A process for forming sigma trenches

Abstract

This invention discloses a process for forming sigma trenches. The method includes: providing a substrate with an initial groove; under condition P1, introducing a deposition and etching gas with a flow rate ratio R1 to grow a first epitaxial layer, wherein the growth rate of the lower and bottom sides is greater than that of the upper sides, to obtain a first trench; under condition P2, introducing a deposition and etching gas with a flow rate ratio R2 to etch the inner wall of the first trench, wherein the etching rate of the lower and bottom sides is greater than that of the upper sides, to obtain a second trench; under condition P3, introducing a deposition and etching gas with a flow rate ratio R3 to grow a second epitaxial layer, wherein the growth rate of the upper sidewall is greater than that of the lower and bottom sides, to obtain a third trench; under condition P4, introducing a deposition and etching gas with a flow rate ratio R4 to grow a third epitaxial layer to backfill the bottom of the third trench, to form a sigma trench; wherein P3 > P1 > P4, and R3 > R1 > R2. The method of this invention can solve the problems of complex TMAH etching processes, poor stability, and difficulty in residual cleaning leading to device performance degradation.

Description

Technical Field

[0001] This invention relates to the field of semiconductor manufacturing technology, and in particular to a process method for forming sigma trenches. Background Technology

[0002] Driven by the trend towards miniaturization and high performance in semiconductor devices, advanced nanoscale processes have become one of the core manufacturing processes for many mid-to-high-end devices due to their excellent balance between performance and cost. Germanium-silicon epitaxial layers are widely used in key device structures of this process because they can effectively enhance carrier mobility and significantly improve device performance.

[0003] In existing technologies, to provide an ideal stress structure for the germanium-silicon epitaxial layer, tetramethylammonium hydroxide (TMAH) is typically used to selectively etch different crystal planes of the semiconductor substrate before the germanium-silicon epitaxial layer is filled. After cleaning, sigma trenches are formed. Sigma trenches can precisely control the stress distribution during subsequent germanium-silicon epitaxial growth, providing favorable conditions for carrier migration and thus improving device performance.

[0004] However, current TMAH-based etching processes suffer from numerous technical drawbacks, severely hindering further improvements in device performance and mass production stability. Firstly, the TMAH etching process is complex, requiring stringent environmental and parameter precision, leading to significant implementation difficulties and relatively high production costs. Secondly, the etching rate is easily affected by factors such as TMAH concentration, processing temperature, and reaction time, resulting in poor process stability. Thirdly, residual etching solution components after TMAH etching are difficult to completely remove through conventional cleaning. These residues introduce impurities during subsequent germanium-silicon epitaxial filling, affecting the quality of the epitaxial layer and adversely impacting subsequent processes, ultimately leading to decreased device performance.

[0005] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art. Summary of the Invention

[0006] The purpose of this invention is to use a new four-step epitaxial growth + etching synergy process to form sigma trenches, replacing the traditional TMAH polycrystalline surface etching + cleaning process, in order to solve the problems of complex TMAH etching process, poor stability and difficulty in residual cleaning leading to device performance degradation.

[0007] To achieve the above objectives, the present invention provides a process for forming sigma trenches, comprising at least the following steps: Step 1, providing a substrate with an initial groove formed thereon; Step 2: Under the first pressure P1, a deposition gas and an etching gas with a flow rate ratio of R1 are introduced to grow a first epitaxial layer in the initial groove, such that the growth rate of the first epitaxial layer on the lower sidewall and bottom wall of the initial groove is greater than its growth rate on the upper sidewall of the initial groove, thereby obtaining the first trench. Step 3: Under the second pressure P2, a deposition gas and an etching gas with a flow rate ratio of R2 are introduced to etch the inner wall of the first trench, so that the etching rate of the lower part of the sidewall and bottom wall of the first trench is greater than the etching rate of the upper part of the sidewall of the first trench, thereby obtaining the second trench. Step 4: Under the third pressure P3, a deposition gas and an etching gas with a flow rate ratio of R3 are introduced to grow a second epitaxial layer in the second trench, such that the growth rate of the second epitaxial layer on the upper sidewall of the second trench is greater than its growth rate on the lower sidewall and bottom wall of the second trench, thereby obtaining a third trench. Step 5: Under the fourth pressure P4, a deposition gas and an etching gas with a flow rate ratio of R4 are introduced to grow a third epitaxial layer to backfill the bottom of the third trench, forming a sigma trench. Among them, P3 > P1 > P4, and R3 > R1 > R2.

[0008] Optionally, the first pressure P1 is 30 torr to 80 torr, the second pressure P2 is 30 torr to 80 torr, the third pressure P3 is 100 torr to 200 torr, and the fourth pressure P4 is 5 torr to 10 torr.

[0009] Optionally, the flow ratio R1 ranges from (0.8 to 1.0):1, the flow ratio R2 ranges from (0.5 to 0.7):1, the flow ratio R3 ranges from (2.0 to 4.0):1, and the flow ratio R4 ranges from (0.9 to 1.0):1.

[0010] Optionally, in step 5, the bottom of the third trench is backfilled until the 001 crystal plane appears, at which point the backfilling stops.

[0011] Optionally, the crystal plane index of the lower part of the bottom wall and sidewall of the first trench is greater than the crystal plane index of the upper part of the sidewall of the first trench; the crystal plane index of the lower part of the bottom wall and sidewall of the second trench is greater than the crystal plane index of the upper part of the sidewall of the second trench; and the crystal plane index of the lower part of the bottom wall and sidewall of the third trench is greater than the crystal plane index of the upper part of the sidewall of the third trench.

[0012] Optionally, the upper part of the sidewalls of the first trench, the second trench, and the third trench are all 100 crystal planes; the lower part of the sidewall of the third trench includes a 611 crystal plane; and the bottom wall of the third trench includes a 311 crystal plane.

[0013] Optionally, the substrate is a silicon substrate, the deposition gas is at least one of dichlorosilane, silane, and silane halide; the etching gas is a mixture of fluorine-containing gas, hydrocarbon and oxygen; and the first epitaxial layer, the second epitaxial layer and the third epitaxial layer are all made of silicon.

[0014] Optionally, the initial groove has a rectangular structure, the lower part of the sidewall and the bottom wall of the first groove form a pointed structure, the lower part of the sidewall and the bottom wall of the second groove form a conical cross-section trap structure, and the lower part of the sidewall and the bottom wall of the third groove form a rhomboid structure.

[0015] Optionally, the opening width of the upper sidewall of the third trench is smaller than the opening width of the upper sidewall of the first trench, and the opening width of the upper sidewall of the first trench is less than or equal to the opening width of the upper sidewall of the second trench.

[0016] Optionally, in step 2, the growth thickness of the first epitaxial layer on the upper sidewall of the initial groove is 3nm~20nm; in step 4, the growth thickness of the second epitaxial layer on the upper sidewall of the second trench is 8nm~25nm.

[0017] Optionally, steps 2-5 may also include: introducing hydrogen gas as a carrier gas, with a hydrogen flow rate of 20 slm to 30 slm.

[0018] Optionally, the process temperature is 850℃~950℃.

[0019] Optionally, the depth of the initial groove is 50nm~200nm and the width is 30nm~100nm.

[0020] Optionally, after step 1 and before step 2, the process further includes: performing a high-temperature annealing pretreatment on the substrate; the annealing temperature is 900℃~1000℃, and the annealing time is 30s~60s.

[0021] Compared with the prior art, the technical solution of the present invention has at least the following beneficial effects: 1) This invention fully utilizes the characteristics of different crystal orientations inside the trench and the anisotropy of epitaxial growth and etching. It adopts a new four-step epitaxial growth + etching synergy process to form sigma trenches, replacing the traditional TMAH polycrystalline etching + cleaning process. It eliminates the steps of TMAH etching solution preparation, crystal selective etching control and complex cleaning, simplifies the process, shortens the single-wafer processing cycle, reduces production costs and improves production efficiency.

[0022] 2) Thanks to the precise control capabilities of current epitaxial equipment, the control accuracy of epitaxial growth parameters (temperature, pressure, gas flow rate, etc.) in the process of this application can reach within ±1%, which is far higher than the traditional TMAH etching process (parameter fluctuations can easily reach ±5%). Furthermore, the process of this application controls the deviation of the shape and size (depth, sidewall angle, etc.) of the sigma trench to ±0.5%, while the deviation of the traditional process is usually ±2%~3%, thereby greatly improving the process stability.

[0023] 3) This invention avoids the problem of TMAH etching solution residue and eliminates the risk of impurities introduced by residual components during the subsequent germanium-silicon epitaxial filling process, thereby reducing the defect density of the germanium-silicon epitaxial layer and ensuring device quality.

[0024] 4) The four-step epitaxial growth + etching synergistic process of the present invention and the subsequent germanium-silicon epitaxial filling can be carried out continuously in situ in the same epitaxial chamber, reducing wafer transfer time and contamination risk. Attached Figure Description

[0025] Figure 1 This is a flowchart of the process for forming sigma trenches according to the present invention.

[0026] Figure 2 This is a schematic diagram of the silicon substrate with an initial groove formed according to the present invention.

[0027] Figure 3 This is a schematic diagram of the structure in which the first trench is formed in the silicon substrate according to the present invention.

[0028] Figure 4 This is a schematic diagram of the structure of forming a second trench in the silicon substrate according to the present invention.

[0029] Figure 5 This is a schematic diagram of the structure in which a third trench is formed in the silicon substrate according to the present invention.

[0030] Figure 6 This is a schematic diagram of the structure of forming sigma trenches in the silicon substrate according to the present invention.

[0031] Figure 7 This is a schematic diagram of the sigma trench structure of the present invention.

[0032] Attached image labels: Silicon substrate 10, initial trench 20, first epitaxial layer 30, first trench 40, sharp corner structure 41, second trench 50, conical cross-section well structure 51, second epitaxial layer 60, third trench 70, rhombic structure 71, third epitaxial layer 80, sigma trench 90, upper sidewall of the first trench 200a, upper sidewall of the second trench 200b, upper sidewall of the third trench 200c, lower sidewall of the first trench 300a, lower sidewall of the second trench 300b, lower sidewall of the third trench 300c, bottom wall of the first trench 400a, bottom wall of the second trench 400b, bottom wall of the third trench 400c. Detailed Implementation

[0033] The following detailed description, in conjunction with the accompanying drawings and specific embodiments, provides a further detailed explanation of the process for forming sigma trenches proposed in this invention. The advantages and features of this invention will become clearer from the following description. It should be noted that the drawings are in a very simplified form and use non-precise proportions, used only to facilitate and clearly illustrate the embodiments of this invention. Please refer to the drawings to make the objectives, features, and advantages of this invention more apparent and understandable. It should be understood that the structures, proportions, sizes, etc., depicted in the accompanying drawings are only for illustrative purposes and to enable those skilled in the art to understand and read them, and are not intended to limit the implementation conditions of this invention. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to the size, without affecting the effects and objectives achieved by this invention, should still fall within the scope of the technical content disclosed in this invention.

[0034] As is well known in the art, the “100 crystal plane” and “001 crystal plane” mentioned herein are often referred to as low-index crystal planes, and the “611 crystal plane” and “311 crystal plane” are often referred to as high-index crystal planes. Among them, the “100 crystal plane” refers to a plane perpendicular to the x-axis, the “001 crystal plane” refers to a plane perpendicular to the y-axis, the “611 crystal plane” refers to a plane at an angle of 13.26° to the y-axis, and the “311 crystal plane” refers to a plane at an angle of 25.24° to the y-axis.

[0035] In this paper, the "upper sidewall" of each trench is a low-index crystal orientation plane (100 crystal plane); by processing the "lower sidewall" of each trench in sequence, a high-index crystal orientation plane (611 crystal plane) can be finally obtained; by processing the "bottom wall" of each trench in sequence, a high-index crystal orientation plane (311 crystal plane) can be finally obtained.

[0036] As mentioned earlier, in existing technologies, tetramethylammonium hydroxide (TMAH) is often used to selectively etch the semiconductor substrate before germanium-silicon epitaxial growth, forming sigma trenches after cleaning, thereby precisely controlling the stress distribution during subsequent germanium-silicon epitaxial growth. However, current TMAH-based etching processes have many technical drawbacks (such as process complexity, stringent requirements, high production costs, and poor process stability), which severely restrict the improvement of semiconductor device performance and mass production stability.

[0037] To overcome these technical shortcomings, this invention provides a novel process for forming sigma trenches. By fully utilizing the different crystal orientations within the trench and the anisotropic nature of epitaxial growth and etching, a new four-step epitaxial growth + etching synergistic process is employed to replace the traditional TMAH polycrystalline etching + cleaning process for forming sigma trenches. This solves the problems of complex processes, poor stability, and difficulty in residual cleaning leading to device performance degradation in existing TMAH etching processes. Furthermore, the control precision of epitaxial growth parameters can be achieved within ±1%, and the shape and size deviation of the sigma trenches is controlled within ±0.5%, significantly improving process stability. The process method of this invention is described in detail below.

[0038] like Figure 1 As shown, the present invention provides a process for forming sigma trenches, comprising at least the following steps: Step 1, provide a substrate with an initial groove formed.

[0039] The substrate is a silicon substrate 10. An initial groove 20 is formed inside the silicon substrate 10 using an inductively coupled plasma (ICP) etching process. The initial groove 20 has a rectangular structure (e.g., ...). Figure 2 As shown in the figure, the etching depth is 50nm~200nm and the etching width is 30nm~100nm. The initial groove 20 lays the structural foundation for the subsequent gradual epitaxy and etching to form sigma trenches.

[0040] After etching to form the initial groove 20, the silicon substrate 10 is cleaned to remove impurities such as polymers generated during the etching process. After cleaning, the silicon substrate 10 with the initial groove 20 is placed in the epitaxial chamber, and hydrogen gas is introduced into the epitaxial chamber to perform high-temperature annealing pretreatment on the silicon substrate 10 to remove the native oxide layer on the surface of the silicon substrate 10; the annealing temperature is 900℃~1000℃, and the annealing time is 30s~60s.

[0041] Step 2: Under the first pressure P1, a deposition gas and an etching gas with a flow rate ratio of R1 are introduced to grow a first epitaxial layer in the initial groove, such that the growth rate of the first epitaxial layer on the lower sidewall and bottom wall of the initial groove is greater than its growth rate on the upper sidewall of the initial groove, thereby obtaining the first trench.

[0042] After the silicon substrate 10 is pretreated, the process temperature is set to 850℃~950℃. Under the first pressure P1 (30 torr~80 torr), a deposition gas and an etching gas with a flow ratio of R1 ((0.8~1.0):1) are introduced into the epitaxial chamber to grow the first epitaxial layer 30 inside the initial groove 20. The growth time is controlled to be T1. Under the synergistic effect of low pressure, deposition gas, and etching gas, the crystal orientations of the upper, lower, and bottom sides of the initial trench 20 are different, resulting in anisotropic epitaxial growth: the growth rate of the upper sidewall (100 crystal plane) is slower, while the lower and bottom sides, after high-temperature baking, form high-index crystal orientation planes and grow at a faster rate. This causes the growth rate G2 of the first epitaxial layer 30 in the lower and bottom sides of the initial trench 20 to be greater than the growth rate G1 of the first epitaxial layer 30 in the upper sidewall of the initial trench 20, thus forming a first trench 40 with a wide, straight upper opening and a narrow V-shaped lower opening. The lower and bottom sides grow faster, forming the lower sidewall 300a and bottom wall 400a of the first trench 40. The lower sidewall 300a and bottom wall 400a form a sharp-angle structure 41 with an angle of 45°~90° and facing the bottom surface of the silicon substrate 10 (e.g., Figure 3 (As shown).

[0043] The sidewall of the first groove 40 includes an upper sidewall 200a and a lower sidewall 300a along the depth direction, and the bottom of the lower sidewall 300a is connected to a bottom wall 400a; the upper sidewall 200a, the lower sidewall 300a and the bottom wall 400a together form a continuous contour surface. The upper sidewall 200a of the first trench 40 extends downward from the top opening of the first trench 40. The upper sidewall 200a is a straight wall surface that extends vertically or inwardly. The inclination direction of the upper sidewall 200a forms a first angle θ1 with the normal direction of the surface of the silicon substrate 10. The range of the first angle θ1 is 0°≤θ1≤10°. For example, the first angle θ1 can be 3°, 5° or 8°. The lower part 300a of the sidewall is connected to the lower end of the upper part 200a of the sidewall and extends inwardly to the bottom wall 400a. The lower part 300a of the sidewall is a continuous inclined inner surface. The inclined direction of the lower part 300a of the sidewall forms a second included angle θ2 with the normal direction of the surface of the silicon substrate 10. The second included angle θ2 is greater than or equal to the first included angle θ1. The range of the second included angle θ2 is: 0°≤θ2<22.5°. For example, the second included angle θ2 can be 10°, 15° or 20°. The bottom wall 400a is located at the bottom of the first trench 40 and has a concave V-shaped surface. The lowest point of the bottom wall 400a is located on the central axis of the first trench 40. The inclined direction of the bottom wall 400a forms a third included angle θ3 with the normal direction of the surface of the silicon substrate 10. The third included angle θ3 is greater than the second included angle θ2. The range of the third included angle θ3 is: 22.5°≤θ3<45°. For example, the third included angle θ3 can be 25°, 30°, 35° or 40°. The bottom wall 400a and the lower part of the side wall 300a form a continuous arc-shaped profile.

[0044] Step 3: Under the second pressure P2, a deposition gas and an etching gas with a flow rate ratio of R2 are introduced to etch the inner wall of the first trench, so that the etching rate of the lower part of the sidewall and bottom wall of the first trench is greater than the etching rate of the upper part of the sidewall of the first trench, thereby obtaining the second trench.

[0045] After the first trench 40 is formed, the process temperature is maintained at 850℃~950℃. Under the second pressure P2 (30 torr~80 torr), a deposition gas and an etching gas with a flow ratio of R2 ((0.5~0.7):1) are introduced into the epitaxial chamber. Since the flow rate of the deposition gas is less than that of the etching gas (the deposition rate is less than the etching rate), this step is mainly etching, mainly etching the inner wall of the first trench 40. The etching time is controlled as T2{T2∈(1.2*T1, 2*T1)}. Under the synergistic effect of low-pressure, low-flow deposition gas and high-flow etching gas, the crystal orientations of the upper 200a, lower 300a, and bottom 400a of the first trench 40 are different, resulting in anisotropy in the etching: the etching rate of the upper 200a (100 crystal plane) is extremely slow (close to 0), with little or no etching, while the etching rate of the lower 300a and bottom 400a (high index crystal planes) is relatively fast. This makes the etching rate E2{E2∈[-1.5*G2,-G2)} of the etching gas on the lower 300a and bottom 400a of the first trench 40 greater than the etching rate E1 (E1≈0) on the upper 200a of the first trench 40, thus forming the second trench 50. The lower sidewall 300a and bottom wall 400a of the first trench 40 are etched relatively quickly, the sharp corner structure 41 disappears, and the lower sidewall 300b and bottom wall 400b of the second trench 50 appear. At this time, the lower sidewall 300b and bottom wall 400b form a tapered cross-section well structure 51. The lateral width of the tapered cross-section well structure 51 is greater than the lateral width of the sharp corner structure 41 (overall enlargement), and the bottom of the tapered cross-section well structure 51 faces the bottom surface of the silicon substrate 10 (e.g., ...). Figure 4 (As shown).

[0046] The sidewall of the second groove 50 includes an upper sidewall 200b and a lower sidewall 300b along the depth direction, and the bottom of the lower sidewall 300b is connected to a bottom wall 400b; the upper sidewall 200b, the lower sidewall 300b and the bottom wall 400b together form a continuous contour surface. The upper sidewall 200b of the second trench 50 extends downward from the top opening of the second trench 50. The upper sidewall 200b is a straight wall surface that extends vertically or inwardly. The inclination direction of the upper sidewall 200b forms a fourth included angle θ4 with the normal direction of the surface of the silicon substrate 10. The range of the fourth included angle θ4 is 0°≤θ4≤10°. For example, the fourth included angle θ4 can be 3°, 5° or 8°. The lower part 300b of the sidewall is connected to the lower end of the upper part 200b of the sidewall and extends outward at an angle to the bottom wall 400b. The lower part 300b of the sidewall is a continuous inclined inner surface. Compared with the lower part 300a of the first groove 40, which is an inwardly inclined "\" shape, the lower part 300b of the sidewall of the second groove 50 is an outwardly inclined " / " shape. The lower part 300b of the sidewall of the second groove 50 is formed by etching the lower part 300a of the sidewall of the first groove 40 outward. The lower part 300a of the sidewall of the first groove 40 is a wall surface that gradually converges inward from top to bottom. After etching, part of the material of the lower part 300a of the sidewall... The material is removed, and the lower part 300b of the sidewall transforms into a wall surface that gradually expands outward from top to bottom; the inclined direction of the lower part 300b of the sidewall forms a fifth included angle θ5 with the normal direction of the surface of the silicon substrate 10; the range of the fifth included angle θ5 is: 10°<θ5≤30°, for example, the fifth included angle θ5 can be 15°, 20°, 25° or 30°; the connection between the lower part 300b of the sidewall and the upper part 200b of the sidewall forms a sixth included angle θ6, the range of the sixth included angle θ6 is 140°≤θ6<170°, for example, the sixth included angle θ6 can be 150°, 155° or 160°; The bottom wall 400b is located at the bottom of the second trench 50 and has a concave V-shaped surface. The lowest point of the bottom wall 400b is located on the central axis of the second trench 50. The bottom wall 400b of the second trench 50 is formed by etching outward from the bottom wall 400a of the first trench 40. The tilt direction of the bottom wall 400b forms a seventh included angle θ7 with the normal direction of the surface of the silicon substrate 10. The seventh included angle θ7 is greater than the third included angle θ3. The range of the seventh included angle θ7 is 45°≤θ7≤60°. For example, the seventh included angle θ7 can be 45°, 50°, 55° or 60°. The connection between the bottom wall 400b and the lower part of the side wall 300b forms an eighth included angle θ8. The range of the eighth included angle θ8 is 90°≤θ8<125°. For example, the eighth included angle θ8 can be 90°, 100°, 110° or 120°.

[0047] Step 4: Under the third pressure P3, a deposition gas and an etching gas with a flow rate ratio of R3 are introduced to grow a second epitaxial layer in the second trench, such that the growth rate of the second epitaxial layer on the upper sidewall of the second trench is greater than its growth rate on the lower sidewall and bottom wall of the second trench, thereby obtaining a third trench.

[0048] After the second trench 50 is formed, the process temperature remains at 850℃~950℃. Under the third pressure P3 (100 torr~200 torr), a deposition gas and an etching gas with a flow ratio of R3 (2.0~4.0:1) are introduced into the epitaxial chamber. Since the flow rate of the deposition gas is greater than that of the etching gas (the deposition rate is greater than the etching rate), deposition is the main process in this step, and the second epitaxial layer 60 is mainly grown inside the second trench 50. The growth time is controlled to be T3. T3∈(0, 0.5*T1]}. Under the synergistic effect of high pressure, large flow rate of deposition gas and small flow rate of etching gas, the crystal orientations of the upper 200b, lower 300b and bottom 400b of the second trench 50 are different, and the epitaxial growth exhibits anisotropy: the growth rate of the upper 200b (100 crystal plane) is faster, while the lower 30b and bottom 400b (high index crystal plane) are affected by the free path of the high pressure gas, resulting in lower gas concentration and slower growth rate. This makes the growth rate G3{ G3∈(G1, G2]} of the second epitaxial layer 60 in the upper 200b of the second trench 50 greater than the growth rate G4{ G3∈(G1, G2]} of the second epitaxial layer 60 in the lower 300b and bottom 400b of the second trench 50. G4∈(0,G3)}, thus forming the third trench 70. The upper sidewall 200b grows rapidly to form the upper sidewall 200c (100 crystal plane) of the third trench 70, while the lower sidewall 300b and bottom wall 400b grow slowly, further regularizing the shape of the conical cross-section well structure 51 (overall reduction), until the lower sidewall 300c and bottom wall 400c of the third trench 70 appear. At this time, the lower sidewall 300c and bottom wall 400c form a rhombus structure 71, with the bottom of the rhombus structure 71 facing the bottom surface of the silicon substrate 10 (e.g., Figure 5 (As shown). The lower part of the sidewall 300c includes a 611 crystal plane, and the bottom wall 400c includes a 311 crystal plane.

[0049] The sidewall of the third groove 70 includes an upper sidewall 200c and a lower sidewall 300c along the depth direction, and the bottom of the lower sidewall 300c is connected to a bottom wall 400c; the upper sidewall 200c, the lower sidewall 300c and the bottom wall 400c together form a continuous contour surface. The upper part 200c of the sidewall of the third trench 70 extends downward from the top opening of the third trench 70. The upper part 200c of the sidewall is a vertical or slightly inclined straight wall surface. The inclination direction of the upper part 200c of the sidewall forms a ninth angle θ9 with the normal direction of the surface of the silicon substrate 10. The range of the ninth angle θ9 is 0°≤θ9≤5°. For example, the ninth angle θ9 can be 0°, 1°, 3° or 5°. The lower sidewall 300c is connected to the lower end of the upper sidewall 200c and extends outward at an angle to the bottom wall 400c. The lower sidewall 300c is a continuous inclined inner surface. Compared to the lower sidewall 300b of the second trench 50, the lower sidewall 300c of the third trench 70 has a greater outward inclination. The lower sidewall 300c of the third trench 70 is formed by depositing material inward from the lower sidewall 300b of the second trench 50. The lower sidewall 300c is a wall surface that gradually expands outward from top to bottom. The inclination direction of the lower sidewall 300c forms a tenth angle θ with the normal direction of the surface of the silicon substrate 10. 10 The tenth included angle θ 10 The range is: 30°≤θ 10 ≤45°, for example, the tenth included angle θ 10 It can be 30°, 35°, 40° or 45°; the connection between the lower part 300c of the sidewall and the upper part 200c of the sidewall forms an eleventh included angle θ. 11 The eleventh included angle θ 11 The range is 130°≤θ 11 ≤150°, for example, the eleventh included angle θ 11 It can be 130°, 140°, or 150°; The bottom wall 400c is located at the bottom of the third trench 70 and has a concave V-shaped surface. The lowest point of the bottom wall 400c is located on the central axis of the third trench 70. The bottom wall 400c of the third trench 70 is formed by depositing material inward from the bottom wall 400b of the second trench 50. The tilt direction of the bottom wall 400c forms a twelfth angle θ with the normal direction of the surface of the silicon substrate 10. 12 The twelfth included angle θ 12 The range is 45°≤θ 12 ≤60°, for example, the twelfth included angle θ 12 It can be 45°, 50°, 55° or 60°; the connection between the bottom wall 400c and the lower part of the side wall 300c forms a thirteenth included angle θ. 13 The thirteenth included angle θ 13 The range is 75°≤θ 13 ≤105°, for example, the thirteenth included angle θ 13It can be 75°, 80°, 90°, 100° or 105°; like Figure 5 As shown in the cross-sectional view, the third groove 70 exhibits a composite geometric configuration of a rectangle at the top and a rhombus at the bottom. Specifically, the upper part 200c of the sidewall extends along the depth direction, forming a rectangular portion; the lower part 300c of the sidewall gradually expands along the depth direction, while the bottom wall 400c gradually narrows along the depth direction, with the lower part 300c and the bottom wall 400c forming a rhombus-shaped portion; the upper end of the rhombus-shaped portion connects to the lower end of the rectangular portion, forming a transition interface; the rectangular portion and the rhombus-shaped portion are arranged coaxially along the depth direction, forming a continuous contour surface; one diagonal of the rhombus-shaped portion is arranged along the depth direction, and the other diagonal is arranged horizontally; the rhombus-shaped portion has a narrowed section with its tip pointing downwards, and the lower tip of the rhombus-shaped portion is located at the deepest point of the third groove 70.

[0050] It is understandable that, in the above process, the etching gas slightly etches or even does not etch the upper sidewall 200a of the first trench 40 to form the upper sidewall 200b of the second trench 50. Therefore, the opening width of the upper sidewall 200a of the first trench 40 is less than or equal to the opening width of the upper sidewall 200b of the second trench 50. The second epitaxial layer 60 grows rapidly on the upper sidewall 200b of the second trench 50 to form the upper sidewall 200c of the third trench 70. Therefore, the opening width of the upper sidewall 200c of the third trench 70 is less than the opening width of the upper sidewall 200a of the first trench 40. In step 2, the growth thickness of the first epitaxial layer 30 on the upper sidewall of the initial groove 20 is 3nm~20nm. In step 4, the growth thickness of the second epitaxial layer 60 on the upper sidewall 200b of the second trench 50 is 8nm~25nm.

[0051] Step 5: Under the fourth pressure P4, a deposition gas and an etching gas with a flow rate ratio of R4 are introduced to grow a third epitaxial layer and backfill the bottom of the third trench to form a sigma trench.

[0052] After the third trench 70 is formed, the process temperature remains at 850℃~950℃. Under the fourth pressure P4 (5 torr~10 torr), a deposition gas and an etching gas with a flow ratio of R4 ((0.9~1.0):1) are introduced into the epitaxial chamber to grow the third epitaxial layer 80 at the bottom of the third trench 70. Under the synergistic effect of the extremely low pressure, deposition gas, and etching gas, the upper 200c and lower 300c of the sidewall do not grow (growth rate G5=0), while the growth rate G6{G6∈[G4,G2]} of the bottom 400c (high index crystal plane 311) is relatively fast. The third epitaxial layer 80 backfills the bottom of the third trench 70 until the 001 crystal plane (low index crystal plane) appears, at which point backfilling stops, ultimately forming the sigma trench 90 (as shown in the image). Figure 6 As shown, the lower tip of the rhomboid portion is filled in.

[0053] The specific structure of Sigma Trench 90 is as follows: Figure 7 As shown, the upper part of the sidewall has a 100 crystal plane, the lower part of the sidewall includes a 611 crystal plane, the bottom wall includes a 311 crystal plane, and the bottom end of the bottom wall includes a 001 crystal plane.

[0054] In some embodiments, the deposition gas is at least one of dichlorosilane, silane, and halogenated silane, and the etching gas is a mixture of fluorine-containing gas, hydrocarbon, and oxygen. By precisely adjusting the growth rate or etching rate, the shapes of the sharp-corner structure 41, the conical cross-section well structure 51, the rhombic structure 71, and the sigma trench 90 can be precisely controlled. The first epitaxial layer 30, the second epitaxial layer 60, and the third epitaxial layer 80 are all made of silicon, which helps to reduce the risk of defects introduced by subsequent processing.

[0055] In some embodiments, during steps 2-5 above, hydrogen is also introduced as a carrier gas simultaneously with the deposition gas and etching gas into the epitaxial chamber, with a hydrogen flow rate of 20 slm to 30 slm. Hydrogen serves as a dilution and transport medium, adjusting the partial pressure and mixing ratio of the process gases to improve the uniformity and stability of the epitaxial layer or etching process.

[0056] The following detailed description of the process for forming sigma trenches according to the present invention is provided in conjunction with specific embodiments.

[0057] Example Step S1: Provide a silicon substrate and use an inductively coupled plasma (ICP) etching process (the etching gas is a mixture of SF6 and O2) to etch an initial groove with a rectangular structure inside the silicon substrate. The initial groove has a depth of 100 nm and a width of 50 nm.

[0058] Step S2: Use deionized water to ultrasonically clean the silicon substrate for 10 minutes to remove impurities such as polymers generated during the etching process.

[0059] Step S3: Place the silicon substrate with the initial groove in the epitaxial chamber, and introduce hydrogen gas into the epitaxial chamber to perform high-temperature annealing pretreatment on the silicon substrate to remove the natural oxide layer on the surface of the silicon substrate. The annealing temperature is 950°C and the annealing time is 50s.

[0060] Step S4: Set the process temperature to 900℃. Under a first pressure of 50 torr, introduce dichlorosilane at a flow rate of 800 sccm, a fluorine-containing gas at a flow rate of 1000 sccm, a mixture of hydrocarbons and oxygen at a flow rate of 1000 sccm, and hydrogen at a flow rate of 25 slm into the epitaxial chamber. G2 is used to grow a first silicon layer in the initial trench, such that the growth rate G2 of the first silicon layer on the lower sidewall and bottom wall of the initial trench is greater than its growth rate G1 on the upper sidewall of the initial trench. The growth thickness of the first silicon layer on the upper sidewall of the initial trench is 10 nm, thus obtaining the first trench. The upper sidewall of the first trench has a 100 crystal plane, and the lower sidewall and bottom wall form a sharp corner structure.

[0061] Step S5: After the first trench is formed, the process temperature is set to 900℃. Under the second pressure of 60 torr, dichlorosilane with a flow rate of 600 sccm, a fluorine-containing gas with a flow rate of 1000 sccm, a mixture of hydrocarbons and oxygen, and hydrogen with a flow rate of 25 slm are introduced into the epitaxial chamber to etch the inner wall of the first trench. The etching rate E2 of the lower part of the sidewall and the bottom wall of the first trench is greater than the etching rate E1 of the upper part of the sidewall of the first trench, thereby obtaining the second trench. The upper part of the sidewall of the second trench is a 100 crystal plane, and the lower part of the sidewall and the bottom wall form a cone-shaped cross-section well structure.

[0062] Step S6: After the second trench is formed, the process temperature is set to 900℃. Under a third pressure of 150 torr, dichlorosilane with a flow rate of 1000 sccm, a mixture of fluorine-containing gas, hydrocarbon and oxygen with a flow rate of 300 sccm, and hydrogen with a flow rate of 25 slm are introduced into the epitaxial chamber. A second silicon layer is grown in the second trench, such that the growth rate G3 of the second silicon layer on the upper sidewall of the second trench is greater than its growth rate G4 on the lower sidewall and bottom wall of the second trench. The growth thickness of the second silicon layer on the upper sidewall of the second trench is 20 nm, thus obtaining the third trench. The upper sidewall of the third trench has a 100 crystal plane, the lower sidewall includes a 611 crystal plane, and the bottom wall includes a 311 crystal plane. The lower sidewall and the bottom wall form a rhombic structure.

[0063] In step S7, after the third trench is formed, the process temperature is set to 900℃. Under the fourth pressure of 7 torr, dichlorosilane with a flow rate of 900 sccm, a fluorine-containing gas with a flow rate of 1000 sccm, a mixture of hydrocarbons and oxygen with a flow rate of 25 slm are introduced into the epitaxial chamber. The third silicon layer is grown and backfilled to the bottom of the third trench until the 001 crystal plane appears, at which point the backfilling is stopped, and finally the sigma trench is formed.

[0064] In summary, this invention fully utilizes the different crystal orientations within the trenches and the anisotropic nature of epitaxial growth and etching. It employs a novel four-step epitaxial growth + etching synergy process to replace the traditional TMAH polycrystalline etching + cleaning process for forming sigma trenches. This addresses the problems of complex processes, poor stability, and performance degradation caused by difficult residual cleaning in existing TMAH etching processes. Furthermore, the precision of epitaxial growth parameter control can reach within ±1%, and the shape and size deviation of the sigma trenches can be controlled within ±0.5%, significantly improving process stability.

[0065] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0066] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0067] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A process for forming sigma trenches, characterized in that, Include at least the following steps: Step 1, providing a substrate with an initial groove formed thereon; Step 2: Under the first pressure P1, a deposition gas and an etching gas with a flow rate ratio of R1 are introduced to grow a first epitaxial layer in the initial groove, such that the growth rate of the first epitaxial layer on the lower sidewall and bottom wall of the initial groove is greater than its growth rate on the upper sidewall of the initial groove, thereby obtaining the first trench. Step 3: Under the second pressure P2, a deposition gas and an etching gas with a flow rate ratio of R2 are introduced to etch the inner wall of the first trench, so that the etching rate of the lower part of the sidewall and bottom wall of the first trench is greater than the etching rate of the upper part of the sidewall of the first trench, thereby obtaining the second trench. Step 4: Under the third pressure P3, a deposition gas and an etching gas with a flow rate ratio of R3 are introduced to grow a second epitaxial layer in the second trench, such that the growth rate of the second epitaxial layer on the upper sidewall of the second trench is greater than its growth rate on the lower sidewall and bottom wall of the second trench, thereby obtaining a third trench. Step 5: Under the fourth pressure P4, a deposition gas and an etching gas with a flow rate ratio of R4 are introduced to grow a third epitaxial layer to backfill the bottom of the third trench, forming a sigma trench. Among them, P3 > P1 > P4, and R3 > R1 > R2.

2. The method as described in claim 1, characterized in that, The first pressure P1 is 30 torr to 80 torr, the second pressure P2 is 30 torr to 80 torr, the third pressure P3 is 100 torr to 200 torr, and the fourth pressure P4 is 5 torr to 10 torr.

3. The method as described in claim 1, characterized in that, The flow ratio R1 ranges from (0.8 to 1.0):1, the flow ratio R2 ranges from (0.5 to 0.7):1, the flow ratio R3 ranges from (2.0 to 4.0):1, and the flow ratio R4 ranges from (0.9 to 1.0):

1.

4. The method as described in claim 1, characterized in that, In step 5, the bottom of the third trench is backfilled until the 001 crystal face appears, at which point the backfilling stops.

5. The method as described in claim 1, characterized in that, The crystal plane indexes of the lower part of the bottom wall and sidewall of the first trench are greater than the crystal plane indexes of the upper part of the sidewall of the first trench; the crystal plane indexes of the lower part of the bottom wall and sidewall of the second trench are greater than the crystal plane indexes of the upper part of the sidewall of the second trench; the crystal plane indexes of the lower part of the bottom wall and sidewall of the third trench are greater than the crystal plane indexes of the upper part of the sidewall of the third trench.

6. The method as described in claim 5, characterized in that, The upper part of the sidewalls of the first trench, the second trench, and the third trench are all 100 crystal planes; the lower part of the sidewall of the third trench includes a 611 crystal plane; and the bottom wall of the third trench includes a 311 crystal plane.

7. The method as described in claim 1, characterized in that, The substrate is a silicon substrate, the deposition gas is at least one of dichlorosilane, silane, and silane halide; the etching gas is a mixture of fluorine-containing gas, hydrocarbon and oxygen; the first epitaxial layer, the second epitaxial layer and the third epitaxial layer are all made of silicon.

8. The method as described in claim 1, characterized in that, The initial groove has a rectangular structure, the lower part of the sidewall and the bottom wall of the first groove form a pointed structure, the lower part of the sidewall and the bottom wall of the second groove form a conical cross-section trap structure, and the lower part of the sidewall and the bottom wall of the third groove form a rhomboid structure.

9. The method as described in claim 1, characterized in that, The opening width of the upper sidewall of the third trench is smaller than the opening width of the upper sidewall of the first trench, and the opening width of the upper sidewall of the first trench is less than or equal to the opening width of the upper sidewall of the second trench.

10. The method as described in claim 9, characterized in that, In step 2, the growth thickness of the first epitaxial layer on the upper sidewall of the initial groove is 3nm~20nm; in step 4, the growth thickness of the second epitaxial layer on the upper sidewall of the second trench is 8nm~25nm.

11. The method as described in claim 1, characterized in that, Steps 2-5 also include: introducing hydrogen gas as a carrier gas, with a hydrogen flow rate of 20 slm to 30 slm.

12. The method as described in claim 1, characterized in that, The process temperature is 850℃~950℃.

13. The method as described in claim 1, characterized in that, The initial groove has a depth of 50nm~200nm and a width of 30nm~100nm.

14. The method as described in claim 1, characterized in that, After step 1 and before step 2, the process also includes: performing a high-temperature annealing pretreatment on the substrate; the annealing temperature is 900℃~1000℃, and the annealing time is 30s~60s.

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