Trench schottky diode with complex p+ structure and manufacturing method thereof

Abstract

The application discloses a trench Schottky diode with a composite P+ structure and a manufacturing method thereof, and belongs to the technical field of semiconductors. The cell structure comprises a substrate, an epitaxial layer on one side surface of the substrate, the epitaxial layer comprising a wave-shaped trench and a plurality of P+ columns, the depth of the wave-shaped trench gradually decreasing from the center to the periphery of the cell structure, the plurality of P+ columns being uniformly distributed around the wave-shaped trench, an anode metal, the inner surface of the wave-shaped trench being deposited with Schottky metal, the Schottky metal forming a wave-shaped Schottky contact with the epitaxial layer, the anode metal being located in the wave-shaped trench covered by the Schottky metal and covering the surface of the epitaxial layer away from the substrate, and a cathode metal located on the surface of the substrate away from the epitaxial layer. The application not only solves the contradiction between the breakdown voltage and the on-voltage of the trench Schottky diode in high-voltage and high-frequency applications, but also improves the surge resistance of the device under extreme conditions.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of semiconductor, and particularly relates to a trench Schottky diode with a composite P+ structure and a manufacturing method thereof. BACKGROUND

[0002] With the development of power electronic systems towards high frequency, high voltage and high power density, the traditional Schottky diode faces severe challenges in terms of reverse recovery loss, high-temperature leakage current and withstand voltage capacity. The planar Schottky diode is limited in breakdown voltage due to the concentration of electric field, and the conventional trench Schottky diode can improve the breakdown voltage, but still has problems such as high reverse recovery charge and electric field peak at the bottom of the trench. Especially in the application scenarios of new energy vehicle OBC (On-Board Charger) and photovoltaic inverter, the excessive reverse recovery charge under high-frequency switching conditions will cause significant switching loss, and the high-temperature leakage current will threaten the system reliability.

[0003] Although the existing solutions such as field plate structure and composite dielectric layer can optimize the electric field distribution to some extent, it is still difficult to simultaneously consider high breakdown voltage (BV), low forward voltage (VF), low reverse leakage current (IR) and high surge resistance. Therefore, it is urgent to provide a Schottky diode with a new trench structure to improve the withstand voltage while ensuring low forward voltage and high surge resistance. SUMMARY

[0004] In order to solve the above problems in the prior art, the present application provides a trench Schottky diode with a composite P+ structure and a manufacturing method thereof. The technical problem to be solved by the present application is solved by the following technical scheme:

[0005] In a first aspect, the present application provides a trench Schottky diode with a composite P+ structure, and the cell structure of the trench Schottky diode comprises:

[0006] a substrate;

[0007] an epitaxial layer located on one side surface of the substrate; the epitaxial layer comprises a wave-shaped trench and a plurality of P+ columns; the depth of the wave-shaped trench gradually decreases from the center to the periphery of the cell structure, and the plurality of P+ columns are uniformly distributed around the wave-shaped trench;

[0008] an anode metal; the inner surface of the wave-shaped trench is deposited with a Schottky metal, which forms a wave-shaped Schottky contact with the epitaxial layer, and the anode metal is located in the wave-shaped trench covered by the Schottky metal and covers the surface of the epitaxial layer away from the substrate;

[0009] a cathode metal located on the surface of the substrate away from the epitaxial layer.

[0010] In one embodiment of the present invention, the epitaxial layer further includes a deep trench extending along a first direction, the deep trench being located at the center of the bottom of the wavy trench, and the first direction is perpendicular to the plane of the substrate and pointing from the epitaxial layer to the substrate.

[0011] In one embodiment of the present invention, a gate oxide layer is deposited on the outer surface of the deep trench.

[0012] In one embodiment of the present invention, a polysilicon layer is deposited inside the deep trench, wherein the Schottky metal forms an ohmic contact with the polysilicon layer.

[0013] In one embodiment of the present invention, in a direction perpendicular to the plane of the substrate, the orthographic projection of the wavy groove is a circle;

[0014] The epitaxial layer includes four P+ columns, and the four P+ columns are equidistant from the circle.

[0015] In one embodiment of the present invention, the Schottky metal includes titanium, molybdenum, chromium or platinum.

[0016] In one embodiment of the present invention, the material of the epitaxial layer includes silicon.

[0017] In a second aspect, the present invention further provides a method for manufacturing a trench Schottky diode with a composite P+ structure, which is used to manufacture the trench Schottky diode with a composite P+ structure described in the first aspect;

[0018] The method comprises:

[0019] Providing a substrate and growing an epitaxial layer on the surface of the substrate;

[0020] A plurality of windows arranged at intervals are formed on the surface of the epitaxial layer away from the substrate; the cross-section of the windows is an inverted trapezoid, and the opening width and depth of each window gradually decrease in a direction from the center of the cell structure to the periphery;

[0021] Each window is rounded by wet isotropic etching to form a wavy groove;

[0022] Etching a deep groove at the center of the bottom of the wavy groove;

[0023] Using an ion implantation process, a plurality of P+ columns are formed around the wavy grooves and are evenly distributed.

[0024] growing a gate oxide layer on an outer surface of the deep trench and depositing a polysilicon layer inside the deep trench;

[0025] Depositing a Schottky metal on the inner surface of the wavy groove, wherein the Schottky metal forms a wavy Schottky contact with the epitaxial layer and an ohmic contact with the polysilicon layer;

[0026] Anode metal is deposited on the surface of the epitaxial layer away from the substrate and in the wavy groove covered by Schottky metal, and cathode metal is deposited on the surface of the substrate away from the epitaxial layer to form an anode and a cathode respectively.

[0027] In one embodiment of the present invention, the step of forming a plurality of windows arranged at intervals on a surface of the epitaxial layer away from the substrate comprises:

[0028] Depositing a silicon dioxide layer on the surface of the epitaxial layer away from the substrate as a hard mask;

[0029] forming a photoresist pattern on the surface of the silicon dioxide layer, and transferring the photoresist pattern to the silicon dioxide layer by etching;

[0030] The epitaxial layer is dry-etched using the patterned silicon dioxide layer as a mask to form a plurality of windows arranged at intervals.

[0031] In one embodiment of the present invention, the step of etching a deep trench at the center of the bottom of the wavy trench comprises:

[0032] Forming a rectangular groove at the center of the bottom of the wavy groove by dry deep silicon etching;

[0033] The rectangular groove is rounded by wet isotropic etching to form a deep groove.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] (1) The present invention provides a trench Schottky diode with a composite P+ structure and a method for manufacturing the same. The cell structure of the trench Schottky diode includes: a substrate and an epitaxial layer located on the surface of the substrate. The surface of the epitaxial layer away from the substrate includes a wavy groove, and the continuously changing curvature radius of the wavy groove can make the electric field diverge uniformly, avoid the formation of a local peak electric field, and thus improve the breakdown voltage of the device.

[0036] (2) The inner surface of the wavy groove is covered with Schottky metal, which can form a Schottky contact with the epitaxial layer. The wavy structure of the sidewall and bottom of the wavy groove can effectively expand the Schottky contact area, which is beneficial to reducing the current density at rated current and the forward conduction voltage of the device.

[0037] (3) The deep trench is located at the center of the bottom of the wavy trench, and a gate oxide layer is deposited on the outer surface, which can further increase the breakdown voltage of the device.

[0038] Therefore, the composite P+ structure trench Schottky diode and its manufacturing method provided in this application can solve the contradiction between the breakdown voltage and the conduction voltage of traditional trench Schottky diodes in high-voltage and high-frequency applications, while improving the device's surge resistance under extreme conditions.

[0039] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 1 is a top view of a cell structure of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0041] Figure 2 is a cross-sectional view of a cellular structure provided by an embodiment of the present invention in the diagonal AA' direction;

[0042] Figure 3 is a schematic diagram of the arrangement of multiple cellular structures provided by an embodiment of the present invention;

[0043] Figure 4 This is a flow chart of a method for manufacturing a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0044] Figure 5 This is a schematic diagram of a manufacturing process of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0045] Figure 6 This is a schematic diagram of a manufacturing process of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0046] Figure 7 This is a schematic diagram of a manufacturing process of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0047] Figure 8 This is a schematic diagram of a manufacturing process of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0048] Figure 9 This is a schematic diagram of a manufacturing process of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0049] Figure 10 This is a schematic diagram of a manufacturing process of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0050] Figure 11 This is a schematic diagram of a manufacturing process of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present invention;

[0051] Figure 12is a schematic diagram of a manufacturing process of the trench Schottky diode with a composite P+ structure provided by the embodiment of the present application. DETAILED DESCRIPTION

[0052] The present application will be further described in conjunction with specific embodiments, but the embodiments of the present application are not limited thereto.

[0053] Figure 1 is a top view of the cell structure of the trench Schottky diode with a composite P+ structure provided by the embodiment of the present application, Figure 2 is a cross-sectional view of the cell structure in the diagonal line AA' direction. As shown in Figures 1 and 2 The embodiment of the present application provides a trench Schottky diode with a composite P+ structure, and the cell structure of the trench Schottky diode includes:

[0054] a substrate 1;

[0055] an epitaxial layer 2 located on one side surface of the substrate 1; the epitaxial layer 2 includes a wave-shaped trench 21 and a plurality of P+ columns 22; the depth of the wave-shaped trench 21 gradually decreases from the center to the periphery of the cell structure, and the plurality of P+ columns 22 are uniformly distributed around the wave-shaped trench 21;

[0056] an anode metal 3; an inner surface of the wave-shaped trench 21 is deposited with a Schottky metal 23, the Schottky metal 23 forms a wave-shaped Schottky contact with the epitaxial layer 2, the anode metal 3 is located in the wave-shaped trench 21 covered by the Schottky metal 23, and covers the surface of the epitaxial layer 2 away from the substrate 1;

[0057] a cathode metal 4 located on the surface of the substrate 1 away from the epitaxial layer 2.

[0058] Specifically, the above-mentioned cell structure of the trench Schottky diode includes a substrate 1 and an epitaxial layer 2 located on the upper surface of the substrate 1, the surface of the epitaxial layer 2 away from the substrate 1 includes a wave-shaped trench 21, in Figure 1 the perspective view, the orthographic projection of the wave-shaped trench 21 is a circle, and in Figure 2 the perspective view, the bottom and the sidewall of the wave-shaped trench 21 are wave-shaped, and the depth thereof in the vertical direction (the direction perpendicular to the plane on which the substrate 1 is located) gradually decreases from the center to the periphery of the cell structure, so that the cross section of the cell structure in the diagonal line direction, i.e. Figure 2 the AA' direction shown in

[0059] In related art, trench Schottky diodes typically use a vertical trench structure with a right-angled or V-shaped trench bottom. This design leads to electric field concentration at the bottom of the trench, especially at locations with geometrical changes (sudden changes in the radius of curvature). Furthermore, because the peak electric field at the bottom of the trench is much higher than the average field strength, it can easily lead to localized premature breakdown, limiting the reverse breakdown voltage of the device. In this embodiment, the introduction of the wavy trench 21 allows its continuously varying radius of curvature to evenly distribute the electric field, avoiding the formation of local peak electric fields and thereby increasing the breakdown voltage of the device.

[0060] Schottky metal 23 such as titanium, molybdenum, chromium, platinum, etc. is also deposited on the inner surface of the wavy groove 21. Silicon can be selectively used as the material of the epitaxial layer 2, so that the Schottky metal 23 can form a Schottky contact with the epitaxial layer 2. The wavy structure of the sidewalls and bottom of the wavy groove 21 can effectively expand the Schottky contact area, which is beneficial to reducing the current density under rated current and the forward conduction voltage of the device.

[0061] Please continue to see Figure 1-2 In the cellular structure, four P+ columns 22 are evenly distributed around the wavy groove 21. In the direction perpendicular to the plane of the substrate 1, the positive projection of the wavy groove is a circle, and the four P+ columns 22 are equidistant from the circle.

[0062] It should be understood that the cell structure is the smallest repeating unit of the trench Schottky diode. Figure 3 is a schematic diagram of the arrangement of multiple cellular structures provided by an embodiment of the present invention, such as Figure 3 As shown, the four cellular structures are arranged in an array. The P+ column 22 can not only improve the surge resistance of the device, but also solve the problem of insufficient depletion in the reverse direction due to the long distance between the wavy grooves 21 in the two adjacent cellular structures, making the reverse electric field distribution more uniform and avoiding premature breakdown.

[0063] The cell structure of the trench Schottky diode also includes an anode metal 3 and a cathode metal 4. The anode metal 3 is located in the wavy trench 21 covered by the Schottky metal 23 and covers the surface of the epitaxial layer 2 away from the substrate 1. The cathode metal 4 is located on the lower surface of the substrate 1.

[0064] To avoid the residual electric field concentration at the bottom of the wavy trench 21, the epitaxial layer 2 further comprises a deep trench 24 extending along a first direction, which is a direction perpendicular to the plane where the substrate 1 is located and directed from the epitaxial layer 2 to the substrate 1. Optionally, the deep trench 24 is located at the center of the bottom of the wavy trench 21. The deep trench 24 further extends the residual electric field lines downward by introducing an additional abrupt radius of curvature point, so as to avoid the center of the bottom of the wavy trench from becoming a new electric field peak area. Based on the above analysis, it can be seen that the trench Schottky diode with the composite P+ structure provided by the present application can solve the contradiction between the breakdown voltage and the on-state voltage of the conventional trench Schottky diode in high-voltage high-frequency applications, and at the same time, the surge resistance of the device under extreme conditions is improved.

[0065] In addition, the outer surface of the deep trench 24 is deposited with a gate oxide layer 25, and the material of the gate oxide layer 25 can be SiO2, and the thickness is 0.1-0.4 μm. It should be understood that the thermally grown SiO2 is a high-quality insulator with dense structure and relatively few defects, and the intrinsic breakdown field strength thereof is very high, about 10 MV / cm, that is, about 10 million volts per centimeter thickness can be withstood. In the present embodiment, the main function of the gate oxide layer 25 is insulation and control of the channel, and the high-quality gate oxide layer 25 can provide reliable electrical isolation, avoiding the low breakdown problem caused by direct metal-semiconductor contact. In addition, the gate oxide layer 25 in combination with the silicon surface state below it helps to form a smoother and more controllable surface potential distribution, and good gate oxide / silicon interface quality (low interface state density, low fixed charge) can reduce the distortion or local enhancement of the electric field at the interface, avoiding the generation of unexpected low breakdown points, thereby improving the breakdown voltage of the device.

[0066] Optionally, a polysilicon layer 26 is deposited in the interior of the deep trench 24, wherein the Schottky metal 23 forms an ohmic contact with the polysilicon layer 26.

[0067] Figure 4 is a flowchart of a manufacturing method of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present application, Figure 5-12 is a schematic diagram of a manufacturing process of a trench Schottky diode with a composite P+ structure provided by an embodiment of the present application. As shown in Figure 2-12 , the present embodiment provides a manufacturing method of a trench Schottky diode with a composite P+ structure for manufacturing a trench Schottky diode with a composite P+ structure;

[0068] The above method comprises:

[0069] S1, as shown in Figure 5 , a substrate 1 is provided, and an epitaxial layer 2 is grown on the surface of the substrate 1.

[0070] S2. A plurality of spaced windows are formed on the surface of the epitaxial layer 2 away from the substrate 1. The cross section of the windows is an inverted trapezoid, and the opening width and depth of each window gradually decrease from the center of the cell structure to the periphery.

[0071] For example, see Figure 6-7 , step S2 includes:

[0072] S201, depositing a silicon dioxide layer on the surface of the epitaxial layer 2 away from the substrate 1 as a hard mask;

[0073] S202, forming a photoresist pattern on the surface of the silicon dioxide layer, and transferring the photoresist pattern to the silicon dioxide layer by etching;

[0074] S203 , using the patterned silicon dioxide layer as a mask, dry-etching the epitaxial layer 2 to form a plurality of windows arranged at intervals.

[0075] It should be noted that the number and size of the windows should be flexibly adjusted according to the required curvature radius and depth of the wavy groove, and this application does not impose any restrictions on this.

[0076] S3, such as Figure 8 As shown, each window is rounded by wet isotropic etching to form a wavy groove 21.

[0077] S4. Etching to form a deep trench 24 at the center of the bottom of the wavy trench 21.

[0078] Specifically, if Figure 9-10 As shown, firstly, a rectangular trench is formed at the center of the bottom of the wavy trench 21 by dry deep silicon etching, and then the rectangular trench is rounded by wet isotropic etching to form a deep trench 24.

[0079] S5, see Figure 11 , using an ion implantation process, a plurality of P+ columns 22 are evenly distributed around the wavy groove 21 .

[0080] S6 , growing a gate oxide layer 25 on the outer surface of the deep trench 24 , and depositing a polysilicon layer 26 inside the deep trench 24 .

[0081] In this step, after the gate oxide layer 25 is grown on the outer surface of the deep trench 24 , a polysilicon layer 26 is deposited on the upper surface of the cell structure, and then the polysilicon layer 26 is etched, leaving only the polysilicon layer 26 inside the deep trench 24 .

[0082] S7 . Depositing a Schottky metal 23 on the inner surface of the wavy groove 21 . The Schottky metal 23 forms a wavy Schottky contact with the epitaxial layer 2 and an ohmic contact with the polysilicon layer 26 .

[0083] S8, depositing anode metal 3 on the surface of the epitaxial layer 2 away from the substrate 1 and in the wavy groove 21 covered by the Schottky metal 23, and depositing cathode metal 4 on the surface of the substrate 1 away from the epitaxial layer 2 to form an anode and a cathode respectively, to obtain Figure 2 The trench Schottky diode with a composite P+ structure is shown.

[0084] It can be seen from the above embodiments that the beneficial effects of the present invention are:

[0085] (1) The present invention provides a trench Schottky diode with a composite P+ structure and a method for manufacturing the same. The cell structure of the trench Schottky diode includes: a substrate and an epitaxial layer located on the surface of the substrate. The surface of the epitaxial layer away from the substrate includes a wavy groove, and the continuously changing curvature radius of the wavy groove can make the electric field diverge uniformly, avoid the formation of a local peak electric field, and thus improve the breakdown voltage of the device.

[0086] (2) The inner surface of the wavy groove is covered with Schottky metal, which can form a Schottky contact with the epitaxial layer. The wavy structure of the sidewall and bottom of the wavy groove can effectively expand the Schottky contact area, which is beneficial to reducing the current density at rated current and the forward conduction voltage of the device.

[0087] (3) The deep trench is located at the center of the bottom of the wavy trench, and a gate oxide layer is deposited on the outer surface, which can further increase the breakdown voltage of the device.

[0088] Therefore, the composite P+ structure trench Schottky diode and its manufacturing method provided in this application can solve the contradiction between the breakdown voltage and the conduction voltage of traditional trench Schottky diodes in high-voltage and high-frequency applications, while improving the device's surge resistance under extreme conditions.

[0089] In the description of the present invention, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0090] Descriptions with reference to the terms "one embodiment," "some embodiments," "examples," "specific examples," or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification.

[0091] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.

Claims

1. A trench Schottky diode with a composite P+ structure, characterized in that: The cell structure of the trench Schottky diode includes: substrate; An epitaxial layer located on one surface of the substrate; the epitaxial layer includes a wavy groove and a plurality of P+ columns; the depth of the wavy groove gradually decreases from the center of the cell structure to the periphery, and the plurality of P+ columns are evenly distributed around the wavy groove; Anode metal; a Schottky metal is deposited on the inner surface of the wavy groove, the Schottky metal forms a wavy Schottky contact with the epitaxial layer, and the anode metal is located in the wavy groove covered by the Schottky metal and covers the surface of the epitaxial layer away from the substrate; The cathode metal is located on the surface of the substrate away from the epitaxial layer.

2. The trench Schottky diode of the composite P+ structure according to claim 1, characterized in that: The epitaxial layer further includes a deep trench extending along a first direction, the deep trench being located at the center of the bottom of the wavy trench. The first direction is perpendicular to the plane of the substrate and pointing from the epitaxial layer to the substrate.

3. The trench Schottky diode of the composite P+ structure according to claim 2, characterized in that: A gate oxide layer is deposited on the outer surface of the deep trench.

4. The trench Schottky diode of the composite P+ structure according to claim 3, characterized in that: A polysilicon layer is deposited inside the deep trench, wherein the Schottky metal forms an ohmic contact with the polysilicon layer.

5. The trench Schottky diode of the composite P+ structure according to claim 1, characterized in that: In a direction perpendicular to the plane of the substrate, the orthographic projection of the wavy groove is a circle; The epitaxial layer includes four P+ columns, and the four P+ columns are equidistant from the circle.

6. The trench Schottky diode of the composite P+ structure according to claim 1, characterized in that: The Schottky metal includes titanium, molybdenum, chromium or platinum.

7. The trench Schottky diode of the composite P+ structure according to claim 1, characterized in that: The material of the epitaxial layer includes silicon.

8. A method for manufacturing a trench Schottky diode with a composite P+ structure, characterized in that: Used to manufacture the composite P+ structure trench Schottky diode according to any one of claims 1 to 7; The method comprises: Providing a substrate and growing an epitaxial layer on the surface of the substrate; A plurality of windows arranged at intervals are formed on the surface of the epitaxial layer away from the substrate; the cross-section of the windows is an inverted trapezoid, and the opening width and depth of each window gradually decrease in a direction from the center of the cell structure to the periphery; Each window is rounded by wet isotropic etching to form a wavy groove; Etching a deep groove at the center of the bottom of the wavy groove; Using an ion implantation process, a plurality of P+ columns are formed around the wavy grooves and are evenly distributed. growing a gate oxide layer on an outer surface of the deep trench and depositing a polysilicon layer inside the deep trench; Depositing a Schottky metal on the inner surface of the wavy groove, wherein the Schottky metal forms a wavy Schottky contact with the epitaxial layer and an ohmic contact with the polysilicon layer; Anode metal is deposited on the surface of the epitaxial layer away from the substrate and in the wavy groove covered by Schottky metal, and cathode metal is deposited on the surface of the substrate away from the epitaxial layer to form an anode and a cathode respectively.

9. The method for manufacturing a trench Schottky diode with a composite P+ structure according to claim 8, characterized in that: The step of forming a plurality of windows arranged at intervals on a surface of the epitaxial layer away from the substrate comprises: Depositing a silicon dioxide layer on the surface of the epitaxial layer away from the substrate as a hard mask; forming a photoresist pattern on the surface of the silicon dioxide layer, and transferring the photoresist pattern to the silicon dioxide layer by etching; The epitaxial layer is dry-etched using the patterned silicon dioxide layer as a mask to form a plurality of windows arranged at intervals.

10. The method for manufacturing a trench Schottky diode with a composite P+ structure according to claim 8, characterized in that: The step of etching a deep groove at the center of the bottom of the wavy groove comprises: Forming a rectangular groove at the center of the bottom of the wavy groove by dry deep silicon etching; The rectangular groove is rounded by wet isotropic etching to form a deep groove.

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