Triangular fillet-based photoconductive switch device and manufacturing and packaging method thereof

Abstract

The invention discloses a photoconductive switching device based on a triangular fillet shape and a manufacturing and packaging method, and mainly solves the problem of low light energy utilization rate of the photoconductive switching device in the prior art. The invention relates to a semi-insulating LED (light emitting diode), which comprises a semi-insulating substrate (1), a reflecting mirror (2), a metal electrode (3), an external connection electrode (4) and a packaging material (5), the semi-insulating substrate adopts a triangular fillet shape, and an included angle alpha is formed between the side surface of the semi-insulating substrate and the reflecting mirror; the length of the reflecting mirror is only half of the bottom length of the triangular substrate, the reflecting mirror is positioned on the left half surface or the right half surface of the triangular substrate, and the other half surface serves as a light incident surface; the triangular substrate carries out total reflection on the entered trigger light, and the light after total reflection is reflected back to the substrate through the reflector for secondary total reflection; the metal electrode is located on the semi-insulating substrate and is connected with the external connection electrode, and the packaging material is poured outside the whole device. The device improves the utilization rate of light energy, is simple in structure, and can be used for an ultra-wideband electromagnetic pulse generator and a solid-state compact pulse power supply.

Description

Technical Field

[0001] The invention belongs to the field of microelectronic technology, and in particular relates to a photoconductive switch device which can be used for an ultra-wideband electromagnetic pulse generator and a solid-state compact pulse power supply. Background Art

[0002] Photoconductive switching devices refer to a new type of device that uses laser pulses of a certain wavelength to stimulate a large number of photogenerated carriers in the body of the semiconductor device, control the conductivity of the material, and thus achieve conduction and shutdown. Photoconductive switches have the advantages of fast closing time, small jitter time, high repetition frequency, no electromagnetic interference, and miniaturization. However, a key problem currently faced by photoconductive switching devices is that the utilization rate of laser energy is not high. Due to the limited absorption of light energy by the substrate material, a considerable part of the light energy penetrates the device and is lost, resulting in low light energy utilization. Improving the utilization rate of triggering laser light energy in photoconductive switching devices can improve the photoelectric conversion efficiency, generate more photogenerated carriers, increase the on-state current, and thus increase the output power of photoconductive switching devices.

[0003] Patent document with application number 202310744520.2 discloses an optical structure for improving the light energy utilization rate of gallium arsenide photoconductive switch. It uses various optical lenses and reflectors to improve the light energy utilization rate of the photoconductive switch. The optical structure consists of a photoconductive switch, a cylindrical lens, four reflectors, and a plano-concave cylindrical mirror; wherein the centers of the first reflector, the cylindrical lens, the photoconductive switch, and the second reflector are arranged horizontally from top to bottom in sequence; the left end face of the cylindrical lens is bonded to the reflective surface of the third reflector, and the right end face is bonded to the reflective surface of the fourth reflector, and the axis passes through the center of the third and fourth reflectors; a plano-concave cylindrical mirror is arranged inside the circular through hole in the middle of the first reflector; the gallium arsenide photoconductive switch, the third reflector, the first reflector, the plano-concave cylindrical mirror, and the fourth reflector constitute a closed space containing a cylindrical lens. This structural design can make the incident pulse laser be reflected through the substrate material multiple times, increase the optical path of the trigger laser in the device, so that the trigger laser is fully absorbed by the photoconductive substrate, improve the utilization rate of the pulse laser energy, and reduce the on-resistance of the photoconductive switch. Although this structure can improve the light energy utilization rate of the photoconductive switch and achieve high photoelectric conversion efficiency to a certain extent, the structure is relatively cumbersome and complex, and the optical path is difficult to align. If the relative position of the lens and the through hole is slightly offset, it will have a greater impact on the output. At the same time, this structure will also increase the volume of the device, which is not conducive to the compactness and integration of the photoconductive switch device.

[0004] The patent document with application number CN202310681018.1 discloses an efficient photoconductive switch optical system based on a grating structure. It uses a grating structure to improve the light energy utilization rate of the photoconductive switch. By preparing a grating structure composed of a series of parallel microgrooves in the light incident area, the absorption of the laser is enhanced by the reflection effect of the grating. At the same time, the photoelectric conversion efficiency can be further improved by changing parameters such as the grating period. Although this system can improve the photoelectric conversion efficiency of the photoconductive switch device, due to the strict design and production of its grating structure, it is easy to cause increased scattering and reflection of light, reducing the utilization rate of light energy by the photoconductive switch.

[0005] In order to improve the utilization rate of light energy, a common solution is to introduce anti-reflection film and reflective film on the light incident surface and backlight surface of the switch respectively to reduce the loss of laser light energy on the light incident surface, and at the same time allow the laser that is not absorbed by the substrate to pass through the conduction area for a second time, increase the optical path of the light in the device body, and thus improve the utilization rate of light. This solution has the following problems: First, there is a problem of mismatch between the film material and the substrate lattice.

[0006] Secondly, during the testing of the photoconductive switch, it is necessary to undergo multiple laser irradiations, which may cause damage to the film material, directly affecting the reliability of the photoconductive switch and even causing failure.

[0007] Third, thermal stress will be generated between the multi-layer stacked structures, which will cause the film to crack and reduce the anti-reflection and anti-transmission effects of the film. Summary of the invention

[0008] The purpose of the present invention is to address the deficiencies of the above-mentioned prior art and provide a photoconductive switch device based on a triangular rounded corner and a manufacturing and packaging method to simplify the device structure and improve the working stability of the device while improving the utilization rate of light energy.

[0009] The key technology to achieve the purpose of the present invention is: involving a triangular rounded substrate structure, utilizing the total reflection of the substrate itself and an external reflector to improve the light energy utilization rate of the photoconductive switch; simplifying the device structure by selecting simple and controllable optical components to improve the stability of the device operation.

[0010] The technical solution of the present invention is achieved in this way:

[0011] 1. A photoconductive switch device based on a triangular rounded corner, comprising a semi-insulating substrate, a reflector, a metal electrode, an external connection electrode and a packaging material, characterized in that:

[0012] The semi-insulating substrate is in a triangular shape with rounded corners, and an angle α is set between the side surface and the reflector;

[0013] The reflector has a length of only half the side length of the triangular substrate and is located on the left or right half of the triangular substrate. The other half serves as the light incident surface and has the same thickness as the semi-insulating substrate.

[0014] The triangular substrate totally reflects the incoming trigger light, and the totally reflected light is then reflected back to the substrate through a reflector for secondary total reflection, thereby improving the utilization rate of light energy of the device.

[0015] Preferably, the triangular substrate is made of any one of the semi-insulating materials such as silicon carbide, gallium nitride, gallium arsenide, gallium oxide or diamond, has a thickness of 0.35mm-1mm, a side length of 6-14mm, and an angle α between the side and the reflector is 1°~5°.

[0016] Preferably, the metal electrode is located in the middle of the upper and lower surfaces of the semi-insulating substrate and is shaped like a rounded triangle. It is an ohmic contact and the material is one of a Ni / Ti / Pt / Au composite metal layer, a Ti / Pt / Au composite metal layer, a W / Ti / Au composite metal layer, a Ni / Ti / Au composite metal layer or a Ni / Au composite metal layer.

[0017] Preferably, the external connection electrode is made of copper metal material, and is in the shape of a rectangular strip with a length of 6mm-10mm, a width of 2mm-4mm, and a thickness of 0.05mm-0.1mm, covering half of the surface of the metal electrode.

[0018] Preferably, the packaging material is epoxy resin, which tightly covers the entire device surface and is dense and air-free.

[0019] 2. A method for manufacturing a triangular rounded photoconductive switch device, characterized in that it comprises the following steps:

[0020] S1: Semi-insulating substrate is cleaned;

[0021] S2: evenly apply photoresist on the front side of the cleaned semi-insulating substrate;

[0022] S3: photolithography and development are performed on the semi-insulating substrate coated with photoresist through a mask to form a plurality of chamfered triangular electrode-shaped groove areas, and metal is evaporated, sputtered, and electroplated on the front side of the semi-insulating substrate to form a metal layer;

[0023] S4: stripping and removing all the metal outside the front electrode area of ​​the semi-insulating substrate to form a metal electrode;

[0024] S5: Clean the back side of the semi-insulating substrate again

[0025] S6: evenly apply photoresist on the back side of the cleaned semi-insulating substrate;

[0026] S7: photolithography and development are performed on the back side of the semi-insulating substrate coated with photoresist through a mask to form a chamfered triangular electrode-shaped groove area, and metal is evaporated, sputtered, and electroplated on the back side of the semi-insulating substrate to form a metal layer;

[0027] S8: removing all the metal outside the metal electrode region on the back side of the semi-insulating substrate to form a metal electrode;

[0028] S9: External connection electrodes are welded on the metal electrodes on the upper and lower surfaces of the semi-insulating substrate respectively, the photoconductive switch device is fixed in the packaging mold through the external connection electrodes, and then the reflector is fixed by glue to encapsulate the entire device.

[0029] S10: Cooling until the epoxy resin glue solidifies, opening the packaging mold, and obtaining the photoconductive switch device.

[0030] 3. A packaging method for a triangular rounded photoconductive switch device, characterized by comprising the following implementation steps:

[0031] 1) Use Teflon material to prepare the packaging mold of the mortise and tenon structure, which has a glue injection hole, and the mold is required to be tightly fastened to ensure that there is no glue leakage;

[0032] 2) Fix the position of the photoconductive switch device by using the external electrode that has been welded on the metal electrode, fix the photoconductive switch device in the middle of the mold, and tightly buckle the mold together;

[0033] 3) Glue is poured through the glue injection holes of the mold. When the glue can just completely immerse the device, the reflector is placed on the side close to the device. It is required that the reflector and the flowing glue surface are kept horizontal and at a slight angle to the device. Then a small amount of packaging glue is poured to completely immerse the entire structure.

[0034] 4) Evacuate the packaging glue, cool it and wait for it to solidify;

[0035] 5) Open the mold to obtain the packaged device.

[0036] 4. A mold for packaging a photoconductive switch device, comprising a bottom part and a top part, wherein the bottom part and the top part are tightly matched with each other through a protruding tenon and a recessed mortise to form a mortise and tenon packaging structure, characterized in that:

[0037] The bottom is provided with a square groove with a depth of 1.5mm-2.0mm close to the bottom edge, and oblique shallow grooves with a depth of 0.05mm-0.1mm are provided on both sides of the groove. The two oblique shallow grooves are both provided with an angle α with the bottom edge so that the device and the reflector maintain an angle of 1° to 5°;

[0038] The top is provided with a square groove with a depth of 0.5mm-0.8mm close to the bottom edge, which corresponds to the square groove at the bottom.

[0039] Compared with the prior art, the present invention has the following advantages:

[0040] Firstly, the present invention improves the light energy utilization rate of the photoconductive switch device by utilizing the total reflection of the photoconductive open tube substrate in the shape of a rounded triangle and the reflector.

[0041] Secondly, the present invention improves the light energy utilization rate of the device by only adding an external reflector, which not only makes the process simple and controllable, but also improves the working stability of the device.

[0042] Thirdly, the present invention avoids the local electric field enhancement caused by the tip edge and corner of the device through the rounded structure design, thereby reducing the risk of device breakdown.

[0043] Fourthly, the present invention designs an oblique shallow groove in the packaging structure to place the external electrode, and utilizes the angle between the photoconductive switch device and the reflector to prevent the risk of the laser being damaged by the reflected light, while also making the trigger light evenly distributed in the substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 Device structure diagram of the present invention;

[0045] Figure 2 Preparation of the present invention Figure 1 Schematic diagram of the device process;

[0046] Figure 3 The present invention encapsulates Figure 1 Device flow chart;

[0047] Figure 4 A structural diagram of the mold used in the packaging of the present invention. DETAILED DESCRIPTION

[0048] The specific examples of the present invention are described in detail below with reference to the accompanying drawings.

[0049] Example 1: A light-guided switch based on a triangular rounded corner

[0050] Reference Figure 1 This example is based on a triangular rounded photoconductive switch, comprising a semi-insulating substrate 1, a reflector 2, a metal electrode 3, an external connection electrode 4 and a packaging material 5, wherein:

[0051] The semi-insulating substrate 1 is made of silicon carbide material and is in the shape of an equilateral triangle with a side length of 10 mm and a thickness of 1 mm. The three corners of the equilateral triangle are chamfered with a radius of 1 mm to form a rounded equilateral triangle shape.

[0052] The reflector 2 is a 5mm×1mm rectangular shape, located on the left half of the side surface of the triangular substrate 1, and maintains an angle of 2° with the side surface of the substrate to ensure that the two do not contact each other.

[0053] The metal electrode 3 is located in the middle of the substrate 1 and is in the shape of a rounded regular triangle with a side length of 6 mm. The material is a Ni / Ti / Pt / Au composite metal layer, and the thickness of each layer is 300nm / 200nm / 200nm / 2000nm respectively;

[0054] The external connection electrode 4 is made of copper material. The external connection electrode on the upper surface covers the right side of the upper surface metal electrode, and the external connection electrode on the lower surface covers the left side of the lower surface metal electrode. The external connection electrode is partially encapsulated in epoxy resin and partially exposed to the air.

[0055] The packaging material 5 is made of epoxy resin glue and covers the entire surface of the semi-insulating substrate, the metal electrode and the reflector.

[0056] Example 2: Fabrication of a light-conductive switch based on a triangular rounded corner

[0057] Reference Figure 2 The method of making a triangular rounded light-conductive switch in this example includes the following steps:

[0058] Step 1: Clean the semi-insulating substrate, such as Figure 2 As shown in a.

[0059] The semi-insulating substrate wafer was sequentially placed in BOE solution, deionized water, acetone, isopropanol, and deionized water for ultrasonic cleaning for 120 s, 180 s, 600 s, 180 s, and 180 s, respectively;

[0060] Step 2: Apply glue to the front side of the semi-insulating substrate, such as Figure 2 As shown in b.

[0061] The photoresist is evenly applied to the front side of the cleaned semi-insulating substrate.

[0062] Step 3: Prepare a metal layer on the front side of the semi-insulating substrate, such as Figure 2 As shown in c.

[0063] The front side of the semi-insulating substrate after the glue coating is photolithographically developed through a mask with a prepared rounded triangle pattern to prepare the required electrode pattern area, and then nickel metal is evaporated and deposited on the front side of the semi-insulating substrate, and then titanium, platinum and gold are sputtered. In this example, it is set but not limited to that the thickness of the evaporated nickel is 300nm, and the thickness of the sputtered titanium, platinum and gold are 200nm, 200nm and 2000nm respectively.

[0064] Step 4: Prepare a metal electrode on the front side of the semi-insulating substrate, such as Figure 2 As shown in d.

[0065] The metal layer outside the metal electrode area on the front side of the semi-insulating substrate is completely stripped off to form a Ni / Ti / Pt / Au metal electrode;

[0066] Step 5: Clean the back side of the semi-insulating substrate, such as Figure 2 As shown in e.

[0067] The back side of the semi-insulating substrate is ultrasonically cleaned with BOE solution, deionized water, acetone, isopropanol, and deionized water in sequence. The cleaning time of each solution is set to be 120s, 180s, 600s, 180s, and 180s respectively in this example, but not limited to;

[0068] Step 6: Apply glue to the back of the semi-insulating substrate, such as Figure 2 f.

[0069] Evenly apply photoresist on the back side of the semi-insulating substrate after back side cleaning;

[0070] Step 7: Prepare a metal layer on the back side of the semi-insulating substrate, such as Figure 2 As shown in g.

[0071] The back side of the semi-insulating substrate after the glue coating is photolithographically developed through a prepared rounded triangle-shaped mask to prepare the required electrode pattern area, and then nickel metal is evaporated and deposited on the back side of the semi-insulating substrate, and then titanium, platinum and gold are sputtered. In this example, but not limited to, the thickness of the evaporated nickel is set to 300nm, and the thickness of the sputtered titanium, platinum and gold is set to 200nm, 200nm and 2000nm respectively.

[0072] Step 8: Prepare a metal electrode on the back side of the semi-insulating substrate, such as Figure 2 h.

[0073] The metal outside the metal electrode area on the back side of the semi-insulating substrate is completely stripped off to form a back side metal electrode;

[0074] Step 9: Fill the packaging mold with glue to fix the device structure, such as Figure 2 As shown in i.

[0075] Weld the external connection electrodes to the right side of the metal electrode on the upper surface of the semi-insulating substrate and the left side of the metal electrode on the lower surface, and then fix the device in the packaging mold through the external connection electrodes. When the epoxy resin glue just covers the semi-insulating substrate, place a reflector on the left side of the semi-insulating substrate, and fill the packaging glue to cover the entire device structure.

[0076] Step 10: After the mold is filled with glue, it is cooled until it solidifies, and then the mold is opened to obtain a photoconductive switch device, such as Figure 2 j shown.

[0077] Example 3: Triangular rounded-corner photoconductive switch package

[0078] Reference Figure 3 The method of packaging a triangular rounded-corner photoconductive switch in this example includes the following steps:

[0079] The first step is to use a packaging mold made of Teflon material with a mortise and tenon structure. The mold is divided into a top and a bottom. At the same time, a glue injection hole is prepared on the side of the packaging mold. The top and bottom of the mold can be tightly buckled without glue leakage;

[0080] The second step is to fix the photoconductive switch device in the middle of the bottom groove of the mold through the external electrode of the photoconductive switch device, and then cover the top part on the bottom part, and tightly buckle the mold together through the mortise and tenon structure;

[0081] The third step is to place the mold on a horizontal plane and inject glue through the injection holes of the mold to ensure that the liquid level of the epoxy resin glue can remain horizontal. When the glue can just completely immerse the device, place the reflector on the left side of the semi-insulating substrate, adjust the position of the reflector so that the reflector faces the left half of the semi-insulating substrate, and then inject a small amount of packaging glue to completely immerse the entire structure.

[0082] The fourth step is to place the entire mold after the glue is poured in a sealed environment, use a vacuum pump to extract the gas in the environment, exhaust the air in the packaging glue, and then cool it until the epoxy resin glue solidifies;

[0083] The fifth step is to open the mold to obtain the packaged device.

[0084] Example 4: Mold for packaging a triangular rounded-corner optical switch

[0085] Reference Figure 4 The mold used for encapsulating the triangular rounded-corner optical switch in this example includes a bottom 1 and a top 2 and is made of Teflon material.

[0086] The bottom 1 is provided with a square groove 12 close to the bottom edge of the bottom; the depth of the groove is 1.5mm, and its two side edges are provided with oblique shallow grooves 11 with a depth of 0.05mm. The two oblique shallow grooves 11 are both set at an angle of 2° with the bottom edge so that the device and the reflector maintain an angle of 2°. Tenons are also provided at the four top corners of the bottom 1.

[0087] The top 2 is provided with a square groove 21 close to the bottom edge of the top, the depth of the groove is 0.5 mm, which corresponds to the square groove 12 of the bottom. Mortises corresponding to the tenons of the bottom are also provided at the four corners of the top 2.

[0088] When in use, the photoconductive switch device is first fixed to the bottom 1 through the external electrode, and then the tenon of the bottom 1 and the mortise of the top 2 are buckled together, and then the subsequent glue filling is carried out.

[0089] The above descriptions are only a few specific examples of the present invention and do not constitute any limitation to the present invention. It is obvious that for professionals in this field, after understanding the content and principles of the present invention, they may make various modifications and changes in form and details without departing from the principles and structures of the present invention. However, these modifications and changes based on the ideas of the present invention are still within the scope of protection of the claims of the present invention.

Claims

1. A photoconductive switch device based on a triangular rounded corner, comprising a semi-insulating substrate (1), a reflector (2), a metal electrode (3), an external connection electrode (4) and a packaging material (5), characterized in that: The semi-insulating substrate (1) is in a triangular shape with rounded corners, and an angle α is provided between the side surface and the reflector; The reflector (2) has a length of only half the side length of the triangular substrate and is located on the left half or right half of the triangular substrate. The other half serves as the light incident surface and has the same thickness as the semi-insulating substrate. The triangular substrate totally reflects the incoming trigger light, and the totally reflected light is then reflected back to the substrate through a reflector for secondary total reflection, thereby improving the utilization rate of light energy of the device.

2. The device according to claim 1, characterized in that The semi-insulating substrate (1) is made of any one of the semi-insulating materials such as silicon carbide, gallium nitride, gallium arsenide, gallium oxide or diamond, has a thickness of 0.35 mm to 1 mm, a side length of 6 to 14 mm, and an angle α between the side surface and the reflector of 1° to 5°.

3. The device according to claim 1, characterized in that The metal electrode (3) is located in the middle of the upper and lower surfaces of the semi-insulating substrate and is in the shape of a rounded triangle. It is an ohmic contact and is made of one of a Ni / Ti / Pt / Au composite metal layer, a Ti / Pt / Au composite metal layer, a W / Ti / Au composite metal layer, a Ni / Ti / Au composite metal layer or a Ni / Au composite metal layer.

4. The device according to claim 1, characterized in that The external connection electrode (4) is made of copper metal material and is in the shape of a rectangular strip with a length of 6mm-10mm, a width of 2mm-4mm, and a thickness of 0.05mm-0.1mm, covering half of the surface of the metal electrode.

5. The device according to claim 1, characterized in that The packaging material (5) is epoxy resin, which tightly covers the entire device surface and is dense and air-free.

6. A method for manufacturing a photoconductive switch device based on a triangular rounded corner, characterized in that: The steps include: S1: Semi-insulating substrate is cleaned; S2: evenly apply photoresist on the front side of the cleaned semi-insulating substrate; S3: photolithography and development are performed on the semi-insulating substrate coated with photoresist through a mask to form a plurality of chamfered triangular electrode-shaped groove areas, and metal is evaporated, sputtered, and electroplated on the front side of the semi-insulating substrate to form a metal layer; S4: stripping and removing all the metal outside the front electrode area of ​​the semi-insulating substrate to form a metal electrode; S5: Cleaning the back side of the semi-insulating substrate again; S6: evenly apply photoresist on the back side of the cleaned semi-insulating substrate; S7: photolithography and development are performed on the back side of the semi-insulating substrate coated with photoresist through a mask to form a chamfered triangular electrode-shaped groove area, and metal is evaporated, sputtered, and electroplated on the back side of the semi-insulating substrate to form a metal layer; S8: removing all the metal outside the metal electrode region on the back side of the semi-insulating substrate to form a metal electrode; S9: welding external connection electrodes to the metal electrodes on the upper and lower surfaces of the semi-insulating substrate respectively, fixing the photoconductive switch device in the packaging mold through the external connection electrodes, and then filling glue to fix the reflector to encapsulate the entire device; S10: Cooling until the epoxy resin glue solidifies, opening the packaging mold, and obtaining the photoconductive switch device.

7. The method according to claim 6, characterized in that The semi-insulating substrate wafer is cleaned by sequentially placing the semi-insulating substrate wafer into BOE solution, deionized water, acetone, isopropanol, and deionized water for ultrasonic cleaning for 120s, 180s, 600s, 180s, and 180s, respectively.

8. The method according to claim 6, characterized in that The material of the mask is made of quartz and chromium, wherein the chromium is a light-proof area and the quartz is a light-transmitting area, and the shape of the light-transmitting area is a chamfered triangle.

9. A packaging method for a triangular rounded-corner photoconductive switch device, characterized in that: The implementation steps include: (1) The packaging mold of the mortise and tenon structure is made of Teflon material, and a glue injection hole is opened on it. The mold is required to fit tightly to ensure that there is no glue leakage; (2) Fixing the position of the photoconductive switch device by using the external electrode that has been welded on the metal electrode of the device, fixing the photoconductive switch device in the middle of the mold, and tightly buckling the mold together; (3) Glue is poured through the glue injection holes of the mold. When the glue can just completely immerse the device, the reflector is placed on the side close to the device. At this time, the reflector and the flowing glue surface are required to be level and maintain a slight angle with the device. Then a small amount of packaging glue is poured to completely immerse the entire structure; (4) Evacuate the encapsulation glue, cool it and wait for it to solidify; (5) Open the mold to obtain the packaged device.

10. A mold for packaging a photoconductive switch device, comprising a bottom part (1) and a top part (2), wherein the two parts are tightly matched by a protruding tenon (11) and a recessed mortise (21) to form a mortise and tenon packaging structure, characterized in that: The bottom (1) is provided with a square groove (12) with a depth of 1.5 mm to 2.0 mm close to the bottom edge, and oblique shallow grooves (13) with a depth of 0.05 mm to 0.1 mm are provided on both sides of the groove, and the two oblique shallow grooves are both provided with an angle α with the bottom edge so that the device and the reflector maintain an angle of 1° to 5°; The top (2) is provided with a square groove (22) with a depth of 0.5mm-0.8mm close to the bottom edge, which corresponds to the square groove at the bottom.

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