET 30 to an HV-MISFET.

[0031] The MISFET 30 is composed of at least one MISFET cell formed in the transistor region 9A. In this embodiment, the MISFET cell includes, in a cross-sectional view, at least one (one in this embodiment) n-type first well region 31, at least one (plurality in this embodiment) p-type second well region 32, at least one (plurality in this embodiment) n-type drain region 33, at least one (plurality in this embodiment) n-type source region 34, at least one (plurality in this embodiment) p-type channel region 35, at least one (plurality in this embodiment) p-type contact region 36, multiple shallow trench structures 37, and at least one (plurality in this embodiment) planar gate structure 38. The shallow trench structure 37 may also be referred to as an "STI (shallow trench isolation) structure."

[0032] The first well region 31 is formed in a surface layer portion of the second layer 7 in the transistor region 9A. The first well region 31 has a higher n-type impurity concentration than the second layer 7. The plurality of second well regions 32 are formed in a surface layer portion of the second layer 7 at intervals from the first well region 31 in the transistor region 9A. One of the second well regions 32 is formed at an interval on one side of the first well region 31 in the first direction X, and the other second well region 32 is formed at an interval on the other side of the first direction X from the first well region 31.

[0033] The drain region 33 is formed in a surface layer portion of the first well region 31 at a distance inward from the periphery of the first well region 31. The plurality of source regions 34 are each formed in a surface layer portion of the corresponding second well region 32 at a distance inward from the periphery of the corresponding second well region 32. The plurality of channel regions 35 are each formed between the second layer 7 and the corresponding source region 34 in the surface layer portion of the corresponding second well region 32. The plurality of contact regions 36 are each formed in a surface layer portion of the corresponding second well region 32 at a distance inward from the periphery of the corresponding second well region 32. The plurality of contact regions 36 are adjacent to the corresponding source region 34.

[0034] The plurality of shallow trench structures 37 are each formed in the second layer 7 at intervals from the third layer 8 in the thickness direction of the second layer 7. The plurality of shallow trench structures 37 are preferably formed at depths spaced apart from the bottoms of the first well region 31 and the second well region 32 toward the first main surface 3. The plurality of shallow trench structures 37 are formed along the periphery of the drain region 33 and separate the drain region 33 from other regions.

[0035] The plurality of shallow trench structures 37 are formed along the outer edges (peripheries on the element isolation structure 10 side) of the plurality of second well regions 32, and separate the plurality of second well regions 32 from other regions. Each of the plurality of shallow trench structures 37 includes a shallow trench 39 and a buried insulator 40. Each shallow trench 39 is formed in the first main surface 3. Each buried insulator 40 is buried in the shallow trench 39.

[0036] The plurality of planar gate structures 38 are formed on the second layer 7 (first main surface 3) so as to cover the corresponding channel regions 35, and control the on / off of the corresponding channel regions 35. In this embodiment, the plurality of planar gate structures 38 are formed so as to straddle the first well region 31 and the corresponding source region 34. The plurality of planar gate structures 38 may cover a portion of the shallow trench structure 37 that defines the drain region 33.

[0037] The plurality of planar gate structures 38 include a gate insulating film 41 and a gate electrode 42 stacked in this order from the second layer 7 side. The gate insulating film 41 may include a silicon oxide film. The gate insulating film 41 preferably includes a silicon oxide film made of an oxide of the semiconductor chip 2. The gate electrode 42 preferably includes conductive polysilicon. The gate electrode 42 preferably includes conductive polysilicon of the same conductivity type as the first layer 6 (i.e., p-type). The p-type impurity of the gate electrode 42 is preferably boron. Of course, the gate electrode 42 may have n-type conductivity.

[0038] Fig. 4 is an enlarged view of the region surrounded by the two-dot chain line IV in Fig. 3, showing a first form of the element isolation structure 10. Fig. 5 is an enlarged view of the region surrounded by the two-dot chain line IV in Fig. 3, showing a second form of the element isolation structure 10. Figs. 6A to 6C are enlarged views of the region surrounded by the two-dot chain line VI in Fig. 4, showing first to third forms of the contact portion 12, respectively. Next, the structure of the element isolation structure 10 will be described in more detail.

[0039] As described above, the element isolation structure 10 includes the trench 13 , the trench insulating film 14 and the trench electrode 15 . The trench insulating film 14 covers the inner wall 16 and the outer wall 17 of the trench 13. Meanwhile, the trench insulating film 14 exposes the semiconductor chip 2 from a bottom wall 18 of the trench 13. The trench insulating film 14 may also be referred to as a pair of sidewall insulating films 19 formed along the inner wall 16 and the outer wall 17, respectively, in the depth direction of the trench 13. The sidewall insulating film 19 may have a first surface 191 and a second surface 192 that are substantially parallel to the inner wall 16 and the outer wall 17. The second surface 192 of the sidewall insulating film 19 may be the surface that contacts the inner wall 16 and the outer wall 17, and the first surface 191 may be the surface opposite thereto.

[0040] The thickness T1 of the sidewall insulating film 19 may be, for example, not less than 2 μm and not more than 6 μm. The thickness T1 may be defined as the thickness in a direction intersecting the depth direction of the trench 13. The pair of sidewall insulating films 19 may be distinguished as a first sidewall insulating film 19A on the inner circumferential wall 16 side and a second sidewall insulating film 19B on the outer circumferential wall 17 side. For example, referring to FIG. 2, the first sidewall insulating film 19A and the second sidewall insulating film 19B are indicated by gray areas. In plan view, the first sidewall insulating film 19A is formed on the inner circumferential wall 16 along the circumferential direction of the annular trench 13. The second sidewall insulating film 19B is formed on the outer circumferential wall 17 along the circumferential direction of the annular trench 13. The first sidewall insulating film 19A and the second sidewall insulating film 19B are formed concentrically with each other. The second sidewall insulating film 19B surrounds the first sidewall insulating film 19A.

[0041] As shown in Figures 4 and 5, trench 13 may be formed in a tapered shape having an opening width that narrows toward bottom wall 18 in a cross-sectional view. Although not shown, trench 13 may be formed in a vertical shape having a substantially constant opening width in a cross-sectional view. As shown in Figures 4 and 5, bottom wall 18 of trench 13 may be formed in a curved shape that bulges in the depth direction of trench 13. Although not shown, bottom wall 18 of trench 13 may have a flat surface parallel to first main surface 3.

[0042] The bottom wall 18 of the trench 13 may include a protrusion 20 that protrudes upward from the lower end of the sidewall insulating film 19 in the depth direction of the trench 13. The protrusion 20 is fitted into the lower end of each of the first sidewall insulating film 19A and the second sidewall insulating film 19B. As a result, a recess 21 corresponding to the shape of the protrusion 20 is formed in the lower end of each of the first sidewall insulating film 19A and the second sidewall insulating film 19B. Referring to FIG. 2 , in plan view, the protrusion 20 is formed in an annular shape along the circumferential direction of the annular first sidewall insulating film 19A and the second sidewall insulating film 19B so as to overlap the first sidewall insulating film 19A and the second sidewall insulating film 19B. Since the protrusion 20 is continuous around the entire periphery of the first sidewall insulating film 19A and the second sidewall insulating film 19B in plan view, it may also be referred to as an "annular protruding stripe." Therefore, the recess 21 corresponding to the shape of the protrusion 20 may also be referred to as an "annular recessed stripe."

[0043] Referring to FIG. 5, the trench insulating film 14 may include a first film portion 141 having a relatively high density and a second film portion 142 having a lower density than the first film portion 141. A clearly defined film interface may or may not exist between the first film portion 141 and the second film portion 142, as shown in FIG. 5. The film density can be compared, for example, by etching the first film portion 141 and the second film portion 142 with a common etching gas or etching solution and comparing the etching rates. For example, when the first film portion 141 and the second film portion 142 are etched with a common etching gas or etching solution, the etching rate of the first film portion 141 having a relatively high density may be slower than the etching rate of the second film portion 142. When the trench insulating film 14 is made of silicon oxide, hydrofluoric acid (HF) can be used as the common etching gas.

[0044] In this embodiment, a second film portion 142, a first film portion 141, a second film portion 142, and a first film portion 141 are formed in this order from the trench electrode 15 toward the inner wall 16 and the outer wall 17 of the trench 13 in a direction intersecting the depth direction of the trench 13. Each of the first film portions 141 and each of the second film portions 142 extends in the depth direction of the trench 13. At least the inner peripheral wall 16 and outer peripheral wall 17, and the bottom wall 18 of the trench 13 are covered with the first film portion 141. Therefore, the protruding portion 20 of the bottom wall 18 of the trench 13 protrudes into the first film portion 141. In the trench insulating film 14, the second film portion 142, the first film portion 141, the second film portion 142, and the first film portion 141 may extend from the first film portion 141 serving as the base film portion 144 covering the bottom wall 18 toward the opening end of the trench 13 (upward). On the other hand, the side surface of the trench electrode 15 may be covered at the lower portion with the first film portion 141 (base film portion 144), and the portion excluding the lower portion with the second film portion 142. The trench electrode 15 may cross a boundary portion 143 between a first film portion 141 and a second film portion 142 covering the bottom wall 18 in the depth direction of the trench 13 .

[0045] 6A and 6B, a region sandwiched between a pair of sidewall insulating films 19 and where the bottom wall 18 of the trench 13 is exposed may be a contact hole 11 in the trench insulating film 14. The trench electrode 15 may include a contact portion 12 connected to the semiconductor chip 2 via the contact hole 11. In this embodiment, the bottom wall 18 of the trench 13 has a recess 23 that is continuous with the contact hole 11. A side surface 111 of the contact hole 11 and a side surface 231 of the recess 23 are continuous and flush with each other. The contact portion 12 of the trench electrode 15 is formed in the recess 23 via the contact hole 11.

[0046] In this embodiment, the contact portion 12 of the trench electrode 15 includes a bottom portion 121 that extends along the bottom wall 18 of the trench 13 and a side portion 122 that extends upward from the bottom portion 121 and crosses the boundary portion 24 between the trench insulating film 14 and the bottom wall 18 of the trench 13. The bottom portion 121 of the contact portion 12 may have a flat shape in a cross-sectional view. The side portion 122 of the contact portion 12 may have a flat shape as shown in FIG. 6A or a curved shape as shown in FIGS. 6B and 6C. The side portion 122 of the contact portion 12 may be curved in a convex shape so as to bulge outward relative to the trench 13, as shown in FIG. 6B, or may be curved in a concave shape so as to bulge inward relative to the trench 13, as shown in FIG. 6C.

[0047] Fig. 7 is a diagram showing a flow of a manufacturing process of the semiconductor device 1. Figs. 8A, 8B to 12A, 12B are schematic diagrams showing some of the manufacturing processes of the semiconductor device 1 in the order of steps. In Figs. 8A, 8B to 12A, 12B, the figures numbered "A" are plan views, and the figures numbered "B" are cross-sectional views. Note that Fig. 7 and Figs. 8A, 8B to 12A, 12B omit some steps, such as the step of forming the impurity region 22.

[0048] 7, 8A, and 8B, to manufacture the semiconductor device 1, a p-type semiconductor wafer 25 (high-concentration layer 6a) that will become the semiconductor chip 2 is prepared, and a p-type epitaxial layer (low-concentration layer 6b) is formed on the semiconductor wafer 25 (step S1). The next step is to form a buried layer (third layer 8) (step S2). For example, n-type impurities (e.g., phosphorus) are implanted into the surface of the low-concentration layer 6b. Next, while introducing the n-type impurities, silicon is epitaxially grown on the low-concentration layer 6b, thereby forming a second layer 7 on the first layer 6. Thereafter, an annealing process is performed, whereby the n-type impurities implanted into the surface of the low-concentration layer 6b diffuse to both sides in the thickness direction of the semiconductor wafer 25. As a result, a third layer 8 (buried layer) is formed between the first layer 6 and the second layer 7. The obtained semiconductor wafer 25 has the first main surface 3 and the second main surface 4 described above.

[0049] Next, a hard mask 26 is formed on the first main surface 3 of the semiconductor wafer 25 (step S3). The hard mask 26 has a first opening 43 and a second opening 44 that respectively correspond to the shapes of a main trench 27 and a sub-trench 28, which will be described later. Next, the semiconductor wafer 25 is etched through the hard mask 26 to form a deep trench 29 in the semiconductor wafer 25 (step S4). The deep trench 29 is formed to penetrate the second layer 7, the third layer 8, and the pn junction J, and reach the first layer 6. The deep trench 29 defines an element region 9 in the semiconductor wafer 25.

[0050] Here, the deep trench 29 includes at least three annular deep trenches 29 that are concentrically arranged and physically separated from one another. Specifically, the deep trench 29 may be a trench group 45 that includes a main trench 27 and a plurality of sub-trenches 28 that are arranged inside and outside the main trench 27 and have a width narrower than that of the main trench 27. In this embodiment, the same number of sub-trenches 28 (one each in FIGS. 8A and 8B ) are formed on both the inside and outside of the main trench 27. The sub-trenches 28 may include an inner sub-trench 28A that is arranged on the device region 9 side (inside) of the main trench 27 and surrounded by the main trench 27, and an outer sub-trench 28B that is arranged on the opposite side of the device region 9 (outside) of the main trench 27 and surrounds the main trench 27. The inner sub-trench 28A and the outer sub-trench 28B may be referred to as a "first sub-trench" and a "second sub-trench," respectively.

[0051] The width W2 of the main trench 27 may be, for example, not less than 2.5 μm and not more than 3 μm, and the width W3 of the sub-trench 28 may be, for example, not less than 1 μm and not more than 1.5 μm. In the trench group 45, the deep trenches 29 are physically separated from one another in annular shape. Therefore, a semiconductor wall portion 46 is formed between adjacent deep trenches 29, using a portion of the semiconductor wafer 25. Referring to FIG. 8A , each semiconductor wall portion 46 is formed in a strip shape along the circumferential direction of the trench group 45 in a plan view, forming a boundary between adjacent deep trenches 29. Referring to FIG. 8B , the semiconductor wall portion 46 may be, for example, a single wide trench 47 that defines the entire deep trench 29 belonging to the trench group 45, and the semiconductor wall portion 46 may be provided on the bottom wall 48 of the trench 47. The semiconductor wall portion 46 faces the sidewall 49 of the trench 47, with a space defined by the sub-trenches 28 sandwiched therebetween. The thickness T2 of the semiconductor wall portion 46 is preferably, for example, 1 μm or less. This allows the semiconductor wall portion 46 to be easily transformed into an insulator wall portion 51 in the subsequent thermal oxidation process.

[0052] Next, referring to FIGS. 9A and 9B, the semiconductor wafer 25 is subjected to a thermal oxidation process. As a result, a first insulating film 50 is formed on the bottom wall 48 and sidewall 49 of the trench 47 (step S5). The first insulating film 50 may also be referred to as a "thermal oxide film" or a "liner oxide film." In FIG. 9A, the first insulating film 50 is indicated by a relatively thick solid line. Through this thermal oxidation, the semiconductor wall portion 46 is oxidized from both the side facing the main trench 27 and the side facing the sub-trench 28, thereby transforming into an insulator, and is formed as an insulator wall portion 51. The insulator wall portion 51 may also be referred to as a boundary insulating film 52 that forms the boundary between adjacent deep trenches 29. Because the insulator wall portion 51 (boundary insulating film 52) is formed by transforming the semiconductor wall portion 46, it may have the same thickness T2 as the semiconductor wall portion 46.

[0053] On the other hand, in this process, the lower part of the semiconductor wall 46 is not partially transformed into an insulator in the depth direction of the trench 47, so that a protrusion 20 is formed that protrudes from the lower end of the insulator wall 51 toward the interior of the insulator wall 51. 10A and 10B, an insulating material is deposited on the semiconductor wafer 25 by, for example, a CVD method. A gas used in the CVD method may be, for example, TEOS (Tetra Ethyl Ortho Silicate) gas. The insulating material backfills the sub-trench 28 and is deposited along the inner surface of the main trench 27. As a result, a buried insulating film 53 is formed in the sub-trench 28, and a second insulating film 54 is formed along the inner surface of the main trench 27 (step S6). In other words, the second insulating film 54 is formed on the sidewalls of the insulator wall portion 51 and the bottom wall 48 of the trench 47. A space 55 surrounded by the second insulating film 54 remains in the main trench 27.

[0054] As a result, in the trench 47, a sidewall insulating film 56 including a first insulating film 50, a buried insulating film 53, an insulator wall portion 51, and a second insulating film 54 which are stacked in this order from the sidewall 49 in a direction intersecting the depth direction of the trench 47, and a bottom wall insulating film 57 including the first insulating film 50 and the second insulating film 54 which are stacked in this order from the bottom wall 48 in the depth direction of the trench 47 are formed. In this state, the trench 47 corresponds to the aforementioned trench 13, and the sidewall insulating film 56 corresponds to the aforementioned trench insulating film 14. Furthermore, the sidewall 49 of the trench 47 corresponds to the aforementioned inner circumferential wall 16 and outer circumferential wall 17, and the bottom wall 48 of the trench 47 corresponds to the aforementioned bottom wall 18.

[0055] 11A and 11B, bottom wall insulating film 57 exposed in space 55 of main trench 27 is selectively removed by etching, thereby forming contact hole 11 that exposes a portion of semiconductor wafer 25 from bottom wall 18 (step S7). 12A and 12B, a conductive material is deposited on the semiconductor wafer 25 by, for example, a CVD method. In this embodiment, the conductive material is polysilicon. The conductive material backfills the space 55 in the main trench 27. This forms a trench electrode 15 in the main trench 27 (step S8). The trench electrode 15 is connected to the semiconductor wafer 25 via the contact hole 11. Thereafter, the hard mask 26 and the second insulating film 54 on the first main surface 3 of the semiconductor wafer 25 are removed. Through the above steps, the element isolation structure 10 is formed.

[0056] The next step is to form a MISFET 30 in the element region 9. For example, a first well region 31 and a second well region 32 are formed in the element region 9 (step S9), and a shallow trench structure 37 is formed (step S10). After that, element structures such as a drain region 33 and a source region 34 are formed (step S11), and a planar gate structure 38 is formed. Then, the semiconductor wafer 25 is divided into the size of each semiconductor chip 2. Thereafter, if necessary, the semiconductor chip 2 is bonded to a lead frame and sealed with a sealing resin, thereby obtaining a semiconductor device 1.

[0057] According to the above method, the boundary insulating film 52 (insulator wall portion 51) constituting part of the sidewall insulating film 56 of the trench 13 is formed by altering the semiconductor wall portion 46 sandwiched between adjacent annular deep trenches 29. Therefore, by increasing the number of sub-trenches 28 and the number of semiconductor wall portions 46, the sidewall insulating film 56 can be selectively thickened among the sidewall insulating film 56 and the bottom wall insulating film 57 within the trench group 45. This prevents the bottom wall insulating film 57 from being thickened in the same manner as the sidewall insulating film 56 is thickened. This allows the bottom wall insulating film 57 to be kept thinner than the sidewall insulating film 56, thereby shortening the time required to form the contact hole 11 in the bottom wall insulating film 57 in the process shown in FIGS. 11A and 11B. This improves the manufacturing efficiency of the semiconductor device 1. Furthermore, since the thickness of the sidewall insulating film 56 can be controlled depending on the number of sub-trenches 28, a desired breakdown voltage can be easily achieved. This achieves both improved manufacturing efficiency and improved breakdown voltage.

[0058] Furthermore, since the four corners of the trench 47 are rounded in plan view, the width of the trench 47 can be made constant all around. This allows the buried insulating film 53 to be buried evenly in the steps of FIGS. 10A and 10B. 8A and 8B to 12A and 12B, one sub-trench 28 is formed on each of the device region 9 side (inside) and the opposite side (outside) of the main trench 27. However, as shown in FIG. 13A, multiple sub-trenches 28 may be formed on each side. This results in multiple semiconductor wall portions 46 being formed on both the inside and outside of the main trench 27. Therefore, as shown in FIG. 13B, multiple insulator wall portions 51 (boundary insulating film 52) can be formed by thermally oxidizing the multiple semiconductor wall portions 46 on each side. As a result, it is possible to form a sidewall insulating film 56 that is thicker than the sidewall insulating film 56 formed in the steps of FIGS. 8A and 8B to 12A and 12B.

[0059] FIG. 14 is a diagram showing the relationship between the thickness of the sidewall insulating film 19 of the element isolation structure 10 and the magnitude of the breakdown voltage. The horizontal axis of FIG. 14 represents the thickness of the sidewall insulating film 19, with the sidewall insulating film 19 becoming thicker toward the right of the horizontal axis. The vertical axis of FIG. 14 represents the magnitude of the breakdown voltage (BV Sub) of the substrate when a reverse voltage is applied between the source and drain, with the breakdown voltage and breakdown voltage increasing toward the upper side of the vertical axis. An examination of FIG. 14 revealed that the thicker the sidewall insulating film 19, the higher the breakdown voltage. Therefore, by forming the sidewall insulating film 19 thick in accordance with the above-described method, the breakdown voltage of the semiconductor device 1 can be improved while suppressing a decrease in manufacturing efficiency.

[0060] Although embodiments of the present disclosure have been described, the present disclosure may be embodied in other forms. For example, in the above-described embodiments, an example was described in which the first conductivity type was p-type and the second conductivity type was n-type. However, the first conductivity type may be n-type and the second conductivity type may be p-type. A specific configuration in this case can be obtained by replacing n-type regions with p-type regions and p-type regions with n-type regions in the above description and accompanying drawings. In the above-described embodiments, examples were described in which the p-type was expressed as the "first conductivity type" and the n-type was expressed as the "second conductivity type." However, these are used to clarify the order of the description, and the p-type may also be expressed as the "second conductivity type" and the n-type as the "first conductivity type."

[0061] The embodiments of the present disclosure are to be considered as illustrative in all respects and not restrictive, and are intended to include modifications in all respects. The following characteristics can be extracted from the description of this specification and the drawings. [Appendix 1-1] a semiconductor chip (2) having a first main surface (3) on one side and a second main surface (4) on the other side; a pn junction (J) extending along the first main surface (3) and formed inside the semiconductor chip (2); a trench (13) that penetrates the pn junction (J) from the first main surface (3) and defines an element region (9, 9A) in the semiconductor chip (2); an insulating film (14) covering the side walls (16, 17) and bottom wall (18) of the trench (13); a buried electrode (15) buried in the trench (13) via the insulating film (14), The semiconductor device (1), wherein a bottom wall (18) of the trench (13) includes a protrusion (20) that protrudes from a lower end of the insulating film (14) toward an upper portion inside the insulating film (14) in the depth direction of the trench (13). [Appendix 1-2] The insulating film (14) has a contact hole (11) that selectively exposes a bottom wall (18) of the trench (13), The semiconductor device (1) according to Appendix 1-1, wherein the embedded electrode (15) includes a contact portion (12) connected to the semiconductor chip (2) via the contact hole (11). [Appendix 1-3] The semiconductor chip (2) has a recess (21) that is continuous with the contact hole (11), The semiconductor device (1) according to appendix 1-2, wherein the contact portion (12) is formed in the recess (21) via the contact hole (11). [Appendix 1-4] The semiconductor device (1) described in Appendix 1-3, wherein the contact portion (12) includes a bottom portion (121) that is along the bottom wall (18) of the trench (13), and a side portion (122) that extends upward from the bottom portion (121) and crosses a boundary portion (24) between the insulating film (14) and the bottom wall (18) of the trench (13). [Appendix 1-5] The semiconductor device (1) according to appendix 1-4, wherein the side portion (122) of the contact portion (12) has a curved shape in cross section. [Appendix 1-6] The semiconductor device (1) according to any one of Supplementary Notes 1-1 to 1-5, wherein the thickness (T1) of the insulating film (14) in a direction intersecting the depth direction of the trench (13) is 2 μm or more and 6 μm or less.

[0062] According to this configuration, the thickness of the insulating film (14) is 2 μm or more and 6 μm or less, so that the breakdown voltage can be made relatively high. [Appendix 1-7] the insulating film (14) includes a first film portion (141) having a relatively high density and a second film portion (142) having a lower density than the first film portion (141); The semiconductor device (1) according to any one of Appendix 1-1 to Appendix 1-6, wherein the second film portion (142), the first film portion (141), the second film portion (142), and the first film portion (141), each extending in the depth direction of the trench (13), are formed in this order from the embedded electrode (15) toward the side walls (16, 17) of the trench (13) in a direction intersecting the depth direction of the trench (13). [Appendix 1-8] The semiconductor device (1) according to appendix 1-7, wherein at least the side walls (16, 17) and bottom wall (18) of the trench (13) are covered with the first film portion (141) of the insulating film (14). [Appendix 1-9] The semiconductor device (1) according to appendix 1-8, wherein the protrusion (20) is formed to protrude into the first film portion (141) that covers the bottom wall (18) of the trench (13). [Appendix 1-10] the trench (13) includes an annular trench (13) surrounding the element region (9, 9A); the insulating film (14) has, in a plan view, an annular portion formed on a side wall (16, 17) of the annular trench (13) along a circumferential direction of the annular trench (13); The semiconductor device (1) according to any one of Supplementary Notes 1-1 to 1-9, wherein the protrusion (20) is formed in a plan view along the circumferential direction of the annular portion of the insulating film (14) so as to overlap the annular portion. [Appendix 1-11] a first step of selectively etching a semiconductor layer (25) having a first main surface (3) on one side and a second main surface (4) on the other side, and having a pn junction (J) formed therein and extending along the first main surface (3), to form a trench (47) that penetrates the pn junction (J) and partitions an element region (9, 9A) in the semiconductor layer (25), and to form a semiconductor wall portion (46) that stands on a bottom wall (48) of the trench (47) and faces a side wall (49) of the trench (47) across a space (28) using a part of the semiconductor layer (25); a second step of forming a first insulating film (50) along the sidewalls (49) and bottom wall (48) of the trench (47) by thermal oxidation, and also transforming the semiconductor wall portion (46) into an insulator by the thermal oxidation to form an insulator wall portion (51) that faces the first insulating film (50) on the sidewall (49) of the trench (47) across the space (28); a third step of depositing an insulating material in the trench (47) to form a buried insulating film (53) that backfills the space (28) and a second insulating film (54) along the sidewalls of the insulator wall portion (51) and the bottom wall (48) of the trench (47) on the opposite side of the space (28), thereby forming a sidewall insulating film (56) including the first insulating film (50), the buried insulating film (53), the insulator wall portion (51), and the second insulating film (54) on the sidewall (49) of the trench (47), and a bottom wall insulating film (57) including the first insulating film (50) and the second insulating film (54) on the bottom wall (48) of the trench (47); and a fourth step of depositing a conductive material in the trench (47) to form a buried electrode (15) that backfills the trench (47).

[0063] According to this method, the insulator wall portion 51 (semiconductor wall portion 46), which constitutes a part of the sidewall insulating film 56, is erected on the bottom wall 48 of the trench 47 so as to follow the sidewall 49 of the trench 47. Therefore, by increasing the number of semiconductor wall portions 46, the sidewall insulating film 56 can be selectively thickened. This prevents the bottom wall insulating film 57 from becoming thicker as the sidewall insulating film 56 increases. This allows the bottom wall insulating film 57 to be kept thinner than the sidewall insulating film 56, thereby reducing the time required for etching the bottom wall insulating film 57. This improves the manufacturing efficiency of the semiconductor device 1. Furthermore, the thickness of the sidewall insulating film 56 can be controlled in accordance with the increased number of semiconductor wall portions 46, making it easy to achieve a desired breakdown voltage. This makes it possible to improve both manufacturing efficiency and pressure resistance. [Appendix 1-12] The method for manufacturing a semiconductor device (1) described in Appendix 1-11, wherein the second step includes a step of forming a protrusion (20) that protrudes from the lower end of the insulator wall portion (51) toward the interior and upper side of the insulator wall portion (51) by preventing the lower portion of the semiconductor wall portion (46) from partially transforming into an insulator in the depth direction of the trench (47). [Appendix 1-13] The method for manufacturing a semiconductor device (1) according to Appendix 1-11 or Appendix 1-12, wherein the thickness (T2) of the semiconductor wall portion (46) is 1 μm or less.

[0064] According to this method, the semiconductor wall portion (46) can be easily transformed into the insulating wall portion (51) by thermal oxidation. [Appendix 1-14] a first step of selectively etching a semiconductor layer (25) having a first main surface (3) on one side and a second main surface (4) on the other side, and having a pn junction (J) formed therein and extending along the first main surface (3), to form a trench group (45) that partitions element regions (9, 9A) in the semiconductor layer (25), the trench group including at least three annular trenches (29) arranged concentrically with one another and penetrating the pn junction (J), the trench group including a main trench (27) and a plurality of sub-trenches (28) arranged inside and outside the main trench (27) and having a width narrower than that of the main trench (27); a second step of forming a first insulating film (50) along the sidewalls and bottom wall (48) of each of the annular trenches (29) belonging to the trench group (45) by thermal oxidation, and also transforming a portion (46) of the semiconductor layer (25) sandwiched between adjacent annular trenches (29) into an insulator by the thermal oxidation, thereby forming a boundary insulating film (52) that forms a boundary between the adjacent annular trenches (29); a third step of depositing an insulating material in the trench group (45) after the second step to form a buried insulating film (53) that backfills the sub-trench (28) and a second insulating film (54) along the inner surface of the main trench (27), thereby forming a sidewall insulating film (56) including the second insulating film (54), the boundary insulating film (52), the buried insulating film (53), and the first insulating film (50) on each of the inside and outside of the main trench (27), and forming a bottom wall insulating film (57) including the first insulating film (50) and the second insulating film (54) on the bottom wall (48) of the main trench (27); a fourth step of selectively removing the bottom wall insulating film (57) in the main trench (27) to form a contact hole (11) that exposes a portion of the semiconductor layer (25) on the bottom wall (48) of the main trench (27); and a fifth step of depositing a conductive material in the main trench (27) to backfill the main trench (27) and form a buried electrode (15) connected to the semiconductor layer (25) via the contact hole (11).

[0065] According to this method, the boundary insulating film (52) constituting a part of the sidewall insulating film (56) is formed by altering a portion of the semiconductor layer (25) sandwiched between adjacent annular trenches (29). Therefore, by increasing the number of sub-trenches (28), the sidewall insulating film (56) can be selectively thickened among the sidewall insulating film (56) and bottom wall insulating film (57) within the trench group (45). This prevents the bottom wall insulating film (57) from becoming thicker as the sidewall insulating film (56) becomes thicker. This allows the bottom wall insulating film (57) to be kept thinner than the sidewall insulating film (56), thereby shortening the time required to form the contact hole (11) in the bottom wall insulating film (57) in the fourth step. This improves the manufacturing efficiency of the semiconductor device (1). Furthermore, since the thickness of the sidewall insulating film (56) can be controlled in accordance with the number of sub-trenches (28), the desired breakdown voltage can be easily achieved. This achieves both improved manufacturing efficiency and improved breakdown voltage. [Appendix 1-15] The method for manufacturing a semiconductor device (1) described in Appendix 1-14, wherein the second step includes a step of forming a protrusion (20) that protrudes upward from the lower end of the boundary insulating film (52) inside the boundary insulating film (52) by preventing a lower portion of the semiconductor layer (25) sandwiched between adjacent annular trenches (29) from partially transforming into an insulator in the depth direction of the trench group (45). [Appendix 1-16] The method for manufacturing a semiconductor device (1) according to Appendix 1-14 or Appendix 1-15, wherein the first step includes forming the same number of sub-trenches (28) inside and outside the main trench (27).

[0066] According to this method, it is possible to form the sidewall insulating film (56) having a uniform thickness on both the inside and outside of the main trench (27). [Appendix 1-17] The method for manufacturing a semiconductor device (1) according to any one of Appendices 1-14 to 1-16, wherein the first step includes forming a plurality of the sub-trenches (28) on both the inside and outside of the main trench (27). [Appendix 1-18] The width (W2) of the main trench (27) is 2.5 μm or more and 3 μm or less, The method for manufacturing a semiconductor device (1) according to any one of Appendixes 1-14 to 1-17, wherein the width (W3) of the sub-trench (28) is 1 μm or more and 1.5 μm or less. [Appendix 1-19] The method for manufacturing a semiconductor device (1) according to any one of Supplementary Notes 1-14 to 1-18, wherein the thickness (T2) of the boundary insulating film (52) in a direction intersecting the depth direction of the trench group (45) is 1 μm or less. [Appendix 1-20] The method for manufacturing the semiconductor device (1) according to any one of Supplementary Note 1-11 to Supplementary Note 1-19, wherein the third step includes a step of depositing the insulating material by a CVD method using TEOS gas. [Explanation of symbols]

[0067] 1: Semiconductor device 2: Semiconductor chip 3: First main surface 4: Second main surface 5A: 1st side 5B: 2nd side 5C: 3rd side 5D: 4th side 6 :1st layer 6a: High concentration layer 6b:Low concentration layer 7: 2nd layer 8:Third layer 8a: Low concentration buried layer 8b: High concentration buried layer 9: Element area 9A: Transistor area 10: Element isolation structure 11: Contact hole 12: Contact part 13: Trench 14: Trench insulating film 15: Trench electrode 16:Inner peripheral wall 17:Outer wall 18: Bottom wall 19: Sidewall insulating film 19A: First sidewall insulating film 19B: Second sidewall insulating film 20:Protrusion 21: Recess 22: Impurity region 23: Recess 24: Boundary 25: Semiconductor wafer 26: Hard mask 27: Main trench 28: Subtrench 28A: Inner sub-trench 28B: Outer sub-trench 29: Deep Trench 30:MISFET 31: First well region 32: Second well region 33: Drain region 34: Source area 35: Channel region 36: Contact area 37: Shallow trench structure 38: Planar gate structure 39: Shallow trench 40: Buried insulator 41: Gate insulating film 42: Gate electrode 43: First opening 44: Second opening 45: Trench group 46: Semiconductor wall 47: Trench 48: Bottom wall 49: Side wall 50: First insulating film 51: Insulator wall 52: Boundary insulating film 53: Buried insulating film 54: Second insulating film 55: Space 56: Sidewall insulating film 57: Bottom wall insulating film 111: Side 121: Bottom 122: Side 141: First membrane part 142: Second membrane part 143: Boundary 144: Base membrane part 191: 1st page 192: 2nd side 231: Side T1: Thickness T2: Thickness W1: Trench width W2: Width W3:Width

Claims

1. a semiconductor chip having a first main surface on one side and a second main surface on the other side; a pn junction extending along the first main surface and formed inside the semiconductor chip; a trench that penetrates the pn junction from the first main surface and defines an element region in the semiconductor chip; an insulating film covering the sidewalls and bottom wall of the trench; a buried electrode buried in the trench via the insulating film, In the insulating film covering the sidewall and bottom wall of the trench, the thickness of the sidewall insulating film covering the sidewall is thicker than the thickness of the bottomwall insulating film covering the bottom wall, a bottom wall of the trench including a protruding portion protruding from a lower end of the insulating film toward an interior upper portion of the insulating film in a depth direction of the trench.

2. the insulating film has a contact hole that selectively exposes a bottom wall of the trench; 2. The semiconductor device according to claim 1, wherein said buried electrode includes a contact portion connected to said semiconductor chip through said contact hole.

3. the semiconductor chip has a recessed portion that is continuous with the contact hole, 3. The semiconductor device according to claim 2, wherein said contact portion is formed in said recess through said contact hole.

4. 4. The semiconductor device according to claim 3, wherein said contact portion includes a bottom portion along the bottom wall of said trench, and a side portion extending upward from said bottom portion and crossing a boundary portion between said insulating film and said bottom wall of said trench.

5. The semiconductor device according to claim 4 , wherein the side of said contact portion has a curved shape in cross section.

6. 6. The semiconductor device according to claim 1, wherein the thickness of said insulating film in a direction intersecting the depth direction of said trench is 2 μm or more and 6 μm or less.

7. the insulating film includes a first film portion having a relatively high density and a second film portion having a density lower than that of the first film portion, 7. The semiconductor device according to claim 1, wherein the second film portion, the first film portion, the second film portion, and the first film portion are formed in order from the buried electrode toward the side wall of the trench in a direction intersecting the depth direction of the trench, each extending in the depth direction of the trench.

8. 8. The semiconductor device according to claim 7, wherein at least the sidewalls and bottom wall of said trench are covered with said first film portion of said insulating film.

9. 9. The semiconductor device according to claim 8, wherein said protruding portion is formed so as to protrude into said first film portion covering the bottom wall of said trench.

10. the trench includes an annular trench surrounding the element region; the insulating film has an annular portion formed on a side wall of the annular trench along a circumferential direction of the annular trench in a plan view; 10. The semiconductor device according to claim 1, wherein the protruding portion is formed so as to overlap the annular portion of the insulating film along a circumferential direction of the annular portion in a plan view.

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