A vertical 4h-sic photoconductive switch

Abstract

The application discloses a vertical 4H-SiC photoconductive switch, which comprises a SiC substrate, a front electrode, a passivation layer and a back electrode, the front electrode is located on the upper surface of the SiC substrate, the passivation layer is located on the upper surface of the SiC substrate and is arranged at the edge of the front electrode, and the back electrode is located on the lower surface of the SiC substrate; wherein the front electrode comprises a ring electrode and a mesh electrode, the mesh electrode is arranged in the inner circle of the ring electrode and is connected with the ring electrode; a plurality of terminal protection rings are further arranged in the SiC substrate below the outer side of the front electrode, and the circle centers of the longitudinal projections of the terminal protection rings coincide with the circle center of the longitudinal projection of the ring electrode. The structure design relieves the electric field concentration effect near the electrode caused by the uneven distribution of the surface electric field of the photoconductive switch, avoids the premature breakdown of the device, and doubles the withstand voltage capacity of the photoconductive switch; meanwhile, the uniformity of the internal current distribution of the photoconductive switch is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics, and particularly relates to a vertical 4H-SiC photoconductive switch. BACKGROUND

[0002] In recent years, as an important component of power electronics technology, power electronic switches are widely used in smart grids, consumer electronics, radars, aviation systems and other fields. Common switch devices include photoconductive switches, spark gaps, back light thyratrons, rectifiers, thyristors and field effect tubes, etc. Among them, photoconductive switches have been widely used in high pulse and high power systems due to their compact structure, small switching jitter, fast switching rate, small inductance and high frequency. SiC crystal, as the third generation semiconductor material, has the advantages of large band gap, high thermal conductivity and high breakdown field strength, among which the comprehensive performance of 4H-SiC is relatively outstanding among several common configurations of SiC, and it is the most promising substrate material in the field of photoconductive switches so far.

[0003] In the design of 4H-SiC photoconductive switches, the optimization of electrode structure is crucial, which directly affects the electrical, thermal characteristics and reliability of the switch. The electrode structure of the conventional 4H-SiC photoconductive switch is mainly divided into two categories: planar type and vertical type, as shown in FIG. 1, wherein the left side is a planar photoconductive switch, and the right side is a vertical photoconductive switch. The planar photoconductive switch is prone to surface breakdown because the conductive channel is distributed on the surface of the electrode side due to the shallow absorption of the trigger laser by SiC. The vertical photoconductive switch can well solve this problem and fully exert the advantage of the large critical breakdown field strength of SiC. Figure 1

[0004] However, the existing vertical 4H-SiC photoconductive switch design still has the following problems:

[0005] Firstly, the peak electric field is concentrated at the edge of the electrode, and the electric field concentration effect near the electrode under high voltage bias makes the electric breakdown advance, which greatly limits the voltage withstand capability of the photoconductive switch; secondly, the sudden peak electric field near the electrode during switching of the device causes dielectric breakdown of the ohmic contact interface, resulting in cracks on the surface of the electrode, affecting the high-temperature stability of the ohmic contact, and increasing the contact resistance; thirdly, the current concentration phenomenon near the electrode causes a large amount of Joule heat, generates thermal stress, and causes thermal breakdown, thereby causing degradation of the device. SUMMARY

[0006] In order to solve the above problems in the prior art, the application provides a vertical 4H-SiC photoconductive switch. The technical problem to be solved by the application is solved by the following technical scheme:

[0007] ​In a first aspect, the present application provides a vertical 4H-SiC photoconductive switch, comprising a SiC substrate, a front electrode, a passivation layer and a back electrode, the front electrode is located on the upper surface of the SiC substrate, the passivation layer is located on the upper surface of the SiC substrate and is arranged at the edge of the front electrode; the back electrode is located on the lower surface of the SiC substrate.

[0008] The front electrode comprises a ring electrode and a mesh electrode, the mesh electrode is arranged in the inner circle of the ring electrode and is connected with the ring electrode.

[0009] A plurality of terminal protection rings are arranged in the SiC substrate below the outer side of the front electrode, and the centers of the longitudinal projections of the plurality of terminal protection rings coincide with the center of the longitudinal projection of the ring electrode.

[0010] In an embodiment of the present application, the inner diameter of the ring electrode is 9600 μm, and the outer diameter is 10600 μm.

[0011] In an embodiment of the present application, the mesh electrode is formed by a plurality of metal wires arranged in a crisscross manner; the width of the metal wire is 30 μm, and the density of the mesh electrode is 4%-5% of the area of the inner circle of the ring electrode.

[0012] In an embodiment of the present application, the width of each of the plurality of terminal protection rings is 3 μm, and the distance from the innermost terminal protection ring to the outer circle of the ring electrode is 1 μm; the distance between adjacent two terminal protection rings is the same, or is in a segmented equal difference distribution from inside to outside.

[0013] In an embodiment of the present application, the number of the plurality of terminal protection rings is 120 or more, and is uniformly distributed with a ring distance of 1 μm.

[0014] In an embodiment of the present application, the number of the plurality of terminal protection rings is 80, and the distance between adjacent two terminal protection rings is:

[0015]

[0016] Wherein, Sn represents the distance between the nth terminal protection ring and the (n-1)th terminal protection ring.

[0017] In an embodiment of the present application, the front electrode adopts a multi-layer electrode structure in the longitudinal direction, which is Ni / Ti / Pt / Au respectively.

[0018] The back electrode adopts a multi-layer electrode structure in the longitudinal direction, which is Ni / Ti / Pt / Ag / Pt / Au respectively.

[0019] In an embodiment of the present application, a transparent conductive film is further arranged above the front electrode, and the material of the transparent conductive film is an oxide semiconductor thin film or graphene.

[0020] In one embodiment of the present application, the thickness of the SiC substrate is 500 nm, the thickness of the passivation layer is 1000 nm, and the thickness of the transparent conductive film is 362 nm.

[0021] The present application has the following advantages:

[0022] The vertical 4H-SiC photoconductive switch provided by the present application comprises a SiC substrate, a front electrode, a passivation layer and a back electrode, the front electrode is located on the upper surface of the SiC substrate, the passivation layer is located on the upper surface of the SiC substrate and is arranged at the edge of the front electrode, and the back electrode is located on the lower surface of the SiC substrate; wherein the front electrode comprises a ring electrode and a mesh electrode, the mesh electrode is arranged in the inner circle of the ring electrode and is connected with the ring electrode; a plurality of terminal protection rings are further arranged in the SiC substrate below the outer side of the front electrode, and the centers of the longitudinal projections of the plurality of terminal protection rings coincide with the center of the longitudinal projection of the ring electrode. The structure design can make each protection ring well divide voltage, thereby increasing the junction curvature radius, which greatly relieves the electrode near-field concentration effect caused by the uneven distribution of the surface electric field of the photoconductive switch, avoids premature breakdown of the device, and greatly improves the voltage resistance of the photoconductive switch; on the other hand, the front electrode structure combining the ring electrode and the mesh electrode is introduced, the electrode area is increased, the heavy frequency reliability problem caused by the excessive current density near the electrode is alleviated, the uniformity of the internal current distribution of the photoconductive switch is greatly improved while the transient output characteristics are taken into account.

[0023] 2. The vertical 4H-SiC photoconductive switch provided by the present application adopts the Ni / Ti / Pt / Au system electrode structure, which not only optimizes the ohmic contact quality, improves the thermal stability and contact reliability of the ohmic contact, but also reduces the specific contact resistance, improves the switching speed and power conversion performance.

[0024] 3. The vertical 4H-SiC photoconductive switch provided by the present application designs a high-efficiency light path structure with positive electrode anti-reflection and negative electrode anti-transmission, specially designs the thickness of the front transparent conductive film according to the refractive index and resistivity, greatly reduces the reflectivity of the laser on the surface, meets the anti-reflection and conductive requirements, and further improves the utilization efficiency of the laser, which has an absolute advantage in realizing low-energy triggering.

[0025] The present application will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a structure schematic view of a planar electrode and a vertical electrode of a conventional 4H-SiC photoconductive switch;

[0027] Figure 2is a front view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present application;

[0028] Figure 3 is a top view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present application;

[0029] Figure 4 is a structural size schematic view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present application;

[0030] Legend of reference signs:

[0031] 1-SiC substrate, 2-front electrode, 21-ring electrode, 22-mesh electrode, 3-passivation layer, 4-back electrode, 5-terminal protection ring. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0033] Research shows that the design of the electrode structure mainly needs to consider the following three problems: first, the uneven distribution of the surface electric field under high voltage bias leads to the electric field concentration effect, which makes the electric breakdown in advance and affects the voltage withstanding performance of the device; second, the peak electric field of the switch instantaneously mutates, which leads to dielectric breakdown of the ohmic contact interface, and cracks are generated on the electrode surface; third, the current concentration phenomenon causes a large amount of Joule heat, generates thermal stress, and leads to thermal breakdown. At present, the electrode structure with excellent performance under the vertical structure is based on the scheme of transparent electrode and laser directly irradiating from the electrode. The electrode shape is a ring electrode. This structure optimizes the internal light intensity distribution of the switch, but sacrifices more electrode contact area, while the mesh electrode can take into account both aspects and has better conduction performance. However, the existence of the mesh electrode also leads to the loss of the irradiation area.

[0034] Based on this, the present application provides a new vertical 4H-SiC photoconductive switch structure.

[0035] Please see Figure 2 and Figure 3 , Figure 2 is a front view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present application, Figure 3is a top view of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present application. The vertical 4H-SiC photoconductive switch provided by the present application mainly comprises a SiC substrate 1, a front electrode 2, a passivation layer 3 and a back electrode 4. The front electrode 2 is located on the upper surface of the SiC substrate 1, the passivation layer 3 is located on the upper surface of the SiC substrate 1 and is arranged at the edge of the front electrode 2, and the back electrode 4 is located on the lower surface of the SiC substrate 1.

[0036] The front electrode 2 comprises a ring-shaped electrode 21 and a mesh-shaped electrode 22. The mesh-shaped electrode 22 is arranged in the inner circle of the ring-shaped electrode 21 and is connected with the ring-shaped electrode 21.

[0037] The SiC substrate 1 below the outer side of the front electrode 2 is further provided with a plurality of terminal protection rings 5 which are concentric, and the centers of the longitudinal projections of the plurality of terminal protection rings 5 coincide with the center of the longitudinal projection of the ring-shaped electrode 21.

[0038] The structure design, on the one hand, makes each protection ring well divide voltage through the design of the plurality of terminal protection rings, thereby increasing the junction curvature radius, which greatly relieves the electric field concentration effect near the electrode caused by the uneven distribution of the electric field on the surface of the photoconductive switch, avoids the premature breakdown of the device, and greatly improves the voltage resistance of the photoconductive switch; on the other hand, the front electrode structure combining the ring-shaped electrode and the mesh-shaped electrode is introduced, the electrode area is increased, the problem of high frequency reliability caused by the excessively large current density near the electrode is alleviated, the uniformity of the internal current distribution of the photoconductive switch is greatly improved while the transient output characteristics are taken into account.

[0039] Further, please refer to Figure 4 , Figure 4 is a structure size schematic diagram of a vertical 4H-SiC photoconductive switch provided by an embodiment of the present application. The lateral size of the vertical 4H-SiC photoconductive switch designed in the embodiment is 12000 μm, and the longitudinal size is 2.35 μm. The thickness of the SiC substrate 1 is 500 nm, the thickness of the passivation layer 3 is 1000 nm, the thickness of the front electrode 2, i.e. the anode, is 450 nm, and the thickness of the back electrode, i.e. the cathode, is 850 nm.

[0040] Optionally, in the embodiment, the inner diameter of the ring-shaped electrode 21 is 9600 μm, and the outer diameter is 10600 μm.

[0041] Optionally, as an implementation manner, the mesh-shaped electrode 22 can be formed by a plurality of metal wires which are arranged in a crisscross manner, as shown in Figure 3 . The width of the metal wire is 30 μm, and the density of the mesh-shaped electrode 22 is 4%-5% of the area of the inner circle of the ring-shaped electrode 21.

[0042] The application increases the electrode area and slows down the problem of heavy frequency reliability caused by too large current density near the electrode by reasonably designing the density of the mesh electrode. The influence of the electrode density on the current transient output characteristics and the electrode surface temperature distribution is analyzed by changing the metal line width of the mesh electrode 22, and a scheme of setting the metal width to 30 mu m and the area to 5% of the inner circle is proposed, which takes into account the transient output characteristics while greatly improving the uniformity of the current distribution inside the photoconductive switch.

[0043] Further, for the terminal protection ring 5, the embodiment can realize N + The N-type terminal protection ring can be understood as that the SiC substrate 1 under the front electrode 2 is subjected to high-concentration N ion implantation to make ohmic contact, and the N + The doping concentration of the N-type terminal protection ring is the same as that of the main junction under the electrode.

[0044] Optionally, as an implementation manner, the width of the terminal protection ring 5 is designed to be 3 mu m, and the distance between the innermost terminal protection ring and the outer circle of the ring electrode 21 is 1 mu m; the distance between the adjacent two terminal protection rings is the same, or is segmentally distributed in equal difference from inside to outside.

[0045] For example, the number of the terminal protection rings 5 can be specifically set to 120 or more, and is uniformly distributed with a ring distance of 1 mu m.

[0046] For example, the number of the terminal protection rings 5 can be specifically set to 80, the distance between the adjacent two protection rings is segmentally distributed in equal difference from inside to outside, and is specifically distributed according to the following formula:

[0047]

[0048] Wherein, Sn represents the distance between the nth terminal protection ring and the n-1th terminal protection ring.

[0049] After the vertical type 4H-SiC photoconductive switch designed in the embodiment introduces the N+-N terminal protection ring, as the applied voltage of the device increases, the space charge region expands to connect the depletion regions between the first ring and the main junction, the curvature radius of the curved junction is increased, and the voltage withstanding capacity of the device is improved. Similarly, the same result will occur between the protection rings until the last ring.

[0050] In addition, the distance between the terminal protection ring and the main junction is designed to be 1 mu m, the width of the protection ring is designed to be 3 mu m, the number of the protection rings is 80, and the protection rings are segmentally distributed in equal difference. While taking into account the process complexity and the size of the device, the voltage withstanding capacity is greatly improved.

[0051] Further, for the front electrode 2 and the back electrode 4, the mainstream metals for preparing the SiC ohmic contact at present are Ni, Ti, Al, TiC, TiN, TiW and other metals and metal combinations thereof, and the specific contact resistance value of the n-type 4H-SiC ohmic contact can reach 1*10 -6 Ω·cm 2 .

[0052] In order to improve the quality of the ohmic contact and relieve the interface dielectric breakdown, the front electrode 2 of the embodiment adopts a multi-layer electrode structure in the longitudinal direction, which is Ni / Ti / Pt / Au, and the thicknesses are 200 / 50 / 100 / 100, respectively, in nm.

[0053] The electrode structure of the Ni / Ti / Pt / Au system adopted by the embodiment not only optimizes the quality of the ohmic contact, improves the thermal stability and contact reliability of the ohmic contact, but also reduces the specific contact resistance, improves the switching speed and power conversion performance.

[0054] In addition, considering that air breakdown is prone to occur at the edge of the electrode, the embodiment increases a silicon dioxide passivation layer at the edge of the electrode for insulation protection, and a new type of nano-silver composite solder paste doped with inorganic and reinforced with fibers is coated on the three-phase point of the device during packaging. The thickness of the passivation layer can be set to 1000 nm in general.

[0055] Further, the embodiment further provides a transparent conductive film (not shown in the figure) above the front electrode 2, and the material of the transparent conductive film is an oxide semiconductor thin film or graphene.

[0056] Optionally, the transparent conductive film can adopt an oxide semiconductor thin film such as AZO (Al-doped ZnO material), ITO (In2O3(SnO2), tin-doped indium oxide), SGO (Ga2O3(SnO2), tin-doped gallium oxide) and the like.

[0057] Taking the AZO conductive thin film as an example, researches show that the transmittance of the AZO thin film is greater than 95% near the wavelength of 532 nm, and the AZO thin film has an anti-reflection and anti-increase function, which can make more light waves enter the substrate; and when the thickness of the AZO thin film is greater than 300 nm, the resistivity is as low as 5.6*10-6Ω·cm, and the conductive capacity approaches that of a metal material, which can meet the conductive requirement of the electrode. Therefore, the AZO thin film with a thickness of 362 nm is magnetron sputtered on the electrode based on the incident laser trigger wavelength of 532 nm, the maximum transmittance function of the thin film is maximized, and the laser energy is more effectively utilized.

[0058] In summary, the vertical 4H-SiC photoconductive switch designed in the application has the following beneficial effects:

[0059] 1、The application increases the curvature radius of the junction by designing the distribution of the terminal protection ring, so that each protection ring can well divide the voltage, thereby greatly relieving the electric field concentration effect near the electrode caused by the uneven distribution of the electric field on the surface of the photoconductive switch, avoiding premature breakdown of the device, and doubling the voltage resistance of the photoconductive switch.

[0060] 2、The application designs a high-efficiency light path structure with positive electrode antireflection and negative electrode reflection, and specially designs the thickness of the front transparent conductive film according to the refractive index and resistivity, greatly reduces the reflectivity of the laser on the surface, meets the requirements of antireflection and conductivity, and further improves the utilization efficiency of the laser, which has an absolute advantage in realizing low-energy triggering.

[0061] 3、The application optimizes the ohmic contact quality, reduces the specific contact resistance, thereby improves the switching speed and power conversion performance, and makes the device have good high-frequency reliability, high-temperature reliability and ablation resistance.

[0062] 4、The application greatly improves the uniformity of the current distribution inside the photoconductive switch by considering the transient output characteristics while reasonably designing the density of the mesh electrode and the line width of the metal lines between the mesh electrodes.

[0063] In the description of the application, it should be understood that the orientations or positional relationships indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise" and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the application.

[0064] In the application, unless otherwise explicitly specified and limited, "on" or "under" of a first feature to a second feature can include that the first and second features are in direct contact, or that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, "on", "above" and "above" of the first feature to the second feature include that the first feature is directly above and obliquely above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The first feature "below", "below" and "below" of the second feature include that the first feature is directly below and obliquely below the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature.

[0065] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the specification, the illustrative description of the above terms does 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 one or more embodiments or examples. In addition, those skilled in the art can combine and combine different embodiments or examples described in the specification.

[0066] The above is a further detailed description of the present application in combination with specific preferred embodiments, and cannot be considered as limiting the specific implementation of the present application to these descriptions. For those skilled in the art, without departing from the concept of the present application, a number of simple deductions or substitutions can be made, which should be considered as falling within the protection scope of the present application.

Claims

1. A vertical 4H-SiC photoconductive switch, characterized by, The SiC substrate (1), the front electrode (2), the passivation layer (3) and the back electrode (4), the front electrode (2) is located on the upper surface of the SiC substrate (1), the passivation layer (3) is located on the upper surface of the SiC substrate (1) and is arranged at the edge of the front electrode (2); the back electrode (4) is located on the lower surface of the SiC substrate (1); Wherein, the front electrode (2) comprises a ring electrode (21) and a mesh electrode (22), the mesh electrode (22) is arranged in the inner circle of the ring electrode (21) and is connected with the ring electrode (21); The SiC substrate (1) below the outer side of the front electrode (2) is also provided with a plurality of terminal protection rings (5) concentrically, and the centers of the longitudinal projections of the plurality of terminal protection rings (5) coincide with the center of the longitudinal projection of the ring electrode (21).

2. A vertical 4H-SiC photoconductive switch according to claim 1, wherein The inner diameter of the ring electrode (21) is 9600μm, and the outer diameter is 10600μm.

3. A vertical 4H-SiC photoconductive switch according to claim 1, wherein The mesh electrode (22) is formed by a plurality of metal wires intersecting longitudinally and transversely; the width of the metal wire is 30μm, and the density of the mesh electrode (22) is 4%-5% of the area of the inner circle of the ring electrode (21).

4. A vertical 4H-SiC photoconductive switch according to claim 1, wherein The width of the plurality of terminal protection rings (5) is 3μm, and the distance from the innermost terminal protection ring to the outer circle of the ring electrode (21) is 1μm; the distance between adjacent two terminal protection rings is the same, or is segmentally distributed in equal difference from inside to outside.

5. A vertical 4H-SiC photoconductive switch according to claim 4, wherein The number of the plurality of terminal protection rings (5) is 120 or more, and is uniformly distributed with a ring distance of 1μm.

6. A vertical 4H-SiC photoconductive switch as claimed in claim 4, wherein, The number of the plurality of terminal protection rings (5) is 80, and the distance between adjacent two protection rings is: Wherein, Sn represents the distance between the nth terminal protection ring and the (n-1)th terminal protection ring.

7. A vertical 4H-SiC photoconductive switch as claimed in claim 1, wherein, The material of the passivation layer (3) is silicon dioxide.

8. A vertical 4H-SiC photoconductive switch as in Claim 1, wherein, The front electrode (2) adopts a multilayer electrode structure in the longitudinal direction, which is Ni / Ti / Pt / Au respectively. The back electrode (4) adopts a multilayer electrode structure in the longitudinal direction, which is Ni / Ti / Pt / Ag / Pt / Au respectively.

9. A vertical 4H-SiC photoconductive switch as in Claim 1, wherein, The front electrode (2) is also provided with a transparent conductive film, and the material of the transparent conductive film is an oxide semiconductor film or graphene.

10. A vertical 4H-SiC photoconductive switch as claimed in claim 9, characterized by The thickness of the SiC substrate (1) is 500nm, the thickness of the passivation layer (3) is 1000nm, and the thickness of the transparent conductive film is 362nm.

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